InfoMagic Standards 1994 January/InfoMagic Standards - January 1994.iso/inet/doc/hitchgde.txt

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           The Hitchhikers Guide to the Internet


                       25 August 1987



                          Ed Krol
                   [e-mail]















This document was produced through funding of the National
Science Foundation.





Copyright (C) 1987, by the Board of Trustees of The Univer-
sity of Illinois.  Permission to duplicate this document, in
whole or part, is granted provided reference is made to the
source and this copyright is included in whole copies.

Purpose and Audience

This document assumes that one is familiar with the workings
of a non-connected simple IP network (e.g. a few 4.2 BSD
systems on an Ethernet not connected to anywhere else).
Appendix A contains remedial information to get one to this
point.  Its purpose is to get that person, familiar with a
simple net, versed in the "oral tradition" of the Internet
to the point that that net can be connected to the Internet
with little danger to either.  It is not a tutorial, it con-
sists of pointers to other places, literature, and hints
which are not normally documented.  Since the Internet is a
dynamic environment, changes to this document will be made
regularly.  The author welcomes comments and suggestions.
This is especially true of terms for the glossary (defini-
tions are not necessary).



What is the Internet?

In the beginning there was the ARPAnet, a wide area experi-
mental network connecting hosts and terminal servers
together.  Procedures were set up to regulate the allocation
of addresses and to create voluntary standards for the net-
work.  As local area networks became more pervasive, many
hosts became gateways to local networks.  A network layer to
allow the interoperation of these networks was developed and
called IP (Internet Protocol).  Over time other groups
created long haul IP based networks (NASA, NSF, states...).
These nets, too, inter-operate because of IP.  The collec-
tion of all of these interoperating networks is the Inter-
net.

Two groups do much of the research and information work of
the Internet (ISI and SRI).  ISI (the Informational Sciences
Institute) does much of the research, standardization, and
allocation work of the Internet.  SRI International provides
information services for the Internet.  In fact, after you
are connected to the Internet most of the information in
this document can be retrieved from the Network Information
Center (NIC) run by SRI.



Operating the Internet

Each network, be it the ARPAnet, NSFnet or a regional net-
work, has its own operations center.  The ARPAnet is run by
BBN, Inc. under contract from DARPA.  Their facility is
called the Network Operations Center or NOC.  Cornell
University temporarily operates NSFnet (called the Network
Information Service Center, NISC).  It goes on to the


                            -2-

regionals having similar facilities to monitor and keep
watch over the goings on of their portion of the Internet.
In addition, they all should have some knowledge of what is
happening to the Internet in total. If a problem comes up,
it is suggested that a campus network liaison should contact
the network operator to which he is directly connected. That
is, if you are connected to a regional network (which is
gatewayed to the NSFnet, which is connected to the
ARPAnet...)  and have a problem, you should contact your
regional network operations center.



RFCs

The internal workings of the Internet are defined by a set
of documents called RFCs (Request for Comments).  The gen-
eral process for creating an RFC is for someone wanting
something formalized to write a document describing the
issue and mailing it to Jon Postel ([e-mail]).  He
acts as a referee for the proposal.  It is then commented
upon by all those wishing to take part in the discussion
(electronically of course).  It may go through multiple
revisions.  Should it be generally accepted as a good idea,
it will be assigned a number and filed with the RFCs.

The RFCs can be divided into five groups: required, sug-
gested, directional, informational and obsolete.  Required
RFC's (e.g. RFC-791, The Internet Protocol) must be imple-
mented on any host connected to the Internet.  Suggested
RFCs are generally implemented by network hosts.  Lack of
them does not preclude access to the Internet, but may
impact its usability.  RFC-793 (Transmission Control Proto-
col) is a suggested RFC.  Directional RFCs were discussed
and agreed to, but their application has never come into
wide use.  This may be due to the lack of wide need for the
specific application (RFC-937 The Post Office Protocol) or
that, although technically superior, ran against other per-
vasive approaches (RFC-891 Hello).  It is suggested that
should the facility be required by a particular site, an
implementation be done in accordance with the RFC.  This
insures that, should the idea be one whose time has come,
the implementation will be in accordance with some standard
and will be generally usable.  Informational RFCs contain
factual information about the Internet and its operation
(RFC-990, Assigned Numbers).  Finally, as the Internet and
technology have grown, some RFCs have become unnecessary.
These obsolete RFCs cannot be ignored, however.  Frequently
when a change is made to some RFC that causes a new one to
be issued obsoleting others, the new RFC only contains
explanations and motivations for the change.  Understanding
the model on which the whole facility is based may involve
reading the original and subsequent RFCs on the topic.


                            -3-

(Appendix B contains a list of what are considered to be the
major RFCs necessary for understanding the Internet).



The Network Information Center

The NIC is a facility available to all Internet users which
provides information to the community.  There are three
means of NIC contact: network, telephone, and mail.  The
network accesses are the most prevalent.  Interactive access
is frequently used to do queries of NIC service overviews,
look up user and host names, and scan lists of NIC docu-
ments.  It is available by using

     %telnet sri-nic.arpa

on a BSD system and following the directions provided by a
user friendly prompter.  From poking around in the databases
provided one might decide that a document named
NETINFO:NUG.DOC (The Users Guide to the ARPAnet) would be
worth having.  It could be retrieved via an anonymous FTP.
An anonymous FTP would proceed something like the following.
(The dialogue may vary slightly depending on the implementa-
tion of FTP you are using).

     %ftp sri-nic.arpa
     Connected to sri-nic.arpa.
     220 SRI_NIC.ARPA FTP Server Process 5Z(47)-6 at Wed 17-Jun-87 12:01
     Name (sri-nic.arpa:myname): anonymous
     331 ANONYMOUS user ok, send real ident as password.
     Password: myname
     230 User ANONYMOUS logged in at Wed 17-Jun-87 12:01 PDT, job 15.
     ftp> get netinfo:nug.doc
     200 Port 18.144 at host 128.174.5.50 accepted.
     150 ASCII retrieve of <NETINFO>NUG.DOC.11 started.
     226 Transfer Completed 157675 (8) bytes transferred
     local: netinfo:nug.doc  remote:netinfo:nug.doc
     157675 bytes in 4.5e+02 seconds (0.34 Kbytes/s)
     ftp> quit
     221 QUIT command received. Goodbye.

(Another good initial document to fetch is NETINFO:WHAT-
THE-NIC-DOES.TXT)!

Questions of the NIC or problems with services can be asked
of or reported to using electronic mail.  The following
addresses can be used:

     [e-mail]         General user assistance, document requests
     [e-mail]   User registration and WHOIS updates
     [e-mail]  Hostname and domain changes and updates
     [e-mail]      SRI-NIC computer operations


                            -4-

     [e-mail] Comments on NIC publication Services


For people without network access, or if the number of docu-
ments is large, many of the NIC documents are available in
printed form for a small charge.  One frequently ordered
document for starting sites is a compendium of major RFCs.
Telephone access is used primarily for questions or problems
with network access.  (See appendix B for mail/telephone
contact numbers).



The NSFnet Network Service Center

The NSFnet Network Service Center (NNSC) is funded by NSF to
provide a first level of aid to users of NSFnet should they
have questions or encounter problems traversing the network.
It is run by BBN Inc.  Karen Roubicek
([e-mail]) is the NNSC user liaison.

The NNSC, which currently has information and documents
online and in printed form, plans to distribute news through
network mailing lists, bulletins, newsletters, and online
reports.  The NNSC also maintains a database of contact
points and sources of additional information about NSFnet
component networks and supercomputer centers.

Prospective or current users who do not know whom to call
concerning questions about NSFnet use, should contact the
NNSC.  The NNSC will answer general questions, and, for
detailed information relating to specific components of the
Internet, will help users find the appropriate contact for
further assistance.  (Appendix B)



Mail Reflectors

The way most people keep up to date on network news is
through subscription to a number of mail reflectors.  Mail
reflectors are special electronic mailboxes which, when they
receive a message, resend it to a list of other mailboxes.
This in effect creates a discussion group on a particular
topic.  Each subscriber sees all the mail forwarded by the
reflector, and if one wants to put his "two cents" in sends
a message with the comments to the reflector....

The general format to subscribe to a mail list is to find
the address reflector and append the string -REQUEST to the
mailbox name (not the host name).  For example, if you
wanted to take part in the mailing list for NSFnet reflected
by [e-mail], one sends a request to


                            -5-

[e-mail].  This may be a wonderful
scheme, but the problem is that you must know the list
exists in the first place.  It is suggested that, if you are
interested, you read the mail from one list (like NSFNET)
and you will probably become familiar with the existence of
others.  A registration service for mail reflectors is pro-
vided by the NIC in the files NETINFO:INTEREST-GROUPS-1.TXT,
NETINFO:INTEREST-GROUPS-2.TXT, and NETINFO:INTEREST-GROUPS-
3.TXT.

The NSFNET mail reflector is targeted at those people who
have a day to day interest in the news of the NSFnet (the
backbone, regional network, and Internet inter-connection
site workers).  The messages are reflected by a central
location and are sent as separate messages to each sub-
scriber.  This creates hundreds of messages on the wide area
networks where bandwidth is the scarcest.

There are two ways in which a campus could spread the news
and not cause these messages to inundate the wide area net-
works.  One is to re-reflect the message on the campus.
That is, set up a reflector on a local machine which for-
wards the message to a campus distribution list.  The other
is to create an alias on a campus machine which places the
messages into a notesfile on the topic.  Campus users who
want the information could access the notesfile and see the
messages that have been sent since their last access.  One
might also elect to have the campus wide area network
liaison screen the messages in either case and only forward
those which are considered of merit.  Either of these
schemes allows one message to be sent to the campus, while
allowing wide distribution within.



Address Allocation

Before a local network can be connected to the Internet it
must be allocated a unique IP address.  These addresses are
allocated by ISI.  The allocation process consists of get-
ting an application form received from ISI.  (Send a message
to [e-mail] and ask for the template for a
connected address).  This template is filled out and mailed
back to hostmaster.  An address is allocated and e-mailed
back to you.  This can also be done by postal mail (Appendix
B).

IP addresses are 32 bits long.  It is usually written as
four decimal numbers separated by periods (e.g.,
192.17.5.100).  Each number is the value of an octet of the
32 bits.  It was seen from the beginning that some networks
might choose to organize themselves as very flat (one net
with a lot of nodes) and some might organize hierarchically


                            -6-

(many interconnected nets with fewer nodes each and a back-
bone).  To provide for these cases, addresses were differen-
tiated into class A, B, and C networks.  This classification
had to with the interpretation of the octets.  Class A net-
works have the first octet as a network address and the
remaining three as a host address on that network.  Class C
addresses have three octets of network address and one of
host.  Class B is split two and two.  Therefore, there is an
address space for a few large nets, a reasonable number of
medium nets and a large number of small nets.  The top two
bits in the first octet are coded to tell the address for-
mat.  All of the class A nets have been allocated.  So one
has to choose between Class B and Class C when placing an
order.  (There are also class D (Multicast) and E (Experi-
mental) formats.  Multicast addresses will likely come into
greater use in the near future, but are not frequently used
now).

In the past sites requiring multiple network addresses
requested multiple discrete addresses (usually Class C).
This was done because much of the software available (not-
ably 4.2BSD) could not deal with subnetted addresses.
Information on how to reach a particular network (routing
information) must be stored in Internet gateways and packet
switches.  Some of these nodes have a limited capability to
store and exchange routing information (limited to about 300
networks).  Therefore, it is suggested that any campus
announce (make known to the Internet) no more than two
discrete network numbers.

If a campus expects to be constrained by this, it should
consider subnetting.  Subnetting (RFC-932) allows one to
announce one address to the Internet and use a  set of
addresses on the campus.  Basically, one defines a mask
which allows the network to differentiate between the net-
work portion and host portion of the address.  By using a
different mask on the Internet and the campus, the address
can be interpreted in multiple ways.  For example, if a
campus requires two networks internally and has the 32,000
addresses beginning 128.174.X.X (a Class B address) allo-
cated to it,  the campus could allocate 128.174.5.X to one
part of campus and 128.174.10.X to another.  By advertising
128.174 to the Internet with a subnet mask of FF.FF.00.00,
the Internet would treat these two addresses as one. Within
the campus a mask of FF.FF.FF.00 would be used, allowing the
campus to treat the addresses as separate entities. (In
reality you don't pass the subnet mask of FF.FF.00.00 to the
Internet, the octet meaning is implicit in its being a class
B address).

A word of warning is necessary.  Not all systems know how to
do subnetting.  Some 4.2BSD systems require additional
software.  4.3BSD systems subnet as released.  Other devices


                            -7-

and operating systems vary in the problems they have dealing
with subnets.  Frequently these machines can be used as a
leaf on a network but not as a gateway within the subnetted
portion of the network.  As time passes and more systems
become 4.3BSD based, these problems should disappear.

There has been some confusion in the past over the format of
an IP broadcast address.  Some machines used an address of
all zeros to mean broadcast and some all ones.  This was
confusing when machines of both type were connected to the
same network. The broadcast address of all ones has been
adopted to end the grief.  Some systems (e.g. 4.2 BSD) allow
one to choose the format of the broadcast address.  If a
system does allow this choice, care should be taken that the
all ones format is chosen.  (This is explained in RFC-1009
and RFC-1010).



Internet Problems

There are a number of problems with the Internet.  Solutions
to the problems range from software changes to long term
research projects. Some of the major ones are detailed
below:

Number of Networks

     When the Internet was designed it was to have about 50
     connected networks.  With the explosion of networking,
     the number is now approaching 300.  The software in a
     group of critical gateways (called the core gateways of
     the ARPAnet) are not able to pass or store much more
     than that number.  In the short term, core reallocation
     and recoding has raised the number slightly.  By the
     summer of '88 the current PDP-11 core gateways will be
     replaced with BBN Butterfly gateways which will solve
     the problem.

Routing Issues

     Along with sheer mass of the data necessary to route
     packets to a large number of networks, there are many
     problems with the updating, stability, and optimality
     of the routing algorithms.  Much research is being done
     in the area, but the optimal solution to these routing
     problems is still years away.  In most cases the the
     routing we have today works, but sub-optimally and
     sometimes unpredictably.

Trust Issues




                            -8-

     Gateways exchange network routing information.
     Currently, most gateways accept on faith that the
     information provided about the state of the network is
     correct.  In the past this was not a big problem since
     most of the gateways belonged to a single administra-
     tive entity (DARPA).  Now with multiple wide area net-
     works under different administrations, a rogue gateway
     somewhere in the net could cripple the Internet.  There
     is design work going on to solve both the problem of a
     gateway doing unreasonable things and providing enough
     information to reasonably route data between multiply
     connected networks (multi-homed networks).

Capacity & Congestion

     Many portions of the ARPAnet are very congested during
     the busy part of the day.  Additional links are planned
     to alleviate this congestion, but the implementation
     will take a few months.


These problems and the future direction of the Internet are
determined by the Internet Architect (Dave Clark of MIT)
being advised by the Internet Activities Board (IAB).  This
board is composed of chairmen of a number of committees with
responsibility for various specialized areas of the Inter-
net.  The committees composing the IAB and their chairmen
are:

        Committee                            Chair
     Autonomous Networks                  Deborah Estrin
     End-to-End Services                  Bob Braden
     Internet Architecture                Dave Mills
     Internet Engineering                 Phil Gross
        EGP2                              Mike Petry
        Name Domain Planning              Doug Kingston
        Gateway Monitoring                Craig Partridge
        Internic                          Jake Feinler
        Performance & Congestion Control  Robert Stine
        NSF Routing                       Chuck Hedrick
        Misc. MilSup Issues               Mike St. Johns
     Privacy                              Steve Kent
     IRINET Requirements                  Vint Cerf
     Robustness & Survivability           Jim Mathis
     Scientific Requirements              Barry Leiner

Note that under Internet Engineering, there are a set of
task forces and chairs to look at short term concerns.  The
chairs of these task forces are not part of the IAB.



Routing


                            -9-

Routing is the algorithm by which a network directs a packet
from its source to its destination.  To appreciate the prob-
lem, watch a small child trying to find a table in a restau-
rant.  From the adult point of view the structure of the
dining room is seen and an optimal route easily chosen.  The
child, however, is presented with a set of paths between
tables where a good path, let alone the optimal one to the
goal is not discernible.

A little more background might be appropriate.  IP gateways
(more correctly routers) are boxes which have connections to
multiple networks and pass traffic  between these nets.
They decide how the packet is to be sent based on the infor-
mation in the IP header of the packet and the state of the
network.  Each interface on a router has an unique address
appropriate to the network to which it is connected.  The
information in the IP header which is used is primarily the
destination address.  Other information (e.g. type of ser-
vice) is largely ignored at this time.  The state of the
network is determined by the routers passing information
among themselves.  The distribution of the database (what
each node knows), the form of the updates, and metrics used
to measure the value of a connection, are the parameters
which determine the characteristics of a routing protocol.

Under some algorithms each node in the network has complete
knowledge of the state of the network (the adult algorithm).
This implies the nodes must have larger amounts of local
storage and enough CPU to search the large tables in a short
enough time (remember this must be done for each packet).
Also, routing updates usually contain only changes to the
existing information (or you spend a large amount of the
network capacity passing around megabyte routing updates).
This type of algorithm has several problems.  Since the only
way the routing information can be passed around is across
the network and the propagation time is non-trivial, the
view of the network at each node is a correct historical
view of the network at varying times in the past.  (The
adult algorithm, but rather than looking directly at the
dining area, looking at a photograph of the dining room.
One is likely to pick the optimal route and find a bus-cart
has moved in to block the path after the photo was taken).
These inconsistencies can cause circular routes (called
routing loops) where once a packet enters it is routed in a
closed path until its time to live (TTL) field expires and
it is discarded.

Other algorithms may know about only a subset of the net-
work.  To prevent loops in these protocols, they are usually
used in a hierarchical network.  They know completely about
their own area, but to leave that area they go to one par-
ticular place (the default gateway).  Typically these are
used in smaller networks (campus, regional...).


                            -10-



Routing protocols in current use:

Static (no protocol-table/default routing)

     Don't laugh.  It is probably the most reliable, easiest
     to implement, and least likely to get one into trouble
     for a small network or a leaf on the Internet.  This
     is, also, the only method available on some
     CPU-operating system combinations. If a host is con-
     nected to an Ethernet which has only one gateway off of
     it, one should make that the default gateway for the
     host and do no other routing.  (Of course that gateway
     may pass the reachablity information somehow on the
     other side of itself).

     One word of warning, it is only with extreme caution
     that one should use static routes in the middle of a
     network which is also using dynamic routing.  The
     routers passing dynamic information are sometimes con-
     fused by conflicting dynamic and static routes.  If
     your host is on an ethernet with multiple routers to
     other networks on it and the routers are doing dynamic
     routing among themselves, it is usually better to take
     part in the dynamic routing than to use static routes.

RIP

     RIP is a routing protocol based on XNS (Xerox Network
     System) adapted for IP networks.  It is used by many
     routers (Proteon, cisco, UB...) and many BSD Unix sys-
     tems.  BSD systems typically run a program called
     routed to exchange information with other systems run-
     ning RIP.  RIP works best for nets of small diameter
     where the links are of equal speed.  The reason for
     this is that the metric used to determine which path is
     best is the hop-count.  A hop is a traversal across a
     gateway.  So, all machines on the same Ethernet are
     zero hops away.  If a router connects connects two net-
     works directly, a machine on the other side of the
     router is one hop away....  As the routing information
     is passed through a gateway, the gateway adds one to
     the hop counts to keep them consistent across the net-
     work.  The diameter of a network is defined as the
     largest hop-count possible within a network.  Unfor-
     tunately, a hop count of 16 is defined as infinity in
     RIP meaning the link is down. Therefore, RIP will not
     allow hosts separated by more than 15 gateways in the
     RIP space to communicate.

     The other problem with hop-count metrics is that if
     links have different speeds, that difference is not


                            -11-

     reflected in the hop-count. So a one hop satellite link
     (with a .5 sec delay) at 56kb would be used instead of
     a two hop T1 connection. Congestion can be viewed as a
     decrease in the efficacy of a link. So, as a link gets
     more congested, RIP will still know it is the best
     hop-count route and congest it even more by throwing
     more packets on the queue for that link.

     The protocol is not well documented.  A group of people
     are working on producing an RFC to both define the
     current RIP and to do some extensions to it to allow it
     to better cope with larger networks.  Currently, the
     best documentation for RIP appears to be the code to
     BSD routed.


Routed

     The routed program, which does RIP for 4.2BSD systems,
     has many options. One of the most frequently used is:
     routed -q (quiet mode) which means listen to RIP infor-
     mation but never broadcast it.  This would be used by a
     machine on a network with multiple RIP speaking gate-
     ways.  It allows the host to determine which gateway is
     best (hopwise) to use to reach a distant network.  (Of
     course you might want to have a default gateway to
     prevent having to pass all the addresses known to the
     Internet around with RIP).

     There are two ways to insert static routes into routed,
     the /etc/gateways file and the route add command.

     Static routes are useful if you know how to reach a
     distant network, but you are not receiving that route
     using RIP.  For the most part the route add command is
     preferable to use.  The reason for this is that the
     command adds the route to that machine's routing table
     but does not export it through RIP.  The /etc/gateways
     file takes precedence over any routing information
     received through a RIP update.  It is also broadcast as
     fact in RIP updates produced by the host without ques-
     tion, so if a mistake is made in the /etc/gateways
     file, that mistake will soon permeate the RIP space and
     may bring the network to its knees.

     One of the problems with routed is that you have very
     little control over what gets broadcast and what
     doesn't.  Many times in larger networks where various
     parts of the network are under different administrative
     controls, you would like to pass on through RIP only
     nets which you receive from RIP and you know are rea-
     sonable.  This prevents people from adding IP addresses
     to the network which may be illegal and you being
     responsible for passing them on to the Internet.  This


                            -12-

     type of reasonability checks are not available with
     routed and leave it usable, but inadequate for large
     networks.


Hello (RFC-891)

     Hello is a routing protocol which was designed and
     implemented in a experimental software router called a
     "Fuzzball" which runs on a PDP-11. It does not have
     wide usage, but is the routing protocol currently used
     on the NSFnet backbone.  The data transferred between
     nodes is similar to RIP (a list of networks and their
     metrics).  The metric, however, is milliseconds of
     delay.  This allows Hello to be used over nets of vari-
     ous link speeds and performs better in congestive
     situations.

     One of the most interesting side effects of Hello based
     networks is their great timekeeping ability.  If you
     consider the problem of measuring delay on a link for
     the metric, you find that it is not an easy thing to
     do.  You cannot measure round trip time since the
     return link may be more congested, of a different
     speed, or even not there.  It is not really feasible
     for each node on the network to have a builtin WWV
     (nationwide radio time standard) receiver.  So, you
     must design an algorithm to pass around time between
     nodes over the network links where the delay in
     transmission can only be approximated.  Hello routers
     do this and in a nationwide network maintain synchron-
     ized time within milliseconds.


Exterior Gateway Protocol (EGP RFC-904)

     EGP is not strictly a routing protocol, it is a reacha-
     bility protocol. It tells only if nets can be reached
     through a particular gateway, not how good the connec-
     tion is.  It is the standard by which gateways to local
     nets inform the ARPAnet of the nets they can reach.
     There is a metric passed around by EGP but its usage is
     not standardized formally.  Its typical value is value
     is 1 to 8 which are arbitrary goodness of link values
     understood by the internal DDN gateways. The smaller
     the value the better and a value of 8 being unreach-
     able.  A quirk of the protocol prevents distinguishing
     between 1 and 2, 3 and 4..., so the usablity of this as
     a metric is as three values and unreachable.  Within
     NSFnet the values used are 1, 3, and unreachable.  Many
     routers talk EGP so they can be used for ARPAnet gate-
     ways.



                            -13-



Gated

     So we have regional and campus networks talking RIP
     among   themselves,  the  NSFnet  backbone  talking
     Hello, and the DDN speaking EGP.

     How do they interoperate?  In the beginning there was
     static routing, assembled into the Fuzzball software
     configured for each site.  The problem with doing
     static routing in the middle of the network is that it
     is broadcast to the Internet whether it is usable or
     not.  Therefore, if a net becomes unreachable and you
     try to get there, dynamic routing will immediately
     issue a net unreachable to you.  Under static routing
     the routers would think the net could be reached and
     would continue trying until the application gave up (in
     2 or more minutes).  Mark Fedor of Cornell
     ([e-mail]) attempted to solve these
     problems with a replacement for routed called gated.

     Gated talks RIP to RIP speaking hosts, EGP to EGP
     speakers, and Hello to Hello'ers.  These speakers fre-
     quently all live on one Ethernet, but luckily (or
     unluckily) cannot understand each others ruminations.
     In addition, under configuration file control it can
     filter the conversion.  For example, one can produce a
     configuration saying announce RIP nets via Hello only
     if they are specified in a list and are reachable by
     way of a RIP broadcast as well.  This means that if a
     rogue network appears in your local site's RIP space,
     it won't be passed through to the Hello side of the
     world.  There are also configuration options to do
     static routing and name trusted gateways.

     This may sound like the greatest thing since sliced
     bread, but there is a catch called metric conversion.
     You have RIP measuring in hops, Hello measuring in mil-
     liseconds, and EGP using arbitrary small numbers.  The
     big questions is how many hops to a millisecond, how
     many milliseconds in the EGP number 3....  Also,
     remember that infinity (unreachability) is 16 to RIP,
     30000 or so to Hello, and 8 to the DDN with EGP.  Get-
     ting all these metrics to work well together is no
     small feat.  If done incorrectly and you translate an
     RIP of 16 into an EGP of 6, everyone in the ARPAnet
     will still think your gateway can reach the unreachable
     and will send every packet in the world your way.  For
     these reasons, Mark requests that you consult closely
     with him when configuring and using gated.




                            -14-

Names

All routing across the network is done by means of the IP
address associated with a packet. Since humans find it dif-
ficult to remember addresses like 128.174.5.50, a symbolic
name register was set up at the NIC where people would say
"I would like my host to be named 'uiucuxc'".  Machines con-
nected to the Internet across the nation would connect to
the NIC in the middle of the night, check modification dates
on the hosts file, and if modified move it to their local
machine.  With the advent of workstations and micros,
changes to the host file would have to be made nightly.  It
would also be very labor intensive and consume a lot of net-
work bandwidth. RFC-882 and a number of others describe
domain name service, a distributed data base system for map-
ping names into addresses.

We must look a little more closely into what's in a name.
First, note that an address specifies a particular connec-
tion on a specific network.  If the machine moves, the
address changes.  Second, a machine can have one or more
names and one or more network addresses (connections) to
different networks.  Names point to a something which does
useful work (i.e. the machine) and IP addresses point to an
interface on that provider.  A name is a purely symbolic
representation of a list of addresses on the network.  If a
machine moves to a different network, the addresses will
change but the name could remain the same.

Domain names are tree structured names with the root of the
tree at the right.  For example:

                      uxc.cso.uiuc.edu

is a machine called 'uxc' (purely arbitrary), within the
subdomains method of allocation of the U of I) and 'uiuc'
(the University of Illinois at Urbana), registered with
'edu' (the set of educational institutions).

A simplified model of how a name is resolved is that on the
user's machine there is a resolver.  The resolver knows how
to contact across the network a root name server. Root
servers are the base of the tree structured data retrieval
system.  They know who is responsible for handling first
level domains (e.g. 'edu').  What root servers to use is an
installation parameter. From the root server the resolver
finds out who provides 'edu' service.  It contacts the 'edu'
name server which supplies it with a list of addresses of
servers for the subdomains (like 'uiuc').  This action is
repeated with the subdomain servers until the final sub-
domain returns a list of addresses of interfaces on the host
in question.  The user's machine then has its choice of
which of these addresses to use for communication.


                            -15-

A group may apply for its own domain name (like 'uiuc'
above).  This is done in a manner similar to the IP address
allocation.  The only requirements are that the requestor
have two machines reachable from the Internet, which will
act as name servers for that domain.  Those servers could
also act as servers for subdomains or other servers could be
designated as such.  Note that the servers need not be
located in any particular place, as long as they are reach-
able for name resolution.  (U of I could ask Michigan State
to act on its behalf and that would be fine).  The biggest
problem is that someone must do maintenance on the database.
If the machine is not convenient, that might not be done in
a timely fashion.  The other thing to note is that once the
domain is allocated to an administrative entity, that entity
can freely allocate subdomains using what ever manner it
sees fit.

The Berkeley Internet Name Domain (BIND) Server implements
the Internet name server for UNIX systems.  The name server
is a distributed data base system that allows clients to
name resources and to share that information with other net-
work hosts.  BIND is integrated with 4.3BSD and is used to
lookup and store host names, addresses, mail agents, host
information, and more.  It replaces the /etc/hosts file for
host name lookup.  BIND is still an evolving program.  To
keep up with reports on operational problems, future design
decisions, etc, join the BIND mailing list by sending a
request to [e-mail].  It can also
be obtained via anonymous FTP from ucbarpa.berkley.edu.

There are several advantages in using BIND.  One of the most
important is that it frees a host from relying on /etc/hosts
being up to date and complete.  Within the .uiuc.edu domain,
only a few hosts are included in the host table distributed
by SRI.  The remainder are listed locally within the BIND
tables on uxc.cso.uiuc.edu (the server machine for most of
the .uiuc.edu domain).  All are equally reachable from any
other Internet host running BIND.

BIND can also provide mail forwarding information for inte-
rior hosts not directly reachable from the Internet.  These
hosts can either be on non-advertised networks, or not con-
nected to a network at all, as in the case of UUCP-reachable
hosts.  More information on BIND is available in the "Name
Server Operations Guide for BIND" in UNIX System Manager's
Manual, 4.3BSD release.

There are a few special domains on the network, like SRI-
NIC.ARPA.  The 'arpa' domain is historical, referring to
hosts registered in the old hosts database at the NIC.
There are others of the form NNSC.NSF.NET.  These special
domains are used sparingly and require ample justification.
They refer to servers under the administrative control of


                            -16-

the network rather than any single organization.  This
allows for the actual server to be moved around the net
while the user interface to that machine remains constant.
That is, should BBN relinquish control of the NNSC, the new
provider would be pointed to by that name.

In actuality, the domain system is a much more general and
complex system than has been described.  Resolvers and some
servers cache information to allow steps in the resolution
to be skipped.  Information provided by the servers can be
arbitrary, not merely IP addresses.  This allows the system
to be used both by non-IP networks and for mail, where it
may be necessary to give information on intermediate mail
bridges.


What's wrong with Berkeley Unix

University of California at Berkeley has been funded by
DARPA to modify the Unix system in a number of ways.
Included in these modifications is support for the Internet
protocols.  In earlier versions (e.g. BSD 4.2) there was
good support for the basic Internet protocols (TCP, IP,
SMTP, ARP) which allowed it to perform nicely on IP ether-
nets and smaller Internets.  There were deficiencies, how-
ever, when it was connected to complicated networks.  Most
of these problems have been resolved under the newest
release (BSD 4.3).  Since it is the springboard from which
many vendors have launched Unix implementations (either by
porting the existing code or by using it as a model), many
implementations (e.g. Ultrix) are still based on BSD 4.2.
Therefore, many implementations still exist with the BSD 4.2
problems.  As time goes on, when BSD 4.3 trickles through
vendors as new release, many of the problems will be
resolved.  Following is a list of some problem scenarios and
their handling under each of these releases.

ICMP redirects

     Under the Internet model, all a system needs to know to
     get anywhere in the Internet is its own address, the
     address of where it wants to go, and how to reach a
     gateway which knows about the Internet.  It doesn't
     have to be the best gateway.  If the system is on a
     network with multiple gateways, and a host sends a
     packet for delivery to a gateway which feels another
     directly connected gateway is more appropriate, the
     gateway sends the sender a message.  This message is an
     ICMP redirect, which politely says "I'll deliver this
     message for you, but you really ought to use that gate-
     way over there to reach this host".  BSD 4.2 ignores
     these messages.  This creates more stress on the gate-
     ways and the local network, since for every packet


                            -17-

     sent, the gateway sends a packet to the originator.
     BSD 4.3 uses the redirect to update its routing tables,
     will use the route until it times out, then revert to
     the use of the route it thinks is should use.  The
     whole process then repeats, but it is far better than
     one per packet.

Trailers

     An application (like FTP) sends a string of octets to
     TCP which breaks it into chunks, and adds a TCP header.
     TCP then sends blocks of data to IP which adds its own
     headers and ships the packets over the network.  All
     this prepending of the data with headers causes memory
     moves in both the sending and the receiving machines.
     Someone got the bright idea that if packets were long
     and they stuck the headers on the end (they became
     trailers), the receiving machine could put the packet
     on the beginning of a page boundary and if the trailer
     was OK merely delete it and transfer control of the
     page with no memory moves involved.  The problem is
     that trailers were never standardized and most gateways
     don't know to look for the routing information at the
     end of the block.  When trailers are used, the machine
     typically works fine on the local network (no gateways
     involved) and for short blocks through gateways (on
     which trailers aren't used).  So TELNET and FTP's of
     very short files work just fine and FTP's of long files
     seem to hang.  On BSD 4.2 trailers are a boot option
     and one should make sure they are off when using the
     Internet.  BSD 4.3 negotiates trailers, so it uses them
     on its local net and doesn't use them when going across
     the network.

Retransmissions

     TCP fires off blocks to its partner at the far end of
     the connection.  If it doesn't receive an acknowledge-
     ment in a reasonable amount of time it retransmits the
     blocks.  The determination of what is reasonable is
     done by TCP's retransmission algorithm.  There is no
     correct algorithm but some are better than others,
     where better is measured by the number of retransmis-
     sions done unnecessarily.  BSD 4.2 had a retransmission
     algorithm which retransmitted quickly and often.  This
     is exactly what you would want if you had a bunch of
     machines on an ethernet (a low delay network of large
     bandwidth).  If you have a network of relatively longer
     delay and scarce bandwidth (e.g. 56kb lines), it tends
     to retransmit too aggressively.  Therefore, it makes
     the networks and gateways pass more traffic than is
     really necessary for a given conversation.  Retransmis-
     sion algorithms do adapt to the delay of the network


                            -18-

     after a few packets, but 4.2's adapts slowly in delay
     situations.  BSD 4.3 does a lot better and tries to do
     the best for both worlds.  It fires off a few
     retransmissions really quickly assuming it is on a low
     delay network, and then backs off very quickly.  It
     also allows the delay to be about 4 minutes before it
     gives up and declares the connection broken.
















































                            -19-




                           Appendix A
               References to Remedial Information


     Quaterman and Hoskins, "Notable Computer Networks",
     Communications of the ACM, Vol 29, #10, pp.
     (October, 1986).

     Tannenbaum, Andrew S., Computer Networks, Prentice
     Hall, 1981.

     Hedrick, Chuck, Introduction to the Internet Protocols,
     Anonymous FTP from topaz.rutgers.edu, directory
     pub/tcp-ip-docs, file tcp-ip-intro.doc.






































                            -20-

                           Appendix B
                       List of Major RFCs


          RFC-768        User Datagram Protocol (UDP)
          RFC-791        Internet Protocol (IP)
          RFC-792        Internet Control Message Protocol (ICMP)
          RFC-793        Transmission Control Protocol (TCP)
          RFC-821        Simple Mail Transfer Protocol (SMTP)
          RFC-822        Standard for the Format of ARPA Internet Text
                           Messages
          RFC-854        Telnet Protocol
          RFC-917 *      Internet Subnets
          RFC-919 *      Broadcasting Internet Datagrams
          RFC-922 *      Broadcasting Internet Datagrams in the Presence
                           of Subnets
          RFC-940 *      Toward an Internet Standard Scheme for Sub-
                           netting
          RFC-947 *      Multi-network Broadcasting within the Internet
          RFC-950 *      Internet Standard Subnetting Procedure
          RFC-959        File Transfer Protocol (FTP)
          RFC-966 *      Host Groups: A Multicast Extension to the
                           Internet Protocol
          RFC-988 *      Host Extensions for IP Multicasting
          RFC-997 *      Internet Numbers
          RFC-1010 *     Assigned Numbers
          RFC-1011 *     Official ARPA-Internet Protocols

     RFC's marked with the asterisk (*) are not included in
     the 1985 DDN Protocol Handbook.

     Note: This list is a portion of a list of RFC's by
     topic retrieved from the NIC under NETINFO:RFC-SETS.TXT
     (anonymous FTP of course).

     The following list is not necessary for connection to
     the Internet, but is useful in understanding the domain
     system, mail system, and gateways:

          RFC-882        Domain Names - Concepts and Facilities
          RFC-883        Domain Names - Implementation
          RFC-973        Domain System Changes and Observations
          RFC-974        Mail Routing and the Domain System
          RFC-1009       Requirements for Internet Gateways












                            -21-




                           Appendix C
             Contact Points for Network Information


Network Information Center (NIC)

     DDN Network Information Center
     SRI International, Room EJ291
     333 Ravenswood Avenue
     Menlo Park, CA 94025
     [phone] or [phone]
     [e-mail]


NSF Network Service Center (NNSC)

     NNSC
     BBN Laboratories Inc.
     10 Moulton St.
     Cambridge, MA 02238
     [phone]
     [e-mail]






























                            -22-




                          Glossary


core gateway   The innermost gateways of the ARPAnet.  These
               gateways have a total picture of the reacha-
               bility to all networks known to the ARPAnet
               with EGP.  They then redistribute reachabil-
               ity information to all those gateways speak-
               ing EGP.  It is from them your EGP agent
               (there is one acting for you somewhere if you
               can reach the ARPAnet) finds out it can reach
               all the nets on the ARPAnet. Which is then
               passed to you via Hello, gated, RIP....

count to infinity The symptom of a routing problem where
               routing information is passed in a circular
               manner through multiple gateways.  Each gate-
               way increments the metric appropriately and
               passes it on.  As the metric is passed around
               the loop, it increments to ever increasing
               values til it reaches the maximum for the
               routing protocol being used, which typically
               denotes a link outage.

hold down      When a router discovers a path in the network
               has gone down announcing that that path is
               down for a minimum amount of time (usually at
               least two minutes).  This allows for the pro-
               pagation of the routing information across
               the network and prevents the formation of
               routing loops.

split horizon  When a router (or group of routers working in
               consort) accept routing information from mul-
               tiple external networks, but do not pass on
               information learned from one external network
               to any others.  This is an attempt to prevent
               bogus routes to a network from being pro-
               pagated because of gossip or counting to
               infinity.












                            -23-