TCP Maintenance and Minor F. Gont
Extensions (tcpm) UK CPNI
Internet-Draft March 30, 2010
Intended status: BCP
Expires: October 1, 2010
Reducing the TIME-WAIT state using TCP timestamps
draft-gont-tcpm-tcp-timestamps-04.txt
Abstract
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.
Status of this Memo
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Improved processing of incoming connection requests . . . . . 3
3. Interaction with various timestamps generation algorithms . . 6
4. Corner-cases . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Connection request after system reboot . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Changes from previous versions of the draft (to
be removed by the RFC Editor before publishing
this document as an RFC) . . . . . . . . . . . . . . 9
A.1. Changes from draft-gont-tcpm-tcp-timestamps-03 . . . . . . 9
A.2. Changes from draft-gont-tcpm-tcp-timestamps-02 . . . . . . 9
A.3. Changes from draft-gont-tcpm-tcp-timestamps-01 . . . . . . 9
A.4. Changes from draft-gont-tcpm-tcp-timestamps-00 . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
The Timestamps option, specified in RFC 1323 [RFC1323], allows a TCP
to include a timestamp value in its segments, that can be used 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].
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. 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.
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
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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. Both the ISN
(Initial Sequence Number) and the timestamp 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
synchronized states should occur as follows:
o If a SYN segment is received for a connection in any synchronized
state other than TIME-WAIT, respond with an ACK, applying rate-
throttling.
o If the corresponding connection is in the TIME-WAIT state, then,
* If the previous incarnation of the connection used timestamps,
then,
+ 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 incarnation of the connection (for that
direction of the data transfer), honour the connection
request (creating a connection in the SYN-RECEIVED state).
+ If TCP timestamps would be enabled for the new incarnation
of the connection, the timestamp contained in the incoming
SYN segment is equal to the last timestamp seen on the
previous incarnation of the connection (for that direction
of the data transfer), and the Sequence Number of the
incoming SYN segment is larger than the last sequence number
seen on the previous incarnation of the connection (for that
direction of the data transfer), then honour the connection
request (creating a connection in the SYN-RECEIVED state).
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+ If TCP timestamps would not be enabled for the new
incarnation of the connection, but the Sequence Number of
the incoming SYN segment is larger 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 state.
* If the previous incarnation of the connection did not use
timestamps, then,
+ If TCP timestamps would be enabled for the new incarnation
of the connection, honour the incoming connection request.
+ If TCP timestamps would not be enabled for the new
incarnation of the connection, but the Sequence Number of
the incoming SYN segment is larger than the last sequence
number seen on the previous incarnation of the connection
(for the same direction of the data transfer), then honour
the incoming connection request (even if the sequence number
of the incoming SYN segment falls within the receive window
of the previous incarnation of the connection).
+ Otherwise, silently drop the incoming SYN segment, thus
leaving the previous incarnation of the connection in the
TIME-WAIT state.
Note:
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
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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 KB/s 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.
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,
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o The TIME_WAIT state is assassinated, with the connection request
being rejected (as specified in [RFC0793]), or,
o 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.
Note:
It should be noted that this algorithm 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].
4. Corner-cases
4.1. Connection request after system reboot
The question was raised on the tcpm mailing-list as to how this
algorithm would operate in case a computer reboots, keeps the same IP
address, looses memory of the previous time stamps, and then tries to
reestablish a previous connection.
Firstly, as specified in [RFC0793], hosts must not establish new
connections for a period of 2*MSL 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", then the
possible scenarios are:
o If the selected timestamp for the new connection is monotonically-
increasing with respect to the last timestamp seen on the previous
incarnation of the connection, the TIME-WAIT state is tossed, and
the new connection request succeeds.
o Otherwise, the connection request may time out or be rejected
(depending on whether the workaround described in [RFC1337] is
implemented or not). This case corresponds to the current state-
of-affairs without the algorithm proposed in this document.
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5. Security Considerations
While the algorithm described in this document for processing
incoming SYN segments would benefit from TCP timestamps that are
monotonically-increasing across connections, this document does not
propose any specific algorithm for generating timestamps, nor does it
require monotonically-increasing timestamps across conenctions.
[CPNI-TCP] contains a detailed discussion of the security
implications of TCP timestamps.
6. IANA Considerations
This document has no actions for IANA.
7. Acknowledgements
The author of this document would like to thank (in alphabetical
order) Mark Allman, Christian huitema, Alfred Hoenes, 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.
8. References
8.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
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.
[RFC1337] Braden, B., "TIME-WAIT Assassination Hazards in TCP",
RFC 1337, May 1992.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[CPNI-TCP]
CPNI, "Security Assessment of the Transmission Control
Protocol (TCP)", http://www.cpni.gov.uk/Docs/
tn-03-09-security-assessment-TCP.pdf, 2009.
[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.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007.
Appendix A. Changes from previous versions of the draft (to be removed
by the RFC Editor before publishing this document as an
RFC)
A.1. Changes from draft-gont-tcpm-tcp-timestamps-03
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.
A.2. Changes from draft-gont-tcpm-tcp-timestamps-02
o Minor edits (the I-D was just about to expire, so it was
resubmitted with almost no changes).
A.3. Changes from draft-gont-tcpm-tcp-timestamps-01
o Version -01 of the draft had expired, and hence the I-D is
resubmitted to make it available again (no changes).
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A.4. Changes from draft-gont-tcpm-tcp-timestamps-00
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.
Author's Address
Fernando Gont
UK Centre for the Protection of National Infrastructure
Email: fernando@gont.com.ar
URI: http://www.cpni.gov.uk
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