Request for Comments: 6897
Alcatel-Lucent Bell Labs
Multipath TCP (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 that is a simple extension of TCP's interface for MPTCP-aware applications.
Status of This Memo
This document is not an Internet Standards Track specification; it is published for informational purposes.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6897.
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Table of Contents
1. Introduction ....................................................3 2. Terminology .....................................................4 3. Comparison of MPTCP and Regular TCP .............................5 3.1. Effect on Performance ......................................5 3.1.1. Throughput ..........................................5 3.1.2. Delay ...............................................6 3.1.3. Resilience ..........................................7 3.2. Potential Problems .........................................8 3.2.1. Impact of Middleboxes ...............................8 3.2.2. Dealing with Multiple Addresses inside Applications ........................................9 3.2.3. Security Implications ..............................10 4. Operation of MPTCP with Legacy Applications ....................10 4.1. Overview of the MPTCP Network Stack .......................10 4.2. Address Issues ............................................11 4.2.1. Specification of Addresses by Applications .........11 4.2.2. Querying of Addresses by Applications ..............12 4.3. MPTCP Connection Management ...............................13 4.3.1. Reaction to Close Call by Application ..............13 4.3.2. Other Connection Management Functions ..............13 4.4. Socket Option Issues ......................................13 4.4.1. General Guideline ..................................13 4.4.2. Disabling of the Nagle Algorithm ...................13 4.4.3. Buffer Sizing ......................................14 4.4.4. Other Socket Options ...............................14 4.5. Default Enabling of MPTCP .................................14 4.6. Summary of Advice to Application Developers ...............15 5. Basic API for MPTCP-Aware Applications .........................15 5.1. Design Considerations .....................................15 5.2. Requirements on the Basic MPTCP API .......................16 5.3. Sockets Interface Extensions by the Basic MPTCP API .......17 5.3.1. Overview ...........................................17 5.3.2. Enabling and Disabling of MPTCP ....................19 5.3.3. Binding MPTCP to Specified Addresses ...............19 5.3.4. Querying the MPTCP Subflow Addresses ...............20 5.3.5. Getting a Unique Connection Identifier .............20 6. Other Compatibility Issues .....................................21 6.1. Usage of TLS over MPTCP ...................................21 6.2. Usage of the SCTP Sockets API .............................21 6.3. Incompatibilities with Other Multihoming Solutions ........21 6.4. Interactions with DNS .....................................22 7. Security Considerations ........................................22 8. Conclusion .....................................................23 9. Acknowledgments ................................................23 10. References ....................................................24 10.1. Normative References .....................................24 10.2. Informative References ...................................24 Appendix A. Requirements on a Future Advanced MPTCP API ...........26 A.1. Design Considerations ......................................26 A.2. MPTCP Usage Scenarios and Application Requirements .........27 A.3. Potential Requirements on an Advanced MPTCP API ............29 A.4. Integration with the SCTP Sockets API ......................30
Multipath TCP adds the capability of using multiple paths to a regular TCP session . The motivations for this extension include increasing throughput, overall resource utilization, and resilience to network failure, and these motivations are discussed, along with high-level design decisions, as part of the multipath TCP architecture . MPTCP  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 effects 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.
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 Portable Operating System Interface (POSIX) standard  as much as possible.
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 API.
There can be interactions or incompatibilities of MPTCP with other APIs or sockets interface extensions, which are discussed later in this document. Some network stack implementations, especially on mobile devices, have centralized connection managers or other higher-level APIs to solve multi-interface issues, as surveyed in . Their interaction with MPTCP is outside the scope of this document.
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.
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 .
This document uses the MPTCP terminology introduced in .
Concerning the API towards applications, the following terms are distinguished:
- Legacy API: The interface towards TCP that is currently used by applications. This document explains the effect of MPTCP for such applications, as well as resulting issues.
- 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.
- Advanced API: An API that offers more fine-grained control over the behavior of MPTCP. Its specification is outside the scope of this document.
3. Comparison of MPTCP and Regular TCP
This section discusses the effect of MPTCP on performance as seen by an application, in comparison to what may be expected from the use of regular TCP.
3.1. Effect on Performance
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 provides a connection that performs at least as well as one using single-path TCP. A corresponding congestion control algorithm is described in . The following sections summarize the performance effect of MPTCP as seen by an application.
The most obvious performance improvement that can be expected from the use of MPTCP is an increase in throughput, since MPTCP will pool more than one path (where available) between two endpoints. This will usually provide as great or greater bandwidth for an application, even though exceptions may exist, e.g., due to differences in the congestion control dynamics. For instance, if a new subflow is started, the short-term throughput can be smaller than the theoretical optimum. If there are shared bottlenecks between the flows, then the congestion control algorithms will in most cases ensure that load is evenly spread amongst regular and multipath TCP sessions, so that no end user receives worse performance than if all were using single-path TCP. There are some known corner cases in which an upgrade to MPTCP can affect other users .
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, 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 account.
The transport of MPTCP signaling information results in a small overhead. The use of MPTCP instead of a single TCP connection therefore results in a smaller goodput. Also, 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 negligible in many usage scenarios, and the protocol contains optimizations in its design so that this overhead is minimal.
The benefits of MPTCP regarding throughput and resilience may come at some cost regarding data delivery delay and delay jitter.
If the delays on the constituent subflows of an MPTCP connection differ, the jitter perceivable to an application may appear higher as the data are spread across the subflows. Although MPTCP will ensure in-order delivery to the application, the data delivery could be more bursty than may be usual with single-path TCP, in particular on highly asymmetric paths.
Applications with high real-time requirements might be affected by such a scenario. One possible remedy is to disable MPTCP for such jitter-sensitive applications, either by using the basic API defined in this document, or by other means, such as system policies.
However, the actual delay and jitter of data transport over MPTCP depend on the scheduling and congestion control algorithms used for sending data, as well as the heuristics to establish and shut down subflows. A sender can implement strategies to minimize the delay jitter seen by applications, but this requires an accurate estimation of the path characteristics. If the scheduling decisions are suboptimal or if assumptions about the path characteristics turn out to be wrong, delay jitter may be increased and affect delay-sensitive applications. In general, for a delay-sensitive application, it would be desirable to select an appropriate congestion control algorithm for its traffic needs.
Alternatively, MPTCP could be used in high-reliability, rather than high-throughput, modes of operation, such as by mirroring traffic on subflows, or by only using additional subflows for hot standby. These methods of traffic scheduling would not cause delay variation in the same way. These additional modes, and the selection of alternative scheduling algorithms, would need to be indicated by an advanced API, the specification of which requires further analysis and is outside the scope of this document.
If data transport on one subflow fails, the retransmissions inside MPTCP could affect the delivery delay to the application. Yet, without MPTCP that data or the whole connection might have been lost, and other reliability mechanisms (e.g., application-level recovery) would likely have an even larger delay impact.
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.
Another performance improvement through the use of MPTCP is better 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.
As one special case, MPTCP can be used with only one active subflow at a given point in time. In that case, resilience compared to single-path TCP is improved. MPTCP also supports make-before-break and break-before-make handovers between subflows. In both cases, the MPTCP connection can survive an unavailability or change of an IP address (e.g., due to shutdown of an interface or handover). MPTCP closes or resets the MPTCP connection separately from the individual subflows, as described in .
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 stability 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 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 falls 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. Therefore, an alternative approach could be to try both MPTCP and regular TCP connection attempts at the same time and respond to whichever replies first, in a fashion similar to the "Happy Eyeballs" mechanism for IPv6 . One could also apply a shorter timeout on the MPTCP attempt and thus reduce the setup delay if fallback to regular TCP is needed.
An MPTCP implementation can learn the rate of MPTCP connection attempt successes or failures to particular hosts or networks, and on particular interfaces, and could therefore learn heuristics of when and when not to use MPTCP. A detailed discussion of the various fallback mechanisms, for failures occurring at different points in the connection, is presented in . It must be emphasized that all such heuristics could also fail, and learning can be difficult in certain environments, e.g., if the host is mobile.
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 , and MPTCP's mechanisms to work around these issues are presented and discussed in .
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. Dealing with Multiple Addresses inside Applications
In regular TCP, there is a one-to-one mapping of the sockets 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.
Whilst this doesn't matter for most applications, some applications may need to adapt to the presence of multiple addresses, because implicit assumptions are outdated. In this section, selected examples for resulting issues are discussed. The question of whether such implicit assumptions matter is an application-level decision, and this document only provides general guidance and a basic API to retrieve relevant information.
A few applications require the transport to be along a single path; they 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.
Certain applications store the IP addresses of TCP connections, e.g., by logging mechanisms. Such logging mechanisms will continue to work with MPTCP, but two important aspects have to be mentioned: First, if the application is not aware of MPTCP, it will use the existing interface to the network stack. This implies that an MPTCP-unaware application will track the IP addresses of the first subflow only. IP addresses used by follow-up subflows will be ignored. Second, an MPTCP-aware application can use the basic API described in this document to monitor the IP addresses of all subflows, e.g., for logging mechanisms. If an MPTCP connection uses several subflows, this will possibly imply that data structures have to be adapted and that the amount of data that has to be logged and stored per connection will increase.
An MPTCP implementation may choose to maintain an MPTCP connection even if the IP address of the original subflow is no longer allocated to a host, depending on the policy concerning the first subflow (fate-sharing; see Section 4.2.2). In this case, the IP address exposed to an MPTCP-unaware application can differ from the addresses actually being used by MPTCP. It is even possible that the IP address gets assigned to another host during the lifetime of an MPTCP connection. As further discussed below, this could be an issue if the IP addresses are exchanged by applications, e.g., inside the application protocol. This issue can be addressed by enabling fate- sharing, at the cost of resilience, because the MPTCP connection then cannot close the initial subflow.
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 as well. A detailed threat analysis of MPTCP is published in .
Impact on Transport Layer Security (TLS) is discussed in Section 6.1.
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 (MPTCP-unaware) applications. TCP interacts with other parts of the network stack via different interfaces. The de facto standard API between TCP and applications is the sockets interface. The position of MPTCP in the protocol stack is illustrated in Figure 1.
+-------------------------------+ | Application | +-------------------------------+ ^ | ~~~~~~~~~~|~Sockets 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 socket 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 are unaware of MPTCP and use the existing API towards TCP without any changes. This is the default case.
- MPTCP-aware applications: These applications indicate support for an enhanced MPTCP interface. This document specifies a minimum set of API extensions for such applications.
In the following sections, it is discussed to what 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 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 are used 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 the expression of further preferences in an MPTCP-compatible way (e.g., binding 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 sockets 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 connection continues to exist. Since the subflow addresses can change, 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 and port numbers of the first subflow of an MPTCP connection, in all circumstances, even if that particular subflow is no longer in use.
As the addresses 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). This fate-sharing avoids the reuse of the pair of IP addresses and ports while an MPTCP connection is still in progress, but at the cost of reducing the utility of MPTCP if IP addresses of the first subflow are not available any more (e.g., mobility events). Whether to close the whole MPTCP connection by default SHOULD be controlled by a local policy. Further experiments are needed to investigate its implications.
The functions getpeername() and getsockname() SHOULD also always 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 the Stream Control Transmission Protocol (SCTP). Instead of getpeername() or getsockname(), MPTCP-aware applications can use new API calls, described in Section 5.3, in order to retrieve the full list of address pairs for the subflows in use.
4.3. MPTCP Connection Management
4.3.1. Reaction to Close Call by Application
As described in , MPTCP distinguishes between the closing of subflows (by TCP FIN) and closing the whole MPTCP connection (by Data FIN).
When an application closes a socket, e.g., by calling the close() function, this indicates that the application has no more data to send, like for single-path TCP. MPTCP will then close the MPTCP connection via Data FIN messages. This is completely transparent for an application.
In summary, the semantics of the close() interface for applications are not changed compared to TCP.
4.3.2. Other Connection Management Functions
In general, an MPTCP connection is maintained separately from individual subflows. MPTCP therefore has internal mechanisms to establish, close, or reset the MPTCP connection . These mechanisms provide equivalent functions like single-path TCP and can be mapped accordingly. Therefore, these MPTCP internals do not affect the application interface.
4.4. Socket Option Issues
4.4.1. General Guideline
The existing sockets API includes options that modify the behavior of sockets and their underlying communications protocols. Various socket options exist on the 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.
4.4.2. Disabling of the Nagle Algorithm
One commonly used TCP socket option (TCP_NODELAY) disables the Nagle algorithm as described in . This option is also specified in the POSIX standard . Applications can use this option in combination with MPTCP in exactly 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 (for instance, only transmitting on one path). Specific algorithms are outside the scope of this document.
4.4.3. Buffer Sizing
Applications can explicitly configure send and receive buffer sizes via 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.
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.
4.4.4. Other Socket Options
TCP features the ability to send "Urgent" data, but its use is not recommended in general, and specifically not with MPTCP .
Some network stacks may provide additional implementation-specific socket options or interfaces that affect TCP's behavior. In such cases, implementers must ensure that these options do not interfere with the MPTCP interface.
4.5. 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.
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 saturated.
4.6. Summary of Advice to Application Developers
- 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.
- Socket buffer dimensioning: Multipath transport requires larger buffers in the receiver for resequencing, as already explained. Applications should use reasonable 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 .
- 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 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 not make sense to enable the TCP_NODELAY socket option, nor is it reasonable to use many small socket send() calls each 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 behavior.
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. But a receiver may also have preferences about data transfer choices, and it may have performance requirements as well. 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:
- 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 sockets 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 handle both 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 be 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 to obtain information on the pairs of 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 replaces the (address, port) pair for a connection identifier in single-path TCP, which is no longer static in MPTCP.
An application can continue to use getpeername() or getsockname() in addition to the basic MPTCP API. Both functions return the corresponding addresses of the first subflow, as already explained.
5.3. Sockets Interface Extensions by the Basic MPTCP API
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.
- TCP_MULTIPATH_ENABLE: Enable/disable MPTCP
- TCP_MULTIPATH_ADD: Bind MPTCP to a set of given local addresses, or add a set of new local addresses to an existing MPTCP connection
- TCP_MULTIPATH_REMOVE: Remove a local address from an MPTCP connection
- TCP_MULTIPATH_SUBFLOWS: Get the pairs of addresses currently used by the MPTCP subflows
- TCP_MULTIPATH_CONNID: Get the local connection identifier for this MPTCP connection
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. The data types are listed for information only. In addition to IP addresses, an application MAY also indicate TCP port numbers, as further detailed below.
+------------------------+-----+-----+------------------------------+ | Name | Get | Set | Data type | +------------------------+-----+-----+------------------------------+ | TCP_MULTIPATH_ENABLE | o | o | boolean | | TCP_MULTIPATH_ADD | | o | list of addresses | | | | | (and ports) | | TCP_MULTIPATH_REMOVE | | o | list of addresses | | | | | (and ports) | | TCP_MULTIPATH_SUBFLOWS | o | | list of pairs of addresses | | | | | (and ports) | | TCP_MULTIPATH_CONNID | o | | integer | +------------------------+-----+-----+------------------------------+
Table 1: MPTCP Socket Operations
There are restrictions on when these new socket operations can be used:
- 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.
- TCP_MULTIPATH_ADD: This operation can be applied both before connection setup and 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.
- TCP_MULTIPATH_REMOVE: This operation can be applied both before connection setup and during a connection. In both cases, it removes an address from the list of local addresses that may be used by subflows.
- TCP_MULTIPATH_SUBFLOWS: This value is read-only and can only be used after connection setup.
- 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 the value "true". In this case, the MPTCP implementation SHOULD try to negotiate MPTCP for that connection. Note that multipath transport will not necessarily be enabled, as it requires support at both end systems, no middleboxes on the path that would prevent any additional signaling, and at least one endpoint with multiple addresses.
Building on the backward compatibility specified in Section 4.2.1, if an application enables MPTCP but binds to a specific address or interface, MPTCP MUST be enabled, but MPTCP MUST respect the application's choice and only use addresses that are explicitly provided by the application. Note that it would be possible for an application to use the legacy bindings and then expand on them by using TCP_MULTIPATH_ADD. Note also that it is possible for more than one local address to be initially available to MPTCP in this case, if an application has bound to a specific interface with multiple addresses.
An application can disable MPTCP by setting TCP_MULTIPATH_ENABLE to a value of "false". 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 "false" then means lack of MPTCP support. A value of "true" means that MPTCP is supported.
5.3.3. Binding MPTCP to Specified Addresses
Before connection establishment, an application can use the TCP_MULTIPATH_ADD function to indicate a set of local IP addresses that MPTCP may bind to. The parameter of the function is a list of addresses in a corresponding data structure. By extension, this operation will also control the list of addresses that can be advertised to the peer via MPTCP signaling.
If an application binds to a specific address or interface, it is not required to use the TCP_MULTIPATH_ADD operation for that address. As explained in Section 5.3.2, MPTCP MUST only use the explicitly specified addresses in that case.
An application MAY also indicate a TCP port number that, if specified, MPTCP MUST attempt to bind to. The port number MAY be different than the one used by existing subflows. If no port number is provided by the application, the port number is automatically selected by the MPTCP implementation, and it is RECOMMENDED that it is the same across all subflows.
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 that can be signaled to the peer. It should be noted that this signal is only a hint, and an MPTCP implementation MAY select only a subset of the addresses.
The TCP_MULTIPATH_REMOVE operation can be used to remove a local address, or 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 removal.
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 in an MPTCP connection by means of the read-only TCP_MULTIPATH_SUBFLOWS operation.
The return value is a list of pairs of tuples of IP address and TCP port number. In one pair, the first tuple refers to the local IP address and the local TCP port, and the second one to the remote IP address and remote TCP port used by the subflow. The list MUST only include established subflows. Both addresses in each pair MUST be either IPv4 or IPv6.
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 SHOULD be an integer number and SHOULD be locally unique (e.g., the MPTCP token).
6. Other Compatibility Issues
6.1. Usage of TLS over MPTCP
Transport Layer Security (TLS)  may be used over MPTCP's basic API. When TLS compares any addresses used by MPTCP against names or addresses present in X.509 certificates  , it MUST only compare them with the address that MPTCP used to start the initial subflow as presented to TLS. The addresses used for subsequent subflows need not to be compared against any TLS certificate information. Finer-grained control would require an advanced API or proactive subflow management via the basic API.
6.2. Usage of the SCTP Sockets API
For dealing with multihoming, several sockets API extensions have been defined for SCTP . As MPTCP realizes multipath transport from and to multihomed end systems, some of these interface function calls are actually applicable to MPTCP in a similar way.
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 than that of 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 sockets 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. Integration with the SCTP API is outside the scope of the basic API.
6.3. 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 . Care should be taken that the use of MPTCP not conflict with the overlapping features of other APIs:
- SHIM API : This API specifies sockets API extensions for the multihoming shim layer.
- HIP API : The Host Identity Protocol (HIP) also results in a new API.
- API for Mobile IPv6 : For Mobile IPv6, a significantly extended sockets API exists as well (in addition to API extensions for IPv6 ).
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 for further study.
6.4. Interactions with DNS
In multihomed or multiaddressed environments, there are various issues that are not specific to MPTCP but have to be considered as well. These problems are summarized in .
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.
7. Security Considerations
This document first defines the behavior of the standard TCP/IP API for MPTCP-unaware applications. In general, enabling MPTCP has some security implications for applications, which are introduced in Section 5.3.3, and these threats are further detailed in . The protocol specification of MPTCP  defines several mechanisms to protect MPTCP against those attacks.
The syntax and semantics of the API for MPTCP-unaware applications does not change. However, assumptions that non-MPTCP-aware applications may make on the data retrieved by the backward- compatible API are discussed in Section 4.2.2. System administrators may wish to disable MPTCP for certain applications that signal addresses, or make security decisions (e.g., opening firewall holes), based on responses to such queries.
In addition, the basic MPTCP API for MPTCP-aware applications defines functions that provide an equivalent level of control and information as exists for regular TCP. This document does not mandate a specific implementation of the basic MPTCP API. The implementation should be designed not to affect memory management assumptions in existing code. Implementors should take into account that data structures will be more complex than for standard TCP, e.g., when multiple subflow addresses have to be stored. When dealing with such data structures, care is needed not to add security vulnerabilities to applications.
New functions enable adding and removing local addresses from an MPTCP connection (TCP_MULTIPATH_ADD and TCP_MULTIPATH_REMOVE). These functions don't add security threats if the MPTCP stack verifies that the addresses provided by the application are indeed available as source addresses for subflows.
However, applications should use the TCP_MULTIPATH_ADD function with care, as new subflows might get established to those addresses. Furthermore, it could result in some form of information leakage since MPTCP might advertise those addresses to the other connection endpoint, which could learn IP addresses of interfaces that are not visible otherwise.
Use of different addresses should not be assumed to lead to use of different paths, especially for security purposes.
MPTCP-aware applications should also take care when querying and using information about the addresses used by subflows (TCP_MULTIPATH_SUBFLOWS). As MPTCP can dynamically open and close subflows, a list of addresses queried once can get outdated during the lifetime of an MPTCP connection. Then, the list may contain invalid entries, i.e., addresses that are not used any more or that might not even be assigned to that host any more. Applications that want to ensure that MPTCP only uses a certain set of addresses should explicitly bind to those addresses.
One specific example is the use TLS on top of MPTCP. Corresponding guidance can be found in Section 6.1.
This document discusses MPTCP's implications and its performance impact on applications. In addition, it specifies a basic MPTCP API. For legacy applications, it is ensured that the existing sockets API continues to work. MPTCP-aware applications can use the basic MPTCP API that provides some control over the transport layer equivalent to regular TCP.
The authors sincerely thank the following people for their helpful comments and reviews of the document: Philip Eardley, Lavkesh Lahngir, John Leslie, Costin Raiciu, Michael Tuexen, and Javier Ubillos.
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 was previously supported by Roke Manor Research and by Trilogy (http://www.trilogy-project.org/), a research project (ICT-216372) partially funded by the European Community under its Seventh Framework Program.
10.1. Normative References
 Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981.  Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Ford, A., Raiciu, C., Handley, M., Barre, S., and J. Iyengar, "Architectural Guidelines for Multipath TCP Development", RFC 6182, March 2011.  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, "TCP Extensions for Multipath Operation with Multiple Addresses", RFC 6824, January 2013.  Bagnulo, M., "Threat Analysis for TCP Extensions for Multipath Operation with Multiple Addresses", RFC 6181, March 2011.  Raiciu, C., Handley, M., and D. Wischik, "Coupled Congestion Control for Multipath Transport Protocols", RFC 6356, October 2011.  "IEEE Standard for Information Technology -- Portable Operating System Interface (POSIX) Base Specifications, Issue 7", IEEE Std. 1003.1-2008, 2008.
10.2. Informative References
 Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, "Advanced Sockets Application Program Interface (API) for IPv6", RFC 3542, May 2003.  Chakrabarti, S. and E. Nordmark, "Extension to Sockets API for Mobile IPv6", RFC 4584, July 2006.  Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto, "Sockets Application Program Interface (API) for Multihoming Shim", RFC 6316, July 2011.  Komu, M. and T. Henderson, "Basic Socket Interface Extensions for the Host Identity Protocol (HIP)", RFC 6317, July 2011.  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V. Yasevich, "Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)", RFC 6458, December 2011.  Blanchet, M. and P. Seite, "Multiple Interfaces and Provisioning Domains Problem Statement", RFC 6418, November 2011.  Wasserman, M. and P. Seite, "Current Practices for Multiple- Interface Hosts", RFC 6419, November 2011.  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with Dual-Stack Hosts", RFC 6555, April 2012.  Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.  Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008.  Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, March 2011.  Sarolahti, P., "Multi-address Interface in the Socket API", Work in Progress, March 2010.  Khalili, R., Gast, N., Popovic, M., and J. Le Boudec, "Performance Issues with MPTCP", Work in Progress, February 2013.  Honda, M., Nishida, Y., Raiciu, C., Greenhalgh, A., Handley, M., and H. Tokuda, "Is it Still Possible to Extend TCP?", Proc. ACM Internet Measurement Conference (IMC), November 2011.
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 offer much control of the MPTCP implementation's behavior. 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 optimized 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 of whether a receiving application could influence the sending policy. A better integration with TLS could be another relevant objective (cf. Section 6.1) that requires further work.
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 uses, e.g., the connectbyname() function) would be useful for numerous purposes, independent of MPTCP.
It has also been suggested that a separate address family called AF_MULTIPATH  be used. This separate address family could be used to exchange multiple addresses between an application and the standard sockets API, but it would be a more fundamental change compared to the basic API described in this document.
This appendix documents a list of potential usage scenarios and requirements for the advanced API. The specification and implementation of a corresponding API are outside the scope of this document.
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 maximizing utilization 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), 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 subflows. 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 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 that can overflow into an additional subflow; and secondly, there is the single path with hot standby, whereby an application may want an alternative backup subflow in order to improve resilience. In case 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 maximizing the use of all subflows.
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:
- High bandwidth
- Low latency and jitter stability
- High reliability
These are not necessarily disjoint, since there are also broadband interactive applications that require both high-speed bulk data traffic and a low latency and jitter.
In an advanced API, applications could provide high-level guidance to the MPTCP implementation concerning these performance requirements, for instance, which requirement is considered to be the most important. The MPTCP stack would then use internal mechanisms to fulfill this abstract indication of a desired service, as far as possible. This would affect the assignment of data (including retransmissions) to existing subflows (e.g., 'use all in parallel', 'use as overflow', 'hot standby', 'duplicate traffic') as well as the decisions regarding when to set up additional subflows to which addresses. In both cases, different policies can exist, which can be expected to be implementation-specific.
Therefore, an advanced API could provide a mechanism for 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 as to 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.
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 to 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 specific 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 the form of a profile. REQ9: An application should be able to indicate communication characteristics, e.g., the expected amount of data to be sent, the expected duration of the connection, or the expected rate at which data is provided. Applications may in some cases be able to forecast such properties. If so, such information could be an additional input parameter for heuristics inside the MPTCP implementation, which could be useful, for example, to decide when to set up additional subflows. REQ10: 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. REQ11: 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. For instance, an application might want to use certain subflows as backup only. REQ12: 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. REQ13: An application should be able to control the use of fate- sharing of the MPTCP connection and the initial subflow, e.g., to overwrite system policies. REQ14: An application should be able to register for callbacks to be informed of changes to subflows on an MPTCP connection. This "push" interface would allow the application to make timely logging and configuration changes, if required, and would avoid frequent polling of information.
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.
A subset of these functions might also be implemented system-wide or by other configuration mechanisms. These implementation details are left for further study.
A.4. Integration with the SCTP Sockets API
The advanced API may also integrate or use the SCTP sockets API. The following functions that are defined for SCTP have functionality similar to the basic MPTCP API:
The syntax and semantics of these functions are described in .
A potential objective for the advanced API is to provide a consistent MPTCP and SCTP interface to the application. This is left for further study.
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