Solved: There have been some

KATTUBAVA E.S · ‎11-19-2015

Hi Experts

Greetings!

What is the default MSS size set in cisco IOS for TCP host communication, I could see from the packet snipper output that the default MSS is set to 536 even though the MTU of the link is 1500. Why cisco IOS is not using 1460 [Discarding TCP & IP header (20+20)] as the default MSS for TCP communication and what is the purpose of choosing a minimal MSS value 536 by deafult?

Thanks in advance

-bava

Cisco Freak · ‎11-19-2015

 There have been some assumptions made about using other than the
   default size for datagrams with some unfortunate results.

      HOSTS MUST NOT SEND DATAGRAMS LARGER THAN 576 OCTETS UNLESS THEY
      HAVE SPECIFIC KNOWLEDGE THAT THE DESTINATION HOST IS PREPARED TO
      ACCEPT LARGER DATAGRAMS.

         This is a long established rule.

   To resolve the ambiguity in the TCP Maximum Segment Size option
   definition the following rule is established:

      THE TCP MAXIMUM SEGMENT SIZE IS THE IP MAXIMUM DATAGRAM SIZE MINUS
      FORTY.

         The default IP Maximum Datagram Size is 576.
         The default TCP Maximum Segment Size is 536.

https://tools.ietf.org/html/rfc879

View solution in original post

Cisco Freak · ‎11-19-2015

 There have been some assumptions made about using other than the
   default size for datagrams with some unfortunate results.

      HOSTS MUST NOT SEND DATAGRAMS LARGER THAN 576 OCTETS UNLESS THEY
      HAVE SPECIFIC KNOWLEDGE THAT THE DESTINATION HOST IS PREPARED TO
      ACCEPT LARGER DATAGRAMS.

         This is a long established rule.

   To resolve the ambiguity in the TCP Maximum Segment Size option
   definition the following rule is established:

      THE TCP MAXIMUM SEGMENT SIZE IS THE IP MAXIMUM DATAGRAM SIZE MINUS
      FORTY.

         The default IP Maximum Datagram Size is 576.
         The default TCP Maximum Segment Size is 536.

https://tools.ietf.org/html/rfc879

Joseph W. Doherty · ‎11-19-2015

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Posting

As the "Cisco Freak" has already noted, the RFC he provided addresses your question.

But, if you want to know how to "fix" on Cisco devices, see if you can enable the global command:

ip tcp path-mtu-discovery

Also, you might be able to improve TCP transmission rate to a Cisco device, but adding the global command:

ip tcp window-size 65535

TCP MSS