The IP over ATM Mailing List Archive by date

[burrows@pa.dec.com (Mike Burrows)]

> There are no documents cited for 16 or 21.  Any idea in which
> documents they are cited, if any, and whether they should be removed
> from the next version of Assigned Numbers?

My apologies.  No one asked me for a document, so I didn't supply one.
We're using type 21, and I cannot see how to replace its functionality
with type 16 or 19.

Here is what the document would say about type 21:

Hardware type 21 is a virtual circuit identifier.  This allows you to
use ARP and associated protocols to map between VCIs and other address
types.

When the hardware protocol type is 21:
	The hardware length field should be set to 4
	The protocol type field should be set to:
	   1 if you are mapping to or from a 48-UID (Ethernet address)
	   x if you are mapping to or from addresses with Ethernet type x

The only protocol type that is known to have been used with
hardware type 21 is 1 (Ethernet address).


We normally use this type as follows.   A host X that wishes to
communicate with a host Y sends a type 21 ARP packet containing host
Y's address.   Initially, this ARP packet is sent on a VCI negotiated
between the host X and the box Z that X is directly connected to.  (Z
is usually a switch, but can be a host in the simplest possible
network.)   If it is possible to reach Y from X, it is the
responsibility of Z either to answer the ARP, or to pass it on to
another host (possibily X) that will answer the ARP.  It no suitable
VCI exists, Z may choose to set one up and place the new VI in the reply.
If a circuit already exists between the hosts, its VCI is returned in
the ARP reply.  We use only bidirectional ABR VCs for this purpose.
Hosts normally cache the results of such ARPs, and periodically refresh
the cache.  We arrange for hosts to respond to these ARP requests, so
that X can refresh its cache entry for Y by sending ARPs directly to Y
using the VCI in the cache entry.

The main virtue of this technique is simplicity.

Mike Burrows