TCP的MSS协商, 什么情况下会不成功

madope

请问TCP的MSS协商，在什么情况下会不成功呢? TCP/IP书上说，如果TCP连接时MSS不能协商成功将用默认大小536。为什么会协商不成功呢？两端的MSS不一样时，只用小的值就可以呀。

Tinnal

下面是从TCP RFC层面去分析，从内核代码层面分析的情况，请自己去看代码。

1.  RFC 879, page 2, Section 3, "The MSS counts only data octets in the segment, it does not count the TCP header or the IP header."

3.  The TCP Maximum Segment Size Option

   TCP provides an option that may be used at the time a connection is
   established (only) to indicate the maximum size TCP segment that can
   be accepted on that connection.  This Maximum Segment Size (MSS)
   announcement (often mistakenly called a negotiation) is sent from the
   data receiver to the data sender and says "I can accept TCP segments
   up to size X". The size (X) may be larger or smaller than the
   default.  The MSS can be used completely independently in each
   direction of data flow.  The result may be quite different maximum
   sizes in the two directions.

   The MSS counts only data octets in the segment, it does not count the
   TCP header or the IP header.

   A footnote:  The MSS value counts only data octets, thus it does not
   count the TCP SYN and FIN control bits even though SYN and FIN do
   consume TCP sequence numbers.

既然是选项，则如果对端的协议没有去实现，就有可能协商不成功了。

2.  RFC 879, page 1, Section 1, "The default TCP Maximum Segment Size is 536."

1.  Introduction

   This memo discusses the TCP Maximum Segment Size and its relation to
   the IP Maximum Datagram Size.  TCP is specified in reference [1].  IP
   is specified in references [2,3].

   This discussion is necessary because the current specification of
   this TCP option is ambiguous.

   Much of the difficulty with understanding these sizes and their
   relationship has been due to the variable size of the IP and TCP
   headers.

   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.

这是536的出处。

mysky0407

学习了，虽然做交换机的，但网络知识还是很欠缺。