Internet Engineering Task Force (IETF) D. Borman
Request for Comments: 6691 Quantum Corporation
Updates: 879, 2385 July 2012
Category: Informational
ISSN: 2070-1721
TCP Options and Maximum Segment Size (MSS)
Abstract
This memo discusses what value to use with the TCP Maximum Segment
Size (MSS) option, and updates RFC 879 and RFC 2385.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6691.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Borman Informational [Page 1]RFC 6691 TCP Options and MSS July 2012
1. Introduction
There has been some confusion as to what value to use with the TCP
MSS option when using IP and TCP options. RFC 879 [RFC879] states:
The MSS counts only data octets in the segment, it does not count
the TCP header or the IP header.
This statement is unclear about what to do about IP and TCP options,
since they are part of the headers. RFC 1122 [RFC1122] clarified the
MSS option, which is discussed in Appendix A, but there still seems
to be some confusion.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. The Short Statement
When calculating the value to put in the TCP MSS option, the MTU
value SHOULD be decreased by only the size of the fixed IP and TCP
headers and SHOULD NOT be decreased to account for any possible IP or
TCP options; conversely, the sender MUST reduce the TCP data length
to account for any IP or TCP options that it is including in the
packets that it sends. The rest of this document just expounds on
that statement, and the goal is to avoid IP-level fragmentation of
TCP packets.
The size of the fixed TCP header is 20 bytes [RFC793], the size of
the fixed IPv4 header is 20 bytes [RFC791], and the size of the fixed
IPv6 header is 40 bytes [RFC2460]. The determination of what MTU
value should be used, especially in the case of multi-homed hosts, is
beyond the scope of this document.
3. MSS in Other RFCs
3.1. RFC 879
RFC 879 [RFC879] discusses the MSS option and other topics. In the
Introduction, it states:
THE TCP MAXIMUM SEGMENT SIZE IS THE IP MAXIMUM DATAGRAM SIZE MINUS
FORTY.
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In Section 13, it states:
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.
These are both correct. However, in the next paragraph -- in
Section 14 -- it then confuses this by stating that the consequence
is:
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).
That is incorrect. The simpler and more correct statement would be:
When TCP is used in a situation where either the IP or TCP headers
are not minimum, the sender must reduce the amount of TCP data in
any given packet by the number of octets used by the IP and TCP
options.
3.2. RFC 2385
RFC 2385 [RFC2385] incorrectly states, in Section 4.3:
As with other options that are added to every segment, the size of
the MD5 option must be factored into the MSS offered to the other
side during connection negotiation. Specifically, the size of the
header to subtract from the MTU (whether it is the MTU of the
outgoing interface or IP's minimal MTU of 576 bytes) is now at
least 18 bytes larger.
This is incorrect. The value for the MSS option is only adjusted by
the fixed IP and TCP headers.
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4. Clarification from the TCP Large Windows Mailing List
The initial clarification was sent to the TCP Large Windows mailing
list in 1993 [Borman93]; this section is based on that message.
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
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
sender must decrease the size of the TCP data accordingly. The
reason for this can be seen in the following table:
+--------------------+--------------------+
| MSS is adjusted | MSS isn't adjusted |
| to include options | to include options |
+--------------------+--------------------+--------------------+
| Sender adjusts | Packets are too | Packets are the |
| packet length | short | correct length |
| for options | | |
+--------------------+--------------------+--------------------+
| Sender doesn't | Packets are the | Packets are too |
| adjust packet | correct length | long |
| length for options | | |
+--------------------+--------------------+--------------------+
The goal is to not send IP datagrams that have to be fragmented, and
packets sent with the constraints in the lower right of this grid
will cause IP fragmentation. Since the sender doesn't know if the
received MSS option was adjusted to include options, the only way to
guarantee that the packets are not too long is for the data sender to
decrease the TCP data length by the size of the IP and TCP options.
It follows, then, that since the sender will be adjusting the TCP
data length when sending IP and TCP options, there is no need to
include the IP and TCP option lengths in the MSS value.
Another argument against including IP or TCP options in the
determination of the MSS value is that the MSS is a fixed value, and
by their very nature the lengths of IP and TCP options are variable,
so the MSS value can never accurately reflect all possible IP and TCP
option combinations. The only one that knows for sure how many IP
and TCP options are in any given packet is the sender; hence, the
sender should be doing the adjustment to the TCP data length to
account for any IP and TCP options.
Borman Informational [Page 4]RFC 6691 TCP Options and MSS July 2012
5. Additional Considerations
5.1. Path MTU Discovery
The TCP MSS option specifies an upper bound for the size of packets
that can be received. Hence, setting the value in the MSS option too
small can impact the ability for Path MTU Discovery to find a larger
path MTU. For more information on Path MTU Discovery, see:
o "Path MTU Discovery" [RFC1191]
o "TCP Problems with Path MTU Discovery" [RFC2923]
o "Packetization Layer Path MTU Discovery" [RFC4821]
5.2. Interfaces with Variable MSS Values
The effective MTU can sometimes vary, as when used with variable
compression, e.g., RObust Header Compression (ROHC) [RFC5795]. It is
tempting for TCP to want to advertise the largest possible MSS, to
support the most efficient use of compressed payloads.
Unfortunately, some compression schemes occasionally need to transmit
full headers (and thus smaller payloads) to resynchronize state at
their endpoint compressors/decompressors. If the largest MTU is used
to calculate the value to advertise in the MSS option, TCP
retransmission may interfere with compressor resynchronization.
As a result, when the effective MTU of an interface varies, TCP
SHOULD use the smallest effective MTU of the interface to calculate
the value to advertise in the MSS option.
5.3. IPv6 Jumbograms
In order to support TCP over IPv6 jumbograms, implementations need to
be able to send TCP segments larger than 64K. RFC 2675 [RFC2675]
defines that a value of 65,535 is to be treated as infinity, and Path
MTU Discovery [RFC1981] is used to determine the actual MSS.
5.4. Avoiding Fragmentation
Packets that are too long will either be fragmented or dropped. If
packets are fragmented, intermediary firewalls or middle boxes may
drop the fragmented packets. In either case, when packets are
dropped, the connection can fail; hence, it is best to avoid
generating fragments.
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6. Security Considerations
This document clarifies how to determine what value should be used
for the MSS option and does not introduce any new security concerns.
7. References
7.1. Normative References
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC879] Postel, J., "The TCP Maximum Segment Size and Related
Topics", RFC 879, November 1983.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6
Jumbograms", RFC 2675, August 1999.
7.2. Informative References
[Borman93] Borman, D., "TCP MSS & Timestamps", Message to the tcplw
mailing list, January 7, 1993.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU
Discovery for IP version 6", RFC 1981, August 1996.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP
MD5 Signature Option", RFC 2385, August 1998.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, September 2000.
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[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The
RObust Header Compression (ROHC) Framework", RFC 5795,
March 2010.
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Appendix A. Details from RFC 793 and RFC 1122
RFC 793 [RFC793] defines the MSS option as follows:
Maximum Segment Size Option Data: 16 bits
If this option is present, then it communicates the maximum
receive segment size at the TCP which sends this segment. This
field must only be sent in the initial connection request
(i.e., in segments with the SYN control bit set). If this
option is not used, any segment size is allowed.
RFC 1122 [RFC1122] provides additional clarification in
Section 4.2.2.6, on pages 85-86. First, it changes the default
behavior when the MSS option is not present:
If an MSS option is not received at connection setup, TCP MUST
assume a default send MSS of 536 (576-40) [TCP:4].
Then, it clarifies how to determine the value to use in the MSS
option:
The MSS value to be sent in an MSS option must be less than or
equal to:
MMS_R - 20
where MMS_R is the maximum size for a transport-layer message that
can be received (and reassembled). TCP obtains MMS_R and MMS_S
from the IP layer; see the generic call GET_MAXSIZES in
Section 3.4.
What is implied in RFC 1122, but not explicitly stated, is that the
20 is the size of the fixed TCP header. The definition of MMS_R is
found in Section 3.3.2, on page 57:
MMS_R is given by:
MMS_R = EMTU_R - 20
since 20 is the minimum size of an IP header.
Borman Informational [Page 8]RFC 6691 TCP Options and MSS July 2012
and on page 56 (also Section 3.3.2):
We designate the largest datagram size that can be reassembled by
EMTU_R ("Effective MTU to receive"); this is sometimes called the
"reassembly buffer size". EMTU_R MUST be greater than or equal to
576, SHOULD be either configurable or indefinite, and SHOULD be
greater than or equal to the MTU of the connected network(s).
What should be noted here is that EMTU_R is the largest datagram size
that can be reassembled, not the largest datagram size that can be
received without fragmentation. Taking this literally, since most
modern TCP/IP implementations can reassemble a full 64K IP packet,
implementations should be using 65535 - 20 - 20, or 65495, for the
MSS option. But there is more to it than that. RFC 1122 also
states, on page 86:
The choice of TCP segment size has a strong effect on performance.
Larger segments increase throughput by amortizing header size and
per-datagram processing overhead over more data bytes; however, if
the packet is so large that it causes IP fragmentation, efficiency
drops sharply if any fragments are lost [IP:9].
Since it is guaranteed that any IP datagram that is larger than the
MTU of the connected network will have to be fragmented to be
received, implementations ignore the "greater than or" part of
"SHOULD be greater than or equal to the MTU of the connected
network(s)". Thus, the MSS value to be sent in an MSS option must be
less than or equal to:
EMTU_R - FixedIPhdrsize - FixedTCPhdrsize
where FixedTCPhdrsize is 20, and FixedIPhdrsize is 20 for IPv4 and 40
for IPv6.
Author's Address
David Borman
Quantum Corporation
Mendota Heights, MN 55120
EMail: david.borman@quantum.com
Borman Informational [Page 9]
