Strange TCP checksum problem with several interconnected networks

[rick at Seismo.ARPA]

From:  Rick Adams <rick at Seismo.ARPA>



Here's the situation:

		 C--------D
		 |
		 |
	A--------B


C & D are on the milnet (sharing the same imp). C & B are on an ethernet.
A & B are connected with a serial line.

A, B & C are on the same Class-B network. D is a vax running 4.2bsd. B & C are
vaxes running 4.1C with the networking code of 4.2 (The 4.1C was extremely
hacked before we got 4.2, so I "lifted" the 4.2 networking code). A is a 68000
running a System III compatible unix with TCP/IP based loosely on 3Comms UNET.

A, C and D can all talk to each other in both directions (C as both a milnet
number and the class-B network number).

Here's where it starts to get strange.

B can talk to A or C with the Class-B number. However, it can NOT talk
to D or C as a Milnet number. (Before you say routing, remember A can talk
to everyone and everyone can talk to A).

The really wierd part is that when B is trying to talk to C or D, the
destination machines (C/D) get TCP CHECKSUM ERRORS! The ip packets are
ok.

			Destination
		     	     Class-B   Milnet
Source		A	B	C	C	D
A		ok	ok	ok	ok	ok
B		ok	ok	ok	NO	NO
C		ok	ok	ok	ok	ok
D		ok	ok	ok	ok	ok


Any ideas on why I am getting TCP checksum errors in this one case
would be greatly appreciated.

---rick