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Versions: 00 01 02 03                                                   
MPTCP Working Group                                     Samar Shailendra
Internet-Draft                                         Hemant Kumar Rath
Intended status: Informational                                 Arpan Pal
Expires: August 02,2018                                          TCS R&I
                                                          Abhijit Mondal
                                                           IIT Kharagpur
                                                        January 29, 2018

   Extended Socket APIs to control subflow priority in Multipath TCP

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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.


   This document provides the extended Socket APIs to control subflow
   priority for Multipath TCP. It also describes an additional data
   structure for MPTCP to make the subflow priority persistent across
   subflow disconnection.

Table of Contents

   1. Introduction...................................................2
   2. Socket APIs for subflow Priority...............................3
      2.1. Control Subflow Priority..................................4
      2.2. Remembering Subflow Priority..............................4
   3. Security Considerations........................................5
   4. IANA Considerations............................................5
   5. Conclusions....................................................5
   6. References.....................................................6
      6.1. Normative References......................................6
      6.2. Informative References....................................6
   7. Authors' Addresses.............................................7

1. Introduction

   Multipath TCP (MPTCP) [RFC6824, RFC6182] has been designed as a
   successor to TCP [RFC0793] with complete backward compatibility i.e.
   it is able to use the same TCP socket APIs as well as communicate
   with the non-MPTCP enabled hosts. MPTCP is included in the mainline
   Linux Kernel [MPLINUX]. It has also been used in various devices such
   as iPhone [I-PHONE] to improve the reliability by concurrently
   connecting the WiFi and the cellular interfaces. Currently, MPTCP has
   implemented four path managers to effectively use multiple paths
   between the source and the destination.

   o  Default: This path manager does nothing more than accepting the
      passive creation of subflows.

   o  Full Mesh: This creates a full-mesh of subflows between all
      available source and destination interfaces. Usually the subflows
      are created by the client.

   o  Ndiffports: This path manager creates multiple subflows between
      every source and destination pair.

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   o  Binder: This is a relatively newer path manager included in the
      list. It is based on Loose Source Routing [BOC2013].

   MPTCP is completely backward compatible with the TCP socket APIs.
   However, these socket APIs do not exploit the complete potential of
   MPTCP as these functionality are not available in TCP [RFC6824].
   Hence, it is not possible to provide fine grained control over MPTCP
   and in some ways it restricts the usability of MPTCP. Since, defining
   new system call is a big ask for MPTCP and application developers are
   more used to controlling the behavior of TCP using socket option APIs
   (setSockOpt and getSockopt), it is quite effective to develop new
   socket APIs for MPTCP as well to control its behavior.

   In their draft [HESID17], Hesmans et al. have proposed MPTCP socket
   APIs to interact with the underlying socket and control the subflow
   behavior e.g. getting the list of existing subflows, creation of new
   subflow, termination of subflows etc.. In this draft, we use their
   draft [HESID17] as the foundation and provide the extended socket
   APIs to allow the application to mark a particular subflow as an
   active or a backup subflow. Such a control is very helpful for the
   applications which intend to send multiple data streams with
   different Quality of Service (QoS) [SHA2017] requirements to another
   application or end host.

   By default MPTCP creates every new subflow as an active subflow.
   Hence, if a backup or low priority subflow gets disconnected and
   connects again, it becomes active i.e. it does not remember its
   previous state. In this document we also provide a new data structure
   to remember the states of subflows. Hence, if a particular subflow
   between any source destination pair is restored after disconnection,
   it restores with the same state.

   The rest of the document is organized as follows: it gives the
   details of the socket APIs and the corresponding getSockOpt and
   setSockopt details. We than provide the details of data structure to
   remember the subflow priority and the corresponding socket APIs and
   the socket options.

2. Socket APIs for subflow Priority

   In this section we present the new socket APIs for the subflow
   priority control. Currently, MPTCP provides two possible priorities
   for each subflow; high priority or active subflow, and low priority
   or backup subflow. These socket APIs can enable the client or the
   server to dynamically set the subflow priority.

   For all socket APIs, the underlying connections are assumed to
   Multipath TCP connections, otherwise these APIs return an error and

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   set the errono as "EOPNOTSUPP".

2.1. Control Subflow Priority

   This document addresses a typical requirement for an application to
   change the priority of a subflow without restarting the entire

   The first important step to set the subflow priority is to get the
   list of available subflows. This can be performed using the
   "MPTCP_GET_SUB_IDS" socket option defined by Hensman et al.[HESID17,

   The next step is to set the priority of a particular subflow. The
   "MPTCP_SET_SUB_PRIO" socket option can be used to set the subflow
   priority identified by its subflow id. This can be called by using
   "setsockopt" with "MPTCP_SET_SUB_PRIO" as the socket option and
   passing a pointer to "mptcp_sub_prio" structure. The "mptcp_sub_prio"
   structure is defined as follows:

   struct mptcp_sub_prio {
          __u8     id;
          __u16    low_prio;

   Here, "id" is the subflow id as returned by the get subflow socket
   API and "low_prio" is priority value to be defined for the subflow.
   Note that a subflow is an active subflow if the low_prio flag is set
   to "0". A typical illustration of this API to set the sublfow
   priority as backup is as follows:

   struct mptcp_sub_prio fp = {5, 1}; //subflow id 5 is set to backup.

   setsockopt(sockfd, IPPROTO_TCP, MPTCP_SET_SUB_PRIO,&fp, sizeof(fp));

   On successful return of the above socket API, MPTCP protocol sends
   this information to remote host using MP_PRIO flag.

2.2. Remembering Subflow Priority

   A new subflow in MPTCP is in active state and immediately starts
   sending the data. If the subflow between a particular source and
   destination is marked as backup using "MPTCP_SET_SUB_PRIO", it
   becomes active if the subflow gets disconnected and connects again or
   a new subflow is created. To handle this situation, two new lists
   "ActiveInterfaceList" and "BackupInterfaceList" are included in MPTCP
   to remember the state of the subflows between two end hosts. To
   resolve any inconsistency between the two lists, the former has been

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   assigned a higher priority than the later.

   To populate the active and backup lists, setsockopt with
   respectively can be used. This requires the application to pass the
   pointer to "MPTCP_SUB_PATH" structure. The "MPTCP_SUB_PATH" is
   defined as follows:

   struct mptcp_sub_path {
      sa_family_t sa_family;
      union {
         struct in_addr sin_addr;
         struct in6_addr sin6_addr;
      union {
         struct in_addr din_addr;
         struct in6_addr din6_addr;

   These lists can be updated any time; however the runtime update will
   not change the state of an existing subflow. Hence, this socket APIs
   must be called before the subflows are created.

3. Security Considerations

   There are no new security considerations for this document.

4. IANA Considerations

   There are no IANA considerations in this document.

5. Conclusions

   This document provides extended socket APIs for MPTCP to control the
   subflow priorities. These are expected to be very useful for those
   applications which want a fine grained control over the data to be
   sent over the available subflows between the end hosts. These APIs
   can increase the versatility of MPTCP subflows and also provide an
   opportunity to the application developers to select the subflows more
   intelligently. This is expected to be useful for different scenarios
   and devices e.g. drones [SHA2017, RAO2017] where it is important to
   segregate control data from user data on different subflow.

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6. References

6.1. Normative References

   [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC793,
             DOI 10.17487/RFC0793, September 1981,

   [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,

   [HESID17] B. Hesmans and O. Bonaventure, "A socket API to control
             Multipath TCP", Internet Draft, July 2017,

   [MPLINUX] "Multipath TCP implementation in the Linux kernel",

6.2. Informative References

   [RFC6182] Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
             Iyengar, "Architectural Guidelines for Multipath TCP
             Development", RFC6182, DOI 10.17487/RFC6182, March 2011,

   [I-PHONE] Apple Inc, "iOS - Multipath TCP Support in iOS7",

   [BOC2013] L. Boccassi, M. M. Fayed, and M. K. Marina, "Binder: A
             System to Aggregate Multiple Internet Gateways in Community
             Networks," in Proceedings of the 2013 ACM MobiCom Workshop
             on Lowest Cost Denominator Networking for Universal Access.
             ACM, 2013, pp. 3-8.

   [SHA2017] S. Shailendra, K. Aniruddh, B. Panigrahi, and A. Simha,
             "Multipath TCP Path Scheduler for Drones - A Segregation of
             Control and User Data," in Proceedings of the 18th  ACM
             International Symposium on Mobile Ad Hoc Networking and
             Computing. ACM, 2017.

   [RAO2017] A. Rao, M. Visali, S. Shailendra, B. Panigrahi, and A.
             Simha, "Reliable Robotic Communication using MPTCP," in 9th
             International Conference on Communication Systems and
             Networks, Jan 2017, pp. 429-430.

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   [HES2017] B. Hesmans and O. Bonaventure, "An enhanced socket API for
             Multipath TCP," in Proceedings of the 2016 Applied
             Networking Research Workshop. ACM, 2016, pp. 1-6.

7. Authors' Addresses

   Samar Shailendra
   TCS Research & Innovation
   Bangalore, India

   Email: s.samar@tcs.com

   Hemant Kumar Rath
   TCS Research & Innovation
   Bhubaneswar, India

   Email: Hemant.rath@tcs.com

   Arpan Pal
   TCS Research & Innovation
   Kolkata, India

   Email: arpan.pal@tcs.com

   Abhijit Mondal
   Indian Institute of Technology Kharagpur
   Kharagpur, India

   Email: abhimondal@iitkgp.ac.in

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