Internet Engineering Task Force (IETF) T. Reddy

Internet Engineering Task Force (IETF) T. Reddy
Request for Comments: 8016 Cisco
Category: Standards Track D. Wing
ISSN: 2070-1721
P. Patil
P. Martinsen
Cisco
November 2016

Mobility with Traversal Using Relays around NAT (TURN)

Abstract

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 that only the local network elements are aware of the changed IP
address and the remote peer is unaware of the changed IP address.

This document 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.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8016.

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Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 4
3. Mobility Using TURN . . . . . . . . . . . . . . . . . . . . . 4
3.1. Creating an Allocation . . . . . . . . . . . . . . . . . 5
3.1.1. Sending an Allocate Request . . . . . . . . . . . . . 5
3.1.2. Receiving an Allocate Request . . . . . . . . . . . . 6
3.1.3. Receiving an Allocate Success Response . . . . . . . 6
3.1.4. Receiving an Allocate Error Response . . . . . . . . 7
3.2. Refreshing an Allocation . . . . . . . . . . . . . . . . 7
3.2.1. Sending a Refresh Request . . . . . . . . . . . . . . 7
3.2.2. Receiving a Refresh Request . . . . . . . . . . . . . 7
3.2.3. Receiving a Refresh Response . . . . . . . . . . . . 9
3.3. New STUN Attribute MOBILITY-TICKET . . . . . . . . . . . 9
3.4. New STUN Error Response Code . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Example of Ticket Construction . . . . . . . . . . . 12
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13

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1. Introduction

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, Locator/ID Separation Protocol (LISP)). Other techniques
exist, which make the change in IP address agnostic to the upper-
layer protocol (e.g., Stream Control Transmission Protocol (SCTP)).
The mechanism described in this document is in that last category.

A server using Traversal Using Relays around NAT (TURN) [RFC5766]
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 and send an updated offer to the remote peer
conveying the newly relayed candidate address. Then, the remote side
has to re-gather all candidates and signal them to the client, and
the endpoints have to perform Interactive Connectivity Establishment
(ICE) [RFC5245] checks. If the ICE continuous nomination procedure
[NOMBIS] is used, then the newly relayed candidate address would have
to be "trickled" (i.e., incrementally provisioned as described in
[TRICKLE-SIP]), and ICE checks would have to be performed according
to [TRICKLE-ICE] by the endpoints to nominate pairs for selection 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, which 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)
[RFC5389]. When there is a change in IP address, the client uses
(D)TLS Session Resumption without Server-Side State as described in
[RFC5077] to resume secure communication with the TURN server, using
the changed client IP address.

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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 document uses terminology defined in [RFC5245] 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's physical cable is disconnected,
radio transmission is turned off, or a client moves 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 reconnected with a physical
cable, radio transmission is turned on, or a client moves into radio
range.

3. 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 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 the 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 the 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.

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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 | |
| | |

Figure 1: Mobility Using TURN

In Figure 1, the client sends an Allocate request with a 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 the client decides to refresh the
allocation, and thus sends a Refresh request to the server with a
MOBILITY-TICKET attribute containing the ticket it 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.

3.1. Creating an Allocation

3.1.1. Sending an Allocate Request

In addition to the process described in Section 6.1 of [RFC5766], the
client includes the MOBILITY-TICKET attribute with a length of zero.
This indicates that the client is a mobile node and wants a ticket.

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3.1.2. 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 a length of
zero, but TURN session mobility is forbidden by local policy, the
server will reject the request with the new error code 405 (Mobility
Forbidden). If the MOBILITY-TICKET attribute is included and has a
non-zero length, then the server will generate an error response with
an error code of 400 (Bad Request). 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 and create an
allocation, the server replies with a success response that includes
a STUN MOBILITY-TICKET attribute. The TURN server can store system-
internal data in 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 the Allocate success response. An example of
ticket construction is discussed in Appendix A. 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 a 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 the published guidance on
STUN UDP fragmentation (Section 7.1 of [RFC5389]).

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.

3.1.3. 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.

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3.1.4. Receiving an Allocate Error Response

If the client receives an Allocate error response with error code 405
(Mobility Forbidden), the error is processed as follows:

405 (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 sending the Allocate request containing the MOBILITY-TICKET
with this server until it believes the problem has been fixed.

All other error responses must be handled as described in [RFC5766].

3.2. Refreshing an Allocation

3.2.1. 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 sends a Refresh
request as described in Section 7.1 of [RFC5766]. If the client's IP
address or source port has changed and the client wants to retain the
existing allocation, the client includes the MOBILITY-TICKET
attribute received in the Allocate success response in the Refresh
request. If there has been no IP address or source port number
change, the client MUST NOT include a MOBILITY-TICKET attribute, as
this would be rejected by the server and the client would need to
retransmit the Refresh request without the MOBILITY-TICKET attribute.

3.2.2. Receiving a Refresh Request

In addition to the process described in Section 7.2 of [RFC5766], 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
forbids mobility, then the server rejects the Refresh request with a
405 (Mobility Forbidden) error 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

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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 the
STUN MOBILITY-TICKET attribute is the same as the stored 5-tuple
allocation, then the TURN server rejects the request with a 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 a MESSAGE-INTEGRITY validation to identify that
it is the same user that had previously created the TURN allocation.
If the above check is not successful, then the 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 either has 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 be either the host transport address or the 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. The 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 a Send Indication or ChannelData message from the
client on the new 5-tuple or a message from the client to close the
old connection (e.g., a TLS fatal alert or TCP RST). After receiving
any of those messages, a TURN server discards 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., the packet is lost if the request is sent over UDP, or
the server is unable to fulfill the request), then the client can
continue to exchange data on the old 5-tuple until it receives the
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 the 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,

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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 the STUN MOBILITY-TICKET
attribute.

To accommodate the potential 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 the
Refresh request with the old STUN MOBILITY-TICKET attribute from the
client.

3.2.3. 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.

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

3.4. New STUN Error Response Code

This document defines the following new error response code:

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

4. IANA Considerations

IANA has added the following attribute to the "STUN Attributes"
registry [IANA-STUN]:

o MOBILITY-TICKET (0x8030, in the comprehension-optional range)

Also, IANA has added a new STUN error code "Mobility Forbidden" with
the value 405 to the "STUN Error Codes" registry [IANA-STUN].

5. Security Considerations

The TURN server MUST always ensure that the ticket is authenticated
and encrypted using strong cryptographic algorithms to prevent
modification or eavesdropping by an attacker. The ticket MUST be
constructed such that it has strong entropy to ensure that nothing
can be gleaned by looking at the ticket alone.

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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 attacker's IP address and port.
The TURN client and server MUST use the STUN long-term credential
mechanism [RFC5389], the STUN Extension for Third-Party Authorization
[RFC7635], or a (D)TLS connection to prevent malicious users from
impersonating the client. With any of those three mechanisms, when
the server receives the Refresh request with the 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. If (D)TLS is not used
or the (D)TLS handshake fails, and authentication also fails, then
the TURN client and server MUST fail and not proceed with TURN
mobility.

Security considerations described in [RFC5766] are also applicable to
this mechanism.

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <http://www.rfc-editor.org/info/rfc5077>.

[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
DOI 10.17487/RFC5245, April 2010,
<http://www.rfc-editor.org/info/rfc5245>.

[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<http://www.rfc-editor.org/info/rfc5389>.

[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,
DOI 10.17487/RFC5766, April 2010,
<http://www.rfc-editor.org/info/rfc5766>.

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RFC 8016 Mobility with TURN November 2016

6.2. Informative References

[IANA-STUN]
IANA, "Session Traversal Utilities for NAT (STUN)
Parameters",
<http://www.iana.org/assignments/stun-parameters>.

[NOMBIS] Uberti, J. and J. Lennox, "Improvements to ICE Candidate
Nomination", Work in Progress,
draft-uberti-mmusic-nombis-00, March 2015.

[RFC7635] Reddy, T., Patil, P., Ravindranath, R., and J. Uberti,
"Session Traversal Utilities for NAT (STUN) Extension for
Third-Party Authorization", RFC 7635,
DOI 10.17487/RFC7635, August 2015,
<http://www.rfc-editor.org/info/rfc7635>.

[TRICKLE-ICE]
Ivov, E., Rescorla, E., Uberti, J., and P. Saint-Andre,
"Trickle ICE: Incremental Provisioning of Candidates for
the Interactive Connectivity Establishment (ICE)
Protocol", Work in Progress, draft-ietf-ice-trickle-04,
September 2016.

[TRICKLE-SIP]
Ivov, E., Stach, T., Marocco, E., and C. Holmberg, "A
Session Initiation Protocol (SIP) usage for Trickle ICE",
Work in Progress, draft-ietf-mmusic-trickle-ice-sip-06,
October 2016.

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Appendix A. Example of Ticket Construction

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 a 256-bit key for HMAC-SHA-256-128 for integrity
protection. The ticket can be structured as follows:

struct {
opaque key_name[16];
opaque iv[16];
opaque encrypted_state<0..2^16-1>;
opaque mac[16];
} ticket;

Figure 2: Ticket Format

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 (which is either self-contained or a
handle) in encrypted_state is encrypted using 128-bit AES in CBC mode
with the given Initialization Vector (IV). The Message
Authentication Code (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).

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RFC 8016 Mobility with TURN November 2016

Acknowledgements

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
Kuehlewind, Jonathan Lennox, and Brandon Williams for review and
comments.

Authors' Addresses

Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India

Email: [email protected]

Dan Wing

Email: [email protected]

Prashanth Patil
Cisco Systems, Inc.
Bangalore
India

Email: [email protected]

Paal-Erik Martinsen
Cisco Systems, Inc.
Philip Pedersens vei 22
Lysaker, Akershus 1325
Norway

Email: [email protected]

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