Internet Engineering Task Force A. Ford
Internet-Draft Roke Manor Research
Intended status: Experimental C. Raiciu
Expires: April 29, 2010 M. Handley
University College London
October 26, 2009
TCP Extensions for Multipath Operation with Multiple Addresses
draft-ford-mptcp-multiaddressed-02
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 29, 2010.
Copyright Notice
Copyright (c) 2009 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
TCP/IP communication is currently restricted to a single path per
Ford, et al. Expires April 29, 2010 [Page 1]
Internet-Draft Multipath TCP October 2009
connection, yet multiple paths often exist between peers. The
simultaneous use of these multiple paths for a TCP/IP session would
improve resource usage within the network, and thus improve user
experience through higher throughput and improved resilience to
network failure.
Multipath TCP provides the ability to simultaneously use multiple
paths between peers. This document presents a set of extensions to
traditional TCP to support multipath operation. The protocol offers
the same type of service to applications as TCP - reliable bytestream
- and provides the components necessary to establish and use multiple
TCP flows across potentially disjoint paths.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Design Assumptions . . . . . . . . . . . . . . . . . . . . 3
1.2. Layered Representation . . . . . . . . . . . . . . . . . . 4
1.3. Operation Summary . . . . . . . . . . . . . . . . . . . . 5
1.4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 6
1.5. Requirements Language . . . . . . . . . . . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Semantic Issues . . . . . . . . . . . . . . . . . . . . . . . 7
4. MPTCP Protocol . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Session Initiation . . . . . . . . . . . . . . . . . . . . 9
4.2. Starting a New Subflow . . . . . . . . . . . . . . . . . . 10
4.3. Address Knowledge Exchange (Path Management) . . . . . . . 11
4.3.1. Adding Addresses . . . . . . . . . . . . . . . . . . . 13
4.3.2. Remove Address . . . . . . . . . . . . . . . . . . . . 14
4.4. General MPTCP Operation . . . . . . . . . . . . . . . . . 14
4.4.1. Receive Window Considerations . . . . . . . . . . . . 16
4.4.2. Congestion Control Considerations . . . . . . . . . . 17
4.4.3. Subflow Policy . . . . . . . . . . . . . . . . . . . . 17
4.4.4. Retransmissions . . . . . . . . . . . . . . . . . . . 18
4.5. Closing a Connection . . . . . . . . . . . . . . . . . . . 19
4.6. Error Handling . . . . . . . . . . . . . . . . . . . . . . 20
5. Security Considerations . . . . . . . . . . . . . . . . . . . 20
6. Interactions with Middleboxes . . . . . . . . . . . . . . . . 21
7. Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
10.2. Informative References . . . . . . . . . . . . . . . . . . 23
Appendix A. Notes on use of TCP Options . . . . . . . . . . . . . 23
Appendix B. Resync Packet . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
Ford, et al. Expires April 29, 2010 [Page 2]
Internet-Draft Multipath TCP October 2009
1. Introduction
Multipath TCP (henceforth referred to as MPTCP) is set of extensions
to regular TCP [2] to allow a transport connection to operate across
multiple paths simultaneously. This document presents the protocol
changes required by Multipath TCP, specifically those for signalling
and setting up multiple paths ("subflows"), managing these subflows,
reassembly of data, and termination of sessions. This is not the
only information required to create a Multipath TCP implementation,
however. This document is complemented by several others:
o Architecture [3], which explains the motivations behind Multipath
TCP and a functional separation through which an extensible MPTCP
implementation can be developed.
o Congestion Control [4], presenting a safe congestion control
algorithm for coupling the behaviour of the multiple paths in
order to "do no harm" to other network users.
o Application Considerations [5], discussing what impact MPTCP will
have on applications, what applications will want to do with
MPTCP, and as a consequence of these factors, what API extensions
an MPTCP implementation should present.
1.1. Design Assumptions
In order to limit the potentially huge design space, the authors
imposed two key constraints on the multipath TCP design presented in
this document:
o It must be backwards-compatible with current, regular TCP, to
increase its chances of deployment
o It can be assumed that one or both endpoints are multihomed and
multiaddressed
To simplify the design we assume that the presence of multiple
addresses at an endpoint is sufficient to indicate the existence of
multiple paths. These paths need not be entirely disjoint: they may
share one or many routers between them. Even in such a situation
making use of multiple paths is beneficial, improving resource
utilisation and resilience to a subset of node failures. The
congestion control algorithms as discussed in [4] ensure this does
not act detrimentally.
There are three aspects to the backwards-compatibility listed above:
Ford, et al. Expires April 29, 2010 [Page 3]
Internet-Draft Multipath TCP October 2009
External Constraints: The protocol must function through the vast
majority of existing middleboxes such as NATs, firewalls and
proxies, and as such must resemble existing TCP as far as possible
on the wire. Furthermore, the protocol must not assume the
segments it sends on the wire arrive unmodified at the
destination: they may be split or coalesced; options may be
removed or duplicated.
Application Constraints: The protocol must be usable with no change
to existing applications that use the standard TCP API (although
it is reasonable that not all features would be available to such
legacy applications).
Fall-back: The protocol should be able to fall back to standard TCP
with no interference from the user, to be able to communicate with
legacy hosts.
Areas for further study:
o In theory, since this is purely a TCP extension, it should be
possible to use MPTCP with both IPv4 and IPv6 on dual-stack hosts,
thus having the additional possible benefit of aiding transition.
o Some features of the design presented here could be extended to
work with non-multi-addressed hosts by using other packet metadata
(such as ports or flow label), packet marking, or partial
(potenitally proxied) multipath.
1.2. Layered Representation
MPTCP operates at the transport layer, and its existence aims to be
transparent to both higher and lower layers. It is a set of
additional features on top of standard TCP, and as such MPTCP is
designed to be usable by legacy applications with no changes. A
possible implementation would be for such a feature to be a system-
wide setting: "Use multipath TCP by default? Y/N". Multipath-aware
applications would be able to use an extended sockets API to have
further influence on the behaviour of MPTCP. Figure 1 illustrates
this layering.
Ford, et al. Expires April 29, 2010 [Page 4]
Internet-Draft Multipath TCP October 2009
+-------------------------------+
| Application |
+---------------+ +-------------------------------+
| Application | | MPTCP |
+---------------+ + - - - - - - - + - - - - - - - +
| TCP | | Subflow (TCP) | Subflow (TCP) |
+---------------+ +-------------------------------+
| IP | | IP | IP |
+---------------+ +-------------------------------+
Figure 1: Comparison of Standard TCP and MPTCP Protocol Stacks
Detailed discussion of an architecture for developing a multipath TCP
implementation, especially regarding the functional separation by
which different components should be developed, is given in [3].
1.3. Operation Summary
This section provides a high-level summary of normal operation of
MPTCP, and is illustrated by the scenario shown in Figure 2. A
detailed description of operation is given in Section 4.
o To a non-MPTCP-aware application, MPTCP will be indistinguishable
from normal TCP. All MPTCP operation is handled by the MPTCP
implementation, although extended APIs could provide additional
control and influence [5]. An application begins by opening a TCP
socket in the normal way.
o An MPTCP connection begins as a single TCP session. This is
illustrated in Figure 2 as being between Addresses A1 and B1 on
Hosts A and B respectively.
o If extra paths are available, additional TCP sessions are created
on these paths, and are combined with the existing session, which
continues to appear as a single connection to the applications at
both ends. The creation of the additional TCP session is
illustrated between Address A2 on Host A and Address B1 on Host B.
o MPTCP identifies multiple paths by the presence of multiple
addresses at endpoints. Combinations of these multiple addresses
equate to the additional paths. In the example, other potential
paths that could be set up are A1<->B2 and A2<->B2. Although this
additional session is shown as being initiated from A2, it could
equally have been initiated from B1.
o The discovery and setup of additional TCP sessions (termed
'subflows') will be achieved through a path management method.
This document describes a mechanism by which an endpoint can
Ford, et al. Expires April 29, 2010 [Page 5]
Internet-Draft Multipath TCP October 2009
initiate new subflows by using its additional addresses, or by
signalling its available addresses to the other endpoint.
o MPTCP adds connection-level sequence numbers in order to
reassemble the data stream in-order from multiple subflows.
Connections are terminated by connection-level FIN packets as well
as those relating to the individual subflows.
Host A Host B
------------------------ ------------------------
Address A1 Address A2 Address B1 Address B2
---------- ---------- ---------- ----------
| | | |
| (initial connection setup) | |
|----------------------------------->| |
|<-----------------------------------| |
| | | |
| (additional subflow setup) |
| |--------------------->| |
| |<---------------------| |
| | | |
| | | |
Figure 2: Example MPTCP Usage Scenario
1.4. Open Issues
This specification is a work-in-progress, and as such there are many
issues that are still to be resolved. This section lists many of the
key open issues within this specification; these are discussed in
more detail in the appropriate sections throughout this document.
o Best handshake mechanisms (Section 4.1). This document contains a
proposed scheme by which connections and subflows can be set up.
It is felt that, although this is "no worse than regular TCP",
there could be opportunities for significant improvements in
security that could be included (potentially optionally) within
this protocol.
o Issues around simulataneous opens, where both ends attempt to
create a new subflow simultaneously, need to be investigated and
behaviour specified.
o Appropriate mechanisms for controlling policy/priority of subflow
usage (specifically regarding controlling incoming traffic,
Section 4.4.3). The ECN signal is currently proposed but other
alternatives, including per subflow receive windows or path
Ford, et al. Expires April 29, 2010 [Page 6]
Internet-Draft Multipath TCP October 2009
property options, could be employed instead.
o How much control do we want over subflows from other subflows
(e.g. closing when interface has failed)? Do we want to
differentiate between subflows and addresses (Section 4.2)?
o Do we want a connection identifier in every packet? E.g. would
make implementation of IDS much easier?
o Best way of ensuring data/subflow sequence numbering mapping
through middleboxes (Section 4.4)?
o Is there any benefit to a data-level acknowlegement?
1.5. Requirements Language
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 RFC 2119 [1].
2. Terminology
Path: A sequence of links between a sender and a receiver, defined
in this context by a source and destination address pair.
Subflow: A stream of TCP packets sent over a path. A subflow is a
component part of a connection between two endpoints.
Connection: A collection of one or more subflows, over which an
application can communicate between two endpoints. There is a
one-to-one mapping between a connection and a socket.
Token: A unique identifier given to a multipath connection by an
endpoint. May also be referred to as a "Connection ID".
Endpoint: A host operating an MPTCP implementation, and either
initiating or terminating a MPTCP connection.
3. Semantic Issues
In order to support multipath operation, the semantics of some TCP
components have changed. To aid clarity, this section collects these
semantic changes as a reference.
Ford, et al. Expires April 29, 2010 [Page 7]
Internet-Draft Multipath TCP October 2009
Sequence Number: The (in-header) TCP sequence number is subflow-
specific. To allow the receiver to reorder application data, an
additional data-level sequence space is used. In this space, the
initial SYN and the final DATA FIN occupy one octet. There is an
explicit mapping of data sequence space to subflow sequence space,
which is signalled through TCP options in data packets.
Receive Window: The receive window exists at the connection level,
rather than at the subflow level, as it tries to regulate the
sending rate of the sender to a slower receiver. With multipath
TCP, each subflow MUST report the same global receive window,
describing the per connection receive buffer.
FIN: The FIN only applies to a subflow, not to a connection. For a
connection-level FIN, use the DATA FIN option.
ACK: The ACK acknowledges the subflow sequence number only, and the
mapping to the data sequence number is handled out-of-band.
RST: The RST only applies to a subflow. There is no connection-
level RST, since it would be impossible to distinguish the two,
i.e. if there is no state about a subflow, the host cannot know to
what connection the subflow is related. A connection is
considered reset if every subflow sends a RST in response.
Address List: The address management is handled per-connection to
permit the application of per-connection local policy.
5-tuple: The 5-tuple (protocol, local address, local port, remote
address, remote port) presented to the application layer in a non-
multipath-aware application is that of the first subflow, even if
the subflow has since been closed and removed from the connection.
These API issues are discussed in more detail in [5].
4. MPTCP Protocol
This section describes the operation of the MPTCP protocol, and is
subdivided into sections for each key part of the protocol operation.
All MPTCP operations are signalled using optional TCP header fields.
These TCP Options will have option numbers allocated by IANA, as
listed in Section 9, and are defined throughout the following
subsections.
Ford, et al. Expires April 29, 2010 [Page 8]
Internet-Draft Multipath TCP October 2009
4.1. Session Initiation
Session Initiation begins with a SYN, SYN/ACK exchange on a single
path. Each of these packets will additionally feature the Multipath
Capable TCP option (Figure 3), which declares the sender's locally
unique 32-bit token for this connection, and contains a version
field.
The "Multipath Capable" option declares an endpoint to be capable of
operating Multipath TCP (or rather, more accurately, a desire to
operate Multipath TCP on this particular connection). As well as
this declaration, this field presents a token, which is used when
adding additional subflows to this connection.
This token is generated by the sender and has local meaning only,
hence it MUST be unique for the sender. The token MUST be difficult
for an attacker to guess, and thus it is recommended it SHOULD be
generated randomly. (However, see further discussions about security
in Section 5, including the possibility of 64-bit tokens.)
This option is only present in packets with the SYN flag set. It is
only used in the first TCP session of a connection, in order to
identify the connection; all following connections will use path
management options (see Section 4.2) to join the existing connection.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------------------------------+
| Kind=OPT_MPC | Length = 7 |(resvd)|Version| Sender Token :
+---------------+---------------+-------------------------------+
: Sender Token (continued - 4 octets total) |
+-----------------------------------------------+
Figure 3: Multipath Capable option
The version field represents the version of MPTCP in use. The
version provided in this specification is 0. The reserved bits may
be used for connection-specific flags in later versions, or may be
used to indicate an authentication method.
If a SYN contains a "multipath capable" option but the SYN/ACK does
not, it is assumed that the recipient is not multipath capable and
thus the MPTCP session will operate as regular, single-path TCP. If
a SYN does not contain a "multipath capable" option, the SYN/ACK MUST
NOT contain one in response.
If these packets are unacknowledged, it is up to local policy to
decide how to respond. It is expected that a sender will eventually
Ford, et al. Expires April 29, 2010 [Page 9]
Internet-Draft Multipath TCP October 2009
fall back to single-path TCP (i.e. without the Multipath Capable
Option), in order to work around middleboxes that may drop packets
with unknown options; however, the number of multipath-capable
attempts that are made first will be up to local policy. In the case
of out-of-order packets, i.e. if a multipath-capable SYN/ACK is
received in response to a multipath-capable SYN, after a standard SYN
has been sent, then once again it is up to the initiator to choose
how to behave. For example, it could respond to new connections
using the previously declared token, or it could simply drop any new
multipath options within the flow.
If an endpoint is known to be multiaddressed (e.g. through multiple
addresses returned in a DNS lookup), alternative destination
addresses SHOULD be tried first, before falling back to regular TCP.
In addition to this option, a Data Sequence Number option (discussed
in Section 4.4) is included to provide an initial data-level sequence
number (and this initial SYN counts as one octet in this space, as
for a regular SYN in single-path TCP). This could also have some
(minor) security benefits, discussed in Section 5.
4.2. Starting a New Subflow
Endpoints have knowledge of their own address(es), and can become
aware of the other endpoint's addresses through signalling exchanges
as described in Section 4.3. Using this knowledge, an endpoint will
initiate a new subflow over a currently unused pair of addresses.
A new subflow is started as a normal TCP SYN/ACK exchange. The
"Join" TCP option (Figure 4) is used to identify of which connection
the new subflow should become a part. The token used is the locally
unique token of the destination for the subflow, as defined by the
Multipath Capable option received in the first SYN/ACK exchange.
It should be noted that, in theory, additional subflows can exist
between any pair of ports; no explicit accept calls or bind calls are
required to open additional subflows. To associate a new subflow to
an existing connection, the token supplied in the subflow's SYN
exchange is used for demultiplexing. This means that port numbers on
subflow SYN exchanges are not important, and any values can be used,
as long as the 5-tuple is unique for each host. In practice, it is
envisaged that most new subflows will connect to a port that is
already in use as the source or destination port of an existing
subflow, in order to have a greater chance of getting through
firewalls and other middleboxes, and to support traffic engineering
of the flows.
Deumultiplexing subflow SYNs MUST be done using the token; this is
Ford, et al. Expires April 29, 2010 [Page 10]
Internet-Draft Multipath TCP October 2009
unlike traditional TCP, where the destination port is used for
demultiplexing SYN packets. Once a subflow is setup, demultiplexing
packets is done using the five-tuple, as in traditional TCP.
The "Join" option includes an "Address ID". This is an identifier,
locally unique to the sender of this option, and with only per-
connection relevance, which identifies the source address of this
packet. This serves two purposes. Firstly, if an address becomes
unexpectedly unavailable on the sender, it can signal this to the
receiver via a remove address option (Section 4.3.2) without needing
to know what the source address actually is (thus allowing the use of
NATs). Secondly, it allows correlation between new connection
attempts and address signalling (Section 4.3.1), to prevent duplicate
subflow initiation.
TBD: Instead of an Address ID, are there any cases where a Subflow ID
(i.e. unique to the subflow) would be useful instead? For example,
two addresses which become NATted to the same address?
This option can only be present when the SYN flag is set.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------------------------------+
| Kind=OPT_JOIN | Length = 7 |Receiver Token (4 octets total):
+---------------+---------------+----------------+--------------+
: Receiver Token (continued) | Address ID |
+-------------------------------+----------------+
Figure 4: Join Connection option
4.3. Address Knowledge Exchange (Path Management)
We use the term "path management" to refer to the exchange of
information about additional paths between endpoints, which in this
design is managed by multiple addresses at endpoints. For more
detail of the architectural thinking behind this design, see the
separate document [3].
This design makes use of two methods of sharing such information,
used simultaneously. The first is the direct setup of new subflows,
already described in Section 4.2, where the initiator has an
additional address. The second method is described in the following
subsections, whereby addresses are signalled explicitly to the other
endpoint, to allow it to initiate new connections. This approach, of
two complementary mechanisms, has been chosen to allow addresses to
change in flight, and thus support operation through NATs, whilst
also allowing the signalling of previously unknown addresses, such as
Ford, et al. Expires April 29, 2010 [Page 11]
Internet-Draft Multipath TCP October 2009
those belonging to other address families (e.g. IPv4 and IPv6).
Here is an example of typical operation of the protocol:
o An endpoint that is multihomed starts an additional TCP session to
an address/port pair that is already in use on the other endpoint,
using a token to identify the flow (Section 4.2). (A multihomed
destination may open a new subflow from its new address to an
existing subflow's source address and port, or a multihomed source
may open a new subflow from its new address to an existing
subflow's destination and port).
o To expand upon this, say a connection is intiated from host "A" on
(address, port) combination A1 to destination (address, port) B1
on host "B". If host A is multihomed, it starts an additional
connection from new (address, port) A2 to B1, using B's previously
declared token. Alternatively, if B is multhomed, it will try to
set up a new TCP connection from B2 to A1, using A's previously
declared token.
o Simultaneously (or after a timeout), an "Add Address" option
(Section 4.3.1) is sent on an existing subflow, informing the
receiver of the sender's alternative address(es). The recipient
can use this information to open a new subflow to the sender's
additional address. Using the previous notation, this would be an
Add Address packet sent from A1 to B1, informing B of address A2.
o The mix of using the SYN-based option and the Add Address option,
including timeouts, is implementation-specific and can be tailored
to agree with local policy.
o If host B successfully receives the first SYN, starting a new
subflow, it can use the Address ID in the Join option to correlate
this with the Add Address option that will also arrive on an
existing subflow. Assuming the endpoint has already responded to
the SYN with a SYN/ACK, it will know to ignore the Add Address
option. Otherwise, if it has not received such a SYN, it will try
to initiate a new subflow from one or more of its addresses to
address A2 (triggered by the Add Address option). This is
intended to permit new sessions to be opened if one endpoint is
behind a NAT. A slight security improvement can be gained if a
host ensures there is a correlated Add Address option before
responding to the SYN.
Other scenarios are valid, however, such as those where entirely new
addresses are signalled, e.g. to allow an IPv6 and an IPv4 path to be
used simultaneously.
Ford, et al. Expires April 29, 2010 [Page 12]
Internet-Draft Multipath TCP October 2009
4.3.1. Adding Addresses
The Add Address TCP Option announces additional addresses on which an
endpoint can be reached (Figure 5), which allows several (ID,
address) pairs to be announced to the other endpoint. Multiple
addresses can be added if there is sufficient TCP option space,
otherwise multiple TCP messages containing this option will be sent.
This option can be used at any time during a connection, depending on
when the sender wishes to enable multiple paths and/or when paths
become available.
Every address has an ID which can be used for address removal, and
therefore endpoints must cache the mapping between ID and address.
This is also used to identify Join Connection options (Section 4.2)
relating to the same address, even when address translators are in
use. The ID must be unique to the sender and connection, per
address, but its mechanism for allocating such IDs is implementation-
specific.
This option is shown for IPv4. For IPv6, the IPVer field will read
6, and the length of the address will be 16 octets not 4, and thus
the length of the option will be 2 + (18 * number_of_entries). If
there is sufficient TCP option space, multiple addresses can be
included, with an ID following on immediately from the previous
address, and their existance can be inferred through the option
length and version fields.
NB: by having a IPVer field, we get four free reserved bits. These
could be used in later versions of this protocol for expressing
sender policy, e.g. one bit for "use now" or similar, to
differentiate between subflows for backup purposes and those for
throughput.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+-------+-------+
| Kind=OPT_ADDR | Length | Address ID | IPVer |(resvd)|
+---------------+---------------+---------------+-------+-------+
| Address (IPv4 - 4 octets) |
+---------------------------------------------------------------+
( ... further ID/Version/Address fields as required ... )
Figure 5: Add Address option (for IPv4)
Ford, et al. Expires April 29, 2010 [Page 13]
Internet-Draft Multipath TCP October 2009
4.3.2. Remove Address
If, during the lifetime of a MPTCP connection, a previously-announced
address becomes invalid (e.g. if the interface disappears), the
affected endpoint should announce this so that the other endpoint can
remove subflows related to this address.
This is achieved through the Remove Address option (Figure 6), which
will remove a previously-added address (or list of addresses) from a
connection and terminate any subflows currently using that address.
The sending and receipt of this message should trigger the sending of
FINs by both endpoints on the affected subflow(s) (if possible), as a
courtesy to cleaning up middlebox state, but endpoints may clean up
their internal state without a long timeout.
Address removal is undertaken by ID, so as to permit the use of NATs
and other middleboxes. If there is no address at the requested ID,
the receiver will silently ignore the request.
The standard way to close a subflow (so long as it is still
functioning) is to use a FIN exchange as in regular TCP - for more
information, see Section 4.5.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+
|Kind=OPT_REMADR| Length = 2+n | Address ID | ...
+---------------+---------------+---------------+
Figure 6: Remove Address option
4.4. General MPTCP Operation
This section discusses operation of MPTCP for data transfer. At a
high level, an MPTCP implementation will take one input data stream
from an application, and split it into one or more subflows. The
data stream as a whole can be reassembled through the use of the Data
Sequence Mapping (Figure 7) option, which defines the mapping from
the data sequence number to the subflow sequence number. This is
used by the receiver to ensure in-order delivery to the application
layer. Meanwhile, the subflow-level sequence numbers (i.e. the
regular sequence numbers in the TCP header) have subflow-only
relevance.
The only acknowledgements are those at the subflow-level, so the
sender must be able to map these acknowledgements to the data
sequence numbers that were contained in the relevant packets. The
Ford, et al. Expires April 29, 2010 [Page 14]
Internet-Draft Multipath TCP October 2009
sender thus knows, if subflow data goes unackowledged, which part of
the original data stream this equates to, and thus what data must be
retransmitted. It is expected (but not mandated) that SACK [6] is
used at the subflow level to improve efficiency.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+------------------------------+
| Kind=OPT_DSN | Length | Data Sequence Number ... :
+---------------+---------------+------------------------------+
: ... ( (length-8) octets ) | Data-level Length (2 octets) |
+-------------------------------+------------------------------+
| Subflow Sequence Number (4 octets) |
+-------------------------------+------------------------------+
Figure 7: Data Sequence Mapping option
This option specifies a full mapping from data sequence number to
subflow sequence number, informing the receiver that there is a one-
to-one correspondence between the two sequence spaces for the
specified length. The purpose of the explicit mapping is to assist
with compatibility with situations where TCP/IP segmentation or
coalescing is undertaken separately from the stack that is generating
the data flow (e.g. through the use of TCP segmentation offloading on
network interface cards, or by middleboxes such as performance
enhancing proxies).
The data sequence number specified in this option is absolute,
whereas the subflow sequence numbering is relative (the SYN at the
start of the subflow has subflow sequence number 1).
A mapping is unique, in that the subflow sequence number is bound to
the data sequence number after the mapping has been processed. It is
not possible to change this mapping afterwards; however, the same
data sequence number can be mapped on different subflows for
retransmission purposes (see Section 4.4.4).
A receiver MUST NOT accept data for which it does not have a mapping
to the data sequence space. To do this, the receiver will not
acknowledge the unmapped data at subflow level. It is better to have
a subflow fail than to accept data in the wrong order. However, if
there was a lost packet in the subflow, the receiver SHOULD wait for
this to be retransmitted before closing the subflow, since the lost
packet may contain the necessary mapping information.
Although it is expected that initial implementations will use 32-bit
data sequence numbers (i.e. 4 octets, so a length field of 12),
Ford, et al. Expires April 29, 2010 [Page 15]
Internet-Draft Multipath TCP October 2009
setting the length field to 16 and including a 64-bit sequence number
(eight octets) MUST be considered valid and processed appropriately.
This may have also have useful security implications, discussed in
Section 5.
As with the standard TCP sequence number, the data sequence number
should not start at zero, but at a random value to make session
hijacking harder. This is done by including a Data Sequence Number
option along with the Multipath Capable option in the initial SYN
(which occupies one octet of data sequence space; see Section 4.1).
In this case, to save option space, neither the data-level length nor
the subflow sequence number fields are present in this option, so the
Length field will be the length of the Data Sequence Number, plus two
octets.
The Data Sequence Mapping does not need to be included in every MPTCP
packet, as long as the subflow sequence space in that packet is
covered by a mapping known at a receiver. This can be used to reduce
overhead in cases where the mapping is known in advance; one such
case is when there is a single subflow between the endpoints, another
is when segments of data are scheduled in larger than packet-sized
chunks.
The MPTCP data and subflow level sequence numbering could be seen to
be analogous to that used in SACK, however there are subtle
differences. The key similarity is that it is possible to have
temporary "holes" in the received data sequence space - later data
may have arrived earlier (most likely on a different subflow), but
does not need to be retransmitted. The "holes" are later filled in.
The key difference, however, is that while SACK can rely on the
regular TCP cumulative acknowledgements to indicate how much data has
been successfully received (with no holes), there is no similar
method in MPTCP. Instead, the sender must keep track of the
acknowledgements to derive what data has been successfully received.
This leads to some oddities especially with session termination (see
Section 4.5).
4.4.1. Receive Window Considerations
Regular TCP advertises a receive window in each packet, telling the
sender how much data the receiver is willing to accept past the
cumulative ack. The receive window is used to implement flow
control, throttling down fast senders when receivers cannot keep up.
MPTCP also uses a unique receive window, shared between the subflows.
The idea is to allow any subflow to send data as long as the receiver
is willing to accept it; the alternative, maintaining per subflow
receive windows, could end-up stalling some subflows while others
Ford, et al. Expires April 29, 2010 [Page 16]
Internet-Draft Multipath TCP October 2009
would not use up their window.
An issue will arise regarding how large a receive buffer to
implement. The lower bound would be the maximum bandwidth/delay
product of all paths, however this could easily fill when a packet is
lost on a slower subflow and needs to be retransmitted (see
Section 4.4.4). The upper bound would be the maximum RTT multiplied
by the maximum total bandwidth available. This will cover most
eventualities, but could easily become very large. It is FFS what
the best approach is.
4.4.2. Congestion Control Considerations
Different subflows in an MPTCP connection have different congestion
windows. To achieve resource pooling, it is necessary to couple the
congestion windows in use on each subflow, in order to push most
traffic to uncongested links. One algorithm for achieving this is
presented in [4]; the algorithm does not achieve perfect resource
pooling but is "safe" in that it is readily deployable in the current
Internet.
It is foreseeable that different congestion controllers will be
implemented for MPTCP, each aiming to achieve different properties in
the resource pooling/fairness/stability design space. Much research
is expected in this area in the near future.
Regardless of the algorithm used, the design of the MPTCP protocol
aims to provide the congestion control implementations sufficient
information to take the right decisions; this information includes,
for each subflow, which packets where lost and when.
4.4.3. Subflow Policy
Within a local MPTCP implementation, a host may use any local policy
it wishes to decide how to share the traffic to be sent over the
available paths.
In the typical use case, where the goal is to maximise throughput,
all available paths will be used simultaneously for data transfer,
using coupled congestion control as described in [4]. It is
expected, however, that other use cases will appear.
For instance, a possibility is an 'all-or-nothing' approach, i.e.
have a second path ready for use in the event of failure of the first
path, but alternatives could include entirely saturating one path
before using an additional path (the 'overflow' case). Such choices
would be most likely based on the monetary cost of links, but may
also be based on properties such as the delay or jitter of links,
Ford, et al. Expires April 29, 2010 [Page 17]
Internet-Draft Multipath TCP October 2009
where stability is more important than throughput. Application
requirements such as this are discussed in detail in [5].
The ability to make effective choices at the sender requires full
knowledge of the path "cost", which is unlikely to be the case.
There is no mechanism in MPTCP for a receiver to signal their own
particular preferences for paths, but this is a necessary feature
since receivers will often be the multihomed party, and may have to
pay for metered incoming bandwidth. Instead of incorporating complex
signalling, it is proposed to use existing TCP features to signal
priority implicitly. If a receiver wishes to keep a path active as a
backup but wishes to prevent data being sent on that path, it could
stop sending ACKs for any data it receives on that path. The sender
would interpret this as severe congestion or a broken path and stop
using it. We do not advocate this method, however, since this is
brutal, naive, and will result in unnecessary retransmissions.
Therefore, a proposal is to use ECN [7] to to provide fake congestion
signals on paths that a receiver wishes to stop being used for data.
This has the benefit of causing the sender to back off without the
need to retransmit data unnecessarily, as in the case of a lost ACK.
This should be sufficient to allow a receiver to express their
policy, although does not permit a rapid increase in throughput when
switching to such a path.
TBD: This is clearly an overload of the ECN signal, and as such other
solutions, such as explicitly signalling path operation preferences
(such as in the reserved bits of certain TCP options, or through
entirely new options) may be a preferred solution.
4.4.4. Retransmissions
This protocol specification does not mandate any mechanisms for
handling retransmissions in the event of path failures, and much will
be dependent upon local policy (as discussed in Section 4.4.3). The
data sequence number, as given in a TCP option, is used to reassemble
the incoming streams before presentation to the application layers,
so a sender is free to re-send data with the same data sequence
number on a different subflow. When doing this, an endpoint must
still retransmit the original data on the original subflow, in order
to preserve the subflow integrity (middleboxes could replay old data,
and/or could reject holes in subflows), and a receiver will ignore
these retransmissions. While this is clearly suboptimal, for
compatibility reasons we feel this is the best behaviour.
Optimisations could be negotiated in future versions of this
protocol.
Of course, retransmissions on alternative subflows will only occur if
Ford, et al. Expires April 29, 2010 [Page 18]
Internet-Draft Multipath TCP October 2009
this is what local policy suggests. Indeed, it may be equally valid
to retransmit on the same subflow if alternative paths have
considerably worse quality of service, or are only kept for backup
purposes. Additionally, it may be possible for some implementations
to signal from lower layers if there are problems with the paths, and
so more appropriate responses could occur.
4.5. Closing a Connection
Under single path TCP, a FIN signifies that the sender has no more
data to send. In order to allow subflows to operate independently,
however, and with as little change from regular TCP as possible, a
FIN in MPTCP will only affect the subflow on which it is sent. This
allows nodes to exercise considerable freedom over which paths are in
use at any one time. The semantics of a FIN remain as for regular
TCP, i.e. it is not until both sides have ACKed each other's FINs
that the subflow is fully closed.
When an application calls close() on a socket, this indicates that it
has no more data to send, and for regular TCP this would result in a
FIN on the connection. For MPTCP, an equivalent mechanism is needed,
and this is the DATA FIN. This option, shown in Figure 8, is
attached to a regular FIN option on a subflow.
A DATA FIN is an indication that the sender has no more data to send,
and as such can be used as a rapid indication of the end of data from
a sender. A DATA FIN, as with the FIN on a regular TCP connection,
is a unidirectional signal.
The DATA FIN is an optimisation to rapidly indicate the end of a data
stream and clean up state associated with a MPTCP connection,
especially when some subflows may have failed. Specifically, when a
DATA FIN has been received, IF all data has been successfully
received, timeouts on all subflows MAY be reduced. Similarly, when
sending a DATA FIN, once all data (including the DATA FIN, since it
occupies one octet of data sequence space) has been acknowledged,
FINs must be sent on every subflow. This applies to both endpoints,
and is required in order to clean up state in middleboxes.
There are complex interactions, however, between a DATA FIN and
subflow properties:
o A DATA FIN MUST only be sent on a packet which also has the FIN
flag set.
o A DATA FIN occupies one octet (the final octet) of Data Sequence
Number space. Therefore, even if there is no user data, a Data
Sequence Number option MUST be added to a packet containing the
Ford, et al. Expires April 29, 2010 [Page 19]
Internet-Draft Multipath TCP October 2009
DATA FIN option. This allows the receiver to easily determine the
last data sequence number that should have been received.
o There is a one-to-one mapping between the DATA FIN and the
subflow's FIN flag (and its associated sequence space and thus its
acknowlegement). In other words, when a subflow's FIN flag has
been acknowledged, the associated DATA FIN is also acknowledged.
o As such, the acknowledgement of a FIN and DATA FIN DOES NOT
indicate that all data has been successfully received. Because
the data level ack is inferred from subflow acks, an endpoint must
use subflow acks to discover when all data up to and including the
DATA FIN has been received.
It should be noted that an endpoint may also send a FIN on an
individual subflow to shut it down, but this impact is limited to the
subflow in question. If all subflows have been closed with a FIN,
that is equivalent to having closed the connection with a DATA FIN.
1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---------------+---------------+
| Kind=OPT_DFIN | Length = 2 |
+---------------+---------------+
Figure 8: DATA FIN option
4.6. Error Handling
TBD
Unknown token in MPTCP SYN should equate to an unknown port, e.g. a
TCP reset? We should make this as silent and tolerant as possible.
Where possible, we should keep this close to the semantics of TCP.
However, some MPTCP-specific issues such as where a data sequence
number is missing from a subflow, will definitely need MPTCP-specific
errors handling in those cases.
5. Security Considerations
TBD
(Token generation, handshake mechanisms, new subflow authentication,
etc...)
A generic threat analysis for the addition of multipath capabilities
to TCP is presented in [8]. The protocol presented here has been
Ford, et al. Expires April 29, 2010 [Page 20]
Internet-Draft Multipath TCP October 2009
designed to minimise or eliminate these identified threats. (A
future version of this document will explicitly address the presented
threats).
The development of a TCP extension such as this will bring with it
many additional security concerns. We have set out here to produce a
solution that is "no worse" than current TCP, with the possibility
that more secure extensions could be proposed later.
The primary area of concern will be around the handshake to start new
subflows which join existing connections. The proposal set out in
Section 4.1 and Section 4.2 is for the initiator of the new subflow
to include the token of the other endpoint in the handshake. The
purpose of this is to indicate that the sender of this token was the
same entity that received this token at the initial handshake.
One area of concern is that the token could be simply brute-forced.
The token must behard to guess, and as such could be randomly
generated. This may still not be strong enough, however, and so the
use of 64 bits for the token would alleviate this somewhat.
Use of these tokens only provide an indication that the token is the
same as at the initial handshake, and does not say anything about the
current sender of the token. Therefore, another approach would be to
bring a new measure of freshness in to the handshake, so instead of
using the initial token a sender could request a new token from the
receiver to use in the next handshake. Hash chains could also be
used for this purpose.
Yet another alternative would be for all SYN packets to include a
data sequence number. This could either be used as a passive
identifier to indicate an awareness of the current data sequence
number (although a reasonable window would have to be allowed for
delays). Or, the SYN could form part of the data sequence space -
but this would cause issues in the event of lost SYNs (if a new
subflow is never established), thus causing unnecessary delays for
retransmissions.
6. Interactions with Middleboxes
TBD
How we get around NATs, firewalls. Problems with TCP proxies. How
to make an MPTCP-aware middlebox, ...
Ford, et al. Expires April 29, 2010 [Page 21]
Internet-Draft Multipath TCP October 2009
7. Interfaces
TBD
Interface with applications, interface with TCP, interface with lower
layers...
Discussion of interaction with applications (both in terms of how
MPTCP will affect an application's assumptions of the transport
layer, and what API extensions an application may wish to use with
MPTCP) are discussed in [5].
8. Acknowledgements
The authors are 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.
The authors gratefully acknowledge significant input into this
document from many members of the Trilogy project, notably Iljitsch
van Beijnum, Lars Eggert, Marcelo Bagnulo Braun, Robert Hancock, Pasi
Sarolahti, Olivier Bonaventure, Toby Moncaster, Philip Eardley and
Andrew McDonald.
9. IANA Considerations
This document will make a request to IANA to allocate new values for
TCP Option identifiers, as follows:
+------------+----------------------+---------------+-------+
| Symbol | Name | Ref | Value |
+------------+----------------------+---------------+-------+
| OPT_MPC | Multipath Capable | Section 4.1 | (tbc) |
| OPT_ADDR | Add Address | Section 4.3.1 | (tbc) |
| OPT_REMADR | Remove Address | Section 4.3.2 | (tbc) |
| OPT_JOIN | Join Connection | Section 4.2 | (tbc) |
| OPT_DSN | Data Sequence Number | Section 4.4 | (tbc) |
| OPT_DFIN | DATA FIN | Section 4.5 | (tbc) |
+------------+----------------------+---------------+-------+
Table 1: TCP Options for MPTCP
Ford, et al. Expires April 29, 2010 [Page 22]
Internet-Draft Multipath TCP October 2009
10. References
10.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
10.2. Informative References
[2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[3] Ford, A., Raiciu, C., Barre, S., Iyengar, J., and B. Ford,
"Architectural Guidelines for Multipath TCP Development",
draft-ford-mptcp-architecture-00 (work in progress),
October 2009.
[4] Raiciu, C., Handley, M., and D. Wischik, "Coupled Multipath-
Aware Congestion Control", draft-raiciu-mptcp-congestion-00
(work in progress), October 2009.
[5] Scharf, M. and A. Ford, "MPTCP Application Interface
Considerations", draft-scharf-mptcp-api-00 (work in progress),
October 2009.
[6] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[7] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001.
[8] Bagnulo, M., "Threat Analysis for Multi-addressed/Multi-path
TCP", draft-bagnulo-mptcp-threat-00 (work in progress),
October 2009.
[9] Eddy, W. and A. Langley, "Extending the Space Available for TCP
Options", draft-eddy-tcp-loo-04 (work in progress), July 2008.
Appendix A. Notes on use of TCP Options
The TCP option space is limited due to the length of the Data Offset
field in the TCP header (4 bits), which defines the TCP header length
in 32-bit words. With the standard TCP header being 20 bytes, this
leaves a maximum of 40 bytes for options, and many of these may
already be used by options such as timestamp and SACK.
Ford, et al. Expires April 29, 2010 [Page 23]
Internet-Draft Multipath TCP October 2009
As such, when doing address list manipulation, not all data may fit.
This can be mitigated in one of two ways:
o Using an option to extend the option space, such as that proposed
in [9], which proposes an option providing a 16-bit header length
field. Such an option could only be used between nodes that
support it, however, and so long options could not be used until a
handshake is complete.
o Alternatively, since at least one IP address option field should
be able to fit per packet, address list manipulation can be
undertaken with one address per packet. One method could be to
wait for data to send, and then append one new address per packet.
This would seem reasonable if the TCP session begins rapidly, but
if it is required that the multipath session is ready before the
first data is to be sent, address list manipulation would be
required on empty data (signalling only) packets. Issues may
arise regarding acknowledged delivery of signalling versus data -
this is discussed in Section 3 below.
Appendix B. Resync Packet
In earlier versions of this draft, we proposed the use of a "re-sync"
option that would be used in certain circumstances when a sender
needs to instruct the receiver to skip over certain subflow sequence
numbers (i.e. to treat the specified sequence space as having been
received and acknowledged).
The typical use of this option will be when packets are retransmitted
on different subflows, after failing to be acknowledged on the
original subflow. In such a case, it becomes necessary to move
forward the original subflow's sequence numbering so as not to later
transmit different data with a previously used sequence number (i.e.
when more data comes to be transmitted on the original subflow, it
would be different data, and so must not be sent with previously-used
(but unacknowledged) sequence numbering).
The rationale for needing to do this is two-fold: firstly, when ACKs
are received they are for the subflow only, and the sender infers
from this the data that was sent - if the same sequence space could
be occupied by different data, the sender won't know whether the
intended data was received. Secondly, certain classes of middleboxes
may cache data and not send the new data on a previously-seen
sequence number.
This option was dropped, however, since some middleboxes may get
confused when they meet a hole in the sequence space, and do not
Ford, et al. Expires April 29, 2010 [Page 24]
Internet-Draft Multipath TCP October 2009
understand the resync option. It is therefore felt that the same
data must continue to be retransmitted on a subflow even if it is
already received after being retransmitted on another. There should
not be a significant performance hit from this since the amount of
data involved and needing to be retransmitted multiple times will be
relatively small.
Therefore, it is necessary to 're-sync' the expected sequence
numbering at the receiving end of a subflow, using the following TCP
option. This packet declares a sequence number space (inclusive)
which the receiving node should skip over, i.e. if the receiver's
next expected sequence number was previously within the range
start_seq_num to end_seq_num, move it forward to end_seq_num + 1.
This option will be used on the first new packet on the subflow that
needs its sequence numbering re-synchronised. It will be continue to
be included on every packet sent on this subflow until a packet
containing this option has been acknowledged (i.e. if subflow
acknowledgements exist for packets beyond the end sequence number).
If the end sequence number is earlier than the current expected
sequence number (i.e. if a resync packet has already been received),
this option should be ignored.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+------------------------------+
|Kind=OPT_RESYNC| Length = 10 | Start Sequence Number :
+---------------+---------------+------------------------------+
: (4 octets) | End Sequence Number :
+---------------+---------------+------------------------------+
: (4 octets) |
+-------------------------------+
Figure 9: Resync option
Authors' Addresses
Alan Ford
Roke Manor Research
Old Salisbury Lane
Romsey, Hampshire SO51 0ZN
UK
Phone: +44 1794 833 465
Email: alan.ford@roke.co.uk
Ford, et al. Expires April 29, 2010 [Page 25]
Internet-Draft Multipath TCP October 2009
Costin Raiciu
University College London
Gower Street
London WC1E 6BT
UK
Email: c.raiciu@cs.ucl.ac.uk
Mark Handley
University College London
Gower Street
London WC1E 6BT
UK
Email: m.handley@cs.ucl.ac.uk
Ford, et al. Expires April 29, 2010 [Page 26]