Network Working Group S. Guha, Ed.
Request for Comments: 5382 Cornell U.
BCP: 142 K. Biswas
Category: Best Current Practice Cisco Systems
B. Ford
MPI-SWS
S. Sivakumar
Cisco Systems
P. Srisuresh
Kazeon Systems
October 2008
NAT Behavioral Requirements for TCP
Status of This Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Abstract
This document defines a set of requirements for NATs that handle TCP
that would allow many applications, such as peer-to-peer applications
and online games to work consistently. Developing NATs that meet
this set of requirements will greatly increase the likelihood that
these applications will function properly.
Guha, et al. Best Current Practice [Page 1]RFC 5382 NAT TCP Requirements October 2008
Table of Contents
1. Applicability Statement . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. TCP Connection Initiation . . . . . . . . . . . . . . . . . . 4
4.1. Address and Port Mapping Behavior . . . . . . . . . . . . 5
4.2. Internally Initiated Connections . . . . . . . . . . . . . 5
4.3. Externally Initiated Connections . . . . . . . . . . . . . 7
5. NAT Session Refresh . . . . . . . . . . . . . . . . . . . . . 10
6. Application Level Gateways . . . . . . . . . . . . . . . . . . 12
7. Other Requirements Applicable to TCP . . . . . . . . . . . . . 12
7.1. Port Assignment . . . . . . . . . . . . . . . . . . . . . 12
7.2. Hairpinning Behavior . . . . . . . . . . . . . . . . . . . 13
7.3. ICMP Responses to TCP Packets . . . . . . . . . . . . . . 13
8. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 14
9. Security Considerations . . . . . . . . . . . . . . . . . . . 16
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1. Normative References . . . . . . . . . . . . . . . . . . . 18
11.2. Informational References . . . . . . . . . . . . . . . . . 18
Guha, et al. Best Current Practice [Page 2]RFC 5382 NAT TCP Requirements October 2008
1. Applicability Statement
This document is adjunct to [BEHAVE-UDP], which defines many terms
relating to NATs, lays out general requirements for all NATs, and
sets requirements for NATs that handle IP and unicast UDP traffic.
The purpose of this document is to set requirements for NATs that
handle TCP traffic.
The requirements of this specification apply to traditional NATs as
described in [RFC2663].
This document only covers the TCP aspects of NAT traversal.
Middlebox behavior that is not necessary for network address
translation of TCP is out of scope. Packet inspection above the TCP
layer and firewalls are out of scope except for Application Level
Gateway (ALG) behavior that may interfere with NAT traversal.
Application and OS aspects of TCP NAT traversal are out of scope.
Signaling-based approaches to NAT traversal, such as Middlebox
Communication (MIDCOM) and Universal Plug and Play (UPnP), that
directly control the NAT are out of scope. Finally, TCP connections
intended for the NAT (e.g., an HTTP or Secure Shell Protocol (SSH)
management interface) and TCP connections initiated by the NAT (e.g.,
reliable syslog client) are out of scope.
2. Introduction
Network Address Translators (NATs) hinder connectivity in
applications where sessions may be initiated to internal hosts.
Readers may refer to [RFC3022] for detailed information on
traditional NATs. [BEHAVE-UDP] lays out the terminology and
requirements for NATs in the context of IP and UDP. This document
supplements these by setting requirements for NATs that handle TCP
traffic. All definitions and requirements in [BEHAVE-UDP] are
inherited here.
[RFC4614] chronicles the evolution of TCP from the original
definition [RFC0793] to present-day implementations. While much has
changed in TCP with regards to congestion control and flow control,
security, and support for high-bandwidth networks, the process of
initiating a connection (i.e., the 3-way handshake or simultaneous-
open) has changed little. It is the process of connection initiation
that NATs affect the most. Experimental approaches such as T/TCP
[RFC1644] have proposed alternate connection initiation approaches,
but have been found to be complex and susceptible to denial-of-
service attacks. Modern operating systems and NATs consequently
primarily support the 3-way handshake and simultaneous-open modes of
connection initiation as described in [RFC0793].
Guha, et al. Best Current Practice [Page 3]
RFC 5382 NAT TCP Requirements October 2008
Recently, many techniques have been devised to make peer-to-peer TCP
applications work across NATs. [STUNT], [NATBLASTER], and [P2PNAT]
describe Unilateral Self-Address Fixing (UNSAF) mechanisms that allow
peer-to-peer applications to establish TCP through NATs. These
approaches require only endpoint applications to be modified and work
with standards compliant OS stacks. The approaches, however, depend
on specific NAT behavior that is usually, but not always, supported
by NATs (see [TCPTRAV] and [P2PNAT] for details). Consequently, a
complete TCP NAT traversal solution is sometimes forced to rely on
public TCP relays to traverse NATs that do not cooperate. This
document defines requirements that ensure 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 [RFC2119].
"NAT" in this specification includes both "Basic NAT" and "Network
Address/Port Translator (NAPT)" [RFC2663]. The term "NAT Session" is
adapted from [NAT-MIB] and is defined as follows.
NAT Session - A NAT session is an association between a TCP session
as seen in the internal realm and a TCP session as seen in the
external realm, by virtue of NAT translation. The NAT session will
provide the translation glue between the two session representations.
This document uses the term "TCP connection" (or just "connection")
to refer to individual TCP flows identified by the 4-tuple (source
and destination IP address and TCP port) and the initial sequence
numbers (ISN).
This document uses the term "address and port mapping" (or just
"mapping") as defined in [BEHAVE-UDP] to refer to state at the NAT
necessary for network address and port translation of TCP
connections. This document also uses the terms "Endpoint-Independent
Mapping", "Address-Dependent Mapping", "Address and Port-Dependent
Mapping", "filtering behavior", "Endpoint-Independent Filtering",
"Address-Dependent Filtering", "Address and Port-Dependent
Filtering", "Port assignment", "Port overloading", "hairpinning", and
"External source IP address and port" as defined in [BEHAVE-UDP].
4. TCP Connection Initiation
This section describes various NAT behaviors applicable to TCP
connection initiation.
Guha, et al. Best Current Practice [Page 4]
RFC 5382 NAT TCP Requirements October 2008
4.1. Address and Port Mapping Behavior
A NAT uses a mapping to translate packets for each TCP connection. A
mapping is dynamically allocated for connections initiated from the
internal side, and potentially reused for certain subsequent
connections. NAT behavior regarding when a mapping can be reused
differs for different NATs as described in [BEHAVE-UDP].
Consider an internal IP address and TCP port (X:x) that initiates a
TCP connection to an external (Y1:y1) tuple. Let the mapping
allocated by the NAT for this connection be (X1':x1'). Shortly
thereafter, the endpoint initiates a connection from the same (X:x)
to an external address (Y2:y2) and gets the mapping (X2':x2') on the
NAT. As per [BEHAVE-UDP], if (X1':x1') equals (X2':x2') for all
values of (Y2:y2), then the NAT is defined to have "Endpoint-
Independent Mapping" behavior. If (X1':x1') equals (X2':x2') only
when Y2 equals Y1, then the NAT is defined to have "Address-Dependent
Mapping" behavior. If (X1':x1') equals (X2':x2') only when (Y2:y2)
equals (Y1:y1), possible only for consecutive connections to the same
external address shortly after the first is terminated and if the NAT
retains state for connections in TIME_WAIT state, then the NAT is
defined to have "Address and Port-Dependent Mapping" behavior. This
document introduces one additional behavior where (X1':x1') never
equals (X2':x2'), that is, for each connection a new mapping is
allocated; in such a case, the NAT is defined to have "Connection-
Dependent Mapping" behavior.
REQ-1: A NAT MUST have an "Endpoint-Independent Mapping" behavior
for TCP.
Justification: REQ-1 is necessary for UNSAF methods to work.
Endpoint-Independent Mapping behavior allows peer-to-peer
applications to learn and advertise the external IP address and
port allocated to an internal endpoint such that external peers
can contact it (subject to the NAT's security policy). The
security policy of a NAT is independent of its mapping behavior
and is discussed later in Section 4.3. Having Endpoint-
Independent Mapping behavior allows peer-to-peer applications to
work consistently without compromising the security benefits of
the NAT.
4.2. Internally Initiated Connections
An internal endpoint initiates a TCP connection through a NAT by
sending a SYN packet. The NAT allocates (or reuses) a mapping for
the connection, as described in the previous section. The mapping
defines the external IP address and port used for translation of all
packets for that connection. In particular, for client-server
Guha, et al. Best Current Practice [Page 5]RFC 5382 NAT TCP Requirements October 2008
applications where an internal client initiates the connection to an
external server, the mapping is used to translate the outbound SYN,
the resulting inbound SYN-ACK response, the subsequent outbound ACK,
and other packets for the connection. This method of connection
initiation corresponds to the 3-way handshake (defined in [RFC0793])
and is supported by all NATs.
Peer-to-peer applications use an alternate method of connection
initiation termed simultaneous-open (Fig. 8, [RFC0793]) to traverse
NATs. In the simultaneous-open mode of operation, both peers send
SYN packets for the same TCP connection. The SYN packets cross in
the network. Upon receiving the other end's SYN packet, each end
responds with a SYN-ACK packet, which also cross in the network. The
connection is considered established once the SYN-ACKs are received.
From the perspective of the NAT, the internal host's SYN packet is
met by an inbound SYN packet for the same connection (as opposed to a
SYN-ACK packet during a 3-way handshake). Subsequent to this
exchange, both an outbound and an inbound SYN-ACK are seen for the
connection. Some NATs erroneously block the inbound SYN for the
connection in progress. Some NATs block or incorrectly translate the
outbound SYN-ACK. 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 connection that conform to valid transitions of the
TCP State-Machine (Fig. 6, [RFC0793]).
REQ-2: A NAT MUST support all valid sequences of TCP packets
(defined in [RFC0793]) for connections initiated both internally
as well as externally when the connection is permitted by the NAT.
In particular:
a) In addition to handling the TCP 3-way handshake mode of
connection initiation, A NAT MUST handle the TCP simultaneous-
open mode of connection initiation.
Justification: The intent of this requirement is to allow standards
compliant TCP stacks to traverse NATs no matter what path the
stacks take through the TCP state-machine and no matter which end
initiates the connection as long as the connection is permitted by
the filtering policy of the NAT (filtering policy is described in
the following section).
a) In addition to TCP packets for a 3-way handshake, A NAT must be
prepared to accept an inbound SYN and an outbound SYN-ACK for
an internally initiated connection in order to support
simultaneous-open.
Guha, et al. Best Current Practice [Page 6]RFC 5382 NAT TCP Requirements October 2008
4.3. Externally Initiated Connections
The NAT allocates a mapping for the first connection initiated by an
internal endpoint to an external endpoint. In some scenarios, the
NAT's policy may allow this mapping to be reused for connections
initiated from the external side to the internal endpoint. Consider
as before an internal IP address and port (X:x) that is assigned (or
reuses) a mapping (X1':x1') when it initiates a connection to an
external (Y1:y1). An external endpoint (Y2:y2) attempts to initiate
a connection with the internal endpoint by sending a SYN to
(X1':x1'). A NAT can choose to either allow the connection to be
established, or to disallow the connection. If the NAT chooses to
allow the connection, it translates the inbound SYN and routes it to
(X:x) as per the existing mapping. It also translates the SYN-ACK
generated by (X:x) in response and routes it to (Y2:y2), and so on.
Alternately, the NAT can disallow the connection by filtering the
inbound SYN.
A NAT may allow an existing mapping to be reused by an externally
initiated connection if its security policy permits. Several
different policies are possible as described in [BEHAVE-UDP]. If a
NAT allows the connection initiation from all (Y2:y2), then it is
defined to have "Endpoint-Independent Filtering" behavior. If the
NAT allows connection initiations only when Y2 equals Y1, then the
NAT is defined to have "Address-Dependent Filtering" behavior. If
the NAT allows connection initiations only when (Y2:y2) equals
(Y1:y1), then the NAT is defined to have "Address and Port-Dependent
Filtering" behavior (possible only shortly after the first connection
has been terminated but the mapping is still active). One additional
filtering behavior defined in this document is when the NAT does not
allow any connection initiations from the external side; in such
cases, the NAT is defined to have "Connection-Dependent Filtering"
behavior. The difference between "Address and Port-Dependent
Filtering" and "Connection-Dependent Filtering" behavior is that the
former permits an inbound SYN during the TIME_WAIT state of the first
connection to initiate a new connection while the latter does not.
REQ-3: If application transparency is most important, it is
RECOMMENDED that a NAT have an "Endpoint-Independent Filtering"
behavior for TCP. If a more stringent filtering behavior is most
important, it is RECOMMENDED that a NAT have an "Address-Dependent
Filtering" behavior.
a) The filtering behavior MAY be an option configurable by the
administrator of the NAT.
b) The filtering behavior for TCP MAY be independent of the
filtering behavior for UDP.
Guha, et al. Best Current Practice [Page 7]RFC 5382 NAT TCP Requirements October 2008
Justification: The intent of this requirement is to allow peer-to-
peer applications that do not always initiate connections from the
internal side of the NAT to continue to work in the presence of
NATs. This behavior also allows applications behind a BEHAVE
compliant NAT to inter-operate with remote endpoints that are
behind non-BEHAVE compliant (legacy) NATs. If the remote
endpoint's NAT does not have Endpoint-Independent Mapping behavior
but has only one external IP address, then an application can
still traverse the combination of the two NATs if the local NAT
has Address-Dependent Filtering. Section 9 contains a detailed
discussion on the security implications of this requirement.
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 to
signal an error to the sender. Signaling an error through ICMP
messages allows the sender to quickly detect that the SYN did not
reach the intended destination. Silently dropping the packet, on the
other hand, allows applications to perform simultaneous-open more
reliably.
Silently dropping the SYN aids simultaneous-open as follows.
Consider that the application is attempting a simultaneous-open and
the outbound SYN from the internal endpoint has not yet crossed the
NAT (due to network congestion or clock skew between the two
endpoints); this outbound SYN would otherwise have created the
necessary mapping at the NAT to allow translation of the inbound SYN.
Since the outbound SYN did not reach the NAT in time, the inbound SYN
cannot be processed. If a NAT responds to the premature inbound SYN
with an error message that forces the external endpoint to abandon
the connection attempt, it hinders applications performing a TCP
simultaneous-open. If instead the NAT silently ignores the inbound
SYN, the external endpoint retransmits the SYN after a TCP timeout.
In the meantime, the NAT creates the mapping in response to the
(delayed) outbound SYN such that the retransmitted inbound SYN can be
routed and simultaneous-open can succeed. The downside to this
behavior is that in the event the inbound SYN is erroneous, the
remote side does not learn of the error until after several TCP
timeouts.
NAT support for simultaneous-open as well as quickly signaling errors
are both important for applications. Unfortunately, there is no way
for a NAT to signal an error without forcing the endpoint to abort a
potential simultaneous-open: TCP RST and ICMP Port Unreachable
packets require the endpoint to abort the attempt while the ICMP Host
and Network Unreachable errors may adversely affect other connections
to the same host or network [RFC1122].
Guha, et al. Best Current Practice [Page 8]RFC 5382 NAT TCP Requirements October 2008
In addition, when an unsolicited SYN is received by the NAT, the NAT
may not know whether the application is attempting a simultaneous-
open (and that it should therefore silently drop the SYN) or whether
the SYN is in error (and that it should notify the sender).
REQ-4: A NAT MUST NOT respond to an unsolicited inbound SYN packet
for at least 6 seconds after the packet is received. If during
this interval the NAT receives and translates an outbound SYN for
the connection the NAT MUST silently drop the original unsolicited
inbound SYN packet. Otherwise, the NAT SHOULD send an ICMP Port
Unreachable error (Type 3, Code 3) for the original SYN, unless
REQ-4a applies.
a) The NAT MUST silently drop the original SYN packet if sending a
response violates the security policy of the NAT.
Justification: The intent of this requirement is to allow
simultaneous-open to work reliably in the presence of NATs as well
as to quickly signal an error in case the unsolicited SYN is in
error. As of writing this memo, it is not possible to achieve
both; the requirement therefore represents a compromise. The NAT
should tolerate some delay in the outbound SYN for a TCP
simultaneous-open, which may be due to network congestion or loose
synchronization between the endpoints. If the unsolicited SYN is
not part of a simultaneous-open attempt and is in error, the NAT
should endeavor to signal the error in accordance with [RFC1122].
a) There may, however, be reasons for the NAT to rate-limit or
omit such error notifications, for example, in the case of an
attack. Silently dropping the SYN packet when under attack
allows simultaneous-open to work without consuming any extra
network bandwidth or revealing the presence of the NAT to
attackers. Section 9 mentions the security considerations for
this requirement.
For NATs that combine NAT functionality with end-host functionality
(e.g., an end-host that also serves as a NAT for other hosts behind
it), REQ-4 above applies only to SYNs intended for the NAT'ed hosts
and not to SYNs intended for the NAT itself. One way to determine
whether the inbound SYN is intended for a NAT'ed host is to allocate
NAT mappings from one port range, and allocate ports for local
endpoints from a different non-overlapping port range. More dynamic
implementations can be imagined.
Guha, et al. Best Current Practice [Page 9]RFC 5382 NAT TCP Requirements October 2008
5. NAT Session Refresh
A NAT maintains state associated with in-progress and established
connections. 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 for which 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 is to preferentially
abandon sessions for crashed endpoints, followed by closed TCP
connections and partially open connections. A NAT can check if an
endpoint for a session has crashed by sending a TCP keep-alive packet
and receiving a TCP RST packet in response. If the NAT cannot
determine whether the endpoint is active, it should not abandon the
session until the TCP connection has been idle for some time. Note
that an established TCP connection can stay idle (but live)
indefinitely; hence, there is no fixed value for an idle-timeout that
accommodates all applications. However, a large idle-timeout
motivated by recommendations in [RFC1122] can reduce the chances of
abandoning a live session.
A TCP connection passes through three phases: partially open,
established, and closing. During the partially open phase, endpoints
synchronize initial sequence numbers. The phase is initiated by the
first SYN for the connection and extends until both endpoints have
sent a packet with the ACK flag set (TCP states: SYN_SENT and
SYN_RCVD). ACKs in both directions mark the beginning of the
established phase where application data can be exchanged
indefinitely (TCP states: ESTABLISHED, FIN_WAIT_1, FIN_WAIT_2, and
CLOSE_WAIT). The closing phase begins when both endpoints have
terminated their half of the connection by sending a FIN packet.
Once FIN packets are seen in both directions, application data can no
longer be exchanged, but the stacks still need to ensure that the FIN
packets are received (TCP states: CLOSING and LAST_ACK).
TCP connections can stay in established phase indefinitely without
exchanging any packets. Some end-hosts can be configured to send
keep-alive packets on such idle connections; by default, such keep-
alive packets are sent every 2 hours if enabled [RFC1122].
Consequently, a NAT that waits for slightly over 2 hours can detect
idle connections with keep-alive packets being sent at the default
rate. TCP connections in the partially open or closing phases, on
the other hand, can stay idle for at most 4 minutes while waiting for
in-flight packets to be delivered [RFC1122].
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RFC 5382 NAT TCP Requirements October 2008
The "established connection idle-timeout" for a NAT is defined as the
minimum time a TCP connection in the established phase must remain
idle before the NAT considers the associated session a candidate for
removal. The "transitory connection idle-timeout" for a NAT is
defined as the minimum time a TCP connection in the partially open or
closing phases must remain idle before the NAT considers the
associated session a candidate for removal. TCP connections in the
TIME_WAIT state are not affected by the "transitory connection idle-
timeout".
REQ-5: If a NAT cannot determine whether the endpoints of a TCP
connection are active, it MAY abandon the session if it has been
idle for some time. In such cases, the value of the "established
connection idle-timeout" MUST NOT be less than 2 hours 4 minutes.
The value of the "transitory connection idle-timeout" MUST NOT be
less than 4 minutes.
a) The value of the NAT idle-timeouts MAY be configurable.
Justification: The intent of this requirement is to minimize the
cases where a NAT abandons session state for a live connection.
While some NATs may choose to abandon sessions reactively in
response to new connection initiations (allowing idle connections
to stay up indefinitely in the absence of new initiations), other
NATs may choose to proactively reap idle sessions. In cases where
the NAT cannot actively determine if the connection is alive, this
requirement ensures that applications can send keep-alive packets
at the default rate (every 2 hours) such that the NAT can
passively determine that the connection is alive. The additional
4 minutes allows time for in-flight packets to cross the NAT.
NAT behavior for handling RST packets, or connections in TIME_WAIT
state is left unspecified. A NAT MAY hold state for a connection in
TIME_WAIT state to accommodate retransmissions of the last ACK.
However, since the TIME_WAIT state is commonly encountered by
internal endpoints properly closing the TCP connection, holding state
for a closed connection may limit the throughput of connections
through a NAT with limited resources. [RFC1337] describes hazards
associated with TIME_WAIT assassination.
The handling of non-SYN packets for connections for which there is no
active mapping is left unspecified. Such packets may be received if
the NAT silently abandons a live connection, or abandons a connection
in TIME_WAIT state before the 4 minute TIME_WAIT period expires. The
decision to either silently drop such packets or to respond with a
TCP RST packet is left up to the implementation.
Guha, et al. Best Current Practice [Page 11]RFC 5382 NAT TCP Requirements October 2008
NAT behavior for notifying endpoints when abandoning live connections
is left unspecified. When a NAT abandons a live connection, for
example due to a timeout expiring, the NAT MAY either send TCP RST
packets to the endpoints or MAY silently abandon the connection.
Sending a RST notification allows endpoint applications to recover
more quickly; however, notifying the endpoints may not always be
possible if, for example, session state is lost due to a power
failure.
6. Application Level Gateways
Application Level Gateways (ALGs) in certain NATs modify IP addresses
and TCP ports embedded inside application protocols. Such ALGs may
interfere with UNSAF methods or protocols that try to be NAT-aware
and must therefore be used with extreme caution.
REQ-6: If a NAT includes ALGs that affect TCP, it is RECOMMENDED
that all of those ALGs (except for FTP [RFC0959]) be disabled by
default.
Justification: The intent of this requirement is to prevent ALGs
from interfering with UNSAF methods. The default state of an FTP
ALG is left unspecified because of legacy concerns: as of writing
this memo, a large fraction of legacy FTP clients do not enable
passive (PASV) mode by default and require an ALG to traverse
NATs.
7. Other Requirements Applicable to TCP
A list of general and UDP-specific NAT behavioral requirements are
described in [BEHAVE-UDP]. A list of ICMP-specific NAT behavioral
requirements are described in [BEHAVE-ICMP]. The requirements listed
below reiterate the requirements from these two documents that
directly affect TCP. The following requirements do not relax any
requirements in [BEHAVE-UDP] or [BEHAVE-ICMP].
7.1. Port Assignment
NATs that allow different internal endpoints to simultaneously use
the same mapping are defined in [BEHAVE-UDP] to have a "Port
assignment" behavior of "Port overloading". Such behavior is
undesirable, as it prevents two internal endpoints sharing the same
mapping from establishing simultaneous connections to a common
external endpoint.
REQ-7: A NAT MUST NOT have a "Port assignment" behavior of "Port
overloading" for TCP.
Guha, et al. Best Current Practice [Page 12]RFC 5382 NAT TCP Requirements October 2008
Justification: This requirement allows two applications on the
internal side of the NAT to consistently communicate with the same
destination.
NAT behavior for preserving the source TCP port range for connections
is left unspecified. Some applications expect the source TCP port to
be in the well-known range (TCP ports from 0 to 1023). The "r"
series of commands (rsh, rcp, rlogin, etc.) are an example. NATs
that preserve the range from which the source port is picked allow
such applications to function properly through the NAT; however, by
doing so the NAT may compromise the security of the application in
certain situations; applications that depend only on the IP address
and source TCP port range for security (the "r" commands, for
example) cannot distinguish between an attacker and a legitimate user
behind the same NAT.
7.2. Hairpinning Behavior
NATs that forward packets originating from an internal address,
destined for an external address that matches the active mapping for
an internal address, back to that internal address are defined in
[BEHAVE-UDP] as supporting "hairpinning". If the NAT presents the
hairpinned packet with an external source IP address and port (i.e.,
the mapped source address and port of the originating internal
endpoint), then it is defined to have "External source IP address and
port" for hairpinning. Hairpinning is necessary to allow two
internal endpoints (known to each other only by their external mapped
addresses) to communicate with each other. "External source IP
address and port" behavior for hairpinning avoids confusing
implementations that expect the external source IP address and port.
REQ-8: A NAT MUST support "hairpinning" for TCP.
a) A NAT's hairpinning behavior MUST be of type "External source
IP address and port".
Justification: This requirement allows two applications behind the
same NAT that are trying to communicate with each other using
their external addresses.
a) Using the external source address and port for the hairpinned
packet is necessary for applications that do not expect to
receive a packet from a different address than the external
address they are trying to communicate with.
7.3. ICMP Responses to TCP Packets
Several TCP mechanisms depend on the reception of ICMP error messages
triggered by the transmission of TCP segments. One such mechanism is
path MTU discovery [RFC1191], which is required for the correct
Guha, et al. Best Current Practice [Page 13]RFC 5382 NAT TCP Requirements October 2008
operation of TCP. The current path MTU discovery mechanism requires
the sender of TCP segments to be notified of ICMP "Datagram Too Big"
responses.
REQ-9: If a NAT translates TCP, it SHOULD translate ICMP Destination
Unreachable (Type 3) messages.
Justification: Translating ICMP Destination Unreachable messages,
particularly the "Fragmentation Needed and Don't Fragment was Set"
(Type 3, Code 4) message avoids communication failures ("black
holes" [RFC2923]). Furthermore, TCP's connection establishment
and maintenance mechanisms also behave much more efficiently when
ICMP Destination Unreachable messages arrive in response to
outgoing TCP segments.
REQ-10: Receipt of any sort of ICMP message MUST NOT terminate the
NAT mapping or TCP connection for which the ICMP was generated.
Justification: This is necessary for reliably performing TCP
simultaneous-open where a remote NAT may temporarily signal an
ICMP error.
8. Requirements
A NAT that supports all of the mandatory requirements of this
specification (i.e., the "MUST") and is compliant with [BEHAVE-UDP],
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 [BEHAVE-UDP] is "fully
compliant with all the mandatory and recommended requirements of this
specification".
REQ-1: A NAT MUST have an "Endpoint-Independent Mapping" behavior
for TCP.
REQ-2: A NAT MUST support all valid sequences of TCP packets
(defined in [RFC0793]) for connections initiated both internally
as well as externally when the connection is permitted by the NAT.
In particular:
a) In addition to handling the TCP 3-way handshake mode of
connection initiation, A NAT MUST handle the TCP simultaneous-
open mode of connection initiation.
REQ-3: If application transparency is most important, it is
RECOMMENDED that a NAT have an "Endpoint-Independent Filtering"
behavior for TCP. If a more stringent filtering behavior is most
important, it is RECOMMENDED that a NAT have an "Address-Dependent
Filtering" behavior.
Guha, et al. Best Current Practice [Page 14]RFC 5382 NAT TCP Requirements October 2008
a) The filtering behavior MAY be an option configurable by the
administrator of the NAT.
b) The filtering behavior for TCP MAY be independent of the
filtering behavior for UDP.
REQ-4: A NAT MUST NOT respond to an unsolicited inbound SYN packet
for at least 6 seconds after the packet is received. If during
this interval the NAT receives and translates an outbound SYN for
the connection the NAT MUST silently drop the original unsolicited
inbound SYN packet. Otherwise, the NAT SHOULD send an ICMP Port
Unreachable error (Type 3, Code 3) for the original SYN, unless
REQ-4a applies.
a) The NAT MUST silently drop the original SYN packet if sending a
response violates the security policy of the NAT.
REQ-5: If a NAT cannot determine whether the endpoints of a TCP
connection are active, it MAY abandon the session if it has been
idle for some time. In such cases, the value of the "established
connection idle-timeout" MUST NOT be less than 2 hours 4 minutes.
The value of the "transitory connection idle-timeout" MUST NOT be
less than 4 minutes.
a) The value of the NAT idle-timeouts MAY be configurable.
REQ-6: If a NAT includes ALGs that affect TCP, it is RECOMMENDED
that all of those ALGs (except for FTP [RFC0959]) be disabled by
default.
The following requirements reiterate requirements from [BEHAVE-UDP]
or [BEHAVE-ICMP] that directly affect TCP. This document does not
relax any requirements in [BEHAVE-UDP] or [BEHAVE-ICMP].
REQ-7: A NAT MUST NOT have a "Port assignment" behavior of "Port
overloading" for TCP.
REQ-8: A NAT MUST support "hairpinning" for TCP.
a) A NAT's hairpinning behavior MUST be of type "External source
IP address and port".
REQ-9: If a NAT translates TCP, it SHOULD translate ICMP Destination
Unreachable (Type 3) messages.
REQ-10: Receipt of any sort of ICMP message MUST NOT terminate the
NAT mapping or TCP connection for which the ICMP was generated.
Guha, et al. Best Current Practice [Page 15]RFC 5382 NAT TCP Requirements October 2008
9. Security Considerations
[BEHAVE-UDP] discusses security considerations for NATs that handle
IP and unicast UDP traffic. Security concerns specific to handling
TCP packets are discussed in this section.
Security considerations for REQ-1: This requirement does not
introduce any TCP-specific security concerns.
Security considerations for REQ-2: This requirement does not
introduce any TCP-specific security concerns. Simultaneous-open
and other transitions in the TCP state machine are by-design and
necessary for TCP to work correctly in all scenarios. Further,
this requirement only affects connections already in progress as
authorized by the NAT in accordance with its policy.
Security considerations for REQ-3: The security provided by the NAT
is governed by its filtering behavior as addressed in
[BEHAVE-UDP]. Connection-Dependent Filtering behavior is most
secure from a firewall perspective, but severely restricts
connection initiations through a NAT. Endpoint-Independent
Filtering behavior, which is most transparent to applications,
requires an attacker to guess the IP address and port of an active
mapping in order to get his packet to an internal host. Address-
Dependent Filtering, on the other hand, is less transparent than
Endpoint-Independent Filtering but more transparent than
Connection-Dependent Filtering; it is more secure than Endpoint-
Independent Filtering as it requires an attacker to additionally
guess the address of the external endpoint for a NAT session
associated with the mapping and be able to receive packets
addressed to the same. While this protects against most attackers
on the Internet, it does not necessarily protect against attacks
that originate from behind a remote NAT with a single IP address
that is also translating a legitimate connection to the victim.
Security considerations for REQ-4: This document recommends that a
NAT respond to unsolicited inbound SYN packets with an ICMP error
delayed by a few seconds. Doing so may reveal the presence of a
NAT to an external attacker. Silently dropping the SYN makes it
harder to diagnose network problems and forces applications to
wait for the TCP stack to finish several retransmissions before
reporting an error. An implementer must therefore understand and
carefully weigh the effects of not sending an ICMP error or rate-
limiting such ICMP errors to a very small number.
Guha, et al. Best Current Practice [Page 16]RFC 5382 NAT TCP Requirements October 2008
Security considerations for REQ-5: This document recommends that a
NAT that passively monitors TCP state keep idle sessions alive for
at least 2 hours 4 minutes or 4 minutes depending on the state of
the connection. If a NAT is under attack, it may attempt to
actively determine the liveliness of a TCP connection or let the
NAT administrator configure more conservative timeouts.
Security considerations for REQ-6: This requirement does not
introduce any TCP-specific security concerns.
Security considerations for REQ-7: This requirement does not
introduce any TCP-specific security concerns.
Security considerations for REQ-8: This requirement does not
introduce any TCP-specific security concerns.
Security considerations for REQ-9: This requirement does not
introduce any TCP-specific security concerns.
Security considerations for REQ-10: This requirement does not
introduce any TCP-specific security concerns.
NAT implementations that modify TCP sequence numbers (e.g., for
privacy reasons or for ALG support) must ensure that TCP packets with
Selective Acknowledgement (SACK) notifications [RFC2018] are properly
handled.
NAT implementations that modify local state based on TCP flags in
packets must ensure that out-of-window TCP packets are properly
handled. [RFC4953] summarizes and discusses a variety of solutions
designed to prevent attackers from affecting TCP connections.
10. Acknowledgments
Joe Touch contributed the mechanism for handling unsolicited inbound
SYNs. Thanks to Mark Allman, Francois Audet, Lars Eggert, Paul
Francis, Fernando Gont, Sam Hartman, Paul Hoffman, Dave Hudson,
Cullen Jennings, Philip Matthews, Tom Petch, Magnus Westerlund, and
Dan Wing for their many contributions, comments, and suggestions.
Guha, et al. Best Current Practice [Page 17]RFC 5382 NAT TCP Requirements October 2008
11. References
11.1. Normative References
[BEHAVE-UDP] Audet, F. and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, January 2007.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC0959] Postel, J. and J. Reynolds, "File Transfer Protocol",
STD 9, RFC 959, October 1985.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery",
RFC 1191, November 1990.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informational References
[BEHAVE-ICMP] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha,
"NAT Behavioral Requirements for ICMP protocol", Work
in Progress, June 2008.
[NAT-MIB] Rohit, R., Srisuresh, P., Raghunarayan, R., Pai, N.,
and C. Wang, "Definitions of Managed Objects for
Network Address Translators (NAT)", RFC 4008,
March 2005.
[NATBLASTER] 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.
[P2PNAT] 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.
[RFC1337] Braden, B., "TIME-WAIT Assassination Hazards in TCP",
RFC 1337, May 1992.
Guha, et al. Best Current Practice [Page 18]RFC 5382 NAT TCP Requirements October 2008
[RFC1644] Braden, B., "T/TCP -- TCP Extensions for Transactions
Functional Specification", RFC 1644, July 1994.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow,
"TCP Selective Acknowledgment Options", RFC 2018,
October 1996.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, September 2000.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[RFC4614] Duke, M., Braden, R., Eddy, W., and E. Blanton, "A
Roadmap for Transmission Control Protocol (TCP)
Specification Documents", RFC 4614, September 2006.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, July 2007.
[STUNT] 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.
[TCPTRAV] 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.
Guha, et al. Best Current Practice [Page 19]RFC 5382 NAT TCP Requirements October 2008
Authors' Addresses
Saikat Guha (editor)
Cornell University
331 Upson Hall
Ithaca, NY 14853
US
Phone: +1 607 255 1008
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
Max Planck Institute for Software Systems
Campus Building E1 4
D-66123 Saarbruecken
Germany
Phone: +49-681-9325657
EMail: baford@mpi-sws.org
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
Kazeon Systems, Inc.
1161 San Antonio Rd.
Mountain View, CA 94043
US
Phone: +1 408 836 4773
EMail: srisuresh@yahoo.com
Guha, et al. Best Current Practice [Page 20]
RFC 5382 NAT TCP Requirements October 2008
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Guha, et al. Best Current Practice [Page 21]
