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Versions: 00 01

Network Working Group                                  M. Boucadair, Ed.
Internet-Draft                                         C. Jacquenet, Ed.
Intended status: Informational                                    Orange
Expires: April 30, 2017                              O. Bonaventure, Ed.
                                                                Tessares
                                                             D. Behaghel
                                                               OneAccess
                                                                S. Secci
                                                                    UPMC
                                                      W. Henderickx, Ed.
                                                    Nokia/Alcatel-Lucent
                                                            R. Skog, Ed.
                                                                Ericsson
                                                           S. Vinapamula
                                                                 Juniper
                                                                  S. Seo
                                                           Korea Telecom
                                                             W. Cloetens
                                                              SoftAtHome
                                                                U. Meyer
                                                                Vodafone
                                                           LM. Contreras
                                                              Telefonica
                                                              B. Peirens
                                                                Proximus
                                                        October 27, 2016


Network-Assisted MPTCP: Use Cases, Deployment Scenarios and Operational
                             Considerations
              draft-nam-mptcp-deployment-considerations-00

Abstract

   Network-Assisted MPTCP deployment models are designed to facilitate
   the adoption of MPTCP for the establishment of multi-path
   communications without making any assumption about the support of
   MPTCP by the communicating peers.  MPTCP Conversion Points (MCPs)
   located in the network are responsible for establishing multi-path
   communications on behalf of endpoints, thereby taking advantage of
   MPTCP capabilities to achieve different goals that include (but are
   not limited to) optimization of resource usage (e.g., bandwidth
   aggregation), of resiliency (e.g., primary/backup communication
   paths), and traffic offload management.

   This document describes Network-Assisted MPTCP uses cases, deployment
   scenarios, and operational considerations.




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Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 30, 2017.

Copyright Notice

   Copyright (c) 2016 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Network-Assisted MPTCP Use Cases  . . . . . . . . . . . . . .   5
   4.  Deployment Scenarios  . . . . . . . . . . . . . . . . . . . .   6
     4.1.  LTE/WLAN Aggregation  . . . . . . . . . . . . . . . . . .   6
     4.2.  Fixed/Wireless Access Aggregation . . . . . . . . . . . .   6
   5.  Deployment & Operational Considerations . . . . . . . . . . .   7
     5.1.  MCP Location  . . . . . . . . . . . . . . . . . . . . . .   7



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     5.2.  MCP Insertion in a Multipath Communication  . . . . . . .   7
       5.2.1.  Explicit Mode (Off-path)  . . . . . . . . . . . . . .   7
       5.2.2.  Implicit Mode (On-path) . . . . . . . . . . . . . . .   8
     5.3.  Authorization . . . . . . . . . . . . . . . . . . . . . .  10
     5.4.  MCP Behaviors . . . . . . . . . . . . . . . . . . . . . .  10
       5.4.1.  Transparent MCP . . . . . . . . . . . . . . . . . . .  11
       5.4.2.  Non-Transparent MCP . . . . . . . . . . . . . . . . .  12
     5.5.  Address Family Considerations . . . . . . . . . . . . . .  14
     5.6.  Policies & Configuration Parameters . . . . . . . . . . .  14
       5.6.1.  Traffic Distribution Scheme . . . . . . . . . . . . .  14
       5.6.2.  Flows Eligible to Multipath Service . . . . . . . . .  14
       5.6.3.  TCP Fragmentation . . . . . . . . . . . . . . . . . .  15
       5.6.4.  DSCP Preservation . . . . . . . . . . . . . . . . . .  15
       5.6.5.  Supported Transport Protocols . . . . . . . . . . . .  15
       5.6.6.  Logging . . . . . . . . . . . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
     7.1.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.2.  Denial-of-Service (DoS) . . . . . . . . . . . . . . . . .  16
     7.3.  Illegitimate MCP  . . . . . . . . . . . . . . . . . . . .  16
     7.4.  High Rate Reassembly  . . . . . . . . . . . . . . . . . .  16
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The overall quality of connectivity services can be enhanced by
   combining several access network links for various purposes -
   resource optimization, better resiliency, etc.  Some transport
   protocols, such as Multipath TCP, can help achieve such better
   quality, but failed to be massively deployed so far.

   The support of multipath transport capabilities by communicating
   hosts remains a privileged target design so that such hosts can
   directly use the available resources provided by a variety of access
   networks they can connect to.  Nevertheless, network operators do not
   control end hosts while the support of MPTCP by content servers
   remains close to zero.

   Network-Assisted MPTCP deployment models are designed to facilitate
   the adoption of MPTCP for the establishment of multi-path
   communications without making any assumption about the support of
   MPTCP capabilities by communicating peers.  Network-Assisted MPTCP
   deployment models rely upon MPTCP Conversion Points (MCPs) that act




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   on behalf of hosts so that they can take advantage of establishing
   communications over multiple paths.

   Such MCPs can be deployed in CPEs (Customer Premises Equipment), as
   well in the network side.  MCPs are responsible for establishing
   multi-path communications on behalf of endpoints.

   This document describes Network-Assisted MPTCP uses cases
   (Section 3), deployment scenarios (Section 4), and operational
   considerations (Section 5).

2.  Terminology

   The reader should be familiar with the terminology defined in
   [RFC6824].

   This document makes use of the following terms:

   o  Client: an endhost that initiates transport flows forwarded along
      a single path.  Such endhost is not assumed to support multipath
      transport capabilities.

   o  Server: an endhost that communicates with a client.  Such endhost
      is not assumed to support multipath transport capabilities.

   o  Multipath Client: a Client that supports multipath transport
      capabilities.

   o  Multipath Server: a Server that supports multipath transport
      capabilities.  Both the client and the server can be single-homed
      or multi-homed.  However, for the use cases discussed in this
      document, the number of interfaces on the endhosts is not
      relevant.

   o  Transport flow: a sequence of packets that belong to a
      unidirectional transport flow and which share at least one common
      characteristic (e.g., the same destination address).  TCP and SCTP
      flows are composed of packets that have the same source and
      destination addresses, the same protocol number and the same
      source and destination ports.

   o  Multipath Conversion Point (MCP): a function that terminates a
      transport flow and relays all data received over it over another
      transport flow.

      MCP is a function provided by the network operator that converts a
      multipath transport flow and relays it over a single path
      transport flow and vice versa.



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3.  Network-Assisted MPTCP Use Cases

   The first use case is a Multipath Client that interacts with a
   Server.  To benefit from the capabilities of its multipath transport
   protocol, the Multipath Client will interact with a Multipath
   Conversion Point (MCP) located in the network as illustrated in
   Figure 1.

        C <===========>MCP <------------> S
        +<============>+

   Legend:
        <===>: MPTCP leg

      Figure 1: A Multipath Client interacts with a Server through a
                        Multipath Conversion Point

   A similar approach consists of a Multipath Server that leverages its
   multipath capabilities when interacting with a Client as shown in
   Figure 2.

        C <---------> MCP <===========> S
                        +<=============>+

      Figure 2: A Client interacts with a Multipath Server through a
                        Multipath Conversion Point

   The third use case is when a Client interacts with a Server and the
   corresponding communication is deemed eligible to multi-path
   forwarding.  In this case, two Multipath Conversion Points are used.
   The upstream MCP converts the (single path) transport flow initiated
   by the Client into a multipath transport flow towards a downstream
   MCP (called, network-located MCP).  This downstream MCP converts the
   multipath transport flow received from the upstream MCP in a single
   path transport flow forwarded to the destination Server.  The end-to-
   end transport flow between the Client and the Server is thus
   decomposed into three flows as shown in Figure 3: A (single path)
   transport flow between the Client and the upstream MCP, a multipath
   transport flow between the upstream and the downstream MCPs, and a
   single path transport flow between the downstream MCP and the Server.

             Upstream          Downstream
        C <---> MCP <===========> MCP <------------> S
                  +<=============>+

     Figure 3: A Client interacts with a Server through two Multipath
                             Conversion Points




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4.  Deployment Scenarios

   This section discusses some deployment scenarios related to Network-
   Assisted MPTCP designs.

4.1.  LTE/WLAN Aggregation

   This deployment scenario is considered by mobile operators so that
   they can propose their customers to aggregate LTE resources with WLAN
   resources.

   As depicted in Figure 4, the mobile terminal (UE, User Equipment) is
   MPTCP-capable.  The MCP function is enabled in the network to
   terminate MPTCP connections (e.g., in the PDN Gateway, a dedicated
   service function located at the (S)Gi interface, co-located with a
   TCP proxy, etc.).

   This deployment scenario is an implementation of the use case
   depicted in Figure 1.

  +------------+        _--------_    +----------------+
  |            |       (    LTE   )   |                |
  |   UE       +=======+          +===+  Backbone      |
  | (MPTCP     |       (_        _)   |   Network      |
  |  Client)   |         (_______)    |+--------------+|
  |            |       IP Network #1  || Concentrator ||------> Internet
  |            |                      ||    (MCP)     ||
  |            |                      |+--------------+|
  |            |       IP Network #2  |                |
  |            |        _--------_    |                |
  |            |       (           )  |                |
  |            +=======+  WLAN     +==+                |
  |            |       (_        _)   |                |
  +------------+        (_______)     +----------------+

     Figure 4: Network-Assisted MPTCP LTE/WLAN Aggregation (Host-based
                                  model)

4.2.  Fixed/Wireless Access Aggregation

   One of the promising deployment scenarios for Multipath TCP is to
   enable a CPE that is connected to multiple access networks (e.g.,
   DSL, LTE, WLAN) to optimize the usage of such resources.  This
   deployment scenario, called Hybrid Access, relies upon MCPs located
   in both the CPE and the network (Figure 5).  The latter plays the
   role of an MPTCP concentrator.  Such concentrator terminates the
   MPTCP sessions established from CPEs, before redirecting traffic into
   legacy TCP sessions.



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   This deployment scenario is an implementation of the use case
   depicted in Figure 2.

  +------------+        _--------_    +----------------+
  |            |       (    LTE   )   |                |
  |   CPE      +=======+          +===+  Backbone      |
  |  (MCP)     |       (_        _)   |   Network      |
  |            |         (_______)    |+--------------+|
  |            |       IP Network #1  || Concentrator ||------> Internet
  |            |                      ||    (MCP)     ||
  |            |                      |+--------------+|
  |            |       IP Network #2  |                |
  |            |        _--------_    |                |
  |            |       (    DSL    )  |                |
  |            +=======+           +==+                |
  |            |       (_        _)   |                |
  +-----+------+        (_______)     +----------------+
        |
  ---- LAN ----
        |
    end-nodes

    Figure 5: Network-Assisted MPTCP Fixed/Wireless Access Aggregation

   For mobile operators that provide CPE-based mobile broadband
   services, LTE and WLAN resources can be aggregated by means of MPTCP.
   In such deployment scenario, the MCP function is enabled in the CPE
   and in the network.

5.  Deployment & Operational Considerations

5.1.  MCP Location

   The location of MCPs is deployment-specific.  Network Providers may
   choose to adopt centralized or distributed designs.  Nevertheless, in
   order to take advantage of MPTCP, the location of an MCP should not
   jeopardize packet forwarding performance overall.

5.2.  MCP Insertion in a Multipath Communication

   Two deployment scenarios can be considered for involving an MCP in
   the communication path.  These scenarios are described below.

5.2.1.  Explicit Mode (Off-path)

   This scenario assumes that the IP reachability information of an MCP
   is explicitly configured on a device, e.g., by means of a specific




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   DHCP option [I-D.boucadair-mptcp-dhc].  A device can be a CPE or a
   host.

   MPTCP connections are established explicitly using the address(es) of
   the MCP (Figure 6).  In order to forward packets to their ultimate
   destination, the MCP is provided during the connection establishment
   with the destination IP address (and optionally destination port
   numbers).  Typically, this is achieved thanks to the use of the
   MP_CONVERT option defined in [I-D.boucadair-mptcp-plain-mode].

   +---+                                     +-----+      +--+
   |H1 |                                     | MCP |      |RM|
   +---+                                     +-----+      +--+
   h1@h2@                                      mcp@        rm@
    | |                                         |           |
    | |src:Hi@                          dst:mcp@|    dst:rm@|
    | |<=================MPTCP Leg=============>|<---TCP -->|
    | |                                         |           |

   Legend:
     H1: A host serviced by an MCP.
     RM: A remote machine.

         Figure 6: Sample Connection Establishment (Explicit Mode)

   This scenario aims to avoid any adherence of the Network-Assisted
   MPTCP procedure and the underlying routing and forwarding policies.
   Furthermore, this scenario allows for more flexibility in terms of
   mounting MPTCP subflows as it does not require any specific order in
   the establishment of subflows among available interfaces.

   Because the MCP's reachability information is explicitly configured
   on the device, means to guarantee successful inbound MPTCP
   connections can be enabled in the device to instruct the MCP to
   maintain active bindings so that incoming packets can be successfully
   redirected towards the appropriate device.

5.2.2.  Implicit Mode (On-path)

   Unlike the explicit mode, the implicit mode assumes that the MCP is
   located on a default forwarding path (primary path).  As such, the
   first subflow must always be placed over that primary path so that
   the MCP can intercept MPTCP flows.  Once intercepted, the MCP
   advertises its reachability information by means of MPTCP signals
   (MP_JOIN or ADD_ADDR).






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   +----+                                     +-----+      +--+
   | H1 |                                     | MCP |      |RM|
   +----+                                     +-----+      +--+
   h1@h2@                                      mcp@        rm@
    | |                                         |           |
    |src:h1@                             dst:rm@|    dst:rm@|
    |<============Initial MPTCP subflow========>|<---TCP -->|
    | |                       ...               |           |
    | |src:h2@                          dst:mcp@|    dst:rm@|
    | |<=========Secondary MPTCP subflow=======>|<---TCP -->|
    | |                       ...               |           |


         Figure 7: Sample Connection Establishment (Implicit Mode)

   Subsequent subflows are then sent directly to the MCP (Figure 7).
   The handling of these subsequent subflows is identical to the one of
   the explicit mode; only the establishment of the initial subflow
   differs.  Concretely, in reference to Figure 8, once the upstream MCP
   intercepts an initial subflow, it adds itself to the MPTCP connection
   by sending ADD_ADDR on the primary subflow.  Then, MP_JOIN is sent to
   the IP address conveyed in ADD_ADDR to create the secondary subflow.

   +----+                                     +-----+      +--+
   | H1 |                                     | MCP |      |RM|
   +----+                                     +-----+      +--+
   h1@h2@                                      mcp@        rm@
    | |                                         |           |
    |<==============ADD_ADDR====================|           |
    | | _______________________________________ |           |
    | |/            Secondary subflow          \|           |
    | |================SYN+MP_JOIN=============>|           |
    | |<============SYN/ACK(MPJOIN)=============|           |
    | |============ACK(MP_JOIN)================>|           |
    | |                       ...               |           |

           Figure 8: Secondary Subflow Creation (Implicit Mode)

5.2.2.1.  Demux Native MPTCP Flows from Proxied MPTCP Flows

   If no MP_CONVERT option ([I-D.boucadair-mptcp-plain-mode]) is
   included in the initial SYN message, the MCP cannot distinguish
   "native" MPTCP connections from "proxied" ones.  The subsequent risk
   is that native MPTCP communications will be reverted to TCP
   connections.  Such risk should be mitigated by enabling the
   MP_CONVERT option to be included in the SYN message to create the
   initial subflow.




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5.3.  Authorization

   The Network Provider that manages the various network attachments
   (including the MCPs) may enforce authentication and authorization
   policies using appropriate mechanisms.  For example, a non-exhaustive
   list of methods to achieve authorization is provided hereafter:

   o  The network provider may enforce a policy based on the
      International Mobile Subscriber Identity (IMSI) to verify that a
      user is allowed to benefit from the aggregation service.  If that
      authorization fails, the Packet Data Protocol (PDP) context
      /bearer will not be mounted.  This method does not require any
      interaction with the MCP.

   o  The network provider may enforce a policy based upon Access
      Control Lists (ACLs), e.g., at a Broadband Network Gateway (BNG)
      to control the CPEs that are authorized to communicate with an
      MCP.  These ACLs may be installed as a result of RADIUS exchanges,
      for instance ([I-D.boucadair-mptcp-radius]).  This method does not
      require any interaction with the MCP.

   o  The MCP may implement an Ident interface [RFC1413] to retrieve an
      identifier that will be used to assess whether that client is
      entitled to make use of the aggregation service.  Ident exchanges
      will take place only when receiving the first subflow from a given
      source IP address.

   o  The device that embeds the MCP may also host a RADIUS client that
      will solicit an AAA server to check whether connections received
      from a given source IP address are authorized or not
      ([I-D.boucadair-mptcp-radius]).

   A first safeguard against the misuse of MCP resources by illegitimate
   users (e.g., users with access networks that are not managed by the
   same service provider that operates the MCP) is to reject MPTCP
   connections received on the Internet-facing interfaces.  Only MPTCP
   connections received on the customer-facing interfaces of an MCP will
   be accepted.

   Because only the CPE is entitled to establish MPTCP connections with
   an MCP, ACLs may be installed on the CPE to avoid that internal
   terminals issue MPTCP connections towards one of the MCPs.

5.4.  MCP Behaviors

   The MCP MUST be provided with instructions about the behavior to
   adopt with regards to the processing of source addresses.  The
   following sub-sections elaborate on various schemes.



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5.4.1.  Transparent MCP

   Transparent Network-Assisted MPTCP deployment is a deployment where
   the visible source address of a packet forwarded by an MCP to a
   remote machine located in the Internet is the IP address of the
   endhost, not an address that is provisioned to the MCP.  In order to
   intercept incoming traffic, specific IPv4/IPv6 routes are injected so
   that traffic is redirected towards the MCP.

   No dedicated IP address pool is required to the MCP for the Network-
   Assisted MPTCP service.

5.4.1.1.  IPv4 Address Preservation

   The MCP can be tweaked to behave in the IPv4 address preservation
   mode.  This is the IPv4 address assigned to the endhost (typically,
   within a mobile deployment context as discussed in Section 4.1) or a
   WAN address of the CPE for the wired case (Section 4.2).

5.4.1.2.  Source IPv6 Prefix Preservation at Network-located MCP

   Some IPv6 deployments may require the preservation of the source IPv6
   prefix (Figure 9).

   This model requires the MCP to support ALGs to accommodate
   applications with IPv6 address referrals.

   +--+      +-----+                                +---+        +--+
   |H1|      | MCP |                                |MCP|        |RM|
   +--+      +--+--+                                +---+        +--+
   IP@s     IP@1, IP@2                              IP@mcf        IP@d
    |           ||                                     |           |
    |src:IP@s   ||src:IP@1                   dst:IP@mcf|src:IP@1   |
    |---------->||------------------------------------>|---------->|
    |   Dst:IP@d||                                     |   dst:IP@d|
    |           ||                                     |           |
    |<--SYN/ACK-||<---------------SYN/ACK--------------|<-SYN/ACK--|
    |---ACK---->||----------------ACK----------------->|---ACK---->|
    |           ||                                     |           |

   Legend:
     mcf: MCP Customer-facing Interface

     Figure 9: Example of Source IPv6 Prefix Preservation at Network-
                       located MCP (Initial subflow)






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5.4.1.3.  Source IPv6 Address Preservation at Network-located MCP

   Some IPv6 deployments may require the preservation of the source IPv6
   address (Figure 10).

   This model avoids the need for the MCP to support ALGs to accommodate
   applications with IPv6 address referrals.

   +--+      +-----+                                +---+        +--+
   |H1|      | MCP |                                |MCP|        |RM|
   +--+      +--++-+                                +---+        +--+
   IP@s     IP@1, IP@2                              IP@mcf        IP@d
    |           ||                                     |           |
    |src:IP@s   ||src:IP@1                   dst:IP@mcf|src:IP@s   |
    |---------->||------------------------------------>|---------->|
    |   Dst:IP@d||                                     |   dst:IP@d|
    |           ||                                     |           |
    |<--SYN/ACK-||<---------------SYN/ACK--------------|<-SYN/ACK--|
    |---ACK---->||----------------ACK----------------->|---ACK---->|
    |           ||                                     |           |
    |           ||                                     |           |

    Figure 10: Example of Outgoing SYN with Source Address Preservation

5.4.2.  Non-Transparent MCP

   Unlike the transport mode, this section focuses on deployments where
   a dedicated IP address pool is provisioned to the MCP for the
   Network-Assisted MPTCP service.

5.4.2.1.  IPv4 Address Sharing at the at the Network-located MCP

   Because of global IPv4 address depletion, optimization of the IPv4
   address usage is mandatory.  This obviously includes the IPv4
   addresses that are assigned by the MCP at its Internet-facing
   interfaces (Figure 11 and Figure 12).  A pool of global IPv4
   addresses is provisioned to the MCP along with possible instructions
   about the address sharing ratio to apply (see Appendix B of
   [RFC6269]).  Adequate forwarding policies are enforced so that
   traffic destined to an address of such pool is intercepted by the
   appropriate MCP.










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   +--+                                            +---+        +--+
   |H1|                                            |MCP|        |RM|
   +--+                                            +---+        +--+
    ||                                                |           |
    ||src:IP@s        MP_CONVERT(D=0,IP@d)  dst:IP@mcf|src:IP@mif |
    ||-----------------------SYN--------------------->|---SYN---->|
    ||                                                |   dst:IP@d|
    ||                                                |           |
    ||<--------------------SYN/ACK--------------------|<-SYN/ACK--|
    ||-----------------------ACK--------------------->|---ACK---->|
    ||                                                |           |

   Legend:
     mcf: MCP Customer-facing Interface
     mif: MCP Internet-facing Interface

         Figure 11: Example of Outgoing SYN without Source Address
                      Preservation (Single-ended MCP)

   +--+      +-----+                                +---+        +--+
   |H1|      | MCP |                                |MCP|        |RM|
   +--+      +--++-+                                +---+        +--+
    |           ||                                     |           |
    |src:IP@s   ||src:IP@1 MP_CONVERT(D=0,IP@d)        |src:IP@mif |
    |---SYN---->||----------------SYN----------------->|---SYN---->|
    |   dst:IP@d||                           dst:IP@mcf|   dst:IP@d|
    |           ||                                     |           |
    |<--SYN/ACK-||<---------------SYN/ACK--------------|<-SYN/ACK--|
    |---ACK---->||----------------ACK----------------->|---ACK---->|
    |           ||                                     |           |


         Figure 12: Example of Outgoing SYN without Source Address
                      Preservation (Dual-ended MCPs)

5.4.2.2.  IPv4 Address 1:1 Translation at the MCP

   For networks that do not face global IPv4 address depletion yet, the
   MCP can be configured so that source IPv4 addresses of the CPE are
   replaced with other (public) IPv4 addresses.  A pool of global IPv4
   addresses is then provisioned to the MCP for this purpose.  Rewriting
   source IPv4 addresses may be used as a means to redirect incoming
   traffic towards the appropriate MCP.








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5.4.2.3.  IPv6 Prefix Sharing (NPTv6) at the Network-located MCP

   Rewriting the source IPv6 prefix ([RFC6296]) may be needed to
   redirect incoming traffic towards the appropriate MCP.  A pool of
   IPv6 prefixes is then provisioned to the MCP for this purpose.

5.5.  Address Family Considerations

   Subflows of a given MPTCP connection can be associated to the same
   address family or may be established with different address families.
   Also, the Network-Assisted MPTCP using MP_CONVERT option, regardless
   of the addressing scheme enforced by each CPE network attachment.  In
   particular, the plain transport mode indifferently accommodates the
   following combinations.

                  LAN Leg  MPTCP Legs TCP Leg towards RM
                  ------- ----------- ------------------
                    IPv4      IPv4           IPv4
                    IPv4      IPv6           IPv4
                    IPv4  IPv6 & IPv4        IPv4
                    IPv6      IPv6           IPv6
                    IPv6      IPv4           IPv6
                    IPv6  IPv6 & IPv4        IPv6

5.6.  Policies & Configuration Parameters

5.6.1.  Traffic Distribution Scheme

   The logic of traffic distribution over multiple paths is deployment-
   specific.  This document does not require nor preclude any particular
   traffic distribution scheme.  Nevertheless, MCPs MUST be configurable
   with a parameter to indicate which traffic distribution scheme to
   enable.  Indeed, policies can be enforced by an MCP instance operated
   by the Network Provider to manage both upstream and downstream
   traffic.  These policies may be subscriber-specific, connection-
   specific, system-wise, or else.

5.6.2.  Flows Eligible to Multipath Service

   The Multipath Client and MCPs may be provided with a set of
   classification policies to help electing flows for the MPTCP service.
   These policies may be provisioned either statically and dynamically
   (or a combination thereof).

   Also, multiple MCPs may serve a given end-user, as a function of the
   nature of the service or the traffic to be forwarded over MPTCP
   connections.  For example, an MCP may be used by a service provider
   to proceed with CPE-targeted maintenance operations, whereas another



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   MCP may be configured to service multi-path communications initiated
   by a set of end-users.

5.6.3.  TCP Fragmentation

   Methods to avoid TCP fragmentation, such as rewriting the TCP Maximum
   Segment Size (MSS) option, must be supported by MCPs.

5.6.4.  DSCP Preservation

   The MCP MAY be configured to preserve the same DSCP marking or
   enforce DSCP re-marking policies.  DSCP preservation MUST be enabled
   by default.

5.6.5.  Supported Transport Protocols

   The MCP supports TCP by design.  Additional transport protocols
   SHOULD be supported.  A configuration parameter MUST be supported by
   the MCP to indicate which transport protocols can be relayed into an
   MPTCP connection.

5.6.6.  Logging

   If the MCP is used in global IPv4 address sharing environments, the
   logging recommendations discussed in Section 4 of [RFC6888] need to
   be considered.  Security-related issues encountered in address
   sharing environments are documented in Section 13 of [RFC6269].  A
   configuration parameter should be supported to enable/disable the
   logging function.

6.  IANA Considerations

   This document does not request any action from IANA.

7.  Security Considerations

   MPTCP-related security threats are discussed in [RFC6181] and
   [RFC6824].  Additional considerations are discussed in the following
   sub-sections.

7.1.  Privacy

   The MCP may have access to privacy-related information (e.g., IMSI,
   link identifier, subscriber credentials, etc.).  The MCP MUST NOT
   leak such sensitive information outside a local domain.






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7.2.  Denial-of-Service (DoS)

   Means to protect the MCP against Denial-of-Service (DoS) attacks MUST
   be enabled.  Such means include the enforcement of ingress filtering
   policies at the network boundaries [RFC2827].

   In order to prevent the exhaustion of MCP's resources by establishing
   a large number of simultaneous subflows for each MPTCP connection,
   the MCP administrator SHOULD limit the number of allowed subflows per
   CPE for a given connection.  Means to protect against SYN flooding
   attacks MUST also be enabled ([RFC4987]).

   Attacks that originate outside of the domain can be prevented if
   ingress filtering policies are enforced.  Nevertheless, attacks from
   within the network between a host and an MCP instance are yet another
   actual threat.  Means to ensure that illegitimate nodes cannot
   connect to a network should be implemented.

7.3.  Illegitimate MCP

   Traffic theft is a risk if an illegitimate MCP is inserted in the
   path.  Indeed, inserting an illegitimate MCP in the forwarding path
   allows traffic intercept and can therefore provide access to
   sensitive data issued by or destined to a host.  To mitigate this
   threat, secure means to discover an MCP should be enabled.

7.4.  High Rate Reassembly

   The MCP may perform packet reassembly.  Some security-related issues
   are discussed in [RFC4963][RFC1858][RFC3128].

8.  Acknowledgements

   Many thanks to Chi Dung Phung, Mingui Zhang, Rao Shoaib, Yoshifumi
   Nishida, and Christoph Paasch for the comments.

   Thanks to Ian Farrer, Mikael Abrahamsson, Alan Ford, Dan Wing, and
   Sri Gundavelli for the fruitful discussions held during the IETF#95
   meeting.

   Special thanks to Pierrick Seite, Yannick Le Goff, Fred Klamm, and
   Xavier Grall for their valuable comments.

   Thanks also to Olaf Schleusing, Martin Gysi, Thomas Zasowski, Andreas
   Burkhard, Silka Simmen, Sandro Berger, Michael Melloul, Jean-Yves
   Flahaut, Adrien Desportes, Gregory Detal, Benjamin David, Arun
   Srinivasan, and Raghavendra Mallya for the discussion.




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9.  References

9.1.  Normative References

   [I-D.boucadair-mptcp-plain-mode]
              Boucadair, M., Jacquenet, C., Behaghel, D.,
              stefano.secci@lip6.fr, s., Henderickx, W., Skog, R.,
              Bonaventure, O., Vinapamula, S., Seo, S., Cloetens, W.,
              Meyer, U., and L. Contreras, "An MPTCP Option for Network-
              Assisted MPTCP Deployments: Plain Transport Mode", draft-
              boucadair-mptcp-plain-mode-08 (work in progress), July
              2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
              <http://www.rfc-editor.org/info/rfc6824>.

9.2.  Informative References

   [I-D.boucadair-mptcp-dhc]
              Boucadair, M., Jacquenet, C., and T. Reddy, "DHCP Options
              for Network-Assisted Multipath TCP (MPTCP)", draft-
              boucadair-mptcp-dhc-06 (work in progress), October 2016.

   [I-D.boucadair-mptcp-radius]
              Boucadair, M. and C. Jacquenet, "RADIUS Extensions for
              Network-Assisted Multipath TCP (MPTCP)", draft-boucadair-
              mptcp-radius-03 (work in progress), October 2016.

   [RFC1413]  St. Johns, M., "Identification Protocol", RFC 1413,
              DOI 10.17487/RFC1413, February 1993,
              <http://www.rfc-editor.org/info/rfc1413>.

   [RFC1858]  Ziemba, G., Reed, D., and P. Traina, "Security
              Considerations for IP Fragment Filtering", RFC 1858,
              DOI 10.17487/RFC1858, October 1995,
              <http://www.rfc-editor.org/info/rfc1858>.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <http://www.rfc-editor.org/info/rfc2827>.



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   [RFC3128]  Miller, I., "Protection Against a Variant of the Tiny
              Fragment Attack (RFC 1858)", RFC 3128,
              DOI 10.17487/RFC3128, June 2001,
              <http://www.rfc-editor.org/info/rfc3128>.

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963,
              DOI 10.17487/RFC4963, July 2007,
              <http://www.rfc-editor.org/info/rfc4963>.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
              <http://www.rfc-editor.org/info/rfc4987>.

   [RFC6181]  Bagnulo, M., "Threat Analysis for TCP Extensions for
              Multipath Operation with Multiple Addresses", RFC 6181,
              DOI 10.17487/RFC6181, March 2011,
              <http://www.rfc-editor.org/info/rfc6181>.

   [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
              P. Roberts, "Issues with IP Address Sharing", RFC 6269,
              DOI 10.17487/RFC6269, June 2011,
              <http://www.rfc-editor.org/info/rfc6269>.

   [RFC6296]  Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
              Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,
              <http://www.rfc-editor.org/info/rfc6296>.

   [RFC6888]  Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
              A., and H. Ashida, "Common Requirements for Carrier-Grade
              NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888,
              April 2013, <http://www.rfc-editor.org/info/rfc6888>.

Authors' Addresses

   Mohamed Boucadair (editor)
   Orange
   Rennes  35000
   France

   Email: mohamed.boucadair@orange.com










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   Christian Jacquenet (editor)
   Orange
   Rennes
   France

   Email: christian.jacquenet@orange.com


   Olivier Bonaventure (editor)
   Tessares
   Belgium

   Email: olivier.bonaventure@tessares.net


   Denis Behaghel
   OneAccess

   Email: Denis.Behaghel@oneaccess-net.com


   Stefano Secci
   Universite Pierre et Marie Curie
   Paris
   France

   Email: stefano.secci@lip6.fr


   Wim Henderickx (editor)
   Nokia/Alcatel-Lucent
   Belgium

   Email: wim.henderickx@alcatel-lucent.com


   Robert Skog (editor)
   Ericsson

   Email: robert.skog@ericsson.com











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   Suresh Vinapamula
   Juniper
   1137 Innovation Way
   Sunnyvale, CA  94089
   USA

   Email: Sureshk@juniper.net


   SungHoon Seo
   Korea Telecom
   Seoul
   Korea

   Email: sh.seo@kt.com


   Wouter Cloetens
   SoftAtHome
   Vaartdijk 3 701
   3018 Wijgmaal
   Belgium

   Email: wouter.cloetens@softathome.com


   Ullrich Meyer
   Vodafone
   Germany

   Email: ullrich.meyer@vodafone.com


   Luis M. Contreras
   Telefonica
   Spain

   Email: luismiguel.contrerasmurillo@telefonica.com


   Bart Peirens
   Proximus

   Email: bart.peirens@proximus.com







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