BEHAVE WG P. Srisuresh
Internet Draft Consultant
Expires: September 5, 2006 B. Ford
M.I.T.
S. Sivakumar
Cisco Systems
S. Guha
Cornell U.
March 2006
NAT Behavioral Requirements for ICMP protocol
<draft-srisuresh-behave-nat-icmp-01.txt>
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Abstract
This document identifies the behavioral properties required of the
Network Address Translator devices (NATs) in conjunction with the
ICMP protocol. The objective of this memo is to make NAT middleboxes
more predictable and compatible with diverse application protocols
that traverse the devices. Companion documents provide behavioral
recommendations specific to TCP and UDP.
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Table of Contents
1. Introduction & Scope ..........................................
2. ICMP Behavioral Requirements ..................................
2.1. ICMP Query Packet Handling .....,,........................
2.2. ICMP Error Packet Handling .....,,........................
2.2.1. NAT Sessions Pertaining to ICMP Error Packets .....
2.3. Rejection of Outbound IP Packets Disallowed by NAT .......
2.4. Path MTU Discovery .......................................
2.4.1. Honoring the DF Bit in IP header ..................
2.4.2. Honoring the "Packet too big" ICMP Error Message ..
3. Summary of Requirements .......................................
4. Security Considerations .......................................
1. Introduction & Scope
This document is adjunct to [BEH-UDP] and [BEH-TCP], which define
requirements for NATs that handle unicast UDP and TCP traffic.
The purpose of this document is to set requirements for NATs with
regard to ICMP messages.
The requirements of this specification apply to Traditional NATs as
described in RFC 2663 [2].
This document only covers the ICMP aspects of NAT traversal.
Traditional NAT inherently mandates a certain level of firewall like
functionality. However, firewall functionality in general or any
other middlebox functionality is out of the scope of this
specification. Application and Operating System (OS) aspects of ICMP
NAT traversal are out-of-scope.
NAT traversal strategies that involve explicit signaling between the
application and the NAT [SOCKS, RSIP, MIDCOM, UPNP] are out of the
scope of this document.
This document focuses strictly on the behavior of the NAT device,
and not on the behavior of applications that traverse NATs.
A separate companion document [BEH-APP] provides recommendations for
application designers on how to make applications work robustly over
NATs that follow the behavioral requirements specified here and the
companion Behave documents.
The following section lists the behavioral recommendations to the
NAT device vendors in conjunction with handling ICMP protocol.
Section 3 summarizes all the requirements in one place.
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2. ICMP Behavioral Requirements
This section lists the behavioral requirements for a NAT device
when processing ICMP packets. Even though ICMP is a transport
protocol on top of IP, ICMP packet processing is often considered
an integral of IP and is independent of other transport
protocols. In the following sub sections, requirements are
discussed in detail along with the rationale behind them.
2.1. ICMP Query Packet Handling
A NAT device SHOULD transparently forward any ICMP query
packets initiated from the nodes behind NAT devices and the
responses to these query packets in the opposite direction. This
involves translating the IP header. The NAPT device SHOULD
additionally modify the query ID and the associated checksum in the
ICMP header if there is an outstanding query with the same ID.
It is to RECOMMENDED that the administrator be allowed to configure
the ICMP session timeout. Typically, the ICMP NAT Session timeout
is set to 30 seconds or longer. Setting the ICMP NAT Session timeout
to a very large duration (say, much larger than 30 secs) longer
could potentially tie up NAT resources. Caution is warranted as
applications (such as ping and traceroute) built on top of ICMP
query messages complete within a few seconds. By setting the timeout
to a large value, the NAT device could be holding up precious NAT
resources such as mappings and NAT Sessions for the whole duration.
REQ-1: A NAT device SHOULD transparently forward any ICMP query
packets initiated from the nodes behind NAT devices and the
responses to these query packets in the opposite direction. It is
RECOMMENDED that the administrator be allowed to configure the
ICMP session timeout.
2.2. ICMP Error Packet Handling
A NAT device MUST transparently forward ICMP error messages ([ICMP])
it receives from intermediate or end nodes in either realm to the
intended end node. Unlike other IP packets, the basis for
translation of an ICMP error packet is the NAT Session to which the
packet embedded within the ICMP error message payload belongs to,
not the IP and ICMP headers in the outer layer.
Consider the following scenario in figure 1. Say, NAT-xy is a
traditional NAT device connecting hosts in private and external
networks. Router-x and Host-x are in the external network. Router-y
and Host-y are in the private network. The subnets in the external
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network are routable from the private as well as the external
domains. Whereas, the subnets in the private network are only
routable within the private domain. When Host-y initiated a session
to Host-x, let us say that the NAT device assigned an IP address of
Host-y' to associate with Host-y in the external network.
Host-x
|
---------------+-------------------
|
+-------------+
| Router-x |
+-------------+
External Network |
--------------------+--------+-------------------
| ^ |
| | (Host-y', Host-x) |
| | v
+-------------+
| NAT-xy |
+-------------+
|
| Private Network
----------------+------------+----------------
|
+-------------+
| Router-y |
+-------------+
|
----------------+-------+--------
| ^ |
| | (Host-y, Host-x) |
| | v
Host-y
Figure 1. NAT topology with routers in private & external realms
Say, a packet from Host-y to Host-x triggered an ICMP error message
from one of Router-x or Host-x (both of which are in the external
domain). Such an ICMP error packet will have one of Router-x or
Host-x as the source IP address and Host-y' as the destination IP
address. When the NAT device receives the ICMP error packet, the
NAT device must use the packet embedded within the ICMP error
message (i.e., the IP packet from Host-y to Host-x) to look up the
NAT Session the embedded packet belongs to and use the NAT Session
to translate the embedded payload. The NAT device must also use the
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NAT Session to translate the outer IP header. In the outer header,
the source IP address will remain unchanged because the originator
of the ICMP error message (Host-x or Router-x) is in external
domain and routable from the private domain. The destination IP
address Host-y' must however be translated to Host-y using the NAT
Session parameters.
Now, say, a packet from Host-x to Host-y triggered an ICMP error
message from one of Router-y or Host-y (both of which are in the
private domain). Such an ICMP error packet will have one of
Router-y or Host-y as the source IP address and Host-x as the
destination IP address. When the NAT device receives the ICMP error
packet, the NAT device must use the packet embedded within the ICMP
error message (i.e., the IP packet from Host-x to Host-y) to look up
the NAT Session the embedded packet belongs to and use the NAT
Session to translate the embedded payload. The NAT device must also
use the NAT Session to translate the outer IP header. In the outer
header, the destination IP address will remain unchanged, as the IP
addresses for Host-x is already in the external domain. If the ICMP
error message is generated by Host-y, the NAT device must simply
use the NAT Session to translate the source IP address Host-y to
Host-y'. However, if the ICMP error message is generated by the
intermediate node Router-y, the NAT device will not have had a
translation entry for Router-y within the NAT Session. The NAT may
also not have an Address mapping in place for Router-y. In such a
case, the NAT device must simply use its own IP address in the
external domain to translate the source IP address.
Changes to ICMP error message ([ICMP]) MUST include changes to IP and
ICMP headers on the outer layer as well as changes to the relevant
IP and transport headers of the packet embedded within the ICMP-error
message payload. Section 4.3 of the RFC 3022 describes the various
items within the ICMP error message that MUST be translated by the
NAT device.
REQ-2: A NAT device MUST transparently forward ICMP error packets
to the target end node, so long as the NAT has active mapping for
for the embedded payload. If the NAT does not have an active
mapping for the embedded payload, the NAT should silently drop the
ICMP error packet. In the case the ICMP error packet is originated
by a node in the private realm for which the NAT device has no
mapping, the NAT device MUST use its own IP address in the public
realm to translate the originating node IP address in the outer
IP header.
2.2.1. NAT Sessions Pertaining to ICMP Error Packets
While processing an ICMP error packet, a NAT device MUST NOT
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refresh or delete the NAT Session that pertains to the embedded
payload within the ICMP error packet. This is in spite of the
fact that the NAT device uses the NAT Session to translate the
embedded payload. By not effecting the NAT Sessions, the NAT
device is able to retain them, even as someone spoofs ICMP error
messages pertaining to the NAT Sessions.
REQ-3: While processing an ICMP error packet, a NAT device MUST NOT
refresh or delete the NAT Session that pertains to the embedded
payload within the ICMP error packet.
2.3. Rejection of Outbound IP Packets Disallowed by NAT
A NAT device typically permits all outbound sessions. However,
a NAT device may disallow some outbound sessions due to resource
constraints. For example, a NAT device may not permit the first
packet of a new outbound session, if the NAT device is out of
resources (out of addresses or TCP/UDP ports or a NAT Session
resource) to set up a state for the session, or, the specific
session may be administratively restricted by the NAT device.
When a packet is prohibited by a NAT device due to resource or
administration considerations, the NAT device SHOULD send ICMP
destination unreachable message, with a code of 10 (Communication
with destination host administratively prohibited) to the sender.
Unfortunately, there is not another ICMP code currently defined to
indicate "Communication with destination host port administratively
prohibited". So, the same code should be used for host as well as
port filtering.
REQ-4: When an outbound packet is prohibited by a NAT device due to
resource/authorization consideration, the NAT device SHOULD send
ICMP destination unreachable message, with a code of 10
(Communication with destination host administratively prohibited)
to the sender prior to dropping the packet.
2.4. Path MTU support
IP fragmentation by intermediate nodes often results in
degraded performance. In some cases, IP fragmentation by the
intermediate nodes could even cause end-to-end communication
to entirely fail. Many applications avoid fragmentation in
the network by originating IP packets that fit within the
maximum Path MTU enroute and setting the DF (Don't Fragment)
bit so the intermediate nodes enroute do not fragment the
packets. For example, a number of TCP connections have the
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DF bit set in the IP header of the TCP segments they
transmit. Likewise, IP based VPN tunnels also often set the
DF bit on the external IP encapsulation.
2.4.1. Honoring the DF Bit in IP header
A NAT device MUST honor the DF bit in the IP header of the
packets that transit the device. If the DF bit is set and the
MTU on the forwarding interface of the NAT device is such that
the IP datagram cannot be forwarded without fragmentation, the
NAT device MUST issue a "packet too big" ICMP message (ICMP
type 3, Code 4) with a suggested MTU back to the sender and
drop the original IP packet. The sender will resend after
taking the appropriate corrective action. If the DF bit is
not set and the MTU on the forwarding interface of the NAT
device mandates fragmentation, the NAT device must simply
send this fragmented, just as any router does [RFC1812]
REQ-5: If DF bit is set on a transit IP packet and the NAT
device cannot forward the packet without fragmentation, the
NAT device MUST send a "Packet too big" ICMP message (ICMP
type 3, Code 4) with a suggested MTU back to the sender and
drop the original IP packet.
2.4.2. Honoring the "Packet too big" ICMP Error Message
This is flip side of the argument for the above section. By
virtue of the address translation NAT performs, NAT may end
up being the recipient of "Packet too big" message.
When NAT device is the recipient of "Packet too big"
ICMP message from the network, the NAT device must simply
forward the ICMP message back to the intended recipient, as
stated in REQ-2.
3. Summary of Requirements
This section summarizes the requirements discussed at length in the
preceding section.
REQ-1: A NAT device SHOULD transparently forward any ICMP query
packets initiated from the nodes behind NAT devices and the
responses to these query packets in the opposite direction. It is
RECOMMENDED that the administrator be allowed to configure the
ICMP session timeout.
REQ-2: A NAT device MUST transparently forward ICMP error packets
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to the target end node, so long as the NAT has active mapping for
for the embedded payload. If the NAT does not have an active
mapping for the embedded payload, the NAT should silently drop the
ICMP error packet. In the case the ICMP error packet is originated
by a node in the private realm for which the NAT device has no
mapping, the NAT device MUST use its own IP address in the public
realm to translate the originating node IP address in the outer
IP header.
REQ-3: While processing an ICMP error packet, a NAT device MUST not
refresh or delete the NAT Session that pertains to the embedded
payload within the ICMP error packet.
REQ-4: When an outbound packet is prohibited by a NAT device due to
resource/authorization consideration, the NAT device SHOULD send
ICMP destination unreachable message, with a code of 10
(Communication with destination host administratively prohibited)
to the sender prior to dropping the packet.
REQ-5: If DF bit is set on a transit IP packet and the NAT
device cannot forward the packet without fragmentation, the
NAT device MUST send a "Packet too big" ICMP message (ICMP
type 3, Code 4) with a suggested MTU back to the sender and
drop the original IP packet.
4. Security Considerations
This memo raises no new security issues.
Normative References
[BEH-UDP] F. Audet and C. Jennings, "NAT Behavioral Requirements for
Unicast UDP", draft-ietf-behave-nat-udp-04.txt (Work In
Progress), September 2005.
[BEH-TCP] Guha, S., Biswas, K., Ford, B., Francis, P., Sivakumar, S.,
and Srisuresh, P., "NAT Behavioral Requirements for
Unicast TCP", draft-ietf-behave-tcp-00.txt (Work In
Progress), February 2006.
[ICMP] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[KEYWORDS] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
Srisuresh et. al. [Page 8]
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[NAT-TERM] P. Srisuresh and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August
1999.
[NAT-TRAD] P. Srisuresh and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001.
[PMTU] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[PRIV-ADDR] Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de Groot, and
E. Lear, "Address Allocation for Private Internets", RFC
1918, February 1996.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
Informative References
[UNSAF] L. Daigle and IAB, "IAB Considerations for UNilateral Self-
Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
Author's Addresses:
Pyda Srisuresh
Consultant
20072 Pacifica Dr.
Cupertino, CA 95014
U.S.A.
Phone: (408)836-4773
E-mail: srisuresh@yahoo.com
Bryan Ford
Computer Science and Artificial Intelligence Laboratory
Massachusetts Institute of Technology
77 Massachusetts Ave.
Cambridge, MA 02139
U.S.A.
Phone: (617) 253-5261
E-mail: baford@mit.edu
Web: http://www.brynosaurus.com/
Senthil Sivakumar
Cisco Systems, Inc.
7100-8 Kit Creek Road
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PO Box 14987
Research Triangle Park, NC 27709-4987
U.S.A.
Phone: +1 919 392 5158
Email: ssenthil@cisco.com
Saikat Guha
Cornell University
331 Upson Hall
Ithaca, NY 14853
U.S.A.
Email: saikat@cs.cornell.edu
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