The Series Transfer Pattern (STP)

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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:¶

• 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.¶
• 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.¶

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)

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.¶

5. Security considerations

TO DO¶

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