IP Network Address Translator (NAT) Terminology and Considerations
RFC 2663
Network Working Group P. Srisuresh
Request for Comments: 2663 M. Holdrege
Category: Informational Lucent Technologies
August 1999
IP Network Address Translator (NAT) Terminology and Considerations
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 (1999). All Rights Reserved.
Preface
The motivation behind this document is to provide clarity to the
terms used in conjunction with Network Address Translators. The term
"Network Address Translator" means different things in different
contexts. The intent of this document is to define the various
flavors of NAT and standardize the meaning of terms used.
The authors listed are editors for this document and owe the content
to contributions from members of the working group. Large chunks of
the document titled, "IP Network Address Translator (NAT)" were
extracted almost as is, to form the initial basis for this document.
The editors would like to thank the authors Pyda Srisuresh and Kjeld
Egevang for the same. The editors would like to thank Praveen
Akkiraju for his contributions in describing NAT deployment
scenarios. The editors would also like to thank the IESG members
Scott Bradner, Vern Paxson and Thomas Narten for their detailed
review of the document and adding clarity to the text.
Abstract
Network Address Translation is a method by which IP addresses are
mapped from one realm to another, in an attempt to provide
transparent routing to hosts. Traditionally, NAT devices are used to
connect an isolated address realm with private unregistered addresses
to an external realm with globally unique registered addresses. This
document attempts to describe the operation of NAT devices and the
associated considerations in general, and to define the terminology
used to identify various flavors of NAT.
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1. Introduction and Overview
The need for IP Address translation arises when a network's internal
IP addresses cannot be used outside the network either because they
are invalid for use outside, or because the internal addressing must
be kept private from the external network.
Address translation allows (in many cases, except as noted in
sections 8 and 9) hosts in a private network to transparently
communicate with destinations on an external network and vice versa.
There are a variety of flavors of NAT and terms to match them. This
document attempts to define the terminology used and to identify
various flavors of NAT. The document also attempts to describe other
considerations applicable to NAT devices in general.
Note, however, this document is not intended to describe the
operations of individual NAT variations or the applicability of NAT
devices.
NAT devices attempt to provide a transparent routing solution to end
hosts trying to communicate from disparate address realms. This is
achieved by modifying end node addresses en-route and maintaining
state for these updates so that datagrams pertaining to a session are
routed to the right end-node in either realm. This solution only
works when the applications do not use the IP addresses as part of
the protocol itself. For example, identifying endpoints using DNS
names rather than addresses makes applications less dependent of the
actual addresses that NAT chooses and avoids the need to also
translate payload contents when NAT changes an IP address.
The NAT function cannot by itself support all applications
transparently and often must co-exist with application level gateways
(ALGs) for this reason. People looking to deploy NAT based solutions
need to determine their application requirements first and assess the
NAT extensions (i.e., ALGs) necessary to provide application
transparency for their environment.
IPsec techniques which are intended to preserve the Endpoint
addresses of an IP packet will not work with NAT enroute for most
applications in practice. Techniques such as AH and ESP protect the
contents of the IP headers (including the source and destination
addresses) from modification. Yet, NAT's fundamental role is to alter
the addresses in the IP header of a packet.
2. Terminology and concepts used
Terms most frequently used in the context of NAT are defined here for
reference.
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2.1. Address realm or realm
An address realm is a network domain in which the network addresses
are uniquely assigned to entities such that datagrams can be routed
to them. Routing protocols used within the network domain are
responsible for finding routes to entities given their network
addresses. Note that this document is limited to describing NAT in
IPv4 environment and does not address the use of NAT in other types
of environment. (e.g. IPv6 environments)
2.2. Transparent routing
The term "transparent routing" is used throughout the document to
identify the routing functionality that a NAT device provides. This
is different from the routing functionality provided by a traditional
router device in that a traditional router routes packets within a
single address realm.
Transparent routing refers to routing a datagram between disparate
address realms, by modifying address contents in the IP header to be
valid in the address realm into which the datagram is routed.
Section 3.2 has a detailed description of transparent routing.
2.3. Session flow vs. Packet flow
Connection or session flows are different from packet flows. A
session flow indicates the direction in which the session was
initiated with reference to a network interface. Packet flow is the
direction in which the packet has traveled with reference to a
network interface. Take for example, an outbound telnet session. The
telnet session consists of packet flows in both inbound and outbound
directions. Outbound telnet packets carry terminal keystrokes and
inbound telnet packets carry screen displays from the telnet server.
For purposes of discussion in this document, a session is defined as
the set of traffic that is managed as a unit for translation.
TCP/UDP sessions are uniquely identified by the tuple of (source IP
address, source TCP/UDP port, target IP address, target TCP/UDP
port). ICMP query sessions are identified by the tuple of (source IP
address, ICMP query ID, target IP address). All other sessions are
characterized by the tuple of (source IP address, target IP address,
IP protocol).
Address translations performed by NAT are session based and would
include translation of incoming as well as outgoing packets belonging
to that session. Session direction is identified by the direction of
the first packet of that session (see sec 2.5).
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Note, there is no guarantee that the idea of a session, determined as
above by NAT, will coincide with the application's idea of a session.
An application might view a bundle of sessions (as viewed by NAT) as
a single session and might not even view its communication with its
peers as a session. Not all applications are guaranteed to work
across realms, even with an ALG (defined below in section 2.9)
enroute.
2.4. TU ports, Server ports, Client ports
For the reminder of this document, we will refer TCP/UDP ports
associated with an IP address simply as "TU ports".
For most TCP/IP hosts, TU port range 0-1023 is used by servers
listening for incoming connections. Clients trying to initiate a
connection typically select a source TU port in the range of 1024-
65535. However, this convention is not universal and not always
followed. Some client stations initiate connections using a source TU
port number in the range of 0-1023, and there are servers listening
on TU port numbers in the range of 1024-65535.
A list of assigned TU port services may be found in RFC 1700 [Ref 2].
2.5. Start of session for TCP, UDP and others
The first packet of every TCP session tries to establish a session
and contains connection startup information. The first packet of a
TCP session may be recognized by the presence of SYN bit and absence
of ACK bit in the TCP flags. All TCP packets, with the exception of
the first packet, must have the ACK bit set.
However, there is no deterministic way of recognizing the start of a
UDP based session or any non-TCP session. A heuristic approach would
be to assume the first packet with hitherto non-existent session
parameters (as defined in section 2.3) as constituting the start of
new session.
2.6. End of session for TCP, UDP and others
The end of a TCP session is detected when FIN is acknowledged by both
halves of the session or when either half receives a segment with the
RST bit in TCP flags field. However, because it is impossible for a
NAT device to know whether the packets it sees will actually be
delivered to the destination (they may be dropped between the NAT
device and the destination), the NAT device cannot safely assume that
the segments containing FINs or SYNs will be the last packets of the
session (i.e., there could be retransmissions). Consequently, a
session can be assumed to have been terminated only after a period of
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4 minutes subsequent to this detection. The need for this extended
wait period is described in RFC 793 [Ref 7], which suggests a TIME-
WAIT duration of 2 * MSL (Maximum Segment Lifetime) or 4 minutes.
Note that it is also possible for a TCP connection to terminate
without the NAT device becoming aware of the event (e.g., in the case
where one or both peers reboot). Consequently, garbage collection is
necessary on NAT devices to clean up unused state about TCP sessions
that no longer exist. However, it is not possible in the general case
to distinguish between connections that have been idle for an
extended period of time from those that no longer exist. In the case
of UDP-based sessions, there is no single way to determine when a
session ends, since UDP-based protocols are application specific.
Many heuristic approaches are used to terminate sessions. You can
make the assumption that TCP sessions that have not been used for
say, 24 hours, and non-TCP sessions that have not been used for a
couple of minutes, are terminated. Often this assumption works, but
sometimes it doesn't. These idle period session timeouts vary a great
deal both from application to application and for different sessions
of the same application. Consequently, session timeouts must be
configurable. Even so, there is no guarantee that a satisfactory
value can be found. Further, as stated in section 2.3, there is no
guarantee that NAT's view of session termination will coincide with
that of the application.
Another way to handle session terminations is to timestamp entries
and keep them as long as possible and retire the longest idle session
when it becomes necessary.
2.7. Public/Global/External network
A Global or Public Network is an address realm with unique network
addresses assigned by Internet Assigned Numbers Authority (IANA) or
an equivalent address registry. This network is also referred as
External network during NAT discussions.
2.8. Private/Local network
A private network is an address realm independent of external network
addresses. Private network may also be referred alternately as Local
Network. Transparent routing between hosts in private realm and
external realm is facilitated by a NAT router.
RFC 1918 [Ref 1] has recommendations on address space allocation for
private networks. Internet Assigned Numbers Authority (IANA) has
three blocks of IP address space, namely 10/8, 172.16/12, and
192.168/16 set aside for private internets. In pre-CIDR notation, the
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first block is nothing but a single class A network number, while the
second block is a set of 16 contiguous class B networks, and the
third block is a set of 256 contiguous class C networks.
An organization that decides to use IP addresses in the address space
defined above can do so without coordination with IANA or any other
Internet registry such as APNIC, RIPE and ARIN. The address space
can thus be used privately by many independent organizations at the
same time. However, if those independent organizations later decide
they wish to communicate with each other or the public Internet, they
will either have to renumber their networks or enable NAT on their
border routers.
2.9. Application Level gateway (ALG)
Not all applications lend themselves easily to translation by NAT
devices; especially those that include IP addresses and TCP/UDP ports
in the payload. Application Level Gateways (ALGs) are application
specific translation agents that allow an application on a host in
one address realm to connect to its counterpart running on a host in
different realm transparently. An ALG may interact with NAT to set up
state, use NAT state information, modify application specific payload
and perform whatever else is necessary to get the application running
across disparate address realms.
ALGs may not always utilize NAT state information. They may glean
application payload and simply notify NAT to add additional state
information in some cases. ALGs are similar to Proxies, in that, both
ALGs and proxies facilitate Application specific communication
between clients and servers. Proxies use a special protocol to
communicate with proxy clients and relay client data to servers and
vice versa. Unlike Proxies, ALGs do not use a special protocol to
communicate with application clients and do not require changes to
application clients.
3. What is NAT?
Network Address Translation is a method by which IP addresses are
mapped from one address realm to another, providing transparent
routing to end hosts. There are many variations of address
translation that lend themselves to different applications. However,
all flavors of NAT devices should share the following
characteristics.
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a) Transparent Address assignment.
b) Transparent routing through address translation.
(routing here refers to forwarding packets, and not
exchanging routing information)
c) ICMP error packet payload translation.
Below is a diagram illustrating a scenario in which NAT is enabled on
a stub domain border router, connected to the Internet through a
regional router made available by a service provider.
\ | / . /
+---------------+ WAN . +-----------------+/
|Regional Router|----------------------|Stub Router w/NAT|---
+---------------+ . +-----------------+\
. | \
. | LAN
. ---------------
Stub border
Figure 1: A typical NAT operation scenario
3.1. Transparent Address Assignment
NAT binds addresses in private network with addresses in global
network and vice versa to provide transparent routing for the
datagrams traversing between address realms. The binding in some
cases may extend to transport level identifiers (such as TCP/UDP
ports). Address binding is done at the start of a session. The
following sub-sections describe two types of address assignments.
3.1.1. Static Address assignment
In the case of static address assignment, there is one-to-one address
mapping for hosts between a private network address and an external
network address for the lifetime of NAT operation. Static address
assignment ensures that NAT does not have to administer address
management with session flows.
3.1.2. Dynamic Address assignment
In this case, external addresses are assigned to private network
hosts or vice versa, dynamically based on usage requirements and
session flow determined heuristically by NAT. When the last session
using an address binding is terminated, NAT would free the binding so
that the global address could be recycled for later use. The exact
nature of address assignment is specific to individual NAT
implementations.
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3.2. Transparent routing
A NAT router sits at the border between two address realms and
translates addresses in IP headers so that when the packet leaves one
realm and enters another, it can be routed properly. Because NAT
devices have connections to multiple address realms, they must be
careful to not improperly propagate information (e.g., via routing
protocols) about networks from one address realm into another, where
such an advertisement would be deemed unacceptable.
There are three phases to Address translation, as follows. Together
these phases result in creation, maintenance and termination of state
for sessions passing through NAT devices.
3.2.1. Address binding
Address binding is the phase in which a local node IP address is
associated with an external address or vice versa, for purposes of
translation. Address binding is fixed with static address assignments
and is dynamic at session startup time with dynamic address
assignments. Once the binding between two addresses is in place, all
subsequent sessions originating from or to this host will use the
same binding for session based packet translation.
New address bindings are made at the start of a new session, if such
an address binding didn't already exist. Once a local address is
bound to an external address, all subsequent sessions originating
from the same local address or directed to the same local address
will use the same binding.
The start of each new session will result in the creation of a state
to facilitate translation of datagrams pertaining to the session.
There can be many simultaneous sessions originating from the same
host, based on a single address binding.
3.2.2. Address lookup and translation
Once a state is established for a session, all packets belonging to
the session will be subject to address lookup (and transport
identifier lookup, in some cases) and translation.
Address or transport identifier translation for a datagram will
result in the datagram forwarding from the origin address realm to
the destination address realm with network addresses appropriately
updated.
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3.2.3. Address unbinding
Address unbinding is the phase in which a private address is no
longer associated with a global address for purposes of translation.
NAT will perform address unbinding when it believes that the last
session using an address binding has terminated. Refer section 2.6
for some heuristic ways to handle session terminations.
3.3. ICMP error packet translation
All ICMP error messages (with the exception of Redirect message type)
will need to be modified, when passed through NAT. The ICMP error
message types needing NAT modification would include Destination-
Unreachable, Source-Quench, Time-Exceeded and Parameter-Problem. NAT
should not attempt to modify a Redirect message type.
Changes to ICMP error message will include changes to the original IP
packet (or portions thereof) embedded in the payload of the ICMP
error message. In order for NAT to be completely transparent to end
hosts, the IP address of the IP header embedded in the payload of the
ICMP packet must be modified, the checksum field of the same IP
header must correspondingly be modified, and the accompanying
transport header. The ICMP header checksum must also be modified to
reflect changes made to the IP and transport headers in the payload.
Furthermore, the normal IP header must also be modified.
4.0. Various flavors of NAT
There are many variations of address translation that lend themselves
to different applications. NAT flavors listed in the following sub-
sections are by no means exhaustive, but they do capture the
significant differences that abound.
The following diagram will be used as a base model to illustrate NAT
flavors. Host-A, with address Addr-A is located in a private realm,
represented by the network N-Pri. N-Pri is isolated from external
network through a NAT router. Host-X, with address Addr-X is located
in an external realm, represented by the network N-Ext. NAT router
with two interfaces, each attached to one of the realms provides
transparent routing between the two realms. The interface to the
external realm is assigned an address of Addr-Nx and the interface to
private realm is assigned an address of Addr-Np. Further, it may be
understood that addresses Addr-A and Addr-Np correspond to N-Pri
network and the addresses Addr-X and Addr-Nx correspond to N-Ext
network.
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________________
( )
( External ) +--+
( Address Realm )-- |__|
( (N-Ext) ) /____\
(________________) Host-X
| (Addr-X)
|(Addr-Nx)
+--------------+
| |
| NAT router |
| |
+--------------+
|(Addr-Np)
|
----------------
( )
+--+ ( Private )
|__|------( Address Realm )
/____\ ( (N-pri) )
Host-A (________________)
(Addr-A)
Figure 2: A base model to illustrate NAT terms.
4.1. Traditional NAT (or) Outbound NAT
Traditional NAT would allow hosts within a private network to
transparently access hosts in the external network, in most cases.
In a traditional NAT, sessions are uni-directional, outbound from the
private network. This is in contrast with Bi-directional NAT, which
permits sessions in both inbound and outbound directions. A detailed
description of Bi-directional NAT may be found in section 4.2.
The following is a description of the properties of realms supported
by traditional NAT. IP addresses of hosts in external network are
unique and valid in external as well as private networks. However,
the addresses of hosts in private network are unique only within the
private network and may not be valid in the external network. In
other words, NAT would not advertise private networks to the external
realm. But, networks from the external realm may be advertised within
the private network. The addresses used within private network must
not overlap with the external addresses. Any given address must
either be a private address or an external address; not both.
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A traditional NAT router in figure 2 would allow Host-A to initiate
sessions to Host-X, but not the other way around. Also, N-Ext is
routable from within N-Pri, whereas N-Pri may not be routable from
N-Ext.
Traditional NAT is primarily used by sites using private addresses
that wish to allow outbound sessions from their site.
There are two variations to traditional NAT, namely Basic NAT and
NAPT (Network Address Port Translation). These are discussed in the
following sub-sections.
4.1.1. Basic NAT
With Basic NAT, a block of external addresses are set aside for
translating addresses of hosts in a private domain as they originate
sessions to the external domain. For packets outbound from the
private network, the source IP address and related fields such as IP,
TCP, UDP and ICMP header checksums are translated. For inbound
packets, the destination IP address and the checksums as listed above
are translated.
A Basic NAT router in figure 2 may be configured to translate N-Pri
into a block of external addresses, say Addr-i through Addr-n,
selected from the external network N-Ext.
4.1.2. Network Address Port Translation (NAPT)
NAPT extends the notion of translation one step further by also
translating transport identifier (e.g., TCP and UDP port numbers,
ICMP query identifiers). This allows the transport identifiers of a
number of private hosts to be multiplexed into the transport
identifiers of a single external address. NAPT allows a set of hosts
to share a single external address. Note that NAPT can be combined
with Basic NAT so that a pool of external addresses are used in
conjunction with port translation.
For packets outbound from the private network, NAPT would translate
the source IP address, source transport identifier and related fields
such as IP, TCP, UDP and ICMP header checksums. Transport identifier
can be one of TCP/UDP port or ICMP query ID. For inbound packets, the
destination IP address, destination transport identifier and the IP
and transport header checksums are translated.
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A NAPT router in figure 2 may be configured to translate sessions
originated from N-Pri into a single external address, say Addr-i.
Very often, the external interface address Addr-Nx of NAPT router is
used as the address to map N-Pri to.
4.2. Bi-directional NAT (or) Two-Way NAT
With a Bi-directional NAT, sessions can be initiated from hosts in
the public network as well as the private network. Private network
addresses are bound to globally unique addresses, statically or
dynamically as connections are established in either direction. The
name space (i.e., their Fully Qualified Domain Names) between hosts
in private and external networks is assumed to be end-to-end unique.
Hosts in external realm access private realm hosts by using DNS for
address resolution. A DNS-ALG must be employed in conjunction with
Bi-Directional NAT to facilitate name to address mapping.
Specifically, the DNS-ALG must be capable of translating private
realm addresses in DNS Queries and responses into their external
realm address bindings, and vice versa, as DNS packets traverse
between private and external realms.
The address space requirements outlined for traditional NAT routers
are applicable here as well.
A Bi-directional NAT router in figure 2 would allow Host-A to
initiate sessions to Host-X, and Host-X to initiate sessions to
Host-A. Just as with traditional NAT, N-Ext is routable from within
N-Pri, but N-Pri may not be routable from N-Ext.
4.3. Twice NAT
Twice NAT is a variation of NAT in that both the source and
destination addresses are modified by NAT as a datagram crosses
address realms. This is in contrast to Traditional-NAT and Bi-
Directional NAT, where only one of the addresses (either source or
destination) is translated. Note, there is no such term as 'Once-
NAT'.
Twice NAT is necessary when private and external realms have address
collisions. The most common case where this would happen is when a
site had (improperly) numbered its internal nodes using public
addresses that have been assigned to another organization.
Alternatively, a site may have changed from one provider to another,
but chosen to keep (internally) the addresses it had been assigned by
the first provider. That provider might then later reassign those
addresses to someone else. The key issue in such cases is that the
address of the host in the external realm may have been assigned the
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same address as a host within the local site. If that address were to
appear in a packet, it would be forwarded to the internal node rather
than through the NAT device to the external realm. Twice-NAT attempts
to bridge these realms by translating both source and destination
address of an IP packet, as the packet transitions realms.
Twice-NAT works as follows. When Host-A wishes to initiate a session
to Host-X, it issues a DNS query for Host-X. A DNS-ALG intercepts the
DNS query, and in the response returned to Host-A the DNS-ALG
replaces the address for Host-X with one that is properly routable in
the local site (say Host-XPRIME). Host A then initiates communication
with Host-XPRIME. When the packets traverse the NAT device, the
source IP address is translated (as in the case of traditional NAT)
and the destination address is translated to Host-X. A similar
translation is performed on return packets coming from Host-X.
The following is a description of the properties of realms supported
by Twice-NAT. Network address of hosts in external network are unique
in external networks, but not within private network. Likewise, the
network address of hosts in private network are unique only within
the private network. In other words, the address space used in
private network to locate hosts in private and public networks is
unrelated to the address space used in public network to locate hosts
in private and public networks. Twice NAT would not be allowed to
advertise local networks to the external network or vice versa.
A Twice NAT router in figure 2 would allow Host-A to initiate
sessions to Host-X, and Host-X to initiate sessions to Host-A.
However, N-Ext (or a subset of N-Ext) is not routable from within N-
Pri, and N-Pri is not routable from N-Ext.
Twice NAT is typically used when address space used in a Private
network overlaps with addresses used in the Public space. For
example, say a private site uses the 200.200.200.0/24 address space
which is officially assigned to another site in the public internet.
Host_A (200.200.200.1) in Private space seeks to connect to Host_X
(200.200.200.100) in Public space. In order to make this connection
work, Host_X's address is mapped to a different address for Host_A
and vice versa. The twice NAT located at the Private site border may
be configured as follows:
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Private to Public : 200.200.200.0/24 -> 138.76.28.0/24
Public to Private : 200.200.200.0/24 -> 172.16.1.0/24
Datagram flow : Host_A(Private) -> Host_X(Public)
a) Within private network
DA: 172.16.1.100 SA: 200.200.200.1
b) After twice-NAT translation
DA: 200.200.200.100 SA: 138.76.28.1
Datagram flow Host_X (Public) -> Host_A (Private)
a) Within Public network
DA: 138.76.28.1 SA: 200.200.200.100
b) After twice-NAT translation, in private network
SA: 200.200.200.1 DA: 172.16.1.100
4.4. Multihomed NAT
There are limitations to using NAT. For example, requests and
responses pertaining to a session must be routed via the same NAT
router, as a NAT router maintains state information for sessions
established through it. For this reason, it is often suggested that
NAT routers be operated on a border router unique to a stub domain,
where all IP packets are either originated from the domain or
destined to the domain. However, such a configuration would turn a
NAT router into a single point of failure.
In order for a private network to ensure that connectivity with
external networks is retained even as one of the NAT links fail, it
is often desirable to multihome the private network to same or
multiple service providers with multiple connections from the private
domain, be it from same or different NAT boxes.
For example, a private network could have links to two different
providers and the sessions from private hosts could flow through the
NAT router with the best metric for a destination. When one of NAT
routers fail, the other could route traffic for all connections.
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Multiple NAT boxes or multiple links on the same NAT box, sharing the
same NAT configuration can provide fail-safe backup for each other.
In such a case, it is necessary for backup NAT device to exchange
state information so that a backup NAT can take on session load
transparently when the primary NAT fails. NAT backup becomes simpler,
when configuration is based on static maps.
5.0. Realm Specific IP (RSIP)
"Realm Specific IP" (RSIP) is used to characterize the functionality
of a realm-aware host in a private realm, which assumes realm-
specific IP address to communicate with hosts in private or external
realm.
A "Realm Specific IP Client" (RSIP client) is a host in a private
network that adopts an address in an external realm when connecting
to hosts in that realm to pursue end-to-end communication. Packets
generated by hosts on either end in such a setup would be based on
addresses that are end-to-end unique in the external realm and do not
require translation by an intermediary process.
A "Realm Specific IP Server" (RSIP server) is a node resident on both
private and external realms, that can facilitate routing of external
realm packets within a private realm. These packets may either have
been originated by an RSIP client or directed to an RSIP-client.
RSIP-Server may also be the same node that assigns external realm
addresses to RSIP-Clients.
There are two variations to RSIP, namely Realm-specific Address IP
(RSA-IP) and Realm-Specific Address and Port IP (RSAP-IP). These
variations are discussed in the following sub-sections.
5.1. Realm Specific Address IP (RSA-IP)
A Realm Specific Address IP (RSA-IP) client adopts an IP address from
the external address space when connecting to a host in external
realm. Once an RSA-IP client assumes an external address, no other
host in private or external domain can assume the same address, until
that address is released by the RSA-IP client.
The following is a discussion of routing alternatives that may be
pursued for the end-to-end RSA-IP packets within private realm. One
approach would be to tunnel the packet to the destination. The outer
header can be translated by NAT as normal without affecting the
addresses used in the internal header. Another approach would be to
set up a bi-directional tunnel between the RSA-IP Client and the
border router straddling the two address realms. Packets to and from
the client would be tunneled, but packets would be forwarded as
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normal between the border router and the remote destination. Note,
the tunnel from the client TO the border router may not be necessary.
You might be able to just forward the packet directly. This should
work so long as your internal network isn't filtering packets based
on source addresses (which in this case would be external addresses).
As an example, Host-A in figure 2 above, could assume an address
Addr-k from the external realm and act as RSA-IP-Client to allow
end-to-end sessions between Addr-k and Addr-X. Traversal of end-to-
end packets within private realm may be illustrated as follows:
First method, using NAT router enroute to translate:
===================================================
Host-A NAT router Host-X
------ ----------- ------
<Outer IP header, with
src=Addr-A, Dest=Addr-X>,
embedding
<End-to-end packet, with
src=Addr-k, Dest=Addr-X>
----------------------------->
<Outer IP header, with
src=Addr-k, Dest=Addr-X>,
embedding
<End-to-end packet, with
src=Addr-k, Dest=Addr-X>
--------------------------->
.
.
.
<Outer IP header, with
src=Addr-X, Dest=Addr-k>,
embedding
<End-to-end packet, with
src=Addr-X, Dest=Addr-k>
<---------------------------------
<Outer IP header, with
src=Addr-X, Dest=Addr-A>,
embedding <End-to-end packet,
with src=Addr-X, Dest=Addr-k>
<--------------------------------------
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Second method, using a tunnel within private realm:
==================================================
Host-A NAT router Host-X
------ ----------- ------
<Outer IP header, with
src=Addr-A, Dest=Addr-Np>,
embedding
<End-to-end packet, with
src=Addr-k, Dest=Addr-X>
----------------------------->
<End-to-end packet, with
src=Addr-k, Dest=Addr-X>
------------------------------->
.
.
.
<End-to-end packet, with
src=Addr-X, Dest=Addr-k>
<--------------------------------
<Outer IP header, with
src=Addr-Np, Dest=Addr-A>,
embedding <End-to-end packet,
with src=Addr-X, Dest=Addr-k>
<----------------------------------
There may be other approaches to pursue.
An RSA-IP-Client has the following characteristics. The collective
set of operations performed by an RSA-IP-Client may be termed "RSA-
IP".
1. Aware of the realm to which its peer nodes belong.
2. Assumes an address from external realm when communicating with
hosts in that realm. Such an address may be assigned statically
or obtained dynamically (through a yet-to-be-defined protocol)
from a node capable of assigning addresses from external realm.
RSA-IP-Server could be the node coordinating external realm
address assignment.
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3. Route packets to external hosts using an approach amenable to
RSA-IP-Server. In all cases, RSA-IP-Client will likely need
to act as a tunnel end-point, capable of encapsulating
end-to-end packets while forwarding and decapsulating in the
return path.
"Realm Specific Address IP Server" (RSA-IP server) is a node resident
on both private and external realms, that facilitates routing of
external realm packets specific to RSA-IP clients inside a private
realm. An RSA-IP-Server may be described as having the following
characteristics.
1. May be configured to assign addresses from external realm to
RSA-IP-Clients, either statically or dynamically.
2. Must be a router resident on both the private and external
address realms.
3. Must be able to provide a mechanism to route external realm
packets within private realm. Of the two approaches described,
the first approach requires RSA-IP-Server to be a NAT router
providing transparent routing for the outer header. This
approach requires the external peer to be a tunnel end-point.
With the second approach, an RSA-IP-Server could be any router
(including a NAT router) that can be a tunnel end-point with
RSA-IP-Clients. It would detunnel end-to-end packets outbound
from RSA-IP-Clients and forward to external hosts. On the
return path, it would locate RSA-IP-Client tunnel, based on the
destination address of the end-to-end packet and encapsulate the
packet in a tunnel to forward to RSA-IP-Client.
RSA-IP-Clients may pursue any of the IPsec techniques, namely
transport or tunnel mode Authentication and confidentiality using AH
and ESP headers on the embedded packets. Any of the tunneling
techniques may be adapted for encapsulation between RSA-IP-Client and
RSA-IP-Server or between RSA-IP-Client and external host. For
example, IPsec tunnel mode encapsulation is a valid type of
encapsulation that ensures IPsec authentication and confidentiality
for the embedded end-to-end packets.
5.2 Realm Specific Address and port IP (RSAP-IP)
Realm Specific Address and port IP (RSAP-IP) is a variation of RSIP
in that multiple private hosts use a single external address,
multiplexing on transport IDentifiers (i.e., TCP/UDP port numbers and
ICMP Query IDs).
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"RSAP-IP-Client" may be defined similar to RSA-IP-Client with the
variation that RSAP-IP-Client assumes a tuple of (external address,
transport Identifier) when connecting to hosts in external realm to
pursue end-to-end communication. As such, communication with external
nodes for an RSAP-IP-Client may be limited to TCP, UDP and ICMP
sessions.
"RSAP-IP-Server" is similar to RSA-IP-Server in that it facilitates
routing of external realm packets specific to RSAP-IP clients inside
a private realm. Typically, an RSAP-IP-Server would also be the one
to assign transport tuples to RSAP-IP-Clients.
A NAPT router enroute could serve as RSAP-IP-Server, when the outer
encapsulation is TCP/UDP based and is addressed between the RSAP-IP-
Client and external peer. This approach requires the external peer to
be the end-point of TCP/UDP based tunnel. Using this approach,
RSAP-IP-Clients may pursue any of the IPsec techniques, namely
transport or tunnel mode authentication and confidentiality using AH
and ESP headers on the embedded packets. Note however, IPsec tunnel
mode is not a valid type of encapsulation, as a NAPT router cannot
provide routing transparency to AH and ESP protocols.
Alternately, packets may be tunneled between RSAP-IP-Client and
RSAP-IP-Server such that RSAP-IP-Server would detunnel packets
outbound from RSAP-IP-Clients and forward to external hosts. On the
return path, RSAP-IP-Server would locate RSAP-IP-Client tunnel,
based on the tuple of (destination address, transport Identifier) and
encapsulate the original packet within a tunnel to forward to RSAP-
IP-Client. With this approach, there is no limitation on the
tunneling technique employed between RSAP-IP-Client and RSAP-IP-
Server. However, there are limitations to applying IPsec based
security on end-to-end packets. Transport mode based authentication
and integrity may be attained. But, confidentiality cannot be
permitted because RSAP-IP-Server must be able to examine the
destination transport Identifier in order to identify the RSAP-IP-
tunnel to forward inbound packets to. For this reason, only the
transport mode TCP, UDP and ICMP packets protected by AH and ESP-
authentication can traverse a RSAP-IP-Server using this approach.
As an example, say Host-A in figure 2 above, obtains a tuple of
(Addr-Nx, TCP port T-Nx) from NAPT router to act as RSAP-IP-Client to
initiate end-to-end TCP sessions with Host-X. Traversal of end-to-
end packets within private realm may be illustrated as follows. In
the first method, outer layer of the outgoing packet from Host-A uses
(private address Addr-A, source port T-Na) as source tuple to
communicate with Host-X. NAPT router enroute translates this tuple
into (Addr-Nx, Port T-Nxa). This translation is independent of RSAP-
IP-Client tuple parameters used in the embedded packet.
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First method, using NAPT router enroute to translate:
====================================================
Host-A NAPT router Host-X
------ ----------- ------
<Outer TCP/UDP packet, with
src=Addr-A, Src Port=T-Na,
Dest=Addr-X>,
embedding
<End-to-end packet, with
src=Addr-Nx, Src Port=T-Nx, Dest=Addr-X>
----------------------------->
<Outer TCP/UDP packet, with
src=Addr-Nx, Src Port=T-Nxa,
Dest=Addr-X>,
embedding
<End-to-end packet, with
src=Addr-Nx, Src Port=T-Nx, Dest=Addr-X>
--------------------------------------->
.
.
.
<Outer TCP/UDP packet with
src=Addr-X, Dest=Addr-Nx,
Dest Port=T-Nxa>,
embedding
<End-to-end packet, with
src=Addr-X, Dest=Addr-Nx,
Dest Port=T-Nx>
<----------------------------------
<Outer TCP/UDP packet, with
src=Addr-X, Dest=Addr-A,
Dest Port=T-Na>,
embedding
<End-to-end packet, with
src=Addr-X, Dest=Addr-Nx,
Dest Port=T-Nx>
<-----------------------------------
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Second method, using a tunnel within private realm:
==================================================
Host-A NAPT router Host-X
------ ----------- ------
<Outer IP header, with
src=Addr-A, Dest=Addr-Np>,
embedding
<End-to-end packet, with
src=Addr-Nx, Src Port=T-Nx,
Dest=Addr-X>
----------------------------->
<End-to-end packet, with
src=Addr-Nx, Src Port=T-Nx,
Dest=Addr-X>
-------------------------------->
.
.
.
<End-to-end packet, with
src=Addr-X, Dest=Addr-Nx,
Dest Port=T-Nx>
<----------------------------------
<Outer IP header, with
src=Addr-Np, Dest=Addr-A>,
embedding
<End-to-end packet, with
src=Addr-X, Dest=Addr-Nx,
Dest Port=T-Nx>
<----------------------------------
6.0. Private Networks and Tunnels
Let us consider the case where your private network is connected to
the external world via tunnels. In such a case, tunnel encapsulated
traffic may or may not contain translated packets depending upon the
characteristics of address realms a tunnel is bridging.
The following subsections discuss two scenarios where tunnels are
used (a) in conjunction with Address translation, and (b) without
translation.
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6.1. Tunneling translated packets
All variations of address translations discussed in the previous
section can be applicable to direct connected links as well as
tunnels and virtual private networks (VPNs).
For example, a private network connected to a business partner
through a VPN could employ traditional NAT to communicate with the
partner. Likewise, it is possible to employ twice NAT, if the
partner's address space overlapped with the private network. There
could be a NAT device on one end of the tunnel or on both ends of the
tunnel. In all cases, traffic across the VPN can be encrypted for
security purposes. Security here refers to security for traffic
across VPNs alone. End-to-end security requires trusting NAT devices
within private network.
6.2. Backbone partitioned private Networks
There are many instances where a private network (such as a corporate
network) is spread over different locations and use public backbone
for communications between those locations. In such cases, it is not
desirable to do address translation, both because large numbers of
hosts may want to communicate across the backbone, thus requiring
large address tables, and because there will be more applications
that depend on configured addresses, as opposed to going to a name
server. We call such a private network a backbone-partitioned private
network.
Backbone-partitioned stubs should behave as though they were a non-
partitioned stub. That is, the routers in all partitions should
maintain routes to the local address spaces of all partitions. Of
course, the (public) backbones do not maintain routes to any local
addresses. Therefore, the border routers must tunnel (using VPNs)
through the backbones using encapsulation. To do this, each NAT box
will set aside a global address for tunneling.
When a NAT box x in stub partition X wishes to deliver a packet to
stub partition Y, it will encapsulate the packet in an IP header with
destination address set to the global address of NAT box y that has
been reserved for encapsulation. When NAT box y receives a packet
with that destination address, it decapsulates the IP header and
routes the packet internally. Note, there is no address translation
in the process; merely transfer of private network packets over an
external network tunnel backbone.
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7.0. NAT operational characteristics
NAT devices are application unaware in that the translations are
limited to IP/TCP/UDP/ICMP headers and ICMP error messages only. NAT
devices do not change the payload of the packets, as payloads tend to
be application specific.
NAT devices (without the inclusion of ALGs) do not examine or modify
transport payload. For this reason, NAT devices are transparent to
applications in many cases. There are two areas, however, where NAT
devices often cause difficulties: 1) when an application payload
includes an IP address, and 2) when end-to-end security is needed.
Note, this is not a comprehensive list.
Application layer security techniques that do not make use of or
depend on IP addresses will work correctly in the presence of NAT
(e.g., TLS, SSL and ssh). In contrast, transport layer techniques
such as IPSec transport mode or the TCP MD5 Signature Option RFC 2385
[Ref 17] do not.
In IPSec transport mode, both AH and ESP have an integrity check
covering the entire payload. When the payload is TCP or UDP, the
TCP/UDP checksum is covered by the integrity check. When a NAT device
modifies an address the checksum is no longer valid with respect to
the new address. Normally, NAT also updates the checksum, but this is
ineffective when AH and ESP are used. Consequently, receivers will
discard a packet either because it fails the IPSec integrity check
(if the NAT device updates the checksum), or because the checksum is
invalid (if the NAT device leaves the checksum unmodified).
Note that IPsec tunnel mode ESP is permissible so long as the
embedded packet contents are unaffected by the outer IP header
translation. Although this technique does not work in traditional NAT
deployments (i.e., where hosts are unaware that NATs are present),
the technique is applicable to Realm-Specific IP as described in
Section 5.0.
Note also that end-to-end ESP based transport mode authentication and
confidentiality are permissible for packets such as ICMP, whose IP
payload content is unaffected by the outer IP header translation.
NAT devices also break fundamental assumptions by public key
distribution infrastructures such as Secure DNS RFC 2535 [Ref 18] and
X.509 certificates with signed public keys. In the case of Secure
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DNS, each DNS RRset is signed with a key from within the zone.
Moreover, the authenticity of a specific key is verified by following
a chain of trust that goes all the way to the DNS root. When a DNS-
ALG modifies addresses (e.g., as in the case of Twice-NAT),
verification of signatures fails.
It may be of interest to note that IKE (Session key negotiation
protocol) is a UDP based session layer protocol and is not protected
by network based IPsec security. Only a portion of the individual
payloads within IKE are protected. As a result, IKE sessions are
permissible across NAT, so long as IKE payload does not contain
addresses and/or transport IDs specific to one realm and not the
other. Given that IKE is used to setup IPSec associations, and there
are at present no known ways of making IPSec work through a NAT
function, it is a future work item to take advantage of IKE through a
NAT box.
One of the most popular internet applications "FTP" would not work
with the definition of NAT as described. The following sub-section is
devoted to describing how FTP is supported on NAT devices. FTP ALG
is an integral part of most NAT implementations. Some vendors may
choose to include additional ALGs to custom support other
applications on the NAT device.
7.1. FTP support
"PORT" command and "PASV" response in FTP control session payload
identify the IP address and TCP port that must be used for the data
session it supports. The arguments to the PORT command and PASV
response are an IP address and a TCP port in ASCII. An FTP ALG is
required to monitor and update the FTP control session payload so
that information contained in the payload is relevant to end nodes.
The ALG must also update NAT with appropriate data session tuples and
session orientation so that NAT could set up state information for
the FTP data sessions.
Because the address and TCP port are encoded in ASCII, this may
result in a change in the size of packet. For instance,
10,18,177,42,64,87 is 18 ASCII characters, whereas
193,45,228,137,64,87 is 20 ASCII characters. If the new size is same
as the previous, only the TCP checksum needs adjustment as a result
of change of data. If the new size is less than or greater than the
previous, TCP sequence numbers must also be changed to reflect the
change in length of FTP control data portion. A special table may be
used by the ALG to correct the TCP sequence and acknowledge numbers.
The sequence number and acknowledgement correction will need to be
performed on all future packet of the connection.
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8.0. NAT limitations
8.1. Applications with IP-address Content
Not All applications lend themselves easily to address translation by
NAT devices. Especially, the applications that carry IP address (and
TU port, in case of NAPT) inside the payload. Application Level
Gateways, or ALGs must be used to perform translations on packets
pertaining to such applications. ALGs may optionally utilize address
(and TU port) assignments made by NAT and perform translations
specific to the application. The combination of NAT functionality and
ALGs will not provide end-to-end security assured by IPsec. However,
tunnel mode IPsec can be accomplished with NAT router serving as
tunnel end point.
SNMP is one such application with address content in payload. NAT
routers would not translate IP addresses within SNMP payloads. It is
not uncommon for an SNMP specific ALG to reside on a NAT router to
perform SNMP MIB translations proprietary to the private network.
8.2. Applications with inter-dependent control and data sessions
NAT devices operate on the assumption that each session is
independent. Session characteristics like session orientation,
source and destination IP addresses, session protocol, and source and
destination transport level identifiers are determined independently
at the start of each new session.
However, there are applications such as H.323 that use one or more
control sessions to set the characteristics of the follow-on sessions
in their control session payload. Such applications require use of
application specific ALGs that can interpret and translate the
payload, if necessary. Payload interpretation would help NAT be
prepared for the follow-on data sessions.
8.3. Debugging Considerations
NAT increases the probability of mis-addressing. For example, same
local address may be bound to different global address at different
times and vice versa. As a result, any traffic flow study based
purely on global addresses and TU ports could be confused and might
misinterpret the results.
If a host is abusing the Internet in some way (such as trying to
attack another machine or even sending large amounts of junk mail or
something) it is more difficult to pinpoint the source of the trouble
because the IP address of the host is hidden in a NAT router.
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8.4. Translation of fragmented FTP control packets
Translation of fragmented FTP control packets is tricky when the
packets contain "PORT" command or response to "PASV" command.
Clearly, this is a pathological case. NAT router would need to
assemble the fragments together first and then translate prior to
forwarding.
Yet another case would be when each character of packets containing
"PORT" command or response to "PASV" is sent in a separate datagram,
unfragmented. In this case, NAT would simply have to let the packets
through, without translating the TCP payload. Of course, the
application will fail if the payload needed to be altered. The
application could still work in a few cases, where the payload
contents can be valid in both realms, without modifications enroute.
For example, FTP originated from a private host would still work
while traversing a traditional NAT or bi-directional NAT device, so
long as the FTP control session employed PASV command to establish
data sessions. The reason being that the address and port number
specified by FTP server in the PASV response (sent as multiple
unfragmented packets) is valid to the private host, as is. The NAT
device will simply view the ensuing data session (also originating
from private host) as an independent TCP session.
8.5. Compute intensive
NAT is compute intensive even with the help of a clever checksum
adjustment algorithm, as each data packet is subject to NAT lookup
and modifications. As a result, router forwarding throughput could
be slowed considerably. However, so long as the processing capacity
of the NAT device exceeds line processing rate, this should not be a
problem.
9.0. Security Considerations
Many people view traditional NAT router as a one-way (session)
traffic filter, restricting sessions from external hosts into their
machines. In addition, when address assignment in NAT router is done
dynamically, that makes it harder for an attacker to point to any
specific host in the NAT domain. NAT routers may be used in
conjunction with firewalls to filter unwanted traffic.
If NAT devices and ALGs are not in a trusted boundary, that is a
major security problem, as ALGs could snoop end user traffic payload.
Session level payload could be encrypted end to end, so long as the
payload does not contain IP addresses and/or transport identifiers
that are valid in only one of the realms. With the exception of RSIP,
end-to-end IP network level security assured by current IPsec
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techniques is not attainable with NAT devices in between. One of the
ends must be a NAT box. Refer section 7.0 for a discussion on why
end-to-end IPsec security cannot be assured with NAT devices along
the route.
The combination of NAT functionality, ALGs and firewalls will provide
a transparent working environment for a private networking domain.
With the exception of RSIP, end-to-end network security assured by
IPsec cannot be attained for end-hosts within the private network
(Refer section 5.0 for RSIP operation). In all other cases, if you
want to use end-to-end IPsec, there cannot be a NAT device in the
path. If we make the assumption that NAT devices are part of a
trusted boundary, tunnel mode IPsec can be accomplished with NAT
router (or a combination of NAT, ALGs and firewall) serving as tunnel
end point.
NAT devices, when combined with ALGs, can ensure that the datagrams
injected into Internet have no private addresses in headers or
payload. Applications that do not meet these requirements may be
dropped using firewall filters. For this reason, it is not uncommon
to find NAT, ALG and firewall functions co-exist to provide security
at the borders of a private network. NAT gateways can be used as
tunnel end points to provide secure VPN transport of packet data
across an external network domain.
Below are some additional security considerations associated with NAT
routers.
1. UDP sessions are inherently unsafe. Responses to a datagram
could come from an address different from the target address
used by sender ([Ref 4]). As a result, an incoming UDP packet
might match the outbound session of a traditional NAT router
only in part (the destination address and UDP port number of
the packet match, but the source address and port number may
not). In such a case, there is a potential security compromise
for the NAT device in permitting inbound packets with partial
match. This UDP security issue is also inherent to firewalls.
Traditional NAT implementations that do not track datagrams on
a per-session basis but lump states of multiple UDP sessions
using the same address binding into a single unified session
could compromise the security even further. This is because,
the granularity of packet matching would be further limited to
just the destination address of the inbound UDP packets.
2. Multicast sessions (UDP based) are another source for security
weakness for traditional-NAT routers. Once again, firewalls face
the same security dilemma as the NAT routers.
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Say, a host on private network initiated a multicast session.
Datagram sent by the private host could trigger responses in the
reverse direction from multiple external hosts. Traditional-NAT
implementations that use a single state to track a multicast
session cannot determine for certain if the incoming UDP packet
is in response to an existing multicast session or the start of
new UDP session initiated by an attacker.
3. NAT devices can be a target for attacks.
Since NAT devices are Internet hosts they can be the target of a
number of different attacks, such as SYN flood and ping flood
attacks. NAT devices should employ the same sort of protection
techniques as Internet-based servers do.
REFERENCES
[1] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot,G. and E.
Lear, "Address Allocation for Private Internets", BCP 5, RFC
1918, February 1996.
[2] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
October, 1994.
[3] Braden, R., "Requirements for Internet Hosts -- Communication
Layers", STD 3, RFC 1122, October 1989.
[4] Braden, R., "Requirements for Internet Hosts -- Application and
Support", STD 3, RFC 1123, October 1989.
[5] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
June 1995.
[6] Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)", STD
9, RFC 959, October 1985.
[7] Postel, J., "Transmission Control Protocol (TCP) Specification",
STD 7, RFC 793, September 1981.
[8] Postel, J., "Internet Control Message Protocol Specification"
STD 5, RFC 792, September 1981.
[9] Postel, J., "User Datagram Protocol (UDP)", STD 6, RFC 768,
August 1980.
[10] Mogul, J. and J. Postel, "Internet Standard Subnetting
Procedure", STD 5, RFC 950, August 1985.
Srisuresh & Holdrege Informational [Page 28]
RFC 2663 NAT Terminology and Considerations August 1999
[11] Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4 Address
Behavior Today", RFC 2101, February 1997.
[12] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[13] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[14] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
[15] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[16] Piper, D., "The Internet IP Security Domain of Interpretation
for ISAKMP", RFC 2407, November 1998.
[17] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[18] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
Authors' Addresses
Pyda Srisuresh
Lucent Technologies
4464 Willow Road
Pleasanton, CA 94588-8519
U.S.A.
Phone: (925) 737-2153
Fax: (925) 737-2110
EMail: srisuresh@lucent.com
Matt Holdrege
Lucent Technologies
1701 Harbor Bay Parkway
Alameda, CA 94502
Phone: (510) 769-6001
EMail: holdrege@lucent.com
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RFC 2663 NAT Terminology and Considerations August 1999
Full Copyright Statement
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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