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Network Working Group                                          X. de Foy
Internet-Draft                                                 M. Perras
Intended status: Informational               InterDigital Communications
Expires: September 3, 2018                                   U. Chunduri
                                                              Huawei USA
                                                               K. Nguyen
                                                               M. Kibria
                                                               K. Ishizu
                                                               F. Kojima
                                                                    NICT
                                                           March 2, 2018


                Considerations for MPTCP operation in 5G
               draft-defoy-mptcp-considerations-for-5g-00

Abstract

   This document describes scenarios where the behavior of the 5G
   mobility management framework is different from earlier systems, and
   may benefit from some form of adaptation of MPTCP implementations
   and/or the 5G system.

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
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on September 3, 2018.

Copyright Notice

   Copyright (c) 2018 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



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Impact of 5G Session and Service Continuity on MPTCP  . . . .   2
     2.1.  SSC mode 1  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  SSC mode 2  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  SSC mode 3  . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  MPTCP with 5G Dual Connectivity . . . . . . . . . . . . . . .   6
   4.  Summary of Requirements and Conclusion  . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   MPTCP [RFC6824] is being deployed and widely adopted in today's smart
   devices, which typically have multiple network interfaces such as
   Cellular and Wifi.  It provides reliability, bandwidth aggregation
   capability, and handover efficiency.

   This document describes scenarios where the behavior of the 5G
   mobility management framework is different from earlier systems, and
   may benefit from some form of adaptation of MPTCP implementations
   and/or the 5G system.

2.  Impact of 5G Session and Service Continuity on MPTCP

   One of the goals of 5G [_3GPP.23.501] is to enable low latency in
   some use cases.  Mobility in the Evolved Packet System (EPS) was
   based on a central mobility solution which could hinder that goal,
   and therefore 5G uses a distributed mobility solution based on
   multiple anchors providing different IP addresses as the device moves
   from one area to another.

   The base scenario in this section is: a 5G device connected to a
   single-homed server is in an area with no usable Wifi coverage.  An
   application using MPTCP sends traffic over a single subflow, over the
   cellular air interface.  Then, as the device moves, the 5G device
   reacts to mobility events.  Additionally, we also discuss briefly



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   cases where switching from Wifi to cellular backup, and cases where
   both MPTCP peers are 5G mobile devices.

   In 5G, every unit of network service (PDU session) can have an IP
   (IPv4 or IPv6), Ethernet or unstructured type.  While session
   continuity is supported for all types, we will focus on IP-type PDU
   sessions primarily.  Different PDU sessions will typically correspond
   to distinct network interfaces on the device (though this is not
   explicit in the standard, and some implementations may possibly
   behave differently).

   In the EPS, session continuity was enabled by having the P-GW and IP
   address of the mobile device's PDU session maintained over time, even
   when the device moved around.  In 5G, different types of session
   continuity can be provided, and are indicated by a "Session and
   Service Continuity" (SSC) mode value of 1, 2 or 3 (defined in
   [_3GPP.23.501] section 5.6.9).  Every PDU session is associated with
   a single SSC mode, which cannot be changed on this PDU session.  The
   following sub-sections will study how 5G handles each SSC mode, and
   potential effects on MPTCP.

   In 5G multiple applications running on a device may end up using
   different PDU sessions (e.g. different network interfaces), for
   example because they require different network slices or SSC modes.
   It is therefore important for an MPTCP implementation to be aware of
   which network interfaces are available to which local applications.
   This mapping information will be known to the 5G stack, and can be
   made available to an MPTCP implementation running on the device.

2.1.  SSC mode 1

   In SSC mode 1 the same network anchor is kept regardless of device
   location.  An application running on the device will therefore be
   able to keep using the same IP address on the same interface.

   Additionally, in SSC mode 1, the network may decide to add and
   remove, dynamically, additional network anchors (and therefore IP
   addresses) to the PDU session, while always keeping the initial one.

   The MPTCP stack will therefore be able to create new subflows and
   benefit from a potentially shorter path, when the device is far from
   its initial network anchor, with the caveats that those additional
   subflows will be available on a temporary basis only.  MPTCP must not
   close the initial subflow in this SSC mode, since this is the only
   one guaranteed to be maintained over time.






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2.2.  SSC mode 2

   SSC mode 2 has a break-before-make behavior.  When the device leaves
   the service area of its first network anchor, the network stops using
   it and starts using a new second network anchor closer to the device.
   (Such service areas may have a highly variable size depending on
   network deployments.)  On the device, this can result in the
   currently used network interface being brought down, and after a
   short time a new network interface being brought up.  The time
   between these 2 events is not standardized and implementation
   dependent.

   Break-before-make within cellular technology

      When MPTCP is active on cellular only, this break-before-make
      behavior is similar to the existing break-before-make capability
      usually used in cellular/Wifi offload (section 3.1.3 of [RFC6897]
      and section 2.2 of [RFC8041]).  A similar MPTCP behavior is
      therefore needed: wait for a given time for a new IP address to be
      configured.  As per [RFC6897], to benefit from this MPTCP
      resilience feature, the application should not request using a
      specific network interface.

   Cellular and Wifi

      Additionally, when Wifi is active and cellular is used as backup,
      MPTCP implementations should also support this break-before-make
      behavior to maintain a usable backup IP address on cellular.  In
      rare cases where a switch-to-cellular backup would be needed
      during a break-before-make transition on cellular, MPTCP's
      existing break-before-make capability can ensure MPTCP waits for a
      new cellular-facing IP address to be available.

2.3.  SSC mode 3

   SSC mode 3 has a make-before-break behavior.  When the device leaves
   the service area of its first network anchor, the network selects a
   second network anchor closer to the device, and either creates a new
   PDU session (i.e. new IP address on new network interface) or share
   the existing PDU session (i.e. new IP address on same network
   interface).  The first network anchor keeps being used for a given
   time period, which is communicated to the device by the network using
   the "valid lifetime" field of a prefix information option in a router
   advertisement ([RFC4861], [RFC4862]).  5G does not mandate a specific
   range for this valid lifetime.  The first/older IP address should not
   be used to create any new traffic ([RFC4862] section 5.5.4).  In some
   implementations, the network (SMF) may decide to release the first
   network anchor as soon as it stops carrying traffic.



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   There is no limit set by the 5G standard for the number of
   concurrently used network anchors.  We expect that in usual cases the
   first network anchor will be released before a third network anchor
   starts being used.  Nevertheless, to our knowledge nothing prevents a
   5G system deployment to allow a third network anchor to be selected
   while the first one is still in use.

   On the 5G device, when using SSC mode 3, mobility will therefore
   result in a new IP address being configured, either on the same
   network interface initially used, or on a different interface.  In
   general an application will see a single cellular-facing IP address,
   and during transient phase it will see 2 IP addresses (with a
   possibility for more than 2 concurrent IP addresses on some 5G system
   implementations).  In cases where the server is single-homed and the
   Wifi interface is down, and assuming a full-mesh path manager policy,
   there will be in general one subflow, and from time to time,
   temporarily 2 subflows (or more on some 5G systems).  In cases where
   two mobile 5G devices are communicating with each other over MPTCP
   and with the same assumptions on Wifi and path manager policy, there
   will be in general one subflow, and from time to time, temporarily 2
   or even more rarely 4 subflows (again, possibly more on some 5G
   systems).

   MPTCP must create new subflows when a new IP address on a same or a
   new cellular-facing network interface becomes available to the
   application.  MPTCP may keep using the first subflow during a
   transient phase.  Here are some considerations related to this
   transient phase:

   o  When compared with simply waiting for the first IP address to be
      brought down, ramping down usage of the first subflow will not
      incur inefficiencies from resending lost segments.  This may
      especially help low-latency applications by avoiding throughput
      drop.

   o  Assuming a lowest-rtt-first scheduling policy is used, after the
      initial TCP slow start, the shortest path subflow should typically
      carry the most traffic.  Ramping down should ideally start after
      the initial slow start is over.

   o  To make sure the ramping down completes before the interface is
      brought down by the network, the MPTCP stack should be aware of
      how long will the first network anchor be kept in use, e.g.
      through configuration or communication with the local 5G stack.

   o  Ramping down and closing flows on the first network anchor as soon
      as possible will help recycling network resources more rapidly.




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      This is especially true in cases where more than 2 network anchors
      may be used concurrently.

   o  There may be some level of contention between subflows during the
      transient phase, since they share the same air interface, and
      especially if they share the same PDU session and QoS marking.
      The shortest path subflow may therefore not reach its full
      capacity during the transient phase.

   o  Additionally, the shortest subflow must not be closed during the
      transient phase (even if it is less efficient for some reason), to
      avoid losing all connectivity at the end of the transient phase.
      To avoid this issue, the MPTCP stack could for example follow a
      policy not to close any subflow created using the latest IP
      address, during the transient period (in SSC mode 3).

   In cases where cellular is used for backup, there is a possibility
   that the switch to using backup occurs during a transient phase.  To
   support this case, MPTCP should keep creating and releasing subflows
   as described above, even when cellular subflows are used as backup,
   to ensure that the backup is always usable.  When a backup event
   occurs during a transient phase, MPTCP should use the subflows
   associated with the most recent cellular-facing IP address, i.e.
   corresponding to the latest/closest network anchor.

3.  MPTCP with 5G Dual Connectivity

   One of the key features of 5G [_3GPP.23.501] is dual connectivity
   (DC).  With DC, a 5G device can be served by two different base
   stations.  DC may play an essential role in leveraging the benefit of
   5G new radio, especially in the evolving architecture with the
   coexistence of 4G and 5G radios.

   On a 5G device with DC, an application is able to send data to the
   destination (e.g., a single-home server) through multiple radio
   links, over one or more PDU sessions.  Some PDU sessions may be over
   a single radio link, while others may have flows over each radio
   link.  Therefore, in a first case, DC can be made visible to
   applications through different IP addresses, while in a second case,
   DC can be used by different flows terminated at the same IP address
   on the device.

   In any of those cases, the issues of out of order delivery and
   diverse latency values need to be supported in DC.  However, such
   reliable communication scenarios have not been addressed in the
   current DC architecture.  Based on the design history of DC in
   earlier systems, the 5G system will need to incorporate features to
   support robustness/reliability (e.g. backup and duplication), that



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   will likely result in added complexity.  On the other hand, in order
   to benefit the most from DC, the 5G device is expected to send/
   receive an optimal amount of traffic over each radio link (e.g., for
   the sake of minimizing overall latency).  Hence, the device needs to
   select dynamically the most suitable path for a given radio
   condition.  Additionally, algorithms for shifting, based on
   congestion, ongoing traffic between paths are also necessary.

   MPTCP, which includes path manager, scheduler, and congestion control
   functions, shows a lot of potential to address the aforementioned
   issues.  MPTCP could therefore be integrated with DC and the 5G
   protocol stack, as an alternative to developing 5G-specific
   solutions.  As part of this integration, the MPTCP stack should be
   aware of the presence of multiple radio links, whether they are
   exposed using multiple IP addresses or hidden under a single IP
   address.  MPTCP's scheduler should optimally partition traffic or
   duplicate a data flow over different links, depending on the
   application's need, network policy and conditions.

4.  Summary of Requirements and Conclusion

   With regards to 5G session continuity mechanism, MPTCP stack behavior
   (including path manager and scheduling) should be updated to achieve
   optimal performance.  As a summary:

   o  MPTCP should obtain information from the local 5G stack (SSC mode,
      mapping between interfaces and applications, valid lifetime on
      first network anchor in SSC mode3)

   o  In SSC mode 3 during the transient period following a mobility
      event, MPTCP should gracefully stop using old cellular-facing
      interface(s), and must not release subflow(s) using the latest
      cellular-facing IP address.

   o  In SSC mode 1 MPTCP must not close the initial subflow.

   o  When cellular is used as backup, MPTCP should actively maintain
      the backup path in SSC mode 2 and 3.

   With regards to dual connectivity, MPTCP can be closely integrated
   with the 5G stack to avoid duplicating its feature in 5G.  As a
   summary:

   o  MPTCP should be aware of the presence of multiple DC radio links,
      which may be exposed as a single or distinct network interfaces/IP
      addresses.





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   o  MPTCP should optimally partition traffic or duplicate a data flow
      over DC links, depending on the application's need, network policy
      and conditions.

5.  IANA Considerations

   This document requests no IANA actions.

6.  Security Considerations

   No new security considerations are identified at this time.

7.  Acknowledgements

   The following people contributed to the present document:

   o  Debashish Purkayastha

   o  Akbar Rahman

   o  Ulises Olvera-Hernandez

8.  Informative References

   [_3GPP.23.501]
              3GPP, "System Architecture for the 5G System", 3GPP
              TS 23.501 1.4.0, 9 2017,
              <http://www.3gpp.org/ftp/Specs/html-info/23501.htm>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [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>.

   [RFC6897]  Scharf, M. and A. Ford, "Multipath TCP (MPTCP) Application
              Interface Considerations", RFC 6897, DOI 10.17487/RFC6897,
              March 2013, <https://www.rfc-editor.org/info/rfc6897>.




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   [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>.

Authors' Addresses

   Xavier de Foy
   InterDigital Communications, LLC
   1000 Sherbrooke West
   Montreal
   Canada

   Email: Xavier.Defoy@InterDigital.com


   Michelle Perras
   InterDigital Communications, LLC
   Montreal
   Canada

   Email: Michelle.Perras@InterDigital.com


   Uma Chunduri
   Huawei USA
   2330 Central Expressway
   Santa Clara, CA  95050
   USA

   Email: uma.chunduri@huawei.com


   Kien Nguyen
   National Institute of Information and Communications Technology
   YRP Center No.1 Building 7F, 3-4 Hikarinooka, Yokosuka
   Kanagawa  239-0847
   Japan

   Email: kienng@nict.go.jp











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   Mirza Golam Kibria
   National Institute of Information and Communications Technology
   YRP Center No.1 Building 7F, 3-4 Hikarinooka, Yokosuka
   Kanagawa  239-0847
   Japan

   Email: mirza.kibria@nict.go.jp


   Kentaro Ishizu
   National Institute of Information and Communications Technology
   YRP Center No.1 Building 7F, 3-4 Hikarinooka, Yokosuka
   Kanagawa  239-0847
   Japan

   Email: ishidu@nict.go.jp


   Fumihide Kojima
   National Institute of Information and Communications Technology
   YRP Center No.1 Building 7F, 3-4 Hikarinooka, Yokosuka
   Kanagawa  239-0847
   Japan

   Email: f-kojima@nict.go.jp


























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