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 draft-dunbar-lsr-5g-edge-compute-04 Abstract 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 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 groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt xxx, et al. Expires July 15, 2022 [Page 1] Internet-Draft LSR Extension for 5G EC Service 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) 2022 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 Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 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 Dunbar, et al. Expires July 15, 2022 [Page 2] Internet-Draft LSR Extension for 5G EC Service 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. Dunbar, et al. Expires July 15, 2022 [Page 3] Internet-Draft LSR Extension for 5G EC Service 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 perspective. 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. Dunbar, et al. Expires July 15, 2022 [Page 4] Internet-Draft LSR Extension for 5G EC Service 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 forwarding. 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 location. Dunbar, et al. Expires July 15, 2022 [Page 5] Internet-Draft LSR Extension for 5G EC Service 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 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 application instances directly attached to Dunbar, et al. Expires July 15, 2022 [Page 6] Internet-Draft LSR Extension for 5G EC Service - 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 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| | ... +-+-+-+-+-+-+-+-+... 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 Dunbar, et al. Expires July 15, 2022 [Page 7] Internet-Draft LSR Extension for 5G EC Service 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 hours. 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 [RFC5308]. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Dunbar, et al. Expires July 15, 2022 [Page 8] Internet-Draft LSR Extension for 5G EC Service | 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 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] 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 FlexAlgo. Dunbar, et al. Expires July 15, 2022 [Page 9] Internet-Draft LSR Extension for 5G EC Service - 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 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. Dunbar, et al. Expires July 15, 2022 [Page 10] Internet-Draft LSR Extension for 5G EC Service 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", 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. Dunbar, et al. Expires July 15, 2022 [Page 11] Internet-Draft LSR Extension for 5G EC Service 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 seamless. Dunbar, et al. Expires July 15, 2022 [Page 12] Internet-Draft LSR Extension for 5G EC Service +--+ |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. Dunbar, et al. Expires July 15, 2022 [Page 13] Internet-Draft LSR Extension for 5G EC Service - 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. Dunbar, et al. Expires July 15, 2022 [Page 14] Internet-Draft LSR Extension for 5G EC Service 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 path. 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 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 Dunbar, et al. Expires July 15, 2022 [Page 15]