Internet Engineering Task Force                                M. Scharf
Internet-Draft                                  Alcatel-Lucent Bell Labs
Intended status: Informational                                   A. Ford
Expires: May 26, 2011                                Roke Manor Research
                                                       November 22, 2010

               MPTCP Application Interface Considerations


   Multipath TCP (MPTCP) adds the capability of using multiple paths to
   a regular TCP session.  Even though it is designed to be totally
   backward compatible to applications, the data transport differs
   compared to regular TCP, and there are several additional degrees of
   freedom that applications may wish to exploit.  This document
   summarizes the impact that MPTCP may have on applications, such as
   changes in performance.  Furthermore, it discusses compatibility
   issues of MPTCP in combination with non-MPTCP-aware applications.
   Finally, the document describes a basic application interface for
   MPTCP-aware applications that provides access to multipath address
   information and a level of control equivalent to regular TCP.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 26, 2011.

Copyright Notice

   Copyright (c) 2010 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
   Provisions Relating to IETF Documents

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   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Comparison of MPTCP and Regular TCP  . . . . . . . . . . . . .  5
     3.1.  Performance Impact . . . . . . . . . . . . . . . . . . . .  6
       3.1.1.  Throughput . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.2.  Delay  . . . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.3.  Resilience . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Potential Problems . . . . . . . . . . . . . . . . . . . .  7
       3.2.1.  Impact of Middleboxes  . . . . . . . . . . . . . . . .  7
       3.2.2.  Outdated Implicit Assumptions  . . . . . . . . . . . .  8
       3.2.3.  Security Implications  . . . . . . . . . . . . . . . .  8
   4.  Operation of MPTCP with Legacy Applications  . . . . . . . . .  8
     4.1.  Overview of the MPTCP Network Stack  . . . . . . . . . . .  8
     4.2.  Address Issues . . . . . . . . . . . . . . . . . . . . . .  9
       4.2.1.  Specification of Addresses by Applications . . . . . .  9
       4.2.2.  Querying of Addresses by Applications  . . . . . . . . 10
     4.3.  Socket Option Issues . . . . . . . . . . . . . . . . . . . 11
       4.3.1.  General Guideline  . . . . . . . . . . . . . . . . . . 11
       4.3.2.  Disabling of the Nagle Algorithm . . . . . . . . . . . 11
       4.3.3.  Buffer Sizing  . . . . . . . . . . . . . . . . . . . . 11
       4.3.4.  Other Socket Options . . . . . . . . . . . . . . . . . 12
     4.4.  Default Enabling of MPTCP  . . . . . . . . . . . . . . . . 12
     4.5.  Summary of Advices to Application Developers . . . . . . . 12
   5.  Basic API for MPTCP-aware Applications . . . . . . . . . . . . 13
     5.1.  Design Considerations  . . . . . . . . . . . . . . . . . . 13
     5.2.  Requirements on the Basic MPTCP API  . . . . . . . . . . . 13
     5.3.  Sockets Interface Extensions by the Basic MPTCP API  . . . 15
       5.3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . 15
       5.3.2.  Enabling and Disabling of MPTCP  . . . . . . . . . . . 16
       5.3.3.  Binding MPTCP to Specified Addresses . . . . . . . . . 17
       5.3.4.  Querying the MPTCP Subflow Addresses . . . . . . . . . 17
       5.3.5.  Getting a Unique Connection Identifier . . . . . . . . 18
   6.  Other Compatibility Issues . . . . . . . . . . . . . . . . . . 18
     6.1.  Usage of the SCTP Socket API . . . . . . . . . . . . . . . 18
     6.2.  Incompatibilities with other Multihoming Solutions . . . . 18
     6.3.  Interactions with DNS  . . . . . . . . . . . . . . . . . . 19
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19

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   9.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 20
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     11.2. Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Requirements on a Future Advanced MPTCP API . . . . . 21
     A.1.  Design Considerations  . . . . . . . . . . . . . . . . . . 21
     A.2.  MPTCP Usage Scenarios and Application Requirements . . . . 22
     A.3.  Potential Requirements on an Advanced MPTCP API  . . . . . 24
   Appendix B.  Change History of the Document  . . . . . . . . . . . 25

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1.  Introduction

   Multipath TCP adds the capability of using multiple paths to a
   regular TCP session [1].  The motivations for this extension include
   increasing throughput, overall resource utilisation, and resilience
   to network failure, and these motivations are discussed, along with
   high-level design decisions, as part of the Multipath TCP
   architecture [4].  The MPTCP protocol [5] offers the same reliable,
   in-order, byte-stream transport as TCP, and is designed to be
   backward compatible with both applications and the network layer.  It
   requires support inside the network stack of both endpoints.

   This document first presents the impacts that MPTCP may have on
   applications, such as performance changes compared to regular TCP.
   Second, it defines the interoperation of MPTCP and applications that
   are unaware of the multipath transport.  MPTCP is designed to be
   usable without any application changes, but some compatibility issues
   have to be taken into account.  Third, this memo specifies a basic
   Application Programming Interface (API) for MPTCP-aware applications.
   The API presented here is an extension to the regular TCP API to
   allow an MPTCP-aware application the equivalent level of control and
   access to information of an MPTCP connection that would be possible
   with the standard TCP API on a regular TCP connection.

   An advanced API for MPTCP is outside the scope of this document.
   Such an advanced API could offer a more fine-grained control over
   multipath transport functions and policies.  The appendix includes a
   brief, non-compulsory list of potential features of such an advanced

   The de facto standard API for TCP/IP applications is the "sockets"
   interface.  This document provides an abstract definition of MPTCP-
   specific extensions to this interface.  These are operations that can
   be used by an application to get or set additional MPTCP-specific
   information on a socket, in order to provide an equivalent level of
   information and control over MPTCP as exists for an application using
   regular TCP.  It is up to the applications, high-level programming
   languages, or libraries to decide whether to use these optional
   extensions.  For instance, an application may want to turn on or off
   the MPTCP mechanism for certain data transfers, or limit its use to
   certain interfaces.  The abstract specification is in line with the
   Posix standard [8] as much as possible.

   There are also various related extensions of the sockets interface:
   [12] specifies sockets API extensions for a multihoming shim layer.
   The API enables interactions between applications and the multihoming
   shim layer for advanced locator management and for access to
   information about failure detection and path exploration.

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   Experimental extensions to the sockets API are also defined for the
   Host Identity Protocol (HIP) [13] in order to manage the bindings of
   identifiers and locator.  Further related API extensions exist for
   IPv6 [10], Mobile IP [11], and SCTP [14].  There can be interactions
   or incompatibilities of these APIs with MPTCP, which are discussed
   later in this document.

   Some network stack implementations, specially on mobile devices, have
   centralized connection managers or other higher-level APIs to solve
   multi-interface issues, as surveyed in [16].  Their interaction with
   MPTCP is outside the scope of this note.

   The target readers of this document are application developers whose
   software may benefit significantly from MPTCP.  This document also
   provides the necessary information for developers of MPTCP to
   implement the API in a TCP/IP network stack.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [3].

   This document uses the MPTCP terminology introduced in [5].

   Concerning the API towards applications, the following terms are

   o  Legacy API: The interface towards TCP that is currently used by
      applications.  This document explains the impact of MPTCP for such
      applications, as well as resulting issues.

   o  Basic API: A simple extension of TCP's interface for applications
      that are aware of MPTCP.  This document abstractly describes this
      interface, which provides access to multipath address information
      and a level of control equivalent to regular TCP.

   o  Advanced API: An API that offers more fine-grained control over
      the MPTCP behaviour.  Its detailed specification is outside scope
      of this document.

3.  Comparison of MPTCP and Regular TCP

   This section discusses the impact that the use of MPTCP will have on
   applications, in comparison to what may be expected from the use of
   regular TCP.

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3.1.  Performance Impact

   One of the key goals of adding multipath capability to TCP is to
   improve the performance of a transport connection by load
   distribution over separate subflows across potentially disjoint
   paths.  Furthermore, it is an explicit goal of MPTCP that it should
   not provide a worse performing connection that would have existed
   through the use of single-path TCP.  A corresponding congestion
   control algorithm is described in [7].  The following sections
   summarize the performance impact of MPTCP as seen by an application.

3.1.1.  Throughput

   The most obvious performance improvement that will be gained with the
   use of MPTCP is an increase in throughput, since MPTCP will pool more
   than one path (where available) between two endpoints.  This will
   provide greater bandwidth for an application.  If there are shared
   bottlenecks between the flows, then the congestion control algorithms
   will ensure that load is evenly spread amongst regular and multipath
   TCP sessions, so that no end user receives worse performance than
   single-path TCP.

   This performance increase additionally means that an MPTCP session
   could achieve throughput that is greater than the capacity of a
   single interface on the device.  If any applications make assumptions
   about interfaces due to throughput (or vice versa), they must take
   this into account (although an MPTCP implementation must always
   respect an application's request for a particular interface).

   Furthermore, the flexibility of MPTCP to add and remove subflows as
   paths change availability could lead to a greater variation, and more
   frequent change, in connection bandwidth.  Applications that adapt to
   available bandwidth (such as video and audio streaming) may need to
   adjust some of their assumptions to most effectively take this into

   The transport of MPTCP signaling information results in a small
   overhead.  If multiple subflows share a same bottleneck, this
   overhead slightly reduces the capacity that is available for data
   transport.  Yet, this potential reduction of throughput will be
   neglectible in many usage scenarios, and the protocol contains
   optimisations in its design so that this overhead is minimal.

3.1.2.  Delay

   If the delays on the constituent subflows of an MPTCP connection
   differ, the jitter perceivable to an application may appear higher as
   the data is spread across the subflows.  Although MPTCP will ensure

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   in-order delivery to the application, the application must be able to
   cope with the data delivery being burstier than may be usual with
   single-path TCP.  Since burstiness is commonplace on the Internet
   today, it is unlikely that applications will suffer from such an
   impact on the traffic profile, but application authors may wish to
   consider this in future development.

   In addition, applications that make round trip time (RTT) estimates
   at the application level may have some issues.  Whilst the average
   delay calculated will be accurate, whether this is useful for an
   application will depend on what it requires this information for.  If
   a new application wishes to derive such information, it should
   consider how multiple subflows may affect its measurements, and thus
   how it may wish to respond.  In such a case, an application may wish
   to express its scheduling preferences, as described later in this

3.1.3.  Resilience

   The use of multiple subflows simultaneously means that, if one should
   fail, all traffic will move to the remaining subflow(s), and
   additionally any lost packets can be retransmitted on these subflows.

   Subflow failure may be caused by issues within the network, which an
   application would be unaware of, or interface failure on the node.
   An application may, under certain circumstances, be in a position to
   be aware of such failure (e.g. by radio signal strength, or simply an
   interface enabled flag), and so must not make assumptions of an MPTCP
   flow's stablity based on this.  An MPTCP implementation must never
   override an application's request for a given interface, however, so
   the cases where this issue may be applicable are limited.

3.2.  Potential Problems

3.2.1.  Impact of Middleboxes

   MPTCP has been designed in order to pass through the majority of
   middleboxes.  Empirical evidence suggests that new TCP options can
   successfully be used on most paths in the Internet.  Nevertheless
   some middleboxes may still refuse to pass MPTCP messages due to the
   presence of TCP options, or they may strip TCP options.  If this is
   the case, MPTCP should fall back to regular TCP.  Although this will
   not create a problem for the application (its communication will be
   set up either way), there may be additional (and indeed, user-
   perceivable) delay while the first handshake fails.  A detailed
   discussion of the various fallback mechanisms, for failures occurring
   at different points in the connection, is presented in [5].

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   There may also be middleboxes that transparently change the length of
   content.  If such middleboxes are present, MPTCP's reassembly of the
   byte stream in the receiver is difficult.  Still, MPTCP can detect
   such middleboxes and then fall back to regular TCP.  An overview of
   the impact of middleboxes is presented in [4] and MPTCP's mechanisms
   to work around these are presented and discussed in [5].

   MPTCP can also have other unexpected implications.  For instance,
   intrusion detection systems could be triggered.  A full analysis of
   MPTCP's impact on such middleboxes is for further study after
   deployment experiments.

3.2.2.  Outdated Implicit Assumptions

   In regular TCP, there is a one-to-one mapping of the socket interface
   to a flow through a network.  Since MPTCP can make use of multiple
   subflows, applications cannot implicitly rely on this one-to-one
   mapping any more.  Applications that require the transport along a
   single path can disable the use of MPTCP as described later in this
   document.  Examples include monitoring tools that want to measure the
   available bandwidth on a path, or routing protocols such as BGP that
   require the use of a specific link.

3.2.3.  Security Implications

   The support for multiple IP addresses within one MPTCP connection can
   result in additional security vulnerabilities, such as possibilities
   for attackers to hijack connections.  The protocol design of MPTCP
   minimizes this risk.  An attacker on one of the paths can cause harm,
   but this is hardly an additional security risk compared to single-
   path TCP, which is vulnerable to man-in-the-middle attacks, too.  A
   detailed thread analysis of MPTCP is published in [6].

4.  Operation of MPTCP with Legacy Applications

4.1.  Overview of the MPTCP Network Stack

   MPTCP is an extension of TCP, but it is designed to be backward
   compatible for legacy applications.  TCP interacts with other parts
   of the network stack by different interfaces.  The de facto standard
   API between TCP and applications is the sockets interface.  The
   position of MPTCP in the protocol stack can be illustrated in
   Figure 1.

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                      |           Application         |
                             ^                 |
                  ~~~~~~~~~~~|~Socket Interface|~~~~~~~~~~~
                             |                 v
                     |             MPTCP             |
                     + - - - - - - - + - - - - - - - +
                     | Subflow (TCP) | Subflow (TCP) |
                     |       IP      |      IP       |

                      Figure 1: MPTCP protocol stack

   In general, MPTCP can affect all interfaces that make assumptions
   about the coupling of a TCP connection to a single IP address and TCP
   port pair, to one sockets endpoint, to one network interface, or to a
   given path through the network.

   This means that there are two classes of applications:

   o  Legacy applications: These applications are unaware of MPTCP and
      use the existing API towards TCP without any changes.  This is the
      default case.

   o  MPTCP-aware applications: These applications indicate support for
      an enhance MPTCP interface.  This document specified a minimum set
      of API extensions for such applications.

   In the following, it is discussed to which extent MPTCP affects
   legacy applications using the existing sockets API.  The existing
   sockets API implies that applications deal with data structures that
   store, amongst others, the IP addresses and TCP port numbers of a TCP
   connection.  A design objective of MPTCP is that legacy applications
   can continue to use the established sockets API without any changes.
   However, in MPTCP there is a one-to-many mapping between the socket
   endpoint and the subflows.  This has several subtle implications for
   legacy applications using sockets API functions.

4.2.  Address Issues

4.2.1.  Specification of Addresses by Applications

   During binding, an application can either select a specific address,
   or bind to INADDR_ANY.  Furthermore, on some systems other socket
   options (e.g., SO_BINDTODEVICE) can be used to bind to a specific

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   interface.  If an application uses a specific address or binds to a
   specific interface, then MPTCP MUST respect this and not interfere in
   the application's choices.  The binding to a specific address or
   interface implies that the application is not aware of MPTCP and will
   disable the use of MPTCP on this connection.  An application that
   wishes to bind to a specific set of addresses with MPTCP must use
   multipath-aware calls to achieve this (as described in
   Section 5.3.3).

   If an application binds to INADDR_ANY, it is assumed that the
   application does not care which addresses to use locally.  In this
   case, a local policy MAY allow MPTCP to automatically set up multiple
   subflows on such a connection.

   The basic sockets API of MPTCP-aware applications allows to express
   further preferences in an MPTCP-compatible way (e.g. bind to a subset
   of interfaces only).

4.2.2.  Querying of Addresses by Applications

   Applications can use the getpeername() or getsockname() functions in
   order to retrieve the IP address of the peer or of the local socket.
   These functions can be used for various purposes, including security
   mechanisms, geo-location, or interface checks.  The socket API was
   designed with an assumption that a socket is using just one address,
   and since this address is visible to the application, the application
   may assume that the information provided by the functions is the same
   during the lifetime of a connection.  However, in MPTCP, unlike in
   TCP, there is a one-to-many mapping of a connection to subflows, and
   subflows can be added and removed while the connections continues to
   exist.  Therefore, MPTCP cannot expose addresses by getpeername() or
   getsockname() that are both valid and constant during the
   connection's lifetime.

   This problem is addressed as follows: If used by a legacy
   application, the MPTCP stack MUST always return the addresses of the
   first subflow of an MPTCP connection, in all circumstances, even if
   that particular subflow is no longer in use.

   As this address may not be valid any more if the first subflow is
   closed, the MPTCP stack MAY close the whole MPTCP connection if the
   first subflow is closed (i.e. fate sharing between the initial
   subflow and the MPTCP connection as a whole).  Whether to close the
   whole MPTCP connection by default SHOULD be controlled by a local
   policy.  Further experiments are needed to investigate its

   The functions getpeername() and getsockname() SHOULD also always

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   return the addresses of the first subflow if the socket is used by an
   MPTCP-aware application, in order to be consistent with MPTCP-unaware
   applications, and, e. g., also with SCTP.  Instead of getpeername()
   or getsockname(), MPTCP-aware applications can use new API calls,
   documented later, in order to retrieve the full list of address pairs
   for the subflows in use.

4.3.  Socket Option Issues

4.3.1.  General Guideline

   The existing sockets API includes options that modify the behavior of
   sockets and their underlying communications protocols.  Various
   socket options exist on socket, TCP, and IP level.  The value of an
   option can usually be set by the setsockopt() system function.  The
   getsockopt() function gets information.  In general, the existing
   sockets interface functions cannot configure each MPTCP subflow
   individually.  In order to be backward compatible, existing APIs
   therefore SHOULD apply to all subflows within one connection, as far
   as possible.

4.3.2.  Disabling of the Nagle Algorithm

   One commonly used TCP socket option (TCP_NODELAY) disables the Nagle
   algorithm as described in [2].  This option is also specified in the
   Posix standard [8].  Applications can use this option in combination
   with MPTCP exactly in the same way.  It then SHOULD disable the Nagle
   algorithm for the MPTCP connection, i.e., all subflows.

   In addition, the MPTCP protocol instance MAY use a different path
   scheduler algorithm if TCP_NODELAY is present.  For instance, it
   could use an algorithm that is optimized for latency-sensitive
   traffic.  Specific algorithms are outside the scope of this document.

4.3.3.  Buffer Sizing

   Applications can explicitly configure send and receive buffer sizes
   by the sockets API (SO_SNDBUF, SO_RCVBUF).  These socket options can
   also be used in combination with MPTCP and then affect the buffer
   size of the MPTCP connection.  However, when defining buffer sizes,
   application programmers should take into account that the transport
   over several subflows requires a certain amount of buffer for
   resequencing in the receiver.  MPTCP may also require more storage
   space in the sender, in particular, if retransmissions are sent over
   more than one path.  In addition, very small send buffers may prevent
   MPTCP from efficiently scheduling data over different subflows.
   Therefore, it does not make sense to use MPTCP in combination with
   small send or receive buffers.

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   An MPTCP implementation MAY set a lower bound for send and receive
   buffers and treat a small buffer size request as an implicit request
   not to use MPTCP.

4.3.4.  Other Socket Options

   Some network stacks also provide other implementation-specific socket
   options or interfaces that affect TCP's behavior.  If a network stack
   supports MPTCP, it must be ensured that these options do not

4.4.  Default Enabling of MPTCP

   It is up to a local policy at the end system whether a network stack
   should automatically enable MPTCP for sockets even if there is no
   explicit sign of MPTCP awareness of the corresponding application.
   Such a choice may be under the control of the user through system

   The enabling of MPTCP, either by application or by system defaults,
   does not necessarily mean that MPTCP will always be used.  Both
   endpoints must support MPTCP, and there must be multiple addresses at
   at least one endpoint, for MPTCP to be used.  Even if those
   requirements are met, however, MPTCP may not be immediately used on a
   connection.  It may make sense for multiple paths to be brought into
   operation only after a given period of time, or if the connection is

4.5.  Summary of Advices to Application Developers

   o  Using the default MPTCP configuration: Like TCP, MPTCP is designed
      to be efficient and robust in the default configuration.
      Application developers should not explicitly configure TCP (or
      MPTCP) features unless this is really needed.

   o  Socker buffet dimensioning: Multipath transport requires larger
      buffers in the receiver for resequencing, as already explained.
      Applications should use reasonably buffer sizes (such as the
      operating system default values) in order to fully benefit from
      MPTCP.  A full discussion of buffer sizing issues is given in [5].

   o  Facilitating stack-internal heuristics: The path management and
      data scheduling by MPTCP is realized by stack-internal algorithms
      that may implicitly try to self-optimize their behavior according
      to assumed application needs.  For instance, an MPTCP
      implementation may use heuristics to determine whether an
      application requires delay-sensitive or bulk data transport, using
      for instance port numbers, the TCP_NODELAY socket options, or the

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      application's read/write patterns as input parameters.  An
      application developer can facilitate the operation of such
      heuristics by avoiding atypical interface use cases.  For
      instance, for long bulk data transfers, it does neither make sense
      to enable the TCP_NODELAY socket option, nor is it reasonable to
      use many small subsequent socket "send()" calls with small amounts
      of data only.

5.  Basic API for MPTCP-aware Applications

5.1.  Design Considerations

   While applications can use MPTCP with the unmodified sockets API,
   multipath transport results in many degrees of freedom.  MPTCP
   manages the data transport over different subflows automatically.  By
   default, this is transparent to the application, but an application
   could use an additional API to interface with the MPTCP layer and to
   control important aspects of the MPTCP implementation's behaviour.

   This document describes a basic MPTCP API.  The API contains a
   minimum set of functions that provide an equivalent level of control
   and information as exists for regular TCP.  It maintains backward
   compatibility with legacy applications.

   An advanced MPTCP API is outside the scope of this document.  The
   basic API does not allow a sender or a receiver to express
   preferences about the management of paths or the scheduling of data,
   even if this can have a significant performance impact and if an
   MPTCP implementation could benefit from additional guidance by
   applications.  A list of potential further API extensions is provided
   in the appendix.  The specification of such an advanced API is for
   further study and may partly be implementation-specific.

   MPTCP mainly affects the sending of data.  Therefore, the basic API
   only affects the sender side of a data transfer.  A receiver may also
   have preferences about data transfer choices, and it may have
   performance requirements, too.  A receiver may also have preferences
   about data transfer choices, and it may have performance
   requirements, too.  Yet, the configuration of such preferences is
   outside of the scope of the basic API.

5.2.  Requirements on the Basic MPTCP API

   Because of the importance of the sockets interface there are several
   fundamental design objectives for the basic interface between MPTCP
   and applications:

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   o  Consistency with existing sockets APIs must be maintained as far
      as possible.  In order to support the large base of applications
      using the original API, a legacy application must be able to
      continue to use standard socket interface functions when run on a
      system supporting MPTCP.  Also, MPTCP-aware applications should be
      able to access the socket without any major changes.

   o  Sockets API extensions must be minimized and independent of an

   o  The interface should both handle IPv4 and IPv6.

   The following is a list of the core requirements for the basic API:

   REQ1:  Turn on/off MPTCP: An application should be able to request to
          turn on or turn off the usage of MPTCP.  This means that an
          application should be able to explicitly request the use of
          MPTCP if this is possible.  Applications should also be able
          to request not to enable MPTCP and to use regular TCP
          transport instead.  This can be implicit in many cases, since
          MPTCP must disabled by the use of binding to a specific
          address.  MPTCP may also be enabled if an application uses a
          dedicated multipath address family (such as AF_MULTIPATH,

   REQ2:  An application should be able to restrict MPTCP to binding to
          a given set of addresses.

   REQ3:  An application should be able obtain information on the
          addresses used by the MPTCP subflows.

   REQ4:  An application should be able to extract a unique identifier
          for the connection (per endpoint).

   The first requirement is the most important one, since some
   applications could benefit a lot from MPTCP, but there are also cases
   in which it hardly makes sense.  The existing sockets API provides
   similar mechanisms to enable or disable advanced TCP features.  The
   second requirement corresponds to the binding of addresses with the
   bind() socket call, or, e.g., explicit device bindings with a
   SO_BINDTODEVICE option.  The third requirement ensures that there is
   an equivalent to getpeername() or getsockname() that is able to deal
   with more than one subflow.  Finally, it should be possible for the
   application to retrieve a unique connection identifier (local to the
   endpoint on which it is running) for the MPTCP connection.  This is
   equivalent to using the (address, port) pair for a connection
   identifier in single-path TCP, which is no longer static in MPTCP.

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   An application can continue to use getpeername() or getsockname() in
   addition to the basic MPTCP API.  In that case, both functions return
   the corresponding addresses of the first subflow, as already

5.3.  Sockets Interface Extensions by the Basic MPTCP API

5.3.1.  Overview

   The abstract, basic MPTCP API consists of a set of new values that
   are associated with an MPTCP socket.  Such values may be used for
   changing properties of an MPTCP connection, or retrieving
   information.  These values could be accessed by new symbols on
   existing calls such as setsockopt() and getsockopt(), or could be
   implemented as entirely new function calls.  This implementation
   decision is out of scope for this document.  The following list
   presents symbolic names for these MPTCP socket settings.

   o  TCP_MULTIPATH_ENABLE: Enable/disable MPTCP

   o  TCP_MULTIPATH_ADD: Bind MPTCP to a set of given local addresses,
      or add a new local address to an existing MPTCP connection

   o  TCP_MUTLIPATH_REMOVE: Remove a local address from a MPTCP

   o  TCP_MULTIPATH_SUBFLOWS: Get the pairs of addresses currently used
      by the MPTCP subflows

   o  TCP_MULTIPATH_CONNID: Get the local connection identifier for this
      MPTCP connection

   Table Table 1 shows a list of the abstract socket operations for the
   basic configuration of MPTCP.  The first column gives the symbolic
   name of the operation.  The second and third columns indicate whether
   the operation provides values to be read ("Get") or takes values to
   configure ("Set").  The fourth column lists the type of data
   associated with this operation.

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    | Name                   | Get | Set |          Data type         |
    | TCP_MULTIPATH_ENABLE   |  o  |  o  |           boolean          |
    | TCP_MULTIPATH_ADD      |     |  o  |      list of addresses     |
    | TCP_MULTIPATH_REMOVE   |     |  o  |      list of addresses     |
    | TCP_MULTIPATH_SUBFLOWS |  o  |     | list of pairs of addresses |
    | TCP_MULTIPATH_CONNID   |  o  |     |       32-bit integer       |

                     Table 1: MPTCP Socket Operations

   There are restrictions when these new socket operations can be used:

   o  TCP_MULTIPATH_ENABLE: This value SHOULD only be set before the
      establishment of a TCP connection.  Its value SHOULD only be read
      after the establishment of a connection.

   o  TCP_MULTIPATH_ADD: This operation can be both applied before
      connection setup or during a connection.  If used before, it
      controls the local addresses that an MPTCP connection can use.  In
      the latter case, it allows MPTCP to use an additional local
      address, if there has been a restriction before connection setup.

   o  TCP_MULTIPATH_REMOVE: This operation only has meaning after
      connection setup.

   o  TCP_MULTIPATH_SUBFLOWS: This value is read-only and SHOULD only be
      used after connection setup.

   o  TCP_MULTIPATH_CONNID: This value is read-only and SHOULD only be
      used after connection setup.

5.3.2.  Enabling and Disabling of MPTCP

   An application can explicitly indicate multipath capability by
   setting TCP_MULTIPATH_ENABLE to a value larger than 0.  In this case,
   the MPTCP implementation SHOULD try to negitiate MPTCP for that
   connection.  Note that multipath transport will not necessarily be
   enabled, as it requires multiple addresses and support in the other
   end-system and potentially also on middleboxes.

   An application can disable MPTCP setting TCP_MUTLIPATH_ENABLE to a
   value of 0.  In that case, MPTCP MUST NOT be used on that connection.

   After connection establishment, an application can get the value of
   TCP_MULTIPATH_ENABLE.  A value of 0 then means lack of MPTCP support.
   Any value equal to or larger than 1 means that MPTCP is supported.

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   As alternative to setting an explicit value, an application could
   also use a new, separate address family called AF_MULTIPATH [9].
   This separate address family can be used to exchange multiple
   addresses between an application and the standard sockets API, and
   additionally acts as an explicit indication that an application is
   MPTCP-aware, i.e., that it can deal with the semantic changes of the
   sockets API, in particular concerning getpeername() and
   getsockname().  The usage of AF_MULTIPATH is also more flexible with
   respect to multipath transport, either IPv4 or IPv6, or both in
   parallel [9].

5.3.3.  Binding MPTCP to Specified Addresses

   Before connection establishment, an application can use
   TCP_MULTIPATH_ADD socket option to indicate a set of local IP
   addresses that MPTCP may bind to.  By extension, this operation will
   also control the list of addresses that can be advertised to the peer
   via MPTCP signalling.

   This operation can also be used to modify the address list in use
   during the lifetime of an MPTCP connection.  In this case, it is used
   to indicate a set of additional local addresses that the MPTCP
   connection can make use of, and which can be signalled to the peer.
   It should be noted that this signal is only a hint, and an MPTCP
   implementation MAY only use a subset of the addresses.

   The TCP_MULTIPATH_REMOVE operation can be used to remove a (set of)
   local addresses from an MPTCP connection.  MPTCP MUST close any
   corresponding subflows (i.e. those using the local address that is no
   longer present), and signal the removal of the address to the peer.
   If alternative paths are available using the supplied address list
   but MPTCP is not currently using them, an MPTCP implementation SHOULD
   establish alternative subflows before undertaking the address

   It should be remembered that these operations SHOULD support both
   IPv4 and IPv6 addresses, potentially in the same call.

5.3.4.  Querying the MPTCP Subflow Addresses

   An application can get a list of the addresses used by the currently
   established subflows by means of the read-only TCP_MULTIPATH_SUBFLOWS
   operation.  The return value is a list of pairs of IP addresses.  In
   one pair, the first address refers to the local IP address, and the
   second one to the remote IP address used by the subflow.  The list
   MUST only include established subflows.  Each address pair MUST be
   either a pair of IPv4 addresses or a pair of IPv6 addresses.

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5.3.5.  Getting a Unique Connection Identifier

   An application that wants a unique identifier for the connection,
   analogous to an (address, port) pair in regular TCP, can query the
   TCP_MULTIPATH_CONNID value to get a local connection identifier for
   the MPTCP connection.

   This is a 32-bit number, and SHOULD be the same as the local
   connection identifier sent in the MPTCP handshake.

6.  Other Compatibility Issues

6.1.  Usage of the SCTP Socket API

   For dealing with multi-homing, several socket API extensions have
   been defined for SCTP [14].  As MPTCP realizes multipath transport
   from and to multi-homed endsystems, some of these interface function
   calls are actually applicable to MPTCP in a similar way.

   The following functions that are defined for SCTP have similar
   functionality to the MPTCP API extensions defined earlier:

   o  sctp_bindx()

   o  sctp_connectx()

   o  sctp_getladdrs()

   o  sctp_getpaddrs()

   The syntax and semantics of these functions are described in [14].

   API developers MAY wish to integrate SCTP and MPTCP calls to provide
   a consistent interface to the application.  Yet, it must be
   emphasized that the transport service provided by MPTCP is different
   to SCTP, and this is why not all SCTP API functions can be mapped
   directly to MPTCP.  Furthermore, a network stack implementing MPTCP
   does not necessarily support SCTP and its specific socket interface
   extensions.  This is why the basic API of MPTCP defines additional
   socket options only, which are a backward compatible extension of
   TCP's application interface.

6.2.  Incompatibilities with other Multihoming Solutions

   The use of MPTCP can interact with various related sockets API
   extensions.  The use of a multihoming shim layer conflicts with
   multipath transport such as MPTCP or SCTP [12].  Care should be taken
   for the usage not to confuse with the overlapping features of other

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   o  SHIM API [12]: This API specifies sockets API extensions for the
      multihoming shim layer.

   o  HIP API [13]: The Host Identity Protocol (HIP) also results in a
      new API.

   o  API for Mobile IPv6 [11]: For Mobile IPv6, a significantly
      extended socket API exists as well.

   In order to avoid any conflict, multiaddressed MPTCP SHOULD NOT be
   enabled if a network stack uses SHIM6, HIP, or Mobile IPv6.
   Furthermore, applications should not try to use both the MPTCP API
   and another multihoming or mobility layer API.

   It is possible, however, that some of the MPTCP functionality, such
   as congestion control, could be used in a SHIM6 or HIP environment.
   Such operation is outside the scope of this document.

6.3.  Interactions with DNS

   In multihomed or multiaddressed environments, there are various
   issues that are not specific to MPTCP, but have to be considered,
   too.  These problems are summarized in [15].

   Specifically, there can be interactions with DNS.  Whilst it is
   expected that an application will iterate over the list of addresses
   returned from a call such as getaddrinfo(), MPTCP itself MUST NOT
   make any assumptions about multiple A or AAAA records from the same
   DNS query referring to the same host, as it is possible that multiple
   addresses refer to multiple servers for load balancing purposes.

   TODO: Elaborate on DNS

7.  Security Considerations

   Will be added in a later version of this document.

8.  IANA Considerations

   No IANA considerations.

9.  Conclusion

   This document discusses MPTCP's application implications and
   specifies a basic MPTCP API.  For legacy applications, it is ensured
   that the existing sockets API continues to work.  MPTCP-aware

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   applications can use the basic MPTCP API that provides some control
   over the transport layer equivalent to regular TCP.  A more fine-
   granular interaction between applications and MPTCP requires an
   advanced MPTCP API, which is not specified in this document.

10.  Acknowledgments

   Authors sincerely thank to the following people for their helpful
   comments to the document: Costin Raiciu, Philip Eardley

   Michael Scharf is supported by the German-Lab project
   ( funded by the German Federal Ministry of
   Education and Research (BMBF).  Alan Ford is supported by Trilogy
   (, a research project (ICT-216372)
   partially funded by the European Community under its Seventh
   Framework Program.  The views expressed here are those of the
   author(s) only.  The European Commission is not liable for any use
   that may be made of the information in this document.

11.  References

11.1.  Normative References

   [1]   Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
         September 1981.

   [2]   Braden, R., "Requirements for Internet Hosts - Communication
         Layers", STD 3, RFC 1122, October 1989.

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

   [4]   Ford, A., Raiciu, C., Handley, M., and J. Iyengar,
         "Architectural Guidelines for Multipath TCP Development",
         draft-ietf-mptcp-architecture-02 (work in progress),
         October 2010.

   [5]   Ford, A., Raiciu, C., and M. Handley, "TCP Extensions for
         Multipath Operation with Multiple Addresses",
         draft-ietf-mptcp-multiaddressed-02 (work in progress),
         October 2010.

   [6]   Bagnulo, M., "Threat Analysis for Multi-addressed/Multi-path
         TCP", draft-ietf-mptcp-threat-03 (work in progress),
         October 2010.

   [7]   Raiciu, C., Handley, M., and D. Wischik, "Coupled Multipath-
         Aware Congestion Control", draft-ietf-mptcp-congestion-00 (work

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         in progress), July 2010.

   [8]   "IEEE Std. 1003.1-2008 Standard for Information Technology --
         Portable Operating System Interface (POSIX). Open Group
         Technical Standard: Base Specifications, Issue 7, 2008.".

11.2.  Informative References

   [9]   Sarolahti, P., "Multi-address Interface in the Socket API",
         draft-sarolahti-mptcp-af-multipath-01 (work in progress),
         March 2010.

   [10]  Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, "Advanced
         Sockets Application Program Interface (API) for IPv6",
         RFC 3542, May 2003.

   [11]  Chakrabarti, S. and E. Nordmark, "Extension to Sockets API for
         Mobile IPv6", RFC 4584, July 2006.

   [12]  Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto, "Socket
         Application Program Interface (API) for Multihoming Shim",
         draft-ietf-shim6-multihome-shim-api-13 (work in progress),
         February 2010.

   [13]  Komu, M. and T. Henderson, "Basic Socket Interface Extensions
         for Host Identity Protocol (HIP)", draft-ietf-hip-native-api-12
         (work in progress), January 2010.

   [14]  Stewart, R., Poon, K., Tuexen, M., Yasevich, V., and P. Lei,
         "Sockets API Extensions for Stream Control Transmission
         Protocol (SCTP)", draft-ietf-tsvwg-sctpsocket-23 (work in
         progress), July 2010.

   [15]  Blanchet, M. and P. Seite, "Multiple Interfaces Problem
         Statement", draft-ietf-mif-problem-statement-04 (work in
         progress), May 2010.

   [16]  Wasserman, M. and P. Seite, "Current Practices for Multiple
         Interface Hosts", draft-ietf-mif-current-practices-01 (work in
         progress), June 2010.

Appendix A.  Requirements on a Future Advanced MPTCP API

A.1.  Design Considerations

   Multipath transport results in many degrees of freedom.  The basic
   MPTCP API only defines a minimum set of the API extensions for the
   interface between the MPTCP layer and applications, which does not

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   offer much control of the MPTCP implementation's behaviour.  A
   future, advanced API could address further features of MPTCP and
   provide more control.

   Applications that use TCP may have different requirements on the
   transport layer.  While developers have become used to the
   characteristics of regular TCP, new opportunities created by MPTCP
   could allow the service provided to be optimised further.  An
   advanced API could enable MPTCP-aware applications to specify
   preferences and control certain aspects of the behavior, in addition
   to the simple control provided by the basic interface.  An advanced
   API could also address aspects that are completely out-of-scope of
   the basic API, for example, the question whether a receiving
   application could influence the sending policy.

   Furthermore, an advanced MPTCP API could be part of a new overall
   interface between the network stack and applications that addresses
   other issues as well, such as the split between identifiers and
   locators.  An API that does not use IP addresses (but, instead e.g. a
   connectbyname() function) would be useful for numerous purposes,
   independent of MPTCP.

   This appendix documents a list of potential usage scenarios and
   requirements for the advanded API.  The specification and
   implementation of a corresponding API is outside the scope of this

A.2.  MPTCP Usage Scenarios and Application Requirements

   There are different MPTCP usage scenarios.  An application that
   wishes to transmit bulk data will want MPTCP to provide a high
   throughput service immediately, through creating and maximising
   utilisation of all available subflows.  This is the default MPTCP use

   But at the other extreme, there are applications that are highly
   interactive, but require only a small amount of throughput, and these
   are optimally served by low latency and jitter stability.  In such a
   situation, it would be preferable for the traffic to use only the
   lowest latency subflow (assuming it has sufficient capacity), maybe
   with one or two additional subflows for resilience and recovery
   purposes.  The key challenge for such a strategy is that the delay on
   a path may fluctuate significantly and that just always selecting the
   path with the smallest delay might result in instability.

   The choice between bulk data transport and latency-sensitive
   transport affects the scheduler in terms of whether traffic should
   be, by default, sent on one subflow or across several ones.  Even if

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   the total bandwidth required is less than that available on an
   individual path, it is desirable to spread this load to reduce stress
   on potential bottlenecks, and this is why this method should be the
   default for bulk data transport.  However, that may not be optimal
   for applications that require latency/jitter stability.

   In the case of the latter option, a further question arises: Should
   additional subflows be used whenever the primary subflow is
   overloaded, or only when the primary path fails (hot-standby)?  In
   other words, is latency stability or bandwidth more important to the
   application?  This results in two different options: Firstly, there
   is the single path which can overflow into an additional subflow; and
   secondly there is single-path with hot-standby, whereby an
   application may want an alternative backup subflow in order to
   improve resilience.  In case that data delivery on the first subflow
   fails, the data transport could immediately be continued on the
   second subflow, which is idle otherwise.

   Yet another complication is introduced with the potential that MPTCP
   introduces for changes in available bandwidth as the number of
   available subflows changes.  Such jitter in bandwidth may prove
   confusing for some applications such as video or audio streaming that
   dynamically adapt codecs based on available bandwidth.  Such
   applications may prefer MPTCP to attempt to provide a consistent
   bandwidth as far as is possible, and avoid maximising the use of all

   A further, mostly orthogonal question is whether data should be
   duplicated over the different subflows, in particular if there is
   spare capacity.  This could improve both the timeliness and
   reliability of data delivery.

   In summary, there are at least three possible performance objectives
   for multipath transport (not necessarily disjoint):

   1.  High bandwidth

   2.  Low latency and jitter stability

   3.  High reliability

   In an advanced API, applications could provide high-level guidance to
   the MPTCP implementation concerning these performance requirements,
   for instance, which is considered to be the most important one.  The
   MPTCP stack would then use internal mechanisms to fulfill this
   abstract indication of a desired service, as far as possible.  This
   would both affect the assignment of data (including retransmissions)
   to existing subflows (e.g., 'use all in parallel', 'use as overflow',

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   'hot standby', 'duplicate traffic') as well as the decisions when to
   set up additional subflows to which addresses.  In both cases
   different policies can exist, which can be expected to be

   Therefore, an advanced API could provide a mechanism how applications
   can specify their high-level requirements in an implementation-
   independent way.  One possibility would be to select one "application
   profile" out of a number of choices that characterize typical
   applications.  Yet, as applications today do not have to inform TCP
   about their communication requirements, it requires further studies
   whether such an approach would be realistic.

   Of course, independent of an advanced API, such functionality could
   also partly be achieved by MPTCP-internal heuristics that infer some
   application preferences e.g. from existing socket options, such as
   TCP_NODELAY.  Whether this would be reliable, and indeed appropriate,
   is for further study, too.

A.3.  Potential Requirements on an Advanced MPTCP API

   The following is a list of potential requirements for an advanced
   MPTCP API beyond the features of the basic API.  It is included here
   for information only:

   REQ5:   An application should be able to establish MPTCP connections
           without using IP addresses as locators.

   REQ6:   An application should be able obtain usage information and
           statistics about all subflows (e.g., ratio of traffic sent
           via this subflow).

   REQ7:   An application should be able to request a change in the
           number of subflows in use, thus triggering removal or
           addition of subflows.  An even finer control granularity
           would be a request for the establishment of a new subflow to
           a provided destination, or a request for the termination of a
           specified, existing subflow.

   REQ8:   An application should be able to inform the MPTCP
           implementation about its high-level performance requirements,
           e.g., in form of a profile.

   REQ9:   An application should be able to control the automatic
           establishment/termination of subflows.  This would imply a
           selection among different heuristics of the path manager,
           e.g., 'try as soon as possible', 'wait until there is a bunch
           of data', etc.

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   REQ10:  An application should be able to set preferred subflows or
           subflow usage policies.  This would result in a selection
           among different configurations of the multipath scheduler.

   REQ11:  An application should be able to control the level of
           redundancy by telling whether segments should be sent on more
           than one path in parallel.

   An advanced API fulfilling these requirements would allow application
   developers to more specifically configure MPTCP.  It could avoid
   suboptimal decisions of internal, implicit heuristics.  However, it
   is unclear whether all of these requirements would have a significant
   benefit to applications, since they are going above and beyond what
   the existing API to regular TCP provides.

Appendix B.  Change History of the Document

   Changes compared to version 03:

   o  Removal of explicit references to "socket options" and getsockopt/


   o  Mention of stability of bandwidth as another potential QoS
      parameter for the advanced API.

   o  Address comments received from Philip Eardley: Explanation of the
      API terminology, more explicit statement concerning applications
      that bind to a specific address, and some smaller editorial fixes

   Changes compared to version 02:

   o  Definition of the behavior of getpeername() and getsockname() when
      being called by an MPTCP-aware application.

   o  Discussion of the possiblity that an MPTCP implementation could
      support the SCTP API, as far as it is applicable to MPTCP.

   o  Various editorial fixes.

   Changes compared to version 01:

   o  Second half of the document completely restructured

   o  Separation between a basic API and an advanced API: The focus of
      the document is the basic API only; all text concerning a

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      potential extended API is moved to the appendix

   o  Several clarifications, e. g., concerning buffer sizeing and the
      use of different scheduling strategies triggered by TCP_NODELAY

   o  Additional references

   Changes compared to version 00:

   o  Distinction between legacy and MPTCP-aware applications

   o  Guidance concerning default enabling, reaction to the shutdown of
      the first subflow, etc.

   o  Reference to a potential use of AF_MULTIPATH

   o  Additional references to related work

Authors' Addresses

   Michael Scharf
   Alcatel-Lucent Bell Labs
   Lorenzstrasse 10
   70435 Stuttgart


   Alan Ford
   Roke Manor Research
   Old Salisbury Lane
   Romsey, Hampshire  SO51 0ZN

   Phone: +44 1794 833 465

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