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Versions: (draft-bonaventure-mptcp-converters) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19

TCPM Working Group                                   O. Bonaventure, Ed.
Internet-Draft                                                  Tessares
Intended status: Experimental                          M. Boucadair, Ed.
Expires: August 16, 2020                                          Orange
                                                           S. Gundavelli
                                                                   Cisco
                                                                  S. Seo
                                                           Korea Telecom
                                                              B. Hesmans
                                                                Tessares
                                                       February 13, 2020


                       0-RTT TCP Convert Protocol
                     draft-ietf-tcpm-converters-16

Abstract

   This document specifies an application proxy, called Transport
   Converter, to assist the deployment of TCP extensions such as
   Multipath TCP.  A Transport Converter may provide conversion service
   for one or more TCP extensions.  The conversion service is provided
   by means of the TCP Convert Protocol (Convert).

   This protocol provides 0-RTT (Zero Round-Trip Time) conversion
   service since no extra delay is induced by the protocol compared to
   connections that are not proxied.  Also, the Convert Protocol does
   not require any encapsulation (no tunnels, whatsoever).

   This specification assumes an explicit model, where the Transport
   Converter is explicitly configured on hosts.

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 https://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 August 16, 2020.



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Copyright Notice

   Copyright (c) 2020 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
   (https://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
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   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
     1.1.  The Problem . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Network-Assisted Connections: The Rationale . . . . . . .   4
   2.  Differences with SOCKSv5  . . . . . . . . . . . . . . . . . .   6
   3.  Conventions and Definitions . . . . . . . . . . . . . . . . .   8
   4.  Architecture & Behaviors  . . . . . . . . . . . . . . . . . .   9
     4.1.  Functional Elements . . . . . . . . . . . . . . . . . . .   9
     4.2.  Theory of Operation . . . . . . . . . . . . . . . . . . .  11
     4.3.  Data Processing at the Transport Converter  . . . . . . .  14
     4.4.  Address Preservation vs. Address Sharing  . . . . . . . .  16
       4.4.1.  Address Preservation  . . . . . . . . . . . . . . . .  16
       4.4.2.  Address/Prefix Sharing  . . . . . . . . . . . . . . .  17
   5.  Sample Examples . . . . . . . . . . . . . . . . . . . . . . .  18
     5.1.  Outgoing Converter-Assisted Multipath TCP Connections . .  18
     5.2.  Incoming Converter-Assisted Multipath TCP Connection  . .  20
   6.  The Convert Protocol (Convert)  . . . . . . . . . . . . . . .  21
     6.1.  The Convert Fixed Header  . . . . . . . . . . . . . . . .  22
     6.2.  Convert TLVs  . . . . . . . . . . . . . . . . . . . . . .  22
       6.2.1.  Generic Convert TLV Format  . . . . . . . . . . . . .  22
       6.2.2.  Summary of Supported Convert TLVs . . . . . . . . . .  23
       6.2.3.  The Info TLV  . . . . . . . . . . . . . . . . . . . .  24
       6.2.4.  Supported TCP Extensions TLV  . . . . . . . . . . . .  24
       6.2.5.  Connect TLV . . . . . . . . . . . . . . . . . . . . .  25
       6.2.6.  Extended TCP Header TLV . . . . . . . . . . . . . . .  28
       6.2.7.  The Cookie TLV  . . . . . . . . . . . . . . . . . . .  28
       6.2.8.  Error TLV . . . . . . . . . . . . . . . . . . . . . .  29
   7.  Compatibility of Specific TCP Options with the Conversion
       Service . . . . . . . . . . . . . . . . . . . . . . . . . . .  32
     7.1.  Base TCP Options  . . . . . . . . . . . . . . . . . . . .  32
     7.2.  Window Scale (WS) . . . . . . . . . . . . . . . . . . . .  33
     7.3.  Selective Acknowledgments . . . . . . . . . . . . . . . .  33



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     7.4.  Timestamp . . . . . . . . . . . . . . . . . . . . . . . .  34
     7.5.  Multipath TCP . . . . . . . . . . . . . . . . . . . . . .  34
     7.6.  TCP Fast Open . . . . . . . . . . . . . . . . . . . . . .  34
     7.7.  TCP-AO  . . . . . . . . . . . . . . . . . . . . . . . . .  35
   8.  Interactions with Middleboxes . . . . . . . . . . . . . . . .  35
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  36
     9.1.  Privacy & Ingress Filtering . . . . . . . . . . . . . . .  36
     9.2.  Authorization . . . . . . . . . . . . . . . . . . . . . .  37
     9.3.  Denial of Service . . . . . . . . . . . . . . . . . . . .  38
     9.4.  Traffic Theft . . . . . . . . . . . . . . . . . . . . . .  38
     9.5.  Authentication Considerations . . . . . . . . . . . . . .  38
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  39
     10.1.  Convert Service Name . . . . . . . . . . . . . . . . . .  39
     10.2.  The Convert Protocol (Convert) Parameters  . . . . . . .  39
       10.2.1.  Convert Versions . . . . . . . . . . . . . . . . . .  40
       10.2.2.  Convert TLVs . . . . . . . . . . . . . . . . . . . .  40
       10.2.3.  Convert Error Messages . . . . . . . . . . . . . . .  41
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  42
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  42
     11.2.  Informative References . . . . . . . . . . . . . . . . .  44
   Appendix A.  Example Socket API Changes to Support the 0-RTT
                Convert Protocol . . . . . . . . . . . . . . . . . .  47
     A.1.  Active Open (Client Side) . . . . . . . . . . . . . . . .  47
     A.2.  Passive Open (Converter Side) . . . . . . . . . . . . . .  47
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  48
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  49
   Change Log  . . . . . . . . . . . . . . . . . . . . . . . . . . .  50
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  52

1.  Introduction

1.1.  The Problem

   Transport protocols like TCP evolve regularly [RFC7414].  TCP has
   been improved in different ways.  Some improvements such as changing
   the initial window size [RFC6928] or modifying the congestion control
   scheme can be applied independently on clients and servers.  Other
   improvements such as Selective Acknowledgments [RFC2018] or large
   windows [RFC7323] require a new TCP option or to change the semantics
   of some fields in the TCP header.  These modifications must be
   deployed on both clients and servers to be actually used on the
   Internet.  Experience with the latter TCP extensions reveals that
   their deployment can require many years.  Fukuda reports in
   [Fukuda2011] results of a decade of measurements showing the
   deployment of Selective Acknowledgments, Window Scale and TCP
   Timestamps.  [ANRW17] describes measurements showing that TCP Fast
   Open (TFO) [RFC7413] is still not widely deployed.




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   There are some situations where the transport stack used on clients
   (or servers) can be upgraded at a faster pace than the transport
   stack running on servers (or clients).  In those situations, clients
   would typically want to benefit from the features of an improved
   transport protocol even if the servers have not yet been upgraded and
   conversely.  Some assistance from the network to make use of these
   features is valuable.  For example, Performance Enhancing Proxies
   [RFC3135], and other service functions have been deployed as
   solutions to improve TCP performance over links with specific
   characteristics.

   Recent examples of TCP extensions include Multipath TCP (MPTCP)
   [RFC6824] or TCPINC [RFC8548].  Those extensions provide features
   that are interesting for clients such as wireless devices.  With
   Multipath TCP, those devices could seamlessly use WLAN (Wireless
   Local Area Network) and cellular networks, for bonding purposes,
   faster hand-overs, or better resiliency.  Unfortunately, deploying
   those extensions on both a wide range of clients and servers remains
   difficult.

   More recently, 5G bonding experimentation has been conducted into
   global range of the incumbent 4G (LTE) connectivity using newly
   devised clients and a Multipath TCP proxy.  Even if the 5G and the 4G
   bonding relying upon Multipath TCP increases the bandwidth, it is as
   well crucial to minimize latency for all the way between endhosts
   regardless of whether intermediate nodes are inside or outside of the
   mobile core.  In order to handle URLLC (Ultra Reliable Low Latency
   Communication) for the next generation mobile network, Multipath TCP
   and its proxy mechanism such as the one used to provide Access
   Traffic Steering, Switching, and Splitting (ATSSS) must be optimized
   to reduce latency [TS23501].

1.2.  Network-Assisted Connections: The Rationale

   This document specifies an application proxy, called Transport
   Converter.  A Transport Converter is a function that is installed by
   a network operator to aid the deployment of TCP extensions and to
   provide the benefits of such extensions to clients.  A Transport
   Converter may provide conversion service for one or more TCP
   extensions.  Which TCP extensions are eligible to the conversion
   service is deployment-specific.  The conversion service is provided
   by means of the 0-RTT TCP Convert Protocol (Convert), that is an
   application-layer protocol which uses a dedicated TCP port number.

   The Convert Protocol provides 0-RTT (Zero Round-Trip Time) conversion
   service since no extra delay is induced by the protocol compared to
   connections that are not proxied.  Particularly, the Convert Protocol
   does not require extra signaling setup delays before making use of



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   the conversion service.  The Convert Protocol does not require any
   encapsulation (no tunnels, whatsoever).

   The Transport Converter adheres to the main principles drawn in
   [RFC1919].  In particular, a Transport Converter achieves the
   following:

   o  Listen for client sessions;

   o  Receive from a client the address of the final target server;

   o  Setup a session to the final server;

   o  Relay control messages and data between the client and the server;

   o  Perform access controls according to local policies.

   The main advantage of network-assisted conversion services is that
   they enable new TCP extensions to be used on a subset of the path
   between endpoints, which encourages the deployment of these
   extensions.  Furthermore, the Transport Converter allows the client
   and the server to directly negotiate TCP extensions for the sake of
   native support along the full path.

   The Convert Protocol is a generic mechanism to provide 0-RTT
   conversion service.  As a sample applicability use case, this
   document specifies how the Convert Protocol applies for Multipath
   TCP.  It is out of scope of this document to provide a comprehensive
   list of all potential conversion services.  Applicability documents
   may be defined in the future.

   This document does not assume that all the traffic is eligible to the
   network-assisted conversion service.  Only a subset of the traffic
   will be forwarded to a Transport Converter according to a set of
   policies.  These policies, and how they are communicated to
   endpoints, are out of scope.  Furthermore, it is possible to bypass
   the Transport Converter to connect directly to the servers that
   already support the required TCP extension(s).

   This document assumes an explicit model in which a client is
   configured with one or a list of Transport Converters (statically or
   through protocols such as [I-D.boucadair-tcpm-dhc-converter]).
   Configuration means are outside the scope of this document.

   The use of a Transport Converter means that there is no end-to-end
   transport connection between the client and server.  This could
   potentially create problems in some scenarios such as those discussed
   in Section 4 of [RFC3135].  Some of these problems may not be



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   applicable, for example, a Transport Converter can inform a client by
   means of Network Failure (65) or Destination Unreachable (97) error
   messages (Section 6.2.8) that it encounters a failure problem; the
   client can react accordingly.  An endpoint, or its network
   administrator, can assess the benefit provided by the Transport
   Converter service versus the risk.  This is one reason why the
   Transport Converter functionality has to be explicitly requested by
   an endpoint.

   This document is organized as follows.  First, Section 2 provides a
   brief overview of the differences between the well-known SOCKS
   protocol and the 0-RTT Convert protocol.  Section 4 provides a brief
   explanation of the operation of Transport Converters.  Then,
   Section 6 describes the Convert Protocol.  Section 7 discusses how
   Transport Converters can be used to support different TCP extensions.
   Section 8 then discusses the interactions with middleboxes, while
   Section 9 focuses on the security considerations.  Appendix A
   describes how a TCP stack would need to support the protocol
   described in this document.

2.  Differences with SOCKSv5

   Several IETF protocols provide proxy services; the closest to the
   0-RTT Convert protocol being the SOCKSv5 protocol [RFC1928].  This
   protocol is already used to deploy Multipath TCP in some cellular
   networks (Section 2.2 of [RFC8041]).

   A SOCKS Client creates a connection to a SOCKS Proxy, exchanges
   authentication information, and indicates the IP address and port
   number of the target Server.  At this point, the SOCKS Proxy creates
   a connection towards the target Server and relays all data between
   the two proxied connections.  The operation of an implementation
   based on SOCKSv5 (without authentication) is illustrated in Figure 1.


















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   Client                SOCKS Proxy               Server
      |                       |                       |
      | --------------------> |                       |
      |         SYN           |                       |
      | <-------------------- |                       |
      |       SYN+ACK         |                       |
      | --------------------> |                       |
      |         ACK           |                       |
      |                       |                       |
      | --------------------> |                       |
      |Version=5, Auth Methods|                       |
      | <-------------------- |                       |
      |       Method          |                       |
      | --------------------> |                       |
      |Auth Request (unless "No auth" method negotiated)
      | <-------------------- |                       |
      |     Auth Response     |                       |
      | --------------------> |                       |
      | Connect Server:Port   | --------------------> |
      |                       |          SYN          |
      |                       | <-------------------- |
      |                       |        SYN+ACK        |
      | <-------------------- |                       |
      |      Succeeded        |                       |
      | --------------------> |                       |
      |       Data1           |                       |
      |                       | --------------------> |
      |                       |         Data1         |
      |                       | <-------------------- |
      |                       |         Data2         |
      | <-------------------- |                       |
      |          Data2        |                       |
                            ...

     Figure 1: Establishment of a TCP Connection through a SOCKS Proxy
                          Without Authentication

   When SOCKS is used, an "end-to-end" connection between a Client and a
   Server becomes a sequence of two TCP connections that are glued
   together on the SOCKS Proxy.  The SOCKS Client and Server exchange
   control information at the beginning of the bytestream on the Client-
   Proxy connection.  The SOCKS Proxy then creates the connection with
   the target Server and then glues the two connections together so that
   all bytes sent by the application (Client) to the SOCKS Proxy are
   relayed to the Server and vice versa.

   The Convert Protocol is also used on TCP proxies that relay data
   between an upstream and a downstream connection, but there are



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   important differences with SOCKSv5.  A first difference is that the
   0-RTT Convert protocol exchanges all the control information during
   the initial RTT.  This reduces the connection establishment delay
   compared to SOCKS which requires two or more round-trip-times before
   the establishment of the downstream connection towards the final
   destination.  In today's Internet, latency is a important metric and
   various protocols have ben tuned to reduce their latency
   [I-D.arkko-arch-low-latency].  A recently proposed extension to SOCKS
   leverages the TFO (TCP Fast Open) option
   [I-D.olteanu-intarea-socks-6] to reduce this delay.

   A second difference is that the Convert Protocol explicitly takes the
   TCP extensions into account.  By using the Convert Protocol, the
   Client can learn whether a given TCP extension is supported by the
   destination Server.  This enables the Client to bypass the Transport
   Converter when the Server supports the required TCP extension(s).
   Neither SOCKSv5 [RFC1928] nor the proposed SOCKSv6
   [I-D.olteanu-intarea-socks-6] provide such a feature.

   A third difference is that a Transport Converter will only confirm
   the establishment of the connection initiated by the Client provided
   that the downstream connection has already been accepted by the
   Server.  If the Server refuses the connection establishment attempt
   from the Transport Converter, then the upstream connection from the
   Client is rejected as well.  This feature is important for
   applications that check the availability of a Server or use the time
   to connect as a hint on the selection of a Server [RFC8305].

   A fourth difference is that the 0-RTT Convert protocol only allows
   the Client to specify the IP address/port number of the destination
   server and not a DNS name.  We evaluated an alternate design that
   included the DNS name of the remote peer instead of its IP address as
   in SOCKS [RFC1928].  However, that design was not adopted because it
   induces both an extra load and increased delays on the Transport
   Converter to handle and manage DNS resolution requests.

3.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.








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4.  Architecture & Behaviors

4.1.  Functional Elements

   The Convert Protocol considers three functional elements:

   o  Clients;

   o  Transport Converters;

   o  Servers.

   A Transport Converter is a network function that proxies all data
   exchanged over one upstream connection to one downstream connection
   and vice versa (Figure 2).  The Transport Converter, thus, maintains
   state that associates one upstream connection to a corresponding
   downstream connection.

   A connection can be initiated from both sides of the Transport
   Converter (Internet-facing interface, customer-facing interface).

                              |
                              :
                              |
                        +------------+
   Client <- upstream ->| Transport  |<- downstream -> Server
            connection  | Converter  |   connection
                        +------------+
                              |
    customer-facing interface : Internet-facing interface
                              |

     Figure 2: A Transport Converter Proxies Data between Pairs of TCP
                                Connections

   "Client" refers to a software instance embedded on a host that can
   reach a Transport Converter via its customer-facing interface.  The
   "Client" can initiate connections via a Transport Converter (referred
   to as outgoing connections).  Also, the "Client" can accept incoming
   connections via a Transport Converter (referred to as incoming
   connections).

   Transport Converters can be operated by network operators or third
   parties.  Nevertheless, this document focuses on the single
   administrative deployment case where the entity offering the
   connectivity service to a client is also the entity which owns and
   operates the Transport Converter.




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   A Transport Converter can be embedded in a standalone device or be
   activated as a service on a router.  How such function is enabled is
   deployment-specific.

   The architecture assumes that new software will be installed on the
   Client hosts to interact with one or more Transport Converters.
   Furthermore, the architecture allows for making use of new TCP
   extensions even if those are not supported by a given server.

   A Client is configured, through means that are outside the scope of
   this document, with the names and/or the addresses of one or more
   Transport Converters and the TCP extensions that they support.  The
   procedure for selecting a Transport Converter among a list of
   configured Transport Converters is outside the scope of this
   document.

   One of the benefits of this design is that different transport
   protocol extensions can be used on the upstream and the downstream
   connections.  This encourages the deployment of new TCP extensions
   until they are widely supported by servers, in particular.

   The architecture does not mandate anything on the Server side.

   Similar to SOCKS, the architecture does not interfere with end-to-end
   TLS connections [RFC8446] between the Client and the Server
   (Figure 3).  In other words, end-to-end TLS is supported in the
   presence of a Converter.

       Client             Transport                Server
          |               Converter                  |
          |                   |                      |
          /==========================================\
         |            End-to-end TLS                  |
          \==========================================/

       * TLS messages exchanged between the Client
         and the Server are not shown.

            Figure 3: End-to-end TLS via a Transport Converter

   It is out of scope of this document to elaborate on specific
   considerations related to the use of TLS in the Client-Converter
   connection leg to exchange Convert messages (in addition to the end-
   to-end TLS connection).







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4.2.  Theory of Operation

   At a high level, the objective of the Transport Converter is to allow
   the use a specific extension, e.g., Multipath TCP, on a subset of the
   path even if the peer does not support this extension.  This is
   illustrated in Figure 4 where the Client initiates a Multipath TCP
   connection with the Transport Converter (packets belonging to the
   Multipath TCP connection are shown with "===") while the Transport
   Converter uses a regular TCP connection with the Server.

       Client             Transport                Server
          |               Converter                  |
          |                   |                      |
          |==================>|--------------------->|
          |                   |                      |
          |<==================|<---------------------|
          |                   |                      |
         Multipath TCP packets   Regular TCP packets

       Figure 4: An Example of 0-RTT Network-Assisted Outgoing MPTCP
                                Connection

   The packets belonging to a connection established through a Transport
   Converter may follow a different path than the packets directly
   exchanged between the Client and the Server.  Deployments should
   minimize the possible additional delay by carefully selecting the
   location of the Transport Converter used to reach a given
   destination.

   When establishing a connection, the Client can, depending on local
   policies, either contact the Server directly (e.g., by sending a TCP
   SYN towards the Server) or create the connection via a Transport
   Converter.  In the latter case (that is, the conversion service is
   used), the Client initiates a connection towards the Transport
   Converter and indicates the IP address and port number of the Server
   within the connection establishment packet.  Doing so enables the
   Transport Converter to immediately initiate a connection towards that
   Server, without experiencing an extra delay.  The Transport Converter
   waits until the receipt of the confirmation that the Server agrees to
   establish the connection before confirming it to the Client.

   The Client places the destination address and port number of the
   Server in the payload of the SYN sent to the Transport Converter to
   minimize connection establishment delays.  The Transport Converter
   maintains two connections that are combined together:

   o  the upstream connection is the one between the Client and the
      Transport Converter.



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   o  the downstream connection is the one between the Transport
      Converter and the Server.

   Any user data received by the Transport Converter over the upstream
   (or downstream) connection is proxied over the downstream (or
   upstream) connection.  In particular, if the initial SYN message
   contains user data in its payload (e.g., [RFC7413]), that data MUST
   be placed right after the Convert TLVs when generating the SYN.

   Figure 5 illustrates the establishment of an outgoing TCP connection
   by a Client through a Transport Converter.

   o  Note: The information shown between brackets in Figure 5 (and
      other figures in the document) refers to Convert Protocol messages
      described in Section 6.

                           Transport
       Client              Converter              Server
          |                   |                      |
          |SYN [->Server:port]|         SYN          |
          |------------------>|--------------------->|
          |<------------------|<---------------------|
          |    SYN+ACK [ ]    |        SYN+ACK       |
          |        ...        |          ...         |

      Figure 5: Establishment of an Outgoing TCP Connection Through a
                            Transport Converter

   The Client sends a SYN destined to the Transport Converter.  The
   payload of this SYN contains the address and port number of the
   Server.  The Transport Converter does not reply immediately to this
   SYN.  It first tries to create a TCP connection towards the target
   Server.  If this upstream connection succeeds, the Transport
   Converter confirms the establishment of the connection to the Client
   by returning a SYN+ACK and the first bytes of the bytestream contain
   information about the TCP options that were negotiated with the
   Server.  Also, a state entry is instantiated for this connection.
   This state entry is used by the Converter to handle subsequent
   messages belonging to the connection.

   The connection can also be established from the Internet towards a
   Client via a Transport Converter (Figure 6).  This is typically the
   case when the Client hosts an application server that listens to a
   specific port number.  When the Converter receives an incoming SYN
   from a remote host, it checks if it can provide the conversion
   service for the destination IP address and destination port number of
   that SYN.  The Transport Converter receives this SYN because it is,
   for example, on the path between the remote host and the Client or it



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   provides address sharing service for the Client.  If the check fails,
   the packet is silently ignored by the Converter.  If the check is
   successful, the Converter tries to initiate a TCP connection towards
   the Client from its own address and using its configured TCP options.
   In the SYN that corresponds to this connection attempt, the Transport
   Convert inserts a TLV message that indicates the source address and
   port number of the remote host.  A transport session entry is created
   by the Converter for this connection.  SYN+ACK and ACK will be then
   exchanged between the Client, the Converter, and remote host to
   confirm the establishment of the connection.  The Converter uses the
   transport session entry to proxy packets belonging to the connection.

     Transport              Remote
       Client              Converter             Host (RH)
          |                   |                      |
          |SYN [<-RH IP@:port]|         SYN          |
          |<------------------|<---------------------|
          |------------------>|--------------------->|
          |    SYN+ACK [ ]    |        SYN+ACK       |
          |        ...        |          ...         |


      Figure 6: Establishment of an Incoming TCP Connection Through a
                            Transport Converter

   Standard TCP ([RFC0793], Section 3.4) allows a SYN packet to carry
   data inside its payload but forbids the receiver from delivering it
   to the application until completion of the three-way-handshake.  To
   enable applications to exchange data in a TCP handshake, this
   specification follows an approach similar to TCP Fast Open [RFC7413]
   and thus removes the constraint by allowing data in SYN packets to be
   delivered to the Transport Converter application.

   As discussed in [RFC7413], such change to TCP semantic raises two
   issues.  First, duplicate SYNs can cause problems for some
   applications that rely on TCP.  Second, TCP suffers from SYN flooding
   attacks [RFC4987].  TFO solves these two problems for applications
   that can tolerate replays by using the TCP Fast Open option that
   includes a cookie.  However, the utilization of this option consumes
   space in the limited TCP header.  Furthermore, there are situations,
   as noted in Section 7.3 of [RFC7413] where it is possible to accept
   the payload of SYN packets without creating additional security risks
   such as a network where addresses cannot be spoofed and the Transport
   Converter only serves a set of hosts that are identified by these
   addresses.

   For these reasons, this specification does not mandate the use of the
   TCP Fast Open option when the Client sends a connection establishment



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   packet towards a Transport Converter.  The Convert Protocol includes
   an optional Cookie TLV that provides similar protection as the TCP
   Fast Open option without consuming space in the TCP header.
   Furthermore, this design allows for the use of longer cookies than
   [RFC7413].

   If the downstream (or upstream) connection fails for some reason
   (excessive retransmissions, reception of an RST segment, etc.), then
   the Converter reacts by forcing the tear-down of the upstream (or
   downstream) connection.

   The same reasoning applies when the upstream connection ends with an
   exchange of FIN packets.  In this case, the Converter should also
   terminate the downstream connection by using FIN packets.  If the
   downstream connection terminates with the exchange of FIN packets,
   the Converter should initiate a graceful termination of the upstream
   connection.

4.3.  Data Processing at the Transport Converter

   As mentioned in Section 4.2, the Transport Converter acts as a TCP
   proxy between the upstream connection (i.e., between the Client and
   the Transport Converter) and the downstream connection (i.e., between
   the Transport Converter and the Server).

   The control messages, discussed in Section 6, establish state
   (called, transport session entry) in the Transport Converter that
   will enable it to proxy between the two TCP connections.

   The Transport Converter uses the transport session entry to proxy
   packets belonging to the connection.  An implementation example of a
   transport session entry for TCP connections is shown in Figure 7.

                      (C,c) <--> (T,t), (S,s), Lifetime

      Where:
        * C and c are the source IP address and source port number
          used by the Client for the upstream connection.
        * S and s are the Server's IP address and port number.
        * T and t are the source IP address and source port number
          used by the Transport Converter to proxy the connection.
        * Lifetime is the validity lifetime of the entry as assigned
          by the Converter.

           Figure 7: An Example of Transport Session Entry (TCP)

   Clients send packets bound to connections eligible to the conversion
   service to the provisioned Transport Converter and destination port



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   number.  This applies for both control messages and data.  Additional
   information is supplied by Clients to the Transport Converter by
   means of Convert messages as detailed in Section 6.  User data can be
   included in SYN or non-SYN messages.  User data is unambiguously
   distinguished from Convert TLVs by a Transport Converter owing to the
   Convert Fixed Header in the Convert messages (Section 6.1).  These
   Convert TLVs are destined to the Transport Convert and are, thus,
   removed by the Transport Converter when proxying between the two
   connections.

   Upon receipt of a packet that belongs to an existing connection
   between a Client and the Transport Converter the Converter proxies
   the user data to the Server using the information stored in the
   corresponding transport session entry.  For example, in reference to
   Figure 7, the Transport Converter proxies the data received from (C,
   c) downstream using (T,t) as source transport address and (S,s) as
   destination transport address.

   A similar process happens for data sent from the Server.  The
   Converter acts as a TCP proxy and sends the data to the Client
   relying upon the information stored in a transport session entry.
   The Converter associates a lifetime with state entries used to bind
   an upstream connection with its downstream connection.

   When Multipath TCP is used between the Client and the Transport
   Converter, the Converter maintains more state (e.g. information about
   the subflows) for each Multipath TCP connection.  The procedure
   described above continues to apply except that the Converter needs to
   manage the establishment/termination of subflows and schedule packets
   among the established ones.  These operations are part of the
   Multipath TCP implementation.  They are independent of the Convert
   protocol that only processes the Convert messages in the beginning of
   the bytestream.

   A Transport Converter may operate in address preservation mode (that
   is, the Converter does not rewrite the source IP address (i.e.,
   C==T)) or address sharing mode (that is, an address pool is shared
   among all Clients serviced by the Converter (i.e., C!=T)); refer to
   Section 4.4 for more details.  Which behavior to use by a Transport
   Converter is deployment-specific.  If address sharing mode is
   enabled, the Transport Converter MUST adhere to REQ-2 of [RFC6888]
   which implies a default "IP address pooling" behavior of "Paired" (as
   defined in Section 4.1 of [RFC4787]) MUST be supported.  This
   behavior is meant to avoid breaking applications that depend on the
   source address remaining constant.






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4.4.  Address Preservation vs. Address Sharing

   The Transport Converter is provided with instructions about the
   behavior to adopt with regards to the processing of source addresses
   of outgoing packets.  The following sub-sections discusses two
   deployment models for illustration purposes.  It is out of the scope
   of this document to make a recommendation.

4.4.1.  Address Preservation

   In this model, the visible source IP address of a packet proxied by a
   Transport Converter to a Server is an IP address of the end host
   (Client).  No dedicated IP address pool is provisioned to the
   Transport Converter, but the the Transport Converter is located on
   the path between the Client and the Server.

   For Multipath TCP, the Transport Converter preserves the source IP
   address used by the Client when establishing the initial subflow.
   Data conveyed in secondary subflows will be proxied by the Transport
   Converter using the source IP address of the initial subflow.  An
   example of a proxied Multipath TCP connection with address
   preservation is shown in Figure 8.

                                          Transport
           Client                        Converter          Server

            @:C1,C2                        @:Tc                @:S
               ||                            |                  |
               |src:C1     SYN         dst:Tc|src:C1       dst:S|
               |-------MPC [->S:port]------->|-------SYN------->|
               ||                            |                  |
               ||dst:C1                src:Tc|dst:C1       src:S|
               |<---------SYN/ACK------------|<-----SYN/ACK-----|
               ||                            |                  |
               |src:C1                 dst:Tc|src:C1       dst:S|
               |------------ACK------------->|-------ACK------->|
               |                             |                  |
               |src:C2          ...    dst:Tc|       ...        |
               ||<-----Secondary Subflow---->|src:C1       dst:S|
               ||                            |-------data------>|
               |               ..            |    ...           |

 Legend:
   Tc: IP address used by the Transport Converter on its customer-facing
       interface.

                 Figure 8: Example of Address Preservation




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   The Transport Converter must be on the forwarding path of incoming
   traffic.  Because the same (destination) IP address is used for both
   proxied and non-proxied connections, the Transport Converter should
   not drop incoming packets it intercepts if no matching entry is found
   for the packets.  Unless explicitly configured otherwise, such
   packets are forwarded according to the instructions of a local
   forwarding table.

4.4.2.  Address/Prefix Sharing

   A pool of global IPv4 addresses is provisioned to the Transport
   Converter along with possible instructions about the address sharing
   ratio to apply (see Appendix B of [RFC6269]).  An address is thus
   shared among multiple clients.

   Likewise, rewriting the source IPv6 prefix [RFC6296] may be used to
   ease redirection of incoming IPv6 traffic towards the appropriate
   Transport Converter.  A pool of IPv6 prefixes is then provisioned to
   the Transport Converter for this purpose.

   Adequate forwarding policies are enforced so that traffic destined to
   an address of such pool is intercepted by the appropriate Transport
   Converter.  Unlike Section 4.4.1, the Transport Converter drops
   incoming packets which do not match an active transport session
   entry.

   An example is shown in Figure 9.
























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                                        Transport
         Client                         Converter          Server

            @:C                        @:Tc|Te                @:S
             |                             |                  |
             |src:C                  dst:Tc|src:Te       dst:S|
             |-------SYN [->S:port]------->|-------SYN------->|
             |                             |                  |
             |dst:C                  src:Tc|dst:Te       src:S|
             |<---------SYN/ACK------------|<-----SYN/ACK-----|
             |                             |                  |
             |src:C                  dst:Tc|src:Te       dst:S|
             |------------ACK------------->|-------ACK------->|
             |                             |                  |
             |              ...            |    ...           |

Legend:
  Tc: IP address used by the Transport Converter for its customer-facing
      interface.
  Te: IP address used by the Transport Converter for its Internet-facing
      interface.

                         Figure 9: Address Sharing

5.  Sample Examples

5.1.  Outgoing Converter-Assisted Multipath TCP Connections

   As an example, let us consider how the Convert Protocol can help the
   deployment of Multipath TCP.  We assume that both the Client and the
   Transport Converter support Multipath TCP, but consider two different
   cases depending on whether the Server supports Multipath TCP or not.

   As a reminder, a Multipath TCP connection is created by placing the
   MP_CAPABLE (MPC) option in the SYN sent by the Client.

   Figure 10 describes the operation of the Transport Converter if the
   Server does not support Multipath TCP.













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                           Transport
       Client              Converter              Server
          |SYN, MPC           |                      |
          |[->Server:port]    |         SYN, MPC     |
          |------------------>|--------------------->|
          |<------------------|<---------------------|
          |  SYN+ACK,MPC [.]  |      SYN+ACK         |
          |------------------>|--------------------->|
          |     ACK, MPC      |          ACK         |
          |        ...        |          ...         |

     Figure 10: Establishment of a Multipath TCP Connection through a
   Transport Converter towards a Server that does not support Multipath
                                    TCP

   The Client tries to initiate a Multipath TCP connection by sending a
   SYN with the MP_CAPABLE option (MPC in Figure 10).  The SYN includes
   the address and port number of the target Server, that are extracted
   and used by the Transport Converter to initiate a Multipath TCP
   connection towards this Server.  Since the Server does not support
   Multipath TCP, it replies with a SYN+ACK that does not contain the
   MP_CAPABLE option.  The Transport Converter notes that the connection
   with the Server does not support Multipath TCP and returns the
   extended TCP header received from the Server to the Client.

   Note that, if the TCP connection is reset for some reason, the
   Converter tears down the Multipath TCP connection by transmitting a
   MP_FASTCLOSE.  Likewise, if the Multipath TCP connection ends with
   the transmission of DATA_FINs, the Converter terminates the TCP
   connection by using FIN segments.  As a side note, given that with
   Multipath TCP, RST only has the scope of the subflow and will only
   close the concerned subflow but not affect the remaining subflows,
   the Converter does not terminate the downstream TCP connection upon
   receipt of an RST over a Multipath subflow.

   Figure 11 considers a Server that supports Multipath TCP.  In this
   case, it replies to the SYN sent by the Transport Converter with the
   MP_CAPABLE option.  Upon reception of this SYN+ACK, the Transport
   Converter confirms the establishment of the connection to the Client
   and indicates to the Client that the Server supports Multipath TCP.
   With this information, the Client has discovered that the Server
   supports Multipath TCP.  This will enable the Client to bypass the
   Transport Converter for the subsequent Multipath TCP connections that
   it will initiate towards this Server.







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                           Transport
       Client              Converter              Server
          |SYN, MPC           |                      |
          |[->Server:port]    |         SYN, MPC     |
          |------------------>|--------------------->|
          |<------------------|<---------------------|
          |SYN+ACK, MPC       |      SYN+ACK, MPC    |
          |[MPC supported]    |                      |
          |------------------>|--------------------->|
          |     ACK, MPC      |        ACK, MPC      |
          |        ...        |          ...         |


     Figure 11: Establishment of a Multipath TCP Connection through a
                 Converter towards an MPTCP-capable Server

5.2.  Incoming Converter-Assisted Multipath TCP Connection

   An example of an incoming Converter-assisted Multipath TCP connection
   is depicted in Figure 12.  In order to support incoming connections
   from remote hosts, the Client may use PCP [RFC6887] to instruct the
   Transport Converter to create dynamic mappings.  Those mappings will
   be used by the Transport Converter to intercept an incoming TCP
   connection destined to the Client and convert it into a Multipath TCP
   connection.

   Typically, the Client sends a PCP request to the Converter asking to
   create an explicit TCP mapping for (internal IP address, internal
   port number).  The Converter accepts the request by creating a TCP
   mapping (internal IP address, internal port number, external IP
   address, external port number).  The external IP address and external
   port number will be then advertised using an out-of-band mechanism so
   that remote hosts can initiate TCP connections to the Client via the
   Converter.  Note that the external and internal information may be
   the same.

   Then, when the Converter receives an incoming SYN, it checks its
   mapping table to verify if there is an active mapping matching the
   destination IP address and destination port of that SYN.  If no entry
   is found, the Converter silently ignores the message.  If an entry is
   found, the Converter inserts an MP_CAPABLE option and Connect TLV in
   the SYN packet, rewrites the source IP address to one of its IP
   addresses and, eventually, the destination IP address and port number
   in accordance with the information stored in the mapping.  SYN+ACK
   and ACK will be then exchanged between the Client and the Converter
   to confirm the establishment of the initial subflow.  The Client can
   add new subflows following normal Multipath TCP procedures.




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                           Transport             Remote
       Client              Converter              Host
         |                     |                    |
         |<--------------------|<-------------------|
         |SYN, MPC             |         SYN        |
         |[Remote Host:port]   |                    |
         |-------------------->|------------------->|
         |      SYN+ACK, MPC   |       SYN+ACK      |
         |<--------------------|<-------------------|
         |       ACK, MPC      |           ACK      |
         |        ...          |          ...       |

     Figure 12: Establishment of an Incoming Multipath TCP Connection
                       through a Transport Converter

   It is out of scope of this document to define specific Convert TLVs
   to manage incoming connections.  These TLVs can be defined in a
   separate document.

6.  The Convert Protocol (Convert)

   This section defines the Convert Protocol (Convert, for short)
   messages that are exchanged between a Client and a Transport
   Converter.

   The Transport Converter listens on a dedicated TCP port number for
   Convert messages from Clients.  That port number is configured by an
   administrator.

   Convert messages MUST be included as the first bytes of the
   bytestream.  All Convert messages starts with a 32 bits long fixed
   header (Section 6.1) followed by one or more Convert TLVs (Type,
   Length, Value) (Section 6.2).

   o  Implementation note 1: Several implementers expressed concerns
      about the use of TFO.  As a reminder, the TFO Cookie protects from
      some attack scenarios that affect open servers like web servers.
      The Convert Protocol is different and, as discussed in RFC7413,
      there are different ways to protect from such attacks.  Instead of
      using a TFO cookie inside the TCP options, which consumes precious
      space in the extended TCP header, the Convert Protocol supports
      the utilization of a Cookie that is placed in the SYN payload.
      This provides the same level of protection as a TFO Cookie in
      environments were such protection is required.

   o  Implementation note 2: Error messages are not included in RST but
      sent in the bytestream.  Implementers have indicated that




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      processing RST on clients was difficult on some platforms.  This
      design simplifies client implementations.

6.1.  The Convert Fixed Header

   The Convert Protocol uses a 32 bits long fixed header that is sent by
   both the Client and the Transport Converter over each established
   connection.  This header indicates both the version of the protocol
   used and the length of the Convert message.

   The Client and the Transport Converter MUST send the fixed-sized
   header, shown in Figure 13, as the first four bytes of the
   bytestream.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+-------------------------------+
   |  Version      |  Total Length |          Unassigned           |
   +---------------+---------------+-------------------------------+

                    Figure 13: The Convert Fixed Header

   The Version is encoded as an 8 bits unsigned integer value.  This
   document specifies version 1.  Version 0 is reserved by this document
   and MUST NOT be used.

   The Total Length is the number of 32 bits word, including the header,
   of the bytestream that are consumed by the Convert messages.  Since
   Total Length is also an 8 bits unsigned integer, those messages
   cannot consume more than 1020 bytes of data.  This limits the number
   of bytes that a Transport Converter needs to process.  A Total Length
   of zero is invalid and the connection MUST be reset upon reception of
   a header with such total length.

   The Unassigned field MUST be set to zero in this version of the
   protocol.  These bits are available for future use.

   The Total Length field unambiguously marks the number of 32 bits
   words that carry Convert TLVs in the beginning of the bytestream.

6.2.  Convert TLVs

6.2.1.  Generic Convert TLV Format

   The Convert Protocol uses variable length messages that are encoded
   using the generic TLV format depicted in Figure 14.





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   The length of all TLVs used by the Convert Protocol is always a
   multiple of four bytes.  All TLVs are aligned on 32 bits boundaries.
   All TLV fields are encoded using the network byte order.

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +---------------+---------------+-------------------------------+
      |     Type      |     Length    |             Value  ...        |
      +---------------+---------------+-------------------------------+
      //              ...   (optional) Value                         //
      +---------------------------------------------------------------+

                   Figure 14: Convert Generic TLV Format

   The Length field covers Type, Length, and Value fields.  It is
   expressed in units of 32 bits words.  If necessary, Value MUST be
   padded with zeroes so that the length of the TLV is a multiple of 32
   bits.

   A given TLV MUST only appear once on a connection.  If a Client
   receives two or more instances of the same TLV over a Convert
   connection, it MUST reset the associated TCP connection.  If a
   Converter receives two or more instances of the same TLV over a
   Convert connection, it MUST return a Malformed Message Error TLV and
   close the associated TCP connection.

6.2.2.  Summary of Supported Convert TLVs

   This document specifies the following Convert TLVs:

   +------+-----+----------+------------------------------------------+
   | Type | Hex |  Length  | Description                              |
   +------+-----+----------+------------------------------------------+
   |   1  | 0x1 |    1     | Info TLV                                 |
   |  10  | 0xA | Variable | Connect TLV                              |
   |  20  | 0x14| Variable | Extended TCP Header TLV                  |
   |  21  | 0x15| Variable | Supported TCP Extensions TLV             |
   |  22  | 0x16| Variable | Cookie TLV                               |
   |  30  | 0x1E| Variable | Error TLV                                |
   +------+-----+----------+------------------------------------------+

             Figure 15: The TLVs used by the Convert Protocol

   Type 0x0 is a reserved value.  If a Client receives a TLV of type
   0x0, it MUST reset the associated TCP connection.  If a Converter
   receives a TLV of type 0x0, it MUST return an Unsupported Message
   Error TLV and close the associated TCP connection.




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   Implementations MUST reset the connection upon reception of messages
   with such TLV.

   The Client typically sends in the first connection it established
   with a Transport Converter the Info TLV (Section 6.2.3) to learn its
   capabilities.  Assuming the Client is authorized to invoke the
   Transport Converter, the latter replies with the Supported TCP
   Extensions TLV (Section 6.2.4).

   The Client can request the establishment of connections to servers by
   using the Connect TLV (Section 6.2.5).  If the connection can be
   established with the final server, the Transport Converter replies
   with the Extended TCP Header TLV (Section 6.2.6).  If not, the
   Transport Converter returns an Error TLV (Section 6.2.8) and then
   closes the connection.  The Transport Converter MUST NOT send a RST
   immediately after the detection of an error to let the Error TLV
   reach the Client.  As explained later, the Client will anyway send a
   RST upon reception of the Error TLV.

   When an error is encountered an Error TLV with the appropriate error
   code MUST be returned by the Transport Converter.

6.2.3.  The Info TLV

   The Info TLV (Figure 16) is an optional TLV which can be sent by a
   Client to request the TCP extensions that are supported by a
   Transport Converter.  It is typically sent on the first connection
   that a Client establishes with a Transport Converter to learn its
   capabilities.  Assuming a Client is entitled to invoke the Transport
   Converter, the latter replies with the Supported TCP Extensions TLV
   described in Section 6.2.4.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+-------------------------------+
   |     Type=0x1  |     Length    |             Zero              |
   +---------------+---------------+-------------------------------+


                          Figure 16: The Info TLV

6.2.4.  Supported TCP Extensions TLV

   The Supported TCP Extensions TLV (Figure 17) is used by a Transport
   Converter to announce the TCP options for which it provides a
   conversion service.  A Transport Converter SHOULD include in this
   list the TCP options that it supports in outgoing SYNs.




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   Each supported TCP option is encoded with its TCP option Kind listed
   in the "TCP Parameters" registry maintained by IANA.

                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +---------------+---------------+-------------------------------+
    |     Type=0x15 |     Length    |           Unassigned          |
    +---------------+---------------+-------------------------------+
    |     Kind #1   |     Kind #2   |           ...                 |
    +---------------+---------------+-------------------------------+
    /                              ...                              /
    /                                                               /
    +---------------------------------------------------------------+

                Figure 17: The Supported TCP Extensions TLV

   TCP option Kinds 0, 1, and 2 defined in [RFC0793] are supported by
   all TCP implementations and thus MUST NOT appear in this list.

   The list of Supported TCP Extensions is padded with 0 to end on a 32
   bits boundary.

   For example, if the Transport Converter supports Multipath TCP,
   Kind=30 will be present in the Supported TCP Extensions TLV that it
   returns in response to Info TLV.

6.2.5.  Connect TLV

   The Connect TLV (Figure 18) is used to request the establishment of a
   connection via a Transport Converter.  This connection can be from or
   to a Client.

   The 'Remote Peer Port' and 'Remote Peer IP Address' fields contain
   the destination port number and IP address of the Server, for
   outgoing connections.  For incoming connections destined to a Client
   serviced via a Transport Converter, these fields convey the source
   port number and IP address of the SYN packet received by the
   Transport Converter from the server.

   The Remote Peer IP Address MUST be encoded as an IPv6 address.  IPv4
   addresses MUST be encoded using the IPv4-Mapped IPv6 Address format
   defined in [RFC4291].  Further, Remote Peer IP address field MUST NOT
   include multicast, broadcast, and host loopback addresses [RFC6890].
   If a Converter receives a Connect TLVs with such invalid addresses,
   it MUST reply with a Malformed Message Error TLV and close the
   associated TCP connection.





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   We distinguish two types of Connect TLV based on their length: (1)
   the Base Connect TLV has a length of 20 bytes and contains a remote
   address and a remote port (Figure 18), (2) the Extended Connect TLV
   spans more than 20 bytes and also includes the optional 'TCP Options'
   field (Figure 19).  This field is used to request the advertisement
   of specific TCP options to the server.

                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +---------------+---------------+-------------------------------+
    |     Type=0xA  |     Length    |      Remote Peer Port         |
    +---------------+---------------+-------------------------------+
    |                                                               |
    |         Remote Peer IP Address (128 bits)                     |
    |                                                               |
    |                                                               |
    +---------------------------------------------------------------+

                      Figure 18: The Base Connect TLV

                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +---------------+---------------+-------------------------------+
    |     Type=0xA  |     Length    |      Remote Peer Port         |
    +---------------+---------------+-------------------------------+
    |                                                               |
    |         Remote Peer IP Address (128 bits)                     |
    |                                                               |
    |                                                               |
    +---------------------------------------------------------------+
    /                          TCP Options (Variable)               /
    /                              ...                              /
    +---------------------------------------------------------------+

                    Figure 19: The Extended Connect TLV

   The 'TCP Options' field is a variable length field that carries a
   list of TCP option fields (Figure 20).  Each TCP option field is
   encoded as a block of 2+n bytes where the first byte is the TCP
   option Kind and the second byte is the length of the TCP option as
   specified in [RFC0793].  The minimum value for the TCP option Length
   is 2.  The TCP options that do not include a length sub-field, i.e.,
   option types 0 (EOL) and 1 (NOP) defined in [RFC0793] MUST NOT be
   placed inside the TCP options field of the Connect TLV.  The optional
   Value field contains the variable-length part of the TCP option.  A
   length of two indicates the absence of the Value field.  The TCP
   options field always ends on a 32 bits boundary after being padded
   with zeros.



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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +---------------+---------------+---------------+---------------+
    |  TCPOpt kind  | TCPOpt Length | Value  (opt)  |  ....         |
    +---------------+---------------+---------------+---------------+
    |                             ....                              |
    +---------------------------------------------------------------+
    |                              ...                              |
    +---------------------------------------------------------------+

                     Figure 20: The TCP Options Field

   Upon reception of a Base Connect TLV, and absent any policy (e.g.,
   rate-limit) or resource exhaustion conditions, a Transport Converter
   attempts to establish a connection to the address and port that it
   contains.  The Transport Converter MUST use by default the TCP
   options that correspond to its local policy to establish this
   connection.  These are the options that it advertises in the
   Supported TCP Extensions TLV.

   Upon reception of an Extended Connect TLV, a Transport Converter
   first checks whether it supports the TCP Options listed in the 'TCP
   Options' field.  If not, it returns an error message (Section 6.2.8).
   If the above check succeeded and absent any rate limit policy or
   resource exhaustion conditions, a Transport Converter MUST attempt to
   establish a connection to the address and port that it contains.  It
   MUST include in the SYN that it sends to the Server the options
   listed in the 'TCP Options' sub-field and the TCP options that it
   would have used according to its local policies.  For the TCP options
   that are included in the TCP Options field without an optional value,
   the Transport Converter MUST generate its own value.  For the TCP
   options that are included in the 'TCP Options' field with an optional
   value, it MUST copy the entire option in the SYN sent to the remote
   server.  This feature is required to support TCP Fast Open.  See
   Section 7 for a detailed discussion of the different types of TCP
   options.

   The Transport Converter may refuse a Connect TLV request for various
   reasons (e.g., authorization failed, out of resources, invalid
   address type, unsupported TCP option).  An error message indicating
   the encountered error is returned to the requesting Client
   (Section 6.2.8).  In order to prevent denial-of-service attacks,
   error messages sent to a Client SHOULD be rate-limited.








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6.2.6.  Extended TCP Header TLV

   The Extended TCP Header TLV (Figure 21) is used by the Transport
   Converter to return to the Client the TCP options that were returned
   by the Server in the SYN+ACK packet.  A Transport Converter MUST
   return this TLV if the Client sent an Extended Connect TLV and the
   connection was accepted by the server.

                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +---------------+---------------+-------------------------------+
    |     Type=0x14 |     Length    |           Unassigned          |
    +---------------+---------------+-------------------------------+
    /               Returned Extended TCP header                    /
    /                              ...                              /
    +---------------------------------------------------------------+

                  Figure 21: The Extended TCP Header TLV

   The Returned Extended TCP header field is a copy of the TCP Options
   that were included in the SYN+ACK received by the Transport
   Converter.

   The Unassigned field MUST be set to zero by the sender and ignored by
   the receiver.

6.2.7.  The Cookie TLV

   The Cookie TLV (Figure 22) is an optional TLV which is similar to the
   TCP Fast Open Cookie [RFC7413].  A Transport Converter may want to
   verify that a Client can receive the packets that it sends to prevent
   attacks from spoofed addresses.  This verification can be done by
   using a Cookie that is bound to, for example, the IP address(es) of
   the Client.  This Cookie can be configured on the Client by means
   that are outside of this document or provided by the Transport
   Converter as follows.

   A Transport Converter that has been configured to use the optional
   Cookie TLV MUST verify the presence of this TLV in the payload of the
   received SYN.  If this TLV is present, the Transport Converter MUST
   validate the Cookie by means similar to those in Section 4.1.2 of
   [RFC7413] (i.e., IsCookieValid).  If the Cookie is valid, the
   connection establishment procedure can continue.  Otherwise, the
   Transport Converter MUST return an Error TLV set to "Not Authorized"
   and close the connection.

   If the received SYN did not contain a Cookie TLV, and cookie
   validation is required, the Transport Converter MAY compute a Cookie



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   bound to this Client address and return a Convert message containing
   the fixed header, an Error TLV set to "Missing Cookie" and the
   computed Cookie and close the connection.  The Client will react to
   this error by first issuing a reset to terminate the connection.  It
   also stores the received Cookie in its cache and attempts to
   reestablish a new connection to the Transport Converter that includes
   the Cookie TLV.

   The format of the Cookie TLV is shown in Figure 22.

                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +---------------+---------------+-------------------------------+
    |     Type=0x16 |     Length    |             Zero              |
    +---------------+---------------+-------------------------------+
    /                        Opaque  Cookie                         /
    /                              ...                              /
    +---------------------------------------------------------------+

                         Figure 22: The Cookie TLV

6.2.8.  Error TLV

   The Error TLV (Figure 23) is meant to provide information about some
   errors that occurred during the processing of a Convert message.
   This TLV has a variable length.  Upon reception of an Error TLV, a
   Client MUST reset the associated connection.

                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +---------------+---------------+----------------+--------------+
    |     Type=0x1E |     Length    |    Error Code  |  Value       |
    +---------------+---------------+----------------+--------------+
    //              ...   (optional) Value                         //
    +---------------------------------------------------------------+

                         Figure 23: The Error TLV

   Different types of errors can occur while processing Convert
   messages.  Each error is identified by an Error Code represented as
   an unsigned integer.  Four classes of error codes are defined:

   o  Message validation and processing errors (0-31 range): returned
      upon reception of an invalid message (including valid messages but
      with invalid or unknown TLVs).






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   o  Client-side errors (32-63 range): the Client sent a request that
      could not be accepted by the Transport Converter (e.g.,
      unsupported operation).

   o  Converter-side errors (64-95 range): problems encountered on the
      Transport Converter (e.g., lack of resources) which prevent it
      from fulfilling the Client's request.

   o  Errors caused by the destination server (96-127 range): the final
      destination could not be reached or it replied with a reset.

   The following error codes are defined in this document:

   o  Unsupported Version (0): The version number indicated in the fixed
      header of a message received from a peer is not supported.

      This error code MUST be generated by a peer (e.g.  Transport
      Converter) when it receives a request having a version number that
      it does not support.

      The value field MUST be set to the version supported by the peer.
      When multiple versions are supported by the peer, it includes the
      list of supported version in the value field; each version is
      encoded in 8 bits.  The list of supported versions should be
      padded with zeros to end on a 32 bits boundary.

      Upon receipt of this error code, the remote peer (e.g., Client)
      checks whether it supports one of the versions returned by the
      peer.  The highest common supported version MUST be used by the
      remote peer in subsequent exchanges with the peer.

   o  Malformed Message (1): This error code is sent to indicate that a
      message received from a peer cannot be successfully parsed and
      validated.

      Typically, this error code is sent by the Transport Converter if
      it receives a Connect TLV enclosing a multicast, broadcast, or
      loopback IP address.

      To ease troubleshooting, the value field MUST echo the received
      message shifted by one byte to keep to original alignment of the
      message.

   o  Unsupported Message (2): This error code is sent to indicate that
      a message type received from a Client is not supported.






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      To ease troubleshooting, the value field MUST echo the received
      message shifted by one byte to keep to original alignment of the
      message.

   o  Missing Cookie (3): If a Transport Converter requires the
      utilization of Cookies to prevent spoofing attacks and a Cookie
      TLV was not included in the Convert message, the Transport
      Converter MUST return this error to the requesting client.  The
      first byte of the value field MUST be set to zero and the
      remaining bytes of the Error TLV contain the Cookie computed by
      the Transport Converter for this Client.

      A Client which receives this error code SHOULD cache the received
      Cookie and include it in subsequent Convert messages sent to that
      Transport Converter.

   o  Not Authorized (32): This error code indicates that the Transport
      Converter refused to create a connection because of a lack of
      authorization (e.g., administratively prohibited, authorization
      failure, invalid Cookie TLV, etc.).  The Value field MUST be set
      to zero.

      This error code MUST be sent by the Transport Converter when a
      request cannot be successfully processed because the authorization
      failed.

   o  Unsupported TCP Option (33): A TCP option that the Client
      requested to advertise to the final Server cannot be safely used.

      The Value field is set to the type of the unsupported TCP option.
      If several unsupported TCP options were specified in the Connect
      TLV, then the list of unsupported TCP options is returned.  The
      list of unsupported TCP options MUST be padded with zeros to end
      on a 32 bits boundary.

   o  Resource Exceeded (64): This error indicates that the Transport
      Converter does not have enough resources to perform the request.

      This error MUST be sent by the Transport Converter when it does
      not have sufficient resources to handle a new connection.  The
      Transport Converter may indicate in the Value field the suggested
      delay (in seconds) that the Client SHOULD wait before soliciting
      the Transport Converter for a new proxied connection.  A Value of
      zero corresponds to a default delay of at least 30 seconds.

   o  Network Failure (65): This error indicates that the Transport
      Converter is experiencing a network failure to proxy the request.




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      The Transport Converter MUST send this error code when it
      experiences forwarding issues to proxy a connection.  The
      Transport Converter may indicate in the Value field the suggested
      delay (in seconds) that the Client SHOULD wait before soliciting
      the Transport Converter for a new proxied connection.  A Value of
      zero corresponds to a default delay of at least 30 seconds.

   o  Connection Reset (96): This error indicates that the final
      destination responded with an RST packet.  The Value field MUST be
      set to zero.

   o  Destination Unreachable (97): This error indicates that an ICMP
      destination unreachable, port unreachable, or network unreachable
      was received by the Transport Converter.  The Value field MUST
      echo the Code field of the received ICMP message.

   Figure 24 summarizes the different error codes.

    +-------+------+-----------------------------------------------+
    | Error | Hex  | Description                                   |
    +-------+------+-----------------------------------------------+
    |    0  | 0x00 | Unsupported Version                           |
    |    1  | 0x01 | Malformed Message                             |
    |    2  | 0x02 | Unsupported Message                           |
    |    3  | 0x03 | Missing Cookie                                |
    |   32  | 0x20 | Not Authorized                                |
    |   33  | 0x21 | Unsupported TCP Option                        |
    |   64  | 0x40 | Resource Exceeded                             |
    |   65  | 0x41 | Network Failure                               |
    |   96  | 0x60 | Connection Reset                              |
    |   97  | 0x61 | Destination Unreachable                       |
    +-------+------+-----------------------------------------------+

                      Figure 24: Convert Error Values

7.  Compatibility of Specific TCP Options with the Conversion Service

   In this section, we discuss how several deployed standard track TCP
   options can be supported through the Convert Protocol.  The other TCP
   options will be discussed in other documents.

7.1.  Base TCP Options

   Three TCP options were initially defined in [RFC0793]: End-of-Option
   List (Kind=0), No-Operation (Kind=1) and Maximum Segment Size
   (Kind=2).  The first two options are mainly used to pad the TCP
   header.  There is no reason for a client to request a Transport




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   Converter to specifically send these options towards the final
   destination.

   The Maximum Segment Size option (Kind=2) is used by a host to
   indicate the largest segment that it can receive over each
   connection.  This value is function of the stack that terminates the
   TCP connection.  There is no reason for a Client to request a
   Transport Converter to advertise a specific MSS value to a remote
   server.

   A Transport Converter MUST ignore options with Kind=0, 1 or 2 if they
   appear in a Connect TLV.  It MUST NOT announce them in a Supported
   TCP Extensions TLV.

7.2.  Window Scale (WS)

   The Window Scale (WS) option (Kind=3) is defined in [RFC7323].  As
   for the MSS option, the window scale factor that is used for a
   connection strongly depends on the TCP stack that handles the
   connection.  When a Transport Converter opens a TCP connection
   towards a remote server on behalf of a Client, it SHOULD use a WS
   option with a scaling factor that corresponds to the configuration of
   its stack.  A local configuration MAY allow for WS option in the
   proxied message to be function of the scaling factor of the incoming
   connection.

   There is no benefit from a deployment viewpoint in enabling a Client
   of a Transport Converter to specifically request the utilization of
   the WS option (Kind=3) with a specific scaling factor towards a
   remote Server.  For this reason, a Transport Converter MUST ignore
   option Kind=3 if it appears in a Connect TLV.  It MUST NOT announce
   it in a Supported TCP Extensions TLV.

7.3.  Selective Acknowledgments

   Two distinct TCP options were defined to support selective
   acknowledgments in [RFC2018].  This first one, SACK Permitted
   (Kind=4), is used to negotiate the utilization of selective
   acknowledgments during the three-way handshake.  The second one, SACK
   (Kind=5), carries the selective acknowledgments inside regular
   segments.

   The SACK Permitted option (Kind=4) MAY be advertised by a Transport
   Converter in the Supported TCP Extensions TLV.  Clients connected to
   this Transport Converter MAY include the SACK Permitted option in the
   Connect TLV.





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   The SACK option (Kind=5) cannot be used during the three-way
   handshake.  For this reason, a Transport Converter MUST ignore option
   Kind=5 if it appears in a Connect TLV.  It MUST NOT announce it in a
   TCP Supported Extensions TLV.

7.4.  Timestamp

   The Timestamp option [RFC7323] can be used during the three-way
   handshake to negotiate the utilization of timestamps during the TCP
   connection.  It is notably used to improve round-trip-time
   estimations and to provide protection against wrapped sequence
   numbers (PAWS).  As for the WS option, the timestamps are a property
   of a connection and there is limited benefit in enabling a client to
   request a Transport Converter to use the timestamp option when
   establishing a connection to a remote server.  Furthermore, the
   timestamps that are used by TCP stacks are specific to each stack and
   there is no benefit in enabling a client to specify the timestamp
   value that a Transport Converter could use to establish a connection
   to a remote server.

   A Transport Converter MAY advertise the Timestamp option (Kind=8) in
   the TCP Supported Extensions TLV.  The clients connected to this
   Transport Converter MAY include the Timestamp option in the Connect
   TLV but without any timestamp.

7.5.  Multipath TCP

   The Multipath TCP options are defined in [RFC6824].  [RFC6824]
   defines one variable length TCP option (Kind=30) that includes a sub-
   type field to support several Multipath TCP options.  There are
   several operational use cases where clients would like to use
   Multipath TCP through a Transport Converter [IETFJ16].  However, none
   of these use cases require the Client to specify the content of the
   Multipath TCP option that the Transport Converter should send to a
   remote server.

   A Transport Converter which supports Multipath TCP conversion service
   MUST advertise the Multipath TCP option (Kind=30) in the Supported
   TCP Extensions TLV.  Clients serviced by this Transport Converter may
   include the Multipath TCP option in the Connect TLV but without any
   content.

7.6.  TCP Fast Open

   The TCP Fast Open cookie option (Kind=34) is defined in [RFC7413].
   There are two different usages of this option that need to be
   supported by Transport Converters.  The first utilization of the TCP
   Fast Open cookie option is to request a cookie from the server.  In



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   this case, the option is sent with an empty cookie by the client and
   the server returns the cookie.  The second utilization of the TCP
   Fast Open cookie option is to send a cookie to the server.  In this
   case, the option contains a cookie.

   A Transport Converter MAY advertise the TCP Fast Open cookie option
   (Kind=34) in the Supported TCP Extensions TLV.  If a Transport
   Converter has advertised the support for TCP Fast Open in its
   Supported TCP Extensions TLV, it needs to be able to process two
   types of Connect TLV.  If such a Transport Converter receives a
   Connect TLV with the TCP Fast Open cookie option that does not
   contain a cookie, it MUST add an empty TCP Fast Open cookie option in
   the SYN sent to the remote server.  If such a Transport Converter
   receives a Connect TLV with the TCP Fast Open cookie option that
   contains a cookie, it MUST copy the TCP Fast Open cookie option in
   the SYN sent to the remote server.

7.7.  TCP-AO

   TCP-AO [RFC5925] provides a technique to authenticate all the packets
   exchanged over a TCP connection.  Given the nature of this extension,
   it is unlikely that the applications that require their packets to be
   authenticated end-to-end would want their connections to pass through
   a converter.  For this reason, we do not recommend the support of the
   TCP-AO option by Transport Converters.  The only use cases where it
   could make sense to combine TCP-AO and the solution in this document
   are those where the TCP-AO-NAT extension [RFC6978] is in use.

   A Transport Converter MUST NOT advertise the TCP-AO option (Kind=29)
   in the Supported TCP Extensions TLV.  If a Transport Converter
   receives a Connect TLV that contains the TCP-AO option, it MUST
   reject the establishment of the connection with error code set to
   "Unsupported TCP Option", except if the TCP-AO-NAT option is used.

8.  Interactions with Middleboxes

   The Convert Protocol is designed to be used in networks that do not
   contain middleboxes that interfere with TCP.  Under such conditions,
   it is assumed that the network provider ensures that all involved on-
   path nodes are not breaking TCP signals (e.g., strip TCP options,
   discard some SYNs, etc.).

   Nevertheless, and in order to allow for a robust service, this
   section describes how a Client can detect middlebox interference and
   stop using the Transport Converter affected by this interference.

   Internet measurements [IMC11] have shown that middleboxes can affect
   the deployment of TCP extensions.  In this section, we focus the



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   middleboxes that modify the payload since the Convert Protocol places
   its messages at the beginning of the bytestream.

   Consider a middlebox that removes the SYN payload.  The Client can
   detect this problem by looking at the acknowledgment number field of
   the SYN+ACK returned by the Transport Converter.  The Client MUST
   stop to use this Transport Converter given the middlebox
   interference.

   Consider now a middlebox that drops SYN/ACKs with a payload.  The
   Client won't be able to establish a connection via the Transport
   Converter.  The case of a middlebox that removes the payload of
   SYN+ACKs or from the packet that follows the SYN+ACK (but not the
   payload of SYN) can be detected by a Client.  This is hinted by the
   absence of a valid Convert message in the response.

   As explained in [RFC7413], some CGNs (Carrier Grade NATs) can affect
   the operation of TFO if they assign different IP addresses to the
   same end host.  Such CGNs could affect the operation of the cookie
   validation used by the Convert Protocol.  As a reminder CGNs, enabled
   on the path between a Client and a Transport Converter, must adhere
   to the address preservation defined in [RFC6888].  See also the
   discussion in Section 7.1 of [RFC7413].

9.  Security Considerations

9.1.  Privacy & Ingress Filtering

   The Transport Converter may have access to privacy-related
   information (e.g., subscriber credentials).  The Transport Converter
   is designed to not leak such sensitive information outside a local
   domain.

   Given its function and its location in the network, a Transport
   Converter has access to the payload of all the packets that it
   processes.  As such, it MUST be protected as a core IP router (e.g.,
   [RFC1812]).

   Furthermore, ingress filtering policies MUST be enforced at the
   network boundaries [RFC2827].

   This document assumes that all network attachments are managed by the
   same administrative entity.  Therefore, enforcing anti-spoofing
   filters at these network ensures that hosts are not sending traffic
   with spoofed source IP addresses.






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9.2.  Authorization

   The Convert Protocol is intended to be used in managed networks where
   end hosts can be identified by their IP address.

   Stronger mutual authentication schemes MUST be defined to use the
   Convert Protocol in more open network environments.  One possibility
   is to use TLS to perform mutual authentication between the client and
   the Converter.  That is, use TLS when a Client retrieves a Cookie
   from the Converter and rely on certificate-based client
   authentication, pre-shared key based [RFC4279] or raw public key
   based client authentication [RFC7250] to secure this connection.

   If the authentication succeeds, the Converter returns a cookie to the
   Client.  Subsequent Connect messages will be authorized as a function
   of the content of the Cookie TLV.

   In deployments where network-assisted connections are not allowed
   between hosts of a domain (i.e., hairpinning), the Converter may be
   instructed to discard such connections.  Hairpinned connections are
   thus rejected by the Transport Converter by returning an Error TLV
   set to "Not Authorized".  Absent explicit configuration otherwise,
   hairpinning is enabled by the Converter (see Figure 25.

             <===Network Provider===>

      +----+ from X1:x1 to X2':x2'   +-----+ X1':x1'
      | C1 |>>>>>>>>>>>>>>>>>>>>>>>>>>>>>--+---
      +----+                         |  v  |
                                     |  v  |
                                     |  v  |
                                     |  v  |
      +----+ from X1':x1' to X2:x2   |  v  | X2':x2'
      | C2 |<<<<<<<<<<<<<<<<<<<<<<<<<<<<<--+---
      +----+                         +-----+
                                    Converter

      Note: X2':x2' may be equal to
            X2:x2

                      Figure 25: Hairpinning Example

   See below for authorization considerations that are specific for
   Multipath TCP.







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9.3.  Denial of Service

   Another possible risk is the amplification attacks since a Transport
   Converter sends a SYN towards a remote Server upon reception of a SYN
   from a Client.  This could lead to amplification attacks if the SYN
   sent by the Transport Converter were larger than the SYN received
   from the Client or if the Transport Converter retransmits the SYN.
   To mitigate such attacks, the Transport Converter SHOULD rate limit
   the number of pending requests for a given Client.  It SHOULD also
   avoid sending to remote Servers SYNs that are significantly longer
   than the SYN received from the Client.  Finally, the Transport
   Converter SHOULD only retransmit a SYN to a Server after having
   received a retransmitted SYN from the corresponding Client.  Means to
   protect against SYN flooding attacks should also be enabled (e.g.,
   Section 3 of [RFC4987]).

9.4.  Traffic Theft

   Traffic theft is a risk if an illegitimate Converter is inserted in
   the path.  Indeed, inserting an illegitimate Converter in the
   forwarding path allows traffic interception and can therefore provide
   access to sensitive data issued by or destined to a host.  Converter
   discovery and configuration are out of scope of this document.

9.5.  Authentication Considerations

   The operator that manages the various network attachments (including
   the Transport Converters) can 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 TCP converter 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 Transport Converter for authorization
      matters.

   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 hosts that are authorized to communicate with a
      Transport Converter.  These ACLs may be installed as a result of
      RADIUS exchanges, e.g., [I-D.boucadair-radext-tcpm-converter].
      This method does not require any interaction with the Transport
      Converter for authorization matters.




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   o  A device that embeds a Transport Converter 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-radext-tcpm-converter].

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

10.  IANA Considerations

   Note to the RFC Editor: Please replace "THISRFC" in the following
   sub-sections with the RFC number to be assigned to this document.

10.1.  Convert Service Name

   IANA is requested to assign a service name for the Convert Protocol
   from the "Service Name and Transport Protocol Port Number Registry"
   available at https://www.iana.org/assignments/service-names-port-
   numbers/service-names-port-numbers.xhtml.

      Service Name:           convert
      Port Number:            N/A
      Transport Protocol(s):  TCP
      Description:            0-RTT TCP Convert Protocol
      Assignee:               IESG <iesg@ietf.org>
      Contact:                IETF Chair <chair@ietf.org>
      Reference:              THISRFC

   Clients may use this service name to fed the procedure defined in
   [RFC2782] to discover the IP address(es) and the port number used by
   the Transport Converters of a domain.

10.2.  The Convert Protocol (Convert) Parameters

   IANA is requested to create a new "The TCP Convert Protocol (Convert)
   Parameters" registry.

   The following subsections detail new registries within "The Convert
   Protocol (Convert) Parameters" registry.

   The Designated Expert is expected to ascertain the existence of
   suitable documentation as described in Section 4.6 of [RFC8126] and
   to verify that the document is permanently and publicly available.



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   The Designated Expert is also expected to check the clarity of
   purpose and use of the requested code points.

   Also, criteria that should be applied by the Designated Experts
   includes determining whether the proposed registration duplicates
   existing functionality, whether it is likely to be of general
   applicability or whether it is useful only for a private use, and
   whether the registration description is clear.  IANA must only accept
   registry updates to the 128-191 range (for both "Convert TLVs" and
   "Convert Error Messages" sub-registries) from the Designated Experts
   and should direct all requests for registration to the review mailing
   list.  It is suggested that multiple Designated Experts be appointed.
   In cases where a registration decision could be perceived as creating
   a conflict of interest for a particular Expert, that Expert should
   defer to the judgment of the other Experts.

10.2.1.  Convert Versions

   IANA is requested to create the "Convert versions" sub-registry.  New
   values are assigned via IETF Review (Section 4.8 of [RFC8126]).

   The initial values to be assigned at the creation of the registry are
   as follows:

    +---------+--------------------------------------+-------------+
    | Version | Description                          | Reference   |
    +---------+--------------------------------------+-------------+
    |    0    | Reserved by this document            |  THISRFC    |
    |    1    | Assigned by this document            |  THISRFC    |
    +---------+--------------------------------------+-------------+

10.2.2.  Convert TLVs

   IANA is requested to create the "Convert TLVs" sub-registry.  The
   procedure for assigning values from this registry is as follows:

   o  The values in the range 1-127 can be assigned via IETF Review.

   o  The values in the range 128-191 can be assigned via Specification
      Required.

   o  The values in the range 192-255 are reserved for Private Use.

   The initial values to be assigned at the creation of the registry are
   as follows:






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    +---------+--------------------------------------+-------------+
    |  Code   | Name                                 | Reference   |
    +---------+--------------------------------------+-------------+
    |    0    | Reserved                             |   THISRFC   |
    |    1    | Info TLV                             |   THISRFC   |
    |   10    | Connect TLV                          |   THISRFC   |
    |   20    | Extended TCP Header TLV              |   THISRFC   |
    |   21    | Supported TCP Extension TLV          |   THISRFC   |
    |   22    | Cookie TLV                           |   THISRFC   |
    |   30    | Error TLV                            |   THISRFC   |
    +---------+--------------------------------------+-------------+

10.2.3.  Convert Error Messages

   IANA is requested to create the "Convert Errors" sub-registry.  Codes
   in this registry are assigned as a function of the error type.  Four
   types are defined; the following ranges are reserved for each of
   these types:

   o  Message validation and processing errors: 0-31

   o  Client-side errors: 32-63

   o  Transport Converter-side errors: 64-95

   o  Errors caused by destination server: 96-127

   The procedure for assigning values from this sub-registry is as
   follows:

   o  0-127: Values in this range are assigned via IETF Review.

   o  128-191: Values in this range are assigned via Specification
      Required.

   o  192-255: Values in this range are reserved for Private Use.

   The initial values to be assigned at the creation of the registry are
   as follows:












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    +-------+------+-----------------------------------+-----------+
    | Error | Hex  | Description                       | Reference |
    +-------+------+-----------------------------------+-----------+
    |    0  | 0x00 | Unsupported Version               |  THISRFC  |
    |    1  | 0x01 | Malformed Message                 |  THISRFC  |
    |    2  | 0x02 | Unsupported Message               |  THISRFC  |
    |    3  | 0x03 | Missing Cookie                    |  THISRFC  |
    |   32  | 0x20 | Not Authorized                    |  THISRFC  |
    |   33  | 0x21 | Unsupported TCP Option            |  THISRFC  |
    |   64  | 0x40 | Resource Exceeded                 |  THISRFC  |
    |   65  | 0x41 | Network Failure                   |  THISRFC  |
    |   96  | 0x60 | Connection Reset                  |  THISRFC  |
    |   97  | 0x61 | Destination Unreachable           |  THISRFC  |
    +-------+------+-----------------------------------+-----------+

                    Figure 26: The Convert Error Codes

11.  References

11.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018,
              DOI 10.17487/RFC2018, October 1996,
              <https://www.rfc-editor.org/info/rfc2018>.

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

   [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, <https://www.rfc-editor.org/info/rfc2827>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4787]  Audet, F., Ed. and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
              2007, <https://www.rfc-editor.org/info/rfc4787>.



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

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

   [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,
              <https://www.rfc-editor.org/info/rfc6824>.

   [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, <https://www.rfc-editor.org/info/rfc6888>.

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,
              <https://www.rfc-editor.org/info/rfc6890>.

   [RFC6978]  Touch, J., "A TCP Authentication Option Extension for NAT
              Traversal", RFC 6978, DOI 10.17487/RFC6978, July 2013,
              <https://www.rfc-editor.org/info/rfc6978>.

   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, Ed., "TCP Extensions for High Performance",
              RFC 7323, DOI 10.17487/RFC7323, September 2014,
              <https://www.rfc-editor.org/info/rfc7323>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <https://www.rfc-editor.org/info/rfc7413>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.







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11.2.  Informative References

   [ANRW17]   Trammell, B., Kuehlewind, M., De Vaere, P., Learmonth, I.,
              and G. Fairhurst, "Tracking transport-layer evolution with
              PATHspider", Applied Networking Research Workshop 2017
              (ANRW17) , July 2017.

   [Fukuda2011]
              Fukuda, K., "An Analysis of Longitudinal TCP Passive
              Measurements (Short Paper)", Traffic Monitoring and
              Analysis. TMA 2011. Lecture Notes in Computer Science, vol
              6613. , 2011.

   [HotMiddlebox13b]
              Detal, G., Paasch, C., and O. Bonaventure, "Multipath in
              the Middle(Box)", HotMiddlebox'13 , December 2013,
              <http://inl.info.ucl.ac.be/publications/multipath-
              middlebox>.

   [I-D.arkko-arch-low-latency]
              Arkko, J. and J. Tantsura, "Low Latency Applications and
              the Internet Architecture", draft-arkko-arch-low-
              latency-02 (work in progress), October 2017.

   [I-D.boucadair-mptcp-plain-mode]
              Boucadair, M., Jacquenet, C., Bonaventure, O., Behaghel,
              D., stefano.secci@lip6.fr, s., Henderickx, W., Skog, R.,
              Vinapamula, S., Seo, S., Cloetens, W., Meyer, U.,
              Contreras, L., and B. Peirens, "Extensions for Network-
              Assisted MPTCP Deployment Models", draft-boucadair-mptcp-
              plain-mode-10 (work in progress), March 2017.

   [I-D.boucadair-radext-tcpm-converter]
              Boucadair, M. and C. Jacquenet, "RADIUS Extensions for
              0-RTT TCP Converters", draft-boucadair-radext-tcpm-
              converter-02 (work in progress), April 2019.

   [I-D.boucadair-tcpm-dhc-converter]
              Boucadair, M., Jacquenet, C., and T. Reddy.K, "DHCP
              Options for 0-RTT TCP Converters", draft-boucadair-tcpm-
              dhc-converter-03 (work in progress), October 2019.

   [I-D.olteanu-intarea-socks-6]
              Olteanu, V. and D. Niculescu, "SOCKS Protocol Version 6",
              draft-olteanu-intarea-socks-6-08 (work in progress),
              November 2019.





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   [I-D.peirens-mptcp-transparent]
              Peirens, B., Detal, G., Barre, S., and O. Bonaventure,
              "Link bonding with transparent Multipath TCP", draft-
              peirens-mptcp-transparent-00 (work in progress), July
              2016.

   [IETFJ16]  Bonaventure, O. and S. Seo, "Multipath TCP Deployment",
              IETF Journal, Fall 2016 , n.d..

   [IMC11]    Honda, K., Nishida, Y., Raiciu, C., Greenhalgh, A.,
              Handley, M., and T. Hideyuki, "Is it still possible to
              extend TCP?", Proceedings of the 2011 ACM SIGCOMM
              conference on Internet measurement conference , 2011.

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,
              <https://www.rfc-editor.org/info/rfc1812>.

   [RFC1919]  Chatel, M., "Classical versus Transparent IP Proxies",
              RFC 1919, DOI 10.17487/RFC1919, March 1996,
              <https://www.rfc-editor.org/info/rfc1919>.

   [RFC1928]  Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
              L. Jones, "SOCKS Protocol Version 5", RFC 1928,
              DOI 10.17487/RFC1928, March 1996,
              <https://www.rfc-editor.org/info/rfc1928>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <https://www.rfc-editor.org/info/rfc2782>.

   [RFC3135]  Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
              Shelby, "Performance Enhancing Proxies Intended to
              Mitigate Link-Related Degradations", RFC 3135,
              DOI 10.17487/RFC3135, June 2001,
              <https://www.rfc-editor.org/info/rfc3135>.

   [RFC4279]  Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
              Ciphersuites for Transport Layer Security (TLS)",
              RFC 4279, DOI 10.17487/RFC4279, December 2005,
              <https://www.rfc-editor.org/info/rfc4279>.

   [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,
              <https://www.rfc-editor.org/info/rfc6269>.




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

   [RFC6887]  Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
              P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
              DOI 10.17487/RFC6887, April 2013,
              <https://www.rfc-editor.org/info/rfc6887>.

   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928,
              DOI 10.17487/RFC6928, April 2013,
              <https://www.rfc-editor.org/info/rfc6928>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <https://www.rfc-editor.org/info/rfc7250>.

   [RFC7414]  Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
              Zimmermann, "A Roadmap for Transmission Control Protocol
              (TCP) Specification Documents", RFC 7414,
              DOI 10.17487/RFC7414, February 2015,
              <https://www.rfc-editor.org/info/rfc7414>.

   [RFC8041]  Bonaventure, O., Paasch, C., and G. Detal, "Use Cases and
              Operational Experience with Multipath TCP", RFC 8041,
              DOI 10.17487/RFC8041, January 2017,
              <https://www.rfc-editor.org/info/rfc8041>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8548]  Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack,
              Q., and E. Smith, "Cryptographic Protection of TCP Streams
              (tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019,
              <https://www.rfc-editor.org/info/rfc8548>.







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   [TS23501]  3GPP (3rd Generation Partnership Project), ., "Technical
              Specification Group Services and System Aspects; System
              Architecture for the 5G System; Stage 2 (Release 16)",
              2019, <https://www.3gpp.org/ftp/Specs/
              archive/23_series/23.501/>.

Appendix A.  Example Socket API Changes to Support the 0-RTT Convert
             Protocol

A.1.  Active Open (Client Side)

   On the client side, the support of the 0-RTT Converter protocol does
   not require any other changes than those identified in Appendix A of
   [RFC7413].  Those modifications are already supported by multiple TCP
   stacks.

   As an example, on Linux, a client can send the 0-RTT Convert message
   inside a SYN by using sendto with the MSG_FASTOPEN flag as shown in
   the example below:

     s = socket(AF_INET, SOCK_STREAM, 0);

     sendto(s, buffer, buffer_len, MSG_FASTOPEN,
                    (struct sockaddr *) &server_addr, addr_len);

   The client side of the Linux TCP TFO can be used in two different
   modes depending on the host configuration (sysctl tcp_fastopen
   variable):

   o  0x1: (client) enables sending data in the opening SYN on the
      client.

   o  0x4: (client) send data in the opening SYN regardless of cookie
      availability and without a cookie option.

   By setting this configuration variable to 0x5, a Linux client using
   the above code would send data inside the SYN without using a TFO
   option.

A.2.  Passive Open (Converter Side)

   The Converter needs to enable the reception of data inside the SYN
   independently of the utilization of the TFO option.  This implies
   that the Transport Converter application cannot rely on the TFO
   cookies to validate the reachability of the IP address that sent the
   SYN.  It must rely on other techniques, such as the Cookie TLV
   described in this document, to verify this reachability.




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   [RFC7413] suggested the utilization of a TCP_FASTOPEN socket option
   the enable the reception of SYNs containing data.  Later, Appendix A
   of [RFC7413], mentioned:

   Traditionally, accept() returns only after a socket is connected.
   But, for a Fast Open connection, accept() returns upon receiving
   SYN with a valid Fast Open cookie and data, and the data is available
   to be read through, e.g., recvmsg(), read().

   To support the 0-RTT Convert Protocol, this behavior should be
   modified as follows:

    Traditionally, accept() returns only after a socket is connected.
    But, for a Fast Open connection, accept() returns upon receiving a
    SYN with data, and the data is available to be read through, e.g.,
    recvmsg(), read(). The application that receives such SYNs with data
    must be able to validate the reachability of the source of the SYN
    and also deal with replayed SYNs.

   The Linux server side can be configured with the following sysctls:

   o  0x2: (server) enables the server support, i.e., allowing data in a
      SYN packet to be accepted and passed to the application before
      3-way handshake finishes.

   o  0x200: (server) accept data-in-SYN w/o any cookie option present.

   However, this configuration is system-wide.  This is convenient for
   typical Transport Converter deployments where no other applications
   relying on TFO are collocated on the same device.

   Recently, the TCP_FASTOPEN_NO_COOKIE socket option has been added to
   provide the same behavior on a per socket basis.  This enables a
   single host to support both servers that require the TFO cookie and
   servers that do not use it.

Acknowledgments

   Although they could disagree with the contents of the document, we
   would like to thank Joe Touch and Juliusz Chroboczek whose comments
   on the MPTCP mailing list have forced us to reconsider the design of
   the solution several times.

   We would like to thank Raphael Bauduin, Stefano Secci, Anandatirtha
   Nandugudi and Gregory Vander Schueren for their help in preparing
   this document.  Nandini Ganesh provided valuable feedback about the
   handling of TFO and the error codes.  Yuchung Cheng and Praveen
   Balasubramanian helped to clarify the discussion on supplying data in



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   SYNs.  Phil Eardley and Michael Scharf's helped to clarify different
   parts of the text.

   Many thanks to Mirja Kuehlewind for the detailed AD review.

   This document builds upon earlier documents that proposed various
   forms of Multipath TCP proxies [I-D.boucadair-mptcp-plain-mode],
   [I-D.peirens-mptcp-transparent] and [HotMiddlebox13b].

   From [I-D.boucadair-mptcp-plain-mode]:

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

   Thanks to Ian Farrer, Mikael Abrahamsson, Alan Ford, Dan Wing, and
   Sri Gundavelli for the fruitful discussions in IETF#95 (Buenos
   Aires).

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

   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.

Contributors

   Bart Peirens contributed to an early version of the document.

   As noted above, this document builds on two previous documents.

   The authors of [I-D.boucadair-mptcp-plain-mode] were:

   o  Mohamed Boucadair

   o  Christian Jacquenet

   o  Olivier Bonaventure

   o  Denis Behaghel

   o  Stefano Secci

   o  Wim Henderickx

   o  Robert Skog




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   o  Suresh Vinapamula

   o  SungHoon Seo

   o  Wouter Cloetens

   o  Ullrich Meyer

   o  Luis M.  Contreras

   o  Bart Peirens

   The authors of [I-D.peirens-mptcp-transparent] were:

   o  Bart Peirens

   o  Gregory Detal

   o  Sebastien Barre

   o  Olivier Bonaventure

Change Log

   This section to be removed before publication.

   o  00 : initial version, designed to support Multipath TCP and TFO
      only

   o  00 to -01 : added section Section 7 describing the support of
      different standard tracks TCP options by Transport Converters,
      clarification of the IANA section, moved the SOCKS comparison to
      the appendix and various minor modifications

   o  01 to -02: Minor modifications

   o  02 to -03: Minor modifications

   o  03 to -04: Minor modifications

   o  04 to -05: Integrate a lot of feedback from implementers who have
      worked on client and server side implementations.  The main
      modifications are the following :

      *  TCP Fast Open is not strictly required anymore.  Several
         implementers expressed concerns about this requirement.  The
         TFO Cookie protects from some attack scenarios that affect open
         servers like web servers.  The Convert Protocol is different



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         and as discussed in RFC7413, there are different ways to
         protect from such attacks.  Instead of using a TFO cookie
         inside the TCP options, which consumes precious space in the
         extended TCP header, this version supports the utilization of a
         Cookie that is placed in the SYN payload.  This provides the
         same level of protection as a TFO Cookie in environments were
         such protection is required.

      *  the Bootstrap procedure has been simplified based on feedback
         from implementers

      *  Error messages are not included in RST segments anymore but
         sent in the bytestream.  Implementers have indicated that
         processing such segments on clients was difficult on some
         platforms.  This change simplifies client implementations.

      *  Many minor editorial changes to clarify the text based on
         implementers feedback.

   o  05 to -06: Many clarifications to integrate the comments from the
      chairs in preparation to the WGLC:

      *  Updated IANA policy to require "IETF Review" instead of
         "Standard Action"

      *  Call out explicitly that data in SYNs are relayed by the
         Converter

      *  Reiterate the scope

      *  Hairpinning behavior can be disabled (policy-based)

      *  Fix nits

   o  07:

      *  Update the text about supplying data in SYNs to make it clear
         that a constraint defined in RFC793 is relaxed following the
         same rationale as in RFC7413.

      *  Nits

      *  Added Appendix A on example Socket API changes

   o  08:

      *  Added short discussion on the termination of connections




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   o  09:

      *  Address various comments received during last call

   o  10-13:

      *  Changes to address the comments from Phil: Add a new section to
         group data plane considerations in one place + add a new
         appendix with more details on address modes + rearrange the
         MPTCP text.

   o  14: fixed nits (the shepherd write-up)

   o  15: Rewrote parts of the text to address the detailed comments
      provided by M.  Kuehlewind

Authors' Addresses

   Olivier Bonaventure (editor)
   Tessares

   Email: Olivier.Bonaventure@tessares.net


   Mohamed Boucadair (editor)
   Orange
   Clos Courtel
   Rennes  35000
   France

   Email: mohamed.boucadair@orange.com


   Sri Gundavelli
   Cisco

   Email: sgundave@cisco.com


   SungHoon Seo
   Korea Telecom

   Email: sh.seo@kt.com








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   Benjamin Hesmans
   Tessares

   Email: Benjamin.Hesmans@tessares.net















































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