Multipath TCP (MPTCP) Application Interface Considerations
RFC 6897
| Document | Type |
RFC - Informational
(March 2013; No errata)
Was draft-ietf-mptcp-api (mptcp WG)
|
|
|---|---|---|---|
| Authors | Michael Scharf , Alan Ford | ||
| Last updated | 2015-10-14 | ||
| Replaces | draft-scharf-mptcp-api | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text html pdf htmlized bibtex | ||
| Reviews | |||
| Stream | WG state | WG Document | |
| Document shepherd | No shepherd assigned | ||
| IESG | IESG state | RFC 6897 (Informational) | |
| Action Holders |
(None)
|
||
| Consensus Boilerplate | Unknown | ||
| Telechat date | |||
| Responsible AD | Martin Stiemerling | ||
| Send notices to | (None) | ||
Internet Engineering Task Force (IETF) M. Scharf
Request for Comments: 6897 Alcatel-Lucent Bell Labs
Category: Informational A. Ford
ISSN: 2070-1721 Cisco
March 2013
Multipath TCP (MPTCP) Application Interface Considerations
Abstract
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.
Scharf & Ford Informational [Page 1]
RFC 6897 MPTCP API March 2013
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................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
Scharf & Ford Informational [Page 2]
RFC 6897 MPTCP API March 2013
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
1. Introduction
Multipath TCP adds the capability of using multiple paths to a
regular TCP session [1]. The motivations for this extension include
increasing throughput, overall resource utilization, and resilience
to network failure, and these motivations are discussed, along with
high-level design decisions, as part of the multipath TCP
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 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
Scharf & Ford Informational [Page 3]
RFC 6897 MPTCP API March 2013
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 [8]. 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
[8] 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
[15]. 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.
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 MPTCP terminology introduced in [5].
Scharf & Ford Informational [Page 4]
RFC 6897 MPTCP API March 2013
Concerning the API towards applications, the following terms are
distinguished:
o 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.
o Basic API: A simple extension of TCP's interface for applications
that are aware of MPTCP. This document abstractly describes this
interface, which provides access to multipath address information
and a level of control equivalent to regular TCP.
o Advanced API: An API that offers more fine-grained control over
the 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
[7]. The following sections summarize the performance effect of
MPTCP as seen by an application.
3.1.1. Throughput
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 [21].
Scharf & Ford Informational [Page 5]
RFC 6897 MPTCP API March 2013
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.
3.1.2. Delay
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
Scharf & Ford Informational [Page 6]
RFC 6897 MPTCP API March 2013
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.
3.1.3. Resilience
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 [5].
Subflow failure may be caused by issues within the network, which an
application would be unaware of, or interface failure on the node.
Scharf & Ford Informational [Page 7]
RFC 6897 MPTCP API March 2013
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 [22]. 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 [16]. 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 [5]. 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 [4], and MPTCP's mechanisms
to work around these issues are presented and discussed in [5].
MPTCP can also have other unexpected implications. For instance,
intrusion detection systems could be triggered. A full analysis of
MPTCP's impact on such middleboxes is for further study after
deployment experiments.
Scharf & Ford Informational [Page 8]
RFC 6897 MPTCP API March 2013
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.
Scharf & Ford Informational [Page 9]
RFC 6897 MPTCP API March 2013
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 [6].
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.
Scharf & Ford Informational [Page 10]
RFC 6897 MPTCP API March 2013
This means that there are two classes of applications:
o Legacy applications: These applications are unaware of MPTCP and
use the existing API towards TCP without any changes. This is the
default case.
o MPTCP-aware applications: These applications indicate support for
an 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).
Scharf & Ford Informational [Page 11]
RFC 6897 MPTCP API March 2013
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.
Scharf & Ford Informational [Page 12]
RFC 6897 MPTCP API March 2013
4.3. MPTCP Connection Management
4.3.1. Reaction to Close Call by Application
As described in [5], 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 [5]. 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 [2]. This option is also specified in the
POSIX standard [8]. 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.
Scharf & Ford Informational [Page 13]
RFC 6897 MPTCP API March 2013
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 [4].
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
Scharf & Ford Informational [Page 14]
RFC 6897 MPTCP API March 2013
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
o Using the default MPTCP configuration: Like TCP, MPTCP is designed
to be efficient and robust in the default configuration.
Application developers should not explicitly configure TCP (or
MPTCP) features unless this is really needed.
o 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 [5].
o Facilitating stack-internal heuristics: The path management and
data scheduling by MPTCP is realized by stack-internal algorithms
that may implicitly try to self-optimize their behavior according
to assumed application needs. For instance, an MPTCP
implementation may use heuristics to determine whether an
application requires delay-sensitive or bulk data transport,
using, for instance, port numbers, the TCP_NODELAY socket options,
or the 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.
Scharf & Ford Informational [Page 15]
RFC 6897 MPTCP API March 2013
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:
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 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.
o Sockets API extensions must be minimized and independent of an
implementation.
o 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
[20]).
REQ2: An application should be able to restrict MPTCP to binding to
a given set of addresses.
Scharf & Ford Informational [Page 16]
RFC 6897 MPTCP API March 2013
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
5.3.1. Overview
The abstract, basic MPTCP API consists of a set of new values that
are associated with an MPTCP socket. Such values may be used for
changing properties of an MPTCP connection or retrieving information.
These values could be accessed by new symbols on existing calls such
as setsockopt() and getsockopt() or could be implemented as entirely
new function calls. This implementation decision is out of scope for
this document. The following list presents symbolic names for these
MPTCP socket settings.
o TCP_MULTIPATH_ENABLE: Enable/disable MPTCP
o TCP_MULTIPATH_ADD: Bind MPTCP to a set of given local addresses,
or add a set of new local addresses to an existing MPTCP
connection
o TCP_MULTIPATH_REMOVE: Remove a local address from an MPTCP
connection
Scharf & Ford Informational [Page 17]
RFC 6897 MPTCP API March 2013
o TCP_MULTIPATH_SUBFLOWS: Get the pairs of addresses currently used
by the MPTCP subflows
o TCP_MULTIPATH_CONNID: Get the local connection identifier for this
MPTCP connection
Table 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:
o TCP_MULTIPATH_ENABLE: This value should only be set before the
establishment of a TCP connection. Its value should only be read
after the establishment of a connection.
o TCP_MULTIPATH_ADD: This operation can be 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.
o 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.
Scharf & Ford Informational [Page 18]
RFC 6897 MPTCP API March 2013
o TCP_MULTIPATH_SUBFLOWS: This value is read-only and can only be
used after connection setup.
o TCP_MULTIPATH_CONNID: This value is read-only and should only be
used after connection setup.
5.3.2. Enabling and Disabling of MPTCP
An application can explicitly indicate multipath capability by
setting TCP_MULTIPATH_ENABLE to 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.
Scharf & Ford Informational [Page 19]
RFC 6897 MPTCP API March 2013
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).
Scharf & Ford Informational [Page 20]
RFC 6897 MPTCP API March 2013
6. Other Compatibility Issues
6.1. Usage of TLS over MPTCP
Transport Layer Security (TLS) [17] 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 [18] [19], 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 [13]. 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 [11]. Care should be taken
that the use of MPTCP not conflict with the overlapping features of
other APIs:
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.
o API for Mobile IPv6 [10]: For Mobile IPv6, a significantly
extended sockets API exists as well (in addition to API extensions
for IPv6 [9]).
Scharf & Ford Informational [Page 21]
RFC 6897 MPTCP API March 2013
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 [14].
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 [6]. The
protocol specification of MPTCP [5] 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
Scharf & Ford Informational [Page 22]
RFC 6897 MPTCP API March 2013
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.
8. Conclusion
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.
9. Acknowledgments
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.
Scharf & Ford Informational [Page 23]
RFC 6897 MPTCP API March 2013
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. References
10.1. 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., Barre, S., and J. Iyengar,
"Architectural Guidelines for Multipath TCP Development",
RFC 6182, March 2011.
[5] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, "TCP
Extensions for Multipath Operation with Multiple Addresses",
RFC 6824, January 2013.
[6] Bagnulo, M., "Threat Analysis for TCP Extensions for Multipath
Operation with Multiple Addresses", RFC 6181, March 2011.
[7] Raiciu, C., Handley, M., and D. Wischik, "Coupled Congestion
Control for Multipath Transport Protocols", RFC 6356,
October 2011.
[8] "IEEE Standard for Information Technology -- Portable Operating
System Interface (POSIX) Base Specifications, Issue 7", IEEE
Std. 1003.1-2008, 2008.
10.2. Informative References
[9] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, "Advanced
Sockets Application Program Interface (API) for IPv6",
RFC 3542, May 2003.
[10] Chakrabarti, S. and E. Nordmark, "Extension to Sockets API for
Mobile IPv6", RFC 4584, July 2006.
Scharf & Ford Informational [Page 24]
RFC 6897 MPTCP API March 2013
[11] Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto, "Sockets
Application Program Interface (API) for Multihoming Shim",
RFC 6316, July 2011.
[12] Komu, M. and T. Henderson, "Basic Socket Interface Extensions
for the Host Identity Protocol (HIP)", RFC 6317, July 2011.
[13] 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.
[14] Blanchet, M. and P. Seite, "Multiple Interfaces and
Provisioning Domains Problem Statement", RFC 6418,
November 2011.
[15] Wasserman, M. and P. Seite, "Current Practices for Multiple-
Interface Hosts", RFC 6419, November 2011.
[16] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, April 2012.
[17] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
Protocol Version 1.2", RFC 5246, August 2008.
[18] 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.
[19] 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.
[20] Sarolahti, P., "Multi-address Interface in the Socket API",
Work in Progress, March 2010.
[21] Khalili, R., Gast, N., Popovic, M., and J. Le Boudec,
"Performance Issues with MPTCP", Work in Progress,
February 2013.
[22] 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.
Scharf & Ford Informational [Page 25]
RFC 6897 MPTCP API March 2013
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 [20] 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.
Scharf & Ford Informational [Page 26]
RFC 6897 MPTCP API March 2013
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.
Scharf & Ford Informational [Page 27]
RFC 6897 MPTCP API March 2013
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:
1. High bandwidth
2. Low latency and jitter stability
3. 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.
Scharf & Ford Informational [Page 28]
RFC 6897 MPTCP API March 2013
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.
Scharf & Ford Informational [Page 29]
RFC 6897 MPTCP API March 2013
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:
o sctp_bindx()
o sctp_connectx()
o sctp_getladdrs()
o sctp_getpaddrs()
o sctp_freeladdrs()
o sctp_freepaddrs()
The syntax and semantics of these functions are described in [13].
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.
Scharf & Ford Informational [Page 30]
RFC 6897 MPTCP API March 2013
Authors' Addresses
Michael Scharf
Alcatel-Lucent Bell Labs
Lorenzstrasse 10
70435 Stuttgart
Germany
EMail: michael.scharf@alcatel-lucent.com
Alan Ford
Cisco
Ruscombe Business Park
Ruscombe, Berkshire RG10 9NN
UK
EMail: alanford@cisco.com
Scharf & Ford Informational [Page 31]