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Versions: 00 01 02 03 04 draft-ietf-behave-tcp
Network Working Group S. Guha
Internet-Draft Cornell U.
Expires: August 5, 2006 K. Biswas
Cisco Systems
B. Ford
M.I.T.
P. Francis
Cornell U.
S. Sivakumar
Cisco Systems
P. Srisuresh
Consultant
Feb 2006
NAT Behavioral Requirements for Unicast TCP
draft-hoffman-behave-tcp-04.txt
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Abstract
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This document defines a set of requirements for NATs that handle TCP
that would allow many applications, such as peer-to-peer applications
and on-line games, to work consistently. Developing NATs that meet
this set of requirements will greatly increase the likelihood that
these applications will function properly.
Table of Contents
1. Applicability Statement . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. TCP Session Setup . . . . . . . . . . . . . . . . . . . . . . 4
4.1 Address and Port Mapping . . . . . . . . . . . . . . . . . 4
4.2 Internally Initiated Sessions . . . . . . . . . . . . . . 4
4.3 Externally Initiated Sessions . . . . . . . . . . . . . . 5
5. TCP Session Refresh . . . . . . . . . . . . . . . . . . . . . 6
6. Application Level Gateways . . . . . . . . . . . . . . . . . . 7
7. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Security considerations . . . . . . . . . . . . . . . . . . . 7
9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 9
11. Normative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . 12
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1. Applicability Statement
This document is adjunct to RFC FIXME-BEHAVE-UDP [1], which defines
many terms relating to NATs, lays out general requirements for all
NATs, and sets requirements for NATs that handle unicast UDP traffic.
The purpose of this document is to set requirements for NATs that
handle TCP traffic (that is, almost every NAT).
The requirements of this specification apply to Traditional NATs as
described in RFC 2663 [2].
This document only covers the TCP aspects of NAT traversal.
Firewalls, and packet inspection above the TCP layer are out-of-
scope. Middle-box behavior that is not necessary for network address
translation of TCP is out-of-scope. Application and OS aspects of
TCP NAT traversal are out-of-scope. Signaling based approaches to
NAT traversal such as Midcom and UPnP that directly control the NAT
are out-of-scope.
2. Introduction
Network Address Translators (NATs) hinder connectivity in
applications where connections may be initiated to internal hosts.
RFC FIXME-BEHAVE-UDP [1] lays out the terminology and requirements
for NATs in the context of UDP. This document supplements these by
setting requirements for NATs that handle TCP traffic. All
definitions and requirements in [1] are inherited here.
Recently, many techniques have been devised to make peer-to-peer TCP
applications work across NATs. STUNT [3], NATBLASTER [4], and P2PNAT
[5] describe UNilateral Self-Address Translation (UNSAF) mechanisms
to establish TCP through NATs by modifying only endpoints. These
approaches depend on specific NAT behavior that is not always
supported (see [6] and [5] for details). Consequently a complete TCP
NAT Traversal solution is sometimes forced to rely on public TCP
Relays. This document defines requirements that ensures that TCP NAT
Traversal approaches are not forced to use data relays.
3. Terminology
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 [7].
This document uses the term "session" as defined in RFC 2663 [2].
"NAT" in this specification includes both "Basic NAT" and "Network
Address/Port Translator (NAPT)" [2].
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This document uses the terms "address and port mapping", "endpoint
independent mapping", "filtering behavior", "endpoint independent
filtering", "address dependent filtering" and "address and port
dependent filtering" as defined in RFC FIXME-BEHAVE-UDP [1].
4. TCP Session Setup
This section describes various NAT behaviors applicable to TCP
session setup.
4.1 Address and Port Mapping
RFC FIXME-BEHAVE-UDP [1] defines the criteria for the re-use of a
mapping for new sessions. The definition presented there is agnostic
of the transport protocol used and applies directly to TCP.
REQ-1: A NAT MUST have an "External NAT mapping is endpoint
independent" behavior.
Justification: REQ-1 is necessary for UNSAF methods to work. Refer
to REQ-1 in [1] for details.
4.2 Internally Initiated Sessions
An internal endpoint initiates a TCP session through a NAT by sending
a SYN packet. The NAT assigns an external IP address and port number
for the session so the resulting SYNACK response can be received,
translated and routed to the internal endpoint. This translation is
used for the subsequent ACK and other packets for the duration of the
session. This corresponds to the 3-Way Handshake mode of session
initiation defined in RFC 793 [8] and is supported by all NATs.
RFC 793 defines an alternate mode of session initiation, termed
Simultaneous-Open, which is used by peer-to-peer applications to
traverse NATs. In the Simultaneous-Open mode of operation, both
endpoints send SYN packets that cross in the network, followed by
SYNACK packets that cross in the network. From the perspective of
the NAT, the internal host's SYN packet is responded by an inbound
SYN packet for the same session (as opposed to a SYNACK packet).
Subsequent to this exchange, both an outbound and inbound SYNACK are
seen for the session. Some NATs block the inbound SYN for the
session; some NATs block or incorrectly translate the outbound
SYNACK. Such behavior breaks TCP Simultaneous-Open and prevents
peer-to-peer applications from functioning correctly behind a NAT.
In order to provide network address translation service for TCP, it
is necessary for a NAT to correctly receive, translate, and forward
all packets for a session that conforms to valid transitions of the
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TCP State-Machine [8].
REQ-2: For a TCP session, a NAT MUST support all valid sequences of
TCP packets as defined in RFC 793. In particular:
a) A NAT MUST support TCP Simultaneous-Open.
Justification: This requirement enables standards compliant TCP
stacks to traverse NATs.
4.3 Externally Initiated Sessions
When an internal endpoint initiates a session, the NAT assigns an
external IP:port. Some peer-to-peer applications let other external
endpoints initiate a TCP session to the internal endpoint by sending
a SYN to the external IP:port allocated. Such applications depend on
the NAT to reuse the mapping and route the SYN to the internal
endpoint. The internal endpoint replies with a SYNACK packet that
the NAT is expected to translate and forward to the external
endpoint, which responds with an ACK to complete the initiation. The
filtering behavior of the NAT, defined in [1], governs which external
endpoints are allowed to send inbound SYN packets for a new session.
REQ-3: A NAT with "Endpoint independent filtering" or "Address
dependent filtering" behavior MUST support TCP session initiations
from the specific external endpoints. Note that this requirement
is not applicable to NATs that have "Address and port dependent
filtering" behavior.
Justification: This is to avoid breaking peer-to-peer applications
which do not always initiate sessions from the internal side of
the NAT.
If the inbound SYN packet is filtered, either because a corresponding
mapping does not exist or because of the NAT's filtering behavior, a
NAT has two basic choices: to ignore the packet silently, or signal
an error to the sender. Ignoring the SYN helps applications perform
TCP Simultaneous-Open in the presence of clock skew and network
congestion where the inbound SYN may arrive at the NAT before the
outbound SYN creates the necessary session state.
REQ-4: It is RECOMMENDED that a NAT silently discard inbound SYN
packets that are filtered or cannot be routed.
Justification: This allows applications to traverse NATs with greater
ease.
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5. TCP Session Refresh
A NAT maintains state associated with new and established sessions.
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 for. 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 sessions is to
preferentially abandon sessions for crashed endpoints, followed by
closed TCP sessions and partially-open sessions. A NAT can check if
an endpoint has crashed by sending a TCP keep-alive packet and
checking for the response. If the NAT cannot determine whether the
session is active, it should not abandon it until the session has
been idle for some time. The time is derived from values recommended
in RFC 1122 [9]. The states of a TCP session that these values
correspond to are defined in RFC 793 [8] and can be inferred by
passively examining the TCP flags of inbound and outbound packets for
that session.
The established session timer is defined as the time a mapping will
stay active for a session over which application data can be
exchanged. Application data can be exchanged over a TCP session in
states: ESTABLISHED, FIN_WAIT_1, FIN_WAIT_2, and CLOSE_WAIT.
The transitory session timer is defined as the time a mapping will
stay active for a session over which application data cannot yet be
exchanged, or can no longer be exchanged. This includes partially-
open TCP sessions in states: SYN_SENT and SYN_RCVD, and closed
sessions in states: CLOSING, LAST_ACK, and TIME_WAIT.
REQ-5: If a NAT cannot determine whether the endpoints of a TCP
session are active, it MAY abandon the session if it has been idle
for some time. A default value of 2 hours for the established
session timer is RECOMMENDED. A default value of 4 minutes for
the transitory session timer is RECOMMENDED.
a) The value of the NAT TCP session timers MAY be configurable.
Justification: If a NAT cannot determine whether the endpoint of an
idle TCP session has crashed, the NAT should assume that the
endpoint is active. However, to defend against DoS attacks, a NAT
can abandon session state under certain circumstances while
minimally impacting active endpoints. For idle TCP sessions where
data can be exchanged (that is, once ACK packets are seen in both
directions, and FIN packets have not been seen in both
directions), some endpoints send keep-alive packets at 2 hour
intervals by default. For idle TCP sessions that are partially-
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open or closed, TCP waits 2xMSL (4 minutes) for in-flight packets
to be delivered and acknowledged. If a NAT passively waits for at
least this interval and does not see any packets for the TCP
session, it can prematurely abandon the session without impacting
most applications. NAT behavior for handling RST packets for a
session is left undefined.
a) Configuration helps troubleshoot and accommodate specific
applications.
6. Application Level Gateways
FIXME OPEN ISSUE: Is this out-of-scope? Do we need to specify all
TCP ALGs should be off by default? What about FTP?
7. Requirements
A NAT that supports all of the mandatory requirements of this
specification (i.e., the "MUST") and is compliant with [1], is
"compliant with this specification." A NAT that supports all of the
requirements of this specification (i.e., included the "RECOMMENDED")
and is fully compliant with [1] is "fully compliant with all the
mandatory and recommended requirements of this specification."
REQ-1: A NAT MUST have an "External NAT mapping is endpoint
independent" behavior.
REQ-2: For a TCP session, a NAT MUST support all valid sequences of
TCP packets as defined in RFC 793. In particular:
a) A NAT MUST support TCP Simultaneous-Open.
REQ-3: A NAT with "Endpoint independent filtering" or "Address
dependent filtering" behavior MUST support TCP session initiations
from the specific external endpoints. Note that this requirement
is not applicable to NATs that have "Address and port dependent
filtering" behavior.
REQ-4: It is RECOMMENDED that a NAT silently discard inbound SYN
packets that are filtered or cannot be routed.
REQ-5: If a NAT cannot determine whether the endpoints of a TCP
session are active, it MAY abandon the session if it has been idle
for some time. A default value of 2 hours for the established
session timer is RECOMMENDED. A default value of 4 minutes for
the transitory session timer is RECOMMENDED.
a) The value of the NAT TCP session timers MAY be configurable.
8. Security considerations
In addition to the security considerations addressed in [1], there
are additional concerns for handling TCP packets and are discussed in
this section.
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Security considerations for REQ-1: This requirement does not
introduce any TCP-specific concerns in addition to those already
addressed in [1].
Security considerations for REQ-2: This document requires that a NAT
accept an inbound SYN packet for a session in response to the
outbound SYN packet. In order to provide extra security, some
NATs require the ACK flag to be set in all inbound packets. This
attempts to protect against attackers that can blindly spoof SYN
packets appearing to come from the external destination, but are
not able to receive, and therefore acknowledge, packets addressed
to the same. REQ-2 in this document does not prevent a NAT from
providing the same security guarantees. Even after the inbound
SYN is accepted, the external endpoint is required by the TCP
specification to explicitly acknowledge the internal endpoint's
sequence number through a subsequent SYNACK or ACK packet. The
NAT can check these subsequent packets to thwart such spoofing
attacks.
Security considerations for REQ-3: The security provided by the NAT
is governed by its filtering behavior as addressed in [1].
Allowing TCP sessions to be initiated by endpoints in accordance
with the filtering behavior does not introduce additional
concerns.
Security considerations for REQ-4: This document recommends that if
an inbound SYN packet is filtered, then the NAT should silently
discard it. Some NATs send TCP RST or ICMP errors in response to
filtered packets. This serves to protect the NAT'ed hosts from
identity-theft attacks. In such an attack, the NAT is the
rightful recipient for an address, but an attacker blindly spoofs
packets from this address. If the NAT silently drops unexpected
inbound packets then the attacker can potentially spoof an entire
TCP session and masquerade as the NAT'ed endpoint. REQ-4 allows a
NAT to respond to such attacks by sending error packets for
unexpected non-SYN packets that follow the SYN packet.
Security considerations for REQ-5: This document recommends that a
NAT that passively monitors session state keep idle TCP sessions
alive for at least 4 minutes for partially-open or closed
sessions, and for at least 2 hours for established sessions by
default. If a NAT is under a DoS attack, the NAT administrator
may configure session timeouts accordingly, or let the NAT
actively determine session state.
NAT implementations that change local state based on TCP flags in
packets must ensure that out-of-window TCP packets are properly
handled. Out-of-window TCP packets are sometimes used in attacks
where an attacker resets arbitrary TCP sessions by guessing only the
endpoint IP addresses and ports. If the window is too large, an
attacker can send a small number of packets with crafted sequence
numbers such that one of these packets is considered an in-window
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packet that resets the session.
9. IANA considerations
This document does not change or create any IANA-registered values.
10. Acknowledgments
Thanks to Paul Hoffman for his many contributions to this document.
11. Normative References
[1] Audet, F. and C. Jennings, "NAT Behavioral Requirements for
Unicast UDP", draft-ietf-behave-nat-udp (work in progress).
[2] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999.
[3] Guha, S. and P. Francis, "NUTSS: A SIP based approach to UDP and
TCP connectivity", Proceedings of the ACM SIGCOMM Workshop on
Future Directions in Network Architecture (Portland, OR),
August 2004.
[4] Biggadike, A., Ferullo, D., Wilson, G., and A. Perrig,
"NATBLASTER: Establishing TCP connections between hosts behind
NATs", Proceedings of the ACM SIGCOMM Asia Workshop (Beijing,
China), April 2005.
[5] Ford, B., Srisuresh, P., and D. Kegel, "Peer-to-peer
communication across network address translators", Proceedings
of the USENIX Annual Technical Conference (Anaheim, CA),
April 2005.
[6] Guha, S. and P. Francis, "Characterization and Measurement of
TCP Traversal through NATs and Firewalls", Proceedings of the
Internet Measurement Conference (Berkeley, CA), October 2005.
[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[8] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[9] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.
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Authors' Addresses
Saikat Guha
Cornell University
331 Upson Hall
Ithaca, NY 14853
US
Email: saikat@cs.cornell.edu
Kaushik Biswas
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134
US
Phone: +1 408 525 5134
Email: kbiswas@cisco.com
Bryan Ford
M.I.T.
Laboratory for Computer Science
77 Massachusetts Ave.
Cambridge, MA 02139
US
Phone: +1 617 253 5261
Email: baford@mit.edu
Paul Francis
Cornell University
4108 Upson Hall
Ithaca, NY 14853
US
Phone: +1 607 255 9223
Email: francis@cs.cornell.edu
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Senthil Sivakumar
Cisco Systems, Inc.
7100-8 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709-4987
US
Phone: +1 919 392 5158
Email: ssenthil@cisco.com
Pyda Srisuresh
Consultant
20072 Pacifica Dr.
Cupertino, CA 95014
US
Phone: +1 408 836 4773
Email: srisuresh@yahoo.com
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