Internet-Draft P. Patil
Intended status: Standards Track T. Reddy
Expires: January 08, 2013 Cisco
July 09, 2012

Mobility with ICE (MICE)


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.

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Table of Contents

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 by using tunnels (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.

To overcome NAT and Firewall traversal issues, hosts may choose to use the services of a TURN server that acts as a relay to exchange packets with other hosts [RFC5766]. Relaying is also used for IPv6 privacy. Clients allocate an IP address and port on the TURN server, called the relayed transport address, for a specific client IP address and port. Both the client and the server remember the 5-tuple used in the Allocate request and all subsequent transactions between the client and the server use the same 5-tuple. When an IP address of a client changes because of movement to a new network, the existing allocation on the TURN server cannot be re-used and the client will have to allocate a second relayed transport address by creating a second allocation using a different 5-tuple. This will break connection between the client and its peer. Communication between the client and its peer can only continue when the new relayed transport address is signaled to the peer.

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.

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].

3. Mobility using ICE

When both endpoints support ICE, ICE itself can provide mobility.

[[more text will be written here.]]

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 server, it will also include a new STUN attribute MOBILITY-REQUEST which indicates that the client is a mobile node. The TURN server provisions a ticket that is sent inside a 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. 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-REQUEST               |             |            
    |                                    |             |             
    |<--------------- Allocate failure --|             |             
    |                 (401 Unauthorized) |             |             
    |                                    |             |             
    |-- Allocate request --------------->|             |             
    |   + MOBILITY-REQUEST               |             |             
    |                                    |             |             
    |<---------- 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-REQUEST attribute indicating that the client is a mobile node.

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-REQUEST attribute is included and the TURN session mobility is forbidden by local policy, the server MUST reject the request with a TBD (Mobility Forbidden) error. Following the rules specified in [RFC5389], if the server does not understand the MOBILITY-REQUEST attribute, it generates an Allocate error response, which includes an ERROR-CODE attribute with 420 (Unknown Attribute) response code. This response will contain an UNKNOWN-ATTRIBUTE attribute listing the unknown MOBILITY-REQUEST 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].

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 response. This attribute will be presented by the client to the server during a subsequent Refresh Request to aid mobility.

4.3. New STUN Attributes


This attribute is used by the client to indicate to the TURN server that it is a mobile node. This attribute has no value part and thus the attribute length field is 0.


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 Codes

This document defines the following new error response codes:

TBD (Mobility Forbidden): Mobility request was valid but cannot be performed due to administrative or similar restrictions.

5. IANA Considerations

This note requests of the IANA to add attributes MOBILITY-REQUEST and MOBILITY-TICKET to the TURN attribute registry. The note also requires reservation of a new error code for error Mobility Forbidden.

6. Security Considerations

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. References

7.1. 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.
[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.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P. and D. Wing, "Session Traversal Utilities for NAT (STUN)", RFC 5389, October 2008.

7.2. Informative References

[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, August 1999.
[RFC5768] Rosenberg, J., "Indicating Support for Interactive Connectivity Establishment (ICE) in the Session Initiation Protocol (SIP)", RFC 5768, April 2010.

Authors' Addresses

Dan Wing Cisco Systems, Inc. 170 West Tasman Drive San Jose, California 95134 USA EMail:
Prashanth Patil Cisco Systems, Inc. Cessna Business Park, Varthur Hobli Sarjapur Marthalli Outer Ring Road Bangalore, Karnataka 560103 India EMail:
Tirumaleswar Reddy Cisco Systems, Inc. Cessna Business Park, Varthur Hobli Sarjapur Marathalli Outer Ring Road Bangalore, Karnataka 560103 India EMail: