Deadline-aware Transport Protocol
draft-shi-quic-dtp-02

Abstract

This document defines Deadline-aware Transport Protocol (DTP) to provide block-based deliver-before-deadline transmission. The intention of this memo is to describe a mechanism to fulfill unreliable transmission based on QUIC as well as how to enhance timeliness of data delivery.

Status of This Memo

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Table of Contents

• 1. Introduction
• 1.1. Conventions
• 2. Motivation
• 3. Design of DTP
• 3.1. Abstraction
• 3.2. Architecture of DTP
• 3.3. Deadline-aware Scheduler
• 3.4. Deadline-aware Redundancy
• 3.5. Loss Detection and Congestion Control
• 4. Extension of QUIC
• 4.1. New Frame: BLOCK_INFO Frame
• 4.2. Adjusted QUIC Frame: Timestamped ACK Frame
• 4.3. Redundancy Packet
• 5. DTP Use Cases
• 5.1. Block Based Real Time Application
• 5.2. API of DTP
• 5.2.1. Data Transmission Functions
• 5.2.2. Feedback Functions
• 6. IANA Considerations
• 7. Security Considerations
• 8. Normative References
• Authors' Addresses

1. Introduction

Many emerging applications have the deadline requirement for their data transmission. However, current transport layer protocol like TCP [RFC0793] and UDP [RFC0768] only provide primitive connection establishment and data sending service. This document proposes a new transport protocol atop QUIC [QUIC] to deliver application data before end-to-end deadline.

1.1. Conventions

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [RFC2119].

2. Motivation

Many applications such as real-time media and online multiplayer gaming have requirements for their data to arrive before a certain time i.e., deadline. For example, the end-to-end delay of video conferencing system should be below human perception (about 100ms) to enable smooth interaction among participants. For Online multiplayer gaming, the server aggregates each player's actions every 60ms and distributes these information to other players so that each player's state can be kept in sync. Data missing the deadline is useless since it will be overwrote by the new data.

These real-time applications have following common features:

• They tend to generate and process the data in block fashion. Each block is a minimal data processing unit. Missing a single byte of data will make the block useless. For example, video/audio encoder produces the encoded streams as a series of block(I,B,P frame or GOP). Decoder consumes the frame into the full image. For online games, the player’s commands and world state will be bundled together as a message.
• They will continuously generate new data. Different from web browsing or file syncing, real-time applications like video conferencing and online multiplayer gaming have uninterruptedly interactions with users, and each interaction requires a bunch of new data to be transmitted.
• They prefer the timeliness of data instead of reliability since data missing deadline are useless to application and will be obsoleted by newer data. For example in multiplayer online games, the gaming server will broadcast the latest player states to every client, and the old information does not matter if it can not be delivered in time. So the meaningful deadline of the application is actually the block completion time i.e., the time between when the block is generated at sender and when the block is submitted to application at receiver.

However, current transport layer protocols lack support for block-based deadline delivery. TCP guarantees reliability so it will waste network resource to transmit stale data and cause fresh data to miss its deadline. UDP is unreliable but it doesn't drop data according to deadline, all data have the same chance to be dropped indeed. QUIC makes several improvements and introduces Stream Prioritization [QUIC] to enhance application performance, but prioritization is not enough for enhancing timeliness.

Insufficiency of existing transport layer forces applications to design their own customized and complex mechanism to meet the deadline requirement. For example, the video bitrate auto-adjustment in most streaming applications. But this is a disruption to the Layered Internet Architecture, forcing applications to worry about network conditions.

This document proposes Deadline-aware Transport Protocol (DTP) to provide deliver-before-deadline transmission. DTP is implemented as an extension of QUIC (Refer to [Section 4]) because QUIC provides many useful features including full encryption, user space deployment, zero-RTT handshake and multiplexing without head-of-line blocking.

3. Design of DTP

The key insight of DTP is that these real-time applications usually have multiple blocks (As shown in Figure 1 below) to be transferred simultaneously and these blocks have diverse impact on user experience(denoted as priority). For example, audio data is more important than video stream in video conferencing. Central region is more important than surrounding region in 360 degree video. Foreground object rendering is more important than the background scene in mobile VR offloading.

The priority difference among multiple blocks makes it possible to drop low priority data to improve timeliness of high priority data delivery, which can enhance the overall QoE if resources allocated to blocks are correctly prioritized. In this section, we describe the mechanism which enables DTP to leverage that insight.

3.1. Abstraction

DTP provides block-based data abstraction for application. Application MUST attach metadata along with the data block to facilitate the scheduling decision, those metadata include:

• Each block has a deadline requirement, meaning if the block cannot arrive before the deadline, then the whole block may become useless because it will be overwrote by newer blocks. The application can mark the deadline timestamp indicating the deadline of its completion time. In the API of DTP, the deadline argument represents the desired block completion time in ms.
• Each block has its own importance to the user experience. The application can assign each block a priority to indicate the importance of the block. The lower the priority value, the more important the block. The priority argument also indicates the reliability requirement of the block. The higher priority, the less likely the block will be dropped by sender.

3.2. Architecture of DTP

The sender side architecture is shown in Figure 1:

                            +-------------+                                 
                            |             |                                 
                            | Application |                                 
                            |             |                                 
                            +-------------+                                
                                   |                                     
                                   |                                       
                                   V                                      
 +------------------------------------------------------------------------+            
 |         Block 0                Block 1                   Block n       |             
 | +--------+----------+  +--------+----------+     +--------+----------+ |             
 | |Metadata|Data Block|  |Metadata|Data Block| ... |Metadata|Data Block| |     
 | +--------+----------+  +--------+----------+     +--------+----------+ |
 |                                                                        |
 | (Metadata includes Deadline and Priority)                              |
 +------------------------------------------------------------------------+
                                   |
                                   |
                                   v
                            +-------------+
                            |             |
                            |  Scheduler  |
                            |             |
                            +-------------+
                                   |
                                   |
                                   v
                            +-------------+ 
                            |             |
                            | Redundancy  | 	
                            | Encoder     |
                            +-------------+
                                   |
                                   |
                                   v								   
                            +-------------+                               
                            |             |                               
                            | Congestion  |         
                            | Control     |                   
                            +-------------+
                                   |
                                   v
                 Figure 1: The Architecture of DTP

In receiver side, the transport layer will receive data and reassemble the block. The process is symmetric with the sender side. It first goes through packet parsing and redundancy processing module. Transport layer also keeps track of the deadline of each block. When receiver calls RECV function (Refer to [Section 5]), the transport layer returns the received in-ordered data to the application.

3.3. Deadline-aware Scheduler

The scheduler will pick the blocks to send and drop stale blocks when the buffer is limited. This section describes the algorithm of DTP scheduler.

Scheduler of DTP takes into account many factors when picking blocks in sender buffer to send. The goal of the scheduler is to deliver as much as high priority data before the deadline and drop obsolete or low-priority blocks. To achieve this, the scheduler utilizes both bandwidth and RTT measurement provided by the congestion control module and the metadata of blocks provided by the application to estimate the block completion time. The scheduler will run each time ACK is received or the application pushes the data.

A simple algorithm which only considers priority cannot get optimal result in transmitting deadline-required data. Suppose the bandwidth reduces and the scheduler chooses not to send the low priority block. Then the bandwidth is restored. The data block with lower priority is closer to the deadline than the high priority block. If in this round the scheduler still chooses to send the high priority block, then the low priority block may miss the deadline next round and become useless. In some cases, the scheduler can choose to send a low priority block because it’s more urgent. But it should do so without causing the high priority stream missing the deadline. This example reveals a fundamental conflict between the application specified priority and deadline implicated priority. DTP needs to take both priorities into consideration when scheduling blocks.

DTP will combine all these factors to calculate real priority of each block. Then the scheduler just picks the block with the highest real priority. Scheduler of DTP will calculate the block remaining transmission time and then compare it to the deadline. The closer to the deadline, the higher real priority. And higher application specified priority will also result in higher real priority. In this way, the scheduler can take both approaching deadline and application-specified priority into account. Blocks which are severely overdue can be dropped accordingly.

3.4. Deadline-aware Redundancy

After the scheduler pick the block to send, the packetizer will break the block into packet streams. Those packet streams will go through the redundancy module. When the link is lossy and deadline is tight, retransmission will cause the block missing the deadline. Redundancy module has the ability of sending redundancy (like FEC Repair Symbols) along with the data that will help to recover the data packets (like FEC Source Symbols), this can avoid retransmission.

We use unencrypted DTP packets as input to Redundancy Module because the loss of a DTP packet exactly corresponds to the loss of one Redundancy Packet. And to perform the coding and decoding with packets of different sizes, some packets may need to be padded with PADDING Frame. The present design of Redundancy Module follows the FEC Framework specified in [arXiv:1809.04822]. Figure 2 illustrates this framework:

 

           | 
           | 
           v 
    +-------------+              
    |             |
    |     DTP     |
    |  Scheduler  |              
    |             |              
    +------+------+              
           |                                                         
        (1)|DTP Packets            
+----------|-------------------------------------------+
|          v                                           |
|  +-------+------+                    +------------+  | 
|  |              | (2)DTP Payload     |            |  | DTP
|  |  Redundancy  |------------------> | Redundancy |  | Redundancy
|  |  Packtizing &|<------------------ | Scheme     |  | Module
|  |  Grouping    | (2)Redundancy Data |            |  |
|  +-------+------+                    +------------+  |
|          |                                           |
+----------|-------------------------------------------+
        (3)|Redundancy
           |Packets
           |
           v
		Figure 2: DTP Redundancy Module

Figure 2 above shows the mechanism of how the Deadline-aware Redundancy module works. (1) Redundancy Module first receives the unencrypted DTP packets from scheduler. (2) The Redundancy Scheme use DTP Payload (similar to FEC Repair Symbols) to generate Redundancy Data (similar to FEC Source Symbols). (3) Redundancy-protected DTP Packets and Redundancy Data will be packtized and grouped. Redundancy Packtizing and Grouping Part will generate FEC Payload INFO (Figure 5) and attach it to the DTP Packets and Redundancy Data, generating Redundancy Packets (a Redundancy Packet with the header shown in Figure 5). Once the protocol receives the Repair Symbols, they are sent to the receiver through the FEC Packets. At the receiver-side, the received Redundancy Packets can be processed immediately. The Redundancy Data is reconstituted from the Redundancy Packtizing and Grouping and passed to the underlying Redundancy Scheme to recover the lost DTP Packets.

Although Redundancy Module allows recovering lost packets without waiting for retransmissions, it consumes more bandwidth than a regular, non-Redundancy-protected transmission. In order to avoid spending additional bandwidth when it is not needed, design of Redundancy MUST allow defining which DTP packets should be considered as Redundancy Packets. Currently we use a F flag from DTP Packet Header to indicate whether a packet is Redundancy-protected or not. The format of header will be described in [Section 4.3] later.

The Redundancy Data generated in Redundancy module MUST be distinguished from application data payload. Redundancy Data should not be transferred to the application upon reception, they are indeed generated by and for the Redundancy Scheme used by the transport protocol. We use Redundancy Packet to transmit Redundancy Data [Section 4.3].

There are multiple Redundancy Scheme candidates. During the handshake process, a scheme will be negotiated for the DTP session, just like encryption scheme negotiation. Currently DTP specifically chooses Reed-Solomon FEC Scheme as described in [arXiv:1809.04822].

3.5. Loss Detection and Congestion Control

This document reuses the congestion control module defined in QUIC [QUIC]. Congestion control module is responsible to send packets, collects ACK and do packet loss detection. Then it will put the lost data back to the retransmission queue of each block. Congestion control module is also responsible to monitor the network status and report the network condition such as bandwidth and RTT to scheduler.

4. Extension of QUIC

DTP is implemented as an extension of QUIC by mapping QUIC stream to DTP block one to one. In that way, DTP can reuse the QUIC stream cancellation mechanism to drop the stale block during transmission. And DTP can also utilize the max stream data size defined by QUIC to negotiate its max block size. Besides, the block id of DTP can also be mapped to QUIC stream id without breaking the QUIC stream id semantic.

DTP endpoints communicate by exchanging packets. And the payload of DTP packets, consists of a sequence of complete frames. As defined in [QUIC], each frame begins with a Frame Type, indicating its type, followed by additional type-dependent fields. Besides the many frame types defined in Section 12.4 of [QUIC], DTP introduces BLOCK_INFO Frame to support timeliness data transmission. And DTP also makes adjustment on QUIC ACK Frame. Another extension is introducing FEC packet to support FEC.

4.1. New Frame: BLOCK_INFO Frame

DTP adds a BLOCK_INFO frame (type=0x20) in the front of each block to inform scheduler of Block Size, Block Priority and Block Deadline. These parameters can be used to do block scheduling. The BLOCK_INFO frame is as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Block Size (i)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Block Priority (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Block Deadline (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   
		Figure 3: BLOCK_INFO Frame Format

• Stream ID: A variable-length integer indicating the stream ID of the stream.
• Block Size: A variable-length integer indicating the size of the block.
• Block Priority: A variable-length integer indicating the priority of the block.
• Block Deadline: A variable-length integer indicating the required transimission deadline.

4.2. Adjusted QUIC Frame: Timestamped ACK Frame

DTP add a new Time Stamp Parameter to QUIC ACK Frame. Timestamped ACK frames are sent by reveiver to inform senders of the time when the packet the peer is acknowledging is received and processed. ACK mechanism of DTP is almost the same with QUIC. The format of the Timestamped ACK frames is similar to that of the standard ACK Frames defined in section 19.3 of [QUIC]:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Largest Acknowledged (i)                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Time Stamp (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          ACK Delay (i)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ACK Range Count (i)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       First ACK Range (i)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          ACK Ranges (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          [ECN Counts]                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     
           Figure 4: Timestamped ACK Frame Format

Using this time stamp parameter we can calculate whether the prior blocks transmitted missing deadline or not, and we can also calculate the block completion rate before deadline.

4.3. Redundancy Packet

We use a F Flags in DTP Packet to distinguish which DTP packets is Redundancy-protected or not. Figure 5 shows the Redundancy Packet Format. If the flag is set, the Redundancy Payload INFO field is appended to the header. The Redundancy Payload INFO is an opaque field for the protocol. It is used by the Redundancy Scheme to identify the redundancy-protected data and communicate information about the encoding and decoding procedures to the receiver-side Redundancy Scheme.

                                   
    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
   +-+-+-+-+-+-+-+-+
   |F|   Flags(7)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Redundancy Payload INFO (i)(if F set)          ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Payload (i)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

		Figure 5: Redundancy Packet Format

• F: A flag indicating whether this DTP packets is FEC-protected or not.
• FEC Payload INFO: A variable-length integer containing the information of Redundancy Group ID and packet index in this Redundancy Group.
• Payload: The payload of the Redundancy Packet, containing DTP Payload or Redundancy Data.

5. DTP Use Cases

5.1. Block Based Real Time Application

DTP can provide deliver-before-deadline service for Block Based Real Time Applications. Applications like real-time media and online multiplayer gaming have deadline requirements for their data transimission. These application also tend to generate and process the data in block fashion, for example, video/audio encoder produces the encoded streams as a series of block (I,B,P frame or GOP). And these real-time applications usually have multiple blocks (As shown in Figure 1) to be transferred simultaneously. DTP can optimize the data transmission of these applications by scheduling which block to be sent first. And Redundancy Module of DTP can reduce retransmission delay.

5.2. API of DTP

DTP extends the send socket API to let application attach metadata along with the data block, and the API of DTP is structured as follows:

5.2.1. Data Transmission Functions

Send

• Format: SEND(connection id, buffer address, byte count, block id, block deadline, block priority) -> byte count
• The return value of SEND is the continuous bytes count which is successfully written. If the transport layer buffer is limited or the flow control limit of the block is reached, application needs to call SEND again.
• Mandatory attributes:
• • connection id - local connection name of an indicated connection.
  • buffer address - the location where the block to be transmitted is stored.
  • byte count - the size of the block data in number of bytes.
  • block id - the identity of the block.
  • block deadline - deadline of the block.
  • block priority - priority of the block.

Update

• Format: UPDATE(connection id, block id, block deadline, block priority) -> result
• The UPDATE function is used to update the metadata of the block. The return value of UPDATE function indicates the success of the action. It will return success code if succeeds, and error code if fails.
• Mandatory attributes:
• • connection id - local connection name of an indicated connection.
  • block id - the identity of the block.
  • block deadline - new deadline of the block.
  • block priority - new priority of the block.

Retreat

• Format: RETREAT(connection id, block id) -> result
• The RETREAT function is used to cancel the block. The return value of RETREAT function indicates the success of the action. It will return success code if succeeds, and error code if fails.
• Mandatory attributes:
• • connection id - local connection name of an indicated connection.
  • block id - the identity of the block.

Receive

• Format: RECV(connection id, buffer address, byte count, [,block id]) -> byte count, fin flag, [,block id]
• The RECV function shall read the first block in-queue into the buffer specified, if there is one available. The return value of RECV is the number of continuous bytes which is successfully read, and fin flag to indicate the ending of the block. If the block is cancelled, the RECV function will return error code BLOCK_CANCELLED. It will also returns the block id on which it receives if application does not specify it.
• If the block size specified in the RECV function is smaller than the size of the receiving block, then the block will be partial copied(indicated by the fin flag). Next time RECV function is called, the remaining block will be copied, and the id will be the same. This fragmentation will give extra burden to applications. To avoid the fragmentation, sender and receiver can negotiate a max block size when handshaking.
• Mandatory attributes:
• • connection id - local connection name of an indicated connection.
  • buffer address - the location where the block received is stored.
  • byte count - the size of the block data in number of bytes.
• Optional attributes:
• • block id - to indicate which block to receive the data on.

5.2.2. Feedback Functions

Status

• Format: STATS(connection id, block id) -> byte count
• The STATS function is used to query the deadline delivery result. The application uses STATS to query the bytes delivered before the deadline to receiver of each block. The information can be used to adjust the block sending rate of each priority. For example, if the application finds that the lowest priority block always get dropped due to the limited bandwidth, the application can stop generating the block to save the computation power. Combined the status of each priority, the application can also get the overall network capacity to facilitate the rate adaptation algorithm.
• Mandatory attributes:
• • connection id - local connection name of an indicated connection.
  • block id - the identity of the block.

Block Completion Time (BCT)

• Format: QUERY_BCT(connection id, block id) -> block completion time
• After receiving the block, application can query the block completion time using QUERY_BCT. This can also facilitate the rate or deadline adaptation of application. For example, if the base RTT of the network is bigger than deadline, then all blocks will miss the deadline. In this case, application may choose to relax its deadline.
• Mandatory attributes:
• • connection id - local connection name of an indicated connection.
  • block id - the identity of the block.

All these functions mentioned above are running in asynchronous mode. An application can use various event driven framework to call those functions.

6. IANA Considerations

This document has no actions for IANA.

7. Security Considerations

See the security considerations in [QUIC] and [QUIC-TLS]; the block-based data of DTP shares the same security properties as the data transmitted within a QUIC connection

8. Normative References

Authors' Addresses