Why do Cisco IOS devices set the MSS to 536 for remote networks?

[Mitrixsen]

Hello, everyone.

I am covering MSS at the moment and I've read that Cisco IOS devices set the MSS to 536 for TCP connections to remote networks.

From R1 to R2
R1#telnet 10.0.0.2
Trying 10.0.0.2 ... Open
*Jun 16 11:39:18.237: TCP0: Connection to 10.0.0.2:23, advertising MSS 2460
*Jun 16 11:39:18.238: TCP: tcb 10E2A330 connection to 10.0.0.2:23, peer MSS 2460, MSS is 2460

From R1 to R3
R1#telnet 10.0.0.6
Trying 10.0.0.6 ... Open
*Jun 16 11:40:16.232: TCP0: Connection to 10.0.0.6:23, advertising MSS 536
*Jun 16 11:40:16.236: TCP: tcb 10E2A330 connection to 10.0.0.6:23, peer MSS 536, MSS is 536

My question is, why does it work like this? One reason I can think of is that when you initiate a TCP connection to a device on the local network, you know what the MTU of the local segment is, while with remote networks, there could be different MTUs along the path so you play it safe by using a lower MSS value.

However, isn't limiting yourself by so many bytes a little unefficient? If your capable of sending that much more..

Thank you!

David

[Joseph W. Doherty]

Oh, as an aside, since OP is using telnet and its efficiency a consideration, what about sending packets for just a single character?

Are you familiar with Nagle algorithm RFC as a possible option?  Or, some header compression RFCs?  ;  )