Network Working Group                                L. Dunbar
Internet Draft                                         H. Chen
Intended status: Standard                            Futurewei
Expires: July 15, 2022                             Aijun Wang
                                                 China Telecom
                                              January 15, 2022

           IGP Extension for 5G Edge Computing Service

   This draft describes using additional site-costs and
   preference related metrics to influence the SPF and using
   Flexible Algorithms to indicate the constraints. The
   purpose is to differentiate multiple paths with similar
   routing distance to one destination in 5G Local Data
   Network (LDN)to achieve optimal performance.

Status of this Memo
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Table of Contents

   1. Introduction........................................... 3
      1.1. Unbalanced Distribution due to UE Mobility........ 4
      1.2. ANYCAST in 5G EC Environment...................... 4
      1.3. Scope of the Document............................. 5

   2. Conventions used in this document...................... 5

   3. Solution Overview...................................... 6

   4. New Flags added to FAD Flags Sub-TLV................... 7

   5. Characteristics of the Site Cost Metric................ 7

   6. "Site-Cost" Advertisement in OSPF...................... 8

   7. "Site-Cost" Advertisement in IS-IS..................... 8

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   8. Alternative method for Distributing Aggregated Cost.... 9

   9. Manageability Considerations........................... 9

   10. Security Considerations............................... 9

   11. IANA Considerations................................... 9

   12. References........................................... 10
      12.1. Normative References............................ 10
      12.2. Informative References.......................... 11

   13. Appendix:5G Edge Computing Background................ 12

   14. 5G EC LDN Characteristics for the Constraint SPF. Error!
   Bookmark not defined.
      14.1. IP Layer Metrics to Gauge EC Server Running Status
      ...................................................... 13
      14.2. App Metrics Constrained Shortest Path First. Error!
      Bookmark not defined.
      14.3. Reason for using IGP Based Solution............. 14
      14.4. Flow Affinity to an ANYCAST server.............. 15

   15. Acknowledgments...................................... 15

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). Those applications instances can be behind one
   or multiple application-layer load balancers. When they
   have relatively short flows that can go to any instance,
   having the cluster of them at different locations share the
   same IP address can minimize the impact to DNS and achieve
   optimal forwarding that leverages network conditions. E.g.,
   Kubernetes for data center networking uses one single
   Virtual IP address for a cluster of instances of
   microservices so that the network can forward via multiple
   paths towards one single destination.

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   This draft describes using additional site costs to
   influence the shortest path computation for a specific set
   of prefixes. As there are a small number of egress routers
   having those prefixes (or destinations) that need to
   incorporate site costs in SPF computation, Flexible
   Algorithms [LSR-FlexAlgo] is used to indicate the need for
   the site costs to be considered for the specific
   topologies. Flexible algorithms provide mechanisms for
   topologies to use different IGP path algorithms.

 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 a group of App servers at one location, which
   can be behind an application-layer load balancer, are
   heavily utilized, the instances for the same application at
   another location can be under-utilized. The difference in
   the routing distance to reach multiple sites where the
   application instances are instantiated might be relatively
   small in 5G LDN environment. The site capacity and
   preferences can be more significant than the routing
   distance from the application's latency and performance

   Since the condition can change in days or weeks, it is
   difficult for the application controller to anticipate the
   moving and adjusting relocation of application instances.

 1.2. ANYCAST in 5G EC Environment

   ANYCAST is assigning the same IP address for multiple
   instances at different locations. Using ANYCAST can
   eliminate the single point of failure and bottleneck at
   load balancers or DNS. Another benefit is removing the
   dependency on how UEs resolve IP addresses for their
   applications. Some UEs (or clients) might use stale cached
   IP addresses for an extended period.

   But having the same IP address at multiple locations of the
   5G Edge Computing environment can be problematic because
   all those locations can be close in proximity. There might
   be a tiny difference in the routing distance to reach an
   application instance attached to a different edge router.

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 1.3. Scope of the Document

   The draft is for scenarios where applications or micro
   services are instantiated at multiple locations behind
   multiple application layer load balancers. They have
   relative short flows that can go to any instances.

   Under this scenario, multiple instances for the same type
   of services can be assigned with the same IP address, so
   that network condition can be utilized to achieve optimal

   From IP network perspective, application layer load
   balancers and app servers all appear as IP addresses.
   Throughout this document, the term "app server" can
   represent the load balancer in front of a cluster of app
   server instances, app server instances, or app server.

   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.
               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

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   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

               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",
   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

3. Solution Overview

   The proposed solution is for the egress edge router (A-ER)
   with the application instances directly attached to

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     - advertise the "Site-Cost" via IP prefix reachability
        TLV associated with the (anycast) prefix.

     - use a Flag in the Flexible Algorithm TLV to indicate
        that "site-cost" needs to be included for the
        constrained SPF to reach the Prefix

   The "Site-Cost" associated with a prefix (i.e., ANYCAST
   prefix) is computed based on the Capacity Index, the
   Preference Index, and other constraints by a consistent
   algorithm across all A-ERs. The capacity and preference
   indexes are configured to the A-ER.

   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

4. New Flags added to FAD Flags Sub-TLV

   A New flag is added to indicate a constrained SPF compute
   method is needed for the prefix.


     0 1 2 3 4 5 6 7...
     |M|P| | ...

   P-flag: Site-Cost Metrics is included in deriving
   Constrained IGP path to the prefix.

5. Characteristics of the Site Cost Metric

   The "Site Cost" associated with a prefix (i.e., ANYCAST
   prefix) can be a value configured on the router to which
   the prefix is attached. The "site Cost" can be computed
   based on an algorithm configured on router for specific
   prefixes. The actual mechanism of assigning "Site Cost" or
   the detailed algorithm is out of the scope of document. It

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   is important that the "site cost" metric doesn't change too
   frequently to avoid route oscillation within the network.

   The "site cost" change rate is comparable with the rate
   that the application controller adds or removes the
   application instances at locations to adjust the workload
   distribution. Typically, the rate of change could be in
   days. On rare occasions, there might need rate changes in

6. "Site-Cost" Advertisement in OSPF

     - IPv4: OSPFv2
        A new Aggregated Cost Sub-TLV needs to be added to
        OSPFv2 Extended Prefix TLV [RFC7684]

     - IPv6: OSPFv3
        A new sub-TLV can be appended to the E-Intra-Area-
        Prefix-LSA, E-Inter-Area-Prefix-LSA, E-AS-External-
        LSA, and E-Type-7-LSA [RFC8362] to carry the
        Aggregated Cost.

 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                        |
           Figure 1: Aggregated cost Advertisement in IS-IS

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

   Aggregated Cost can be appended as subTLV to the Extended
   IP Reachability TLV 135 for IPv4 [RFC5305] and 236 for IPv6

   For Multi-Topology with non-zero IDs, the Aggregated Cost
   SubTLV can be carried by Multi-topology TLV 235 for IPv4
   and 237 for IPv6 [RFC5120].

 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        |

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|              AggCost to the App Server                        |
             Figure 2: Aggregated cost Advertisement in IS-IS

8. Alternative method for Distributing Aggregated Cost

   Section 6 and Section 7 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

   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-

9. Manageability Considerations

     To be added.

10. Security Considerations

   To be added.

11. IANA Considerations

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

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

   P-flag added to FAD Flags Sub-TLV to indicate that the
   Site-Cost Metrics is included in deriving Constrained IGP
   path to the prefix.

12. References

 12.1. 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

   [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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 12.2. 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",
             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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13. Appendix A: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

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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

 13.1. Metrics to change traffic flow patterns

   When UEs pattern changes, the Application controller can
   instantiate more instances at certain locations to
   accommodate higher demand.

   However, network layer can offer a simpler solution. By
   adjusting the site cost for the prefix at specific egress
   routers, IGP distribution of those site cost plus the flex
   algorithm can increase (or decrease) flows for the specific
   prefixes towards the certain locations.

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     - 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.

 13.2. Reason for using IGP Based Solution

    Here are some benefits of using IGP to propagate the IP
    Layer App-Metrics:
    - Intermediate routers can utilize 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.

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 13.3. 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
   Flow Label extracted from the IPv6 Header along the same

14. Acknowledgments

   Acknowledgements to Peter Psenak, Les Ginsberg, Robert
   Raszuk, Acee Lindem, Shraddha Hegde, Tony Li, Gyan Mishra,
   Jeff Tantsura, and Donald Eastlake for their review and

   This document was prepared using

Authors' Addresses

   Linda Dunbar

   Huaimo Chen

   Aijun Wang
   China Telecom

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