Internet DRAFT - draft-williams-peer-redirect
draft-williams-peer-redirect
Network Working Group B. Williams
Internet-Draft Akamai
Intended status: Standards Track T. Reddy
Expires: May 5, 2016 Cisco
November 2, 2015
Peer-specific Redirection for Traversal Using Relays around NAT (TURN)
draft-williams-peer-redirect-04
Abstract
This specification describes a peer-specific redirection method that
allows the TURN server to redirect a client for the purpose of
improving communication with a specific peer without negatively
affecting communication with other peers.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 5, 2016.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Redirection for Performance . . . . . . . . . . . . . . . 3
1.2. Redirection for Load Balancing . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Peer-specific Server Redirect Mechanism . . . . . . . . . . . 4
3.1. Forming an Allocate Request . . . . . . . . . . . . . . . 5
3.2. Receiving an Allocate Request . . . . . . . . . . . . . . 5
3.3. Forming a CreatePermission or ChannelBind Request . . . . 5
3.4. Receiving a CreatePermission or ChannelBind Request . . . 6
3.5. Forming a Redirect Indication . . . . . . . . . . . . . . 7
3.6. Receiving a Redirect Indication . . . . . . . . . . . . . 8
4. ICE Interactions . . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5.1. Permission Flood . . . . . . . . . . . . . . . . . . . . 9
5.2. Unsolicited or Invalid Redirect Indication . . . . . . . 10
5.3. Replayed Redirect Indication . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Change History . . . . . . . . . . . . . . . . . . . 12
A.1. Changes from version 03 to 04 . . . . . . . . . . . . . . 13
A.2. Changes from version 02 to 03 . . . . . . . . . . . . . . 13
A.3. Changes from version 01 to 02 . . . . . . . . . . . . . . 13
A.4. Changes from version 00 to 01 . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
A Traversal Using Relay around NAT (TURN) [RFC5766] service provider
may provide multiple candidate TURN servers for use by a host, but it
might not be possible to determine which candidate TURN server will
provide the best performance until both peers have been identified.
This could be true for a variety of reasons, including:
o Using the selected relay for a specific peer results in a sub-
optimal end-to-end Internet path.
o Load conditions on the selected relay have changed since the
allocation was established such that it cannot support the new
data flow.
At the same time, the above conditions might apply to one peer but
not another, such that it would be best to selectively use the
existing relay allocation for peers that will receive reasonable
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performance and redirect data flows for other peers to an alternate
server. These scenarios are discussed in greater detail below.
The Session Traversal Utilities for NAT (STUN) protocol
[I-D.ietf-tram-stunbis] defines an ALTERNATE-SERVER mechanism with
which a server can redirect a client to another server by replying to
a request message with an error response with error code 300 (Try
Alternate). The TURN protocol describes error code 300 as one of the
possible error codes for an Allocate error response.
This specification describes an additional use of the ALTERNATE-
SERVER STUN attribute for TURN that allows the TURN server to
redirect a client for the purpose of improving communication with a
specific peer without negatively affecting communication with other
peers.
1.1. Redirection for Performance
Consider the following example:
Boston
Peer C
Chicago [PC]
Peer B /
TURN Relay A ----------[PB]-------------[TC]
San Francisco ----------/ TURN Relay C
[TA]----------/ New York
|
[PA]
Peer A
Los Angeles
When Peer B wishes to communicate with either Peer A or Peer C, it
performs a DNS lookup and discovers TURN Relay C, the nearest of the
candidate TURN servers. Peer B then sends a TURN Allocate request to
TURN Relay C to determine the reflexive and relay candidates to
offer. After the reflexive candidate has been chosen, Peer B sends a
ChannelBind request to TURN Relay C to establish a channel for
communication with the peer. If Peer C is the remote peer, the
existing allocation will perform reasonably well, but if Peer A is
the remote peer, the latency for relayed packets will be nearly twice
as long as if TURN Relay A had been selected as the relay candidate.
The problem is worse if Peer B wishes to communicate with both Peer A
and Peer C, since there is no single relay candidate that would
provide optimum performance for both peers.
If TURN Relay C and TURN Relay A are part of a common TURN service,
it would be possible for TURN Relay C to determine that TURN Relay A
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will provide optimal service for communication between Peer B and
Peer A. This allows the TURN service to redirect just the data
channel between Peer A and Peer B to TURN relay A, thus providing
optimal performance for both relay channels.
The above example describes the problem in terms of physical
geography instead of network geography in order to help clarify the
discussion. However, readers should note that the problem of
selecting a relay server to achieve optimal end-to-end routing is
much more complicated than the above description suggests, requiring
a detailed real-time view of network connectivity characteristics and
the peering relationships between autonomous systems. A naive
approach based solely on the physical location of the hosts involved
is just as likely to produce negative results as positive ones.
That said, a relay service provider with a broadly distributed system
for actively monitoring network performance across the relevant parts
of the Internet could make use of the resulting data set to select
the optimal relay for each peer pair.
1.2. Redirection for Load Balancing
At the point when a relay allocation is first established, it can be
difficult to determine how much aggregate concurrent load could
eventually be associated with that allocation. The initiating peer
could attempt to use that allocation for any number of peer-to-peer
data flows over an extended period of time, during which time load
conditions on the relay could change substantially, such that quality
of service for already established flows would degrade if the relay
were to accept additional flows.
Under these conditions, a TURN service provider with multiple relay
hosts and distributed capacity could improve service quality by
redirecting data flows to a different host that has more available
capacity.
2. Terminology
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].
3. Peer-specific Server Redirect Mechanism
This specification describes a new STUN indication type, Redirect,
which is used by a TURN server to notify a TURN client when better
service could be available through an alternate TURN server. The
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Redirect indication contains an ALTERNATE-SERVER attribute to provide
the address for the alternate TURN server.
This specification also defines two new comprehension-optional STUN
attributes: CHECK-ALTERNATE and XOR-OTHER-ADDRESS. The CHECK-
ALTERNATE attribute is used by the client to request that the server
perform peer-specific redirection. The XOR-OTHER-ADDRESS is used by
the client to provide an alternate peer address for location
identification in the event that the XOR-PEER-ADDRESS attribute in
the CreatePermission or ChannelBind request is not expected to
reliably serve this purpose.
3.1. Forming an Allocate Request
When forming an Allocate request, a TURN client includes a CHECK-
ALTERNATE STUN attribute to signal to the TURN server that peer-
specific redirection is both supported and desired.
When forming a CHECK-ALTERNATE attribute, the STUN Type is TBD-CA.
To maintain backward compatibility, this type is in the
comprehension-optional range, which means that an [RFC5766] compliant
TURN server can safely ignore it.
The CHECK-ALTERNATE attribute has no value part and thus the
attribute length field is 0.
3.2. Receiving an Allocate Request
When a server receives an Allocate request, it first processes the
request as per the TURN specification [RFC5766]. After determining
that a success response will be prepared, a TURN server that supports
peer-specific redirection checks for a CHECK-ALTERNATE attribute. If
one exists, the server stores this information as part of the
allocation state. There is no need for the server to indicate that
the attribute was accepted in the success response.
3.3. Forming a CreatePermission or ChannelBind Request
When sending a CreatePermission or a ChannelBind request, the XOR-
OTHER-ADDRESS STUN attribute allows the TURN client to provide an
alternate peer address that can be used by the server to identify the
network geographic location of the peer when performing the peer-
specific redirection check. Use of this attribute is only necessary
if the XOR-PEER-ADDRESS already contained in the CreatePermission or
ChannelBind request does not adequately serve this purpose, which
should only be true when both peers require a TURN relay for end-to-
end data flow. In this case, the TURN CreatePermission or
ChannelBind request will provide the peer's TURN relay address as the
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XOR-PEER-ADDRESS value. If the RTT between the peer and its TURN
relay server is very small, the TURN relay address might still be an
appropriate address to use for the peer-specific redirection check.
As the RTT grows, the TURN relay address will become less suitable
for this purpose. For this reason, it is generally the case that the
peer's public address (i.e. its host or reflexive address) is a
better indication of its network geographic location than its TURN
relay address.
When forming an XOR-OTHER-ADDRESS attribute, the STUN Type is TBD-
XOA. To support backward compatibility, this type is in the
comprehension-optional range, which means that an [RFC5766] compliant
TURN server can safely ignore it.
The XOR-OTHER-ADDRESS value specifies an address and port suitable
for identification of the peer's network geographic location. It is
encoded in the same way as XOR-MAPPED-ADDRESS
[I-D.ietf-tram-stunbis].
A CreatePermission request is allowed to contain multiple XOR-PEER-
ADDRESS attributes. When multiple peer addresses are provided in a
CreatePermission request, it would be difficult for the TURN server
to associate an XOR-OTHER-ADDRESS attribute with the correct XOR-
PEER-ADDRESS. For this reason, a TURN client MUST form a separate
CreatePermission request for an XOR-PEER-ADDRESS request when an XOR-
OTHER-ADDRESS attribute will be included in the request.
The XOR-OTHER-ADDRESS attribute SHOULD NOT be included in a request
if its value will be identical to the request's XOR-PEER-ADDRESS
attribute. Its value would be redundant and a waste of space in the
message.
3.4. Receiving a CreatePermission or ChannelBind Request
When a TURN server receives a CreatePermission or ChannelBind request
for an allocation that included the CHECK-ALTERNATE attribute, it
processes the request as per the TURN specification [RFC5766] plus
the specific rules mentioned here.
If an XOR-OTHER-ADDRESS attribute is present, the server validates
the number of XOR-PEER-ADDRESS attributes. If there is more than one
XOR-PEER-ADDRESS attribute in the request, the server MUST reject the
request with an error response using error code 400 (Bad Request).
If there is only one XOR-PEER-ADDRESS attribute, the request is
accepted and the value of the XOR-OTHER-ADDRESS attribute is stored
with the permission state for use when checking for an alternate
server.
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The mechanism for deciding when and how to check for an alternate
server is implementation dependent. This activity could be timer
driven (e.g. check for an alternate server once every 120 seconds),
it could be event driven (e.g. check for an alternate server on every
permission or binding refresh), or another mechanism appropriate for
the internal implementation could be chosen. Likewise, the decision
of which specific address(es) to check for alternate servers is also
implementation dependent. A server could check for alternates for
all active permissions, it could check just for permissions that have
relayed non-ICE data, or another selection method appropriate for the
implementation could be chosen.
When checking for an alternate server for a permission where the XOR-
OTHER-ADDRESS attribute was provided, the server SHOULD use this
address for peer location identification. Otherwise, the server
SHOULD use the XOR-PEER-ADDRESS value.
The TURN client will retransmit a CreatePermission or ChannelBind
request if the response is not received. [I-D.ietf-tram-stunbis]
recommends that the retransmit timeout be greater than 500 ms, but
does not require this, so it is important to avoid unnecessary delays
in request processing. For this reason, the mechanism for driving
alternate server checks SHOULD be asynchronous relative to processing
of the associated CreatePermission or ChannelBind request and SHOULD
NOT delay transmission of the response message.
3.5. Forming a Redirect Indication
When an alternate server for a specific permission has been
identified, the server notifies the client using a Redirect
indication. The server MUST NOT send Redirect indications if the
client did not indicate support by including a CHECK-ALTERNATE
attribute in its Allocate request.
The codepoint for Redirect indication is TBD-RI.
A Redirect indication MUST contain a single ALTERNATE-SERVER
attribute to provide the address for the alternate server. The
message MAY contain one or more XOR-PEER-ADDRESS attributes to
indicate a subset of peer addresses for redirection. If all peer
addresses are to be redirected, no XOR-PEER-ADDRESS attribute is
required. The message MUST contain either a MESSAGE-INTEGRITY or
MESSAGE-INTEGRITY-SHA256 attribute (see Section 5 for an explanation
of the rationale).
Because this message codepoint is for indications only, the TURN
client will not send a success response, and the TURN server will
have no way to determine whether the message was received. For
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improved reliability, the TURN server MAY retransmit the indication
multiple times, following the request retransmission semantics
described in [I-D.ietf-tram-stunbis]. Retransmission requirements
for indications might differ from those for requests, since requests
are only retransmitted if no response was received. For this reason,
an implementation that retransmits Redirect indications SHOULD
provide separate configuration settings to control the maximum number
of Redirect retransmissions and the minimum RTO.
3.6. Receiving a Redirect Indication
When a TURN client receives a Redirect indication, it checks that the
indication contains both an ALTERNATE-SERVER attribute and one of
either a MESSAGE-INTEGRITY or a MESSAGE-INTEGRITY-SHA256 attribute
and discards it if it does not. If XOR-PEER-ADDRESS attributes are
present, it checks that all specified addresses are recognized peers
for the allocation and discards the indication if any addresses are
not recognized. Finally, it verifies the integrity attribute's value
and discards the message if that value is invalid.
After validating the message, the ALTERNATE-SERVER value and
associated peer addresses are delivered to the ICE implementation.
Interactions with ICE are described below (Section 4).
See Section 5 below for discussion of how the client should respond
when receiving a Redirect indication when redirection was not
requested.
4. ICE Interactions
With "Vanilla" ICE as defined in [RFC5245], candidate gathering is
complete before the offer/answer exchange. When a client using
standard ICE receives a valid Redirect indication, it first checks
whether it already has an active allocation on the specified server.
If not, it adds the new relay to its list of servers and forms an
allocation. This generates a new relayed candidate to be added to
the local ICE candidates list, which requires ICE restart.
With trickle ICE as defined in [I-D.ietf-ice-trickle], if the end-of-
candidates announcement has not yet been sent, it could be possible
to complete the TURN Allocate request and include the new relayed
candidate in the candidates list for the current ICE negotiation.
However, on some implementations, it could be true that candidate
gathering is already complete, even though the end-of-candidates
announcement hasn't been sent. In other words, a trickle ICE agent
might need to restart ICE in order to restart candidate gathering.
If the end-of-candidates announcement has already been sent, ICE
restart is necessary in order to add the new relayed candidate.
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It is possible for a TURN relay to send multiple Redirect indications
on the same allocation within a short time frame, each for a
different set of peers. For example, consider the case of a remote
peer with two interfaces, one wifi and the other 4G on a different
Internet service provider. It is possible that the network geography
for each interface requires a different relay for best performance
and therefore a unique Redirect indication. After receiving a
Redirect indication that does not apply to all peers associated with
the allocation, it might be beneficial for the ICE agent to delay ICE
restart by a small interval in order to avoid restarting ICE multiple
times within a short time frame. However, selecting a good delay
interval could be difficult, since the time between indications could
vary due to packet loss and retransmission timeouts.
The authors have considered alternative Redirect indication formats
that would allow all concurrent redirects to be provided in a single
indication, which would avoid the above described issue. Feedback is
desired on the question of whether the problem of minimizing ICE
restarts is important enough to add greater complexity to the
building and parsing of Redirect indications.
5. Security Considerations
This section considers attacks that are possible in a TURN deployment
through the specified protocol extension, and discusses how they are
mitigated by mechanisms in the protocol or recommended practices in
the implementation.
The specified mechanism affects the use of TURN CreatePermission
request messages, ChannelBind request messages, and Redirect
indication messages. Each of these TURN message types requires a
STUN message integrity attribute (either MESSAGE-INTEGRITY or
MESSAGE-INTEGRITY-SHA256), which limits attacks that attempt to make
use of the specified mechanism to authenticated clients and servers.
5.1. Permission Flood
A compromised TURN client could send a large number of
CreatePermission or ChannelBind request messages with distinct peer
address values, which would drive increased load on the TURN server.
The mechanism described in this document does not make such an attack
more likely, though it could make it possible to increase the impact
of such an attack due to the additional load associated with
determining whether an alternate server should be used by the client.
The TURN server MAY be configured to disable or rate limit alternate
server checks under some conditions in order to limit the associated
load. The conditions under which it is appropriate for a TURN server
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to ignore disable or rate limit such checks are implementation
dependent.
5.2. Unsolicited or Invalid Redirect Indication
A compromised TURN server could send Redirect indications for
allocations that did not include the CHECK-ALTERNATE attribute. For
a client that does not support this mechanism, receiving such
indications is no worse than receiving messages with any other
unrecognized message type. A client that recognizes the message type
MUST ignore Redirect indications for allocations where CHECK-
ALTERNATE was not specified, and in particular, to avoid unnecessary
authentication overhead, the client SHOULD drop such indications
before attempting to validate the message integrity attribute.
A compromised TURN server could send an invalid ALTERNATE-SERVER
attribute value in a Redirect indication message, where the value
refers to an unaffiliated TURN server to which the sending TURN
server is not allowed to redirect traffic. Such an attack is already
allowed by the use of Try Alternate errors in response to Allocate
request messages. Use of the ALTERNATE-SERVER attribute in the
context of peer-specific redirection does not make such an attack
more likely, though it could make it possible to increase the scale
of such an attack by allowing multiple ALTERNATE-SERVER attributes to
each client, one per requested permission or binding. A client
SHOULD ignore all future Redirect indications received from the TURN
server after an authentication failure with any server identified via
an ALTERNATE-SERVER attribute. A client MAY discontinue use of the
associated TURN allocation after an authentication failure with any
server identified via an ALTERNATE-SERVER attribute.
An external attacker could send an invalid ALTERNATE-SERVER attribute
value in a Redirect indication message. The client must have some
way to detect when this occurs, which is the purpose of including a
message integrity attribute in the Redirect indication. Without the
message integrity attribute, it would be possible for an attacker to
spoof a Redirect indication from the TURN server and drive the client
to attempt to connect to a bad relay server.
5.3. Replayed Redirect Indication
An in-path attacker could capture and replay Redirect indications.
If the client has been redirected again after the replayed Redirect
indication was received, the replay could drive the client to carry
out unnecessary work to establish a new allocation and restart ICE if
the results of the previous Redirect indication have since been
discarded. It could also drive unexpected load from the client to a
server that has since become overloaded, potentially degrading
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performance for not only the target client but also all others now
connected to the alternate server.
Multiple potential mitigations for this attack exist. For example, a
client that maintains a complete list of all TURN servers used
throughout the life of the session could keep track of the
Transaction ID for each Redirect indication received, which would
allow the client to recognize and reject a replayed indication.
Alternatively, a client could rate-limit its responses to Redirect
indications, requiring a configurable interval to expire between
Redirect indications before accepting a new one.
The authors have also considered the option of adding a timestamp
attribute to the Redirect indication message. The timestamp could be
used to minimize the window of opportunity for a Redirect indication
replay attack. However, such use of timestamps is fragile in the
presence of potential clock skew problems between the client and the
server and so has not been included in the specification.
6. IANA Considerations
[Paragraphs below in braces should be removed by the RFC Editor upon
publication]
[The CHECK-ALTERNATE attribute requires that IANA allocate a value in
the "STUN Attributes Registry" from the comprehension-optional range
(0x8000-0xFFFF), to be replaced for TBD-CA throughout this document]
This document defines the CHECK-ALTERNATE STUN attribute, described
in Section 3.1. IANA has allocated the comprehension-optional
codepoint TBD-CA for this attribute.
[The XOR-OTHER-ADDRESS attribute requires that IANA allocate a value
in the "STUN Attributes Registry" from the comprehension-optional
range (0x8000-0xFFFF), to be replaced for TBD-XOA throughout this
document]
This document defines the XOR-OTHER-ADDRESS STUN attribute, described
in Section 3.3. IANA has allocated the comprehension-optional
codepoint TBD-XOA for this attribute.
[The Redirect indication codepoint requires that IANA allocate a
value in the "STUN Methods Registry", to be replace for TBD-RI
throughout this document.]
This document defines the Redirect indication method type, described
in Section 3.5. IANA has allocated the codepoint TBD-RI for this
method type.
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7. Acknowledgements
Many thanks to J. Uberti for his suggestions regarding ICE
interactions.
8. References
8.1. Normative References
[I-D.ietf-tram-stunbis]
Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
D., Mahy, R., and P. Matthews, "Session Traversal
Utilities for NAT (STUN)", draft-ietf-tram-stunbis-04
(work in progress), March 2015.
[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>.
[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-ice-trickle]
Ivov, E., Rescorla, E., Uberti, J., and P. Saint-Andre,
"Trickle ICE: Incremental Provisioning of Candidates for
the Interactive Connectivity Establishment (ICE)
Protocol", draft-ietf-ice-trickle-00 (work in progress),
October 2015.
[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>.
Appendix A. Change History
[Note to RFC Editor: Please remove this section prior to
publication.]
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A.1. Changes from version 03 to 04
Introduced Redirect indication to redefine the mechanism as push-like
notification.
Moved CHECK-ALTERNATE to Allocation request.
Added a short section on ICE interactions.
Changed STUN reference to STUNbis, since the doc now references
STUNbis content. Left other references as they are.
A.2. Changes from version 02 to 03
Minor copy-editing.
A.3. Changes from version 01 to 02
Add warning about the difference between physical geography and
network geography.
Add load balancing use case.
A.4. Changes from version 00 to 01
Expand discussion of when/how to use CHECK-ALTERNATE and XOR-OTHER-
ADDRESS.
Authors' Addresses
Brandon Williams
Akamai, Inc.
8 Cambridge Center
Cambridge, MA 02142
USA
Email: brandon.williams@akamai.com
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: tireddy@cisco.com
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