Internet Warrior 1993 July/Internet Warrior No. 1 July 1993 - Austin Code Works.ISO/docs/tcpip/admin.doc

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                             Introduction
                                  to
                            Administration
                                of an
                            Internet-based
                            Local Network



                      C                       R

                              C       S
                  Computer Science Facilities Group
                              C       I

                      L                       S


                               RUTGERS
                  The State University of New Jersey
            Center for Computers and Information Services
               Laboratory for Computer Science Research


                             24 July 1988

This  is an introduction for people who intend to set up or administer
a network based on the Internet networking protocols (TCP/IP).

Copyright (C) 1988, Charles L. Hedrick.   Anyone  may  reproduce  this
document,  in  whole  or  in  part,  provided  that:   (1) any copy or
republication of the entire document must show Rutgers  University  as
the  source,  and  must  include this notice; and (2) any other use of
this material must reference this manual and Rutgers  University,  and
the fact that the material is copyright by Charles Hedrick and is used
by permission.



Unix is a trademark of AT&T Technologies, Inc.



                          Table of Contents


   1. The problem                                                    1
   2. Routing and Addressing                                         2
   3. Choosing an addressing structure                               3
       3.1 Should you subdivide your address space?                  5
       3.2 Subnets vs. multiple network numbers                      5
       3.3 How to allocate subnet or network numbers                 6
           3.3.1 Dealing with multiple "virtual" subnets  on  one    7
                 network
       3.4 Choosing an address class                                 8
   4. Setting up routing for an individual computer                  9
       4.1 How datagrams are routed                                 11
       4.2 Fixed routes                                             13
       4.3 Routing redirects                                        14
       4.4 Other ways for hosts to find routes                      16
           4.4.1 Spying on Routing                                  16
           4.4.2 Proxy ARP                                          17
           4.4.3 Moving to New Routes After Failures                22
   5. Bridges and Gateways                                          24
       5.1 Alternative Designs                                      25
           5.1.1 A mesh of point to point lines                     26
           5.1.2 Circuit switching technology                       27
           5.1.3 Single-level networks                              27
           5.1.4 Mixed designs                                      28
       5.2 An introduction to alternative switching technologies    29
           5.2.1 Repeaters                                          29
           5.2.2 Bridges and gateways                               30
           5.2.3 More about bridges                                 32
           5.2.4 More about gateways                                34
       5.3 Comparing the switching technologies                     34
           5.3.1 Isolation                                          35
           5.3.2 Performance                                        36
           5.3.3 Routing                                            37
           5.3.4 Network management                                 39
           5.3.5 A final evaluation                                 41
       5.4 Configuring routing for gateways                         44

















                                  i



This  document is intended to help people who planning to set up a new
network based on the Internet protocols, or to administer an  existing
one.    It  assumes  a  basic  familiarity  with the TCP/IP protocols,
particularly the structure of Internet addresses.  A companion  paper,
"Introduction  to  the  Internet  Protocols", may provide a convenient
introduction.  This document does not  attempt  to  replace  technical
documentation  for  your  specific  TCP/IP implementation.  Rather, it
attempts to give overall  background  that  is  not  specific  to  any
particular implementation.  It is directed specifically at networks of
"medium" complexity.  That  is,  it  is  probably  appropriate  for  a
network  involving  several dozen buildings.  Those planning to manage
larger networks will need more preparation than you can get by reading
this document.

In  a  number  of  cases,  commands  and output from Berkeley Unix are
shown.  Most computer  systems  have  commands  that  are  similar  in
function to these.  It seemed more useful to give some actual examples
that to limit myself to general talk, even if the  actual  output  you
see is slightly different.



1. The problem


This document will emphasize primarily "logical" network architecture.
There are many documents and articles in the trade press that  discuss
actual  network  media,  such  as  Ethernet, Token Ring, etc.  What is
generally not made clear in these  articles  is  that  the  choice  of
network  media  is  generally  not  all  that critical for the overall
design of a network.  What can be done by  the  network  is  generally
determined more by the network protocols supported, and the quality of
the implementations.  In practice, media are normally chosen based  on
purely  pragmatic  grounds: what media are supported by the particular
types of computer that you have to connect.  Generally this means that
Ethernet is used for medium-scale systems, Ethernet or a network based
on twisted-pair wiring for micro networks, and specialized  high-speed
networks  (typically  token  ring)  for campus-wide backbones, and for
local  networks  involving  super-computer  and   other   very   high-
performance applications.

Thus  this  document  assumes  that  you  have  chosen  and  installed
individual networks such as Ethernet or token ring,  and  your  vendor
has  helped  you connect your computers to these network.  You are now
faced with the interrelated problems of

   - configuring the software on your computers

   - finding a way to connect individual Ethernets, token rings, etc.,
     to form a single coherent network

   - connecting your networks to the outside world

My  primary  thesis in this document is that these decisions require a
bit  of  advance  thought.    In   fact,   most   networks   need   an
                                  1



"architecture".   This consists of a way of assigning addresses, a way
of doing routing, and various choices about how  hosts  interact  with
the  network.  These decisions need to be made for the entire network,
preferably when it is first being installed.



2. Routing and Addressing


Many of the decisions that you need  to  make  in  setting  up  TCP/IP
depend upon routing, so it will be best to give a bit of background on
that topic now.  I will return to routing  in  a  later  section  when
discussing  gateways  and  bridges.    In  general,  IP datagrams pass
through many networks while they are  going  between  the  source  and
destination.    Here's  a  typical  example.    (Addresses used in the
examples are taken from Rutgers University.)

              network 1               network 2     network 3
               128.6.4                 128.6.21      128.121
        ============================  ==========  ================
          |              |        |    |      |    |         |
       ___|______   _____|____  __|____|__  __|____|____  ___|________
       128.6.4.2    128.6.4.3   128.6.4.1   128.6.21.1    128.121.50.2
                                128.6.21.2  128.121.50.1
       __________   __________  __________  ____________  ____________
       computer A   computer B   gateway R    gateway S    computer C


This diagram shows three normal computer systems,  two  gateways,  and
three  networks.  The networks might be Ethernets, token rings, or any
other sort.  Network 2 could even be a  single  point  to  point  line
connecting gateways R and S.

Note  that computer A can send datagrams to computer B directly, using
network 1.  However it can't reach computer  C  directly,  since  they
aren't  on  the  same  network.    There  are  several ways to connect
separate networks.  This diagram assumes that gateways are used. (In a
later section, we'll look at an alternative.)  In this case, datagrams
going between A and C must be sent through gateway R, network  2,  and
gateway   S.   Every  computer  that  uses  TCP/IP  needs  appropriate
information and algorithms to allow it to know when datagrams must  be
sent through a gateway, and to choose an appropriate gateway.

Routing  is  very  closely tied to the choice of addresses.  Note that
the address of each computer begins with the  number  of  the  network
that  it's  attached  to.    Thus  128.6.4.2 and 128.6.4.3 are both on
network 128.6.4.  Next, notice that gateways, whose job is to  connect
networks,  have  an  address  on each of those networks.  For example,
gateway R connects networks 128.6.4 and 128.6.21.  Its  connection  to
network  128.6.4 has the address 128.6.4.1.  Its connection to network
128.6.21 has the address 128.6.21.2.

Because of this association between addresses  and  networks,  routing
decisions  can  be  based  strictly  on  the  network  number  of  the
                                  2



destination.  Here's what the routing information for computer A might
look like:

       network    gateway     metric

       128.6.4    none        0
       128.6.21   128.6.4.1   1
       128.121    128.6.4.1   2

From  this  table, computer A can tell that datagrams for computers on
network 128.6.4 can be sent directly, and datagrams for  computers  on
networks  128.6.21  and  128.121  need  to  be  sent  to gateway R for
forwarding.  The "metric" is used by  some  routing  algorithms  as  a
measure  of how far away the destination is.  In this case, the metric
simply indicates how many gateways the datagram  has  to  go  through.
(This is often referred to as a "hop count".)

When  computer  A  is  ready  to  send  a  datagram,  it  examines the
destination address.  The network number is taken from  the  beginning
of  the  address  and looked up in the routing table.  The table entry
indicates  whether  the  packet  should  be  sent  directly   to   the
destination or to a gateway.

Note  that  a  gateway  is  simply a computer that is connected to two
different networks, and is prepared to forward packets  between  them.
In  many  cases  it is most efficient to use special-purpose equipment
designed for use as a gateway.  However it is  perfectly  possible  to
use ordinary computers as gateways, as long as they have more than one
network  interface,  and  their  software  is  prepared   to   forward
datagrams.       Most   major   TCP/IP   implementations   (even   for
microcomputers) are designed  to  let  you  use  your  computer  as  a
gateway.  However some of this software has limitations that can cause
trouble for your network.



3. Choosing an addressing structure


The first comment to make about addresses is  a  warning:  Before  you
start  using  a  TCP/IP  network,  you  must  get one or more official
network numbers.  TCP/IP addresses look like this:  128.6.4.3.    This
address is used by one computer at Rutgers University.  The first part
of it, 128.6, is a network number, allocated to Rutgers by  a  central
authority.    Before you start allocating addresses to your computers,
you must get an official network number.  Unfortunately,  some  people
set  up  networks  using  either a randomly-chosen number, or a number
taken from examples in vendor documentation.  While this may  work  in
the  short  run,  it is a very bad idea for the long run.  Eventually,
you will want to connect your network  to  some  other  organization's
network.    Even  if  your  organization  is  highly  secret  and very
concerned about security, somewhere  in  your  organization  there  is
going  to  be  a  research  computer that ends up being connected to a
nearby university.  That university will probably be  connected  to  a
large-scale  national  network.    As  soon  as  one of your datagrams
                                  3



escapes your local network, the organization you  are  talking  to  is
going  to  become  very confused, because the addresses that appear in
your datagrams are probably officially allocated to someone else.

The solution to this is simple: get your own network number  from  the
beginning.    It  costs nothing.  If you delay it, then sometime years
from now you are going to be faced with  the  job  of  changing  every
address on a large network.  Network numbers are currently assigned by
the DDN Network Information Center, SRI International, 333  Ravenswood
Avenue,  Menlo  Park, California 94025 (telephone: [phone]).  You
can get a network number no matter what your  network  is  being  used
for.    You  do  not need authorization to connect to the Defense Data
Network in order to get a number.  The main piece of information  that
will  be  needed  when  you apply for a network number is that address
class that you want.  See below for a discussion of this.

In many ways, the most important decision you have to make in  setting
up  a  network  is  how  you  will  assign  Internet addresses to your
computers.  This choice should be made with a view of how your network
is  likely  to grow.  Otherwise, you will find that you have to change
addresses.  When you have several hundred computers,  address  changes
can be nearly impossible.

Addresses  are  critical  because Internet datagrams are routed on the
basis of their address.  For example, addresses at Rutgers  University
have  a  2-level structure.  A typical address is 128.6.4.3.  128.6 is
assigned to Rutgers University by a central authority.  As far as  the
outside  world  is  concerned,  128.6  is  a  single  network.   Other
universities send any packet whose address begins with  128.6  to  the
nearest  Rutgers  gateway.    However within Rutgers, we divide up our
address space into "subnets".  We use the next 8 bits  of  address  to
indicate  which  subnet  a  computer belongs to.  128.6.4.3 belongs to
subnet 128.6.4.  Generally subnets correspond  to  physical  networks,
e.g.  separate  Ethernets,  although as we will see later there can be
exceptions.  Systems inside Rutgers,  unlike  those  outside,  contain
information  about the Rutgers subnet structure.  So once a packet for
128.6.4.3 arrives at Rutgers, the Rutgers network will route it to the
departmental Ethernet, token ring, or whatever, that has been assigned
subnet number 128.6.4.

When you start a network, there are several addressing decisions  that
face you:

   - Do you subdivide your address space?

   - If so, do you use subnets or class C addresses?

   - You do you allocate subnets or class C networks?

   - How big an address space do you need?





                                  4



3.1 Should you subdivide your address space?


It  is  not  necessary  to  use  subnets  at  all.   There are network
technologies that allow an entire campus or company to act as a single
large  logical Ethernet, so that no internal routing is necessary.  If
you use this technology, then  you  do  not  need  to  subdivide  your
address  space.    In that case, the only decision you have to make is
what class address to apply for.  However we recommend using either  a
subnet approach or some other method of subdividing your address space
in all cases:

   - In section 5.2 we will argue that internal gateways are desirable
     for networks of any degree of complexity.

   - Even if you do not need gateways now, you may find later that you
     need to  use  them.  Thus  it  probably  makes  sense  to  assign
     addresses  as  if  each  Ethernet or token ring was going to be a
     separate subnet.  This will allow for conversion to real  subnets
     later if it proves necessary.

   - For  network  maintenance  purposes,  it  is  convenient  to have
     addresses whose structure corresponds to  the  structure  of  the
     network.    That  is,  when  you  see  a stray packet from system
     128.6.4.3, it is nice to know that all addresses  beginning  with
     128.6.4 are in a particular building.



3.2 Subnets vs. multiple network numbers


Suppose  that  you have been convinced that it's a good idea to impose
some structure on your addresses.  The  next  question  is  what  that
structure should be.  There are two basic approaches.  One is subnets.
The other is multiple network numbers.

The Internet standards specify what constitutes a network number.  For
numbers  beginning with 128 through 191 (the most common numbers these
days), the first  two  octets  form  the  network  number.    E.g.  in
140.3.50.1, 140.3 is the network number.  Network numbers are assigned
to a particular organization.  What you do with the next two octets is
up  to  you.    You  could  choose  to make the next octet be a subnet
number, or you could use some other scheme entirely.  Gateways  within
your  organization  will  be set up to know the subnetting scheme that
you are using.  However outside your organization, no  one  will  know
that 140.3.50 is one subnet and 140.3.51 is another.  They will simply
know that 140.3 is your organization.  Unfortunately, this ability  to
add additional structure to the address via subnets was not present in
the original TCP/IP specifications.  Thus some software  is  incapable
of being told about subnets.

If  enough of the software that you are using has this problem, it may
be impractical for you to use subnets.  Some organizations have used a
different  approach.   It is possible for an organization to apply for
                                  5



several network numbers.  Instead of dividing a single network number,
say  140.3,  into  several subnets, e.g. 140.3.1 through 140.3.10, you
could apply for 10 different network  numbers.    Thus  you  might  be
assigned  the  range  140.3  through 140.12.  All TCP/IP software will
know that these are different network numbers.

While using separate network numbers will work just fine  within  your
organization,  it  has two very serious disadvantages.  The first, and
less serious, is that it wastes address space.  There are  only  about
16,000  possible  class  B addresses.  We cannot afford to waste 10 of
them on your organization, unless it is very large.  This objection is
less  serious because you would normally ask for class C addresses for
this  purpose,  and  there  are  about  2  million  possible  class  C
addresses.

The  more  serious  problem  with using several network numbers rather
than subnets is that it overloads the routing tables in  the  rest  of
the Internet.  As mentioned above, when you divide your network number
into subnets, this division is known within your organization, but not
outside  it.    Thus  systems  outside your organization need only one
entry in their tables in order to be able to reach you.    E.g.  other
universities  have entries in their routing tables for 128.6, which is
the Rutgers network number.  If you use a  range  of  network  numbers
instead  of  subnets,  that  division  will  be  visible to the entire
Internet.  If we used 128.6  through  128.16  instead  of  subdividing
128.6, other universities would need entries for each of those network
numbers in their routing tables.   As  of  this  writing  the  routing
tables  in many of the national networks are exceeding the size of the
current  routing  technology.    It  would  be  considered   extremely
unfriendly  for  any organization to use more than one network number.
This may not be a problem if your network is going  to  be  completely
self-contained,  or if only one small piece of it will be connected to
the  outside  world.    Nevertheless,  most  TCP/IP  experts  strongly
recommend  that  you  use  subnets rather than multiple networks.  The
only reason for considering multiple networks is to deal with software
that  cannot  handle subnets.  This was a problem a few years ago, but
is currently less serious.   As  long  as  your  gateways  can  handle
subnets,  you  can deal with a few individual computers that cannot by
using "proxy ARP" (see below).



3.3 How to allocate subnet or network numbers


Now that you have decided to use subnets or multiple network  numbers,
you  have  to  decide  how  to allocate them.  Normally this is fairly
easy.  Each physical network, e.g. Ethernet or token ring, is assigned
a  separate  subnet  or  network  number.    However  you do have some
options.

In some cases it may make sense to assign several subnet numbers to  a
single  physical  network.   At Rutgers we have a single Ethernet that
spans three buildings, using repeaters.  It is very clear to  us  that
as  computers  are  added  to this Ethernet, it is going to have to be
                                  6



split into several separate Ethernets.  In order to  avoid  having  to
change  addresses when this is done, we have allocated three different
subnet numbers to this Ethernet, one per building.    (This  would  be
handy  even  if  we didn't plan to split the Ethernet, just to help us
keep track of where computers are.)  However before doing  this,  make
very  sure  that  the  software  on all of your computers can handle a
network that has three different network numbers on it.

You also have to choose a "subnet mask".  This is used by the software
on  your  systems to separate the subnet from the rest of the address.
So far we have always assumed  that  the  first  two  octets  are  the
network  number, and the next octet is the subnet number.  For class B
addresses, the standards specify that the first  two  octets  are  the
network  number.    However we are free to choose the boundary between
the subnet number and the rest of the address.  It's  very  common  to
have  a  one-octet  subnet  number,  but  that's not the only possible
choice.  Let's look again at a class B address, e.g. 128.6.4.3.  It is
easy to see that if the third octet is used for a subnet number, there
are 256 possible subnets and within each subnet there are 256 possible
addresses.    (Actually,  the  numbers  are more like 254, since it is
generally a bad idea to use 0 or 255 for subnet numbers or addresses.)
Suppose you know that you will never have more than 128 computers on a
given subnet, but you are afraid you might need more than 256 subnets.
(For  example,  you might have a campus with lots of small buildings.)
In that case, you could define 10 bits for the subnet number,  leaving
6  bits for addresses within each subnet.  This choice is expressed by
a bit mask, using ones for the bits used by  the  network  and  subnet
number,  and  0's  for  the  bits  used for individual addresses.  Our
normal subnet choice is given as 255.255.255.0.  If we  chose  10  bit
subnet  numbers  and  6  bit  addresses,  the  subnet  mask  would  be
255.255.255.192.

Generally it is possible to specify the subnet mask for each  computer
as part of configuring its TCP/IP software.  The TCP/IP protocols also
allow for computers to send a query asking what the  subnet  mask  is.
If  your network supports broadcast queries, and there is at least one
computer or gateway on the network that knows the subnet mask, it  may
be  unnecessary  to  set  it on the other computers.  (This capability
brings with it a whole new set of possible problems.   One  well-known
TCP/IP  implementation  would  answer  with the wrong subnet mask when
queried, thus leading causing every other computer on the  network  to
be misconfigured.)



3.3.1 Dealing with multiple "virtual" subnets on one network


Most  software  is written under the assumption that every computer on
the local network has the same subnet number.  When traffic  is  being
sent  to  a  machine with a different subnet number, the software will
generally expect to find  a  gateway  to  handle  forwarding  to  that
subnet.  Let's look at the implications.  Suppose subnets 128.6.19 and
128.6.20 are on the same Ethernet.  Consider the way things look  from
the point of view of a computer with address 128.6.19.3.  It will have
                                  7



no problem sending to other machines with addresses 128.6.19.x.   They
are on the same subnet, and so our computer will know to send directly
to them on the local Ethernet.  However suppose it is asked to send  a
packet to 128.6.20.2.  Since this is a different subnet, most software
will expect to find a gateway that handles forwarding between the  two
subnets.  Of course there isn't a gateway between subnets 128.6.19 and
128.6.20, since they are on the  same  Ethernet.    Thus  it  must  be
possible  to  tell your software that 128.6.20 is actually on the same
Ethernet.

For the most common TCP/IP implementations, it  is  possible  to  deal
with  more  than  one subnet on a network.  For example, Berkeley Unix
allows you to define gateways using a command "route  add".    Suppose
that  you  get  from subnet 128.6.19 to subnet 128.6.4 using a gateway
whose address is 128.6.19.1.  You would use the command

  route add 128.6.4.0 128.6.19.1 1

This says that to reach subnet 128.6.4, traffic should be sent via the
gateway  at  128.6.19.1, and that the route only has to go through one
gateway.  The "1" is referred to as the "routing metric".  If you  use
a  metric  of  0, you are saying that the destination subnet is on the
same network, and no gateway is needed.  In  our  example,  on  system
128.6.19.3, you would use

  route add 128.6.20.0 128.6.19.1 0

The  actual  address  used  in place of 128.6.19.1 is irrelevant.  The
metric of 0 says that no gateway is actually going to be used, so  the
gateway  address  is  not used.  However it must be a legal address on
the local network.

Note that the commands in this section are simply examples. You should
look  in  the  documentation for your particular implementation to see
how to configure your routing.



3.4 Choosing an address class


When you apply for an official network number, you will be asked  what
class  of network number you need.  The possible answers are A, B, and
C. This affects how large an address  space  you  will  be  allocated.
Class  A addresses are one octet long, class B addresses are 2 octets,
and class C addresses are 3  octets.    This  represents  a  tradeoff:
there are a lot more class C addresses than class A addresses, but the
class C addresses don't allow as many hosts.  The idea was that  there
would  be  a few very large networks, a moderate number of medium-size
ones, and a lot of mom-and-pop stores that would have small  networks.
Here is a table showing the distinction:

   class  range of first octet   network   rest  possible addresses
     A       1 - 126               p      q.r.s    16777214
     B       128 - 191             p.q      r.s    65534
                                  8



     C       192 - 223             p.q.r      s    254

For  example  network  10,  a  class  A network, has addresses between
10.0.0.1 and 10.255.255.254.  So it allows 254**3, or about 16 million
possible  addresses.    (Actually,  network 10 has allocated addresses
where some of the octets are zero, so there are a  few  more  networks
possible.)    Network  192.12.88,  a class C network has hosts between
192.12.88.1 and 128.12.88.254, i.e. 254 possible hosts.

In general, you will be expected to choose the lowest class that  will
provide  you with enough addresses to handle your growth over the next
few years.  In general  organizations  that  have  computers  in  many
buildings  will  probably  need  and be able to get a class B address,
assuming that they are going to use subnetting.  (If you are going  to
use many separate network numbers, you would ask for a number of class
C addresses.)  Class A addresses are  normally  used  only  for  large
public networks and for a few very large corporate networks.



4. Setting up routing for an individual computer


All  TCP/IP  implementations require some configuration for each host.
In some cases this is done in a "system generation".  In other  cases,
various  startup and configuration files must be set up on the system.
Still other systems get configuration information across  the  network
from  a  "server".    While  the  details  differ,  the  same kinds of
information need to  be  supplied  for  most  implementations.    This
includes

   - parameters  describing the specific machine, such as its Internet
     address.

   - parameters describing the network, such as the  subnet  mask  (if
     any)

   - routing software and the tables that drive it

   - startup of various programs needed to handle network tasks

Before  a  machine  is installed on your network, a coordinator should
assign it a host name and Internet address.  If the machine  has  more
than  one  network  interface,  you  must  assign  a separate Internet
address for each.  (In most cases, the same host  name  can  be  used.
The  name  goes  with  the  machine as a whole, whereas the address is
associated with the connection to a specific network.)    The  address
should begin with the network number for the network to which it is to
be attached.  We recommend that you assign addresses starting from  1.
Should  you  find  that you need more subnets than your current subnet
mask allows, you may later need to expand the subnet mask to use  more
bits.  If all addresses use small numbers, this will be possible.

Generally  the  Internet  address  must be specified individually in a
configuration  file  on  each  computer.    However   some   computers
                                  9



(particularly  those  without  permanent  disks on which configuration
information could be  stored)  find  out  their  Internet  address  by
sending  a broadcast request over the network.  In that case, you will
have to make sure that some other system is configured to  answer  the
request.    When  a  system  asks  for  its  Internet  address, enough
information must be put into the request to allow  another  system  to
recognize  it  and  to  send  a  response back.  For Ethernet systems,
generally the  request  will  include  the  Ethernet  address  of  the
requesting  system.    Ethernet addresses are assigned by the computer
manufacturers, and are guaranteed to be unique.  Thus they are a  good
way  of  identifying the computer.  And of course the Ethernet address
is also needed in order to send the response back.  If it is  used  as
the basis for address lookup, the system that is to answer the request
will need a table of Ethernet addresses and the corresponding Internet
address.   The only problem in constructing this table will be finding
the Ethernet address for each  computer.    Generally,  computers  are
designed  so  that  they  print  the  Ethernet  address on the console
shortly after being turned on.  However in some cases you may have  to
type a command that displays information about the Ethernet interface.

Generally  the subnet mask should be specified in a configuration file
associated with the computer.    (For  Unix  systems,  the  "ifconfig"
command is used to specify both the Internet address and subnet mask.)
However there are provisions in the IP protocols  for  a  computer  to
broadcast a request asking for the subnet mask.  The subnet mask is an
attribute of the network.  That is, it is the same for  all  computers
on   a  given  subnet.    Thus  there  is  no  separate  subnet  table
corresponding to the Ethernet/Internet address mapping table  used  to
answer  address  queries.    Generally any machine on the network that
believes it knows the subnet mask will  answer  any  query  about  the
subnet mask.  For that reason, an incorrect subnet mask setting on one
machine can cause confusion throughout the network.

Normally the configuration files do roughly the following things:

   - enable each of the network interfaces (Ethernet interface, serial
     lines,  etc.)    Normally  this  involves  specifying an Internet
     address and subnet mask for each, as well as other  options  that
     will be described in your vendor's documentation.

   - establish  network  routing  information, either by commands that
     add fixed routes, or by starting  a  program  that  obtains  them
     dynamically.

   - turn  on  the  name server (used for looking up names and finding
     the corresponding Internet address --  see  the  section  on  the
     domain system in the Introduction to TCP/IP).

   - set various other information needed by the system software, such
     as the name of the system itself.

   - start various "daemons".  These are programs that provide network
     services  to  other  systems on the network, and to users on this
     system.

                                  10



It is not practical to document these  steps  in  detail,  since  they
differ for each vendor.  This section will concentrate on a few issues
where your choice will depend upon overall decisions  about  how  your
network  is  to  operate.   These overall network policy decisions are
often not as well documented by the vendors as the details of  how  to
start  specific  programs.    Note that some care will be necessary to
integrate commands that you add for routing, etc.,  into  the  startup
sequence  at  the  right  point.  Some of the most mysterious problems
occur when network routing is not set up before  a  program  needs  to
make  a  network  query,  or when a program attempts to look up a host
name before the name server has finished loading all of the names from
a master name server.



4.1 How datagrams are routed


If your system consists of a single Ethernet or similar medium, you do
not need to give routing much attention.   However  for  more  complex
systems,  each  of  your  machines  needs a routing table that lists a
gateway and interface to use for every possible  destination  network.
A  simple  example of this was given at the beginning of this section.
However it is now necessary to describe the way routing works in a bit
more  detail.  On most systems, the routing table looks something like
the following. (This example was taken from a system running  Berkeley
Unix,  using  the  command  "netstat  -n -r".  Some columns containing
statistical information have been omitted.)

    Destination          Gateway              Flags       Interface

    128.6.5.3            128.6.7.1            UHGD        il0
    128.6.5.21           128.6.7.1            UHGD        il0
    127.0.0.1            127.0.0.1            UH          lo0
    128.6.4              128.6.4.61           U           pe0
    128.6.6              128.6.7.26           U           il0
    128.6.7              128.6.7.26           U           il0
    128.6.2              128.6.7.1            UG          il0
    10                   128.6.4.27           UG          pe0
    128.121              128.6.4.27           UG          pe0
    default              128.6.4.27           UG          pe0

The example system is connected to two Ethernets:

      controller  network   address     other networks
         il0      128.6.7   128.6.7.26    128.6.6
         pe0      128.6.4   128.6.4.61    none

The first column shows the designation  for  the  controller  hardware
that  connects the computer to that Ethernet.  (This system happens to
have controllers from two different vendors.  The first one is made by
Interlan,  the  second  by Pyramid.)  The second column is the network
number for the network.  The third column is this computer's  Internet
address  on  that  network.   The last column shows other subnets that
share the same physical network.
                                  11



Now let's look at the routing table.  For the moment,  let  us  ignore
the  first  3  lines.   The majority of the table consists of a set of
entries describing networks.    For  each  network,  the  other  three
columns  show  where  to send datagrams destined for that network.  If
the "G" flag is present  in  the  third  column,  datagrams  for  that
network  must  be sent through a gateway.  The second column shows the
address of the gateway to be used.  If the "G" flag  is  not  present,
the  computer  is  directly  connected to the network in question.  So
datagrams for that network should be sent using the  controller  shown
in  the  third  column.    The  "U"  flag  in  the third column simply
indicates that the route specified by that line is up,  i.e.  that  no
errors have occured indicating that the path is unusable.

The  first  3  lines  show "host routes", indicated by the "H" flag in
column three.    Routing  tables  normally  have  entries  for  entire
networks or subnets.  For example, the entry

    128.6.2              128.6.7.1            UG          il0

indicates  that  datagrams  for  any computer on network 128.6.2 (i.e.
addresses 128.6.2.1 through 128.6.2.254) should  be  sent  to  gateway
128.6.7.1  for  forwarding.   However sometimes routes apply only to a
specific computer, rather than to a whole network.  In  that  case,  a
host  route  is used.  The first column then shows a complete address,
and the "H" flag is present in column 3.  E.g. the entry

    128.6.5.21           128.6.7.1            UHGD        il0

indicates that datagrams for the specific address 128.6.5.21 should be
sent  to  the gateway 128.6.7.1.  As with network routes, the "G" flag
is used for routes that involve a gateway.   The  "D"  flag  indicates
that  the  route  was  added  dynamically,  based  on an ICMP redirect
message from a gateway.  (See below for details.)

The following route is special:

    127.0.0.1            127.0.0.1            UH          lo0

127.0.0.1 is the address of the "loopback device".  This  is  a  dummy
software  module.  Any datagram sent out through that "device" appears
immediately as input.  It can be  used  for  testing.    The  loopback
address  is  also  handy  for  sending  queries  to  programs that are
designed to respond to network queries, but happen to  be  running  on
the  same  computer.    (Why  bother  to use your network to talk to a
program that is on the same machine you are?)

Finally, there are "default" routes, e.g.

    default              128.6.4.27           UG          pe0

This route is used for datagrams that don't match any other entry.  In
this case, they are sent to a gateway with address 128.6.4.27.

In  most  systems,  datagrams are routed by looking up the destination
address in a table such as the one just described.    If  the  address
                                  12



matches  a  specific  host route, then that is used.  Otherwise, if it
matches a network route, that is used.  If no other route  works,  the
default  is  used.   If there is no default, normally the user gets an
error message such as "network is unreachable".

The following sections will describe several ways of setting up  these
routing  tables.    Generally, the actual operation of sending packets
doesn't depend upon which method you use to set up the routes.  When a
packet  is to be sent, its destination is looked up in the table.  The
different routing methods are simply more and less sophisticated  ways
of setting up and maintaining the tables.



4.2 Fixed routes


The  simplest  way  of  doing  routing  is  to have your configuration
contain commands to set up the routing  table  at  startup,  and  then
leave  it  alone.    This  method  is  practical  for relatively small
networks, particularly if they don't change very often.

Most computers automatically set up  some  routing  entries  for  you.
Unix  will  add  an  entry  for the networks to which you are directly
connected.  For example, your startup file might contain the commands

      ifconfig ie0 128.6.4.4 netmask 255.255.255.0
      ifconfig ie1 128.6.5.35 netmask 255.255.255.0

These  specify  that  there  are  two  network  interfaces,  and  your
addresses on them.  The system will automatically create routing table
entries

    128.6.4              128.6.4.4            U           ie0
    128.6.5              128.6.5.35           U           ie1

These specify that  datagrams  for  the  local  subnets,  128.6.4  and
128.6.5, should be sent out the corresponding interface.

In  addition  to  these,  your startup files would contain commands to
define routes to whatever other networks you wanted  to  reach.    For
example,

      route add 128.6.2.0 128.6.4.1  1
      route add 128.6.6.0 128.6.5.35 0

These  commands  specify  that  in  order  to reach network 128.6.2, a
gateway at address 128.6.4.1 should be used, and that network  128.6.6
is  actually  an  additional  network  number for the physical network
connected to interface 128.6.5.35.   Some  other  software  might  use
different  commands  for these cases.  Unix differentiates them by the
"metric", which is the number at the end of the command.   The  metric
indicates  how  many  gateways the datagram will have to go through to
get to the destination.  Routes with metrics of 1 or  greater  specify
the  address of the first gateway on the path.  Routes with metrics of
                                  13



0 indicate that no gateway  is  involved  --  this  is  an  additional
network number for the local network.

Finally, you might define a default route, to be used for destinations
not listed explicitly.  This would normally  show  the  address  of  a
gateway   that   has   enough   information  to  handle  all  possible
destinations.

If your network has only one gateway attached to it,  then  of  course
all  you  need is a single entry pointing to it as a default.  In that
case, you need not worry further about  setting  up  routing  on  your
hosts.    (The  gateway  itself needs more attention, as we will see.)
The following sections are intended to provide  help  for  setting  up
networks where there are several different gateways.



4.3 Routing redirects


Most  Internet  experts  recommend  leaving  routing  decisions to the
gateways.  That is, it is probably a bad  idea  to  have  large  fixed
routing  tables  on each computer.  The problem is that when something
on the network changes, you have to go around to  many  computers  and
update  the  tables.    If  changes  happen  because a line goes down,
service may not be restored until someone has a chance to  notice  the
problem and change all the routing tables.

The  simplest way to keep routes up to date is to depend upon a single
gateway to update your routing tables.  This gateway should be set  as
your  default.  (On Unix, this would mean a command such as "route add
default  128.6.4.27  1",  where  128.6.4.27  is  the  address  of  the
gateway.)   As described above, your system will send all datagrams to
the default when it doesn't have any better route.    At  first,  this
strategy  does  not sound very good if you have more than one gateway.
After all, if all you have is a single default  entry,  how  will  you
ever  use  the other gateways in the cases where they are better?  The
answer is that most gateways are able to send  "redirects"  when  they
get  datagrams  for  which  there  is a better route.  A redirect is a
specific kind of message using  the  ICMP  (Internet  Control  Message
Protocol).    It contains information that generally translates to "In
the future, to get to address XXXXX, please use gateway YYYYY  instead
of  me".    Correct  TCP/IP implementations use these redirects to add
entries to their routing table.  Suppose your routing table starts out
as follows:

    Destination          Gateway              Flags       Interface

    127.0.0.1            127.0.0.1            UH          lo0
    128.6.4              128.6.4.61           U           pe0
    default              128.6.4.27           UG          pe0

This  contains  an entry for the local network, 128.6.4, and a default
pointing to the gateway 128.6.4.27.  Suppose there is also  a  gateway
128.6.4.30,  which  is the best way to get to network 128.6.7.  How do
                                  14



you find it?  Suppose you have datagrams to send to 128.6.7.23.    The
first  datagram  will go to the default gateway, since that's the only
thing in the routing table.  However the default gateway,  128.6.4.27,
will  notice  that 128.6.4.30 would really be a better route.  (How it
does that is up to the gateway.  However there are some fairly  simple
methods  for a gateway to determine that you would be better off using
a  different  one.)    Thus  128.6.4.27  will  send  back  a  redirect
specifying  that packets for 128.6.7.23 should be sent via 128.6.4.30.
Your TCP/IP software will add a routing entry

    128.6.7.23           128.6.4.30           UDHG         pe0

Any future datagrams for 128.6.7.23  will  be  sent  directly  to  the
appropriate gateway.

This  strategy  would  be a complete solution, if it weren't for three
problems:

   - It requires each computer to have  the  address  of  one  gateway
     "hardwired" into its startup files, as the initial default.

   - If a gateway goes down, routing table entries using it may not be
     removed.

   - If your network uses subnets, and your TCP/IP implementation does
     not handle them, this strategy will not work.

How  serious  the  first  problem is depends upon your situation.  For
small networks, there is no problem modifying startup  files  whenever
something  changes.   But some organizations can find it very painful.
If network topology changes, and a gateway  is  removed,  any  systems
that  have  that  gateway  as their default must be adjusted.  This is
particularly serious if the people who maintain the  network  are  not
the  same  as  those  maintaining  the individual systems.  One simple
appoach is to make sure that the default address never changes.    For
example,  you might adopt the convention that address 1 on each subnet
is the default gateway for  that  subnet.    For  example,  on  subnet
128.6.7,  the  default  gateway  would  always  be 128.6.7.1.  If that
gateway is ever removed, some other gateway  is  given  that  address.
(There  must  always  be  at least one gateway left to give it to.  If
there isn't, you are completely cut off anyway.)

The biggest problem with the description given so far is that it tells
you how to add routes but not how to get rid of them.  What happens if
a gateway goes down?  You want traffic to  be  redirected  back  to  a
gateway  that is up.  Unfortunately, a gateway that has crashed is not
going to issue Redirects.  One solution is  to  choose  very  reliable
gateways.  If they crash very seldom, this may not be a problem.  Note
that Redirects can be used to handle some kinds  of  network  failure.
If  a  line goes down, your current route may no longer be a good one.
As long as the gateway to which  you  are  talking  is  still  up  and
talking  to you, it can simply issue a Redirect to the gateway that is
now the best one.  However you still need a way to detect  failure  of
one of the gateways that you are talking to directly.

                                  15



The  best  approach  for  handling  failed gateways is for your TCP/IP
implementation to detect routes  that  have  failed.    TCP  maintains
various timers that allow the software to detect when a connection has
broken.  When this happens, one good approach is  to  mark  the  route
down, and go back to the default gateway.  A similar approach can also
be used to handle failures in the default gateway.  If you  have  mark
two  gateways  as  default,  then  the  software  should be capable of
switching  when  connections  using  one  of   them   start   failing.
Unfortunately,  some  common TCP/IP implementations do not mark routes
as down and change to new ones.  (In particular Berkeley 4.2 Unix does
not.)    However  Berkeley 4.3 Unix does do this, and as other vendors
begin to base products  on  4.3  rather  than  4.2,  this  ability  is
expected to be more common.



4.4 Other ways for hosts to find routes


As  long  as  your  TCP/IP  implementations handle failing connections
properly, establishing one or more default routes in the configuration
file  is  likely  to  be  the simplest way to handle routing.  However
there are two other routing approaches that are worth considering  for
special situations:

   - spying on the routing protocol

   - using proxy ARP



4.4.1 Spying on Routing


Gateways  generally  have  a  special  protocol  that  they  use among
themselves.    Note  that  redirects  cannot  be  used  by   gateways.
Redirects  are  simply ways for gateways to tell "dumb" hosts to use a
different gateway.  The  gateways  themselves  must  have  a  complete
picture of the network, and a way to compute the optimal route to each
subnet.    Generally  they  maintain  this   picture   by   exchanging
information  among  themselves.    There are several different routing
protocols in use for this purpose.  One way for  a  computer  to  keep
track  of  gateways  is  for  it  to listen to the gateways' messages.
There is software available for this purpose for most  of  the  common
routing  protocols.    When  you  run  this  software,  it maintains a
complete picture of the  network,  just  as  the  gateways  do.    The
software  is  generally  designed  to maintain your computer's routing
tables dynamically, so that datagrams are always sent  to  the  proper
gateway.  In effect, the routing software issues the equivalent of the
Unix "route add" and "route delete" commands as the  network  topology
changes.    Generally this results in a complete routing table, rather
than one that depends upon default routes.   (This  assumes  that  the
gateways  themselves  maintain  a  complete table.  Sometimes gateways
keep track of your campus network completely, but use a default  route
for all off-campus networks, etc.)
                                  16



Running  routing  software on each host does in some sense "solve" the
routing problem.  However there are several reasons why  this  is  not
normally  recommended  except  as  a  last  resort.   The most serious
problem is that this reintroduces configuration options that  must  be
kept  up to date on each host.  Any computer that wants to participate
in the protocol among the gateways will need to configure its software
compatibly   with   the   gateways.      Modern  gateways  often  have
configuration options that are  complex  compared  with  those  of  an
individual host.  It is undesirable to spread these to every host.

There  is  a  somewhat  more  specialized problem that applies only to
diskless computers.  By its very nature, a diskless  computer  depends
upon the network and file servers to load programs and to do swapping.
It is dangerous for  diskless  computers  to  run  any  software  that
listens  to  network  broadcasts.   Routing software generally depends
upon broadcasts.  For example,  each  gateway  on  the  network  might
broadcast  its  routing  tables  every  30  seconds.  The problem with
diskless nodes is that the software to listen to these broadcasts must
be loaded over the network.  On a busy computer, programs that are not
used for a few seconds will be swapped or paged out.   When  they  are
activated  again,  they  must  be  swapped  or  paged  in.  Whenever a
broadcast is sent, every computer on the network needs to activate the
routing  software  in order to process the broadcast.  This means that
many diskless computers will be doing swapping or paging at  the  same
time.    This  is likely to cause a temporary overload of the network.
Thus it is very unwise for diskless machines to run any software  that
requires them to listen to broadcasts.



4.4.2 Proxy ARP


Proxy  ARP  is  an alternative technique for letting gateways make all
the routing decisions.  It is applicable to any broadcast network that
uses  ARP  or  a similar technique for mapping Internet addresses into
network-specific  addresses  such  as  Ethernet   addresses.      This
presentation  will  assume  Ethernet.    Other  network  types  can be
acccomodated if you replace "Ethernet address"  with  the  appropriate
network-specific  address,  and ARP with the protocol used for address
mapping by that network type.

In many ways proxy ARP it is similar to  using  a  default  route  and
redirects, however it uses a different mechanism to communicate routes
to the host.  With redirects, a full routing table is used.    At  any
given moment, the host knows what gateways it is routing datagrams to.
With proxy ARP, you dispense with  explicit  routing  tables,  and  do
everything  at the level of Ethernet addresses.  Proxy ARP can be used
for all destinations, only for destinations within your network, or in
various  combinations.   It will be simplest to explain it as used for
all addresses.  To do this, you instruct  the  host  to  pretend  that
every  computer  in  the  world  is  attached  directly  to your local
Ethernet.  On Unix, this would be done using a command

      route add default 128.6.4.2 0
                                  17



where 128.6.4.2 is assumed to be the Internet address  of  your  host.
As  explained  above,  the  metric of 0 causes everything that matches
this route to be sent directly on the local Ethernet.

When a datagram is to be sent to a local  Ethernet  destination,  your
computer  needs  to  know the Ethernet address of the destination.  In
order to find that, it uses something generally called the ARP  table.
This  is  simply  a mapping from Internet address to Ethernet address.
Here's a typical ARP table.  (On our system, it is displayed using the
command "arp -a".)

    FOKKER.RUTGERS.EDU (128.6.5.16) at 8:0:20:0:8:22 temporary
    CROSBY.RUTGERS.EDU (128.6.5.48) at 2:60:8c:49:50:63 temporary
    CAIP.RUTGERS.EDU (128.6.4.16) at 8:0:8b:0:1:6f temporary
    DUDE.RUTGERS.EDU (128.6.20.16) at 2:7:1:0:eb:cd temporary
    W20NS.MIT.EDU (18.70.0.160) at 2:7:1:0:eb:cd temporary
    OBERON.USC.EDU (128.125.1.1) at 2:7:1:2:18:ee temporary
    gatech.edu (128.61.1.1) at 2:7:1:0:eb:cd temporary
    DARTAGNAN.RUTGERS.EDU (128.6.5.65) at 8:0:20:0:15:a9 temporary

Note  that  it  is  simply  a  list  of  Internet  addresses  and  the
corresponding Ethernet address.  The "temporary"  indicates  that  the
entry  was added dynamically using ARP, rather than being put into the
table manually.

If there is an entry for the address in the ARP table, the datagram is
simply  put  on  the Ethernet with the corresponding Ethernet address.
If not, an "ARP request" is broadcast, asking for the destination host
to  identify  itself.   This request is in effect a question "will the
host with Internet  address  128.6.4.194  please  tell  me  what  your
Ethernet address is?".  When a response comes back, it is added to the
ARP table, and future datagrams  for  that  destination  can  be  sent
without delay.

This  mechanism  was  originally  designed  only  for  use  with hosts
attached directly to a single Ethernet.  If you need to talk to a host
on  a different Ethernet, it was assumed that your routing table would
direct you to a gateway.    The  gateway  would  of  course  have  one
interface  on  your Ethernet.  Your computer would then end up looking
up the address of that gateway using  ARP.    It  would  generally  be
useless  to  expect  ARP to work directly with a computer on a distant
network.  Since it isn't on the same  Ethernet,  there's  no  Ethernet
address you can use to send datagrams to it.  And when you send an ARP
request for it, there's nobody to answer the request.

Proxy ARP is based on the  concept  that  the  gateways  will  act  as
proxies  for  distant  hosts.    Suppose  you  have  a host on network
128.6.5, with address 128.6.5.2.  (computer A  in  diagram  below)  It
wants  to send a datagram to host 128.6.4.194, which is on a different
Ethernet (subnet 128.6.4). (computer C in diagram below)  There  is  a
gateway  connecting  the  two subnets, with address 128.6.5.1 (gateway
R):



                                  18



              network 1               network 2
               128.6.5                 128.6.4
        ============================  ==================
          |              |        |    |      |    |
       ___|______   _____|____  __|____|__  __|____|____
       128.6.5.2    128.6.5.3   128.6.5.1   128.6.4.194
                                128.6.4.1
       __________   __________  __________  ____________
       computer A   computer B   gateway R   computer C


Now suppose computer A sends an ARP request for computer  C.  C  isn't
able  to  answer  for  itself.  It's on a different network, and never
even sees the ARP request.  However gateway R can act on  its  behalf.
In  effect,  your  computer  asks "will the host with Internet address
128.6.4.194 please tell me what your Ethernet address  is?",  and  the
gateway   says  "here  I  am,  128.6.4.194  is  2:7:1:0:eb:cd",  where
2:7:1:0:eb:cd is actually the Ethernet address of the gateway.    This
bit  of  illusion  works  just  fine.    Your  host  now  thinks  that
128.6.4.194  is  attached  to  the   local   Ethernet   with   address
2:7:1:0:eb:cd.    Of  course it isn't.  But it works anyway.  Whenever
there's a datagram to be sent to 128.6.4.194, your host  sends  it  to
the specified Ethernet address.  Since that's the address of a gateway
R, the  gateway  gets  the  packet.    It  then  forwards  it  to  the
destination.

Note that the net effect is exactly the same as having an entry in the
routing table saying  to  route  destination  128.6.4.194  to  gateway
128.6.5.1:

    128.6.4.194          128.6.5.1           UGH          pe0

except  that  instead  of  having the routing done at the level of the
routing table, it is done at the level of the ARP table.

Generally it's better to use the routing  table.    That's  what  it's
there for.  However here are some cases where proxy ARP makes sense:

   - when you have a host that does not implement subnets

   - when you have a host that does not respond properly to redirects

   - when you do not want to have to choose a specific default gateway

   - when your software is unable to recover from a failed route

The  technique  was first designed to handle hosts that do not support
subnets.  Suppose that you have a subnetted network.  For example, you
have  chosen  to break network 128.6 into subnets, so that 128.6.4 and
128.6.5 are separate.  Suppose you  have  a  computer  that  does  not
understand  subnets.    It  will  assume that all of 128.6 is a single
network.  Thus it will be difficult to establish routing table entries
to  handle  the  configuration  above.    You  can't tell it about the
gateway explicitly using "route add 128.6.4.0 128.6.5.1  1"  Since  it
thinks  all of 128.6 is a single network, it can't understand that you
                                  19



are trying to tell it where to send  one  subnet.    It  will  instead
interpret  this command as an attempt to set up a host route to a host
who address is 128.6.4.0.  The only thing that would work would be  to
establish  explicit  host  routes  for  every individual host on every
other subnet.  You can't depend upon default gateways and redirects in
this  situation either.  Suppose you said "route add default 128.6.5.1
1".  This would establish the gateway 128.6.5.1 as a default.  However
the  system wouldn't use it to send packets to other subnets.  Suppose
the host is 128.6.5.2, and wants to send a  datagram  to  128.6.4.194.
Since  the destination is part of 128.6, your computer considers it to
be on the same network as itself, and doesn't bother  to  look  for  a
gateway.

Proxy  ARP  solves  this  problem by making the world look the way the
defective implementation expects it to look.  Since  the  host  thinks
all  other  subnets  are part of its own network, it will simply issue
ARP requests for them.  It expects to get  back  an  Ethernet  address
that  can  be used to establish direct communications.  If the gateway
is practicing proxy ARP, it will respond with the  gateway's  Ethernet
address.    Thus  datagrams  are  sent  to the gateway, and everything
works.

As you can see, no specific configuration is need  to  use  proxy  ARP
with a host that doesn't understand subnets.  All you need is for your
gateways to implement proxy ARP.    In  order  to  use  it  for  other
purposes, you must explicitly set up the routing table to cause ARP to
be used.  By default, TCP/IP implementations will  expect  to  find  a
gateway  for any destination that is on a different network.  In order
to make them issue ARP's, you must explicitly  install  a  route  with
metric 0, as in the example "route add default 128.6.5.2 0".

It  is  obvious  that  proxy ARP is reasonable in situations where you
have hosts that don't understand subnets.  Some comments may be needed
on  the  other situations.  Generally TCP/IP implementations do handle
ICMP redirects properly.  Thus it is normally practical to  set  up  a
default  route  to  some gateway, and depend upon the gateway to issue
redirects for  destinations  that  should  use  a  different  gateway.
However in case you ever run into an implementation that does not obey
redirects, or cannot be configured to have a default gateway, you  may
be  able  to  make things work by depending upon proxy ARP.  Of course
this requires that you be able to configure the host  to  issue  ARP's
for  all  destinations.    You  will  need  to  read the documentation
carefully to see exactly what  routing  features  your  implementation
has.

Sometimes  you  may  choose  to depend upon proxy ARP for convenience.
The problem with routing tables is that you have  to  configure  them.
The simplest configuration is simply to establish a default route, but
even there you have to supply some  equivalent  to  the  Unix  command
"route  add  default  ...".    Should you change the addresses of your
gateways, you have to modify this command on all  of  your  hosts,  so
that  they  point to the new default gateway.  If you set up a default
route that depends upon proxy ARP (i.e. has metric 0), you won't  have
to  change  your configuration files when gateways change.  With proxy
ARP, no gateway addresses are  given  explicitly.    Any  gateway  can
                                  20



respond to the ARP request, no matter what its address.

In  order  to  save  you  from having to do configuration, some TCP/IP
implementations default to using ARP when they have  no  other  route.
The  most  flexible implementations allow you to mix strategies.  That
is, if you have specified a route  for  a  particular  network,  or  a
default route, they will use that route.  But if there is no route for
a destination, they will treat it as local, and issue an ARP  request.
As  long as your gateways support proxy ARP, this allows such hosts to
reach any destination without any need for routing tables.

Finally, you may choose to use proxy ARP because  it  provides  better
recovery  from  failure.  This choice is very much dependent upon your
implementation.  The next section will discuss the tradeoffs  in  more
detail.

In  situations  where  there  are  several  gateways  attached to your
network, you may wonder how proxy ARP allows you to  choose  the  best
one.    As  described  above,  your  computer simply sends a broadcast
asking for the Ethernet address for a destination.   We  assumed  that
the  gateways  would be set up to respond to this broadcast.  If there
is more than one  gateway,  this  requires  coordination  among  them.
Ideally,  the  gateways  will  have  a complete picture of the network
topology.  Thus they are able to determine the best  route  from  your
host  to any destination.  If the gateway coordinate among themselves,
it should be possible for the best gateway  to  respond  to  your  ARP
request.    In  practice,  it  may  not always be possible for this to
happen.  It is fairly easy to design algorithms to  prevent  very  bad
routes.  For example, consider the following situation:

          1             2            3
        -------  A  ----------  B ----------

1,  2, and 3 are networks.  A and B are gateways, connecting network 2
to 1 or 3.  If a host on network 2 wants to talk to a host on  network
1,  it  is  fairly  easy  for  gateway  A to decide to answer, and for
gateway B to decide not to.  Here's  how:  if  gateway  B  accepted  a
datagram  for  network 1, it would have to forward it to gateway A for
delivery.  This would mean that it would take a packet from network  2
and  send it right back out on network 2.  It is very easy to test for
routes that involve this sort of circularity.  It is  much  harder  to
deal with a situation such as the following:

                         1
                  ---------------
                    A        B
                    |        | 4
                    |        |
                  3 |        C
                    |        |
                    |        | 5
                    D        E
                  ---------------
                         2

                                  21



Suppose  a  computer  on  network 1 wants to send a datagram to one on
network 2.  The route via A and D is probably better, because it  goes
through  only one intermediate network (3).  It is also possible to go
via B, C, and E, but that path  is  probably  slightly  slower.    Now
suppose  the  computer  on  network  1  sends  an  ARP  request  for a
destination on 2.  It is likely that A and B will both respond to that
request.    B  is not quite as good a route as A. However it is not so
bad as the case above.  B won't have to send the datagram  right  back
out  onto  network  1.    It  is unable to determine there is a better
alternative  route  without  doing  a  significant  amount  of  global
analysis  on  the network.  This may not be practical in the amount of
time available to process an ARP request.



4.4.3 Moving to New Routes After Failures


In principle, TCP/IP routing is capable of handling line failures  and
gateway  crashes.    There  are  various  mechanisms to adjust routing
tables and ARP tables to keep them up to date.    Unfortunately,  many
major  implementations  of  TCP/IP  have  not implemented all of these
mechanisms.  The net result is that you have to look carefully at  the
documentation  for  your  implementation,  and  consider what kinds of
failures are most likely.  You then have to  choose  a  strategy  that
will  work  best  for your site.  The basic choices for finding routes
have all been listed above:  spying on the gateways' routing protocol,
setting  up  a  default  route and depending upon redirects, and using
proxy ARP.  These methods all have their own  limitations  in  dealing
with a changing network.

Spying on the gateways' routing protocol is theoretically the cleanest
solution.  Assuming that the gateways use good routing technology, the
tables  that  they  broadcast  contain  enough information to maintain
optimal routes to all destinations.  Should something in  the  network
change  (a  line  or  a  gateway  goes down), this information will be
reflected in the tables, and the routing  software  will  be  able  to
update the hosts' routing tables appropriately.  The disadvantages are
entirely practical.  However in some situations the robustness of this
approach may outweight the disadvantages.  To summarize the discussion
above, the disadvantages are:

   - If  the  gateways  are  using  sophisticated  routing  protocols,
     configuration may be fairly complex.  Thus you will be faced with
     setting up and maintaining configuration files on every host.

   - Some gateways use proprietary routing protocols.  In  this  case,
     you  may  not  be  able  to  find  software  for  your hosts that
     understands them.

   - If your hosts are diskless, there can be very serious performance
     problems associated with listening to routing broadcasts.

Some  gateways  may  be  able  to  convert from their internal routing
protocol to a simpler one for use by your hosts.  This  could  largely
                                  22



bypass  the  first two disadvantages.  Currently there is no known way
to get around the third one.

The problems with default routes/redirects  and  with  proxy  ARP  are
similar:  they  both  have trouble dealing with situations where their
table entries no longer apply.   The  only  real  difference  is  that
different  tables  are involved.  Suppose a gateway goes down.  If any
of your current routes are using that gateway, you may be in  trouble.
If  you  are depending upon the routing table, the major mechanism for
adjusting routes is the redirect.  This works fine in two  situations:

   - where  the  default  gateway  is not the best route.  The default
     gateway can direct you to a better gateway

   - where a distant line or gateway fails.  If this changes the  best
     route,  the  current gateway can redirect you to the gateway that
     is now best

The case it does not protect you against is where the gateway that you
are currently sending your datagrams to crashes.  Since it is down, it
is unable to redirect you to another gateway.  In many cases, you  are
also  unprotected  if  your  default  gateway  goes  down, since there
routing starts by sending to the default gateway.

The situation with proxy ARP is similar.  If the  gateways  coordinate
themselves  properly,  the  right  one  will  respond  initially.   If
something elsewhere in  the  network  changes,  the  gateway  you  are
currently  issuing  can  issue  a  redirect  to  a new gateway that is
better.  (It is usually possible to use redirects to  override  routes
established  by  proxy  ARP.)    Again, the case you are not protected
against is where the gateway you are currently using crashes.    There
is  no  equivalent  to failure of a default gateway, since any gateway
can respond to the ARP request.

So the big problem is that failure of a gateway you are using is  hard
to  recover  from.   It's hard because the main mechanism for changing
routes is the redirect,  and  a  gateway  that  is  down  can't  issue
redirects.    Ideally,  this  problem should be handled by your TCP/IP
implementation, using timeouts.  If a computer stops getting response,
it  should  cancel the existing route, and try to establish a new one.
Where you are using a  default  route,  this  means  that  the  TCP/IP
implementation  must  be  able  to  declare a route as down based on a
timeout.  If you have been redirected to a  non-default  gateway,  and
that  route is declared down, traffic will return to the default.  The
default gateway can then begin handling the traffic, or redirect it to
a  different  gateway.    To  handle  failure of a default gateway, it
should be possible to have more than one default.  If one is  declared
down,  another  will  be used.  Together, these mechanisms should take
care of any failure.

Similar mechanisms can be used by systems that depend upon proxy  ARP.
If a connection is timing out, the ARP table entry that it uses should
be cleared.  This will cause a new ARP request, which can  be  handled
by a gateway that is still up.  A simpler mechanism would simply be to
time out all ARP entries after some period.  Since making  a  new  ARP
                                  23



request  has  a very low overhead, there's no problem with removing an
ARP entry even if it is still good.  The next time a datagram is to be
sent,  a  new  request  will  be  made.  The response is normally fast
enough that users will not even notice the delay.

Unfortunately,  many  common  implementations   do   not   use   these
strategies.  In Berkeley 4.2, there is no automatic way of getting rid
of any kind of entry, either routing or ARP.  They do  not  invalidate
routes  on  timeout  nor  ARP  entries.  ARP entries last forever.  If
gateway crashes are a significant problem, there may be no choice  but
to  run  software  that  listens to the routing protocol.  In Berkeley
4.3, routing entries are removed when  TCP  connections  are  failing.
ARP  entries  are  still  not  removed.   This makes the default route
strategy more attractive for 4.3 than proxy ARP.  Having more than one
default  route  may  also allow for recovery from failure of a default
gateway.  Note however that 4.3 only handles timeout  for  connections
using TCP.  If a route is being used only by services based on UDP, it
will not recover from gateway failure.  While the "traditional" TCP/IP
services  use  TCP,  network  file  systems  generally  do  not.  Thus
4.3-based systems still  may  not  always  be  able  to  recover  from
failure.

In  general,  you  should  examine  your  implementation  in detail to
determine what sort of error recovery strategy it uses.  We hope  that
the  discussion in this section will then help you choose the best way
of dealing with routing.

There is one more strategy that some older implementations use.  It is
strongly  discouraged,  but we mention it here so you can recognize it
if you see it.  Some implementations detect gateway failure by  taking
active  measure to see what gateways are up.  The best version of this
is based on a list of all gateways that are currently in use.    (This
can  be  determined  from  the routing table.)  Every minute or so, an
echo request datagram is sent to each such  gateway.    If  a  gateway
stops responding to echo requests, it is declared down, and all routes
using it revert to the default.   With  such  an  implementation,  you
normally supply more than one default gateway.  If the current default
stops responding, an alternate is chosen.  In some cases,  it  is  not
even  necessary  to  choose an explicit default gateway.  The software
will  randomly  choose  any  gateway  that  is   responding.      This
implementation  is  very  flexible  and  recovers  well from failures.
However a large network full of such implementations will waste a  lot
of  bandwidth  on  the  echo  datagrams  that are used to test whether
gateways  are  up.    This  is  the  reason  that  this  strategy   is
discouraged.



5. Bridges and Gateways


This  section  will  deal  in  more detail with the technology used to
construct larger networks.  It  will  focus  particularly  on  how  to
connect  together  multiple  Ethernets,  token rings, etc.  These days
most networks are hierarchical.  Individual hosts attach to local-area
                                  24



networks  such  as  Ethernet or token ring.  Then those local networks
are connected via some combination of backbone networks and  point  to
point  links.    A  university might have a network that looks in part
like this:

     ________________________________
     |   net 1      net 2    net 3  |        net 4            net 5
     | ---------X---------X-------- |      --------         --------
     |                         |    |         |                 |
     |  Building A             |    |         |                 |
     |               ----------X--------------X-----------------X
     |                              |  campus backbone network  :
     |______________________________|                           :
                                                         serial :
                                                           line :
                                                         -------X-----
                                                             net  6

Nets 1, 2 and 3 are in one building.  Nets 4 and 5  are  in  different
buildings  on  the  same  campus.  Net 6 is in a somewhat more distant
location.  The diagram above shows nets 1, 2, and  3  being  connected
directly,  with switches that handle the connections being labelled as
"X".  Building A is connected to  the  other  buildings  on  the  same
campus  by  a backbone network.  Note that traffic from net 1 to net 5
takes the following path:

   - from 1 to 2 via the direct connection between those networks

   - from 2 to 3 via another direct connection

   - from 3 to the backbone network

   - across the backbone network from building A to  the  building  in
     which net 5 is housed

   - from the backbone network to net 5

Traffic  for  net  6 would additionally pass over a serial line.  With
the setup as shown, the same switch  is  being  used  to  connect  the
backbone  network  to net 5 and to the serial line.  Thus traffic from
net 5 to net 6 would not need to go through the backbone, since  there
is a direct connection from net 5 to the serial line.

This section is largely about what goes in those "X"'s.



5.1 Alternative Designs


Note  that  there  are alternatives to the sort of design shown above.
One is to use point to point lines or switched lines directly to  each
host.   Another is to use a single-level of network technology that is
capable of handling both local and long-haul networking.

                                  25



5.1.1 A mesh of point to point lines


Rather than connecting hosts to a local network such as Ethernet,  and
then   interconnecting  the  Ethernets,  it  is  possible  to  connect
long-haul serial lines directly to the individual computers.  If  your
network   consists   primarily  of  individual  computers  at  distant
locations, this might make sense.  Here would be  a  small  design  of
that type.

          computer 1                computer 2             computer 3
              |                         |                      |
              |                         |                      |
              |                         |                      |
          computer 4 -------------- computer 5 ----------- computer 6

In  the design shown earlier, the task of routing datagrams around the
network is handled by special-purpose switching units shown as  "X"'s.
If  you  run lines directly between pairs of hosts, your hosts will be
doing this sort of routing and switching,  as  well  as  their  normal
computing.    Unless  you  run  lines  directly  between every pair of
computers, some systems will end up handling traffic for  others.  For
example,  in this design, traffic from 1 to 3 will go through 4, 5 and
6.  This is certainly possible, since most TCP/IP implementations  are
capable of forwarding datagrams.  If your network is of this type, you
should think of your hosts as also acting as gateways.   Much  of  the
discussion  below  on  configuring  gateways will apply to the routing
software that you run on your hosts.  This sort  of  configuration  is
not as common as it used to be, for two reasons:

   - Most large networks have more than one computer per location.  In
     this case it is less expensive to set up a local network at  each
     location than to run point to point lines to each computer.

   - Special-purpose  switching  units have become less expensive.  It
     often makes sense to offload the routing and communications tasks
     to a switch rather than handling it on the hosts.

It is of course possible to have a network that mixes the two kinds of
techology.  In this case,  locations  with  more  equipment  would  be
handled  by  a hierarchical system, with local-area networks connected
by switches.  Remote locations with a single computer would be handled
by  point  to  point lines going directly to those computers.  In this
case the routing software used on the remote computers would  have  to
be  compatible  with that used by the switches, or there would need to
be a gateway between the two parts of the network.

Design decisions of this type are typically made after  an  assessment
of  the  level  of network traffic, the complexity of the network, the
quality of routing software available for the hosts, and  the  ability
of the hosts to handle extra network traffic.




                                  26



5.1.2 Circuit switching technology


Another  alternative  to  the hierarchical LAN/backbone approach is to
use circuit switches connected to each individual computer.   This  is
really  a  variant  of  the  point  to point line technique, where the
circuit switch allows each system to have what  amounts  to  a  direct
line to every other system.  This technology is not widely used within
the TCP/IP community, largely because the TCP/IP protocols assume that
the  lowest  level  handles  isolated  datagrams.    When a continuous
connection  is  needed,  higher  network  layers  maintain  it   using
datagrams.    This  datagram-oriented  technology  does  not  match  a
circuit-oriented environment very closely.  In order  to  use  circuit
switching  technology,  the IP software must be modified to be able to
build and tear down virtual circuits as appropriate.  When there is  a
datagram  for a given destination, a virtual circuit must be opened to
it.  The virtual circuit would  be  closed  when  there  has  been  no
traffic  to  that  destination  for  some time.  The major use of this
technology is for  the  DDN  (Defense  Data  Network).    The  primary
interface  to  the  DDN is based on X.25.  This network appears to the
outside as a distributed X.25 network.  TCP/IP software  intended  for
use with the DDN must do precisely the virtual circuit management just
described.     Similar   techniques   could   be   used   with   other
circuit-switching  technologies, e.g. ATT's DataKit, although there is
almost no software currently available to support this.



5.1.3 Single-level networks


In some cases new developments in wide-area networks can eliminate the
need  for hierarchical networks.  Early hierarchical networks were set
up because the only convenient  network  technology  was  Ethernet  or
other  LAN's, and those could not span distances large enough to cover
an entire campus.  Thus it  was  necessary  to  use  serial  lines  to
connect  LAN's  in  various  locations.    It  is now possible to find
network technology whose characteristics are similar to Ethernet,  but
where  a  single  network  can  span a campus.  Thus it is possible to
think of using a single large network, with no hierarchical structure.

The  primary  limitations  of  a  large   single-level   network   are
performance  and  reliability  considerations.  If a single network is
used  for  the  entire  campus,  it  is  very  easy  to  overload  it.
Hierarchical   networks  can  handle  a  larger  traffic  volume  than
single-level networks if traffic patterns have a reasonable amount  of
locality.  That is, in many applications, traffic within an individual
department tends to be greater than traffic among departments.

Let's look at a concrete example.  Suppose there are  10  departments,
each of which generate 1 Mbit/sec of traffic.  Suppose futher than 90%
of that traffic is to other systems within the  department,  and  only
10%  is to other departments.  If each department has its own network,
that network only needs to handle 1 Mbit/sec.   The  backbone  network
connecting  the  department also only needs 1 Mbit/sec capacity, since
                                  27



it is handling 10% of 1 Mbit from each department.  In order to handle
this  situation  with  a  single wide-area network, that network would
have  to  be  able  to  handle  the  simultaneous  load  from  all  10
departments, which would be 10 Mbit/sec.

The   second  limitation  on  single-level  networks  is  reliability,
maintainability and security.  Wide-area networks are  more  difficult
to  diagnose  and  maintain than local-area networks, because problems
can be introduced from any building to which the network is connected.
They  also  make traffic visible in all locations.  For these reasons,
it is often sensible to handle local  traffic  locally,  and  use  the
wide-area  network  only  for  traffic  that  actually must go between
buildings.  However if you have a situation where  each  location  has
only  one  or  two  computers, it may not make sense to set up a local
network at each location, and a single-level network may make sense.



5.1.4 Mixed designs


In practice,  few  large  networks  have  the  luxury  of  adopting  a
theoretically pure design.

It is very unlikely that any large network will be able to avoid using
a hierarchical design.  Suppose we  set  out  to  use  a  single-level
network.  Even if most buildings have only one or two computers, there
will be some location where there are enough that a local-area network
is justified.  The result is a mixture of a single-level network and a
hierachical network.  Most buildings have  their  computers  connected
directly  to  the  wide-area  network, as with a single-level network.
However in one building there is a local-area network which  uses  the
wide-area  network  as  a  backbone,  connecting to it via a switching
unit.

On the other side of the story, even network designers with  a  strong
commitment  to  hierarchical networks are likely to find some parts of
the network where it simply doesn't make economic sense to  install  a
local-area  network.    So  a  host  is put directly onto the backbone
network, or tied directly to a serial line.

However you should think carefully before  making  ad  hoc  departures
from  your  design  philosophy in order to save a few dollars.  In the
long run, network maintainability is going to depend upon your ability
to make sense of what is going on in the network.  The more consistent
your technology is, the more likely you are to be able to maintain the
network.








                                  28



5.2 An introduction to alternative switching technologies


This  section will discuss the characteristics of various technologies
used to switch datagrams between networks.  In effect, we  are  trying
to  fill  in  some  details  about the black boxes assumed in previous
sections.  There are three basic types of switches, generally referred
to as repeaters, bridges, and gateways, or alternatively as level 1, 2
and 3 switches (based on the level of the  ISO  model  at  which  they
operate).    Note however that there are systems that combine features
of more than one of these, particularly bridges and gateways.

The most important dimensions on which switches  vary  are  isolation,
performance, routing and network management facilities.  These will be
discussed below.

The most serious difference is between repeaters  and  the  other  two
types  of  switch.    Until recently, gateways provided very different
services from bridges.  However these two technologies are now  coming
closer  together.  Gateways are beginning to adopt the special-purpose
hardware that has characterized bridges in  the  past.    Bridges  are
beginning to adopt more sophisticated routing, isolation features, and
network management, which have characterized  gateways  in  the  past.
There  are  also systems that can function as both bridge and gateway.
This means that at the moment, the crucial  decision  may  not  be  to
decide  whether  to  use  a  bridge  or  a gateway, but to decide what
features you want in a switch  and  how  it  fits  into  your  overall
network design.



5.2.1 Repeaters


A repeater is a piece of equipment that connects two networks that use
the same technology.  It receives every data packet on  each  network,
and retransmits it onto the other network.  The net result is that the
two networks have exactly the same  set  of  packets  on  them.    For
Ethernet or IEEE 802.3 networks there are actually two different kinds
of repeater.  (Other network technologies may not need  to  make  this
distinction.)

A  simple  repeater  operates at a very low level indeed.  Its primary
purpose is to get around limitations in cable length caused by  signal
loss or timing dispersion.  It allows you to construct somewhat larger
networks than you would otherwise be able to construct.    It  can  be
thought  of  as  simply  a two-way amplifier.  It passes on individual
bits in the signal, without doing any processing at the packet  level.
It even passes on collisions.  That is, if a collision is generated on
one of  the  networks  connected  to  it,  the  repeater  generates  a
collision  on  the  other  network.  There is a limit to the number of
repeaters that you can use in a network.  The  basic  Ethernet  design
requires  that signals must be able to get from one end of the network
to the other within a specified amount of time.    This  determines  a
maximum  allowable length.  Putting repeaters in the path does not get
                                  29



around this limit.  (Indeed each repeater adds some delay, so in  some
ways  a repeater makes things worse.)  Thus the Ethernet configuration
rules limit the number of repeaters that can be in any path.

A "buffered repeater" operates at the level  of  whole  data  packets.
Rather  than passing on signals a bit at a time, it receives an entire
packet from one network into an internal buffer and  then  retransmits
it  onto  the other network.  It does not pass on collisions.  Because
such low-level features  as  collisions  are  not  repeated,  the  two
networks continue to be separate as far as the Ethernet specifications
are concerned.  Thus there  are  no  restrictions  on  the  number  of
buffered  repeaters  that can be used.  Indeed there is no requirement
that both of the networks be of  the  same  type.    However  the  two
networks  must  be sufficiently similar that they have the same packet
format.  Generally this means that  buffered  repeaters  can  be  used
between two networks of the IEEE 802.x family (assuming that they have
chosen the same address length), or two networks of some other related
family.    A  pair  of  buffered  repeaters can be used to connect two
networks via a serial line.

Buffered repeaters share with simple repeaters the most basic feature:
they  repeat every data packet that they receive from one network onto
the other.  Thus the two networks end up with exactly the same set  of
packets on them.



5.2.2 Bridges and gateways


A  bridge  differs from a buffered repeater primarily in the fact that
it exercizes some selectivity as to what packets it  forwards  between
networks.    Generally  the  goal  is  to increase the capacity of the
system by keeping local traffic confined to the network  on  which  it
originates.    Only  traffic  intended  for the other network (or some
other network accessed through it) goes through the bridge.    So  far
this  description would also apply to a gateway.  Bridges and gateways
differ in the way they determine what packets to forward.    A  bridge
uses  only  the  ISO level 2 address.  In the case of Ethernet or IEEE
802.x networks, this is the 6-byte Ethernet or MAC-level address. (The
term  MAC-level  address  is  more  general.   However for the sake of
concreteness, examples in this section will assume  that  Ethernet  is
being  used.    You  may generally replace the term "Ethernet address"
with the equivalent MAC-level address for other similar technologies.)
A bridge does not examine the packet itself, so it does not use the IP
address or its equivalent for  routing  decisions.    In  contrast,  a
gateway  bases  its decisions on the IP address, or its equivalent for
other protocols.

There are several reasons why it matters which kind of address is used
for  decisions.    The  most basic is that it affects the relationship
between the  switch  and  the  upper  layers  of  the  protocol.    If
forwarding is done at the level of the MAC-level address (bridge), the
switch will be invisible to the protocols.  If it is done  at  the  IP
level,  the  switch will be visible.  Let's give an example.  Here are
                                  30



two networks connected by a bridge:

              network 1          network 2
               128.6.5            128.6.4
        ==================  ================================
          |            |      |            |             |
       ___|______    __|______|__   _______|___   _______|___
       128.6.5.2        bridge       128.6.4.3     128.6.4.4
       __________    ____________   ___________   ___________
       computer A                   computer B    computer C


Note that the bridge does not have an IP address.  As far as computers
A,  B,  and  C  are  concerned,  there  is a single Ethernet (or other
network) to which they are all attached.  This means that the  routing
tables  must  be  set up so that computers on both networks treat both
networks as local.  When computer A opens a connection to computer  B,
it  first  broadcasts  an ARP request asking for computer B's Ethernet
address.  The bridge must  pass  this  broadcast  from  network  1  to
network  2.  (In general, bridges must pass all broadcasts.)  Once the
two computers know each other's Ethernet addresses, communications use
the  Ethernet  address  as the destination.  At that point, the bridge
can start exerting some selectivity.  It will only pass packets  whose
Ethernet  destination  address  is for a machine on the other network.
Thus a packet from B to A will be passed from network 2 to  1,  but  a
packet from B to C will be ignored.

In  order  to  make  this  selection,  the  bridge needs to know which
network each machine is on.  Most modern bridges build up a table  for
each  network,  listing the Ethernet addresses of machines known to be
on that network.  They do this by watching all of the packets on  both
networks.   When a packet first appears on network 1, it is reasonable
to conclude that the Ethernet source address corresponds to a  machine
on network 1.

Note  that a bridge must look at every packet on the Ethernet, for two
different reasons.  First, it may use  the  source  address  to  learn
which  machines  are  on  which  network.  Second, it must look at the
destination address in order to decide whether it needs to forward the
packet to the other network.

As  mentioned  above,  generally bridges must pass broadcasts from one
network to the other.  Broadcasts are often used to locate a resource.
The ARP request is a typical example of this.  Since the bridge has no
way of knowing what host is going to answer  the  broadcast,  it  must
pass   it   on  to  the  other  network.    Some  newer  bridges  have
user-selectable filters.  With them, it  is  possible  to  block  some
broadcasts  and  allow  others.  You might allow ARP broadcasts (which
are  essential  for  IP  to  function),  but  confine  less  essential
broadcasts  to one network.  For example, you might choose not to pass
rwhod broadcasts, which some systems use to keep track of  every  user
logged  into  every  other  system.    You  might  decide  that  it is
sufficient for rwhod to know about the systems on a single segment  of
the network.

                                  31



Now let's take a look at two networks connected by a gateway

              network 1                   network 2
               128.6.5                     128.6.4
        ====================      ==================================
          |              |          |              |             |
       ___|______    ____|__________|____   _______|___   _______|___
       128.6.5.2     128.6.5.1  128.6.4.1    128.6.4.3     128.6.4.4
       __________    ____________________   ___________   ___________
       computer A           gateway           computer B    computer C


Note  that  the  gateway  has IP addresses assigned to each interface.
The  computers'  routing  tables  are  set  up  to   forward   through
appropriate  address.    For  example,  computer A has a routing entry
saying that it should use the  gateway  128.6.5.1  to  get  to  subnet
128.6.4.

Because  the  computers  know  about the gateway, the gateway does not
need to scan all the packets on the Ethernet.  The computers will send
packets to it when appropriate.  For example, suppose computer A needs
to send a message to computer B. Its routing table will tell it to use
gateway  128.6.5.1.    It  will issue an ARP request for that address.
The gateway will respond to the ARP request, just as any  host  would.
From then on, packets destinated for B will be sent with the gateway's
Ethernet address.



5.2.3 More about bridges


There are several advantages to using  the  Mac-level  address,  as  a
bridge  does.   First, every packet on an Ethernet or IEEE network has
such an address.  The address is in the same place for  every  packet,
whether  it  is  IP,  DECnet,  or  some  other  protocol.   Thus it is
relatively fast to get the address from the packet.   A  gateway  must
decode  the  entire IP header, and if it is to support protocols other
than IP, it must have software for each such  protocol.    This  means
that  a bridge automatically supports every possible protocol, whereas
a gateway requires specific provisions for  each  protocol  it  is  to
support.

However  there  are  also disadvantages.  The one that is intrinsic to
the design of a bridge is

   - A bridge must look at every packet on the network, not just those
     addressed  to  it.    Thus it is possible to overload a bridge by
     putting it on a very busy network, even if very little traffic is
     actually going through the bridge.

However  there  are another set of disadvantages that are based on the
way bridges are usually built.  It is possible in principle to  design
bridges  that do not have these disadvantages, but I don't know of any
plans to do so.  They all stem from the fact that bridges do not  have
                                  32



a complete routing table that describes the entire system of networks.
They simply have a list of the Ethernet addresses that lie on each  of
its two networks. This means

   - A  bridge  can  handle only two network interfaces.  At a central
     site, where many networks converge, this normally means that  you
     set  up  a backbone network to which all the bridges connect, and
     then buy a separate bridge to connect each other network to  that
     backbone.  Gateways often have between 4 and 8 interfaces.

   - Networks  that  use  bridges cannot have loops in them.  If there
     were a loop,  some  bridges  would  see  traffic  from  the  same
     Ethernet address coming from both directions, and would be unable
     to decide which table to put that address  in.    Note  that  any
     parallel  paths  to  the  same direction constitute a loop.  This
     means  that  multiple  paths  cannot  be  used  for  purposes  of
     splitting the load or providing redundancy.

There  are  some  ways  of  getting around the problem of loops.  Many
bridges allow configurations with redundant connections, but turn  off
links  until  there are no loops left.  Should a link fail, one of the
disabled ones is then brought back into service.  Thus redundant links
can  still  buy  you  extra  reliability.    But they can't be used to
provide extra capacity.  It is also possible to build  a  bridge  that
will  make  use  of  parallel point to point lines, in the one special
case where those lines go between a  single  pair  of  bridges.    The
bridges  would  treat  the two lines as a single virtual line, and use
them alternately in round-robin fashion.

The process of disabling redundant  connections  until  there  are  no
loops  left  is  called  a "spanning tree algorithm".  This name comes
from the fact that a tree is defined as a pattern of connections  with
no loops.  Thus one wants to disable connections until the connections
that are left form a tree that "spans" (includes) all of the  networks
in  the  system.  In order to do this, all of the bridges in a network
system must communicate among themselves.  There is an  IEEE  proposal
to  standardize  the protocol for doing this, and for constructing the
spanning tree.

Note that there is a tendency  for  the  resulting  spanning  tree  to
result  in  high  network  loads  on certain parts of the system.  The
networks near the "top of the tree" handle all traffic between distant
parts  of  the  network.  In a network that uses gateways, it would be
possible to put in an extra link between parts  of  the  network  that
have  heavy  traffic between them.  However such extra links cannot be
used by a set of bridges.









                                  33



5.2.4 More about gateways


Gateways have their own advantages and disadvantages.   In  general  a
gateway  is more complex to design and to administer than a bridge.  A
gateway must participate in all of the protocols that it  is  designed
to  forward.  For example, an IP gateway must respond to ARP requests.
The IP standards also require it to completely process the IP  header,
decrementing the time to live field and obeying any IP options.

Gateways  are  designed to handle more complex network topologies than
bridges.  As such, they have a different (and  more  complex)  set  of
decisions  to make.  In general a bridge has only a binary decision to
make: does it or does it not pass a given packet from one  network  to
the  other?    However  a gateway may have several network interfaces.
Furthermore, when it forwards a packet, it must decide  what  host  or
gateway to send the packet to next.  It is even possible for a gateway
to decide to send a packet back onto the same network  it  came  from.
If  a  host  is  using the gateway as its default, it may send packets
that really should go to some  other  gateway.    In  that  case,  the
gateway will send the packet on to the right gateway, and send back an
ICMP redirect to the host.  Many gateways  can  also  handle  parallel
paths.   If there are several equally good paths to a destination, the
gateway will alternate among them in round-robin fashion.

In order to handle these decisions, a gateway will  typically  have  a
routing  table  that  looks  very  much  like  a host's.  As with host
routing tables, a gateway's table contains an entry for each  possible
network  number.    For  each network, there is either an entry saying
that that network is connected directly to the gateway, or there is an
entry saying that traffic for that network should be forwarded through
some other gateway  or  gateways.    We  will  describe  the  "routing
protocols"  used to build up this information later, in the discussion
on how to configure a gateway.



5.3 Comparing the switching technologies


Repeaters, buffered repeaters, bridges, and gateways form a  spectrum.
Those  devices  near  the  beginning  of the list are best for smaller
networks.  They are less expensive, and easier to  set  up,  but  less
general.    Those  near  the end of the list are suitable for building
more complex networks.  Many networks will contain a mixture of switch
types,  with  repeaters  being  used  to  connect a few nearby network
segments, bridges used for slightly larger areas  (particularly  those
with low traffic levels), and gateways used for long-distance links.

Note  that  this  document  so  far has assumed that only gateways are
being used.  The section on setting up a host described how to set  up
a  routing  table  listing  the  gateways  to  use  to  get to various
networks.  Repeaters and bridges are invisible to IP.  So  as  far  as
previous  sections are concerned, networks connected by them are to be
considered a single network.  Section 3.3.1 describes how to configure
                                  34



a  host  in  the  case  where  several subnets are carried on a single
physical network.  The same configuration should be used when  several
subnets are connected by repeaters or bridges.

As  mentioned  above,  the most important dimensions on which switches
vary are isolation,  performance,  routing,  network  management,  and
performing auxilliary support services.



5.3.1 Isolation


Generally  people  use switches to connect networks to each other.  So
they are normally thinking  of  gaining  connectivity,  not  providing
isolation.  However isolation is worth thinking about.  If you connect
two networks and  provide  no  isolation  at  all,  then  any  network
problems  on  other  networks suddenly appear on yours as well.  Also,
the two networks together may have enough traffic  to  overwhelm  your
network.  Thus it is well to think of choosing an appropriate level of
protection.

Isolation comes in  two  kinds:  isolation  against  malfunctions  and
traffic  isolation.  In order to discuss isolation of malfunctions, we
have to have a taxonomy of malfunctions.  Here are the  major  classes
of malfunctions, and which switches can isolate them:

   - Electrical  faults,  e.g.    a short in the cable or some sort of
     fault that distorts the signal.  All types of switch will confine
     this  to  one  side  of  the switch: repeater, buffered repeater,
     bridge, gateway.  These are worth  protecting  against,  although
     their frequency depends upon how often your cables are changed or
     disturbed.  It is rare for this sort of fault  to  occur  without
     some disturbance of the cable.

   - Transceiver and controller problems that general signals that are
     valid electrically but nevertheless incorrect (e.g. a continuous,
     infinitely  long  packet,  spurious  collisions,  never  dropping
     carrier).  All except the  simple  repeater  will  confine  this:
     buffered  repeater, bridge, gateway.  (Such problems are not very
     common.)

   - Software malfunctions that  lead  to  excessive  traffic  between
     particular  hosts  (i.e.  not  broadcasts).  Bridges and gateways
     will isolate these.  (This type of failure is fairly rare.   Most
     software and protocol problems generate broadcasts.)

   - Software  malfunctions  that lead to excessive broadcast traffic.
     Gateways will isolate these.  Generally bridges will not, because
     they  must pass broadcasts.  Bridges with user-settable filtering
     can protect against some  broadcast  malfunctions.    However  in
     general  bridges  must  pass ARP, and most broadcast malfunctions
     involve ARP.    This  problem  is  not  severe  on  single-vendor
     networks  where  software  is  under  careful  control.   However
     research sites generally see problems of this sort regularly.
                                  35



Traffic isolation is provided by bridges and gateways.  The most basic
decision  is  how  many  computers  can  be put onto a network without
overloading its capacity.  This requires knowledge of the capacity  of
the  network,  but  also  how  the hosts will use it.  For example, an
Ethernet may support hundreds of systems if all the  network  is  used
for  is remote logins and an occasional file transfer.  However if the
computers are diskless, and use the network for swapping, an  Ethernet
will  support  between  10 and 40, depending upon their speeds and I/O
rates.

When you have to put more computers onto a network than it can handle,
you split it into several networks and put some sort of switch between
them.  If you do the split correctly, most  of  the  traffic  will  be
between machines on the same piece.  This means putting clients on the
same network as their servers, putting terminal servers  on  the  same
network as the hosts that they access most commonly, etc.

Bridges  and  gateways  generally  provide  similar degrees of traffic
isolation.  In both cases, only traffic bound for hosts on  the  other
side of the switch is passed.  However see the discussion on routing.



5.3.2 Performance


This  is  becoming  less  of  an  issue  as  time  goes  on, since the
technology is improving.  Generally  repeaters  can  handle  the  full
bandwidth  of  the  network.  (By their very nature, a simple repeater
must be able to do so.) Bridges and gateways  often  have  performance
limitations  of  various sorts.  Bridges have two numbers of interest:
packet scanning rate and throughput.  As  explained  above,  a  bridge
must  look  at every packet on the network, even ones that it does not
forward.  The number of packets per second that it can  scan  in  this
way  is  the packet scanning rate.  Throughput applies to both bridges
and gateways.  This is the rate at which  they  can  forward  traffic.
Generally  this  depends  upon  packet  size.   Normally the number of
packets per second that a unit can handle will be  greater  for  short
packets  than  long  ones.    Early models of bridge varied from a few
hundred packets per second to around 7000.  The higher speeds are  for
equipment  that  uses special-purpose hardware to speed up the process
of scanning packets.  First-generation  gateways  varied  from  a  few
hundred packets per second to 1000 or more.  However second-generation
gateways are now available, using special-purpose hardware of the same
sophistication  as that used by bridges.  They can handle on the order
of 10000 packets per second.   Thus  at  the  moment  high-performance
bridges  and gateways can switch most of the bandwidth of an Ethernet.
This means that performance should no longer be a basis  for  choosing
between types of switch.  However within a given type of switch, there
are still specific models with higher or lower capacity.

Unfortunately there  is  no  single  number  on  which  you  can  base
performance estimates.  The figure most commonly quoted is packets per
second.  Be aware that most vendors count a packet  only  once  as  it
goes  through  a gateway, but that one prominent vendor counts packets
                                  36



twice.  Thus their switching rates must be deflated by a factor of  2.
Also,  when  comparing  numbers make sure that they are for packets of
the same size.  A simple performance model is

    processing time = switching time + packet size * time per byte

That is, the time to switch a packet is normally a constant  switching
time, representing interrupt latency, header processing, routing table
lookup,  etc.,  plus  a  component  proportional   to   packet   size,
representing the time needed to do any packet copying.  One reasonable
approach to reporting performance is to give packets  per  second  for
minimum  and  maximum  size  packets.    Another is to report limiting
switching speed in packets per second  and  throughput  in  bytes  per
second, i.e.  the two terms of the equation above.



5.3.3 Routing


Routing refers to the technology used to decide where to send a packet
next.  Of course for a repeater this is not an issue, since  repeaters
forward every packet.

Bridges  are  almost  always  constructed with exactly two interfaces.
Thus routing turns into two decisions: (1) whether the  bridge  should
function  at  all,  and  (2)  whether it should forward any particular
packet.  The second decision is usually based on a table of  MAC-level
addresses.    As described above, this is built up by scanning traffic
on both sides of the bridge.  The goal is  to  forward  those  packets
whose  destination is on the other side of the bridge.  This algorithm
requires that the network configuration have  no  loops  or  redundant
lines.    Less  sophisticated  bridges  leave  this  up  to the system
designer.  With these bridges, you must set up your  network  so  that
there  are no loops in it.  More sophisticated bridges allow arbitrary
topology, but disable links until no  loops  remain.    This  provides
extra  reliability.    If  a  link  fails, an alternative link will be
turned on automatically.  Bridges that work  this  way  have  protocol
that  allows them to detect when a unit must be disabled or reenabled,
so that at any instant the set  of  active  links  forms  a  "spanning
tree".   If you require the extra reliability of redundant links, make
sure that the bridges you use can disable  and  enable  themselves  in
this  way.    There is currently no official standard for the protocol
used among bridges, although there  is  a  standard  in  the  proposal
stage.    If you buy bridges from more than one vendor, make sure that
their spanning-tree protocols will interoperate.

Gateways generally allow arbitrary network topologies, including loops
and  redundant  links.    Because  gateways  may  have  more  than two
interfaces, they must decide not only when to forward  a  packet,  but
where  to  send  it  next.  They do this by maintaining a model of the
entire network topology.  Different routing techniques maintain models
of greater or lesser complexity, and use the data with varying degrees
of sophistication.   Gateways  that  handle  TCP/IP  should  generally
support  the  two  Internet  standard  routing protocols: RIP (Routing
                                  37



Information Protocol) and EGP (External Gateway Protocol).  EGP  is  a
special-purpose protocol for use in networks where there is a backbone
under a separate administration.  It allows exchange  of  reachability
information  with  the  backbone  in  a  controlled way.  If you are a
member of such a network, your gateway must  support  EGP.    This  is
becoming  common  enough  that it is probably a good idea to make sure
that all gateways support EGP.

RIP is a protocol designed to handle routing within small to  moderate
size networks, where line speeds do not differ radically.  Its primary
limitations are:

   - It cannot be used with networks where any path goes through  more
     than  15  gateways.  This range may be further reduced if you use
     an optional feature for giving a slow line a weight  larger  than
     one.

   - It  cannot  share  traffic  between parallel lines (although some
     implementations allow this if the lines are between the same pair
     of gateways).

   - It cannot adapt to changes in network load.

   - It  is  not well suited to situations where there are alternative
     routes through lines of very different speeds.

   - It may not be stable in networks where lines or gateways change a
     lot.

Some  vendors supply proprietary modifications to RIP that improve its
operation with EGP or increase the maximum path length beyond 15,  but
do  not  otherwise modify it very much.  If you expect your network to
involve gateways from more  than  one  vendor,  you  should  generally
require  that  all of them support RIP, since this is the only routing
protocol that is generally available.  If you expect  to  use  a  more
sophisticated  protocol  in  addition,  the  gateways  must  have some
ability to translate between their own protocol and RIP.  However  for
very large or complex networks, there may be no choice but to use some
other protocol throughout.

More sophisticated routing protocols are possible.  The  primary  ones
being considered today are cisco System's IGRP, and protocols based on
the SPF (shortest-path first) algorithms.  In general these  protocols
are designed for larger or more complex networks.  They are in general
stable under a wider  variety  of  conditions,  and  they  can  handle
arbitrary combinations of line type and speed.  Some of them allow you
to  split  traffic  among  parallel  paths,  to  get  better   overall
throughput.    Some newer technologies may allow the network to adjust
to take into account paths that are overloaded.  However at the moment
I  do  not  know of any commercial gateway that does this.  (There are
very serious problems with maintaining stable  routing  when  this  is
done.) There are enough variations among routing technology, and it is
changing rapidly enough, that you should discuss your proposed network
topology  in  detail with all of the vendors that you are considering.
Make sure that their technology can  handle  your  topology,  and  can
                                  38



support  any  special  requirements  that you have for sharing traffic
among parallel lines, and for adjusting topology to take into  account
failures.    In  the  long  run,  we expect one or more of these newer
routing protocols to attain the status of a standard, at least on a de
facto basis.  However at the moment, there is no generally implemented
routing technology other than RIP.

One additional routing topic to consider is policy-based routing.   In
general routing protocols are designed to find the shortest or fastest
possible path for every packet.  In some cases, this is  not  desired.
For  reasons  of  security, cost accountability, etc., you may wish to
limit certain paths to certain uses.   Most  gateways  now  have  some
ability to control the spread of routing information so as to give you
some administrative control over the way routes are used.    Different
gateways  vary  in the degree of control that they support.  Make sure
that you discuss any requirements that you have for control  with  all
prospective gateway vendors.



5.3.4 Network management


Network  management  covers  a  wide variety of topics.  In general it
includes gathering statistical data and status information about parts
of  your network, and taking action as necessary to deal with failures
and other changes.  There are several things that a switch can  do  to
make this process easier.  The most basic is that it should have a way
of gathering and reporting statistics.  These should  include  various
sorts  of packet counts, as well as counts of errors of various kinds.
This data is likely to be  most  detailed  in  a  gateway,  since  the
gateway  classifies  packets using the protocols, and may even respond
to certain types of packet itself.  However bridges and even  buffered
repeaters  can  certainly  have counts of packets forwarded, interface
errors, etc.  It should be  possible  to  collect  this  data  from  a
central monitoring point.

There is now an official Internet approach to network monitoring.  The
first stages use a related set of protocols, SGMP and SNMP.   Both  of
these  protocols  are designed to allow you to collect information and
to make changes in configuration parameters  for  gateways  and  other
entities on your network.  You can run the matching interface programs
on any host in your network.    SGMP  is  now  available  for  several
commercial  gateways,  as  well as for Unix systems that are acting as
gateways.  There is a  limited  set  of  information  which  any  SGMP
implementation  is  required to supply, as well as a uniform mechanism
for vendors to add information of their own.  By late 1988, the second
generation  of  this  protocol, SNMP, should be in service.  This is a
slightly more sophisticated protocol.  It has with it a more  complete
set  of  information that can be monitored, called the MIB (Management
Information Base).  Unlike the somewhat  ad  hoc  collection  of  SGMP
variables,  the  MIB is the result of numerous committee deliberations
involving a number of vendors and users.  Eventually  it  is  expected
that  there  will  be  a  TCP/IP  equivalent  of CMIS, the ISO network
monitoring service.  However CMIS, and its protocols,  CMIP,  are  not
                                  39



yet  official  ISO  standards,  so  they are still in the experimental
stages.

In general terms all of these protocols  accomplish  the  same  thing:
They  allow you to collect criticial information in a uniform way from
all vendors' equipment.  You send commands as  UDP  datagrams  from  a
network  management  program  running  on  some  host in your network.
Generally the interaction is fairly simple,  with  a  single  pair  of
datagrams exchanged: a command and a response.  At the moment security
is fairly simple.  It  is  possible  to  require  what  amounts  to  a
password  in  the  command.   (In SGMP it is referred to as a "session
name", rather  than  a  password.)  More  elaborate,  encryption-based
security is being developed.

You  will  probably  want to configure the network management tools at
your disposal to do several different things.  For short-term  network
monitoring,  you will want to keep track of switches crashing or being
taken down for maintenance, and of failure of communications lines and
other  hardware.  It is possible to configurate SGMP and SNMP to issue
"traps" (unsolited messages) to a specified host or list of hosts when
some of these critical events occur (e.g. lines up and down).  However
it is unrealistic to expect a switch to notify you  when  it  crashes.
It  is  also  possible  for  trap  messages  to be lost due to network
failure or  overload.    Thus  you  should  also  poll  your  switches
regularly  to  gather  information.    Various displays are available,
including a map of your network where  items  change  color  as  their
status  changes, and running "strip charts" that show packet rates and
other items through selected switches.  This software is still in  its
early  stages,  so  you  should  expect  to  see a lot of change here.
However at the very least you should expect to be notified in some way
of  failures.    You  may  also  want  to  be  able to take actions to
reconfigure the system in  response  to  failures,  although  security
issues make some mangers nervous about doing that through the existing
management protocols.

The second type of monitoring you are likely  to  want  to  do  is  to
collect information for use in periodic reports on network utilization
and  performance.    For  this,  you  need  to  sample   each   switch
perodically,  and  retrieve numbers of interest.  At Rutgers we sample
hourly, and get the number of packets forwarded for IP and  DECnet,  a
count  of reloads, and various error counts.  These are reported daily
in some detail.  Monthly summaries are produced giving traffic through
each  gateway,  and a few key error rates chosen to indicate a gateway
that is being overloaded (packets dropped in input and output).

It should be possible to use monitoring techniques of this  kind  with
most  types  of switch.  At the moment, simple repeaters do not report
any statistics.  Since they do not generally have processors in  them,
doing  so  would  cause  a  major  increase in their cost.  However it
should be possible to do network management  for  buffered  repeaters,
bridges,  and  gateways.    Gateways  are  the  most likely to contain
sophisticated network management software.  Most gateway vendors  that
handle  TCP/IP  are  expected  to  implement  the monitoring protocols
described above.    Many  bridge  vendors  make  some  provisions  for
collecting performance data.  Since bridges are not protocol-specific,
                                  40



most  of  them  do  not  have  the  software  necessary  to  implement
TCP/IP-based  network management protocols.  In some cases, monitoring
can be done only by typing commands to  a  directly-attached  console.
(We have seen one case where it is necessary to take the bridge out of
service to gather this data.) In other cases, it is possible to gather
data  via  the  network, but the monitoring protocol is ad hoc or even
proprietary.

Except for very small networks, you should probably insist that all of
the devices on your network collect statistics and provide some way of
querying them remotely.  In the long run,  you  can  expect  the  most
software  to be available for standard protocols such as SGMP/SNMP and
CMIS.  However proprietary monitoring tools may be sufficient as  long
as they work with all of the equipment that you have.



5.3.5 A final evaluation


Here  is  a summary of the places where each kind of switch technology
is normally used:

   - Repeaters are normally confined to a single building.  Since they
     provide  no traffic isolation, you must make sure that the entire
     set of networks connected by repeaters can carry the traffic from
     all  of  the  computers  on  it.  Since they generally provide no
     network monitoring tools, you will not want to use repeaters  for
     a link that is likely to fail.

   - Bridges  and gateways should be placed sufficiently frequently to
     break your network into pieces for which the  traffic  volume  is
     manageable.   You may want to place bridges or gateways in places
     where traffic would  not  require  them  for  network  monitoring
     reasons.

   - Because  bridges must pass broadcast packets, there is a limit to
     the size network you can construct using them.  It is probably  a
     good  idea to limit the network connected by bridges to a hundred
     systems or so.  This number can be increased somewhat for bridges
     with good facilities for filtering.

   - Because  certain  kinds  of  network  misbehavior will be passed,
     bridges should be used only among portions of the network where a
     single group is responsible for diagnosing problems.  You have to
     be crazy to use a bridge  between  networks  owned  by  different
     organizations.    Portions  of your network where experiments are
     being done in network technology should always be  isolated  from
     the rest of the network by gateways.

   - For  many  applications  it is more important to choose a product
     with the right combination  of  performance,  network  management
     tools,  and  other  features  than  to  make the decision between
     bridges and gateways.

                                  41



@section(Configuring Gateways)

This section deals with configuration  issues  that  are  specific  to
gateways.   Gateways than handle TCP/IP are themselves Internet hosts.
Thus the  discussions  above  on  configuring  addresses  and  routing
information  apply to gateways as well as to hosts.  The exact way you
configure a gateway will depend upon the vendor.  In some  cases,  you
edit  files  stored  on  a  disk  in  the gateway itself.  However for
reliability reasons most gateways do not have disks of their own.  For
them, configuration information is stored in non-volatile memory or in
configuration files that are uploaded from one or more  hosts  on  the
network.

At  a  minimum, configuration involves specifying the Internet address
and address mask for  each  interface,  and  enabling  an  appropriate
routing   protocol.    However  generally  a  few  other  options  are
desirable.  There are often parameters in  addition  to  the  Internet
address that you should set for each interface.

One important parameter is the broadcast address.  As explained above,
older software may react badly when broadcasts are sent using the  new
standard  broadcast  address.  For this reason, some vendors allow you
to choose a broadcast address to be used on each interface.  It should
be  set  using  your  knowledge  of  what computers are on each of the
networks.  In general if the computers  follow  current  standards,  a
broadcast  address  of  255.255.255.255 should be used.  However older
implementations may behave better with other  addresses,  particularly
the  address  that  uses  zeros for the host number.  (For the network
128.6 this would be 128.6.0.0.  For compatibility with  software  that
does  not  implement subnets, you would use 128.6.0.0 as the broadcast
address even for a subnet such as  128.6.4.)  You  should  watch  your
network  with  a  network  monitor  and  see  the  results  of several
different broadcast address choices.  If you make a bad choice,  every
time  the  gateway  sends a routing update broadcast, many machines on
your network will respond with ARP's or ICMP errors.  Note  that  when
you  change  the  broadcast  address  in  the gateway, you may need to
change it on the individual computers as well.  Generally the idea  is
to  change  the  address on the systems that you can configure to give
behavior that is compatible with systems that you can't configure.

Other interface parameters may be necessary to deal with peculiarities
of  the  network  it is connected to.  For example, many gateways test
Ethernet interfaces to make sure that the cable is connected  and  the
transceiver  is  working correctly.  Some of these tests will not work
properly with the older Ethernet version 1 transceivers.  If  you  are
using  such  a  transceiver,  you would have to disable this keepalive
testing.  Similarly, gateways connected by a serial line  normally  do
regular  testing  to  make sure that the line is still working.  There
can be situations where this needs to be disabled.

Often you will have to enable features of the software that  you  want
to  use.    For  example, it is often necessary to turn on the network
management protocol explicitly, and to give it the name or address  of
a host that is running software to accept traps (error messages).

                                  42



Most  gateways  have  options  that relate to security.  At a minimum,
this may include setting password for making changes remotely (and the
"session  name"  for  SGMP).  If you need to control access to certain
parts of your network, you will also need  to  define  access  control
lists or whatever other mechanism your gateway uses.

Gateways  that load configuration information over the network present
special issues.   When  such  a  gateway  boots,  it  sends  broadcast
requests of various kinds, attempting to find its Internet address and
then to load configuration information.  Thus it is necessary to  make
sure  that there is some computer that is prepared to respond to these
requests.  In some cases, this is a dedicated  micro  running  special
software.   In other cases, generic software is available that can run
on a variety of machines.  You should consult your vendor to make sure
that  this  can be arranged.  For reliability reasons, you should make
sure that there is  more  than  one  host  with  the  information  and
programs  that  your  gateways  need.   In some cases you will have to
maintain several different files.  For example, the gateways  used  at
Rutgers use a program called "bootp" to supply their Internet address,
and they then load the code and configuration information using  TFTP.
This  means  that  we  have to maintain a file for bootp that contains
Ethernet and Internet addresses for each gateway, and a set  of  files
containing  other configuration information for each gateway.  If your
network is large, it is worth taking some trouble to  make  sure  that
this  information remains consistent.  We keep master copies of all of
the configuration information on a single computer, and distribute  it
to  other  systems  when it changes, using the Unix utilities make and
rdist.    If  your  gateway  has  an  option  to  store  configuration
information  in  non-volatile memory, you will eliminate some of these
logistical headaches.  However this presents its own  problems.    The
contents  of  non-volatile  memory should be backed up in some central
location.  It will also be harder for network management personnel  to
review  configuration  information  if  it  is  distributed  among the
gateways.

Starting  a  gateway  is  particularly   challenging   if   it   loads
configuration  information  from  a  distant  portion  of the network.
Gateways that  expect  to  take  configuration  information  from  the
network  generally  issue broadcast requests on all of the networks to
which they are connected.  If there is a  computer  on  one  of  those
networks  that  is  prepared  to  respond  to  the request, things are
straightforward.  However some gateways may  be  in  remote  locations
where  there  are  no  nearby  computer  systems  that can support the
necessary protocols.  In this case, it is necessary to arrange for the
requests  to  be  routed  back  to network where there are appropriate
computers.  This requires what is strictly speaking a violation of the
basic  design philosophy for gateways.  Generally a gateway should not
allow broadcasts from one network  to  pass  through  to  an  adjacent
network.    In  order  to  allow  a  gateway to get information from a
computer on a different network, at  least  one  of  the  gateways  in
between  will  have  to  be configured to pass the particular class of
broadcasts used to retrieve this information.  If you have  this  sort
of  configuration,  you should test the loading process regularly.  It
is not unusual to find that gateways do not  come  up  after  a  power
failure  because  someone changed the configuration of another gateway
                                  43



and made it impossible to load some necessary information.



5.4 Configuring routing for gateways


The final topic to be considered is configuring routing.  This is more
complex  for  a gateway than for a normal host.  Most Internet experts
recommend that routing be left to the gateways.  Thus hosts may simply
have  a  default  route that points to the nearest gateway.  Of course
the gateways themselves can't get by with this.   They  need  to  have
complete routing tables.

In  order to understand how to configure a gateway, we have to look in
a bit more detail at how gateways communicate routes.  When you  first
turn  on a gateway, the only networks it knows about are the ones that
are  directly  connected  to  it.    (They  are   specified   by   the
configuration  information.)   In order to find out how to get to more
distant parts of the network, it engages  in  some  sort  of  "routing
protocol".    A routing protocol is simply a protocol that allows each
gateway to advertise which networks it can get to, and to spread  that
information  from  one  gateway to the next.  Eventually every gateway
should know how to get to every network.  There are  different  styles
of routing protocol.  In one common type, gateways talk only to nearby
gateways.  In  another  type,  every  gateway  builds  up  a  database
describing  every  other  gateway  in  the system.  However all of the
protocols have some way for each gateway in the system to find out how
to get to every destination.

A  metric is some number or set of numbers that can be used to compare
routes.  The routing table is  constructed  by  gathering  information
from other gateways.  If two other gateways claim to be able to get to
the same destination, there must be some way of deciding which one  to
use.   The metric is used to make that decision.  Metrics all indicate
in some general sense the "cost" of a route.  This may be  a  cost  in
dollars of sending packets over that route, the delay in milliseconds,
or some other measure.  The simplest metric is just  a  count  of  the
number  of  gateways  along  the  path.  This is referred to as a "hop
count".  Generally this metric  information  is  set  in  the  gateway
configuration files, or is derived from information appearing there.

At  a minimum, routing configuration is likely to consist of a command
to enable the routing protocol that you want to  use.    Most  vendors
will have a prefered routing protocol.  Unless you have some reason to
choose another, you should use that.  The normal reason  for  choosing
another  protocol  is  for  compatibility with other kinds of gateway.
For example, your network may be  connected  to  a  national  backbone
network  that  requires  you to use EGP (exterior gateway protocol) to
communicate routes with it.  EGP is only appropriate for that specific
case.    You  should  not use EGP within your own network, but you may
need to use it  in  addition  to  your  regular  routing  protocol  to
communicate  with a national network.  If your own network has several
different types of gateway, then  you  may  need  to  pick  a  routing
protocol  that  all of them support.  At the moment, this is likely to
                                  44



be RIP (Routing Information Protocol).  Depending upon the  complexity
of  your  network,  you  could  use  RIP  throughout it, or use a more
sophisticated protocol among the gateways that support it, and use RIP
only at the boundary between gateways from different vendors.

Assuming  that  you  have  chosen a routing protocol and turned it on,
there are some additional decisions that you may need to make.  One of
the  more  basic configuration options has to do with supplying metric
information.  As indicated above, metrics are numbers which  are  used
to decide which route is the best.  Unsophisticated routing protocols,
e.g. RIP, normally just count hops.  So a route that passes through  2
gateways  would  be  considered better than one that passes through 3.
Of course if the latter route used 1.5Mbps lines and the  former  9600
bps  lines,  this  would  be  the  wrong  decision.  Thus most routing
protocols allow you to set parameters to take this sort of thing  into
account.  With RIP, you would arrange to treat the 9600 bps line as if
it were several hops.  You would  increase  the  effective  hop  count
until  the  better route was chosen.  More sophisticated protocols may
take the bit rate of the line into account automatically.  However you
should  be on the lookout for configuration parameters that need to be
set.    Generally  these  parameters  will  be  associated  with   the
particular  interface.   For example, with RIP you would have to set a
metric value for the interface connected to the 9600 bps line.    With
protocols  that  are  based on bit rate, you might need to specify the
speed  of  each  line  (if  the   gateway   cannot   figure   it   out
automatically).

Most  routing  protocols  are  designed  to let each gateway learn the
topology of the entire network, and to choose the best possible  route
for  each  packet.    In some cases you may not want to use the "best"
route.  You may want traffic to stay out of a certain portion  of  the
network  for  security  or  cost  reasons.   One way to institute such
controls is by specifying routing options.  These options  are  likely
to be different for different vendors.  But the basic strategy is that
if the rest of the network doesn't know about a  route,  it  won't  be
used.    So  controls normally take the form of limiting the spread of
information about routes whose use you want to control.

Note that there  are  ways  for  the  user  to  override  the  routing
decisions made by your gateways.  If you really need to control access
to a certain network, you will have to do two separate  things:    Use
routing  controls  to  make sure that the gateways use only the routes
you want them to.  But also use access control lists on  the  gateways
that are adjacent to the sensitive networks.  These two mechanisms act
at different levels.  The routing controls affect what happens to most
packets:  those  where  the  user  has not specified routing manually.
Your routing mechanism must be set up to choose  an  acceptable  route
for  them.   The access control list provides an additional limitation
which prevents users from supplying their own  routing  and  bypassing
your controls.

For  reliability  and  security reasons, there may also be controls to
allow you to list the gateways from which you will accept information.
It  may  also  be possible to rank gateways by priority.  For example,
you might decide to listen to routes from within your own organization
                                  45



before   routes  from  other  organizations  or  other  parts  of  the
organization.  This would  have  the  effect  of  having  traffic  use
internal  routes  in preference to external ones, even if the external
ones appear to be better.

If you use several different routing protocols, you will probably have
some  decisions  to  make regarding how much information to pass among
them.  Since multiple routing  protocols  are  often  associated  with
multiple  organizations,  you  must be sure to make these decisions in
consultation  with  management  of  all  of  the  relevant   networks.
Decisions  that  you  make may have consequences for the other network
which are not immediately obvious.  You might think it would  be  best
to  configure  the gateway so that everything it knows is passed on by
all routing protocols.  However here are some reasons why you may  not
want to do so:

   - The  metrics  used  by  different  routing  protocols  may not be
     comparable.  If you  are  connected  to  two  different  external
     networks,  you  want to specify that one should always be used in
     preference to the other, or that the nearest one should be  used,
     rather  than  attempting  to  compare metric information received
     from the two networks to see which has the better route.

   - EGP is particularly sensitive, because the  EGP  protocol  cannot
     handle  loops.    Thus  there  are  strict  rules  governing what
     information may be communicated to a backbone that uses EGP.   In
     situations  where  EGP  is being used, management of the backbone
     network should help you configure your routing.

   - If you have slow lines in your network (9600 bps or slower),  you
     may  prefer  not  to send a complete routing table throughout the
     network.  If you are connected to an external  network,  you  may
     prefer  to treat it as a default route, rather than to inject all
     of its routing information into your routing protocol.





















                                  46