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TCP Maintenance and Minor Extensions                            M. Amend
Internet-Draft                                                        DT
Intended status: Experimental                                    J. Kang
Expires: 8 January 2021                                           Huawei
                                                             7 July 2020


        Multipath TCP Extension for Robust Session Establishment
                     draft-amend-tcpm-mptcp-robe-00

Abstract

   Multipath TCP extends the plain, single-path limited, TCP towards the
   capability of multipath transmission.  This greatly improves the
   reliability and performance of TCP communication.  For backwards
   compatibility reasons the Multipath TCP was designed to setup
   successfully an initial path first, after which subsequent paths can
   be added for multipath transmission.  For that reason the Multipath
   TCP has the same limitations as the plain TCP during connection
   setup, in case the selected path is not functional.

   This document proposes a set of implementations and possible
   combinations thereof, that provide a more Robust Establishment (RobE)
   of MPTCP sessions.  It includes RobE_TIMER, RobE_SIM, RobE_eSIM and
   RobE_IPS.

   RobE_TIMER is designed to stay close to MPTCP in that standard
   functionality is used wherever possible.  Resiliency against network
   outages is achieved by modifying the SYN retransmission timer: If one
   path is defective, another path is used.

   RobE_SIM and RobE_eSIM provides the ability to simultaneously use
   multiple paths for connection setup.  They ensure connectivity if at
   least one functional path out of a bunch of paths is given and offers
   beside that the opportunity to significantly improve loading times of
   Internet services.

   RobE_IPS provides a heuristic to select properly an initial path for
   connection establishment with a remote host based on empirical data
   derived from previous connection information.

   In practice, these independent solutions can be complementary used.
   This document also presents the design and protocol procedure for
   those combinations in addition to the respective stand-alone
   solutions.






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Status of This Memo

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   This Internet-Draft will expire on 8 January 2021.

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/
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Implementation without MPTCP protocol adaptation  . . . . . .   7
     2.1.  Re-transmission Timer(RobE_TIMER) . . . . . . . . . . . .   7
     2.2.  Simultaneous Initial Paths Simple Version (RobE_SIM)  . .   8
     2.3.  Heuristic Initial Path Selection (RobE_IPS) . . . . . . .   9
       2.3.1.  Architecture  . . . . . . . . . . . . . . . . . . . .   9
       2.3.2.  Typical Scenarios . . . . . . . . . . . . . . . . . .  10
       2.3.3.  Path decision information . . . . . . . . . . . . . .  13
       2.3.4.  Initial Path Selection use local RTT information  . .  14
     2.4.  Combination of RobE_SIM and RobE_IPS  . . . . . . . . . .  14
     2.5.  Combination of RobE_TIMER and RobE_IPS  . . . . . . . . .  15
   3.  Implementation with Bi-directional MPTCP Support  . . . . . .  16
     3.1.  Simultaneous Initial Paths Extended Version
           (RobE_eSIM) . . . . . . . . . . . . . . . . . . . . . . .  17



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       3.1.1.  RobE_eSIM implicit Negotiation and Procedure  . . . .  17
       3.1.2.  RobE_eSIM explicit Negotiation and Procedure  . . . .  19
       3.1.3.  Protocol Adaptation . . . . . . . . . . . . . . . . .  19
       3.1.4.  Fallback Mechanisms . . . . . . . . . . . . . . . . .  20
       3.1.5.  Comparison Robe_SIM and RobE_eSIM . . . . . . . . . .  22
       3.1.6.  Security Consideration  . . . . . . . . . . . . . . .  23
     3.2.  Heuristic Initial Path Selection with remote RTT
           Measurement . . . . . . . . . . . . . . . . . . . . . . .  23
       3.2.1.  Description . . . . . . . . . . . . . . . . . . . . .  24
       3.2.2.  Protocol Adaptation . . . . . . . . . . . . . . . . .  24
       3.2.3.  Fallback Mechanism  . . . . . . . . . . . . . . . . .  25
       3.2.4.  Security Consideration  . . . . . . . . . . . . . . .  25
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .  26
     5.2.  Informative References  . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   Multipath TCP Robust Session Establishment (MPTCP RobE) is a set of
   extensions to regular MPTCP [RFC6824] and its next version [RFC8684],
   which releases single path limitations during the initial connection
   setup.  Several scenarios require and benefit from a reliable and in
   time connection setup which is not covered by [RFC6824] and [RFC8684]
   so far.  MPTCP was designed to be compliant with the TCP standard
   [RFC0793] and introduced therefore the concept of an initial TCP flow
   while adding subsequent flows after successful multipath negotiation
   on the initial path.  While fulfilling its purpose, MPTCP is however
   fully dependent on the transmission characteristics of the
   communication link selected for initiating MPTCP.

   Figure 1 shows the traditional way of MPTCP handshaking with a
   MP_CAPABLE exchanged first, followed when successful negotiated by
   additional flows engaging MP_JOIN.  [RFC6824] and the next MPTCP
   [RFC8684] differ in that a Key-A is sent with the first MP_CAPABLE or
   not.














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               Host A                                  Host B
        ------------------------                       ----------
        Address A1    Address A2                       Address B1
        ----------    ----------                       ----------
            |             |                                |
            |            SYN + MP_CAPABLE(Key-A[*])        |
            |--------------------------------------------->|
            |<---------------------------------------------|
            |          SYN/ACK + MP_CAPABLE(Key-B)         |
            |             |                                |
            |        ACK + MP_CAPABLE(Key-A, Key-B)        |
            |--------------------------------------------->|
            |             |                                |
            |             |   SYN + MP_JOIN(Token-B, R-A)  |
            |             |------------------------------->|
            |             |<-------------------------------|
            |             | SYN/ACK + MP_JOIN(HMAC-B, R-B) |
            |             |                                |
            |             |     ACK + MP_JOIN(HMAC-A)      |
            |             |------------------------------->|
            |             |<-------------------------------|
            |             |             ACK                |

            [*] Key-A in the first MP-capable is related to
                RFC6824 only and does not exist in RFC8684.

                      Figure 1: MPTCP connection setup

   Multipath TCP itself enables hosts to exchange packets belonging to a
   single connection over several paths.  Implemented in mobile phones
   (UEs), these paths are usually assigned to different network
   interfaces within the UE and corresponds to different networks such
   as cellular and WiFi.  The path or network interface for initiating
   the initial subflow setup is most often provided by the operation
   system of the UE.  For example, if a cellular connection and WiFi are
   present in a mobile phone, WiFi is usually the interface offered to
   initiate the MPTCP session.














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   This design falls short in situations where the default path does not
   provide the best performance compared to other available paths.  In a
   worst case the default path is not even capable of setting up the
   initial flow letting any other functional path unused.  For example,
   if the WiFi signal is weak, broken or cannot forward traffic to the
   destination, the establishment of the subflow will be delayed or
   impossible.  This in turn, leads to a longer startup delay or no
   communication at all for services using MPTCP even if other
   functional paths are available.  Even in scenarios where all paths
   are functional but services would benefit from a setup over the path
   with the lowest latency, MPTCP has no mean to support this demand.

   It can be concluded, that sequential path establishment relying with
   an initial path establishment over an external given default route
   will result in experience reduction when using MPTCP.  So this
   document proposes solutions to overcome the aforementioned
   limitations and provides a more robust connection setup compared to
   traditional MPTCP.

   RobE_SIM and RobE_eSIM aims to overcome the limitations of [RFC6824]
   and [RFC8684], using one initial flow and introduces the concept of
   potential initial flows triggered simultaneously.  Potential initial
   flows gives the freedom to use more than one path to request
   multipath capability and select the initial flow at a later point.
   Potential initial flow mechanisms and the gain of robustness and
   performance over the traditional MPTCP connection setup are evaluated
   in [RobE_slides] and [RobE_paper].  RobE_SIM is a break-before-make
   mechanism, guaranteeing at least the robust connection establishment,
   however the RobE_eSIM reuses every potential initial flow request to
   combine it with less overhead and accelerated multipath availability,
   leveraging a new MPTCP option MP_JOIN_CAP.  From a standardization
   perspective, the RobE_SIM is fully compliant with [RFC6824] and
   [RFC8684] and is herein more of a descriptive and procedural nature.
   The RobE_eSIM requires a new MPTCP option but with the potential to
   significantly improve the MPTCP experience.

   For the limitation of the default initial path, RobE_IPS makes no
   changes to standard MPTCP procedure and improves the performance of
   connection establishment by introducing an initial path selection
   strategy and required algorithms.  The input for strategy and
   algorithms is the transmission status information which represents
   the transmission performance of each available path or network
   interface.  The transmission status information is characterized by
   at least one of the parameters: signal strength, throughput, round-
   trip time (RTT) and link success rate.  In this way, a path with
   better transmission performance can be learned and determined and the
   respective network interface can be used for connection
   establishment.



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   The most simple approach for a robust MPTCP session establishment is
   RobE_TIMER, iterating the process of initial path establishment over
   all available paths, if the previous try has failed.  Triggering a
   new try on a next path is depending on an expiration timer,
   preferably re-use TCP's in-built expiration timer.

   Table 1 summarizes the impact of RobE_TIMER, RobE_SIM, RobE_eSIM and
   RobE_IPS compared to [RFC6824] and [RFC8684].

   +============+==========+=====================+==========+================+==========+
   | Scenario   | MPTCP    | RobE_TIMER          | RobE_SIM | RobE_eSIM      | RobE_IPS |
   +============+==========+=====================+==========+================+==========+
   | IP packet  | Delayed  |In the scope of timer|No impact | No impact      | Delayed  |
   | loss       |connection|                     |          |                |connection|
   +------------+----------+---------------------+----------+----------------+----------+
   | IP broken  | No       |In the scope of timer|No impact | No impact      | No       |
   |            |connection|                     |          |                |connection|
   +------------+----------+---------------------+----------+----------------+----------+
   | IP setup   | Default  |Default route (+ path| Fastest  | Fastest path   | Selected |
   | duration   | route    | 1..n)               | path     |                | path     |
   | dependency |          |                     |          |                |          |
   +------------+----------+---------------------+----------+----------------+----------+
   | MP         |MP_CAPABLE|sum_1..n(MP_CAPABLE_n|MP_CAPABLE|max(MP_CAPABLE_1|MP_CAPABLE|
   |availability| HS +     | HS) + MP_JOIN HS    | HS +     |.. MP_CAPABLE_n | HS +     |
   | duration   |MP_JOIN HS|                     |MP_JOIN HS| HS)            |MP_JOIN HS|
   +------------+----------+---------------------+----------+----------------+----------+
   |Guaranteeing|Depends on| Yes                 | Yes      | Yes            |Depends on|
   | session    | the      |                     |          |                |selection |
   | setup      | default  |                     |          |                |          |
   |            | route    |                     |          |                |          |
   +------------+----------+---------------------+----------+----------------+----------+

      Table 1: Overview RobE features during initial connection setup

      |  IP: Initial Path; MP: Multi-Path; HS: Handshake

1.1.  Terminology

   This document makes use of a number of terms that are either MPTCP-
   specific or have defined meaning in the context of MPTCP, as follows:

   Path:  A sequence of links between a sender and a receiver, defined
      in this context by a 4-tuple of source and destination address/
      port pairs.

   Subflow:  A flow of TCP segments operating over an individual path,
      which forms part of a larger MPTCP connection.  A subflow is
      started and terminated similar to a regular TCP connection.



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2.  Implementation without MPTCP protocol adaptation

   RobE_TIMER, RobE_SIM and RobE_IPS are compatible with the current
   MPTCP protocol definitions in [RFC6824] and [RFC8684] but may be lack
   of the full optimization potential which require protocol adaptation
   in Section 3.  Following sections will describe them in detail.

2.1.  Re-transmission Timer(RobE_TIMER)

   In RobE_TIMER, a new connection is initiated by sending a
   SYN+MP_CAPABLE along the initial path.  If this path is functional,
   the solution will perform identical to classic MPTCP: the initial
   flow will be established, and subsequent flows can be created
   afterwards.  If however the initial path is faulty, the
   retransmission will be triggered on another path.  This path might
   circumvent the dysfunctional network, and allow the client to create
   an initial subflow.  The first path is now seen as a subsequent path
   and the client sends SYN+MP_JOIN messages to create a subsequent
   flow.

   In high latency networks, the initial SYN+MP_CAPABLE might be delayed
   until the client retries on another path.  Once the second SYN
   arrives at the server, it will try to complete the three-way
   handshake.  If the first SYN was delayed by more than the
   retransmission time plus half a Round Trip Time (RTT) of the second
   path, it will arrive at the server after the second SYN.  The server
   could now treat the segment as obsolete and drop it.

                Host A                                    Host B
         ------------------------                       ----------
         Address A1    Address A2                       Address B1
         ----------    ----------                       ----------
             |             |                                |
             |            SYN + MP_CAPABLE(Key-A[*])        |
             |Timer---------------------------------------->|
             |             |   SYN + MP_CAPABLE(Key-A'[*])  |
             |             |------------------------------->|
             |             |   SYN/ACK+MP_CAPABLE(Key-B')   |
             |             |<-------------------------------|
             |             | ACK + MP_CAPABLE(Key-A',Key-B')|
             |             |------------------------------->|
             |             |   SYN + MP_JOIN(Token-B',R-A)  |
             |--------------------------------------------->|
             |   Subflow will be set up as normal MPTCP     |
             |                                              |

              [*] Key-A in the first MP-capable is related to
               RFC6824 only and does not exist in RFC8684.



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                     Figure 2: The Robe_TIMER Solution

   Immediately after sending the final ACK of the initial handshake,
   subflows are established on the remaining paths as defined in
   [RFC6824] and [RFC8684]

   [Notes: How to set the Timer is TBD.  If there is the case that the
   first SYN on default path arrives earlier than that from the second
   path, the MPTCP connection will be initialized on the path of the
   first SYN.  The server could treat the second SYN as obsolete and
   drop it.]

2.2.  Simultaneous Initial Paths Simple Version (RobE_SIM)

   RobE_SIM is a sender only implementation and no negotiation is
   required.  In RobE_SIM, the MPTCP connection setup benefits from the
   fastest path.  As shown in Figure 3, host A initiates the connection
   handshake on more than one path independently (SA1 and SA2).  The
   paths selected for RobE_SIM and referred to as potential initial
   flows, can belong to the number of interfaces on the device or a
   subset selected on experience.  When Host A receives the first SYN/
   ACK back from Host B (SA3), the path carrying this message is
   identified as the normal initial path.  Host A sends then immediately
   a TCP RST message (SA6.1) on any other path used for simultaneous
   connection setup causing an immediate termination of assigned flows
   (break-before-make).  The terminated ones are merged as subsequent
   subflows following the JOIN procedure described in [RFC6824] and
   [RFC8684].  The process is equivalent to any other scenario where the
   SYN/ACK arrives on an other path than depicted in Figure 3.






















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                   Host A                                  Host B
           ------------------------                       ----------
           Address A1    Address A2                       Address B1
           ----------    ----------                       ----------
               |             |                                |
               |            SYN + MP_CAPABLE(Key-A[*])        |
       (SA1)   |--------------------------------------------->|  (SB1)
               |             |    SYN + MP_CAPABLE(Key-A'[*]) |
       (SA2)   |             |------------------------------->|  (SB2)
               |             |                                |
       (SA3)   |<---------------------------------------------|  (SB3)
               |          SYN/ACK + MP_CAPABLE(Key-B)         |
       (SA4)   |             |<-------------------------------|  (SB4)
               |             |   SYN/ACK + MP_CAPABLE(Key-B') |
               |             |                                |
               |        ACK + MP_CAPABLE(Key-A, Key-B)        |
       (SA5)   |--------------------------------------------->|  (SB5)
               |             |             RST                |
       (SA6.1) |             |------------------------------->|  (SB6.1)
      RobE SIM |             |   SYN + MP_JOIN(Token-B, R-A)  |
      (robust) |             |------------------------------->|
               |             |         MP_JOIN Process...     |

               [*] Key-A in the first MP-capable is related to
                   RFC6824 only and does not exist in RFC8684.

                 Figure 3: MPTCP RobE_SIM Connection Setup

2.3.  Heuristic Initial Path Selection (RobE_IPS)

2.3.1.  Architecture

   Figure 4 provides the architecture for RobE_IPS and employs an
   "Initial Path Selection" logic which can be integrated into the MPTCP
   stack or exists as an isolated module in the terminal.  The IPS logic
   has access to a set of transmission status information for each
   available path or its belonging network interfaces.  When an
   application starts a first communication, IPS selects based on the
   available path transmission characteristics the path with the highest
   probability to succeed.











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         +-------------------+              +-------------------+
         |     Terminal      |              |      Server       |
         |  +-------------+  |              |  +-------------+  |
         |  |Application n|  |              |  |Application n|  |
         |  +-------------+  |              |  +-------------+  |
         |        |          |              |         |         |
         |  +-------------+  |              |         |         |
         |  |Initial-path |  |-------+      |         |         |
         |  |  Selection  |  |       |      |         |         |
         |  +-------------+  |       |      |         |         |
         |        |          |  +--------+  |         |         |
         |  +-------------+  |--|Internet|--|  +-------------+  |
         |  | MPTCP Stack |  |--+--------+--|  | MPTCP Stack |  |
         |  +-------------+  |              |  +-------------+  |
         +-------------------+              +-------------------+

             Figure 4: Architecture for Initial-path Selection

2.3.2.  Typical Scenarios

   Two typical RobE_IPS scenarios are presented in this section.
   Figure 5 shows that the "Initial Path Selection" logic executed for
   each MPTCP connection establishment.  On the other hand Figure 6
   describes that "Initial Path Selection" in case no path information
   are available.  Considering the fact that no heuristics are given
   before a recent MPTCP connection was established, the default initial
   path can be adopted.  Further combinations and implementations with
   more or less sophisticated heuristics are possible.























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                           +---------------+
                           |  Application  |
                           |    Request    |
                           +---------------+
                                   |
                                   V
                           +---------------+
                      +--->| Initial-path  |<---+
                      |    |   Selection   |    |
                      |    +---------------+    |
                      |            |            |
                      |            V            |Info
                      |    +---------------+    |
                      |    |  Set initial  |----+
                      |    |     path      |
                      |    |   for MPTCP   |
                      |    +---------------+
                      |            |
                      |            V
                      |    +---------------+
                      |No  |Establish MPTCP|
                      +----|  Connection   |
                           +---------------+
                                   |Yes
                                   V

            Figure 5: RobE_IPS for each connection establishment
























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                        +--------------+
                        |  Application |
                        |    Request   |
                        +--------------+
                                |
                                V
                        +--------------+Yes
                        |     First    |------------+
                        |  Connection? |            |
                        +--------------+            |
                                |No                 |
                                V                   |
                        +--------------+            V
                 +----->| Initial-path |<-+     +-------+
                 |      |   Selection  |  |     |Default|
                 |      +--------------+  |     |initial|
                 |              |         |     |  path |
                 |              |         |     +-------+
                 |              V         |Info     |
                 |      +--------------+  |         |
                 |      |  Set initial |--+         |
                 |      |     path     |            |
                 |      |   for MPTCP  |            |
                 |      +--------------+            |
                 |              |                   |
                 |              V                   |
                 |No    +--------------+            |
                 +------| Successful? |<-----------+
                        +--------------+
                                |Yes
                                V

         Figure 6: RobE_IPS using default route when no meaningful
                            heuristic available

   Figure 7 shows the process flow of "Initial Path Selection".  Upon a
   request from an application, the IPS logic will acquire transmission
   status information which represents the transmission performance of
   each available path or network interface and evaluate it.  The
   transmission status information is characterized by at least one of
   the parameters: signal strength, throughput, round-trip time (RTT)
   and link success rate.  In this way, the path with the best
   transmission performance can be determined and used for connection
   establishment.







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                                    |
                                    V
                       +---------------------------+
                       |Acquire transmission status|
                       | info for available paths  |
                       +---------------------------+
                                    |
                                    V
                       +---------------------------+
                       |   Evaluating the status   |
                       |    for available paths    |
                       +---------------------------+
                                    |No
                                    V
                       +---------------------------+
                       | Determining an available  |
                       |     path with better      |
                       |      transmission         |
                       |      performance          |
                       +---------------------------+
                                    |
                                    V
                       +---------------------------+
                       |     Using the network     |
                       |         interface         |
                       |corresponding to the path |
                       | with better transmission  |
                       |performance for connection |
                       |       establishment       |
                       +---------------------------+
                                    |
                                    V

        Figure 7: Implementation process for Initial Path Selection

2.3.3.  Path decision information

   The level of heuristic can be mainly divided into three layer:
   application level, transport-layer level and link-layer level based
   on the information acquisition method.  For example, RTT can be
   calculated for each path within a MPTCP connection and belongs
   thereof to the transport-layer level.  The transmission status
   information for each available path SHOULD be characterized by at
   least one of the parameters: signal strength, throughput, RTT and
   link success rate.  Application level information are more seen for
   statistical purposes.





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   *  Application level: application name, domain name, port number and
      location.

   *  Transport-layer level: RTT, CWND, Error rate.

2.3.4.  Initial Path Selection use local RTT information

   Figure 8 presents a "Initial Path Selection" logic based on RTT, e.g.
   assuming two paths over LTE and WiFi access.  RTT calculation on the
   transport layer usually reflect the time when an information is sent
   and an related acknowledgment received.  For an asymmetric usage
   (e.g. download only) of a communication it might happen that recent
   RTT calculation is only available on sender side which is possibly
   not the side which employs the IPS logic.  A solution for this can be
   found in Section 3.2.  Instead of using the most recent RTT value of
   a path a filtered value consisting of several measured RTTs can be
   used.  A RTT can also be derived from link layer information but may
   has a limited meaning when it does not picture the end-to-end
   latency.

                +-------------------+
                |    New Session    |
                +-------------------+
                          |
                          V
                +-------------------+ No
                |Running Connections|-----------+
                |(LTE.RTT<WiFi.RTT) |           |
                +-------------------+           |
                          |Yes                  |
                          V                     V
                +-------------------+   +----------------+
                |     Set LTE as    |   |   Set WiFi as  |
                |    initial path   |   |  initial path  |
                +-------------------+   +----------------+

               Figure 8: Initial-path Selection based on RTT

2.4.  Combination of RobE_SIM and RobE_IPS

   In an implementation, a single solution may not be sufficient to get
   an expected behavior.  Combination of approaches to improve
   robustness is recommended therefore.  Figure 9 shows the combination
   of RobE_SIM and RobE_IPS.  RobE_SIM can be used at the very beginning
   when the sender is absent of path information followed by RobE_IPS
   for consecutive connections.





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                         +--------------+
                         |  Application |
                         |    Request   |
                         +--------------+
                                 |
                                 V
                         +--------------+
                  +----->| Any data for | No
                  |      | Initial Path |----------+
                  |      |  Selection?  |          |
                  |      +--------------+          |
                  |              |                 |
                  |              V                 V
                  |      +--------------+     +--------+
                  |      | Initial-path |     |RobE_SIM|
                  |      |  Selection   |<-+  +--------+
                  |      +--------------+  |       |
                  |              |         |       |
                  |              V         |Info   |
                  |      +---------------+ |       |
                  |No    |Establish MPTCP|-+       |
                  +------|  Connection   |<--------+
                         +---------------+
                                 |
                                 V
                   No    +---------------+
                  <------|  Successful ? |
                  Network+---------------+
                  Problem        |Yes
                                 V

               Figure 9: Combination of RobE_SIM and RobE_IPS

2.5.  Combination of RobE_TIMER and RobE_IPS

   Since RobE_IPS solely does not guarantee that session can be set up
   on the selection of initial path, it can also be combined with
   RobE_TIMER which generates less overhead compared to the combination
   with RobE_SIM in Section 2.4 and guarantees session setup.
   RobE_TIMER can be introduced to optimize the control of path
   switching when the initial path selected by RobE_IPS is
   dysfunctional.  When the system enables RobE_IPS and uses the
   selected initial path for session establishment,it sets the timer for
   path switching.  When timer is expired, the system will change to
   another path to re-establish connection according to Section 2.1.






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                                +---------------+
                                |  Application  |
                                |    Request    |
                                +---------------+
                                        |
                                        V
                                +---------------+
                                |  Initial Path |
                         |----->|   Selection   |
                         |      | and Set Timer |
                         |      +---------------+
                         |              |
                         |              V
                         |Yes   +---------------+
                         +------| Timer is up?  |
                                +---------------+
                                        |No
                                        V
                                +---------------+
                                |Establish MPTCP|
                                |  Connection   |
                                +---------------+
                                        |
                                        V
                          No    +---------------+
                         <------|  Successful?  |
                         Network+---------------+
                         Problem        |Yes
                                        V

             Figure 10: Combination of RobE_Timer and RobE_IPS

3.  Implementation with Bi-directional MPTCP Support

   Solutions which requires bi-directional support between two MPTCP
   hosts promise to have better and possibly more features.  However,
   they cannot be defined without extending current standards in
   [RFC6824] and [RFC8684].  The RobE_SIM and RobE_IPS approach are both
   capable of profit from an explicit support of the remote end host and
   defined within this section.











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3.1.  Simultaneous Initial Paths Extended Version (RobE_eSIM)

   RobE_eSIM extends RobE_SIM by reusing the potential initial flows.
   This eliminates the overhead from RobE_SIM by introducing a new
   option MP_JOIN_CAP and accelerate the transmission speed by early
   availability of multiple paths.  Further it relaxes the dependency on
   a reliable third ACK of the 3-way handshake in [RFC8684].  Remote
   endpoint support can be negotiated in two ways, an implicit in
   Section 3.1.1 or explicit in Section 3.1.2.

3.1.1.  RobE_eSIM implicit Negotiation and Procedure

   Similar to RobE_SIM in Section 2.2, the establishment process of
   [RFC6824] or [RFC8684] is applied independently on multiple path
   simultaneously.  In Figure 11 this is shown in SA1 and SA2.  The
   first path which returns a SYN/ACK (e.g.  SA3) is selected as the
   initial path and proceed with the traditional establishment process
   (SA5).  Any other path which has to send the final ACK of the 3-way
   handshake includes a new option MP_JOIN_CAP (see definition in
   Section 3.1.3.2) instead of a MP_CAPABLE (SA6.2).

                   Host A                                  Host B
           ------------------------                       ----------
           Address A1    Address A2                       Address B1
           ----------    ----------                       ----------
               |             |                                |
               |            SYN + MP_CAPABLE(Key-A[*])        |
       (SA1)   |--------------------------------------------->|  (SB1)
               |             |    SYN + MP_CAPABLE(Key-A'[*]) |
       (SA2)   |             |------------------------------->|  (SB2)
               |             |                                |
       (SA3)   |<---------------------------------------------|  (SB3)
               |          SYN/ACK + MP_CAPABLE(Key-B)         |
       (SA4)   |             |<-------------------------------|  (SB4)
               |             |   SYN/ACK + MP_CAPABLE(Key-B') |
               |             |                                |
               |        ACK + MP_CAPABLE(Key-A, Key-B)        |
       (SA5)   |--------------------------------------------->|  (SB5)
               |             |                                |
       (SA6.2) |             |                                |  (SB6.2)
      RobE EXT |             | ACK + MP_JOIN_CAP(Key-A, HMAC) |
      (+fast)  |             |------------------------------->|

               [*] Key-A in the first MP-capable is related to
                   RFC6824 only and does not exist in RFC8684.

            Figure 11: MPTCP RobE_eSIM implicit Connection Setup




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   Following the possible process in Figure 11, two further
   constellations are imaginable and elaborated below.

   1.  In the flow diagram Figure 11, A1<->B1 is assumed to be the
       initial flow.  A2<->B1 shall be recycled and the ACK is sent with
       MP_JOIN_CAP.  Furthermore, the MP_CAPABLE arrives first at Host B
       (SB5) and the MP_JOIN_CAP afterwards (SB6.2).  When the
       MP_JOIN_CAP is received, Host B has to iterate over the
       connection list once (like MP_JOIN) and check for Key-A
       availability.  If a Key-A connection is found, this one is
       validated against the HMAC value.  The validation has two
       reasons: first, several Key-A can exist, because different hosts
       may choose the same Key-A by accident.  Furthermore, no one can
       join a connection by just recording/brute-forcing Key-A and
       duplicating the request.

   2.  Like above, but MP_JOIN_CAP arrives before last MP_CAPABLE at
       Host B

       *  [RFC8684]; Based on Key-A, Host B will iterate over the
          connection list, but it will not find a match, because Key-A
          of the previous selected initial flow (SA3, SA5) has not
          arrived yet.  So it will continue with a fast iteration only
          over the connections which are still in establishment phase
          using the 10 bit Key-B fast hash (crc16(Key-B) & 0x3FF).  If
          it matches against a (precomputed) existing Key-B_fast_hash in
          the connection list, it will validate the request using the
          HMAC(Key-A+B+B') to ensure legitimation.  If successful, both,
          the initial flow and the MP_JOIN_CAP flow, can be immediately
          established.  This is true, because without the knowledge of
          Key-B, Host A could not calculate the HMAC.  So it is clear,
          that Host A had received the SYN/ACK (SB3).  This also
          mitigates the exchange of a reliable ACK during the handshake
          process.  MPTCP sends the Key-A only with the last ACK and
          therefore prevents subsequent flow establishment until
          successful reception at Host B.  Using RobE_EXT, the reception
          of a MP_JOIN_CAP ([RFC8684]) is sufficient to establish both,
          the path carrying Key-B and Key-B'.

       *  [RFC6824]; Can match based on Key-A, same effort as for a MP
          JOIN.

   3.  A2<->B1 is selected as initial flow, because the respective SYN/
       ACK returns earlier at Host A.  It is the same as above, just the
       other way round.






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3.1.2.  RobE_eSIM explicit Negotiation and Procedure

   The process of an explicit negotiation of RobE_eSIM follows Figure 11
   but uses the ROBE_eSIM_EN option Figure 13 additionally during the
   handshake procedure.

               Host A                                        Host B
      ------------------------                             ----------
      Address A1    Address A2                             Address B1
      ----------    ----------                             ----------
          |             |                                        |
          |      SYN+MP_CAPABLE+ROBE_eSIM_EN(Key-A[*])           |
          |----------------------------------------------------->|
          |             | SYN+MP_CAPABLE+ROBE_eSIM_EN(Key-A'[*]) |
          |             |--------------------------------------->|
          |      SYN/ACK+MP_CAPABLE+ROBE_eSIM_EN(Key-B)          |
          |<---------------------------------------------------->|
          |             | SYN/ACK+MP_CAPABLE+ROBE_eSIM_EN(Key-B')|
          |             |<---------------------------------------|
          |      ACK+MP_CAPABLE(Key-A,Key-B)                     |
          |----------------------------------------------------->|
          |             |                                        |
          |             |  ACK+MP_JOIN_CAP(Key-A,HMAC)           |
          |             |--------------------------------------->|
          |             |                                        |

            [*] Key-A in the first MP-capable is related to
                RFC6824 only and does not exist in RFC8684.

            Figure 12: MPTCP RobE_eSIM explicit Connection Setup

3.1.3.  Protocol Adaptation

3.1.3.1.  ROBE_eSIM_EN Option

                        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
     +---------------+---------------+-------+-------+---------------+
     |     Kind      |    Length     |Subtype|    (reserved)         |
     +---------------+---------------+-------+-------+---------------+

                       Figure 13: ROBE_eSIM_EN_OPTION

3.1.3.2.  MP_JOIN_CAP Option







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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
   +---------------+---------------+-------+-------+---------------+
   |     Kind      |    Length     |Subtype|       |    ADDR_ID    |
   +---------------+---------------+-------+-------+---------------+
   |                    Sender's Key-A (64 bits)                   |
   |                                                               |
   |                                                               |
   +---------------------------------------------------------------+
   |                        HMAC (>=96 bits)                       |
   |                                                               |
   |                                                               |
   :                                                               :
   +---------------------------------------------------------------+
    Key-B_fast_hash = crc16(Key-B) & 0x3FF          -> (10bit)
    HMAC_keys =  HMAC(Key-A+Key-B+Key-B')           -> (>=96bit)
    HMAC =  (HMAC_keys & ~0x3FF) | Key-B_fast_hash  -> (size HMAC_keys)

                           Figure 14: MP_JOIN_CAP

   Computational effort on receiver side is most often expected to be
   the same as with MP_JOIN.  Key-A ensures identification of related
   flows Key-B_fast_hash enables MP session even when selected initial
   flow is not fully established yet (slight computational overhead).
   HMAC authenticates relationship of initial and potential initial
   flows.

3.1.4.  Fallback Mechanisms

3.1.4.1.  Fallback mechanism for implicit RobE_eSIM

   [TBD]

3.1.4.2.  Fallback mechanism for explicit RobE_eSIM

   This mechanism considers that both sides support MPTCP capability but
   the receiver does not equipped with RobE_eSIM.  MPTCP session with
   RobE_eSIM negotiation will seamlessly fallback to normal MPTCP
   process.

   [Requires further check how an unaware Host B reacts on possible
   ROBE_eSIM_EN; Ignore or RST?  See also RFC6824 Sec. 3.6 "Should
   fallback [...] the path does not support the MPTCP options"]








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                    Host A                              Host B
          ------------------------                    ----------
          Address A1    Address A2                    Address B1
          ----------    ----------                    ----------
              |             |                             |
              |      SYN+MP_CAPABLE+ROBE_eSIM_EN          |
              |------------------------------------------>|
              |             | SYN+MP_CAPABLE+ROBE_eSIM_EN |
              |             |---------------------------->|
              |         SYN/ACK+MP_CAPABLE                |
              |<----------------------------------------->|
              |             |    SYN/ACK+MP_CAPABLE       |
              |             |<----------------------------|
              |       ACK+MP_CAPABLE                      |
              |------------------------------------------>|
              |             |          RST                |
              |             |---------------------------->|
              |             |       SYN+MP_JOIN           |
              |             |---------------------------->|
              |             |     MP_JOIN Process...      |
              |             |                             |

        Figure 15: Fallback to MPTCP when missing RobE_eSIM support

3.1.4.3.  Fallback to regular TCP when missing MPTCP support

   When the receiver is not MPTCP enabled, MPTCP session with RobE_eSIM
   negotiation will seamlessly fallback to regular process which is
   illustrated in this section.






















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                    Host A                              Host B
          ------------------------                    ----------
          Address A1    Address A2                    Address B1
          ----------    ----------                    ----------
              |             |                             |
              |      SYN+MP_CAPABLE+ROBE_eSIM_EN          |
              |------------------------------------------>|
              |             | SYN+MP_CAPABLE+ROBE_eSIM_EN |
              |             |---------------------------->|
              |         SYN/ACK                           |
              |<----------------------------------------->|
              |             |    SYN/ACK                  |
              |             |<----------------------------|
              |           ACK                             |
              |------------------------------------------>|
              |             |          RST                |
              |             |---------------------------->|
              |             |   Regular TCP Process...    |
              |             |                             |

              Figure 16: Fallback to TCP without MPTCP support

3.1.5.  Comparison Robe_SIM and RobE_eSIM

   Potential initial flows in RobE_SIM Section 2.2 and RobE_eSIM
   Section 3.1 guarantee MPTCP session establishment if at least one
   selected path for session establishment is functional.  Figure 17
   makes the differences between both approaches visible and points to
   the latest decision possibility during session setup when RobE_SIM or
   RobE_eSIM can be selected.  Until SA5 in Figure 17 traditional MPTCP
   connection setup is independently applied on multiple paths
   simultaneously and offers to select the initial flow later (potential
   initial flows).  The final decision which path is selected as the
   main one and the handling of the remaining flow(s) differs in SA6.1
   when RobE_SIM is applied or instead SA6.2 RobE_eSIM.
















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                Host A                                  Host B
        ------------------------                       ----------
        Address A1    Address A2                       Address B1
        ----------    ----------                       ----------
            |             |                                |
            |            SYN + MP_CAPABLE(Key-A[*])        |
    (SA1)   |--------------------------------------------->|  (SB1)
            |             |    SYN + MP_CAPABLE(Key-A'[*]) |
    (SA2)   |             |------------------------------->|  (SB2)
            |             |                                |
    (SA3)   |<---------------------------------------------|  (SB3)
            |          SYN/ACK + MP_CAPABLE(Key-B)         |
    (SA4)   |             |<-------------------------------|  (SB4)
            |             |   SYN/ACK + MP_CAPABLE(Key-B') |
            |             |                                |
            |        ACK + MP_CAPABLE(Key-A, Key-B)        |
    (SA5)   |--------------------------------------------->|  (SB5)
            |             |             RST                |
    (SA6.1) |             |------------------------------->|  (SB6.1)
   RobE SIM |             |                                |
   (robust) |             |                                |
   -------------------------------------------------------------------
   RobE EXT |             |                                |
   (+fast)  |             | ACK + MP_JOIN_CAP(Key-A, HMAC) |
    (SA6.2) |             |------------------------------->|  (SB6.2)

            [*] Key-A in the first MP-capable is related to
                RFC6824 only and does not exist in RFC8684.

          Figure 17: MPTCP RobE_SIM and RobE_eSIM connection setup

3.1.6.  Security Consideration

   [Tbd, however no differences to [RFC6824] and {{RFC8684} are
   expected]

3.2.  Heuristic Initial Path Selection with remote RTT Measurement














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3.2.1.  Description

   Usually the path RTT can be determined by a time difference between
   sending a package and an ACK and is integrated into the TCP protocol.
   For asymmetric transmission, the latest RTT for TCP flows is
   calculated by the side which sends data at latest and possible does
   not correspond to the site which employs RobE_IPS.  This problem is
   already elaborated in Section 2.3.4 and can be solved by transmitting
   the RTT information per subflow.  The negotiation procedure is
   depicted in Figure 18 and uses the MPTCP option L_RTT_EN defined in
   Section 3.2.2.

                  Host A                                Host B
          ------------------------                    ----------
          Address A1    Address A2                    Address B1
          ----------    ----------                    ----------
              |             |                             |
              |           SYN+MP_CAPABLE+L_RTT_EN         |
              |------------------------------------------>|
              |         SYN/ACK+MP_CAPABLE+L_RTT_EN       |
              |<------------------------------------------|
              |               ACK+MP_CAPABLE              |
              |------------------------------------------>|
              |      ACK+DSS+L_RTT_EN(latest RTT)+Data    |
              |<------------------------------------------|
              |             |       SYN+MP_JOIN           |
              |             |---------------------------->|
              |             |     MP_JOIN Process...      |
              |             |                             |

             Figure 18: Negotiation procedure for RTT exchange

   A successful negotiation allows the exchange of the measured RTT
   value from one subflow of a MPTCP host to another using the "Latest
   RTT" field within the L_RTT_EN option.

3.2.2.  Protocol Adaptation

   Calculating the "Latest RTT" by a remote host in an asymmetry
   transmission scenario should be transferred from remote host to the
   client running RobE_IPS.  So a new MPTCP subtype option named
   L_RTT_EN is allocated for this function.  During the three-way
   handshake L_RTT_EN is used for negotiation of remote RTT measurement
   capability between client and server (in Section 3.2.1).  When both
   parts support the usage of remote RTT measurement, the "Latest RTT"
   field in L_RTT_EN is applied for carrying the value of latest RTT
   computed by the remote host.




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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
     +---------------+---------------+-------+-------+---------------+
     |     Kind      |    Length     |Subtype|     (reserved)        |
     +---------------+---------------+-------+-------+---------------+
     |                    Latest RTT (32 bits)                       |
     |                                                               |
     |                                                               |
     +---------------------------------------------------------------+

                      Figure 19: ROBE_L_RTT_EN OPTION

3.2.3.  Fallback Mechanism

   When the receiver is not L_RTT_EN capable, MPTCP session with
   L_RTT_EN negotiation will seamlessly fallback to normal MPTCP
   process.

   [TBD, Need same checks as Section 3.1.4.2]

                       Host A                           Host B
          ------------------------                    ----------
          Address A1    Address A2                    Address B1
          ----------    ----------                    ----------
              |             |                             |
              |        SYN+MP_CAPABLE+L_RTT_EN            |
              |------------------------------------------>|
              |         SYN/ACK+MPTCP_CAPABLE             |
              |<------------------------------------------|
              |           ACK+MPTCP_CAPABLE               |
              |------------------------------------------>|
              |             |         SYN+MP_JOIN         |
              |             |---------------------------->|
              |             |       MP_JOIN Process...    |
              |             |                             |

               Figure 20: Fallback to MPTCP without RobE_IPS

3.2.4.  Security Consideration

   [Tbd]

4.  IANA Considerations

   This document defines three new values to MPTCP Option Subtype as
   following.





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      +=======+==============+=======================+=============+
      | Value | Symbol       | Name                  | Reference   |
      +=======+==============+=======================+=============+
      | TBD   | ROBE_eSIM_EN | RobE_eSIM enabled     | Section 3.1 |
      +-------+--------------+-----------------------+-------------+
      | TBD   | MP_JOIN_CAP  | Join connection       | Section 3.1 |
      |       |              | directly in RobE_eSIM |             |
      +-------+--------------+-----------------------+-------------+
      | TBD   | L_RTT_EN     | Server RTT enabled    | Section 3.2 |
      +-------+--------------+-----------------------+-------------+

                      Table 2: RobE Option Subtypes

5.  References

5.1.  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>.

   [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>.

   [RFC8684]  Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
              Paasch, "TCP Extensions for Multipath Operation with
              Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
              2020, <https://www.rfc-editor.org/info/rfc8684>.

5.2.  Informative References

   [RobE_paper]
              Amend, M., Rakocevic, V., Matz, A.P., and E. Bogenfeld,
              "RobE: Robust Connection Establishment for Multipath TCP",
              ANRW '18 p. 76-82, 16 July 2018,
              <http://doi.acm.org/10.1145/3232755.3232762>.

   [RobE_slides]
              Amend, M., Matz, A.P., and E. Bogenfeld, "A proposal for
              MPTCP Robust session Establishment (MPTCP RobE)", IETF
              99 Multipath TCP WG session, 18 July 2017,
              <https://datatracker.ietf.org/meeting/99/materials/slides-
              99-mptcp-a-proposal-for-mptcp-robust-session-
              establishment-mptcp-robe-01>.





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Authors' Addresses

   Markus Amend
   Deutsche Telekom
   Deutsche-Telekom-Alle 9
   64295 Darmstadt
   Germany

   Email: Markus.Amend@telekom.de


   Jiao Kang
   Huawei
   D2-03,Huawei Industrial Base
   Shenzhen
   Guangdong, 518129
   China

   Email: kangjiao@huawei.com
































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