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Internet Engineering Task Force                                    Q. Wu
Internet-Draft                                                     H. Li
Intended status: Informational                                  Q. Zhang
Expires: May 4, 2020                                              J. Liu
                                                     Tsinghua University
                                                        November 1, 2019


              Predictable Multipath TCP (MPTCP) extension
                      draft-qian-mptcp-predict-00

Abstract

   Multipath TCP (MPTCP) adds the capability of using multiple paths to
   a regular TCP session, which is quite suitable for integrated network
   scenarios where multiple paths almost always exist.  However, link
   handover and link on-off switching happen frequently in network
   systems that integrate different systems (especially the systems that
   continually move), which defeats the quality of MPTCP connections.
   Information about the link handover and on-off switching in above-
   mentioned scenarios can be predicted in advance, but MPTCP is not
   capable of utilizing the prediction information.  This document
   suggests MPTCP be extended with the capacity of obtaining and
   utilizing the prediction information.  Furthermore, the document
   describes one possible way to enhance MPTCP with prediction
   information, which proposes a modified MPTCP scheduler utilizing link
   on-off prediction information.

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
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   This Internet-Draft will expire on May 4, 2020.







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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Problem: How do handover and on-off switching weaken MPTCP? .   4
     3.1.  How Link Handover weakens MPTCP . . . . . . . . . . . . .   4
     3.2.  How Link On-off switching weakens MPTCP . . . . . . . . .   4
   4.  Prediction Information  . . . . . . . . . . . . . . . . . . .   5
     4.1.  Predictability  . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Predictable Information . . . . . . . . . . . . . . . . .   5
   5.  An example: Scheduler Utilizing Prediction Information  . . .   5
     5.1.  Scenario  . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.2.  Modification to Scheduler: How to utilize Prediction
           Information . . . . . . . . . . . . . . . . . . . . . . .   5
     5.3.  Enhancement . . . . . . . . . . . . . . . . . . . . . . .   6
     5.4.  Corresponding extension for path management . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Multipath TCP (MPTCP) adds the capability of using multiple paths to
   a regular TCP session [RFC793].  The motivations for this extension
   include increasing throughput, overall resource utilization, and
   resilience to network failure, and these motivations are discussed,
   along with high-level design decisions, as part of the multipath TCP
   architecture [RFC6182].

   In integrated networks that integrate different systems (especially
   the systems that continually move), like Integrated Satellite-



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   Territorial Network (ISTN) and aviation aircrafts network that
   simultaneously using satellite communications and ATG (Air to Ground)
   Broadband, multiple paths between different hosts almost always exist
   and therefore it is quite suitable to apply MPTCP in such scenarios
   to increase throughput and resilience to network failure.  However,
   link handover and link on-off switching happen frequently in those
   scenarios and they severely decrease the quality of MPTCP
   transmission connections.

   On the other hand, some information about the link handover and on-
   off switching in above-mentioned scenarios can be predicted in
   advance according to prior knowledge including satellite orbit,
   preset route of airlines, the fixed position of the ATG ground
   stations and so on.  But MPTCP is not capable of obtaining or
   utilizing the prediction information, which leads to a contradiction
   between the existence of prediction information and the disability of
   utilizing the information in MPTCP.

   The main idea of this document is to suggest that MPTCP be extended
   with the capacity of obtaining and utilizing the prediction
   information.  This document first describes how link handover and on-
   off switching weaken MPTCP and how some information about the link
   changes can be predicted.  Finally, this document shows an example of
   utilizing prediction information in MPTCP.

   The target readers of this document are protocol designers and
   network researchers who work on MPTCP.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED","MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   This document uses the MPTCP terminology introduced in
   [I-D.ietf-mptcp-rfc6824bis] [RFC6824].

   RTT: Round-Trip Time;

   ISTN: Integrated Satellite-Territorial Network;

   QoS: Quality of Service;

   LEO: Low Earth Orbit;

   GEO: Geostationary Orbit;

   ATG: Air To Ground.



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3.  Problem: How do handover and on-off switching weaken MPTCP?

   In integrated networks, multiple and heterogeneous paths almost
   always exist between different hosts.  However, link handover and on-
   off switching happen frequently in these networks.  This part
   describes how link handover and on-off switching weaken MPTCP
   performance.

3.1.  How Link Handover weakens MPTCP

   QoS including bandwidth, RTT of paths exist in different network
   systems are quite different from each other.  In integrated networks,
   link handover between different systems often happen.  For example,
   when the satellite link of an aircraft handovers from a LEO satellite
   to a GEO satellite, the end-to-end delay will increase from tens of
   milliseconds (ms) to more than 500 ms.  If the size of congestion
   window doesn't increase with the end-to-end delay, it will limit and
   decrease the transmission rate of the sender.  What's more, the large
   delay results in a slow congestion window increase, which means the
   sender will stay at low transmission rate for a long period.  On the
   other hand, MPTCP scheduler usually schedules packets to different
   paths based on the paths' RTT, therefore the end-to-end delay changes
   will also result in more out-of-order packets at the receiver.

3.2.  How Link On-off switching weakens MPTCP

   On-off switching of links also happen in integrated networks.  For
   example, when an aviation aircraft connects to Internet with
   satellite communications and ATG (Air to Ground) Broadband link
   simultaneously, the ATG link may switch between on and off states as
   the aircraft flies, due to the discontinuity of ATG Broadband
   resulted from complicated topography (like mountains and lakes).  As
   a transport layer protocol, the only way that MPTCP detects
   disconnection and reconnection of links is to analyze the packets
   that the receiver replied based on timers.  Therefore, it usually
   takes MPTCP a long time to detects link disconnection and
   reconnection.  When a link suddenly disconnects, the unsent data
   scheduled to that link will not be transmitted in time and it will
   take MPTCP scheduler a long time to find the link off and retransmit
   the data, which results in out-of-order packets at the receiver.
   What's more, if the sender can't receive ACKs for quite a long time,
   the send window of all the sub-flows will get exhausted.  When a link
   reconnects, it will also take MPTCP a period to find the link on and
   schedule some data to the link.







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4.  Prediction Information

4.1.  Predictability

   In emerging integrated networks, some new nodes including LEO
   satellites, GEO satellites, aviation aircrafts, ground base stations,
   ships on the ocean and high-speed railways are involved.  They are
   different: some of them, like ground base stations, are at fixed
   positions on the earth while others keep moving at expectable speed
   on their pre-defined orbits or routines on earth, on ocean or in the
   space.  These characteristics mean that the relative positions
   between them are predictable and furthermore, the state of the
   network links between them are predictable.

4.2.  Predictable Information

   For link handover, predictable information include the time handover
   happens, link bandwidth and end-to-end delay changes after handover
   and so on.  For on-off switching, predictable information include the
   time link disconnection and reconnection happens, link bandwidth and
   end-to-end delay when link reconnects and so on.

5.  An example: Scheduler Utilizing Prediction Information

   This part shows an example of utilizing prediction information to
   enhance MPTCP under a link on-off switching scenario.

5.1.  Scenario

   It's suitable for Aviation aircrafts to connect to Internet with
   MPTCP, where ATG Broadband and Satellite Communications are
   simultaneously available.  However, the complicated topography (like
   mountains and lakes) leads to the discontinuity of ATG Broadband, and
   therefore defeats the quality of aircraft network service (Refer to
   Section 3.2).  On the other hand, the on-off connection state of the
   ATG Broadband access link of the aircrafts can be predicted from the
   preset route of airlines and the position of the ATG ground stations
   in advance of the conversion of link connection states.

5.2.  Modification to Scheduler: How to utilize Prediction Information

   The default scheduling principle of MPTCP scheduler is "min-RTT"
   principle, which schedules each packet to the path with minimum RTT
   within the paths whose send window is not full.  The modified version
   is called "MPTCP-P", which introduces prediction information to
   calculate a modified round trip time called "RTT-P" and the
   calculating details are as follows:




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   The prediction information that MPTCP-P requests include a) the time
   when the next disconnection of the ATG link happens, b) the time when
   the next reconnection of the ATG link happens, which are respectively
   denoted as time_down and time_up.  The current time is denoted as
   time_now.  The RTT calculated by default method of TCP is denoted as
   RTT_Orignal.

   When the ATG link of the aircraft is connected (i.e. time_now <
   time_down):

   RTT-P = RTT_Orignal;

   When the ATG link of the aircraft is disconnected (i.e. time_down <
   time_now < time_up):

   RTT-P = RTT_Orignal + time_up - time_now;

   MPTCP-P schedules packets in "min-RTT-P" principle, which schedules
   each packet to the path with minimum RTT-P within the paths whose
   send window is not full.

5.3.  Enhancement

   Without prediction information about link on-off switching, original
   MPTCP detects link connection state based on received packets and
   timers.  Therefore, it takes time for MPTCP to find link disconnected
   or reconnected and then accordingly schedule packets to different
   paths.

   With prediction information about link on-off switching, MPTCP-P
   could a) reduce the packets to be allocated to a path and cancel
   timers for the packets sent through a path when the predictable link
   of the path is going to disconnect, b) allocate packets to a
   reconnecting path at the moment it gets reconnected according to the
   prediction information.

5.4.  Corresponding extension for path management

   In order to keep the temporarily disconnected subflow valid in case
   it is in a quite long disconnected period, it might be necessary to
   a) setting it into backup state [I-D.ietf-mptcp-rfc6824bis] [RFC6824]
   or b) inform the other end the link on-off state through newly
   extended MPTCP option.  What's more, some extension to TCP might be
   needed to keep the corresponding paths valid.







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6.  Acknowledgements

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Security Considerations

9.  Informative References

   [I-D.ietf-mptcp-rfc6824bis]
              Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
              Paasch, "TCP Extensions for Multipath Operation with
              Multiple Addresses", June 2019,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mptcp-
              rfc6824bis-18>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC6182]  Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
              Iyengar, "Architectural Guidelines for Multipath TCP
              Development", RFC 6182, DOI 10.17487/RFC6182, March 2011,
              <https://www.rfc-editor.org/info/rfc6182>.

   [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>.

   [RFC793]   Information Sciences Institute,University of Southern
              California, "Transmission Control Protocol", Sept 1981,
              <https://datatracker.ietf.org/doc/rfc793/>.

Authors' Addresses

   Qian Wu
   Tsinghua University
   Institute for Network Sciences and Cyberspace, Tsinghua University
   Beijing  100084
   China

   Email: wuqian@cernet.edu.cn






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   Hewu Li
   Tsinghua University
   Institute for Network Sciences and Cyberspace, Tsinghua University
   Beijing  100084
   China

   Email: lihewu@cernet.edu.cn


   Qi Zhang
   Tsinghua University
   Institute for Network Sciences and Cyberspace, Tsinghua University
   Beijing  100084
   China

   Email: qi-zhang19@mails.tsinghua.edu.cn


   Jun Liu
   Tsinghua University
   Institute for Network Sciences and Cyberspace, Tsinghua University
   Beijing  100084
   China

   Email: juneliu@tsinghua.edu.cn


























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