Network Working Group C. Jennings
Internet-Draft S. Nandakumar
Intended status: Standards Track Cisco
Expires: August 3, 2018 January 30, 2018
Snowflake - A Lighweight, Asymmetric, Flexible, Receiver Driven
Connectivity Establishment
draft-jennings-dispatch-snowflake-00
Abstract
Interactive Connectivity Establishment (ICE) (RFC5245) defines
protocol machinery for two peers to discover each other and establish
connectivity in order to send and receive Media Streams.
This draft raises some issues inherent in the assumptions with ICE
and proposes a lightweight receiver driven protocol for asymmetric
connecitivity establishment.
Status of This Memo
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This Internet-Draft will expire on August 3, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
4. Snowflake for connectivity establishment . . . . . . . . . . 4
4.1. System Components . . . . . . . . . . . . . . . . . . . . 4
4.2. Protocol Workings . . . . . . . . . . . . . . . . . . . . 5
4.3. Advantages of Snowflake . . . . . . . . . . . . . . . . . 8
4.3.1. Diagnostics . . . . . . . . . . . . . . . . . . . . . 8
4.3.2. Timing . . . . . . . . . . . . . . . . . . . . . . . 8
4.3.3. Asymmetric Media . . . . . . . . . . . . . . . . . . 8
4.3.4. Fast Start . . . . . . . . . . . . . . . . . . . . . 8
5. IANA Consideration . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
ICE was designed over a decade and certain assumptions about the
network topology, timing considerations, application complexity have
drastically changed since then. Newer additions/clarifications to
ICE in [I-D.ietf-ice-rfc5245bis] and Trickle ICE
[I-D.ietf-ice-trickle] have indeed help improve its performance and
the way the connectivity checks are performed.
However, enforcing stringent global pacing requirements, coupled
timing dependencies between the ICE agents, the need for symmetric
connection setup, for example, has rendered the protocol inflexible
for innovation and increasingly difficult to apply and debug in a
dynamic network and evolving application contexts.
This specification defines Snowflake, where, like ICE, both sides
gather a set of address candidates that may work for communication.
However, instead of both sides trying to synchronize connectivity
checks in time-coupled fashion, the sending side acts as a slave and
sends STUN packets wherever the receiving side tells it to and when
it is told to do so. The receiving side is free to choose whatever
algorithm and timing it wants to find a path that works. The sender
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and receiver roles are reversed for media flow in the opposite
direction.
The current version of this draft builds on its original
instantiation submitted in year 2015 as
2. Terminology
In this document, the key words "MUST", "MUST NOT", "SHOULD", "SHOULD
NOT", "MAY", and "OPTIONAL" are to be interpreted as described in RFC
2119 [RFC2119] and indicate requirement levels for compliant
implementations.
3. Problem Statement
ICE was developed roughly ten years ago and several things have been
learned that could be improved:
1. It is spectacularly difficult to debug and analyze failures or
successes in ICE or develop good automated tests. Many
implementations have had significant bugs for long periods of
time. This is further complicated by the timing dependency as
explained next.
2. It is timing dependent. It relies on both sides to to do
something (candidate pairing, validation) at roughly the same
time and that ability to do this goes down with the number of
interfaces and candidates being handled. Mobile interfaces, dual
stack agents make this situation worse.
3. Differences in interpretation and implementation of the protocol
with respect to aggressive vs normal nomination may hinder rapid
convergence or may end up in agents choosing suboptimal routes.
4. It does not discover asymmetric routes. For example UDP leaving
a device may work just fine even though UDP coming into that
device does not work at all.
5. Many deployments consider using a TURN/Media Router in their
topology today in order to support fast session start or ensuring
reliable connection (although with small latency overhead). At
the time ICE was designed it was not understood if this would be
too expensive or not so. ICE works without TURN but better with
it.
6. The asymmetric nature of the controlling / controlled roles has
caused many interoperability problems and bugs. Also Role
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conflicts might lead to degrade connection setup depending on
which side gets the the controlling role.
7. Priorities are complicated in dual stack world and ICE is brittle
to changes in this part of the algorithm. Although there are
advises in dual-stack-fairness specification that might help
here.
4. Snowflake for connectivity establishment
Snowflake is a light weight, asymmetric, flexible and receiver
controlled protocol for end points to establish connectivity between
them.
The following subsections go into further details of its working
4.1. System Components
A typical Snowflake operating model has the following components
o Sender Agent: A Software agent interested in sending data
stream(s) to a remote receiver.
o Receiver Agent: A Software agent capable of receiving data
stream(s).
o Snowflake Agent: A software agent that is expected to have a STUN
Client implementation at the minimum for gathering candidates and
performing connectivity checks.
o Signaling Server: Publicly reachable Server in the cloud
accessible by both the Sender and the receiver agents, acts as
backchannel/message bus for carrying signals between the Snowflake
agents.
o STUN Server: Optional component for determining the public facing
transport address of an agent behind NAT.
o TURN Server/Media Router: Recommended component acting as media
relay between the agents. A TURN Server can also act as
backchannel in certain instantiations.
o BackChannel: A dedicated channel used by the agents to convey
Snowflake messages, can be a Signaling Server/Turn Server that can
be reached publicly by the agents.
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4.2. Protocol Workings
The basic principle here is, each side (Receiver Agent) is
responsible for discovering a viable path for it's incoming media.
It does so by indicating the addresses for the Sender to verify the
connectivity. Once a viable path is established, the Sender Agent
continues to transmit the media. This processs deviates from ICE by
negating the need for agent's role (controlled vs controlling),
nomination procedures (aggressive vs passive) and tightly coupled
symmetric checklists validation.
As a precursor to connectivity establishment, the protocol assumes
that there exists a dedicated backchannel that a Receiver Agent uses
to invoke operations on the Sender Agent to trigger test for
connectivity or perform updates for the same as the session
progresses.
The protocol starts with the Sender Agent conveying its intention to
send media via the backchannel to the Receiver agent. The Sender can
provide additional details on type of media, its quality information,
as part of its "Media Send Intention" message.
On receiving the Sender's media send intention message, the Receiver
Agent gathers the candidates defined by its local policies or
previous knowledge of connectivity checks. The candidate(s) along
with additional attributes (priority, type for example) are then
exchanged by invoking an appropriate operation on the Sender Agent.
An message of type "Test Candidates" is sent with encapsualted
candidate information. This is equivalent to the way the ICE
candidates are trickled in the Trickle ICE via a signaling server.
On the Sender Agent, the candidates thus obtained (i,e in the Test
Candidates message) is used by the STUN client implementation to
carry out the connectivity checks towards the receiver. The
connectivity checks are performed along the media path as its done
with ICE today. This opens up the required local pinholes and are
further maintained by the Sender for the duration of the session.
The Sender Agent also requests the Receiver Agent to send it a "STUN
Ping" message from a given address (source of connectivity check) to
a specific candidate provided in the "Test Candidates" message. This
is done so that Sender Agent can verify the connectivity status
results over the backchannel. This mechanism is beneficial
especially for one-sided media scenarios where the Receiver Agent
can't send the STUN response to the sender or if the response to STUN
connectivity response was lost in transmission. The Sender does this
by sending a "Stun Ping Request" message and populates the
aforementioned information. To reciprocate, the Receiver Agent
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follows up with a "Stun Ping" message populated with the results for
which STUN Connectivity checks was received successfully. If a
successful response were received from either of the flows, there is
a viable path for the Sender to transmit the media.
The above set of procedures can be continuously performed during the
lifetime of the session as and when the Receiver Agent determines a
better candidate for receiving the media. Such a decision is totally
defined by the local policies and can be performed independently of
the other side.
Also to ensure Receiver Agent's consent for receiving the media, the
sender should follow the procedures in [RFC7675] to get the consent.
It is also RECOMMENDED that the consent be verified over the
backchannel as well. In order to do so, the Sender Agent sends "STUN
Ping Request" message with the candidate information for which
consent needs to be obtained. In response, the Receiver Agent sends
"STUN Ping" message indicating the consent status.
Below picture captures one instance of protocol exchange where the
Receiver Agent indicates the Sender Agent to carry out the
connectivity checks. One can envision multiple executions of the
protocol as and when receiver has updated its knowledge of addresses
or priorities or bandwidth availability.
Snowflake Information Flow Model
---------------------------------------
Sender Agent BackChannel Receiver Agent
| | |
| | |
| | |
|(1) connect to backchannel |
|.............................|
| | |
| | |
|(2) Media Send Intention (via backchannel)
|---------------------------->|
| | |
| | |
| | |Gather candidate address(es)
| | |
| | |
| | |
| |(3) Test Candidate(s) [address,priority..]
| |<-------------|
| | |
| | |
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|(4) Test Candidate(s) [address,priority..]
|<-------------| |
| | |
| | |
|(5) STUN connectivity check over media path
|.............................|
| | |
| | |
|(6) STUN Ping Request (candidates checked)
|------------->| |
| | |
| | |
| |(7) STUN Ping Request (candidates checked)
| |------------->|
| | |
| | |
| |(8) STUN Ping (connectivity results)
| |<-------------|
| | |
| | |
|(9) STUN Ping (connectivity results)
|<-------------| |
| | |
| | |
|(10) Found a viable path, ask for consent
|.............................|
| | |
| | |
|(11) STUN Ping Request (candidate consent)
|------------->| |
| | |
| | |
|(12) STUN Ping Request (candidate consent)
|<-------------| |
| | |
| | |
| |(13) STUN Ping (consent result)
| |<-------------|
| | |
| | |
|(14) STUN Ping (consent result)
|<-------------| |
| | |
| | |
|(15) Consent Appproved for Sending Media
|.............................|
| | |
| | |
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Notes:
Steps 6 - 9 is optional and media path based connectivity check
might suffice.
Steps 11 - 14 can happen exclusively on media path and backchannel
may be used for reliability
4.3. Advantages of Snowflake
4.3.1. Diagnostics
This makes it very easy to see which outbound connection were sent
from the Sender Agent to open a pin hole. Then when the Sender asked
the Receiver Agent to send a test STUN Ping, the connectivity can be
easily verified. It becomes easier to set up a client with an
automated test jig that tests all the combinations and makes sure
they work as you only need to test receiving capability and sender
capability independently.
4.3.2. Timing
This more or less removes the timing complexity by allowing both
sides to be responsible for their own timing. If it turns out that
we can pace things much faster than 50ms then this allows us to take
advantage of that without both sides upgrading at the same time.
If we end up with a lot more candidates due to v6, mobile etc, this
removes the issue we have today where a path might have worked but
the two sides did not find it due to timing issues.
4.3.3. Asymmetric Media
This allows media to be sent in one direction over a path that does
not work in the reverse direction.
4.3.4. Fast Start
Given there exists a dedicated backchannel, this protocol can speed
up the media flow by using TURN server for the backchannel, for
example. Once either agents learns more about the candidates, each
can update the other side to ensure a better low latency path is used
for media.
5. IANA Consideration
TODO
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6. Security Considerations
TODO
7. Acknowledgements
TODO
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, .
8.2. Informative References
[I-D.ietf-ice-rfc5245bis]
Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", draft-ietf-ice-
rfc5245bis-16 (work in progress), January 2018.
[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-15 (work in progress),
November 2017.
[RFC7675] Perumal, M., Wing, D., Ravindranath, R., Reddy, T., and M.
Thomson, "Session Traversal Utilities for NAT (STUN) Usage
for Consent Freshness", RFC 7675, DOI 10.17487/RFC7675,
October 2015, .
Authors' Addresses
Cullen Jennings
Cisco
Email: fluffy@iii.ca
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Suhas Nandakumar
Cisco
Email: snandaku@cisco.com
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