Just share my experience:

I have a number of HP machines (hp-ux 10.01, 10.20 & 11.0 with adsm v2 & v3
clients), and AIX server v2.1.5.13 with FDDI as network.

For HP-UX 10.20 with v3 client I have the following setting,
   TCPBuffsize        32
   TCPWindowsize       64
   TXNBYTE            25600

If I increase TCPWindowsize (eg to 128), there will be error message in
dsmerror.log:
11/19/98   09:00:11 TcpOpen(): setsockopt(SO_SNDBUF): errno = 233
11/19/98   09:00:11 TcpOpen(): setsockopt(SO_RCVBUF): errno = 233
11/19/98   09:00:11 TcpOpen(): setsockopt failed on one or more options

For the HP-UX 11 machine, I can use    TCPWindowsize       1024

Regards,
Eric Tang