NAT Working Group P. Srisuresh
INTERNET-DRAFT Lucent Technologies
Category: Informational Matt Holdrege
Expire in six months Ascend Communications
October 1998
IP Network Address Translator (NAT) Terminology and Considerations
<draft-ietf-nat-terminology-01.txt>
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also
distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-
Drafts as reference material or to cite them other than as
"work in progress."
To view the entire list of current Internet-Drafts, please check
the "1id-abstracts.txt" listing contained in the Internet-Drafts
Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net
(Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au
(Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu
(US West Coast).
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 draft, 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 ADs, Scott
Bradner and Vern Paxson, for their detailed review of the document.
Srisuresh & Holdrege [Page 1]
Internet Draft NAT Terminology and Considerations October 1998
Abstract
Network Address Translation is a method by which IP addresses are
mapped from one realm to another, providing transparent routing to
hosts. Traditionally, NATs are used to connect an isolated routing
realm with private unregistered addresses to an external routing
realm with globally unique registered addresses. This document
attempts to describe the operation of NATs in general and to
define the terminology used to identify various flavors of NAT.
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 for privacy
reasons or because they are invalid for use outside the network.
Address translation would allow hosts in a private network to
transparently access 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 NATs in general.
Note, however, this document is not intended to describe the
operations of individual NAT variations or the applicability
of NATs.
NATs provide transparent routing solution to end hosts trying to
communicate from disparate routing realms. This transparent routing
is achieved by modifying end node addresses en-route and
maintaining state for these updates so that datagrams pertaining
to a session are transparently routed to the right end-node in
either realm. This solution works best when the end user identifier
(such as host name) is different from the address used to locate
the end user. IPsec techniques which are intended to guarantee the
end-to-end security of an IP packet cannot be assumed to transit NAT.
Techniques such as AH and ESP secure IP header address contents of
the end host packets. Yet, NAT's fundamental role is to alter the
addresses in the IP header of a packet.
NATs cannot by themselves 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.
Srisuresh & Holdrege [Page 2]
Internet Draft NAT Terminology and Considerations October 1998
2. Terminology and concepts used
2.1. NAT terminology
Terms most frequently used in the context of NATs are defined here
for reference.
2.1.1. 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 2.1.3).
2.1.2. 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 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 TU port number
in the range of 0-1023, and there are servers listening on TU
port numbers in the range of 1024-65535.
Srisuresh & Holdrege [Page 3]
Internet Draft NAT Terminology and Considerations October 1998
A list of assigned TU port services may be found in [Ref 2].
2.1.3. 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-existant
session parameters (as defined in section 2.1.1) as constituting
the start of new session.
2.1.4. 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 sets RST bit in
TCP flags field. Within a short period (say, a couple of seconds)
after one of the session partners sets RST bit, the session can
be safely assumed to have been terminated. However, it is
possible to have TCP sessions hung forever. As for non-TCP
sessions, there is no deterministic way of identifying session
end unless you know the application protocol.
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
say, 1 minute, are terminated. Often this assumption works, but
sometimes it doesn't. These idle period session timeouts may vary
considerably across the board and may be made user configurable.
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.
2.1.5. Routing realm
A routing 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. Although NATs may be used with IPv6, this document is
limited to describing NATs in a IPv4 environment.
2.1.6. Public/Global/External network
Srisuresh & Holdrege [Page 4]
Internet Draft NAT Terminology and Considerations October 1998
A Global or Public Network is a routing 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.
2.1.7. Private/Local network
A private network is a routing realm independent of external
routing network. 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 device.
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 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, with NAT enabled on their
border routers.
2.1.8. Application Level gateway (ALG)
Not all applications lend themselves easily to translation by NATs;
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
routing realm to connect to its counterpart running on a host in
different realm. An ALG may interact with NAT to set up state,
use NAT state information, modify application specific payload and
perform whatever else is required to get the application running
across disparate routing 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. Just as with proxies,
ALGs could be transparent as well as non-transparent. Proxies
relay client data to servers and vice versa, by using a special
protocol to communicate with proxy clients. Unlike Proxies, ALGs
Srisuresh & Holdrege [Page 5]
Internet Draft NAT Terminology and Considerations October 1998
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 routing 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 NATs should share the following characteristics.
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 routing 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.
Srisuresh & Holdrege [Page 6]
Internet Draft NAT Terminology and Considerations October 1998
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 detected 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.
3.2. Transparent routing
NATs translate addresses in IP header to contain routable addresses,
so that each routing realm can use routing protocols appropriate to
the realm to route datagrams. NATs should be careful to not
advertise networks in a routing realm, 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 terminations of
soft state for sessions passing through NATs.
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 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 bind 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.
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
Srisuresh & Holdrege [Page 7]
Internet Draft NAT Terminology and Considerations October 1998
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 routing realm
to the destination routing realm with network addresses
appropriately updated.
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. When the last session using an address bind is
terminated, it is safe to do the address unbinding. Refer section
2.1 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
Srisuresh & Holdrege [Page 8]
Internet Draft NAT Terminology and Considerations October 1998
is located in 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.
________________
( )
( External ) +--+
( Routing Realm )-- |__|
( (N-Ext) ) /____\
(________________) Host-X
| (Addr-X)
|(Addr-Nx)
+--------------+
| |
| NAT router |
| |
+--------------+
|(Addr-Np)
|
----------------
( )
+--+ ( Private )
|__|------( Routing Realm )
/____\ ( (N-pri) )
Host-A (________________)
(Addr-A)
Figure 2: A base model to illustrate NAT terms.
4.1. Traditional NAT
Traditional NAT would allow hosts within a private network to
transparently access hosts in the external network. In a
traditional NAT, sessions are uni-directional, outbound from
the private network. Sessions in the opposite direction may be
allowed on an exceptional basis using static address maps for
pre-selected hosts.
In this setup, network address 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
Srisuresh & Holdrege [Page 9]
Internet Draft NAT Terminology and Considerations October 1998
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.
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 to connect to the Internet sites
which are RFC1918 addressed or sites with addresses that have
private enterprise significance. This is also used to avoid address
renumbering when changing service providers, even as the addressing
within the private network is IANA assigned.
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)
With NAPT, a single external address is set aside for translating
sessions originated by hosts in a private domain. This is made
possible by multiplexing transport level identifiers of multiple
private hosts simultaneously into the transport identifiers of
a single assigned external address. For this reason, only the
applications supported by transport protocols TCP, UDP and ICMP
are supported by NAPT. TCP and UDP protocols contain source and
destination port numbers. ICMP query based applications are also
supported as the queries contain a Query Identifier to correlate
Srisuresh & Holdrege [Page 10]
Internet Draft NAT Terminology and Considerations October 1998
responses with requests.
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.
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. Two-Way NAT or Bi-directional 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 unique.
The address space requirements outlined for traditional NATs 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
routing realms. Typically, twice NAT would be deployed on an
interface that attempts to "address isolate" private space from
the public Internet.
In this setup, the 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
Srisuresh & Holdrege [Page 11]
Internet Draft NAT Terminology and Considerations October 1998
or vice versa.
Sessions are allowed to be initiated from hosts in private network
to hosts in public network or vice versa. The name space (i.e.,
Fully Qualified Domain Names) between hosts in private and external
networks is assumed to be unique.
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 :
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
Srisuresh & Holdrege [Page 12]
Internet Draft NAT Terminology and Considerations October 1998
4.4. Host NAT
A "Host NAT Client" is a host in private network that adopts an
address in 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.
However, a routing mechanism would be required to deliver the
end-to-end packets within private realm. One approach would be
to embed these packets within an IP packet such that the outer
packet is addressed between Host-NAT-Client's private address and
the external peer. In such a case, NAT router in between could
provide transparent routing of the outer packet by translating the
outer IP header en-route. Another approach would be to embed the
end-to-end packet inside a tunnel while traversing in the private
network, such that the tunnel is addressed between
Host-NAT-Client's private address and a router resident on both
realms.
As an example, Host-A in figure 2 above, could assume an address
Addr-k from the external realm and act as Host-NAT-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>
--------------------------->
.
Srisuresh & Holdrege [Page 13]
Internet Draft NAT Terminology and Considerations October 1998
.
.
<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>
<--------------------------------------
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>
<----------------------------------
Srisuresh & Holdrege [Page 14]
Internet Draft NAT Terminology and Considerations October 1998
There may be other approaches to pursue. Common to all, suffices
to say, there exists a need for a node that is resident on both
private and external realms that can facilitate routing of
external realm packets within private realm. Such a node is
termed, "Host NAT Server". Host-NAT-Server could also be the same
node that assigns external addresses to Host-NAT-Clients.
A Host-NAT-Client has the following characteristics. The collective
set of operations performed by Host-NAT-Client is termed "Host-NAT".
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 from a node capable of assigning
addresses from external realm. Host-NAT-Server could be the
node coordinating external realm address assignment.
3. Route packets to external hosts using an approach amenable to
Host-NAT-Server. In all cases, Host-NAT-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.
A Host-NAT-Server may be described as having the following
characteristics.
1. May be configured to assign addresses from external realm to
Host-NAT-Clients, either statically or dynamically.
2. Must be a router resident on both the private and external
routing 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 Host-NAT-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, a Host-NAT-Server could be any router
(including a NAT router) that can be a tunnel end-point with
Host-NAT-Clients. It would detunnel end-to-end packets outbound
from Host-NAT-Clients and forward to external hosts. On the
return path, it would locate Host-NAT-Client tunnel, based on the
destination address of the end-to-end packet and encapsulate the
packet in a tunnel to forward to Host-NAT-Client.
Srisuresh & Holdrege [Page 15]
Internet Draft NAT Terminology and Considerations October 1998
Host-NAT-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 Host-NAT-Client
and Host-NAT-Server or between Host-NAT-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.
4.4.1. Host NAPT
Host Network Address Port Translation (Host NAPT) is a variation
of Host-NAT in that multiple private hosts use a single external
address, multiplexing on transport IDentifiers (i.e., TCP/UDP
port numbers and ICMP Query IDs).
"Host NAPT Client" may be defined similar to Host-NAT-Client with
the variation that Host-NAPT-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 a Host-NAPT-Client is limited to TCP, UDP
and ICMP sessions.
"Host-NAPT-Server" is similar to Host-NAT-Server in that it
facilitates routing of the end-to-end packets inside private realm.
Typically, a Host-NAPT-Server would also be the one to assign
transport tuples to Host-NAPT-Clients.
A NAPT router enroute could serve as Host-NAPT-Server, when the
outer encapsulation is TCP/UDP based (such as PPTP, L2TP tunneling)
and is addressed between the Host-NAPT-Client and external peer.
This approach requires the external peer to be the end-point of
TCP/UDP based tunnel. Using this approach, Host-NAPT-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 Host-NAPT-Client and
Host-NAPT-Server such that host-NAPT-Server would detunnel packets
outbound from Host-NAPT-Clients and forward to external hosts. On
the return path, Host-NAPT-Server would locate Host-NAPT-Client
tunnel, based on the tuple of (destination address, transport
Identifier) and encapsulate the original packet within a tunnel
to forward to Host-NAPT-Client. With this approach, there is no
limitation on the tunneling technique employed between
Srisuresh & Holdrege [Page 16]
Internet Draft NAT Terminology and Considerations October 1998
Host-NAPT-Client and Host-NAPT-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 Host-NAPT-Server
must be able to examine the destination transport Identifier in
order to identify the Host-NAPT-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
Host-NAPT-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
Host-NAPT-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 Host-NAPT-Client tuple parameters
used in the embedded packet.
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
Srisuresh & Holdrege [Page 17]
Internet Draft NAT Terminology and Considerations October 1998
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>
<-----------------------------------
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
Srisuresh & Holdrege [Page 18]
Internet Draft NAT Terminology and Considerations October 1998
src=Addr-Np, Dest=Addr-A>,
embedding
<End-to-end packet, with
src=Addr-X, Dest=Addr-Nx,
Dest Port=T-Nx>
<----------------------------------
4.5. 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 NATs 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 box 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 NATs fail, the other could route traffic for all connections.
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 would be desirable for backup NATs 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. 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 routing 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.
Srisuresh & Holdrege [Page 19]
Internet Draft NAT Terminology and Considerations October 1998
5.1. 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 NATs within private network.
5.2. 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.
6.0. NAT operational characteristics
Srisuresh & Holdrege [Page 20]
Internet Draft NAT Terminology and Considerations October 1998
NATs are application independent in that the translations are
limited to IP/TCP/UDP/ICMP headers and ICMP error messages only.
NATs do not change the payload of the packets, as payloads tend
to be application specific.
Due to their application independence, NATs are not considered
a hindrance to applications pursuing end-to-end transport and
application layer security. Applications that include IP
addresses in payload are an exception to this. However,
end-to-end IP network level security assured by current IPsec
techniques is not possible for the most part, as NATs modify
the IP header contents in transit. As an exception, IPsec
tunnel mode encryption and authentication are permissible so
long as the embedded packet contents are unaffected by the
outer IP header translation. Refer section 4.4 for details on
the use of IPsec with Host-NAT.
IPsec assumes the traditional IP address as the globally unique
ID and requires IP addresses to be unique. Yet, NATs fundamentally
operate on the premise of modifying the IP addresses. This
strongly restricts the use of IPsec and any other protocol which
includes an IP address in an end-to-end security association.
NATs also break the same fundamental assumption by public
key distribution infrastructures such as secure DNS and X.509
certificates with signed public keys. Integrity of a Security
Association (SA), identified by the tuple of (Destination Address,
SPI, secure protocol) may be jeopardized by the manipulation of
addresses by NAT. Tampering of addresses along the way by NAT
could break the authenticity of signed data and confidentiality
of encrypted data. For example, altering the IP header would
most certainly break the authentication assured by the AH header.
It would also break the authentication and confidentiality assured
by ESP header for the TCP/UDP packets. However, authentication and
confidentiality may be ascertained for non-TCP/UDP packets using
ESP header, so long as the payload is routing realm neutral.
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 a realm
and not the other.
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
Srisuresh & Holdrege [Page 21]
Internet Draft NAT Terminology and Considerations October 1998
may choose to include additional ALGs to custom support other
applications on the NAT device.
6.1. 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.
7.0. NAT limitations
7.1. Applications with IP-address Content
Not All applications lend themselves easily to address translation
by NATs. 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 NATs and ALGs will
not provide end-to-end security assured by IPsec. However, tunnel
mode IPsec can be accomplished with NAT 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.
7.2. Applications with inter-dependent control and data sessions
Srisuresh & Holdrege [Page 22]
Internet Draft NAT Terminology and Considerations October 1998
NATs 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.
7.3. 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.
7.4. 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. One option would be to drop
the fragments and send an ICMP error message to packet
originator. Alternately, NAT router could attempt to assemble
fragments 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.
7.5. 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
Srisuresh & Holdrege [Page 23]
Internet Draft NAT Terminology and Considerations October 1998
processing capacity of the NAT device exceeds line processing
rate, this should not be a problem.
8.0. Security Considerations
Many people view traditional NAT as a one-way (session) traffic
filter, restricting sessions from external hosts into their
machines. In addition, when address assignment in NAT is done
dynamically, that makes it harder for an attacker to point to
any specific host in the NAT domain. NATs may be used in
conjunction with firewalls to filter unwanted traffic.
If NATs 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 Host-NAT, end-to-end IP network level security
assured by current IPsec techniques is not attainable with NATs
in between. One of the ends must be a NAT box. Refer section 6.0
for a discussion on why end-to-end IPsec security cannot be
assured with NAT devices along the route.
The combination of NATs, ALGs and firewalls will provide a
transparent working environment for a private networking domain.
With the exception of Host-NAT, end-to-end network security
assured by IPsec cannot be attained for end-hosts within the
private network (Refer section 4.4 for Host-NAT 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 NATs
are part of a trusted boundary, tunnel mode IPsec can be
accomplished with NAT (or a combination of NAT, ALGs and
firewall) serving as tunnel end point.
NATs, 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 that NATs, ALGs and firewalls 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
Srisuresh & Holdrege [Page 24]
Internet Draft NAT Terminology and Considerations October 1998
could come from an address different from the target address
used by sender ([Ref 4]). NAT implementations that do not track
datagrams on a per-session basis but lump states of multiple UDP
sessions into a single state could compromise the security even
further.
2. Multicast sessions (UDP based) are another source for security
weaknesses.
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. NAT
implementations that use a single state to track the multicast
responses in a multicast session could potentially be the
target of security attacks. This multicast specific security
concern, however, is not unique to NAT implementations, and
exists across all hosts supporting multicast applications.
3. NATs can be a target for attacks.
Since NATs are Internet hosts they can be the target of a
number of different attacks, such as SYN flood and ping flood
attacks. NATs should employ the same sort of protection
techniques as Internet-based servers do.
REFERENCES
[1] Rekhter, Y., Moskowitz, B., Karrenberg, D., G. de Groot, and,
Lear, E. "Address Allocation for Private Internets", RFC 1918
[2] J. Reynolds and J. Postel, "Assigned Numbers", RFC 1700
[3] R. Braden, "Requirements for Internet Hosts -- Communication
Layers", RFC 1122
[4] R. Braden, "Requirements for Internet Hosts -- Application
and Support", RFC 1123
[5] F. Baker, "Requirements for IP Version 4 Routers", RFC 1812
[6] J. Postel, J. Reynolds, "FILE TRANSFER PROTOCOL(FTP)", RFC 959
[7] "TRANSMISSION CONTROL PROTOCOL (TCP) SPECIFICATION", RFC 793
[8] J. Postel, "INTERNET CONTROL MESSAGE PROTOCOL SPECIFICATION",
RFC 792
Srisuresh & Holdrege [Page 25]
Internet Draft NAT Terminology and Considerations October 1998
[9] J. Postel, "User Datagram Protocol (UDP)", RFC 768
[10] J. Mogul, J. Postel, "Internet Standard Subnetting Procedure",
RFC 950
[11] Brian carpenter, Jon Crowcroft, Yakov Rekhter, "IPv4 Address
Behavior Today", RFC 2101
[12] S. Kent, R. Atkinson, "Security Architecture for the Internet
Protocol", <draft-ietf-ipsec-arch-sec-05.txt> - Work in
progress.
[13] S. Kent, R. Atkinson, "IP Encapsulating Security Payload
(ESP)", <draft-ietf-ipsec-esp-v2-05.txt> - Work in progress.
[14] S. Kent, R. Atkinson, "IP Authentication Header",
<draft-ietf-ipsec-auth-header-06.txt> - Work in progress.
[15] D. Harkins, D. Carrel, "The Internet Key Exchange (IKE)",
<draft-ietf-ipsec-isakmp-oakley-08.txt> - Work in progress.
[16] D. Piper, "The Internet IP Security Domain of Interpretation
for ISAKMP", <draft-ietf-ipsec-ipsec-doi-09.txt> - Work in
progress.
Authors' Addresses
Pyda Srisuresh
Lucent technologies
4464 Willow Road
Pleasanton, CA 94588-8519
U.S.A.
Voice: (925) 737-2153
Fax: (925) 737-2110
EMail: suresh@ra.lucent.com
Matt Holdrege
Ascend Communications, Inc.
One Ascend Plaza
1701 Harbor Bay Parkway
Alameda, CA 94502
Voice: (510) 769-6001
Fax: (510) 814-2300
EMail: matt@ascend.com
Srisuresh & Holdrege [Page 26]
