Network Working Group Network Working Group
Internet-Draft D. Borman
Updates: 879, 2385 Wind River Systems
Intended Status: Informational March 25, 2010
File: draft-ietf-tcpm-tcpmss-03.txt
TCP Options and MSS
Abstract
This memo discusses what value to use with the TCP MSS option.
Status of This Memo
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Copyright
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
1. Introduction
There has been some confusion as to what value should be filled in
the TCP MSS option when using IP and TCP options. RFC-879 [RFC879]
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stated:
The MSS counts only data octets in the segment, it does not
count the TCP header or the IP header.
which 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 (see 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, the size of the fixed
IPv4 header is 20 bytes, and the size of the fixed IPv6 header is 40
bytes. 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 to other RFCs
3.1 RFC-879
RFC-897 [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."
and 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."
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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 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.
4. Clarification from the TCP Large Windows mailing list
Additional clarification was sent to the TCP Large Windows mailing
list in 1993 [Borman93].
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:
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+--------------------+--------------------+
| 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 | | |
+--------------------+--------------------+--------------------+
Since 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, the only way to guarantee
that this doesn't happen 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 length 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.
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:
RFC-1191 "Path MTU Discovery" [RFC1191]
RFC-2923 "TCP Problems with Path MTU Discovery" [RFC2923]
RFC-4821 "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., ROHC [RFC5795]. It is tempting for TCP to want
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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 compressor/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.
6. Security Considerations
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.
7. IANA Considerations
This document has no actions for IANA.
8. References
Informative References
[Borman93] Borman, D., "TCP MSS & Timestamps", Message to the
tcplw mailing list, Jan 7, 1993.
[RFC793] Postel, J., "Transmission Control Protocol," RFC-793,
September 1981.
[RFC879] Postel, J., "The TCP Maximum Segment Size and Related
Topics", RFC-879, ISI, November 1983.
[RFC1122] Braden, R., editor, "Requirements for Internet Hosts --
Communication Layers", RFC-1122, October, 1989.
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[RFC1191] Mogul, J.C., Deering, S.E., "Path MTU discovery"
RFC-1191, November 1990.
[RFC1981] McCann, J., Deering, S. and Mogul, J.,, "Path MTU
Discovery for IP Version 6", RFC-1981, August 1986.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC-2119, March 1997.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP
MD5 Signature Option", RFC-2385, August 1988.
[RFC2675] Borman, D., Deering, S., Hinden, R., "IPv6 Jumbograms",
RFC-2675, August, 1999.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC-2923, September 2000.
[RFC4821] Mathis, M., Heffner, J., "Packetization Layer Path MTU
Discovery", RFC-4821, March 2007.
[RFC5795] Sandlund, K., Gelletier, G. and Jonsson, L-E., "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:
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, 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, 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.
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
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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, in RFC-1122 on page 86 it
also states:
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.
Authors' Addresses
David Borman
Wind River Systems
Mendota Heights, MN 55120
Phone: (651) 454-3052
Email: david.borman@windriver.com
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