\
Network Working Group                                L. Dunbar
Internet Draft                                         H. Chen
Intended status: Standard                            Futurewei
Expires: May 8, 2022                             Aijun Wang
                                                 China Telecom
                                              November 8, 2021

           IGP Extension for 5G Edge Computing Service
               draft-dunbar-lsr-5g-edge-compute-02

Abstract
   Routers in 5G Local Data Network (LDN) can use additional
   site-costs, preference, and other application related
   metrics on top of the network condition to compute
   constraint-based SPF within the 5G LDN to enhance
   performance for selected services. This draft describes
   those application server related metrics to be used in
   Flexible Algorithms.

Status of this Memo
   This Internet-Draft is submitted in full conformance with
   the provisions of BCP 78 and BCP 79.

   This Internet-Draft is submitted in full conformance with
   the provisions of BCP 78 and BCP 79. This document may not
   be modified, and derivative works of it may not be created,
   except to publish it as an RFC and to translate it into
   languages other than English.

   Internet-Drafts are working documents of the Internet
   Engineering Task Force (IETF), its areas, and its working
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   than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt




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   The list of Internet-Draft Shadow Directories can be
   accessed at http://www.ietf.org/shadow.html

   This Internet-Draft will expire on April 7, 2021.

Copyright Notice

   Copyright (c) 2021 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
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   (http://trustee.ietf.org/license-info) in effect on the
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   in the Simplified BSD License.

Table of Contents


   1. Introduction........................................... 3
      1.1. Unbalanced Distribution due to UE Mobility........ 3
      1.2. ANYCAST in 5G EC Environment...................... 4
   2. Conventions used in this document...................... 4
   3. Solution Overview...................................... 6
   4. FAD sub-TLV for Constrained SPF with Site Cost......... 7
      4.1. New Flags added to FAD Flags Sub-TLV.............. 7
   5. "Site-Cost" Advertisement in OSPF...................... 8
      5.1. OSPFv3 LSA to Carry the Aggregated Cost........... 8
      5.2. OSPFv2 LSA to Carry the Aggregated Cost........... 8
      5.3. AppMetaData Sub-TLV in OSPF....................... 9
         5.3.1. OSPFv2 Extended Prefix Opaque LSA Extension.. 9
         5.3.2. OSPFv3 Extended LSA to carry the AppMetaData 10
   6. "Site-Cost" Advertisement in IS-IS.................... 10
      6.1. Aggregated Cost Advertisement in IS-IS........... 10
      6.2. AppMetaData Advertisement in IS-IS............... 11
   7. IP Layer App-Metrics (AppMetaData) SubSub-TLVs........ 12
   8. AppMetaData Metric Advertisement...................... 14
   9. Alternative method for Distributing Aggregated Cost... 15
   10. Manageability Considerations......................... 15


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   11. Security Considerations.............................. 15
   12. IANA Considerations.................................. 15
   13. References........................................... 16
      13.1. Normative References............................ 16
      13.2. Informative References.......................... 17
   14. Appendix:5G Edge Computing Background................ 18
   15. 5G EC LDN Characteristics for the Constraint SPF..... 19
      15.1. IP Layer Metrics to Gauge EC Server Running Status
      ...................................................... 19
      15.2. App Metrics Constrained Shortest Path First..... 21
      15.3. Reason for using IGP Based Solution............. 22
      15.4. Flow Affinity to an ANYCAST server.............. 22
   16. Acknowledgments...................................... 23

1. Introduction

   In 5G Edge Computing (EC) environment, it is common for an
   application that needs low latency to be instantiated on
   multiple servers close in proximity to UEs (User
   Equipment). When those multiple server instances share one
   IP address (ANYCAST), the transient network and load
   conditions can be incorporated in selecting an optimal path
   among server instances and UEs.

   Flexible algorithms provide mechanisms for topologies to
   use different IGP path algorithms. This draft describes
   using Flexible Algorithms [LSR-FlexAlgo] to indicate the
   desired constrained SPF behavior for a subset of prefixes,
   in addition to the encodings for advertising the IP Layer
   App related metrics that can impact application servers'
   performance.

 1.1. Unbalanced Distribution due to UE Mobility

   UEs' frequent moving from one 5G site to another can make
   it difficult to plan where the App Servers should be
   hosted. When one App server is heavily utilized, other App
   servers of the same address close by can be under-utilized.
   The difference in the routing distance to reach multiple
   Application Servers might be relatively small. The traffic
   load at the router where the App Server is attached and the
   site capacity, when combined, can be more significant from
   the latency and performance perspective.




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   Since the condition can be short-lived, it is difficult for
   the application controller to anticipate the moving and
   adjusting.

 1.2. ANYCAST in 5G EC Environment

   ANYCAST makes it possible to load balance across server
   locations based on network conditions dynamically. With
   multiple servers having the same IP address, it eliminates
   the single point of failure and bottleneck at the
   application layer load balancers. Another benefit of using
   ANYCAST address is removing the dependency on how UEs get
   the IP addresses for their applications. Some UEs (or
   clients) might use stale cached IP addresses for an
   extended period.

   But having multiple locations of the same IP address in the
   5G Edge Computing environment can be problematic because
   all those locations can be close in proximity. There might
   be very small difference in the routing distance to reach
   an Application Server attached to a different edge router,
   which can cause packets from one flow to be forwarded to
   different locations, resulting in service glitches.

   Note: for the ease of description, the EC (Edge Computing)
   server, Application server, App server are used
   interchangeably throughout this document.



2. Conventions used in this document


   A-ER:       Egress Edge 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 hosts the software system
               for the application.

   Application Server Location: Represent a cluster of servers
               at one location serving the same Application.



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               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 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. 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

   LDN:        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


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   be interpreted as described in BCP 14 [RFC2119] [RFC8174]
   when, and only when, they appear in all capitals, as shown
   here.


3. Solution Overview

   The proposed solution is for the egress edge router (A-ER)
   with the EC Servers directly attached to advertise the
   "Site-Cost" [Section 15.1] via IP prefix reachability TLV
   associated with the (anycast) prefix and use the Flexible
   algorithms [LSR-FlexAlgo] to advertise the desired
   constrained SPF behavior, so that constrained IGP path can
   be computed as desired.

   There are two types of "Site-Cost":

     a) The IP Layer App related metrics, such as the Load
       Measurement, the Capacity Index, and the Preference
       Index that are collected by the egress routers for an
       attached EC server (i.e., ANYCAST prefix), as
       described in Section 15.1.

     b) The aggregated cost associated with an EC server
       (i.e., ANYCAST prefix).

       The aggregated cost is computed based on the Load
       Measurement, the Capacity Index, the Preference Index,
       and other constraints by a consistent algorithm across
       all A-ERs.

       This document, "AppMetaData" refers to the encoding of
       the IP Layer App related metrics carried by IP Prefix
       Reachability TLVs.

   The solution assumes that the 5G EC controller or
   management system is aware of the EC ANYCAST addresses that
   need optimized forwarding. To minimize the processing, only
   the addresses that match with the ACLs configured by the 5G
   EC controller will have their Site-Cost collected and
   advertised.









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4. FAD sub-TLV for Constrained SPF with Site Cost

   In order for Site-Cost to be considered in IGP path
   computation, a new Calc-Type, Metric Type and FAD Flag need
   to be indicated in the FAD sub-TLV [LSR-FlexAlgo].




    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      | Length          |Flex-Algorithm | Metric-Type   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Calc-Type   | Priority        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Constrained SPF Flags Sub-TLVs                  |
   +                                                               +
   |                     ...                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          Figure 1: IS-IS Flex Algo Definition Sub-TLV

   Flex-Algorithm: SPF.

   Metric-Type:
     A new value to be assigned by IANA to indicate the "Site-
     Cost" included in computing the constrained SPF.

   Calc-Type:
     A value chosen by the IGP operator to indicate a specific
     constrained SPF algorithm that takes the "Site-Cost" into
     the SPF computation across the routers in the 5G LDN.



 4.1. New Flags added to FAD Flags Sub-TLV

   New flags are added to indicate the constrained SPF compute
   methods in the IS-IS FAD Flags Sub-TLV (Section 6.4 of
   [LSR-FlexAlgo]) or the OSPF FAD Flags Sub-TLV (Section 7.4
   of [[LSR-FlexAlgo]), respectively.

   Flags:

     0 1 2 3 4 5 6 7...
     +-+-+-+-+-+-+-+-+...
     |M|T|A| ...
     +-+-+-+-+-+-+-+-+...



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     T-flag: Site-Cost Metrics is only used as tiebreaker.

     A-flag: Site-Cost Metrics are the additional metrics for
     the Calc-Type indicated.



5. "Site-Cost" Advertisement in OSPF

 5.1. OSPFv3 LSA to Carry the Aggregated Cost

   For EC servers using IPv6 address, the aggregated cost
   computed by the A-ERs can be encoded in the Metric field
   [the interface cost] of Intra-Area-Prefix-LSA specified by
   Section 3.7 of the [ RFC5340].
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     6 (Intra-Area Prefix)     |         TLV Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          0    |  Aggregated Cost to the EC Server             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | PrefixLength  | PrefixOptions |             0                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Address Prefix                      |
     |                               ...                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 2: Aggregated Cost to EC Server



 5.2. OSPFv2 LSA to Carry the Aggregated Cost

   For EC servers in IPv4 address, the aggregated cost can be
   encoded in the "Metric" field of the Stub Link LSA [Link
   type =3] specified by Section 12.4 of the [RFC2328].















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 5.3. AppMetaData Sub-TLV in OSPF

 5.3.1. OSPFv2 Extended Prefix Opaque LSA Extension

   For IPv4 network, IP layer App related metrics,
   AppMetaData, can be carried by the OSPFv2 Extended Prefix
   Opaque LSA with the extended Prefix TLV [RFC7684].

     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                          | Length                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Route Type    | Prefix Length | AF            | Flags         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Address Prefix (variable)                                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Load Measurement Sub-TLV                                      |
     ~                                                               ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | capacity Index Sub-TLV                                        |
     ~                                                               ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Site Preference Sub-TLV                                       |
     ~                                                               ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



























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 5.3.2. OSPFv3 Extended LSA to carry the AppMetaData

   For IPv6, OSPFv3 Extended LSA with the Intra-Area-Prefix
   Address TLV [RFC8362] can be extended to carry the IP Layer
   App related Metrics as below:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |AppMetaDataType                | Length                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            IPv6 AppServer (ANYCAST) address                   |
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Load measurement SubSub-TLV                        |
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Capability SubSub-TLV                              |
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Preference SubSub-TLV                             |
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 3: AppMetaData sub TLV

   AppMetaData Type (TBD1): AppMetaData-OSPF-IPv6.



6. "Site-Cost" Advertisement in IS-IS

 6.1. Aggregated Cost Advertisement in IS-IS

   Egress routers can append the Aggregated Cost to the IP
   Reachability TLVs.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |AggCostSubTLV  | Length        | AggCost to the App Server     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | PrefixLength  | PrefixOptions |             0                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Address Prefix                      |
     |                               ...                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 4: Aggregated cost to EC server




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 6.2. AppMetaData Advertisement in IS-IS

   The IP Layer App related Metrics are encoded in the
   AppMetaData Advertisement Sub-TLV carried by IP Prefix
   Reachability TLV 128, 130, or 135.

   Here is the AppMetaData Sub-TLV encoding for IS-IS:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |AppMetaDataType| Length        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            IPv6 or IPv4  AppServer (ANYCAST) address          |
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Load measurement SubSub-TLV                        |
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Capability SubSub-TLV                              |
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Preference SubSub-TLV                             |
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 5: AppMetaData sub TLV



   AppMetaData Type (TBD1): ISIS-IPv4 or ISIS-IPv6.





















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7. IP Layer App-Metrics (AppMetaData) SubSub-TLVs

   Two types of Load Measurement SubSub-TLVs are specified:

   a) The Aggregated Load Index based on a weighted
     combination of the collected measurements.
   b) The raw measurements of packets/bytes to/from the App
     Server address. The raw measurement is useful when the
     egress edge routers cannot be configured with a
     consistent algorithm to compute the aggregated load
     index or the raw measurements are needed by a central
     analytic system.


   The Aggregated Load 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 (TBD2)           |               Length          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Measurement Period                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Aggregated Load Index to reach the App Server       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 6: Aggregated Load Index Sub-TLV

     Type=TBD2 (to be assigned by IANA) indicates that the
     sub-TLV carries the 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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   The 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 7: Raw Load Measurement Sub-TLV



     Type= TBD3 (to be assigned by IANA) indicates that the
     sub-TLV carries the Raw measurements of packets/bytes
     to/from the App Server ANYCAST 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 (TBD4)         |               Length          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Capacity Index                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 8: Capacity Index Sub-TLV













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   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 (TBD5)         |               Length          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Preference Index                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 9: Preference Index Sub-TLV


   Note: "Capacity Index" and "Site preference" can be
   different for different attached server addresses. For
   Figure 1, the address S1 can have higher Site Preference
   when attached to R1 than R2.

8. AppMetaData Metric Advertisement

   With Flex-Algorithm, the network administrator can define a
   function that compute the SPF with consideration of the
   AppMetaData metrics advertised by the routers to which the
   EC servers are directly attached.

   This document defines a new standard metric type,
   AppMetaData, for this purpose. The AppMetaData Metric MAY
   be advertised in the Generic Metric sub-TLV with the metric
   type set to "AppMetaData Metric". ISIS and OSPF will
   advertise this new type of metric in their link
   advertisements. AppMetaData metric is a prefix attribute
   and for advertisement and processing of this attribute for
   Flex-algorithm purposes, MUST follow the section 12 of [I-
   D.ietf-lsr-flex-algo]

   Flex-Algorithm uses this metric type by specifying the
   AppMetaData as the metric type in a FAD TLV. A FAD TLV may
   also specify an automatic computation of the AppMetaData
   metric based on a links advertised bandwidth. An explicit
   advertisement of a link's AppMetaData metric using the
   Generic Metric sub-TLV overrides this automatic
   computation. The automatic AppMetaData metric calculation
   sub-TLVs are advertised in FAD TLV and these parameters are
   applicable to applications such as Flex-algorithm that make
   use of the FAD TLV.






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9. Alternative method for Distributing Aggregated Cost

   Section 7 and Section 8 demonstrate different ways for
   OSPFv2, OSPFv3, and ISIS to propagate the aggregated cost.
   It would be better if the aggregated cost could be
   advertised the same way, regardless of OSPFv2, OSPFv3, or
   ISIS.

   Draft [draft-wang-lsr-stub-link-attributes] introduces the
   Stub-Link TLV for OSPFv2/v3 and ISIS protocol respectively.
   Considering the interfaces on an edge router that connects
   to the EC servers are normally configured as passive
   interfaces, these IP-layer App-metrics can also be
   advertised as the attributes of the passive/stub link. The
   associated prefixes can then be advertised in the "Stub-
   Link TLV" that is defined in [draft-wang-lsr-stub-link-
   attributes]. All the associated prefixes share the same
   characteristic of the link. Other link related sub-TLVs
   defined in [RFC8920] can also be attached and applied to
   the calculation of path to the associated prefixes."

   Section 6 for the advertisement of AppMetaData Metric can
   also utilize the Stub-Link TLV that defined in [draft-wang-
   lsr-stub-link-attributes]

10. Manageability Considerations

     To be added.

11. Security Considerations


   To be added.

12. IANA Considerations

   The following Sub-TLV types need to be added by IANA to
   FlexAlgo.

          - AppMetaData Type for ISIS or OSPF (TBD1): IPv4 or
            IPv6





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   The following SubSub-TLV types need to be added by IANA, to
   be included in FAD sub-TLV, ISIS Extended-LSA Sub-TLVs, or
   OSPFv2 Extended Link Opaque LSA TLVs Registry.

          - Aggregated Load Index Sub-TLV type (TBD2)
          - Raw Load Measurement Sub-TLV type (TBD3)
          - Capacity Index Sub-TLV type (TBD4)
          - Preference Index Sub-TLV type (TBD5)



13. References


 13.1. Normative References

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

   [RFC2328] J. Moy, "OSPF Version 2", RFC 2328, April 1998.

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

   [RFC7684] P. Psenak, et al, "OSPFv2 Prefix/Link Attribute
             Advertisement", RFC 7684, Nov. 2015.

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

   [RFC8326] A. Lindem, et al, "OSPFv3 Link State
             advertisement (LSA0 Extensibility", RFC 8362,
             April 2018.

   [RFC9012] E. Rosen, et al "The BGP Tunnel Encapsulation
             Attribute", April 2021.






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 13.2. 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-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.

   [BGP-5G-AppMetaData] L. Dunbar, K. Majumdar, H. Wang, "BGP
             App Metadata for 5G Edge Computing Service",
             draft-dunbar-idr-5g-edge-compute-app-meta-data-
             03, work-in-progress, Sept 2020.

   [LSR-Flex-Algo] P. Psenak, et al, "IGP Flexible Algorithm",
             draft-ietf-lsr-flex-algo-17, July 2021.

   [LSR-Flex-Algo-BW] S. Hegde, et al, "Flexible Algorithms:
             Bandwidth, Delay, Metrics and Constraints",
             draft-ietf-lsr-flex-algo-bw-con-01, July 2021.

   [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.














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14. Appendix:5G Edge Computing Background

   The network connecting the 5G EC servers with the 5G Base
   stations consists of a small number of dedicated routers
   that form the 5G Local Data Network (LDN) to enhance the
   performance of the EC services.

   When a User Equipment (UE) initiates application packets
   using the destination address from a DNS reply or its
   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, performs NAT sometimes, before handing the
   packets from the UEs to the adjacent router, also known as
   the ingress router to the EC LDN, which 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), the handover procedure is
   initiated, which includes the 5G SMF (Session Management
   Function) selecting a new UPF-PSA [3GPP TS 23.501 and TS
   23.502]. When the handover process is complete, the IP
   point of attachment is to the new UPF-PSA. The UE's IP
   address stays the same unless moving to different operator
   domain. 5GC may maintain a path from the old UPF to the new
   UPF for a short time for SSC [Session and Service
   Continuity] mode 3 to make the handover process more
   seamless.






















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   +--+
   |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 10: App Servers in different edge DCs





15. 5G EC LDN Characteristics for the Constraint SPF

 15.1. IP Layer Metrics to Gauge EC Server Running Status

   Most applications do not expose their internal logic to the
   network. Their communications are generally encrypted. Most
   of them do not even respond to PING or ICMP messages
   initiated by routers.




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   Here are some IP Layer App related Metrics that can gauge
   the servers running status and environment:

     - Capacity Index:
       a numeric number, configured on all A-ERs in the
       domain consistently, is used to represent the capacity
       of an EC server attached to an A-ER. The IP addresses
       exposed to the A-ER can be the App Layer Load
       balancers that have many instances attached.  At other
       sites, the IP address exposed is the server itself.
     - Site preference index:
       Is used to describe some sites are more preferred than
       others. For example, a site with less leasing cost has
       a higher preference value. Note: the preference value
       is configured on all A-ERs in the domain consistently
       by the Domain Controller.

     - Load Measurement for gauging the load of the attached
       prefix (i.e., EC Server):
       The Load Measurement for an EC Server is a weighted
       combination of the number of packets/bytes to the EC
       server (i.e., its IP address) and the number of
       packets/bytes from the EC server. The Load Measurement
       are collected by the A-ER that has the EC Server
       directly attached.

       An A-ER only collects those measurement for the
       prefixes instructed by the Domain Controller.

   For ease of description, those metrics with more to be
   added later are called IP Layer App Metrics (or Site-Cost)
   throughout the document.













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 15.2. App Metrics Constrained Shortest Path First

   The main benefit of using ANYCAST is to leverage the
   network layer information to balance the traffic among
   multiple locations of one application server.

   For the 5G EC environment, the routers in the LDN need to
   take consideration of various measurements of the EC
   servers attached to each A-ER in addition to TE metrics to
   compute ECMP paths to the servers.

   Here is one algorithm that computes the cost to reach the
   App Servers attached to Site-i relative to another site,
   say Site-j. When the reference site, Site-j, 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 the reference site (Site-j).

               CP-j * Load-i                Pref-j * Network-Delay-i
  Cost-i= (w *(----------------) + (1-w) *(-------------------------))
              CP-i * Load-j                Pref-i * Network-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 site i, a higher value means
      higher capacity.

      Network Delay-i: Network latency measurement (RTT) to
      the A-ER that has the Application Server attached at the
      site-i.
      Noted: Ingress nodes can easily measure RTT to all the
      egress edge nodes by existing IPPM metrics. But it is
      not so easy for ingress nodes to measure RTT to all the
      App Servers. Therefore, "Network-Delay-i", a.k.a.
      Network latency measurement (RTT), is between the
      Ingress and egress edge nodes. The cost for the egress
      edge nodes to reach to their attached servers is
      embedded in the "capacity index".

      Pref-i: Preference index for site-i, a higher value
      means higher preference. Preference can be derived from
      the total path cost to reach the A-ER [RFC5305], as
      calculated below: 1/(total-path-cost).


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      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.

 15.3. Reason for using IGP Based Solution

    Here are some benefits of using IGP to propagate the IP
    Layer App-Metrics:
    - Intermediate routers can derive the aggregated cost to
      reach the EC Servers attached to different egress edge
      nodes, especially:
        - The path to the optimal egress edge node can be
           more accurate or shorter.
        - Convergence is shorter when there is any failure
           along the way towards the optimal ANYCAST server.
        - When there is any failure at the intended ANYCAST
           server, all the packets in transit can be optimally
           forwarded to another App Server attached to a
           different egress edge router.
    - Doesn't need the ingress nodes to establish tunnels with
      egress edge nodes.

    There are limitations of using IGP too, such as:

    - The IGP approach might not suit well to 5G EC LDN
      operated by multiple ISPs.
      For LDN operated by multiple IPSs, BGP should be used.
      [BGP-5G-AppMetaData] describes the BGP UPDATE message to
      propagate IP Layer App-Metrics crossing multiple ISPs.

 15.4. Flow Affinity to an ANYCAST server

   When multiple servers with the same IP address (ANYCAST)
   are attached to different A-ERs, Flow Affinity means
   routers sending the packets of the same flow to the same A-
   ER even if the cost towards the A-ER is no longer optimal.

   Many commercial routers support some forms of flow affinity
   to ensure packets belonging to one flow be forwarded along
   the same path.

   Editor's note: for IPv6 traffic, Flow Affinity can be
   achieved by routers forwarding the packets with the same



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   Flow Label extracted from the IPv6 Header along the same
   path.



16. Acknowledgments

   Acknowledgements to Peter Psenak, Acee Lindem, Shraddha
   Hegde, Tony Li, Gyan Mishra, Jeff Tantsura, and Donald
   Eastlake for their review and suggestions.

   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

   Aijun Wang
   China Telecom
   Email: wangaj3@chinatelecom.cn


















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