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SPRING and DMM                                             S. Matsushima
Internet-Draft                                                  SoftBank
Intended status: Standards Track                             C. Filsfils
Expires: January 18, 2018                            Cisco Systems, Inc.
                                                           July 17, 2017


                       SRv6 for Mobile User-Plane
           draft-matsushima-spring-dmm-srv6-mobile-uplane-01

Abstract

   This document discusses the applicability of SRv6 (Segment Routing
   IPv6) to user-plane of mobile networks that SRv6 source routing
   capability with its programmability can fulfill the user-plane
   functions, such as access and anchor functions.  It takes advantage
   of underlying layer awareness and flexibility to deploy user-plane
   functions that enables optimizing data-path for applications.
   Network slicing and an interworking way between SRv6 and existing
   mobile user-plane are also discussed in this document.

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
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   This Internet-Draft will expire on January 18, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   2
   3.  Motivations . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Mobile User-Plane . . . . . . . . . . . . . . . . . . . . . .   3
   5.  User-Plane Functions  . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Access Node Segment . . . . . . . . . . . . . . . . . . .   5
     5.2.  Layer-2 Anchor Segment  . . . . . . . . . . . . . . . . .   6
     5.3.  Layer-3 Anchor Segment  . . . . . . . . . . . . . . . . .   6
     5.4.  Stateless Interworking Segment  . . . . . . . . . . . . .   7
     5.5.  Rate Limit Segment  . . . . . . . . . . . . . . . . . . .   8
   6.  Network Slicing Considerations  . . . . . . . . . . . . . . .   8
   7.  Control Plane Considerations  . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   In mobile networks, mobility management systems provide connectivity
   while mobile nodes move around.  While the control-plane of the
   system signals movements of a mobile node, user-plane establishes
   tunnel between the mobile node and anchor node over IP based backhaul
   and core networks.

   This document discusses the applicability of SRv6 (Segment Routing
   IPv6) to those mobile networks.  SRv6 provides source routing to
   networks where operators can explicitly indicate route for the
   packets from and to the mobile node.  SRv6 endpoint nodes act as
   roles of anchor of mobile user-plane.

2.  Conventions and Terminology

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






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   All segment routing and SRv6 network programming terms are defined in
   [I-D.ietf-spring-segment-routing] and
   "[I-D.filsfils-spring-srv6-network-programming].

3.  Motivations

   Today's and future applications are requiring highly optimized data-
   path between mobile nodes and the entities of those applications in
   perspectives of latency, bandwidth, etc,. However current
   architecture of mobile management is agnostic about underlying
   topologies of transport layer.  It rigidly fragments the user-plane
   in radio access, core and service networks and connects them by
   tunneling techniques through the user-plane functions such as access
   and anchor nodes.  Those agnostic and rigidness make it difficult for
   the operator to optimize the data-path.

   While the mobile network industry has been trying to solve that,
   applications shift to use IPv6 data-path and network operators adopt
   it as their IP transport as well.  SRv6 integrates both application
   data-path and underlying transport layer in data-path optimization
   aspects that does not require any other techniques.

   SRv6 source routing capability with programmable functions
   [I-D.filsfils-spring-srv6-network-programming] could fulfills the
   user-plane functions of mobility management.  It takes advantage of
   underlying layer awareness and flexibility to deploy user-plane
   functions.  Those are the motivations to adopt SRv6 for mobile user-
   plane.

4.  Mobile User-Plane

   This section describes user-plane using SRv6 for mobile networks.
   This clarifies mobile user-plane functions to which SRv6 endpoint
   applied.

   Figure 1 shows mobile user-plane functions which are connected
   through SRv6 enabled IPv6 networks.  In the Figure 1, an mobile node
   (MN) connects to an SRv6 endpoint serving access node role for the
   MN.  When the endpoint receives packets from the MN, it pushes SRH to
   the packets.  The segment list in the SRH indicates the rest of user-
   plane segments which are L2 and L3 anchors respectively.  Then the
   endpoint send the packets to the SRv6 enabled IPv6 network.  In
   opposite direction, when an SRv6 endpoint serving L3 anchor role for
   the MN receives packets to it, the endpoint push SRH consist of the
   L2 anchor and access node segments to the packets.






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                                User-plan
                                Function
                               <L2 Anchor>
                                O------O
                                | SRv6 |
                                | End  |
                                | Point|
                                O------O
              User-plan            ||           User-plan
    [MN]      Function        _____||_____      Function
      |     <Access Node>    /            \    <L3 Anchor>
   ___v___     O------O     /              \     O------O     ________
  /  Radio \   | SRv6 |    /      SRv6      \    | SRv6 |    /        \
 /  Access  \==| End  |===/      enabled     \===| End  |===/ Service  \
 \    NW    /  | Point|   \       IPv6       /   | Point|   \    NW    /
  \________/   O------O    \       NW       /    O------O    \________/
                            \              /
                             \____________/


                   Figure 1: Mobile User-plane with SRv6

   An SRv6 segment represents those each function, such as Access Node,
   Layer-2 (L2) Anchor and Layer-3 (L3) Anchor.  This makes mobile
   networks highly flexible to deploy any user-plane functions to which
   nodes in user flow basis.  An SRv6 segment can represent a set of
   flows in any granularity of aggregation even though it is just for a
   single flow.

   Figure 2 shows that an SRv6 endpoint connects existing IPv4 mobile
   user-plane, which is defined in [RFC5213] and [TS.29281].  An SRv6
   segment in the endpoint represents interworking function which
   enables interworking between existing access node and SRv6 anchor
   segment, or SRv6 access node segment and existing anchor node.

   Existing mobile user-plane with IPv6 underlay is expected to be
   widely deployed.  As IPv6 network should be interoperable with SRv6
   enabled network and SRv6 endpoints can be accommodated on it,
   interworking with existing IPv6 network is out of scope of this
   document.











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     ________                                                 _______
    /        \    O------O                                   /       \
   / Service  \===|L2/L3 |                                  / Service \
   \    NW    /   |Anchor|         User-plan                \    NW   /
    \________/    |Node  |         Function                  \_______/
                  O------O       <Interworking>                  ||
                      \\_______     O------O     ________     O------O
                      /        \    | SRv6 |    /  SRv6  \    | SRv6 |
                     / Existing \===| End  |===/  enabled \===| End  |
                     \  IPv4 NW /   | Point|   \  IPv6 NW /   | Point|
       [MN]           \________/    O------O    \________/    O------O
        |             //                                         ||
     ___v____    O------O                                     ___||__
    / Radio  \   |      |                                    / Radio  \
   /  Access  \==|Access|                            [MN]~~~/  Access  \
   \    NW    /  |Node  |                                   \    NW    /
    \________/   O------O                                    \________/





           Figure 2: Interworking with Existing Mobile Networks

   The detail of SRv6 segments representing user-plane functions are
   described in Section 5.

5.  User-Plane Functions

   This section describes mobile user-plane functions applied to SRv6
   segment.  SRv6 endpoint is capable to do that.  Terminology of
   endpoint functions refers to the net-pgm draft.  SRv6 endpoint
   functions are considered for each direction, such as uplink (UL) from
   mobile node to the correspondent node, and downlink (DL) from the
   correspondent node to mobile node.

5.1.  Access Node Segment

   Access node segment provides SRv6 endpoint the role to which mobile
   node is connected directly. eNodeB could be referenced as an entity
   implementing the access node in 3GPP term.  The applicable SRv6
   functions for the access nodes are following:

   UL:  T.Insert, or T.Encaps

   DL:  End.X, or End.DX2/4





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   In uplink, an endpoint of an access node segment does T.Insert
   process for the receiving packets from mobile node when the packets
   matched to the policy bound to the segment in case IPv6 is the outer
   header of the packet.  When the header is IPv4, or other types, the
   endpoint does T.Encap process for the packets.  The segment list
   represents policy of data-path for the flows from mobile node.

   In downlink, the endpoint does End.X process for receiving packets
   when the active segment represents access node to the mobile node.
   When the segment left of receiving packets is zero, and IPv4 is
   indicated as the effective next header (ENH), the endpoint does
   End.DX4 process for the packets.  When the ENH indicates layer-2
   type, the endpoint does End.DX2 process for it.

5.2.  Layer-2 Anchor Segment

   Layer-2 anchor segment provides SRv6 endpoint the role to be anchor
   point while mobile node move around within a serving area which could
   be assumed as a layer-2 network.  Serving Gateway (SGW) could be
   referenced as an entity implementing the layer-2 anchor in 3GPP term.
   The applicable SRv6 functions for the layer-2 anchor are following:

   UL:  End, or End.B6

   DL:  Same as UL

   In uplink, an endpoint of a L2 anchor segment does End process for
   the packets when the active segment represents the L2 anchor.  When
   the segment is bound to another policy of data-path, the endpoint
   does End.B6 process for the packets to be inserted a SRH which
   consists of segment list along with the policy.  The last segment of
   the inserted SRH must be the edge endpoint of the SRv6 domain of
   mobile network.

   In downlink, there's no difference between the uplink behavior.

5.3.  Layer-3 Anchor Segment

   Layer-3 anchor segment provides SRv6 endpoint the role to be anchor
   point across a mobile network consists of multiple serving areas.
   Packet data network gateway (PGW) could be referenced as an entity
   implementing the layer-3 anchor.  The applicable SRv6 functions for
   the layer-3 anchor are following:

   UL:  End.T, End.DT4, or End.DX2

   DL:  T.Insert, or T.Encaps




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   In uplink, an endpoint of a L3 anchor segment does End.T process for
   the packets when the active segment represents the L3 anchor and
   specific IPv6 routing table is bound to it.  Those routing tables may
   be accommodated in the endpoint for networks which require individual
   routing policies.  When the segment left of receiving packet is zero
   and IPv4 is indicated as the effective next header (ENH), the
   endpoint does End.DT4 process for the packets with specific IPv4
   routing table bound to the segment.  When the ENH indicates layer-2
   type, the endpoint does End.DX2 process for it.

   In downlink, an endpoint of a L3 anchor segment does T.Insert process
   for the packets when the IPv6 flow of packets matched to the policy
   bound to the segment.  When the matched flow is IPv4, or other types,
   the endpoint does T.Encap process for the packets.  The segment list
   represents policy of data-path for the flows.

5.4.  Stateless Interworking Segment

   Interworking segment provides SRv6 endpoint the role to be
   interworking point between existing mobile user-plane and SRv6 mobile
   user-plane.  It is expected that the endpoint of interworking segment
   could be unaware from the control-plane of the mobility management.
   While there are combinations of interworking either existing or SRv6
   network in which user-plane functions accommodate, interworking
   segment should cover all combinations without mobility state.

   To fulfill the above requirements, SID for interworking segment is
   encoding identifiers of corresponding tunnel in existing network as
   argument of the SID.  Tunnel encoding format in SID is following:


              +----------------------+------+-------+-------+
              |Locater of interwork  |  DA  |  SA   | Tun-ID|
              +----------------------+------+-------+-------+
                      128-a-b-c          a      b       c


               Figure 3: Stateless Interworking SID Encoding

   In SRv6 to existing network direction, an endpoint of an interworking
   segment allocate a node local SID prefix to interworking segments.
   When the endpoint receives packet and the active segment of the
   packet indicates the SID, the endpoint pop the SRH of the SID, and
   then the endpoint encaps the payload with the encoded information in
   the SID which are tunnel identifier of tunnel header, source and
   destination IPv4 address of IPv4 header described in Figure 3.  Then
   the endpoint send out the packet to the existing network along with
   its routing policy.



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   In existing network to SRv6 network direction, existing network
   allocates IPv4 address spaces routed to interworking SRv6 network.
   SRv6 network allocates a domain-wise SID prefix for interworking
   segments.  When a SRv6 endpoint connects to existing network receives
   packet destined to the allocated IPv4 address, the endpoint decaps
   outer IPv4 and tunnel header.  And then the endpoint does T.Insert
   process with the SID which consists of the allocated domain-wise SID
   prefix, source and destination addresses, and tunnel identifier as
   described in Figure 3.  Then the endpoint send out the packet to the
   SRv6 network along with its routing policy.

   In case of IPv4 flow packet over the user-plane, the endpoint does
   IPv6 header encaps and decaps instead of SRH insert and pop process.
   The IPv6 header includes interworking segment SID in the SRH.

   Noted that to make sure stateless interworking, entities of control-
   plane in mobile management should cooperate with SRv6 user-plane
   settings.  Further control-plane consideration is discussed in
   Section 7.

5.5.  Rate Limit Segment

   Mobile user-plane requires rate-limit feature.  SID is able to encode
   limiting rate as an argument in SID.  Multiple flows of packets
   should have same group identifier in SID when those flows are in an
   same AMBR group.  This helps to keep user-plane stateless.  That
   enables SRv6 endpoint nodes which are unaware from the mobile
   control-plane information.  Encoding format of rate limit segment SID
   is following:


              +----------------------+----------+-----------+
              | Locater of rate-limit| group-id | limit-rate|
              +----------------------+----------+-----------+
                        128-i-j            i          j


               Figure 4: Stateless Interworking SID Encoding

6.  Network Slicing Considerations

   Mobile network may be required to create a network slicing that
   represent a set of network resources and isolate those resource from
   other slices.  User-plane functions represented as SRv6 segments
   would be part of a slice.

   To represent a set of user-plane function segments for a slice,
   sharing same prefix through those SIDs within the slice could be a



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   straightforward way.  SIDs in a network slice may include other type
   of functions in addition to the mobile user-plane functions described
   in this document, and underlay integration to meet SLA and quality
   requirements.

   While network slicing has been discussed in the IETF and other
   standardization bodies, what functionalities are required for network
   slicing in mobile user-plane is further study item and to be
   discussed.

7.  Control Plane Considerations

   Mobile control-plane entities must allocate SIDs to user-plane
   function segments in case of those entities are distributed to
   accommodate in the SRv6 endpoints, or those are separated from the
   endpoint but each of them corresponds to each SRv6 endpoint.  In
   latter case, control-plane entity must advertise allocated SID to the
   endpoint through some means.

   When a centralized controller interfaces to mobile control-planes is
   capable to allocate SIDs to the controlling SRv6 endpoints, the
   mobile control-planes just need to indicate the endpoint nodes and
   their user-plane roles to the controller.  In this case, the
   controller must allocate appropriate SIDs for the user-plane roles to
   the indicated SRv6 endpoints.  The controller must advertise
   allocated SIDs to the endpoints.

   To indicate endpoints and their user-plane functions from mobile
   control-plane to user-plane, the endpoint or the controller could
   take advantage of [I-D.ietf-dmm-fpc-cpdp].  It provides interface to
   the control-plane to manage the user-plane of mobile networks.

   In case of stateless interworking, SID allocating entity needs to be
   aware SID prefix which interworking SRv6 endpoint and SRv6 domain
   allocate discussed in Section 5.4.  The mobile control-plane also
   need to allocate tunnel endpoint IPv4 address to which corresponding
   interworking segment destined from existing user-plane that is also
   discussed in Section 5.4.

8.  Security Considerations

   TBD

9.  IANA Considerations

   This document has no actions for IANA.





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10.  References

10.1.  Normative References

   [I-D.filsfils-spring-srv6-network-programming]
              Filsfils, C., Leddy, J., daniel.voyer@bell.ca, d.,
              daniel.bernier@bell.ca, d., Steinberg, D., Raszuk, R.,
              Matsushima, S., Lebrun, D., Decraene, B., Peirens, B.,
              Salsano, S., Naik, G., Elmalky, H., Jonnalagadda, P.,
              Sharif, M., Ayyangar, A., Mynam, S., Henderickx, W.,
              Bashandy, A., Raza, K., Dukes, D., Clad, F., and P.
              Camarillo, "SRv6 Network Programming", draft-filsfils-
              spring-srv6-network-programming-01 (work in progress),
              June 2017.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
              and R. Shakir, "Segment Routing Architecture", draft-ietf-
              spring-segment-routing-12 (work in progress), June 2017.

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

10.2.  Informative References

   [I-D.ietf-dmm-fpc-cpdp]
              Matsushima, S., Bertz, L., Liebsch, M., Gundavelli, S.,
              Moses, D., and C. Perkins, "Protocol for Forwarding Policy
              Configuration (FPC) in DMM", draft-ietf-dmm-fpc-cpdp-07
              (work in progress), March 2017.

   [RFC5213]  Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
              Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
              RFC 5213, DOI 10.17487/RFC5213, August 2008,
              <http://www.rfc-editor.org/info/rfc5213>.

   [TS.29281]
              3GPP, , "General Packet Radio System (GPRS) Tunnelling
              Protocol User Plane (GTPv1-U)", 3GPP TS 29.281 10.3.0,
              September 2011.

Authors' Addresses







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   Satoru Matsushima
   SoftBank
   Tokyo
   Japan

   Email: satoru.matsushima@g.softbank.co.jp


   Clarence Filsfils
   Cisco Systems, Inc.
   Belgium

   Email: cf@cisco.com






































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