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Versions: 00 01 02 03 RFC 2663
NAT Working Group P. Srisuresh
INTERNET-DRAFT Lucent Technologies
Category: Informational Matt Holdrege
Expire in six months Ascend Communications
June 1999
IP Network Address Translator (NAT) Terminology and Considerations
<draft-ietf-nat-terminology-03.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026. 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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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 IESG members
Scott Bradner, Vern Paxson and Thomas Narten for their detailed
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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.
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
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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.
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
a IPv4 environment and does not address the use of NAT in any 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
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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).
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.
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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 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.
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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 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,
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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.
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.
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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.
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.
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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.
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.
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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.
A NAPT router in figure 2 may be configured to translate sessions
originated from N-Pri into a single external address, say Addr-i.
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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
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
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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 :
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
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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.
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.
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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 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).
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Internet Draft NAT Terminology and Considerations June 1999
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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Internet Draft NAT Terminology and Considerations June 1999
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.
5.1. 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
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Internet Draft NAT Terminology and Considerations June 1999
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.
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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Internet Draft NAT Terminology and Considerations June 1999
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
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translation.
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 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 DNS, each DNS RRset is signed with a key from within the
zone. Moreover, the authenticity of a specific key is verified by
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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
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
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Internet Draft NAT Terminology and Considerations June 1999
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 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.
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Internet Draft NAT Terminology and Considerations June 1999
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.
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., 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
[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
Srisuresh & Holdrege [Page 29]
Internet Draft NAT Terminology and Considerations June 1999
Behavior Today", RFC 2101
[12] S. Kent, R. Atkinson, "Security Architecture for the Internet
Protocol", RFC 2401
[13] S. Kent, R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406
[14] S. Kent, R. Atkinson, "IP Authentication Header", RFC 2402
[15] D. Harkins, D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409
[16] D. Piper, "The Internet IP Security Domain of Interpretation
for ISAKMP", RFC 2407
[17] A. Heffernan, "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385
[18] D. Eastlake, "Domain Name System Security Extensions" RFC 2535
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
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