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.
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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).
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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]).
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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:
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.
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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):
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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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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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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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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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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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.
