BEHAVE P. Srisuresh
Internet-Draft Consultant
Expires: April 23, 2006 S. Sivakumar
K. Biswas
Cisco Systems
B. Ford
M.I.T
October 20, 2005
NAT Behavioral Requirements for TCP
draft-sivakumar-behave-nat-tcp-req-02
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
NAT devices are available from a number of vendors and are in use by
several residential and enterprise users. Yet, there is much
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variation in how the NAT devices work. Application developers,
network administrators and users of NAT devices seek some level of
uniformity and predictability in how various of the NAT devices
operate. The objective of this document is to specify the
operational and behavioral requirements on the NAT devices while
processing TCP packets. A NAT device that conforms to the
requirements listed in the document will bring predictability in how
NATs operate with regard to TCP packet processing. A NAT device is
said to be IETF behave compliant when it complies with the
requirements outlined in this document and other companion BEHAVE
documents like [BEH-UDP], which outlines the requirements for
processing UDP and some generic requirements.
Table of Contents
1. Introduction & Scope . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. TCP requirements . . . . . . . . . . . . . . . . . . . . . . . 3
3.1 Address Mapping and/or TCP Port Mapping . . . . . . . . . 3
3.2 Timeouts for TCP NAT Sessions . . . . . . . . . . . . . . 4
3.3 Handling Inbound SYN packets for an exisitng NAT TCP
Session . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.4 Handling Inbound SYN packets for non-existent TCP
connections . . . . . . . . . . . . . . . . . . . . . . . 5
3.5 Denial of Service (DoS) attacks . . . . . . . . . . . . . 6
3.6 NAT initiated TCP keep-alives . . . . . . . . . . . . . . 7
3.7 NAT initiated RST packets . . . . . . . . . . . . . . . . 7
4. Hints to implementers . . . . . . . . . . . . . . . . . . . . 8
4.1 Light weight TCP state machine is a common practice . . . 8
4.2 TCP segment processing in NATs supporting ALGs . . . . . . 9
4.3 Adjusting Sequence Acknowledgement Numbers . . . . . . . . 10
5. TCP behavioral requirements summary . . . . . . . . . . . . . 10
6. Security considerations . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1 Normative References . . . . . . . . . . . . . . . . . . . 11
8.2 Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 14
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1. Introduction & Scope
NAT implementations vary amongst vendors in how they handle TCP
packets. This document defines the operational and behavioral
requirements that the NAT devices should comply with while processing
TCP packets. In addition, a section is devoted to describing hints
to implementers in deciphering some of the requirements.
The requirements outlined here are applicable across all NAT types
identified in [RFC2663], most importantly the Traditional NAT, as
described in [RFC3022]. This document does not mandate a specific
implementation choice. Behavioral requirements for UDP are covered
in [BEH-UDP].
Application Layer Gateways (ALGs) are out of scope for this document.
However, hints on how a NAT could be extended to support ALGs are
discussed under the hints section. Permissible ALGs are listed in
[BEH-UDP] .
2. Terminology
Definitions for the NAT terms used throughout the document may be
found in [RFC2663]. TCP terms used in the document are as per the
definitions given in [TCP].
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 [RFC2119].
3. TCP requirements
This section lists the behavioral requirements of a NAT device when
processing TCP packets. Associated with each requirement, the
rationale behind the requirement is discussed in detail.
3.1 Address Mapping and/or TCP Port Mapping
NAT provides transparent routing between address realms by assigning
realm-specific endpoint locator(s), as packets pertaining to a
session cross realm boundaries. Several applications use the same
endpoint to establish multiple simultaneous sessions. Many
peer-to-peer applications use the public endpoint registration of
peering hosts to initiate sessions into.
In order to support peer-to-peer applications and applications that
entertain multiple simultaneous sessions using the same TCP endpoint,
NAT MUST retain the association it assigned to an endpoint at the
start of the first session and reuse the same endpoint association
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when multiple sessions using the same endpoint are routed through the
NAT device. Such a mapping between endpoints can occur when a NAT
device maintains Address Mappings and/or TCP Port Mappings.
REQ-1: A NAT device MUST maintain Address and/or TCP Port Mappings
3.2 Timeouts for TCP NAT Sessions
As may be noted from [TCP], an end-to-end TCP session in its lifetime
goes through three phases, namely Connecting, Established, and
Closing. Each end-to-end TCP session is managed through a separate
NAT Session within NAT. The NAT Session must be capable of
identifying the current phase of the end-to-end TCP session it
represents and use an idle timeout period that is appropriate for the
current phase.
Connecting Phase: An end-to-end TCP session is said to enter the
Connecting Phase when either of the endpoints sends the first SYN for
the TCP session and exit the phase upon completion of 3-way SYN
handshake. The idle timeout used by the NAT Session during this
phase is called the SYN timeout. SYN timeout needs to be relatively
short, so NAT can protect itself (and, potentially, the hosts behind
it) from SYN flood attacks. A NAT session is freed when the SYN
timeout expires
Established Phase: An end-to-end TCP session is said to enter the
Established Phase upon completion of 3-way handshake and exit the
phase upon seeing the first FIN or RST for the session. The idle
timeout used by the NAT during this phase of the end-to-end TCP
session is called the Session timeout. Session timeout needs to be
relatively long, so the NAT Session can retain state of the
end-to-end TCP Session within itself even after long periods of
inactivity in the session. Long periods of inactivity is not
uncommon with applications such as telnet and ftp. When Session
timer expires, the corresponding NAT Session may be freed (or) the
NAT Session may assume the TCP connection to have transitioned into
the Closing phase.
Closing Phase: An end-to-end TCP session is said to enter the Closing
Phase when either of the endpoitns sends the first FIN or RST for the
session. Alternately, the NAT Session may deem the TCP session to
have entered this phase when the TCP Session timer expires. The idle
timeout used by the NAT Session during this phase is called the Close
timeout. Close timeout is relatively short to ensure that the ACKs
for the final FINs on a gracefully-closed TCP session had a chance to
propagate in both directions, and to allow time for either endpoint
to re-open a recently closed or reset TCP session if desired. A NAT
device MAY opt to have different Close timeouts depending upon
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whether the Closing phase is triggered by FIN or not. Once the Close
timer expires, the NAT Session will be freed.
The following requirements apply to the NAT's timeouts:
REQ-2: A NAT device MUST be capable of identifying the current phase
of an end-to-end TCP session and use different idle timeout periods
for each phase of the TCP Session. The timeouts used for each phase
SHOULD be admin configurable. The recommended value for SYN timeout
is 30 seconds. The recommended value for TCP session timeout is 30
minutes. Lastly, the recommended value for close timeout is 2 x MSL
(Maximum Segment Lifetime) or 4 minutes.
3.3 Handling Inbound SYN packets for an exisitng NAT TCP Session
A NAT device might allow sessions to be initiated in just one
direction and not the other. However, once a NAT session is created
for a permitted TCP session, and the TCP session is in Connecting
phase, the NAT device MUST let the SYN packets through in either
direction. This is because TCP protocol fundamentally permits
simultaneous TCP Open from either end. A number of TCP based
Peer-to-peer applications utilize the simultaneous TCP open technique
to establish peer-to-peer connections.
If the TCP session is in established or Closing phases and a new SYN
packet arrives, the NAT device should assume that the TCP session has
re-entered the Connecting phase and initiate SYN timer. In summary,
a NAT device MUST let the SYN packets through once a NAT Session is
established for the TCP connection.
The following requirement applies to SYN packets arriving during
Connecting, Established or Closing phases of a TCP connection.
REQ-3: A NAT device MUST let the SYN packets through when the SYN
Packets are received on a TCP connection which is in one of
Connecting, Established or Closing phases.
3.4 Handling Inbound SYN packets for non-existent TCP connections
Inbound SYN packets MUST be permitted on all NAT devices so long as
an outbound SYN packet has been initiated first on the same
connection tuple via the NAT device (and the NAT device has a NAT
Session created for it). Inbound SYN packets are permitted, even
without an outbound SYN on bi-directional NAT and load share NAT
devices. In addition, NAT devices configured with a static
inbound/birectional address or port mapping that matches the
connection tuple of the inbound SYN packets MUST also permit the SYN
packets.
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In all other cases, the NAT devices MUST simply drop the inbound SYN
packets. The NAT device should not send RST packet or an ICMP error
packet back to the sender. However, there is an exception to this
behavior. A NAT device sharing the TCP ports of the external IP
address between NAT function and its own endhost applications may
choose to respond with a RST when an inbound SYN is directed to one
of the endhost TCP ports. However, when the inbound SYN is directed
to one of the NAT function TCP ports, the response should be as
described earlier for a NAT device.
The following requirement applies to incoming SYN packets.
REQ-4: A NAT device MUST let the incoming SYN packets through so long
as an outbound SYN packet has been initiated first on the same
connection tuple via the NAT device. NATs MUST permit inbound SYN
even without an outbound SYN on bi-directional NAT & load share NAT
devices. NATs MUST NOT generate RST or ICMP error packet back to the
sender upon inbound SYN processing.
3.5 Denial of Service (DoS) attacks
Since NAT devices are Internet hosts, they can be the target of a
number of different DoS attacks, such as SYN floods and RST attacks.
NAT devices SHOULD employ the same sort of protection techniques as
Internet-based servers do.
Let us examine two types of Dos attacks that are well known with
regard to TCP connections. A SYN flood attack is a DoS attack in
which one or more external entities initiate a number of simultaneous
TCP connections using a SYN packet, but donot complete the 3-way
handshake. Naturally, a NAT device is prone to this type of attack
when the NAT device is in the traversal path of the SYN attacks. One
technique to defend against this type of attack is to ensure that the
NAT device employs a short SYN timeout and reduce the timeout even
further when it determines it is under SYN flood attack.
RST attack is another well known DoS attack. An attacker could
simply forge a number of RST packets for a variety of Established TCP
connections and cause the NAT sessions to be reset and freed. One
technique to defend against this type of attack is to validate the
RST packet and not let the packet through unless the sequence number
used in the RST packet is within the expected TCP window size of the
TCP Session.
REQ-5: A NAT device SHOULD employ necessary techniques to defend
against well known DoS attacks the endhosts are subject to.
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3.6 NAT initiated TCP keep-alives
When session timer expires for a NAT session, that indicates that the
associated TCP session has been idle with no activity for the period
matching the TCP Session timeout. Sensing no activity, NAT could
free up the NAT Session and remove the state associated with the TCP
connection within the NAT device. However, doing so would violate
the end-to-end reliability of the IP network. Ideally speaking, IP
network is not supposed to retain any hard state. Unfortunately, a
NAT device retains session state within itself (via the NAT Sessions)
and this information should not be dropped without confirming that
one or both halfs of the TCP session are alive.
A NAT device may validate the liveness of a TCP client by sending
keep-alive packets to the TCP client using the technique described in
section 4.2.3.6 of [HOST]. If the NAT device receives an ACK or
other traffic from the internal endpoint, it resets the session timer
and assumes the connection to be in ôEstablished" phase. If the NAT
device receives a RST from the TCP client, the NAT device transitions
the TCP connection into the "closing" phase and initiates Close
timeout for the session. If the NAT device receives no response from
the internal endpoint after sending several keep-alive packets, the
NAT assumes that the internal endpoint is dead and again assumes that
the TCP connection has entered the "closing" phase.
Below is the keep-alive requirement on NAT devices
REQ-6: When session timer expires for an established TCP connection,
the NAT device MAY initiate sending TCP keep-alives to the clients
prior to freeing up the Session state within NAT.
3.7 NAT initiated RST packets
When session timer expires for a NAT session, it is an indication
that the associated TCP connection has been idle with no activity for
the duration matching the TCP Session timeout. Sensing no activity,
NAT could free up the NAT Session and remove the state associated
with the TCP connection. When this happens, the two TCP endpoints in
the network, which might potentially be alive, may be unable to
resume activity on the connection because the NAT device enroute no
longer has the state information pertaining to the end-to-end TCP
connection. This can be problematic for application servers that
impose limits on the number of connections a user might be allowed to
setup in a given period of time. After a few zombie sessions, the
server might deny access to its clients, when the connection count on
the server exceeds the set limit. The Server has no way to know that
some of the client sessions it retains are zombies and shouldnt be
counted as real. In such a situation, the NAT device sending a RST
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packet to both parties will alert the two parties of the connection
going away. And, the application servers are not fooled with zombie
sessions. Note, a NAT device may choose to send RST packets after it
probed the TCP client with TCP Keep-alive packets.
Below is the requirement on sending TCP RST packet
REQ-7: When Session timer expires on an Established TCP connection,
the NAT device SHOULD send a RST packet to both halfs of a TCP
connection and enter Closing state on the connection prior to freeing
up the NAT Session.
4. Hints to implementers
4.1 Light weight TCP state machine is a common practice
Unlike UDP, TCP sessions are fundamentally unicast in nature and
multiple NAT Sessions cannot be aggregated. NAT devices maintain a
separate NAT Session to track each end-to-end TCP connection that
traverses the NAT device. A NAT device needs to be able to track the
current phase of a TCP session at any given time so an idle timer for
a duration appropriate for the phase is initiated. Further, a NAT
device defending against even the most trivial type of DoS attack
will require the knowledge of TCP sequence number and window Size to
defend itself against such attacks. As such, many vendors use a
light-weight state machine within the NAT Session to track the
current state of a TCP connection. Items tracked within the state
machine would include the last acknowledged sequence number from
either half of the TCP session, TCP window size, and the TCP
connection phase.
The State machine within a NAT Session enters the Connecting state
when NAT sees the first SYN packet for that session. The state
machine transitions from the Connecting to Established state once the
3-way handshake is completed. The state machine transitions from the
Established state to the Closing state when the NAT observes a
FIN/FIN ACK sequence, representing graceful shutdown reached
cooperatively by both endpoints, or when the NAT observes a RST from
either endpoint, representing a non-graceful connection reset forced
by one endpoint. The NAT device deletes the NAT session after the
Close timer expires while the TCP connection is in the Closing state.
In addition to the basic state information, NAT devices also record
information about TCP sequence numbers and acknowledgment numbers on
the traversing TCP packets, so as to defend against DOS attacks
(REQ-5). If the NAT contains built-in ALGs that can change the
payload length of TCP packets, effectively inserting or removing
bytes from the TCP stream in one or both directions, then the NAT
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MUST adjust the sequence numbers in all subsequent packets exchanged
in either direction to reflect these inserted or removed bytes.
4.2 TCP segment processing in NATs supporting ALGs
The following discussion on TCP segment processing is relevant only
when a NAT device includes support for one or more embedded ALGs.
Many NAT devices have the ALG for FTP enabled by default. A NAT
device may receive payload relevant to an ALG in multiple TCP
segments. Consider the following diagram where the MSS is set to 536
bytes in each endpoint of the TCP connection.
+-------------------+ +-------------------+
| Application-Layer | | Application-Layer |
+-------------------+ +-------------------+
| TCP [MSS = 536] | | TCP [MSS = 536] |
+-------------------+ +-------------------+
| IP | | IP |
+-------------------+ +-------------------+
| Lower-Layer | | Lower-Layer |
| | | |
+-------------------+ +-------------------+
End-host -1 End-host -2
| +--------+ |
+-------------------| NAT |----------------+
+--------+
Say the application layer on end-host-1 is sending a payload of size
600 bytes. The payload will be sent to the recipient in 2 TCP
segments as follows, because the MSS is set to 536 bytes.
TCP Segment 1:
+---------+------------------------------------------+----------+
|IP header| TCP header[Payload-Len=536,PUSH flag = 0]|Appl-data1|
+---------+------------------------------------------+----------+
TCP Segment 2:
+---------+-----------------------------------------+----------+
|IP header| TCP header[Payload-Len=64,PUSH flag = 1]|Appl-data2|
+---------+-----------------------------------------+----------+
A NAT device enroute may receive the TCP segments either in order or
out of order. In either case, the NAT device needs to assemble the
individual segments into a contiguous payload and make the complete
payload available for the ALG to process prior to forwarding the
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segments transparently to another realm.
In order to do this, a NAT device is required to enforce some type of
queuing mechanism such that when all relevant segments of a payload
are received, it is able to reassemble the TCP segments and make the
contiguous payload available for ALG processing.
For in-order segments, the NAT device needs to send a TCP ACK for the
initial segments it received, but didnt forward to the recipient
enpoint. This is done so the NAT device can prompt the sending
endpoint to continue to send the remaining TCP segments.
4.3 Adjusting Sequence Acknowledgement Numbers
The following discussion on adjusting Sequence and Acknowledgement
numbers is relevant only when a NAT device includes support for one
or more embedded ALGs.
When the embedded ALG on a NAT device modifies the TCP payload, the
corresponding payload may increase or decrease in size. As a result,
the NAT device is expected to remember the delta change and adjust
sequence/acknowledgement numbers in all subsequent TCP packets within
the session. Implementors of NAT devices often keep the delta
changes in payload due to ALG processing within the NAT Session as an
extension of the state information the NAT device keeps.
5. TCP behavioral requirements summary
Below is a summary of all the TCP behavioral requirements.
REQ-1: A NAT device MUST maintain Address and/or TCP Port Mappings.
REQ-2: A NAT device MUST be capable of identifying the current phase
of an end-to-end TCP session and use different idle timeout periods
for each phase of the TCP Session.
The timeouts used for each phase SHOULD be admin configurable. The
recommended value for SYN timeout is 30 seconds. The recommended
value for TCP session timeout is 30 minutes. Lastly, the recommended
value for close timeout is 2 x MSL (Maximum Segment Lifetime) or 4
minutes.
REQ-3: A NAT device MUST let the SYN packets through when the SYN
Packets are received on a TCP connection which is in one of
Connecting or Closing phase.
REQ-4: A NAT device MUST let the incoming SYN packets through so long
as an outbound SYN packet has been initiated first on the same
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connection tuple via the NAT device. NATs MUST permit inbound SYN
even without an outbound SYN on bi-directional NAT & load share NAT
devices. NATs MUST NOT generate RST or ICMP error packet back to the
sender upon inbound SYN processing.
REQ-5: A NAT device SHOULD employ necessary techniques to defend
against well known DoS attacks the endhosts are subject to.
REQ-6: When session timer expires for an Established TCP connection,
the NAT device MAY initiate sending TCP keep-alives to the clients
prior to freeing up the Session state within NAT.
REQ-7: When session timer expires for an Established TCP connection,
the NAT device SHOULD send a RST packet to both halfs of a TCP
connection and enter Closing state on the connection prior to freeing
Up the NAT Session.
6. Security considerations
The security considerations described in [RFC2663] for all
variations of NATs are applicable here. The recommendations and
requirements in this document do not effect the security properties
of the NAT devices adversely.
7. Acknowledgements
The authors would like to thank Nagendra Modadugu, Saikat Guha and
other BEHAVE workgroup members for their valuable feedback and
comments.
8. References
8.1 Normative References
[HOST] Braden, R., "Requirements for Internet-Hosts -
Communication Layers", RFC 1122, October 1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, March 1997.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
2001.
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[TCP] Postel, J., "Transmission Control Protocol (TCP)
Specification", RFC 793, STD 7, September 1981.
8.2 Informative References
[ASND] Reynolds, J. and J. Postel, "Assigned numbers", RFC 923,
October 1984.
[BEH-UDP] Audet, F. and C. Jennings, "NAT Behavioral Requirements
for Unicast UDP, draft-ietf-behave-nat-00.txt
(work-in-progress)", January 2005.
[ICMP] Postel, J., "Internet Control Message Protocol", RFC 792,
September 1981.
[NAT-CHK] Ford, B. and D. Andersen, "Nat Check Web Site:
http://midcom-p2p.sourceforge.net", June 2004.
[NAT-CMPL]
Holdrege, M. and P. Srisuresh, "Protocol Complications
with the IP Network Address Translator", RFC 3027,
January 2001.
[UNSAF] Daigle, L. and IAB, "IAB Considerations for Unilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[V4-REQ] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
Authors' Addresses
Pyda Srisuresh
Consultant
20072 Pacifica Dr.
Cupertino, CA 95014
USA
Phone: (408)836-4773
Email: srisuresh@yahoo.com
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Senthil Sivakumar
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134
USA
Phone:
Email: ssenthil@cisco.com
Kaushik Biswas
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134
USA
Phone: +1 408 525 5134
Email: kbiswas@cisco.com
Bryan Ford
M.I.T.
Laboratory for Computer Science
77 Massachusetts Ave.
Cambridge, MA 02139
USA
Phone: 1-617-253-5261
Email: baford@mit.edu
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