Internet DRAFT - draft-jones-tsvwg-transport-for-satellite
draft-jones-tsvwg-transport-for-satellite
Network Working Group T. Jones
Internet-Draft G. Fairhurst
Intended status: Informational University of Aberdeen
Expires: 28 April 2022 N. Kuhn
CNES
J. Border
Hughes Network Systems, LLC
E. Stephan
Orange
25 October 2021
Enhancing Transport Protocols over Satellite Networks
draft-jones-tsvwg-transport-for-satellite-02
Abstract
IETF transport protocols such as TCP, SCTP and QUIC are designed to
function correctly over any network path. This includes networks
paths that utilise a satellite link or network. While transport
protocols function, the characteristics of satellite networks can
impact performance when using the defaults in standard mechanisms,
due to the specific characteristics of these paths.
[RFC2488] and [RFC3135] describe mechanisms that enable TCP to more
effectively utilize the available capacity of a network path that
includes a satellite system. Since publication, both application and
transport layers and satellite systems have evolved. Indeed, the
development of encrypted protocols such as QUIC challenges currently
deployed solutions, for satellite systems the capacity has increased
and commercial systems are now available that use a range of
satellite orbital positions.
This document follows the terminology proposed in
[I-D.irtf-panrg-path-properties] to describe the current
characterises of common satellite paths. This document also
describes considerations when implementing and deploying reliable
transport protocols that are intended to work efficiently over paths
that include a satellite system. It discusses available network
mitigations and offers advice to designers of protocols and operators
of satellite networks.
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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. SATCOM terminology . . . . . . . . . . . . . . . . . . . . . 5
3. Satellite Systems . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Geosynchronous Earth Orbit (GEO) . . . . . . . . . . . . 7
3.2. Low Earth Orbit (LEO) . . . . . . . . . . . . . . . . . . 8
3.3. Medium Earth Orbit (MEO) . . . . . . . . . . . . . . . . 8
3.4. Hybrid Network Paths . . . . . . . . . . . . . . . . . . 9
4. Satellite System Characteristics . . . . . . . . . . . . . . 9
4.1. Impact of delay . . . . . . . . . . . . . . . . . . . . . 11
4.1.1. Larger Bandwidth Delay Product . . . . . . . . . . . 11
4.1.2. Variable Link Delay . . . . . . . . . . . . . . . . . 12
4.1.3. Impact of delay on protocol feedback . . . . . . . . 12
4.2. Intermittent connectivity . . . . . . . . . . . . . . . . 12
5. On-Path Mitigations . . . . . . . . . . . . . . . . . . . . . 12
5.1. Link-Level Forward Error Correction and ARQ . . . . . . . 12
5.2. PMTU Discovery . . . . . . . . . . . . . . . . . . . . . 12
5.3. Quality of Service (QoS) . . . . . . . . . . . . . . . . 13
5.4. Split-TCP PEP . . . . . . . . . . . . . . . . . . . . . . 13
5.5. Application Proxies . . . . . . . . . . . . . . . . . . . 14
6. Generic Transport Protocol Mechanisms . . . . . . . . . . . . 14
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6.1. Transport Initialization . . . . . . . . . . . . . . . . 15
6.2. Getting up to Speed . . . . . . . . . . . . . . . . . . . 15
6.3. Sizing of Maxium Congestion Window . . . . . . . . . . . 15
6.4. Reliability (Loss Recovery/Repair) . . . . . . . . . . . 16
6.4.1. Packet Level Forward Error Correction . . . . . . . . 16
6.5. Flow Control . . . . . . . . . . . . . . . . . . . . . . 16
6.6. ACK Traffic Reduction . . . . . . . . . . . . . . . . . . 17
6.7. Multi-Path . . . . . . . . . . . . . . . . . . . . . . . 17
7. Protocol Specific Mechanisms . . . . . . . . . . . . . . . . 18
7.1. TCP Protocol Mechanisms . . . . . . . . . . . . . . . . . 18
7.1.1. Transport Initialization . . . . . . . . . . . . . . 18
7.1.2. Getting Up To Speed . . . . . . . . . . . . . . . . . 18
7.1.3. Size of Windows . . . . . . . . . . . . . . . . . . . 18
7.1.4. Reliability . . . . . . . . . . . . . . . . . . . . . 18
7.1.5. ACK Reduction . . . . . . . . . . . . . . . . . . . . 18
7.2. QUIC Protocol Mechanisms . . . . . . . . . . . . . . . . 18
7.2.1. Transport initialization . . . . . . . . . . . . . . 18
7.2.2. Getting up to Speed . . . . . . . . . . . . . . . . . 18
7.2.3. Size of Windows . . . . . . . . . . . . . . . . . . . 18
7.2.4. Reliability . . . . . . . . . . . . . . . . . . . . . 18
7.2.5. Asymmetry . . . . . . . . . . . . . . . . . . . . . . 18
7.2.6. Packet Level Forward Error Correction . . . . . . . . 19
7.2.7. Split Congestion Control . . . . . . . . . . . . . . 19
8. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. Mitigation Summary . . . . . . . . . . . . . . . . . . . 19
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
10. Security Considerations . . . . . . . . . . . . . . . . . . . 20
11. Informative References . . . . . . . . . . . . . . . . . . . 20
Appendix A. Example Network Profiles . . . . . . . . . . . . . . 23
A.1. LEO . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
A.2. MEO . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
A.3. GEO . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
A.3.1. Small public satellite broadband access . . . . . . . 24
A.3.2. Medium public satellite broadband access . . . . . . 24
A.3.3. Congested medium public satellite broadband access . 25
A.3.4. Variable medium public satellite broadband access . . 26
A.3.5. Loss-free large public satellite broadband access . . 26
A.3.6. Lossy large public satellite broadband access . . . . 27
Appendix B. Revision Notes . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
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1. Introduction
Satellite communications (SATCOM) systems have long been used to
support point-to-point links and specialised networks. The
predominate current use today is to support Internet Protocols.
Typical example applications include: use as an access technology for
remote locations, backup and rapid deployment of new services,
transit networks, backhaul of various types of IP networks, and
provision to mobile environments (maritime, aircraft, etc.).
In most scenarios, the satellite IP network segment forms only one
part of the end-to-end path used by an Internet transport protocol.
This means that user traffic can experience a path that includes a
satellite network combined 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 a satellite
service, a typical user does not deploy special software or
applications when a satellite network is being used. Users are often
unaware of the technologies underpinning the links forming a network
path.
Satellite path characteristics have an effect on the operation of
transport protocols, such as TCP, SCTP or QUIC. Transport Protocol
performance can be affected by the magnitude and variability of the
network delay. When transport protocols perform poorly the link
utilization can be low. Techniques and recommendations have been
made that can improve the performance of transport protocols when the
path includes as satellite network.
The end-to-end performance of an application using an Internet path
can be impacted by the path characteristics, such as the Bandwidth-
Delay Product (BDP) of the links and network devices forming the
path. It can also be impacted by underlying mechanisms used to
manage the radio resources.
Performance can be impacted at several layers. For instance, the
page load time for a complex page can be much larger when a path
includes a satellite system. A significant contribution to the
reduced performance arises from the initialisation and design of
transport mechanisms.
Although mechanisms are designed for use across Internet paths, not
all designs are performant when used over the wide diversity of path
characteristics that can occur. This document therefore considers
the implications of Internet paths that include a satellite system.
The analysis and conclusions might also apply to other network
systems that also result in characteristics that differ from typical
Internet paths.
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RFC2488 specifies an Internet Best Current Practices for the Internet
Community, relating to use of the standards-track Transmission
Control Protocol (TCP) mechanisms over satellite channels [RFC2488].
A separate RFC,[RFC2760], identified research issues and proposed
mitigations for satellite paths.
Since the publication of these RFCs many TCP mechanisms have become
widely used. In particular, this includes a series of mitigation
based on Performance Enhancing Proxies (PEPs) [RFC3135] that split
the protocol at the transport layer. Although PEPs are now a common
component of satellite systems, their use slows the deployment of new
transport protocols and mechanisms (each of which demands an update
to the PEP functionality). This has made it difficult for new
protocol extensions to achieve comparable performance over satellite
channels. In addition, protocols with strong requirements on
authentication and privacy such as QUIC [RFC9000] are not able to be
split using a PEP and mitigation, and need to therefore use other
methods.
XXX Note from the current authors: This document currently focuses on
Geosynchronous Earth Orbit (GEO) satellite systems, the authors
solicit feedback and experience from users and operators of satellite
systems using other orbits. XXX
2. SATCOM terminology
This section follows the terminology proposed in
[I-D.irtf-panrg-path-properties] to describe a generic SATCOM system
for broadband access. This description is inline with the one
proposed in [RFC8975].
A generic SATCOM system could contain the following entities:
* A: A Host providing the end service (e.g. web server);
* B: A Node being the point-of-presence for the SATCOM system;
* C: A Node gathering network functions (e.g. firewall, PEP, caching
services, etc.);
* D: A Node gathering MAC and PHY functionnalities (a.k.a. the
satellite gateway);
* E: A Node being one of the satellite (if there are several
satellites) (this node could include network layer functions);
* F: A Node receiving the signal from the satellite (a.k.a. the
satellite terminal);
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* G: A Host providing the end service (e.g. web browser).
These entities would be interconnected with path elements which
properties differ from one SATCOM system to another.
[I-D.irtf-panrg-path-properties] provides properties that can be
discussed to describe the path. These properties are exploited
throughout the whole document to describe SATCOM systems.
While the paths interconnecting the entities (1) A to B, (2) B to C,
(3) C to D and (4) F to G are quite generic for all the systems, and
not specific for SATCOM systems, some properties need to be
discussed:
* Protocol Features available
* Transport Protocols available
* Transparency
The paths (1) D to E and (2) D to F are quite specific to SATCOM
systems. In particular, the following elements, provided by
[I-D.irtf-panrg-path-properties], are in the scope of this document
and deserve some description:
* Symmetric Path
* Disjointness
* Transparency
* Link Capacity
* Link Usage
* One-Way Delay
* One-Way Delay Variation
* One-Way Packet Loss
3. Satellite Systems
Satellite communications systems have been deployed using many space
orbits, including low earth orbit, medium earth orbits,
geosynchronous orbits, elliptical orbits and more. This document
considers the characteristics of all satellite networks.
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* Many communications satellites are located at Geostationary Orbit
(GEO) with an altitude of approximately 36,000 km [Sta94]. At
this altitude the orbit period is the same as the Earth's rotation
period. Therefore, each ground station is always able to "see"
the orbiting satellite at the same position in the sky. The
propagation time for a radio signal to travel twice that distance
(corresponding to a ground station directly below the satellite)
is 239.6 milliseconds (ms) [Mar78]. For ground stations at the
edge of the view area of the satellite, the distance traveled is 2
x 41,756 km for a total propagation delay of 279.0 ms [Mar78].
These delays are for one ground station-to-satellite-to-ground
station route (or "hop"). Therefore, the propagation delay for a
packet and the corresponding reply (one round-trip time or RTT)
could be at least 558 ms. The RTT is not based solely due to
satellite propagation time and will be increased by other factors,
such as the serialisation time, including any FEC encoding/ARQ
delay and propagation time of other links along the network path
and the queueing delay in network equipment. The delay is
increased when multiple hops are used (i.e. communications is
relayed via a gateway) or if inter-satellite links are used. As
satellites become more complex and include on-board processing of
signals, additional delay can be added.
* Communications satellites can also be built to use a Low Earth
Orbit (LEO) [Stu95] [Mon98]. The lower orbits require the use of
constellations of satellites for constant coverage. In other
words, as one satellite leaves the ground station's sight, another
satellite appears on the horizon and the channel is switched to
it. The propagation delay to a LEO orbit ranges from several
milliseconds when communicating with a satellite directly
overhead, to as much as 20 ms when the satellite is on the
horizon. Some of these systems use inter-satellite links and have
variable path delay depending on routing through the network.
* Another orbital position use a Medium Earth Orbit (MEO) [Mar78].
These orbits lie between LEO and GEO.
3.1. Geosynchronous Earth Orbit (GEO)
The characteristics of systems using Geosynchronous Earth Orbit (GEO)
satellites differ from paths only using terrestrial links in their
path characteristics:
* A large propagation delay of at least 250ms one-way delay;
* Use of radio resource management (often using techniques similar
to cellular mobile or DOCSIS cable networks, but differ to
accommodate the satellite propagation delay);
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* Links can be highly asymmetric (in terms of capacity, one-way
delay and in their cost of operation, see Appendix A for example
scenarios).
As an example. GEO systems use the DVB-S2 specifications [EN 302
307-1], published by the European Telecommunications Standards
Institute (ETSI), where the key concept is to ensure both a good
usage of the satellite resource and a Quasi Error Free (QEF) link.
These systems typically monitor the link quality in real-time, with
the help of known symbol sequences, included along with regular
packets, which enable an estimation of the current signal-to-noise
ratio. This estimation is then feedback allowing the transmitting
link to adapt its coding rate and modulation to the actual
transmission conditions.
3.2. Low Earth Orbit (LEO)
There is an important variability of LEO systems. Depending on the
locations of the gateways on the ground, routing within the
constellation may be necessary to bring to packets down to the
ground. Depending on the routes currently available for an end user,
high levels of jitter may occur (from 40ms to 140ms with the Iridium
constellation). This may lead to out-of-order delivery of packets.
XXX The authors solicit feedback and experience from users and
operators of satellite systems in LEO orbits. XXX
3.3. Medium Earth Orbit (MEO)
MEO systems such as O3B combines advantages and drawbacks from both
LEO and GEO systems.
MEO systems can have a large coverage and with limited number of
satellites required providing a broad service. The usage of powerful
satellites enables provision of high data rates.
MEO systems have the drawback, from a transport protocol perspective,
that the BDP can be very high due to the altitude of such
constellations (8 063 km for O3B) and there may be delay variations
when the satellite changes (every 45 minutes with O3B). The latter
can be dealt with by means of double antennas terminals.
XXX The authors solicit feedback and experience from users and
operators of satellite systems in MEO orbits. XXX
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3.4. Hybrid Network Paths
XXX The authors solicit feedback and experience from users and
operators of satellite systems in hybrid network scenarios. XXX
4. Satellite System Characteristics
There is an inherent delay in the delivery of a packet over a
satellite system due to the finite speed of light and the altitude of
communications satellites.
Satellite links are dominated by two fundamental characteristics, as
described below:
* Packet Loss: The strength of any radio signal falls in proportion
to the square of the distance traveled. For a satellite link the
square of the distance traveled is large and so the signal becomes
weak before reaching its destination. This results in a low
signal-to-noise ratio. Some frequencies are particularly
susceptible to atmospheric effects such as rain attenuation. For
mobile applications, satellite channels are especially susceptible
to multi-path distortion and shadowing (e.g., blockage by
buildings). Typical bit error ratios (BER) for a satellite link
today are on the order of 1 error per 10 million bits (1 x 10^-7)
or less frequent. Advanced error control coding (e.g., Reed
Solomon or LDPC) can be added to existing satellite services and
is currently being used by many services. Satellite performance
approaching fiber will become more common using advanced error
control coding in new systems. However, many legacy satellite
systems will continue to exhibit higher physical layer BER than
newer satellite systems. TCP uses all packet drops as signals of
network congestion and reduces its window size in an attempt to
alleviate the congestion. In the absence of knowledge about why a
packet was dropped (congestion or corruption), TCP must assume the
drop was due to network congestion to avoid congestion collapse
[Jac88] [FF98]. Therefore, packets dropped due to corruption
cause TCP to reduce the size of its sliding window, even though
these packet drops do not signal congestion in the network.
* Bandwidth: The radio spectrum is a limited natural resource, there
is a restricted amount of bandwidth available to satellite systems
which is typically controlled by licenses. This scarcity makes it
difficult to trade bandwidth to solve other design problems.
Satellite-based radio repeaters are known as transponders.
Traditional C-band transponder bandwidth is typically 36 MHz to
accommodate one color television channel (or 1200 voice channels).
Ku-band transponders are typically around 50 MHz. Furthermore,
one satellite may carry a few dozen transponders. Not only is
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bandwidth limited by nature, but the allocations for commercial
communications are limited by international agreements so that
this scarce resource can be used fairly by many different
communications applications. Typical carrier frequencies for
current, point- to-point, commercial, satellite services are 6 GHz
(uplink) and 4 GHz (downlink), also known as C-band, and 14/12 GHz
(Ku band). Services also utilise higher bands, including 30/20
GHz (Ka-band). XXX JB: I think we need add Ka-band details. You
cannot get 250 Mbps out of a C-band or Ku-band transponder.
Outbound Ka-band transponders range from 100 to 500 MHz. Inbound
Ka-band transponders range from 50 to 250 MHz.XXX
* Link Design: It is common to consider the satellite network
segment composed of a forward link and a return link. The forward
link (also called "downlink") is the link from the ground station
of the satellite to the user terminal. The return link (also
called "uplink") is the link in the opposite direction. These two
links can have different capacities and employ different
technologies to carry the IP packets. On the forward link, the
satellite gateway often manages all the available capacity,
possibly with several carriers, to communicate with a set of
remote terminals. A carrier is a single Time-Division-
Multiplexing (TDM) channel that multiplexes packets addressed to
specific terminals. There are trade-offs in terms of overall
system efficiency and performance observed by a user. Most
systems incur additional delay to ensure overall system
performance.
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* Shared Medium Access: In common with other radio media, satellite
capacity can be assigned for use by a link for a period of time,
for the duration of communication, for a per-packet or per burst
of packets, or accessed using contention mechanisms. Packets sent
over a shared radio channels need to be sent in frames that need
to be allocated resources (bandwidth, power, time) for their
transmission. This results in a range of characteristics that are
very different to a permanently assigned medium (such as an
Ethernet link using an optical fibre). On the return link,
satellite resource is typically dynamically shared among the
terminals. Two access methods can be distinguished: on-demand
access or contention access. In the former, a terminal receives
dedicated transmission resources (usually to send to the gateway).
In the latter, some resources are reserved for contention access,
where a set of terminals are allowed to compete to obtain
transmission 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].
Satellite systems have several characteristics that differ from most
terrestrial channels. These characteristics may degrade the
performance of TCP. These characteristics include:
4.1. Impact of delay
Even for characteristics shared with terrestrial paths, the impact on
a satellite link could be amplified by the path RTT. For example,
paths using a satellite system can also exhibit a high loss-rate
(e.g., a mobile user or a user behind a Wi-Fi link), where the
additional delay can impact transport mechanisms.
4.1.1. Larger Bandwidth Delay Product
Although capacity is often less than in many terrestrial systems, the
bandwidth delay product (BDP) defines the amount of data that a
protocol is permitted to have "in flight" at any one time to fully
utilize the available capacity. In flight means data that is
transmitted, but not yet acknowledged.
The delay used in this equation is the path RTT and the bandwidth is
the capacity of the bottleneck link along the network path. Because
the delay in some satellite environments is higher, protocols need to
keep a larger number of packets in flight.
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This also impacts the size of window/credit needed to avoid flow
control mechanisms throttling the sender rate.
4.1.2. Variable Link Delay
In some satellite environments, such as some Low Earth Orbit (LEO)
constellations, the propagation delay to and from the satellite
varies over time.
Even when the propagation delay varies only very slightly, the
effects of medium access methods can result in significant variation
in the link delay. Whether or not this will have an impact on
performance of a well-designed transport is currently an open
question.
4.1.3. Impact of delay on protocol feedback
The link delay of some satellite systems may require more time for a
transport sender to determine whether or not a packet has been
successfully received at the final destination. This delay impacts
interactive applications as well as loss recovery, congestion
control, flow control, and other algorithms (see Section 6).
4.2. Intermittent connectivity
In some non-GEO satellite orbit configurations, from time to time
Internet connections need to be transferred from one satellite to
another or from one ground station to another. This hand-off might
cause excessive packet loss or reordering if not properly performed.
5. On-Path Mitigations
This section describes mitigations that operate on the path, rather
than with the transport endpoints.
5.1. Link-Level Forward Error Correction and ARQ
XXX Common, but includes adaptive ModCod and sometimes ARQ - which
can reduce the loss at the expense of decreasing the available
capacity. XXX
5.2. PMTU Discovery
XXX Packet Size can impact performance and mitigations (such as PEP/
Application Proxy) can interact with end-to-end PMTUD XXX
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5.3. Quality of Service (QoS)
Links where packets are sent over radio channels exhibit various
trade-offs in the way the signal is sent on the communications
channel. These trade-offs are not necessarily the same for all
packets, and network traffic flows can be optimised by mapping these
onto different types of lower layer treatment (packet queues,
resource management requests, resource usage, and adaption to the
channel using FEC, ARQ, etc). Many systems differentiate classes of
traffic to mange these QoS trade-offs.
5.4. Split-TCP PEP
High BDP networks commonly break the TCP end-to-end paradigm to adapt
the transport protocol. Splitting a TCP connection allows adaptation
for a specific use-case and to address the issues discussed in
Section 2. Satellite communications commonly deploy Performance
Enhancing Proxy (PEP) for compression, caching and TCP acceleration
services [RFC3135] . Their deployment can result in significant
performance improvement (e.g., a 50% page load time reduction in a
SATCOM use-case [ICCRG100] .
[NCT13] and [RFC3135] describe the main functions of a SATCOM TCP
split solution. For traffic originated at a gateway to an endpoint
connected via a satellite terminal, the TCP split proxy intercepts
TCP SYN packets, acting on behalf of the endpoint and adapts the
sending rate to the SATCOM scenario. The split solution can
specifically tune TCP parameters to the satellite link (latency,
available capacity).
When a proxy is used on each side of the satellite link, the
transport protocol can be replaced by a protocol other than TCP,
optimized for the satellite link. This can be tuned using a priori
information about the satellite system and/or by measuring the
properties of the network segment that includes the satellite system.
Split connections can also recover from packet loss that is local to
the part of the connection on which the packet losses occur. This
eliminates the need for end-to-end recovery of lost packets.
One important advantage of a TCP split solution is that it does not
require any end-to-end modification and is independent of both the
client and server sides.
Split-TCP comes with a significant drawback: TCP splitters are often
unable to track end-to-end improvements in protocol mechanisms (e.g.,
RACK, ECN, TCP Fast Open) or new protocols that share a wire format
with TCP (MPTCP [RFC6824]). The set of methods configured in a split
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proxy usually continue to be used, until the split solution is
finally updated. This can delay/negate the benefit of any end-to-end
improvements, contributing to ossification of the transport system.
5.5. Application Proxies
Authenticated proxies:
* Split some functions, so the proxy needs to agree on the formats/
semantics of the protocol info that is changed
* Need a trust relationship - need to be authenticated, and
understand what is modified
* Proxy needs to be on-path, which places constraints on the routing
via the box
* Need to discover the device, which might vary by user - by service
- etc.
6. Generic Transport Protocol Mechanisms
This section outlines transport protocol mechanisms that may be
necessary to tune or optimize in satellite or hybrid satellite/
terrestrial networks to better utilize the available capacity of the
link. These mechanisms may also be needed to fully utilize fast
terrestrial channels. Furthermore, these mechanisms do not
fundamentally hurt performance in a shared terrestrial network. Each
of the following sections outlines one mechanism and why that
mechanism may be needed.
* Transport initialization: the connection handshake (in TCP the
3-way exchange) takes a longer time to complete, delaying the time
to send data (several transport protocol exchanges may be needed,
such as TLS);
* Size of congestion window required: to fully exploit the
bottleneck capacity, a high BDP requires a larger number of in-
flight packets;
* Size of receiver (flow control) window required: to fully exploit
the bottleneck capacity, a high BDP requires a larger number of
in-flight packets;
* Reliability: transport layer loss detection and repair can incur a
single or multiple RTTs (the performance of end-to-end
retransmission is also impacted when using a high RTT path);
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* Getting up to speed: many congestion control methods employ an
exponential increase in the sending rate during slow start (for
path capacity probing), a high RTT will increase the time to reach
the maximum possible rate;
* Asymmetry: when the links are asymmetric the return path may
modify the rate and/timing of transport acknowledgment traffic,
potentially changing behaviour (e.g., limiting the forward sending
rate).
6.1. Transport Initialization
Many transport protocols now deploy 0-RTT mechanisms [REF] to reduce
the number of RTTs required to establish a connection. QUIC has an
advantage that the TLS and TCP negotiations can be completed during
the transport connection handshake. This can reduce the time to
transmit the first data.
6.2. Getting up to Speed
Results of [IJSCN19] illustrate that it can still take many RTTs for
a CC to increase the sending rate to fill the bottleneck capacity.
The delay in getting up to speed can dominate performance for a path
with a large RTT, and requires the congestion and flow controls to
accommodate the impact of path delay.
One relevant solution is 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] . Such a solution requires using sender pacing to
avoid generating bursts of packets in a network.
6.3. Sizing of Maxium Congestion Window
Size of windows required: to fully exploit the bottleneck capacity, a
high BDP requires a larger number of in-flight packets.
The number of in-flight packets required to fill a bottleneck
capacity, is dependent on the 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.
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6.4. Reliability (Loss Recovery/Repair)
The time for end systems to perform packet loss detection and
recovery/repair is a function of the path RTT.
The RTT also determines the time needed by a server to react to a
congestion event. Both can impact the user experience. For example,
when a user uses a Wi-Fi link to access the Internet via SATCOM
terminal.
A solution could be to opportunistically retransmit packets even if
they have not been detected as lost but the congestion control allows
to transmit more packets.
6.4.1. Packet Level Forward Error Correction
XXX Packet level FEC can mitigate loss/re-ordering, with a trade-off
in capacity. XXX
End-to-end packet Forward Error Correction offers an alternative to
retransmission with different trade offs in terms of utilised
capacity and repair capability.
The benefits of introducing FEC need to weighed against the
additional overhead introduced by end-to-end FEC and the opportunity
to use link-local ARQ and/or link-adaptive FEC. A transport
connections can suffer link-related losses from a particular link
(e.g., Wi-Fi), but also congestion loss (e.g. router buffer overflow
in a satellite operator ground segment or along an Internet path).
6.5. Flow Control
Flow Control mechanisms allow the receiver to control the amount of
data a sender can have in flight at any time. Flow Control allows
the receiver to allocate the smallest buffer sizes possible improving
memory usage on receipt.
The sizing of initial receive buffers requires a balance between
keeping receive memory allocation small while allowing the send
window to grow quickly to help ensure high utilization. The size of
receive windows and their growth can govern the performance of the
protocol if updates are not timely.
Many TCP implementations deploy Auto-scaling mechanisms to increase
the size of the largest receive window over time. If these increases
are not timely then sender traffic can stall while waiting to be
notified of an increase in receive window size. XXX QUIC? XXX
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Multi-streaming Protocols such as QUIC implement Flow Control using
credit-based mechanisms that allow the receiver to prioritise which
stream is able to send and when. Credit-based systems, when flow
credit allocations are not timely, can stall sending when credit is
exhausted.
6.6. ACK Traffic Reduction
When the links are asymmetric, for various reasons, the return path
may modify the rate and/timing of transport acknowledgment traffic,
potentially changing behaviour (e.g., limiting the forward sending
rate).
Asymmetry in capacity (or in the way capacity is granted to a flow)
can lead to cases where the transmission in one direction of
communication is restricted by the transmission of the acknowledgment
traffic flowing in the opposite direction. A network segment could
present limitations in the volume of acknowledgment traffic (e.g.,
limited available return path capacity) or in the number of
acknowledgment packets (e.g., when a radio-resource management system
has to track channel usage), or both.
TCP Performance Implications of Network Path Asymmetry [RFC3449]
describes a range of mechanisms that have been used to mitigate the
impact of path asymmetry, primarily targeting operation of TCP.
Many mitigations have been deployed in satellite systems, often as a
mechanism within a PEP. Despite their benefits over paths with high
asymmetry, 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 (e.g., using an IP VPN).
One simple mitigation is for the remote endpoint to send compound
acknowledgments less frequently. A rate of one ACK for every RTT/4
can significantly reduce this traffic. The QUIC transport
specification may evolve to allow the ACK Ratio to be adjusted.
6.7. Multi-Path
XXX This includes between different satellite systems and between
satellite and terrestrial paths XXX
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7. Protocol Specific Mechanisms
7.1. TCP Protocol Mechanisms
7.1.1. Transport Initialization
7.1.2. Getting Up To Speed
One relevant solution is tuning of the initial window described in
[I-D.irtf-iccrg-sallantin-initial-spreading][RFC6928], which has been
shown to improve performance both for high BDP and more common BDP
[CONEXT15] [ICC16]. This requires sender pacing to avoid generating
bursts of packets to the network.
7.1.3. Size of Windows
7.1.4. Reliability
7.1.5. ACK Reduction
Mechanisms are being proposed in TCPM for TCP [REF].
7.2. QUIC Protocol Mechanisms
7.2.1. Transport initialization
QUIC has an advantage that the TLS and transport protocol
negotiations can be completed during the transport connection
handshake. This can reduce the time to transmit the first data.
Moreover, using 0-RTT may further reduce the connection time for
users reconnecting to a server.
7.2.2. Getting up to Speed
Getting up to speed may be easier with the usage of the 0-RTT-BDP
extension proposed in [I-D.kuhn-quic-0rtt-bdp].
7.2.3. Size of Windows
7.2.4. Reliability
7.2.5. Asymmetry
The QUIC transport specification may evolve to allow the ACK Ratio to
be adjusted.
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Default could be adapted following [I-D.fairhurst-quic-ack-scaling]
or using extensions to tune acknowledgement strategies
[I-D.iyengar-quic-delayed-ack].
7.2.6. Packet Level Forward Error Correction
Network coding as proposed in [I-D.swett-nwcrg-coding-for-quic] and
[I-D.roca-nwcrg-rlc-fec-scheme-for-quic] could help QUIC recover from
link or congestion loss.
Another approach could utilise QUIC tunnels such as proposed in the
MASQUE WG to apply packet FEC to all or a part of the end-to-end path
or enable local retransmissions.
7.2.7. Split Congestion Control
Splitting the congestion control requires the deployment of
application proxies.
8. Discussion
Many of the issues identified for high BDP paths already exist when
using an encrypted transport service over a path that employs
encryption at the IP layer. This includes endpoints that utilise
IPsec at the network layer, or use VPN technology over a satellite
network segment. Users 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 different solution
could be to deploy and maintain a bespoke protocol tailored to high
BDP environments. In the future, we anticipate that fall-back to TCP
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.
8.1. Mitigation Summary
XXX A Table will be inserted here XXX
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9. Acknowledgments
The authors would like to thank Mark Allman, Daniel R. Glover and
Luis A. Sanchez the authors of RFC2488 from which the format and
descriptions of satellite systems in this document have taken
inspiration.
The authors would like to thank Christian Huitema, Igor Lubashev,
Alexandre Ferrieux, Francois Michel, Emmanuel Lochin, github user
sedrubal and the participants of the IETF106 side-meeting on QUIC for
high BDP for their useful feedback.
10. 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.
11. Informative References
[CONEXT15] Li, Q., Dong, M., and P B. Godfrey, "Halfback: Running
Short Flows Quickly and Safely", ACM CoNEXT , 2015.
[FF98] Floyd, S. and K. Fall, "Promoting the Use of End-to-End
Congestion Control in the Internet", IEEE Transactions on
Networking 10.1109/90.79302, 1999.
[I-D.fairhurst-quic-ack-scaling]
Fairhurst, G., Custura, A., and T. Jones, "Changing the
Default QUIC ACK Policy", Work in Progress, Internet-
Draft, draft-fairhurst-quic-ack-scaling-04, 15 March 2021,
<https://www.ietf.org/archive/id/draft-fairhurst-quic-ack-
scaling-04.txt>.
[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", Work in Progress,
Internet-Draft, draft-irtf-iccrg-sallantin-initial-
spreading-00, 15 January 2014,
<https://www.ietf.org/archive/id/draft-irtf-iccrg-
sallantin-initial-spreading-00.txt>.
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[I-D.irtf-panrg-path-properties]
Enghardt, T. and C. Krähenbühl, "A Vocabulary of Path
Properties", Work in Progress, Internet-Draft, draft-irtf-
panrg-path-properties-03, 9 July 2021,
<https://www.ietf.org/archive/id/draft-irtf-panrg-path-
properties-03.txt>.
[I-D.iyengar-quic-delayed-ack]
Iyengar, J. and I. Swett, "Sender Control of
Acknowledgement Delays in QUIC", Work in Progress,
Internet-Draft, draft-iyengar-quic-delayed-ack-02, 2
November 2020, <https://www.ietf.org/archive/id/draft-
iyengar-quic-delayed-ack-02.txt>.
[I-D.kuhn-quic-0rtt-bdp]
Kuhn, N., Stephan, E., Fairhurst, G., Jones, T., and C.
Huitema, "Transport parameters for 0-RTT connections",
Work in Progress, Internet-Draft, draft-kuhn-quic-0rtt-
bdp-09, 7 June 2021, <https://www.ietf.org/archive/id/
draft-kuhn-quic-0rtt-bdp-09.txt>.
[I-D.roca-nwcrg-rlc-fec-scheme-for-quic]
Roca, V., Michel, F., Swett, I., and M. Montpetit,
"Sliding Window Random Linear Code (RLC) Forward Erasure
Correction (FEC) Schemes for QUIC", Work in Progress,
Internet-Draft, draft-roca-nwcrg-rlc-fec-scheme-for-quic-
03, 9 March 2020, <https://www.ietf.org/archive/id/draft-
roca-nwcrg-rlc-fec-scheme-for-quic-03.txt>.
[I-D.swett-nwcrg-coding-for-quic]
Swett, I., Montpetit, M., Roca, V., and F. Michel, "Coding
for QUIC", Work in Progress, Internet-Draft, draft-swett-
nwcrg-coding-for-quic-04, 9 March 2020,
<https://www.ietf.org/archive/id/draft-swett-nwcrg-coding-
for-quic-04.txt>.
[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.
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[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.
[Jac88] Jacobson, V., "Congestion Avoidance and Control", ACM
SIGCOMM 88, 1988.
[Mar78] Martin, J., "Communications Satellite Systems", Prentice
Hall 78, 1978.
[Mon98] Montpetit, M.J., "TELEDESIC: Enabling The Global Community
Interaccess", International Wireless Symposium 98, 1998.
[NCT13] Pirovano, A. and F. Garcia, "A new survey on improving TCP
performances over geostationary satellite link", Network
and Communication Technologies , 2013.
[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>.
[RFC2760] Allman, M., Ed., Dawkins, S., Glover, D., Griner, J.,
Tran, D., Henderson, T., Heidemann, J., Touch, J., Kruse,
H., Ostermann, S., Scott, K., and J. Semke, "Ongoing TCP
Research Related to Satellites", RFC 2760,
DOI 10.17487/RFC2760, February 2000,
<https://www.rfc-editor.org/info/rfc2760>.
[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>.
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[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<https://www.rfc-editor.org/info/rfc6824>.
[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>.
[RFC8975] Kuhn, N., Ed. and E. Lochin, Ed., "Network Coding for
Satellite Systems", RFC 8975, DOI 10.17487/RFC8975,
January 2021, <https://www.rfc-editor.org/info/rfc8975>.
[RFC9000] 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/info/rfc9000>.
[Sta94] Stallings, W., "Data and Computer Communications",
MacMillian 4th edition, 1994.
[Stu95] Sturza, M.A., "Architecture of the TELEDESIC Satellite
System", International Mobile Satellite Conference 95,
1995.
Appendix A. Example Network Profiles
This proposes sampler profiles and a set of regression tests to
evaluate transport protocols over SATCOM links and discusses how to
ensure acceptable protocol performance.
XXX These test profiles currently focus on the measuring performance
and testing for regressions in the QUIC protocol. The authors
solicit input to adapt these tests to apply to more transport
protocols. XXX
A.1. LEO
A.2. MEO
A.3. GEO
This section proposes a set of regression tests for QUIC that
consider high BDP scenarios. We define by:
* Download path: from Internet to the client endpoint;
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* Upload path: from the client endpoint to a server (e.g., in the
Internet).
A.3.1. Small public satellite broadband access
The tested scenario has the following path characteristics:
* Satellite downlink path: 10 Mbps
* Satellite uplink path: 2 Mbps
* No emulated packet loss
* RTT: 650 ms
* Buffer size : BDP
During the transmission of 100 MB on both download and upload paths,
the test should report the upload and download time of 2 MB, 10 MB
and 100 MB.
Initial thoughts of the performance objectives for QUIC are the
following:
* 3 s for downloading 2 MB
* 10 s for downloading 10 MB
* 85 s for downloading 100 MB
* 10 s for uploading 2 MB
* 50 s for uploading 10 MB
* 420 s for uploading 100 MB
A.3.2. Medium public satellite broadband access
The tested scenario has the following path characteristics:
* Satellite downlink path: 50 Mbps
* Satellite uplink path: 10 Mbps
* No emulated packet loss
* RTT: 650 ms
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* Buffer size : BDP
During the transmission of 100 MB on the download path, the test
should report the download time for 2 MB, 10 MB and 100 MB. Then, to
assess the performance of QUIC with the 0-RTT extension and its
variants, after 10 seconds, repeat the transmission of 100 MB on the
download path where the download time for 2 MB, 10 MB and 100 MB is
recorded.
Initial thoughts of the performance objectives for QUIC are the
following:
* 3 s for the first downloading 2 MB
* 5 s for the first downloading 10 MB
* 20 s for the first downloading 100 MB
* TBD s for the second downloading 2 MB
* TBD s for the second downloading 10 MB
* TBD s for the second downloading 100 MB
A.3.3. Congested medium public satellite broadband access
There are cases where the uplink path is congested or where the
capacity of the uplink path is not guaranteed.
The tested scenario has the following path characteristics:
* Satellite downlink path: 50 Mbps
* Satellite uplink path: 0.5 Mbps
* No emulated packet loss
* RTT: 650 ms
* Buffer size : BDP
During the transmission of 100 MB on the download path, the test
should report the download time for 2 MB, 10 MB and 100 MB.
Initial thoughts of the performance objectives for QUIC are the
following:
* 3 s for downloading 2 MB
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* 5 s for downloading 10 MB
* 20 s for downloading 100 MB
A.3.4. Variable medium public satellite broadband access
There are cases where the downlink path is congested or where, due to
link layer adaptations to rain fading, the capacity of the downlink
path is variable.
The tested scenario has the following path characteristics:
* Satellite downlink path: 50 Mbps - wait 5s - 10 Mbps
* Satellite uplink path: 10 Mbps
* No emulated packet loss
* RTT: 650 ms
* Buffer size : BDP
During the transmission of 100 MB on the download path, the test
should report the download time for 2 MB, 10 MB and 100 MB.
Initial thoughts of the performance objectives for QUIC are the
following:
* TBD s for downloading 2 MB
* TBD s for downloading 10 MB
* TBD s for downloading 100 MB
A.3.5. Loss-free large public satellite broadband access
The tested scenario has the following path characteristics:
* Satellite downlink path: 250 Mbps
* Satellite uplink path: 6 Mbps
* No emulated packet loss
* RTT: 650 ms
* Buffer size : BDP
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During the transmission of 100 MB on the download path, the test
should report the download time for 2 MB, 10 MB and 100 MB. Then, to
assess the performance of QUIC with the 0-RTT extension and its
variants, after 10 seconds, repeat the transmission of 100 MB on the
download path where the download time for 2 MB, 10 MB and 100 MB is
recorded.
Initial thoughts of the performance objectives for QUIC are the
following:
* 3 s for the first downloading 2 MB
* 5 s for the first downloading 10 MB
* 8 s for the first downloading 100 MB
* TBD s for the second downloading 2 MB
* TBD s for the second downloading 10 MB
* TBD s for the second downloading 100 MB
A.3.6. Lossy large public satellite broadband access
The tested scenario has the following path characteristics:
* Satellite downlink path: 250 Mbps
* Satellite uplink path: 6 Mbps
* Emulated packet loss on both downlink and uplink paths:
- Uniform random transmission link losses: 1%
* RTT: 650 ms
* Buffer size : BDP
During the transmission of 100 MB on the download path, the test
should report the download time for 2 MB, 10 MB and 100 MB.
Initial thoughts of the performance objectives for QUIC are the
following:
* 3 s for downloading 2 MB (uniform transmission link losses)
* 6 s for downloading 10 MB (uniform transmission link losses)
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* 10 s for downloading 100 MB (uniform transmission link losses)
Appendix B. Revision Notes
Note to RFC-Editor: please remove this entire section prior to
publication.
Individual draft -00:
* Comments and corrections are welcome directly to the authors or
via the https://github.com/uoaerg/draft-jones-transport-for-
satellite github repo in the form of pull requests and issues.
Individual draft -01:
* Explained Terms Forward and return link
* Rearranged text to help the document read better
* Fix typos and inaccuracies
* Added a mention of MPTCP
Authors' Addresses
Tom Jones
University of Aberdeen
Email: tom@erg.abdn.ac.uk
Godred Fairhurst
University of Aberdeen
Email: gorry@erg.abdn.ac.uk
Nicolas Kuhn
CNES
Email: nicolas.kuhn@cnes.fr
John Border
Hughes Network Systems, LLC
Email: border@hns.com
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Emile Stephan
Orange
Email: emile.stephan@orange.com
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