Hello Misc, I am consistently having trouble sending data to one particular host. After quite a lot of troubleshooting and reviewing config on my side, I noticed that during the TCP handshake the host in question was not sending any MSS in its SYN/ACK packet. My OpenBSD box then continues to transmit data with the MSS of 1460 that it defaults to, and when it sends a full packet it is getting dropped because the receiving side cannot receive a packet this large. I am also not getting back any ICMP Must Fragment or anything of the like.
I understand the host I am trying to communicate with has its own set of issues, but my question to Misc is that I was under the belief that if either side did not explicitly send a MSS during the handshake the required behavior was to default to 576 for the send MSS. My understanding was this was the required behavior even with PMTU enabled, however it does not seem to be the case. Am I misunderstanding how this is supposed to work? If I disable PMTU, my box does reduce the Send MSS as I would expect. Thanks! Matthew -- CONFIDENTIALITY NOTICE: This email and any files transmitted with it are confidential and are intended solely for the use of the individual or entity to which they are addressed. This communication may contain material protected by HIPAA legislation (45 CFR, Parts 160 & 164) or by 42 CFR Part 2. If you are not the intended recipient, be advised that you have received this email in error and that any use, dissemination, forwarding, printing or copying of this email is strictly prohibited. If you have received this email in error, please notify the sender by reply email and destroy all copies of the original message.
