Network Working Group P. Hoffman
Internet-Draft Cybersecurity Association
Expires: January 18, 2006 S. Guha
P. Francis
Cornell University
July 17, 2005
NAT Behavioral Requirements for Unicast TCP
draft-hoffman-behave-tcp-02.txt
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Copyright (C) The Internet Society (2005).
Abstract
This document defines a set of requirements for NATs that handle
unicast TCP that would allow many applications, such as peer-to-peer
reliable 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.
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1. Introduction
[BEHAVE-UDP] defines many terms relating to NATs, lays out general
requirements for all NATs, and sets requirements for NATs that handle
unicast UDP traffic. This document is an adjunct to [BEHAVE-UDP] and
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 and outgoing SYNACK 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. In some cases, it may require a SYN packet to be
received after a SYN packet is sent, or a SYNACK packet to be sent
after a SYN packet it sent. Normal TCP stacks correctly handle such
cases, in accordance with [RFC-TCP].
REQ-1: A NAT MUST allow all TCP packets in either direction on any
open NAT mapping that is a TCP connection.
Justification for REQ-1: This allows end-hosts to follow any valid
transition defined in TCP.
3. Incoming SYN packets to non-existent mappings
NATs have two basic choices when they receive a SYN packet for a
mapping that does not exist: ignore it or respond to it with a RST or
ICMP packet. A NAT that receives a SYN packet from the external side
for an address and port mapping that does not exist must decide
whether or not to respond. Ignoring the SYN instead of responding
with an RST can help some applications that start multiple TCP
connections. Sending an ICMP soft-error does not affect other NATs
or OS stacks and allows the application to decide whether or not to
abort the connection attempt.
REQ-2: A NAT MUST NOT send a RST packet in response to a SYN packet
sent from the external side to an address and port mapping that
does not exist.
a) A NAT MAY send an ICMP Host Unreachable (Type=3, Code=1) in
response to such a SYN packet.
Justification for REQ-2: Not sending an RST helps applications that
open multiple TCP connections and attempt to trigger a TCP
Simultaneous-open. Sending an ICMP soft error allows applications
to choose to abort a TCP attempt without having to wait for a TCP
timeout.
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4. Resource exhaustion and timers
A NAT maintains state associated with the mappings that it maintains.
Because of this, a NAT is susceptible to a resource-exhaustion attack
whereby an attacker (or virus) on the internal side attempts to cause
the NAT to create more state than it has resources. To prevent such
an attack, a NAT needs to abandon mappings 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. The states of a TCP
connection are defined in [RFC-TCP] and can be inferred by examining
the TCP flags of incoming and outgoing packets for that mapping. The
mapping timer is defined as the time a mapping will stay active
without packets traversing the NAT. The timer values for various
states are mentioned in [RFC-1122].
REQ-3: A NAT TCP mapping timer SHOULD NOT expire in less than 4
minutes for a TCP connection in SYN_SENT, SYN_RCVD, CLOSING,
LAST_ACK or TIME_WAIT state. A NAT TCP mapping timer SHOULD NOT
expire in less than 2 hours for a TCP connection in any other
state except CLOSED.
a) When a NAT closes an open TCP mapping, it MUST send RST packets
to each end of the connection.
b) To maximize application transparency, a NAT MAY expire mappings
only when it is low on resources.
Justification for REQ-3: TCP Keep-alive packets, when enabled, are
sent at 2 hour intervals on an idle connection by default. During
connection setup or teardown, TCP waits 2xMSL (4 minutes) for in-
flight packets to be delivered and acknowledged. This requirement
can be easily implemented by examining the ACK flag in both
directions during connection setup and examining the FIN flag in
both directions during connection teardown.
5. Requirements
A NAT that supports all of the mandatory requirements of this
specification (i.e., the "MUST"), is "compliant with this
specification." A NAT that supports all of the requirements of this
specification (i.e., included the "RECOMMENDED") is "fully compliant
with all the mandatory and recommended requirements of this
specification."
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REQ-1: A NAT MUST allow all TCP packets in either direction on any
open NAT mapping that is a TCP connection.
REQ-2: A NAT MUST NOT send a RST packet in response to a SYN packet
sent from the external side to an address and port mapping that
does not exist.
a) A NAT MAY send an ICMP Host Unreachable (Type=3, Code=1) in
response to such a SYN packet.
REQ-3: A NAT TCP mapping timer SHOULD NOT expire in less than 4
minutes for a TCP connection in SYN_SENT, SYN_RCVD, CLOSING,
LAST_ACK or TIME_WAIT state. A NAT TCP mapping timer SHOULD NOT
expire in less than 2 hours for a TCP connection in any other
state except CLOSED.
a) When a NAT closes an open TCP mapping, it MUST send RST packets
to each end of the connection.
b) To maximize application transparency, a NAT MAY expire mappings
only when it is low on resources.
Requirements inherited from [BEHAVE-UDP]:
[[Once finalized, BEHAVE-UDP requirement list with UDP changed to TCP
where applicable.]]
6. 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? ]]
[[If the NAT is examining TCP flags, then it needs to track sequence
numbers and the TCP timestamp option to correctly determine out-of-
window packets.]]
7. IANA considerations
This document does not change or create any IANA-registered values.
8. Normative References
[BEHAVE-UDP]
Audet, F. and C. Jennings, "NAT Behavioral Requirements
for Unicast UDP", draft-ietf-behave-nat-udp (work in
progress).
[RFC-1122]
Braden, R., "Requirements for Internet Hosts --
Communication Layers", RFC 793.
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[RFC-TCP] Postel, J., "Transmission Control Protocol", RFC 793.
Authors' Addresses
Paul Hoffman
Cybersecurity Association
127 Segre Place
Santa Cruz, CA 95060
USA
Email: paul.hoffman@cybersecurity.org
Saikat Guha
Cornell University
331 Upson Hall
Ithaca, NY 14853
USA
Email: saikat@cs.cornell.edu
Paul Francis
Cornell University
4108 Upson Hall
Ithaca, NY 14853
USA
Email: francis@cs.cornell.edu
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