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draft-ietf-atm-nbma-00.txt
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IP over ATM Working Group Juha Heinanen
Reguest for Comments: DRAFT Telecom Finland
Expires August 12, 1993 February 12, 1993
NBMA Address Resolution Protocol (NBMA ARP)
Abstract
This document contains an informal description of the NBMA ARP
protocol and is only intended as a basis for a discussion. The
section describing the protocol procedures in detail will be filled
in later.
1. Introduction
The NBMA Address Resolution Protocol (NBMA ARP) allows a source
terminal (a host or router) connected to a non-broadcast, multiaccess
link layer network (called NBMA for short) to resolve a network layer
address of a destination terminal to its link layer address in the
same NBMA. The destination terminal can be connected to the NBMA
either directly or indirectly via a router. Once the address
resolution has been completed, the source terminal may either start
sending network layer Protocol Data Units (PDUs) to the destination
(connectionless NBMA) or may first establish a connection to the
destination with a desired bandwidth and other QOS characteristics
(connection oriented NBMA).
Note that an NBMA can be non-broadcast either because it technically
doesn't support broadcasting (e.g. an X.25 network) or because
broadcasting is not feasible for one reason or another (e.g. an SMDS
broadcast group would be too large).
2. Protocol Overview
The NBMA ARP protocol is based on interaction between terminals
(hosts and routers) and so-called NBMA ARP servers (ARP servers for
short). A terminal generates an NBMA ARP Request (ARP Request for
short) to one of its ARP servers. The server then either responds
with an NBMA ARP Reply (ARP Reply for short) or forwards the request
to another ARP server. Forwarding of the requests and replies is
based on the network layer destination address found in the ARP PDU
(see section 4).
The forwarding function requires that ARP servers are able to route
each supported network layer protocol. ARP servers don't, however,
need to be able to switch network layer PDUs belonging to those
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RFC DRAFT NBMA ARP February 1993
protocols. Exceptions are ARP servers acting as proxys for terminals
connected to other link layer networks. Such servers must also be
able to switch network layer PDUs between the NBMA and the other link
layer networks.
Each terminal is associated with one or more ARP servers to which the
terminal advertizes its network and NBMA addresses and which the
terminal uses to resolve network layer addresses of other terminals.
When a terminal wants to resolve a network layer address of a
destination terminal, it sends an ARP Request to one of its ARP
servers. After receiving an ARP Request, an ARP server checks if it
is serving the destination terminal. If so, it generates a positive
ARP Reply that contains the NBMA address of the destination terminal
and forwards it towards the network address of the source terminal.
If the ARP server is not serving the destination terminal, it looks
up from its forwarding table the next hop towards the destination.
If no entry is found, the ARP server generates a negative ARP Reply
and forwards it towards the network address of the source terminal.
If a forwarding entry is found, the ARP server checks if the next hop
is behind its NBMA interface. If so, it forwards the ARP Request to
the next hop ARP server. If the next hop is behind some other
interface, the ARP server may be willing to act as a proxy for the
destination terminal, in which case it generates a positive ARP Reply
containing its own NBMA address as the link layer address of the
destination terminal.
When a positive ARP Reply reaches the source terminal, it may need to
establish a connection to the given NBMA address before starting to
send network layer PDUs to the destination. If communication attempt
fails and the ARP reply was non-authoritative, the terminal may,
before giving up, send another ARP Request, this time asking for an
authoritative answer. A positive ARP Reply will be non-authoritative
if an ARP server is replying based on cached information.
3. Configuration
Terminals
A terminal connected to an NBMA is configured with its network and
NBMA addresses as well as with an NBMA address of each of its ARP
servers. The configuration may be manual or automatic depending on
the network layer protocol and the properties of the NBMA. For
example, an ATM network may help a connected terminal to determine
its ATM address and may also define one or more predefined ATM
addresses of ARP servers.
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RFC DRAFT NBMA ARP February 1993
Once the addresses are configured, the terminal may set up a
connection to each of its ARP servers (unless a permanent connection
has already been configured or the NBMA is connectionless). Then the
terminal can inform the ARP servers about its network and NBMA
addresses using a suitable discovery protocol. The discovery
protocol depends on the network layer protocol. For example, an IP
terminal may use InARP [RFC 1923] and a CLNP terminal may use ES-IS
[ISO 9542].
If a terminal is connected to more than one link layer network, it
may also need to be configured to receive routing information from
its ARP servers. The terminal can then use the routing information
to find out which network address prefixes are reachable via which
link layer networks and at which cost.
ARP Servers
An ARP server is configured with a set of network address prefixes
that correspond to the network addresses of the terminals it is
prepared to serve. An IP ARP server, for example, must belong to the
same IP networks as the served terminals. Similarly, a CLNP ARP
server must share common area addresses with the terminals it is
serving.
An ARP server must also be configured with whatever information is
needed so that it can exchange routing information with its
neighboring ARP servers (if any). Routing information is exchanged
using regular intra- and/or inter-domain routing protocols such as
OSPF, (dual) IS-IS, BGP, or IDRP. As mentioned above, it may also
need to be configured to advertize its routing information to the
served terminals.
An ARP server acting as a proxy for terminals connected to other link
layer networks, must, in addition to the above, be configured to
redistribute routing information between the NBMA and the other link
layer networks.
4. PDU Format
The NBMA ARP PDU format is a modification of the standard ARP [RFC
826] PDU format. In both formats, the operation code field is
located at the same offset from the beginning of the PDU, which makes
it possible to use the same protocol identification codes (e.g.
EtherType) for both the standard ARP and the NBMA ARP. The format of
the NBMA ARP PDU is shown below:
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RFC DRAFT NBMA ARP February 1993
nbma-arp$lnk 2 bytes Link layer type
nbma-arp$net 2 bytes Network layer type
nbma-arp$sll 1 byte Length of source link layer
address
nbma-arp$snl 1 byte Length of source network
layer address
nbma-arp$op 1 byte Operation code
nbma-arp$dll 1 byte Length of destination link
layer address
nbma-arp$dnl 1 byte Length of destination network
layer address
nbma-arp$sla m bytes Source link layer address
nbma-arp$sna n bytes Source network layer address
nbma-arp$dla o bytes Destination link layer
address
nbma-arp$dna p bytes Destination network layer
address
The possible values for the Link layer type and Network layer type
fields are the same as for the Hardware type and Protocol type of the
standard ARP and may be found in the current Assigned Numbers RFC.
All Length fields give the length of the corresponding address in
bits. An empty address field is indicated with the length value 0.
The operation code indicates the type of the message. The assigned
values are:
NBMA ARP Request = 10
NBMA ARP Request for Authoritative Information = 11
NBMA ARP Positive, Authoritative Reply = 12
NBMA ARP Positive, Non-Authoritative Reply = 13
NBMA ARP Negative, Authoritative Reply = 14
These values were chosen so as not to conflict with other ARP
extensions.
In requests and negative replies, the Destination link layer address
field is always empty. All fields are present in all other messages.
The difference between NBMA Request and NBMA Request for
Authoritative Information is that in the former case an NBMA ARP
server is not allowed to reply based on cached information. There
also exists two versions of positive replies so that the requester
knows if a reply to an NBMA ARP Request was based on authoritative or
cached information.
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RFC DRAFT NBMA ARP February 1993
5. Protocol Operation
A detailed specification of the NBMA ARP protocol will be added
later.
6. Discussion
The result of an ARP Request depends on how routing is configured
among the ARP servers. If the destination terminal is directly
connected to the NBMA and the ARP servers always prefer NBMA routes
over routes via other link layer networks, then the ARP Reply always
returns the NBMA address of the destination terminal itself rather
than the NBMA address of some proxy ARP server. For destinations
outside the NBMA, routing between proxy ARP servers and routers in
the other link layer networks should be organized so that the desired
proxy ARP server is always found.
In addition to ARP servers, an NBMA terminal could also be associated
with one or more regular routers that could act as "connectionless
servers" for the terminal. Then the terminal could choose whether to
try to resolve the network layer address of a destination to its NBMA
address or whether just to send the network layer PDUs to one of the
terminal's connectionless servers. The latter option may be
desirable if communication with the destination is short lived and/or
doesn't require much network resources. The connectionless servers
could, of course, be physically integrated in the ARP servers by
augmenting them with network layer switching functionality.
Although the above description of the NBMA ARP protocol deals with
resolving network layer addresses to link layer addresses, the
protocol can as well be applied to resolving a network or link layer
address to another network or link layer address. Such a capability
may be needed if a single logical network consists of subnetworks
implementing different addressing schemes. An example could be an
ATM network consisting of both NSAP and E.164 addressed subnetworks.
Several levels of ARPing may, however, add a considerable delay to
the connection set up process and should thus be avoided.
Acknowledgements
I would like to thank Dennis Ferguson of ANS, Joel Halpern of Network
Systems, and Paul Tsuchiya of Bellcore for their valuable insight and
comments to earlier versions of this draft.
Author's Address
Juha Heinanen Telecom Finland, PO Box 228, SF-33101 Tampere, Finland
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RFC DRAFT NBMA ARP February 1993
Phone: +358 49 500 958
Email: Juha.Heinanen@datanet.tele.fi
Heinanen Expires August 12, 1993 [Page 6]