TCP Maintenance and Minor F. Gont
Extensions (tcpm) Consultant
Internet-Draft October 28, 2008
Intended status: BCP
Expires: May 1, 2009
On the generation of TCP timestamps
draft-gont-tcpm-tcp-timestamps-00.txt
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Abstract
This document describes an algorithm for selecting the timestamps (TS
value) used for TCP connections that use the TCP timestamp option,
such that the resulting timestamps are monotonically-increasing
across connections that involve the same four-tuple {local IP
address, local TCP port, remote IP address, remote TCP port}. The
properties of the algorithm are such the possibility of an attacker
guessing the exact value is reduced. Additionally, it describes an
algorithm for processing incoming SYN segments to allow a higher
connection establishment rates to any TCP end-point.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Proposed algorithm . . . . . . . . . . . . . . . . . . . . . . 3
3. Improved processing of incoming connection requests . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 8
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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
Protect 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
ISNs for the same purpose, as allowed by RFC 1122 [RFC0793] itself,
and has been incorporated in a number of TCP implementations, such as
that included in the Linux kernel. [Linux]
In order to avoid the security implications of predictable
timestamps, the proposed algorithm generates timestamps such such
that the possibility of an attacker guessing the exact value is
reduced.
Section 2 proposes the aforementioned algorithm for generating TCP
timestamps. Section 3 describes an improved processing of incomming
connection requests, that may allow higher connection-establishment
rates to any TCP end-point.
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. Proposed algorithm
It is RECOMMENDED that timestamps are generated with a similar
algorithm to that introduced by RFC 1948 [RFC1948] for the generation
of Initial Sequence Numbers (ISNs). That is,
timestamp = T() + F(localhost, localport, remotehost, remoteport,
secret_key)
where the result of T() is a global system clock that complies with
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the requirements of Section 4.2.2 of RFC 1323 [RFC1323], and F() is a
function that should not be computable from the outside. Therefore,
we suggest F() to be a cryptographic hash function of the
connection-id and some secret data.
F() provides an offset that will be the same for all incarnations of
a connection between the same two endpoints, while T() provides the
monotonically increasing values that are needed for PAWS.
3. 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 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 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 TIME-WAIT
state should occur as follows:
o If the previous incarnation of the connection used timestamps,
then,
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* If the incoming SYN segment contains a timestamp option, and
the timestamp is greater than the last timestamp seen on the
previous incarnation of the connection, honor the connection
request (creating a connection in the SYN-RECEIVED state).
* If the incoming SYN segment contains a timestamp option, and
the timestamp 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 honor the connection request.
* If the incoming SYN segment does not contain a timestamp
option, 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), honor the connection request.
* Otherwise, silently drop the incoming SYN segment, thus leaving
the previous connection in the TIME-WAIT state.
o If the previous incarnation of the connection did not use
timestamps, then,
* If the incoming connection request contains a timestamp option,
and timestamps will be enabled for the new incarnation of the
connection (i.e., the SYN/ACK segment will contain a timestamp
option), honor the incoming connection request.
* If the incoming connection request does not contain a timestamp
option, 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 honor 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).
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
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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 depends on the timestamps to be monotonically
increasing across connections between the same two TCP endpoints.
Therefore, we strongly advice to generate timestamps as described in
Section 2.
4. Security Considerations
This document describes an algorithm that can be used to obfuscate
the timestamp value used for new connections, such that the
possibility of an attacker guessing the exact value is reduced.
Some implementations are known to maintain a global timestamp clock,
which is used for all connections. This is undesirable, as an
attacker that can establish a connection with a host would learn the
timestamp used for all the other connections maintained by that host,
which could be useful for performing any attacks that require the
attacker to forge TCP segments. Some implementations are known to
initialize their global timestamp clock to zero when the system is
bootstrapped. This is undesirable, as the timestamp clock would
disclose the system uptime.
The algorithm discussed in this document for generating the TCP
timestamps avoids these problems by generating timestamps as
monotonically-increasing function with a per-connection-id random
offset. [CPNI-TCP]
5. IANA Considerations
This document has no actions for IANA.
6. Acknowledgements
Yet to be added
7. References
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7.1. 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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7.2. Informative References
[CPNI-TCP]
CPNI, "Security Assessment of the Transmission Control
Protocol (TCP)", (to be published) .
[Linux] The Linux Project, "http://www.kernel.org".
[RFC1948] Bellovin, S., "Defending Against Sequence Number Attacks",
RFC 1948, May 1996.
Author's Address
Fernando Gont
Consultant
Email: fernando@gont.com.ar
URI: http://www.gont.com.ar
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