Internet DRAFT - draft-shi-quic-dtp
draft-shi-quic-dtp
QUIC Y. Cui
Internet-Draft C. Ma
Intended status: Informational Tsinghua University
Expires: 31 July 2024 H. Shi
K. Zheng
W. Wang
Huawei
28 January 2024
Deadline-aware Transport Protocol
draft-shi-quic-dtp-09
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.
About This Document
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-shi-quic-dtp/.
Source for this draft and an issue tracker can be found at
https://github.com/STAR-Tsinghua/DTP-draft.
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 31 July 2024.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions and Definitions . . . . . . . . . . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Design of DTP . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Abstraction . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Architecture of DTP . . . . . . . . . . . . . . . . . . . 5
3.3. Deadline-aware Scheduler . . . . . . . . . . . . . . . . 7
3.3.1. Block dropping mechanism . . . . . . . . . . . . . . 7
3.4. Deadline-aware Redundancy . . . . . . . . . . . . . . . . 8
3.5. Loss Detection and Congestion Control . . . . . . . . . . 9
4. Extension of QUIC . . . . . . . . . . . . . . . . . . . . . . 10
4.1. New Frame: BLOCK_INFO Frame . . . . . . . . . . . . . . . 10
4.2. New Frame: Timestamped ACK Frame . . . . . . . . . . . . 11
4.3. New Packet: Redundancy Packet . . . . . . . . . . . . . . 13
5. DTP Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Block Based Real Time Application . . . . . . . . . . . . 14
5.2. API of DTP . . . . . . . . . . . . . . . . . . . . . . . 14
5.2.1. Data Transmission Functions . . . . . . . . . . . . . 14
5.2.2. Feedback Functions . . . . . . . . . . . . . . . . . 16
5.3. Collaborate with upper layer protocols . . . . . . . . . 17
6. Design Considerations . . . . . . . . . . . . . . . . . . . . 17
6.1. Clock Synchronization . . . . . . . . . . . . . . . . . . 17
6.2. Block Dependency . . . . . . . . . . . . . . . . . . . . 17
6.3. Automatic Block Info . . . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9. Normative References . . . . . . . . . . . . . . . . . . . . 18
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
Many emerging applications have the deadline requirement for their
data transmission. However, current transport layer protocols 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 and Definitions
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.
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,
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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.
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3.1. Abstraction
DTP provides block-based data abstraction for application. A 'block'
is a piece of continuous data. A partial delivered block is useless
for applications, and each block can be independently processed.
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.
The sender can actively drop any block. DTP SHOULD transmit every
undropped block reliably.
3.2. Architecture of DTP
The sender side architecture is shown in Figure 1:
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+-------------+
| |
| 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 |
| |
+-------------+
|
|
v
+-------------+
| |
| Congestion |
| Control |
+-------------+
|
v
Figure 1: The Architecture of DTP
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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 is 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.
3.3.1. Block dropping mechanism
DTP allows the sender side to cancel sending several blocks in the
transport layer, and this action is called 'drop'. By dropping some
stale blocks, DTP can enhance the timeliness of other sending blocks
and save bandwidth. DTP SHOULD implement some strategies on the
sender side to determine which 'block' should be dropped. On the
receiver side, DTP SHOULD be able to check which block is dropped and
MAY have functions to inform the application about the canceled
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blocks.
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
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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 6) and attach it to the DTP Packets and Redundancy Data,
generating Redundancy Packets (a Redundancy Packet with the header
shown in Figure 6). 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.
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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 implements its block dropping mechanism by leveraging QUIC's
stream cancellation function. DTP only defines the drop action on
the *sender side* to cancel stale blocks. DTP leaves the decisions
to the application layer on the receiver side to determine whether to
accept an overdue block. However, because QUIC allows to cancel
streams on both sides and DTP is an extension of QUIC, DTP MAY cancel
the block from the receiver side. It requires mechanisms to measure
each receiving block's importance and drop it.
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 two kinds of BLOCK_INFO frames (type=0x20, 0x21). Either of
these frames SHOULD be attached in the front of each block to inform
the 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:
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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) = 0x20..0x21 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream ID (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Size (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Priority (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Block Deadline (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Start timestamp (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. Dealine should be a duration in
microseconds.
* Start timestamp: An optional parameter to inform the receiver
about the starting time of this block. This parameter may be
helpful when the receiver wants to use the deadline information.
The timestamp parameter SHOULD be the same format as Unix
timestamp (https://en.wikipedia.org/wiki/Unix_time). The sender
and receiver SHOULD do clock synchronization if they use this
parameter.
4.2. New Frame: Timestamped ACK Frame
DTP adds a new Timestamped ACK Frame, containing a timestamp to carry
the timeliness information. The receiver sends Timestamped ACK Frame
to inform the sender when a packet is received and processed. ACK
mechanism of DTP is almost the same as QUIC. The format of
Timestamped ACK frames is similar to the standard ACK Frames defined
in section 19.3 of [QUIC] (As shown in Figure 4):
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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) = 0x02..0x03 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Largest Acknowledged (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Delay (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Range Count (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First ACK Range (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Ranges (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [ECN Counts] ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Standard QUIC ACK Frame Format
DTP appends a timestamp parameter after the original QUIC ACK Frame
Format and defines the type of this new frame 0x22..0x23 (As shown in
Figure 5). The timestamp parameter can be regarded as an optional
parameter of the QUIC ACK Frame while using an extension frame type.
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) = 0x22..0x23 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Largest Acknowledged (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Delay (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Range Count (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First ACK Range (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Ranges (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [ECN Counts] ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Timestamped ACK Frame Format
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Using this timestamp parameter, we can calculate whether the prior
blocks transmitted misses the deadline or not, and we can also
calculate the block completion rate before the deadline. The
timestamp parameter SHOULD be in the same format as Unix timestamp
(https://en.wikipedia.org/wiki/Unix_time).
The Timestamped ACK is adequate to inform the sender about the
timeliness information from the receiver side. To fully use the
deadline information in the block, the sender and the receiver SHOULD
do clock synchronization.
4.3. New Packet: Redundancy Packet
We use a F Flags in DTP Packet to distinguish which DTP packets is
Redundancy-protected or not. Figure 6 shows the Redundancy Packet
Format. If the flag is set, the Redundancy Group ID, m, n, index
field is appended to the header. They are used by the Redundancy
Scheme(Forward-Error-Correction) 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 Group ID (i)(if F set) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| m (i)(if F set) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| n (i)(if F set) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| index (i)(if F set) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Redundancy Packet Format
* F: A flag indicating whether this DTP packets is FEC-protected or
not.
* Redundancy Group ID: A variable-length integer indicating the id
of the redundancy group which the packet belongs to.
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* m: A variable-length integer indicating the number of original
packets of the redundancy group.
* n: A variable-length integer indicating the number of redundancy
packets of the redundancy group.
* index: A variable-length integer indicating the location of the
packet inside the 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
5.2.1. Data Transmission Functions
5.2.1.1. 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.
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* 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.
5.2.1.2. 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.
5.2.1.3. 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.
5.2.1.4. Receive
Format: RECV(connection id, buffer address, byte count, [,block id])
-> byte count, fin flag, [,block id]
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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
5.2.2.1. on_dropped
Format: ON_DROPPED(connection id) -> block id, deadline, priority,
goodbytes
The ON_DROPPED function is called when a block is dropped. The
metadata of the dropped block such as block id, deadline, priority is
attached. The number of bytes delivered before its
deadline(goodbytes) is returned.
Mandatory attributes:
* connection id - local connection name of an indicated connection.
5.2.2.2. on_delivered
Format: ON_DELIVERED(connection id) -> block id, deadline, priority,
delta, goodbytes
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The ON_DELIVERED function is called when a block is delivered. The
metadata of the delivered block such as block id, deadline, priority
is attached. The number of bytes delivered before its deadline
(goodbytes) and the difference between the block completion time and
the deadline (delta) are returned.
Mandatory attributes:
* connection id - local connection name of an indicated connection.
All these functions mentioned above are running in asynchronous mode.
An application can use various event driven framework to call those
functions.
5.3. Collaborate with upper layer protocols
Application protocol on top of DTP may benefit from the block info
and detail metric of the transport layer. DTP MAY expose the block
information to the receiver side application and the status of the
congestion control and buffer status to both sender side and receiver
side application. This information will enable multiple DTP relay
node working together to improve the deadline-delivery performance
end-to-end.
6. Design Considerations
6.1. Clock Synchronization
The fundamental design of DTP relies on precise clock
synchronization. The block scheduler requires high clock precision
to accurately perform block canceling functions and efficient
scheduling. Timestamped ACKs also necessitate high clock precision
to enable the server and client to utilize deadline information
effectively. However, achieving high precision clock synchronization
across the web poses challenges. Further discussions are required to
explore how to best utilize the deadline information in such
circumstances.
6.2. Block Dependency
Video streams often exhibit decoding dependencies among their frames.
To address this, it would be beneficial to include block dependencies
as critical metadata in the block info. Our basic design involves
adding an integer field to the block info frame, indicating the
stream id on which the current block depends. This enhancement may
facilitate efficient block processing and playback, ensuring that
frames are correctly ordered and decoded based on their dependencies.
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6.3. Automatic Block Info
DTP receives block priorities and block deadlines from the send and
update API. However, determining appropriate values for these
parameters can be challenging for applications. Even in cases where
applications, such as RTC publishers, aim for transport delays below
100ms, they may not get the ideal transport result by setting the
block deadline parameter to 100ms. To address this, it might be
necessary to devise an automatic method that can recognize the
application's requirements and assign rational parameter values
accordingly. Implementing such an automatic mechanism can streamline
the configuration process for applications, freeing developers from
the burden of manually fine-tuning parameters and ensuring optimal
data transmission with minimal human intervention.
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. IANA Considerations
This document has no IANA actions.
9. Normative References
[arXiv_1809.04822]
Michel, F., Coninck, Q., and O. Bonaventure, "Adding
Forward Erasure Correction to QUIC", September 2018,
<https://arxiv.xilesou.top/pdf/1809.04822.pdf>.
[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
[QUIC-TLS] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/rfc/rfc9001>.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/rfc/rfc768>.
[RFC0793] Postel, J., "Transmission Control Protocol", RFC 793,
DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/rfc/rfc793>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
Acknowledgments
We sincerely thank Z. Liu (liu-zw20@mails.tsinghua.edu.cn) and J.
Zhang (zhangjie19@mails.tsinghua.edu.cn) for contributing to the DTP
project. They provided a lot of advice and revisions to the draft
and actively helped advance the relevant progress of DTP
standardization.
Authors' Addresses
Yong Cui
Tsinghua University
30 Shuangqing Rd
Beijing
China
Email: cuiyong@tsinghua.edu.cn
Chuan Ma
Tsinghua University
30 Shuangqing Rd
Beijing
China
Email: mc21@mails.tsinghua.edu.cn
Hang Shi
Huawei
Email: shihang9@huawei.com
Kai Zheng
Huawei
Email: kai.zheng@huawei.com
Wei Wang
Huawei
Email: wangwei375@huawei.com
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