Internet Engineering Task Force A. Ford, Ed.
Internet-Draft Roke Manor Research
Intended status: Informational C. Raiciu
Expires: December 24, 2010 University College London
S. Barre
Universite catholique de
Louvain
J. Iyengar
Franklin and Marshall College
June 22, 2010
Architectural Guidelines for Multipath TCP Development
draft-ietf-mptcp-architecture-01
Abstract
Endpoints are often connected by multiple paths, but TCP restricts
communications to a single path per transport connection. Resource
usage within the network would be more efficient were these multiple
paths able to be used concurrently. This should enhance user
experience through improved resilience to network failure and higher
throughput.
This document outlines architectural guidelines for the development
of a Multipath Transport Protocol, with references to how these
architectural components come together in the Multipath TCP (MPTCP)
protocol. This document also lists certain high level design
decisions that provide foundations for the MPTCP design, based upon
these architectural requirements.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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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."
This Internet-Draft will expire on December 24, 2010.
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
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This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Reference Scenario . . . . . . . . . . . . . . . . . . . . 5
2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Functional Goals . . . . . . . . . . . . . . . . . . . . . 6
2.2. Compatibility Goals . . . . . . . . . . . . . . . . . . . 7
2.2.1. Application Compatibility . . . . . . . . . . . . . . 7
2.2.2. Network Compatibility . . . . . . . . . . . . . . . . 7
2.2.3. Compatibility with other network users . . . . . . . . 8
3. An Architectural Basis For MPTCP . . . . . . . . . . . . . . . 9
4. A Functional Decomposition of MPTCP . . . . . . . . . . . . . 10
5. High-Level Design Decisions . . . . . . . . . . . . . . . . . 12
5.1. Sequence Numbering . . . . . . . . . . . . . . . . . . . . 12
5.2. Reliability . . . . . . . . . . . . . . . . . . . . . . . 13
5.3. Buffers . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.4. Signalling . . . . . . . . . . . . . . . . . . . . . . . . 15
5.5. Path Management . . . . . . . . . . . . . . . . . . . . . 15
5.6. Connection Identification . . . . . . . . . . . . . . . . 16
5.7. Network Layer Compatibility . . . . . . . . . . . . . . . 16
5.8. Congestion Control . . . . . . . . . . . . . . . . . . . . 17
6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. Interactions with Applications . . . . . . . . . . . . . . . . 17
9. Interactions with Middleboxes . . . . . . . . . . . . . . . . 18
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 19
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
13.1. Normative References . . . . . . . . . . . . . . . . . . . 20
13.2. Informative References . . . . . . . . . . . . . . . . . . 20
Appendix A. Implementation Architecture . . . . . . . . . . . . . 21
A.1. Functional Separation . . . . . . . . . . . . . . . . . . 21
A.1.1. Application to default MPTCP protocol . . . . . . . . 21
A.1.2. Generic architecture for MPTCP . . . . . . . . . . . . 24
A.2. PM/MPS interface . . . . . . . . . . . . . . . . . . . . . 25
Appendix B. Changelog . . . . . . . . . . . . . . . . . . . . . . 26
B.1. Changes since draft-ietf-mptcp-architecture-00 . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
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1. Introduction
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 constraints both on the end-
systems and within the network. If these resources could instead be
used concurrently, end user experience could be greatly improved.
Such enhancements would also reduce the necessary expenditure on
network infrastructure which would otherwise be needed to create an
equivalent improvement in user experience.
By the application of resource pooling[2], these available resources
can be 'pooled' such that they appear as a single logical resource to
the user. The purpose of a multipath transport, therefore, is to
make use of multiple available paths, through resource pooling, to
bring two key benefits:
o To increase the resilience of the connectivity by providing
multiple paths, protecting end hosts from the failure of one.
o To increase the efficiency of the resource usage, and thus
increase the network capacity available to end hosts.
Multipath TCP (MPTCP)[3] is a set of extensions for TCP[4] that
implements a multipath transport and achieves these goals by pooling
multiple paths within a transport connection, transparent to the
application. While multihoming and multipath functions have been
implemented in transport protocols previously, notably SCTP[5], MPTCP
is distinct in recognizing application and network compatibility
goals that we believe are important for deployability of a multipath
transport; we discuss these goals in more detail later in Section 2.
This document makes three contributions: (i) it describes goals for a
multipath transport - goals that MPTCP is designed to meet; (ii) it
lays out an architectural basis for MPTCP's design - a discussion
that applies to other multipath transports as well; and (iii) it
discusses and documents high-level design decisions made in MPTCP's
development, and considers their implications.
Companion documents to this architectural overview are those which
provide details of the protocol extensions[3], congestion control
algorithms[6], and application-level considerations[7]. Put
together, these components specify a complete Multipath TCP design.
We note that specific components are replaceable with other protocols
in accordance with the layer and functional decompositions discussed
in this document.
Please note this document is a work-in-progress and covers several
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topics, some of which may be more appropriately moved to separate
documents as this work evolves.
1.1. 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].
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.
Endpoint: A host either initiating or terminating a MPTCP
connection.
Multipath TCP (MPTCP): A modified version of the TCP [4] protocol
that supports the simultaneous use of multiple paths between
endpoints.
Subflow: A flow of TCP packets operating over an individual path,
which forms part of a larger MPTCP connection.
MPTCP Connection: A set of one or more subflows combined to provide
a single Multipath TCP service to an application at an endpoint.
1.3. Reference Scenario
The diagram shown in Figure 1 illustrates a typical usage scenario
for MPTCP. Two hosts, A and B, are communicating with each other.
These endpoints are multi-homed and multi-addressed, providing two
disjoint connections to the Internet. The addresses on each endpoint
are referred to as A1, A2, B1 and B2. There are therefore up to four
different paths between the two endpoints: A1-B1, A1-B2, A2-B1,
A2-B2.
+------+ __________ +------+
| |A1 ______ ( ) ______ B1| |
| Host |--/ ( ) \--| Host |
| | ( Internet ) | |
| A |--\______( )______/--| B |
| |A2 (__________) B2| |
+------+ +------+
Figure 1: Simple MPTCP Usage Scenario
The scenario could have any number of addresses (1 or more) on each
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endpoint, so long as the number of paths available between the two
endpoints is 2 or more (i.e. num_addr(A) * num_addr(B) > 1). The
paths created by these address combinations through the Internet need
not be entirely disjoint - shared bottlenecks will be addressed by
the MPTCP congestion controller. Furthermore, the paths through the
Internet may be interrupted by any number of middleboxes including
NATs and Firewalls. Finally, although the diagram refers to the
Internet, MPTCP may be used over any network where there are multiple
paths that could be used concurrently.
TBD - what further detail here would be useful?
2. Goals
This section outlines primary goals that Multipath TCP aims to meet.
These are broadly broken down into functional goals, which steer
services and features that MPTCP must provide, and compatibility
goals, which determine how MPTCP should appear to entities that
interact with it.
2.1. Functional Goals
In providing the use of multiple paths, MPTCP has the following two
functional goals.
o Improve Throughput: MPTCP MUST support the concurrent use of
multiple paths. To meet the minimum performance incentives for
deployment, an MPTCP connection over multiple paths SHOULD achieve
no lesser throughput than a single TCP connection over the best
constituent path.
o Improve Resilience: MPTCP MUST support the use of multiple paths
interchangeably for resilience purposes, by permitting packets to
be sent and re-sent on any available path. It follows that, in
the worst case, the protocol MUST be no less resilient than legacy
TCP.
As distribution of traffic among available paths and responses to
congestion are done in accordance with resource pooling
principles[2], a secondary effect of meeting these goals is that
widespread use of MPTCP over the Internet should optimize overall
network utility by shifting load away from congested bottlenecks and
by taking advantage of spare capacity wherever possible.
Furthermore, MPTCP SHOULD feature automatic negotiation of its use.
A host supporting Multipath TCP that requires the other endpoint to
do so too must be able to detect reliably whether this endpoint does
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in fact support the next-generation protocol, using it if so, and
otherwise automatically falling back to the legacy protocol.
2.2. Compatibility Goals
In addition to the functional goals listed above, a Multipath TCP
must meet a number of compatibility goals in order to support
deployment in today's Internet. These goals fall into the following
categories:
2.2.1. Application Compatibility
Application compatibility refers to the appearance of MPTCP to the
application both in terms of the API that can be used and the
expected service model that is provided.
MPTCP MUST follow the same service model as TCP [4]: in-order,
reliable, and byte-oriented delivery. Furthermore, an MPTCP
connection SHOULD provide the application with no worse throughput
than it would expect from running a single TCP connection over any
one of its available paths.
A multipath-capable equivalent of TCP SHOULD retain backward
compatibility with existing TCP APIs, so that existing applications
can use the newer transport merely by upgrading the operating systems
of the end-hosts. This does not preclude the use of an advanced API
to permit multipath-aware applications to specify preferences, nor
for users to configure their systems in a different way from the
default, for example switching on or off the automatic use of MPTCP.
2.2.2. Network Compatibility
Traditional Internet architecture slots network devices in the
network layer and lower layers of the OSI 7-layer stack, where the
layers above the network layer - the transport layer and upper layers
- are instantiated only at the end-hosts. While this architecture,
shown in Figure 2, was largely adhered to earlier, this layering no
longer reflects the "ground truth" in the Internet with the
proliferation of middleboxes[8]. Middleboxes routinely interpose on
the transport layer; sometimes even completely terminating transport
connections, thus leaving the application layer as the first real
end-to-end layer, as shown in Figure 3.
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+-------------+ +-------------+
| Application |<------------ end-to-end ------------->| Application |
+-------------+ +-------------+
| Transport |<------------ end-to-end ------------->| Transport |
+-------------+ +-------------+ +-------------+ +-------------+
| Network |<->| Network |<->| Network |<->| Network |
+-------------+ +-------------+ +-------------+ +-------------+
End Host Router Router End Host
Figure 2: Traditional Internet Architecture
+-------------+ +-------------+
| Application |<------------ end-to-end ------------->| Application |
+-------------+ +-------------+ +-------------+
| Transport |<------------------->| Transport |<->| Transport |
+-------------+ +-------------+ +-------------+ +-------------+
| Network |<->| Network |<->| Network |<->| Network |
+-------------+ +-------------+ +-------------+ +-------------+
Firewall,
End Host Router NAT, or Proxy End Host
Figure 3: Internet Reality
Middleboxes that interpose on the transport layer result in loss of
"fate-sharing"[9], that is, they often hold "hard" state that, when
lost or corrupted, results in loss or corruption of the end-to-end
transport connection.
MPTCP MUST remain backward compatible with the Internet as it exists
today, including being able to traverse predominant middleboxes such
as firewalls, NATs, and performance enhancing proxies[8]. This
requirement comes from recognizing middleboxes as a significant
deployment bottleneck for any transport that is not TCP, and
constrains MPTCP to appear as TCP does on the wire and to use
established TCP extensions where necessary. To ensure end-to-endness
of the transport, we further require MPTCP to preserve fate-sharing
without making any assumptions about middlebox behavior.
2.2.3. Compatibility with other network users
As a corollary to both network and application compatibility, the
architecture must enable new Multipath TCP flows to coexist
gracefully with existing legacy TCP flows, competing for bandwidth
neither unduly aggressively or unduly timidly (unless low-precedence
operation is specifically requested by the application, such as with
LEDBAT). The use of multiple paths MUST not unduly harm users using
single path TCP at shared bottlenecks, beyond the impact that would
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occur from another single legacy TCP flow.
3. An Architectural Basis For MPTCP
We now present one possible transport architecture that we believe
can effectively support MPTCP's goals. The new Internet model
described here is based on ideas proposed earlier in Tng ("Transport
next-generation") [10]. While by no means the only possible
architecture supporting multipath transport, Tng incorporates many
lessons learned from previous transport research and development
practice, and offers a strong starting point from which to consider
the extant Internet architecture and its bearing on the design of any
new Internet transports or transport extensions.
+------------------+
| Application |
+------------------+ ^ Application-oriented transport
| | | functions (Semantic Layer)
+ - - Transport - -+ ----------------------------------
| | | Network-oriented transport
+------------------+ v functions (Flow+Endpoint Layer)
| Network |
+------------------+
Existing Layers Tng Decomposition
Figure 4: Decomposition of Transport Functions
Tng loosely splits the transport layer into "application-oriented"
and "network-oriented" layers, as shown in Figure 4. The
application-oriented "Semantic" layer implements functions driven
primarily by concerns of supporting and protecting the application's
end-to-end communication, while the network-oriented "Flow+Endpoint"
layer implements functions such as endpoint identification (using
port numbers) and congestion control. These network-oriented
functions, while traditionally located in the ostensibly "end-to-end"
Transport layer, have proven in practice to be of great concern to
network operators and the middleboxes they deploy in the network to
enforce network usage policies[11] [12] or optimize communication
performance[13]. Figure 5 shows how middleboxes interact with
different layers in this decomposed model of the transport layer: the
application-oriented layer operates end-to-end, while the network-
oriented layer operates "segment-by-segment" and can be interposed
upon by middleboxes.
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+-------------+ +-------------+
| Application |<------------ end-to-end ------------->| Application |
+-------------+ +-------------+
| Semantic |<------------ end-to-end ------------->| Semantic |
+-------------+ +-------------+ +-------------+ +-------------+
|Flow+Endpoint|<->|Flow+Endpoint|<->|Flow+Endpoint|<->|Flow+Endpoint|
+-------------+ +-------------+ +-------------+ +-------------+
| Network |<->| Network |<->| Network |<->| Network |
+-------------+ +-------------+ +-------------+ +-------------+
Firewall Performance
End Host or NAT Enhancing Proxy End Host
Figure 5: Middleboxes in the new Internet model
MPTCP's architectural design follows Tng's decomposition as shown in
Figure 6. The MPTCP component, which provides application
compatibility through the preservation of TCP-like semantics of
global ordering of application data and reliability, is an
instantiation of the "application-oriented" Semantic layer; whereas
the legacy-TCP component, which provides network compatibility by
appearing and behaving as a TCP flow in network, is an instantiation
of the "network-oriented" Flow+Endpoint layer.
+--------------------------+ +-------------------------+
| Application | | Application |
+--------------------------+ +-------------------------+
| Semantic | | MPTCP |
|--------------------------| + - - - - - + - - - - - +
| Flow+Endpt | Flow+Endpt | | TCP | TCP |
+--------------------------+ +-------------------------+
| Network | Network | | IP | IP |
+--------------------------+ +-------------------------+
Figure 6: MPTCP mapping to Tng
As a protocol extension to TCP, MPTCP thus explicitly acknowledges
middleboxes in its design, and specifies a protocol that operates at
two scales: the MPTCP component operates end-to-end, while it allows
the TCP component to operate segment-by-segment.
4. A Functional Decomposition of MPTCP
Having laid out the goals to be met and the architectural basis for
MPTCP, we now provide a functional decomposition MPTCP's design.
The MPTCP component relies upon (what appear to the network to be)
standard TCP sessions, termed "subflows", to provide the underlying
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transport per path, and as such these retain the network
compatibility desired. MPTCP as described in [3] carries MPTCP-
specific information in a TCP-compatible manner, although this
mechanism is separate from the actual information being transferred
so could evolve in future revisions. Figure 7 illustrates the
layered architecture.
+-------------------------------+
| Application |
+---------------+ +-------------------------------+
| Application | | MPTCP |
+---------------+ + - - - - - - - + - - - - - - - +
| TCP | | Subflow (TCP) | Subflow (TCP) |
+---------------+ +-------------------------------+
| IP | | IP | IP |
+---------------+ +-------------------------------+
Figure 7: Comparison of Standard TCP and MPTCP Protocol Stacks
Situated below the application, the MPTCP extension manages multiple
TCP subflows below it and must implement the following functions:
o Path Management: This is the function to detect and use multiple
paths between two endpoints. In the case of the MPTCP design [3],
this feature is implemented using multiple IP addresses at least
one of the endpoints. Although this does not guarantee path
diversity, and there may be shared bottlenecks, this is a simple
mechanism that can be used with no additional features in the
network. The path management features of the MPTCP protocol are
the mechanisms to signal alternative addresses to endpoints, and
mechanisms to set up new subflows attached to an existing MPTCP
connection.
o Packet Scheduling: This function breaks the bytestream received
from the application into segments which are transmitted on one of
the available lower subflows. The MPTCP design makes use of a
data sequence mapping, associating packets sent on different
subflows to a connection-level sequence numbering, thus allowing
packets sent on different subflows to be correctly re-ordered at
the receiver. The packet scheduler is dependent upon information
about the availability of paths exposed by the path management
component, and then makes use of the subflows to transmit these
packets.
o Subflow (single-path TCP) Interface: A subflow component takes
segments from the packet-scheduling component and transmits them
over the specified path, ensuring detectable delivery to the
endpoint. Detection of delivery is necessary to allow the
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congestion control protocol to attribute packet delivery or loss
to the right path. Note that the packet scheduling component does
not embed enough information in packets to allow this to happen:
segments with the same connection-level sequence number can be
transmitted over multiple paths, i.e. as retransmissions or just
to increase redundancy. MPTCP uses TCP underneath for network
compatibility; TCP ensures in-order, reliable delivery. TCP adds
its of sequence numbers to the segments; these are used to detect
and retransmit lost packets.
o Congestion Control: This function manages congestion control
across the subflows. As specified, this congestion control
algorithm must ensure that a MPTCP connection does not unfairly
take more bandwidth than a single path TCP flow would take at a
shared bottlneck. An algorithm to support this is specified in
[6].
These functions fit together as follows. The Path Management looks
after the discovery (and if necessary, initialisation) of multiple
paths between two endpoints. The Packet Scheduler then receives
packets from the application for the network and does the necessary
operations on them (such as adding a data-level sequence number)
before sending to a subflow. The subflow then adds its own sequence
number, acks, and passes them to network. The receiving subflow re-
orders data and passes it to the MPTCP component, which performs
connection level re-ordering, removes the segment boundaries and
sends it to the application. Finally, the congestion control
component exists as part of the packet scheduling, in order to
schedule which packets should be sent at what rate on which subflow.
5. High-Level Design Decisions
There is seemingly a wide range of choices when designing a multipath
extension to TCP. However, the goals as discussed earlier in this
document constrain the possible solutions, leaving relative little
choice in many areas. Here, we outline high-level design choices
that draw from the architectural basis discussed earlier in
Section 3, and their implications for the MPTCP design.
5.1. Sequence Numbering
MPTCP uses two levels of sequence spaces: a connection level sequence
number, and another sequence number for each subflow. This permits
connection-level segmentation and reassembly, and retransmission of
the same part of connection-level sequence space on different
subflow-level sequence space.
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The alternative approach would be to use a single connection level
sequence number, which gets sent on multiple subflows. This has two
problems: first, the individual subflows will appear to the network
as TCP sessions with gaps in the sequence space; this in turn may
upset certain middleboxes such as intrusion detection systems, or
certain transparent proxies, and would go against the network
compatibility goal. Second, the sender cannot attribute packet
losses or receptions to the correct path when the same packet is sent
on multiple paths, in the case of retransmissions.
The sender must be able to tell the receiver how to reorder the data,
for delivery to the application. The sender does so by telling the
receiver how subflow-level data (carying subflow sequence numbers)
maps at connection level, which we refer to as Data Sequence Mapping.
This mapping takes the form (data seq, subflow seq, length), i.e. for
a given number of bytes (the length), the subflow sequence space
beginning at the given sequence number maps to the connection-level
sequence space (beginning at the given data seq number).
This architecture does not mandate a mechanism for signalling such
information, and it could conceivably have various sources.
One option would be to use existing fields in the TCP segment (such
as subflow seqno, length) and only add the data sequence number to
each segment, for instance as a TCP option. This is, however,
vulnerable to middleboxes that resegment or assemble data, since
there is no specified behaviour for coalescing TCP options. If one
signalled (data seqno, length), this would still be vulnerable to
middleboxes that coalesce segments and do not correctly coalesce the
options. Because of these potential issues, the current
specification of MPTCP mandates that the full mapping should be sent
to the other end.
To reduce the overhead, it would be permissable for the mapping to be
sent periodically and cover more than a single segment. It could
also be excluded entirely in the case of a connection before more
than one subflow is used, where the data-level and subflow-level
sequence space is the same.
5.2. Reliability
Under normal behaviour, MPTCP can use the data sequence mapping and
subflow ACKs to decide when a connection-level segment was received.
This has certain implications on end-to-end semantics. It means that
once a packet is acked at subflow level it cannot be discarded in the
re-order buffer at the connection level. Secondly, unlike in
standard TCP, a receiver cannot simply drop out-of-order segments if
needed (for instance, due to memory pressure).
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Furthermore, it is possible to conceive of some cases where
connection-level acknowledgements could improve robustness. Consider
a subflow traversing a transparent proxy: if the proxy acks a segment
and then crashes, the sender will not retransmit the lost segment on
another subflow, as it thinks the segment has been received. The
connection grinds to a halt despite having other working subflows,
and the sender would be unable to determine the cause of the problem.
Finally, as an optimisation, it may be feasible for a connection-
level acknowledgement to be transmitted over the shortest RTT path,
potentially reducing send buffer requirements (see Section 5.3).
Therefore, to provide a fully robust multipath TCP solution, MPTCP
SHOULD feature explicit connection-level acknowledgements.
Regarding retransmissions, it MUST be possible for a packet to be
retransmitted on a different subflow to that on which it was
originally sent. This is one of MPTCP's core goals, in order to
maintain integrity during temporary or permanent subflow failure, and
this is enabled by the dual sequence number space.
The scheduling of retransmissions will have significant impact on
MPTCP user experience. The current MPTCP specification suggests that
data outstanding on subflows that have timed out should be
rescheduled for transmission on different subflows. This behaviour
aims to minimize disruption when a path breaks, and uses the first
timeout as indicators. More conservative versions would be to use
second or third timeouts for the same packet.
When packet loss is detected and corrected with fast retransmit,
retransmission on different subflows may still be desirable in
certain cases, for instance to reduce the receive buffer
requirements. However, in all cases with retransmissions on
different subflows, the lost packets SHOULD still be sent on the path
that lost them. This is currently believed to be necessary to
maintain subflow integrity, as per the network compatiblity goal. By
doing this, throughput will be wasted, and it is unclear at this
point what the optimal retransmit strategy is.
5.3. Buffers
Receive Buffer: ideally, a subflow failing should not affect the
throughput of other working subflows. However, the receive buffer
has limited size: if a flow times out, the other subflows will
quickly fill the receive buffer with out-of-order data, and will
stall. Hence, receive buffer sizing is important for both robustness
and throughput.
The smallest receive buffer we need to avoid stalling under any
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circumstances is max(RTO)*sum(BW). This is, for most multipath
connections, too expensive. A more reasonable size is proportional
to max(RTT)*sum(BW) which ensures subflows don't stall when fast
retransmit works. Also, depending on how the implementation behaves,
an additional sum(RTT*BW) might be needed for the individual re-order
buffers of the TCP subflows.
Send Buffer: the smallest send buffer we need is sum(BDP) across all
paths; this is to hold data until it's acked at subflow level. If we
didn't use a subflow level ack, and relied on a data-level ack, the
send buffer would need to be as big as the receive buffer of the
connection, max(RTT)*sum(BW). In practice, the senders will be web
servers and receivers will be desktops or mobile servers. The send
buffer size matters particularly for servers, which must be able to
maintain a large number of ongoing connections.
5.4. Signalling
Since MPTCP will use regular TCP streams as its transport mechanism,
a MPTCP connection will also begin as a single TCP stream.
Nevertheless, it must signal to the peer that it supports MPTCP and
wishes to use it on this connection. As such, a TCP Option will be
used to transmit this information, since this is the established
mechanism for indicating additional functionality on a TCP session.
On top of this, however, is signalling required during the operation
of an MPTCP session, such as that for reassembly for multiple
subflows, and for informing the other endpoint about potential other
available addresses. It is not mandated by the architecture in what
format this signalling should be transmitted.
The current MPTCP protocol proposal suggests the use of TCP options
for this signalling, however another approach would be to embed such
information in the payload, and use type-length-value (TLV) encoding
to separate signalling and payload data.
5.5. Path Management
Currently, the network does not expose multiple paths between
endpoints. Multipath TCP will use multiple addresses at one or both
endpoints to get different paths to the destination. The hope is
that these paths, whilst not necesarily entirely non-overlapping,
will be sufficiently disjoint to allow multipath achieve improved
throughput and robustness.
Multiple different (source, destination) address pairs will thus be
used as path selectors. Each path will be identified by a TCP
4-tuple (i.e. source address, destination address, source port,
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destination port), thus allowing the extension of MPTCP to use such
4-tuples as path selectors if the network will route different ports
over different paths (which may be the case with technologies such as
ECMP).
For increased chance of successfully setting up additional subflows
(such as when one end is behind a firewall, NAT, or other restrictive
middlebox), either endpoint should be able to add new subflows to a
MPTCP connection.
The modularity of path management will permit alternative mechanisms
to be employed if appropriate in the future.
5.6. Connection Identification
Therefore, each MPTCP connection should have a connection identifier
at each endpoint, which is locally unique within that endpoint. In
many ways, this is analogous to a port number in regular TCP. The
manifestation and purpose of such an identifier is out of the scope
of this architecture document.
Legacy applications will not, however, have access to this identifier
and in such cases a MPTCP connection will be identified by the
5-tuple of the first TCP subflow. It is out of the scope of this
document, however, to define the behaviour of the MPTCP
implementation if the first TCP subflow later fails. If there are
legacy applications that make assumptions about continued existance
of the initial address pair, their behaviour could be disrupted by
carrying on regardless. It is expected that this is a very small,
possibly negligible, set of applications, however. In the case of
applications that have specifically asked to be bound to a particular
address or interface, MPTCP will not be used.
Since the requirements of applications are not clear at this stage,
however, it is as yet unconfirmed what the best behaviour is. It
will be an implementation-specific solution, however, and as such the
behaviour is expected to be chosen by implementors once more research
has been undertaken to determine its impact.
5.7. Network Layer Compatibility
MPTCP's modifications remain at the transport layer, although some
knowledge of the underlying network layer is required. MPTCP MUST
work with IPv4 and IPv6 interchangeably, i.e. one MPTCP connection
may operate over both IPv4 and IPv6 networks.
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5.8. Congestion Control
As already documented in network-layer compatibility requirements,
the congestion control algorithms used by an MPTCP implementation
must not harm other legacy users on shared bottlenecks. To achieve
this, the congestion control algorithms on use on each subflow must
be coupled in some way - a proposal for this is given in [6].
6. Summary
This document has provided a summary of the components that have been
identified to provide a Multipath TCP solution, and described the
high-level design decisions that have been used as a basis of the
MPTCP specification.
The suite of drafts that specify a complete MPTCP implementation, on
top of this architectural overview, are as follows:
o A specification of the MPTCP protocol [3], describing the on- and
off-the-wire differences to regular TCP.
o A specification of a coupled congestion control algorithm [6],
that can be applied to the above protocol while meeting the goals
for such an algorithm as specified in this document.
o A document [7] that builds upon the application compatibility
issues discussed in this document, explaining in more detail what
if any changes an application may experience through the use of
MPTCP. This document also provides a proposed API through which
an application can influence the behaviour of the MPTCP protocol,
as specified in the above drafts.
7. Security Considerations
Please see [14] for a threat analysis of Multipath TCP. The threats
analysed in this companion document are addressed as appropriate in
the protocol design [3].
8. Interactions with Applications
Interactions with applications - incuding, but not limited to,
performances changes that may be expected, semantic changes, and new
features that may be requested of an API, are presented in [7].
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9. Interactions with Middleboxes
As discussed in Section 2.2, it is a goal of MPTCP to be deployable
today and thus compatible with the majority of middleboxes. This
section summarises the issues that may arise with NATs, firewalls,
proxies, intrusion detection systems, and other middleboxes that, if
not considered in the protocol design, may hinder its deployment.
This section is intended primarily as a description of options and
considerations only. Protocol-specific solutions to these issues
will be given in the companion documents.
Multipath TCP will be deployed in a network that no longer provides
just basic datagram delivery. A miriad of middleboxes are deployed
to optimize various perceived problems with the Internet protocols:
NATs primarily address space shortage [11], Performance Enhancing
Proxies (PEPs) optimize TCP for different link characteristics [13],
firewalls [12] and intrusion detection systems try to block malicious
content from reaching a host, and traffic normalizers [15] ensure a
consistent view of the traffic stream to IDSes and hosts.
All these middleboxes optimize current applications at the expense of
future applications. In effect, future applications must mimic
existing ones if they want to be deployed. Further, the precise
behaviour of all these middleboxes is not clearly specified, and
implementation errors make matters worse, raising the bar for the
deployment of new technologies.
The following list of middlebox classes documents behaviour that
could impact the use of MPTCP. This list is used in [3] to describe
the features of the MPTCP protocol that are used to mitigate the
impact of these middlebox behaviours.
o NATs: Network Address Translators decouple the endpoint's local IP
address with that which is seen in the wider Internet when the
packets are transmitted through a NAT. This adds complexity, and
reduces the chances of success, when signalling IP addresses.
o PEPs: Performance Enhancing Proxies, which aim to improve the
performance of protocols over low-performance (e.g. high latency
or high error rate) links. As such, they may "split" a TCP
connection and behaviour such as proactive ACKing may occur. As
with NATs, it is no longer guaranteed that one endpoint is
communicating directly with another.
o Traffic Normalizers: These aim to eliminate ambiguities and
potential attacks at the network level, and amongst other things
are unlikely to permit holes in sequence space.
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o TCP Options: many middleboxes are in a position to drop packets
with unknown TCP options, or strip those options from the packets.
o Segmentation/Colescing: middleboxes (or even something as close to
the end host as TCP Segmentation Offloading) may change the packet
boundaries from those which the sender intended. It may do this
by splitting packets, or coalescing them together. This leads to
two major impacts: we cannot guarantee where a packet boundary
will be, and we cannot say for sure what a middlebox will do with
TCP options in these cases (they may be repeated, dropped, or sent
only once).
o Firewalls: on top of preventing incoming connections, firewalls
may also attempt additional protection such as sequence number
randomization.
o Intrusion Detection Systems: IDSs may look for traffic patterns to
protect a network, and may have false positives with MPTCP and
drop the connections during normal operation. For future MPTCP-
aware middleboxes, they will require the ability to correlate the
various paths in use.
10. Acknowledgements
Alan Ford, Costin Raiciu and Sebastien Barre 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.
11. Contributors
The authors would like to acknowledge the contributions of Mark
Handley and Bryan Ford to this document.
12. IANA Considerations
None.
13. References
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13.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
13.2. Informative References
[2] 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>.
[3] Ford, A., Raiciu, C., and M. Handley, "TCP Extensions for
Multipath Operation with Multiple Addresses",
draft-ietf-mptcp-multiaddressed-00 (work in progress),
June 2010.
[4] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[5] Stewart, R., "Stream Control Transmission Protocol", RFC 4960,
September 2007.
[6] Raiciu, C., Handley, M., and D. Wischik, "Coupled Multipath-
Aware Congestion Control", draft-raiciu-mptcp-congestion-01
(work in progress), March 2010.
[7] Scharf, M. and A. Ford, "MPTCP Application Interface
Considerations", draft-scharf-mptcp-api-01 (work in progress),
March 2010.
[8] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues",
RFC 3234, February 2002.
[9] Carpenter, B., "Internet Transparency", RFC 2775,
February 2000.
[10] Ford, B. and J. Iyengar, "Breaking Up the Transport Logjam",
ACM HotNets, October 2008.
[11] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001.
[12] Freed, N., "Behavior of and Requirements for Internet
Firewalls", RFC 2979, October 2000.
[13] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
Shelby, "Performance Enhancing Proxies Intended to Mitigate
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Link-Related Degradations", RFC 3135, June 2001.
[14] Bagnulo, M., "Threat Analysis for Multi-addressed/Multi-path
TCP", draft-ietf-mptcp-threat-02 (work in progress),
March 2010.
[15] Handley, M., Paxson, V., and C. Kreibich, "Network Intrusion
Detection: Evasion, Traffic Normalization, and End-to-End
Protocol Semantics", Usenix Security 2001, 2001, <http://
www.usenix.org/events/sec01/full_papers/handley/handley.pdf>.
Appendix A. Implementation Architecture
This section provides suggestions for an architecture to implement an
extensible, modular multipath transport protocol.
A.1. Functional Separation
This section describes a generic view of the internal implementation
of a Multipath TCP, through which the technical components specified
in the companion documents can fit together. It shows how an
implementation could be built that permits extensibility between
components without changing the external representation.
We first show the functional decomposition of an MPTCP solution that
is completely contained in the transport layer. That solution is
described in more details in [3]. Then we generalize the approach to
allow good extensibility of that solution.
A.1.1. Application to default MPTCP protocol
Although, in the default approach, MPTCP is fully contained in the
transport layer, it can still be divided into two main modules. One
manages the scheduling of packets as well as congestion control. The
other one manages the control of paths. The interface between the
two is dealt with thanks to a Path Index. As shown in Figure 8, the
Path Manager announces to the MultiPath Scheduler what paths can be
used trough path indices, and maintains the mapping between that
value and the particular action that it must apply to use the path
(an example of such a mapping is in Table 1). In the case of the
built-in Path Manager, the action is to replace an address/port pair
with another one, in such a way that another path is used across the
Internet to forward that packet.
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Control plane <-- | --> Data plane
+---------------------------------------------------------------+
| Multipath Scheduler (MPS) |
+---------------------------------------------------------------+
^ | |
| | [A1,B1,|pA1,pB1]
|For conn_id | |
|<A1,B1,pA1,pB1> | +-------------+
|Paths 1->4 can be | | Data packet |<--Path idx:3
|used. | +-------------+ attached
| | | by MPS
| | V
+--------------------------------------------\------------------+
| Path Manager (PM) \[A1,B1]->[A1,B2] |
+--------------------------------------------------\------------+
/ \ | \
/-----------------------------\ | /"\ /"\ /"\ /"\
| rewriting table: || | | | | | | | |
| Subflow id <--> network_id || | | | | | | | |
| || | | | | | | | |
| [see table below] || | | | | | | | |
| || \./ \./ \./ \./
+------------------------------+| path1 path2 path3 path4
Figure 8: Functional separation of MPTCP in the transport layer
The MultiPath Scheduler only deals with abstract paths, represented
by numbers. It only sees one address pair throughout the
communication, that we call the connection identifier. However, the
MultiPath Scheduler must be able to perform per-subflow congestion
control, and thus to distinguish between the subflows. This leads to
define a subflow identifier, that consists of the usual transport
identifier extended with the path index:
<addr_src,psrc,addr_dst,pdst,path_index>. The following options,
described in [3], are managed by the MultiPath Scheduler.
o MULTIPATH CAPABLE (MPC): Tell the peer that we support MPTCP.
Note that the MPC option also holds a token, which is necessary
only if the built-in Path Manager is used. In the next section we
describe the generalized case, where the token can be ignored by
the receiver if another path manager is used.
o DATA SEQUENCE NUMBER (DSN): Identify the position of a set of
bytes in the meta-flow.
o DATA FIN (DFIN): Terminate a meta-flow.
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An implementation MUST use those options even if another Path Manager
than the default one is implemented.
The Path manager applies a particular technology to give the MPS the
possibility to use several paths. The built-in MPTCP Path Manager
uses multiple IPv4 addresses as its mean to influence the forwarding
of packets through the Internet.
When the MPS starts a new connection, the PM chooses a token that
will be used to identify the connection. This is necessary to allow
the PM applying the correct path index to incoming packets. An
example mapping table is given hereafter:
+-----------------+---------------+---------+-----------------+
| connection id | subflow id | token | Network id |
+-----------------+---------------+---------+-----------------+
| <A1,B1,pA1,pB1> | <conn_id,pi1> | token_1 | <A1,B1,pA1,pB1> |
| <A1,B1,pA1,pB1> | <conn_id,pi2> | token_1 | <A2,B2,pA1,pB2> |
| <A1,B1,pA1,pB1> | <conn_id,pi3> | token_1 | <A1,B2,pA1,pB2> |
| <A1,B1,pA1,pB1> | <conn_id,pi4> | token_1 | <A2,B1,pA1,pB1> |
| <A1,B1,pA1,pB3> | <conn_id,pi1> | token_2 | <A1,B1,pA1,pB3> |
| <A1,B1,pA1,pB3> | <conn_id,pi2> | token_2 | <A2,B1,pA1,pB3> |
+-----------------+---------------+---------+-----------------+
Table 1: Example mapping table for built-in PM
Table 1 shows an example where two connections are ongoing. One is
identified by token_1, the other one with token_2. Since addresses
are rewritten by the path manager, the attachment to the right
connection is achieved thanks to the token, which is used at
connection establishment and subflow establishment. It is then
remembered. The first column holds the information that is exposed
to the applications, while the last column shows the information that
is actually written in packets that will fly through the network. We
note that additionnally to the addresses, ports can be rewritten,
which contributes to supporting NATs. The table also shows the role
of the token, which is to attach various combinations of ports and
addresses to a single connection. The token is specific to the
built-in path manager, and can be ignored if another path manager is
used. An implementation of the built-in path manager MUST implement
the following options (defined in more details in [3]):
o Add Address (ADDR): Announce a new address we own
o Remove Addresse (REMADDR): Withdraw a previously announced address
o Join Connection (JOIN): Attach a new subflow to the current
connection
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Those options form the default MPTCP Path Manager, 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.1.2. Generic architecture for MPTCP
Now that the functional decomposition has been shown for MPTCP with
the built-in Path Manager, we show how that architecture can be
generalized to allow the implementation of other Path Managers for
MPTCP. A general overview of the architecture is provided in
Figure 9. 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.
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Control plane <-- | --> Data plane
+---------------------------------------------------------------+
| Multipath Scheduler (MPS) |
+---------------------------------------------------------------+
^ | |
| | [A1,B1,|pA1,pB1]
| | |
|Announcing new | +-------------+
|paths. (referred | | Data packet |<--Path idx:3
|to as path indices) | +-------------+ attached
| | | by MPS
| | V
+--------------------------------------------\------------------+
| Path Manager (PM) \__________zzzzz |
+--------------------------------------------------------\------+
/ \ | \
/---------------------------\ | /"\ /"\ /"\
| subflow_id Action | | | | | | | |
|<A1,B1,pA1,pB1,1> xxxxx | | | | | | | |
|<A1,B1,pA1,pB1,2> yyyyy | | \./ \./ \./
|<A1,B1,pA1,pB1,3> zzzzz | | path1 path2 path3
+---------------------------+
Figure 9: Overview of MPTCP architecture
From then on, it is possible for the MPS to associate a Path Index
with its packets, so that the Path Manager can map this Path Index to
a particular action (see table in the lower left part of Figure 9).
The particular action depends on the network mechanism used to select
a path. Examples are address rewriting, tunnelling or setting a path
selector value inside the packet. Note that the Path Index is not
supposed to be written inside the packet, but instead associated with
it, internally to the implementation.
The applicability of the architecture is not limited to the MPTCP
protocol. While we define in this document an MPTCP MPS (MPTCP
Multipath Scheduler), other Multipath Schedulers can be defined. For
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 MPTCP case,
however.
A.2. 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
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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 each 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 MPTCP to know
what outgoing path is acknowledged by an incoming packet.
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 removal. Additionnaly, a PM MAY provide
complementary path information when available, such as link
quality or preference level.
Appendix B. Changelog
B.1. Changes since draft-ietf-mptcp-architecture-00
o Added middlebox compatibility discussion (Section 9).
o Clarified path identification (TCP 4-tuple) in Section 5.5.
o Added brief scenario and diagram to Section 1.3.
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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
Gower Street
London WC1E 6BT
UK
Email: c.raiciu@cs.ucl.ac.uk
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
Janardhan Iyengar
Franklin and Marshall College
Mathematics and Computer Science
PO Box 3003
Lancaster, PA 17604-3003
USA
Phone: 717-358-4774
Email: jiyengar@fandm.edu
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