\
Network Working Group                                L. Dunbar
Internet Draft                                         H. Chen
Intended status: Standard                            Futurewei
Expires: June 16, 2022                             Aijun Wang
                                                 China Telecom
                                               January 7, 2022

           IGP Extension for 5G Edge Computing Service
               draft-dunbar-lsr-5g-edge-compute-03

Abstract
   Routers in 5G Local Data Network (LDN) can use additional
   site-costs, preference, and other application related
   metrics in addition to the network routing distance to
   compute constraint-based SPF within the 5G LDN to enhance
   performance for selected services. This draft describes
   using application server related metrics to influence the
   SPF and Flexible Algorithms to indicate the constraints.

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

Copyright Notice

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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. New Flags added to FAD Flags Sub-TLV................... 6

   5. "Site-Cost" Advertisement in OSPF...................... 7

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

   7. Alternative method for Distributing Aggregated Cost.... 7

   8. Manageability Considerations........................... 8


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   9. Security Considerations................................ 8

   10. IANA Considerations................................... 8

   11. References............................................ 8
      11.1. Normative References............................. 9
      11.2. Informative References........................... 9

   12. Appendix:5G Edge Computing Background................ 11

   13. 5G EC LDN Characteristics for the Constraint SPF..... 12
      13.1. IP Layer Metrics to Gauge EC Server Running Status
      ...................................................... 12
      13.2. App Metrics Constrained Shortest Path First..... 14
      13.3. Reason for using IGP Based Solution............. 15
      13.4. Flow Affinity to an ANYCAST server.............. 15

   14. Acknowledgments...................................... 16

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 for 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.


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   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 than
   the routing distance from the latency and performance
   perspective.

   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 is assigning the same IP address for multiple
   servers in 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 in multiple locations of the
   5G Edge Computing environment can be problematic because
   all those locations can be close in proximity. There might
   be a very small difference in the routing distance to reach
   an Application Server attached to a different edge router.

   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.



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   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
               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





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   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. Solution Overview

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

     - 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 an EC server (i.e., ANYCAST
   prefix) is computed based on the IP layer App-related
   metrics [Section 12.1], such as Load Measurement, the
   Capacity Index, the Preference Index, and other constraints
   by a consistent algorithm across all A-ERs.

   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.


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.

   Flags:

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




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

5. "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


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

   Aggregated Cost appended to the IP Reachability TLV: 128,
   130, or 135.

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 2: Aggregated cost Advertisement in IS-IS


7. Alternative method for Distributing Aggregated Cost

   Section 6 and Section 7 demonstrate different ways for
   OSPFv2, OSPFv3, and ISIS to propagate the aggregated cost.


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   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]

8. Manageability Considerations

     To be added.

9. Security Considerations


   To be added.

10. IANA Considerations

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

     - 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.

11. References






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 11.1. 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.




 11.2. Informative References

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

   [5G-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.


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   [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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12. 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





13. 5G EC LDN Characteristics for the Constraint SPF

 13.1. 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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 13.2. 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.

 13.3. 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.

 13.4. 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.



14. 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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