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Deadline-aware Transport Protocol

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Hang Shi , Yong Cui , Zhiwen Liu
Last updated 2019-11-16
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QUIC                                                              H. Shi
Internet-Draft                                                    Y. Cui
Intended status: Informational                                    Z. Liu
Expires: May 20, 2020                                Tsinghua University
                                                       November 17, 2019

                   Deadline-aware Transport Protocol


   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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 20, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Design of DTP . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Abstraction . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Architecture of DTP . . . . . . . . . . . . . . . . . . .   4
     3.3.  Deadline-aware Scheduler  . . . . . . . . . . . . . . . .   6
     3.4.  Deadline-aware Redundancy . . . . . . . . . . . . . . . .   6
     3.5.  Loss Detection and Congestion Control . . . . . . . . . .   7
   4.  Extension of QUIC . . . . . . . . . . . . . . . . . . . . . .   7
   5.  API of DTP  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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

   they appear in this document, are to be interpreted as described in

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.

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   These real-time applications have following common features:

   o  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.

   o  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.

   o  They prefer the timeliness of data instead of reliability since
      blocks 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, since application is not supposed 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 building blocks including full encryption, user space
   deployment, zero-RTT handshake and multiplexing without head-of-line

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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 to
   facilitate the scheduling decision.  Application can attach metadata
   along with the data block, those metadata include:

   o  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.

   o  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:

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                       |             |
                       | Application |
                       |             |
   |        0               1                    n       |
   | +----+---+---+  +----+---+---+       +----+---+---+ |
   | |Data| D | P |  |Data| D | P |  ...  |Data| D | P | |<--+ Lost
   | +----+---+---+  +----+---+---+       +----+---+---+ |   | Packet
   |        D: Deadline         P: Priority              |   |
   +-----------------------------------------------------+   |
                              |                              |
                              |                              |
                              V                              |
                       +-------------+                       |
                       |             |                       | Bandwidth
                       |  Scheduler  |<----------------------+ &
                       |             |                       | RTT
                       +-------------+                       |
                              |                              |
                              |                              |
                              V                              |
                       +-------------+                       |
                       |             |                       |
                       | Redundancy  |<----------------------+ Loss
                       | Encoder     |                       | Rate
                       +-------------+                       |
                              |                              |
                              |                              |
                              V                              |
                       +-------------+                       |
                       |             |                       |
                       | Congestion  +-----------------------+
                       | Control     |

             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

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   transport layer returns the received in-ordered data to the

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 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.  The

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   redundancy module can help mitigate that problem by generating the
   redundancy packets to avoid retransmission.  Since only
   retransmission of tail packets of the block will increase the block
   completion time so the redundancy is only applied to tail packets of
   each block.  The tail packets is defined within the Bandwidth-Delay
   Product range of the block.  And blocks with higher priority also get
   more redundancy.

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

5.  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

   A) Data transmission functions


      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:

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      *  connection id - local connection name of an indicated

      *  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.


      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

      *  block id - the identity of the block.

      *  block deadline - new deadline of the block.

      *  block priority - new priority of the block.


      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:

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      *  connection id - local connection name of an indicated

      *  block id - the identity of the block.


      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

      *  buffer address - the location where the block received is

      *  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.

   B) Feedback functions


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      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

      *  block id - the identity of the block.

   Block Completion Time (BCT)

      Format: QUERY_BCT(connection id, block id) -> block completion

      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

      *  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

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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

   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-20 (work
              in progress), April 2019.

              Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
              draft-ietf-quic-tls-20 (work in progress), April 2019.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

Authors' Addresses

   Hang Shi
   Tsinghua University
   30 Shuangqing Rd


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   Yong Cui
   Tsinghua University
   30 Shuangqing Rd


   Zhiwen Liu
   Tsinghua University
   30 Shuangqing Rd


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