Network Working Group M. Holdrege
Request for Comments: 3027 ipVerse
Category: Informational P. Srisuresh
Jasmine Networks
January 2001
Protocol Complications with the IP Network Address Translator
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
Many internet applications can be adversely affected when end nodes
are not in the same address realm and seek the assistance of an IP
Network Address Translator (NAT) enroute to bridge the realms. The
NAT device alone cannot provide the necessary application/protocol
transparency in all cases and seeks the assistance of Application
Level Gateways (ALGs) where possible, to provide transparency. The
purpose of this document is to identify the protocols and
applications that break with NAT enroute. The document also attempts
to identify any known workarounds. It is not possible to capture all
applications that break with NAT in a single document. This document
attempts to capture as much information as possible, but is by no
means a comprehensive coverage. We hope the coverage provides
sufficient clues for applications not covered.
Table of Contents
1.0 Introduction .............................................. 2
2.0 Common Characteristics of Protocols broken by NAT ......... 2
3.0 Protocols that cannot work with NAT enroute ............... 4
4.0 Protocols that can work with the aid of an ALG ............ 8
5.0 Protocols designed explicitly to work with NAT enroute .... 16
6.0 Acknowledgements .......................................... 17
7.0 Security Considerations ................................... 17
8.0 References ................................................ 17
9.0 Authors' Addresses ........................................ 19
10.0 Full Copyright Statement ................................ 20
Holdrege & Srisuresh Informational [Page 1]
RFC 3027 Protocol Complications with NAT January 2001
1.0 Introduction
This document requires the reader to be familiar with the terminology
and function of NAT devices as described in [NAT-TERM]. In a
nutshell, NAT attempts to provide a transparent routing solution to
end hosts requiring communication to disparate address realms. NAT
modifies end node addresses (within the IP header of a packet) en-
route and maintains state for these updates so that datagrams
pertaining to a session are transparently routed to the right end-
node in either realm. Where possible, application specific ALGs may
be used in conjunction with NAT to provide application level
transparency. Unlike NAT, the function of ALG is application
specific and would likely require examination and recomposition of IP
payload.
The following sections attempt to list applications that are known to
have been impacted by NAT devices enroute. However, this is by no
means a comprehensive list of all known protocols and applications
that have complications with NAT - rather just a subset of the list
gathered by the authors. It is also important to note that this
document is not intended to advocate NAT, but rather to point out the
complications with protocols and applications when NAT devices are
enroute.
2.0 Common Characteristics of Protocols broken by NAT
[NAT-TERM] and [NAT-TRAD] have sections listing the specific nature
of problems and limitations to NAT devices. Some of these
limitations are being restated in this section to summarize
characteristics of protocols that are broken by NAT.
2.1 Realm-specific IP address information in payload
A wide range of applications fail with NAT enroute when IP packets
contain realm-specific IP address or port information in payload. An
ALG may be able to provide work around in some cases. But, if the
packet payload is IPsec secured (or secure by a transport or
application level security mechanisms), the application is bound to
fail.
2.2 Bundled session applications
Bundled session applications such as FTP, H.323, SIP and RTSP, which
use a control connection to establish a data flow are also usually
broken by NAT devices enroute. This is because these applications
exchange address and port parameters within control session to
establish data sessions and session orientations. NAT cannot know
the inter-dependency of the bundled sessions and would treat each
Holdrege & Srisuresh Informational [Page 2]
RFC 3027 Protocol Complications with NAT January 2001
session to be unrelated to one another. Applications in this case
can fail for a variety of reasons. Two most likely reasons for
failures are: (a) addressing information in control payload is
realm-specific and is not valid once packet crosses the originating
realm, (b) control session permits data session(s) to originate in a
direction that NAT might not permit.
When DNS names are used in control payload, NAT device in conjunction
with a DNS-ALG might be able to offer the necessary application level
transparency, if NAT has no contention with data session orientation.
However, using DNS names in place of realm-specific IP addresses may
not be an option to many of these applications (e.g., FTP).
When realm-specific addressing is specified in payload, and the
payload is not encrypted, an ALG may in some cases be able to provide
the work around necessary to make the applications run transparently
across realms. The complexity of ALG depends on the application
level knowledge required to process payload and maintain state.
2.3 Peer-to-peer applications
Peer-to-peer applications more than client-server based applications
are likely to break with NAT enroute. Unlike Client-server
applications, Peer-to-peer applications can be originated by any of
the peers. When peers are distributed across private and public
realms, a session originated from an external realm is just as likely
as the session from a host in private realm. External peers will be
able to locate their peers in private realm only when they know the
externally assigned IP address or the FQDN ahead of time. FQDN name
to assigned address mapping can happen only so long as the enroute
NAT device supports DNS-ALG. Examples of Peer-to-peer applications
include interactive games, Internet telephony and event-based
protocols (such as Instant-Messaging).
This is particularly a problem with traditional NAT and may be less
of an issue with bi-directional NAT, where sessions are permitted in
both directions.
A possible work-around for this type of problem with traditional-NAT
is for private hosts to maintain an outbound connection with a server
acting as their representative to the globally routed Internet.
2.4 IP fragmentation with NAPT enroute
IP fragmentation with NAPT enroute is not an issue with any single
application, but pervades across all TCP/UDP applications. The
problem is described in detail in [NAT-TRAD]. Briefly, the problem
goes as follows. Say, two private hosts originated fragmented
Holdrege & Srisuresh Informational [Page 3]
RFC 3027 Protocol Complications with NAT January 2001
TCP/UDP packets to the same destination host. And, they happened to
use the same fragmentation identifier. When the target host receives
the two unrelated datagrams, carrying same fragmentation id, and from
the same assigned host address, the target host is unable to
determine which of the two sessions the datagrams belong to.
Consequently, both sessions will be corrupted.
2.5 Applications requiring retention of address mapping
NAT will most likely break applications that require address mapping
to be retained across contiguous sessions. These applications
require the private-to-external address mapping to be retained
between sessions so the same external address may be reused for
subsequent session interactions. NAT cannot know this requirement
and may reassign external address to different hosts between
sessions.
Trying to keep NAT from discarding an address mapping would require a
NAT extension protocol to the application that would allow the
application to inform the NAT device to retain the mappings.
Alternately, an ALG may be required to interact with NAT to keep the
address mapping from being discarded by NAT.
2.6 Applications requiring more public addresses than available
This is a problem when the number of private hosts is larger than the
external addresses available to map the private addresses into. Take
for example the rlogin service initiated from a host in private realm
supported by NAPT. Rlogin service clients use well-known rlogin port
512 as their TCP port ID. No more than one host in private realm can
initiate the service. This is a case of trying to use a service that
fundamentally requires more public addresses than are available. NAT
devices can conserve addresses, but they cannot create more
addresses.
3.0 Protocols that cannot work with NAT enroute
3.1 IPsec and IKE
NAT fundamentally operates by modifying end node addresses (within
the IP header) en-route. The IPsec AH standard [IPsec-AH] on the
other hand is explicitly designed to detect alterations to IP packet
header. So when NAT alters the address information enroute in IP
header, the destination host receiving the altered packet will
invalidate the packet since the contents of the headers have been
altered. The IPsec AH secure packet traversing NAT will simply not
reach the target application, as a result.
Holdrege & Srisuresh Informational [Page 4]
RFC 3027 Protocol Complications with NAT January 2001
IPsec ESP ([IPsec-ESP]) encrypted packets may be altered by NAT
device enroute only in a limited number of cases. In the case of
TCP/UDP packets, NAT would need to update the checksum in TCP/UDP
headers, when an address in IP header is changed. However, as the
TCP/UDP header is encrypted by the ESP, NAT would not be able to make
this checksum update. As a result, TCP/UDP packets encrypted in
transport mode ESP, traversing a NAT device will fail the TCP/UDP
checksum validation on the receiving end and will simply not reach
the target application.
Internet Key Exchange Protocol IKE can potentially pass IP addresses
as node identifiers during Main, Aggressive and Quick Modes. In
order for an IKE negotiation to correctly pass through a NAT, these
payloads would need to be modified. However, these payloads are
often protected by hash or obscured by encryption. Even in the case
where IP addresses are not used in IKE payloads and an IKE
negotiation could occur uninterrupted, there is difficulty with
retaining the private-to-external address mapping on NAT from the
time IKE completed negotiation to the time IPsec uses the key on an
application. In the end, the use of end-to-end IPsec is severely
hampered anyway, as described earlier.
For all practical purposes, end-to-end IPsec is impossible to
accomplish with NAT enroute.
3.2 Kerberos 4
Kerberos 4 tickets are encrypted. Therefore, an ALG cannot be
written. When the KDC receives a ticket request, it includes the
source IP address in the returned ticket. Not all Kerberos 4
services actually check source IP addresses. AFS is a good example
of a Kerberos 4 service which does not. Services which don't check
are not picky about NAT devices enroute. Kerberos tickets are tied
to the IP address that requested the ticket and the service with
which to use the ticket.
The K4 ticket (response) contains a single IP address describing the
interface used by the client to retrieve the ticket from the TGT
from the perspective of KDC. This works fine if the KDC is across a
NAT gateway and as long as all of the Kerberos services are also
across a NAT gateway. The end user on private network will not
notice any problems.
There is also the caveat that NAT uses the same address mapping for
the private host for the connection between the client and the KDC as
for the connection between the client and the application server. A
work around this problem would be to keep an arbitrary connection
open to remote server during throughout the ticket lifetime, so as
Holdrege & Srisuresh Informational [Page 5]
RFC 3027 Protocol Complications with NAT January 2001
not to let NAT drop the address binding. Alternately, an ALG will
need to be deployed to ensure that NAT would not change address
bindings during the lifetime of a ticket and between the time a
ticket is issued to private host and the time the ticket is used by
private host.
But, the ticket will be valid from any host within the private realm
of NAPT. Without NAPT, an attacker needs to be able to spoof the
source IP addresses of a connection that is being made in order to
use a stolen ticket on a different host. With NAPT, all the attacker
needs to do from the private realm of NAPT is to simply gain
possession of a ticket. Of course, this assumes, NAPT private domain
is not a trusted network - not surprisingly, since many attacks occur
from inside the organization.
3.3 Kerberos 5
Just as with Kerberos 4, Kerberos 5 tickets are encrypted.
Therefore, an ALG cannot be written.
In Kerberos 5, the client specifies a list of IP addresses which the
ticket should be valid for, or it can ask for a ticket valid for all
IP addresses. By asking for an all-IP-addresses ticket or a ticket
containing the NAPT device address, you can get krb5 to work with an
NAPT device, although it isn't very transparent (it requires the
clients to behave differently than they otherwise would). The MIT
krb5 1.0 implementation didn't have any configurability for what IP
addresses the client asked for (it always asked for the set of its
interface addresses) and did not interact well with NAT. The MIT
krb5 1.1 implementation allows you to put "noaddresses" somewhere in
krb5.conf to request all-IP-addresses-valid tickets.
The K5 ticket (response) contains IP addresses, as requested by the
client node, from which the ticket is to be considered valid. If the
services being accessed with Kerberos authentication are on the
public side of the NAT, then the Kerberos authentication will fail
because the IP address used by the NAT (basic NAT or NAPT) is not in
the list of acceptable addresses.
There are two workarounds in Kerberos 5 both of which reduce the
security of the tickets. The first is to have the clients in NAPT
private realm specify the public IP address of the NAPT in the
ticket's IP list. But this leads to the same security problem
detailed for K4. Plus, it is not obvious for the client in the
private domain to find out the public IP Address of the NAPT. That
would be a change of application behavior on end-host.
Holdrege & Srisuresh Informational [Page 6]
RFC 3027 Protocol Complications with NAT January 2001
The second method is to remove all IP addresses from the K5 tickets
but this now makes theft of the ticket even worse since the tickets
can be used from anywhere. Not just from within the private network.
3.4 The X Windowing System and X-term/Telnet
The X Windowing system is TCP based. However, the client-server
relationship with these applications is reverse compared to most
other applications. The X server or Open-windows server is the
display/mouse/keyboard unit (i.e., the one that controls the actual
Windows interface). The clients are the application programs driving
the Windows interface.
Some machines run multiple X Windows servers on the same machine.
The first X Windows server is at TCP port 6000. The first Open
Windows server can be at port 6000 or port 2000 (more flexible). We
will mainly refer X windowing system for illustration purposes here.
X-term Transmits IP addresses from the client to the server for the
purpose of setting the DISPLAY variable. When set the DISPLAY
variable is used for subsequent connections from X clients on the
host to an X server on the workstation. The DISPLAY variable is sent
inline during the TELNET negotiations as
DISPLAY=<local-ip-addr>:<server>.<display>
where the <local-ip-addr> is retrieved by looking at the local ip
address associated with the socket used to connect to <server>. The
<server> determines which port (6000 + <server>) should be used to
make the connection. <display> is used to indicate which monitor
attached to the X server should be used but is not important to this
discussion.
The <local-ip-addr> used is not a DNS name because:
. The is no ability for the local machine to know its DNS name
without performing a reverse DNS lookup on the local-ip-addr
. There is no guarantee that the name returned by a reverse
DNS lookup actually maps back to the local IP address.
. Lastly, without DNSSEC, it may not be safe to use DNS addresses
because they can easily be spoofed. NAT and DNS-ALG cannot work
unless DNSSEC is disabled.
A common use of this application is people dialing in to corporate
offices from their X terminals at home. Say, the X client is running
on a host on the public side of the NAT and X server is running on a
Holdrege & Srisuresh Informational [Page 7]
RFC 3027 Protocol Complications with NAT January 2001
host on the private side of the NAT. The DISPLAY variable is
transmitted inline to the host the X client is running in some way.
The process transmitting the contents of the DISPLAY variable does
not know the address of the NAT.
If the channel transmitting the DISPLAY variable is not encrypted,
NAT device might solicit the help of an ALG to replace the IP address
and configure a port in the valid display range (ports 6000 and
higher) to act as a gateway. Alternately, NAT may be configured to
listen for incoming connections to provide access to the X Server(s),
without requiring an ALG. But, this approach increases the security
risk by providing access to the X server that would not otherwise be
available. As the ALG tampers with the IP addresses it will also not
be possible for X Authorization methods other than MIT-MAGIC-COOKIE-1
to be used. MIT-MAGIC-COOKIE-1 is the least secure of all the
documented X Authorization methods.
When START_TLS is used there may be client certificate verification
problems caused by the NAT depending on the information provided in
the certificate.
3.5 RSH/RLOGIN
RSH uses multiple sessions to support separate streams for stdout and
stderr. A random port number is transmitted inline from the client
to the server for use as stderr port. The stderr socket is a
connection back from the server to the client. And unlike FTP, there
is no equivalent to PASV mode. For traditional NAT, this is a
problem as traditional NAT would not permit incoming sessions.
RLOGIN does not use multiple sessions. But the Kerberos protected
versions of RSH and RLOGIN will not work in a NAT environment due to
the ticket problems and the use of multiple sessions.
4.0 Protocols that can work with the aid of an ALG
This document predominantly addresses problems associated with
Traditional NAT, especially NAPT.
4.1 FTP
FTP is a TCP based application, used to reliably transfer files
between two hosts. FTP uses bundled session approach to accomplish
this.
FTP is initiated by a client accessing a well-known port number 21 on
the FTP server. This is called the FTP control session. Often, an
additional data session accompanies the control session. By default,
Holdrege & Srisuresh Informational [Page 8]
RFC 3027 Protocol Complications with NAT January 2001
the data session would be from TCP port 20 on server to the TCP port
client used to initiate control session. However, the data session
ports may be altered within the FTP control sessions using ASCII
encoded PORT and PASV commands and responses.
Say, an FTP client is in a NAT supported private network. An FTP ALG
will be required to monitor the FTP control session (for both PORT
and PASV modes) to identify the FTP data session port numbers and
modify the private address and port number with the externally valid
address and port number. In addition, the sequence and
acknowledgement numbers, TCP checksum, IP packet length and checksum
have to be updated. Consequently the sequence numbers in all
subsequent packets for that stream must be adjusted as well as TCP
ACK fields and checksums.
In rare cases, increasing the size of the packet could cause it to
exceed the MTU of a given transport link. The packet would then have
to be fragmented which could affect performance. Or, if the packet
has the DF bit set, it would be ICMP rejected and the originating
host would then have to perform Path MTU Discovery. This could have
an adverse effect on performance.
Note however, if the control command channel is secured, it will be
impossible for an ALG to update the IP addresses in the command
exchange.
When AUTH is used with Kerberos 4, Kerberos 5, and TLS, the same
problems that occur with X-Term/Telnet occur with FTP.
Lastly, it is of interest to note section 4 of RFC 2428 (FTP
extensions for IPv6 and NATs) which describes how a new FTP port
command (EPSV ALL) can be used to allow NAT devices to fast-track the
FTP protocol, eliminating further processing through ALG, if the
remote server accepts "EPSV ALL".
4.2 RSVP
RSVP is positioned in the protocol stack at the transport layer,
operating on top of IP (either IPv4 or IPv6). However, unlike other
transport protocols, RSVP does not transport application data but
instead acts like other Internet control protocols (for example,
ICMP, IGMP, routing protocols). RSVP messages are sent hop-by-hop
between RSVP-capable routers as raw IP datagrams using protocol
number 46. It is intended that raw IP datagrams should be used
between the end systems and the first (or last) hop router. However,
this may not always be possible as not all systems can do raw network
I/O. Because of this, it is possible to encapsulate RSVP messages
within UDP datagrams for end-system communication. UDP-encapsulated
Holdrege & Srisuresh Informational [Page 9]
RFC 3027 Protocol Complications with NAT January 2001
RSVP messages are sent to either port 1698 (if sent by an end system)
or port 1699 (if sent by an RSVP-enabled router). For more
information concerning UDP encapsulation of RSVP messages; consult
Appendix C of RFC 2205.
An RSVP session, a data flow with a particular destination and
transport-layer protocol, is defined by:
Destination Address - the destination IP address for the data
packets. This may be either a unicast or a multicast address.
Protocol ID - the IP protocol ID (for example, UDP or TCP).
Destination Port - a generalized destination port that is used for
demultiplexing at a layer above the IP layer.
NAT devices are presented with unique problems when it comes to
supporting RSVP. Two issues are:
1. RSVP message objects may contain IP addresses. The result is that
an RSVP-ALG must be able to replace the IP addresses based upon the
direction and type of the message. For example, if an external
sender were to send an RSVP Path message to an internal receiver, the
RSVP session will specify the IP address that the external sender
believes is the IP address of the internal receiver. However, when
the RSVP Path message reaches the NAT device, the RSVP session must
be changed to reflect the IP address that is used internally for the
receiver. Similar actions must be taken for all message objects that
contain IP addresses.
2. RSVP provides a means, the RSVP Integrity object, to guarantee the
integrity of RSVP messages. The problem is that because of the first
point, a NAT device must be able to change IP addresses within the
RSVP messages. However, when this is done, the RSVP Integrity object
is no longer valid as the RSVP message has been changed. Therefore
an RSVP-ALG will not work when RSVP Integrity Object is used.
4.3 DNS
DNS is a TCP/UDP based protocol. Domain Names are an issue for hosts
which use local DNS servers in NAT private realm. DNS name to
address mapping for hosts in private domain should be configured on
an authoritative name server within private domain. This server
would be accessed by external and internal hosts alike for name
resolutions. A DNS-ALG would be required to perform address to name
conversions on DNS queries and responses. [DNS-ALG] describes DNS-
ALG
Holdrege & Srisuresh Informational [Page 10]
RFC 3027 Protocol Complications with NAT January 2001
in detail. If DNS packets are encrypted/authenticated per DNSSEC,
then DNS_ALG will fail because it won't be able to perform payload
modifications.
Applications using DNS resolver to resolve a DNS name into an IP
address, assume availability of address assignment for reuse by the
application specific session. As a result, DNS-ALG will be required
to keep the address assignment (between private and external
addresses) valid for a pre-configured period of time, past the DNS
query.
Alternately, if there isn't a need for a name server within private
domain, private domain hosts could simply point to an external name
server for external name lookup. No ALG is required when the name
server is located in external domain.
4.4 SMTP
SMTP is a TCP based protocol ([SMTP]), used by Internet email
programs such as sendmail to send TCP-based email messages to well-
known port 25. The mail server may be located within or outside
private domain. But, the server must be assigned a global name and
address, accessible by external hosts. When mail server is located
within private domain, inbound SMTP sessions must be redirected to
the private host from its externally assigned address. No special
mapping is required when Mail server is located in external domain.
Generally speaking, mail systems are configured such that all users
specify a single centralized address (such as fooboo@company.com),
instead of including individual hosts (such as
fooboo@hostA.company.com). The central address must have an MX
record specified in the DNS name server accessible by external hosts.
In the majority of cases, mail messages do not contain reference to
private IP addresses or links to content data via names that are not
visible to outside. However, some mail messages do contain IP
addresses of the MTAs that relay the message in the "Received: "
field. Some mail messages use IP addresses in place of FQDN for
debug purposes or due to lack of a DNS record, in "Mail From: "
field.
If one or more MTAs were to be located behind NAT in a private
domain, and the mail messages are not secured by signature or
cryptographic keys, an SMTP-ALG may be used to translate the IP
address information registered by the MTAs. If the MTAs have static
address mapping, the translation would be valid across realms for
long periods of time.
Holdrege & Srisuresh Informational [Page 11]
RFC 3027 Protocol Complications with NAT January 2001
The ability to trace the mail route may be hampered or prevented by
NAT alone, without the ALG. This can cause problems when debugging
mail problems or tracking down abusive users of mail.
4.5 SIP
SIP (Refer [SIP]) can run on either TCP or UDP, but by default on the
same port 5060.
When used with UDP, a response to a SIP request does not go to the
source port the request came from. Rather the SIP message contains
the port number the response should be sent to. SIP makes use of
ICMP port unreachable errors in the response to request
transmissions. Request messages are usually sent on the connected
socket. If responses are sent to the source port in the request,
each thread handling a request would have to listen on the socket it
sent the request on. However, by allowing responses to come to a
single port, a single thread can be used for listening instead.
A server may prefer to place the source port of each connected socket
in the message. Then each thread can listen for responses
separately. Since the port number for a response may not go to the
source port of the request, SIP will not normally traverse a NAT and
would require a SIP-ALG.
SIP messages carry arbitrary content, which is defined by a MIME
type. For multimedia sessions, this is usually the Session
Description Protocol (SDP RFC 2327). SDP may specify IP addresses or
ports to be used for the exchange of multimedia. These may loose
significance when traversing a NAT. Thus a SIP-ALG would need the
intelligence to decipher and translate realm-relevant information.
SIP carries URL's in its Contact, To and From fields that specify
signaling addresses. These URL's can contain IP addresses or domain
names in the host port portion of the URL. These may not be valid
once they traverse a NAT.
As an alternative to an SIP-ALG, SIP supports a proxy server which
could co-reside with NAT and function on the globally significant NAT
port. Such a proxy would have a locally specific configuration.
4.6 RealAudio
In default mode, RealAudio clients (say, in a private domain) access
TCP port 7070 to initiate conversation with a real-audio server (say,
located an external domain) and to exchange control messages during
playback (ex: pausing or stopping the audio stream). Audio session
parameters are embedded in the TCP control session as byte stream.
Holdrege & Srisuresh Informational [Page 12]
RFC 3027 Protocol Complications with NAT January 2001
The actual audio traffic is carried in the opposite direction on
incoming UDP based packets (originated from the server) directed to
ports in the range of 6970-7170.
As a result, RealAudio is broken by default on a traditional NAT
device. A work around for this would be for the ALG to examine the
TCP traffic to determine the audio session parameters and selectively
enable inbound UDP sessions for the ports agreed upon in the TCP
control session. Alternately, the ALG could simply redirect all
inbound UDP sessions directed to ports 6970-7170 to the client
address in the private domain.
For bi-Directional NAT, you will not need an ALG. Bi-directional NAT
could simply treat each of the TCP and UDP sessions as 2 unrelated
sessions and perform IP and TCP/UDP header level translations.
The readers may contact RealNetworks for detailed guidelines on how
their applications can be made to work, traversing through NAT and
firewall devices.
4.7 H.323
H.323 is complex, uses dynamic ports, and includes multiple UDP
streams. Here is a summary of the relevant issues:
An H.323 call is made up of many different simultaneous connections.
At least two of the connections are TCP. For an audio-only
conference, there may be up to 4 different UDP 'connections' made.
All connections except one are made to ephemeral (dynamic) ports.
Calls can be initiated from the private as well as the external
domain. For conferencing to be useful, external users need to be
able to establish calls directly with internal users' desktop
systems.
The addresses and port numbers are exchanged within the data stream
of the 'next higher' connection. For example, the port number for
the H.245 connection is established within the Q.931 data stream.
(This makes it particularly difficult for the ALG, which will be
required to modify the addresses inside these data streams.) To make
matters worse, it is possible in Q.931, for example, to specify that
the H.245 connection should be secure (encrypted). If a session is
encrypted, it is impossible for the ALG to decipher the data stream,
unless it has access to the shared key.
Most of the control information is encoded in ASN.1 (only the User-
User Information within Q.931 Protocol Data Units, or PDUs, is
Holdrege & Srisuresh Informational [Page 13]
RFC 3027 Protocol Complications with NAT January 2001
ASN.1-encoded (other parts of each Q.931 PDU are not encoded). For
those unfamiliar with ASN.1, suffice it to say that it is a complex
encoding scheme, which does not end up with fixed byte offsets for
address information. In fact, the same version of the same
application connecting to the same destination may negotiate to
include different options, changing the byte offsets.
Below is the protocol exchange for a typical H.323 call between User
A and User B. A's IP address is 88.88.88.88 and B's IP address is
99.99.99.99. Note that the Q.931 and H.245 messages are encoded in
ASN.1 in the payload of an RTP packet. So to accomplish a connection
through a NAT device, an H.323-ALG will be required to examine the
packet, decode the ASN.1, and translate the various H.323 control IP
addresses.
User A User B
A establishes connection to B on well-
known Q.931 port (1720)
----------------------------------------------->
Q.931 Setup caller address = 88.88.88.88
caller port = 1120
callee address = 99.99.99.99
callee port = 1720
<-----------------------------------------------
Q.931 Alerting
<-----------------------------------------------
Q.931 Connect H.245 address = 99.99.99.99
H.245 port = 1092
User A establishes connection to User B at
99.99.99.99, port 1092
<---------------------------------------------->
Several H.245 messages are exchanged (Terminal
Capability Set, Master Slave Determination and
their respective ACKs)
<-----------------------------------------------
H.245 Open Logical Channel, channel = 257
RTCP address = 99.99.99.99
RTCP port = 1093
----------------------------------------------->
H.245 Open Logical Channel Ack, channel = 257
RTP address = 88.88.88.88
RTP port = 2002
(This is where User A would like RTP
data sent to)
Holdrege & Srisuresh Informational [Page 14]
RFC 3027 Protocol Complications with NAT January 2001
RTCP address = 88.88.88.88
RTCP port = 2003
----------------------------------------------->
H.245 Open Logical Channel, channel = 257
RTCP address = 88.88.88.88
RTCP port = 2003
<-----------------------------------------------
H.245 Open Logical Channel Ack, channel = 257
RTP address = 99.99.99.99
RTP port = 1092
(This is where User B would like RTP data
sent to)
RTCP address = 99.99.99.99
RTP port = 1093
Also note that if an H.323 Gateway resided inside a NAT boundary, the
ALG would have to be cognizant of the various gateway discovery
schemes and adapt to those schemes as well. Or if just the H.323
host/terminal was inside the NAT boundary and tried to register with
a Gatekeeper, the IP information in the registration messages would
have to be translated by NAT.
4.8 SNMP
SNMP is a network management protocol based on UDP. SNMP payload may
contain IP addresses or may refer IP addresses through an index into
a table. As a result, when devices within a private network are
managed by an external node, SNMP packets transiting a NAT device may
contain information that is not relevant in external domain. In some
cases, as described in [SNMP-ALG], an SNMP ALG may be used to
transparently convert realm-specific addresses into globally unique
addresses. Such an ALG assumes static address mapping and bi-
directional NAT. It can only work for the set of data types (textual
conventions) understood by the SNMP-ALG implementation and for a
given set of MIB modules. Furthermore, replacing IP addresses in the
SNMP payload may lead to communication failures due to changes in
message size or changes in the lexicographic ordering.
Making SNMP ALGs completely transparent to all management
applications is not an achievable task. The ALGs will run into
problems with SNMPv3 security features, when authentication (and
optionally privacy) is enabled, unless the ALG has access to security
keys. [NAT-ARCH] also hints at potential issues with SNMP management
via NAT.
Alternately, SNMP proxies, as defined in [SNMP-APPL], may be used in
conjunction with NAT to forward SNMP messages to external SNMP
engines (and vice versa). SNMP proxies are tailored to the private
Holdrege & Srisuresh Informational [Page 15]
RFC 3027 Protocol Complications with NAT January 2001
domain context and can hence operate independent of the specific
managed object types being accessed. The proxy solution will require
the external management application to be aware of the proxy
forwarder and the individual nodes being managed will need to be
configured to direct their SNMP traffic (notifications and requests)
to the proxy forwarder.
5.0 Protocols designed explicitly to work with NAT enroute
5.1 Activision Games
Activision Games were designed to be NAT-friendly so as not to
require an ALG for the games to work transparently through
traditional NAT devices. Game players within a private domain can
play with other players in the same domain or external domain.
Activision gaming protocol is proprietary and is based on UDP. The
address server uses UDP port number 21157 and is expected to be
located in the global address realm.
Game players connect to the address server first, and send their
private IP address information (such as private IP address and UDP
port number) in the initial connect message. The server notes
private address information from the connect message and external
address information from the IP and UDP headers. The server then
sends both the private and external address information of the game
player to all the other peer players. At this point, each game
player knows the private and public address information of every
other peer. Subsequent to this, each client opens up symmetrical
direct connection to each other and uses whichever address (private
or external) works first.
Now, the clients can have a session directly with other clients (or)
they can have session with other clients via the gaming server. The
key is to allow reuse of the same (global address, assigned UDP port)
tuple used for initial connection to the game server for all
subsequent connections to the client. A game player is recognized by
one of (private address, UDP port) or (global address, assigned UDP
port) by all other peer players. So, the binding between tuples
should remain unchanged on NAT, so long as the gaming player is in
session with one or multiple other players.
Opening a connection to a game server in external realm from a
private host is no problem. All NAT would have to do is provide
routing transparency and retain the same private-to-external address
binding so long as there is a minimum of one gaming session with an
external node. But, an NAPT configuration must allow multiple
simultaneous UDP connections on the same assigned global
address/port.
Holdrege & Srisuresh Informational [Page 16]
RFC 3027 Protocol Complications with NAT January 2001
The above approach has some problems. For example, a client could
try contacting a private address, but that private address could be
in use locally, when the private address at some other realm is
meant. If the node that was contacted wrongfully has some other
service or no service registered for the UDP port, the Activision
connect messages are expected to be simply dropped. In the unlikely
event, a registered application chooses to interpret the message, the
results can be unpredictable.
The readers may refer to Activision for the proprietary, detailed
information on the function and design of this protocol.
6.0 Acknowledgements
The authors would like to express sincere thanks to Bernard Aboba,
Bill Sommerfield, Dave Cridland, Greg Hudson, Henning Schulzrine,
Jeffrey Altman, Keith Moore, Thomas Narten, Vernon Shryver and others
that had provided valuable input in preparing this document. Special
thanks to Dan Kegel for sharing the Activision games design
methodology.
7.0 Security Considerations
The security considerations outlined in [NAT-TERM] are relevant to
all NAT devices. This document does not warrant additional security
considerations.
8.0 References
[NAT-TERM] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.
[NAT-TRAD] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
2001.
[H.323] ITU-T SG16 H.323, Intel white paper, "H.323 and
Firewalls", Dave Chouinard, John Richardson, Milind
Khare (with further assistance from Jamie Jason).
[SNMP-ALG] Raz, D., Schoenwaelder, J. and B. Sugla, "An SNMP
Application Level Gateway for Payload Address
Translation", RFC 2962, October 2000.
[SNMP-APPL] Levi, D., Meyer, P. and B. Stewart, "SNMP Applications",
RFC 2573, April 1999.
Holdrege & Srisuresh Informational [Page 17]
RFC 3027 Protocol Complications with NAT January 2001
[NAT-ARCH] Hain, T. "Architectural Implications of NAT", RFC 2993,
November 2000.
[SMTP] Postel, J., "Simple Mail Transfer Protocol", STDl 10,
RFC 821, August 1982.
[FTP] Postel, J. and J. Reynolds, "File Transfer Protocol
(FTP)", STD 9, RFC 959, October 1985.
[SIP] Handley, M., Schulzrinne, H., Schooler, E. and J.
Rosenberg, "SIP: Session Initiation Protocol", RFC 2543,
March 1999.
[X Windows] Scheifler, B., "FYI on the X Window System", FYI 6, RFC
1198, January 1991.
[RSVP] Braden, R., Zhang. L., Berson. S., Herzog, S. and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version
1 Functional Specification", RFC 2205, September 1997.
[DNS-TERMS] Mockapetris, P., "Domain Names - Concepts and
Facilities", STD 13, RFC 1034, November 1987.
[DNS-IMPL] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC 1035, November 1987.
[DNS-ALG] Srisuresh, P., Tsirtsis, G., Akkiraju, P. and A.
Heffernan, "DNS extensions to Network Address
Translators (DNS_ALG)", RFC 2694, September 1999.
[IPsec] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[IPsec-ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[IPsec-AH] Kent, S. and R. Atkinson, "IP Authentication Header",
RFC 2402, November 1998.
[IPsec-DOCS] Thayer, R., Doraswamy, N. and R. Glenn, "IP Security
Document Roadmap", RFC 2411, November 1998.
[NAT-SEC] Srisuresh, P., "Security Model with Tunnel-mode IPsec
for NAT Domains", RFC 2709, October 1999.
[PRIV-ADDR] Rekhter, Y., Moskowitz, B., Karrenberg, D., G. de Groot,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
Holdrege & Srisuresh Informational [Page 18]
RFC 3027 Protocol Complications with NAT January 2001
[ADDR-BEHA] Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4
Address Behaviour Today", RFC 2101, February 1997.
Authors' Addresses
Matt Holdrege
ipVerse
223 Ximeno Ave.
Long Beach, CA 90803
EMail: matt@ipverse.com
Pyda Srisuresh
Jasmine Networks, Inc.
3061 Zanker Road, Suite B
San Jose, CA 95134
U.S.A.
Phone: (408) 895-5032
EMail: srisuresh@yahoo.com
Holdrege & Srisuresh Informational [Page 19]
RFC 3027 Protocol Complications with NAT January 2001
Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Holdrege & Srisuresh Informational [Page 20]
|
