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MMUSIC D. Wing
Internet-Draft P. Patil
Intended status: Standards Track T. Reddy
Expires: January 17, 2013 Cisco
July 16, 2012
Mobility with ICE (MICE)
draft-wing-mmusic-ice-mobility-01
Abstract
This specification describes how endpoint mobility can be achieved
using ICE. Two mechanisms are shown, one where both endpoints
support ICE and another where only one endpoint supports ICE.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 17, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. Mobility using ICE . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Gaining an Interface . . . . . . . . . . . . . . . . . . . 4
3.2. Receiving ICE Mobility event . . . . . . . . . . . . . . . 5
3.3. Losing an Interface . . . . . . . . . . . . . . . . . . . 6
3.4. New STUN Attributes MOBILITY-EVENT and MOBILITY-SUPPORT . 6
4. Mobility using TURN . . . . . . . . . . . . . . . . . . . . . 6
4.1. Creating an Allocation . . . . . . . . . . . . . . . . . . 7
4.1.1. Sending an Allocate Request . . . . . . . . . . . . . 7
4.1.2. Receiving an Allocate Request . . . . . . . . . . . . 8
4.1.3. Receiving an Allocate Success Response . . . . . . . . 8
4.1.4. Receiving an Allocate Error Response . . . . . . . . . 8
4.2. Refreshing an Allocation . . . . . . . . . . . . . . . . . 9
4.2.1. Sending a Refresh Request . . . . . . . . . . . . . . 9
4.2.2. Receiving a Refresh Request . . . . . . . . . . . . . 9
4.2.3. Receiving a Refresh Response . . . . . . . . . . . . . 9
4.3. New STUN Attribute MOBILITY-TICKET . . . . . . . . . . . . 10
4.4. New STUN Error Response Code . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6.1. Considerations for ICE mechanism . . . . . . . . . . . . . 10
6.2. Considerations for TURN mechanism . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
When moving between networks, an endpoint has to change its IP
address. This change breaks upper layer protocols such as TCP and
RTP. Various techniques exist to prevent this breakage, all tied to
making the endpoint's IP address static (e.g., Mobile IP, Proxy
Mobile IP, LISP). Other techniques exist, which make the upper layer
protocol ambivalent to IP address changes (e.g., SCTP). The
mechanisms described in this document are in that last category.
ICE [RFC5245] ensures two endpoints have a working media path between
them, and is typically used by Internet-connected interactive media
systems (e.g., SIP endpoints). ICE does not expect either the local
host or the remote host to change their IP addresses. Although ICE
does allow an "ICE restart", this is done by sending a re-INVITE
which goes over the SIP signaling path. The SIP signaling path is
often slower than the media path (which needs to be recovered as
quickly as possible), consumes an extra half round trip, and incurs
an additional delay if the mobility event forces the endpoint to re-
connect with its SIP proxy. Thus, this document attempts to perform
mobility entirely on the media path.
A TURN [RFC5766] server relays media packets and is used for a
variety of purposes, including overcoming NAT and firewall traversal
issues and IP address privacy. The existing TURN specification does
not allow the client address to change, especially if multiple
clients share the same TURN username (e.g., the same credentials are
used on multiple devices).
This document proposes two mechanisms to achieve RTP mobility: a
mechanism where both endpoints support ICE, and a mechanism where
only one endpoint supports ICE. When both endpoints support ICE, ICE
itself can be used to provide mobility. When only one endpoint
supports ICE, a TURN server provides mobility. Both mobility
techniques work across and between network types (e.g., between 3G
and wired Internet access), so long as the client can still access
the remote ICE peer or TURN server.
Readers are assumed to be familiar with ICE [RFC5245].
2. Notational Conventions
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 [RFC2119].
This note uses terminology defined in [RFC5245].
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3. Mobility using ICE
When both endpoints support ICE, ICE itself can provide mobility
functions. One of the primary aspects of ICE is its address
gathering, wherein ICE has each endpoint determine all of the IP
addresses and ports that might be usable for that endpoint and
communicate that list of addresses and ports to its peer, usually
over SDP. That enables the next primary aspect of ICE, which is its
connectivity checks: each ICE endpoint sends a connectivity check to
that list of addresses and ports. A connectivity check may
unknowingly traverse a NAT, which means the ICE endpoint receiving
the connectivity check cannot validate the source IP address or port
of the connectivity against the list of IP addresses and ports
provided by the ICE peer. In fact, if the source IP address and port
is not known to the ICE endpoint, it is added to the list of
candidates (Section 7.2.1.3 of [RFC5245].
ICE Mobility takes advantage of that existent ICE functionality.
Media can be switched to the new interface before or after the
previous interface is lost.
When an interface is lost, media traffic might or might not be
utilizing that interface. If media traffic is currently traversing
the interface, this is considered a "break before make", because the
host has not already moved its media traffic to a different
interface.
Endpoints that support ICE Mobility perform ICE normally, and MUST
also include the MOBILITY-SUPPORT attribute in all of their STUN
requests and their STUN responses. The inclusion of this attribute
allows the ICE peer to determine if it can achieve mobility using ICE
or needs to use TURN (or needs to use some other mechanism, such as
Mobile IP). To force the use of TURN to achieve ICE mobility, the
ICE endpoint SHOULD NOT respond to ICE connectivity checks that have
an IP address and port different from the TURN server, unless those
connectivity checks contain the MOBILITY-SUPPORT attribute. In this
way, the remote peer will think those other candidates are invalid
(because its connectivity checks did not succeed).
After concluding ICE and moving to the ICE completed state (see
Section 8 of [RFC5245] either endpoint or both endpoints can initiate
ICE Mobility, no matter if it was the Controlling Agent or the
Controlled Agent during normal ICE processing.
3.1. Gaining an Interface
When gaining an interface which is suitable to send media by the
host's policy (if any), the ICE endpoint performs ICE Mobility. ICE
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Mobility is performed by:
1. The ICE endpoint clears its ICE check list.
2. The ICE endpoint initiating an ICE connectivity check on the new
interface, with the MOBILITY-EVENT attribute.
3. If this interface is the only suitable interface for media (that
is, other suitable interfaces have been lost), then the
connectivity check from the previous step SHOULD also include the
USE-CANDIDATE attribute to signals an aggressive nomination (see
Section 2.6 of [RFC5245]), and media MAY immediately begin
flowing over that interface.
4. The ICE endpoint performs Sections 7.2.1.3, 7.2.1.4, and 7.2.1.5
of [RFC5245].
5. If the ICE connectivity check succeeds then ICE agents creates a
new pair, adds the pair to the valid list and marks it as
selected. The ICE agent can now send media using the newly
selected candidate pair, even if it is running in Regular
Nomination mode.
6. Once ICE connectivity checks for all of the media streams are
completed, the controlling ICE endpoint follows the procedures in
Section 11.1 of [RFC5245], specifically to send updated offer if
the candidates in the m and c lines for the media stream (called
the DEFAULT CANDIDATES) do not match ICE's SELECTED CANDIDATES
(also see Appendix B.9 of [RFC5245]).
3.2. Receiving ICE Mobility event
A STUN Binding Request containing the MOBILITY-EVENT attribute MAY be
received by an ICE endpoint. If this is received before the endpoint
is in the ICE Concluded state, it should be silently discarded.
The agent remembers the highest-priority nominated pairs in the Valid
list for each component of the media stream, called the previous
selected pairs. It continues sending media to that address until it
finishes with the steps described below. Because those packets might
not be received due to the mobility event, it MAY cache a copy of
those packets.
The ICE endpoint clears its ICE check list.
The ICE endpoint performs Sections 7.2.1.3, 7.2.1.4, and 7.2.1.5 of
[RFC5245].
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3.3. Losing an Interface
When an interface is lost, the SDP MAY be updated, so that the remote
ICE host does not waste its efforts with connectivity checks to that
address, as those checks will fail. Because it can be argued that
this is merely an optimization, and that the interface loss might be
temporary (and soon regained), and that ICE has reasonable
accommodation for candidates where connectivity checks timeout, this
specification does not strongly encourage updating the SDP to remove
a lost interface. Likewise, this specification recommends that ICE
candidate addresses be maintained actively, subject to the host's
policy. For example, battery operated hosts have a strong incentive
to not maintain mappings to TURN servers, as that maintenance
requires periodic keepalive messages. As another example, a host
that is receiving media over IPv6 may not want to persist with
keeping a NATted IPv4 mapping alive (because that consumes a NAT
mapping that could be more useful to a host actively utilizing the
mapping for real traffic).
Note: this differs from Section 8.3 of [RFC5245], which encourages
abandoning un-used candidates.
Note: A future version of this document will have more normative
language in this section.
3.4. New STUN Attributes MOBILITY-EVENT and MOBILITY-SUPPORT
Two new attributes are defined by this section: MOBILITY-EVENT and
MOBILITY-SUPPORT.
The MOBILITY-EVENT attribute indicate the sender experienced a
mobility event. This attribute has no value, thus the attribute
length field MUST always be 0. Rules for sending and interpretation
of receiving are described above.
The MOBILITY-SUPPORT attribute indicates the sender supports ICE
Mobility, as defined in this document. This attribute has no value,
thus the attribute length field MUST always be 0. Rules for sending
and interpretation of receiving are described above.
4. Mobility using TURN
To achieve mobility, a TURN client should be able to retain an
allocation on the TURN server across changes in the client IP address
as a consequence of movement to other networks.
When the client sends the initial Allocate request to the TURN
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server, it will also include the new STUN attribute MOBILITY-TICKET
(with zero length value), which indicates that the client is capable
of mobility and desires a ticket. The TURN server provisions a
ticket that is sent inside the new STUN attribute MOBILITY-TICKET in
the Allocate Success response to the client. The ticket will be used
by the client when it wants to refresh the allocation but with a new
client IP address and port. It also ensures that the allocation can
only be refreshed this way by the same client. When a client's IP
address changes due to mobility, it presents the previously obtained
ticket in a Refresh Request to the TURN server. If the ticket is
found to be valid, the TURN server will retain the same relayed
address/port for the new IP address/port allowing the client to
continue using previous channel bindings -- thus, the TURN client
does not need to obtain new channel bindings. Any data from external
peer will be delivered by the TURN server to this new IP address/port
of the client. The TURN client will continue to send application
data to its peers using the previously allocated channelBind
Requests.
TURN TURN Peer
client server A
|-- Allocate request --------------->| |
| + MOBILITY-TICKET (length=0) | |
| | |
|<--------------- Allocate failure --| |
| (401 Unauthorized) | |
| | |
|-- Allocate request --------------->| |
| + MOBILITY-TICKET (length=0) | |
| | |
|<---------- Allocate success resp --| |
| + MOBILITY-TICKET | |
... ... ...
(changes IP address)
| | |
|-- Refresh request ---------------->| |
| + MOBILITY-TICKET | |
| | |
|<----------- Refresh success resp --| |
| + MOBILITY-TICKET | |
| | |
4.1. Creating an Allocation
4.1.1. Sending an Allocate Request
In addition to the process described in Section 6.1 of [RFC5766], the
client includes the MOBILITY-TICKET attribute with length 0. This
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indicates the client is a mobile node and wants a ticket.
4.1.2. Receiving an Allocate Request
In addition to the process described in Section 6.2 of [RFC5766], the
server does the following:
If the MOBILITY-TICKET attribute is included, and has length zero,
and the TURN session mobility is forbidden by local policy, the
server MUST reject the request with the new Mobility Forbidden error
code. Following the rules specified in [RFC5389], if the server does
not understand the MOBILITY-TICKET attribute, it ignores the
attribute.
If the server can successfully process the request create an
allocation, the server replies with a success response that includes
a STUN MOBILITY-TICKET attribute. TURN server stores it's session
state, such as 5-tuple and NONCE, into a ticket that is encrypted by
a key known only to the TURN server and sends the ticket in the STUN
MOBILITY-TICKET attribute as part of Allocate success response.
The ticket is opaque to the client, so the structure is not subject
to interoperability concerns, and implementations may diverge from
this format. TURN Allocation state information is encrypted using
128-bit key for Advance Encryption Standard (AES) and 256-bit key for
HMAC-SHA-256 for integrity protection.
4.1.3. Receiving an Allocate Success Response
In addition to the process described in Section 6.3 of [RFC5766], the
client will store the MOBILITY-TICKET attribute, if present, from the
response. This attribute will be presented by the client to the
server during a subsequent Refresh request to aid mobility.
4.1.4. Receiving an Allocate Error Response
If the client receives an Allocate error response with error code TBD
(Mobility Forbidden), the error is processed as follows:
o TBD (Mobility Forbidden): The request is valid, but the server is
refusing to perform it, likely due to administrative restrictions.
The client considers the current transaction as having failed. The
client MAY notify the user or operator and SHOULD NOT retry the same
request with this server until it believes the problem has been
fixed.
All other error responses must be handled as described in [RFC5766].
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4.2. Refreshing an Allocation
4.2.1. Sending a Refresh Request
If a client wants to refresh an existing allocation and update its
time-to-expiry or delete an existing allocation, it will send a
Refresh Request as described in Section 7.1 of [RFC5766]. If the
client wants to retain the existing allocation in case of IP change,
it will include the MOBILITY-TICKET attribute received in the
Allocate Success response. If a Refresh transaction was previously
made, the MOBILITY-TICKET attribute received in the Refresh Success
response of the transaction must be used.
4.2.2. Receiving a Refresh Request
In addition to the process described in Section 7.2 of [RFC5766], the
client does the following:
If the STUN MOBILITY-TICKET attribute is included in the Refresh
Request then the server will not retrieve the 5-tuple from the packet
to identify an associated allocation. Instead TURN server will
decrypt the received ticket, verify the ticket's validity and
retrieve the 5-tuple allocation from the contents of the ticket. If
this 5-tuple obtained from the ticket does not identify an existing
allocation then the server MUST reject the request with an error.
If the source IP address and port of the Refresh Request is different
from the stored 5-tuple allocation, the TURN server proceeds with
checks to see if NONCE in the Refresh request is the same as the one
provided in the ticket. The TURN server also uses MESSAGE-INTEGRITY
validation to identify the that it is the same user which had
previously created the TURN allocation. If the above checks are not
successful then server MUST reject the request with a 441 (Wrong
Credentials) error.
If all of the above checks pass, the TURN server understands that the
client has moved to a new network and acquired a new IP address. The
source IP address of the request could either be the host transport
address or server-reflexive transport address. The server then
updates it's 5-tuple with the new client IP address and port. TURN
server calculates the ticket with the new 5-tuple and sends the new
ticket in the STUN MOBILITY-TICKET attribute as part of Refresh
Success response.
4.2.3. Receiving a Refresh Response
In addition to the process described in Section 7.3 of [RFC5766], the
client will store the MOBILITY-TICKET attribute, if present, from the
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response. This attribute will be presented by the client to the
server during a subsequent Refresh Request to aid mobility.
4.3. New STUN Attribute MOBILITY-TICKET
This attribute is used to retain an Allocation on the TURN server.
It is exchanged between the client and server to aid mobility. The
value is encrypted and identifies identifies session state such as
5-tuple and NONCE. The value of MOBILITY-TICKET is a variable-length
value.
4.4. New STUN Error Response Code
This document defines the following new error response code:
Mobility Forbidden: Mobility request was valid but cannot be
performed due to administrative or similar restrictions.
5. IANA Considerations
IANA is requested to add the following attributes to the STUN
attribute registry [iana-stun],
o MOBILITY-TICKET (0x802E, in the comprehension-optional range)
o MOBILITY-EVENT (0x802, in the comprehension-required range)
o MOBILITY-SUPPORT (0x8000, in the comprehension-optional range)
and to add a new STUN error code "Mobility Forbidden" with the value
501 to the STUN Error Codes registry [iana-stun].
6. Security Considerations
6.1. Considerations for ICE mechanism
A mobility event only occurs after both ICE endpoints have exchanged
their ICE information. Thus, both username fragments are already
known to both endpoints. Each endpoint contributes at least 24 bits
of randomness to the ice-ufrag (Section 15.4 of [RFC5245]), which
provides 48 bits of randomness. An off-path attacker would have to
guess those 48 bits to cause the endpoints to perform HMAC-SHA1
validation of the MESSAGE-INTEGRITY attribute.
An attacker on the path between the ICE endpoints will see both ice-
ufrags, and can cause the endpoints to perform HMAC-SHA1 validation
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by sending messages from any IP address.
6.2. Considerations for TURN mechanism
TURN server MUST use strong encryption and integrity protection for
the ticket to prevent an attacker from using a brute force mechanism
to obtain the ticket's contents or refreshing allocations.
Security considerations described in [RFC5766] are also applicable to
this mechanism.
7. Acknowledgements
Thanks to Alfred Heggestad for his review and comments.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
8.2. Informative References
[iana-stun]
IANA, "IANA: STUN Attributes", April 2011,
<http://www.iana.org/assignments/stun-parameters/stun-pa
rameters.xml>.
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Authors' Addresses
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
USA
Email: dwing@cisco.com
Prashanth Patil
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marthalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: praspati@cisco.com
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: tireddy@cisco.com
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