Cisco Systems, Inc.

Cessna Business Park, Varthur Hobli Sarjapur Marathalli Outer Ring Road Bangalore Karnataka 560103 India tireddy@cisco.com

Cisco Systems, Inc.

170 West Tasman Drive San Jose California 95134 USA dwing@cisco.com

Cisco Systems, Inc.

Bangalore India praspati@cisco.com

Cisco Systems, Inc.

Philip Pedersens vei 22 Lysaker Akershus 1325 Norway palmarti@cisco.com

TRAM It is desirable to minimize traffic disruption caused by changing IP address during a mobility event. One mechanism to minimize disruption is to expose a shorter network path to the mobility event so only the local network elements are aware of the changed IP address but the remote peer is unaware of the changed IP address. This draft provides such an IP address mobility solution using Traversal Using Relays around NAT (TURN). This is achieved by allowing a client to retain an allocation on the TURN server when the IP address of the client changes.

When moving between networks, the endpoint's IP address can change or (due to NAT) the endpoint's public IP address can change. Such a change of IP address 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 change in IP address agnostic to the upper layer protocol (e.g., SCTP). The mechanism described in this document are in that last category. A Traversal Using Relays around NAT (TURN) server relays media packets and is used for a variety of purposes, including overcoming NAT and firewall traversal issues. The existing TURN specification does not permit a TURN client to reuse an allocation across client IP address changes. Due to this, when the IP address of the client changes, the TURN client has to request a new allocation, create permissions for the remote peer, create channels etc. In addition the client has to re-establish communication with its signaling server, send an updated offer to the remote peer conveying the new relayed candidate address, remote side has to regather all candidates and signal them to the client and then the endpoints have to perform Interactive Connectivity Establishment (ICE) connectivity checks. If ICE continuous nomination procedure is used then new relayed candidate address would have to be trickled and ICE connectivity checks have to be performed by the endpoints to nominate pairs that will be selected by ICE. This specification describes a mechanism to seamlessly reuse allocations across client IP address changes without any of the hassles described above. A critical benefit of this technique is that the remote peer does not have to support mobility, or deal with any of the address changes. The client, that is subject to IP address changes, does all the work. The mobility technique works across and between network types (e.g., between 3G and wired Internet access), so long as the client can still access the TURN server. The technique should also work seamlessly when (D)TLS is used as a transport protocol for Session Traversal Utilities for NAT (STUN) . When there is a change in IP address, the client uses (D)TLS Session Resumption without Server-Side State as described in to resume secure communication with the TURN server, using the changed client IP address.

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 . This note uses terminology defined in , and the following additional terminology: Break Before Make: The old communication path is broken ("break") before new communication can be created ("make"). Such changes typically occur because a network is disconnected with a physical cable, turning radio off, or moving out of radio range. Make Before Break: A new communication path is created ("make") before the old communication path is broken ("break"). Such changes typically occur because a network is connected with a physical cable, turning radio on, or moving into radio range.

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 include a 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. This ensures that an allocation can only be refreshed by the same client that allocated relayed transport address. 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.

| | | + 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 | | | | | ]]>

In , the client sends an Allocate request with an MOBILITY-TICKET attribute to the server without credentials. Since the server requires that all requests be authenticated using STUN's long-term credential mechanism, the server rejects the request with a 401 (Unauthorized) error code. The client then tries again, this time including credentials (not shown). This time, the server accepts the Allocate request and returns an Allocate success response and a ticket inside the MOBILITY-TICKET attribute. Sometime later, the client IP address changes and decides to refresh the allocation and thus sends a Refresh request to the server with MOBILITY-TICKET attribute containing the ticket it had received from the server. The refresh is accepted and the server replies with a Refresh success response and a new ticket inside the MOBILITY-TICKET attribute.

In addition to the process described in Section 6.1 of , the client includes the MOBILITY-TICKET attribute with length 0. This indicates the client is a mobile node and wants a ticket.

In addition to the process described in Section 6.2 of , the server does the following: If the MOBILITY-TICKET attribute is included, and has length zero, but TURN session mobility is forbidden by local policy, the server will reject the request with the new Mobility Forbidden error code. If the MOBILITY-TICKET attribute is included and has non-zero length then the server will generate an error response with an error code of 400 (Bad Request). Following the rules specified in , if the server does not understand the MOBILITY-TICKET attribute, it ignores the attribute. If the server can successfully process the request and create an allocation, the server replies with a success response that includes a STUN MOBILITY-TICKET attribute. TURN server can store system internal data into the 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. An example for ticket construction is discussed in .The ticket is opaque to the client, so the structure is not subject to interoperability concerns, and implementations may diverge from this format. The client could be roaming across networks with different path MTU and from one address family to another (e.g. IPv6 to IPv4). The TURN server to support mobility must assume that the path MTU is unknown and use a ticket length in accordance with published guidance on STUN UDP fragmentation (Section 7.1 of ). Note: There is no guarantee that the fields in the ticket are going to be decodable to a client, and therefore attempts by a client to examine the ticket are unlikely to be useful.

In addition to the process described in Section 6.3 of , 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.

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 can notify the user or operator. The client SHOULD NOT retry to send Allocate request containing MOBILITY-TICKET with this server until it believes the problem has been fixed. All other error responses must be handled as described in .

If a client wants to refresh an existing allocation and update its time-to-expiry or delete an existing allocation, it MUST send a Refresh Request as described in Section 7.1 of and MUST NOT include a MOBILITY-TICKET attribute. 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.

In addition to the process described in Section 7.2 of , the server does the following: If the STUN MOBILITY-TICKET attribute is included in the Refresh Request and the server configuration changed to forbid mobility or the server transparently fails-over to another server instance that does not understand the MOBILITY-TICKET attribute then the server rejects the Refresh request with a Mobility Forbidden error code and the client starts afresh with a new allocation. 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 the TURN server will decrypt the received ticket, verify the ticket's validity and retrieve the 5-tuple allocation using the ticket. If this 5-tuple obtained does not identify an existing allocation then the server MUST reject the request with a 437 (Allocation Mismatch) error. If the ticket is invalid then the server MUST reject the request with a 400 (Bad Request) error. If the source IP address and port of the Refresh Request with STUN MOBILITY-TICKET attribute is same as the stored 5-tuple allocation then the TURN server rejects the request with 400 (Bad Request) 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 MESSAGE-INTEGRITY validation to identify the that it is the same user which had previously created the TURN allocation. If the above check is 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 either moved to a new network and acquired a new IP address (Break Before Make) or is in the process of switching to a new interface (Make Before Break). The source IP address of the request could either be the host transport address or server-reflexive transport address. The server then updates its state data with the new client IP address and port but does not discard the old 5-tuple from its state data. 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. The new ticket sent in the refresh response MUST be different from the old ticket. The TURN server MUST continue receiving and processing data on the old 5-tuple and MUST continue transmitting data on the old-5 tuple until it receives an Send Indication or ChannelData message from the client on the new 5-tuple or an message from the client to close the old connection (e.g., a TLS fatal alert, TCP RST). After receiving any of those messages, a TURN server discards the the old ticket and the old 5-tuple associated with the old ticket from its state data. Data sent by the client to the peer is accepted on the new 5-tuple and data received from the peer is forwarded to the new 5-tuple. If the refresh request containing the MOBILITY-TICKET attribute does not succeed (e.g., packet lost if the request is sent over UDP, or the server being unable to fulfill the request) then the client can continue to exchange data on the old 5-tuple until it receives Refresh success response. The old ticket can only be used for the purposes of retransmission. If the client wants to refresh its allocation with a new server-reflexive transport address, it MUST use the new ticket. If the TURN server has not received a Refresh Request with STUN MOBILITY-TICKET attribute but receives Send indications or ChannelData messages from a client, the TURN server MAY discard or queue those Send indications or ChannelData messages (at its discretion). Thus, it is RECOMMENDED that the client avoid transmitting a Send indication or ChannelData message until it has received an acknowledgement for the Refresh Request with STUN MOBILITY-TICKET attribute. To accommodate for loss of Refresh responses, a server must retain the old STUN MOBILITY-TICKET attribute for a period of at least 30 seconds to be able to recognize a retransmission of Refresh request with the old STUN MOBILITY-TICKET attribute from the client.

In addition to the process described in Section 7.3 of , 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.

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 of MOBILITY-TICKET is encrypted and is of variable-length.

This document defines the following new error response code: TBD Mobility Forbidden: Mobility request was valid but cannot be performed due to administrative or similar restrictions.

[Note to RFC editor: Please update sections 3.1.4 and 3.4 with the error number.] IANA is requested to add the following attributes to the STUN attribute registry, MOBILITY-TICKET (0x8030, in the comprehension-optional range) and to add a new STUN error code "Mobility Forbidden" with the value 405 to the STUN Error Codes registry.

[Note to RFC Editor: Please remove this section and reference to prior to publication.] This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in . The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. According to , "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".

This is a public project, the full list of authors and contributors here: http://turnserver.open-sys.org/downloads/AUTHORS A mature open-source TURN server specs implementation (RFC 5766, RFC 6062, RFC 6156, etc) designed for high-performance applications, especially geared for WebRTC. http://code.google.com/p/rfc5766-turn-server/ The Mobile ICE feature implementation can be qualified as "production" - it is well tested and fully implemented, but not widely used, yet.. Fully implements Mobility with TURN. BSD: http://turnserver.open-sys.org/downloads/LICENSE Mobility with TURN implementation is somewhat challenging for a multi-threaded performance-oriented application (because the mobile ticket information must be shared between the threads) but it is doable. Oleg Moskalenko <mom040267@gmail.com>.

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. The ticket MUST be constructed such that it has strong entropy to ensure nothing can be gleaned by looking at the ticket alone. An attacker monitoring the traffic between the TURN client and server can impersonate the client and refresh the allocation using the ticket issued to the client with the attackers IP address and port. This attack can be prevented by using STUN long-term credential mechanism or STUN Extension for Third-Party Authorization or (D)TLS connection between the TURN client and the TURN server. With any of those three mechanisms, when the server receives Refresh Request with STUN MOBILITY-TICKET attribute from the client it identifies that it is indeed the same client but with a new IP address and port using the ticket it had previously issued to refresh the allocation. Security considerations described in are also applicable to this mechanism.

Thanks to Alfred Heggestad, Lishitao, Sujing Zhou, Martin Thomson, Emil Ivov, Oleg Moskalenko, Dave Waltermire, Pete Resnick, Antoni Przygienda, Alissa Cooper, Ben Campbell, Suresh Krishnan, Mirja Kühlewind and Brandon Williams for review and comments.

The TURN server uses two different keys: one 128-bit key for Advance Encryption Standard (AES) in Cipher Block Chaining (CBC) mode (AES_128_CBC) and 256-bit key for HMAC-SHA-256-128 for integrity protection. The ticket can be structured as follows:

; opaque mac[16]; } ticket; ]]>

Here, key_name serves to identify a particular set of keys used to protect the ticket. It enables the TURN server to easily recognize tickets it has issued. The key_name should be randomly generated to avoid collisions between servers. One possibility is to generate new random keys and key_name every time the server is started. The TURN state information (self-contained or handle) in encrypted_state is encrypted using 128-bit AES in CBC mode with the given IV. The MAC is calculated using HMAC-SHA-256-128 over key_name (16 octets)and IV (16 octets), followed by the length of the encrypted_state field (2 octets) and its contents (variable length).