Internet Engineering Task Force M. Scharf
Internet-Draft Alcatel-Lucent Bell Labs
Intended status: Informational A. Ford
Expires: September 9, 2010 Roke Manor Research
March 8, 2010
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.
Status of This Memo
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Copyright Notice
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Copyright (c) 2010 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Comparison of MPTCP and Regular TCP . . . . . . . . . . . . . 5
3.1. Performance Impact . . . . . . . . . . . . . . . . . . . . 5
3.1.1. Throughput . . . . . . . . . . . . . . . . . . . . . . 5
3.1.2. Delay . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1.3. Resilience . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Potential Problems . . . . . . . . . . . . . . . . . . . . 6
3.2.1. Impact of Middleboxes . . . . . . . . . . . . . . . . 7
3.2.2. Outdated Implicit Assumptions . . . . . . . . . . . . 7
3.2.3. Security Implications . . . . . . . . . . . . . . . . 7
4. Operation of MPTCP with Legacy Applications . . . . . . . . . 7
4.1. Overview of the MPTCP Network Stack . . . . . . . . . . . 7
4.2. Usage of Addresses Inside Applications . . . . . . . . . . 8
4.3. Usage of Existing Socket Options . . . . . . . . . . . . . 9
4.4. Default Enabling of MPTCP . . . . . . . . . . . . . . . . 10
4.5. Known Remaining Issues with Legacy Applications . . . . . 10
5. Minimal API Enhancements for MPTCP-aware Applications . . . . 11
5.1. Indicating MPTCP Awareness . . . . . . . . . . . . . . . . 11
5.2. Modified Address Handling . . . . . . . . . . . . . . . . 11
5.3. Usage of a New Address Family . . . . . . . . . . . . . . 11
6. Extended MPTCP API . . . . . . . . . . . . . . . . . . . . . . 11
6.1. MPTCP Usage Scenarios and Application Requirements . . . . 11
6.2. Requirements on API Extensions . . . . . . . . . . . . . . 13
6.3. Design Considerations . . . . . . . . . . . . . . . . . . 15
6.4. Overview of Sockets Interface Extensions . . . . . . . . . 15
6.5. Detailed Description . . . . . . . . . . . . . . . . . . . 16
6.5.1. TCP_MP_ENABLE . . . . . . . . . . . . . . . . . . . . 16
6.5.2. TCP_MP_SUBFLOWS . . . . . . . . . . . . . . . . . . . 16
6.5.3. TCP_MP_PROFILE . . . . . . . . . . . . . . . . . . . . 16
6.6. Usage examples . . . . . . . . . . . . . . . . . . . . . . 17
6.7. Interactions and Incompatibilities with other
Multihoming Solutions . . . . . . . . . . . . . . . . . . 17
6.8. Other Advice to Application Developers . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
9. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 18
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1. Normative References . . . . . . . . . . . . . . . . . . . 18
11.2. Informative References . . . . . . . . . . . . . . . . . . 19
Appendix A. Change History of the Document . . . . . . . . . . . 19
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1. Introduction
Multipath TCP (MPTCP) adds the capability of using multiple paths to
a regular TCP session [1]. The motivations for this extension
include increasing throughput, overall resource utilisation, and
resilience to network failure, and these motivations are discussed,
along with high-level design decisions, as part of the MPTCP
architecture [4]. MPTCP [5] offers the same reliable, in-order,
byte-stream transport as TCP, and is designed to be backward-
compatible with both applications and the network layer. It requires
support inside the network stack of both endpoints. This document
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]
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]. Their interaction with
MPTCP is outside the scope of this note.
There are also various related extensions of the sockets interface:
[11] 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] in order to manage the bindings of
identifiers and locator. Other related API extensions exist for IPv6
[10] and SCTP [13]. There can be interactions or incompatibilities
of these APIs with MPTCP, which are discussed later in this document.
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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].
This document uses the terminology introduced in [5].
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]. 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
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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
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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].
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
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API between TCP and applications is the sockets interface. The
position of MPTCP in the protocol stack can be illustrated in
Figure 1.
+-------------------------------+
| Application |
+-------------------------------+
^ |
~~~~~~~~~~~|~Socket Interface|~~~~~~~~~~~
| v
+-------------------------------+
| MPTCP |
+ - - - - - - - + - - - - - - - +
| Subflow (TCP) | Subflow (TCP) |
+-------------------------------+
| IP | IP |
+-------------------------------+
Figure 1: MPTCP protocol stack
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:
o Legacy applications: These applications use the existing API
towards TCP without any changes. This is the default case.
o MPTCP-aware applications: These applications indicate support for
an enhance MPTCP interface.
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,
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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
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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]. This option is also specified in the
Posix standard [8]. Applications can use this option in combination
with MPTCP exactly in the same way. It then 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.
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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]. 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].
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.
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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
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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:
o Consistency with existing sockets APIs must be maintained as far
as possible. In order to support the large base of applications
using the original API, a legacy application must be able to
continue to use standard socket interface functions when run on a
system supporting MPTCP. Also, MPTCP-aware applications should be
able to access the socket without any major changes.
o Sockets API extensions must be minimized and independent of an
implementation.
o 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.
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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?)
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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]
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:
o TCP_MP_ENABLE: Enable/disable MPTCP
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o TCP_MP_SUBFLOWS: Get the addresses currently used by the MPTCP
subflows, optionally complemented by further information such as
usage ratio
o TCP_MP_PROFILE: Get/set the MPTCP profile
o ...
Table Table 1 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.
+-----------------+-----+-----+-----------+
| Option name | Get | Set | Data type |
+-----------------+-----+-----+-----------+
| TCP_MP_ENABLE | o | o | int |
| TCP_MP_SUBFLOWS | o | | *1 |
| TCP_MP_PROFILE | o | o | int |
| ... | | | |
+-----------------+-----+-----+-----------+
*1: Data structure containing the addresses of each subflow, plus
further information
Table 1: Socket options for MPTCP
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
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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:
o SHIM API [11]: This API specifies sockets API extensions for the
multihoming shim layer.
o HIP API [12]: 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]. 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
o 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.
o 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.
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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
[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[2] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Ford, A., Raiciu, C., Barre, S., and J. Iyengar, "Architectural
Guidelines for Multipath TCP Development",
draft-ietf-mptcp-architecture-00 (work in progress),
March 2010.
[5] Ford, A., Raiciu, C., and M. Handley, "TCP Extensions for
Multipath Operation with Multiple Addresses",
draft-ford-mptcp-multiaddressed-02 (work in progress),
October 2009.
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[6] Bagnulo, M., "Threat Analysis for Multi-addressed/Multi-path
TCP", draft-ietf-mptcp-threat-00 (work in progress),
February 2010.
[7] Raiciu, C., Handley, M., and D. Wischik, "Coupled Multipath-
Aware Congestion Control", draft-raiciu-mptcp-congestion-00
(work in progress), October 2009.
[8] "IEEE Std. 1003.1-2008 Standard for Information Technology --
Portable Operating System Interface (POSIX). Open Group
Technical Standard: Base Specifications, Issue 7, 2008.".
11.2. Informative References
[9] Sarolahti, P., "Multi-address Interface in the Socket API",
draft-sarolahti-mptcp-af-multipath-01 (work in progress),
March 2010.
[10] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, "Advanced
Sockets Application Program Interface (API) for IPv6",
RFC 3542, May 2003.
[11] Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto, "Socket
Application Program Interface (API) for Multihoming Shim",
draft-ietf-shim6-multihome-shim-api-13 (work in progress),
February 2010.
[12] Komu, M. and T. Henderson, "Basic Socket Interface Extensions
for Host Identity Protocol (HIP)", draft-ietf-hip-native-api-12
(work in progress), January 2010.
[13] Stewart, R., Poon, K., Tuexen, M., Yasevich, V., and P. Lei,
"Sockets API Extensions for Stream Control Transmission
Protocol (SCTP)", draft-ietf-tsvwg-sctpsocket-21 (work in
progress), February 2010.
[14] Wasserman, M., "Current Practices for Multiple Interface
Hosts", draft-ietf-mif-current-practices-00 (work in progress),
October 2009.
Appendix A. Change History of the Document
Changes compared to version 00:
o Distinction between legacy and MPTCP-aware applications
o Guidance concerning default enabling, reaction to the shutdown of
the first sub-flow, etc.
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o Reference to a potential use of AF_MULTIPATH
o Additional references to related work
Authors' Addresses
Michael Scharf
Alcatel-Lucent Bell Labs
Lorenzstrasse 10
70435 Stuttgart
Germany
EMail: michael.scharf@alcatel-lucent.com
Alan Ford
Roke Manor Research
Old Salisbury Lane
Romsey, Hampshire SO51 0ZN
UK
Phone: +44 1794 833 465
EMail: alan.ford@roke.co.uk
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