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rfc:rfc6182

Internet Engineering Task Force (IETF) A. Ford Request for Comments: 6182 Roke Manor Research Category: Informational C. Raiciu ISSN: 2070-1721 M. Handley

                                             University College London
                                                              S. Barre
                                      Universite catholique de Louvain
                                                            J. Iyengar
                                         Franklin and Marshall College
                                                            March 2011
       Architectural Guidelines for Multipath TCP Development

Abstract

 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
 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 development of a
 Multipath TCP (MPTCP).  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 document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6182.

Ford, et al. Informational [Page 1] RFC 6182 MPTCP Architecture March 2011

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Ford, et al. Informational [Page 2] RFC 6182 MPTCP Architecture March 2011

Table of Contents

 1. Introduction ....................................................4
    1.1. Requirements Language ......................................5
    1.2. Terminology ................................................5
    1.3. Reference Scenario .........................................6
 2. Goals ...........................................................6
    2.1. Functional Goals ...........................................6
    2.2. Compatibility Goals ........................................7
         2.2.1. Application Compatibility ...........................7
         2.2.2. Network Compatibility ...............................8
         2.2.3. Compatibility with Other Network Users .............10
    2.3. Security Goals ............................................10
    2.4. Related Protocols .........................................10
 3. An Architectural Basis for Multipath TCP .......................11
 4. A Functional Decomposition of MPTCP ............................12
 5. High-Level Design Decisions ....................................14
    5.1. Sequence Numbering ........................................14
    5.2. Reliability and Retransmissions ...........................15
    5.3. Buffers ...................................................17
    5.4. Signaling .................................................18
    5.5. Path Management ...........................................19
    5.6. Connection Identification .................................20
    5.7. Congestion Control ........................................21
    5.8. Security ..................................................21
 6. Software Interactions ..........................................23
    6.1. Interactions with Applications ............................23
    6.2. Interactions with Management Systems ......................23
 7. Interactions with Middleboxes ..................................23
 8. Contributors ...................................................25
 9. Acknowledgements ...............................................25
 10. Security Considerations .......................................26
 11. References ....................................................26
    11.1. Normative References .....................................26
    11.2. Informative References ...................................26

Ford, et al. Informational [Page 3] RFC 6182 MPTCP Architecture March 2011

1. Introduction

 As the Internet evolves, demands on Internet resources are ever-
 increasing, but often these resources (in particular, bandwidth)
 cannot be fully utilized due to protocol constraints both on the end-
 systems and within the network.  If these resources could 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.
 Multipath transport aims to realize some of the goals of resource
 pooling by simultaneously making use of multiple disjoint (or
 partially disjoint) paths across a network.  The two key benefits of
 multipath transport are the following:
 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 is a modified version of TCP [1] that implements a
 multipath transport and achieves these goals by pooling multiple
 paths within a transport connection, transparently to the
 application.  Multipath TCP is primarily concerned with utilizing
 multiple paths end-to-end, where one or both of the end hosts are
 multihomed.  It may also have applications where multiple paths exist
 within the network and can be manipulated by an end host, such as
 using different port numbers with Equal Cost MultiPath (ECMP) [4].
 MPTCP, defined in [5], is a specific protocol that instantiates the
 Multipath TCP concept.  This document looks both at general
 architectural principles for a Multipath TCP fulfilling the goals
 described in Section 2, as well as the key design decisions behind
 MPTCP, which are detailed in Section 5.
 Although multihoming and multipath functions are not new to transport
 protocols (Stream Control Transmission Protocol (SCTP) [6] being a
 notable example), MPTCP aims to gain wide-scale deployment by
 recognizing 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 a service
 equivalent to TCP for non-MPTCP-aware applications).

Ford, et al. Informational [Page 4] RFC 6182 MPTCP Architecture March 2011

 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 that
 provide details of the protocol extensions [5], congestion control
 algorithms [7], and application-level considerations [8].  Put
 together, these components specify a complete Multipath TCP design.
 Note that specific components are replaceable in accordance with the
 layer and functional decompositions discussed in this document.

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 [2].

1.2. Terminology

 Regular/Single-Path TCP:  The standard version of TCP [1] in use
    today, operating between a single pair of IP addresses and ports.
 Multipath TCP:  A modified version of the TCP protocol that supports
    the simultaneous use of multiple paths between hosts.
 Path:  A sequence of links between a sender and a receiver, defined
    in this context by a source and destination address pair.
 Host:  An end host either initiating or terminating a Multipath TCP
    connection.
 MPTCP:  The proposed protocol extensions specified in [5] to provide
    a Multipath TCP implementation.
 Subflow:  A flow of TCP segments operating over an individual path,
    which forms part of a larger Multipath TCP connection.
 (Multipath TCP) Connection:  A set of one or more subflows combined
    to provide a single Multipath TCP service to an application at a
    host.

Ford, et al. Informational [Page 5] RFC 6182 MPTCP Architecture March 2011

1.3. Reference Scenario

 The diagram shown in Figure 1 illustrates a typical usage scenario
 for Multipath TCP.  Two hosts, A and B, are communicating with each
 other.  These hosts are multihomed 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 Multipath TCP Usage Scenario
 The scenario could have any number of addresses (1 or more) on each
 host, as 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 -- potential fairness issues introduced by
 shared bottlenecks need to be handled by the Multipath TCP congestion
 controller.  Furthermore, the paths through the Internet often do not
 provide a pure end-to-end service, and instead may be affected by
 middleboxes such as NATs and firewalls.

2. Goals

 This section outlines primary goals that Multipath TCP aims to meet.
 These are broadly broken down into the following: 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 worse throughput than a single TCP connection over the
    best constituent path.

Ford, et al. Informational [Page 6] RFC 6182 MPTCP Architecture March 2011

 o  Improve Resilience: Multipath TCP MUST support the use of multiple
    paths interchangeably for resilience purposes, by permitting
    segments 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 improve overall network
 utility by shifting load away from congested bottlenecks and by
 taking advantage of spare capacity wherever possible.
 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
 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
 categories.

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, a
 Multipath TCP connection SHOULD provide the application with no worse
 throughput or resilience than it would expect from running a single
 TCP connection over any one of its available paths.  A Multipath TCP
 may not, however, be able to provide the same level of consistency of
 throughput and latency as a single TCP connection.  These, and other,
 application considerations are discussed in detail in [8].
 A multipath-capable equivalent of TCP MUST retain some level of
 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

Ford, et al. Informational [Page 7] RFC 6182 MPTCP Architecture March 2011

 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.
 It is possible for regular TCP sessions today to survive brief breaks
 in connectivity by retaining state at end hosts before a timeout
 occurs.  It would be desirable to support similar session continuity
 in MPTCP; however, the circumstances could be different.  Whilst in
 regular TCP the IP addresses will remain constant across the break in
 connectivity, in MPTCP a different interface may appear.  It is
 desirable (but not mandated) to support this kind of "break-before-
 make" session continuity.  This places constraints on security
 mechanisms, however, as discussed in Section 5.8.  Timeouts for this
 function would be locally configured.

2.2.2. Network Compatibility

 In the traditional Internet architecture, network devices operate at
 the network layer and lower layers, with the layers above the network
 layer 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 [9].  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.
 +-------------+                                       +-------------+
 | 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

Ford, et al. Informational [Page 8] RFC 6182 MPTCP Architecture March 2011

 +-------------+                                       +-------------+
 | 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" [10], 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 retain 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 [9].  This requirement comes from recognizing
 middleboxes as a significant deployment bottleneck for any transport
 that is not TCP or UDP, 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, Multipath
 TCP MUST preserve fate-sharing without making any assumptions about
 middlebox behavior.
 A detailed analysis of middlebox behavior 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.
 Middleboxes may also cause some TCP features to be able to exist on
 one subflow but not another.  Typically, these will be at the subflow
 level (such as selective acknowledgment (SACK) [11]) and thus do not
 affect the connection-level behavior.  In the future, any proposed
 TCP connection-level extensions should consider how they can coexist
 with MPTCP.
 The modifications to support Multipath TCP remain at the transport
 layer, although some knowledge of the underlying network layer is
 required.  Multipath TCP SHOULD work with IPv4 and IPv6
 interchangeably, i.e., one connection may operate over both IPv4 and
 IPv6 networks.

Ford, et al. Informational [Page 9] RFC 6182 MPTCP Architecture March 2011

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 nor 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 similar fairness to that which occurs at a
 shared bottleneck with single-path TCP.

2.3. Security Goals

 The extension of TCP with multipath capabilities will bring with it a
 number of new threats, analyzed in detail in [12].  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.

2.4. Related Protocols

 There are several similarities between SCTP [6] and MPTCP, in that
 both can make use of multiple addresses at end hosts to give some
 multipath capability.  In SCTP, the primary use case is to support
 redundancy and mobility for multihomed hosts (i.e., a single path
 will change one of its end host addresses); the simultaneous use of
 multiple paths is not supported.  Extensions are proposed to support
 simultaneous multipath transport [13], but these are yet to be
 standardized.  By far the most widely used stream-based transport
 protocol is, however, TCP [1], and SCTP does not meet the network and
 application compatibility goals specified in Section 2.2.  For
 network compatibility, there are issues with various middleboxes
 (especially NATs) that are unaware of SCTP and consequently end up
 blocking it.  For application compatibility, applications need to
 actively choose to use SCTP, and with the deployment issues, very few
 choose to do so.  MPTCP's compatibility goals are in part based on
 these observations of SCTP's deployment issues.

Ford, et al. Informational [Page 10] RFC 6182 MPTCP Architecture March 2011

3. An Architectural Basis for Multipath TCP

 This section presents one possible transport architecture that the
 authors believe can effectively support the goals for Multipath TCP.
 The new Internet model described here is based on ideas proposed
 earlier in Tng ("Transport next-generation") [14].  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 [15] [16] or optimize communication
 performance [17].  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.

Ford, et al. Informational [Page 11] RFC 6182 MPTCP Architecture March 2011

 +-------------+                                       +-------------+
 | 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.  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
 "application-oriented" Semantic layer; whereas the subflow TCP
 component, which provides network compatibility by appearing and
 behaving as a TCP flow in the 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: Relationship between Tng (Left) and MPTCP (Right)
 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

 The previous two sections have discussed the goals for a Multipath
 TCP design, and provided a basis for decomposing the functions of a
 transport protocol in order to better understand the form a solution
 should take.  This section builds upon this analysis by presenting
 the functional components that are used within the MPTCP design.

Ford, et al. Informational [Page 12] RFC 6182 MPTCP Architecture March 2011

 MPTCP 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:
 o  Path Management: This is the function to detect and use multiple
    paths between two hosts.  MPTCP uses the presence of multiple IP
    addresses at one or both of the hosts as an indicator of this.
    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 byte stream received
    from the application into segments to be transmitted on one of the
    available 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 segments.
    This function is also responsible for connection-level re-ordering
    on receipt of packets from the TCP subflows, according to the
    attached data sequence mappings.
 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.

Ford, et al. Informational [Page 13] RFC 6182 MPTCP Architecture March 2011

    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.  On receipt, the subflow passes its
    reassembled data to the packet scheduling component for
    connection-level reassembly; the data sequence mapping from the
    sender's packet scheduling component allows re-ordering of the
    entire byte stream.
 o  Congestion Control: This function coordinates congestion control
    across the subflows.  As specified, this congestion control
    algorithm MUST ensure that an MPTCP connection does not unfairly
    take more bandwidth than a single path TCP flow would take at a
    shared bottleneck.  An algorithm to support this is specified in
    [7].
 These functions fit together as follows.  The path management looks
 after the discovery (and if necessary, initialization) 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,
 in order to schedule which segments 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.  This section outlines high-level design
 choices that draw from the architectural basis discussed earlier in
 Section 3, which the design of MPTCP [5] takes into account.

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.

Ford, et al. Informational [Page 14] RFC 6182 MPTCP Architecture March 2011

 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 segment is sent on multiple paths (i.e., in the case of
 retransmissions).
 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 (carrying subflow
 sequence numbers) maps at the connection level.  This is referred to
 as the "data sequence mapping".  This mapping can be represented as a
 tuple of (data sequence number, subflow sequence number, 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 sequence number).
 This information could conceivably have various sources.
 One option to signal the data sequence mapping would be to use
 existing fields in the TCP segment (such as subflow sequence number,
 length) and add only the data sequence number to each segment, for
 instance, as a TCP option.  This would be vulnerable, however, to
 middleboxes that re-segment or assemble data, since there is no
 specified behavior for coalescing TCP options.  If one signaled (data
 sequence number, length), this would still be vulnerable to
 middleboxes that coalesce segments and do not understand MPTCP
 signaling so do not correctly rewrite the options.
 Because of these potential issues, the design decision taken in the
 MPTCP protocol 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 permissible 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.

Ford, et al. Informational [Page 15] RFC 6182 MPTCP Architecture March 2011

 Under normal behavior, MPTCP could 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 would mean that once a segment 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, it may be
 desirable to drop segments 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 optimization, it may be feasible for a
 connection-level acknowledgement to be transmitted over the shortest
 Round-Trip Time (RTT) path, potentially reducing send buffer
 requirements (see Section 5.3).
 Therefore, to provide a fully robust multipath TCP solution given the
 above constraints, MPTCP for use on the public Internet MUST 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 specification [5].
 Regarding retransmissions, it MUST be possible for a segment to be
 retransmitted on a different subflow from 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 behavior

Ford, et al. Informational [Page 16] RFC 6182 MPTCP Architecture March 2011

 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 segment.
 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 segments 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 compatibility goal.
 By doing this, some efficiency is lost, 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, MPTCP 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
 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 MPTCP, 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 segment 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.

Ford, et al. Informational [Page 17] RFC 6182 MPTCP Architecture March 2011

 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., 100 Mb/s, 10 ms RTT) are used in conjunction with slow paths
 (e.g., 1 Mb/s, 1000 ms RTT).  In that case, the required receive
 buffer would be 12.5 MB, which is likely too big.  In extreme cases
 such as this example, it may be prudent to only use some of the
 fastest available paths for the MPTCP connection, potentially using
 the slow path(s) for backup only.
 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 locally store 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. Signaling

 Since MPTCP uses TCP as its subflow transport mechanism, an 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
 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 signaling is required during the operation of an
 MPTCP session, such as that for reassembly across multiple subflows,
 and for informing the other host about other available IP addresses.
 The MPTCP protocol design will use TCP options for this additional
 signaling.  This has been chosen as the mechanism most fitting in
 with the goals as specified in Section 2.  With this mechanism, the
 signaling required 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
 entities.

Ford, et al. Informational [Page 18] RFC 6182 MPTCP Architecture March 2011

 This decision is the consensus of the Working Group (following
 detailed discussions at IETF78), and the main reasons for this are as
 follows:
 o  TCP options are the traditional signaling method for TCP;
 o  A TCP option on a SYN is the most compatible way for an end host
    to signal it is MPTCP capable;
 o  If connection-level ACKs are signaled in the payload, then they
    may suffer from packet loss and may be congestion-controlled,
    which may affect the data throughput in the forward direction and
    could lead to head-of-line blocking;
 o  Middleboxes, such as NAT traversal helpers, can easily parse TCP
    options, e.g., to rewrite addresses.
 On the other hand, the main drawbacks of TCP options compared to TLV
 encoding in the payload are the following:
 o  There is limited space for signaling messages;
 o  A middlebox may, potentially, drop a packet with an unknown
    option;
 o  The transport of control information in options is not necessarily
    reliable.
 The detailed design of MPTCP alleviates these issues as far as
 possible by carefully considering the size of MPTCP options and
 seamlessly falling back to regular TCP on the loss of control data.
 Both option and payload encoding may interfere with offloading of TCP
 processing to high-speed network interface cards, such as
 segmentation, checksumming, and reassembly.  For network cards
 supporting MPTCP, signaling in TCP options should simplify offloading
 due to the separate handling of MPTCP signaling and data.

5.5. Path Management

 Currently, the network does not expose path diversity between pairs
 of IP addresses.  In order to achieve path diversity from today's IP
 networks, in the typical case, MPTCP uses multiple addresses at one
 or both hosts to infer different paths across the network.  It is
 expected that these paths, whilst not necessarily entirely non-
 overlapping, will be sufficiently disjoint to allow multipath to

Ford, et al. Informational [Page 19] RFC 6182 MPTCP Architecture March 2011

 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 in most cases.  However, each path will be
 identified by a standard five-tuple (i.e., source address,
 destination address, source port, destination port, protocol), which
 can allow the extension of MPTCP to use ports as well as addresses as
 path selectors.  This will allow hosts to use port-based load
 balancing with MPTCP, for example, if the network routes different
 ports over different paths (which may be the case with technologies
 such as Equal Cost MultiPath (ECMP) routing [4]).  It should be
 noted, however, that ISPs often undertake traffic engineering in
 order to optimize resource utilization within their networks, and
 care should be taken (by both ISPs and developers) that MPTCP using
 broadly similar paths does not adversely interfere with this.
 For an 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 an 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 path management is a separate function from the packet
 scheduling, subflow interface, and congestion control functions of
 MPTCP, as documented in Section 4.  As such, it would be feasible to
 replace this IP-address-based design with an alternative path
 selection mechanism in the future, with no significant changes to the
 other functional components.

5.6. Connection Identification

 Since an MPTCP connection may not be bound to a traditional 5-tuple
 (source address and port, destination address and port, protocol
 number) for the entirety of its existence, 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 an ephemeral port number in regular TCP.  The
 manifestation and purpose of such an identifier is out of the scope
 of this architecture document.

Ford, et al. Informational [Page 20] RFC 6182 MPTCP Architecture March 2011

 Non-MPTCP-aware applications will not, however, have access to this
 identifier and in such cases an 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 behavior of the MPTCP
 implementation if the first TCP subflow later fails.  If there are
 MPTCP-unaware applications that make assumptions about continued
 existence of the initial address pair, their behavior could be
 disrupted by carrying on regardless.  It is expected that this is a
 very small, possibly negligible, set of applications, however.  MPTCP
 MUST NOT be used for applications that request to bind to a specific
 address or interface, since such applications are making a deliberate
 choice of path in use.
 Since the requirements of applications are not clear at this stage,
 however, it is as yet unconfirmed whether carrying on in the event of
 the loss of the initial address pair would be a damaging assumption
 to make.  This behavior will be an implementation-specific solution,
 and as such it 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
 achieve these goals, the congestion control algorithms on each
 subflow must be coupled in some way.  A proposal for a suitable
 congestion control algorithm is given in [7].

5.8. Security

 A detailed threat analysis for Multipath TCP is presented in a
 separate document [12].  That document focuses on flooding attacks
 and hijacking attacks that can be launched against a Multipath TCP
 connection.
 The basic security goal of Multipath TCP, as introduced in
 Section 2.3, can be stated as: "provide a solution that is no worse
 than standard TCP".

Ford, et al. Informational [Page 21] RFC 6182 MPTCP Architecture March 2011

 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 do the following:
 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 (DoS) attacks.  For
 example, there are various mechanisms to protect against TCP reset
 attacks [18], and Multipath TCP should continue to support similar
 protection.  In addition, TCP SYN Cookies [19] 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.
 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' (as well as a 'make-
    before-break') approach to adding subflows (within a limited time
    period) implies that a host cannot rely on using a pre-existing
    subflow to support the addition of a new one.
 The MPTCP protocol will be designed with these security requirements
 in mind, and the protocol specification [5] will document how these
 are met.

Ford, et al. Informational [Page 22] RFC 6182 MPTCP Architecture March 2011

6. Software Interactions

6.1. Interactions with Applications

 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.  This is because the applications are indicating a clear
 choice of path to use and thus will have expectations of behavior
 that must be maintained, in order to adhere to the application
 compatibility goals.
 Interactions with applications are presented in [8] -- including, but
 not limited to, performances changes that may be expected, semantic
 changes, and new features that may be requested through an enhanced
 API.
 TCP features the ability to send "Urgent" data, the delivery of which
 to the application may or may not be out-of-band.  The use of this
 feature is not recommended due to security implications and
 implementation differences [20].  MPTCP requires contiguous data to
 support its data sequence mapping over multiple segments, and
 therefore the Urgent pointer cannot interrupt an existing mapping.
 An MPTCP implementation MAY choose to support sending Urgent data,
 and if it does, it SHOULD send the Urgent data on the soonest
 available unassigned subflow sequence space.  Incoming Urgent data
 SHOULD be mapped to connection-level sequence space and delivered to
 the application analogous to Urgent data in regular TCP.

6.2. Interactions with Management Systems

 To enable interactions between TCP and network management systems,
 the TCP [21] and TCP Extended Statistics (ESTATS) [22] MIBs have been
 defined.  MPTCP should share these MIBs for aspects that are designed
 to be transparent to the application.
 It is anticipated that an MPTCP MIB will be defined in the future,
 once experience of experimental MPTCP deployments is gathered.  This
 MIB would provide access to MPTCP-specific properties such as whether
 MPTCP is enabled and the number and properties of the individual
 paths in use.

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 summarizes 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.

Ford, et al. Informational [Page 23] RFC 6182 MPTCP Architecture March 2011

 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 myriad of middleboxes are deployed
 to optimize various perceived problems with the Internet protocols:
 NATs primarily address IP address space shortage [15], Performance
 Enhancing Proxies (PEPs) optimize TCP for different link
 characteristics [17], firewalls [16] and intrusion detection systems
 try to block malicious content from reaching a host, and traffic
 normalizers [23] ensure a consistent view of the traffic stream to
 Intrusion Detection Systems (IDS) 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 behavior
 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 behavior that could
 impact the use of MPTCP.  This list is used in [5] to describe the
 features of the MPTCP protocol that are used to mitigate the impact
 of these middlebox behaviors.
 o  NATs: Network Address Translators decouple the host's local IP
    address (and, in the case of NAPTs, port) 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 signaling 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 behavior such as proactive ACKing may occur, and
    therefore it is no longer guaranteed that one host is
    communicating directly with another.  PEPs, firewalls, or other
    middleboxes may also change the declared receive window size.
 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 (which
    has an impact on MPTCP's retransmission and subflow sequence
    numbering design choices).

Ford, et al. Informational [Page 24] RFC 6182 MPTCP Architecture March 2011

 o  Firewalls: on top of preventing incoming connections, firewalls
    may also attempt additional protection such as sequence number
    randomization (so a sender cannot reliably know what TCP sequence
    number the receiver will see).
 o  IDSs: Intrusion Detection Systems may look for traffic patterns to
    protect a network and may have false positives with MPTCP and drop
    the connections during normal operation.  Future MPTCP-aware
    middleboxes will require the ability to correlate the various
    paths in use.
 o  Content-Aware Firewalls: Some middleboxes may actively change data
    in packets, such as rewriting URIs in HTTP traffic.
 In addition, all classes of middleboxes may affect TCP traffic in the
 following ways:
 o  TCP Options: some middleboxes may drop packets with unknown TCP
    options or strip those options from the packets.
 o  Segmentation and Coalescing: middleboxes (or even something as
    close to the end host as TCP Segmentation Offloading (TSO) on a
    Network Interface Card (NIC)) may change the packet boundaries
    from those that the sender intended.  It may do this by splitting
    packets or coalescing them together.  This leads to two major
    impacts: where a packet boundary will be cannot be guaranteed and
    what a middlebox will do with TCP options in these cases (they may
    be repeated, dropped, or sent only once) cannot be said for sure.

8. Contributors

 The authors would like to acknowledge the contributions of 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, Philip Eardley, Michael Scharf, Lars Eggert, Cullen
 Jennings, Joel Halpern, Juergen Quittek, Alexey Melnikov, David
 Harrington, Jari Arkko, and Stewart Bryant.

9. Acknowledgements

 Alan Ford, Costin Raiciu, Mark Handley, 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.

Ford, et al. Informational [Page 25] RFC 6182 MPTCP Architecture March 2011

10. 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 [12] 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 2.3, whilst
 Section 5.8 provides more detailed discussion of security
 requirements and design decisions which are applied in the MPTCP
 protocol design [5].

11. References

11.1. 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.

11.2. 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,
       <http://ccr.sigcomm.org/online/files/p47-handleyA4.pdf>.
 [4]   Hopps, C., "Analysis of an Equal-Cost Multi-Path Algorithm",
       RFC 2992, November 2000.
 [5]   Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, "TCP
       Extensions for Multipath Operation with Multiple Addresses",
       Work in Progress, March 2011.
 [6]   Stewart, R., "Stream Control Transmission Protocol", RFC 4960,
       September 2007.
 [7]   Raiciu, C., Handley, M., and D. Wischik, "Coupled Congestion
       Control for Multipath Transport Protocols", Work in Progress,
       March 2011.
 [8]   Scharf, M. and A. Ford, "MPTCP Application Interface
       Considerations", Work in Progress, March 2011.
 [9]   Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues",
       RFC 3234, February 2002.

Ford, et al. Informational [Page 26] RFC 6182 MPTCP Architecture March 2011

 [10]  Carpenter, B., "Internet Transparency", RFC 2775,
       February 2000.
 [11]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
       Selective Acknowledgment Options", RFC 2018, October 1996.
 [12]  Bagnulo, M., "Threat Analysis for TCP Extensions for Multipath
       Operation with Multiple Addresses", RFC 6181, March 2011.
 [13]  Becke, M., Dreibholz, T., Iyengar, J., Natarajan, P., and M.
       Tuexen, "Load Sharing for the Stream Control Transmission
       Protocol (SCTP)", Work in Progress, December 2010.
 [14]  Ford, B. and J. Iyengar, "Breaking Up the Transport Logjam",
        ACM HotNets, October 2008.
 [15]  Srisuresh, P. and K. Egevang, "Traditional IP Network Address
       Translator (Traditional NAT)", RFC 3022, January 2001.
 [16]  Freed, N., "Behavior of and Requirements for Internet
       Firewalls", RFC 2979, October 2000.
 [17]  Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
       Shelby, "Performance Enhancing Proxies Intended to Mitigate
       Link-Related Degradations", RFC 3135, June 2001.
 [18]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
       Robustness to Blind In-Window Attacks", RFC 5961, August 2010.
 [19]  Eddy, W., "TCP SYN Flooding Attacks and Common Mitigations",
       RFC 4987, August 2007.
 [20]  Gont, F. and A. Yourtchenko, "On the Implementation of the TCP
       Urgent Mechanism", RFC 6093, January 2011.
 [21]  Raghunarayan, R., "Management Information Base for the
       Transmission Control Protocol (TCP)", RFC 4022, March 2005.
 [22]  Mathis, M., Heffner, J., and R. Raghunarayan, "TCP Extended
       Statistics MIB", RFC 4898, May 2007.
 [23]  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>.

Ford, et al. Informational [Page 27] RFC 6182 MPTCP Architecture March 2011

Authors' Addresses

 Alan Ford
 Roke Manor Research
 Old Salisbury Lane
 Romsey, Hampshire  SO51 0ZN
 UK
 Phone: +44 1794 833 465
 EMail: alan.ford@roke.co.uk
 Costin Raiciu
 University College London
 Gower Street
 London  WC1E 6BT
 UK
 EMail: c.raiciu@cs.ucl.ac.uk
 Mark Handley
 University College London
 Gower Street
 London  WC1E 6BT
 UK
 EMail: m.handley@cs.ucl.ac.uk
 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

Ford, et al. Informational [Page 28]

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