Internet-Draft The Series Transfer Pattern (STP) April 2020
Bormann & Hartke Expires 10 October 2020 [Page]
Network Working Group
Intended Status:
C. Bormann
Universität Bremen TZI
K. Hartke

The Series Transfer Pattern (STP)


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

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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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 10 October 2020.

Table of Contents

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:

[I-D.birkholz-yang-push-coap-problemstatement] is a problem statement that will require the design of another scheme to transfer Series-shaped data.

2. Objectives

Series transfer applications may have rather different objectives.

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:

i i m e 1 i m m 1 1 t t 1 t e 0 e
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):

: t i i t i e a : k t 1 l l e a n m m i e 9 m g a g m : : 2 e k 1 e t e p p 1 i n p 1 e e m 8 M i 1 0 e g m t : i 2 i t
Figure 2: A paginated collection of items

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.

8 p m i : i g e 2 e : t a l 1 g 1 t t e M i e k e 2 i 0 k e m m a t : l n : p p m 9 n m e i a e 1 1 : i 1 e g i t i t m
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:

m 1 a n n e 2 i e : : g k i e i a t : e m : e m a i m p e k d p 1 1 M l h 1 l i e t 0 : i t g 1 t
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.

      • How does the server know who are the observers? E.g., what to do with newly joining observers?
      • 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.

e g 0 n t i t e e i 1 l a 1 : : m p e k t : M k a m 1 i : i e m i l t t 1 m 1 : 2 a i e i g l n p
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.

5. Security considerations


6. Informative References

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, , <>.
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, , <>.
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, , <>.
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, , <>.
Nottingham, M., Ed. and R. Sayre, Ed., "The Atom Syndication Format", RFC 4287, DOI 10.17487/RFC4287, , <>.
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, , <>.
Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, , <>.
Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests", RFC 7232, DOI 10.17487/RFC7232, , <>.
Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, , <>.
Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard, E., and A. Tripathy, "Subscription to YANG Notifications", RFC 8639, DOI 10.17487/RFC8639, , <>.
Clemm, A. and E. Voit, "Subscription to YANG Notifications for Datastore Updates", RFC 8641, DOI 10.17487/RFC8641, , <>.
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, , <>.
WHATWG, "HTML Living Standard -- 9.2 Server-sent events", , <>.


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
Klaus Hartke
Torshamnsgatan 23
16483 Stockholm