rtgwg                                                             P. Liu
Internet-Draft                                                  T. Jiang
Intended status: Informational                              China Mobile
Expires: 8 September 2022                                     P. Eardley
                                                         British Telecom
                                                              D. Trossen
                                                                   C. Li
                                                     Huawei Technologies
                                                            7 March 2022

                 Dynamic-Anycast (Dyncast) Requirements


   This draft provides requirements for an architecture addressing the
   problems outlined in the use case and problem statement draft for
   Dyncast[I-D.liu-dyncast-ps-usecases] .

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   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definition of Terms . . . . . . . . . . . . . . . . . . . . .   3
   3.  Desirable System Characteristics and Requirements . . . . . .   3
     3.1.  Anycast-based Service Addressing Methodology  . . . . . .   3
     3.2.  Instance Affinity . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Proper Runtime-state Granularity and Keeping  . . . . . .   5
     3.4.  Encoding Metrics  . . . . . . . . . . . . . . . . . . . .   5
     3.5.  Signaling Metrics . . . . . . . . . . . . . . . . . . . .   6
     3.6.  Using Metrics in Routing Decisions  . . . . . . . . . . .   6
     3.7.  Supporting Service Dynamism . . . . . . . . . . . . . . .   7
   4.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Computing service instances instantiated at multiple geographical
   edge sites are used to better realize an edge computing service in
   edge computing use cases, as shown in[I-D.liu-dyncast-ps-usecases].
   To optimally deliver the service request to the most appropriate
   service instance is the fundamental requirement in such deployments.
   As shown in [I-D.liu-dyncast-ps-usecases], choosing the most
   appropriate service instance should take both, the computing
   resources available and the network path quality, into consideration.
   "Optimal" here additionally means the architecture and overall
   mechanism should be efficient, support high dynamism, while
   maintaining instance affinity, as shown in

   This draft provides the requirements to realize the potential dynamic
   anycast architecture by alleviating the problems of existing
   solutions outlined in [I-D.liu-dyncast-ps-usecases]

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2.  Definition of Terms

   Service:  A monolithic functionality that is provided by an endpoint
     according to the specification for said service.  A composite
     service can be built by orchestrating monolithic services.

   Service instance:  Running environment (e.g., a node) that makes the
     functionality of a service available.  One service can have several
     instances running at different network locations.

   Service identifier:  Used to uniquely identify a service, at the same
     time identifying the whole set of service instances that each
     represent the same service behaviour, no matter where those service
     instances are running.

   Anycast:  An addressing and packet sending methodology that assign an
     "anycast" identifier for one or more service instances to which
     requests to an "anycast" identifier could be routed, following the
     definition in [RFC4786] as anycast being "the practice of making a
     particular Service Address available in multiple, discrete,
     autonomous locations, such that datagrams sent are routed to one of
     several available locations".

   Dyncast:  Dynamic Anycast, taking the dynamic nature of computing
     resource metrics into account to steer an anycast-like decision in
     sending an incoming service request.

3.  Desirable System Characteristics and Requirements

   In the following, we outline the desirable characteristics of a
   system to overcome the observed problems in
   [I-D.liu-dyncast-ps-usecases] for the realization of the use cases
   described in that document.

3.1.  Anycast-based Service Addressing Methodology

   A unique service identifier is used by all the service instances for
   a specific service no matter which edge it attaches to.  An anycast
   like addressing and routing methodology among multiple edges makes
   sure the data packet can potentially reach any of the edges with the
   service instance attached.  At the same time, each service instance
   has its own unicast address to be used by the attaching edge to
   access the service.Since a client will use the service identifier as
   the destination addressing, mapping of the service identifier to the
   unicast address will need to happen in-band, considering the metrics
   for selection to make this selection service-specific.  From an
   addressing perspective, a desirable system for the realization of the
   use cases described in that document.

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   o MUST provide a discovery and mapping methodology for the in-band
   mapping of the service identifier (an anycast address) to a specific
   unicast address.

3.2.  Instance Affinity

   A routing relation between a client and a service exists not at the
   packet but at the service request level in the sense that one or more
   service requests, possibly consisting of one or many more routing-
   level packets, must be ensured to be sent to said service.Each
   service may be provided by one or more service instances, each
   providing equivalent service functionality to their respective
   clients, while those service instances may be deployed at different
   locations in the network.  With that, the routing problem becomes one
   between the client and a selected service instance for at least the
   duration of the service-level request, but possibly more than just
   one request.

   This relationship between the client and the chosen service instance
   is described as "instance affinity" in the following, where the
   "affinity" spans across the aforementioned one or more service
   requests.  This impacts the routing decision to be taken in that the
   normal packet level communication, i.e., each packet is forwarded
   individually based on the forwarding table at the time, will need
   extending with the notion of instance affinity since otherwise
   individual packets may be sent to different places when the network
   status changes, possibly segmenting individual requests and breaking
   service-level semantics.

   The nature of this affinity is highly dependent on the nature of the
   specific service.  The minimal affinity of a single request
   represents a stateless service, where each service request may be
   responded to without any state being held at the service instance for
   fulfilling the request.  Providing any necessary information/state
   in-band as part of the service request, e.g., in the form of a multi-
   form body in an HTTP request or through the URL provided as part of
   the request, is one way to achieve such stateless nature.
   Alternatively, the affinity to a particular service instance may span
   more than one request, as in our VR example in
   [I-D.liu-dyncast-ps-usecases], where previous client input is needed
   to render subsequent frames.  Therefore, a desirable system

   o MUST maintain "instance affinity" which MAY span one or more
   service requests, i.e., all the packets from the same flow MUST go to
   the same service instance.

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3.3.  Proper Runtime-state Granularity and Keeping

   The instance affinity, as outlined in Section 3.2, requires a client
   and the chosen service instance to keep persistent relationship
   across one or more service requests.  For a multi-request session,
   this determines that the mapping logic has to consistently pick up
   the same service instance.  This type of affinity can be normally
   achieved by deploying a mapping device to keep in-place all the
   necessary states.  However, a client, e.g., a mobile UE, has
   generally many applications running.  If all, or majority, of the
   applications request the dyncast-like services, then the runtime
   states that need to be created and accordingly maintained would
   require high granularity.  In the extreme scenario, this granular
   requirement could reach the level of per-UE per-APP per-(sub)flow
   with regard to a service instance.

   Evidently, these fine-granular runtime states can potentially become
   heavy burden for network devices if they have to dynamically create
   and maintain them.  On the other hand, it is not appropriate either
   to place the state-keeping task on clients themselves.  Therefore, a
   desirable system

   o MUST avoid keeping fine runtime-state granularity in network nodes
   in order to achieve instance affinity.

   o MUST provide mechanism to free clients from maintaining granular
   runtime-states in order to achieve instance affinity.

3.4.  Encoding Metrics

   As outlined in the scenarios in [I-D.liu-dyncast-ps-usecases],
   metrics can have many different semantics, particularly if considered
   to be service- specific.  Even the notion of a "computing load"
   metric may be computed in many different ways.  What is crucial,
   however, is the representation and encoding of that metric when being
   conveyed to the routing fabric in order for the routing elements to
   act upon those metrics.  Such representation may entail information
   on the semantics of the metric or it may be purely one or more
   semantic-free numerals.  Agreement of the chosen representation among
   all service and network elements participating in the service-
   specific routing decision is important.  Specifically, a desirable

   o MUST agree on the service-specific metrics and their representation
   between service elements in the participating edges in the network
   and network elements acting upon them.

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   o MAY obfuscate the specific semantic of the metric to preserve
   privacy of the service provider information towards the network

   o MAY include routing protocol metrics

3.5.  Signaling Metrics

   The aforementioned representation of metrics needs conveyance to the
   network elements that will need to act upon them.  Depending on the
   service-specific decision logic, one or more metrics will need to be
   conveyed.  Problems to be addressed here may be that of loop
   avoidance of any advertisement of metrics as well as the frequency of
   such conveyance and therefore the overall load that the signaling may
   add to the overall network traffic.  While existing routing protocols
   may serve as a baseline for signaling metrics, other means to convey
   the metrics can equally be realized.  Specifically, a desirable

   o MUST provide mechanisms to signal the metrics for using in routing

   o MUST realize means for rate control for signaling of metrics

   o MUST implement mechanisms for loop avoidance in signaling metrics,
   when necessary

3.6.  Using Metrics in Routing Decisions

   Metrics being conveyed, as outlined in Section 3.4, in the agreed
   manner, as outlined in Section 3.3, will ultimately need suitable
   action in the routers of the network.  Routing decisions can be
   manifold, possibly including (i) min or max over all metrics, (ii)
   extending previous action with a random or first choice when more
   than one min/max entry found, (iii) weighted round robin of all
   entries, among others.  It is important for the proper work of the
   service-specific routing decision, that it is understood to both
   network and service provider, which action (out of a possible set of
   supported actions) is to be used for a particular set of metrics.
   Specifically, a desirable system

   Further, different network nodes, e.g., routers, switches, etc., bear
   diversified capabilities even in the same routing domain, let alone
   in different administrative domains.  So, the service-specific
   metrics that have been adopted by some nodes might not be supported
   by others, either due to technical reasons, administrative reasons,
   or something else.  There could be some scenario that a node
   supporting service-specific metrics might prefer some type of metrics

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   to others [3GPP-TR22.847], or, in another scenario, even not utilize
   any at all.  Therefore, there must exist flexibility in term of
   metrics handling and routing decisions in a network.

   o MUST specify a default action to be taken, if more than one action

   o MUST allow a network node not supporting service-specific metrics
   to interoperate with the supporting ones, i.e., providing backward

   o SHOULD allow the prioritization of using the service-specific
   metrics when compared to the currently widely-used networking
   metrics, like bandwidth, delay, loss, etc.

   o SHOULD enable other alternative actions to be taken. (1)Any
   solution MUST provide appropriate signaling of the desired action to
   the router.  For this, the action MAY be signaled in combination with
   signaling the metric (see Section 3.4). (2)Any solution SHOULD allow
   associating the desired action to a specific service identifier.

3.7.  Supporting Service Dynamism

   Network cost in the current routing system usually does not change
   very frequently.  However, computing load and service-specific
   metrics in general can be highly dynamic, e.g., changing rapidly with
   the number of sessions, CPU/GPU utilization and memory space.  It has
   to be determined at what interval or events such information needs to
   be distributed among edges.  More frequent distribution of more
   accurate synchronization may result in more overhead in terms of

   Choosing the least path cost is the most common rule in routing.
   However, the logic does not work well when routing should be aware of
   service-specific metrics.  Choosing the least computing load may
   result in oscillation.  The least loaded edge can quickly be flooded
   by the huge number of new computing demands and soon become
   overloaded with tidal effects possibly following.

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   Generally, a single instance may have very dynamic resource
   availability over time in order to serve service requests.  This
   availability may be affected by computing resource capability and
   load, network path quality, and others.  The balancing mechanisms
   should adapt to the service dynamism quickly and seamlessly.  With
   this, the relationship between a single client and the set of
   possible service instances may possibly be very dynamic in that one
   request that is being dispatched to instance A may be followed by a
   request that is being dispatched to instance B and so on, generally
   within the notion of the service-specific service affinity discussed
   before in Section 3.2.  With this in mind, a desirable system

   o MUST support the dynamics of metrics changing on, e.g., a per flow
   basis, without violating the metrics defined in the selection of the
   specific service instance, while taking into account the requirements
   for the signaling of metrics and routing decision (see Section 3.4
   and 3.5).

4.  Conclusion

   This document presents high-level requirements for solutions to
   Dyncast, where the architecture should address how to distribute the
   resource information and how to assure instance affinity in an
   anycast based service addressing environment, while realizing
   appropriate routing actions to satisfy the metrics provided.

5.  Security Considerations


6.  IANA Considerations

   No IANA action is required so far.

7.  Contributors

   The following people have substantially contributed to this document:

           Peter Willis

8.  Informative References

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
              December 2006, <https://www.rfc-editor.org/info/rfc4786>.

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              Liu, P., Willis, P., Trossen, D., and C. Li, "Dynamic-
              Anycast (Dyncast) Use Cases & Problem Statement", Work in
              Progress, Internet-Draft, draft-liu-dyncast-ps-usecases-
              02, 17 January 2022, <https://www.ietf.org/archive/id/

   [TR22.874] 3GPP, "Study on traffic characteristics and performance
              requirements for AI/ML model transfer in 5GS (Release
              18)", 2020.


   The author would like to thank Yizhou Li, Luigi IANNONE and Geng
   Liang for their valuable suggestions to this document.

Authors' Addresses

   Peng Liu
   China Mobile
   Email: liupengyjy@chinamobile.com

   Tianji Jiang
   China Mobile
   Email: jiangtianji@chinamobile.com

   Philip Eardley
   British Telecom
   Email: philip.eardley@bt.com

   Dirk Trossen
   Huawei Technologies
   Email: dirk.trossen@huawei.com

   Cheng Li
   Huawei Technologies
   Email: c.l@huawei.com

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