Design Space Analysis of MoQ

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Table of Contents

1. Introduction

Media over QUIC aims to provide low-latency, efficient media delivery solution for use cases including live streaming, gaming, and video conferencing. The latency requirement and the transmission pattern are analyzed in [moq-req]. To scale efficiently, relay can be used to optimize the delivery performance by caching, selective dropping, etc. However, how to accomplish that remains unclear. Lots of factors of the relay and protocol design choice can affect the performance gain of leveraging relay. This document aims to provide analysis of those design choices.¶

2. Terminology

• Relay: An element which participates in the forwarding of the media content. Possibly support caching, selective dropping to optimize the media transmission performance.¶

• Producer: An endpoint which generate the media stream. Could be the original content producer (a live streaming uploader) or the re-encoder in the cloud.¶

• Consumer: An endpoint which receive the media stream. Could be the live stream viewer or the re-encoder in the cloud.¶

3. Design Choice 1: Static Tree Topology versus Dynamic Mesh Topology

The first question of using relay to forward the media between the producer and the consumer is the topology of relays. In traditional CDN network, each CDN site can be viewed as a relay. Those relays are organized in a tree (see Figure 1). The producer and the consumer are usually connected to the edge node of the CDN which is the leaf node in the tree. In this case, the path for media in live streaming is usually producer - edge node 1 (relay 1) - parent node 1 (relay 2) - origin node (relay 3) - parent node 2 (relay 4) - edge node 2 (relay 5) - consumer, i.e. the media need to first go up to the root of the tree, then go down to another leaf node, traversing multiple (at least 3) relays if the CDN hierarchy is deep or the producer and the consumer is highly distributed. The tree topology is simple to build since the path of the stream is fixed and the leaf node can be lightweight and deployed closely to user. The computing intensive process can be put in the much more powerful root servers.¶

[QUICR-arch] is similar to the tree topology of CDN with one improvement: the relay can shortcut the media transmission. If the producer and the consumer share a parent relay, the media will be forwarded in the relay instead of the root of the tree (called Origin in QUICR's term).¶

                               +----------+
                  +----------->|   Root   +-----------+
                  |            +----------+           |
                  |                                   |
             +----+-----+                        +----v-----+
      +----->| Parent-1 |                        | Parent-2 +--------+
      |      +----------+                        +----------+        |
      |                                                              |
 +----+-----+                                                   +----v-----+
 |  Edge-1  |                                                   |  Edge-2  |
 +----^-----+                                                   +----------+
      |                                                              |
      |                                                              |
+-----+----+                                                     +---v------+
| Producer |                                                     | Consumer |
+----------+                                                     +----------+

Another approach is to connect the relays in a dynamic mesh instead of a static hierarchy. Alibaba's low-latency live streaming network builds on a flat CDN overlay [LiveNet]. A centralized controller collects the latency between each relay periodically and calculates the optimal path (latency-wise) for each media stream dynamically. Alibaba claims the flat topology reduce the latency by half compared to static hierarchy. An example is shown in Figure 2, the media stream is forwarded through relay 1 and relay 4, only 2 hops. If the network path between relay 1 and relay 4 are congested, relay 1 - relay 2/3 - relay 4 maybe used to provide lower-latency forwarding.¶

The controller can be connected with 3rd party application provider and manages the interactive media communication between producer and consumer for the application provider. The interactive media can be delivered to other consumers via certain relays of the live streaming network. A request of interactive media communication can be triggered by a consumer. The request is sent to the application provider which then attempts to pull related media of corresponding consumer and producer from the live streaming network. The application provider merges the media containing the producer and the consumer and delivers the merged media to the live streaming network. The live steaming network conducts the media switching for the corresponding producer, consumer and other consumers via the corresponding relays upon the receipt of the media switching request.¶

              +-----------------------------------------+
              |              Controller                 |
              +-----------------------------------------+
              |                                         |
              |              +---------+                |
              |      +-------> Relay-2 +---------+      |
              |      |       +----+----+ path 2  |      |
              |      |            |              |      |
+----------+  | +----+----+       |         +----v----+ |   +----------+
| Producer +--+>| Relay-1 +-------+---------> Relay-4 +-+-->| Consumer |
+----------+  | +----+----+       | path 1  +---------+ |   +----------+
              |      |            |              |      |
              |      |            |              |      |
              |      |       +----+----+         |      |
              |      +-------+ Relay-3 +---------+      |
              |              +---------+                |
              |                                         |
              +-----------------------------------------+

4. Design Choice 2: QUIC hop-by-hop versus end-to-end

The media flow sending from the producer to the consumer will go through several relays. The media content will be encrypted using QUIC encryption as requested in charter. But whether the relay node will terminate the QUIC connection remains open. There are following two options to implement the MoQ protocol stack.¶

The first option is to running the entire MoQ protocol inside QUIC encryption, including the media metadata which is needed by relay (see Figure 3). Thus the relay has to terminate the QUIC connection, decrypting the QUIC payload. This will require each relay node hold a valid CA certificate and run the CA verification process. Just like what the CDN node does nowadays.¶

        Media (Metadata + Content)
----------------------------------------------
    Protocol header  |  Protocol payload           <-------- MoQ
----------------------------------------------
                   QUIC                            <-------- Transport

The second option is to only encrypt the media content using QUIC encryption but leave the metadata to other mechanism (see Figure 4). In this way, the QUIC connection is from producer to consumer. The relay does not need to decrypt the QUIC, saving the computing power. As the charter put it: "Even when media content is end-to-end encrypted, the relays can access metadata. Hence a new mechanism to convey the metadata to the relay is needed, similar to SDP for RTP, or m3u8 file for HLS.¶

      Media metadata     |  Media content            <-----\
-------------------------|-----------------------           \
     Protocol header     |  Protocol payload         <------ MoQ
-------------------------|-----------------------           /
         Other           |    QUIC                   <-----/

5. Security Considerations

When the metadata is not carried inside the QUIC payload, it should be protected from unauthorized third-part access to to protect the privacy. Relay should be authenticated to access the metadata.¶

Authors' Addresses