rtgwg                                                              Y. Li
Internet-Draft                                                L. Iannone
Intended status: Informational                                D. Trossen
Expires: 25 July 2022                                Huawei Technologies
                                                                  P. Liu
                                                            China Mobile
                                                                   C. Li
                                                     Huawei Technologies
                                                         21 January 2022

                      Dynamic-Anycast Architecture


   This document describes a proposal for an architecture for the
   Dynamic-Anycast (Dyncast).  It includes an architecture overview,
   main components that shall exist, and the workflow.  An example of
   workflow is provided, focusing on the load-balance multi-edge based
   service use-case, where load is distributed in terms of both
   computing and networking resources through the dynamic anycast

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on 25 July 2022.

Copyright Notice

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

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definition of Terms . . . . . . . . . . . . . . . . . . . . .   3
   3.  Architecture Main Concepts  . . . . . . . . . . . . . . . . .   4
   4.  Dyncast Architecture Workflow . . . . . . . . . . . . . . . .   8
     4.1.  Service Notification/Metrics Update . . . . . . . . . . .   8
     4.2.  Service Demand Dispatch and Instance Affinity . . . . . .  10
       4.2.1.  Service Demand Dispatch and Instance Affinity on
               D-Routers ingress . . . . . . . . . . . . . . . . . .  10
       4.2.2.  Service Demand Dispatch and Instance Affinity on
               D-Forwarders ingress  . . . . . . . . . . . . . . . .  11
   5.  Dyncast Control-plane vs Data-plane operations  . . . . . . .  13
   6.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     9.1.  Informative References  . . . . . . . . . . . . . . . . .  15
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Edge computing is expanding from a single edge nodes to multiple
   networked collaborating edge nodes to solve the issues like response
   time, resource optimization, and network efficiency.

   The current network architecture in edge computing provides
   relatively static service dispatching, for example, to the closest
   edge from an IGP perspective, or to the server with the most
   computing resources without considering the network status, and even
   sometimes just based on static configuration.

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   Networking taking into account computing resource metrics seems to be
   an interesting paradigm that fits numbers of use-cases that would
   benefit from such capability [I-D.liu-dyncast-ps-usecases].  Yet,
   more investigation is still needed in key areas for this paradigm
   and, to this end, this document aims at providing an architectural
   framework, which will enable service notification, status update, and
   service dispatch in edge computing..

   The Dyncast architecture presents an anycast based service and access
   model addressing the problematic aspects of existing network layer
   edge computing service deployment, including the unawareness of
   computing resource information of service, static edge selection,
   isolated network and computing metrics and/or slow refresh of status.

   Dyncast assumes that there are multiple equivalent service instances
   running on different edge nodes, globally providing (from a logical
   point of view) one single service.  A single edge may have limited
   computing resources available, and different edges likely have
   different resources available, such as CPU or GPU.  The main
   principle of Dyncast is that multiple edge nodes are interconnected
   and collaborate with each other to achieve a holistic objective,
   namely to dispatch service demands taking into account both service
   instances status as well as network state (e.g., paths length and
   their congestion).  For this, computing resources available to serve
   a request is one of the top metrics to be considered.  At the same
   time, the quality of the network path to an edge node may vary over
   time and may hence be another key attribute to be considered for said
   dispatching of service demands.

2.  Definition of Terms

   Dyncast:  As defined in [I-D.liu-dyncast-ps-usecases], Dynamic
     Anycast, taking the dynamic nature of computing resource metrics
     into account to steer an anycast routing decision.

   Service:  As defined in [I-D.liu-dyncast-ps-usecases], a service
     represents a defined endpoint of functionality encoded according to
     the specification for said service.

   Service instance:  As defined in [I-D.liu-dyncast-ps-usecases], one
     service can have several instances running on different nodes.
     Service instance is a running environment (e.g., a node) that makes
     the functionality of a service available.

   D-Router:  A node supporting Dyncast functionalities as described in

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     this document.  Namely it is able to understand both network-
     related and service-instances-related metrics, take forwarding
     decision based upon and manitain instance affinity, i.e., forwards
     packets belonging to the same service demand to the same instance.

   D-MA:  Dyncast Metric Agent (D-MA): A dyncast specific agent able to
     gather and send metric updates (from both network and instance
     prespective) but not performing forwarding decisions.  May run on a
     D-Router, but it can be also implementated as a separate module
     (e.g., a software library) collocated with a service instance.

   D-Forwarder:  An optional element able to forward packets towards a
     service instance, while not receiving any metric and as such not
     being able to make any decision when a new service demand arrives.
     it relies on a D-Router for the decision, it only guarantees
     instance affinity.

   D-SID:  Dyncast Service ID, an identifier representing a service,
     which the clients use to access said service.  Such identifier
     identifies all of the instances of the same service, no matter on
     where they are actually running.  D-SID is independent of which
     service instance serves the service demand.  Usually multiple
     instances provide a (logically) single service, and service demands
     are dispatched to the different instance through an anycast model,
     i.e., choosing one instance among all available instances.

   D-BID:  Dyncast Binding D-Node, an address to reach a service
     instance for a given D-SID.  It is usually a unicast IP where
     service instances are attached.  Different service instances
     provide the same service identified through D-SID but with
     different Dyncast Binding IDs.

   Service demand:  The demand for a specific service and addressed to a
     specific D-SID.

   Service request:  The request for a specific service and addressed to
     a specific service instance identified with D-BID.

3.  Architecture Main Concepts

   Edge sites (edges for short) are normally the sites where edge
   computing is performed.  Service instances are initiated at different
   edge sites.  Thus, a single service can actually have a significant
   number of instances running on different edges.  A Dyncast Service ID
   (D-SID) is used to uniquely identify a service (e.g., a matrix
   computation for face recognition, or a game server).  Service
   instances can be hosted on servers, virtual machines, access routers
   or gateway in edge data center.

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   Close to (one or more) Service instances is the Dyncast Metric Agent
   (D-MA).  This element has the task to gather information about
   resources and status of the different instances as well as network-
   related information.  Such element may also run in a dyncast-enable
   router (named D-Router), while other deployement scenarios may lead
   to this element running separately on edge nodes.

   A D-Router is actually the main element in a Dyncast network,
   providing the capability to exchange the information about the
   computing resources information of service instances which have been
   gathered through D-MAs.  A D-Router can also be a service access
   point for clients.  When a service demand arrives, it will be
   delivered to the most appropriate service instance.  A service demand
   may be the first packet of a data flow rather than an explicit out of
   band service request.  This achitectural document does not make any
   specific assumption on this matter.  This documents only assumes

   *  D-Routers are able to identify new service demands.  The Dyncast
      architecture presented in this document allows then to deliver
      such a packet to the most appropriate service instance according
      to information received from D-MAs and other D-Routers.

   *  D-Router are able to identify packets belonging to an existing
      service demand.  The Dyncast architecture presented in this
      document allows to deliver these packets always to the same
      service instance selected at the initial service demand.  We term
      this capability as 'instance affinity'.

   The element introduced above are depicted in Figure 1, which shows
   the proposed Dyncast architecture.  In Figure 1, the "infrastructure"
   indicates the general IP infrastrucutre that does not necessarily
   need to suppoort Dyncats, i.e., not all routers of the infrastructure
   need to be D-Routers.

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       edge site 1          edge site 2            edge site 3

        +------------+                          +------------+
      +------------+ |                        +------------+ |
      |  service   | |                        |  service   | |
      |  instance  |-+                        |  instance  |-+
      +------------+                          +------------+
            |                                        |
       +----------+                                  |
       |   D-MA   |                                  |
       +----------+                             +----------+
            |           +-----------------+     |   D-MA   |
       +----------+     |                 |     +----------+
       |D-Router 1| ----|  Infrastructure |---- |D-Router 3|
       +----------+     |                 |     +----------+
            |           +-----------------+          |
            |                    |                   |
            |                    |                   |
      +-----------+         +----------+             |
      |D-Forwarder|         |D-Router 2|             |
      +-----------+         +----------+             |
            |                    |                   |
            |                    |                   |
         +-----+              +------+           +------+
       +------+|            +------+ |         +------+ |
       |client|+            |client|-+         |client|-+
       +------+             +------+           +------+

                      Figure 1: Dyncast Architecture.

   Figure 2 shows an example of Dyncast deployement, with 2 service
   instatiated twice (2 instances) on two different edges, namely edge
   site 2 and 3.  Those service instances utilize different D-BIDs to
   serve service demands.  The edge site 3 uses a standalone D-MA to
   report its metrics to the Dyncast system and, since no client is
   present at that edge, there is no need of a D-Router.  Edge site 2
   instead, collocates the D-MA with a D-router since client are

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    D-SID: Dyncast Service ID
    D-BID: Dyncast Binding ID

            Service/Metrics Information
            (D-SID 1, D-BID 21, <metrics>)
            (D-SID 2, D-BID 22, <metrics>)

                               +-------+ |           D-SID 1
                               |Clients|-+         +--------+
                               +-------+        +--|D-BID 21| instance 1
                                   |            |  +--------+
                             +----------+----+  |              Edge 2
                             |D-Router 2|D-MA|--|    D-SID 2
                             +----------+----+  |  +--------+
                                   |            +--|D-BID 22| instance 2
                           +----------------+      +--------+
                           |                |
                           |                |
   +------+  +----------+  |                |
   |Client|--|D-Router 1|--| Infrastructure |
   +------+  +----------+  |                |
                           |                |       D-SID 2
                           |                |      +--------+
                           +----------------+  +---|D-BID 32| instance 3
                                   |           |   +--------+
                                   |       +------+            Edge 3
                                   +-------| D-MA |
                                           +------+  D-SID 1
                                               |   +--------+
                                               +---|D-BID 31| instance 4

              (D-SID 2, D-BID 32, <metrics>)
              (D-SID 1, D-BID 31, <metrics>)
               Service/Metrics Information

                   Figure 2: Dyncast deployment example.

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   In Figure 2, the Dyncast Service ID (D-SID) follows an anycast
   semantic, such as provided through an IP anycast address.  It is used
   to access a specific service no matter which service instance
   eventually handles the service demand of the client.  Clients or
   other entities which want to access a service need to know about its
   D-SID in advance.  It can be achieved in different ways, for example,
   using a special range of addresses associated to a certain service or
   coding of anycast IP address as D-SID, or using DNS.

   The Dyncast Binding ID (D-BID) is a unicast IP address.  It is
   usually the interface IP address through to reach a specific service
   instance.  Mapping and binding a D-SID to a D-BID is dynamic and
   depends on the computing and network status at the time the service
   demand first arrives (see Section 4.1 for the reporting of such
   status).  To ensure instance affinity, D-Routers are requested to
   remember the instance that has been selected (e.g., by storing the
   mapping) for delivering all packets to the same instance (see
   Section 4.2 for discussing this aspect).

4.  Dyncast Architecture Workflow

   The following subsections provide an overview of how the
   architectural elements introduced in the previous section do work

4.1.  Service Notification/Metrics Update

   When a service instance is instantiated/terminated the service
   information consisting in the mapping between the D-SID and the D-BID
   has to be updated/deletetd as well.  An update can also be triggered
   by a change in relevant metrics (e.g., an instance becomes
   overloaded).  Computing resource information of service instance is
   key information in Dyncast.  Some of them may be relatively static
   like CPU/GPU capacity, and some may be very dynamic, for example,
   CPU/GPU utilization, number of sessions associated, number of queuing
   requests.  Changes in service-related relavant information has to be
   collected by D-MA associated to each service instance.  Various ways
   can be used, for example, via routing protocols like EBGP or via an
   API of a management system.  Conceptually a D-Router collects
   information coming from D-MA and keeps track of the IDs and computing
   metrics of all service instances.

   Figure 2 shows an example of information shared by the Dyncast
   elements.  The D-MA which is deployed with D-Router2 shares binding
   information concerning the two instances of the two services running
   on edge 2 (upper right hand side of the figure).  These information

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   *  (D-SID 1, D-BID 21, metrics)

   *  (D-SID 2, D-BID 22, metrics)

   The D-MA which is deployed as a separate module on edge 3 (lower
   right hand side of the figure) shares binding information concerning
   the two instances of the two services running on edge 3.  These
   information is:

   *  (D-SID 1, D-BID 31, metrics)

   *  (D-SID 2, D-BID 32, metrics)

   Dyncast nodes share among themselves the service information
   including the associated computing metrics for the service instances
   attached to them.  As a network node, a D-Router can also monitor the
   network cost or metrics (e.g., congestion) to reach other D-Routers.
   This is the focus of Dyncast control plane.  Different mechanisms can
   be used to share such information, for instance BGP ([RFC4760]), an
   IGP, or a controller based mechanism.  The specific mechanism is
   beyond the scope of this document.  The architecture assumes that the
   Dyncast elements are able to share relevant information.

   If, for instance, the client on the left hand side of Figure 2 sends
   a service demand for D-SID1, D-Router1 has the knowledge of the
   status of the service instance on both edge 2 and edge 3 and can make
   a decision toward which D-BID to forward the demand.

   There are different ways to represent the computing metrics.  A
   single digitalized value calculated from weighted attributes like
   CPU/GPU consumption and/or number of sessions associated may be used
   for simplicity reasons.  However, it may not accurately reflect the
   computing resources of interest.  Multi-dimensional values give finer
   information.  This architectural document does not make any specific
   assumption about metrics and how to encode or even use them.  As
   stated in Section 3, the only assumption is that a D-Node is able to
   use such metrics so to take a decision when a service demand arrives
   in order to map the demand onto a suitable service request.

   As explained in the problem statement document
   [I-D.liu-dyncast-ps-usecases], computing metrics may change very
   frequently, when and how frequent such information should be
   exchanged among Dyncats elements should be determined also in
   accordance with the distribution protocol used for such purpose.  A
   spectrum of approaches can be employed,such as interval based
   updates, threshhold triggered updates, policy based updates, etc.

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4.2.  Service Demand Dispatch and Instance Affinity

   This is the focus of the Dyncast data plane.  When a new flow
   (representing a service demand) arrives at a Dyncast ingress, such
   ingress node selects the most appropriate egress according to the
   network status and the computing resource of the attached service

   In the Dyncast Architecture there are two possible type of ingress,
   namely D-Routers and D-Forwarders, which are discussed in the

4.2.1.  Service Demand Dispatch and Instance Affinity on D-Routers

   Instance affinity is one of the key features that Dyncast should
   support.  It means that packets from the same 'flow' for a service
   should always be sent to the same egress to be processed by the same
   service instance.  The affinity is determined at the time of newly
   formulated service demand.

   It is worth noting that different services may have different notions
   of what constitutes a 'flow' and may thus identify a flow
   differently.  Typically a flow is identified by the 5-tuple value.
   However, for instance, an RTP video streaming may use different port
   numbers for video and audio, and it may be identified as two flows if
   5-tuple flow identifier is used.  However they certainly should be
   treated by the same service instance.  Therefore a 3-tuple based flow
   identifier is more suitable for this case.  Hence, it is desired to
   provide certain level of flexibility in identifying flows, or from a
   more general perspective, in identifying the set of packets for which
   to apply instance affinity.  More importantly, the means for
   identifying a flow for the purpose of ensuring instance affinity must
   be application-independent to avoid the need for service-specific
   instance affinity methods.

   Specifically, Instance affinity information should be configurable on
   a per-service basis.  For each service, the information can include
   the flow/packets identification type and means, affinity timeout
   value, and etc.  For instance, the affinity configuration can
   indicate what are the values, e.g., 5-tuple or 3-tuple, to be used as
   the flow identifier.

   When the most appropriate egress and service instance is determined
   when a new flow for a service demand arrives, a binding table should
   save this association between new service demand and service instance
   selection.  The information in such binding table may include flow/
   packets identification, affinity timeout value, etc.  The subsequent

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   packets matching the entry are forwarded based on the table.
   Figure 3 shows a possible example of flow binding table at the
   ingress D-Router.

   |       Flow/Packets Identifier           |                |        |
   +------+--------+---------+--------+------+  D-BID egress  | timeout|
   |src_IP| dst_IP |src_port |dst_port|proto |                |        |
   | X    | D-SID 2|   -     |  8888  | tcp  |    D-BID  32   |  xxx   |
   | Y    | D-SID 2|   -     |  8888  | tcp  |    D-BID  12   |  xxx   |

          Figure 3: Example of what a binding table can look like.

4.2.2.  Service Demand Dispatch and Instance Affinity on D-Forwarders

   When a D-Router maintains the binding table, the memory consumed is
   determined by the number of different service demands that a Dyncast
   ingress node handles.  The ingress node can be an edge data center
   gateway, hence it may cover hundreds of thousands of users and each
   user may have tens of flows, creating a concern regarding the memory
   space consumption for the binding table at the Dyncast ingress node.
   To alleviate that concern, the Dyncast Forwarder (D-Forwarder for
   short) can be used and take an active role.

   The D-Forwarder is deployed closer to the clients and it normally
   handles the traffic and service demands of a single or a few clients.
   In this case, the memory required by the binding table will be much
   smaller since the number of entries is now limited to the number of
   local clients only.  Furthermore, the D-Forwarder is not a D-Router,
   that is to say, it does not participate in the status update about
   network and computing metrics among D-Routers.  A D-Forwarder does
   not determine the best egress to forward packets when there is a new
   service demand.  Instead, it has to learn such information from a
   D-Router and maintains it to ensure the instance affinity for
   subsequent packets.  In this way, the D-routers may be relieved from
   binding table maintenance.

   Figure 4 shows the interaction between D-Forwarders and D-Routers.
   The figures show a scenario similar to Figure 2, with the addition of
   a D-Forwarder in front of D-Router1.  When a new service demand
   arrives at a D-Forwarder, the latter has no suitable entry in its
   binding table that allows forwarding the packet to an egress.  As a
   consequence, the D-Forwarder forwards the service demand to a
   D-Router, while marking the 'miss' of matching the demand onto a

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   suitable binding address in the forwarded packet.  Upon receiving the
   service demand, the D-Router, having access to all of the relevant
   metric information, will select the most suitable egress, i.e.,
   service instance, and forward the packet as a service request to the
   chosen service instance.  Based on the 'miss' indication in the
   received service demand, the D-Router will also inform the
   D-Forwarder about the selected egress.  This will allow the
   D-Forwarder to maintain the binding table to ensure the mapping of
   any subsequent service demand.

   The control messages exchange between the D-Forwarder and its
   corresponding D-Router needs to be defined, but is out of the scope
   of this document.  D-Routers have to be also able to inform
   D-Forwarders if there is any issue concerning packet delivery.  For
   instance, an ingress D-Router may find out that the traffic from the
   D-Forwarder is going to an unreachable egress, e.g., due to node
   failure.  In such a case, it should inform the D-Forwarder about the
   issue as soon as possible.  The information exchange may also contain
   possible countermeasures.

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    D-SID: Dyncast Service ID
   D-BID: Dyncast Binding ID
                                       +-------+ |           D-SID 1
                                       |Clients|-+         +--------+
                                       +-------+        +--|D-BID 21|
                                           |            |  +--------+
                                     +----------+----+  |
                                     |D-Router 2|D-MA|--|    D-SID 2
   +------+                          +----------+----+  |  +--------+
   |Client|                                |            +--|D-BID 22|
   +------+                        +----------------+      +--------+
      |    Service Demand          |                |
      |   (Flow X, D-SID 2)        |                |
      |   --------------->         |                |
   +-----------+     +----------+  |                |
   |D-Forwarder|-----|D-Router 1|--| Infrastructure |
   +-----------+     +----------+  |                |
      |   <---------------         |                |
      |(Flow X, D-SID 2, D-BID 32) |                |       D-SID 2
      |     Binding Info           |                |      +--------+
   +------+                        +----------------+  +---|D-BID 32|
   |Client|                                |           |   +--------+
   +------+                                |       +------+
                                           +-------| D-MA |
                                                   +------+  D-SID 1
                                                       |   +--------+
                                                       +---|D-BID 31|

           Figure 4: Service Demand in presence of a D-Forwarder

5.  Dyncast Control-plane vs Data-plane operations

   In summary, Dyncast consists of the following Control-plane and Data-
   plane operations:

   *  Dyncast Control Plane:

      -  Dyncast Service ID Notification: the D-SID, an anycast IP
         address, should be available and known.  This can be achieved
         in different ways.  For example, use a special range or coding
         of anycast IP address as service IDs or using the DNS.

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      -  Dyncast Binding ID Notification: the mapping of (D-SID, D-BID),
         i.e., service ID and the binding address, should be notified to
         the D-Routers when the service instance starts (or stops).
         Various ways can be used, for example, EBGP or management
         system notification.

      -  Metrics Notification: D-MA have to be able to share the metrics
         for a service and its binding ID so that D-Routers can select
         the "best" instance for each new service demand.

      -  Mapping Update Notification: D-Router notifies D-Forwarder of
         incoming service demand of mapping from service ID to binding
         IP according to the local metric information.  This
         notification is sent upon receiving a service demand (from
         D-Forwarder) with 'miss' indication.

   *  Dyncast Data Plane:

      -  New service demand: an ingress D-Router selects the most
         appropriate egress in terms of the network status and the
         computing resources of the instances of the requested service.
         An ingress D-Forwarder selects the binding address information
         for the received service ID, if available.  Otherwise, the
         service demand is forwarded with 'miss' indication set.

      -  Instance Affinity: Make sure the subsequent packets of an
         existing service demand are always delivered to the same
         service instance so that they can be served by the same service

6.  Summary

   This draft introduces a Dyncast architecture that enables the service
   demand to be sent to an optimal service instance.  It can dynamically
   adapt to the computing resources consumption and network status
   change.  Dyncast is a network based architecture that supports a
   large number of edges and is independent of the applications or
   services hosted on the edge.

   More discussion and input on control plane and data plane approach
   are welcome.

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7.  Security Considerations

   The computing resource information changes over time very frequent
   with the creation and termination of service instance.  When such
   information is carried in routing protocol, too many updates can make
   the network fluctuate.  Control plane approach should take it into

   More thorough security analysis to be provided in future revisions.

8.  IANA Considerations

   This document does not make any request to IANA.

9.  References

9.1.  Informative References

              Liu, P., Willis, P., and D. Trossen, "Dynamic-Anycast
              (Dyncast) Use Cases and Problem statement", Work in
              Progress, Internet-Draft, draft-li-dyncast-ps-usecases-01,
              February 2021, <https://datatracker.ietf.org/doc/html/

9.2.  Informative References

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,



Authors' Addresses

   Yizhou Li
   Huawei Technologies

   Email: liyizhou@huawei.com

   Luigi Iannone
   Huawei Technologies

   Email: Luigi.iannone@huawei.com

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   Dirk Trossen
   Huawei Technologies

   Email: dirk.trossen@huawei.com

   Peng Liu
   China Mobile

   Email: liupengyjy@chinamobile.com

   Cheng Li
   Huawei Technologies

   Email: c.l@huawei.com

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