Network Working Group P. Hoffman
Internet-Draft Cybersecurity Association
Expires: December 3, 2005 June 2005
NAT Behavioral Requirements for Unicast TCP
draft-hoffman-behave-tcp-00.txt
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Abstract
This document defines a set of requirements for NATs that handle
unicast TCP that would allow many applications, such as multimedia
communications or on-line gaming, to work consistently. Developing
NATs that meet this set of requirements will greatly increase the
likelihood that these applications will function properly.
1. Introduction
[BEHAVE-UDP] defines many terms relating to NATs, lays out general
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requirements for all NATs, and sets requirements for NATs that handle
unicast UDP traffic. This document is an adjunct to [BEHAVE-UDP]
that sets requirements for NATs that handle unicast TCP traffic (that
is, almost every NAT). All definitions and requirements in [BEHAVE-
UDP] are inherited here.
2. Incoming SYN packets on an open TCP connection
Some TCP-based protocols attempt to start multiple TCP connections
for a single flow of traffic. Such protocols can sometimes work
through NATs. Normal TCP stacks accept incoming SYN packets, of
course.
REQ-1 A NAT MUST allow SYN packets in either direction on any open
NAT session that is a TCP connection.
3. Incoming SYN packets to non-existent sessions
TCP-handling have two basic choices when they receive a SYN packet
for a session that they do not intend to set up: ignore it or respond
to it with a RST packet. A NAT that receives a SYN packet from the
outside for a connection that does not exist must decide whether or
not to respond with a RST packet. Ignoring such a SYN packet causes
no traffic overhead, and does not expose the NAT to any known
security vulnerability. Ignoring the SYN instead of responding with
an RST can help some applications that start multiple TCP
connections.
REQ-2 A NAT MUST NOT reply to a SYN packet sent from the outside to a
connection that is not established.
4. Resource exhaustion and timers
A NAT maintains state associated with the sessions that it maintains.
Because of this, a NAT is susceptible to a resource-exhaustion attack
whereby the attacker attempts to cause the NAT to create more state
that it has resources. To prevent such an attack, a NAT needs to
abandon sessions in order to free the state resources.
A common method that is applicable only to TCP connections is to look
for partially-open or partially closed TCP connections and abandon
those based on timers specific to those states. There is a serious
problem with this method: the timers used by the NAT are unlikely to
be the same as the timers that are used by the two end systems. This
is particularly true for end systems that do not use timers for
partially-open or partially-closed TCP connections, but instead use
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resource-based methods. A NAT that uses such timers is inherently
non-deterministic.
REQ-3 A NAT MUST NOT abandon a TCP connection simply because it has
been in a partially-open or partially-closed state for a long
time, or because it has been open for a long time without
traffic.
A NAT that is facing resource exhaustion MAY abandon TCP connections.
This is the only time that a NAT may abandon a TCP connection, even
one that is partially-open.
REQ-4 In order to cause the least unexpected behavior, a NAT that is
abandoning TCP connections to prevent resource exhaustion
SHOULD first abandon partially-closed TCP sessions; then, only
if needed, it SHOULD abandon partially-open TCP sessions; then,
only if needed, it SHOULD abandon open TCP sessions that have
had no traffic for a relatively long time. A NAT is allowed to
change the order of the sessions to abandon based on particular
attack patterns.
REQ-5 When a NAT closes an open TCP session to avoid resource
exhaustion, it MUST send RST packets to each end of the
session.
5. Security considerations
The security considerations for this document are the same as for
[BEHAVE-UDP]. The fact that this document covers unicast TCP does
not change any of the security considerations there. [[ Is this
statement actually true? ]]
6. IANA considerations
This document does not change or create any IANA-registered values.
7. Normative References
[BEHAVE-UDP]
Audet, F. and C. Jennings, "NAT Behavioral Requirements
for Unicast UDP", draft-ietf-behave-nat-udp (work in
progress).
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Author's Address
Paul Hoffman
Cybersecurity Association
127 Segre Place
Santa Cruz, CA 95060
USA
Email: paul.hoffman@cybersecurity.org
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