Internet Engineering Task Force                             A. Ford, Ed.
Internet-Draft                                       Roke Manor Research
Intended status: Informational                                 C. Raiciu
Expires: April 19, 2011                                       M. Handley
                                               University College London
                                                              J. Iyengar
                                           Franklin and Marshall College
                                                        October 16, 2010

         Architectural Guidelines for Multipath TCP Development


   Hosts 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

   This document outlines architectural guidelines for the development
   of a Multipath Transport Protocol, with references to how these
   architectural components come together in the development of a
   Multipath TCP protocol.  This document lists certain high level
   design decisions that provide foundations for the design of the MPTCP
   protocol, 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
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   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 April 19, 2011.

Copyright Notice

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   Copyright (c) 2010 IETF Trust and the persons identified as the
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   described in the Simplified BSD License.

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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 . . . . . . . .  9
       2.2.4.  Security Goals . . . . . . . . . . . . . . . . . . . .  9
   3.  An Architectural Basis For MPTCP . . . . . . . . . . . . . . .  9
   4.  A Functional Decomposition of MPTCP  . . . . . . . . . . . . . 11
   5.  High-Level Design Decisions  . . . . . . . . . . . . . . . . . 13
     5.1.  Sequence Numbering . . . . . . . . . . . . . . . . . . . . 13
     5.2.  Reliability and Retransmissions  . . . . . . . . . . . . . 14
     5.3.  Buffers  . . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.4.  Signalling . . . . . . . . . . . . . . . . . . . . . . . . 16
     5.5.  Path Management  . . . . . . . . . . . . . . . . . . . . . 17
     5.6.  Connection Identification  . . . . . . . . . . . . . . . . 18
     5.7.  Congestion Control . . . . . . . . . . . . . . . . . . . . 18
     5.8.  Security . . . . . . . . . . . . . . . . . . . . . . . . . 19
   6.  Interactions with Applications . . . . . . . . . . . . . . . . 20
   7.  Interactions with Middleboxes  . . . . . . . . . . . . . . . . 20
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 22
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 22
     12.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Changelog . . . . . . . . . . . . . . . . . . . . . . 24
     A.1.  Changes since draft-ietf-mptcp-architecture-01 . . . . . . 24
     A.2.  Changes since draft-ietf-mptcp-architecture-00 . . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24

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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 that would otherwise be needed to create an
   equivalent improvement in user experience.

   By the application of resource pooling[3], 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.

   MPTCP [4] is a set of extensions for TCP[1] that implements a
   multipath transport and achieves these goals by pooling multiple
   paths within a transport connection, transparent to the application.
   Although multihoming and multipath functions are not new to transport
   protocols, MPTCP aims to gain wide-scale deployment by recognising
   the importance of application and network compatibility goals.  These
   goals, discussed in detail in Section 2, relate to the appearance of
   MPTCP to the network (so non-MPTCP-aware entities see it as TCP) and
   to the application (through providing an equivalent service to TCP to
   non-MPTCP-aware applications).

   This document has three key purposes: (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[4], congestion control
   algorithms[5], and application-level considerations[6].  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.

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1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [2].

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.

   Path Identifier:  Within the context of a multi-addressed multipath
      TCP, a path is defined by the source and destination (address,
      port) pairs (i.e. a 4-tuple).

   Host:  An end host either initiating or terminating a MPTCP

   Multipath TCP:  A modified version of the TCP [1] protocol that
      supports the simultaneous use of multiple paths between hosts.

   MPTCP:  The proposed protocol extensions specified in [4] to provide
      a Multipath TCP implementation.

   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 a host.

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 hosts are multi-homed and multi-addressed, providing two
   disjoint connections to the Internet.  The addresses on each host are
   referred to as A1, A2, B1 and B2.  There are therefore up to four
   different paths between the two hosts: A1-B1, A1-B2, A2-B1, A2-B2.

     +------+           __________           +------+
     |      |A1 ______ (          ) ______ B1|      |
     | Host |--/      (            )      \--| Host |
     |      |        (   Internet   )        |      |
     |  A   |--\______(            )______/--|   B  |
     |      |A2        (__________)        B2|      |
     +------+                                +------+

                   Figure 1: Simple MPTCP Usage Scenario

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   The scenario could have any number of addresses (1 or more) on each
   host, so long as the number of paths available between the two hosts
   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.

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 Multipath TCP must provide; and
   compatibility goals, which determine how Multipath TCP should appear
   to entities that interact with it.

2.1.  Functional Goals

   In supporting the use of multiple paths, Multipath TCP has the
   following two functional goals.

   o  Improve Throughput: Multipath TCP MUST support the concurrent use
      of multiple paths.  To meet the minimum performance incentives for
      deployment, a Multipath TCP connection over multiple paths SHOULD
      achieve no lesser throughput than a single TCP connection over the
      best constituent path.

   o  Improve Resilience: Multipath TCP 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 regular single-path TCP.

   As distribution of traffic among available paths and responses to
   congestion are done in accordance with resource pooling
   principles[3], a secondary effect of meeting these goals is that
   widespread use of Multipath TCP over the Internet should optimize
   overall network utility by shifting load away from congested
   bottlenecks and by taking advantage of spare capacity wherever

   Furthermore, Multipath TCP SHOULD feature automatic negotiation of
   its use.  A host supporting Multipath TCP that requires the other
   host to do so too must be able to detect reliably whether this host

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   does in fact support the required extensions, using them if so, and
   otherwise automatically falling back to single-path TCP.

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

2.2.1.  Application Compatibility

   Application compatibility refers to the appearance of Multipath TCP
   to the application both in terms of the API that can be used and the
   expected service model that is provided.

   Multipath TCP MUST follow the same service model as TCP [1]: in-
   order, reliable, and byte-oriented delivery.  Furthermore, an
   Multipath TCP 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
   multipath extensions.

2.2.2.  Network Compatibility

   Traditional Internet architecture slots network devices in the
   network layer and lower layers, where the layers above the network
   layer are instantiated only at the end-hosts.  While this
   architecture, shown in Figure 2, was initially largely adhered to,
   this layering no longer reflects the "ground truth" in the Internet
   with the proliferation of middleboxes[7].  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   |
   +-------------+   +-------------+   +-------------+   +-------------+
      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"[8], that is, they often hold "hard" state that, when
   lost or corrupted, results in loss or corruption of the end-to-end
   transport connection.

   The network compatibility goal requires that the multipath extension
   to TCP retains compatibility with the Internet as it exists today,
   including making reasonable efforts to be able to traverse
   predominant middleboxes such as firewalls, NATs, and performance
   enhancing proxies[7].  This requirement comes from recognizing
   middleboxes as a significant deployment bottleneck for any transport
   that is not TCP, and constrains Multipath TCP 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
   Multipath TCP to preserve fate-sharing without making any assumptions
   about middlebox behavior.

   A detailed analysis of middlebox behaviour and the impact on the
   Multipath TCP architecture is presented in Section 7.  In addition,
   network compatibility must be retained to the extent that Multipath
   TCP MUST fall back to regular TCP if there are insurmountable
   incompatibilities for the multipath extension on a path.

   MPTCP's modifications remain at the transport layer, although some

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   knowledge of the underlying network layer is required.  MPTCP SHOULD
   work with IPv4 and IPv6 interchangeably, i.e. one MPTCP connection
   may operate over both IPv4 and IPv6 networks.

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 single-path 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 occur from another single-path TCP flow.  Multiple
   Multipath TCP flows on a shared bottleneck MUST share bandwidth
   between each other with the similar fairness to that which occurs
   with a shared bottleneck with single-path TCP.

2.2.4.  Security Goals

   The extension of TCP with multipath capabilities will bring with it a
   number of new threats, analysed in detail in [9].  The security goal
   for Multipath TCP is to provide a service no less secure than
   regular, single-path TCP.  This will be achieved through a
   combination of existing TCP security mechanisms (potentially modified
   to align with the Multipath TCP extensions) and of protection against
   the new multipath threats identified.  The design decisions derived
   from this goal are presented in Section 5.8.

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.

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          |    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.

   +-------------+                                       +-------------+
   | Application |<------------ end-to-end ------------->| Application |
   +-------------+                                       +-------------+
   |  Semantic   |<------------ end-to-end ------------->|  Semantic   |
   +-------------+   +-------------+   +-------------+   +-------------+
   +-------------+   +-------------+   +-------------+   +-------------+
   |   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.  MPTCP, which provides application compatibility through
   the preservation of TCP-like semantics of global ordering of
   application data and reliability, is an instantiation of the

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   "application-oriented" Semantic layer; whereas the subflow 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  |    | Subflow (TCP) | Subflow (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

   MPTCP, as described in [4], makes use of (what appear to the network
   to be) standard TCP sessions, termed "subflows", to provide the
   underlying transport per path, and as such these retain the network
   compatibility desired.  MPTCP-specific information is carried 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 in turn manages
   multiple TCP subflows below it.  In order to do this, it must
   implement the following functions:

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   o  Path Management: This is the function to detect and use multiple
      paths between two hosts.  In the case of the MPTCP design [4],
      this feature is implemented using multiple IP addresses at one or
      both of the hosts.  The path management features of the MPTCP
      protocol are the mechanisms to signal alternative addresses to
      hosts, and mechanisms to set up new subflows joined to an existing
      MPTCP connection.

   o  Packet Scheduling: This function breaks the bytestream received
      from the application into segments to be transmitted on one of the
      available lower subflows.  The MPTCP design makes use of a data
      sequence mapping, associating segments sent on different subflows
      to a connection-level sequence numbering, thus allowing segments
      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 queued

   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 host.
      MPTCP uses TCP underneath for network compatibility; TCP ensures
      in-order, reliable delivery.  TCP adds its own sequence numbers to
      the segments; these are used to detect and retransmit lost packets
      at the subflow layer.  The connection-level sequence numbering
      from the packet scheduling component allows re-ordering of the
      entire bytestream.

   o  Congestion Control: This function coordinates 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

   These functions fit together as follows.  The Path Management looks
   after the discovery (and if necessary, initialisation) of multiple
   paths between two hosts.  The Packet Scheduler then receives a stream
   of data from the application destined for the network, and undertakes
   the necessary operations on it (such as segmenting the data into
   connection-level segments, and adding a connection-level sequence
   number) before sending it on to a subflow.  The subflow then adds its
   own sequence number, acks, and passes them to network.  The receiving
   subflow re-orders data (if necessary) and passes it to the packet
   scheduling component, which performs connection level re-ordering,
   and sends the data stream to the application.  Finally, the
   congestion control component exists as part of the packet scheduling,

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   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 [4].

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.

   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 thus go against the network
   compatibility goal.  Second, the sender would not be able to
   attribute packet losses or receptions to the correct path when the
   same packet is sent on multiple paths (i.e. in the case of

   The sender must be able to tell the receiver how to reassemble the
   data, for delivery to the application.  In order to achieve this, the
   receiver must determine how subflow-level data (carying subflow
   sequence numbers) maps at the connection level.  We refer to this as
   the 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 the
   Data Sequence Mapping, 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,

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   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 understand MPTCP
   signalling so do not correctly rewrite the options.

   Because of these potential issues, the design decision taken in the
   MPTCP protocol [4] is that whenever a mapping for subflow data needs
   to be conveyed to the other host, all three pieces of data (data seq,
   subflow seq, length) must be sent.  To reduce the overhead, it would
   be permissable for the mapping to be sent periodically and cover more
   than a single segment.  Further experimentation is required to
   determine what tradeoffs exist regarding the frequency at which
   mappings should be sent.  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 and Retransmissions

   MPTCP features acknowledgements at connection-level as well as
   subflow-level acknowledgements, in order to provide a robust service
   to the application.

   Under normal behaviour, MPTCP can use the data sequence mapping and
   subflow ACKs to decide when a connection-level segment was received.
   The transmission of TCP ACKs for a subflow are handled entirely at
   the subflow level, in order to maintain TCP semantics and trigger
   subflow-level retransmissions.  This has certain implications on end-
   to-end semantics.  It means that once a packet is acked at the
   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).  Under certain circumstances, therefore, it may
   be desirable to be able to drop packets after acknowledgement on the
   subflow but before delivery to the application, and this can be
   facilitated by a connection-level acknowledgement.

   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.
   An example situation where this may occur would be mobility between
   wireless access points, each of which operates a transport-level
   proxy.  Finally, as an optimisation, it may be feasible for a
   connection-level acknowledgement to be transmitted over the shortest

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   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, in
   addition to subflow-level acknowledgements.  A connection-level
   acknowledgement would only be required in order to signal when the
   receive window moves forward; the heuristics for using such a signal
   are discussed in more detail in the protocol specificiation [4].

   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.

   Typically, fast retransmit on an individual subflow will not trigger
   retransmission on another subflow, although this 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.

   Large-scale experiments are therefore required in order to determine
   the most appropriate retransmission strategy, and recommendations
   will be refined once more information is available.

5.3.  Buffers

   To ensure in-order delivery, Multipath TCP must use a connection
   level receive buffer, where segments are placed until they are in
   order and can be read by the application.

   In regular, single-path TCP, it is usually recommended to set the
   receive buffer to 2*BDP (Bandwidth-Delay Product, i.e.  BDP = BW*RTT,
   where BW = Bandwidth and RTT = Round-Trip Time).  One BDP allows
   supporting reordering of segments by the network.  The other BDP

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   allows the connection to continue during fast retransmit: when a
   segment is fast retransmitted, the receiver must be able to store
   incoming data during one more RTT.

   For Multipath TCP, the story is a bit more complicated.  The ultimate
   goal is that a subflow packet loss or subflow failure should not
   affect the throughput of other working subflows; the receiver should
   have enough buffering to store all data until the missing packet is
   re-transmitted and reaches the destination.

   The worst case scenario would be when the subflow with the highest
   RTT/RTO (Round-Trip Time or Retransmission TimeOut) experiences a
   timeout; in that case the receiver has to buffer data from all
   subflows for the duration of the RTO.  Thus, the smallest connection-
   level receive buffer that would be needed to avoid stalling with
   subflow failures is sum(BW_i)*RTO_max, where BW_i = Bandwidth for
   each subflow and RTO_max is the largest RTO across all subflows.

   This is an order of magnitude more than the receive buffer required
   for a single connection, and is probably too expensive for practical
   purposes.  A more sensible requirement is to avoid stalls in the
   absence of timeouts.  Therefore, the RECOMMENDED receive buffer is
   2*sum(BW_i)*RTT_max, where RTT_max is the largest RTT across all
   subflows.  This buffer sizing ensures subflows do not stall when fast
   retransmit is triggered on any subflow.

   The resulting buffer size should be small enough for practical use.
   However, there may be extreme cases where fast, high throughput paths
   (e.g. 100Mb/s, 10ms RTT) are used in conjunction with slow paths
   (e.g. 1Mb/s, 1000ms RTT).  In that case the required receive buffer
   would be 12.5MB, which is likely too big.  In these cases a Multipath
   TCP scheduler SHOULD use only the fast path, potentially falling back
   to the slow path if the fast path fails.

   Send Buffer: The RECOMMENDED send buffer is the same size as the
   recommended receive buffer i.e., 2*sum(BW_i)*RTT_max.  This is
   because the sender must store locally the segments sent but
   unacknowledged by the connection level ACK.  The send buffer size
   matters particularly for hosts that maintain a large number of
   ongoing connections.  If the required send buffer is too large, a
   host can choose to only send data on the fast subflows, using the
   slow subflows only in cases of failure.

5.4.  Signalling

   Since MPTCP uses TCP as its subflow transport mechanism, a MPTCP
   connection will also begin as a single TCP connection.  Nevertheless,
   it must signal to the peer that it supports MPTCP and wishes to use

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   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.

   In addition, further signalling is required during the operation of
   an MPTCP session, such as that for reassembly for multiple subflows,
   and for informing the other host about potential other available
   addresses.  It is not mandated by the architecture in what format
   this signalling should be transmitted.

   The MPTCP protocol design [4] continues to use TCP Options for this
   signalling.  This has been chosen as the mechanism most fitting in
   with the goals as specified in Section 2.  With this mechanism, the
   signalling requires to operate MPTCP is transported separately from
   the data, allowing it to be created and processed separately from the
   data stream, and retaining architectural compatibility with network

5.5.  Path Management

   Currently, the network does not expose multiple paths between hosts.
   Multipath TCP will use multiple addresses at one or both hosts to
   infer different paths across the network.  The hope is that these
   paths, whilst not necesarily entirely non-overlapping, will be
   sufficiently disjoint to allow multipath to achieve improved
   throughput and robustness.  The use of multiple IP addresses is a
   simple mechanism that requires no additional features in the network.

   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,
   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
   Equal Cost MultiPath (ECMP) routing, e.g. [14]).

   For increased chance of successfully setting up additional subflows
   (such as when one end is behind a firewall, NAT, or other restrictive
   middlebox), either host SHOULD be able to add new subflows to a MPTCP
   connection.  MPTCP MUST be able to handle paths that appear and
   disappear during the lifetime of a connection (for example, through
   the activation of an additional network interface).

   The modularity of path management will permit alternative mechanisms
   to be employed if appropriate in the future.

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5.6.  Connection Identification

   Since an MPTCP connection may not be bound to a traditional 5-tuple
   (source addr and port, destination addr and port, protocol number)
   for the entirity of its existance, it is desirable to provide a new
   mechanism for connection identification.  This will be useful for
   MPTCP-aware applications, and for the MPTCP implementation (and
   MPTCP-aware middleboxes) to have a unique identifier with which to
   associate the multiple subflows.

   Therefore, each MPTCP connection requires a connection identifier at
   each host, which is locally unique within that host.  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
   MPTCP-unaware 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 used an existing API call to bind
   to a specific address or interface, the MPTCP extension MUST NOT be
   used, since the applications are indicating a clear choice of path to
   use and thus will have expectations of behaviour that must be
   maintained, in order to adhere to the application compatibility

   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.  Congestion Control

   As discussed in network-layer compatibility requirements
   Section 2.2.3, there are three goals for the congestion control
   algorithms used by an MPTCP implementation: improve throughput (at
   least as well as a single-path TCP connection would perform); do no
   harm to other network users (do not take up more capacity on any one
   path than if it was a single path flow using only that route - this
   is particularly relevant for shared bottlenecks); and balance
   congestion by moving traffic away from the most congested paths.  To

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   achieve these goals, the congestion control algorithms on use on each
   subflow must be coupled in some way.  A proposal for a suitable
   congestion control algorithm is given in [5].

5.8.  Security

   A detailed threat analysis for Multipath TCP is presented in a
   separate document [9].  This focuses on flooding attacks and
   hijacking attacks that can be launched against a Multipath TCP

   The basic security goal of Multipath TCP, as introduced in
   Section 2.2.4, can be stated as: "provide a solution that is no worse
   than standard TCP".

   From the threat analysis, and with this goal in mind, three key
   security requirements can be identified.  A multi-addressed Multipath
   TCP SHOULD be able to:

   o  Provide a mechanism to confirm that the parties in a subflow
      handshake are the same as in the original connection setup (e.g.
      require use of a key exchanged in the initial handshake in the
      subflow handshake, to limit the scope for hijacking attacks).

   o  Provide verification that the peer can receive traffic at a new
      address before adding it (i.e. verify that the address belongs to
      the other host, to prevent flooding attacks).

   o  Provide replay protection, i.e. ensure that a request to add/
      remove a subflow is 'fresh'.

   Additional mechanisms have been deployed as part of standard TCP
   stacks to provide resistance to Denial-of-Service attacks.  For
   example, there are various mechanisms to protect against TCP reset
   attacks [15], and Multipath TCP should continue to support similar
   protection.  In addition, TCP SYN Cookies [16] were developed to
   allow a TCP server to defer the creation of session state in the
   SYN_RCVD state, and remain stateless until the ESTABLISHED state had
   been reached.  Multipath TCP should, ideally, continue to provide
   such functionality and, at a minimum, avoid significant computational
   burden prior to reaching the ESTABLISHED state (of the Multipath TCP
   connection as a whole).

   It should be noted that aspects of the Multipath TCP design space
   place constraints on the security solution:

   o  The use of TCP options significantly limits the amount of
      information that can be carried in the handshake.

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   o  The need to work through middleboxes results in the need to handle
      mutability of packets.

   o  The desire to support a 'break-before-make' approach to adding
      subflows removes the ability to actively use a pre-existing
      subflow to support the addition of a new one.

   The MPTCP protocol design [4] aims to meet these security
   requirements, and the protocol specification will document how these
   are met.

6.  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 [6].

7.  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 [17] 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 will often need
   to behave in a similar fashion to existing ones, in order to increase
   the chances of successful deployment.  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

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   The following list of middlebox classes documents behaviour that
   could impact the use of MPTCP.  This list is used in [4] 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 host'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 host 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 TCP-level sequence space.

   o  Firewalls: on top of preventing incoming connections, firewalls
      may also attempt additional protection such as sequence number

   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.

   In addition, all classes of middleboxes may affect TCP traffic in the
   following ways:

   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).

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8.  Contributors

   The authors would like to acknowledge the contributions of Sebastien
   Barre, Andrew McDonald, and Bryan Ford to this document.

   The authors would also like to thank the following people for
   detailed reviews: Olivier Bonaventure, Gorry Fairhurst, Iljitsch van
   Beijnum, and Philip Eardley.

9.  Acknowledgements

   Alan Ford, Costin Raiciu and Mark Handley are supported by Trilogy
   (, 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.

10.  IANA Considerations


11.  Security Considerations

   This informational document provides an architectural overview for
   Multipath TCP and so does not, in itself, raise any security issues.
   A separate threat analysis [9] lists threats that can exist with a
   Multipath TCP.  However, a protocol based on the architecture in this
   document will have a number of security requirements.  The high level
   goals for such a protocol are identified in Section 2.2.4, whilst
   Section 5.8 provides more detailed discussion of security
   requirements and design decisions which are applied in the MPTCP
   protocol design [4].

12.  References

12.1.  Normative References

   [1]   Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
         September 1981.

   [2]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

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12.2.  Informative References

   [3]   Wischik, D., Handley, M., and M. Bagnulo Braun, "The Resource
         Pooling Principle", ACM SIGCOMM CCR vol. 38 num. 5, pp. 47-52,
         October 2008,

   [4]   Ford, A., Raiciu, C., and M. Handley, "TCP Extensions for
         Multipath Operation with Multiple Addresses",
         draft-ietf-mptcp-multiaddressed-01 (work in progress),
         July 2010.

   [5]   Raiciu, C., Handley, M., and D. Wischik, "Coupled Multipath-
         Aware Congestion Control", draft-ietf-mptcp-congestion-00 (work
         in progress), July 2010.

   [6]   Scharf, M. and A. Ford, "MPTCP Application Interface
         Considerations", draft-scharf-mptcp-api-02 (work in progress),
         July 2010.

   [7]   Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues",
         RFC 3234, February 2002.

   [8]   Carpenter, B., "Internet Transparency", RFC 2775,
         February 2000.

   [9]   Bagnulo, M., "Threat Analysis for Multi-addressed/Multi-path
         TCP", draft-ietf-mptcp-threat-02 (work in progress),
         March 2010.

   [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
         Link-Related Degradations", RFC 3135, June 2001.

   [14]  Hopps, C., "Analysis of an Equal-Cost Multi-Path Algorithm",
         RFC 2992, November 2000.

   [15]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
         Robustness to Blind In-Window Attacks", RFC 5961, August 2010.

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   [16]  Eddy, W., "TCP SYN Flooding Attacks and Common Mitigations",
         RFC 4987, August 2007.

   [17]  Handley, M., Paxson, V., and C. Kreibich, "Network Intrusion
         Detection: Evasion, Traffic Normalization, and End-to-End
         Protocol Semantics", Usenix Security 2001, 2001, <http://>.

Appendix A.  Changelog

   (For removal by the RFC Editor)

A.1.  Changes since draft-ietf-mptcp-architecture-01

   o  Responded to review comments.

   o  Added security sections.

A.2.  Changes since draft-ietf-mptcp-architecture-00

   o  Added middlebox compatibility discussion (Section 7).

   o  Clarified path identification (TCP 4-tuple) in Section 5.5.

   o  Added brief scenario and diagram to Section 1.3.

Authors' Addresses

   Alan Ford (editor)
   Roke Manor Research
   Old Salisbury Lane
   Romsey, Hampshire  SO51 0ZN

   Phone: +44 1794 833 465

   Costin Raiciu
   University College London
   Gower Street
   London  WC1E 6BT


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   Mark Handley
   University College London
   Gower Street
   London  WC1E 6BT


   Janardhan Iyengar
   Franklin and Marshall College
   Mathematics and Computer Science
   PO Box 3003
   Lancaster, PA  17604-3003

   Phone: 717-358-4774

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