Internet DRAFT - draft-bormann-t2trg-affinity
draft-bormann-t2trg-affinity
dyncast C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Informational 30 August 2021
Expires: 3 March 2022
Providing Instance Affinity in Dyncast
draft-bormann-t2trg-affinity-00
Abstract
Dyncast support in a network provides a client with a fresh optimal
path to a service provider instance, where optimality includes both
path and service provider characteristics. As a service invocation
usually takes more than one packet, dyncast needs to provide instance
affinity for each service invocation. Naive implementations of
instance affinity require per-application, per service-invocation
state in the network.
The present short document defines a way to provide instance affinity
that does not require, but also does not rule out per-application
state.
It also discusses how the information that an application needs to
operate this mechanism can be provided via the discovery mechanisms
offered by a CoRE (Constrained RESTful Environments) server, either
in "/.well-known/core" or via the CoRE resource directory.
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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This Internet-Draft will expire on 3 March 2022.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Approach . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Details . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8. Legacy IP Considerations . . . . . . . . . . . . . . . . . . 6
9. CoRE Discovery . . . . . . . . . . . . . . . . . . . . . . . 6
10. Security Considerations . . . . . . . . . . . . . . . . . . . 7
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
12.1. Normative References . . . . . . . . . . . . . . . . . . 7
12.2. Informative References . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Dyncast support in a network provides a client with a fresh optimal
path to a service provider instance, where optimality includes both
path and service provider characteristics. As a service invocation
usually takes more than one packet, dyncast needs to provide instance
affinity for each service invocation. Naive implementations of
instance affinity require per-application, per service-invocation
state in the network.
The present short document defines a way to provide instance affinity
that does not require, but also does not rule out per-application
state.
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It also discusses how the information that an application needs to
operate this mechanism can be provided via the discovery mechanisms
offered by a CoRE (Constrained RESTful Environments) server, either
in "/.well-known/core" or via the CoRE resource directory.
[I-D.liu-dyncast-ps-usecases] lists use cases of dyncast. The
present document does not discuss the specifics of how the network
provides dyncast, such as the way service instance metrics enter path
computations.
2. Terminology
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.
This document uses the terminology of [I-D.liu-dyncast-ps-usecases],
in particular _Service_ and _Service Instance_ (the latter often
abbreviated to "Instance"). It also defines the following terms:
Client: The system that requests a service.
Service invocation: A single transaction between client and a
service instance. The client is interested in talking to the same
service instance throughout one service invocation. Subsequent
and parallel service invocations can use different service
instances without a problem and therefore do not require affinity.
Instance Affinity: The ability of the network to send all the
packets of a service invocation to the same service instance.
(Note that this doesn't necessarily imply path affinity -- the
client does not care about the path, only about getting to the
same service instance.)
Service period: The temporal granularity (rhythm) in which the
network updates the optimal paths it provides for a service.
Service stretch: The maximum amount of time that the network plans
to provide instance affinity for a service invocation.
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3. Assumptions
This document makes a number of assumptions, some of which are
fundamental to its technical approach, but some of which are only
required for the exposition chosen in this document. A future
version of this document will clearly separate these two kinds of
assumptions.
Due to experience with overly eager load-based updates to routing
metrics, we assume that metrics will be updated on the scale of tens
of seconds. To simplify exposition we therefore set the service
period to 10 seconds (assumptions of this kind are intended to be
possible without loss of generality, but should not be wildly off).
We assume the affinity processing for the entire network will be on a
rhythm that is consistent with the service period. Updates take
effect at the start of a new service period. The entire network is
loosely synchronized on this rhythm. The clients are also aware of
this rhythm.
We assume the service stretch will be quite limited, on the order of
(a generous) five minutes or less. As a result, any service
invocation covers less than 32 service periods. Services that do
need longer service stretches will need to renew the service
invocation regularly (by checking whether the service instance has
changed upon such a renewal, any handover effort needed can be
minimized).
Service identifiers take the form of IPv6 addresses, or more
typically, IPv6 prefixes. The client is able to complete the prefix
with application information. (In a pinch, the client can obtain a
complete current address via DNS lookup.)
4. Objectives
Dyncast needs to provide instance affinity. The present document
outlines how to achieve this without creating per application, or
worse, per invocation state in the network.
The network does not provide any signaling to the clients beyond what
is expected in an IPv6 environment.
In summary, the objective of this draft is to define a stable client
interface to the instance affinity mechanism (and to motivate why
this interface is useful). This interface is designed to remain
stable even while the network support for this mechanism is evolving.
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5. Approach
We number the service periods with a cyclic numbering system that
wraps around about every two service stretches. The network and the
clients are aware of the current service period number; the
synchronization requirement between them is that clients typically
aren't ahead of the network.
When starting a new service invocation, the client builds an IPv6
address out of the service identifier and its view of the current
service period number (or it obtains this address using a DNS
lookup), essentially filling in 6 bits (for the numbers assumed
here). Service requests and the resulting communication within the
invocation are addressed to this current address. The client stores
the current address with the service invocation when initializing it;
it is not ever updated for this invocation.
The network keeps its path optimization state relative to (or indexed
by) the current period number. Routing updates can be processed at
any time but do not lead to an update of the path optimization state
for any service period. The result is that the path chosen after a
routing update may no longer be optimal, but that instance affinity
is kept. For each service, a pointer for the best service instance
is kept for the current and the last 32 service periods.
6. Discussion
The approach presented provides instance affinity without requiring
per application or per invocation state in the network. It does
require up to 32 copies of what are essentially host routes per
service instance. The state scales with the number of service
instances, and not with the number of clients.
The approach is based on IPv6. It can be made to work in an IPv4
network, if there are plentiful IPv4 addresses available (see also
Section 8).
7. Details
The service period number could simply be inserted in the service
identifier, or more complex computation could be performed to make
the current addresses generated this way stand out in a forwarding
engine.
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Naïve clients will start a service invocation with a DNS lookup.
This allows the insertion of the period number to be performed in a
specialized DNS server for the service. Of course, this requires
short time to live (TTL) values and clients that do not on their own
cache the look up results.
So the preferred variant is for the client to be aware of the current
service period number and to do the insertion by itself on each new
service invocation.
8. Legacy IP Considerations
To make this work with IPv4 addresses as service identifiers, we
would need 6 bits that can be varied over time. This is likely too
expensive for many applications. An alternative approach is to use
the port number for the 6 bits. This would mean that the network
would need to look up paths both on destination IP address and
destination port number (48-bit addressing). For IPv4, this should
be good enough.
9. CoRE Discovery
For use with IPv6, this document defines target attributes to enable
CoAP clients [RFC7252] to discover the availability of affinity
addressing and where in the address it is intended to be applied.
The target attributes are:
* "affinity-pos": The starting bit position (counting from most
significant bit first) of the sequence of bits where the service
period number can be inserted into the IPv6 address given.
* "affinity-len": The number of bits of the sequence of bits where
the service period number can be inserted into the IPv6 address
given.
* "affinity-period": The number of seconds a service period spans.
"affinity-period" is used as a divisor of the synchronized time in
seconds, yielding an incremented quotient for the next service
period, the lower "affinity-len" bits are then used as the service
period number.
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Because of general availability of this time scale, the synchronized
time is interpreted according to POSIX [TIME_T]. (POSIX time is also
known as "UNIX Epoch time".) Note that leap seconds are handled
specially by POSIX time and this results in a 1 second discontinuity
several times per decade, which should be of rather limited
consequence for service affinity.
Using the example at the end of Section 5 of [RFC6690], a server
providing a large resource into a dyncast (anycast) pool could
include in its "/.well-known/core":
REQ: GET /.well-known/core?rt=firmware
RES: 2.05 Content
</firmware/v2.1>;rt="firmware";sz=262144;affinity-pos=122;
affinity-len=6;affinity-period=10
(Additional line break for exposition. Obviously, more complex
services than simple retrieval of a large object could be offered.)
This link could turn up in a resource directory
[I-D.ietf-core-resource-directory] entry that looks like:
<coap://[2001:db8:3::123]/firmware/v2.1>;rt="firmware";sz=262144;
affinity-pos=122;affinity-len=6;affinity-period=10
Note that the address given here has a number of bits set in the
section to be overwritten by the service period number to be
inserted.
10. Security Considerations
TBD
11. IANA Considerations
No IANA action is required for this concept draft.
Currently, CoRE target attributes are not subject to registration;
this draft defines three new target attributes as per Section 9.
12. References
12.1. Normative References
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[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[I-D.ietf-core-resource-directory]
Amsüss, C., Shelby, Z., Koster, M., Bormann, C., and P. V.
D. Stok, "CoRE Resource Directory", Work in Progress,
Internet-Draft, draft-ietf-core-resource-directory-28, 7
March 2021, <https://www.ietf.org/archive/id/draft-ietf-
core-resource-directory-28.txt>.
[TIME_T] The Open Group Base Specifications, "Open Group Standard:
Vol. 1: Base Definitions, Issue 7", Section 4.16 'Seconds
Since the Epoch', IEEE Std 1003.1, 2018 Edition, 2018,
<http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
V1_chap04.html#tag_04_16>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
12.2. Informative References
[I-D.liu-dyncast-ps-usecases]
Liu, P., Willis, P., and D. Trossen, "Dynamic-Anycast
(Dyncast) Use Cases & Problem Statement", Work in
Progress, Internet-Draft, draft-liu-dyncast-ps-usecases-
01, 15 February 2021, <https://www.ietf.org/archive/id/
draft-liu-dyncast-ps-usecases-01.txt>.
Author's Address
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
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Phone: +49-421-218-63921
Email: cabo@tzi.org
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