Internet DRAFT - draft-reddy-tram-stun-path-data
draft-reddy-tram-stun-path-data
TRAM T. Reddy
Internet-Draft D. Wing
Intended status: Standards Track P. Martinsen
Expires: July 24, 2015 Cisco
V. Singh
callstats.io
January 20, 2015
Discovery of path characteristics using STUN
draft-reddy-tram-stun-path-data-01
Abstract
A host with multiple interfaces needs to choose the best interface
for communication. Oftentimes, this decision is based on a static
configuration and does not consider the path characteristics, which
may affect the user experience.
This document describes a mechanism for an endpoint to discover the
path characteristics using Session Traversal Utilities for NAT (STUN)
messages. The measurement information can then be used to influence
the endpoint's Interactive Connectivity Establishment (ICE) candidate
pair selection algorithm.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 24, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. Path characteristics determination mechanism . . . . . . . . 3
3.1. The PATH-CHARACTERISTIC attribute in request . . . . . . 4
3.2. The PATH-CHARACTERISTIC attribute in response . . . . . . 5
3.3. Example Operation . . . . . . . . . . . . . . . . . . . . 6
4. Usecases . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The ICE [RFC5245] mechanism uses a prioritization formula to order
the candidate pairs and perform connectivity checks, in which the
most preferred address pairs are tested first and when a sufficiently
good pair is discovered, that pair is used for communications and
further connectivity tests are stopped. This approach works well for
an endpoint with a single interface, but is too simplistic for
endpoints with multiple interfaces, wherein a candidate pair with a
lower priority might infact have better path characteristics (e.g.,
round-trip time, loss, etc.). The ICE connectivity checks can assist
in measuring the path characteristics, but as currently defined, the
STUN responses to re-transmitted requests are indistinguishable from
each other.
This draft extends STUN [RFC5389] to distinguish STUN responses to
re-transmitted requests and this assists the client in determining
the path characteristics like round-trip time (RTT) and packet loss
in each direction between endpoints. These metrics can then be used
by the controlling agent to influence the ICE candidate pair
priorities.
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The PATH-CHARACTERISTICS attribute introduced in this specification
can be used in ICE connectivity checks (STUN Binding request and
response). When multiple TURN servers are discovered then this new
attribute can also be used with Allocate request to determine the
priority amongst the relayed candidates.
This specification can be used with the regular nomination procedure
defined in ICE, wherein ICE connectivity checks need to be performed
on all or subset of the chosen candidate pairs. Finalizing an
approporiate candidate pair based on the path characteristics depends
on the number of candidate pairs, time interval for pacing ICE
connectivity checks and the corresponding RTO values. By picking
appropriate values the endpoints will not observe any noticeable
impact to the media setup time.
TODO: Add details of ICE continous nomination procedure discussed in
mmusic WG which allows picking better candidate pairs as and when
they appear. http://juberti.github.io/draughts/nombis/draft-uberti-
mmusic-nombis.html explains simplifying and improving the procedures
for candidate nomination in ICE to make dynamic decisions.
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 ICE [RFC5245] and STUN
[RFC5389].
3. Path characteristics determination mechanism
When multiple paths are available for communication, the endpoint
sends ICE connectivity checks across each path (candidate pair) and
perhaps chooses the path with the lowest round trip time. Choosing
the path with the lowest round trip time is a reasonable approach,
but re-transmits can cause an otherwise-good path to appear flawed.
However, STUN's retransmission algorithm [RFC5389] cannot determine
the round-trip time (RTT) if a STUN request packet is re-transmitted,
because each request and retransmission packet is identical.
Further, several STUN requests may be sent before the connectivity
between candidate pairs is ascertained (see Section 16 of [RFC5245]).
To resolve the issue of identical request and response packets in a
STUN transaction, this document changes that retransmission behavior
for idempotent packets. In addition to determining RTT, it is also
desirable to detect which path direction caused packet loss,
described as "bi-directional path characteristics," below. This is
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achieved by defining a new STUN attribute and requires compliant STUN
(TURN, ICE) endpoints to count request packets.
This specification defines a new comprehension-optional STUN
attribute PATH-CHARACTERISTIC. PATH-CHARACTERISTIC will have a STUN
Type TBD-CA. This type is in the comprehension-optional range, which
means that STUN agents can safely ignore the attribute if they do not
understand it.
If a client wishes to determine the path characteristics, it inserts
the PATH-CHARACTERISTIC attribute in a STUN request. In the PATH-
CHARACTERISTIC attribute client sends the number of times the STUN
request is retransmitted with the same Transaction ID. The server
would echo back the retransmission count in the response so that
client can distinguish STUN responses from the re-transmitted
requests. Hence, the endpoint can use the STUN requests and
responses to determine the round-trip time (RTT). The server may
also convey the number of times it received the request with the same
Transaction ID and the number of responses it has sent for the STUN
request to the client. Further, this information enables the client
to determine packet loss in each direction.
3.1. The PATH-CHARACTERISTIC attribute in request
The PATH-CHARACTERISTIC attribute in a STUN request takes a 1-byte
Value, which means that the Length is 1 and 3 bytes of padding are
required after the value (Section 15 of [RFC5389]). When sending a
STUN request, the PATH-CHARACTERISTIC attribute allows a client to
indicate to the server that it wants to determine path
characteristics. If the client receives a STUN response with error
code 420 (Unknown Attribute) and PATH-CHARACTERISTIC is listed in the
UNKNOWN-ATTRIBUTE attribute of the message, the client SHOULD
retransmit the original request without the PATH-CHARACTERISTIC
attribute. However this case is not expected to occur, due to the
use of the comprehension-optional attribute type.
This specification updates one the STUN message structuring rules
explained in Section 6 of [RFC5389] that resends of the same request
reuse the same transaction ID and are bit-wise identical to the
previous request. For idempotent packets the ReTransCnt in the PATH-
CHARACTERISTIC attribute will be incremented by 1 for every re-
transmission and the re-transmitted STUN request MUST be bit-wise
identical to the previous request except for the ReTransCnt value.
The format of the value in PATH-CHARACTERISTIC attribute in the
request is:
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0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| ReTransCnt |
+-+-+-+-+-+-+-+-+
Figure 1: PATH-CHARACTERISTIC attribute in request
The field is described below:
ReTransCnt: Number of times request is re-transmitted with the same
transaction ID to the server.
3.2. The PATH-CHARACTERISTIC attribute in response
When a server receives a STUN request that includes a PATH-
CHARACTERISTIC attribute, it processes the request as per the STUN
specification [RFC5389] plus the specific rules mentioned here. The
server checks the following:
o If the PATH-CHARACTERISTIC attribute is not recognized, ignore the
attribute because its type indicates that it is comprehension-
optional. This should be the existing behavior as explained in
section 3.1 of [RFC5389].
o The server that supports PATH-CHARACTERISTIC attribute MUST echo
back ReTransCnt in the response.
o If the server is stateless or does not want to remember the
transaction ID then it would populate value 0 for the ReqTransCnt
and RespTransCnt fields in PATH-CHARACTERISTIC attribute sent in
the response .If the server is stateful then it populates
ReqTransCnt with the number of times it received the STUN request
with the same transaction ID and RespTransCnt with the number of
responses it has sent for the STUN request.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0| ReTransCnt |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ReqTransCnt | RespTransCnt |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: PATH-CHARACTERISTIC attribute in response
The fields are described below:
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ReTransCnt: Copied from request.
ReqTransCnt: Number of times request is received from the client
with the same transaction ID.
RespTransCnt: Number of responses sent to the client for the same
transaction ID.
3.3. Example Operation
The operation is described in Figure 3. In the first case, all the
requests and responses are received correctly. In the upstream loss
case, the first request is lost, but the second one is received
correctly, the client on receiving the response notes that while 2
requests were sent, only one was received by the server, also the
server realizes that the RespTransCnt does not match the ReTransCnt,
therefore 1 request was lost. This may also occur at startup in the
presence firewalls or NATs that block unsolicited incoming traffic.
In the downstream loss case, the responses get lost, client expecting
multiple response notes that while the server responded to 3 requests
but only 1 response was received. In the both loss case, requests
and responses get lost in tandem, the server notes one request packet
was not received, while the client expecting 3 responses received
only one, it notes that one request and response packets were lost.
Normal | Upstream loss | Downstream loss| Both loss |
Client Server | Client Server | Client Server | Client Server |
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
1 1,1 | 1 x | 1 1,1 | 1 x |
1,1 | | x | |
2 2,2 | 2 2,1 | 2 2,2 | 2 2,1 |
2,2 | 2,1 | x | x |
3 3,3 | 3 3,2 | 3 3,3 | 3 3,2 |
3,3 | 3,2 | 3,3 | 3,2 |
Figure 3: Retransmit Operation between client and Server
4. Usecases
The STUN attribute defined in this specification can be used by
applications in the following scenarios:
o When an endpoint has multiple interfaces (for example 3G, 4G,
WiFi, VPN, etc.), an ICE agent can choose the interfaces for media
streams according to the path characteristics. After STUN
responses to STUN checks are received, the ICE agent using regular
nomination can sort the ICE candidate pairs according to the path
characteristics (loss and RTT) discovered using STUN. The
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controlling agent can then assign the highest priority to
candidate pair which best fulfills the desired path
characteristics. However, it should be noted that the path
capacity or throughput is not determined by these STUN checks. If
an endpoint needs to pick paths based on capacity, it would have
to send media on those paths.
o When a host has multiple interfaces available an MPRTP
[I-D.ietf-avtcore-mprtp] application can choose the interfaces for
the corresponding subflows according to the path characteristics
discovered using STUN. For example, the scheduling algorithm
described in [ACM-MPRTP] uses path capacity, latency, and loss
rate for choosing the most suitable subset of paths.
o The STUN extension proposed in this specification can also be used
to choose a TURN server that provides the best user experience
(section 3.1 of [I-D.patil-tram-turn-serv-selection]).
5. IANA Considerations
[Paragraphs in braces should be removed by the RFC Editor upon
publication]
[The PATH-CHARACTERISTIC 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 PATH-CHARACTERISTIC STUN attribute,
described in Section 3. IANA has allocated the comprehension-
optional codepoint TBD-CA for this attribute.
6. Security Considerations
Security considerations discussed in [RFC5389] are to be taken into
account. STUN requires the 96 bits transaction ID to be uniformly
and randomly chosen from the interval 0 .. 2**96-1, and be
cryptographically strong. This is good enough security against an
off-path attacker. An on-path attacker can either inject a fake
response or modify the values in PATH-CHARACTERISTIC attribute to
mislead the client and server, this attack can be mitigated using
STUN authentication. As PATH-CHARACTERISTIC is expected to be used
between peers using ICE, and ICE uses STUN short-term credential
mechanism the risk of on-path attack influencing the messages is
minimal. However, an attacker could corrupt, remove, or delay an ICE
request or response, in order to discourage that path from being
used. Unauthenticated STUN message MUST NOT include the PATH-
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CHARACTERISTIC attribute in order to prevent on-path attacker from
influencing decision-making.
7. Acknowledgements
Thanks to Brandon Williams, Simon Perreault, Aijun Wang for valuable
inputs 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, March 1997.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
8.2. Informative References
[ACM-MPRTP]
Singh, V., Ahsan, S., and J. Ott, "MPRTP: multipath
considerations for real-time media", in Proc. of ACM
Multimedia Systems, MMSys, 2013.
[I-D.ietf-avtcore-mprtp]
Singh, V., Karkkainen, T., Ott, J., Ahsan, S., and L.
Eggert, "Multipath RTP (MPRTP)", draft-ietf-avtcore-
mprtp-00 (work in progress), December 2014.
[I-D.patil-tram-turn-serv-selection]
Patil, P., Reddy, T., and G. Salgueiro, "Traversal Using
Relays around NAT (TURN) Server Selection", draft-patil-
tram-turn-serv-selection-00 (work in progress), October
2014.
Authors' Addresses
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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
Paal-Erik Martinsen
Cisco Systems, Inc.
Philip Pedersens vei 22
Lysaker, Akershus 1325
Norway
Email: palmarti@cisco.com
Varun Singh
Nemu Dialogue System Oy
Espoo 02235
Finland
Email: varun@callstats.io
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