Internet DRAFT - draft-bormann-t2trg-stp
draft-bormann-t2trg-stp
Network Working Group C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Informational K. Hartke
Expires: 10 October 2020 Ericsson
8 April 2020
The Series Transfer Pattern (STP)
draft-bormann-t2trg-stp-03
Abstract
Many applications make use of Series of data items, i.e., an array of
data items where new items can be added over time. Where such Series
are to be made available using REST protocols such as CoAP or HTTP,
the Series has to be mapped into a structure of one or more resources
and a protocol for a client to obtain the Series and to learn about
new items.
Various protocols have been standardized that make Series-shaped data
available, with rather different properties and objectives. The
present document is an attempt to extract a common underlying pattern
and to define media types and an access scheme that can be used right
away for further protocols that provide Series-shaped data.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 10 October 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. A REST Series Transfer Pattern (STP) . . . . . . . . . . . . 4
3.1. Basic collections . . . . . . . . . . . . . . . . . . . . 4
3.2. Pagination and Observing linked lists . . . . . . . . . . 5
3.3. The "cursor" pattern . . . . . . . . . . . . . . . . . . 7
4. IANA considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security considerations . . . . . . . . . . . . . . . . . . . 9
6. Informative References . . . . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
(TO DO: Insert an extended form of the abstract first here, expanding
the reference to [RFC7230] and [RFC7252] in the process.)
Examples for protocols that provide Series-shaped data are:
* The Atom Syndication Format [RFC4287] defines _feeds_ as Series of
_entries_ (links plus some metadata, which can often be much of
the content of an entry), where a feed is represented by a
collection resource that contains just a small number of the most
recent entries. By polling a feed, a client can contain a fresh
view of the Series, with a focus on recent items. If the client
does not poll often enough, it will _miss_ items.
* Messaging protocols such as XMPP or SIMPLE transfer series of what
is often called "Instant Messages". A publish/subscribe mechanism
allows a client to select sequences of messages that it is
interested in.
* Mail servers that provide interactive access to stored messages
present a Series to their clients. Obviously, loss of messages is
frowned upon.
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* CoAP Observe allows a client to observe a resource as it changes;
the client can collect the changes into a Series. Observe is
focused on eventual consistency: a fresher data items simply
overwrites an older one. The present document uses the observe
pattern to build a more general Series Transfer Pattern.
* Syslog is an interesting case of a Series Transfer.
* [RFC8641], [I-D.voit-netmod-yang-notifications2], [RFC8639],
[I-D.ietf-netconf-notification-messages], [RFC8650].
* An RTP stream can be viewed as an (somewhat extreme) case of a
Series; new items are just sent inside separate UDP packets. A
sequence number allows to detect (but not normally ask for
retransmission of) missing items. A timestamp as well as source
data (SSRC, CSRC) provide further common metadata that aid in the
processing of the Series items.
* An example of an ad-hoc design of a series transfer protocol is
[I-D.ietf-netconf-udp-pub-channel].
* Server-sent events [sse] are a somewhat bizarre version of a
series transfer protocol.
* The Interface for Metadata Access Points (IF-MAP) specified by the
Trusted Computing Group and emerging derivatives of that protocol
create a series of updates to a graph representation of related
network-related security information. The requests created by IF-
MAP clients are bundled operations of updates to a MAP Graph,
which compose a Series Transfer Pattern of bundled atomic
operations that ensure the integrity of the MAP Graph. [Henk
Birkholz]
* netflow/IPFIX was defined to transfer a series of data items about
flows. Information about PDU flows accounted by network
interfaces of endpoints is emitted in a series of counter bundles
via the IPFIX protocol. Only a series of these continuous Flow
Records creates a meaningful bigger picture about the current
traffic in the network topology of an administrative domain.
Depending on the characteristics measured, loss of a Flow Record
can range from harmless to missing the only vital counter
measurement. [Henk Birkholz]
* TO DO: Add more items.
[I-D.birkholz-yang-push-coap-problemstatement] is a problem statement
that will require the design of another scheme to transfer Series-
shaped data.
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2. Objectives
Series transfer applications may have rather different objectives.
* The completeness of the Series transfer may be of utmost
importance (e.g., if each item represents a sale), it may be
desirable but can be jettisoned in an overload situation, or it
may just be a likely outcome with a very active client (e.g., with
Atom). Note that there is never a way to _guarantee_ completeness
unless all of the rate and size of incoming new items, the
transfer capacity available, and the processing capabilities of
the client are controlled; however, system designs may want to
give the illusion of "reliability".
* Minimizing the latency of the transfer may be important, as may be
limiting it below a defined maximum (note that these are different
objectives). The latter can be supported in a polling system by
polling at least as often as that maximum latency; this may be
considered inefficient and "push" mechanisms may be developed.
Mail environments have developed "push" services to enable
minimizing the latency. Where latency requirements go below the
time that might be needed for an end-to-end retransmission, error
concealment may provide an acceptable user experience (e.g., in
RTP).
In general, minimizing latency and ensuring completeness are
competing objectives.
Series transfer environments sometimes centralize information
distribution functions, leading to "broker" architectures (often
combined with the "publish/subscribe" pattern). With brokers, Series
publishers may use an entirely different interface to the brokers
from that used by the receiving clients, or the interfaces can be
designed so they are similar for all the forwarding steps.
3. A REST Series Transfer Pattern (STP)
3.1. Basic collections
A series of items can be represented by a single collection resource:
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_____________
| |
| item 11 |
|_____________|
| |
| item 10 |
|_____________|
| . |
| . |
| . |
|_____________|
| |
| item 1 |
|_____________|
Figure 1: A collection of items
While this is adequate in many cases, it has a number of limitations:
* Each retrieval fetches the entire collection
- As long as the collection does not change, this can be
mitigated with ETags (Section 5.10.6 of[RFC7252], Section 2.3
of [RFC7232]).
* When the collection becomes too large, the server has to prune
older items. These then no longer can be retrieved, and there is
even no way for the server to indicate that there used to be older
items.
3.2. Pagination and Observing linked lists
In the Browser Web, it is usual to provide _Pagination_ for
collection resources that can grow large (e.g., search results):
_____________ _____________ _____________
| | | | | |
| item 11 | +--->| item 9 | +--->| item 2 |
|_____________| | |_____________| | |_____________|
| | | | | | |
| item 10 | | | item 8 | ::::: | item 1 |
|_____________| | |_____________| |_____________|
| | | | | | page M
| link -----------+ | link -----------+
|_____________| |_____________|
page 1 page 2
Figure 2: A paginated collection of items
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Without modification, this does not work well for resources that
actually change by themselves: Once a new page needs to be added,
what previously was page 1 now becomes page 2. Obviously, the naming
of pages better remains unchanged with new pages added a the front.
_____________ _____________ _____________
| | | | | |
| item 11 | +--->| item 9 | +--->| item 2 |
|_____________| | |_____________| | |_____________|
| | | | | | |
| item 10 | | | item 8 | ::::: | item 1 |
|_____________| | |_____________| |_____________|
| | | | | | page 1
| link -----------+ | link -----------+
|_____________| |_____________|
page M page 2
Figure 3: Pagination with stable names
However, now the client has no idea what initial page to request to
get the freshest items and the head of the list. It is easy to add a
link to the freshest page:
_____________ _____________ _____________
| | | | | |
| link --------------->| item 11 | +--->| item 2 |
|_____________| |_____________| | |_____________|
head | | | |
| item 10 | ::::: | item 1 |
|_____________| |_____________|
| | | page 1
| link -----------+
|_____________|
page M
Figure 4: Pagination with stable names
The head of the linked list can now be simply observed; the addition
of pages will then be notified to the observer.
As usual in series transfer, the following considerations remain:
* When can the server decide to no longer retain older items?
- There may be a desire for an observer to be able to catch all
items in the series.
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o How does the server know who are the observers? E.g., what
to do with newly joining observers?
o How does an observer signal that it has caught up (to a
specific item)?
* What to do when the decision to remove items from the list cannot
be made and there is no room for new items?
The link head can also include items that have so far not been added
to pages; this can be used to fill up pages evenly without them ever
changing. Obviously, the best number of items to prenotify in this
way as well as the best time to open a new page are different for
different applications.
3.3. The "cursor" pattern
A GET on a resource representing a Series may return a collection
item that contains the following pieces of information
* An array of Series items, either as an array of media-typed
objects in a suitable representation format (e.g., CBOR, MIME) or
by using an array-like media type (e.g., SenML).
- Items may be full items or limit themselves to some metadata
and a link; the client can then follow that link if it is
interested in the data (possibly basing that decision on the
metadata and/or a measure of load).
* A "cursor" that can then be used as a parameter in further GET
requests (see below) in order to receive only newer items than
those received with this transfer.
* A "more bit" that indicates whether such further items already
exist but could not be returned in the present response.
In Figure 5, the cursor is implemented as a URI that can be used as a
link to the next page.
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_____________ _____________ _____________
| | | | | |
| item 10 |<---+ | item 1 |<--------------- link |
|_____________| | |_____________| |_____________|
| | | | tail
| item 11 | ::::: | item 2 |
|_____________| |_____________|
page M | | |
+----------- link |
|_____________|
page 1
Figure 5: Cursor pattern pictured as pagination
A GET may be enhanced with additional parameters (possibly turning it
into a FETCH):
* The cursor.
* A "wait bit" that indicates whether a (possibly empty) reply
should be given right away or the server should wait for new items
to become available. (To avoid the equivalence of the "silly
window syndrome", the wait bit may be enhanced by a minimum number
of items and a timeout after which even a smaller number is made
available.) In effect, this requests a form of "long polling";
see [RFC6202] for some considerations for this in HTTP.
A server may implement a form of custody transfer by interpreting the
cursor as an acknowledgement that the client has received all data up
to the cursor. This is not necessarily acting as an unsafe request
("destructive GET"), as other clients may be active that have not yet
received all these data. To implement a full custody semantics, the
server needs to be aware of all the clients that expect a full Series
Transfer (a classical group management problem).
(Explain how Observe can help. Can it?)
4. IANA considerations
This memo registers a number of media types: TO DO.
* A media type for FETCH selectors (Section 3):
- An alternative way to encode this information into the URI of a
GET should also be available.
* A Series media type as alluded to in Section 3.
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5. Security considerations
TO DO
6. Informative References
[I-D.birkholz-yang-push-coap-problemstatement]
Birkholz, H., Zhou, T., Liu, X., and E. Voit, "YANG Push
Operations for CoMI", Work in Progress, Internet-Draft,
draft-birkholz-yang-push-coap-problemstatement-00, 18
October 2017, <http://www.ietf.org/internet-drafts/draft-
birkholz-yang-push-coap-problemstatement-00.txt>.
[I-D.ietf-netconf-notification-messages]
Voit, E., Jenkins, T., Birkholz, H., Bierman, A., and A.
Clemm, "Notification Message Headers and Bundles", Work in
Progress, Internet-Draft, draft-ietf-netconf-notification-
messages-08, 17 November 2019, <http://www.ietf.org/
internet-drafts/draft-ietf-netconf-notification-messages-
08.txt>.
[I-D.ietf-netconf-udp-pub-channel]
Zheng, G., Zhou, T., and A. Clemm, "UDP based Publication
Channel for Streaming Telemetry", Work in Progress,
Internet-Draft, draft-ietf-netconf-udp-pub-channel-05, 11
March 2019, <http://www.ietf.org/internet-drafts/draft-
ietf-netconf-udp-pub-channel-05.txt>.
[I-D.voit-netmod-yang-notifications2]
Voit, E., Bierman, A., Clemm, A., and T. Jenkins, "YANG
Notification Headers and Bundles", Work in Progress,
Internet-Draft, draft-voit-netmod-yang-notifications2-00,
24 February 2017, <http://www.ietf.org/internet-drafts/
draft-voit-netmod-yang-notifications2-00.txt>.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <https://www.rfc-editor.org/info/rfc4287>.
[RFC6202] Loreto, S., Saint-Andre, P., Salsano, S., and G. Wilkins,
"Known Issues and Best Practices for the Use of Long
Polling and Streaming in Bidirectional HTTP", RFC 6202,
DOI 10.17487/RFC6202, April 2011,
<https://www.rfc-editor.org/info/rfc6202>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
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RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
DOI 10.17487/RFC7232, June 2014,
<https://www.rfc-editor.org/info/rfc7232>.
[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>.
[RFC8639] Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard,
E., and A. Tripathy, "Subscription to YANG Notifications",
RFC 8639, DOI 10.17487/RFC8639, September 2019,
<https://www.rfc-editor.org/info/rfc8639>.
[RFC8641] Clemm, A. and E. Voit, "Subscription to YANG Notifications
for Datastore Updates", RFC 8641, DOI 10.17487/RFC8641,
September 2019, <https://www.rfc-editor.org/info/rfc8641>.
[RFC8650] Voit, E., Rahman, R., Nilsen-Nygaard, E., Clemm, A., and
A. Bierman, "Dynamic Subscription to YANG Events and
Datastores over RESTCONF", RFC 8650, DOI 10.17487/RFC8650,
November 2019, <https://www.rfc-editor.org/info/rfc8650>.
[sse] WHATWG, "HTML Living Standard -- 9.2 Server-sent events",
n.d., <https://html.spec.whatwg.org/multipage/server-sent-
events.html#server-sent-events>.
Acknowledgements
The need for a Series Transfer Pattern has been made clear by a
number of people that contribute to the IRTF Thing-to-Thing Research
Group (T2TRG), e.g. Matthias Kovatsch and Henk Birkholz (both of
whom also provided feedback on an early draft). Henk also
contributed further examples for the use of Series Transfers in
protocols.
Authors' Addresses
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
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Phone: +49-421-218-63921
Email: cabo@tzi.org
Klaus Hartke
Ericsson
Torshamnsgatan 23
SE-16483 Stockholm
Sweden
Email: klaus.hartke@ericsson.com
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