| Internet-Draft | MoQ relay for deadline | September 2023 |
| Cui, et al. | Expires 14 March 2024 | [Page] |
This draft specifies the behavior of MoQ relays for delivering media before the deadline to decrease end-to-end latency and save transport costs in media transmission. To achieve this, the draft introduces deadline-aware actions prioritizing media streams with earlier deadlines, ensuring timely transmission while minimizing costs.¶
This note is to be removed before publishing as an RFC.¶
Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ma-moq-relay-for-deadline/.¶
Source for this draft and an issue tracker can be found at https://github.com/STAR-Tsinghua/draft-moq-for-deadline.¶
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Media over QUIC (MoQ) is a transport system designed to provide efficient media transport. However, some use cases, such as live streaming, online meetings, and gaming, require the client to receive their media before a specific time, referred to as the 'deadline.' Exceeding the deadline results in dropped data, which can increase latency and negatively affect user experience.¶
To address this issue, a deliver-before-deadline transport service can be provided, which is the goal of the Deadline-aware Transport Protocol (DTP) proposed in [I-D.draft-shi-quic-dtp]. DTP leverages stream-level scheduling, active stream canceling, and redundancy coding to prioritize urgent data and prevent outdated data from blocking later data.¶
This document proposes the behavior of deadline-aware actions on MoQ relay nodes, extending the basic MoQ relay to provide deliver-before-deadline transmission. The relay design utilizes scheduling, data canceling, and redundancy coding to decrease queuing time, prevent unnecessary re-transmission of overdue data, and ultimately reduce end-to-end latency. By providing better data delivery strategies, MoQ relays with deadline-aware actions can significantly enhance overall user experience in media transport.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
+===========================+
: /=======\ :
: Deadline :Block 1: :
: -aware :DDL:xxx: :
: Canceling \=======/ :
/-------\ : ^ :
|Block 1| : : :
|DDL:xxx| : +---------+ : :
+---------+ \-------/ : |Deadline | : :
|Publisher|-----------:--->| -aware |==: :
+---------+ /-------\ : | Relay | :
|Block 2| : +---------+ :
|DDL:yyy| : | /-----------\ :
\-------/ : | | Block 2 | :
: | | DDL:yyy | :
: | |-----------| :
: | | Redundancy| :
: | | Coding | :
: v \-----------/ :
: +---------+ :
: |Deadline | : +----------+
: | -aware |------------:--->|Subscriber|
: | Relay | /-------\: +----------+
: +---------+ |Block 2|:
: |DDL:yyy|:
: \-------/:
+===========================+
Deadline-aware MoQ Relay nodes
Figure 1 illustrates the fundamental architecture of Deadline-aware MoQ. This architecture involves the extension of MoQ Publishers and Subscribers, which transport block-like data and add 'Deadline' as a component of Metadata within the header. Relay nodes within this system are equipped with deadline-aware actions, including deadline-aware scheduling, canceling, and redundancy coding. The relay may schedule the data blocks, cancel the overdue ones, and add redundancy code to avoid re-transmission. The relays receive block-like data from the publisher, transfer between relays, make deadline-aware actions, and transmit it to the subscriber.¶
The main focus of this draft is the extension of MoQ relays or the 'Deadline-aware MoQ Relay.' The Deadline-aware MoQ Relay SHOULD send data in a block-like style to enable deadline-aware actions. A Block is a basic data unit in the MoQ system, like the Object in [MOQT]. A Block SHOULD contain, at a minimum, a Block ID field in the its header to distinguish it from others.¶
Depending on the MoQ implementation, the relay implementation may map the block transmission to different mechanisms of [QUIC], such as matching a Block to multiple QUIC datagrams or a single QUIC stream. Deadline-aware MoQ Relay SHOULD support various MoQ transport implementations. When the relay receives data without any deadline-related information from the endpoint, it SHOULD forward it without utilizing any deadline-aware actions.¶
The Deadline-aware MoQ Relay SHOULD support various relay topologies, as discussed in [I-D.draft-shi-moq-design-space-analysis-of-moq]. Each relay topology may require a different MoQ architecture or implementation. Therefore, the deadline-aware actions should act as a plugin that relays can quickly implement regardless of the topology and architecture.¶
In this draft, we utilize the Block as the fundamental unit for data transmission. A Block comprises two essential components: metadata and payload. The payload of a Block is a sequence of bytes that carries the basic unit in media transport, such as a video frame. Meanwhile, a Block's metadata encompasses deadline-related information necessary for enabling Deadline-aware Actions(see Section 4.2). It's worth noting that the metadata of a Block can remain unencrypted, whereas the payload of a Block SHOULD be encrypted.¶
The Block serves as a model exclusively for data transmission within the MoQ framework. Its purpose is to support the design principles of data units within MoQ, like the Object or the Group in MOQT. Importantly, it is crucial that the Block model does not supersede or alter the original data transmission model and should adapt to different designs in MoQ.¶
For Deadline-aware MoQ Relay, data block metadata is required to enable deadline-aware actions. Both the endpoint and the relay SHOULD attach the following metadata to each data block when using deadline-aware actions:¶
The relay SHOULD maintain track of the metadata of a block until the block misses its deadline.¶
Additionally, if the endpoint does not offer metadata in the header of a data block, relays MAY implement other mechanisms to acquire and synchronize deadline-related metadata.¶
e.x. The specific methodology for encapsulating metadata needs to wait until the MoQ specification is standardized.¶
TODO: At present, we set the priority as a relative value within a session. However, the determination of object priority in MOQT remains a subject of ongoing discussion. We are contemplating deferring this aspect until the Working Group reaches a consensus on the matter.¶
Deadlines can be defined in two distinct manners: End-to-End Deadline and Hop-by-Hop Deadline.¶
The End-to-End Deadline indicates the expected end-to-end delay of the application, beyond which a data block is considered obsolete. Conversely, the Hop-by-Hop Deadline represents the anticipated delay between two nodes within each hop. It defines the tolerance for delay between relay nodes but does not convey the end-to-end latency requirement.¶
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Duration (i)/Timestamp (i)] ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 demonstrates a potential implementation of the Deadline field. The Type field specifies the function of these two fields. The last bit of Type (0x1) indicates whether the Block uses Hop-by-Hop Deadline. The least significant bit of Type (0x1) indicates whether the Block utilizes the Hop-by-Hop Deadline. The second-to-last bit of Type (0x2) indicates whether the Block employs a time duration as the maximum delay tolerance or a Unix timestamp as the expiration time for the data. A Type value of 0x4 signifies that the Block has no Deadline requirement.¶
The Deadline-aware MoQ Relay SHOULD implement stratergies to manage both End-to-End Deadline and Hop-by-Hop Deadline requirements.¶
When implementing deadline-aware actions, the Deadline-aware MoQ Relay can utilize the block metadata for scheduling blocks at the block-level. The scheduler SHOULD minimize the total time of queuing and try to meet the deadline requirements of as many high-priority blocks as possible.¶
If a block misses its deadline, Deadline-aware MoQ Relay MAY cancel it. In such cases, the endpoints SHOULD be able to accept partially received data and not request for data re-transmission when a block is dropped. Additionally, the relays MAY inform the endpoints and other relays about the cancellation of these blocks.¶
To improve reliability and decrease latency, Deadline-aware MoQ Relay MAY introduce redundancy data to blocks close to their deadline or transmitted over a network with a high loss rate. This redundancy can help to prevent the need for re-transmission. If the first relay adds redundancy coding to the data, other relay nodes in the network may benefit from it.¶
When redundancy coding is enabled, at least two nodes SHOULD implement a pair of encoder and decoder that comply with the redundancy coding method. The endpoint MAY also implement a redundancy encoder and decoder to utilize the relay's redundancy coding function fully.¶
The first node that encodes the data with redundancy coding MUST add redundancy-related information to the metadata of the data block.¶
In situations where a data block is dropped by a relay due to a missed deadline or other reasons, sending an explicit dropping message to other relays and both endpoints can be helpful in notifying them of the data loss. The dropping message may include information about the block, such as its ID and metadata. However, the effective method for notifying other nodes and the decision regarding whether to send drop notifications to other relays are still pending discussion. Broadcasting drop notifications could potentially lead to network flooding and requires further consideration.¶
Further discussion is required to determine if the relay should implement a buffer for data blocks during forwarding and how such a buffer should be implemented. This buffer may be used for re-transmission purposes and may benefit users with larger delay tolerance, among other potential uses.¶
To enable accurate deadline-aware actions, it is recommended that all endpoints and relays perform clock synchronization. Nevertheless, achieving high-precision clock synchronization over the Internet can be a formidable task, and it is also impractical to synchronize all devices in a single MoQ session. The challenge then becomes how to carry out deadline-aware actions in the presence of imprecise clock accuracy, which is a critical question that needs to be addressed.¶
Access to the metadata of the Deadline-aware MoQ Relay SHOULD be limited to selected relays. The relay SHOULD NOT access the content of the data block.¶
This document has no IANA actions.¶
We sincerely thank Wei Cao for his advice and revisions to this draft.¶