OSPF extension for 5G Edge Computing Service
draft-dunbar-lsr-5g-edge-compute-ospf-ext-01

Network Working Group                                L. Dunbar
Internet Draft                                         H. Chen
Intended status: Standard                            Futurewei
Expires: May 2, 2021

                                              November 2, 2020

          OSPF extension for 5G Edge Computing Service
          draft-dunbar-lsr-5g-edge-compute-ospf-ext-01

Abstract

   This draft describes an OSPF extension that can distribute
   the 5G Edge Computing App running status and environment,
   so that other routers in the 5G Local Data Network can make
   intelligent decision on optimized forwarding of flows from
   UEs. The goal is to improve latency and performance for 5G
   Edge Computing services.

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Table of Contents

   1. Introduction........................................... 3
      1.1. 5G Edge Computing Background...................... 3
      1.2. Problem#1: ANYCAST in 5G EC Environment........... 4
      1.3. Problem #2: Unbalanced Anycast Distribution due to
      UE Mobility............................................ 5
      1.4. Problem 3: Application Server Relocation.......... 6
   2. Conventions used in this document...................... 6
   3. OSPF Extension for 5G EC............................... 7
      3.1. Solution Overview................................. 7
      3.2. IP Layer Metrics to Gauge Application Behavior.... 9
      3.3. To Equalize among Multiple ANYCAST Locations..... 10
      3.4. OSPF Protocol Extension to advertise Load &
      Capacity.............................................. 11
      3.5. Reason for using IGP Based Solution:............. 11
      3.6. OSPF Extension Using TE Stub Link................ 12
      3.7. Aggregated Link Cost Solution.................... 15
   4. Soft Anchoring of an ANYCAST Flow..................... 16

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   5. Manageability Considerations.......................... 18
   6. Security Considerations............................... 18
   7. IANA Considerations................................... 18
   8. References............................................ 18
      8.1. Normative References............................. 18
      8.2. Informative References........................... 19
   9. Acknowledgments....................................... 20

1. Introduction

   This document describes an OSPF extension that can
   distribute the 5G Edge Computing App running status and
   environment, so that other routers in the 5G Local Data
   Network can make intelligent decision on optimized
   forwarding of flows from UEs. The goal is to improve
   latency and performance for 5G Edge Computing services..

1.1. 5G Edge Computing Background

   As described in [5G-EC-Metrics], one Application can have
   multiple Application Servers hosted in different Edge
   Computing data centers that are close in proximity. Those
   Edge Computing (mini) data centers are usually very close
   to, or co-located with, 5G base stations, with the goal to
   minimize latency and optimize the user experience.

   When a UE (User Equipment) initiates application packets
   using the destination address from a DNS reply or from its
   own cache, the 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 the Ingress router of the Edge Computing (EC) Local
   Data Network (LDN). The LDN for 5G EC, which is the IP
   Networks from 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) (gNB1), handover procedures are
   initiated and the 5G SMF (Session Management Function) also
   selects a new UPF-PSA. The standard handover procedures

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   described in 3GPP TS 23.501 and TS 23.502 are followed.
   When the handover process is complete, the UE has a new IP
   address and the IP point of attachment is to the new UPF-
   PSA. 5GC may maintain a path from the old UPF to new the
   UPF for a short period of time for SSC [Session and Service
   Continuity] mode 3 to make the handover process more
   seamless.
   +--+
   |UE|---\+---------+                 +------------------+
   +--+    |  5G     |    +---------+  |   S1: aa08::4450 |
   +--+    | Site +--++---+         +----+                |
   |UE|----|  A   |PSA| 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   |PSA| Rb |        | R2 | S2: aa08::4460 |
   +--+    |      +--++----+        +----+                |
   +--+    |         |  +-----------+  |  S3: aa08::4470  |
   |UE|---/+---------+                 +------------------+
   +--+                                     L-DN2
           Figure 1: App Servers in different edge DCs

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

   Increasingly, Anycast is used extensively by various
   application providers and CDNs because ANYCAST makes it

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   possible to dynamically load balance across server
   locations based on network conditions.

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

   But, having multiple locations of the same ANYCAST address
   in 5G Edge Computing environment can be problematic because
   all those edge computing Data Centers can be close in
   proximity.  There might be very little difference in the
   routing cost to reach the Application Servers in different
   Edge DCs.

   BGP is an integral part in the way IP Anycast usually
   functions. Within BGP routing there are multiple routes for
   the same IP address which are pointing to different
   locations. When many Edge DCs are within one IGP domain,
   there could be no routing cost differentiation by BGP. Same
   routing cost to multiple ANYCAST locations can cause
   packets from one flow to be forwarded to different
   locations, which can cause service glitches.

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

   Another problem of using ANYCAST address for multiple
   Application Servers of the same application in 5G
   environment is that UEs' frequent moving from one 5G site
   to another, which can make it difficult to plan where the
   App Server should be hosted. When one App server is heavily
   utilized, other App servers of the same address 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.

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1.4. Problem 3: Application Server Relocation

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

   Note: for the 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 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 host 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
               external IP network, in front of a set of
               application servers. From IP network
               perspective, this whole group of servers are
               considered as the Application server at the
               location.

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

   EC:         Edge Computing

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   Edge Hosting Environment: An environment providing 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. It might be co-
               located with 5G Base Station and not only host
               5G core functions, but also host frequently
               used Edge server instances.

   gNB         next generation Node B

   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. OSPF Extension for 5G EC

 3.1. Solution Overview

   From IP Layer, the Application Servers are identified by
   their IP (ANYCAST) addresses. The 5G Edge Computing
   controller or management system is aware of the ANYCAST
   addresses of the Applications that need optimized
   forwarding in 5G EC environment. The 5G Edge Computing
   controller or management system can configure the ACLs to

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   filter out those applications on routers, especially the
   routers adjacent to the 5G PSA and the routers to which the
   Application Servers are directly attached.

   The proposed solution is for the routers, i.e. A-ER, that
   have direct links, i.e. the stub links as described in
   RFC2328, 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).

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 3.2. IP Layer Metrics to Gauge Application Behavior

   There are many available network techniques and protocols
   to optimize forwarding or ensuring QoS for applications,
   such as DSCP/DiffServ, Traffic Engineered (TE) solutions,
   Segment Routing, etc. But the reality is that most
   application servers don't expose their internal logics to
   network operators. Their communications are generally
   encrypted. Most of them do not even respond to PING or ICMP
   messages initiated by routers or network gears.

   [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/bites 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
     Capacity Index is used to differentiate the running
     environment of the application server. Some data centers
     can have hundreds, or thousands, of servers behind an
     Application Server's App Layer Load Balancer that is
     reachable from external world. Other data centers can
     have very small number of servers for the application
     server. "Capacity Index", which is a numeric number, is
     used to represent the capacity of the application server
     in a specific location.
   - Site preference index:
     [IPv6-StickyService] describes a scenario that some
     sites are more preferred for handling an application
     server than others for flows from a specific UE.

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   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 5G EC environment, the
   A-ER can be the gateway router to the EC DC where multiple
   Application servers' instance 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. To Equalize among Multiple ANYCAST Locations

   The main benefit of using ANYCAST is to leverage the
   network layer information to equalize the traffic among
   multiple Application Server locations of the same
   Application, which is identified by its ANYCAST addresses.

   For 5G Edge Computing environment, the ingress routers to
   the LDN needs 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.

   [5G-EC-Metrics] describes the algorithms that can be used
   by the routers directly attached to the 5G PSA to compare
   the cost to reach the App Servers between the Site-i or
   Site-j:

               Load-i * CP-j               Pref-j * Delay-i
Cost-i=min(w *(----------------) + (1-w) *(------------------))
              Load-j * CP-i               Pref-i * Delay-j

      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 the site I, 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.

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      Pref-i: Preference index for the site-i, 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. OSPF Protocol Extension to advertise Load & Capacity

   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

   - Propagate the Site Preference Index to other routers in
     the LDN.

   The OSPF extension takes the approach of TE solution:

   - Each mini-DC gateway router announces the stub networks
     of the attached Server addresses with the Load Index Sub-
     TLV.
     Only need to advertise the addresses for the applications
     that needs the network to optimize the forwarding.
     Therefore, A-ER do not need to advertise all attached
     subnets.
   - Load Index and Capacity Index are encoded in a Sub-TLV
     added to the LSA.

3.5. Reason for using IGP Based Solution:

   For scenario of multiple mini data centers within one AS
   domain, there are benefits of using IGP approach:
   - Intermediate routers can forward packets optimally as
     they can derive the load status for the Application

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     Servers at different data centers by the IGP protocol,
     especially:
       - the path to the expected destination can be more
          accurate or shorter
       - Can quickly converge on faulty links and routers
       - When a mini data center has failures, all the
          packets in the fly can be optimally forwarded to an
          App Server in another DC.
   - Doesn't need ingress node to establish tunnels with
     egress nodes.
   - The operations in this approach from users' point of view
     may be simpler.

   Drawback of using IGP:

   - This approach might not suit well to 5G EC LDN operated
     by multiple ISPs networks.
     Using BGP, as specified in AppMetaData NLRI Path
     Attribute [5G-AppMetaData], App Server related metrics
     can be easily propagated crossing multiple ISPs.

 3.6. OSPF Extension Using TE Stub Link

   A new link type in Link TLV of TE LSA is defined for the
   stub links, in addition to the existing P2P and Multi-
   Access link types. A Stub Link is the address of the
   Application Server attached.

   For a stub link, a Link TLV comprises a Link Type sub-TLV
   with stub link type, a Link ID sub-TLV with the address of
   the attached App Server (stub-link), Link Data Sub-TLV and
   some new sub-TLVs, such as, Load measurement sub-TLV,
   Capacity sub-TLV and Preference sub-TLV.

   An example of Link TLV for a stub link is illustrated
   below:

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  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             Type = 2          |               Length          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Link type sub-TLV for stub link                    |
 ~                                                               ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Link ID sub-TLV for the AppServer address          |
 ~                                                               ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Link data sub-TLV for the mask of the AppServer address  |
 ~                                                               ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Load measurement sub-TLV                           |
 ~                                                               ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Capability sub-TLV                                 |
 ~                                                               ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             Preference sub-TLV                                |
 ~                                                               ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 2: A new link type in Link TLV of TE LSA

   The Type value of the Link Type sub-TLV for the stub link:
   3 (to be assigned by IANA).

   Note: [RFC3630] has specified: Type=1 for P2P and Type=2
   used for Multiaccess.

   The Link Data Sub-TLV for the stub network is defined as:

  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            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 AppServer IP address mask                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 2: The Link Data Sub-TLV

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   Two types of Load Measurement Sub-TLVs are specified. One
   is to carry the aggregated cost Index based on 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 the
   egress routers cannot be configured with a consistent
   algorithm to compute the aggregated load index and the raw
   measurements are needed by a central analytic system.

     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                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Aggregated Load Index to reach the App Server       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 3: Aggregated Load Index Sub-TLV

   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 (TBD3)         |               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 =TBD2: 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;

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     Type= TBD3: Raw measurements of packets/bytes to/from the
     App Server address;

     Measurement Period: A user specified period in seconds,
     default is 3600 seconds.

   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                              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        Figure 6: Capacity Index Sub-TLV

   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                            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         Figure 7: Preference Index Sub-TLV

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

3.7. Aggregated Link Cost Solution

   Another solution is to reuse the field for link cost of a
   stub link, if all the egress routers to which the App
   Servers are attacjed can have one consistent algorithm to
   compute the Aggregated Cost based on the App Server's Load
   Index, the Capacity Index and Site Preference Index.

   For a router with an Application Server directly attached,
   its router LSA can contain a stub link for the App Server's
   address [RFC2328]. This solution is using a formula to

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   calculate the aggregated cost value to the directly
   attached App Server and assigned the cost value to the
   Metric field of the stub link in the LSA in RFC2328.
   The cost to the attached Application Server plays a
   significant role on where the flows should be routed. See
   Section 4 for soft anchoring a flow to a specific location
   when the UEs move.
   In this solution, the values of the Link ID, Link data and
   link cost for the stub link are as follows:

        - The Link ID is the IP address of the Application
          Server,
        - The Link Data is the network mask of the
          Application Server address,
        - The Link cost is the aggregated cost reach the
          attached Application Server.

   In this solution, every router connected to an Application
   Server MUST use the same formula to compute the cost of the
   Application Server.
   No new protocol code point is needed.

4. Soft Anchoring of an ANYCAST Flow
   This section describes a solution that can anchor an
   application flow from a UE to a specific ANYCAST Server
   location even when the UE moves from one 5G Site to
   another. This is called "Sticky Service" in the 3GPP Edge
   Computing specification.

   Lets' 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. When there is failure at 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

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   other sites can be chosen when the preferred site
   encounters failure.

   Here is details of this solution:

      - 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 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 higher cost, say 30.
           o ANYCAST L2 has the lowest cost when attached to
             R2, higher cost when attached to R1, R3, R4
             respectively.
           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 replies 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).

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   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 replies the ANYCAST address G1,
        which has the same cost regardless 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 respond back 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.

5. Manageability Considerations

     To be added.

6. Security Considerations

   To be added.

7. IANA Considerations

       To be added.

8. References

8.1. 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.

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   [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

8.2. 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-AppMetaData] L. Dunbar, K. Majumdar, H. Wang, "BGP NLRI
             App Meta Data for 5G Edge Computing Service",
             draft-dunbar-idr-5g-edge-compute-app-meta-data-
             01, work-in-progress, Nov 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-01, work-in-progress, Nov 2020.

   [5G-StickyService] 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.

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   [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.

9. Acknowledgments

   Acknowledgements to Donald Eastlake for their review and
   contributions.

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

Authors' Addresses

   Linda Dunbar
   Futurewei
   Email: ldunbar@futurewei.com

   HuaiMo Chen
   Futurewei
   Email: huaimo.chen@futurewei.com

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