BGP Update for 5G Edge Computing Service Metadata
draft-dunbar-idr-5g-edge-compute-app-meta-data-04

Network Working Group                                   L. Dunbar
Internet Draft                                          Futurewei
Intended status: Standard                             K. Majumdar
Expires: June 10, 2022                                  CommScope
                                                          H. Wang
                                                           Huawei
                                                December 10, 2021

         BGP Update for 5G Edge Computing Service Metadata
         draft-dunbar-idr-5g-edge-compute-app-meta-data-04

Abstract
   This draft describes a new AppMetaData subTLV carried by
   Tunnel Encap[RFC9012] Path Attribute for egress router to
   advertise the running status and environment for the directly
   attached 5G Edge Computing (EC) servers. The AppMetaData can
   be used by the ingress routers in the 5G Local Data Network to
   make path selection not only based on the routing distance but
   also the running environment of the destinations. The goal is
   to improve latency and performance for 5G EC services.

   The extension enables an EC server at one specific location to
   be more preferred than the others with the same IP address to
   receive data flows from a specific source (UE).

Status of this Memo

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Table of Contents

   1. Introduction.............................................. 3
      1.1. 5G Edge Computing Background......................... 3
      1.2. 5G Edge Computing Network Properties................. 4
      1.3. Problem#1: ANYCAST in 5G EC Environment.............. 6
      1.4. Problem #2: Unbalanced Anycast Distribution due to UE
      Mobility.................................................. 7
      1.5. Problem 3: Application Server Relocation............. 7
   2. Conventions used in this document......................... 8
   3. Usage of App-Meta-Data for 5G Edge Computing.............. 9
      3.1. Assumptions.......................................... 9

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      3.2. IP Layer Metrics to Gauge Application Behavior....... 9
      3.3. AppMetaData Constrained Optimal Path Selection...... 10
      3.4. BGP Protocol Extension to advertise Load & Capacity. 11
      3.5. Ingress Node BGP Path Selection Behavior............ 12
         3.5.1. AppMetaData Influenced BGP Path Selection...... 12
         3.5.2. Forwarding Behavior............................ 12
         3.5.3. Forwarding Behavior after a UE moving to a new 5G
         Site.................................................. 13
   4. The Sub-TLVs for App-Meta-Data........................... 14
      4.1. Load Measurement sub-TLV format..................... 14
      4.2. Capacity Index sub-TLV format....................... 15
      4.3. The Site Preference Index sub-TLV format............ 16
   5. AppMetaData Propagation Scope............................ 16
   6. Metrics Change Rate Consideration........................ 16
   7. Soft Anchoring of an ANYCAST Flow........................ 17
   8. Manageability Considerations............................. 18
   9. Security Considerations.................................. 19
   10. IANA Considerations..................................... 19
   11. References.............................................. 19
      11.1. Normative References............................... 19
      11.2. Informative References............................. 20
   12. Acknowledgments......................................... 20

1. Introduction

   This document describes a new subTLV, AppMetaData, for egress
   routers to advertise the running status and environment for
   the directly attached Edge Computing (EC) servers. The
   AppMetaData can be used by the ingress routers in the 5G Local
   Data Network to make path selection not only based on the
   routing distance but also the running environment of the
   destinations. The goal is to improve latency and performance
   for 5G Edge Computing services.

 1.1. 5G Edge Computing Background

   In 5G Edge Computing (EC), one Application can be hosted on
   multiple Application Servers in different EC data centers that
   are close in proximity. The network connecting the EC data
   centers with the 5G Base stations consists of small number of
   routers dedicated for the 5G Local Data Network (LDN), to
   minimize latency and optimize the user experience.

   When a User Equipment (UE) initiates application packets using
   the destination address from a DNS reply or its cache, the

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   packets from the UE are carried in a PDU session through 5G
   Core [5GC] to the 5G UPF-PSA (User Plan Function - PDU Session
   Anchor). The UPF-PSA decapsulates the 5G GTP outer header and
   forwards the packets from the UEs to its directly connected
   Ingress router of the 5G LDN. The LDN for 5G EC, which is the
   IP Networks from the 5GC perspective, is responsible for
   forwarding the packets to the intended destinations.

   When the UE moves out of coverage of its current gNB (next-
   generation Node B)and anchors to a new gNB, the 5G SMF
   (Session Management Function) could select the same UPF or a
   new UPF for the UE per standard handover procedures described
   in 3GPP TS 23.501 and TS 23.502. If the UE is anchored to a
   new UPF-PSA when the handover process is complete, the packets
   to/from the UE is carried by a GTP tunnel to the new UPF-PSA.
   Per TS 23.501-h20 Section 5.8.2, the UE may maintain its IP
   address when anchored to a new UPF-PSA unless the new UFP-PSA
   belongs to different mobile operators. 5GC may maintain a path
   from the old UPF to the new UPF for a short time for the SSC
   [Session and Service Continuity] mode 3 to make the handover
   process more seamless.

  1.2. 5G Edge Computing Network Properties

   In this document, 5G Edge Computing Network refers to multiple
   Local IP Data Networks (LDN) in one region that interconnect
   the Edge Computing data centers. Those IP LDN networks are the
   N6 interfaces from 3GPP 5G perspective.

   The ingress routers to the 5G Edge Computing Network are the
   routers directly connected to 5G UPFs. The egress routers to
   the 5G Edge Computing Network are the routers that have a
   direct link to the Edge Computing servers. The servers and the
   egress routers are co-located. Some of those Edge Computing
   Data centers may have Virtual switches or Top of Rack switches
   between the egress routers and the servers. But transmission
   delay between the egress routers and the Edge Computing
   servers is too small to be considered in this document.

   When one EC data center has multiple EC Servers attached to
   one App Layer Load Balancer, only the App Layer Load Balancer

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   is visible to the 5G Edge Computing Network. How the App Layer
   Load balancer manages the individual servers is out of the
   scope of the network layer.

   The 5G EC Services are specially managed services optimized by
   utilizing the network topology and multiple servers with the
   same IP address (ANYCAST) in multiple EC Data Centers. Many
   services by the UEs are not part of the registered 5G EC
   Services.

   +--+
   |UE|---\+---------+                 +------------------+
   +--+    |  5G     |     +--------+  |   S1: aa08::4450 |
   +--+    | Site +--+-+---+        +----+                |
   |UE|----|  A   |PSA1| Ra|        | R1 | S2: aa08::4460 |
   +--+    |      +----+---+        +----+                |
  +---+    |         |  |           |  |   S3: aa08::4470 |
  |UE1|---/+---------+  |           |  +------------------+
  +---+                 |IP Network |       L-DN1
                        |(3GPP N6)  |
     |                  |           |  +------------------+
     | UE1              |           |  |  S1: aa08::4450  |
     | moves to         |          +----+                 |
     | Site B           |          | R3 | S2: aa08::4460  |
     v                  |          +----+                 |
                        |           |  |  S3: aa08::4470  |
                        |           |  +------------------+
                        |           |      L-DN3
   +--+                 |           |
   |UE|---\+---------+  |           |  +------------------+
   +--+    |  5G     |  |           |  |  S1: aa08::4450  |
   +--+    | Site +--+--+---+       +----+                |
   |UE|----|  B   |PSA2| Rb |       | R2 | S2: aa08::4460 |
   +--+    |      +--+-+----+       +----+                |
   +--+    |         |  +-----------+  |  S3: aa08::4470  |
   |UE|---/+---------+                 +------------------+
   +--+                                     L-DN2
            Figure 1: App Servers in different edge DCs

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   1.3. Problem#1: ANYCAST in 5G EC Environment

   Increasingly, Anycast is used by various application providers
   and CDNs because Anycast provides better and faster resiliency
   to failover events than GEO database DNS-based load balancing,
   which relies on DNS to provide a different IP based on source
   address.

   Anycast address leverages the proximity information present in
   the network (routing) layer. It eliminates the single point of
   failure and bottleneck at the DNS resolvers and application
   layer load balancers. Another benefit of using the ANYCAST
   address is removing the dependency on UEs. Some UEs (or
   clients) might use their cached IP addresses for an extended
   period instead of querying DNS.

   Client using Virtual IP address is a common practice in Cloud
   Native networking, e.g., Kubernetes, to scale dynamic changes
   of app servers' instantiations. However, Virtual IP requires
   the destination node to perform address translation for return
   traffic, which is unsuitable for underlay network nodes with
   millions of flows passing by.

   Having multiple locations of the same ANYCAST address in the
   5G EC LDC can be problematic if path selection is solely based
   on routing cost as the difference in the routing cost to reach
   the Application Servers attached to different egress routers
   can be very small. This list elaborates the issues in detail:

     a) Path Selection: When a new flow comes to an ingress node
        (Ra), how to avoid instability with Anycast flipping
        between paths to the same address.  The problem is more
        so with BGP multipath and picking the optimal path
        depending on close metrics.

     b) Ingress node forwards the packets from one flow to the
        same ANYCAST server.

        a.k.a. Flow Affinity, or Flow-based load balancing.
        Almost all vendors have supported flow or session based
        ECMP load balancing and not per packet to avoid out of
        order packets                           for decades.  When a flow is hashed to an
        ECMP path, the flow remains on that path for the life of
        the flow until the flow ends.

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        The ingress node, (Ra/Rb), can use Flow ID (in IPv6
        header) or UDP/TCP port number combined with the source
        address to enforce packets in one flow being placed in
        one tunnel to one Egress router.  No new features are
        needed.

     c) When a UE moves to a new Cell Tower, a method is needed
        to stick the flow to the same ANYCAST server, which is
        required by 5G Edge Computing: 3GPP TR 23.748.

        Soft-Anchoring in Section 7 describes one method to
        achieve stickiness. [5g-edge-compute-sticky-service]
        describes several approaches to achieve stickiness in the
        IPv6 domain.

   From BGP perspective, the multiple servers with the same IP
   address (ANYCAST)attached to different egress routers is the
   same as multiple next hops for the IP address.

   This draft describes the BGP UPDATE to enable ingress routers
   to take the App Server load, the capacity index, and the
   location preference into consideration when computing the
   optimal path to egress routers.

 1.4. Problem #2: Unbalanced Anycast Distribution due to UE
     Mobility

   UEs frequent moving from one 5G site to another can make it
   difficult to plan where and how many to deploy the App
   servers. When one App server is heavily utilized, other
   servers of the same App close-by can be very underutilized.
   Since the condition can be short-lived, it is difficult for
   the application controller to anticipate the move and adjust.

  1.5. Problem 3: Application Server Relocation

   When an Application Server is added to, moved, or deleted from
   a 5G EC Data Center, the routing protocol needs to propagate
   the changes to 5G PSA or the PSA adjacent routers.  After the
   change, the cost associated with the site might change as
   well.

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   Note: for ease of description, the Edge Application Server and
   Application Server are used interchangeably throughout this
   document.

2. Conventions used in this document

   A-ER:       Egress Router to an Application Server, [A-ER] is
               used to describe the last router that the
               Application Server is attached. For a 5G EC
               environment, the A-ER can be the gateway router to
               a (mini) Edge Computing Data Center.

   Application Server: An application server is a physical or
               virtual server that hosts the software system for
               the application.

   Application Server Location: Represent a cluster of servers at
               one location serving the same Application. One
               application may have a Layer 7 Load balancer,
               whose address(es) are reachable from an external
               IP network, in front of a set of application
               servers. From an IP network perspective, this
               whole group of servers is considered as the
               Application server at the location.

   Edge Application Server: used interchangeably with Application
               Server throughout this document.

   EC:         Edge Computing

   Edge Hosting Environment: An environment providing the support
               required for Edge Application Server's execution.

               NOTE: The above terminologies are the same as
               those used in 3GPP TR 23.758

   Edge DC:    Edge Data Center, which provides the Edge
               Computing Hosting Environment. An Edge DC might
               host 5G core functions in addition to the
               frequently used application servers.

   gNB         next generation Node B

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   L-DN:       Local Data Network

   PSA:        PDU Session Anchor (UPF)

   SSC:        Session and Service Continuity

   UE:         User Equipment

   UPF:        User Plane Function

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

3. Usage of App-Meta-Data for 5G Edge Computing

  3.1. Assumptions

   From IP Layer, the Application servers are identified by their
   IP (ANYCAST) addresses. Here are some assumptions about the 5G
   EC services:
     - Only the registered EC services, which are only a small
        portion of the services, need to include the AppMetadata
        in path selection.
     - The 5G EC controller or management system push down the
        policies (e.g., ACLs) on the relevant routers to filter
        out those registered EC services.
     - The ingress routers' local BGP path compute algorithm
        includes a special plugin that can compute the path to
        the optimal Next Hop (egress router) based on the BGP
        AppMetaData TLV received for the registered EC services.

   The proposed solution is for the egress routers, i.e. A-ER,
   that have direct links to the Application Servers to collect
   various measurements about the Servers' running status [5G-EC-
   Metrics] and advertise the metrics to other routers in 5G EC
   LDN (Local Data Network).

  3.2. IP Layer Metrics to Gauge Application Behavior

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   [5G-EC-Metrics] describes the IP Layer Metrics that can gauge
   the application servers running status and environment:

   - IP-Layer Metric for App Server Load Measurement:
     The Load Measurement to an App Server is a weighted
     combination of the number of packets/bytes to the App Server
     and the number of packets/bytes from the App Server which
     are collected by the A-ER to which the App Server is
     directly attached.
     The A-ER is configured with an ACL that can filter out the
     packets for the Application Server.
   - Capacity Index
     a numeric number, configured on all A-ERs in the domain
     consistently, is used to represent the capacity of the
     application server attached to an A-ER. At some sites, the
     IP address exposed to the A-ER is the App Layer Load
     balancer that have many instances attached.  At other sites,
     the IP address exposed is the server instance itself.
   - Site preference index:
     is used to describe some sites are more preferred than
     others. For example, a site with higher bandwidth has a
     higher preference number than other.

   In this document, the term "Application Server Egress Router"
   [A-ER] is used to describe the last router that an Application
   Server is attached. For the 5G EC environment, the A-ER can be
   the gateway router to the EC DC where multiple Application
   servers are hosted.

   From IP Layer, an Application Server is identified by its IP
   (ANYCAST) Address. Those IP addresses are called the
   Application Server IDs throughout this document.

 3.3. AppMetaData Constrained Optimal Path Selection

   The main benefit of using ANYCAST is to leverage the network
   layer information to select an optimal path among multiple
   application Server locations of the same application
   identified by its ANYCAST addresses.

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   For the 5G EC environment, the ingress routers to the LDN need
   to be notified of the Load Index and Capacity Index of the App
   Servers at different EC data centers to make the intelligent
   decision on where to forward the traffic for the application
   from UEs.

   Here is an algorithm that computes the cost to reach the App
   Servers attached to Site-i relative to another site, say Site-
   b. When the reference site, Site-b, is plugged in the formula,
   the cost is 1. So, if the formula returns a value less than 1,
   the cost to reach Site-i is less than reaching Site-b.

               CP-b * Load-i                Pref-b * Network-Delay-i
  Cost-i= (w *(----------------) + (1-w) *(-------------------------))
              CP-i * Load-b                Pref-i * Network-Delay-b

      Load-i: Load Index at Site-i, it is the weighted
      combination of the total packets or/and bytes sent to and
      received from the Application Server at Site-i during a
      fixed time period.

      CP-i: capacity index at Site-i, a higher value means higher
      capacity.

      Delay-i: Network latency measurement (RTT) to the A-ER that
      has the Application Server attached at the site-i.

      Pref-i: Preference index for the Site-i, a higher value
      means higher preference.

      w: Weight for load and site information, which is a value
      between 0 and 1. If smaller than 0.5, Network latency and
      the site Preference have more influence; otherwise, Server
      load and its capacity have more influence.

 3.4. BGP Protocol Extension to advertise Load & Capacity

    The goal of the protocol extension:
    - Propagate the Load Measurement Index for the attached App
      Servers to other routers in the LDN.

    - Propagate the Capacity Index, and

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    - Propagate Site Preference Index.

    The BGP extension is to include the Load Index Sub-TLV,
    Capacity Sub-TLV, and the Site Preference Sub-TLV in the
    Tunnel Encap Path Attribute associated with the routes.

 3.5. Ingress Node BGP Path Selection Behavior

 3.5.1. AppMetaData Influenced BGP Path Selection

   In this scenario, an ingress router will receive one ANYCAST
   address's multiple routes from different egress routers that
   have the direct links to the ANYCAST servers. The ingress
   router's BGP engine will do path selection, select the best
   route, and download to FIB. And BGP engine will also download
   the other paths to FIB that with the AppMetaData taken into
   the consideration.

   Assume that both Ra and Rb in Figure-1 have BGP Multipath
   enabled. As a result, Dst Address: S1:aa08::4450 is resolved
   via multiple NextHop: R1, R2, R3.

   Suppose the local BGP special Plugin for AppMetaData finds R1
   is the best for the flow towards S1:aa08::4450. Then this
   special Plugin can insert a higher weight for the path R1 so
   that BGP Best Path is locally influenced by the weight
   parameter based on the local decision.

 3.5.2. Forwarding Behavior

   When the ingress router receives a packet and lookup the FIB,
   it gets the destination prefix's whole path and AppMetaData.
   The Forwarding Plane will do computing for the packet and
   choose the suitable path as the result of the computing. Then
   the Forwarding Plane encapsulates the packet destined towards
   the optimal egress node.

   For subsequent packets belonging to the same flow, the ingress
   router needs to forward them to the same egress router unless
   the selected egress router is no longer reachable. Keeping
   packets from one flow to the same egress router, a.k.a. Flow
   Affinity, is supported by many commercial routers.

   How Flow Affinity is implemented is out of the scope for this
   document. Here is one example to illustrate how Flow Affinity
   can be achieved. This illustration is not to be standardized.

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     For the registered EC services, the ingress node keeps a
     table of
     -   Service ID (i.e., ANYCAST address)
     -   Flow-ID
     -   Sticky Egress ID (egress router loopback address)
     -   A timer

     The Flow-ID in this table is to identify a flow, initialized
     to NULL. How Flow-ID is constructed is out of the scope for
     this document. Here is one example of constructing the Flow-
     ID:
       - For IPv6, the Flow-ID can be the Flow-ID extracted from
          the IPv6 packet header with or without the source
          address.
       - For IPv4, the Flow-ID can be the combination of the
          Source Address with or without the TCP/UDP Port number.

     The Sticky Egress ID is the egress node address for the same
     flow. [5G-Sticky-Service] describes several methods to
     derive the Sticky Egress ID.

     The Timer is always refreshed when a packet with the
     matching EC Service ID (ANYCAST address) is received by the
     node.

     If there is no Stick Egress ID present in the table for the
     EC Service ID, the forwarding plane computes the optimal
     path to an egress (NextHop) with the AppMetaData taken into
     consideration. The forwarding plane encapsulates the packet
     with a tunnel to the chosen egress (NextHop). The chosen
     NextHop and the Flow ID are recorded in the table entry of
     the EC Service ID.

   When the selected optimal egress router is no longer
   reachable, refer to Section 6 Soft Anchoring on how another
   path is selected.

 3.5.3. Forwarding Behavior after a UE moving to a new 5G Site

   When a UE moves to a new 5G eNB which is anchored to the same
   UPF, the packets from the UE traverse to the same ingress
   router. Path selection and forwarding behavior are same as
   before.

   When the new eNB is anchored to a different UPF, the packets
   from the UE traverse a different ingress router. If the UE
   source IP address has been changed, indicating the new UPF

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   might belongs to a different administrative domain, the new
   ingress router treats the packets from the UE as a new flow
   and select the optimal path based on the configured policies.
   If the UE maintains the same IP address when anchored to a new
   UPF, the directly connected ingress router might use the pre-
   computed Egress Router which is passed from the neighboring
   router. [5G-Edge-Sticky] describes the method for the ingress
   router connected to the UPF in the new site to take into
   consideration the information passed from other ingress
   routers in selecting the optimal paths. The detailed algorithm
   is out of the scope of this document.

4. The Sub-TLVs for App-Meta-Data

   The App-Meta-Data attribute is encoded in an optional subTLV
   within the Tunnel Encap [RFC9012] Path Attribute.

   All values in the Sub-TLVs are unsigned 32 bits integers.

4.1. Load Measurement sub-TLV format

   Two types of Load Measurement Sub-TLVs are specified. One is
   to carry the aggregated cost Index based on a weighted
   combination of the collected measurements; another one is to
   carry the raw measurements of packets/bytes to/from the App
   Server address. The raw measurement is useful when ingress
   routers have embedded analytics relying on the raw
   measurements.

     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Type (TBD1)           |               Length          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Measurement Period                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Aggregated Load Index to reach the App Server       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 2: Aggregated Load Index Sub-TLV

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   Raw Load Measurement sub-TLV has the following format:

     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Type (TBD2)         |               Length          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Measurement Period                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           total number of packets to the AppServer            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           total number of packets from the AppServer          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           total number of bytes to the AppServer              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           total number of bytes from the AppServer            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 5: Raw Load Measurement Sub-TLV

     Type =TBD1: Aggregated Load Measurement Index derived from
     the Weighted combination of bytes/packets sent to/received
     from the App server:

     Index=w1*ToPackets+w2*FromPackes+w3*ToBytes+w4*FromBytes

     Where wi is a value between 0 and 1; w1+ w2+ w3+ w4 = 1.

     Type= TBD2: Raw measurements of packets/bytes to/from the
     App Server address.

     Measure Period: BGP Update period or user-specified period.

  4.2. Capacity Index sub-TLV format

   The Capacity Index sub-TLV has the following format:

      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Type (TBD3)         |               Length          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Capacity Index                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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   Note: "Capacity Index" can be more stable for each site. If
   those values are configured to nodes, they might not need to
   be included in every BGP UPDATE.

  4.3. The Site Preference Index sub-TLV format

   The site Preference Index is used to achieve Soft Anchoring
   [Section 5] an application flow from a UE to a specific
   location when the UE moves from one 5G site to another.

   The Preference Index sub-TLV has the following format:

      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Type (TBD4)         |               Length          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Preference Index                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Note: "Site Preference Index" can be more stable for each
   site. If those values are configured to nodes, they might not
   need to be included in every BGP UPDATE.

5. AppMetaData Propagation Scope

   AppMetaData is only to be distributed to the relevant ingress
   nodes of the 5G EC local data networks. Only the ingress
   routers that are configured with the 5G EC services ACLs need
   to receive the AppMetaData for specific services.

   For each registered EC service, a corresponding filter group
   can be formed on RR to represent the interested ingress
   routers that are interested in receiving the corresponding
   AppMetaData information.

6. Metrics Change Rate Consideration

   As the metrics change can impact the path selection, it is
   recommended to control the update frequency to avoid rapid
   route oscillations.

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7. Soft Anchoring of an ANYCAST Flow
   "Sticky Service" in the 3GPP Edge Computing specification
   (3GPP TR 23.748) is about flows from a UE sticking to a
   specific ANYCAST location when the UE moves from one 5G Site
   to another.

   "Soft Anchoring" is a mechanism for ingress routers to apply
   preference to the path towards the previous server location
   when the UE is anchored to a new UPF and continue using its
   cached IP for the App server.

   Let's assume one application "App.net" is instantiated on
   four servers that are attached to four different routers R1,
   R2, R3, and R4 respectively. It is desired for packets to the
   "App.net" from UE-1 to stick with one server, say the App
   Server attached to R1, even when the UE moves from one 5G
   site to another. However, when there is a failure reaching R1
   or the Application Server attached to R1, the packets of the
   flow "App.net" from UE-1 need to be forwarded to the
   Application Server attached to R2, R3, or R4.

   We call this kind of sticky service "Soft Anchoring", meaning
   that anchoring to the site of R1 is preferred, but other
   sites can be chosen when the preferred site encounters a
   failure.

   Here are the detailed steps:

      - Assign a group of ANYCAST addresses to one application.
        For example, "App.net" is assigned with 4 ANYCAST
        addresses, L1, L2, L3, and L4. L1/L2/L3/L4 represents
        the location preferred ANYCAST addresses.
      - For the App.net Server attached to a router, the router
        has four Stub links to the same Server, L1, L2, L3, and
        L4 respectively. The cost to L1, L2, L3, and L4 is
        assigned differently for different egress routers. For
        example,
           o When attached to R1, the L1 has the lowest cost,
             say 10, when attached to R2, R3, and R4, the L1 can
             have a higher cost, say 30.
           o ANYCAST L2 has the lowest cost when attached to R2,
             higher cost when attached to R1, R3, R4
             respectively.

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           o ANYCAST L3 has the lowest cost when attached to R3,
             higher cost when attached to R1, R2, R4
             respectively, and
           o ANYCAST L4 has the lowest cost when attached to R4,
             higher cost when attached to R1, R2, R3
             respectively
      - When a UE queries for the "App.net" for the first time,
        the DNS reply has the location preferred ANYCAST
        address, say L1, based on where the query is initiated.
      - When the UE moves from one 5G site-A to Site-B, UE
        continues sending packets of the "App.net" to ANYCAST
        address L1. The routers will continue sending packets to
        R1 because the total cost for the App.net instance for
        ANYCAST L1 is lowest at R1. If any failure occurs making
        R1 not reachable, the packets of the "App.net" from UE-1
        will be sent to R2, R3, or R4 (depending on the total
        cost to reach each of them).

   If the Application Server supports the HTTP redirect, more
   optimal forwarding can be achieved.

      - When a UE queries for the "App.net" for the first time,
        the global DNS reply has the ANYCAST address G1, which
        has the same cost regardless of where the Application
        servers are attached.
      - When the UE initiates the communication to G1, the
        packets from the UE will be sent to the Application
        Server that has the lowest cost, say the Server attached
        to R1. The Application server is instructed with HTTPs
        Redirect to reply with a location-specific URL, say
        App.net-Loc1. The client on the UE will query the DNS
        for App.net-Loc1 and get the response of ANYCAST L1. The
        subsequent packets from the UE-1 for App.net are sent to
        L1.

8. Manageability Considerations

     To be added.

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9. Security Considerations

   To be added.

10. IANA Considerations

   Here are new Sub-TLV types requiring IANA registration:

   Type = TBD1: Aggregated Load Measurement Index derived from
   the Weighted combination of bytes/packets sent to/received
   from the App server.

   Type = TBD2: Raw measurements of packets/bytes to/from the
   App Server address.

   Type = TBD3: Capacity value sub-TLV

   Type = TBD4: Site preference value sub-TLV

11. References

  11.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4364] E. rosen, Y. Rekhter, "BGP/MPLS IP Virtual Private
             networks (VPNs)", Feb 2006.

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

   [RFC8200] s. Deering R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification", July 2017

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  11.2. Informative References

   [3GPP-EdgeComputing] 3GPP TR 23.748, "3rd Generation
             Partnership Project; Technical Specification Group
             Services and System Aspects; Study on enhancement of
             support for Edge Computing in 5G Core network
             (5GC)", Release 17 work in progress, Aug 2020.

   [5G-EC-Metrics] L. Dunbar, H. Song, J. Kaippallimalil, "IP
             Layer Metrics for 5G Edge Computing Service", draft-
             dunbar-ippm-5g-edge-compute-ip-layer-metrics-00,
             work-in-progress, Oct 2020.

   [5G-Edge-Sticky] L. Dunbar, J. Kaippallimalil, "IPv6 Solution
             for 5G Edge Computing Sticky Service", draft-dunbar-
             6man-5g-ec-sticky-service-00, work-in-progress, Oct
             2020.

   [RFC5521] P. Mohapatra, E. Rosen, "The BGP Encapsulation
             Subsequent Address Family Identifier (SAFI) and the
             BGP Tunnel Encapsulation Attribute", April 2009.

   [BGP-SDWAN-Port] L. Dunbar, H. Wang, W. Hao, "BGP Extension
             for SDWAN Overlay Networks", draft-dunbar-idr-bgp-
             sdwan-overlay-ext-03, work-in-progress, Nov 2018.

   [SDWAN-EDGE-Discovery] L. Dunbar, S. Hares, R. Raszuk, K.
             Majumdar, "BGP UPDATE for SDWAN Edge Discovery",
             draft-dunbar-idr-sdwan-edge-discovery-00, work-in-
             progress, July 2020.

   [Tunnel-Encap] E. Rosen, et al "The BGP Tunnel Encapsulation
             Attribute", draft-ietf-idr-tunnel-encaps-10, Aug
             2018.

12. Acknowledgments

   Acknowledgements to Donald Eastlake for their review and
   contributions.

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   This document was prepared using 2-Word-v2.0.template.dot.

Authors' Addresses

   Linda Dunbar
   Futurewei
   Email: ldunbar@futurewei.com

   Kausik Majumdar
   CommScope
   350 W Java Drive, Sunnyvale, CA 94089
   Email:  kausik.majumdar@commscope.com

   Haibo Wang
   Huawei
   Email: rainsword.wang@huawei.com

   Gyan Mishra
   Verizon
   Email: gyan.s.mishra@verizon.com

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