Internet Engineering Task Force N. Kuhn
Internet-Draft CNES
Intended status: Informational G. Fairhurst
Expires: May 6, 2020 University of Aberdeen
J. Border
Hughes Network Systems, LLC
E. Stephan
Orange
November 3, 2019
QUIC for SATCOM
draft-kuhn-quic-4-sat-02
Abstract
QUIC has been designed for use across Internet paths. Initial
designs of QUIC have focussed on common deployment scenarios for web
traffic and have not focussed on the performance when using a path
with a large Bandwidth-Delay Product (BDP). A path can combine
satellites network segment together with a wide variety of other
network technologies (Ethernet, cable modems, WiFi, cellular, radio
links, etc): this complicates the characteristics of the end-to-end
path. One example of such a scenario occurs when a satellite
communication (SATCOM) system is used to provide all or a part of the
end-to-end path. If this is not addressed, the end-to-end quality of
experience can be degraded.
This memo identifies the characteristics of a SATCOM link that impact
the operation of the QUIC transport protocol and proposes current
practice to ensure acceptable protocol performance.
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 May 6, 2020.
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Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Operating over a path with a large BDP . . . . . . . . . . . 3
3. TCP Split Solution . . . . . . . . . . . . . . . . . . . . . 5
4. Mechanisms that improve the performance of QUIC for SATCOM . 5
4.1. Getting up to speed . . . . . . . . . . . . . . . . . . . 5
4.2. Maximum window . . . . . . . . . . . . . . . . . . . . . 6
4.3. Reliability . . . . . . . . . . . . . . . . . . . . . . . 6
4.4. ACK ratio . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The end-to-end performance of an application using an Internet path
can be impacted by the Bandwidth-Delay Product (BDP) of the links and
network devices forming the path. For instance, the page load time
for a complex page can be much larger when the path includes a
satellite link. A significant contribution to this reduced
performance arises from the initialisation and design of transport
mechanisms. QUIC's default congestion control is based on TCP
NewReno [I-D.ietf-quic-recovery] and the recommended initial window
is defined by [RFC6928]. Although QUIC's CC and recovery have been
designed for use across Internet Paths, the initial design could not
optimise for the wide diversity of path characteristics that can
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occur. This document therefore considers the specific implications
of paths with a significant BDP.
Satellite communications (SATCOM) systems have long been used to
support point-to-point links and specialised networks. The
predominate current use is as a link-layer for Internet Protocols.
Typical example applications include: use as an access technology for
remote locations, backup and rapid deployment of new services,
transit networks and backhaul of various types of IP networks, and
provision to mobile (maritime, aircraft, etc.). In most scenarios,
the satellite IP network segment usually only forms one part of the
end-to-end path. This means user traffic can experience a path that
includes satellite link together with a wide variety of other network
technologies (Ethernet, cable modems, WiFi, cellular, radio links,
etc). Although a user can sometimes know the presence of the
satellite service, a typical user does not deploy special software or
applications because they expect a satellite network is being used.
Often a user is unaware of the technologies underpinning the links
forming the network path.
This memo identifies the characteristics of a SATCOM link that impact
the operation of the QUIC transport protocol and proposes best
current practice to ensure acceptable protocol performance.
2. Operating over a path with a large BDP
The characteristics of systems using Geosynchronous Earth Orbit (GEO)
satellites differ from paths only using terrestrial links in their
path characteristics:
o A large propagation delay of at least 250ms one-way delay;
o Employ radio resource management (often using techniques similar
to cellular mobile or DOCSIS cable networks, but differing to
accommodate the satellite propagation delay);
o Links can be highly asymmetric (in terms of capacity and one-way
delay).
Many systems use the DVB-S2 specifications, where the key concept is
to ensure both a good usage of the satellite resource and a Quasi
Error Free (QEF) link. It consists in monitoring the link quality in
real-time, with the help of known symbols sequences, included along
regular packets, on which an estimation of the current signal-to-
noise ratio can be done. Then, this estimation is send back to the
transmitter that can adapt its coding rate and modulation order to
best fit the actual transmission conditions.
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It is common to consider the satellite network segment composed of a
forward link and a return link. The two links can have different
capacities and employ different technologies to carry the IP packets.
On the forward link, the satellite gateway uses all the available
bandwidth, possibly with several carriers, to communicate with the
remote terminals. A carrier is a single Time-Division-Multiplexing
where packets addressed to terminals are multiplexed. On the return
link, the satellite resource is shared among the users. Two access
methods can be distinguished: on-demand access or contention access.
In the former, a terminal receives dedicated resources on its own to
communicate with the gateway. In the latter, some resources are
reserved for contention access, where several terminals can compete
to obtain the resource. Dedicated access, which is more common in
currently deployed systems, can be through a Demand Assigned Multiple
Access (DAMA) mechanism, while contention access techniques are
usually based on Slotted Aloha (SA) and its numerous derivatives.
More information on satellite links characteristics can be found in
[RFC2488][IJSCN17].
Beyond that, even for characteristics shared with terrestrial links,
the impact on a satellite link could be more and can be amplified by
the large RTT. For example, systems can exhibit a high loss-rate
(e.g. mobile users or users behind a Wi-Fi link) which would impact
loss recovery mechanisms and congestion control reaction to such loss
events. The characteristics of a GEO SATCOM system impact the
performance of congestion control:
o Transport initialization: the 3-way handshake takes a long time to
complete, delaying the time at which actual data can be
transmitted;
o Size of windows required: to fully exploit the bottleneck
capacity, a high BDP will increase the number of in-flights
packets;
o Reliability: packet loss detection and correction is slow (the
performance of end-to-end retransmission is also impacted when
using a high RTT path);
o Getting up to speed: the exponential increase of the data rate
during slow start for a channel capacity probing is slowed down
when the RTT is high;
o Asymetry : when the links are asymmetric, for various reasons, the
sizing of the return link may induce modifications of the
transport level acknowledgement traffic.
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3. TCP Split Solution
High BDP networks commonly break the TCP end-to-end paradigm to adapt
the transport protocol. Splitting TCP allows adaptations to this
specific use-case and assessing the issues discussed in section
Section 2. Satellite communications commonly deploy Performance
Enhancement Proxy (PEP) for compression, caching and TCP acceleration
services [RFC3135]. Their deployment can result in 50% page load
time reduction in a SATCOM use-case [ICCRG100].
[NCT13] and [RFC3135] describe the main functions of SATCOM TCP split
solutions. Shortly, for traffic originated at a gateway to an
endpoint connected via a satellite terminal, the TCP split intercepts
TCP SYN packets to act on behalf of the endpoint and adapt the data
rate transmission to the SATCOM scenario. The split solution
specifically tunes the TCP parameters to the context (latency,
available capacity) of each link. When a PEP is used on each side of
the satellite link, a protocol other than TCP, optimized for the
satellite link, may be used. The tuning can be achieved using a
priori information about the satellite system and/or by measuring the
properties of the network segment that includes the satellite system.
One important advantage of a TCP split solution is that it does not
require any end-to-end modifications and is independent for both
client and server sides. That being said, this comes with a
drawback: TCP splitters often are unable to track the most recent
end-to-end improvements in protocol mechanisms (e.g., RACK, ECN, TCP
Fast Open support) contributing to ossification of the transport
system. The methods configured in the split proxy usually continue
to be used until a split solution is finally updated. This can
delay/negate the benefit of any end-to-end improvements.
4. Mechanisms that improve the performance of QUIC for SATCOM
4.1. Getting up to speed
The advantage of using QUIC is that it includes the TLS and TCP
negociations that reduce the time at which the data can be
transmitted. That being said, results of [IJSCN19] illustrate that
it will take many RTTs for the congestion controller to increase the
rate before it fills the bottleneck capacity. This dominates
performance when a path has a large RTT (as in GEO SATCOM networks).
There is an issue in QUIC getting up to speed in a SATCOM context.
The tuning of the initial window described in
[I-D.irtf-iccrg-sallantin-initial-spreading] which has been shown to
improve performance both for high BDP and more common BDP
[CONEXT15][ICC16] could be a relevant solution. That being said,
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such solution requires the usage of pacing to avoid important bursts
of packets in the network that does not have a large BDP.
4.2. Maximum window
A large number of in-flight packets are prewired to fully exploit the
bottleneck capacity, when there is a large BDP. Default values of
maximum windows may not be suitable for a SATCOM context.
Such as presented in [PANRG105], only increasing the initial
congestion window is not the only way that can improve QUIC
performance in a SATCOM context: increasing maximum congestion
windows can also result in much better performance. Other protocol
mechanisms also need to be considered, such as flow control at the
stream level in QUIC.
4.3. Reliability
Packet loss detection and loss repair take additional time on paths
with a larger RTT. This increases the time that a server needs to
react to a congestion event. Both can impact the user experience.
This happens when a user uses a Wi-Fi link to access a SATCOM
terminal. Although the benefits needed to weighed against the
additional capacity in introducing end-to-end FEC and the potential
to use link-local ARQ and/or link-adaptive FEC. End-to-end
connections may not only suffer from losses in the Wi-Fi segment but
also from congestion losses in the satellite operator ground segment.
Using the mechanisms proposed in
[I-D.ferrieux-hamchaoui-quic-lossbits], congestion losses have been
identified on the ground segment.
Introducing network coding in QUIC such as proposed in
[I-D.swett-nwcrg-coding-for-quic] and
[I-D.roca-nwcrg-rlc-fec-scheme-for-quic] could help in recovering
from the residual Wi-Fi or congestion losses. Another solution would
be the usage of QUIC tunnels [I-D.schinazi-masque].
4.4. ACK ratio
Asymmetry in capacity (or in the way capacity is granted to a flow)
can lead to cases where the throughput in one direction of
communication is restricted by the acknowledgement traffic flowing in
the opposite direction. The limitations of specific underlying
networks could be in terms of the volume of acknowledgement traffic
(limited return path capacity) or in the number of acknowledgement
packets (e.g., when a radio-resource management system has to track
channel usage) or both.
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TCP Performance Implications of Network Path Asymmetry [RFC3449]
describes a range of mechanisms that can mitigate the impact of path
asymmetry. One simple method is to tell the remote endpoint to send
compound acknowledgments less frequently. A rate of one ACK every
RTT/4 can significantly reduce this traffic.
Many of these mitigations have been deployed in satellite systems,
often as a mechanism within a PEP. Despite their benefits over paths
with a high asymmetry of capacity, most mechanisms rely on being able
to inspect and/or modify the transport layer header information of
TCP ACK packets. This is not possible when the transport layer
information is encrypted. The QUIC transport specification may
evolve to allow the ACK Ratio to be adjusted.
5. Discussion
Many of the issues identified already exist for any encrypted
transport service that uses a path that employs encryption at the IP
layer. This includes endpoints that utilise IPsec at the network
layer, or use VPN technology over the satellite network segment.
These uses are unable to benefit from enhancement within the
satellite network segment, and often the user is unaware of the
presence of the satellite link on their path, except through
observing the impact it has on the performance they experience.
One solution would be to provide PEP functions at the termination of
the security association (e.g., in a VPN client). Another solution
could be to fall-back to using TCP (possibly with TLS or similar
methods being used on the transport payload). A final solution could
be to deploy and maintain a bespoke protocol tailored to high BDP
environments. In the future, we anticipate that fall-back will
become less desirable, and methods that rely upon bespoke
configurations or protocols will be unattractive. In parallel, new
methods such as QUIC will become widely deployed. The opportunity
therefore exists to ensure that the new generation of protocols offer
acceptable performance over high BDP paths without requiring
operating tuning or specific updates by users.
6. Acknowledgements
TBD
7. Contributors
TBD
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8. IANA Considerations
TBD
9. Security Considerations
This document does not propose changes to the security functions
provided by the QUIC protocol. QUIC uses TLS encryption to protect
the transport header and its payload. Security is considered in the
"Security Considerations" of cited IETF documents.
10. References
10.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
10.2. Informative References
[CONEXT15]
Li, Q., Dong, M., and P. Godfrey, "Halfback: Running Short
Flows Quickly and Safely", ACM CoNEXT , 2015.
[I-D.ferrieux-hamchaoui-quic-lossbits]
Ferrieux, A. and I. Hamchaoui, "The QUIC Loss Bits",
draft-ferrieux-hamchaoui-quic-lossbits-00 (work in
progress), April 2019.
[I-D.ietf-quic-recovery]
Iyengar, J. and I. Swett, "QUIC Loss Detection and
Congestion Control", draft-ietf-quic-recovery-23 (work in
progress), September 2019.
[I-D.ietf-quic-tls]
Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
draft-ietf-quic-tls-23 (work in progress), September 2019.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-23 (work
in progress), September 2019.
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[I-D.ietf-tls-ticketrequests]
Pauly, T., Schinazi, D., and C. Wood, "TLS Ticket
Requests", draft-ietf-tls-ticketrequests-03 (work in
progress), October 2019.
[I-D.irtf-iccrg-sallantin-initial-spreading]
Sallantin, R., Baudoin, C., Arnal, F., Dubois, E., Chaput,
E., and A. Beylot, "Safe increase of the TCP's Initial
Window Using Initial Spreading", draft-irtf-iccrg-
sallantin-initial-spreading-00 (work in progress), January
2014.
[I-D.roca-nwcrg-rlc-fec-scheme-for-quic]
Roca, V., Swett, I., and M. Montpetit, "Sliding Window
Random Linear Code (RLC) Forward Erasure Correction (FEC)
Schemes for QUIC", draft-roca-nwcrg-rlc-fec-scheme-for-
quic-01 (work in progress), February 2019.
[I-D.schinazi-masque]
Schinazi, D., "The MASQUE Protocol", draft-schinazi-
masque-01 (work in progress), July 2019.
[I-D.swett-nwcrg-coding-for-quic]
Swett, I., Montpetit, M., Roca, V., and F. Michel, "Coding
for QUIC", draft-swett-nwcrg-coding-for-quic-03 (work in
progress), July 2019.
[ICC16] Sallantin, R., Baudoin, C., Chaput, E., Arnal, F., Dubois,
E., and A-L. Beylot, "Reducing web latency through TCP IW:
Be smart", IEEE ICC , 2016.
[ICCRG100]
Kuhn, N., "MPTCP and BBR performance over Internet
satellite paths", IETF ICCRG 100, 2017.
[IJSCN17] Ahmed, T., Dubois, E., Dupe, JB., Ferrus, R., Gelard, P.,
and N. Kuhn, "Software-defined satellite cloud RAN",
International Journal of Satellite Communications and
Networking , 2017.
[IJSCN19] Thomas, L., Dubois, E., Kuhn, N., and E. Lochin, "Google
QUIC performance over a public SATCOM access",
International Journal of Satellite Communications and
Networking , 2019.
[NCT13] Pirovano, A. and F. Garcia, "A new survey on improving TCP
performances over geostationary satellite link", Network
and Communication Technologies , 2013.
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[PANRG105]
Kuhn, N., Stephan, E., Border, J., and G. Fairhurst, "QUIC
Over In-sequence Paths with Different Characteristics",
IRTF PANRG 105, 2019.
[RFC2488] Allman, M., Glover, D., and L. Sanchez, "Enhancing TCP
Over Satellite Channels using Standard Mechanisms",
BCP 28, RFC 2488, DOI 10.17487/RFC2488, January 1999,
<https://www.rfc-editor.org/info/rfc2488>.
[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
Shelby, "Performance Enhancing Proxies Intended to
Mitigate Link-Related Degradations", RFC 3135,
DOI 10.17487/RFC3135, June 2001,
<https://www.rfc-editor.org/info/rfc3135>.
[RFC3449] Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
Sooriyabandara, "TCP Performance Implications of Network
Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
December 2002, <https://www.rfc-editor.org/info/rfc3449>.
[RFC6349] Constantine, B., Forget, G., Geib, R., and R. Schrage,
"Framework for TCP Throughput Testing", RFC 6349,
DOI 10.17487/RFC6349, August 2011,
<https://www.rfc-editor.org/info/rfc6349>.
[RFC6928] Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
"Increasing TCP's Initial Window", RFC 6928,
DOI 10.17487/RFC6928, April 2013,
<https://www.rfc-editor.org/info/rfc6928>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
Authors' Addresses
Nicolas Kuhn
CNES
Email: nicolas.kuhn@cnes.fr
Godred Fairhurst
University of Aberdeen
Email: gorry@erg.abdn.ac.uk
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John Border
Hughes Network Systems, LLC
Email: border@hns.com
Emile Stephan
Orange
Email: emile.stephan@orange.com
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