Network Working Group H. Levkowetz

Network Working Group H. Levkowetz
Request for Comments: 3519 ipUnplugged
Category: Standards Track S. Vaarala
Netseal
April 2003

Mobile IP Traversal of Network Address Translation (NAT) Devices

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

Mobile IP's datagram tunnelling is incompatible with Network Address
Translation (NAT). This document presents extensions to the Mobile
IP protocol and a tunnelling method which permits mobile nodes using
Mobile IP to operate in private address networks which are separated
from the public internet by NAT devices. The NAT traversal is based
on using the Mobile IP Home Agent UDP port for encapsulated data
traffic.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Problem description . . . . . . . . . . . . . . . . . . 3
1.3 Assumptions . . . . . . . . . . . . . . . . . . . . . . 4
2. NAT Traversal Overview. . . . . . . . . . . . . . . . . . . . 5
2.1 Basic Message Sequence. . . . . . . . . . . . . . . . . 5
3. New Message Formats . . . . . . . . . . . . . . . . . . . . . 6
3.1 UDP Tunnel Request Extension. . . . . . . . . . . . . . 6
3.1.1 F (Force) Flag. . . . . . . . . . . . . . . . . . 7
3.1.2 R (Registration through FA Required) flag . . . . 8
3.1.3 Reserved Fields . . . . . . . . . . . . . . . . . 8
3.1.4 Encapsulation . . . . . . . . . . . . . . . . . . 8
3.1.5 Mobile IP Registration Bits . . . . . . . . . . . 9
3.2 UDP Tunnel Reply Extension. . . . . . . . . . . . . . . 9
3.2.1 Reply Code. . . . . . . . . . . . . . . . . . . . 10

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3.3 MIP Tunnel Data Message . . . . . . . . . . . . . . . . 10
3.4 UDP Tunnelling Flag in Agent Advertisements . . . . . . 11
3.5 New Registration Reply Codes. . . . . . . . . . . . . . 12
4. Protocol Behaviour. . . . . . . . . . . . . . . . . . . . . . 12
4.1 Relation to standard MIP tunnelling . . . . . . . . . . 12
4.2 Encapsulating IP Headers in UDP . . . . . . . . . . . . 13
4.3 Decapsulation . . . . . . . . . . . . . . . . . . . . . 15
4.4 Mobile Node Considerations. . . . . . . . . . . . . . . 15
4.5 Foreign Agent Considerations. . . . . . . . . . . . . . 16
4.6 Home Agent Considerations . . . . . . . . . . . . . . . 18
4.6.1 Error Handling. . . . . . . . . . . . . . . . . . 19
4.7 MIP signalling versus tunnelling. . . . . . . . . . . . 20
4.8 Packet fragmentation. . . . . . . . . . . . . . . . . . 21
4.9 Tunnel Keepalive. . . . . . . . . . . . . . . . . . . . 21
4.10 Detecting and compensating for loss of NAT mapping. . . 22
4.11 Co-located registration through FA. . . . . . . . . . . 24
5. Implementation Issues . . . . . . . . . . . . . . . . . . . . 24
5.1 Movement Detection and Private Address Aliasing . . . . 24
5.2 Mobility Binding Lifetime . . . . . . . . . . . . . . . 25
6. Security Considerations . . . . . . . . . . . . . . . . . . . 26
6.1 Traffic Redirection Vulnerabilities . . . . . . . . . . 27
6.1.1 Manipulation of the Registration
Request Message . . . . . . . . . . . . . . . . . 27
6.1.2 Sending a Bogus Keepalive Message . . . . . . . . 27
6.2 Use of IPsec. . . . . . . . . . . . . . . . . . . . . . 28
6.3 Firewall Considerations . . . . . . . . . . . . . . . . 28
7. UNSAF Considerations. . . . . . . . . . . . . . . . . . . . . 28
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
9. Intellectual Property Rights. . . . . . . . . . . . . . . . . 30
10. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . 31
11. Normative References. . . . . . . . . . . . . . . . . . . . . 31
12. Informative References. . . . . . . . . . . . . . . . . . . . 32
13. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . 33
14. Full Copyright Statement. . . . . . . . . . . . . . . . . . . 34

1. Introduction

1.1 Terminology

The Mobile IP related terminology described in RFC 3344 [10] is used
in this document. In addition, the following terms are used:

Forward Tunnel
A tunnel that forwards packets towards the mobile node. It starts
at the home agent, and ends at the mobile node's care-of address.

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Reverse Tunnel
A tunnel that starts at the mobile node's care-of address and
terminates at the home agent.

NAT
Network Address Translation. "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." -- RFC
2663 [11]. Basic NAT and NAPT are two varieties of NAT.

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." -- RFC 2663 [11].

NAPT
Network Address Port Translation. "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." -- RFC 2663 [11].

In this document, the more general term NAT is used to cover both
NATs and NAPTs. In most deployment cases today, we believe that the
NATs used are of the NAPT variety.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [6].

1.2 Problem description

A basic assumption that Mobile IP [10] makes is that mobile nodes and
foreign agents are uniquely identifiable by a globally routable IP
address. This assumption breaks down when a mobile node attempts to
communicate from behind a NAT.

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Mobile IP relies on sending traffic from the home network to the
mobile node or foreign agent through IP-in-IP tunnelling. IP nodes
which communicate from behind a NAT are reachable only through the
NAT's public address(es). IP-in-IP tunnelling does not generally
contain enough information to permit unique translation from the
common public address(es) to the particular care-of address of a
mobile node or foreign agent which resides behind the NAT; in
particular there are no TCP/UDP port numbers available for a NAT to
work with. For this reason, IP-in-IP tunnels cannot in general pass
through a NAT, and Mobile IP will not work across a NAT.

Mobile IP's Registration Request and Reply will on the other hand be
able to pass through NATs and NAPTs on the mobile node or foreign
agent side, as they are UDP datagrams originated from the inside of
the NAT or NAPT. When passing out, they make the NAT set up an
address/port mapping through which the Registration Reply will be
able to pass in to the correct recipient. The current Mobile IP
protocol does however not permit a registration where the mobile
node's IP source address is not either the CoA, the Home Address, or
0.0.0.0.

What is needed is an alternative data tunnelling mechanism for Mobile
IP which will provide the means needed for NAT devices to do unique
mappings so that address translation will work, and a registration
mechanism which will permit such an alternative tunnelling mechanism
to be set up when appropriate.

This mechanism will address 3 different scenarios:

- A mobile node with co-located address behind a NAT; no FA

- A mobile node registered with an FA where both the mobile node and
the FA are behind the same NAT

- A mobile node with co-located address using an FA which demands
that registrations pass through the FA (sets the "R" bit) where
both the mobile node and the FA are behind the same NAT

1.3 Assumptions

The primary assumption in this document is that the network allows
communication between an UDP port chosen by the mobile node and the
home agent UDP port 434. If this assumption does not hold, neither
Mobile IP registration nor data tunnelling will work.

This document does NOT assume that mobility is constrained to a
common IP address space. On the contrary, the routing fabric between
the mobile node and the home agent may be partitioned into a

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"private" and a "public" network, and the assumption is that some
mechanism is needed in addition to vanilla Mobile IP according to RFC
3344 [10] in order to achieve mobility within disparate IP address
spaces.

For a more extensive discussion of the problems with disparate
address spaces, and how they may be solved, see RFC 3024 [9].

The reverse tunnels considered here are symmetric, that is, they use
the same configuration (encapsulation method, IP address endpoints)
as the forward tunnel.

2. NAT Traversal Overview

This section gives a brief overview of the MIP UDP tunnelling
mechanism which may be used to achieve NAT traversal for Mobile IP.

In MIP UDP tunnelling, the mobile node may use an extension
(described below) in its Registration Request to indicate that it is
able to use Mobile IP UDP tunnelling instead of standard Mobile IP
tunnelling if the home agent sees that the Registration Request seems
to have passed through a NAT. The home agent may then send a
registration reply with an extension indicating acceptance (or
denial). After assent from the home agent, MIP UDP tunnelling will
be available for use for both forward and reverse tunnelling. UDP
tunnelled packets sent by the mobile node use the same ports as the
registration request message. In particular, the source port may
vary between new registrations, but remains the same for all
tunnelled data and re-registrations. The destination port is always
434. UDP tunnelled packets sent by the home agent uses the same
ports, but in reverse.

2.1 Basic Message Sequence

The message sequence diagram below exemplifies setting up and using a
Mobile IP UDP tunnel as described in this document. The tunnel is
set up by the use of specific extensions in the initial Mobile IP
Registration Request and Reply exchange. Thereafter, any traffic
from the home agent to the mobile node is sent through the UDP
tunnel. The mobile node may at its discretion use the UDP tunnel for
reverse tunnelling or not, although in most cases where MIP UDP
tunnelling is needed, reverse tunnelling will also be needed.

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mobile node NAT home agent
| | |
| | |
| Registration | |
| Request with | |
|-----------------///--------------->>|
|UDP Tunnel Request| |
| | +
| | || IP Source and
| | || CCoA address
| | || discrepancy
| | || seen
| | Registration +
| | Reply with |
|<<---------------///-----------------|
| | UDP Tunnel Reply.|
| | |
| UDP tunnelled pkg| |
|=================///===============>>|
| | UDP tunnelled pkg|
|<<===============///=================|
| | ||absence of
| | ||traffic for
| | ||UDP keepalive
| UDP keepalive | ||period
|-----------------///--------------->>+
. . +
. . .
. . .

3. New Message Formats

3.1 UDP Tunnel Request Extension

This extension is a skippable extension. It signifies that the
sender is capable of handling MIP UDP tunnelling, and optionally that
a particular encapsulation format is requested in the MIP UDP tunnel.
The format of this extension is as shown below. It adheres to the
short extension format described in [10].

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type | Reserved 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|R| Reserved 2| Encapsulation | Reserved 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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Type 144

Length 6. Length in bytes of this extension, not
including the Type and Length bytes.

Sub-Type 0

Reserved 1 Reserved for future use. MUST be set to 0 on
sending, MUST be ignored on reception.

F F (Force) flag. Indicates that the mobile
node wants to force MIP UDP tunnelling to be
established.

R R (Registration through FA Required) flag.
Indicates that the R bit was set in the FA's
Agent Advertisement. Registration is being
made using a co-located care-of address, but
through the FA.

Reserved 2 Reserved for future use. MUST be set to 0 on
sending, MUST be ignored on reception.

Encapsulation Indicates the type of tunnelled data, using
the same numbering as the IP Header Protocol
Field.

Reserved 3 Reserved for future use. MUST be set to 0 on
sending, MUST be verified as 0 on receipt;
otherwise the extension must be handled as not
understood and silently skipped.

3.1.1 F (Force) Flag

Indicates that the mobile node wants to use traversal regardless of
the outcome of NAT detection performed by the home agent. This is
useful if the route between the mobile node and the home agent works
for Mobile IP signalling packets, but not for generic data packets
(e.g., because of firewall filtering rules). If the home agent
supports this protocol, it SHOULD either accept the traversal and
reply with a UDP Tunnel Reply Extension or reject the Registration
Request. In case of the registration failing, the Home Agent SHOULD
send a Registration Reply with Code field set to 129
("administratively prohibited").

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If the HA does not understand the UDP Tunnel Request Extension, it
will silently discard it, and if everything else is fine, it will
reply with a registration reply with reply code 0 (registration
accepted), but without any UDP Tunnel Reply Extension. In this case,
the mobile node MUST NOT use MIP UDP tunnelling.

3.1.2 R (Registration through FA Required) flag

This flag MUST be set by the mobile node when it is using a co-
located address, but registering through an FA because it has
received an Agent Advertisement with the 'R' bit set.

3.1.3 Reserved Fields

The 'Reserved 1' and 'Reserved 2' fields must be ignored on receipt,
while the 'Reserved 3' field must be 0 on receipt, otherwise this
extension MUST be handled as not understood and silently skipped.
This permits future additions to this extension to be made which
either can co-exist with old implementations, or will force a
rejection of the extension from an old implementation.

3.1.4 Encapsulation

The 'Encapsulation' field defines the mode of encapsulation requested
if MIP UDP tunnelling is accepted by the home agent. This field uses
the same values as the IP header Protocol field:

4 IP header (IP-in-UDP tunnelling) RFC 2003 [4]

47 GRE Header (GRE-in-UDP tunnelling) RFC 2784 [8]

55 Minimal IP encapsulation header RFC 2004 [5]

If the home agent finds that UDP tunnelling is not needed, the
encapsulation will be determined by the 'M' and 'G' flags of the
registration request; but if the home agent finds that MIP UDP
tunnelling should be done, the encapsulation is determined from the
value of the Encapsulation field. If the value of this field is
zero, it defaults to the value of 'M' and 'G' fields in the
Registration Request message, but if it is non-zero, it indicates
that a particular encapsulation is desired.

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3.1.5 Mobile IP Registration Bits

The Mobile IP registration bits S, B, D, M, G and T retain their
meaning as described in RFC 3344 [10] and RFC 3024 [9] (except that
the significance of the M and G bits may be modified by the
Encapsulation field when MIP UDP tunnelling is used, as described
above). The use of the M and G bits together with MIP UDP tunnelling
is also touched upon in Section 4.1.

When the MN requests MIP UDP tunnelling, the 'D' bit (Decapsulation
by the mobile node) MUST be set, otherwise UDP tunnelling would not
be meaningful.

Both the MN and the FA SHOULD set the 'T' bit when requesting MIP UDP
tunnelling, even if not all traffic will be reverse tunnelled. This
ensures that a HA which is not prepared to accept reverse tunnelling
will not accept a registration which may later turn out to be
unusable. Also see the discussion of use of the 'T' bit in Foreign
Agent Considerations (Section 4.5).

3.2 UDP Tunnel Reply Extension

This extension is a non-skippable extension. It is sent in reply to
a UDP Tunnel Request extension, and indicates whether or not the HA
will use MIP UDP tunnelling for the current mobility binding. The
format of this extension is as shown below.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type | Reply Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F| Reserved | Keepalive Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type 44

Length 6. Length in bytes of this extension, not
including the Type and Length bytes.

Sub-Type 0

Reply Code Indicates whether the HA assents or declines
to use UDP tunnelling for the current mobility
binding. See Section 3.2.1 below.

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F F (Forced) flag. Indicates that tunnelling is
being forced because the F flag was set in the
tunnelling request, irrespective of the
detection of a NAT or not.

Keepalive Interval Specifies the NAT keepalive interval that the
mobile node SHOULD use. A keepalive packet
SHOULD be sent if Keepalive Interval seconds
have elapsed without any signalling or data
traffic being sent. If this field is set to
0, the mobile node MUST use its default
configured keepalive interval.

Reserved Reserved for future use. MUST be set to 0 on
sending, MUST be ignored on reception.

3.2.1 Reply Code

The Reply Code field of the UDP Tunnel Reply Extension indicates if
UDP tunnelling have been accepted and will be used by the HA. Values
in the range 0 .. 63 indicate assent, i.e., that tunnelling will be
done, while values in the range 64 .. 255 indicate that tunnelling
should not be done. More information may be given by the value of
the response code.

The following response codes are defined for use in the code field of
the UDP Tunnel Reply Extension:

0 Will do tunnelling

64 Tunnelling declined, reason unspecified

3.3 MIP Tunnel Data Message

This MIP message header serves to differentiate traffic tunnelled
through the well-known port 434 from other Mobile IP messages, e.g.,
Registration Requests and Registration Replies.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Next Header | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type 4

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Next Header Indicates the type of tunnelled data, using
the same numbering as the IP Header Protocol
Field.

Reserved Reserved for future use. MUST be set to 0 on
sending, MUST be ignored on reception.

The Next Header field uses the same values as the IP header Protocol
field. Immediately relevant for use with Mobile IP are the following
values:

4 IP header (IP-in-UDP tunnelling) RFC 2003 [4]

47 GRE Header (GRE-in-UDP tunnelling) RFC 2784 [8]

55 Minimal IP encapsulation header RFC 2004 [5]

The receiver of a tunnelled packet MUST check that the Next Header
value matches the tunnelling mode established for the mobility
binding with which the packet was sent. If a discrepancy is
detected, the packet MUST be dropped. A log entry MAY be written,
but in this case the receiver SHOULD ensure that the amount of log
entries written is not excessive.

In addition to the encapsulation forms listed above, the MIP UDP
tunnelling can potentially support other encapsulations, by use of
the Next Header field in the MIP Tunnel Data Header and the
Encapsulation Header field of the UDP Tunnel Request Extension
(Section 3.1).

3.4 UDP Tunnelling Flag in Agent Advertisements

The only change to the Mobility Agent Advertisement Extension defined
in RFC 3344 [10] is a flag indicating that the foreign agent
generating the Agent Advertisement supports MIP UDP Tunnelling. The
flag is inserted after the flags defined in [10].

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |R|B|H|F|M|G|r|T|U| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| zero or more Care-of Addresses |
| ... |

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The flag is defined as follows:

U UDP Tunnelling support. This Agent supports MIP UDP Tunnelling
as specified in this document. This flag SHOULD be set in
advertisements sent by a foreign agent which supports MIP UDP
tunnelling and is situated behind a NAT. It MUST NOT be set in
advertisements from foreign agents which are not situated
behind a NAT, and thus has no need to advertise the capability.

3.5 New Registration Reply Codes

One new registration reply code is defined:

ERROR_HA_UDP_ENCAP_UNAVAIL Requested UDP tunnel encapsulation
unavailable

This is used by the HA to distinguish the registration denial caused
by an unavailable UDP tunnel encapsulation mode from a denial caused
by unavailable standard tunnel encapsulation requested by use of the
'T' bit together with either 'M' or 'G' bit.

4. Protocol Behaviour

4.1 Relation to standard MIP tunnelling

The default encapsulation mode for MIP UDP tunnelling is IP-in-UDP
encapsulation. The mobile node MAY request alternative forms of
encapsulation to be used with UDP tunnelling by setting the 'M' bit
and/or the 'G' bit of a Mobile IP registration request, or by
explicitly requesting a particular encapsulation for the MIP UDP
tunnel by using the Encapsulation field. The M and G bits retain the
meaning as described in RFC 3344 [10] within the context of MIP UDP
tunnelling. The UDP tunnelling version of the classic MIP
encapsulation methods can be summarised as:

IP in UDP. When Mobile IP UDP tunnelling is used, this is the
default encapsulation type. Any home agent and mobile node that
implements Mobile IP UDP tunnelling MUST implement this
encapsulation type.

GRE in UDP. If the 'G' bit is set in a registration request and
the Encapsulation field is zero, the mobile node requests that its
home agent use GRE encapsulation [3] for datagrams tunnelled to
the mobile node. If MIP UDP tunnelling is also requested and
accepted, GRE-in-UDP encapsulation SHALL be used in the same cases
as GRE in IP encapsulation would be used if the MIP UDP tunnelling
had not been requested.

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Minimal encapsulation in UDP. If the 'M' bit is set and the
Encapsulation field is zero, the mobile node requests that its
home agent use minimal encapsulation [5] for datagrams tunnelled
to the mobile node. If MIP UDP tunnelling is also used, minimal
encapsulation in UDP SHALL be used in the same cases as minimal
encapsulation according to RFC 2004 [5] would be used if the MIP
UDP tunnelling had not been requested.

When the Encapsulation field is non-zero, a particular encapsulation
format is requested for the MIP UDP tunnel. If tunnelling is
indicated, the request MUST either be accepted using the requested
encapsulation, or rejected with the error code
ERROR_HA_UDP_ENCAP_UNAVAIL, "Requested UDP tunnel encapsulation
unavailable" defined in Section 3.5. On receipt of this error, the
mobile node MAY choose to send a new Registration Request with
different requirements on MIP UDP tunnelling encapsulation.

4.2 Encapsulating IP Headers in UDP

MIP IP-in-UDP tunnelling, or UDP tunnelling for short, is done in a
manner analogous to that described for IP-in-IP tunnelling in RFC
2003 [4], with the exception of the addition of an UDP header [1] and
MIP Message header [10] between the outer and inner IP header.

Mobile IP Registration Requests and Registration Replies are already
in the form of UDP messages, and SHALL NOT be tunnelled even when MIP
IP-in-UDP tunnelling is in force.

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To encapsulate an IP datagram using MIP IP-in-UDP encapsulation, an
outer IP header [2], UDP header [1] and MIP Message header [10] is
inserted before the datagram's existing IP header, as follows:

+---------------------------+
| |
| Outer IP Header |
+---------------------------+
| |
| UDP Header |
+---------------------------+
| MIP Tunnel Data |
| Message Header |
+---------------------------+ +---------------------------+
| | | |
| IP Header | | IP Header |
+---------------------------+ ====> +---------------------------+
| | | |
| IP Payload | | IP Payload |
| | | |
| | | |
+---------------------------+ +---------------------------+

The outer IP header Source Address and Destination Address, together
with the UDP header Source Port and Destination Port, identify the
"endpoints" of the tunnel. The inner IP header Source Address and
Destination Addresses identify the original sender and the recipient
of the datagram, respectively. The inner IP header is not changed by
the encapsulator, except to decrement the TTL by one if the
tunnelling is being done as part of forwarding the datagram as noted
in RFC 2003 [4], and remains unchanged during its delivery to the
tunnel exit point. No change to IP options in the inner header
occurs during delivery of the encapsulated datagram through the
tunnel. Note that the security options of the inner IP header MAY
affect the choice of security options for the encapsulating (outer)
IP header.

Minimal Encapsulation and GRE encapsulation is done in an analogous
manner, following RFC 2004 [5] for Minimal Encapsulation and RFC 2784
[8] for GRE Encapsulation, but using outer IP, UDP and MIP headers in
place of the outer IP header.

All other provisions and requirements of RFC 2003 [4] and RFC 3024
[9] are in force, except in one respect, as covered in Packet
Fragmentation (Section 4.8).

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4.3 Decapsulation

Before decapsulation is actually done, the decapsulating node MUST
verify that the outer IP addresses and UDP port numbers exactly match
the values used for the tunnel, with the exception of tunnels that
are "half bound" (as described in Section 4.11) where the source UDP
port can change.

IP-in-UDP encapsulated traffic is decapsulated simply by stripping
off the outer IP, UDP and MIP header, which leaves the original IP
packet which is forwarded as is.

Minimal IP encapsulation is processed by the receiver conceptually as
follows. First, the UDP and the Mobile IP headers are removed from
the packet, and the Protocol field of the IP header replaced with the
Next Header field in the MIP Tunnel Data header. Second, the
remaining IP header total length and checksum are adjusted to match
the stripped packet. Third, ordinary minimal IP encapsulation
processing is done.

GRE encapsulated traffic is processed according to RFC 2784 [8] and
RFC 1701 [3], with the delivery header consisting of the outer IP,
UDP and MIP headers.

4.4 Mobile Node Considerations

The UDP Tunnel Request Extension MAY be used in a Mobile IP
Registration Request from the mobile node to the home agent when the
mobile node uses a co-located care-of address. It SHALL NOT be used
by the mobile node when it is registering with a foreign agent care-
of address.

The purpose of this extension is to indicate to the home agent that
the mobile node is able to accept MIP UDP tunnelling if the home
agent has an indication that the mobile node resides behind a NAT or
NAPT. It thus functions as a conditional solicitation for the use of
MIP UDP tunnelling.

As per Section 3.2 and 3.6.1.3 of RFC 3344 [10], the mobile node MUST
place this Extension before the Mobile-Home Authentication Extension
in registration messages, so that it is covered by the Mobile-Home
Authentication Extension.

If the mobile node includes the UDP Tunnel Request extension in a
registration request, but receives a registration reply without a UDP
Tunnel Reply extension, it MUST assume that the HA does not

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understand this extension, and it MUST NOT use UDP tunnelling. If
the mobile node is in fact behind a NAT, the registration may then
succeed, but traffic will not be able to traverse the NAT.

When the mobile node sends MIP UDP tunnelled data, it MUST use the
same UDP source port as was used for the most recent registration
request.

When the mobile node re-registers without having moved, it SHOULD
take care to use the same source port as was used for the original
registration of the current mobility binding. Otherwise, while the
home agent would change destination port on acceptance of the new
registration, and the mobile node would presumably start listening on
the new port, the packets in flight from the home agent at the time
of change will be dropped when arriving at the mobile node's old
port. (This does not mean that the home agent should refuse a
registration request using MIP UDP tunnelling where a new port have
been used, as this might be the result of the NAT dropping state, the
mobile node re-booting, changing interface, etc.)

If a mobile node is registering through a foreign agent but using a
co-located care-of address, and the agent advertisement from the
foreign agent had the 'U' bit set, the mobile node MUST set the 'R'
flag in its UDP Tunnel Request Extension, in order to make the HA use
MIP UDP tunnelling. In this case, the mobile node also MUST send a
keepalive as soon as its registration has been accepted.

If a mobile node is registering through a foreign agent but using a
co-located care-of address, and the agent advertisement from the
foreign agent does not have the 'U' bit set, the mobile node MUST NOT
include a UDP Tunnel Request Extension in the registration request.

4.5 Foreign Agent Considerations

The UDP Tunnel Request Extension MAY be used by a foreign agent when
it is forwarding a Mobile IP Registration Request to a home agent,
when the foreign agent is situated behind a NAT or has some other
compelling reason to require MIP UDP tunnelling.

The purpose of this extension is to indicate to the home agent that
the foreign agent is able to accept MIP UDP tunnelling if the home
agent has an indication that the foreign agent resides behind a NAT
or NAPT. It thus functions as a conditional solicitation for the use
of MIP UDP tunnelling.

A foreign agent which requires the mobile node to register through a
foreign agent by setting the 'R' bit in the agent advertisement, MUST
NOT add the UDP Tunnel Request Extension when forwarding a

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registration request which uses a co-located care-of address, as this
will lead to a UDP tunnel being set up from the home agent to the
foreign agent instead of to the mobile node.

As per Section 3.2 and 3.7.2.2 of RFC 3344 [10], the foreign agent
when using this extension MUST place it after the Mobile-Home
Authentication Extension in registration messages. If the foreign
agent shares a mobility security association with the home agent and
therefore appends a Foreign-Home Authentication Extension, the UDP
Tunnel Request Extension MUST be placed before the Foreign-Home
Authentication Extension.

As the home agent detects the presence of a NAT in the path between
the sender and itself by seeing a mismatch between the IP source
address and the care-of address given in the registration request, it
is REQUIRED that the foreign agent, when using this extension, sends
the registration request with an IP source address matching the
care-of address.

A foreign agent using MIP UDP tunnelling to a home agent because the
FA is situated behind a NAT may be configured to encourage reverse
tunnelling, or be neutral about it, depending on the characteristics
of the NAT. If the NAT translates all source addresses of outgoing
packets to its own public address, it will not be possible to
maintain sessions when moving away from this network if the mobile
node has used triangular routing instead of reverse tunnelling. On
the other hand, if it is known that the NAT is smart enough to not
translate publicly routable source addresses, AND does not do ingress
filtering, triangular routing may succeed. The leg from the home
agent to the foreign agent will still use MIP UDP tunnelling to pass
through the NAT.

Therefore, if it is known when configuring a foreign agent behind a
NAT that the NAT will translate public as well as private addresses,
or it is known that ingress filtering is being done between the
private and public networks, the foreign agent SHOULD reply to
registration requests which don't have the 'T' bit set with a reply
code 75, "reverse tunnel is mandatory and 'T' bit not set".

Conversely, if it is known that the NAT is smart about not
translating public addresses, and no ingress filtering is done, so it
is reasonable to believe that a mobile node with a publicly routable
address may be able to keep up sessions when moving to or from this
network, the foreign agent MAY be configured to forward registration
requests even if they don't have the 'T' bit set.

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If the behaviour of the NAT is unknown in this respect, it SHOULD be
assumed that it will translate all addresses, thus the foreign agent
SHOULD be configured to reply to registration requests which don't
have the 'T' bit set with a reply code 75, "reverse tunnel is
mandatory and 'T' bit not set".

4.6 Home Agent Considerations

The purpose of the MIP UDP Tunnel Reply Extension is to indicate
whether or not the home agent accepts the use of MIP UDP tunnelling
for this mobility binding, and to inform the mobile node or foreign
agent of the suggested tunnel keepalive interval to be used.

The UDP Tunnel Reply Extension MUST be used in a Mobile IP
Registration Reply from the home agent to the mobile node when it has
received and accepted a UDP Tunnel Request (Section 3.1) from a
mobile node.

The home agent MUST use a mismatch between source IP address and
care-of address in the Mobile IP Registration Request message as the
indication that a mobile node may reside behind a NAT. If the
Registration Request also contains the UDP Tunnel Request extension
without the 'R' flag set, and the home agent is capable of, and
permits MIP UDP tunnelling, the home agent SHALL respond with a
registration reply containing an assenting UDP Tunnel Reply Extension
as described in Section 3.2. If the 'R' flag is set, special
considerations apply, as described below.

If the home agent receives a Registration Request with matching
source IP address and co-located care-of address which contains a MIP
UDP Tunnel Request Extension, the home agent SHOULD respond with a
Registration Reply containing a declining UDP Tunnel Reply - unless
tunnelling has been explicitly requested by the mobile node using the
'F' flag as described in Section 3.1.

If the home agent assents to UDP tunnelling, it MUST use the source
address of the registration request as the effective care-of address,
rather than the care-of address given in the registration request,
except in the case where the 'R' flag is set in the UDP Tunnel
Request Extension.

If the home agent receives a Registration Request with the 'R' flag
set in the UDP Tunnel Request Extension, it SHOULD reply with an
assenting UDP Tunnel Reply Extension if it is capable of, and permits
MIP UDP tunnelling. In this case, however, the source address and
port of the registration request may be a NAT'ed version of the
foreign agent source address and port. In order to direct tunnelled
traffic correctly to the mobile node, the home agent MUST wait for

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the first keepalive packet from the mobile node to arrive, before it
can send traffic back to the correct NAT port (the one which is
mapped to the MN). In this case, the home agent MUST check that the
outer source address (but not the port) of this keepalive packet is
identical with the source address of the corresponding registration
request. The inner source address (that of the encapsulated ICMP
echo request) MUST be the home address of the mobile node, and the
inner destination address MUST be that of the home agent. If all
this holds, the outer source address and port of this keepalive
packet SHALL be used by the HA as the outer destination address and
port of the MIP UDP tunnel when forwarding traffic to the mobile
node.

The home agent SHOULD be consistent in acknowledging support for UDP
tunnelling or not. A home agent which understands the UDP Tunnel
Request Extension and is prepared to respond positively to such a
request SHOULD also respond with a UDP Tunnel Reply Extension
containing a declining reply code if use of MIP UDP tunnelling is not
indicated for a session. The mobile node MUST NOT assume such
behaviour from the home agent, since the home agent may undergo a
software change with reboot, a policy change or a replacement; and
consequently a change of behaviour.

4.6.1 Error Handling

The following actions take place when things go wrong.

The HA does not support the UDP Tunnel Request extension:

The home agent ignores the extension and proceeds normally, which
would be to refuse the registration if the IP source address does
not match the care-of address, the home address or 0.0.0.0. Even
if the HA mistakenly does accept the registration, the mobile node
will not be able to receive forward tunnelled data if it is behind
a NAT.

(It would be beneficial to have the mobile node de-register in
this case. The mobile node, however, normally has no way of
telling that it is behind a NAT if it does not receive a UDP
Tunnelling Reply.)

NAT detected by home agent, but traversal not allowed:

In some cases the home agent may disable NAT traversal even though
it supports the UDP Tunnel Request extension and a NAT is
detected. In this case, the home agent SHOULD send a Registration
Reply with the Code field set to 129, "administratively
prohibited".

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NAT not detected, 'F' flag set, but home agent does not allow forced
use of MIP UDP tunnelling:

The home agent SHOULD send a Registration Reply with the Code
field set to 129, "administratively prohibited".

UDP Tunnel Request extension sent by the mobile node (placed before
the MN-HA authentication extension), but 'D' bit in registration
request header not set:

The home agent SHOULD send a Registration Reply with the Code
field set to 134, "poorly formed Request".

UDP Tunnel Request extension sent by the foreign agent (placed after
the MN-HA authentication extension), but 'D' bit is set:

The home agent SHOULD send a Registration Reply with the Code
field set to 134, "poorly formed Request".

Reserved 3 field of UDP Tunnel Request extension is nonzero:

The home agent SHOULD send a Registration Reply with the Code
field set to 134, "poorly formed Request".

Encapsulation type requested in UDP Tunnel Request extension is
unsupported:

The home agent SHOULD send a Registration Reply with the Code
field set to ERROR_HA_UDP_ENCAP_UNAVAIL, "Requested UDP tunnel
encapsulation unavailable" defined in Section 3.5.

4.7 MIP signalling versus tunnelling

UDP tunnelling SHALL be used only for data packets, and only when the
mobility binding used for sending was established using the UDP
Tunnel Request, and accepted by an UDP Tunnel Reply from the home
agent. After MIP UDP tunnelling has been established for a mobility
binding, data packets that are forward or reverse tunnelled using
this mobility binding MUST be tunnelled using MIP UDP tunnelling, not
IP-in-IP tunnelling or some other tunnelling method.

As a consequence:

- Mobile IP signalling is never tunnelled.

- When using simultaneous bindings, each binding may have a
different type (i.e., UDP tunnelling bindings may be mixed with
non-UDP tunnelling bindings).

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- Tunnelling is only allowed for the duration of the binding
lifetime.

4.8 Packet fragmentation

From RFC 3022 [12]:

"Translation of outbound TCP/UDP fragments (i.e., those originating
from private hosts) in NAPT set-up are doomed to fail. The reason is
as follows. Only the first fragment contains the TCP/UDP header that
would be necessary to associate the packet to a session for
translation purposes. Subsequent fragments do not contain TCP/UDP
port information, but simply carry the same fragmentation identifier
specified in the first fragment. Say, two private hosts originated
fragmented TCP/UDP packets to the same destination host. And, they
happened to use the same fragmentation identifier. When the target
host receives the two unrelated datagrams, carrying same
fragmentation id, and from the same assigned host address, it is
unable to determine which of the two sessions the datagrams belong
to. Consequently, both sessions will be corrupted."

Because of this, if the mobile node or foreign agent for any reason
needs to send fragmented packets, the fragmentation MUST be done
prior to the encapsulation. This differs from the case for IP-in-IP
tunnelling, where fragmentation may be done before or after
encapsulation, although RFC 2003 [4] recommends doing it before
encapsulation.

A similar issue exists with some firewalls, which may have rules that
only permit traffic on certain TCP and UDP ports, and not arbitrary
outbound (or inbound) IP traffic. If this is the case and the
firewall is not set to do packet reassembly, a home agent behind a
firewall will also have to do packet fragmentation before MIP UDP
encapsulation. Otherwise, only the first fragment (which contains
the UDP header) will be allowed to pass from the home agent out
through the firewall.

For this reason, the home agent SHOULD do packet fragmentation before
it does MIP UDP encapsulation.

4.9 Tunnel Keepalive

As the existence of the bi-directional UDP tunnel through the NAT is
dependent on the NAT keeping state information associated with a
session, as described in RFC 2663 [11], and as the NAT may decide
that the session has terminated after a certain time, keepalive
messages may be needed to keep the tunnel open. The keepalives
should be sent more often than the timeout value used by the NAT.

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This timeout may be assumed to be a couple of minutes, according to
RFC 2663 [11], but it is conceivable that shorter timeouts may exist
in some NATs.

For this reason the extension used to set up the UDP tunnel has the
option of setting the keepalive message interval to another value
than the default value, see Section 3.2.

The keepalive message sent MUST consist of a properly UDP
encapsulated ICMP echo request directed to the home agent.

For each mobility binding which has UDP tunnelling established, the
non-HA endpoint of the Mobile-IP UDP tunnel MUST send a keepalive
packet if no other packet to the HA has been sent in K seconds. Here
K is a parameter with a default value of 110 seconds. K may be set
to another value by the HA as described in the UDP tunnelling reply
extension (Section 3.2).

Except for the case where the mobile node registers with a co-located
address through an FA (see Section 4.11) MIP UDP tunnelling is done
using the same ports that have already been used for the registration
request / reply exchange. The MN or FA will send its first keepalive
message at the earliest K seconds after the registration request was
sent. The same UDP source port MUST be used for the keepalive
messages as was used for the original Registration Messages and for
data messages.

The remote UDP tunnel endpoint MUST use two-way keepalives consisting
of UDP encapsulated ICMP Echo Request/Reply messages. The rationale
for using two-way keepalives is two-fold:

1. Two-way keepalives allow the mobile node to detect loss of a NAT
mapping. Detection of NAT mapping loss in turn allows the MN to
compensate by re-registering and using a shorter keepalive to
avoid loss of NAT mappings in the future.

2. One-way keepalives (keepalives sent by MN or FA, but without any
reply from the home agent) actually cause more keepalive traffic
overhead; the keepalive messages have to be sent more frequently
to compensate for occasional loss of keepalive messages. In
contrast, two-way keepalives are acknowledged, and retransmissions
occur only when a response is not received for a keepalive request
within a reasonable time.

4.10 Detecting and compensating for loss of NAT mapping

When a mobile node is using UDP encapsulated ICMP Echo Request/Reply
messages as keepalives, it will have to deal with the possibility

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that a NAT mapping is lost by a NAT device. The crucial thing here
is of course not the loss of the NAT mapping in itself; but rather
that the home agent, in the absence of a Registration Request through
the new mapping, will continue to send traffic to the NAT port
associated with the old mapping.

If the mobile node does not get a reply to its UDP encapsulated ICMP
Echo Request even after a number of retransmissions, and is still
connected to the same router that was used to establish the current
mobility binding, the mobile node SHOULD re-register with the home
agent by sending an Registration Request. This lets the HA know
about the new NAT mapping and restores connectivity between mobile
node and home agent.

Having established a new mobility binding, the mobile node MAY use a
shorter keepalive interval than before the NAT mapping was lost; in
particular, the mobile node MAY deviate from the keepalive interval
assigned by the home agent. If the binding loss continues to occur,
the mobile node may shorten the keepalive interval each time it re-
registers, in order to end up with a keepalive interval that is
sufficient to keep the NAT mapping alive. The strategy used to
arrive at a keepalive interval when a NAT mapping is lost is
implementation dependent. However, the mobile node MUST NOT use a
keepalive less than 10 seconds.

Note that the above discussion only applies when the mobile node is
re-registering through the same router, and thus presumably through
the same NAT device that lost a NAT mapping earlier. If the MN moves
and still finds itself behind a NAT, it SHOULD return to its original
keepalive interval (the default value, or as assigned by the home
agent) and it SHOULD NOT do any keepalive interval compensation
unless it discovers a loss of NAT mapping in the new environment.

The home agent MUST NOT attempt to detect or compensate for NAT
binding loss by dynamically changing the keepalive interval assigned
in the Registration Reply; the home agent does not have enough
information to do this reliably and should thus not do it at all.
The mobile node is in a much better position to determine when a NAT
mapping has actually been lost. Note also that a MN is allowed to
let a NAT mapping expire if the MN no longer needs connectivity.

The discussion above does only in a limited sense apply to a foreign
agent which is situated behind a NAT and using MIP UDP tunnelling.
In this case, it is a matter of permanently configuring the FA to use
a keepalive interval which is lower than the NAT mapping lifetime,
rather than trying to dynamically adapt to the binding lifetimes of
different NATs.

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4.11 Co-located registration through FA

This section summarizes the protocol details which have been
necessary in order to handle and support the case when a mobile node
registers with a co-located address through a foreign agent, due to
the FA advertisements having the 'R' bit set. It gives background
information, but lists no new requirements.

When a mobile registers a co-located care-of address through an FA,
the registration request which reaches the HA will have a different
care-of address in the registration request compared to the source
address in the registration request IP-header. If the registration
request also contains a UDP Tunnel Request Extension, the HA will
erroneously set up a UDP tunnel, which will go to the FA instead of
the MN. For this reason, as mentioned in Section 4.4, the mobile
node must not include a UDP Tunnel Request Extension in the
registration if it is registering a co-located address through an FA
which does not have the 'U' bit set in its advertisements.

In order to still be able to use MIP UDP tunnelling in this case,
foreign agents which are situated behind a NAT are encouraged to send
advertisements which have the 'U' bit set, as described in Section
3.4.

If the FA advertisement has the 'U' bit set, indicating that it is
behind a NAT, and also the 'R' bit set, and the mobile node wishes to
use a co-located care-of address, it MUST set the 'R' flag in the UDP
Tunnel Request Extension, in order to inform the HA of the situation
so that it may act appropriately, as described in Section 4.4.

Because the UDP tunnel is now taking another path than the
registration requests, the home agent, when handling registrations of
this type, must wait till the arrival of the first keepalive packet
before it can set up the tunnel to the correct address and port. To
reduce the possibility of tunnel hijacking by sending a keepalive
with a phony source address, it is required that only the port of the
keepalive packet may be different from that of the registration
request; the source address must be the same. This means that if the
FA and MN are communicating with the HA through different NATs, the
connection will fail.

5. Implementation Issues

5.1 Movement Detection and Private Address Aliasing

In providing a mobile node with a mechanism for NAT traversal of
Mobile IP traffic, we expand the address space where a mobile node
may function and acquire care-of addresses. With this comes a new

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problem of movement detection and address aliasing. We here have a
case which may not occur frequently, but is mentioned for
completeness:

Since private networks use overlapping address spaces, they may be
mistaken for one another in some situations; this is referred to as
private address aliasing in this document. For this reason, it may
be necessary for mobile nodes implementing this specification to
monitor the link layer address(es) of the gateway(s) used for sending
packets. A change in the link layer address indicates probable
movement to a new network, even if the IP address remains reachable
using the new link layer address.

For instance, a mobile node may obtain the co-located care-of address
10.0.0.1, netmask 255.0.0.0, and gateway 10.255.255.254 using DHCP
from network #1. It then moves to network #2, which uses an
identical addressing scheme. The only difference for the mobile node
is the gateway's link layer address. The mobile node should store
the link layer address it initially obtains for the gateway (using
ARP, for instance). The mobile node may then detect changes in the
link layer address in successive ARP exchanges as part of its
ordinary movement detection mechanism.

In rare cases the mobile nodes may not be able to monitor the link
layer address of the gateway(s) it is using, and may thus confuse one
point of attachment with another. This specification does not
explicitly address this issue. The potential traffic blackout caused
by this situation may be limited by ensuring that the mobility
binding lifetime is short enough; the re-registration caused by
expiration of the mobility binding fixes the problem (see Section
5.2).

5.2 Mobility Binding Lifetime

When responding to a registration request with a registration reply,
the home agent is allowed to decrease the lifetime indicated in the
registration request, as covered in RFC 3344 [10]. When using UDP
tunnelling, there are some cases where a short lifetime is
beneficial.

First, if the NAT mapping maintained by the NAT device is dropped, a
connection blackout will arise. New packets sent by the mobile node
(or the foreign agent) will establish a new NAT mapping, which the
home agent will not recognize until a new mobility binding is
established by a new registration request.

A second case where a short lifetime is useful is related to the
aliasing of private network addresses. In case the mobile node is

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not able to detect mobility and ends up behind a new NAT device (as
described in Section 5.1), a short lifetime will ensure that the
traffic blackout will not be exceedingly long, and is terminated by a
re-registration.

The definition of "short lifetime" in this context is dependent on
the requirements of the usage scenario. Suggested maximum lifetime
returned by the home agent is 60 seconds, but in case the
abovementioned scenarios are not considered a problem, longer
lifetimes may of course be used.

6. Security Considerations

The ordinary Mobile IP security mechanisms are also used with the NAT
traversal mechanism described in this document. However, there is
one noticeable change: the NAT traversal mechanism requires that the
HA trust unauthenticated address (and port) fields possibly modified
by NATs.

Relying on unauthenticated address information when forming or
updating a mobility binding leads to several redirection attack
vulnerabilities. In essence, an attacker may do what NATs do, i.e.,
modify addresses and ports and thus cause traffic to be redirected to
a chosen address. The same vulnerabilities apply to both MN-HA and
FA-HA NAT traversal.

In more detail: without a NAT, the care-of address in the
registration request will be directly used by the HA to send traffic
back to the MN (or the FA), and the care-of address is protected by
the MN-HA (or FA-HA) authentication extension. When communicating
across a NAT, the effective care-of address from the HA point of view
is that of the NAT, which is not protected by any authentication
extension, but inferred from the apparent IP source address of
received packets. This means that by using the mobile IP
registration extensions described in this document to enable
traversal of NATs, one is opening oneself up to having the care-of
address of a MN (or a FA) maliciously changed by an attacker.

Some, but not all, of the attacks could be alleviated to some extent
by using a simple routability check. However, this document does not
specify such a mechanism for simplicity reasons and because the
mechanism would not protect against all redirection attacks. To
limit the duration of such redirection attacks, it is RECOMMENDED to
use a conservative (that is, short) mobility binding lifetime when
using the NAT traversal mechanism specified in this document.

The known security issues are described in the sections that follow.

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6.1 Traffic Redirection Vulnerabilities

6.1.1 Manipulation of the Registration Request Message

An attacker on the route between the mobile node (or foreign agent)
and the home agent may redirect mobility bindings to a desired
address simply by modifying the IP and UDP headers of the
Registration Request message. Having modified the binding, the
attacker no longer needs to listen to (or manipulate) the traffic.
The redirection is in force until the mobility binding expires or the
mobile node re-registers.

This vulnerability may be used by an attacker to read traffic
destined to a mobile node, and to send traffic impersonating the
mobile node. The vulnerability may also be used to redirect traffic
to a victim host in order to cause denial-of-service on the victim.

The only defense against this vulnerability is to have a short time
between re-registrations, which limits the duration of the
redirection attack after the attacker has stopped modifying
registration messages.

6.1.2 Sending a Bogus Keepalive Message

When registering through an FA using a co-located care-of address,
another redirection vulnerability opens up. Having exchanged
Registration Request/Reply messages with the HA through the FA, the
MN is expected to send the first keepalive message to the HA, thus
finalizing the mobility binding (the binding will remain in a "half
bound" state until the keepalive is received).

Having observed a Registration Request/Reply exchange, an attacker
may send a bogus keepalive message assuming that the mobility binding
is in the "half bound" state. This opens up a similar redirection
attack as discussed in Section 6.1.1. Note, however, that the
attacker does not need to be able to modify packets in flight; simply
being able to observe the Registration Request/Reply message exchange
is sufficient to mount the attack.

With this in mind, the home agent MUST NOT accept a keepalive message
from a different source IP address than where the Registration
Request came from, as specified in Section 4.6. This requirement
limits the extent of the attack to redirecting the traffic to a bogus
UDP port, while the IP address must remain the same as in the initial
Registration Request.

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The only defenses against this vulnerability are: (1) to have a short
time between re-registrations, which limits the duration of the
redirection attack after the attacker has stopped sending bogus
keepalive messages, and (2) to minimize the time the binding is in a
"half bound" state by having the mobile node send the first keepalive
message immediately after receiving an affirmative registration
reply.

6.2 Use of IPsec

If the intermediate network is considered insecure, it is recommended
that IPsec be used to protect user data traffic. However, IPsec does
not protect against the redirection attacks described previously,
other than to protect confidentiality of hijacked user data traffic.

The NAT traversal mechanism described in this document allows all
IPsec-related traffic to go through NATs without any modifications to
IPsec. In addition, the IPsec security associations do not need to
be re-established when the mobile node moves.

6.3 Firewall Considerations

This document does not specify a general firewall traversal
mechanism. However, the mechanism makes it possible to use only a
single address and a port for all MN-HA (or FA-HA) communication.
Furthermore, using the same port for the MIP UDP tunnelled traffic as
for control messages makes it quite probable that if a MIP
registration can reach the home agent, MIP tunnelling and reverse
tunnelling using the described mechanism will also work.

7. UNSAF Considerations

The mechanism described in this document is not an "UNilateral Self-
Address Fixing" (UNSAF) mechanism. Although the mobile node makes no
attempt to determine or use the NAT translated address, the mobile
node through the registration process does attempt to keep the NAT
mapping alive through refresh messages. This section attempts to
address issues that may be raised through this usage through the
framework of the unsaf considerations IAB document [13].

1. Precise definition.
This proposal extends the Mobile IP v4 registration process to
work across intervening NATs. The Home Agent detects the presence
of the NAT by examining the source address in the packet header
and comparing it with the address contained in the registration
message.

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The NAT address and port detected by the home agent are not
exported or communicated to any other node anywhere.

2. Exit strategy.
This mechanism will go out of use as IPv6 and Mobile IP v6 is
deployed, obviating the need for MIPv4 NAT traversal.

It can also be noted that this mechanism makes no changes to the
base MIPv4 protocol which makes it dependent on the presence of
NATs or the current extensions - i.e., no additional protocol
changes would be needed if NATs were to go away.

3. Issues making systems more brittle.
The specific issue which is relevant here is that the effective
care-of address (being the source address in the IP header
received by the HA) is not protected by the Mobile IP
authentication extension, and therefore may be spoofed. This is
discussed in some detail in Section 6, Security Considerations.

4. Requirements for longer term solutions.
The trivial long term solution is a transition to an environment
where NATs are not required. The most obvious such environment
would be an IPv6 based internet.

In the presence of NATs, an improved solution would require

* the ability to discover the translations done by each NAT along
the route

* the ability to validate the authority of each NAT to do those
translations

* communicating as part of the signed registration request the
address of the NAT closest to the HA, for use as the effective
care-of address from the viewpoint of the HA.

* configuration of all intermediate NATs to accept only packets
from the neighbour NATs.

5. Impact on existing, deployed NATs.
One precursor of the mechanism described here has been used
successfully across deployed NATs in Sweden, Germany, England,
Japan and the USA, without necessitating neither adjustments of
the NATs in question, nor adjustment of any protocol parameters.
At the time of writing, little experience exist with the exact
implementation proposed in this document, but effort has been put
into making this mechanism even more robust and adaptive than its
precursors.

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RFC 3519 NAT Traversal for Mobile IP April 2003

With respect to the base Mobile IP specification, the impact of
this document is that an increased frequency of registration
requests is recommended from a security perspective when the NAT
traversal mechanism is used.

8. IANA Considerations

The numbers for the extensions defined in this document have been
taken from the numbering space defined for Mobile IP messages,
registration extensions and error codes defined in RFC 3344 [10].
This document proposes one new message, two new extensions and a new
error code that require type numbers and an error code value that
have been assigned by IANA. The two new extensions also introduce
two new sub-type numbering spaces to be managed by IANA.

Section 3.1 defines a new Mobile IP extension, the UDP Tunnel Request
Extension. The type number for this extension is 144. This
extension introduces a new sub-type numbering space where the value 0
has been assigned to this extension. Approval of new Tunnel Request
Extension sub-type numbers is subject to Expert Review, and a
specification is required [7].

Section 3.2 defines a new Mobile IP extension, the UDP Tunnel Reply
Extension. The type value for this extension is 44. This extension
introduces a new sub-type numbering space where the value 0 has been
assigned to this extension. Approval of new Tunnel Reply Extension
sub-type numbers is subject to Expert Review, and a specification is
required [7].

Section 3.3 defines a new Mobile IP message, the Tunnel Data message.
The type value for this message is 4.

Section 3.5 defines a new error code, ERROR_HA_UDP_ENCAP_UNAVAIL:
"Requested UDP tunnel encapsulation unavailable", from the numbering
space for values defined for use with the Code field of Mobile IP
Registration Reply Messages. Code number 142 has been assigned from
the subset "Error Codes from the Home Agent".

The values for the Next Header field in the MIP Tunnel Data Message
(Section 3.3) shall be the same as those used for the Protocol field
of the IP header [2], and requires no new number assignment.

9. Intellectual Property Rights

The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights (www.ietf.org/ipr.html).

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RFC 3519 NAT Traversal for Mobile IP April 2003

10. Acknowledgements

Much of the text in Section 4.2 has been taken almost verbatim from
RFC 2003, IP Encapsulation within IP [4].

Adding support for the FA case was suggested by George Tsirtsis and
Frode B. Nilsen. Roy Jose pointed out a problem with binding
updates, and Frode, Roy and George pointed out that there are cases
where triangular routes may work, and suggested that reverse
tunnelling need not be mandatory. Roy and Qiang Zhang drew attention
to a number of sections which needed to be corrected or stated more
clearly.

Phil Roberts helped remove a number of rough edges. Farid Adrangi
pointed out the DoS issue now covered in Security Considerations
(Section 6). Francis Dupont's helpful comments made us extend the
Security Considerations section to make it more comprehensive and
clear. Milind Kulkarni and Madhavi Chandra pointed out the required
match between the FA source and care-of addresses when the mechanism
is used by an FA, and also contributed a number of clarifications to
the text.

Thanks also to our co-workers, Ilkka Pietikainen, Antti Nuopponen and
Timo Aalto at Netseal and Hans Sjostrand, Fredrik Johansson and Erik
Liden at ipUnplugged. They have read and re-read the text, and
contributed many valuable corrections and insights.

11. Normative References

[1] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
1980.

[2] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.

[3] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing
Encapsulation (GRE)", RFC 1701, October 1994.

[4] Perkins, C., "IP Encapsulation within IP", RFC 2003, October
1996.

[5] Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
October 1996.

[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[7] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

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RFC 3519 NAT Traversal for Mobile IP April 2003

[8] Farinacci, D., Li, T., Hanks, S., Meyer, D. and P. Traina,
"Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.

[9] Montenegro, G., "Reverse Tunneling for Mobile IP, revised", RFC
3024, January 2001.

[10] Perkins, C., "IP Mobility Support for IPv4", RFC 3344, August
2002.

12. Informative References

[11] Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999.

[12] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001.

[13] Daigle, L., Editor, and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF)", RFC 3424, November 2002.

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RFC 3519 NAT Traversal for Mobile IP April 2003

13. Authors' Addresses

Henrik Levkowetz
ipUnplugged AB
Arenavagen 23
Stockholm S-121 28
SWEDEN

Phone: +46 708 32 16 08
EMail: [email protected]

Sami Vaarala
Netseal
Niittykatu 6
Espoo 02201
FINLAND

Phone: +358 9 435 310
EMail: [email protected]

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RFC 3519 NAT Traversal for Mobile IP April 2003

14. Full Copyright Statement

Copyright (C) The Internet Society (2003). All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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