Internet Engineering Task Force G. Montenegro, Editor
INTERNET DRAFT Sun Microsystems, Inc.
March 2000
Reverse Tunneling for Mobile IP, revised
draft-ietf-mobileip-rfc2344-bis-01.txt
Status of This Memo
This document is a submission by the Mobile IP Working Group of
the Internet Engineering Task Force (IETF). Comments should be
submitted to the Mobile IP mailing list at
"MOBILE-IP@STANDARDS.NORTELNETWORKS.COM".
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.
Abstract
Mobile IP uses tunneling from the home agent to the mobile
node's care-of address, but rarely in the reverse direction.
Usually, a mobile node sends its packets through a router on the
foreign network, and assumes that routing is independent of
source address. When this assumption is not true, it is
convenient to establish a topologically correct reverse tunnel
from the care-of address to the home agent.
This document proposes backwards-compatible extensions to Mobile
IP to support topologically correct reverse tunnels. This
document does not attempt to solve the problems posed by
firewalls located between the home agent and the mobile node's
care-of address.
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Table of Contents
1. Introduction ................................................... 3
1.1. Terminology .................................................. 4
1.2. Assumptions .................................................. 4
1.3. Justification ................................................ 5
2. Overview ....................................................... 5
3. New Packet Formats ............................................. 5
3.1. Mobility Agent Advertisement Extension ....................... 5
3.2. Registration Request ......................................... 6
3.3. Encapsulating Delivery Style Extension ....................... 7
3.4. New Registration Reply Codes ................................. 8
4. Changes in Protocol Behavior ................................... 9
4.1. Mobile Node Considerations ................................... 9
4.1.1. Sending Registration Requests to the Foreign Agent ......... 9
4.1.2. Receiving Registration Replies from the Foreign Agent ...... 9
4.2. Foreign Agent Considerations ................................. 10
4.2.1. Receiving Registration Requests from the Mobile Node ....... 10
4.2.2. Relaying Registration Requests to the Home Agent ........... 11
4.3. Home Agent Considerations .................................... 11
4.3.1. Receiving Registration Requests from the Foreign Agent ..... 11
4.3.2. Sending Registration Replies to the Foreign Agent .......... 12
5. Mobile Node to Foreign Agent Delivery Styles ................... 13
5.1. Direct Delivery Style ........................................ 13
5.1.1. Packet Processing .......................................... 13
5.1.2. Packet Header Format and Fields ............................ 13
5.2. Encapsulating Delivery Style ................................. 14
5.2.1 Packet Processing ........................................... 14
5.2.2. Packet Header Format and Fields ............................ 15
5.3. Support for Broadcast and Multicast Datagrams ................ 16
5.4. Selective Reverse Tunneling .................................. 16
6. Security Considerations ........................................ 17
6.1. Reverse-tunnel Hijacking and Denial-of-Service Attacks ....... 17
6.2. Ingress Filtering ............................................ 18
6.3. Reverse Tunneling for Disparate Address Spaces ............... 18
Appendix: Disparate Address Space Support ......................... 18
A.1. Scope of the Reverse Tunneling Solution ................... 19
A.2. Terminating Forward Tunnels at the Foreign Agent .......... 22
A.3. Initiating Reverse Tunnels at the Foreign Agent ........... 24
7. Acknowledgements ............................................... 24
References ........................................................ 24
Editor and Chair Addresses ........................................ 25
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1. Introduction
Section 1.3 of the Mobile IP specification [1] lists the following
assumption:
It is assumed that IP unicast datagrams are routed based on the
destination address in the datagram header (i.e., not by source
address).
Because of security concerns (for example, IP spoofing attacks), and
in accordance with RFC 2267 [8] and CERT [3] advisories to this
effect, routers that break this assumption are increasingly more
common.
In the presence of such routers, the source and destination IP
address in a packet must be topologically correct. The forward
tunnel complies with this, as its endpoints (home agent address and
care-of address) are properly assigned addresses for their
respective locations. On the other hand, the source IP address of a
packet transmitted by the mobile node does not correspond to the
network prefix from where it emanates.
This document discusses topologically correct reverse tunnels.
Mobile IP does dictate the use of reverse tunnels in the context of
multicast datagram routing and mobile routers. However, the source
IP address is set to the mobile node's home address, so these
tunnels are not topologically correct.
Notice that there are several uses for reverse tunnels regardless of
their topological correctness:
- Mobile routers: reverse tunnels obviate the need for recursive
tunneling [1].
- Multicast: reverse tunnels enable a mobile node away from home
to (1) join multicast groups in its home network, and (2)
transmit multicast packets such that they emanate from its home
network [1].
- The TTL of packets sent by the mobile node (for example, when
sending packets to other hosts in its home network) may be so
low that they might expire before reaching their destination.
A reverse tunnel solves the problem as it represents a TTL
decrement of one [5].
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1.1. Terminology
The discussion below uses terms defined in the Mobile IP
specification. Additionally, it uses the following terms:
Forward Tunnel
A tunnel that shuttles packets towards the mobile node. It
starts at the home agent, and ends at the mobile node's
care-of address.
Reverse Tunnel
A tunnel that starts at the mobile node's care-of address and
terminates at the home agent.
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 RFC 2119 [9].
1.2. Assumptions
Mobility is constrained to a common IP address space (that is, the
routing fabric between, say, the mobile node and the home agent is
not partitioned into a "private" and a "public" network).
This document does not attempt to solve the firewall traversal
problem. Rather, it assumes one of the following is true:
- There are no intervening firewalls along the path of the
tunneled packets.
- Any intervening firewalls share the security association
necessary to process any authentication [6] or encryption [7]
headers which may have been added to the tunneled packets.
The reverse tunnels considered here are symmetric, that is, they use
the same configuration (encapsulation method, IP address endpoints)
as the forward tunnel. IP in IP encapsulation [2] is assumed unless
stated otherwise.
Route optimization [4] introduces forward tunnels initiated at a
correspondent host. Since a mobile node may not know if the
correspondent host can decapsulate packets, reverse tunnels in that
context are not discussed here.
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1.3. Justification
Why not let the mobile node itself initiate the tunnel to the home
agent? This is indeed what it should do if it is already operating
with a topologically correct co-located care-of address.
However, one of the primary objectives of the Mobile IP
specification is not to require this mode of operation.
The mechanisms outlined in this document are primarily intended for
use by mobile nodes that rely on the foreign agent for forward
tunnel support. It is desirable to continue supporting these mobile
nodes, even in the presence of filtering routers.
2. Overview
A mobile node arrives at a foreign network, listens for agent
advertisements and selects a foreign agent that supports reverse
tunnels. It requests this service when it registers through the
selected foreign agent. At this time, and depending on how the
mobile node wishes to deliver packets to the foreign agent, it also
requests either the Direct or the Encapsulating Delivery Style
(section 5).
In the Direct Delivery Style, the mobile node designates the foreign
agent as its default router and proceeds to send packets directly to
the foreign agent, that is, without encapsulation. The foreign
agent intercepts them, and tunnels them to the home agent.
In the Encapsulating Delivery Style, the mobile node encapsulates
all its outgoing packets to the foreign agent. The foreign agent
decapsulates and re-tunnels them to the home agent, using the
foreign agent's care-of address as the entry-point of this new
tunnel.
3. New Packet Formats
3.1. Mobility Agent Advertisement Extension
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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|V|T| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| zero or more Care-of Addresses |
| ... |
The only change to the Mobility Agent Advertisement Extension [1] is
the additional 'T' bit:
T Agent offers reverse tunneling service.
A foreign agent that sets the 'T' bit MUST support the two delivery
styles currently supported: Direct and Encapsulating Delivery Style
(section 5).
Using this information, a mobile node is able to choose a foreign
agent that supports reverse tunnels. Notice that if a mobile node
does not understand this bit, it simply ignores it as per [1].
3.2. Registration Request
Reverse tunneling support is added directly into the Registration
Request by using one of the "rsvd" bits. If a foreign or home agent
that does not support reverse tunnels receives a request with the
'T' bit set, the Registration Request fails. This results in a
registration denial (failure codes are specified in section 3.4).
Most home agents would not object to providing reverse tunnel
support, because they "SHOULD be able to decapsulate and further
deliver packets addressed to themselves, sent by a mobile node"
[1]. In the case of topologically correct reverse tunnels, the
packets are not sent by the mobile node as distinguished by its home
address. Rather, the outermost (encapsulating) IP source address on
such datagrams is the care-of address of the mobile node.
Nevertheless, home agents probably already support the required
decapsulation and further forwarding.
In Registration Requests sent by a mobile node, the Time to Live
field in the IP header MUST be set to 255. This limits a denial of
service attack in which malicious hosts send false Registration
Requests (see Section 6).
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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 |S|B|D|M|G|V|T|-| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Agent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Care-of Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
The only change to the Registration Request packet is the additional
'T' bit:
T If the 'T' bit is set, the mobile node asks its home
agent to accept a reverse tunnel from the care-of
address. Mobile nodes using a foreign agent care-of
address ask the foreign agent to reverse-tunnel its
packets.
3.3. Encapsulating Delivery Style Extension
The Encapsulating Delivery Style Extension MAY be included by the
mobile node in registration requests to further specify reverse
tunneling behavior. It is expected to be used only by the foreign
agent. Accordingly, the foreign agent MUST consume this extension
(that is, it must not relay it to the home agent or include it in
replies to the mobile node). As per Section 3.6.1.3 of [1], the
mobile node MUST include the Encapsulating Delivery Style Extension
after the Mobile-Home Authentication Extension, and before the
Mobile-Foreign Authentication Extension, if present.
The Encapsulating Delivery Style Extension MUST NOT be included if
the 'T' bit is not set in the Registration Request.
If this extension is absent, Direct Delivery is assumed.
Encapsulation is done according to what was negotiated for the
forward tunnel (that is, IP in IP is assumed unless specified
otherwise). For more details on the delivery styles, please refer to
section 5.
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
130
Length
0
3.4. New Registration Reply Codes
Foreign and home agent registration replies MUST convey if the
reverse tunnel request failed. These new reply codes are defined:
Service denied by the foreign agent:
74 requested reverse tunnel unavailable
75 reverse tunnel is mandatory and 'T' bit not set
76 mobile node too distant
and
Service denied by the home agent:
137 requested reverse tunnel unavailable
138 reverse tunnel is mandatory and 'T' bit not set
139 requested encapsulation unavailable
In response to a Registration Request with the 'T' bit set, mobile
nodes may receive (and MUST accept) code 70 (poorly formed request)
from foreign agents and code 134 (poorly formed request) from home
agents. However, foreign and home agents that support reverse
tunneling MUST use codes 74 and 137, respectively.
Absence of the 'T' bit in a Registration Request MAY elicit denials
with codes 75 and 138 at the foreign agent and the home agent,
respectively.
Forward and reverse tunnels are symmetric, that is, both are able to
use the same tunneling options negotiated at registration. This
implies that the home agent MUST deny registrations if an
unsupported form of tunneling is requested (code 139). Notice that
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Mobile IP [1] already defines the analogous failure code 72 for use
by the foreign agent.
4. Changes in Protocol Behavior
Unless otherwise specified, behavior specified by Mobile IP [1] is
assumed. In particular, if any two entities share a mobility
security association, they MUST use the appropriate Authentication
Extension (Mobile-Foreign, Foreign-Home or Mobile-Home
Authentication Extension) when exchanging registration protocol
datagrams. The Mobile-Home Authentication Extension MUST always be
present.
Reverse tunneling imposes additional protocol processing
requirements on mobile entities. Differences in protocol behavior
with respect to Mobile IP [1] are specified in the subsequent
sections.
4.1. Mobile Node Considerations
This section describes how the mobile node handles registrations
that request a reverse tunnel.
4.1.1. Sending Registration Requests to the Foreign Agent
In addition to the considerations in [1], a mobile node sets the 'T'
bit in its Registration Request to petition a reverse tunnel.
The mobile node MUST set the TTL field of the IP header to 255. This
is meant to limit the reverse tunnel hijacking attack (Section 6).
The mobile node MAY optionally include an Encapsulating Delivery
Style Extension.
4.1.2. Receiving Registration Replies from the Foreign Agent
Possible valid responses are:
- A registration denial issued by either the home agent or the
foreign agent:
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a. The mobile node follows the error checking guidelines in
[1], and depending on the reply code, MAY try modifying the
registration request (for example, by eliminating the
request for alternate forms of encapsulation), and issuing
a new registration.
b. Depending on the reply code, the mobile node MAY try
zeroing the 'T' bit, eliminating the Encapsulating Delivery
Style Extension (if one was present), and issuing a new
registration. Notice that after doing so the registration
may succeed, but due to the lack of a reverse tunnel data
transfer may not be possible.
- The home agent returns a Registration Reply indicating that the
service will be provided.
In this last case, the mobile node has succeeded in establishing a
reverse tunnel between its care-of address and its home agent. If
the mobile node is operating with a co-located care-of address, it
MAY encapsulate outgoing data such that the destination address of
the outer header is the home agent. This ability to selectively
reverse-tunnel packets is discussed further in section 5.4.
If the care-of address belongs to a separate foreign agent, the
mobile node MUST employ whatever delivery style was requested
(Direct or Encapsulating) and proceed as specified in section 5.
A successful registration reply is an assurance that both the
foreign agent and the home agent support whatever alternate forms of
encapsulation (other than IP in IP) were requested. Accordingly, the
mobile node MAY use them at its discretion.
4.2. Foreign Agent Considerations
This section describes how the foreign agent handles registrations
that request a reverse tunnel.
4.2.1. Receiving Registration Requests from the Mobile Node
A foreign agent that receives a Registration Request with the 'T'
bit set processes the packet as specified in the Mobile IP
specification [1], and determines whether it can accomodate the
forward tunnel request. If it cannot, it returns an appropriate
code. In particular, if the foreign agent is unable to support the
requested form of encapsulation it MUST return code 72.
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The foreign agent MAY reject Registration Requests without the 'T'
bit set by denying them with code 75 (reverse tunnel is mandatory
and 'T' bit not set).
The foreign agent MUST verify that the TTL field of the IP header is
set to 255. Otherwise, it MUST reject the registration with code 76
(mobile node too distant). The foreign agent MUST limit the rate at
which it sends these registration replies to a maximum of one per
second.
As a last check, the foreign agent verifies that it can support a
reverse tunnel with the same configuration. If it cannot, it MUST
return a Registration Reply denying the request with code 74
(requested reverse tunnel unavailable).
4.2.2. Relaying Registration Requests to the Home Agent
Otherwise, the foreign agent MUST relay the Registration Request to
the home agent.
Upon receipt of a Registration Reply that satisfies validity checks,
the foreign agent MUST update its visitor list, including indication
that this mobile node has been granted a reverse tunnel and the
delivery style expected (section 5).
While this visitor list entry is in effect, the foreign agent MUST
process incoming traffic according to the delivery style,
encapsulate it and tunnel it from the care-of address to the home
agent's address.
4.3. Home Agent Considerations
This section describes how the home agent handles registrations that
request a reverse tunnel.
4.3.1. Receiving Registration Requests from the Foreign Agent
A home agent that receives a Registration Request with the 'T' bit
set processes the packet as specified in the Mobile IP specification
[1] and determines whether it can accomodate the forward tunnel
request. If it cannot, it returns an appropriate code. In
particular, if the home agent is unable to support the requested
form of encapsulation it MUST return code 139 (requested
encapsulation unavailable).
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The home agent MAY reject registration requests without the 'T' bit
set by denying them with code 138 (reverse tunnel is mandatory and
'T' bit not set).
As a last check, the home agent determines whether it can support a
reverse tunnel with the same configuration as the forward tunnel. If
it cannot, it MUST send back a registration denial with code 137
(requested reverse tunnel unavailable).
Upon receipt of a Registration Reply that satisfies validity checks,
the home agent MUST update its mobility bindings list to indicate
that this mobile node has been granted a reverse tunnel and the type
of encapsulation expected.
4.3.2. Sending Registration Replies to the Foreign Agent
In response to a valid Registration Request, a home agent MUST issue
a Registration Reply to the mobile node.
After a successful registration, the home agent may receive
encapsulated packets addressed to itself. Decapsulating such packets
and blindly injecting them into the network is a potential security
weakness (section 6.1). Accordingly, the home agent MUST implement,
and, by default, SHOULD enable the following check for encapsulated
packets addressed to itself:
The home agent searches for a mobility binding whose care-of
address is the source of the outer header, and whose mobile node
address is the source of the inner header.
If no such binding is found, or if the packet uses an encapsulation
mechanism that was not negotiated at registration the home agent
MUST silently discard the packet and SHOULD log the event as a
security exception.
Home agents that terminate tunnels unrelated to Mobile IP (for
example, multicast tunnels) MAY turn off the above check, but this
practice is discouraged for the aforementioned reasons.
While the registration is in effect, a home agent MUST process each
valid reverse tunneled packet (as determined by checks like the
above) by decapsulating it, recovering the original packet, and then
forwarding it on behalf of its sender (the mobile node) to the
destination address (the correspondent host).
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5. Mobile Node to Foreign Agent Delivery Styles
This section specifies how the mobile node sends its data traffic
via the foreign agent. In all cases, the mobile node learns the
foreign agent's link-layer address from the link-layer header in the
agent advertisement.
5.1. Direct Delivery Style
This delivery mechanism is very simple to implement at the mobile
node, and uses small (non-encapsulated) packets on the link between
the mobile node and the foreign agent (potentially a very slow
link). However, it only supports reverse-tunneling of unicast
packets, and does not allow selective reverse tunneling (section
5.4).
5.1.1. Packet Processing
The mobile node MUST designate the foreign agent as its default
router. Not doing so will not guarantee encapsulation of all the
mobile node's outgoing traffic, and defeats the purpose of the
reverse tunnel. The foreign agent MUST:
- detect packets sent by the mobile node, and
- modify its forwarding function to encapsulate them before
forwarding.
5.1.2. Packet Header Format and Fields
This section shows the format of the packet headers used by the
Direct Delivery style. The formats shown assume IP in IP
encapsulation [2].
Packet format received by the foreign agent (Direct Delivery
Style):
IP fields:
Source Address = mobile node's home address
Destination Address = correspondent host's address
Upper Layer Protocol
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Packet format forwarded by the foreign agent (Direct Delivery
Style):
IP fields (encapsulating header):
Source Address = foreign agent's care-of address
Destination Address = home agent's address
Protocol field: 4 (IP in IP)
IP fields (original header):
Source Address = mobile node's home address
Destination Address = correspondent host's address
Upper Layer Protocol
These fields of the encapsulating header MUST be chosen as follows:
IP Source Address
Copied from the Care-of Address field within the Registration
Request.
IP Destination Address
Copied from the Home Agent field within the most recent
successful Registration Reply.
IP Protocol Field
Default is 4 (IP in IP [2]), but other methods of
encapsulation MAY be used as negotiated at registration time.
5.2. Encapsulating Delivery Style
This mechanism requires that the mobile node implement
encapsulation, and explicitly directs packets at the foreign agent
by designating it as the destination address in a new outermost
header. Mobile nodes that wish to send either broadcast or
multicast packets MUST use the Encapsulating Delivery Style.
5.2.1 Packet Processing
The foreign agent does not modify its forwarding function.
Rather, it receives an encapsulated packet and after verifying that
it was sent by the mobile node, it:
- decapsulates to recover the inner packet,
- re-encapsulates, and sends it to the home agent.
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If a foreign agent receives an un-encapsulated packet from a mobile
node which had explicitly requested the Encapsulated Delivery Style,
then the foreign agent MUST NOT reverse tunnel such a packet and
rather MUST forward it using standard, IP routing mechanisms.
5.2.2. Packet Header Format and Fields
This section shows the format of the packet headers used by the
Encapsulating Delivery style. The formats shown assume IP in IP
encapsulation [2].
Packet format received by the foreign agent (Encapsulating Delivery
Style):
IP fields (encapsulating header):
Source Address = mobile node's home address
Destination Address = foreign agent's address
Protocol field: 4 (IP in IP)
IP fields (original header):
Source Address = mobile node's home address
Destination Address = correspondent host's address
Upper Layer Protocol
The fields of the encapsulating IP header MUST be chosen as
follows:
IP Source Address
The mobile node's home address.
IP Destination Address
The address of the agent as learned from the IP source address
of the agent's most recent successful registration reply.
IP Protocol Field
Default is 4 (IP in IP [2]), but other methods of
encapsulation MAY be used as negotiated at registration time.
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Packet format forwarded by the foreign agent (Encapsulating Delivery
Style):
IP fields (encapsulating header):
Source Address = foreign agent's care-of address
Destination Address = home agent's address
Protocol field: 4 (IP in IP)
IP fields (original header):
Source Address = mobile node's home address
Destination Address = correspondent host's address
Upper Layer Protocol
These fields of the encapsulating IP header MUST be chosen as
follows:
IP Source Address
Copied from the Care-of Address field within the Registration
Request.
IP Destination Address
Copied from the Home Agent field within the most recent
successful Registration Reply.
IP Protocol Field
Default is 4 (IP in IP [2]), but other methods of
encapsulation MAY be used as negotiated at registration time.
5.3. Support for Broadcast and Multicast Datagrams
If a mobile node is operating with a co-located care-of address,
broadcast and multicast datagrams are handled according to Sections
4.3 and 4.4 of the Mobile IP specification [1]. Mobile nodes using a
foreign agent care-of address MAY have their broadcast and multicast
datagrams reverse-tunneled by the foreign agent. However, any
mobile nodes doing so MUST use the encapsulating delivery style.
This delivers the datagram only to the foreign agent. The latter
decapsulates it and then processes it as any other packet from the
mobile node, namely, by reverse tunneling it to the home agent.
5.4. Selective Reverse Tunneling
Packets destined to local resources (for example, a nearby printer)
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might be unaffected by ingress filtering. A mobile node with a
co-located care-of address MAY optimize delivery of these packets by
not reverse tunneling them. On the other hand, a mobile node using
a foreign agent care-of address MAY use this selective reverse
tunneling capability by requesting the Encapsulating Delivery Style,
and following these guidelines:
Packets NOT meant to be reversed tunneled:
Sent using the Direct Delivery style. The foreign agent
MUST process these packets as regular traffic: they MAY be
forwarded but MUST NOT be reverse tunneled to the home agent.
Packets meant to be reverse tunneled:
Sent using the Encapsulating Delivery style. The foreign agent
MUST process these packets as specified in section 5.2: they
MUST be reverse tunneled to the home agent.
6. Security Considerations
The extensions outlined in this document are subject to the security
considerations outlined in the Mobile IP specification [1].
Essentially, creation of both forward and reverse tunnels involves
an authentication procedure, which reduces the risk for attack.
6.1. Reverse-tunnel Hijacking and Denial-of-Service Attacks
Once the tunnel is set up, a malicious node could hijack it to
inject packets into the network. Reverse tunnels might exacerbate
this problem, because upon reaching the tunnel exit point packets
are forwarded beyond the local network. This concern is also present
in the Mobile IP specification, as it already dictates the use of
reverse tunnels for certain applications.
Unauthenticated exchanges involving the foreign agent allow a
malicious node to pose as a valid mobile node and re-direct an
existing reverse tunnel to another home agent, perhaps another
malicious node. The best way to protect against these attacks is by
employing the Mobile-Foreign and Foreign-Home Authentication
Extensions defined in [1].
If the necessary mobility security associations are not available,
this document introduces a mechanism to reduce the range and
effectiveness of the attacks. The mobile node MUST set to 255 the
TTL value in the IP headers of Registration Requests sent to the
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foreign agent. This prevents malicious nodes more than one hop away
from posing as valid mobile nodes. Additional codes for use in
registration denials make those attacks that do occur easier to
track.
With the goal of further reducing the attacks the Mobile IP Working
Group considered other mechanisms involving the use of
unauthenticated state. However, these introduce the possibilities of
denial-of-service attacks. The consensus was that this was too much
of a trade-off for mechanisms that guarantee no more than weak
(non-cryptographic) protection against attacks.
6.2. Ingress Filtering
There has been some concern regarding the long-term effectiveness of
reverse-tunneling in the presence of ingress filtering. The
conjecture is that network administrators will target
reverse-tunneled packets (IP in IP encapsulated packets) for
filtering. The ingress filtering recommendation spells out why this
is not the case [8]:
Tracking the source of an attack is simplified when the source is
more likely to be "valid."
6.3. Reverse Tunneling for Disparate Address Spaces
There are security implications involved with the foreign
agent's using link-layer information to select the proper
reverse tunnel for mobile node packets (section A.3).
Unauthenticated link-layers allow a malicious mobile node to
misuse another's existing reverse tunnel, and inject packets
into the network.
For this solution to be viable, the link-layer MUST securely
authenticate traffic received by the foreign agent from the
mobile nodes. Unauthenticated link-layer technologies (for
example shared ethernet) are not recommended to implement
disparate address support.
Appendix: Disparate Address Space Support
Mobile IP [1] assumes that all the entities involved (mobile
node, foreign agent and home agent) have addresses within the
same globally routable address space. In many deployment
scenarios, when a mobile node leaves its home network it may
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wander into a region where its home address is not routable or
known by the local routing fabric. Similarly, the IP addresses
of the foreign agent and the home agent may belong to disparate
address spaces, which precludes their exchanging registration
protocol messages directly. These issues are possible
particularly if the entities involved use addresses from the
ranges specified in RFC1918 to support private networks.
Accurately speaking, the use of private addresses is not the
only cause. It may, in fact, be the most common, but the root of
the problem lies in the use of disparate address spaces. For
example, corporations often have several properly allocated
address ranges. They typically advertise reachability to only a
subset of those ranges, leaving the others for use exclusively
within the corporate network. Since these ranges are not
routable in the general Internet, their use leads to the same
problems encountered with "private" addresses, even though they
are not taken from the ranges specified in RFC1918.
Even if the mobile node, home agent and foreign agent all reside
within the same address space, problems may arise if the
correspondent node does not. However, this problem is not
specific to Mobile IP, and is beyond the scope of this
document. The next section limits even further the scope of the
issues relevant to this document. A subsequent section explains
how reverse tunneling may be used to tackle them.
A.1. Scope of the Reverse Tunneling Solution
Reverse tunneling (as defined in this document) may be used to
cope with disparate address spaces, within the following
constraints:
- There are no provisions to solve the case in which the
correspondent node and the mobile node are in disparate
address spaces. This limits the scope of the problem to
only those issues specific to Mobile IP.
- The foreign agent and the home agent are directly reachable
to each other by virtue of residing in the same address
space. This limits the scope of the problem to only the
simplest of cases. This also implies that the registration
protocol itself has a direct path between the foreign
agent and the home agent, and, in this respect, is not
affected by disparate address spaces. This restriction
also applies to mobile nodes operating with a co-located
care-of address. In this case, reverse tunneling is a
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complete and elegant solution.
- There are no additional protocol elements beyond those
defined by Mobile IP [1] and reverse tunneling. In
particular, additional extensions to the registration
requests or replies, or additional bits in the
header--although potentially useful--are outside the scope
of this document.
In spite of the limitations, reverse tunneling may be used to
solve the most common issues. The range of problems that can be
solved are best understood by looking at some simple diagrams:
Figure A1: NON-ROUTABLE PACKETS IN DISPARATE ADDRESS SPACES
Mc Fa Fb Hb Hc Yc
[MN]-----------------[FA]----------------[HA]---------------[Y]
Addr space A Addr space B Addr space C
In this diagram, there are three disparate address spaces: A, B
and C. The home agent (HA) has one address each on address
spaces B and C, and the foreign agent (FA), on address spaces A
and B. The mobile node's (MN) has a permanent address, Mc,
within address space C.
In the most common scenario both A and C are "private" address
spaces, and B is the public Internet.
Suppose MN sends a packet to correspondent node (Y) in its home
network. Presumably, MN has no difficulties delivering this
packet to the FA, because it does so using layer 2 mechanisms.
Somehow, the FA must realize that this packet must be reverse
tunneled, and it must fetch the proper binding to do so.
Possible mechanisms are outlined in section A.3.
However, once the packet is in address space B it becomes
non-routable. Note that ingress filtering only exacerbates the
problem, because it adds a requirement of topological
significance to the source IP address in addition to the that of
the destination address. As Mobile IP matures, others entities
may be defined (for example, AAA servers). Their addition places
even more requirements on the address spaces in use.
Reverse tunneling adds a topologically significant IP header to
the packet (source IP address of Fb, destination of Hb) during
its transit within address space B. Assuming IP in IP
encapsulation (although others, like GRE are also possible),
this is what the packet looks like:
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Figure A2: IP IN IP REVERSE TUNNELED PACKET FROM FA TO HA
+-----------------+
| +-------+|
| Fb->Hb | Mc->Yc||
| +-------+|
+--------+--------+
HA receives this packet, recovers the original packet, and since
it is cognizant of address space C, delivers it to the
appropriate interface.
Of course, for this to happen, the care-of address registered by
the MN is not the usual Fa, but Fb. How this happens is outside
the scope of this document. Some possible mechanisms are:
- FA recognizes mobile nodes whose addresses fall within
the private address ranges specified by RFC1918. In this
case, the foreign agent could force the use of Fb as
the care-of address, perhaps by rejecting the initial
registration request with an appropriate error message
and supplemental information.
- FA could be configured to always advertise Fb as long
as H->Fb and Fb->H are guaranteed to be valid forward
and reverse tunnels, respectively, for all values of H.
Here, H is the address of any home agent whose mobile
nodes may register via FA.
- FA could indicate that it supports disparate address spaces
via a currently undefined 'P' bit in its advertisements,
and an indication of the relevant address space for any or
all of its care-of addressed by including an NAI [11] or a
realm indicator (perhaps a variant of the NAI). Alter-
natively, mobile nodes so configured could solicit the NAI or
realm indicator information in response to advertisements with
the 'P' bit set.
Additionally, the mobile node needs to supply the appropriate
address for its home agent: Hb instead of the usual Hc. How this
happens is outside the scope of this document. Some possible
mechanisms are:
- This determination could be triggered in response to using
the foreign agent's Fb as the care-of address.
- The mobile node could always use Hb as its home agent
address, specially (1) if Hb is routable within address
space C, or (2) if MN is certain never to be at home (in
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some configurations, the mobile nodes are always roaming).
- The mobile node could be configured with different home
agent addresses and their corresponding address space
(perhaps indicated via an NAI [11] or a variant of it).
Another major issue introduced by private addresses is that of
two or more mobile nodes with the same numeric IP address:
Figure A3: MOBILE NODES WITH CONFLICTING ADDRESSES
Mc=M H1b H1c
[MN1]-------+ +----[HA1]----+---------
| | | Address
| | | space C
Address | | Address +----------
Space Fa-[FA]-Fb Space
A | | B +---------
| | | Address
| | | space D
[MN2]-------+ +----[HA2]----+---------
Md=M H2b H2d
Suppose there are two address spaces A and B, and a foreign
agent (FA) with interfaces on both. There are two home agents
(HA1 and HA2) in address space B, with addresses H1b and H2b,
respectively. Each of the home agents has an interface in a
private address space in addition to address space B: HA1 has
H1c on C, and HA2 has H2d on D. MN1 and MN2 are two mobile
nodes with home addresses Mc and Md, corresponding to address
space C and D, respectively.
If Mc and Md are private addresses as defined in RFC1918, they
may be numerically equivalent (both equal to M). Because of
this, the foreign agent can no longer rely on only the mobile
node's home address to disambiguate amongst its different
bindings.
A.2. Terminating Forward Tunnels at the Foreign Agent
In figure A1, suppose the correspondent node Y sends a packet to
the mobile node at address Mc. The packet is intercepted by the
home agent at Hc and tunneled towards the mobile node via
address Fb.
Once the packet reaches FA (via address Fb), the foreign agent
must identify which of its registered mobile nodes is the
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ultimate destination for the internal packet. In order to do
so, it needs to identify the proper binding via a tuple
guaranteed to be unique among all of its mobile nodes.
The unique tuple sufficient for demultiplexing IP in IP packets
[IPIP] (protocol 4) is:
- destination IP address of the encapsulated (internal)
header
This is mobile node MN's home address (Mc in the above
example). At first glance, it seems like this is unique
among all mobile nodes, but as mentioned above, with
private addresses another mobile may have an address Md
numerically equivalent to Mc.
- source IP address of the external header
This, the remote end of the tunnel, is Hb in the above
example.
- destination IP address of the external header
This, the local end of the tunnel, is Fb in the above
example.
The three values above are learned from a successful
registration and are the mobile node's home address, the home
agent's address and the care-of address. Thus, it is possible to
identify the right binding. Once FA identifies the ultimate
destination of the packet, Mc, it delivers the internal packet
using link layer mechanisms.
GRE packets [10] (protocol 47) are only handled if their
Protocol Type field has a value of 0x800 (other values are
outside the scope of this document), and are demultiplexed based
on the same tuple as IP in IP packets. In GRE terminology, the
tuple is:
- destination IP address of the payload (internal) packet
- source IP address of the delivery (external) packet
- destination IP address of the delivery (external) packet
Notice that the Routing, Sequence Number, Strict Source Route
and Key fields have been deprecated from GRE.
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The above tuples work for IP-in-IP or GRE encapsulation, and
assume that the inner packet is in the clear. Encapsulations
which encrypt the inner packet header are outside the scope of
this document.
A.3. Initiating Reverse Tunnels at the Foreign Agent
In figure A3, suppose mobile node M1 sends a packet to a
correspondent node in its home address space, C, and mobile node
M2 sends a packet to a correspondent node in its home address
space, D.
At FA, the source addresses for both packets will be seen as M,
thus this is not sufficient information. The unique tuple
required to identify the proper binding is:
- link-layer information related to the MN
This may be in the form of a MAC address, a PPP session (or
incoming interface) or channel coding for a digital
cellular service. Device ID's can also be used in this
context.
- source IP address of the IP header.
As was pointed out, this by itself is not guaranteed to be
unique.
This information must be established and recorded at
registration time. The above items are sufficient for the
foreign agent to select the proper binding to use. This, in
turn, produces the address of the home agent, and the reverse
tunneling options negotiated during the registration process.
The foreign agent can now proceed with reverse tunneling.
7. Acknowledgements
The encapsulating style of delivery was proposed by Charlie
Perkins. Jim Solomon has been instrumental in shaping this
document into its present form. Thanks to Samita Chakrabarti for
helpful comments on disparate address space support.
References
[1] C. Perkins. "IP Mobility Support, revised," (work in
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progress), draft-ietf-mobileip-rfc2002-bis-01.txt, January
2000.
[2] C. Perkins. IP Encapsulation within IP. RFC 2003, October
1996.
[3] Computer Emergency Response Team (CERT), "IP Spoofing Attacks
and Hijacked Terminal Connections", CA-95:01, January 1995.
Available via anonymous ftp from info.cert.org in
/pub/cert_advisories.
[4] D. Johnson and C. Perkins. Route Optimization in Mobile IP
(work in progress), draft-ietf-mobileip-optim-09.txt,
February 2000.
[5] Manuel Rodriguez, private communication, August 1995.
[6] S. Kent, R. Atkinson, "IP Authentication Header," RFC 2402,
November 1998 (obsoletes RFC 1826, August 1995).
[7] S. Kent, R. Atkinson, "IP Encapsulating Payload," RFC 2406,
November 1998 (obsoletes RFC 1827, August 1995).
[8] P. Ferguson and D. Senie. Network Ingress Filtering: Defeating
Denial of Service Attacks which employ IP Source Address
Spoofing. RFC 2267, January 1998.
[9] S. Bradner. Key words for use in RFCs to Indicate Requirement
Levels. RFC 2119, March 1997.
[10] Farinacci, D., Li, T., Hanks, S., Meyer, D., Traina, P.,
"Generic Routing Encapsulation (GRE)," (work in progress),
draft-meyer-gre-update-03.txt, January 2000.
[11] B. Aboba and M. Beadles. The Network Access Identifier. RFC
2486, January 1999.
Editor and Chair Addresses
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Questions about this document may be directed at:
Gabriel E. Montenegro
Sun Microsystems, Inc.
901 San Antonio Road
Mailstop UMPK 15-214
Mountain View, California 94303
Voice: +1-650-786-6288
Fax: +1-650-786-6445
E-Mail: gab@sun.com
The working group can be contacted via the current chairs:
Basavaraj Patil Phil Roberts
Nokia Networks Motorola
6000 Connection Drive 1501 West Shure Drive
Irving, TX 75039 Arlington Heights, IL 60004
USA USA
Phone: +1 972-894-6709 Phone: +1 847-632-3148
Fax : +1 972-894-5349 EMail: QA3445@email.mot.com
EMail: Raj.Patil@nokia.com
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