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MPTCP                                                            F. Song
Internet-Draft                                                  H. Zhang
Intended status: Informational               Beijing Jiaotong University
Expires: September 14, 2017                                      H. Chan
                                                                  A. Wei
                                                     Huawei Technologies
                                                          March 13, 2017

                One Way Latency Considerations for MPTCP


   This document discusses the use of One Way Latency (OWL) for
   enhancing multipath TCP (MPTCP).  Several use cases of OWL, such as
   retransmission policy and crucial data scheduling are analyzed.  Two
   kinds of OWL measurement approaches are also provided and compared.
   More explorations related with OWL will be helpful to the performance
   of MPTCP.

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 September 14, 2017.

Copyright Notice

   Copyright (c) 2017 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
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  Potential Usages of OWL in MPTCP  . . . . . . . . . . . . . .   3
     3.1.  Crucial Data Scheduling . . . . . . . . . . . . . . . . .   3
     3.2.  Congestion Control  . . . . . . . . . . . . . . . . . . .   4
     3.3.  Packet Retransmission . . . . . . . . . . . . . . . . . .   5
     3.4.  Bandwidth Estimation  . . . . . . . . . . . . . . . . . .   6
     3.5.  Shared Bottleneck Detection . . . . . . . . . . . . . . .   6
   4.  OWL Measurements in TCP . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Both end hosts and the intermediate devices in the Internet have
   basically been equipping with more and more physical network
   interfaces.  Whereas multiple interfaces had been widely used in
   packet forwarding, traffic engineering, etc., the importance of these
   interfaces at the end hosts had been confirmed and utilized
   [RFC6419].  Moreover, the increased capacity provided by the multiple
   paths created by multiple interfaces is leveraged to aggregate more
   bandwidths, to decrease packet delay and to provide better services.
   Unlike traditional TCP [RFC0793], many transport layer protocols,
   such as MPTCP [RFC6182] [RFC6824] enable the end hosts to
   concurrently transfer data on top of multiple paths to greatly
   increase the overall throughput.

   Round-trip time (RTT) is commonly used in congestion control and loss
   recovery mechanism for data transmission.  Yet the key issue for data
   transmission is simply the delay of the data transmission along a
   path which does not include the return.  The latency for uplink and
   downlink between two peers may be very different.  RTT, which cannot
   accurately reflect the delay of the data transmission along a path,
   can be easily influenced by the latency in the opposite direction
   along that path.  Therefore, the use of One Way Latency (OWL) is
   proposed to describe the exact latency from the time that data is
   sent to the time data is received.

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   This document explains that the performance of current practices of
   MPTCP can be further improved by fully taking advantage of One Way
   Latency (OWL) during the transmission.  The OWL components in the
   forward and reverse directions of a RTT may be asymmetric so that it
   can provide a better measure to the user such as for congestion
   control even with the regular TCP.  The benefits will be more when
   there are multiple paths to choose from.

   This document discusses the necessary considerations of OWL in MPTCP.
   The structure of this document is as follows: Firstly, several use
   cases of OWL in MPTCP are analyzed.  Secondly, two kinds of OWL
   measurements are listed and compared.  The considerations related
   with security and IANA are given at the end.

   The potential targeted audience of this document are application
   programmer whose products may significantly benefit from MPTCP.  This
   document also provides the necessary information for the developers
   of MPTCP to implement the new version API into the TCP/IP network

2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "GLUIRED", "SHALL","SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   One Way Latency (OWL):  the propagation delay between a sender and a
      receiver from the time a signal is sent to the time the signal is

3.  Potential Usages of OWL in MPTCP

   There are a number of OWL use cases when MPTCP is enabled by the
   sender and receiver. .  Although only 5 use cases are illustrated in
   this document, more explorations are still needed.

3.1.  Crucial Data Scheduling

   During a transmission process, there are often some crucial data that
   need to be immediately sent to the destination.  Examples of such
   data include the key frame of multimedia and high priority chunk of
   emergency communication.  One cannot guarantee the arrival sequence
   by using the RTTs alone of the multiple paths.

   The data rate in any given link can be asymmetric.  In addition, the
   delay in a given direction can change according to the amount of
   packet queue.  Therefore delay in a forward direction in a path is

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   not necessarily the same as that in the reverse direction as
   exemplified in Figure 1.

        --------   OWL(s-to-c,path1)=16ms   <--------
      /                                               \
     |     ----->  OWL(c-to-s,path1)= 5ms    -----     |
     |   /            RTT(path1)=21ms              \   |
     |  |                                           |  |
   +------+                                       +------+
   |      |----->  OWL(c-to-s,path2)= 8ms    -----|      |
   |Client|                                       |Server|
   |      |-----   OWL(s-to-c,path2)= 8ms   <-----|      |
   +------+           RTT(path2)=16ms             +------+
     |  |                                           |  |
     |   \                                         /   |
     |     ----->  OWL(c-to-s,path3)=10ms    -----     |
      \                                               /
        --------   OWL(s-to-c,path3)= 8ms   <--------

   Figure 1.  Example with 3 paths between the client and the server
   with OWL as indicated in the figure.  RTT information alone would
   indicate to the client that the fastest path to the server is path 2,
   followed by path 3, and then followed by path 1.  path 2 is the
   fastest, whereas OWL indicates to the client that the fastest path to
   the server is path 1, followed by path 2, and then followed by path

   Using the results of OWL measurement, the sender can easily select
   the faster path, in terms of forward latency, for crucial data
   transmission.  Moreover, the acknowledgements of these crucial data
   could be sent on the path with minimum reverse latency.  Piggyback is
   also useful when duplex communication mode is adopted.

3.2.  Congestion Control

   Congestion in a given direction does not necessarily imply congestion
   also in the reverse direction.

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        --------   No congestion (path 1)   <--------
      /                                               \
     |     ----->  Congestion    (path 1)    -----     |
     |   /                                         \   |
     |  |                                           |  |
   +------+                                       +------+
   |Client|                                       |Server|
   +------+                                       +------+
     |  |                                           |  |
     |   \                                         /   |
     |     ----->  No congestion (path 2)    -----     |
      \                                               /
        --------   Congestion    (path 2)   <--------

   Figure 2.  Example of a congestion situation with 2 paths between the
   client and the server.  There is congestion from client to server
   along path 1 and also from server to client along path 2.  RTT
   information alone will indicate congestion in both paths, whereas OWL
   information will show the client that path 2 is the more lightly
   loaded path to get to the server.

   Network congestion in a given direction can be better described using
   OWL rather than using RTT.  Especially when the congestion can be a
   situation in a unidirectional path, the congestion in the path from a
   client to a server is different from the congestion in the path from
   the server to the client.  The RTT cannot accurately reflect the
   delay of interest for data transmission along a path.  For MPTCP, the
   client needs to choose a more lightly loaded path to send packets
   [RFC6356].  It will then be unwise to compare the RTT among different
   paths, and it should instead compare the OWL among the paths.

   Current version of MPTCP includes different kinds of congestion
   control mechanisms [RFC6356].  By reasonably utilizing OWL, the
   network congestion situation in a single direction could be better

3.3.  Packet Retransmission

   Continuous Multipath Transmission (CMT) increases throughput by
   concurrently transferring new data from a source to a destination
   host via multiple paths.  However, when packet is identified as lost
   by triple duplicated acknowledgements or timeout, the sender needs to
   select a suitable path for retransmission.  Due to the popular
   mechanisms of sequence control in reliable transport protocols,
   outstanding packets on multiple paths may reach to the destination
   disorderly and trigger Receive Buffer Blocking (RBB) problem
   (Figure 3) which will further affect the transmission performance.

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               Packet with octets sequence #   0- 499(lost)
        -----> Packet with octets sequence #1000-1499(rcvd) ------
      /        Packet with octets sequence #2000-2499(rcvd)        \
     |                                                              |
   +------+                                                  +--------+
   |Sender|                                                  |Receiver|
   +------+                                                  +--------+
     |                                                              |
      \        Packet with octets sequence # 501- 999(lost)        /
        -----> Packet with octets sequence #1501-1999(lost)  -----
               Packet with octets sequence #2501-2999(lost)

   Figure 3.  Example of Receive Buffer Blocking: The packet containing
   octets 0-499 is lost.  On the other hand the packets containing
   Octets 500-999, 1000-1499, 1500-1999, 2000-2499, and 2500-2999 have
   all been received.  The octets 500-2999 are then all buffered at the
   receiver, and are blocked by the missing octets 0-499.

   Using the results of OWL measurement, the sender can quickly
   determine the specific path with minimum latency in the forward
   direction.  RBB can be relieved as soon as the receiver obtains the
   most needed packet(s) and submits them all to the upper layer.

3.4.  Bandwidth Estimation

   Understanding the bandwidth condition is beneficial for data packet
   scheduling, and load balancing, etc.  OWL could be integrated with
   bandwidth estimation approaches without interrupting the regular
   transmission of packets.

3.5.  Shared Bottleneck Detection

   Fairness is critical especially when MPTCP and ordinary TCP coexist
   in the same network.  The sender could treat OWL measurements as the
   sample process of shared bottleneck detection and accordingly adjust
   the volume of data packet on multiple paths.

4.  OWL Measurements in TCP

   The timestamp option in TCP [RFC7323] may be invoked to estimate
   latency.  When sending data, the time (TSval) of sending the data is
   provided in the option.  The receiver acknowledges the receipt of
   this data by echoing this time (TSecr) and also provides the time
   (TSval) of sending this acknowledgment.  The difference of these
   times in the acknowledgment of data from the sender can help to
   estimate the OWL from the sender to the receiver.  There are two
   problems though.

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   First, there may be delay from the time the receiver has received the
   data to the time the acknowledgment is sent.  The above number may
   then be an upper bound of OWL.

   Second, the clocks may not be synchronized between the sender and the
   receiver.  The above measure can show the OWL in different paths only
   if the clocks synchronized.  Without clock synchronization, the
   comparison of OWLs among different paths is limited to showing the
   OWL differences among them.

   Two kinds of OWL measurement approaches are available: absolute value
   measurement and relative value measurement.

   To obtain the absolute value of OWL, the primary condition of
   measurement is clock synchronization.  Using Network Time Protocol
   (NTP) [RFC5905], end hosts can calibrate the local clock with the
   remote NTP server.  The additional information or optional
   capabilities can even be added via extension fields in the standard
   NTP header [RFC7822].  The calibration accuracy can reach to the
   millisecond level in less congested situations.  The obvious burden
   here is to persuade the end hosts to initialize the NTP option.

   Obtaining the relative value of OWL is more than enough in some
   circumstances to establish applications on top of it.  When
   retransmission is needed, for example, the sender may only care about
   which path has the minimum forward latency.  When bandwidth is being
   estimated, the difference of forward latency, i.e. delta latency,
   among all available paths is needed.  By exchanging with
   correspondent end host the local timestamps of receiving and sending
   the packets, both sides could obtain the relative value of OWL.

   While absolute value measurement of OWL is more convenient for the
   applications, the overheads are the extra protocol requirement and
   synchronization accuracy.  On the contrary, relative value
   measurement does not need to worry about the accuracy whereas the
   overhead is to add timestamps into the original protocol stack.

5.  Security Considerations

   This document does not contain any security considerations.  However,
   future applications of OWL in MPTCP will definitely need to establish
   relevant mechanisms to improve security.

6.  IANA Considerations

   This document presents no IANA considerations.

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7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

7.2.  Informative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,

   [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,

   [RFC6356]  Raiciu, C., Handley, M., and D. Wischik, "Coupled
              Congestion Control for Multipath Transport Protocols",
              RFC 6356, DOI 10.17487/RFC6356, October 2011,

   [RFC6419]  Wasserman, M. and P. Seite, "Current Practices for
              Multiple-Interface Hosts", RFC 6419, DOI 10.17487/RFC6419,
              November 2011, <http://www.rfc-editor.org/info/rfc6419>.

   [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,

   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, Ed., "TCP Extensions for High Performance",
              RFC 7323, DOI 10.17487/RFC7323, September 2014,

   [RFC7822]  Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
              (NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
              March 2016, <http://www.rfc-editor.org/info/rfc7822>.

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Authors' Addresses

   Fei Song
   Beijing Jiaotong University
   Beijing, 100044
   P. R. China

   Email: fsong@bjtu.edu.cn

   Hongke Zhang
   Beijing Jiaotong University
   Beijing, 100044
   P. R. China

   Email: hkzhang@bjtu.edu.cn

   H Anthony Chan
   Huawei Technologies
   5340 Legacy Dr. Building 3
   Plano, TX 75024

   Email: h.a.chan@ieee.org

   Anni Wei
   Huawei Technologies
   Xin-Xi Rd. No. 3, Haidian District
   Beijing, 100095
   P. R. China

   Email: weiannig@huawei.com

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