TCP/IP over FDDI


Vernon Schryver (sgi!vjs%rhyolite.SGI.COM@ucbvax.Berkeley.EDU)
7 Nov 88 23:07:17 GMT


Some time ago, I asked if "everyone" is going to use 802.2 LLC and
RFC-1042 to encapsulate TCP/IP over FDDI. Only a couple of people
responded, and they said yes.

As I understand (or, more accurately, misunderstand) things, in the
next MAC document 48-bit addresses will be required & 16-bit optional
(good news!). That means, we have a 13 byte MAC header in front of every
FDDI packet. If we add an 8-byte 802.2 LLC header in the style of
RFC-1042, then the IP header starts at the wonderous offset of 21.

A (mostly) standard 4.3BSD+VanJacobson/Karels TCP/IP on a machine which
insists on natural alignment requires either byte-copying or aligned
packets. Byte-copying is generally not a formula for speed. Copying
from 1-byte to 4-byte alignment is "optimal" if you want to avoid
saturating the network.

If we used an 802.2 LLC encapsulation with a length of 3(mod 4) (e.g.
7 or 11), the IP header would be aligned. (It would also be aligned
if you use 16-bit addresses with 5-byte MAC headers.) The only other
evident solutions involve extra hardware.

So, my current questions are: Does "everyone" still think RFC 1042 is a
        good idea? What have I misunderstood?

Vernon Schryver
Silicon Graphics
vjs@sgi.com



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