dyncast P. Liu
Internet-Draft China Mobile
Intended status: Informational P. Willis
Expires: August 15, 2021 BT
D. Trossen
Huawei
February 15, 2021
Dynamic-Anycast (Dyncast) Requirements
draft-liu-dyncast-reqs-00
Abstract
This draft provides requirements for an architecture addressing the
problems outlined in the use case and problem statement draft for
Dyncast [DYNCAST-PS].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definition of Terms . . . . . . . . . . . . . . . . . . . . . 2
3. Desirable System Characteristics and Requirements . . . . . . 3
3.1. Anycast-based Service Addressing Methodology . . . . . . . 3
3.2. Instance Affinity . . . . . . . . . . . . . . . . . . . . 3
3.3. Encoding Metrics . . . . . . . . . . . . . . . . . . . . . 4
3.4. Signaling Metrics . . . . . . . . . . . . . . . . . . . . 5
3.5. Using Metrics in Routing Decisions . . . . . . . . . . . . 5
3.6. Supporting Service Dynamism . . . . . . . . . . . . . . . 6
4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Informative References . . . . . . . . . . . . . . . . . . . . 7
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
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 [DYNCAST-PS]. To optimally
deliver the service request to the most appropriate service instance
is the fundamental requirement in such deployments. As shown in
[DYNCAST-PS], 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 [DYNCAST-
PS].
This draft provides the requirements to realize the potential dynamic
anycast architecture by alleviating the problems of existing
solutions outlined in [DYNCAST-PS].
2. Definition of Terms
Service: A service represents a defined endpoint of functionality
encoded according to the specification for said service.
Service instance: One service can have several instances running on
different nodes. Service instance is a running environment (e.g.,
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a node) that makes the functionality of a service available.
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 [dyncast-ps] 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
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-
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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 [DYNCAST-PS], 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.
3.3. Encoding Metrics
As outlined in the scenarios in [DYNCAST-PS], 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
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elements participating in the service-specific routing decision is
important. Specifically, a desirable system
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.
o MAY obfuscate the specific semantic of the metric to preserve
privacy of the service provider information towards the network
provider.
o MAY include routing protocol metrics
3.4. 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 system
o MUST provide mechanisms to signal the metrics for using in routing
decisions
o MUST realize means for rate control for signaling of metrics
o MUST implement mechanisms for loop avoidance in signaling metrics,
when necessary
3.5. 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
o MUST specify a default action to be taken, if more than one action
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possible
o SHOULD enable other alternative actions to be taken.
o 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).
o Any solution SHOULD allow associating the desired action to a
specific service identifier.
3.6. 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 signaling.
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.
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
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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
TBD
6. IANA Considerations
No IANA action is required so far.
7. Informative References
[DYNCAST-PS] P. Liu, P. Willis, D. Trossen, "Dynamic-Anycast
(Dyncast) Use Cases and Problem Statement", draft-liu-dyncast-ps-
usecases-01 (work in progress), February 2021.
[RFC4786] J. Abley, K. Lingqvist, "Operation of Anycast Services",
RFC4786, December 2006, https://tools.ietf.org/html/rfc4786
Acknowledgements
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
Peter Willis
BT
Email: peter.j.willis@bt.com
Dirk Trossen
Huawei
Email: dirk.trossen@huawei.com
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