Source routing

Jay Elinsky (
30 July 1987, 23:33:38 EDT

Here's a copy of a note I sent to IBMTCP-L, on the subject of source routing
in the IBM TCP/IP code. My apologies in advance if it shows up twice --
someone else may have also sent it, but it hasn't shown up on our database at
Yorktown. I want to make sure it shows up on this mailing list, since the
note that started it all appeared here.

Date: Wednesday, 29 Jul 1987 17:54:27 EDT
From: Jay Elinsky <From: Jay Elinsky <ELINSKY@YKTVMX>
Subject: Source routing

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

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