rfc826.txt

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Network Working Group                                   David C. Plummer 
Request For Comments:  826                                  (DCP@MIT-MC)
                                                           November 1982


	     An Ethernet Address Resolution Protocol
                            -- or --
              Converting Network Protocol Addresses
                   to 48.bit Ethernet Address
                       for Transmission on
                        Ethernet Hardware





			    Abstract

The implementation of protocol P on a sending host S decides,
through protocol P's routing mechanism, that it wants to transmit
to a target host T located some place on a connected piece of
10Mbit Ethernet cable.  To actually transmit the Ethernet packet
a 48.bit Ethernet address must be generated.  The addresses of
hosts within protocol P are not always compatible with the
corresponding Ethernet address (being different lengths or
values).  Presented here is a protocol that allows dynamic
distribution of the information needed to build tables to
translate an address A in protocol P's address space into a
48.bit Ethernet address.

Generalizations have been made which allow the protocol to be
used for non-10Mbit Ethernet hardware.  Some packet radio
networks are examples of such hardware.

--------------------------------------------------------------------

The protocol proposed here is the result of a great deal of
discussion with several other people, most notably J. Noel
Chiappa, Yogen Dalal, and James E. Kulp, and helpful comments
from David Moon.




[The purpose of this RFC is to present a method of Converting
Protocol Addresses (e.g., IP addresses) to Local Network
Addresses (e.g., Ethernet addresses).  This is a issue of general
concern in the ARPA Internet community at this time.  The
method proposed here is presented for your consideration and
comment.  This is not the specification of a Internet Standard.]

Notes:
------       

This protocol was originally designed for the DEC/Intel/Xerox
10Mbit Ethernet.  It has been generalized to allow it to be used
for other types of networks.  Much of the discussion will be
directed toward the 10Mbit Ethernet.  Generalizations, where
applicable, will follow the Ethernet-specific discussion.

DOD Internet Protocol will be referred to as Internet.

Numbers here are in the Ethernet standard, which is high byte
first.  This is the opposite of the byte addressing of machines
such as PDP-11s and VAXes.  Therefore, special care must be taken
with the opcode field (ar$op) described below.

An agreed upon authority is needed to manage hardware name space
values (see below).  Until an official authority exists, requests
should be submitted to
	David C. Plummer
	Symbolics, Inc.
	243 Vassar Street
	Cambridge, Massachusetts  02139
Alternatively, network mail can be sent to DCP@MIT-MC.

The Problem:
------------

The world is a jungle in general, and the networking game
contributes many animals.  At nearly every layer of a network
architecture there are several potential protocols that could be
used.  For example, at a high level, there is TELNET and SUPDUP
for remote login.  Somewhere below that there is a reliable byte
stream protocol, which might be CHAOS protocol, DOD TCP, Xerox
BSP or DECnet.  Even closer to the hardware is the logical
transport layer, which might be CHAOS, DOD Internet, Xerox PUP,
or DECnet.  The 10Mbit Ethernet allows all of these protocols
(and more) to coexist on a single cable by means of a type field
in the Ethernet packet header.  However, the 10Mbit Ethernet
requires 48.bit addresses on the physical cable, yet most
protocol addresses are not 48.bits long, nor do they necessarily
have any relationship to the 48.bit Ethernet address of the
hardware.  For example, CHAOS addresses are 16.bits, DOD Internet
addresses are 32.bits, and Xerox PUP addresses are 8.bits.  A
protocol is needed to dynamically distribute the correspondences
between a <protocol, address> pair and a 48.bit Ethernet address.

Motivation:
-----------

Use of the 10Mbit Ethernet is increasing as more manufacturers
supply interfaces that conform to the specification published by
DEC, Intel and Xerox.  With this increasing availability, more
and more software is being written for these interfaces.  There
are two alternatives: (1) Every implementor invents his/her own
method to do some form of address resolution, or (2) every
implementor uses a standard so that his/her code can be
distributed to other systems without need for modification.  This
proposal attempts to set the standard.

Definitions:
------------

Define the following for referring to the values put in the TYPE
field of the Ethernet packet header:
	ether_type$XEROX_PUP,
	ether_type$DOD_INTERNET,
	ether_type$CHAOS, 
and a new one:
	ether_type$ADDRESS_RESOLUTION.  
Also define the following values (to be discussed later):
	ares_op$REQUEST (= 1, high byte transmitted first) and
	ares_op$REPLY   (= 2), 
and
	ares_hrd$Ethernet (= 1).

Packet format:
--------------

To communicate mappings from <protocol, address> pairs to 48.bit
Ethernet addresses, a packet format that embodies the Address
Resolution protocol is needed.  The format of the packet follows.

    Ethernet transmission layer (not necessarily accessible to
	 the user):
	48.bit: Ethernet address of destination
	48.bit: Ethernet address of sender
	16.bit: Protocol type = ether_type$ADDRESS_RESOLUTION
    Ethernet packet data:
	16.bit: (ar$hrd) Hardware address space (e.g., Ethernet,
			 Packet Radio Net.)
	16.bit: (ar$pro) Protocol address space.  For Ethernet
			 hardware, this is from the set of type
			 fields ether_typ$<protocol>.
	 8.bit: (ar$hln) byte length of each hardware address
	 8.bit: (ar$pln) byte length of each protocol address
	16.bit: (ar$op)  opcode (ares_op$REQUEST | ares_op$REPLY)
	nbytes: (ar$sha) Hardware address of sender of this
			 packet, n from the ar$hln field.
	mbytes: (ar$spa) Protocol address of sender of this
			 packet, m from the ar$pln field.
	nbytes: (ar$tha) Hardware address of target of this
			 packet (if known).
	mbytes: (ar$tpa) Protocol address of target.


Packet Generation:
------------------

As a packet is sent down through the network layers, routing
determines the protocol address of the next hop for the packet
and on which piece of hardware it expects to find the station
with the immediate target protocol address.  In the case of the
10Mbit Ethernet, address resolution is needed and some lower
layer (probably the hardware driver) must consult the Address
Resolution module (perhaps implemented in the Ethernet support
module) to convert the <protocol type, target protocol address>
pair to a 48.bit Ethernet address.  The Address Resolution module
tries to find this pair in a table.  If it finds the pair, it
gives the corresponding 48.bit Ethernet address back to the
caller (hardware driver) which then transmits the packet.  If it
does not, it probably informs the caller that it is throwing the
packet away (on the assumption the packet will be retransmitted
by a higher network layer), and generates an Ethernet packet with
a type field of ether_type$ADDRESS_RESOLUTION.  The Address
Resolution module then sets the ar$hrd field to
ares_hrd$Ethernet, ar$pro to the protocol type that is being
resolved, ar$hln to 6 (the number of bytes in a 48.bit Ethernet
address), ar$pln to the length of an address in that protocol,
ar$op to ares_op$REQUEST, ar$sha with the 48.bit ethernet address
of itself, ar$spa with the protocol address of itself, and ar$tpa
with the protocol address of the machine that is trying to be
accessed.  It does not set ar$tha to anything in particular,
because it is this value that it is trying to determine.  It
could set ar$tha to the broadcast address for the hardware (all
ones in the case of the 10Mbit Ethernet) if that makes it
convenient for some aspect of the implementation.  It then causes
this packet to be broadcast to all stations on the Ethernet cable
originally determined by the routing mechanism.



Packet Reception:
-----------------

When an address resolution packet is received, the receiving
Ethernet module gives the packet to the Address Resolution module
which goes through an algorithm similar to the following.
Negative conditionals indicate an end of processing and a
discarding of the packet.

?Do I have the hardware type in ar$hrd?
Yes: (almost definitely)
  [optionally check the hardware length ar$hln]
  ?Do I speak the protocol in ar$pro?
  Yes:
    [optionally check the protocol length ar$pln]
    Merge_flag := false
    If the pair <protocol type, sender protocol address> is
        already in my translation table, update the sender
	hardware address field of the entry with the new
	information in the packet and set Merge_flag to true. 
    ?Am I the target protocol address?
    Yes:
      If Merge_flag is false, add the triplet <protocol type,
          sender protocol address, sender hardware address> to
	  the translation table.
      ?Is the opcode ares_op$REQUEST?  (NOW look at the opcode!!)
      Yes:
	Swap hardware and protocol fields, putting the local
	    hardware and protocol addresses in the sender fields.
	Set the ar$op field to ares_op$REPLY
	Send the packet to the (new) target hardware address on
	    the same hardware on which the request was received.

Notice that the <protocol type, sender protocol address, sender
hardware address> triplet is merged into the table before the
opcode is looked at.  This is on the assumption that communcation
is bidirectional; if A has some reason to talk to B, then B will
probably have some reason to talk to A.  Notice also that if an
entry already exists for the <protocol type, sender protocol
address> pair, then the new hardware address supersedes the old
one.  Related Issues gives some motivation for this.

Generalization:  The ar$hrd and ar$hln fields allow this protocol