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MPTCP                                                           F. Song
Internet Draft                                                 H. Zhang
Intended status: Informational              Beijing Jiaotong University
Expires: June 14, 2018                                          H. Chan
                                                                 A. Wei
                                                    Huawei Technologies
                                                           Dec 13, 2017


                 One Way Latency Considerations for MPTCP
                          draft-song-mptcp-owl-03


Status of this Memo

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   This Internet-Draft will expire on June 14, 2018.

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Abstract

   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.

Table of Contents


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

1. Introduction

   Both end hosts and the intermediate devices in the Internet have
   basically been equipped with more and more physical network
   interfaces. The importance of interfaces, which had been widely used
   in packet forwarding at the end hosts, had been confirmed and
   utilized [RFC6419]. Moreover, to aggregate more bandwidths, to
   decrease packet delay and to provide better services, the increased
   capacity provided by the multiple paths created by multiple
   interfaces is leveraged. 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. It may be very different
   of the latency for uplink and downlink between two peers. Latency in


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   the opposite direction along a path can easily influence RTT, which
   cannot accurately reflect the delay of the data transmission 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.

   The performance of current practices of MPTCP can be further
   improved by fully taking advantage of One Way Latency (OWL) during
   the transmission is explained in this document. It may be asymmetric
   of the OWL components in the forward and reverse directions of a RTT
   so that it can provide a better measure to the user such as for
   congestion control even with the regular TCP. It will be more
   benefits 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, it analyzed
   several use cases of OWL in MPTCP. Secondly, two kinds of OWL
   measurements are listed and compared. The considerations related
   with security and IANA are given at the end.

   The application programmer whose products may significantly benefit
   from MPTCP will be the potential targeted audience of this document.
   The necessary information for the developers of MPTCP to implement
   the new version API into the TCP/IP network stack is also provided
   in this document.



2. Conventions and Terminology

   The key words "MUST", "MUST NOT", "GLUIRED", "SHALL","SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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
   received.

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.






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3.1. Crucial Data Scheduling

   During a transmission process, some crucial data often 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. No one can 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,
   according to the amount of packet queue, the delay in a given
   direction can change. Therefore, the same as that in the reverse
   direction as exemplified in Figure 1, delay in a forward direction
   in a path is not necessary.

        --------  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  <--------
                      RTT(path3)=18ms

   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 3.

   The sender can easily select the faster path by using the results of
   OWL measurement, 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. When duplex
   communication mode is adopted, piggyback is also useful.



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3.2. Congestion control

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

        --------  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.

   It can be better described the network congestion in a given
   direction 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 delay
   of interest for data transmission along a path cannot be accurately
   reflected by the RTT. For MPTCP, the client needs to choose a more
   lightly loaded path to send packets [RFC6356]. Instead of comparing
   the RTT among different paths, it should compare the OWL among the
   paths.

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






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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, the sender needs to select a
   suitable path for retransmission, when packet is identified as lost
   by triple duplicated acknowledgements or timeout. 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, due to the popular
   mechanisms of sequence control in reliable transport protocols.

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

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

3.4. Bandwidth Estimation

   It's beneficial to understand the bandwidth condition for data
   packet scheduling, and load balancing, etc. OWL could be integrated
   with bandwidth estimation approaches without interrupting the
   regular transmission of packets.





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3.5. Shared Bottleneck Detection

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

4. OWL Measurements in TCP

   The timestamp option in TCP [RFC7323] may be invoked to estimate
   latency. The time (TSval) of sending the data is provided in the
   option when sending data. The receiver acknowledges the receipt of
   this data by echoing this time (TSecr), and also the time (TSval) of
   sending this acknowledgment is provided. Although there are two
   problems, 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.

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

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

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

   In order to obtain the absolute value of OWL, the primary condition
   of measurement is clock synchronization. End hosts can calibrate the
   local clock with the remote NTP server by using network time
   protocol (NTP) [RFC5905]. 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.

   It's more than enough to obtain the relative value of OWL in some
   circumstances to establish applications on top of it. For example,
   the sender may only care about which path has the minimum forwarding
   latency when retransmission is needed. When bandwidth is being
   estimated, the difference of forward latency, i.e. delta latency,
   among all available paths is needed. Both sides could obtain the



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   relative value of OWL by exchanging with correspondent end host the
   local timestamps of receiving and sending the packets.

   The overheads are the extra protocol requirement and synchronization
   accuracy, while absolute value measurement of OWL is more convenient
   for the applications. On the contrary, it's no need for relative
   value 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,
   the relevant mechanisms definitely need to be established by future
   applications of OWL in MPTCP to improve security.

6. IANA Considerations

   This document presents no IANA considerations.

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, <http://www.rfc-
             editor.org/info/rfc2119>.

7.2. Informative Reference

   [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
             793, DOI 10.17487/RFC0793, September 1981,
             <http://www.rfc-editor.org/info/rfc793>.

   [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,
             <http://www.rfc-editor.org/info/rfc5905>.

   [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,
             <http://www.rfc-editor.org/info/rfc6182>.






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   [RFC6356] Raiciu, C., Handley, M., and D. Wischik, "Coupled
             Congestion Control for Multipath Transport Protocols", RFC
             6356, DOI 10.17487/RFC6356, October 2011, <http://www.rfc-
             editor.org/info/rfc6356>.

   [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,
             <http://www.rfc-editor.org/info/rfc6824>.

   [RFC7323]Borman, D., Braden, B., Jacobson, V., and R. Scheffenegger,
             Ed., "TCP Extensions for High Performance", RFC 7323, DOI
             10.17487/RFC7323, September 2014, <http://www.rfc-
             editor.org/info/rfc7323>.

   [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>.

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









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

   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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