MPTCP Application Interface Considerations
draft-scharf-mptcp-api-01

Abstract

Multipath TCP (MPTCP) adds the capability of using multiple paths to a regular TCP session. Even though it is designed to be totally backwards 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 describes an optional extended application interface that provides access to multipath information and enables control of some aspects of the MPTCP implementation's behaviour.

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Table of Contents

1.  Introduction
2.  Terminology
3.  Comparison of MPTCP and Regular TCP
    3.1.  Performance Impact
        3.1.1.  Throughput
        3.1.2.  Delay
        3.1.3.  Resilience
    3.2.  Potential Problems
        3.2.1.  Impact of Middleboxes
        3.2.2.  Outdated Implicit Assumptions
        3.2.3.  Security Implications
4.  Operation of MPTCP with Legacy Applications
    4.1.  Overview of the MPTCP Network Stack
    4.2.  Usage of Addresses Inside Applications
    4.3.  Usage of Existing Socket Options
    4.4.  Default Enabling of MPTCP
    4.5.  Known Remaining Issues with Legacy Applications
5.  Minimal API Enhancements for MPTCP-aware Applications
    5.1.  Indicating MPTCP Awareness
    5.2.  Modified Address Handling
    5.3.  Usage of a New Address Family
6.  Extended MPTCP API
    6.1.  MPTCP Usage Scenarios and Application Requirements
    6.2.  Requirements on API Extensions
    6.3.  Design Considerations
    6.4.  Overview of Sockets Interface Extensions
    6.5.  Detailed Description
        6.5.1.  TCP_MP_ENABLE
        6.5.2.  TCP_MP_SUBFLOWS
        6.5.3.  TCP_MP_PROFILE
    6.6.  Usage examples
    6.7.  Interactions and Incompatibilities with other Multihoming Solutions
    6.8.  Other Advice to Application Developers
7.  Security Considerations
8.  IANA Considerations
9.  Conclusion
10.  Acknowledgments
11.  References
    11.1.  Normative References
    11.2.  Informative References
Appendix A.  Change History of the Document

1.  Introduction

Multipath TCP (MPTCP) adds the capability of using multiple paths to a regular TCP session [1] (Postel, J., “Transmission Control Protocol,” September 1981.). 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 MPTCP architecture [4] (Ford, A., Raiciu, C., Barre, S., and J. Iyengar, “Architectural Guidelines for Multipath TCP Development,” March 2010.). MPTCP [5] (Ford, A., Raiciu, C., and M. Handley, “TCP Extensions for Multipath Operation with Multiple Addresses,” October 2009.) 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 presents the impacts that MPTCP may have on applications, such as performance changes compared to regular TCP. Furthermore, it specifies an extended Application Programming Interface (API) describing how applications can exploit additional features of multipath transport. MPTCP is designed to be usable without any application changes. The specified API is an optional extension that provides access to multipath information and enables control of some aspects of the MPTCP implementation's behaviour, for example switching on or off the automatic use of MPTCP.

The de facto standard API for TCP/IP applications is the "sockets" interface. This document defines experimental MPTCP-specific extensions, in particular additional socket options. 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 provide some guidance concerning its usage (and thus the service the application receives). The syntax and semantics of the specification is in line with the Posix standard [8] (, “IEEE Std. 1003.1-2008 Standard for Information Technology -- Portable Operating System Interface (POSIX). Open Group Technical Standard: Base Specifications, Issue 7, 2008.,” .) as much as possible.

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 [14] (Wasserman, M., “Current Practices for Multiple Interface Hosts,” October 2009.). Their interaction with MPTCP is outside the scope of this note.

There are also various related extensions of the sockets interface: [11] (Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto, “Socket Application Program Interface (API) for Multihoming Shim,” February 2010.) 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. Other experimental extensions to the sockets API are defined for the Host Identity Protocol (HIP) [12] (Komu, M. and T. Henderson, “Basic Socket Interface Extensions for Host Identity Protocol (HIP),” January 2010.) in order to manage the bindings of identifiers and locator. Other related API extensions exist for IPv6 [10] (Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, “Advanced Sockets Application Program Interface (API) for IPv6,” May 2003.) and SCTP [13] (Stewart, R., Poon, K., Tuexen, M., Yasevich, V., and P. Lei, “Sockets API Extensions for Stream Control Transmission Protocol (SCTP),” February 2010.). There can be interactions or incompatibilities of these APIs with MPTCP, which are discussed later in this document.

The target readers of this document are application programmers who develop application software that 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [3] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

This document uses the terminology introduced in [5] (Ford, A., Raiciu, C., and M. Handley, “TCP Extensions for Multipath Operation with Multiple Addresses,” October 2009.).

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.

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 legacy, single-path TCP. A corresponding congestion control algorithm is described in [7] (Raiciu, C., Handley, M., and D. Wischik, “Coupled Multipath-Aware Congestion Control,” October 2009.). 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.

Furthermore, this 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.

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 striped across the subflows. Although MPTCP will ensure 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 document.

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. MPTCP will 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, for example through its ability to open subflows in either direction, and through its use of a data-level sequence number.

Nevertheless some middleboxes may still refuse to pass MPTCP messages due to the presence of 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.

Empirical evidence suggests that new TCP options can successfully be used on most paths in the Internet. But they can also have other unexpected implications. For instance, intrusion detection systems could be triggered. Full analysis of MPTCP's impact on such middleboxes is for further study.

3.2.2.  Outdated Implicit Assumptions

MPTCP overcomes the one-to-one mapping of the socket interface to a flow through the network. As a result, 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] (Bagnulo, M., “Threat Analysis for Multi-addressed/Multi-path TCP,” February 2010.).

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 (MPTCP protocol stack).

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

In general, MPTCP can affect all interfaces that rely on 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:

• Legacy applications: These applications use the existing API towards TCP without any changes. This is the default case.
• MPTCP-aware applications: These applications indicate support for an enhance MPTCP interface.

In the following, it is discussed to which extent MPTCP affects legacy applications using the existing sockets API.

4.2.  Usage of Addresses Inside Applications

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.

During binding, an application can either select a specific address, or bind to INADDR_ANY. Furthermore, the SO_BINDTODEVICE socket option can be used to bind to a specific interface. If an application uses a specific address, or sets the SO_BINDTODEVICE socket option to bind to a specific interface, then MPTCP MUST respect this and not interfere in the application's choices. 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 extended sockets API will allow applications to express specific preferences in an MPTCP-compatible way (e.g. bind to a subset of interfaces only).

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 (fate sharing). Whether to close the whole MPTCP connection by default SHOULD be controlled by a local policy. Further experiments are needed to investigate its implications.

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.  Usage of Existing Socket Options

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.

One commonly used TCP socket option (TCP_NODELAY) disables the Nagle algorithm as described in [2] (Braden, R., “Requirements for Internet Hosts - Communication Layers,” October 1989.). This option is also specified in the Posix standard [8] (, “IEEE Std. 1003.1-2008 Standard for Information Technology -- Portable Operating System Interface (POSIX). Open Group Technical Standard: Base Specifications, Issue 7, 2008.,” .). Applications can use this option in combination with MPTCP exactly in the same way. It then disables the Nagle algorithm for the MPTCP connection, i.e., all subflows.

TODO: Setting this option could also trigger a different path scheduler algorithm - specifically, that which is designed for latency-sensitive traffic, as described in a later section.

Applications can also 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. Therefore, it does not make sense to use MPTCP in combination with very small receive buffers. Small send buffers may prevent MPTCP from efficiently scheduling data over different subflows. It may be appropriate for an MPTCP implementation to set a lower bound for such buffers, or alternatively treat a small buffer size request as an implicit request not to use MPTCP.

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 interfere.

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 preferences.

4.5.  Known Remaining Issues with Legacy Applications

TODO: Future experiments will show whether legacy applications could break despite the backward-compatible API of MPTCP.

5.  Minimal API Enhancements for MPTCP-aware Applications

5.1.  Indicating MPTCP Awareness

While applications can use MPTCP with the unmodified sockets API, a clean interface requires small semantic changes compared to the existing sockets API. Even if these changes do not affect most applications, they are only enabled if an application explicitly signals that it supports multipath transport and the enhanced interface, in order to maintain backward compatibility with legacy applications. An application can explicitly indicate multipath capability by setting the TCP_MP_ENABLE option described below.

5.2.  Modified Address Handling

The main change of the sockets API for MPTCP-aware applications is as follows: If a socket is MPTCP-aware and thus does not use the backward-compatibility mode, the functions getpeername() and getsockname() SHOULD fail with a new error code EMULTIPATH. Due to their ambiguity, an MPTCP-aware application should not use these two functions. Instead, the information about the addresses in use can be accessed by the extended sockets API, if needed.

5.3.  Usage of a New Address Family

As alternative to setting a socket option, an application can also use a new, separate address family called AF_MULTIPATH [9] (Sarolahti, P., “Multi-address Interface in the Socket API,” March 2010.). 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] (Sarolahti, P., “Multi-address Interface in the Socket API,” March 2010.).

6.  Extended MPTCP API

6.1.  MPTCP Usage Scenarios and Application Requirements

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 extended API enables MPTCP-aware applications to specify preferences and control certain aspects of the behavior, in addition to the simple controls already discussed, such as switching on or off the automatic use of MPTCP.

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 case.

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), with one or two additional subflows for resilience and recovery purposes.

The choice between these two options affects the scheduler in terms of whether traffic should be, by default, sent on one subflow or across both. Even if 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. It is recognised, however, that this may not benefit all applications that require latency/jitter stability, so the other (single path) option is provided.

In the case of the latter option, however, 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?

We therefore divide this option into two: 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.

In summary, there are three different "application profiles" concerning the use of MPTCP:

1. Bulk data transport
2. Latency-sensitive transport (with overflow)
3. Latency-sensitive transport (hot-standby)

These different application profiles affect both the management of subflows, i.e., the decisions when to set up additional subflows to which addresses as well as the assignment of data (including retransmissions) to the existing subflows. In both cases different policies can exist.

These profiles have been defined to cover the common application use cases. It is not possible to cover all application requirements, however, and as such applications may wish to have finer control over subflows and packet scheduling. A set of requirements is listed below.

Although it is intended that such functionality will be achieved through new MPTCP-specific options, it may also be possible to infer some application preferences from existing socket options, such as TCP_NODELAY. Whether this would be reliable, and indeed appropriate, is for further study.

6.2.  Requirements on API Extensions

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

• 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.
• Sockets API extensions must be minimized and independent of an implementation.
• The interface should both handle IPv4 and IPv6.

The following is a list of specific requirements from applications:

TODO: This list of requirements is preliminary and requires further discussion. Some requirements have to be removed.

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, e.g., since MPTCP must disabled by the use of binding to a specific address, or may be enabled if an application uses AF_MULTIPATH.

REQ2:

An application will want to be able to restrict MPTCP to binding to a given set of addresses or interfaces.

REQ3:

An application should be able to know if multiple subflows are in use.

REQ4:

An application should be able to enumerate all subflows in use, obtain information on the addresses used by a subflow, and obtain a subflow's usage (e.g., ratio of traffic sent via this subflow).

REQ5:

An application should be able to extract a unique identifier for the connection (per endpoint), analogous to a port, i.e., it should be able to retrieve MPTCP's connection identifier. (TODO)

REQ6:

Set/get the application profile, as discussed in the previous section.

The above requirements are seen as having fairly clear benefits to applications. Although in some cases they are going above and beyond what regular TCP would provide, they are allowing an application to make optimal use of the new features that MPTCP provides.

The following requirements are more specific, and could mostly be implied through more generic options, such as the application profile selection. They are currently included here as potential discussion points, however, as they may have use to application developers as more specific configuration options, beyond being an implicit part of a profile selection.

REQ7:

Constrain the maximum number of subflows to be used by an MPTCP connection.

REQ8:

Request a change in scheduling between subflows.

REQ9:

Request a change in the number of subflows in use, thus triggering removal or addition of subflows. (A finer control granularity would be: Request the establishment of a new subflow to a provided destination, and request the termination of a specified, existing subflow.)

REQ10:

Control automatic establishment/termination of subflows? There could be different configurations of the path manager, e.g., 'try ASAP', 'wait until there is a bunch of data, etc. (Tied to application profile?)

REQ11:

Set/get preferred subflows or subflow usage policies? There could be different configurations of the multipath scheduler, e.g., 'all-or-nothing', 'overflow', etc. (Again, tied to application profile?)

REQ12:

Get/set redundancy, i.e., to send segments on more than one path in parallel.

REQ13:

An application should be able to modify the MPTCP configuration while communication is ongoing, i.e., after establishment of the MPTCP connection.

6.3.  Design Considerations

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 applications can use the sockets API extensions defined in this section to interface with the MPTCP layer and to control important aspects of the MPTCP implementation's behaviour. The API uses non-mandatory socket options and is designed to be as light-weight as possible.

MPTCP mainly affects the sending of data. Therefore, most of the new socket options must be set in the sender side of a data transfer in order to take effect. Nevertheless, it is also possible for a receiver to have preferences about data transfer choices, as it may too have performance requirements. (TODO) It is for further study as to whether it is feasible for a receiving application to influence sending policy, and if so, how this could be implemented.

As this document specifies sockets API extensions, it is written so that the syntax and semantics are in line with the Posix standard [8] (, “IEEE Std. 1003.1-2008 Standard for Information Technology -- Portable Operating System Interface (POSIX). Open Group Technical Standard: Base Specifications, Issue 7, 2008.,” .) as much as possible.

6.4.  Overview of Sockets Interface Extensions

The extended MPTCP API consist of several new socket options that are specific to MPTCP. All of these socket options are defined at TCP level (IPPROTO_TCP). These socket options can be used either by the getsockopt() or by the setsockopt() system call.

The new API functions can be classified into general configuration and more advanced configuration. The new socket options for the general configuration of MPTCP are:

• TCP_MP_ENABLE: Enable/disable MPTCP
• TCP_MP_SUBFLOWS: Get the addresses currently used by the MPTCP subflows, optionally complemented by further information such as usage ratio
• TCP_MP_PROFILE: Get/set the MPTCP profile
• ...

Table Table 1 (Socket options for MPTCP) shows a list of the socket options for the general configuration of MPTCP. The first column gives the name of the option. The second and third columns indicate whether the option can be handled by the getsockopt() system call and/or by the setsockopt() system call. The fourth column lists the type of data structure specified along with the socket option.

*1: Data structure containing the addresses of each subflow, plus further information

TODO: More options may be added in a future version of this note.

6.5.  Detailed Description

6.5.1.  TCP_MP_ENABLE

TODO: Description

6.5.2.  TCP_MP_SUBFLOWS

TODO: Description

6.5.3.  TCP_MP_PROFILE

TODO: Description

6.6.  Usage examples

TODO: Example C code for one or more API functions

6.7.  Interactions and Incompatibilities with other Multihoming Solutions

The use of MPTCP can interact with various related sockets API extensions. Care should be taken for the usage not to confuse with the overlapping features:

• SHIM API [11] (Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto, “Socket Application Program Interface (API) for Multihoming Shim,” February 2010.): This API specifies sockets API extensions for the multihoming shim layer.
• HIP API [12] (Komu, M. and T. Henderson, “Basic Socket Interface Extensions for Host Identity Protocol (HIP),” January 2010.): The Host Identity Protocol (HIP) also results in a new API.

The use of a multihoming shim layer conflicts with multipath transport such as MPTCP or SCTP [11] (Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto, “Socket Application Program Interface (API) for Multihoming Shim,” February 2010.). In order to avoid any conflict, multiaddressed MPTCP SHOULD not be enabled if a network stack uses SHIM6 or HIP. Furthermore, applications should not try to use both the MPTCP API and a multihoming shim layer API. It is feasible, 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.

6.8.  Other Advice to Application Developers

• Using the default MPTCP configuration: MPTCP is designed to be efficient and robust in the default configuration. Application developers should not explicitly configure features unless this is really needed.
• Socker buffer 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.

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 an extended API. From an architectural point of view, MPTCP offers additional degrees of freedom concerning the transport of data. The extended sockets API allows MPTCP-aware applications to have additional control of some aspects of the MPTCP implementation's behaviour and to obtain information about its usage. The new socket options for MPTCP can be used by getsockopt() and/or setsockopt() system calls. But it is also ensured that the existing sockets API continues to work for legacy applications.

10.  Acknowledgments

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

Michael Scharf is supported by the German-Lab project (http://www.german-lab.de/) funded by the German Federal Ministry of Education and Research (BMBF). Alan Ford is supported by Trilogy (http://www.trilogy-project.org/), 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

11.2. Informative References

Appendix A.  Change History of the Document

Changes compared to version 00:

• Distinction between legacy and MPTCP-aware applications
• Guidance concerning default enabling, reaction to the shutdown of the first sub-flow, etc.
• Reference to a potential use of AF_MULTIPATH
• Additional references to related work

Authors' Addresses