Re: 802.3


John Leong (leong%ANDREW@PT.CS.CMU.EDU)
Tue, 28 Jan 86 17:25:59 EST


We have TCP-IP running on the PC's (PC, PC-XT, PC-AT and the PC-RT)
using the IBM (802.5 superset) token ring.

During the implementation, we encountered the problem of assigning SAP
to ARP. After a number of exchanges with Postel and Ira Winston (aurthor
of RFC 948), we came to a conclusion that this is not going to get resolved
in a hurry, we just pick a random number for ARP for our implementation.

The way I will like to see it done is as follows : [using the bit ordering
convention as defined in fig.3-2a of IEEE 802.2-1985 spec]

For IP packets :

DSAP 01 10 0000 SSAP 00 10 0000

For ARP packets :

DSAP 01 10 0000 SSAP 00 01 0000

This is really to say, in view of the fact that SAP is really used as
protocol ID, having two SAP fields is pretty redundant. (Can't image
an SNA SAP meaningfully sending to an IP SAP). In which case, we bastardise
the SSAP field to denote whether the IP 'family' packet [as denoted
by the DSAP] is for IP or ARP or IP trailer or whatever .....

This does not conflict with any standard since (1) 802.2 does not define
the relationship between DSAP and SSAP and (2) our SSAP encoding does
not conflict with 802.2 standard.

I will like to receive comment from on the above and if I do not get
jump upon too heavily, I may submit it as an RFC (amendment to 948).

John Leong@*
leong@@h.cs.cmu.edu

P.S. In RFC943 (Assigned Number), the DSAP for IP is given as binary
0110000 or decimal 96. It is actually incorrect. It is really 01100000
in IEEE terminology or 00000110 in binary or 6 in decimal.



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