QUIC H. Chan
Internet-Draft A. Wei
Intended status: Informational Huawei Technologies
Expires: September 14, 2017 F. Song
H. Zhang
Beijing Jiaotong University
March 13, 2017
One Way Latency Considerations for Multipath in QUIC
draft-chan-quic-owl-01
Abstract
This document discusses the use of One Way Latency (OWL) for
enhancing multipath transmission in QUIC. Several representative
usages of OWL, such as congestion control mechanism, retransmission
policy, crucial data scheduling are analyzed. Two kinds of OWL
measurement approaches are also provided and compared. More
explorations related with OWL will be researched to improve the
performance of QUIC.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. Potential Usages of OWL in QUIC . . . . . . . . . . . . . . . 3
3.1. Crucial Data Scheduling . . . . . . . . . . . . . . . . . 3
3.2. Congestion Control . . . . . . . . . . . . . . . . . . . 4
3.3. Packet Retransmission . . . . . . . . . . . . . . . . . . 5
4. OWL Measurement . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Round-trip time (RTT) is commonly used in congestion control and loss
recovery mechanism for data transmission. Yet the key issue for data
transmission is simply the delay of the data transmission along a
path which does not include the return. The latency for uplink and
downlink between two peers may be very different. RTT, which cannot
accurately reflect the delay of the data transmission along a path,
can be easily influenced by the latency in the opposite direction
along that path. Therefore, the use of One Way Latency (OWL)
[I-D.song-mptcp-owl] is proposed to describe the exact latency from
the time that data is sent to the time data is received.
Using the timestamps information in the ACK Frame of QUIC
[I-D.ietf-quic-transport], the One Way Latency can be calculated in
absolute value or in relative value. As multipath will be supported
by QUIC, path selection based on One Way Latency can improve the
performance of multipath in QUIC in several situations, such as
congestion control, packet retransmission, crucial data scheduling,
etc.
We suggest discussing the necessary considerations of OWL in QUIC.
In the following, possible usages of OWL in QUIC are analyzed, and
then two kinds of OWL measurements are listed and compared.
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2. Conventions and Terminology
The key words "MUST", "MUST NOT", "GLUIRED", "SHALL","SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
One Way Latency (OWL): the propagation delay between a sender and a
receiver from the time a signal is sent to the time the signal is
received.
3. Potential Usages of OWL in QUIC
There are a number of potential uses of OWL, especially for multipath
in QUIC. Although only 3 significant aspects are illustrated in this
document, more explorations are still needed.
3.1. Crucial Data Scheduling
During a transmission process, there are often some crucial data that
need to be sent to the destination immediately. Examples of such
crucial data are the key frame in multimedia, the high priority chunk
of emergency communication, etc. One cannot guarantee the sequence
of data arrival along multiple paths if only the RTTs of the multiple
paths are used.
The data rate in any given link can be asymmetric. In addition, the
delay in a given direction can change according to the amount of
packet queue. Therefore delay in a forward direction in a path is
not necessarily the same as that in the reverse direction as
exemplified in Figure 1.
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-------- OWL(s-to-c,path1)=16ms <--------
/ \
| -----> OWL(c-to-s,path1)= 5ms ----- |
| / RTT(path1)=21ms \ |
| | | |
+------+ +------+
| |-----> OWL(c-to-s,path2)= 8ms -----| |
|Client| |Server|
| |----- OWL(s-to-c,path2)= 8ms <-----| |
+------+ RTT(path2)=16ms +------+
| | | |
| \ / |
| -----> OWL(c-to-s,path3)=10ms ----- |
\ /
-------- OWL(s-to-c,path3)= 8ms <--------
RTT(path3)=18ms
Figure 1. Example with 3 paths between the client and the server
with OWL as indicated in the figure. RTT information alone would
indicate to the client that the fastest path to the server is path 2,
followed by path 3, and then followed by path 1. path 2 is the
fastest, whereas OWL indicates to the client that the fastest path to
the server is path 1, followed by path 2, and then followed by path
3.
Using the results of OWL measurement, the sender can easily select
the faster path, in terms of the latency in the forward direction,
for crucial data transmission. Moreover, the acknowledgements of
these crucial data can be sent on the path with minimum latency in
the reverse direction. Piggyback is then also useful when in duplex
communication mode.
3.2. Congestion Control
Congestion in a given direction does not necessarily imply congestion
also in the reverse direction.
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-------- No congestion (path 1) <--------
/ \
| -----> Congestion (path 1) ----- |
| / \ |
| | | |
+------+ +------+
|Client| |Server|
+------+ +------+
| | | |
| \ / |
| -----> No congestion (path 2) ----- |
\ /
-------- Congestion (path 2) <--------
Figure 2. Example of a congestion situation with 2 paths between the
client and the server. There is congestion from client to server
along path 1 and also from server to client along path 2. RTT
information alone will indicate congestion in both paths, whereas OWL
information will show the client that path 2 is the more lightly
loaded path to get to the server.
Network congestion in a given direction can be better described using
OWL rather than using RTT. Especially when the congestion can be a
situation in a unidirectional path, the congestion in the path from a
client to a server is different from the congestion in the path from
the server to the client. The RTT cannot accurately reflect the
delay of interest for data transmission along a path. For multipath
in QUIC, the client needs to choose a more lightly loaded path to
send packets [RFC6356]. It will then be unwise to compare the RTT
among different paths, and it should instead compare the OWL among
the paths.
3.3. Packet Retransmission
Continuous Multipath Transmission (CMT) increases throughput by
concurrently transferring new data from a source to a destination
host via multiple paths. However, when a packet is lost, Receive
Buffer Blocking (RBB) will occur as illustrated in Figure 3.
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Stream 5, Offset 0, Length 500 (lost)
-----> Stream 5, Offset 1000, Length 500 (rcvd) -----
/ Stream 5, Offset 2000, Length 500 (rcvd) \
| |
+------+ +--------+
|Sender| |Receiver|
+------+ +--------+
| |
\ Stream 5, Offset 500, Length 500 (rcvd) /
-----> Stream 5, Offset 1500, Length 500 (rcvd) -----
Stream 5, Offset 2500, Length 500 (rcvd)
Figure 3. Example of Receive Buffer Blocking: The packet containing
octets 0-499 in Stream ID=5 is lost. On the other hand the packets
containing Octets 500-999, 1000-1499, 1500-1999, 2000-2499 in Stream
ID=5 have all been received. The octets 500-2000 are then all
buffered at the receiver, and are blocked by the missing octets
0-499.
Therefore, the sender needs to select a suitable path to retransmit
ASAP. Using the results of OWL measurement, the sender can quickly
determine the specific path with minimum forward latency. RBB can
then be relieved as soon as the receiver obtains the most needed
frames in the retransmitted packet(s) and submits them to the upper
layer.
4. OWL Measurement
Two kinds of OWL measurement approaches are available: absolute value
measurement and relative value measurement.
To obtain the absolute value of OWL, the primary condition of
measurement is clock synchronization. Using Network Time Protocol
(NTP) [RFC5905], end hosts can calibrate the local clock with the
remote NTP server. The additional information or optional
capabilities can even be added via extension fields in the standard
NTP header [RFC7822]. The calibration accuracy can reach to the
millisecond level in less congested situations. The obvious burden
here is to persuade the end hosts to initialize the NTP option.
Obtaining the relative value of OWL is more than enough in some
circumstances to establish applications on top of it. When
retransmission is needed, for example, the sender may only care about
which path has the minimum forward latency. When bandwidth is being
estimated, the difference of forward latency, i.e. delta latency,
among all available paths is needed. By exchanging with
correspondent end host the local timestamps of receiving and sending
the packets, both sides could obtain the relative value of OWL.
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The considerations to obtain the absolute values are the extra
protocol requirement and synchronization accuracy. However, using
the absolute values is more convenient for its applications. On the
contrary, the relative measurement only needs to send timestamps in
the acknowledgment and there is no need to worry about the clock
synchronization.
5. Security Considerations
TBD
6. IANA Considerations
This document presents no IANA considerations.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
7.2. Informative References
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-01 (work
in progress), January 2017.
[I-D.song-mptcp-owl]
Song, F. and H. Zhang, "One Way Latency Considerations for
MPTCP", draft-song-mptcp-owl-01 (work in progress),
December 2016.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<http://www.rfc-editor.org/info/rfc5905>.
[RFC6356] Raiciu, C., Handley, M., and D. Wischik, "Coupled
Congestion Control for Multipath Transport Protocols",
RFC 6356, DOI 10.17487/RFC6356, October 2011,
<http://www.rfc-editor.org/info/rfc6356>.
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[RFC7822] Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
(NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
March 2016, <http://www.rfc-editor.org/info/rfc7822>.
Authors' Addresses
H Anthony Chan
Huawei Technologies
5340 Legacy Dr. Building 3
Plano, TX 75024
USA
Email: h.a.chan@ieee.org
Anni Wei
Huawei Technologies
Xin-Xi Rd. No. 3, Haidian District
Beijing, 100095
P. R. China
Email: weiannig@huawei.com
Fei Song
Beijing Jiaotong University
Beijing, 100044
P. R. China
Email: fsong@bjtu.edu.cn
Hongke Zhang
Beijing Jiaotong University
Beijing, 100044
P. R. China
Email: hkzhang@bjtu.edu.cn
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