rfc879.txt

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   installations.  Gateways must be prepared to accept datagrams as
   large as can be sent in the maximum packets of the directly attached
   networks.  Gateway implementations should be easily configured for
   installation in different circumstances.

   A footnote:  The MTUs of some popular networks (note that the actual
   limit in some installations may be set lower by administrative
   policy):

      ARPANET, MILNET = 1007
      Ethernet (10Mb) = 1500
      Proteon PRONET  = 2046

9.  Source Fragmentation

   A source host would not normally create datagram fragments.  Under
   normal circumstances datagram fragments only arise when a gateway
   must send a datagram into a network with a smaller maximum packet
   size than the datagram.  In this case the gateway must fragment the
   datagram (unless it is marked "don't fragment" in which case it is
   discarded, with the option of sending an ICMP message to the source
   reporting the problem).

   It might be desirable for the source host to send datagram fragments


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RFC 879                                                    November 1983
TCP Maximum Segment Size                                                


   if the maximum segment size (default or negotiated) allowed by the
   data receiver were larger than the maximum packet size allowed by the
   directly attached network.  However, such datagram fragments must not
   combine to a size larger than allowed by the destination host.

      For example, if the receiving TCP announced that it would accept
      segments up to 5000 octets (in cooperation with the receiving IP)
      then the sending TCP could give such a large segment to the
      sending IP provided the sending IP would send it in datagram
      fragments that fit in the packets of the directly attached
      network.

   There are some conditions where source host fragmentation would be
   necessary.

      If the host is attached to a network with a small packet size (for
      example 256 octets), and it supports an application defined to
      send fixed sized messages larger than that packet size (for
      example TFTP [5]).

      If the host receives ICMP Echo messages with data it is required
      to send an ICMP Echo-Reply message with the same data.  If the
      amount of data in the Echo were larger than the packet size of the
      directly attached network the following steps might be required:
      (1) receive the fragments, (2) reassemble the datagram, (3)
      interpret the Echo, (4) create an Echo-Reply, (5) fragment it, and
      (6) send the fragments.

10. Gateway Fragmentation

   Gateways must be prepared to do fragmentation.  It is not an optional
   feature for a gateway.

   Gateways have no information about the size of datagrams destination
   hosts are prepared to accept.  It would be inappropriate for gateways
   to attempt to keep such information.

   Gateways must be prepared to accept the largest datagrams that are
   allowed on each of the directly attached networks, even if it is
   larger than 576 octets.

   Gateways must be prepared to fragment datagrams to fit into the
   packets of the next network, even if it smaller than 576 octets.

   If a source host thought to take advantage of the local network's
   ability to carry larger datagrams but doesn't have the slightest idea
   if the destination host can accept larger than default datagrams and
   expects the gateway to fragment the datagram into default size


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RFC 879                                                    November 1983
TCP Maximum Segment Size                                                


   fragments, then the source host is misguided.  If indeed, the
   destination host can't accept larger than default datagrams, it
   probably can't reassemble them either. If the gateway either passes
   on the large datagram whole or fragments into default size fragments
   the destination will not accept it.  Thus, this mode of behavior by
   source hosts must be outlawed.

   A larger than default datagram can only arrive at a gateway because
   the source host knows that the destination host can handle such large
   datagrams (probably because the destination host announced it to the
   source host in an TCP MSS option).  Thus, the gateway should pass on
   this large datagram in one piece or in the largest fragments that fit
   into the next network.

   An interesting footnote is that even though the gateways may know
   about know the 576 rule, it is irrelevant to them.

11. Inter-Layer Communication

   The Network Driver (ND) or interface should know the Maximum
   Transmission Unit (MTU) of the directly attached network.

   The IP should ask the Network Driver for the Maximum Transmission
   Unit.

   The TCP should ask the IP for the Maximum Datagram Data Size (MDDS).
   This is the MTU minus the IP header length (MDDS = MTU - IPHdrLen).

   When opening a connection TCP can send an MSS option with the value
   equal MDDS - TCPHdrLen.

   TCP should determine the Maximum Segment Data Size (MSDS) from either
   the default or the received value of the MSS option.

   TCP should determine if source fragmentation is possible (by asking
   the IP) and desirable.

      If so TCP may hand to IP segments (including the TCP header) up to
      MSDS + TCPHdrLen.

      If not TCP may hand to IP segments (including the TCP header) up
      to the lesser of (MSDS + TCPHdrLen) and MDDS.

   IP checks the length of data passed to it by TCP.  If the length is
   less than or equal MDDS, IP attached the IP header and hands it to
   the ND.  Otherwise the IP must do source fragmentation.




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RFC 879                                                    November 1983
TCP Maximum Segment Size                                                


12. What is the Default MSS ?

   Another way of asking this question is "What transmitted value for
   MSS has exactly the same effect of not transmitting the option at
   all?".

   In terms of the previous section:

      The default assumption is that the Maximum Transmission Unit is
      576 octets.

         MTU = 576

      The Maximum Datagram Data Size (MDDS) is the MTU minus the IP
      header length.

         MDDS = MTU - IPHdrLen = 576 - 20 = 556

      When opening a connection TCP can send an MSS option with the
      value equal MDDS - TCPHdrLen.

         MSS = MDDS - TCPHdrLen = 556 - 20 = 536

      TCP should determine the Maximum Segment Data Size (MSDS) from
      either the default or the received value of the MSS option.

         Default MSS = 536, then MSDS = 536

      TCP should determine if source fragmentation is possible and
      desirable.

         If so TCP may hand to IP segments (including the TCP header) up
         to MSDS + TCPHdrLen (536 + 20 = 556).

         If not TCP may hand to IP segments (including the TCP header)
         up to the lesser of (MSDS + TCPHdrLen (536 + 20 = 556)) and
         MDDS (556).













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RFC 879                                                    November 1983
TCP Maximum Segment Size                                                


13. The Truth

   The rule relating the maximum IP datagram size and the maximum TCP
   segment size is:

      TCP Maximum Segment Size = IP Maximum Datagram Size - 40

   The rule must match the default case.

      If the TCP Maximum Segment Size option is not transmitted then the
      data sender is allowed to send IP datagrams of maximum size (576)
      with a minimum IP header (20) and a minimum TCP header (20) and
      thereby be able to stuff 536 octets of data into each TCP segment.

   The definition of the MSS option can be stated:

      The maximum number of data octets that may be received by the
      sender of this TCP option in TCP segments with no TCP header
      options transmitted in IP datagrams with no IP header options.

14. The Consequences

   When TCP is used in a situation when either the IP or TCP headers are
   not minimum and yet the maximum IP datagram that can be received
   remains 576 octets then the TCP Maximum Segment Size option must be
   used to reduce the limit on data octets allowed in a TCP segment.

      For example, if the IP Security option (11 octets) were in use and
      the IP maximum datagram size remained at 576 octets, then the TCP
      should send the MSS with a value of 525 (536-11).




















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RFC 879                                                    November 1983
TCP Maximum Segment Size                                                


15. References

   [1]  Postel, J., ed., "Transmission Control Protocol - DARPA Internet
        Program Protocol Specification", RFC 793, USC/Information
        Sciences Institute, September 1981.

   [2]  Postel, J., ed., "Internet Protocol - DARPA Internet Program
        Protocol Specification", RFC 791, USC/Information Sciences
        Institute, September 1981.

   [3]  Postel, J., "Internet Control Message Protocol - DARPA Internet
        Program Protocol Specification", RFC 792, USC/Information
        Sciences Institute, September 1981.

   [4]  Plummer, D., "An Ethernet Address Resolution Protocol or
        Converting Network Protocol Addresses to 48-bit Ethernet
        Addresses for Transmission on Ethernet Hardware", RFC 826,
        MIT/LCS, November 1982.

   [5]  Sollins, K., "The TFTP Protocol (Revision 2)", RFC 783, MIT/LCS,
        June 1981.





























Postel                                                         [Page 11]