MIP6 H. Tschofenig
Internet-Draft Nokia Siemens Networks
Intended status: Standards Track G. Bajko
Expires: December 29, 2007 Nokia
June 27, 2007
Mobile IP Interactive Connectivity Establishment (M-ICE)
draft-tschofenig-mip6-ice-00.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document describes how the Interactive Connectivity
Establishment (ICE) methodology can be used for Mobile IP to
determine whether end-to-end communication is possible. ICE makes
use of the Session Traversal Utilities for NAT (STUN) protocol in
addition to mechanisms for checking connectivity between peers.
After running the ICE the two MIP end points will be able to
communicate directly or through a relay via Network Address
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Translators (NATs), Network Address and Port Translators (NAPTs) and
firewalls.
This document addresses also the problems raised in RFC 4487 "Mobile
IPv6 and Firewalls: Problem Statement".
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Gathering Candidate Addresses . . . . . . . . . . . . . . 5
1.2. Connectivity Checks . . . . . . . . . . . . . . . . . . . 5
1.3. Sorting Candidates . . . . . . . . . . . . . . . . . . . . 6
1.4. Frozen Candidates . . . . . . . . . . . . . . . . . . . . 6
1.5. Security for Checks . . . . . . . . . . . . . . . . . . . 6
1.6. Concluding M-ICE . . . . . . . . . . . . . . . . . . . . . 6
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Design Choices . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Sending the Initial Offer . . . . . . . . . . . . . . . . . . 8
5. Receiving the Initial Offer . . . . . . . . . . . . . . . . . 8
6. Receipt of the Initial Answer . . . . . . . . . . . . . . . . 9
7. Performing Connectivity Checks . . . . . . . . . . . . . . . . 9
8. Concluding ICE Processing . . . . . . . . . . . . . . . . . . 9
9. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . . 9
10. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11. Attribute Encoding . . . . . . . . . . . . . . . . . . . . . . 10
12. Demultiplexing MIP and STUN messages . . . . . . . . . . . . . 12
13. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
14. Security Considerations . . . . . . . . . . . . . . . . . . . 16
14.1. Attacks on Connectivity Checks . . . . . . . . . . . . . . 16
14.2. Attacks on Address Gathering . . . . . . . . . . . . . . . 18
14.3. Attacks on the Offer/Answer Exchanges . . . . . . . . . . 19
14.4. Insider Attacks . . . . . . . . . . . . . . . . . . . . . 19
14.4.1. MIP Amplification Attack . . . . . . . . . . . . . . 19
14.4.2. STUN Amplification Attack . . . . . . . . . . . . . . 20
15. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 20
15.1. Problem Definition . . . . . . . . . . . . . . . . . . . . 21
15.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 21
15.3. Brittleness Introduced by M-ICE . . . . . . . . . . . . . 22
15.4. Requirements for a Long Term Solution . . . . . . . . . . 23
15.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 23
16. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 23
17. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
18.1. Normative References . . . . . . . . . . . . . . . . . . . 24
18.2. Informative References . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . . . . 27
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1. Introduction
In a typical Mobile IP deployment, there are two endpoints, mobile
node and correspondent nodes, which want to communicate. They are
able to communicate indirectly via a combination of Mobile IP
signaling and reverse tunneling. A couple of different extensions
are available for Mobile IP that allow multiple care-of addresses,
IPv4/IPv6 interworking and different routes to be used through the
network.
Unfortunately, it is likely that some of the available paths do not
work due to connectivity problems caused by firewalling behavior.
The VoIP community has investigated these problems extensively and
developed a protocols and a methodology to systematically perform
connectivity checks in order to determine a working path between the
two end points. The Interactive Connectivity Establishment (ICE)
specification describes how the Session Traversal Utilities for NAT
(STUN) protocol can be used to execute these checks. This document
suggests to utilize the ICE methodology and if possible STUN for
Mobile IP, both Mobile IPv4 and Mobile IPv6. We call this usage
Mobile IP - ICE, M-ICE for short.
This document, however, concentrates on Mobile IPv6 as a starting
point. A future version of this document will also describe the
operation using Mobile IPv4. The ideal outcome is that the best
available path through the network can be chosen when Mobile IP is
used regardless of the MIP version and the environmental problems
the two end points are facing.
At the beginning of the M-ICE process, the end points are ignorant of
their own topologies. They might or might not be behind a NAT (or
multiple tiers of NATs) and might be behind firewalls that limit the
ability to communicate in different ways between the end points.
M-ICE allows these end points to discover enough information about
their topologies to potentially find one or more paths by which they
can communicate.
Figure 1 shows a typical environment for M-ICE deployment. The two
end points are labelled L and R (for left and right, which helps
visualize call flows). Both L and R are behind their own respective
NATs or firewalls though they may not be aware of it. The type of
NAT or firewall and their properties are also unknown. L and R are
capable of engaging in an end-to-end mobility protocol exchange.
This exchange will occur through mobility anchor points, such as Home
Agents.
In this architecture the ICE functionality of TURN servers is
provided by the Home Agent via reverse tunneling. In this document
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we assume that the STUN server is co-located with the Home Agent
since it is convenient from a security and configuration point of
view even though it is, from a solution point of view, not necessary.
+--------+ Mobility +--------+
| Home | Signalling | Home |
| Agent/ |----------------------------| Agent/ |
| STUN | | STUN |
| Server | | Server |
+--------+ +--------+
^ ^
| |
| |
Mobility | |Mobility
Signalling| |Signalling
| |
| |
+---v----+ +---v----+
| FW/NAT | | FW/NAT |
+---^----+ +---^----+
| |
| |
v v
+--------+ +--------+
| Agent | | Agent |
| L | | R |
+--------+ +--------+
Figure 1: Overview
The basic idea behind M-ICE is as follows: each end point has a
variety of candidate TRANSPORT ADDRESSES (combination of IP address,
transport protocol (UDP), and port) it could use to communicate with
the other end point.
To avoid unnecessary UDP encapsulation of end-to-end traffic in
case there is no need todo so, it is also possible to consider
using IP addresses rather than focusing exclusively on TRANSPORT
ADDRESSES. For example, two MIP hosts behind the same NAT do not
need to use UDP encapsulation. If there is no NAT or firewall
between the two communicating nodes then there is again no need to
provide support for UDP encapsulation. A future version of this
document will provide support for this functionality.
Potentially, any of L's candidate transport addresses can be used to
communicate with any of R's candidate transport addresses. In
practice, however, many combinations do not work. For instance, if L
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and R are both behind NATs, their directly attached interface
addresses (e.g., 192.168.1.100) are unlikely to be able to
communicate. The purpose of M-ICE is to discover which pairs of
addresses will work. The way that M-ICE does this is to
systematically try all possible pairs (in a carefully sorted order)
until it finds one or more that works. Once found, the best pair is
used for subsequent communication between the hosts.
1.1. Gathering Candidate Addresses
In order to execute ICE, an agent has to identify all of its address
candidates. A CANDIDATE is a transport address - a combination of IP
address and port for a particular transport protocol.
This document uses three types of candidates:
1. One viable candidate is a transport address obtained directly
from a local interface. Such a candidate is called a HOST
CANDIDATE.
2. Translated addresses on the public side of a NAT (called SERVER
REFLEXIVE CANDIDATES). This address is obtained via STUN.
3. Addresses obtained via relaying traffic through a Home Agent,
called RELAYED CANDIDATES.
1.2. Connectivity Checks
Once L has gathered all of its candidates, it orders them in highest
to lowest priority and sends them to R over the signalling channel.
We refer to the signaling channel to the end-to-end MIP exchange.
The extension to exchange candidates can be found in Section 11.
When R receives the L's MIP message, R performs the same candidate
gathering process and responds with its own list of candidates. At
the end of this process, each agent has a complete list of both its
candidates and its peer's candidates. It pairs them up, resulting in
CANDIDATE PAIRS. To see which pairs work, each agent schedules a
series of connectivity CHECKS. Each check is a STUN transaction that
the client will perform on a particular candidate pair by sending a
STUN request from the local candidate to the remote candidate; a
response indicates there is connectivity to the peer using that
candidate address.
It is important to note that the STUN requests are sent to and from
the exact same IP addresses and ports that will be used for
subsequent data traffic.
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1.3. Sorting Candidates
Because the algorithm above searches all candidate pairs, if a
working pair exists it will eventually find it no matter what order
the candidates are tried in. In order to produce faster (and better)
results, the candidates are sorted in a specified order. The
resulting list of sorted candidate pairs is called the CHECK LIST.
1.4. Frozen Candidates
The concept of frozen candidates is not applied when ICE is applied
to MIP. [Editor's Note: More investigations are needed to evaluate
whether this is indeed true and the concept of frozen candidates can
be ignored.]
1.5. Security for Checks
Because the ICE algorithm is used to discover which addresses can be
used to send traffic between two end points, it is important to
ensure that the process cannot be hijacked to send traffic to the
wrong location. Each STUN connectivity check is covered by a message
authentication code (MAC). There are two ways to generate the keying
material for this MAC. Either keying material is derived from the
keying material generated by the return routability procedure or new
keying material is distributed separately as excercised in ICE.
This document currently uses the latter technique without a strong
preference.
In any case, this MAC provides message integrity and data origin
authentication, thus stopping an attacker from forging or modifying
connectivity check messages.
1.6. Concluding M-ICE
ICE checks are performed in a specific sequence, so that high
priority candidate pairs are checked first, followed by lower
priority ones.
2. Terminology
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 [RFC2119].
This document heavily relies on the terminology introduced in
[I-D.ietf-mmusic-ice].
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3. Design Choices
The work in this document is guided by the following design choices,
namely:
o The offer/answer exchange described in ICE [I-D.ietf-mmusic-ice]
is mapped to the end-to-end MIP signaling exchange. For end-to-
end communication this document assumes that MIPv6 signaling are
allowed to be exchanged between the mobile node and the
correspondent node. When Network Address Translators and
firewalls are located along the path then direct end-to-end
communication between the two end points is typically not possible
and hence this protocol interaction is provided via MIP Home
Agents. The functionality described in [I-D.bajko-mip6-rrtfw] is
used.
o We assume that MIP initiators and MIP responders implement and use
STUN. For performing connectivity checks a couple of other
alternatives are, however, possible:
It would be possible to utilize the SHIM6 REAchability Protocol
(REAP) [I-D.ietf-shim6-failure-detection] but STUN provides the
same support with a more likely chance for widespread
deployment. REAP currently only provides IPv6 support. It it
obviously possible to turn a protocol in any other one.
Custom MIP messages could be created.
o If one peer does not support STUN then the optimal results of
M-ICE cannot be provided. There is, however, the ability to make
use of STUN LITE when a host is on the public address space and
known not to be behind a firewall.
o Obtaining Relay Addresses from STUN [I-D.ietf-behave-turn],
formerly known as TURN, is intentionally not used in this
document. For MIP, the Home Agent tunneling functionality is used
instead of TURN.
o This document makes use of the UDP-encapsulated of MIP packets, as
specified in [I-D.ietf-mip6-nemo-v4traversal].
o This document focuses only on the data exchange between the two
end points rather than on the communication between a mobile node
and the Home Agent or on the ability to allow MIP signaling
messages to traverse NATs and firewalls.
o Each STUN connectivity check is covered by a message
authentication code (MAC) generated based on keying material
derived from information carried in MIP messages, see Section 11.
Alternatively, keying material could be derived from the return
routability test procedure.
Note that the ICE description assumes usage within a VoIP environment
where individual flows are controlled. However, the protocol
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interaction described in this document operates at a lower layer
where application specific message flows are not visible. When a
CANDIDATE PAIR, consisting of two TRANSPORT ADDRESSES, is created
then it will typically refer to multiple flows then traffic between
two end points experiences UDP encapsulation (due to the need to
traverse a NAT or a stateful packet filtering firewall).
The descriptions in the ICE specification related to SIP, ANAT, RTP,
RTCP, third party call control, preconditions, forking, etc. are not
applicable to MIP and are not included in this document.
From an editorial point of view it would be possible to copy-and-
paste relevant parts of the ICE specification and to remove VoIP
specific descriptions but for this version of the document we did
not follow this approach.
The main accomplishment of this document is the reuse of the well-
established ICE specification that builds on STUN. STUN enjoys
widespread implementation support and maximum code re-use was one of
the design criteria for this document.
4. Sending the Initial Offer
In order to send the initial offer in an offer/answer exchange, an
agent must (1) gather candidates, (2) prioritize them, (3) choose
default candidates, and then (4) formulate and send them to the other
peer.
Section 4 of ICE [I-D.ietf-mmusic-ice] is applicable to this document
with the following two exceptions: First, TURN is not used in this
document but instead similar functionality is accomplished via a Home
Agent. Second, the description regarding encoding of candidates in
SDP is not applicable and replaced by a MIP specific encoding
described in Section 11.
5. Receiving the Initial Offer
When an agent receives an initial offer, it will check if the offerer
supports sufficient ICE functionality to proceed (i.e., if both
offerer and answerer are lite implementations, ICE cannot proceed),
determine its own role, gather candidates, prioritize them, choose
default candidates, encode and send an answer, and for full
implementations, form the check lists and begin connectivity checks.
Again, the description regarding encoding of candidates in SDP is not
applicable to this document and is replaced by a MIP specific
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encoding described in Section 11. Note that only the encoding is
different but not the semantic. As such, the description in Section
5 of [I-D.ietf-mmusic-ice] is applicable to this document.
6. Receipt of the Initial Answer
Section 6 of ICE [I-D.ietf-mmusic-ice] describes the procedures that
an agent follows when it receives the answer from the peer. It
verifies that its peer supports ICE, determines its role, and for
full implementations, forms the check list and begins performing
periodic checks.
7. Performing Connectivity Checks
Section 7 of ICE [I-D.ietf-mmusic-ice] describes how connectivity
checks are performed using STUN [I-D.ietf-behave-rfc3489bis] and the
content of that section is fully applicable to this document.
8. Concluding ICE Processing
The description in Section 8 of ICE [I-D.ietf-mmusic-ice] illustrates
processing rules that apply only to full implementations. Concluding
ICE involves nominating pairs by the controlling agent and updating
of state machinery
9. Subsequent Offer/Answer Exchanges
Either agent may generate a subsequent offer at any time. The rules
in Section 9 of ICE [I-D.ietf-mmusic-ice] will cause the controlling
agent to send an updated offer at the conclusion of ICE processing
when ICE has selected different candidate pairs from the default
pairs. Section 9 of ICE [I-D.ietf-mmusic-ice] defines rules for
construction of subsequent offers and answers.
Note that the term "media stream" in Section 9 of ICE
[I-D.ietf-mmusic-ice] translates to an individual UDP-encapsulated
data flow exchanged between the two MIP end points.
10. Keepalives
Section 10 of ICE [I-D.ietf-mmusic-ice] describes a keepalive
mechanism. The RTP description, such as RTP No-Op and RTP comfort
noise, is not applicable to this document. Other useful keepalive
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techniques are described in [I-D.marjou-behave-app-rtp-keepalive] and
may be useful for MIP; a recommendation will be made in a subsequent
version of this document.
11. Attribute Encoding
To accomplish the same functionality this specification needs to
reuse the semantic, but not necessarily the encoding, of seven
attributes defined in the ICE specification [I-D.ietf-mmusic-ice],
namely "candidate", "remote-candidates", "ice-lite", "ice-mismatch",
"ice-ufrag", "ice-pwd" and "ice-options".
Section 15.1 to Section 15.5 of ICE [I-D.ietf-mmusic-ice] describe
the semantic of the attributes.
MIP-ICE Mobility Options Format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Header Len |# of candidates|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |L|M| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Ice-pwd |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ice-ufrag | ice-options (var) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Candidate 1 .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Candidate 2 .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Candidate n .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where L: ice-lite
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M: ice-mismatch
# of candidates: the number of candidates carried by this option
Ice-options:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Candidate:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ver | Length | type | comp-id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| transport | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Connection-address .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| port | rel-port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Rel-address .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields have the following meaning:
ver: IP address version contained in Connection-address and
Rel-address fields
type: cand-type as defined in ICE
comp-id: component-id as defined in ICE
transport: transport address
priority: sender priority assigned to the connection-address, as
defined in ICE
connection-address: IP address, 32 bit if ver=4 and 128 bit if
ver=6
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port: port number
rel-port: port number, as defined in ICE
rel-address: IP address, as defined in ICE
Figure 2: Attribute Encoding
12. Demultiplexing MIP and STUN messages
When MIP and STUN messages are run over the same port it is necessary
to demultiplex them. For this usage it is necessary to have a
FINGERPRINT attribute in place, as defined in
[I-D.ietf-behave-rfc3489bis].
A STUN packet always has the fixed value 0x2112A442 in its Magic
Cookie field (bits 32-64 from the beginning of the UDP payload).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0| STUN Message Type | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 1 0 0 0 0 1 0 0 0 1 0 0 1 0 1 0 1 0 0 1 0 0 0 1 0 0 0 0 1 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. . .
Figure 3: STUN Header
In this same offset from the UDP header, the MIP header has the
Checksum field and the start of the Message Data field. The
concatenation of the Checksum field and the first 16 bits of the
Message Data field may coincide with the STUN Magic Cookie.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Proto | Header Len | MH Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
. .
. Message Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
. . .
Figure 4: MIP Header
When the value in that place equals the value of the STUN Magic
Cookie, the presence of the STUN FINGERPRINT attribute tells
unambigously whether this is a STUN message or not.
A future version will also discuss the demultiplexing when UDP
encapsulation is not used.
13. Example
The subsequent example shows a minimal message. For editorial
reasons middleboxes, such as NATs and firewalls along the path
between L and R are not depicted.
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L STUN R
_______|_______ | |
| (1) gather | | |
| candidates | | |
--------------- | |
| (2) HoTI | |
|---------------------------->|
| (3) CoTI (candidates) |
|---------------------------->|
| | |
| | ______|________
| | | (4) gather |
| | | candidates |
| | ---------------
|(5) HoT | |
|<----------------------------|
| (6) CoT (Candidates) |
|<----------------------------|
|(7) Bind Req | |
|S=$L-CoA-1 | |
|D=$R-CoA | |
|USE-CAND | |
|---------------------------->|
|(8) Bind Res | |
|S=$R-CoA | |
|D=$L-CoA-1 | |
|<----------------------------|
|data flows | |
| | |
|(9) Bind Req | |
|S=$R-CoA | |
|D=$L-CoA-1 | |
|<----------------------------|
| | |
| | |
|(10) Bind Res | |
|S=$L-CoA-1 | |
|D=$R-CoA | |
|---------------------------->|
| | |data flows
Figure 5: Example Message Flow
Lets assume two agents, L and R, where L is a mobile nodes with one
CoA and one HoA, and R is a node. L starts with gathering its host
(CoA) and server relayed (HoA) candidates. Agent L sets a type
preference of 126 for the host candidate and 100 for the server
relayed. The local preference is 65535. Based on this, the priority
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for the host candidate is 2130706178 and for the server relayed
candidate is 1694498562. It chooses its host candidate as the
default candidate and encodes the candidates into the MIP signalling
messages.
The candidate is received at agent R. Agent R will also start
gathering its own candidates, but it only has one host candidate.
Type and local preferences are assigned by R in the same way as L,
and the priority for the candidate will have the same value as L's
host candidate. R also chooses its host candidate as the default
candidate and encodes the candidates into the MIP signalling
messages.
Since neither side indicated that they are lite, the agent which
initiated the signalling that began ICE processing (agent L) becomes
the controlling agent.
Agents L and R both pair up the candidates. They both have two pairs
with the pair priorities 4.57566E+18 and 3.63891E+18, respectively.
Agent R begins its connectivity check for the first pair (between the
two host candidates). Since R is the controlled agent for this
session, the check omits the USE-CANDIDATE attribute.
When agent L gets the answer, it performs a connectivity check. It
implements the aggressive nomination algorithm, and thus includes a
USE-CANDIDATE attribute in this check. Since the check succeeds,
agent L creates a new pair and is added to the valid list. In
addition, it is marked as selected since the Binding Request
contained the USE-CANDIDATE attribute. Since there is a selected
candidate in the Valid list for the one component of this media
stream, ICE processing for this stream moves into the Completed
state. Agent L can now send media if it so chooses.
If agent R is behind a firewall, then the Binding Request from agent
L will be dropped. The ICE draft recommends that agents send STUN
request for the candidate pairs every 20ms. Thus, for instance if
the first Binding Request will be dropped, then next one will
succeed, as agent R also sends a Binding Request using the same 5
tuple selectors and open the pinhole in the firewall.
Upon receipt of the STUN Binding Request from agent L, agent R will
generate its triggered check, from its host candidate to agent L's
host candidate. This check will succeed. Consequently, agent R
constructs a new candidate pair using the host address from the
response as the local candidate and the destination of the request
L-CoA-1 as the remote candidate. This pair is added to the Valid
list for that media stream. Since the check was generated in the
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reverse direction of a check that contained the USE-CANDIDATE
attribute, the candidate pair is marked as selected. Consequently,
processing for this stream moves into the Completed state, and agent
R can also send media.
14. Security Considerations
There are several types of attacks possible in an M-ICE system. This
section considers these attacks and their countermeasures.
14.1. Attacks on Connectivity Checks
An attacker might attempt to disrupt the STUN connectivity checks.
Ultimately, all of these attacks fool an agent into thinking
something incorrect about the results of the connectivity checks.
The possible false conclusions an attacker can try and cause are:
False Invalid:
An attacker can fool a pair of agents into thinking a candidate
pair is invalid, when it isn't. This can be used to cause an
agent to prefer a different candidate (such as one injected by the
attacker), or to disrupt a call by forcing all candidates to fail.
False Valid:
An attacker can fool a pair of agents into thinking a candidate
pair is valid, when it isn't. This can cause an agent to proceed
with a session, but then not be able to receive any data traffic.
False Peer-Reflexive Candidate:
An attacker can cause an agent to discover a new peer reflexive
candidate, when it shouldn't have. This can be used to redirect
data traffic to a DoS target or to the attacker, for eavesdropping
or other purposes.
False Valid on False Candidate:
An attacker has already convinced an agent that there is a
candidate with an address that doesn't actually route to that
agent (for example, by injecting a false peer reflexive candidate
or false server reflexive candidate). It must then launch an
attack that forces the agents to believe that this candidate is
valid.
Of the various techniques for creating faked STUN messages described
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in [I-D.ietf-behave-rfc3489bis], many are not applicable for the
connectivity checks. Compromises of STUN servers are not much of a
concern, since the STUN servers are embedded in endpoints and
distributed throughout the network. Thus, compromising the peer's
embedded STUN server is equivalent to compromising the end point, and
if that happens, far more problematic attacks are possible than those
against ICE.
Injection of fake responses and relaying modified requests all can be
handled in ICE with the countermeasures discussed below.
To force the false invalid result, the attacker has to wait for the
connectivity check from one of the agents to be sent. When it is,
the attacker needs to inject a fake response with an unrecoverable
error response, such as a 600. However, since the candidate is, in
fact, valid, the original request may reach the peer agent, and
result in a success response. The attacker needs to force this
packet or its response to be dropped, through a DoS attack, layer 2
network disruption, or other technique. If it doesn't do this, the
success response will also reach the originator, alerting it to a
possible attack. Fortunately, this attack is mitigated completely
through the STUN message integrity mechanism. The attacker needs to
inject a fake response, and in order for this response to be
processed, the attacker needs the password. If the candidates are
exchange in MIP messages and therefore secured, the attacker will not
have the password.
Forcing the fake valid result works in a similar way. The agent
needs to wait for the Binding Request from each agent, and inject a
fake success response. The attacker won't need to worry about
disrupting the actual response since, if the candidate is not valid,
it presumably wouldn't be received anyway. However, like the fake
invalid attack, this attack is mitigated completely through the STUN
message integrity and offer/answer security techniques.
Forcing the false peer reflexive candidate result can be done either
with fake requests or responses, or with replays. We consider the
fake requests and responses case first. It requires the attacker to
send a Binding Request to one agent with a source IP address and port
for the false candidate. In addition, the attacker must wait for a
Binding Request from the other agent, and generate a fake response
with a XOR-MAPPED-ADDRESS attribute containing the false candidate.
Like the other attacks described here, this attack is mitigated by
the STUN message integrity mechanisms and secure offer/answer
exchanges.
Forcing the false peer reflexive candidate result with packet replays
is different. The attacker waits until one of the agents sends a
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check. It intercepts this request, and replays it towards the other
agent with a faked source IP address. It must also prevent the
original request from reaching the remote agent, either by launching
a DoS attack to cause the packet to be dropped, or forcing it to be
dropped using layer 2 mechanisms. The replayed packet is received at
the other agent, and accepted, since the integrity check passes (the
integrity check cannot and does not cover the source IP address and
port). It is then responded to. This response will contain a XOR-
MAPPED-ADDRESS with the false candidate, and will be sent to that
false candidate. The attacker must then receive it and relay it
towards the originator.
The other agent will then initiate a connectivity check towards that
false candidate. This validation needs to succeed. This requires
the attacker to force a false valid on a false candidate. Injecting
of fake requests or responses to achieve this goal is prevented using
the integrity mechanisms of STUN and the offer/answer exchange.
Thus, this attack can only be launched through replays. To do that,
the attacker must intercept the check towards this false candidate,
and replay it towards the other agent. Then, it must intercept the
response and replay that back as well.
This attack is very hard to launch unless the attacker is identified
by the fake candidate. This is because it requires the attacker to
intercept and replay packets sent by two different hosts. If both
agents are on different networks (for example, across the public
Internet), this attack can be hard to coordinate, since it needs to
occur against two different endpoints on different parts of the
network at the same time.
If the attacker them self is identified by the fake candidate the
attack is easier to coordinate. However, since MIP utilizes IPsec
ESP to protect the data traffic end-to-end, the attacker will not be
able to inspect any application data, they will only be able to
discard them. However, this attack requires the agent to disrupt
packets in order to block the connectivity check from reaching the
target. In that case, if the goal is to disrupt the end-to-end
communication, its much easier to just disrupt it with the same
mechanism, rather than attack ICE.
14.2. Attacks on Address Gathering
ICE endpoints make use of STUN for gathering candidates from a STUN
server in the network. This is corresponds to the Binding Discovery
usage of STUN described in [I-D.ietf-behave-rfc3489bis]. As a
consequence, the attacks against STUN itself that are described in
that specification can still be used against the binding discovery
usage when utilized with ICE.
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However, the additional mechanisms provided by ICE actually
counteract such attacks, making binding discovery with STUN more
secure when combined with ICE.
Consider an attacker which is able to provide an agent with a faked
mapped address in a STUN Binding Request that is used for address
gathering. This is the primary attack primitive described in
[I-D.ietf-behave-rfc3489bis]. This address will be used as a server
reflexive candidate in the ICE exchange. For this candidate to
actually be used for media, the attacker must also attack the
connectivity checks, and in particular, force a false valid on a
false candidate. This attack is very hard to launch if the false
address identifies a fourth party (neither the offerer, answerer, or
attacker), since it requires attacking the checks generated by each
agent in the session.
If the attacker elects not to attack the connectivity checks, the
worst it can do is prevent the server reflexive candidate from being
used. However, if the peer agent has at least one candidate that is
reachable by the agent under attack, the STUN connectivity checks
themselves will provide a peer reflexive candidate that can be used
for the exchange of media. Peer reflexive candidates are generally
preferred over server reflexive candidates. As such, an attack
solely on the STUN address gathering will normally have no impact on
a session at all.
14.3. Attacks on the Offer/Answer Exchanges
An attacker that can modify or disrupt the offer/answer exchanges
themselves can readily launch a variety of attacks with M-ICE. They
could direct data traffic to a target of a DoS attack, they could
insert themselves into the data exchange, and so on. The security
considerations of MIP apply.
14.4. Insider Attacks
In addition to attacks where the attacker is a third party trying to
insert fake offers, answers or STUN messages, there are several
attacks possible with ICE when the attacker is an authenticated and
valid participant in the M-ICE exchange.
14.4.1. MIP Amplification Attack
In this attack, the attacker initiates communication to other agents,
and maliciously includes the IP address and port of a DoS target as
the destination for data traffic signaled in the MIP exchange.
This could causes substantial amplification; a single offer/answer
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exchange can create a continuing flood of data packets, possibly at
high rates (consider video sources). This attack is not specific to
ICE, but ICE can help provide remediation.
Specifically, if ICE is used, the agent receiving the malicious SDP
will first perform connectivity checks to the target of media before
sending media there. If this target is a third party host, the
checks will not succeed, and media is never sent.
Unfortunately, ICE doesn't help if its not used, in which case an
attacker could simply send the offer without the ICE parameters.
However, in environments where the set of clients are known, and
limited to ones that support ICE, the server can reject any offers or
answers that don't indicate ICE support.
14.4.2. STUN Amplification Attack
The STUN amplification attack is similar to the MIP amplification
attack. However, instead of data packets being directed to the
target, STUN connectivity checks are directed to the target. The
attacker sends an offer with a large number of candidates, say 50.
The answerer receives the offer, and starts its checks, which are
directed at the target, and consequently, never generate a response.
The answerer will start a new connectivity check every 20ms, and each
check is a STUN transaction consisting of 7 transmissions of a
message 65 bytes in length (plus 28 bytes for the IP/UDP header) that
runs for 7.9 seconds, for a total of 58 bytes/second per transaction
on average. In the worst case, there can be 395 transactions in
progress at once (7.9 seconds divided by 20ms), for a total of 182
kbps, just for STUN requests.
It is impossible to eliminate the amplification, but the volume can
be reduced through a variety of heuristics. Agents SHOULD limit the
total number of connectivity checks they perform to 100.
Additionally, agents MAY limit the number of candidates they'll
accept in an offer or answer.
15. IAB Considerations
The IAB has studied the problem of "Unilateral Self Address Fixing",
which is the general process by which a agent attempts to determine
its address in another realm on the other side of a NAT through a
collaborative protocol reflection mechanism [RFC3424]. M-ICE is an
example of a protocol that performs this type of function.
Interestingly, the process for M-ICE is not unilateral, but
bilateral, and the difference has a significant impact on the issues
raised by IAB. M-ICE can be considered a B-SAF (Bilateral Self-
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Address Fixing) protocol, rather than an UNSAF protocol. Regardless,
the IAB has mandated that any protocols developed for this purpose
document a specific set of considerations. This section meets those
requirements.
15.1. Problem Definition
From RFC 3424 [RFC3424] any UNSAF proposal must provide:
Precise definition of a specific, limited-scope problem that is to be
solved with the UNSAF proposal. A short term fix should not be
generalized to solve other problems; this is why "short term fixes
usually aren't".
The specific problems being solved by M-ICE are:
Provide a means for two peers to determine the set of transport
addresses which can be used for communication.
Provide a means for resolving many of the limitations of other UNSAF
mechanisms by wrapping them in an additional layer of processing (the
M-ICE methodology).
Provide a means for a agent to determine an address that is reachable
by another peer with which it wishes to communicate.
15.2. Exit Strategy
From RFC 3424, any UNSAF proposal must provide:
Description of an exit strategy/transition plan. The better short
term fixes are the ones that will naturally see less and less use as
the appropriate technology is deployed.
M-ICE itself doesn't easily get phased out. However, it is useful
even in a globally connected Internet, to serve as a means for
detecting whether communication paths are disrupted. M-ICE also
helps prevent certain security attacks which have nothing to do with
NAT. However, what M-ICE does is help phase out other UNSAF
mechanisms. M-ICE effectively selects amongst those mechanisms,
prioritizing ones that are better, and deprioritizing ones that are
worse. Local IPv6 addresses can be preferred. As NATs begin to
dissipate as IPv6 is introduced, server reflexive and relayed
candidates (both forms of UNSAF mechanisms) simply never get used,
because higher priority connectivity exists to the native host
candidates. Therefore, the servers get used less and less, and can
eventually be remove when their usage goes to zero.
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Indeed, M-ICE can assist in the transition from IPv4 to IPv6. It can
be used to determine whether to use IPv6 or IPv4 when two dual-stack
hosts communicate. It can also allow a network with both 6to4 and
native v6 connectivity to determine which address to use when
communicating with a peer.
15.3. Brittleness Introduced by M-ICE
From RFC3424, any UNSAF proposal must provide:
Discussion of specific issues that may render systems more "brittle".
For example, approaches that involve using data at multiple network
layers create more dependencies, increase debugging challenges, and
make it harder to transition.
M-ICE uses ICE that is utilizes [I-D.ietf-behave-rfc3489bis] instead
of traditional STUN, RFC 3489 [RFC3489]). RFC 3489 has several
points of brittleness. One of them is the discovery process which
requires a agent to try and classify the type of NAT it is behind.
This process is error-prone. With M-ICE, that discovery process is
simply not used. Rather than unilaterally assessing the validity of
the address, its validity is dynamically determined by measuring
connectivity to a peer. The process of determining connectivity is
very robust.
Another point of brittleness in traditional STUN is that it assumes
that the STUN server is on the public Internet. Interestingly, with
M-ICE, that is not necessary. There can be a multitude of STUN
servers in a variety of address realms. ICE will discover the one
that has provided a usable address.
The most troubling point of brittleness in traditional STUN is that
it does not work in all network topologies. In cases where there is
a shared NAT between each agent and the STUN server, traditional STUN
may not work. With ICE, that restriction is removed.
Traditional STUN also introduces some security considerations.
Fortunately, those security considerations are also mitigated by ICE.
Consequently, ICE serves to repair the brittleness introduced in
other UNSAF mechanisms, and does not introduce any additional
brittleness into the system.
With M-ICE Home Agents are used and they are assumed to be located on
the public Internet to allow MIP to work.
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15.4. Requirements for a Long Term Solution
From RFC 3424, any UNSAF proposal must provide:
Identify requirements for longer term, sound technical solutions --
contribute to the process of finding the right longer term solution.
M-ICE provides a long term solution by utilizing ICE concepts that
have received a lot of peer review in the VoIP community and to apply
them to MIP. The only other possible long term solutions are (a) to
get rid of middleboxes, such as NATs and firewalls or to (b) interact
with them. Regarding (b) extensions for STUN to allow the protocol
to be deployed on NATs and firewalls is currently being investigated
in [I-D.wing-behave-nat-control-stun-usage].
15.5. Issues with Existing NAPT Boxes
From RFC 3424, any UNSAF proposal must provide:
Discussion of the impact of the noted practical issues with existing,
deployed NA[P]Ts and experience reports.
A number of NAT boxes are now being deployed into the market which
try and provide "generic" ALG functionality. These generic ALGs hunt
for IP addresses, either in text or binary form within a packet, and
rewrite them if they match a binding. This interferes with
traditional STUN. However, the update to STUN
[I-D.ietf-behave-rfc3489bis] uses an encoding which hides these
binary addresses from generic ALGs.
Existing NAPT boxes have non-deterministic and typically short
expiration times for UDP-based bindings. This requires
implementations to send periodic keepalives to maintain those
bindings. ICE uses a default of 15s, which is a very conservative
estimate. Eventually, over time, as NAT boxes become compliant to
behave [RFC4787], this minimum keepalive will become deterministic
and well-known, and the ICE timers can be adjusted. Having a way to
discover and control the minimum keepalive interval would be far
better still.
16. Contributors
We would like to thank Thomas Schreck for his contributions to
various aspects in this document.
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17. Acknowledgments
The authors would like to thank Jonathan Rosenberg for his work on
the ICE specification. This document copy-and-pastes text from the
ICE specification. Hence, all the credits go to Jonathan.
Finally, Dan Wing and Philip Matthews helped us with the work on HIP-
ICE.
18. References
18.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", March 1997.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-16 (work in progress), June 2007.
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
18.2. Informative References
[I-D.ietf-behave-turn]
Rosenberg, J., "Obtaining Relay Addresses from Simple
Traversal Underneath NAT (STUN)",
draft-ietf-behave-turn-03 (work in progress), March 2007.
[I-D.ietf-shim6-failure-detection]
Arkko, J. and I. Beijnum, "Failure Detection and Locator
Pair Exploration Protocol for IPv6 Multihoming",
draft-ietf-shim6-failure-detection-08 (work in progress),
June 2007.
[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
March 2003.
[I-D.ietf-behave-rfc3489bis]
Rosenberg, J., "Session Traversal Utilities for (NAT)
(STUN)", draft-ietf-behave-rfc3489bis-06 (work in
progress), March 2007.
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[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[I-D.wing-behave-nat-control-stun-usage]
Wing, D. and J. Rosenberg, "Discovering, Querying, and
Controlling Firewalls and NATs using STUN",
draft-wing-behave-nat-control-stun-usage-02 (work in
progress), June 2007.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[I-D.ietf-monami6-multiplecoa]
Wakikawa, R., "Multiple Care-of Addresses Registration",
draft-ietf-monami6-multiplecoa-02 (work in progress),
March 2007.
[I-D.ietf-mip6-nemo-v4traversal]
Soliman, H., "Mobile IPv6 support for dual stack Hosts and
Routers (DSMIPv6)", draft-ietf-mip6-nemo-v4traversal-04
(work in progress), March 2007.
[I-D.ietf-mip4-dsmipv4]
Tsirtsis, G., "Dual Stack Mobile IPv4",
draft-ietf-mip4-dsmipv4-02 (work in progress), May 2007.
[I-D.marjou-behave-app-rtp-keepalive]
Marjou, X., "Application Mechanism for maintaining alive
the Network Address Translator (NAT) mappings associated
to RTP flows.", draft-marjou-behave-app-rtp-keepalive-01
(work in progress), February 2007.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[I-D.bajko-mip6-rrtfw]
Bajko, G., "Firewall friendly RTT for MIPv6",
draft-bajko-mip6-rrtfw-01 (work in progress),
October 2006.
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Authors' Addresses
Hannes Tschofenig
Nokia Siemens Networks
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Hannes.Tschofenig@nsn.com
URI: http://www.tschofenig.com
Gabor Bajko
Nokia
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