Mobility with TURNCisco Systems, Inc.170 West Tasman DriveSan JoseCalifornia95134USAdwing@cisco.comCisco Systems, Inc.BangaloreIndiapraspati@cisco.comCisco Systems, Inc.Cessna Business Park, Varthur HobliSarjapur Marathalli Outer Ring RoadBangaloreKarnataka560103Indiatireddy@cisco.comCisco Systems, Inc.Philip Pedersens vei 22LysakerAkershus1325Norwaypalmarti@cisco.comTRAMIt 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 for
a new allocation, create permissions for the remote peer, create
channels etc. In addition the client has to re-establish communication
with its SIP 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 tricked 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 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 the new communication can be made ("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 in of 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.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 MUST 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 MUST 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 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. The ticket is opaque
to the client, so the structure is not subject to interoperability
concerns, and implementations may diverge from this format. An
example for ticket construction is discussed in . 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 MUST ensure that the
ticket length is restricted to avoid UDP fragmentation (Section 7.1
of ). Clients MUST NOT examine the
ticket under the assumption that it complies with this document.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 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 .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 . 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.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 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 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 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 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 it's
state data with the new client IP address and port but does not
discard the old 5-tuple from it's 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 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, TURN server updates its 5-tuple
with the new client IP address and port. 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 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: Mobility Forbidden: Mobility request was valid but cannot be
performed due to administrative or similar restrictions.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/AUTHORSA 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 MICE with TURN
protocol.BSD:
http://turnserver.open-sys.org/downloads/LICENSEMICE 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 and Brandon Williams for review and
comments.IANA: STUN AttributesThe 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: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).