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

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 August 3, 2018.

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Problem Statement
• 4. Snowflake for connectivity establishment
• 4.1. System Components
• 4.2. Protocol Workings
• 4.3. Advantages of Snowflake
• 4.3.1. Diagnostics
• 4.3.2. Timing
• 4.3.3. Asymmetric Media
• 4.3.4. Fast Start
• 5. IANA Consideration
• 6. Security Considerations
• 7. Acknowledgements
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

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 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 <https://datatracker.ietf.org/doc/draft-jennings-mmusic-ice-fix/>

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 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

• Sender Agent: A Software agent interested in sending data stream(s) to a remote receiver.
• Receiver Agent: A Software agent capable of receiving data stream(s).
• Snowflake Agent: A software agent that is expected to have a STUN Client implementation at the minimum for gathering candidates and performing connectivity checks.
• 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.
• STUN Server: Optional component for determining the public facing transport address of an agent behind NAT.
• TURN Server/Media Router: Recommended component acting as media relay between the agents. A TURN Server can also act as backchannel in certain instantiations.
• 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.

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 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..]
          |              |<-------------|
          |              |              |
          |              |              |
          |(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
          |.............................|
          |              |              |
          |              |              |

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

6. Security Considerations

TODO

7. Acknowledgements

TODO

8. References

8.1. Normative References

8.2. Informative References

Authors' Addresses