MPTCP                                                           F. Song
Internet Draft                                                 H. Zhang
Intended status: Informational              Beijing Jiaotong University
Expires: Dec 18, 2019                                           H. Chan
                                                                 A. Wei
                                                    Huawei Technologies
                                                          June 19, 2019

                 One Way Latency Considerations for MPTCP

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   This document discusses the use of One Way Latency (OWL) for
   enhancing multipath TCP (MPTCP). Several usages 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 contribute 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

   The terminal hosts and the intermediate devices in the Internet both
   have been equipped with more and more physical networkinterfaces
   basically. The efficiency of interfaces, which had been widely
   used in packet forwarding at the terminal hosts, had been confirmed
   and utilized [RFC6419]. In addition,in order to aggregate more
   bandwidths,reduce packet delays, and provide better service, the
   increased capacity provided by multiple paths created by multiple
   interfaces is used. Unlike traditional TCP [RFC0793],
   many transport layer protocols, such as MPTCP [RFC6182] [RFC6824]
   enable the terminal 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. However,the key issue
   with data transmission is simply the delay in data transmission along
   the path that does not include the return. The uplink and downlink
   delays between two peers can be very different. RTT can be

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   easily influenced by the latency in the oppsite direction along a
   path, which does not accurately reflect the delay in data
   transmission along the path. Therefore, it is recommended to use
   One Way Latency (OWL) to describe the exact latency from sending data
   to receiving time data.

   It is explained in this document that the full use of One Way Latency
   (OWL) in the transmission process can further improve the performance
   of the current practice of MPTCP. It may be asymmetric
   of the OWL components in the forward and reverse direction 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.

   The necessary considerations of OWL in MPTCP has been discussed in
   this document. The structure of this document is as follows: First,
   the use cases of several OWLs in MPTCP are analyzed. Second, two
   OWL measurements are listed and compared. Finally, the precautions
   related to security and IANA are given.

   The application programmers whose products may benefit from MPTCP
   will be potential target audiences for this document significantly.
   This document also demonstrates the necessary information
   for MPTCP developers to implement the new version of the API in the
   TCP/IP network stack.

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 many OWL use cases when the sender and receiver enable
   MPTCP. 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 the transfer process, it is usually necessary to send some
   basic data to the destination immediately. Examples of such data
   include multimedia key frames and high priority appearance
   communication blocks. No one can guarantee the order of arrival
   by using an RTT with multiple paths alone.

   The data rate in any given link can be asymmetric. In addition,
   the delay in a given direction can vary depending on the number
   of packet queues. Therefore, the same as the opposite direction
   illustrated in Figure 1, the positive delay in the path is not

        --------  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. Although
   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.

   For critical data transfers, the sender can easily select a faster
   path by using OWL measurements (forward delay). In addition, the
   confirmation of these critical data can be sent on the path with
   minimal reverse delay. If the duplex communication mode is adopted,
   the piggybacking is also useful.

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3.2. Congestion control

   Congestion in a given direction does not necessarily imply the
   congestion 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 both path 1 and 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.

   We can use OWL instead of RTT to better describe network congestion
   in a given direction. Especially when congestion can be the case in
   a one-way path, congestion in the path from the client to the server
   is different from congestion in the path from the server to the
   client. The delay of interest for data transmission along a path
   cannot be reflected by the RTT accurately. 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 use
   OWL to compare 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

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3.3. Packet Retransmission

   Continuous Multi-Path Transport (CMT) improves throughput by
   simultaneously transmitting new data from the source to the target
   host through multiple paths. However, when the packet is identified
   as lost by three repeated acknowledgments or timeouts, the sender
   needs to select the appropriate retransmission path. 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.

   By using the results of the OWL measurements, the sender can quickly
   determine the specific path of the positive minimum delay. Once the
   receiver gets the most needed packet, the RBB can be released and
   submitted to the upper layer.

3.4. Bandwidth Estimation

   Understanding bandwidth conditions such as packet scheduling and
   load balancing is critical. OWL can be integrated with bandwidth
   estimation methods without disrupting the regular transmission of

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3.5. Shared Bottleneck Detection

   Fairness is essential when MPTCP and normal TCP coexist in the same
   network. The sender can treat the OWL measurement as a sample
   process for shared bottleneck detection, and the sender adjusts the
   amount of packets 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, these differences in time when validating data from the
   sender can help estimate the OWL from the sender to the receiver.

   First, there may be a delay from the time the recipient of the data
   is received until the time the acknowledgment is sent. Then, the
   above value can be the upper limit of the OWL.

   Second, the clock between the sender and the receiver may not be
   synchronized. OWL can only be displayed in different paths by the
   above measures when the clock is synchronized. The comparison of
   OWL between different paths is limited to showing the OWL
   difference between 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.

   In some cases it is more than enough to get the relative value of
   OWL to build an application on it. For example, the sender may only
   care which path has the minimum forwarding delay when retransmission
   is required. When estimating bandwidth, the difference in forward
   latency in all available paths is required, ie incremental latency.

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   Both sides could obtain therelative value of OWL by exchanging
   with correspondent end host the local timestamps of receiving and
   sending the packets.

   Overhead is an additional protocol requirement and synchronization
   accuracy, while OWL's absolute value measurement is more convenient
   for the application. Instead, relative values are not needed to
   worry about accuracy, and overhead is to add timestamps to the
   original protocol stack.

5. Security Considerations

   This document does not contain any safety precautions. However, the
   future application of OWL in MPTCP definitely needs to establish
   related mechanisms 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-

7.2. Informative Reference

   [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,

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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-

   [RFC6419] Wasserman, M. and P. Seite, "Current Practices for
             Multiple-Interface Hosts", RFC 6419, DOI 10.17487/RFC6419,
             November 2011, <>.

   [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, <http://www.rfc-

   [RFC7822] Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
             (NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
             March 2016, <>.

Authors' Addresses

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


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


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


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


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