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Versions: (draft-gont-tcpm-tcp-timestamps) 00 01   Best Current Practice
02 03 04 rfc6191
TCP Maintenance and Minor                                        F. Gont
Extensions (tcpm)                                                UK CPNI
Internet-Draft                                          February 4, 2011
Intended status: BCP
Expires: August 8, 2011
Reducing the TIME-WAIT state using TCP timestamps
draft-ietf-tcpm-tcp-timestamps-04.txt
This document describes an algorithm for processing incoming SYN
segments that allows higher connection-establishment rates between
any two TCP endpoints when a TCP timestamps option is present in the
incoming SYN segment.  This document only modifies processing of SYN
segments received for connections in the TIME-WAIT state; processing
in all other states is unchanged.
This Internet-Draft will expire on August 8, 2011.
Gont                     Expires August 8, 2011                 [Page 1]
Internet-Draft  Reducing TIME-WAIT state with timestamps   February 2011
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
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material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
2.  Improved processing of incoming connection requests  . . . . .  3
3.  Interaction with various timestamps generation algorithms  . .  6
4.  Interaction with various ISN generation algorithms . . . . . .  7
6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
8.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
8.2.  Informative References . . . . . . . . . . . . . . . . . .  9
Appendix A.  Behavior of the proposed mechanism in specific
scenarios . . . . . . . . . . . . . . . . . . . . . . 10
A.1.  Connection request after system reboot . . . . . . . . . . 10
Appendix B.  Changes from previous versions of the draft (to
be removed by the RFC Editor before publishing
this document as an RFC)  . . . . . . . . . . . . . . 10
B.1.  Changes from draft-ietf-tcpm-tcp-timestamps-03 . . . . . . 10
B.5.  Changes from draft-gont-tcpm-tcp-timestamps-04 . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
The Timestamps option, specified in RFC 1323 [RFC1323], allows a TCP
to include a timestamp value in its segments, that can be used to
perform two functions: Round-Trip Time Measurement (RTTM), and
Protection Against Wrapped Sequences (PAWS).
For the purpose of PAWS, the timestamps sent on a connection are
required to be monotonically increasing.  While there is no
requirement that timestamps are monotonically increasing across TCP
connections, the generation of timestamps such that they are
monotonically increasing across connections between the same two
endpoints allows the use of timestamps for improving the handling of
SYN segments that are received while the corresponding four-tuple is
in the TIME-WAIT state.  That is, the timestamp option could be used
to perform heuristics to determine whether to allow the creation of a
new incarnation of a connection that is in the TIME-WAIT state.
This use of TCP timestamps is simply an extrapolation of the use of
Initial Sequence Numbers (ISNs) for the same purpose, as allowed by
RFC 1122 [RFC1122], and it has been incorporated in a number of TCP
implementations, such as that included in the Linux kernel [Linux].
document are to be interpreted as described in RFC 2119 [RFC2119].
2.  Improved processing of incoming connection requests
In a number of scenarios a socket pair may need to be reused while
the corresponding four-tuple is still in the TIME-WAIT state in a
remote TCP peer.  For example, a client accessing some service on a
host may try to create a new incarnation of a previous connection,
while the corresponding four-tuple is still in the TIME-WAIT state at
the remote TCP peer (the server).  This may happen if the ephemeral
port numbers are being reused too quickly, either because of a bad
policy of selection of ephemeral ports, or simply because of a high
connection rate to the corresponding service.  In such scenarios, the
establishment of new connections that reuse a four-tuple that is in
the TIME-WAIT state would fail.  This problem is discussed in detail
in [INFOCOM-99].
In order to avoid this problem, RFC 1122 [RFC1122] (in Section
4.2.2.13) states that when a connection request is received with a
four-tuple that is in the TIME-WAIT state, the connection request
could be accepted if the sequence number of the incoming SYN segment
is greater than the last sequence number seen on the previous
incarnation of the connection (for that direction of the data
transfer).  This requirement aims at avoiding the sequence number
space of the new and old incarnations of the connection to overlap,
thus avoiding old segments from the previous incarnation of the
connection to be accepted as valid by the new connection.
The same policy may be extrapolated to TCP timestamps.  That is, when
a connection request is received with a four-tuple that is in the
TIME-WAIT state, the connection request could be accepted if the
timestamp of the incoming SYN segment is greater than the last
timestamp seen on the previous incarnation of the connection (for
that direction of the data transfer).
The following paragraphs summarize the processing of SYN segments
received for connections in the TIME-WAIT state.  The processing of
SYN segments received for connections in all other states is
unchanged.  Both the ISN (Initial Sequence Number) and the timestamps
option (if present) of the incoming SYN segment are included in the
heuristics performed for allowing a high connection-establishment
rate.
Processing of SYN segments received for connections in the TIME-WAIT
state SHOULD occur as follows:
o  If the previous incarnation of the connection used timestamps,
*  If TCP timestamps would be enabled for the new incarnation of
the connection, and the timestamp contained in the incoming SYN
segment is greater than the last timestamp seen on the previous
transfer), honour the connection request (creating a connection
in the SYN-RECEIVED state).
the connection, the timestamp contained in the incoming SYN
segment is equal to the last timestamp seen on the previous
transfer), and the Sequence Number of the incoming SYN segment
*  If TCP timestamps would not be enabled for the new incarnation
of the connection, but the Sequence Number of the incoming SYN
segment is greater than the last sequence number seen on the
previous incarnation of the connection (for the same direction
of the data transfer), honour the connection request (creating
a connection in the SYN-RECEIVED state).
*  Otherwise, silently drop the incoming SYN segment, thus leaving
the previous incarnation of the connection in the TIME-WAIT
o  If the previous incarnation of the connection did not use
timestamps, then,
the connection, honour the incoming connection request
(creating a connection in the SYN-RECEIVED state).
of the data transfer), honour the incoming connection request
In the above explanation, the phrase "TCP timestamps would be
enabled for the new incarnation for the connection" means that the
incoming SYN segment contains a TCP Timestamps option (i.e., the
client has enabled TCP timestamps), and that the SYN/ACK segment
that would be sent in response to it would also contain a
Timestamps option (i.e., the server has enabled TCP timestamps).
In such a scenario, TCP timestamps would be enabled for the new
incarnation of the connection.
The "last sequence number seen on the previous incarnation of the
connection (for the same direction of the data transfer)" refers
to the last sequence number used by the previous incarnation of
the connection (for the same direction of the data transfer), and
not to the last value seen in the Sequence Number field of the
corresponding segments.  That is, it refers to the sequence number
corresponding to the FIN flag of the previous incarnation of the
connection, for that direction of the data transfer.
Many implementations do not include the TCP timestamp option when
performing the above heuristics, thus imposing stricter constraints
on the generation of Initial Sequence Numbers, the average data
transfer rate of the connections, and the amount of data transferred
with them.  RFC 793 [RFC0793] states that the ISN generator should be
incremented roughly once every four microseconds (i.e., roughly
250000 times per second).  As a result, any connection that transfers
more than 250000 bytes of data at more than 250 kilobytes/second
could lead to scenarios in which the last sequence number seen on a
connection that moves into the TIME-WAIT state is still greater than
the sequence number of an incoming SYN segment that aims at creating
a new incarnation of the same connection.  In those scenarios, the
4.4BSD heuristics would fail, and therefore the connection request
would usually time out.  By including the TCP timestamp option in the
heuristics described above, all these constraints are greatly
relaxed.
It is clear that the use of TCP timestamps for the heuristics
described above benefit from timestamps that are monotonically
increasing across connections between the same two TCP endpoints.
The upcoming revision of RFC 1323, [I-D.ietf-tcpm-1323bis],
recommends the selection of timestamps such that they are
monotonically-increasing across connections.  An example of such a
Timestamps generation scheme can be found in
[I-D.gont-timestamps-generation].
3.  Interaction with various timestamps generation algorithms
The algorithm proposed in Section 2 clearly benefits of timestamps
that are monotonically-increasing across connections to the same end-
point.  In particular, generation of timestamps such that they are
monotonically-increasing timestamps are important for TCPs that
perform the active open, as those are the timestamps that will be
used for the proposed algorithm.
While monotonically-increasing timestamps ensure that the proposed
algorithm will be able to reduce the TIME-WAIT state of a previous
incarnation of a connection, implementation of the algorithm does not
imply by itself a requirement on the timestamps generation algorithm
of other TCPs.
In the worst-case scenario, an incoming SYN corresponding to a new
incarnation of a connection in the TIME-WAIT contains a timestamp
that is smaller than the last timestamp seen on the previous
incarnation of the connection, the heuristics fail, and the result is
no worse than the current state-of-affairs.  That is, the SYN segment
is ignored (as specified in [RFC1337]), and thus the connection
request times out, or is accepted after future retransmissions of the
SYN.
Some stacks may implement timestamps generation algorithms that do
not lead to monotonically-increasing timestamps across connections
with the same remote endpoint.  An example of such algorithms is the
one described in [RFC4987] and [Opperman], that allows the
implementation of extended TCP SYN cookies.
It should be noted that the "extended TCP SYN cookies" could co-
exist with an algorithm for generating timestamps such that they
are monotonically-increasing.  Monotonically increasing timestamps
could be generated for TCPs that perform the active open, while
timestamps for TCPs that perform the passive open could be
generated according to [Opperman].
Some stacks (notably OpenBSD) implement timestamps randomization
algorithms which do not result in monotonically-increasing ISNs
across connections.  As noted in [Silbersack], such randomization
schemes may prevent the mechanism proposed in this document from
recycling connections that are in the TIME-WAIT state.  However, as
noted earlier in this section, in the worst-case scenario the
heuristics fail, and the result is no worse than the current state-
of-affairs.
4.  Interaction with various ISN generation algorithms
[RFC0793] suggests that the ISNs of TCP connections be generated from
a global timer, such that they are monotonically-increasing across
connections.  However, this ISN-generation scheme leads to
predictable ISNs, which have well-known security implications
[CPNI-TCP].  [RFC1948] proposes an alternative ISN-generation scheme
which results in monotonically-increasing ISNs across connections
that are not easily-predictable by an off-path attacker.
Some stacks (notably OpenBSD) implement ISN randomization algorithms
which do not result in monotonically-increasing ISNs across
connections.  As noted in [Silbersack], such ISN randomization
schemes break the BSD improved handling of SYN segments received for
connections that are in the TIME-WAIT state.
An implementation of the mechanism proposed in this document would
enable recycling of the TIME-WAIT state even in the presence of ISNs
that are not monotonically-increasing across connections, except when
the timestamp contained in the incoming SYN is equal to the last
timestamp seen on the connection in the TIME-WAIT state (for that
direction of the data transfer).
[I-D.ietf-tcpm-tcp-security] contains a detailed discussion of the
security implications of TCP timestamps and of different Timestamps
generation algorithms.
This document has no actions for IANA.
7.  Acknowledgements
This document is based on part of the contents of the technical
report "Security Assessment of the Transmission Control Protocol
(TCP)" [CPNI-TCP] written by Fernando Gont on behalf of the United
Kingdom's Centre for the Protection of National Infrastructure (UK
CPNI).
The author of this document would like to thank (in alphabetical
order) Mark Allman, Francis Dupont, Wesley Eddy, Lars Eggert, Alfred
Hoenes, John Heffner, Christian Huitema, Eric Rescorla, Joe Touch,
and Alexander Zimmermann for providing valuable feedback on an
earlier version of this document.
Additionally, the author would like to thank David Borman for a
fruitful discussion on TCP timestamps at IETF 73.
Finally, the author would like to thank the United Kingdom's Centre
for the Protection of National Infrastructure (UK CPNI) for their
continued support.
RFC 793, September 1981.
[RFC1122]  Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1323]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[CPNI-TCP]
CPNI, "Security Assessment of the Transmission Control
Protocol (TCP)", 2009, <http://www.cpni.gov.uk/Docs/
tn-03-09-security-assessment-TCP.pdf>.
[I-D.gont-timestamps-generation]
Gont, F. and A. Oppermann, "On the generation of TCP
timestamps", draft-gont-timestamps-generation-00 (work in
[I-D.ietf-tcpm-1323bis]
Borman, D., Braden, R., and V. Jacobson, "TCP Extensions
for High Performance", draft-ietf-tcpm-1323bis-01 (work in
progress), March 2009.
[I-D.ietf-tcpm-tcp-security]
Gont, F., "Security Assessment of the Transmission Control
Protocol (TCP)", draft-ietf-tcpm-tcp-security-02 (work in
progress), January 2011.
[INFOCOM-99]
Faber, T., Touch, J., and W. Yue, "The TIME-WAIT state in
TCP and Its Effect on Busy Servers", Proc. IEEE Infocom,
1999, pp. 1573-1583 .
[Linux]    The Linux Project, "http://www.kernel.org".
[Opperman]
Oppermann, A., "FYI: Extended TCP syncookies in FreeBSD-
current",  Post to the tcpm mailing-list. Available at: ht
tp://www.ietf.org/mail-archive/web/tcpm/current/
msg02251.html, 2006.
[RFC1337]  Braden, B., "TIME-WAIT Assassination Hazards in TCP",
RFC 1337, May 1992.
[RFC1948]  Bellovin, S., "Defending Against Sequence Number Attacks",
[RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007.
[Silbersack]
Silbersack, M., "Improving TCP/IP security through
randomization without sacrificing interoperability",
EuroBSDCon 2005 Conference .
Appendix A.  Behavior of the proposed mechanism in specific scenarios
A.1.  Connection request after system reboot
This section clarifies how this algorithm would operate in case a
computer reboots, keeps the same IP address, looses memory of the
previous timestamps, and then tries to reestablish a previous
Firstly, as specified in [RFC0793], hosts must not establish new
connections for a period of 2*MSL (Maximum Segment Lifetime) after
they boot (this is the "quiet time" concept).  As a result, specs-
wise, this scenario should never occur.
If a host does not comply with the "quiet time concept", a connection
request might be sent to a remote host while there is a previous
incarnation of the same connection in the TIME-WAIT state at the
remote host.  In such a scenario, as a result of having lost memory
of previous time stamps, the resulting timestamps might not be
monotonically-increasing, and hence the proposed algorithm might be
unable to recycle the previous incarnation of the connection that is
in the TIME-WAIT state.  This case corresponds to the current state-
of-affairs without the algorithm proposed in this document.
Appendix B.  Changes from previous versions of the draft (to be removed
by the RFC Editor before publishing this document as an
RFC)
B.1.  Changes from draft-ietf-tcpm-tcp-timestamps-03
o  Addresses Tim Polk's DISCUSS.
o  Addresses COMMENTs received during IESG review, and maybe Tim
Polk's DISCUSS.
o  Addresses AD-review comments by Lars Eggert.
Gont                     Expires August 8, 2011                [Page 10]
o  Addresses WG Last call comments received from Wesley Eddy, John
Heffner and Joe Touch.
o  Minor editorial fix (reported by Wes Eddy).
B.5.  Changes from draft-gont-tcpm-tcp-timestamps-04
o  Draft resubmitted as draft-ietf.
o  Changed the document title
o  Removed all the text related to the algorithm earlier proposed for
timestamps generation.
o  Addresses comments received from Alexander Zimmermann, Christian
Huitema, Joe Touch, and others.
o  Minor edits (the I-D was just about to expire, so it was
resubmitted with almost no changes).
o  Version -01 of the draft had expired, and hence the I-D is
resubmitted to make it available again (no changes).
o  Fixed author's affiliation.
o  Addressed feedback submitted by Alfred Hoenes (see:
http://www.ietf.org/mail-archive/web/tcpm/current/msg04281.html),
plus nits sent by Alfred off-list.
Fernando Gont
UK Centre for the Protection of National Infrastructure
Email: fernando@gont.com.ar
URI:   http://www.cpni.gov.uk