Internet Engineering Task Force                                 T. Jones
Internet-Draft                                              G. Fairhurst
Intended status: Informational                    University of Aberdeen
Expires: 26 August 2021                                          N. Kuhn
                                                                    CNES
                                                               J. Border
                                             Hughes Network Systems, LLC
                                                              E. Stephan
                                                                  Orange
                                                        22 February 2021


         Enhancing Transport Protocols over Satellite Networks
              draft-jones-tsvwg-transport-for-satellite-00

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.

   RFC 2488 and RFC 3135 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 describes the current characterises of common satellite
   paths and 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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute




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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 26 August 2021.

Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Satellite Systems . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Geosynchronous Earth Orbit (GEO)  . . . . . . . . . . . .   6
     2.2.  Low Earth Orbit (LEO) . . . . . . . . . . . . . . . . . .   7
     2.3.  Medium Earth Orbit (MEO)  . . . . . . . . . . . . . . . .   7
     2.4.  Hybrid Network Paths  . . . . . . . . . . . . . . . . . .   7
     2.5.  Convergence with Mobile Cellular  . . . . . . . . . . . .   8
   3.  Satellite System Characteristics  . . . . . . . . . . . . . .   8
     3.1.  Impact of Delay . . . . . . . . . . . . . . . . . . . . .  10
       3.1.1.  Larger Bandwidth Delay Product  . . . . . . . . . . .  10
       3.1.2.  Variable Link Delay . . . . . . . . . . . . . . . . .  10
       3.1.3.  Impact of delay on protocol feedback  . . . . . . . .  10
     3.2.  Intermittent connectivity . . . . . . . . . . . . . . . .  11
   4.  On-Path Mitigations . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Link-Level Forward Error Correction and ARQ . . . . . . .  11
     4.2.  PMTU Discovery  . . . . . . . . . . . . . . . . . . . . .  11
     4.3.  Quality of Service (QoS)  . . . . . . . . . . . . . . . .  11
     4.4.  Split-TCP PEP . . . . . . . . . . . . . . . . . . . . . .  11
     4.5.  Application Proxies . . . . . . . . . . . . . . . . . . .  12
   5.  Generic Transport Protocol Mechanisms . . . . . . . . . . . .  13
     5.1.  Getting up to Speed . . . . . . . . . . . . . . . . . . .  14
     5.2.  Sizing of Maxium Congestion Window  . . . . . . . . . . .  14



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     5.3.  Reliability (Loss Recovery/Repair)  . . . . . . . . . . .  14
       5.3.1.  Packet Level Forward Error Correction . . . . . . . .  15
     5.4.  Flow Control  . . . . . . . . . . . . . . . . . . . . . .  15
     5.5.  ACK Traffic Reduction . . . . . . . . . . . . . . . . . .  16
     5.6.  Multi-Path  . . . . . . . . . . . . . . . . . . . . . . .  16
   6.  Protocol Specific Mechanisms  . . . . . . . . . . . . . . . .  16
     6.1.  TCP Protocol Mechanisms . . . . . . . . . . . . . . . . .  16
       6.1.1.  Transport Initialization  . . . . . . . . . . . . . .  16
       6.1.2.  Getting Up To Speed . . . . . . . . . . . . . . . . .  17
       6.1.3.  Size of Windows . . . . . . . . . . . . . . . . . . .  17
       6.1.4.  Reliability . . . . . . . . . . . . . . . . . . . . .  17
       6.1.5.  ACK Reduction . . . . . . . . . . . . . . . . . . . .  17
     6.2.  QUIC Protocol Mechanisms  . . . . . . . . . . . . . . . .  17
       6.2.1.  Transport initialization  . . . . . . . . . . . . . .  17
       6.2.2.  Getting up to Speed . . . . . . . . . . . . . . . . .  17
       6.2.3.  Size of Windows . . . . . . . . . . . . . . . . . . .  17
       6.2.4.  Reliability . . . . . . . . . . . . . . . . . . . . .  17
       6.2.5.  Asymmetry . . . . . . . . . . . . . . . . . . . . . .  17
       6.2.6.  Packet Level Forward Error Correction . . . . . . . .  18
       6.2.7.  Split Congestion Control  . . . . . . . . . . . . . .  18
   7.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  Mitigation Summary  . . . . . . . . . . . . . . . . . . .  18
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   10. Informative References  . . . . . . . . . . . . . . . . . . .  19
   Appendix A.  Example Network Profiles . . . . . . . . . . . . . .  22
     A.1.  LEO . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
     A.2.  MEO . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
     A.3.  GEO . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
       A.3.1.  Small public satellite broadband access . . . . . . .  23
       A.3.2.  Medium public satellite broadband access  . . . . . .  23
       A.3.3.  Congested medium public satellite broadband access  .  24
       A.3.4.  Variable medium public satellite broadband access . .  25
       A.3.5.  Loss-free large public satellite broadband access . .  25
       A.3.6.  Lossy large public satellite broadband access . . . .  26
   Appendix B.  Revision Notes . . . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

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 and mobile
   networks, and service provision to moving terminals (maritime,
   aircraft, etc.).



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   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 can
   therefore be often unaware of the technologies underpinning the links
   forming a network path.

   Satellite path characteristics have an effect on the operation of
   Internet 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.  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.  A significant contribution to the
   reduced performance can arise from the initialisation and design of
   transport mechanisms.  The analysis and conclusions might also apply
   to other network systems that also result in characteristics that
   differ from typical Internet paths.

   RFC 2488 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



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   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 [I-D.ietf-quic-transport] are
   not able to be split using a PEP and mitigation, and need to
   therefore use other methods.

   XXX Authors Note: 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

   The remainder of this document is divided as follows:

   *  Section 2 identifies common characteristics of a SATCOM network
      that can impact the operation of the transport protocols.  This
      complements the description of [RFC2488].

   *  Section 3 discusses specific characteristics that need to be
      considered when implementing and deploying transport protocols and
      highlights key changes since the publication of [RFC2488].

   *  Section 4 outlines existing deployed mitigations that operate
      below the transport protocol layer.  This offers advice to
      designers and operators of satellite networks.

   *  Section 5 outlines transport protocol mechanisms defined that may
      benefit with satellite networks specific tuning and optimization.
      In particular it discusses on end-to-end considerations, and the
      mechanisms that impact performance of encrypted transports.

   *  Finally, Section 6 provides a summary of the features recommended
      for modern transport protocols.

2.  Satellite Systems

   This document considers the characteristics of satellite
   communications systems.  Satellite systems are being deployed using
   many space orbits, including low earth orbit, medium earth orbits,
   geosynchronous orbits, elliptical orbits and more.

   *  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



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      (corresponding to a ground station directly below the satellite)
      is 239.6 milliseconds (ms) [Mar78].  For ground stations at the
      edge of the coverage of a 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 delay to send a packet
      and receive the corresponding reply (one round-trip time or RTT)
      could be at least 558 ms.  This RTT is not solely due to satellite
      signal 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 also
      increased when multiple hops are used (i.e. communications is
      relayed via a gateway) or in systems using inter-satellite links.
      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 same satellite is on the
      horizon.  Some LEO systems use inter-satellite links, where the
      path delay depends on the routing through the network.

   *  Another orbital position use a Medium Earth Orbit (MEO) [Mar78].
      These orbits lie between LEO and GEO.

2.1.  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);

   *  Links can be highly asymmetric in terms of capacity, the one-way
      delay and their cost of operation.

   As an example, many GEO systems are build using the DVB-S2
   specifications [EN 302 307-1], published by the European



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   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, and known symbol sequences, included along with
   regular packets enable an estimation of the current signal-to-noise
   ratio, that can fed back allowing the transmitting link to adapt its
   coding rate and modulation to the current transmission conditions.

2.2.  Low Earth Orbit (LEO)

   There are many designs of LEO systems.  Depending on the locations of
   the gateways on the ground, routing within the constellation can be
   necessary to forward packets down to a ground terminal.  Capacity can
   vary significantly between systems.

   Depending on the routes currently available - especially upon whether
   Inter-Satellite Links (ISL) are used, additional jitter may occur
   (from 40ms to 140ms with the Iridium constellation).  Some systems
   can also experience either out-of-order delivery of packets or
   additional delay due to buffering.  Other systems have very different
   designs.

   XXX The authors solicit feedback and experience from users and
   operators of satellite systems in LEO orbits.  XXX

2.3.  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 coverage requires handover to another MEO satellite (e.g. every
   45 minutes with O3B).  This can be mitigated by diversity techniques
   (e.g. double antennas at terminals).

   XXX The authors solicit feedback and experience from users and
   operators of satellite systems in MEO orbits.  XXX

2.4.  Hybrid Network Paths

   XXX The authors solicit feedback and experience from users and
   operators of satellite systems in hybrid network scenarios.  XXX



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2.5.  Convergence with Mobile Cellular

   XXX This section should look at IP convergence with 5G systems and
   emerging specs 3GPP non terrestrial networks (NTN).  XXX

3.  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
      applications with moving terminals, satellite channels are
      especially susceptible to multi-path distortion and shadowing
      (e.g., blockage by buildings).  A typical modern satellite link
      can have a bit error ratio (BER) of 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 regulated by ITU-R and usually controlled by
      licenses.  This scarcity makes it difficult to increase 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



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      typically around 50 MHz.  Furthermore, one satellite may carry a
      few dozen transponders.  Not only is 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 a satellite network segment
      as composed of a forward link and a return link.  The two links
      usually have different capacities and employ different
      technologies to carry IP packets.  On the forward link, a
      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.  On the return link, satellite resource is typically
      dynamically shared among the terminals.

   *  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).  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.  Dynamic
      access is more common in currently deployed systems and 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].




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   Satellite systems have several characteristics that differ from most
   terrestrial channels.  These characteristics may degrade the
   performance of TCP.  These characteristics include:

3.1.  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.

3.1.1.  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" (data transmitted, but not
   yet acknowledged) at any one time to fully utilize the available
   capacity.

   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 larger, protocols need to
   keep a larger number of packets "in flight" (that is, sent but not
   yet acknowledged).

   This also impacts the size of window/credit needed to avoid flow
   control mechanisms throttling the sender rate.

3.1.2.  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.

3.1.3.  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 5).



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3.2.  Intermittent connectivity

   For systems using non-GEO satellites, from time to time Internet
   connections need to be transferred from one satellite to another or
   from one ground station to another.  This hand-over can be made
   without interrupting the service, but in some system designs might
   cause packet loss or reordering.

4.  On-Path Mitigations

   This section describes mitigations that operate on the path, rather
   than with the transport endpoints.

4.1.  Link-Level Forward Error Correction and ARQ

   XXX Common.  This includes Adaptive Coding and Modulation (ACM) and
   sometimes link ARQ - which can reduce the loss at the expense of
   decreasing the available capacity.  XXX

4.2.  PMTU Discovery

   XXX Packet size can impact performance and mitigations (such as PEP/
   Application Proxy) can interact with end-to-end PMTUD.  XXX

4.3.  Quality of Service (QoS)

   Links were 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.

4.4.  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



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   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.  This also comes with a drawback: split-TCP
   PEPs can ossify the protocol stack being used because they are often
   unable to track improvements in end-to-end protocol mechanisms (e.g.,
   RACK, ECN, TCP Fast Open).  The set of methods configured in a split
   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.

4.5.  Application Proxies

   Authenticated proxies:

   *  The existence of Application Proxies requires a discovery device,
      which might vary by user - by service - etc.;

   *  Application Proxies can split key functions, but this requires
      agreement between endpoints and the proxy on the formats/semantics
      of the protocol info that is to be changed;

   *  With the common use of security functions (such as TLS), there
      also needs to be a trust relationship - a proxy needs to be
      authenticated;

   *  A proxy needs to remain on the path, which can place constraints
      on the routing infrastructure - handover between proxies is
      possible, but is generally complex.







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5.  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);

   *  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
      a specific 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).














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5.1.  Getting up to Speed

   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.  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.

5.2.  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
   might be unsuitable in 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.

5.3.  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.

   End-to-end packet Forward Error Correction (FEC) offers an
   alternative to retransmission with different trade offs in terms of
   utilised capacity and repair capability.




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   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
   [I-D.schinazi-masque] to apply FEC to all or a part of the end-to-end
   path.

   The benefits of introducing FEC need to weighed against the
   additional capacity 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).
   Mechanisms have been proposed in
   [I-D.ferrieux-hamchaoui-quic-lossbits] , to identify congestion
   losses in the ground segment.

5.3.1.  Packet Level Forward Error Correction

   XXX Packet level FEC can mitigate loss/re-ordering, with a trade-off
   in capacity.  XXX

5.4.  Flow Control

   Flow Control mechanisms allow the receiver to control the amount of
   data a send 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

   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.







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5.5.  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.

5.6.  Multi-Path

   XXX This includes between different satellite systems and between
   satellite and terrestrial paths XXX

6.  Protocol Specific Mechanisms

6.1.  TCP Protocol Mechanisms

6.1.1.  Transport Initialization









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6.1.2.  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.

6.1.3.  Size of Windows

6.1.4.  Reliability

6.1.5.  ACK Reduction

   Mechanisms are being proposed in TCPM for TCP [REF].

6.2.  QUIC Protocol Mechanisms

6.2.1.  Transport initialization

   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.  Moreover, using 0-RTT may
   further reduce the connection time for users reconnecting to a
   server.

6.2.2.  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].

6.2.3.  Size of Windows

6.2.4.  Reliability

   Mechanisms have been proposed in
   [I-D.ferrieux-hamchaoui-quic-lossbits] , to identify congestion
   losses in the ground segment.

6.2.5.  Asymmetry

   The QUIC transport specification may evolve to allow the ACK Ratio to
   be adjusted.

   Default could be adapted following [I-D.fairhurst-quic-ack-scaling]
   or using extensions to tune acknowledgement strategies
   [I-D.iyengar-quic-delayed-ack].




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6.2.6.  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 [I-D.schinazi-masque] to
   apply packet FEC to all or a part of the end-to-end path or enable
   local retransmissions.

6.2.7.  Split Congestion Control

   Splitting the congestion control requires the deployment of
   application proxies.

7.  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.

7.1.  Mitigation Summary

   XXX A Table will be inserted here XXX









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8.  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 and the
   participants of the IETF106 side-meeting on QUIC for high BDP for
   their useful feedback.

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.  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".

   [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-03, 14 September
              2020, <http://www.ietf.org/internet-drafts/draft-
              fairhurst-quic-ack-scaling-03.txt>.

   [I-D.ferrieux-hamchaoui-quic-lossbits]
              Ferrieux, A. and I. Hamchaoui, "The QUIC Loss Bits", Work
              in Progress, Internet-Draft, draft-ferrieux-hamchaoui-
              quic-lossbits-00, 9 April 2019, <http://www.ietf.org/
              internet-drafts/draft-ferrieux-hamchaoui-quic-lossbits-
              00.txt>.

   [I-D.ietf-quic-recovery]
              Iyengar, J. and I. Swett, "QUIC Loss Detection and
              Congestion Control", Work in Progress, Internet-Draft,
              draft-ietf-quic-recovery-34, 14 January 2021,
              <http://www.ietf.org/internet-drafts/draft-ietf-quic-
              recovery-34.txt>.



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   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-34, 14 January 2021,
              <http://www.ietf.org/internet-drafts/draft-ietf-quic-
              transport-34.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, <http://www.ietf.org/
              internet-drafts/draft-irtf-iccrg-sallantin-initial-
              spreading-00.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, <http://www.ietf.org/internet-drafts/draft-
              iyengar-quic-delayed-ack-02.txt>.

   [I-D.kuhn-quic-0rtt-bdp]
              Kuhn, N., Emile, S., Fairhurst, G., and T. Jones,
              "Transport parameters for 0-RTT connections", Work in
              Progress, Internet-Draft, draft-kuhn-quic-0rtt-bdp-07, 18
              May 2020, <http://www.ietf.org/internet-drafts/draft-kuhn-
              quic-0rtt-bdp-07.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, <http://www.ietf.org/internet-drafts/
              draft-roca-nwcrg-rlc-fec-scheme-for-quic-03.txt>.

   [I-D.schinazi-masque]
              Schinazi, D., "The MASQUE Protocol", Work in Progress,
              Internet-Draft, draft-schinazi-masque-02, 8 January 2020,
              <http://www.ietf.org/internet-drafts/draft-schinazi-
              masque-02.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,



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              <http://www.ietf.org/internet-drafts/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.

   [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. In ACM
              SIGCOMM, 1988".

   [Mar78]    Martin, J., "Communications Satellite Systems. Prentice
              Hall, 1978.".

   [Mon98]    Montpetit, M.J., "TELEDESIC: Enabling The Global Community
              Interaccess. In Proc. of the International Wireless
              Symposium, May 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,




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              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>.

   [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>.

   [Sta94]    Stallings, W., "Data and Computer Communications.
              MacMillian, 4th edition, 1994.".

   [Stu95]    Sturza, M.A., "Architecture of the TELEDESIC Satellite
              System. In Proceedings of the International Mobile
              Satellite Conference, 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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Internet-Draft      Internet Transport for Satellite       February 2021


   *  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.

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


   Emile Stephan
   Orange

   Email: emile.stephan@orange.com








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