RFC 4685
Atom Threading Extensions
RFC 4685: Atom Threading Extensions 💖🧵
Network Working Group · Request for Comments: 4685 · September 2006
Okay so this one's written by J. Snell 🎀, and it's a Standards Track doc, so it's a real protocol spec, not just vibes. The title block doesn't list anything it obsoletes or updates, so it's a fresh start ✨.
Status of This Memo 📋
okay so this doc is an Internet standards track protocol for the Internet community, and it's basically asking everyone to chime in with discussion and suggestions for improvements 🫶. For the current standardization state and status of this protocol, check the "Internet Official Protocol Standards" (STD 1). And distribution of this memo is unlimited, so share away 💅.
Copyright Notice 🔒
Copyright (C) The Internet Society (2006).
Abstract 🌸
this memo is all about a mechanism that lets feed publishers show threaded discussions inside the Atom Syndication Format 🧵✨.
Table of Contents 🗂️
1. Introduction ....................................................1
2. Notational Conventions ..........................................2
3. The 'in-reply-to' Extension Element .............................2
4. The 'replies' Link Relation .....................................5
5. The 'total' Extension Element ...................................6
6. Considerations for Using thr:count, thr:updated, and total ......7
7. Security Considerations .........................................8
8. IANA Considerations .............................................9
9. References ......................................................9
9.1. Normative References .......................................9
9.2. Informative References ....................................10
Appendix A. Acknowledgements .....................................111. Introduction 🦋
so this doc defines an extension for expressing threaded discussions inside the Atom Syndication Format [RFC4287]. It's giving "reply chains, but make it a standard" 🙈.
2. Notational Conventions 📝
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, [RFC2119], as scoped to those conformance targets.
the XML Namespaces URI [W3C.REC-xml-names-19990114] for the XML elements and attributes described in this specification is:
http://purl.org/syndication/thread/1.0In this document, the namespace prefix thr: stands for that Namespace URI. The prefix is arbitrary, so the choice of prefix isn't semantically significant 💜.
this spec borrows some shorthand from the XML Infoset [W3C.REC-xml-infoset-20040204]. The phrase "Information Item" gets dropped when naming Element and Attribute Information Items. So when this spec says "element," it means an Element Information Item in Infoset terms, and when it says "attribute," it means an Attribute Information Item 🧠.
This specification allows IRIs [RFC3987] to be used. Every URI [RFC3986] is also an IRI, so you can use a URI anywhere an IRI is named. When an IRI that isn't also a URI is given for dereferencing, it MUST be mapped to a URI using the steps in Section 3.1 of [RFC3987]. When an IRI is serving as an identifier, it MUST NOT be so mapped 🔁.
Some sections of this spec are illustrated with a non-normative RELAX NG Compact schema [RELAXNG]. In those sections, this specification uses the atomCommonAttributes, atomMediaType, and atomURI patterns, which are defined in [RFC4287] ✨.
However, the text of this specification is the only thing that defines conformance. The pictures and schemas are just helpers, bestie 🎀.
3. The 'in-reply-to' Extension Element 💬
The "in-reply-to" element is used to say that an entry is a response to another resource. The element MUST contain a "ref" attribute identifying the resource that is being responded to 🫶.
it's not unlike the references and in-reply-to email message headers, defined by [RFC2822]. But here's the tea ☕: unlike the in-reply-to header, the "in-reply-to" element is required to identify the unique identifier of only a single parent resource. If the entry is a response to multiple resources, additional "in-reply-to" elements MAY be used. There's no direct equivalent to the references header, which lists the unique identifiers of each preceding message in a thread 🧵💅.
The grammar goes like this:
in-reply-to =
element thr:in-reply-to {
atomCommonAttributes,
ref,
href?,
source?,
type?,
( undefinedContent )
}
ref = attribute ref { atomURI }
href = attribute href { atomURI }
type = attribute type { atomMediaType }
source = attribute source { atomURI }The "ref" attribute specifies the persistent, universally unique identifier of the resource being responded to. The value MUST conform to the same construction and comparison rules as the value of the atom:id element, as defined in Section 4.2.6 of [RFC4287]. Even though the IRI might use a dereferenceable scheme, processors MUST NOT assume that it can be dereferenced 👀.
If the resource being responded to doesn't have a persistent, universally unique identifier, the publisher MUST assign an identifier that satisfies all the considerations in Section 4.2.6 of [RFC4287] to use as the value of the "ref" attribute. In that case, if a representation of the resource can be retrieved from an IRI that can be used as a valid atom:id value, then this IRI SHOULD be used as the value of both the "ref" and "href" attributes 🔗.
The "source" attribute MAY be used to specify the IRI [RFC3987] of an Atom Feed or Entry Document containing an atom:entry with an atom:id value equal to the value of the "ref" attribute. The IRI you specify, once it's appropriately mapped to a corresponding URI, MUST be dereferenceable 🧚.
The "href" attribute specifies an IRI that may be used to retrieve a representation of the resource being responded to. The IRI you specify, once it's appropriately mapped to a corresponding URI, MUST be dereferenceable 🌈.
The "type" attribute MAY be used to give the client a hint about the media type [RFC4288] of the resource identified by the "href" attribute. The "type" attribute only means something if a corresponding "href" attribute is also provided 🍓.
This specification gives no significance to the order in which multiple "in-reply-to" elements appear within an entry 🙃.
Here's an example of an entry with a response:
<feed xmlns="http://www.w3.org/2005/Atom"
xmlns:thr="http://purl.org/syndication/thread/1.0">
<id>http://www.example.org/myfeed</id>
<title>My Example Feed</title>
<updated>2005-07-28T12:00:00Z</updated>
<link href="http://www.example.org/myfeed" />
<author><name>James</name></author>
<entry>
<id>tag:example.org,2005:1</id>
<title>My original entry</title>
<updated>2006-03-01T12:12:12Z</updated>
<link
type="application/xhtml+xml"
href="http://www.example.org/entries/1" />
<summary>This is my original entry</summary>
</entry>
<entry>
<id>tag:example.org,2005:1,1</id>
<title>A response to the original</title>
<updated>2006-03-01T12:12:12Z</updated>
<link href="http://www.example.org/entries/1/1" />
<thr:in-reply-to
ref="tag:example.org,2005:1"
type="application/xhtml+xml"
href="http://www.example.org/entries/1"/>
<summary>This is a response to the original entry</summary>
</entry>
</feed>To help Atom processors that don't know about the in-reply-to extension still see that there's a relationship between the entry and the resource it's responding to, publishers are advised to consider including a "related" link that references a representation of the resource identified by the in-reply-to element. Those links probably won't get processed as a reference to a predecessor in a threaded conversation, but they're still super helpful for establishing a semantically meaningful relationship between the linked resources 💖.
For example:
<feed xmlns="http://www.w3.org/2005/Atom"
xmlns:thr="http://purl.org/syndication/thread/1.0">
<id>http://www.example.org/myfeed</id>
<title>My Example Feed</title>
<updated>2005-07-28T12:00:00Z</updated>
<link href="http://www.example.org/myfeed" />
<author><name>James</name></author>
<entry>
<id>tag:example.org,2005:1,1</id>
<title>A response to the original</title>
<updated>2006-03-01T12:12:12Z</updated>
<link href="http://www.example.org/entries/1/1" />
<thr:in-reply-to
ref="tag:example.org,2005:1,0"
type="application/xhtml+xml"
href="http://www.example.org/entries/1"
source="http://www.example.org/myfeed" />
<link
rel="related"
type="application/xhtml+xml"
href="http://www.example.org/entries/1" />
<summary>This is a response to the original entry</summary>
</entry>
</feed>4. The 'replies' Link Relation ✨
okay so an Atom link element with rel="replies" is how you point people to the spot where the responses to an entry live 💬 and if you leave off the type attribute, it's assumed to be application/atom+xml 🎀
a replies link sitting inside the feed or source element means the linked resource probably has responses to any of that feed's entries 🌸 and a replies link inside an entry means the linked resource probably has responses meant for that one entry specifically 💅
an atom:link using rel="replies" MAY have a thr:count attribute 🔢 its value is an unsigned, non-negative integer written in the canonical representation of the XML Schema nonNegativeInteger datatype [W3C.REC-xmlschema-2-20041028] 🧚 it's just a hint for clients about roughly how many replies the linked resource has, and it's advisory, so it might not match the real number 🙈
the link MAY also have a thr:updated attribute 🦋 its value is a [RFC3339] date-time stamp that follows the same construction rules as the Atom Date Construct from [RFC4287] ⏰ it's another hint, this time about when the newest reply showed up, and again it might not be accurate 🫶
here's an example feed to vibe with 👑
<feed xmlns="http://www.w3.org/2005/Atom"
xmlns:thr="http://purl.org/syndication/thread/1.0">
<id>http://www.example.org/myfeed</id>
<title>My Example Feed</title>
<updated>2005-07-28T12:00:00Z</updated>
<link href="http://www.example.org/myfeed" />
<author><name>James</name></author>
<entry>
<id>tag:entries.com,2005:1</id>
<title>My original entry</title>
<updated>2006-03-01T12:12:12Z</updated>
<link href="http://www.example.org/entries/1" />
<link rel="replies"
type="application/atom+xml"
href="http://www.example.org/mycommentsfeed.xml"
thr:count="10" thr:updated="2005-07-28T12:10:00Z" />
<summary>This is my original entry</summary>
</entry>
</feed>so Atom feed, entry, and source elements MAY each hold as many replies links as they want 💖 but this spec doesn't give any meaning to whether those links show up or what order they're in 🤷♀️ if an entry has multiple replies links, they might point at different versions of the same set of responses, or they might point at totally separate resources with separate responses 🌈 either way, processors MUST NOT assume that multiple replies links are pointing at different representations of the same resource, and they MUST handle each replies link on its own, independent of the others 🔥
5. The 'total' Extension Element
the total element tells you the total number of unique responses to an entry that the publisher knows about 🍓 its content MUST be an unsigned non-negative integer that follows the canonical representation of the XML Schema nonNegativeInteger datatype [W3C.REC-xmlschema-2-20041028] 💜
total = element thr:total { xsd:nonNegativeInteger }atom entries MAY have a total element, but they MUST NOT have more than one 🙅♀️
and there's no implied relationship between an entry's total value and any individual or combined thr:count values on its replies links 🙃 they're separate things, bestie, don't mix them up 💅
6. Considerations for Using thr:count, thr:updated, and total
okay so thr:count, thr:updated, and total all add extra metadata about the discussion thread attached to an entry 🧵 the whole point is to let feed readers show basic context about the thread without having to fetch, parse, and analyze every linked resource 🚀 but there are a few things implementors need to keep in mind 👀
first up, these extensions MUST NOT be assumed to be completely accurate about the thread 😬 for example, the real number of responses in a linked resource MAY be different from what thr:count says 🙈 publishers SHOULD try their best to keep the values accurate 🎯 the fact that this kind of "external reference metadata" (like the attributes on replies links) isn't authoritative is covered in more detail in Section 3.3 of the W3C document "Tag Finding 12: Authoritative Metadata" [TAG12] 📚
second, these values are volatile and can change faster than the entry they belong to 🌪️ if they get updated a lot, or if any part of the Atom document changes a lot, that could hurt how well the document can be cached, which means more bandwidth gets used overall 😭
publishers SHOULD treat a change to the values of thr:count, thr:updated, and total as an "insignificant" update in terms of [RFC4287] ✨ that means the containing feed, entry, or source element's atom:updated value SHOULD NOT be affected by a change to these extensions 💖
lastly, implementors should know that even though the Atom specification [RFC4287] explicitly lets the link element carry arbitrary extensions, it does not require implementations to support them 🫠 for extensions in general, Atom only requires feed consumers to ignore any extension they don't understand, and nothing stricter than that 🧚 so some implementations MAY NOT be able to fully use the extensions defined here, or in any other spec 🙈
7. Security Considerations
okay this part is serious, so let's get into it 🫡 this spec extends the Atom Syndication Format, so it's subject to the same security considerations defined in [RFC4287] 🔐
now, feeds that use these mechanisms could be crafted to make a consumer start an excessive number of network requests, or even an endless chain of them 😱 that can cause denial of service for the consumer, the target server, and/or the networks in between 💀 consumers can reduce this risk by requiring user intervention after a certain number of requests, or by limiting requests with a hard limit or with heuristics 🛡️
these mechanisms can also be used to build threaded conversations that span resources spread across multiple domains 🌐 for example, someone posting an entry to a weblog on one Internet domain could mark that entry as a response to an entry from a weblog on a different domain 👀 implementors should be aware that attackers can take advantage of these distributed responses to attach inappropriate or unwanted content to a discussion 🚨 to prevent or reduce this, users can be allowed to explicitly configure which sources responses may be retrieved from, or the legitimacy of a response source can be checked with heuristics 🧐
implementors should also watch out for malicious content publishers who edit or change content after it's been published 😤 in closed, centralized comment systems, after-the-fact editing usually isn't a problem, because those changes are easy to prevent, detect, or track 🔒 but with the distributed comments you get from the thr:in-reply-to extension, those changes are much harder to spot, which opens the door to serious attribution and repudiation problems 💔 XML Digital Signatures, as specified in Section 5.1 of [RFC4287], are one possible way to help with those concerns 🖋️ but a valid XML Digital Signature inside an entry is not, by itself, a reliable defense against repudiation issues 🙅♀️
8. IANA Considerations
this spec defines one new Atom link relation type, and it's been registered in the IANA Registry of Link Relation, which is set up by [RFC4287] 📋
Attribute Value: replies
Description: (see Section 4)
Expected display characteristics: (see Section 4)
Security considerations: (see Section 5)9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 🔑
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, July 2002. ⏰
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. 🌍
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, January 2005. 🌏
[RFC4287] Nottingham, M. and R. Sayre, "The Atom Syndication Format", RFC 4287, December 2005. 🌸
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and Registration Procedures", BCP 13, RFC 4288, December 2005. 🎀
[W3C.REC-xml-infoset-20040204] Tobin, R. and J. Cowan, "XML Information Set (Second Edition)", W3C REC REC-xml-infoset-20040204, February 2004. 🧾
[W3C.REC-xml-names-19990114] Hollander, D., Bray, T., and A. Layman, "Namespaces in XML", W3C REC REC-xml-names-19990114, January 1999. 🏷️
[W3C.REC-xmlschema-2-20041028] Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes Second Edition", W3C REC REC-xmlschema-2-20041028, October 2004. 🧮
9.2. Informative References
[RELAXNG] Clark, J., "RELAX NG Compact Syntax", December 2001, http://www.oasis-open.org/committees/relax-ng/compact-20021121.html. 📝
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April 2001. ✉️
[TAG12] Fielding, R. and I. Jacobs, "Tag Finding 12: Authoritative Metadata", http://www.w3.org/2001/tag/doc/mime-respect-20060412. 🏷️
Appendix A. Acknowledgements
the author says a huge thank you to Antone Roundy, Aristotle Pagaltzis, Byrne Reese, David Powell, Eric Scheid, James Holderness, John Panzer, Lisa Dusseault, M. David Peterson, Sam Ruby, Sylvain Hellegouarch, and the rest of the Atom Publishing Format and Protocol working group for their feedback while this spec was being developed 🫶 and any fault or weakness in the definition of this extension is solely the author's fault 🙈
some portions of text in this document were adapted from [RFC4287] so the two specs stay stylistically and technically aligned ✨
Author's Address
James M Snell 💜
EMail: jasnell@gmail.com URI: http://www.snellspace.com 🦋
Full Copyright Statement ✨
Okay bestie, the legal bits 💅 Copyright (C) The Internet Society (2006). 🦋
This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 💜
This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 😭
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Acknowledgement 🌸
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