TRAM T. Reddy
Internet-Draft D. Wing
Intended status: Standards Track P. Patil
Expires: March 5, 2017 P. Martinsen
Cisco
September 1, 2016
Mobility with TURN
draft-ietf-tram-turn-mobility-08
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 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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 5, 2017.
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
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(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
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 . . . . . . . . 6
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 . . . . . . . . . . . . 8
3.3. New STUN Attribute MOBILITY-TICKET . . . . . . . . . . . 9
3.4. New STUN Error Response Code . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. Implementation Status . . . . . . . . . . . . . . . . . . . . 9
5.1. open-sys . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Example ticket construction . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
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, 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) [RFC5766] server relays
media packets and is used for a variety of purposes, including
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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) [RFC5245] connectivity checks. If ICE continuous nomination
procedure [I-D.uberti-mmusic-nombis] is used then new relayed
candidate address would have to be trickled
[I-D.ietf-mmusic-trickle-ice] 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)
[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.
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], 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
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typically occur because a network is connected with a physical cable,
turning radio on, or moving 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 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.
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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 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.
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 length 0. This
indicates 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 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 [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. 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 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 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 [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.
3.1.4. 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
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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 [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 MUST send a
Refresh Request as described in Section 7.1 of [RFC5766] 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.
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
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.
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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.
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
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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 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:
TBD Mobility Forbidden: Mobility request was valid but cannot be
performed due to administrative or similar restrictions.
4. IANA Considerations
[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 [iana-stun],
o 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 [iana-stun].
5. Implementation Status
[Note to RFC Editor: Please remove this section and reference to
[RFC6982] 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 [RFC6982].
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.
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According to [RFC6982], "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".
5.1. open-sys
Organization: This is a public project, the full list of authors
and contributors here: http://turnserver.open-sys.org/downloads/
AUTHORS
Description: A mature open-source TURN server specs implementation
(RFC 5766, RFC 6062, RFC 6156, etc) designed for high-performance
applications, especially geared for WebRTC.
Implementation: http://code.google.com/p/rfc5766-turn-server/
Level of maturity: The Mobile ICE feature implementation can be
qualified as "production" - it is well tested and fully
implemented, but not widely used, yet..
Coverage: Fully implements Mobility with TURN.
Licensing: BSD: http://turnserver.open-sys.org/downloads/LICENSE
Implementation experience: 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.
Contact: Oleg Moskalenko <mom040267@gmail.com>.
6. Security Considerations
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 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.
TURN client and server MUST use STUN long-term credential mechanism
[RFC5389] or STUN Extension for Third-Party Authorization
[RFC7635][RFC7635] or (D)TLS connection to avoid malicious users
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trying to impersonate the client. 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. If (D)TLS
is not used or (D)TLS handshake fails, and authentication also fails
then TURN client and server MUST fail, and not proceed with TURN
mobility.
Security considerations described in [RFC5766] are also applicable to
this mechanism.
7. 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 and Brandon Williams for 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,
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>.
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[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>.
8.2. Informative References
[I-D.ietf-mmusic-trickle-ice]
Ivov, E., Rescorla, E., and J. Uberti, "Trickle ICE:
Incremental Provisioning of Candidates for the Interactive
Connectivity Establishment (ICE) Protocol", draft-ietf-
mmusic-trickle-ice-02 (work in progress), January 2015.
[I-D.uberti-mmusic-nombis]
Uberti, J. and J. Lennox, "Improvements to ICE Candidate
Nomination", draft-uberti-mmusic-nombis-00 (work in
progress), March 2015.
[iana-stun]
IANA, , "IANA: STUN Attributes", April 2011,
<http://www.iana.org/assignments/stun-parameters/stun-pa
rameters.xml>.
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982,
DOI 10.17487/RFC6982, July 2013,
<http://www.rfc-editor.org/info/rfc6982>.
[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>.
Appendix A. Example 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 256-bit key for HMAC-SHA-256-128 for integrity
protection. The ticket can be structured as follows:
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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 (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).
Authors' Addresses
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: tireddy@cisco.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
USA
Email: dwing@cisco.com
Prashanth Patil
Cisco Systems, Inc.
Bangalore
India
Email: praspati@cisco.com
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Paal-Erik Martinsen
Cisco Systems, Inc.
Philip Pedersens vei 22
Lysaker, Akershus 1325
Norway
Email: palmarti@cisco.com
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