Internet Engineering Task Force A. Ford, Ed.
Internet-Draft Roke Manor Research
Intended status: Experimental C. Raiciu
Expires: November 8, 2009 M. Handley
University College London
S. Barre
Universite catholique de
Louvain
May 7, 2009
TCP Extensions for Multipath Operation with Multiple Addresses
draft-ford-mptcp-multiaddressed-00
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Abstract
Often endpoints are connected by multiple paths, but the nature of
TCP/IP restricts communications to a single path per socket.
Resource usage within the network would be more efficient were these
multiple paths able to be used concurrently. This should enhance
user experience through higher throughput and improved resilience to
network failure. This document presents extensions to TCP in order
to transparently provide this multi-path functionality at the
transport layer, if at least one endpoint is multi-addressed.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivations . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Design Assumptions . . . . . . . . . . . . . . . . . . . . 4
1.3. Layered Representation . . . . . . . . . . . . . . . . . . 5
1.4. Operation Summary . . . . . . . . . . . . . . . . . . . . 6
1.5. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 7
1.6. Requirements Language . . . . . . . . . . . . . . . . . . 8
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Semantic Issues . . . . . . . . . . . . . . . . . . . . . . . 9
4. MPTCP Protocol . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Session Initiation . . . . . . . . . . . . . . . . . . . . 10
4.2. Address Knowledge Exchange (Path Management) . . . . . . . 11
4.2.1. Explicit Path Management . . . . . . . . . . . . . . . 11
4.2.1.1. Adding Addresses . . . . . . . . . . . . . . . . . 11
4.2.1.2. Remove Address . . . . . . . . . . . . . . . . . . 12
4.2.2. Implicit Path Management . . . . . . . . . . . . . . . 13
4.2.2.1. Request-SYN . . . . . . . . . . . . . . . . . . . 14
4.2.2.2. Request-FIN (Remove Address) . . . . . . . . . . . 15
4.3. Starting a New Subflow . . . . . . . . . . . . . . . . . . 15
4.4. General MPTCP Operation . . . . . . . . . . . . . . . . . 16
4.4.1. Subflow Policy . . . . . . . . . . . . . . . . . . . . 17
4.4.2. Retransmissions . . . . . . . . . . . . . . . . . . . 19
4.4.3. Resync Packet . . . . . . . . . . . . . . . . . . . . 19
4.5. Closing a Connection . . . . . . . . . . . . . . . . . . . 20
4.6. Error Handling . . . . . . . . . . . . . . . . . . . . . . 22
5. Security Considerations . . . . . . . . . . . . . . . . . . . 22
6. Interactions with Middleboxes . . . . . . . . . . . . . . . . 23
7. Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.1. Normative References . . . . . . . . . . . . . . . . . . . 24
10.2. Informative References . . . . . . . . . . . . . . . . . . 24
Appendix A. Functional Separation . . . . . . . . . . . . . . . . 25
A.1. Motivations . . . . . . . . . . . . . . . . . . . . . . . 25
A.2. TCP Performance . . . . . . . . . . . . . . . . . . . . . 26
A.3. Architecture overview . . . . . . . . . . . . . . . . . . 26
A.4. PM/MPS interface . . . . . . . . . . . . . . . . . . . . . 28
Appendix B. Notes on use of TCP Options . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
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1. Introduction
This section describes the motivations behind the design of Multipath
TCP (henceforth referred to as MPTCP), a set of extensions for
regular TCP [RFC0793] to allow one TCP connection to be spread across
multiple paths. The following sections go on to describe the
extensions themselves, and its operation.
1.1. Motivations
As the Internet evolves, demands on Internet resources are ever-
increasing, but often these resources (in particular, bandwidth)
cannot be fully utilised due to protocol constrains on both the end-
systems and within the network. By the application of resource
pooling [WISCHIK], these resources can be 'pooled' such that they
appear as a single logical resource to the user. Multipath TCP
achieves resource pooling by combining multiple TCP sessions running
over multiple paths, and presenting them as a single TCP connection
to the application.
This form of resource pooling bring two key benefits:
o To increase the efficiency of the resource usage, and thus
increase the network capacity available to end hosts.
o To increase the resilience of the connectivity by providing
multiple paths, protecting end hosts from the failure of one.
The protocol presented in this document still follows the same
service model as TCP [RFC0793]: byte oriented, in order reliable
delivery. This leads to a high level goal of the resulting protocol,
where 'subflows' on different paths will function independently of
one another, i.e. failure of one path should not result in reduced
throughput on the other paths.
1.2. 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
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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 will improve resource utilisation.
There are three aspects to the backwards-compatibility listed above:
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. In addition, therefore, we cannot rely on the TCP
packets (both headers and payloads) remaining unchanged end-to-
end.
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 packet marking or
partial multipath.
o Some features of the design presented here could be combined with
mechanisms such as shim6 [I-D.ietf-shim6-proto].
It is important to note that this document deliberately avoids any
discussion of algorithms for coupling congestion windows in order to
achieve optimum performance. Work in this area is ongoing and will
be presented in separate documents; considerable discussion can be
found in [I-D.van-beijnum-1e-mp-tcp-00]
1.3. 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
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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 architecture.
+-------------------------------+
| Application |
+---------------+ +-------------------------------+
| Application | | MPTCP |
+---------------+ + - - - - - - - + - - - - - - - +
| TCP | | TCP | 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
Appendix A.
1.4. Operation Summary
This section will, very briefly, provide a high-level summary of the
normal operation case 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. An application begins by opening a TCP socket in the
normal way.
o An MPTCP connection begins as a single TCP session. This
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
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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') can be achieved through alternative mechanisms, two of
which are described in this document for comment.
o The exact properties of these TCP sessions that are logically
bonded are dependent upon the congestion and flow control
characteristics of the endpoints' MPTCP implementation.
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
1.5. 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 Congestion control, and especially mechanisms by which congestion
windows should be coupled to best respond to congestion on a path.
This is also related to retransmission algorithms, in particular
how to decide when to retransmit packets on the same or different
paths.
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o Correct path/address management scheme. There are two schemes
(implicit and explicit) presented in this document. The authors
generally tend towards the implicit scheme for simplicity, however
both are presented to solicit feedback. Other alternatives are
also welcome!
o Best handshake mechanisms. 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 of subflow usage.
The ECN signal is currently proposed but other alternatives,
including path property options, could be employed instead.
1.6. 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 [RFC2119].
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.
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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.
Sequence Number: The TCP sequence number is subflow-specific, with a
data sequence number used for reassembly for higher layers.
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, as
the link between a subflow and a connection is established at the
SYN handshake. 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.
IP Address: The IP address presented to the application layer in a
non-multipath-aware application is that of the first address
connected to, even if that address has since been removed from the
connection.
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
discussed in Section 9, and are defined throughout the following
subsections.
This document currently presents two alternatives for management of
addresses to set up additional subflows:
Explicit Path Management: Each endpoint shares a list of addresses
on which it can be reached. Either endpoint can then initiate new
subflows between any pair of these addresses.
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Implicit Path Management: A multihomed endpoint starts additional
subflows, by connecting from an address not currently in use in
the connection to a destination that is in use in an existing
subflow.
We present these alternatives in order to solicit feedback on the
most appropriate mechanism to use for maximum compatibility and thus
liklihood of take-up.
Briefly, the key differences are that explicit path management
provides additional flexibility in the ability of endpoints to use
any combination of addresses (not just those already active), whereas
implicit path management is relatively simpler (requiring fewer TCP
options), and also has functionality to work around NATs.
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 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, but it must be
unique for the sender. The token should be difficult for an attacker
to guess, and thus it is recommended to be generated randomly.
(However, see further discussions about security in Section 5.)
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 techniques 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
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version provided in this specification is 0. The reserved bits may
be used for connection-specific flags in later versions.
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
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 sender to choose how
to behave. For example, the sender 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.
4.2. Address Knowledge Exchange (Path Management)
This section presents two alternative path management techniques, as
introduced at the start of Section 4.
4.2.1. Explicit Path Management
With explicit path management, the addresses over which a host is
accessible are announced to the other party through in-band
signalling, and then hosts can set up new TCP subflows on any subset
of combinations of (source, destination) address pairs. Either
endpoint can initiate the creation of a new subflow.
4.2.1.1. Adding Addresses
Announcing additional addresses that an endpoint can be reached on
will be undertaken by the Add Address TCP Option (Figure 4), where an
(index, address) pair can be announced to the other endpoint.
Several addresses can be added if there is sufficient TCP option
space, otherwise multiple TCP messages containing this option must be
sent. This option can be used at any time during a connection; not
just at the initial SYN/ACK exchange.
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The Add Address option announces a list of alternative IP addresses,
beyond the current one in use, that the sender can be contacted on.
This option can be used multiple times until all available addresses
have been announced, in order to get around TCP option space limits.
It should be noted that every address has an index which can be used
for address removal, and therefore endpoints must cache the mapping
between index and address. The index must be unique to the sender,
and although it is expected to be sequential this is not mandated.
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).
Multiple addresses can be included, with an index 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, 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 | Index | IPVer |(resvd)|
+---------------+---------------+---------------+-------+-------+
| Address (IPv4 - 4 octets) |
+---------------------------------------------------------------+
( ... further Index/Version/Address fields as required ... )
Figure 4: Add Address option (for IPv4)
If an index is already in use, it should be treated as a request to
remove the existing address (see Section 4.2.1.2) followed by a new
addition at that new index.
4.2.1.2. 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 5), which
will remove a previously-added address (or list of addresses) from a
connection and terminate any subflows currently using it.
The sending and receipt of this message should trigger the sending of
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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.
If there is no address at the requested indices, the receiver will
silently ignore the request.
Address removal is undertaken by index, so as to permit the use of
(MPTCP-aware) NATs and other middleboxes, in the cases where new
connections have been initiated but now want to be removed.
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 | Index | ...
+---------------+---------------+---------------+
Figure 5: Remove Address option
4.2.2. Implicit Path Management
As opposed to the explicit path management presented above, this
method does not exchange a list of addresses which are then
(independently) used to set up subflows. Instead, knowledge of an
endpoint's alternative addresses is obtained only when an additional
subflow is being set up (see Section 4.3). Subflows are started,
joining a pre-existing connection, with no pre-negotiation. A
"Request-SYN" option can also used to request a SYN in the reverse
direction, in order to get around middleboxes, notably NATs.
The implicit mechanism makes use of SYNs and connection identifiers
in order to add new subflows to an existing connection. The
following is an example of how this should work:
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 4.3). (A multihomed
destination may open a new subflow from its new address to the
source address and port, or a multihomed source may open a new
subflow from its new address another connection to the existing
destination and port).
o To expand upon this, say a connection is intiated from host "A" on
(address, port) combination A1 to desintation (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
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declared token.
o Simultaneously, a "Request-SYN" option is sent on an existing TCP
connection, asking the recipient to try to open a connection to
the sender's additional address. This is intended to permit new
sessions to be opened if one endpoint is behind a NAT.
o Using the previous notation, this would be a Request-SYN packet
sent from A1 to B1 requesting a SYN to be sent from B1 to A2.
As can be seen, the implicit path management is designed for ease of
deployment and operation through middleboxes such as NATs. The main
drawback is that new subflows can only be started with one of the two
addresses being part of an existing subflow, since there is no
separate exchange of addresses. This improves security and
simplicity but limits the flexibility and speed of being able to set
up entirely disjoint subflows immediately on an address list
exchange. However, once multiple addresses exist at one endpoint,
the other endpoint can target new connections at any or all of these.
4.2.2.1. Request-SYN
This packet requests the recipient to send a SYN (with a join option,
discussed in Section 4.3) to the presented IP address to initiate a
new subflow. The motivation for this is to get around NATs and
firewalls that may block SYN packets in the forward direction. This
packet could be seen as fulfilling the same function as "Add Address"
for explicit path management.
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 19.
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_REQSYN| Length | IPVer |(resvd)| Address... :
+---------------+---------------+-------+-------+---------------+
: ... Address (4 octets - IPv4 version only) |
+-----------------------------------------------+
Figure 6: Request-SYN option (IPv4 version)
OPEN ISSUES: Must the recipient reply from the same address? Can a
nonce be used for security here by echoing it in the "join" option in
the SYN? Do we need anything to prevent DoS here? We also will need
to define the logic of responding to this versus having already sent
a SYN (related to the simultaneous open issue).
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4.2.2.2. Request-FIN (Remove Address)
OPEN ISSUE: Do we want to be able to do Request-FIN? It would be
used to do cleanups of other subflows, e.g. when an interface becomes
unavailable (i.e. like "Remove Address" for explicit path
management). Assuming we need this option, somehow we need to be
able to identify the existing subflows. This is particularly
difficult when there is no subflow identifier.
The primary reason for this message is to allow a sender to tell its
receiver that a particular inferface has been unexpectedly lost, and
thus it should close any connections associated with it. Although
this is purely an efficiency and not essential to the operation of
the protocol, it would nevertheless be useful to deploy such a
mechanism. As currently proposed, this option will not work through
non-MPTCP-aware NATs, and so it should not be expected to be
received.
This option works by a sender identifying the source address that is
no longer valid. A Request-FIN requests the recipient to send a FIN
on the affected subflow(s), and then it can close the subflows with a
short timeout. The sender should also send FINs, however the
Request-FIN is used to help clean up state on middleboxes on subflows
that have unexpectedly broken.
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 19.
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_REQFIN| Length | IPVer |(resvd)| Address... :
+---------------+---------------+-------+-------+---------------+
: ... Address (4 octets - IPv4 version only) |
+-----------------------------------------------+
Figure 7: Request-FIN option (IPv4 version)
4.3. Starting a New Subflow
Endpoints have knowledge of their own multiple addresses, and can
become aware of the other endpoint's addresses through a path
management technique as described in Section 4.2. Once this
knowledge has been gathered, an endpoint will want to initiate a new
subflow over a currently unused pair of addresses.
A new subflow is started as a normal TCP SYN/ACK exchange, to (or
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from) a different address to one already in use. The following TCP
option is used to identify which connection the new subflow should
become part of. The token used is the locally unique token of the
destination for the connection, 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, and as such it is this token that is used
for demuxing at the receiver. A receiver must store some mapping
state, of (source_addr, dest_addr, source_port, dest_port) to its
token, using information from the initial SYN exchange, in order to
enable this. In practice, however, 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.
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 = 6 |Receiver Token (4 octets total):
+---------------+---------------+-------------------------------+
: Receiver Token (continued) |
+-------------------------------+
Figure 8: Join Connection option
4.4. General MPTCP Operation
This section discusses operation of MPTCP for data transfer,
independent of the path management mechanism used.
At a high level, the 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 Number (Figure 9) option, which defines the sequence in
the data stream of the first octet of the packet's payload, and this
is used by the receiver to ensure in-order delivery to th
applicationlayers. Meanwhile, the subflow-level sequence numbers
(i.e. the regular TCP header sequence numbers) 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
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 [RFC2018]
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is used as an efficiency at the subflow level. Each subflow will
maintain its own congestion widow.
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-2) octets ) |
+-------------------------------+
Figure 9: Data Sequence Number option
As a TCP option contains a length field, the length of the Data
Sequence Number can be declared implicitly. Although it is expected
that initial implementations will use 32-bit sequence numbers (i.e. 4
octets, so a length field of 6), setting the length field to 10 and
including a 64-bit sequence number (of four octets) MUST be
considered valid and processed appropriately. This may have also
have useful security implications, discussed in Section 5.
As wth the standard TCP sequence number, the data sequence number
should not start at zero, but at a random value to make session hi-
jacking harder.
The Data Sequence Number is included in every MPTCP packet that
contains data (or a DATA FIN, see Section 4.5), even if only one path
is in use, so long as the MPTCP handshake has been completed and the
endpoints have therefore agreed to use MPTCP.
The MPTCP data and subflow level sequence numbering could be said 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. Subflow Policy
Within a local MPTCP implementation, a host may use any policy it
wishes to decide how to share the traffic to be sent over the
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available paths.
In the typical use case, where the goal is to maximise throughput, it
is necessary to couple the congestion windows in use on each subflow,
in order to react in the most appropriate way to congestion on
subflows. This is the subject of significant theoretical and
practical research outside the scope of this document.
In other use cases, a user may split traffic across available
subflows according to local policy. Typically such cases would be 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 delay or bandwidth, in cases where the additional paths are
significantly worse and not worth including in the base operation.
Other metrics such as this could be wrapped into an overall "cost"
metric for a link.
The ability to make effective choices at the sender requires full
knowledge of the path characteristics, 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, such as in the
case of laptop computers with wired and wireless connectivity.
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, this could be achieved by the receiver not
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, it is proposed to use ECN [RFC3168] 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. A potential solution to this would be
that, if there is significant congestion, or the set of available
paths has changed, MPTCP should wipe all subflow state and restart
the multiplicative increase on all paths that appear uncongested.
ECN will stop any paths that are still not required immediately,
while the receiver's desired backup path will be in use and
throughput will increase quickly. This proposal should be no worse
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than current TCP.
4.4.2. 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 4.4.1). 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, it may be necessary to use the re-sync packet
(Section 4.4.3) in order to skip over the subflow sequence numbers
that were not retransmitted on the original subflow. Of course, such
a retransmission will only occur if 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. Similarly, local
implementation/policy will also determine how to modify the treatment
of paths after packet loss - for example, how long to wait until
returning to treating it as the preferred path. 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
can occur.
4.4.3. Resync Packet
The resync packet is 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
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sequence number.
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 10: Resync option
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 11, 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,
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and as such can be used as a rapid indication of the end of data from
a sender. Therefore, it is an optimisation to 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 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 can 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
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; this must
wait for all subflows to acknowledge.
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 11: DATA FIN option
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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.
The amount of error handling required may also have an impact on the
choice of path management schemes. Issues may include odd cases
where a data sequence number is missing from a subflow. Will
definitely need errors in those cases.
5. Security Considerations
TBD
(Token generation, handshake mechanisms, new subflow authentication,
etc...)
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 4.3 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.
Yet another alternative would be for the SYN packet 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).
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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.
The "Request-FIN" option (if included) is possibly vulnerable to TCP-
Reset style attacks, however the presense of the subflow and data-
level sequence numbers should provide some level of freshness
verification.
6. Interactions with Middleboxes
TBD
How we get around NATs, firewalls. Problems with TCP proxies. How
to make an MPTCP-aware middlebox, ...
7. Interfaces
TBD
Interface with applications, interface with TCP, interface with lower
layers...
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:
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+------------+-----------------+----------+-----------------+-------+
| Symbol | Name | PM | Ref | Value |
+------------+-----------------+----------+-----------------+-------+
| OPT_MPC | Multipath | - | Section 4.1 | (tbc) |
| | Capable | | | |
| OPT_ADDR | Add Address | Explicit | Section 4.2.1.1 | (tbc) |
| OPT_REMADR | Remove Address | Explicit | Section 4.2.1.2 | (tbc) |
| OPT_REQSYN | Request-SYN | Implicit | Section 4.2.2.1 | (tbc) |
| OPT_REQFIN | Request-FIN | Implicit | Section 4.2.2.2 | (tbc) |
| OPT_JOIN | Join Connection | - | Section 4.3 | (tbc) |
| OPT_DSN | Data Sequence | - | Section 4.4 | (tbc) |
| | Number | | | |
| OPT_RESYNC | Re-sync | - | Section 4.4.3 | (tbc) |
| OPT_DFIN | DATA FIN | - | Section 4.5 | (tbc) |
+------------+-----------------+----------+-----------------+-------+
Table 1: TCP Options for MPTCP
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
10.2. Informative References
[I-D.eddy-tcp-loo]
Eddy, W. and A. Langley, "Extending the Space Available
for TCP Options", draft-eddy-tcp-loo-04 (work in
progress), July 2008.
[I-D.ietf-shim6-proto]
Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", draft-ietf-shim6-proto-12 (work
in progress), February 2009.
[I-D.van-beijnum-1e-mp-tcp-00]
van Beijnum, I., "One-ended Multipath TCP",
draft-van-beijnum-1e-mp-tcp-00 (work in progress),
May 2009.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
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[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061,
September 2007.
[RFC5062] Stewart, R., Tuexen, M., and G. Camarillo, "Security
Attacks Found Against the Stream Control Transmission
Protocol (SCTP) and Current Countermeasures", RFC 5062,
September 2007.
[WISCHIK] Wischik, D., Handley, M., and M. Bagnulo Braun, "The
Resource Pooling Principle", ACM SIGCOMM CCR vol. 38 num.
5, pp. 47-52, October 2008,
<http://ccr.sigcomm.org/online/files/p47-handleyA4.pdf>.
Appendix A. Functional Separation
[Potential to move to separate architectural document]
This section describes the functional separation that drives the
design of the MPTCP protocol. Its main goal is to separate MPTCP in
two parts that communicate through a well defined interface. We
first provide the motivations for this functional separation, then we
describe in more details the two main components of the MPTCP
architecture.
A.1. Motivations
The major goal behind MPTCP is to send data over different paths in
the same time. This assumes that an MPTCP implementation must be
able to discover and use the multiple paths that connect two given
hosts, when they exist. However, different mechanisms can be
envisioned for multipath discovery and use. Examples are as follows:
Use multiple addresses: This is the method currently proposed in
this document - if hosts are multi-addressed, different address
pairs may take different routes.
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Use a path selector value: An end-host might be able to tag packets
with a path selector value, or "colour". If some network nodes
are able to read the colour and use it as a path selector, the
host can influence the outgoing path of the packet.
Next-hop selection: In a network configuration where multiple next-
hops can offer to forward packets, a host may decide to send some
of its packets through one next-hop, and some through another.
The above list is not exhaustive, and could grow as new network
technologies are deployed.
A.2. TCP Performance
In addition to purely sending data over multiple paths, MTCP must do
it in a way that will not affect TCP performance. This raises the
need for an efficient multipath congestion control algorithm. While
this specification does not mandate the use of any particular
algorithm for congestion control, it ensures that the protocol is
designed in such a way that any CC algorithm can be designed,
independently of the particular path management mechanism available
to the host. Consequently our architecture for MTCP decouples the
policy from the mechanism. The policy is the decision of what path
to use for each packet to send. It is mainly driven by the
implementation-dependent congestion control algorithm. The mechanism
is the technology used to ensure that a packet will be sent on the
desired path. This separation is intended to be relatively future-
proof by allowing these components to evolve at different speeds.
A.3. Architecture overview
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Control plane <-- | --> Data plane
+---------------------------------------------------------------+
| Multipath Scheduler (MPS) |
+---------------------------------------------------------------+
^ | |
| | |
|Announcing new | +-------------+
|paths. (referred | | Data packet |<--Path idx:3
|to as path indices) | +-------------+ attached
| | | by MPS
| | V
+--------------------------------------------\------------------+
| Path Manager (PM) \__________zzzzz |
+--------------------------------------------------------\------+
/ \ | \
/---------------------\ | /"\ /"\ /"\
| Path key Action | | | | | | | |
| 1 xxxxx | | | | | | | |
| 2 yyyyy | | \./ \./ \./
| 3 zzzzz | | path1 path2 path3
+---------------------+
Figure 12: Overview of MTCP architecture
A general overview of the architecture is provided in Figure 12. The
Multipath Scheduler (MPS) learns about the number of available paths
through notifications received from the Path Manager (PM). From the
point of view of the Multipath Scheduler, a path is just a number,
called a Path Index. Notifications from the PM to the MPS MAY
contain supporting information about the paths, if relevant, so that
the MPS can make more intelligent decisions about where to route
traffic. When the Multipath Scheduler initiates a communication to a
new host, it can only send the packets to the default path. But
since the Path manager is layered below the MPS, it can detect that a
new communication is happening, and tell the MPS about the other
paths it knows about.
From then on, it is possible for the MPS to attach a Path Index to
the control structure of its packets (internal to the MTCP
implementation), so that the Path Manager can map this Path Index to
the corresponding action. (see table in the lower left part of
Figure 12). The particular action depends on the network mechanism
used to select a path. Examples are address rewriting, tunnelling or
setting a path selector valude inside the packet.
The applicability of the architecture is not limited to the MTCP
protocol. While we define in this document an MTCP MPS (MTCP
Multipath Scheduler), other Multipath Schedulers can be defined. For
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example, if an appropriate socket interface is designed, applications
could behave as a Multipath Scheduler and decide where to send any
particular data. In this document we concentrate on the MTCP case,
however.
In this specification, we define the core protocol for Multipath TCP.
The core protocol is not dependent on the Path Management technique
that is chosen, and MUST be implemented in any MTCP MPS. We also
provide a default Path Manager that is based on declaring IP
addresses, and carries control information in TCP options. An
implementation of Multipath TCP can use any Path Manager, but it MUST
be able to fallback to the default PM in case the other end does not
support the custom PM. Alternative Path Managers may be specified in
separate documents in the future.
A.4. PM/MPS interface
The minimal set of requirement for a Path Manager is as follows:
o Outgoing untagged packets: Any outgoing packet flowing through the
Path Manager is either tagged or untagged (by the MPS) with a path
index. If it is untagged, the packet is sent normally to the
Internet, as if no multi-path support were present. Untagged
packets can be used to trigger a path discovery procedure, that
is, a Path Manager can listen to untagged packets and decide at
some time to find if any other path than the default one is
useable for the corresponding host pair. Note that any other
criteria could be used to decide when to start discovering
available paths. Note also that MPS scheduling will not be
possible until the Path Manager has notified the available paths.
The PM is thus the first entity coming into action.
o Outgoing tagged packets: The Path Manager maintains a table
mapping path indices to actions. The action is the operation that
allows using a particular path. Examples of possible actions are
route selection, interface selection or packet transformation.
When the PM sees a packet tagged with a path index, it looks up
its table to find the appropriate action for that packet. The tag
is purely local. It is removed before the packet is transmitted.
o Incoming packets: A Path Manager MUST ensure that incoming path is
mapped unambiguously to exactly one outgoing path. Note that this
requirement implies that the same number of incoming/outgoing
paths must be established. Moreover, a PM MUST tag any incoming
path with the same Path Index as the one used for the
corresponding outgoing path. This is necessary for MTCP to know
what outgoing path in acknowledged by an incoming packet.
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o Module interface: A PM MUST be able to notify the MPS about the
number of available paths. Such notifications MUST contain the
path indices that are legal for use by the MPS. In case the PM
decides to stop providing service for one path, it MUST notify the
MPS about path deletion. Additionnaly, a PM MAY provide
complementary path information when available, such as link
quality or preference level.
Appendix B. 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.
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 [I-D.eddy-tcp-loo], 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.
Ford, et al. Expires November 8, 2009 [Page 29]
Internet-Draft Multipath TCP May 2009
Authors' Addresses
Alan Ford (editor)
Roke Manor Research
Old Salisbury Lane
Romsey, Hampshire SO51 0ZN
UK
Phone: +44 1794 833 465
Email: alan.ford@roke.co.uk
Costin Raiciu
University College London
Email: c.raiciu@cs.ucl.ac.uk
Mark Handley
University College London
Sebastien Barre
Universite catholique de Louvain
Pl. Ste Barbe, 2
Louvain-la-Neuve 1348
Belgium
Phone: +32 10 47 91 03
Email: sebastien.barre@uclouvain.be
Ford, et al. Expires November 8, 2009 [Page 30]