draft-ietf-tcpm-tcpmss-01.txt   draft-ietf-tcpm-tcpmss-02.txt 
Network Working Group Network Working Group Network Working Group Network Working Group
Internet-Draft D. Borman Internet-Draft D. Borman
Intended Status: Informational Wind River Systems Intended Status: Informational Wind River Systems
File: draft-ietf-tcpm-tcpmss-01.txt July 13, 2009 File: draft-ietf-tcpm-tcpmss-02.txt July 13, 2009
TCP Options and MSS TCP Options and MSS
Status of This Memo Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
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There has been some confusion as to what value should be filled in There has been some confusion as to what value should be filled in
the TCP MSS option when using TCP options. RFC-879 [Postel83] the TCP MSS option when using TCP options. RFC-879 [Postel83]
stated: stated:
The MSS counts only data octets in the segment, it does not The MSS counts only data octets in the segment, it does not
count the TCP header or the IP header. count the TCP header or the IP header.
which is unclear about what to do about TCP options. RFC-1122 which is unclear about what to do about TCP options. RFC-1122
[Braden89] attempted to clarify this in section 4.2.2.6, but there [Braden89] attempted to clarify this in section 4.2.2.6, but there
still seems to be confusion. still seems to be confusion. Clarification was first sent to the TCP
Large Windows mailing list [Borman93] in 1993.
2. TCP Options and MSS 2. TCP Options and MSS
The MSS value to be sent in an MSS option should be equal to the The MSS value to be sent in an MSS option should be equal to the
effective MTU minus the fixed IP and TCP headers. By ignoring both effective MTU minus the fixed IP and TCP headers. By ignoring both
IP and TCP options when calculating the value for the MSS option, if IP and TCP options when calculating the value for the MSS option, if
there are any IP or TCP options to be sent in a packet, then the there are any IP or TCP options to be sent in a packet, then the
sender must decrease the size of the TCP data accordingly. The sender must decrease the size of the TCP data accordingly. The
reason for this can be seen in the following table: reason for this can be seen in the following table:
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5. References 5. References
Informative References Informative References
[Braden89] Braden, R., editor, "Requirements for Internet Hosts -- [Braden89] Braden, R., editor, "Requirements for Internet Hosts --
Communication Layers", RFC-1122, October, 1989 Communication Layers", RFC-1122, October, 1989
[Postel83] Postel, J., "The TCP Maximum Segment Size and Related [Postel83] Postel, J., "The TCP Maximum Segment Size and Related
Topics", RFC-879, ISI, November 1983. Topics", RFC-879, ISI, November 1983.
[Borman93] Borman, D., "TCP MSS & Timestamps", Message to tcplw
mailing list, Jan 7, 1993.
Authors' Addresses Authors' Addresses
David Borman David Borman
Wind River Systems Wind River Systems
Mendota Heights, MN 55120 Mendota Heights, MN 55120
Phone: (651) 454-3052 Phone: (651) 454-3052
Email: david.borman@windriver.com Email: david.borman@windriver.com
Full Copyright Statement Full Copyright Statement
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