Source routing

Jay Elinsky (
Wednesday, 29 Jul 1987 17:54:27 EDT

This is from Jay Elinsky at IBM Yorktown, originally sent on the Bitnet-
based IBM TCP/IP For VM Mailing list. He asked that it be forwarded to
the TCP/IP mailing list. - Mark Bodenstein, Cornell University

----------------------------Original message----------------------------
I'm one of the developers of the VM code at Yorktown, and I want to
clarify some possible misconceptions on this subject:

1) We use source routing only on Token Ring. Even on Token Ring, we can
certainly also route through a router, if the destination host is on
a different net or subnet. If the destination host is on a different
ring with the same net or subnet number, we do indeed use source routing.
I believe that this is consistent with the architecture of the Token
Ring. Is it inconsistent with any existing implementation of TCP/IP
over Token Ring? If so, I'd certainly like to know about it.

2) Re "No RIF field, no function": If a host is on the same ring, the
ARP code will recognize it, and subsequent packets between the hosts have
no routing field at all.

3) Our TCP/IP certainly is inter-operable with other TCP/IP implement-
ations. That is our reason for supporting TCP/IP. To repeat what I
said in (1), the source routing issue is only on Token Ring, and to
the best of my knowledge our implementation is consistent with other
TCP/IP Token Ring implementations, and with Token Ring architecture.

4) In the VM code, the Token Ring routing information is kept as part
of the address translation cache. I think that's logical.

                                       Jay Elinsky
                                       IBM T.J. Watson Research Center
                                       Yorktown Heights, NY

This archive was generated by hypermail 2.0b3 on Thu Mar 09 2000 - 14:38:48 GMT