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MPTCP Working Group                                           B. Hesmans
Internet-Draft                                            O. Bonaventure
Intended status: Informational                                 UCLouvain
Expires: January 4, 2018                                   July 03, 2017

                 A socket API to control Multipath TCP


   This document proposes an enhanced socket API to allow applications
   to control the operation of a Multipath TCP stack.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Basic operation . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Multipath TCP Socket API  . . . . . . . . . . . . . . . . . .   5
     3.1.  Subflow list  . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Open subflow  . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Close subflow . . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Get subflow tuple . . . . . . . . . . . . . . . . . . . .   9
     3.5.  Subflow socket option . . . . . . . . . . . . . . . . . .  10
   4.  Multipath-TCP events  . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Subflow creation  . . . . . . . . . . . . . . . . . . . .  12
     4.2.  Subflow deletion  . . . . . . . . . . . . . . . . . . . .  12
     4.3.  New local address available . . . . . . . . . . . . . . .  12
     4.4.  New remote address available  . . . . . . . . . . . . . .  12
     4.5.  Prio changed  . . . . . . . . . . . . . . . . . . . . . .  12
   5.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  12
   6.  Security considerations . . . . . . . . . . . . . . . . . . .  12
   7.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  13
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Multipath TCP [RFC6824] was designed as an incrementally deployable
   [RFC6182] extension to TCP [RFC0793].  One of its design objectives
   was to remain backward compatible with the traditional socket API to
   enable applications to benefit from Multipath TCP without requiring
   any modification.  This solution has been adopted by the Multipath
   TCP implementation in the Linux kernel [MultipathTCP-Linux].  In this
   implementation, once Multipath TCP has been enabled, all TCP
   applications automatically use it.  It is possible to turn Multipath
   TCP off on a per socket basis, but this is rarely used.  The
   Multipath TCP stack contains a module, called the path manager, that
   controls the utilisation of the different paths.  Three path managers
   have been implemented :

   o  the "full mesh" path manager, which is the default one, tries to
      create subflows in full mesh among all the client addresses and
      all addresses advertised by the server.  All subflows are created
      by the client because the server assumes that the client is often
      behind a NAT or firewall

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   o  the "ndiffports" path manager was designed for single-homed hosts.
      It creates n parallel subflows between the client and the server.
      It has been defined notably for datacenters [SIGCOMM11]

   o  the "user space" path manager [CONEXT15] uses Netlink to expose
      events to specific applications and enables them to control the
      operation of the underlying MPTCP stack.

   However, discussions with users of the Multipath TCP implementation
   in the Linux kernel indicate that they would often want a finer
   control on the underlying stack and more precisely on the utilisation
   of the different subflows.  Smartphone applications are a typical
   example.  Measurements indicate that with the default path manager,
   there are many subflows that are created without being used [PAM2016]
   [COMMAG2016].  This increases energy consumption and could be avoided
   on Multipath-TCP aware applications.

   The Multipath TCP implementation used in Apple smartphones, tablets
   and laptops [Apple-MPTCP] took a different approach.  This MPTCP
   stack is not exposed by default to the applications.  To use MPTCP,
   they need to use a specific address family and special system calls

   Using a new address family and new system calls is a major
   modification and application developers may not agree to maintain
   different versions of their applications that run above regular TCP
   and Multipath TCP.  In this document, we propose a simple but
   powerful API that relies only on socket options and the existing
   system calls to interact with the MPTCP stack.  Application
   developers are already used to manipulate socket options and could
   thus easily extend their applications to better utilize the
   underlying MPTCP stack when available.  This approach is similar to
   the API outlined in [RFC6897], but to our knowledge, this API has
   never been implemented.  We also note that during the last decade the
   socket API exposed by SCTP evolved to use more socket options

   This document is organised as follows.  We first describe the basic
   operation of our enhanced API in section Section 2.  We then show in
   section Section 3 how the "getsockopt" and "setsockopt" system calls
   can be used to control the underlying Multipath TCP stack.  We focus
   on basic operations like retrieving the list of subflows that compose
   a Multipath TCP connection, establishing a new subflow or terminating
   an existing subflow in this first version of the document.  We will
   address in the next revision of this document more advanced topics
   such as non-blocking I/O and the utilisation of the "recvmsg" and
   "sendmsg" system calls.

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2.  Basic operation

   In this section, we briefly describe the basic utilisation of the
   enhanced socket API for Multipath TCP.  As an illustration, we
   consider a dual-homed smartphone having a WiFi and a cellular
   interface that interacts with a single homed server.

   We assume for simplicity in this example that the server is passive.
   It creates a listening socket and accepts incoming connections
   through the following system calls :

   o  "socket()"

   o  "bind()"

   o  "listen()"

   Then data can be sent (resp. received) with the "send()" (resp.
   "recv()") system calls and the connection can be terminated by using
   the "close()" or "shutdown()" system calls.

   On the client side, the following system calls are used to create a
   Multipath TCP connection :

   o  "socket()"

   o  "connect()"

   The "connect()" system call succeeds once the initial subflow of the
   Multipath TCP connection has been established.  We assume here that
   Multipath TCP has been negotiated successfully.  The client can then
   send and receive data by using the "send()" and "recv()" system

   The enhanced socket API enables the client (and also the server since
   the protocol is symmetrical, but we ignore this in this section) to
   control the utilisation of the different subflows.  This control is
   performed by setting and retrieving socket options through the
   "setsockopt()" and "getsockopt()" system calls.  Four main socket
   options are defined to control the subflows used by the underlying
   Multipath TCP connection :

   o  "MPTCP_GET_SUB_IDS" can only be used by "getsockopt()".  It is
      used to retrieve the current list of the subflows that compose the
      underlying Multipath TCP connection.  In this list, each one
      identifier is associated with each subflow.

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   o  "MPTCP_GET_SUB_TUPLE".  This socket option is equivalent to the
      "getpeername()" system call with regular TCP, but on a per subflow
      basis.  When used with "getsockopt()", it allows to retrieve the
      IP addresses and ports of the two endpoints of a particular

   o  "MPTCP_OPEN_SUB_TUPLE".  This socket option is the equivalent to
      the "connect()" system call, but it operates on subflows.  It
      allows to attempt to establish a new subflow by specifying its
      (remote and optionally local) endpoints.

   o  "MPTCP_CLOSE_SUB_ID".  This socket option allows to close a
      specific subflow.

   As an example, consider a smartphone application that creates a
   Multipath TCP connection.  This connection is established by using
   the "connect()" system call.  The MPTCP stack selects the outgoing
   interface based on its routing table.  Let us assume that the initial
   subflow is established over the cellular interface.  This is the only
   subflow used for this connection at this time.  To perform a
   handover, the smartphone application would use "MPTCP_OPEN_SUB_TUPLE"
   to create a new subflow over the WiFi interface.  It can then use
   "MPTCP_GET_SUB_TUPLE" to retrieve the local and remote addresses of
   this subflow.  Now that the WiFi subflow is active, the application
   can use "MPTCP_CLOSE_SUB_ID" to close the cellular subflow.

3.  Multipath TCP Socket API

   From an application viewpoint, the interaction with the underlying
   stack is awlays performed through a single socket.  This unique
   socket is used even if a Multipath TCP stack is used and many
   subflows have been established.  This single socket abstraction is
   important because the applications exchange data through a bytestream
   with both TCP and Multipath TCP.  We preserve this abstraction in the
   proposed enhanced socket API but expose some details of the
   underlying MPTCP stack to the application.

   For all the socket options presented bellow, we assume that the
   underlying Multipath TCP connection is still a Multipath TCP
   connection.  Otherwise (e.g. after a fallback), they return an error
   and set errno to "EOPNOTSUPP" is returned.

3.1.  Subflow list

   The first important information that a stack can expose are the
   different subflows that are combined within a given Multipath TCP
   connection.  For this, we need a data structure that represents the
   different subflows that compose a connection.  The "mptcp_sub_ids"

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   structure shown in figure Figure 1 contains an array with the status
   of the different subflows that compose a given connection.  The
   actual size of the array depends on the number of subflows and is
   defined with the "sub_count" field.  The "mptcp_sub_status" structure
   reflects the status of each subflow.  A subflow is identified by its
   "id".  In addition to the "id" of the subflow, the "mptcp_sub_status"
   structure contains one flag : the "low\_prio" flag.  It is set to 1
   when the subflow is defined as a back-up subflow.  Other flags could
   be exposed through this structure in the future.

   struct mptcp_sub_status {
       __u8     id;
       __u16    low_prio:1;

   struct mptcp_sub_ids {
       __u8             sub_count;
       struct mptcp_sub_status sub_status[];

        Figure 1: The mptcp_sub_ids and mptcp_sub_status structures

   This structure is used by the "MPTCP_GET_SUB_IDS" socket option.
   More precisely, the "getsockopt", when used with the
   "MPTCP_GET_SUB_IDS" socket option can retrieve the "mptcp_sub_ids" of
   the underlying Multipath TCP connection.  This call may return an
   empty array if the connection does not contain any subflow.  This can
   happen with Multipath TCP when the last subflow composing the
   connection has been terminated abruptly.

   The "id" that is returned in the "mptcp_sub_ids" structure is
   important because it identifies the subflow and is used as an
   identifier by the other socket options.

   The call may return the error "EINVAL" if the buffer passed by the
   application is too small to copy the array of subflow status.

   A simple example of its utilisation is presented in figure Figure 2.

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   int i;
   unsigned int optlen;
   struct mptcp_sub_ids *ids;

   optlen = 42;

   ids = malloc(optlen);

   getsockopt(sockfd, IPPROTO_TCP, MPTCP_GET_SUB_IDS, ids, &optlen);

   for(i = 0; i < ids->sub_count; i++){
       printf("Subflow id : %i\n",  ids->sub_status[i].id);

      Figure 2: Sample code for the utilisation of MPTCP_GET_SUB_IDS

3.2.  Open subflow

   Another important part of the API is to enable an application to open
   new subflows.  This is possible through the "MPTCP_OPEN_SUB_TUPLE"
   socket option.  This option uses the "mptcp_sub_tuple" structure
   shown in figure Figure 3 to pass the priority, local and remote
   endpoints of the new subflow.

   struct mptcp_sub_tuple {
       __u8    id;
       __u8    prio;
       __u8    addrs[0];
       int     if_idx;

                  Figure 3: The mptcp_sub_tuple structure

   The "id" field is an output.  This is the "id" of the created
   subflow.  The "prio" field indicates if the new subflow should be
   considered as back-up or not.  The "addrs" must be a pair array of
   size two.  The first address must be the address of the source and
   the second address must be the address of the destination.  The
   actual structure passed must be either "sockaddr_in" or
   "sockaddr_in6", but the two elements of the array must be of the same
   type.  The struct "sockaddr" can be used to determine which one is
   actually passed.

   The caller can also set the source address to be either "INADDR_ANY"
   for IPv4 or "in6addr_any" for IPv6.  In this case, the kernel chooses
   the source address to be used for the new subflow.

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   If a single source address is used for multiple interfaces, the
   caller may choose the interface to be used by setting the "if_idx"
   field.  If this field is set to zero the kernel will choose the
   default interface.

   Errors returned by either "bind()" or "connect()" are returned if an
   error occurred during the process.

   An example is provided in figure Figure 4.

   unsigned int optlen;
   struct mptcp_sub_tuple *sub_tuple;
   struct sockaddr_in *addr;
   int error;

   optlen = sizeof(struct mptcp_sub_tuple) +
            2 * sizeof(struct sockaddr_in);
   sub_tuple = malloc(optlen);

   sub_tuple->id = 0;
   sub_tuple->prio = 0;

   addr = (struct sockaddr_in*) &sub_tuple->addrs[0];

   addr->sin_family = AF_INET;
   addr->sin_port = htons(12345);
   inet_pton(AF_INET, "", &addr->sin_addr);


   addr->sin_family = AF_INET;
   addr->sin_port = htons(1234);
   inet_pton(AF_INET, "", &addr->sin_addr);

   error =  getsockopt(sockfd, IPPROTO_TCP, MPTCP_OPEN_SUB_TUPLE,
                       sub_tuple, &optlen);

         Figure 4: Sample code to establish an additional subflow

3.3.  Close subflow

   To close a subflow, the socket option "MPTCP_CLOSE_SUBFLOW" is used.
   This option used the "mptcp_close_sub_id" structure defined in figure
   Figure 5.

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   struct mptcp_close_sub_id {
       __u8    id;
       int     how;

                Figure 5: The mptcp_close_sub_id structure

   In the above structure, "id" is the identifier of the subflow that
   needs to be closed.  If the "id" is invalid, "EINVAL" is returned.

   The "how" field is used to define how to subflow should be
   terminated.  It recognises the same set of constant that are used by
   "shutdown()".  In addition to this set, "RST" can be used to
   indicates that the subflow should be terminated by sending an "RST".

3.4.  Get subflow tuple

   An application may also be interested by the addresses and ports that
   are used by a given subflow.  To retrieve this information, the
   socket option "MPTCP_GET_SUB_TUPLE" is used in combination with the
   "mptcp_sub_tuple" structure shown in figure Figure 6.

   struct mptcp_sub_tuple {
       __u8    id;
       __u8    addrs[0];

                  Figure 6: The mptcp_sub_tuple structure

   This is the same structure as the one used to open a subflow but in
   this context, "id" is the input and "addrs" is the output.

   A sample code is provided in figure Figure 7.

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   unsigned int optlen;
   struct mptcp_sub_tuple *sub_tuple;

   optlen = 100;

   sub_tuple = malloc(optlen);

   sub_tuple->id = sub_id;
   getsockopt(sockfd, IPPROTO_TCP, MPTCP_GET_SUB_TUPLE, sub_tuple,

   sin = (struct sockaddr_in*) &sub_tuple->addrs[0];

   printf("\tip src : %s src port : %hu\n", inet_ntoa(sin->sin_addr),


   printf("\tip dst : %s dst port : %hu\n", inet_ntoa(sin->sin_addr),

        Figure 7: Sample code using the MPTCP_GET_SUB_TUPLE option

3.5.  Subflow socket option

   TCP/IP implementations support different socket options.  Some of
   them can be applied to the TCP layer while others can be applied to
   the IP layer.  To be able to issue a socket option on a specific
   subflow, we define the "MPTCP_SUB_GETSOCKOPT" and
   "MPTCP_SUB_SETSOCKOPT" options.  These two socket options use
   respectively the structures presented in figure Figure 8.

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   struct mptcp_sub_getsockopt {
       __u8        id;
       int        level;
       int        optname;
       char __user    *optval;
       unsigned int __user    *optlen;

   struct mptcp_sub_setsockopt {
       __u8        id;
       int        level;
       int        optname;
       char __user    *optval;
       unsigned int    optlen;

       Figure 8: Structures used by the ``MPTCP_SUB_GETSOCKOPT`` and
                     ``MPTCP_SUB_SETSOCKOPT`` options

   In the two structures "id" indicates to which subflow the socket
   option should be redirected.  The end of each structure contains the
   information needed to perform the socket option call on the subflow.

   Figure Figure 9 illustrates how the IP_TSO socket option can be
   applied on a particular subflow.

   unsigned int optlen, sub_optlen;
   struct mptcp_sub_setsockopt sub_sso;
   int val = 12;

   optlen = sizeof(struct mptcp_sub_setsockopt);
   sub_optlen = sizeof(int);
   sub_sso.id = sub_id;
   sub_sso.level = IPPROTO_IP;
   sub_sso.optname = IP_TOS;
   sub_sso.optlen = sub_optlen;
   sub_sso.optval = (char *) &val;

   setsockopt(sockfd, IPPROTO_TCP, MPTCP_SUB_SETSOCKOPT, &sub_sso,

                      Figure 9: Example socket option

4.  Multipath-TCP events

   In some cases, Multipath-TCP connections should be able to react upon
   events.  Applications that are aware of Multipath-TCP must receive
   those events in order to take their decisions and react if needed.

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   The list of events that could be produced by Multipath-TCP stack,
   upon subscription to those events, are presented bellow.

   _** This section is still an early draft and will be updated to add
   more details. **_

4.1.  Subflow creation

   If the stack or the user requests a new subflow, the user will
   receive this event if the subflow has been fully established and is
   available to send new data.  The event should contain the
   identification of the subflow.

4.2.  Subflow deletion

   If one of the subflow is deleted, an event that contain the
   identification of the subflow must be triggered.  Moreover, the
   reason of the deletion must be stated.

4.3.  New local address available

   When a new local address is available, the application should receive
   all the informations needed to use the new address.

4.4.  New remote address available

   See previous section.

4.5.  Prio changed

   When one of the subflow receives a change in prio.  The event must
   contain the identification of the subflow and the new prio.

5.  IANA considerations

   There are no IANA considerations in this document.

6.  Security considerations

   TCP and UDP implementations usually reserve port numbers below 1024
   for privileged users.  On such implementations, Multipath TCP should
   restrict the ability of the users to create subflows on privileged
   ports through the "MPTCP_OPEN_SUB_TUPLE".

   For similar reasons, the "MPTCP_SUB_SETSOCKOPT" socket option should
   not enable an unprivileged user to retrieve or modify a socket option
   on a subflow if he is not allowed to perform such actions on a
   regular TCP connection.

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   Applications requiring strong security should implement cryptographic
   protocols such as TLS [RFC5246] or ssh [RFC4251].  The proposed API
   enables such application to better control their utilisation of the
   underlying interfaces by managing the different subflows.

7.  Conclusion

   In this document, we have documented an enhanced socket API that
   enables applications to control the creation and the release of
   subflows by the underlying Multipath TCP stack.  We expect that a
   standardised API supported by different implementations will be an
   important stop for the deployment of Multipath TCP aware applications
   on both multihomed hosts such as smartphones as well as on servers.
   This enhanced API has already been implemented on the Multipath TCP
   implementation in the Linux kernel.  Future versions of this document
   will address more advanced utilisations of the socket API such as
   non-blocking I/O and the "sendmsg()" and "recvmsg()" system calls.

8.  Acknowledgements

   We would like to thank Christoph Paasch, Quentin De Coninck Rao
   Shoaib for their comments on an early version of this document.

9.  References

9.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, 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,

9.2.  Informative References

              Hesmans, B. and O. Bonaventure, "An enhanced socket API
              for Multipath TCP", 2016, <https://irtf.org/anrw/2016/

              Apple, Inc, ., "iOS - Multipath TCP Support in iOS 7",
              n.d., <https://support.apple.com/en-us/HT201373>.

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              De Coninck, Q., Baerts, M., Hesmans, B., and O.
              Bonaventure, "Observing Real Smartphone Applications over
              Multipath TCP", IEEE Communications Magazine , March 2016,

              Hesmans, B., Detal, G., Barre, S., Bauduin, R., and O.
              Bonaventure, "SMAPP - Towards Smart Multipath TCP-enabled
              APPlications", Proc. Conext 2015, Heidelberg, Germany ,
              December 2015, <http://inl.info.ucl.ac.be/publications/

              Paasch, C., Barre, S., and . et al, "Multipath TCP
              implementation in the Linux kernel", n.d.,

   [PAM2016]  De Coninck, Q., Baerts, M., Hesmans, B., and O.
              Bonaventure, "A First Analysis of Multipath TCP on
              Smartphones", 17th International Passive and Active
              Measurements Conference (PAM2016) , March 2016,

   [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
              January 2006, <http://www.rfc-editor.org/info/rfc4251>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [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,

   [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
              Yasevich, "Sockets API Extensions for the Stream Control
              Transmission Protocol (SCTP)", RFC 6458,
              DOI 10.17487/RFC6458, December 2011,

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   [RFC6897]  Scharf, M. and A. Ford, "Multipath TCP (MPTCP) Application
              Interface Considerations", RFC 6897, DOI 10.17487/RFC6897,
              March 2013, <http://www.rfc-editor.org/info/rfc6897>.

              Raiciu, C., Barre, S., Pluntke, C., Greenhalgh, A.,
              Wischik, D., and M. Handley, "Improving datacenter
              performance and robustness with multipath TCP",
              Proceedings of the ACM SIGCOMM 2011 conference , 2011,

Authors' Addresses

   Benjamin Hesmans

   Email: Benjamin.Hesmans@uclouvain.be

   Olivier Bonaventure

   Email: Olivier.Bonaventure@uclouvain.be

Hesmans & Bonaventure    Expires January 4, 2018               [Page 15]

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