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rfc:rfc879

Network Working Group J. Postel Request for Comments: 879 ISI

                                                         November 1983
                    The TCP Maximum Segment Size
                         and Related Topics

This memo discusses the TCP Maximum Segment Size Option and related topics. The purposes is to clarify some aspects of TCP and its interaction with IP. This memo is a clarification to the TCP specification, and contains information that may be considered as "advice to implementers".

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.

Postel [Page 1]

RFC 879 November 1983 TCP Maximum Segment Size

2. The IP Maximum Datagram Size

 Hosts are not required to reassemble infinitely large IP datagrams.
 The maximum size datagram that all hosts are required to accept or
 reassemble from fragments is 576 octets.  The maximum size reassembly
 buffer every host must have is 576 octets.  Hosts are allowed to
 accept larger datagrams and assemble fragments into larger datagrams,
 hosts may have buffers as large as they please.
 Hosts must not send datagrams larger than 576 octets unless they have
 specific knowledge that the destination host is prepared to accept
 larger datagrams.

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.

4. The Relationship of TCP Segments and IP Datagrams

 TCP segment are transmitted as the data in IP datagrams.  The
 correspondence between TCP segments and IP datagrams must be one to
 one.  This is because TCP expects to find exactly one complete TCP
 segment in each block of data turned over to it by IP, and IP must
 turn over a block of data for each datagram received (or completely
 reassembled).

Postel [Page 2]

RFC 879 November 1983 TCP Maximum Segment Size

5. Layering and Modularity

 TCP is an end to end reliable data stream protocol with error
 control, flow control, etc.  TCP remembers many things about the
 state of a connection.
 IP is a one shot datagram protocol.  IP has no memory of the
 datagrams transmitted.  It is not appropriate for IP to keep any
 information about the maximum datagram size a particular destination
 host might be capable of accepting.
 TCP and IP are distinct layers in the protocol architecture, and are
 often implemented in distinct program modules.
 Some people seem to think that there must be no communication between
 protocol layers or program modules.  There must be communication
 between layers and modules, but it should be carefully specified and
 controlled.  One problem in understanding the correct view of
 communication between protocol layers or program modules in general,
 or between TCP and IP in particular is that the documents on
 protocols are not very clear about it.  This is often because the
 documents are about the protocol exchanges between machines, not the
 program architecture within a machine, and the desire to allow many
 program architectures with different organization of tasks into
 modules.

6. IP Information Requirements

 There is no general requirement that IP keep information on a per
 host basis.
 IP must make a decision about which directly attached network address
 to send each datagram to.  This is simply mapping an IP address into
 a directly attached network address.
 There are two cases to consider:  the destination is on the same
 network, and the destination is on a different network.
    Same Network
       For some networks the the directly attached network address can
       be computed from the IP address for destination hosts on the
       directly attached network.
       For other networks the mapping must be done by table look up
       (however the table is initialized and maintained, for
       example, [4]).

Postel [Page 3]

RFC 879 November 1983 TCP Maximum Segment Size

    Different Network
       The IP address must be mapped to the directly attached network
       address of a gateway.  For networks with one gateway to the
       rest of the Internet the host need only determine and remember
       the gateway address and use it for sending all datagrams to
       other networks.
       For networks with multiple gateways to the rest of the
       Internet, the host must decide which gateway to use for each
       datagram sent.  It need only check the destination network of
       the IP address and keep information on which gateway to use for
       each network.
 The IP does, in some cases, keep per host routing information for
 other hosts on the directly attached network.  The IP does, in some
 cases, keep per network routing information.
 A Special Case
    There are two ICMP messages that convey information about
    particular hosts.  These are subtypes of the Destination
    Unreachable and the Redirect ICMP messages.  These messages are
    expected only in very unusual circumstances.  To make effective
    use of these messages the receiving host would have to keep
    information about the specific hosts reported on.  Because these
    messages are quite rare it is strongly recommended that this be
    done through an exception mechanism rather than having the IP keep
    per host tables for all hosts.

7. The Relationship between IP Datagram and TCP Segment Sizes

 The relationship between the value of the maximum IP datagram size
 and the maximum TCP segment size is obscure.  The problem is that
 both the IP header and the TCP header may vary in length.  The TCP
 Maximum Segment Size option (MSS) is defined to specify the maximum
 number of data octets in a TCP segment exclusive of TCP (or IP)
 header.
 To notify the data sender of the largest TCP segment it is possible
 to receive the calculation of the MSS value to send is:
    MSS = MTU - sizeof(TCPHDR) - sizeof(IPHDR)
 On receipt of the MSS option the calculation of the size of segment
 that can be sent is:
    SndMaxSegSiz = MIN((MTU - sizeof(TCPHDR) - sizeof(IPHDR)), MSS)

Postel [Page 4]

RFC 879 November 1983 TCP Maximum Segment Size

 where MSS is the value in the option, and MTU is the Maximum
 Transmission Unit (or the maximum packet size) allowed on the
 directly attached network.
 This begs the question, though.  What value should be used for the
 "sizeof(TCPHDR)" and for the "sizeof(IPHDR)"?
 There are three reasonable positions to take: the conservative, the
 moderate, and the liberal.
 The conservative or pessimistic position assumes the worst -- that
 both the IP header and the TCP header are maximum size, that is, 60
 octets each.
    MSS = MTU - 60 - 60 = MTU - 120
    If MTU is 576 then MSS = 456
 The moderate position assumes the that the IP is maximum size (60
 octets) and the TCP header is minimum size (20 octets), because there
 are no TCP header options currently defined that would normally be
 sent at the same time as data segments.
    MSS = MTU - 60 - 20 = MTU - 80
    If MTU is 576 then MSS = 496
 The liberal or optimistic position assumes the best -- that both the
 IP header and the TCP header are minimum size, that is, 20 octets
 each.
    MSS = MTU - 20 - 20 = MTU - 40
    If MTU is 576 then MSS = 536
    If nothing is said about MSS, the data sender may cram as much as
    possible into a 576 octet datagram, and if the datagram has
    minimum headers (which is most likely), the result will be 536
    data octets in the TCP segment.  The rule relating MSS to the
    maximum datagram size ought to be consistent with this.
 A practical point is raised in favor of the liberal position too.
 Since the use of minimum IP and TCP headers is very likely in the
 very large percentage of cases, it seems wasteful to limit the TCP
 segment data to so much less than could be transmitted at once,
 especially since it is less that 512 octets.

Postel [Page 5]

RFC 879 November 1983 TCP Maximum Segment Size

    For comparison:  536/576 is 93% data, 496/576 is 86% data, 456/576
    is 79% data.

8. Maximum Packet Size

 Each network has some maximum packet size, or maximum transmission
 unit (MTU).  Ultimately there is some limit imposed by the
 technology, but often the limit is an engineering choice or even an
 administrative choice.  Different installations of the same network
 product do not have to use the same maximum packet size.  Even within
 one installation not all host must use the same packet size (this way
 lies madness, though).
 Some IP implementers have assumed that all hosts on the directly
 attached network will be the same or at least run the same
 implementation.  This is a dangerous assumption.  It has often
 developed that after a small homogeneous set of host have become
 operational additional hosts of different types are introduced into
 the environment.  And it has often developed that it is desired to
 use a copy of the implementation in a different inhomogeneous
 environment.
 Designers of gateways should be prepared for the fact that successful
 gateways will be copied and used in other situation and
 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

Postel [Page 6]

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

Postel [Page 7]

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.

Postel [Page 8]

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).

Postel [Page 9]

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).

Postel [Page 10]

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]

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