Internet Engineering Task Force M. Scharf
Internet-Draft Alcatel-Lucent Bell Labs
Intended status: Informational A. Ford
Expires: April 18, 2010 Roke Manor Research
October 15, 2009
MPTCP Application Interface Considerations
draft-scharf-mptcp-api-00
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Abstract
Multipath TCP (MPTCP) adds the capability of using multiple paths to
a regular TCP session. Even though it is designed to be totally
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backward compatible, the data transport differs to the existing TCP,
and there are several additional degrees of freedom that affect
applications. 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Impact of MPTCP on Applications . . . . . . . . . . . . . . . 4
3.1. Performance Improvement . . . . . . . . . . . . . . . . . 4
3.1.1. Throughput . . . . . . . . . . . . . . . . . . . . . . 4
3.1.2. Delay . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1.3. Resilience . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Potential Problems . . . . . . . . . . . . . . . . . . . . 5
3.2.1. Impact of Middleboxes . . . . . . . . . . . . . . . . 5
3.2.2. Outdated Implicit Assumptions . . . . . . . . . . . . 5
4. Implications of MPTCP on Existing Interfaces . . . . . . . . . 6
4.1. Overview of the Network Stack . . . . . . . . . . . . . . 6
4.2. Impact on the Use of Socket Options . . . . . . . . . . . 6
4.3. Impact on Existing Other System-wide Settings . . . . . . 7
4.4. Impact on Existing API Calls . . . . . . . . . . . . . . . 8
4.5. Impact on Existing Sockets API Enhancements . . . . . . . 8
5. Application Requirements . . . . . . . . . . . . . . . . . . . 8
5.1. MPTCP Usage Scenarios . . . . . . . . . . . . . . . . . . 8
5.2. Requirements on API Extensions . . . . . . . . . . . . . . 10
6. Specification of API Extensions for MPTCP . . . . . . . . . . 11
6.1. Design Considerations . . . . . . . . . . . . . . . . . . 11
6.2. Overview of Sockets Interface Extensions . . . . . . . . . 12
6.3. Detailed Description . . . . . . . . . . . . . . . . . . . 12
6.3.1. TCP_MP_ENABLE . . . . . . . . . . . . . . . . . . . . 12
6.3.2. TCP_MP_MAXSUBFLOW . . . . . . . . . . . . . . . . . . 12
6.4. Usage examples . . . . . . . . . . . . . . . . . . . . . . 12
6.5. Discussion of Interactions . . . . . . . . . . . . . . . . 12
6.6. Advice to Application Developers . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . . 14
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1. Introduction
Multipath TCP (MPTCP) [4] adds the capability of using multiple paths
to a regular TCP session [1]. MPTCP offers the same reliable, in-
order, byte-stream transport like TCP and is designed to be backward-
compatible. 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. Furthermore, it specifies
an extended Application Programming Interface (API) describing how
applications can exploit additional features of multipath transport.
While MPTCP needs to be usable without any application changes, this
API is an optional extension that provides access to multipath
information and enables control of some aspects of the MPTCP
implementation's behaviour.
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, or 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. The syntax and semantics of the specification is in line with
the Posix standard [5] as much as possible.
There are various related extensions of the sockets interface: [7]
specifies sockets API extensions for the multihoming shim layer. The
API enables interactions between applications and the multihoming
shim layer for advanced locator management, and access to information
about failure detection and path exploration. Other experimental
extensions to the sockets API are defined for the Host Identity
Protocol (HIP) [8] in order to manage the bindings of identifiers and
locator. There can be interactions of these APIs with MPTCP. Other
related API extensions exist for IPv6 [6]. The MPTCP API also has
some similarity to the SCTP socket API [9].
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 [4].
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3. Impact of MPTCP on Applications
3.1. Performance Improvement
One of the key goals of adding multipath capability to TCP is to
improve the performance of a transport connection. 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.
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 correctly spread and the end user receives
no 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.
A small overhead will be present, through the use of MPTCP options,
and as such the impact of this when there are multiple subflows over
a shared bottleneck (or bottlenecks) should be considered, but is
FFS, and will be part of the definition of a suitable congestion
control algorithm.
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 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 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
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consider how multiple subflows may affect its measurements, and thus
how it may wish to respond. In such a case, an application may wish
to express its scheduling preferences, as described later in this
document.
3.1.3. Resilience
The use of multiple subflows simultaneously means that, if one should
fail, all traffic will move to the remaining subflow(s), and
additionally any lost packets can be retransmitted on these subflows.
Subflow failure may be caused by issues within the network, which an
application would be unaware of, or interface failure on the node.
An application may, under certain circumstances, be in a position to
be aware of such failure (e.g. by radio signal strength, or simply an
interface enabled flag), and so must not make assumptions of an MPTCP
flow's stablity based on this. MPTCP will never override an
application's request for a given interface, however, so the cases
where this issue may be applicable are limited.
3.2. Potential Problems
3.2.1. Impact of Middleboxes
MPTCP has been designed in order to pass through the majority of
middleboxes, for example through its ability to open subflows in
either direction, and through its use of a data-level sequence
number.
Nevertheless some middleboxes may still refuse to pass MPTCP messages
due to the presence of TCP options. If this is the case, MPTCP
should fall back to regular TCP. Although this will not create a
problem for the application (its communication will be set up either
way), there may be additional (and indeed, user-perceivable) delay
while the first handshake fails.
Empirical evidence suggests that new TCP options can successfully be
used on most paths in the Internet. But they can also have other
unexpected implications. For instance, intrusion detection systems
could be triggered.
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. One example are
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monitoring tools that want to measure the available bandwidth on a
path.
Security implications: TODO
4. Implications of MPTCP on Existing Interfaces
4.1. Overview of the Network Stack
MPTCP is an extension of TCP. TCP interacts with other parts of the
network stack by different interfaces. The de facto standard API
between TCP and applications is the sockets interface. The position
of MPTCP in the protocol stack can be illustrated as follows:
+-------------------------------+
| Application |
+-------------------------------+
^ |
~~~~~~~~~~~|~Socket Interface|~~~~~~~~~~~
| v
+-------------------------------+
| MPTCP |
+ - - - - - - - + - - - - - - - +
| Subflow (TCP) | Subflow (TCP) |
+-------------------------------+
| IP | IP |
+-------------------------------+
MPTCP protocol stack
In general, MPTCP affects 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.
A design objective of MPTCP is that applications can continue to use
the established sockets API without any changes. Still, some aspects
have to be taken into account: In MPTCP, there is a one-to-many
mapping between the socket endpoint and the subflows. As a
consequence, the existing sockets interface functions cannot
configure each subflow individually. In order to be backward
compatible, existing APIs therefore should apply to all subflows
within one connection, as far as possible.
4.2. Impact on the Use of Socket Options
The sockets API includes options that modify the behavior of sockets
and their underlying communications protocols. Various socket
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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.
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 [5]. 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 is assumed that any application that binds to INADDR_ANY does not
care which addresses are in use locally, and so MPTCP can freely set
up multiple subflows on such a connection. 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. The extended sockets API
will allow applications to express such preferences in an MPTCP-
compatible way (e.g. bind to a subset of devices only).
Some network stacks also provide other implementation-specific socket
options that affect TCP's behavior. If a network stack supports
MPTCP, it must be ensured that these options do not interfere.
4.3. Impact on Existing Other System-wide Settings
TODO: Socker buffer dimensioning: Requirement of larger resequencing
buffer space
TODO: Could also affect interface configuration, information in local
routing table, buffer management, etc.
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4.4. Impact on Existing API Calls
There is an issue, to be resolved, regarding what data should be
returned on a getpeername() or getsockname() request on the socket,
i.e. to retrieve the IP address of the peer or of the local socket.
Our initial thinking is that it should return the IP address pair
that was first connected to, in all circumstances, even if that
particular subflow is no longer in use. 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.
4.5. Impact on Existing Sockets API Enhancements
The use of MPTCP can interact with various related sockets API
extensions:
o SHIM API [7]: This API specifies sockets API extensions for the
multihoming shim layer. TODO: Potential interactions will be
addressed in a future revision of this memo.
o HIP API [8]: The Host Identity Protocol (HIP) also results in a
new API. TODO: Potential interactions will be addressed in a
future revision of this memo.
o IPv6 API [6]: The API for IPv6 leaves open the interaction with
TCP.
5. Application Requirements
5.1. MPTCP Usage Scenarios
Applications that use TCP 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 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.
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The choice between these two options affects the scheduler in terms
of whether traffic should be, by default, sent on one subflow or
across both. Even if the total bandwidth required is less than that
available on an individual path, it is desirable to spread this load
to reduce stress on potential bottlenecks, and this is why this
method should be the default. It is recognised, however, that this
may not benefit all applications that require latency/jitter
stability, so the other (single path) option is provided.
In the case of the latter option, however, a further question arises:
should additional subflows be used whenever the primary subflow is
overloaded, or only when the primary path fails (hot-standby)? In
other words, is latency stability or bandwidth more important to the
application?
We therefore divide this option into two: Firstly, there is the
single path which can overflow into an additional subflow; and
secondly there is single-path with hot-standby, whereby an
application may want an alternative backup subflow in order to
improve resilience. In case that data delivery on the first subflow
fails, the data transport could immediately be continued on the
second subflow, which is idle otherwise.
In summary, there are three different "application profiles"
concerning the use of MPTCP:
1. Bulk data transport
2. Latency-sensitive transport (with overflow)
3. Latency-sensitive transport (hot-standby)
These different application profiles affect both the management of
subflows, i. e., the decisions when to set up additional subflows to
which addresses as well as the assignment of data (including
retransmissions) to the existing subflows. In both cases different
policies can exist.
These profiles have been defined to cover the common application use
cases. It is not possible to cover all application requirements,
however, and as such applications should additionally have finer
control over subflow and scheduling should they require.
Requirements are TBD.
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,
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is FFS.
5.2. 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. In
order to support the large base of applications using the original
API, an application must be able to continue to use all standard
socket interface functions when run on a system supporting MPTCP.
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., by the use of binding to a specific address versus all
addresses).
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 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.
REQ5: 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).
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REQ6: Set/get application profile, as discussed in the previous
section.
REQ7: Constrain the maximum number of subflows to be used by an
MPTCP connection. (Or just infer from application profile?)
REQ8: Request a change in scheduling between subflows? (i.e. a more
granular version of application profile?)
REQ9: Request a change in the number of subflows in use, thus
triggering removal or addition of subflows. (A finer control
granularity would be: Request the establishment of a new
subflow to a provided destination, and request the
termination of a specified, existing subflow.)
REQ10: Control automatic establishment/termination of subflows?
There could be different configurations of the path manager,
e.g., 'try ASAP', 'wait until there is a bunch of data, etc.
(Tied to application profile?)
REQ11: Set/get preferred subflows or subflow usage policies? There
could be different configurations of the multipath scheduler,
e.g., 'all-or-nothing', 'overflow', etc. (Again, tied to
application profile).
REQ12: Set/get sporadic sending of segments on unused paths
("keepalives").
REQ13: An application should be able to modify the MPTCP
configuration while communication is ongoing, i.e., after
establishment of the MPTCP connection.
6. Specification of API Extensions for MPTCP
6.1. 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. TODO: Any control on the receiver side?
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As this document specifies sockets API extensions, it is written so
that the syntax and semantics are in line with the Posix standard [5]
as much as possible.
6.2. 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.
o TCP_MP_ENABLE: MPTCP enabled/disabled
o TCP_MP_MAXSUBFLOWS: Get/set maximum number of paths
o ...
TODO: Table of socket options
6.3. Detailed Description
6.3.1. TCP_MP_ENABLE
TODO: Description
6.3.2. TCP_MP_MAXSUBFLOW
TODO: Description
6.4. Usage examples
TODO: Example C code for one or more API functions
6.5. Discussion of Interactions
TODO: Some of the socket options defined in this document are
overlapping with existing sockets API and care should be taken for
the usage not to confuse with the overlapping features.
TODO: Interactions with system-wide settings?
6.6. Advice to Application Developers
TODO: E. g. use primary addresses and connection identifiers in a
tuple instead of the traditional 5-tuple
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7. Security Considerations
Will be added in a later version of this document.
8. IANA Considerations
No IANA considerations.
9. Conclusion
This document discusses MPTCP's application implications and
specifies an extended API. From an architectural point of view,
MPTCP offers additional degrees of freedom concerning the transport
of data. The extended sockets API allows 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.
10. Acknowledgments
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., Handley, M., and S. Barre, "TCP Extensions
for Multipath Operation with Multiple Addresses",
draft-ford-mptcp-multiaddressed-01 (work in progress),
July 2009.
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[5] "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
[6] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, "Advanced
Sockets Application Program Interface (API) for IPv6", RFC 3542,
May 2003.
[7] Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto, "Socket
Application Program Interface (API) for Multihoming Shim",
draft-ietf-shim6-multihome-shim-api-09 (work in progress),
July 2009.
[8] Komu, M. and T. Henderson, "Basic Socket Interface Extensions
for Host Identity Protocol (HIP)", draft-ietf-hip-native-api-09
(work in progress), September 2009.
[9] Stewart, R., Poon, K., Tuexen, M., Yasevich, V., and P. Lei,
"Sockets API Extensions for Stream Control Transmission Protocol
(SCTP)", draft-ietf-tsvwg-sctpsocket-19 (work in progress),
February 2009.
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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