dorner@pequod.cso.uiuc.edu (Steve Dorner) (12/08/89)
We've found a problem with MacTCP. Specifically, it advertises a bad
TCP Maximum Segment Size (MSS) to some hosts that are not on the same
class B subnet as our Gatorbox.
THE FACTS
Our setup is as follows:
gatorbox is on a class B subnet, 128.174.33. Macintoshes are connected
to the gatorbox by PhoneNet. Pequod (a NeXT machine, but that doesn't
matter) is also on 128.174.33. Uxc (a 4.3bsd-tahoe VAX, but that
doesn't matter, either) is on 128.174.5, a subnet a gateway or two
away.
And this is what we found (by snatching packets off the ethernet):
To pequod, MacTCP advertised an MSS of 546. Add 40 bytes for TCP/IP
headers, and you wind up with a 586 byte IP datagram, which is the
maximum possible ip datagram that can be forwarded on the PhoneNet.
And, in fact, pequod sent maximal packets of 586 bytes on its
connections with MacTCP. So far, so good.
To uxc, however, MacTCP advertised an MSS of 576. Add 40 bytes for
TCP/IP headers, and you get a 616 byte IP datagram, which is TOO big.
Since gatorboxes don't fragment oversize IP datagrams, a packet that
size will never be received by MacTCP, and the connection will
eventually die. And this is exactly what we saw. Connections worked
fine until there was a lot of data to be sent to the Mac, at which
point uxc sent a 616 byte datagram, and the connection died.
While we did not do packet analysis on all the connections, we saw hung
connections to every single host we tried (11 different hosts, from
many different vendors) that was on a subnet of 128.174, but NOT on
128.174.33. The only host on 128.174.33 worked fine, as did both the
hosts we tried that were on totally different networks (i.e., not
128.174).
THE SPECULATION
MacTCP advertises too large an MSS to hosts which are on the same class
B network, but not on the same class B subnet. It advertises proper
values to hosts either on the same class B subnet, or on entirely
different networks.
THE HACK
Fortunately, a little disassembly and trial and error led to a patch
that makes the problem go away. Find the hex byte string
"337c02040014" (at an offset of around 0x6500, give or take 0x200) in
the MacTCP document, and change it to "337c01010014".
This has caused MacTCP to function adequately for our setup for all the
hosts I tried. I'm not POSITIVE what this patch does (I suspect it
just nukes the MSS advertisement altogether, but I haven't verified
that). It resolves our problem, and I'll look no further.
THE REAL SOLUTION
1. Apple needs to fix MacTCP to advertise the MSS correctly to
different subnets. This is the OPTIMAL fix, since IP fragmentation is
unpleasant at best.
2. Cayman needs to fix the GatorSoftware to fragment large IP
datagrams. (Michael Haag [from Cayman Technical Support] assures me
Cayman has done so in the next release, due out 1Q90.)
--
Steve Dorner, U of Illinois Computing Services Office
Internet: s-dorner@uiuc.edu UUCP: {convex,uunet}!uiucuxc!dorner
IfUMust: (217) 244-1765veizades@apple.com (John Veizades) (12/08/89)
In article <1989Dec7.225154.28695@ux1.cso.uiuc.edu> dorner@pequod.cso.uiuc.edu (Steve Dorner) writes: > We've found a problem with MacTCP. Specifically, it advertises a bad > TCP Maximum Segment Size (MSS) to some hosts that are not on the same > class B subnet as our Gatorbox. This is a known problem and is fixed in the 1.0.1 version of MacTCP due out in 1Q90. Check your source of MacTCP for when this is available. John Veizades... MacTCP Engineer