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Network Working Group                                         J. Iyengar
Internet-Draft                             Franklin and Marshall College
Intended status: Informational                                   B. Ford
Expires: January 7, 2010               Max Planck Institute for Software
                                                                 Systems
                                                            July 6, 2009


           A Next Generation Transport Services Architecture
                     draft-iyengar-ford-tng-00.txt

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   Copyright (c) 2009 IETF Trust and the persons identified as the
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Abstract

   While there is substantial community interest in next-generation



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   multipath-capable Internet transports, evolutionary pressures have
   gradually eroded the simplicity of the Internet's original transport
   architecture to a point where it is no longer realistically
   applicable to new tranports.  This document proposes a new
   architectural framework for next-generation multipath-capable
   transport protocols, focusing immediately on multipath TCP but taking
   care to allow for generalization to other multipath-capable
   transports.  The architecture places emphasis on enabling new
   multipath features in a safe, TCP-friendly, and backward-compatible
   fashion, retaining full interoperability with both existing
   applications and existing network infrastructure, and enabling reuse
   of existing protocols as much as possible while providing incremental
   deployment paths to new, more powerful and/or more efficient
   protocols.  The architecture re-establishes the long-lost principles
   of end-to-end reliability and fate sharing, in the presence of
   existing and future network middleboxes, and enables the deployment
   of transport-neutral end-to-end protection without interfering with
   these policy-enforcing or performance-enhancing middleboxes.  This
   document describes architecture goals, a layering model supporting
   these goals, abstract properties of the interfaces between the
   architecture's new layers, general approaches to multipath congestion
   control and how they fit into the architecture, realistic protocol
   design and incremental deployment paths, and ways in which this
   document complements and relates to ongoing protocol design
   activities in the IETF.


























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Architecture Goals . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Functional Goals . . . . . . . . . . . . . . . . . . . . .  6
     2.2.  Performance/Efficiency Goals . . . . . . . . . . . . . . .  7
   3.  Architecture Overview  . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Addressing Operational Concerns  . . . . . . . . . . . . .  9
     3.2.  Endpoint Layer . . . . . . . . . . . . . . . . . . . . . . 12
     3.3.  The Flow Regulation Layer  . . . . . . . . . . . . . . . . 13
     3.4.  The Isolation Layer  . . . . . . . . . . . . . . . . . . . 14
     3.5.  The Semantic Layer . . . . . . . . . . . . . . . . . . . . 15
   4.  Multipath Congestion Control . . . . . . . . . . . . . . . . . 15
   5.  Protocol Implementation and Incremental Deployment Paths . . . 17
   6.  How Tng Complements Current Projects . . . . . . . . . . . . . 20
   7.  Experiences with Running Code  . . . . . . . . . . . . . . . . 22
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     11.2. Informative References . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28




























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1.  Introduction

   While there is substantial community interest in developing next-
   generation Internet transports supporting concurrent multipath
   communication and other new features, the Internet's evolution over
   the past decades have left us in a situation in which the Internet's
   traditional transport architecture is not well-suited to support new
   transports in general, or multipath transports in particular.  This
   document attempts to address this problem by defining a new
   conceptual framework for Internet transports, which is both
   compatible with the pragmatic reality of the Internet today and the
   backward compatibility constraints new transports must face, while
   effectively enabling the deployment of compelling new features and
   the further long-term evolution of Internet transports.

   The Internet's transport layer traditionally combines many functions,
   whose combination in one layer has led to numerous pragmatic
   challenges to transport evolution.  Concurrent multipath transport
   requires decoupling congestion control state from application-visible
   transport instances and associating one transport instance with
   multiple per-path congestion control contexts [iyengar06concurrent].
   Avoiding an explosion in congestion control contexts in the presence
   of applications like HTTP [RFC2616], which frequently use multiple
   transport instances in parallel, similarly requires decoupling and
   sharing congestion control contexts among transport instances
   [RFC2140] [RFC3124].

   Some transport functions have pragmatically proven to be of concern
   to network operators and the policy-enforcing and performance-
   enhancing devices they now frequently deploy within the network,
   despite the the transport's original role as a purely "end-to-end"
   layer.  For example, already-ubiquitous firewalls [RFC2979], NATs
   [RFC3022], and flow-aware routers [roberts03next] need access to
   transport layer port numbers to enforce application-sensitive access
   and traffic management policies, and performance enhancing proxies
   (PEPs) [RFC3135] need to interact with the transport's congestion
   control mechanisms [RFC2581] to optimize communication performance
   across diverse network technologies such as high-speed [RFC3649] and
   wireless [balakrishnan97comparison] links.  Middlebox traversal
   challenges have rendered the traditional Network/Transport layer
   interface effectively useless for deploying new transports
   [I-D.rosenberg-internet-waist-hourglass], and the TCP header's
   limited 40-byte option space is almost consumed by options now
   considered "essential", such as SACK [RFC2018], leaving little leeway
   for further extending TCP in-place.  Deploying end-to-end security is
   problematic because IP-level mechanisms such as IPsec [RFC4301] and
   HIP [RFC4423] interfere with middleboxes [RFC3947] [RFC5207], and
   transport-level mechanisms such as TLS [RFC5246] must be modified for



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   each transport [RFC4347] [RFC5238].

   All of these challenges point to a pressing need to decompose the
   Internet's traditional monolithic transport layer; this document
   proposes such a decomposition as a guide for current and future
   transport evolution efforts.

   The proposed architecture, which we refer to as Tng ("Transport next-
   generation"), decomposes traditional transport functions into four
   layers, described in detail in Section 3.  Tng factors out endpoint-
   related functions into a separate Endpoint Layer, and performance-
   related transport layer functions such as congestion-control into a
   Flow Layer, leaving end-to-end semantic functions such as reliability
   and ordering in a Semantic Layer, and end-to-end security functions
   to an optional Security/Identity Layer.

   The architecture described here is based on ideas proposed earlier
   [ford08breaking] [iyengar09flow], and builds on previous research in
   concurrent multipath transfer [iyengar06concurrent], congestion
   control state sharing [RFC2140] [RFC3124], and transport efficiency
   [RFC1644] [ford07structured].  We argue that addition of new non-
   trivial functionality to existing transport protocols must recognize,
   at least implicitly, the existence of these distinct components
   within the transport layer; we illustrate this point in Section 6 by
   juxtaposing the multipath TCP work currently underway with the Tng
   model.  Our intent is for this architecture to define a framework
   into which specific protocols can fit in a modular fashion; the
   architecture further permits existing transport protocols to be
   reused and combined in new ways with only minor modifications to
   support a wide variety of important deployment scenarios.

   The rest of this draft is organized as follows: Section Section 2
   outlines the most important architectural goals for a next-generation
   transport architecture, with a particular emphasis on multipath
   transport; Section Section 3 describes our Tng architecture with
   specific details on how this architecture cleanly satisfies the goals
   identified above; Section Section 4 discusses the applicability of
   multipath congestion control work and its place in Tng; Section
   Section 5 identifies specific protocol stack designs that are
   compelling instantiations of Tng, with discussions about incremental
   deployability; and finally, Section Section 6 discusses Tng's
   relevance and potential contributions to current projects in and
   outside the IETF.


2.  Architecture Goals

   This section outlines what we perceive to be the most important goals



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   for a next-generation transport architecture, and for multipath-
   capable transport protocols conforming to such an architecture.  We
   divide these goals into two categories: functional goals and
   performance/efficiency goals.  We acknowledge that these goals and
   their categorization is subjective and invite debate on what the true
   goals and priorities should be.

2.1.  Functional Goals

   o  Multihoming: The transport architecture must allow for a logical
      transport endpoint as seen by the application to correspond to
      multiple physical network attachment points, such as multiple IP
      addresses on the same or different network interfaces.

   o  Application Compatibility: The architecture must enable next-
      generation equivalents of existing transports, such as concurrent
      multipath versions of TCP, SCTP, or DCCP, to serve as "drop-in"
      replacements for the corresponding existing transports
      transparently to existing applications using those transports,
      merely by upgrading the operating systems of the end hosts.

   o  Network Compatibility: The architecture must enable next-
      generation transport instances to remain backward compatible with
      the Internet as it exists today, including being able to traverse
      predominant existing middleboxes such as firewalls, NATs, and
      performance enhancing proxies.

   o  Automatic Negotiation: A host supporting a next-generation
      equivalent of an existing transport must be able to detect
      reliably whether a new communication partner supports the next-
      generation protocol, using it if so, and otherwise automatically
      falling back to the existing protocol (e.g., standard TCP, SCTP,
      or DCCP).

   o  Naming/Addressing Neutrality: The transport architecture should
      cleanly abstract over specific naming/addressing schemes for
      hosts, endpoints, and services, with a goal of permitting
      transports conforming to this architecture to support multiple
      endpoint address formats (e.g., IPv4 and IPv6), cryptographic
      endpoint identities [RFC4423] [ford06persistent], and/or name-
      based endpoint identification [cheriton00triad]

   o  Coexistence with End-to-End Security: The architecture must allow
      for the optional deployment of end-to-end authentication and/or
      privacy protection in a transport-neutral but network-compatible
      fashion: i.e., supporting any transport conforming to this
      architecture without requiring modifications specific to each
      transport, while maintaining compatibility with legacy



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      middleboxes.

2.2.  Performance/Efficiency Goals

   o  Multihoming and Multipath Capable: The architecture must enable a
      next-generation transport to detect and utilize multiple available
      paths between two logical endpoints, either one path at a time
      (fail-over multipath) or several at once (concurrent multipath).

   o  Resource Pooling: Transports should be able to balance traffic
      among available paths, optimizing network utility in a global
      sense by shifting load away from congested bottlenecks and taking
      advantage of spare capacity wherever it may be located
      [wischik08resource].

   o  Congestion State Sharing: Since popular applications such as HTTP
      often use multiple transport instances between the same pair of
      hosts, the architecture must avoid multiplicative explosions in
      multipath congestion control contexts - i.e., N transport
      instances times M multipath flows each - by enabling a multipath
      "bundle" of congestion control contexts to be shared cleanly among
      application-visible transport instances.

   o  TCP Friendliness: The architecture must enable new multipath
      transport flows to coexist gracefully with predominant existing
      transport flows, competing for bandwidth neither unduly
      aggressively or unduly timidly (unless low-precedence operation is
      specifically requested by the application, such as with LEDBAT
      [I-D.shalunov-ledbat-congestion]).

   o  Support Small Transactions: Recognizing that many applications
      today make heavy use of frequent small communications, such as
      HTTP conditional GET transactions or streaming media frames, next-
      generation transports should minimize the performance costs of
      supporting these common application behaviors, including by
      minimizing unnecessary protocol overhead on small packets and by
      unnecessary round-trip delays or state maintenance costs when
      applications use short transactions.

   o  Simplicity: The architecture should enable implementations of
      next-generation transports to be as simple as possible while
      remaining consistent with the above goals.  In particular, end
      host implementations that never need certain transport features
      (e.g., embedded hosts) should ideally not have to incur the costs
      of implementing and validating those features.






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3.  Architecture Overview

   This section presents an overview of one possible transport
   architecture that we believe can effectively support the above goals
   and provides a clean framework for next-generation multipath-capable
   transport protocols.  We refer to this architecture for now simply as
   Tng, for "Transport next-generation".  Tng as described here is based
   on ideas we proposed earlier [ford08breaking] [iyengar09flow].  While
   by no means the only possible architecture supporting multipath
   transport, our proposal incorporates many lessons learned from
   previous transport research and development practice, should fit well
   with the protocol design and congestion control work already in
   progress in the SCTP and multipath TCP communities, and we believe
   offers a strong starting point from which to develop a long-term
   architecture for next-generation Internet transports.

   Tng is based on a decomposition of the Internet's traditional
   Transport Layer into up to four new layers, as illustrated below in
   Figure 1:

                                   +---------------+
                                   |  Application  |  ^
                               --> +---------------+  |
        +---------------+     /    |   Semantic    |  |
        |   Application |    /     +---------------+  | Application-
        +---------------+ <--      |   Isolation   |  | Oriented
        |   Transport   |          +---------------+  --------------
        +---------------+ <--      |     Flow      |  | Network-
        |    Network    |    \     +---------------+  | Oriented
        +---------------+     \    |   Endpoint    |  |
                               --> +---------------+  |
                                   |    Network    |  v
                                   +---------------+

         Existing Layers              Tng Layers

                                 Figure 1

   We first summarize Tng's new layers briefly here, then further
   discuss their functions and interactions below.

   o  Semantic Layer: implements whatever communication abstractions are
      to be made available to applications, including providing the end-
      to-end reliability and ordering properties of abstractions like
      TCP's byte streams or SCTP's message-based multi-streams.

   o  Isolation Layer (optional): provides end-to-end protection of the
      application's communication and of the end-to-end reliability



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      mechanisms it builds on.

   o  Flow Regulation Layer or Flow Layer: implements congestion control
      and other performance management functions.

   o  Endpoint Layer: Implements endpoint/service identification
      functions (e.g., port numbers) that network operators and their
      middleboxes require to enforce network access and security
      policies.

   We loosely classify Tng's layers into "application-oriented" and
   "network-oriented" layers, as shown in Figure 1.  We describe the top
   two layers as "application-oriented" because their functions are
   driven primarily by concerns of supporting and protecting the
   application's end-to-end communication.  We consider the bottom two
   layers "network-oriented" because they represent functions that,
   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 [RFC3022] [RFC2979] or optimize communication
   performance [RFC3135].

   Not all of Tng's functional layers need be present in the form
   described above in a particular instantiation of the architecture:
   e.g., the Isolation Layer may be omitted whenever strong end-to-end
   authentication or encryption are not needed.  Similarly, layers may
   be recombined when pragmatic considerations require: e.g., we discuss
   later in Section Section 5 how legacy TCP connections may serve as
   one combined implementation of the Endpoint and Flow Layers for
   maximum compatibility with legacy networks and middleboxes.  The
   layering diagram above is intended to serve as a conceptual model
   that helps appropriately place new functionality in the transport
   suite, and not a protocol design straitjacket.

3.1.  Addressing Operational Concerns

   Tng's decomposition of traditional transport functions serves several
   pragmatically important purposes at once concerning the operation of
   next-generation transports on today's and tomorrow's Internet:

   o  Multihoming and Multipath Transfer: By separating congestion
      control state from the semantic state of application-visible
      transport instances such as reliable byte streams, one transport
      instance may be associated with multiple congestion control (CC)
      contexts representing individual physical network paths to
      implement concurrent multipath transfer cleanly, as illustrated in
      Figure 2 below.  The congestion control contexts for different
      paths may still interact, for example to achieve global resource



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      pooling as discussed later in Section Section 4, but the ability
      to maintain per-path congestion control state is a basic
      prerequisite for concurrent multipath operation on the Internet
      [iyengar06concurrent].

                                +-----------------------+
         Application            |    HTTP, SMTP, etc.   |
                                +-----------+-----------+
                                            |
                                +-----------------------+
         Semantic               |      Byte Stream      |
                                +-----------+-----------+
                                            |
                        Path 1      Path 2  |   Path 3      Path 4
                          /-----------/-----+-----\-----------\
                          |           |           |           |
                      +------+    +------+    +------+    +------+
                      | Per- |    | Per- |    | Per- |    | Per- |
         Flow         | Path |    | Path |    | Path |    | Path |
                      |  CC  |    |  CC  |    |  CC  |    |  CC  |
                      +------+    +------+    +------+    +------+
                          |           |           |           |
                      +------+    +------+    +------+    +------+
         Endpoint     | Port |    | Port |    | Port |    | Port |
                      +------+    +------+    +------+    +------+
                          |           |           |           |
                      +------------------------------------------+
         Network      |               IP Forwarding              |
                      +------------------------------------------+

                                   Figure 2

   o  Congestion State Sharing: Separating congestion control state from
      semantic state further enables multiple application-visible
      transport instances to share a single underlying congestion
      control context, or a single multipath bundle of congestion
      control contexts, as illustrated in Figure 3 below.  Compelling
      even in the absence of multipath transport [RFC2140] [RFC3124]
      [ford07structured], congestion state sharing becomes essential in
      the presence of multipath transfer, to avoid an otherwise N x M
      explosion in congestion control contexts resulting from
      applications like HTTP that use N concurrent transport instances,
      each using M communication paths.








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                      +------------------------------------------+
         Application  |                    HTTP                  |
                      +----------+---------------------+---------+
                                 |                     |
                          +------+------+       +------+------+
         Semantic         |   Stream 1  |       |   Stream 2  |
                          +------+------+       +------+------+
                                 |                     |
                                 \----------+----------/
                                            |
                        Path 1      Path 2  |   Path 3      Path 4
                          /-----------/-----+-----\-----------\
                          |           |           |           |
                      +------+    +------+    +------+    +------+
                      | Per- |    | Per- |    | Per- |    | Per- |
         Flow         | Path |    | Path |    | Path |    | Path |
                      |  CC  |    |  CC  |    |  CC  |    |  CC  |
                      +------+    +------+    +------+    +------+
                          |           |           |           |
                      +------+    +------+    +------+    +------+
         Endpoint     | Port |    | Port |    | Port |    | Port |
                      +------+    +------+    +------+    +------+
                          |           |           |           |
                      +------------------------------------------+
         Network      |               IP Forwarding              |
                      +------------------------------------------+

                                   Figure 3

   o  Middlebox Compatibility: Decomposing the "network-oriented"
      functions of endpoint identification and congestion control
      enables middleboxes to enforce network policies and optimize
      performance without interfering with end-to-end reliability
      [saltzer84endtoend], fate sharing [clark88design], or end-to-end
      security [RFC3135].  In particular, the architecture allows
      middleboxes to interpose on the lower network-oriented layers, as
      illustrated below in Figure 4, without having to understand or
      interfere with the end-to-end application-oriented layers,
      yielding a variety of practical benefits [ford08breaking].
      Through suitable reuse of existing protocols, a Tng protocol stack
      can even remain fully compatible with existing middleboxes, as
      described below in Section 5.









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   +-------------+                                       +-------------+
   | Application |<------------ end-to-end ------------->| Application |
   +-------------+                                       +-------------+
   |  Semantic   |<------------ end-to-end ------------->|  Semantic   |
   +-------------+                                       +-------------+
   |  Isolation  |<------------ end-to-end ------------->|  Isolation  |
   +-------------+                     +-------------+   +-------------+
   |    Flow     |<------------------->|    Flow     |<->|    Flow     |
   +-------------+   +-------------+   +-------------+   +-------------+
   |  Endpoint   |<->|  Endpoint   |<->|  Endpoint   |<->|  Endpoint   |
   +-------------+   +-------------+   +-------------+   +-------------+
   |   Network   |<->|   Network   |<->|   Network   |<->|   Network   |
   +-------------+   +-------------+   +-------------+   +-------------+
                        Firewall         Performance
      End Host           or NAT        Enhancing Proxy      End Host

                                   Figure 4

   o  End-to-End Security: By locating the Isolation Layer precisely on
      the boundary between "network-oriented" and "application-oriented"
      functions, in a position effectively corresponding to somewhere in
      the middle of the classical Transport Layer, end-to-end security
      mechanisms at this position avoid interfering with network
      middleboxes as described above, while retaining IPsec's benefit of
      operating in a "transport neutral" fashion and protecting all
      application communication regardless of transport, without having
      to be modified for each new transport as application-level
      solutions such as TLS/DTLS does [RFC5246] [RFC4347].

   Tng's contribution as an architecture is not to find a perfect or
   complete decomposition of the transport layer, but to identify
   specific transport functions that have proven in practice to be
   "network-oriented" contrary to their traditional placement in the
   transport layer, and to construct a new but incrementally deployable
   layering that reflects this reality and restores the "end-to-endness"
   of the remaining application-oriented functions.

   The following subsections outline each Tng layer in more detail,
   including a brief summary of the interface each layer provides to
   higher layers.

3.2.  Endpoint Layer

   As in the OSI model, TCP/IP traditionally breaks application endpoint
   identifiers into Network Layer (IP address) and Transport Layer (port
   number) components, including only the former in the IP header on the
   assumption that the network need know only how to route to a given
   host, and leaving port numbers to be parsed and demultiplexed by the



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   transport.  As the Internet's size and diversity exploded, however,
   network operators needed to enforce access policies that depend on
   exactly who is communicating--not just which hosts, but which
   applications and users.  Now-ubiquitous middleboxes such as
   Firewalls, traffic shapers, and NATs must therefore understand
   transport headers in order to enforce these network policies.  Since
   middleboxes cannot forward traffic for transports whose headers they
   do not understand, new transports have become effectively
   undeployable other than atop TCP or UDP
   [I-D.rosenberg-internet-waist-hourglass].

   Recognizing that communicating rich endpoint information is a
   network-oriented function relevant to in-network policy enforcement,
   Tng factors this function into its Endpoint Layer so that middleboxes
   can extract this information without having to understand
   application-oriented headers.  Tng reinterprets UDP as an initial
   Endpoint Layer protocol already supported by most middleboxes, but
   TCP can be used as well.  In the longer term, we envision the
   Endpoint Layer evolving in a backward-compatible fashion to
   incorporate ideas from NAT traversal mechanisms [ford05p2p] [RFC5389]
   [I-D.ietf-mmusic-ice] and delegation-friendly architectures
   [walfish04middleboxes] [guha07end], thereby incorporating these
   mechanisms as required into the protocol stack and eventually
   eliminating the need for applications to manage connectivity
   challenges like these.

   The Endpoint Layer's interface to higher layers is almost identical
   to the Network Layer's interface - it provides a simple best-effort
   packet delivery service - but with richer endpoint names than IP
   addresses alone provide.  These endpoint names will initially consist
   simply of (IP address, port number) pairs for compatibility with TCP
   and UDP, but may later evolve in a backward-compatible fashion to
   include extensions such as service/port names
   [I-D.touch-tcp-portnames].

3.3.  The Flow Regulation Layer

   As Tng's Endpoint Layer factors out endpoint identification, the Flow
   Regulation Layer similarly factors out performance related functions
   such as congestion control, with the recognition that these functions
   have likewise become "network-oriented" in practice.  The Flow Layer
   assumes that the underlying Endpoint Layer provides only best-effort
   packet delivery between application endpoints, and builds a flow-
   regulated best-effort delivery service for higher layers to build on.
   In particular, the Flow Layer's interface to higher layers includes
   an explicit signal indicating when the higher layer may transmit new
   packets.




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   To perform this flow regulation, the Flow Layer may either implement
   standard TCP-like congestion control, or, as we discuss in
   [iyengar09flow], may use more specific knowledge of an underlying
   network technology or administrative domain.  In order to support
   multipath transports effectively, the Flow Layer also needs to enable
   higher layers to indicate sets of flows or "bundles" representing a
   single logical communication activity, and adjust the Flow Layer's
   congestion control algorithms to perform resource pooling as
   discussed later in Section 4.  Tng's flow layer might eventually
   incorporate other performance-enhancing mechanisms as well, such as
   forward error correction.

3.4.  The Isolation Layer

   Having factored out network-oriented transport functions into the
   Endpoint and Flow Layers, the optional Isolation Layer "isolates" the
   application from the network, and protects the "end-to-endness" of
   higher layers.  This isolation includes two elements.  First, the
   Isolation Layer protects the application's end-to-end communication
   from interference or eavesdropping within the path, via transport-
   neutral cryptographic security as in IPsec.  Second, the Isolation
   Layer protects the application and end-to-end transport from
   unnecessary exposure to details of network topology and attachment
   points, by implementing location-independent endpoint identities as
   in HIP [RFC4423] or UIA [ford06persistent], which remain stable even
   as devices move or the network reconfigures.

   The Isolation Layer's interface to higher layers is functionally
   equivalent to the interface exported by the Flow Layer, but with
   transformed packet payloads and/or endpoint identities.  We believe
   the Isolation Layer represents a suitable location for end-to-end
   security precisely because it defines the boundary between network-
   oriented and application-oriented functions, thus ensuring integrity
   and security of the latter, while allowing middleboxes to interact
   with the former.  In contrast with SSL/TLS, the Isolation layer is
   neutral to transport semantics and does not need to be adapted to
   each transport.  In contrast with IPsec's standard location
   immediately above IP, the Isolation Layer does give up the ability to
   protect Endpoint and Flow Layer mechanisms from off-path DoS attacks
   as IPsec protects TCP's signaling mechanisms, but if standard non-
   cryptographic defenses against such attacks are deemed insufficient,
   then IPsec authentication can still be deployed in Tng underneath the
   flow layer, ideally via a delegation-friendly scheme permitting
   controlled interposition by middleboxes.







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3.5.  The Semantic Layer

   Tng's Semantic Layer implements the remaining application-oriented
   end-to-end transport functions, particularly end-to-end reliability.
   In the case of TCP, these functions are all those in the original TCP
   protocol [RFC0793] except port numbers, including acknowledgment and
   retransmission, order preservation, and receive window management.
   Other application-visible semantics, such as RDP's reliable datagrams
   and SCTP's message-based multistreaming [RFC4960], could fit equally
   well into Tng's Semantic Layer as distinct protocols.

   The Semantic Layer's interface to lower layers differs from that of
   traditional Internet transports in two ways.  First, a Tng semantic
   protocol uses the Endpoint Layer's endpoint identities (possibly
   transformed by the Isolation Layer) instead of implementing its own
   port number demultiplexing.  Second, a Tng semantic protocol
   implements no congestion control but relies on the underlying Flow
   Layer to signal when packets may be transmitted.  The Semantic
   Layer's interface to higher layers (e.g., the application) depends on
   the transport semantics it implements, but need not differ in any
   application-visible way from existing transport APIs--a fact that
   could aid deployment as discussed later in Section 5.


4.  Multipath Congestion Control

   Tng's decomposition of the Flow Layer from the Semantic Layer
   provides a natural decoupling for per-path congestion state in the
   Flow Layer from the per-transport-instance state in the Semantic
   Layer, allowing for a one-to-one, one-to-many, many-to-one, or many-
   to-many relationship between flows and transport instances in support
   of efficient multipath transport.  An important outstanding issue,
   however, is how and whether multipath operation should affect the
   congestion control algorithm implemented in the Flow Layer.  In
   particular, when a host is transmitting packets from one logical
   transport instance over multiple flows concurrently, how should the
   host adjust its congestion control behavior for each path?  We
   briefly summarize here a few alternative high-level approaches, while
   making no attempt to explore the details or identify the "right"
   approach:

   o  No Adjustment: Each flow independently runs a standard, unmodified
      TCP congestion control algorithm [RFC2581].  This approach is
      simple and inherits all the well-understood properties of TCP
      congestion control, but creates fairness concerns: if M flows
      happen to share a congestion bottleneck, such as a home broadband
      link, then the multipath aggregate competes at that bottleneck M
      times as aggressively as a single-path TCP connection.  The



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      effects of this approach are essentially the same - no better but
      also no worse - as the effects that BitTorrent clients cause as
      they utilize many parallel streams at application level.

   o  Static Weight Adjustments: Each flow independently runs standard
      TCP congestion control, but with each flow's aggressiveness
      lowered equally using known techniques [crowcroft98differentiated]
      so that the multipath aggregate as a whole exhibits the same
      aggressiveness as a single traditional TCP stream (or perhaps
      equivalent to two traditional TCP streams, if we take HTTP's
      allowance for two parallel streams [RFC2616] as a "de facto"
      fairness benchmark).  This approach addresses the above fairness
      issue, at the cost of potentially underutilizing network capacity
      when some paths are more congested than others.

   o  Dynamic Weight Adjustments: As above, but dynamically adjust the
      relative weights of each flow to take greater advantage of lightly
      loaded paths while maintaining aggregate fairness
      [honda09multipath].  Such an approach can provide both fairness
      and resource pooling [wischik08resource], but it is not clear that
      these adjustments will converge to stability as many multipath
      aggregates dynamically shifting their flows' weights in response
      to each others' effects on the network.

   o  New Response Functions: The basic congestion control response
      function can be modified in a multipath-sensitive fashion so that
      the flows comprising all multipath aggregates provably converge
      toward stable and fair use of the network [kelly05stability].  As
      with most new congestion control schemes that modify the basic
      congestion response function, however, it is unclear how such a
      new scheme will behave in competition with existing TCP flows in
      the existing Internet, especially in the presence of connections
      with varying round-trip times.

   Addressing the question of the right approach to adjusting congestion
   control for multipath operation will require further research and
   experimentation, which we consider to be independent of and
   orthogonal to Tng as an architecture.  We assume that we will develop
   multipath transport protocols in parallel with this continuing
   research in multipath congestion control, and that multipath
   transports will for the time being need to support multiple
   alternative multipath congestion control schemes, in the same way
   that mature single-path transport implementations such as Linux's
   already support a variety of selectable single-path schemes.

   To support such a "plug-and-play" selection of multipath congestion
   control schemes, Tng's Flow Layer will need one new architectural
   feature: a way for higher layers to indicate which flows should be



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   logically "tied together" and treated as a single logical aggregate
   for congestion control purposes.  For this purpose, the interface
   that Tng's Flow Layer provides to higher layers provides an operation
   allowing higher layers to tie multiple flows into a "flow bundle".
   If we consider this operation to be a function, e.g.,
   flow_bundle(flow1,flow2), and a Semantic Layer protocol opens three
   flows A, B, and C, to support a particular logical communication
   session, then the Semantic Layer protocol might call flow_bundle(A,B)
   followed by flow_bundle(A,C) to tie the three flows together into a
   bundle.  Exactly what effect this action would have depends of course
   on the specific congestion control algorithm in effect: with the "No
   Adjustment" approach above, for example, tying together flows will
   have no effect on congestion control behavior; with the "Static
   Weight Adjustments" approach, the flow_bundle() calls will cause the
   Flow Layer to recompute the weights of all the newly-bundled flows so
   that the bundle as a whole exhibits the same effective aggressiveness
   as that of one (or two) single-path TCP connections.  The Semantic
   Layer does not generally need to know exactly how these flow_bundle()
   calls are affecting congestion control, since it merely depends on
   the Flow Layer to indicate when each individual flow is ready to
   accept new packets for transmission.

   While the Tng Flow Layer's flow_bundle operation is primarily
   intended for use by a multipath-capable Semantic Layer, there is no
   fundamental reason this operation could not or should not be exported
   further upwards, all the way to the application.  Given a sockets API
   extension providing access to this operation, for example, an HTTP
   client could use it to indicate that the multiple concurrent HTTP
   streams it opens are logically part of the same application-level
   communication session, mitigating the fairness concerns that HTTP
   clients currently cause if they open more than one or two concurrent
   HTTP connections at once.  A flow_bundle operation could probably be
   considered "safe" to export to untrusted applications since bundling
   can only cause flows to become less aggressive, not more.
   Nevertheless, exporting a flow bundling operation to applications is
   outside of Tng's immediate scope and purposes, so we leave it for
   future exploration.


5.  Protocol Implementation and Incremental Deployment Paths

   Any new Internet transport, or even any refactoring of an existing
   Internet transport, faces major deployment hurdles due to the
   Internet's inertia, and Tng is no exception.  However, we find
   several reasons for optimism that an architecture incorporating the
   principles described here could overcome these deployment hurdles.
   This section therefore outlines potential approaches to overcoming
   these hurdles and facilitating deployment of next-generation



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   multipath-capable transports.

   There are many possible protocol stack designs that would be
   architecturally consistent with Tng and its goals; we make no attempt
   to specify such a protocol stack design here, but leave that to be
   decided and standardized by the Internet transport community.  Here
   we merely point out certain compelling possibilities that may be
   particularly worthy of further exploration, a few of which are
   illustrated in Figure 1 and discussed below:


                   +------+     +-------+     +-------+     +-------+
      Semantic     | TCP+ |     | SCTP+ |     | DCCP+ |     |  ???  |
                   +------+     +-------+     +-------+     +-------+
                        \------------\-----+------/-------------/
                                           |
                       /------------/------+------\-------------\
                   +-------+    +---------+    +-------+    +-------+
      Isolation    | DTLS  |    |  IPsec  |    |  TLS  |    |  ???  |
                   +-------+    +---------+    +-------+    +-------+
                       |             |             |            |
                   +---------------------+     +-------+    +-------+
      Flow         |        DCCP         |     |       |    |  ???  |
                   +---------------------+     |       |    +-------+
                              |                |  TCP  |        |
                   +---------------------+     |       |    +-------+
      Endpoint     |         UDP         |     |       |    |  ???  |
                   +---------------------+     +-------+    +-------+
                              |                    |            |
                   +------------------------------------------------+
      Network      |                       IP                       |
                   +------------------------------------------------+

                                 Figure 5

   o  Transport Reuse in the Semantic Layer: Any existing Internet
      transport may be adapted into a Tng Semantic Layer protocol merely
      by relocating it atop suitable Isolation (optional), Flow, and
      Endpoint Layers, and disabling the legacy transport's built-in
      congestion control, if any, in favor of using the Flow Layer's
      congestion control facilities.  Figure 1 above illustrates TCP and
      SCTP protocols modified in this way, which we refer to as "TCP+"
      and "SCTP+" respectively to emphasize that they are identical in
      application-visible semantics but not identical in implementation
      to the corresponding original protocols.  The "DCCP+" protocol
      illustrated in the figure provides congestion controlled datagram
      delivery like DCCP does, hence its name, but in fact needs to do
      little other than provide an application interface to the lower



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      layers, since the Flow Layer implements congestion control.  This
      "DCCP+" protocol is thus likely to be closest to DCCP in
      application-visible function, but closest to UDP in
      implementation.

   o  Application Transparency: A Semantic Layer protocol in Tng can
      offer applications an API similar or identical to that of any
      legacy Internet transport.  Our current Tng prototype already
      includes a Semantic Layer protocol providing a reliable stream
      abstraction compatible with TCP's, although it currently operates
      in user space and does not implement some TCP features such as
      urgent data.  With careful design, a system-level implementation
      of a Tng protocol stack could replace TCP or another legacy
      transport completely transparently to applications: a host
      initiating a connection would dynamically probe the remote host
      for Tng support, use the new protocol stack if possible
      transparently to the application, and fall back on standard TCP
      (again transparently to the application) otherwise.

   o  Transport Reuse in Lower Layers: A protocol stack fully conforming
      to the Tng architectural model could be composed entirely of
      existing protocols "top-to-bottom": e.g., TCP with congestion
      control disabled as the Semantic Layer as described above, either
      DTLS or IPsec as the Isolation Layer, DCCP as the Flow Layer, and
      UDP as the Endpoint Layer (see the left two columns of Figure 1.)
      This approach may not necessarily yield the most far-reaching
      benefits without allowing some modifications to the original
      protocols, and may incur overheads due to redundancies between
      layers.  Nevertheless, reuse could mitigate the difficulty of new
      protocol development and standardization.

   o  Compatibility with Existing Firewalls, NATs, and PEPs: While a
      DCCP-like protocol is most suited to Tng's Flow Layer, a Tng stack
      use standard TCP as a fallback "Flow Layer," atop which the Tng
      stack's true Isolation and Semantic Layer protocols would run as
      if a TCP "application (see the TLS/TCP column in Figure 1).  While
      TCP's overhead and ordering constraints may incur a performance
      cost, encapsulation in legacy TCP flows would make the new stack
      even more compatible with existing networks and capable of
      benefiting from existing TCP-based PEPs, and could still restore
      end-to-end fate-sharing by ensuring that the new Semantic Layer
      retains all end-to-end "hard state" and can restart failed Flow
      Layer TCP flows.

   o  Single-Ended Operation: If only one end host supports the new Tng
      protocol stack, maintaining compatibility with legacy TCP hosts
      constrains the other host to use legacy TCP on the wire as well,
      making it impossible for the on-the-wire format to be decomposed



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      according to the Tng layering model.  On the other hand, even in
      this case, the Tng layering model can still help guide and
      conceptually organize protocol stack implementations in the end
      hosts.  For example, the single-ended multipath TCP protocol
      currently under development [I-D.van-beijnum-1e-mp-tcp], while
      avoiding any changes to TCP's wire protocol for compatibility
      reasons, still requires a clean internal separation between per-
      path congestion control functions and TCP semantic functions such
      as reliability and ordering, as illustrated in Figure 2.
      Organizing next-generation transports according to this model,
      regardless of whether the wire protocol matches the model or not,
      may simplify the design and implementation of end hosts that wish
      to support both single-ended and double-ended multipath operation,
      for example.


6.  How Tng Complements Current Projects

   Two drafts are being considered in the IETF as multipath transport
   solutions: Single-Ended MPTCP [I-D.van-beijnum-1e-mp-tcp], which is a
   sender-only based multipath TCP, and Two-Ended MPTCP
   [I-D.ford-mptcp-multiaddressed], which requires receiver
   participation as well.  Having discussed how Tng applies to the One-
   Ended proposal in Section Section 5, we now focus on how Tng applies
   to the Two-Ended MPTCP proposal.

   Two-Ended MPTCP, functionally, divides the Transport Layer into two
   components: the MPTCP part, which is responsible for global ordering
   of application data and for reliability; and the "legacy TCP" part,
   which is essentially responsible for congestion control on each path.
   MPTCP suggests adding new paths by creating separate connections and
   adding them into the MPTCP pool and also acknowledges the possibility
   that new connections may be established from a sender to a multihomed
   receiver on different ports, since the MPTCP connection is ultimately
   identified by the MPTCP Sender Token.  For all practical purposes,
   the upper MPTCP component maps to our Semantic Layer with the lower
   TCP implementing our Flow and Endpoint Layers.  We juxtapose the Two-
   Ended MPTCP architecture from [I-D.ford-mptcp-multiaddressed] with
   Tng's architecture below:












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        +---------------------------+     +--------------------------+
        |       Application         |     |      Application         |
        +---------------------------+     +--------------------------+
        |         MPTCP             |     |        Semantic          |
        + - - - - -  +  - - - - - - +     +--------------------------+
        |    TCP     |     TCP      |     | Flow+Endp  |  Flow+Endp  |
        +---------------------------+     +--------------------------+
        |     IP     |     IP       |     |     IP     |     IP      |
        +---------------------------+     +--------------------------+

                                 Figure 6

   There are significant functional and structural similarities between
   the Tng architecture and MPTCP, and it is not by accident.  We argue
   that adding any new non-trivial functionality into an existing
   transport must come to terms with this separation of concerns
   necessary within the transport layer.

   While [I-D.ford-mptcp-multiaddressed] identifies the same components
   as we do for implementing a multipath transport protocol, Tng
   separates these components and places them in a clean framework in
   which other Semantic protocols (e.g., SCTP) or other Flow protocols
   (e.g., DCCP over UDP) can also fit and interoperate together.  Our
   work is complementary to the current MPTCP effort, since it describes
   the architecturally clean space within which MPTCP can exist, and
   describes clear directions in which this architecture can evolve.
   Tng defines an architectural blueprint in which MPTCP is one
   instantiation, providing a TCP-compatible byte streams at the
   Semantic Layer and legacy TCP as a backward-compatible Flow Layer.

   We now turn our attention to other multihoming- and multipath-capable
   transport proposals, showing in turn how their design, explicitly or
   implicitly, embodies Tng's architectural separation of concerns:

   o  SCTP [RFC4960] has sophisticated mechanisms for multihoming and
      for managing addition and deletion of endpoints within an end-to-
      end "association" [RFC5061].  The SCTP protocol design makes a
      clear distinction between the application-oriented ordering
      through streams and separate Stream Sequence Numbers (SSNs); this
      "upper" component maps to our Semantic Layer.  SCTP uses a
      separate Transmission Sequence Number (TSN) space for the flow-
      level function of congestion control, which maps to our Flow
      Layer.  SCTP does use flow-level TSNs for reliability; and some
      form of global per-flow sequencing information will be important
      to enable collusion among streams for loss recovery --- this
      information does not have to be explicit in packets, but can be
      signaling between the Semantic and Flow Layers as discussed in
      Section 4.



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   o  LS-SCTP [al04ls-sctp] and pTCP [hsieh02ptcp] are examples of
      multipath transports that require both sender- and receiver-side
      modifications to SCTP and TCP, respectively.  Both use a separate
      sequence space for global ordering while using a per-path sequence
      space for flow functions.  Similar to Two-Ended MPTCP, these
      protocols can be divided into upper and lower parts that map to
      our Semantic and Flow Layers.

   o  CMT [iyengar06concurrent] and mTCP [zhang04transport] are examples
      of multipath transports that require modifications only to the
      sender-side in SCTP and TCP, respectively.  Similar to One-Ended
      MPTCP, as discussed in Section Section 5, such protocols may still
      benefit from a cleaner conceptualization of functions within the
      transport suite.

   o  SST [ford07structured] is a new transport protocol that implements
      Tng's separation of concerns internally in addition to other new
      application-visible features.  SST uses a separate Stream Protocol
      with stream sequence numbers for application-oriented ordering and
      reliability functions, a separate Channel Protocol with transmit
      sequence numbers for security and for network-oriented congestion
      control, and a separate Endpoint Layer implemented by UDP.  Our
      first prototype of Tng builds on the SST implementation.

   It should also be possible to implement Tng by adapting other
   established protocols as well.  DCCP [RFC4340] or CM [RFC3124], could
   implement our Flow Layer; IPsec and/or HIP, modified to run atop the
   Flow Layer, could provide our Isolation Layer; and TCP, SCTP, or
   another existing transport, modified to disable its internal
   congestion control functions and instead use those of the underlying
   Flow Layer, could provide our Semantic Layer.  There would be costs
   to this approach: the existing protocols were not designed for Tng,
   and they each independently re-implement considerable functionality;
   MPTCP is an integrated design that reuses or shares synergistically
   between layers.  Thus, while incremental deployment is possible with
   Tng, one can build integrated designs for specific protocol
   combinations that are tighter and better optimized for bit-space.

   Finally, we note that Tng's Endpoint Layer, being responsible for
   interaction with NATs, provides an architecturally clean space for
   NAT traversal work, such as STUN [RFC5389] and TURN
   [I-D.ietf-behave-turn], that are in progress in the IETF BEHAVE
   working group.


7.  Experiences with Running Code

   We have a working prototype implementation of a protocol stack



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   conforming to Tng's layering model and providing the basic structural
   benefits described above.  The prototype and experiments with it are
   described in more detail elsewhere [iyengar09flow].  More to be
   written (perhaps).


8.  Security Considerations

   To be written.


9.  IANA Considerations

   To be written: endpoint ids, transport protocol numbers, ...


10.  Acknowledgments

   To be written.  Discussions and participants on the multipathtcp
   mailing list.


11.  References

11.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC1644]  Braden, B., "T/TCP -- TCP Extensions for Transactions
              Functional Specification", RFC 1644, July 1994.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018, October 1996.

   [RFC2140]  Touch, J., "TCP Control Block Interdependence", RFC 2140,
              April 1997.

   [RFC2581]  Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
              Control", RFC 2581, April 1999.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2979]  Freed, N., "Behavior of and Requirements for Internet
              Firewalls", RFC 2979, October 2000.




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   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [RFC3124]  Balakrishnan, H. and S. Seshan, "The Congestion Manager",
              RFC 3124, June 2001.

   [RFC3135]  Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
              Shelby, "Performance Enhancing Proxies Intended to
              Mitigate Link-Related Degradations", RFC 3135, June 2001.

   [RFC3649]  Floyd, S., "HighSpeed TCP for Large Congestion Windows",
              RFC 3649, December 2003.

   [RFC3947]  Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
              "Negotiation of NAT-Traversal in the IKE", RFC 3947,
              January 2005.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

   [RFC4423]  Moskowitz, R. and P. Nikander, "Host Identity Protocol
              (HIP) Architecture", RFC 4423, May 2006.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061,
              September 2007.

   [RFC5207]  Stiemerling, M., Quittek, J., and L. Eggert, "NAT and
              Firewall Traversal Issues of Host Identity Protocol (HIP)
              Communication", RFC 5207, April 2008.

   [RFC5238]  Phelan, T., "Datagram Transport Layer Security (DTLS) over
              the Datagram Congestion Control Protocol (DCCP)",
              RFC 5238, May 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.



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   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

11.2.  Informative References

   [I-D.ford-mptcp-multiaddressed]
              Ford, A., Raiciu, C., Handley, M., and S. Barre, "TCP
              Extensions for Multipath Operation with Multiple
              Addresses", draft-ford-mptcp-multiaddressed-00 (work in
              progress), May 2009.

   [I-D.ietf-behave-turn]
              Rosenberg, J., Mahy, R., and P. Matthews, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)",
              draft-ietf-behave-turn-16 (work in progress), July 2009.

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), October 2007.

   [I-D.rosenberg-internet-waist-hourglass]
              Rosenberg, J., "UDP and TCP as the New Waist of the
              Internet Hourglass",
              draft-rosenberg-internet-waist-hourglass-00 (work in
              progress), February 2008.

   [I-D.shalunov-ledbat-congestion]
              Shalunov, S., "Low Extra Delay Background Transport
              (LEDBAT)", draft-shalunov-ledbat-congestion-00 (work in
              progress), March 2009.

   [I-D.touch-tcp-portnames]
              Touch, J., "A TCP Option for Port Names",
              draft-touch-tcp-portnames-00 (work in progress),
              April 2006.

   [I-D.van-beijnum-1e-mp-tcp]
              Beijnum, I., "One-ended multipath TCP",
              draft-van-beijnum-1e-mp-tcp-00 (work in progress),
              May 2009.

   [al04ls-sctp]
              Al, A., Saadawi, T., and M. Lee, "LS-SCTP: A Bandwidth
              Aggregation Technique For Stream Control Transmission



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              Protocol", Computer Communications Vol. 27, No. 10,
              June 2004.

   [balakrishnan97comparison]
              Balakrishnan, H., Padmanabhan, V., Seshan, S., and R.
              Katz, "A Comparison of Mechanisms for Improving TCP
              Performance over Wireless Links", IEEE/ACM Transactions on
              Networking Vol. 5, No. 6, December 1997.

   [cheriton00triad]
              Cheriton, D. and M. Gritter, "TRIAD: A New Next-Generation
              {Internet} Architecture", Online at:
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Authors' Addresses

   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


   Bryan Ford
   Max Planck Institute for Software Systems
   Saarbrucken,
   Germany

   Email: baford@mpi-sws.org

























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