Internet DRAFT - draft-amsuess-core-accept-any
draft-amsuess-core-accept-any
CoRE C. Amsuess
Internet-Draft March 25, 2019
Updates: 7641 (if approved)
Intended status: Standards Track
Expires: September 26, 2019
Accept-Any option for CoAP
draft-amsuess-core-accept-any-00
Abstract
This memoy defines the Accept-Any option, which provides a more
flexible content negotiation than the one originally specified for
the Constrained Application Protocol (CoAP) in [RFC7252]. As this is
particularly useful with but ruled out in CoAP observation
([RFC7641]), that is updated to allow it.
Status of This Memo
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This Internet-Draft will expire on September 26, 2019.
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the Trust Legal Provisions and are provided without warranty as
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1. Introduction
[ This document is being developed in git at
https://github.com/chrysn/accept-any [1]. ]
When CoAP content format defined in [RFC7252], the choice was made to
have the initial content negotiation allow the client to only pick
zero or one content format. This is a good choice for many
situations and ensures that proxies can cache as much as possible
without added complexity. A client that does not know a usable
content format would either leave the request field absent,
indicating it would accept any response, or try several acceptble
values in a series of requests.
In line with that choice, observation ([RFC7641]) required
notification responses to carry the same content format in the
notification as in the original response.
For applications that expect a representation to change its
representability to change during an observation's lifetime,
[I-D.ietf-core-multipart-ct] introduces a way of wrapping responses
in an application/multipart-core response. That approach is
convenient for bags of representations, but lacks actual content
negotiation and produces representations that are not directly usable
but need to be processed from inside the multipart representation.
This document introduces an additional way for the client to indicate
its set of acceptable content formats, and removes the same-content-
format limitation during observation.
2. The Accept-Any option
A new option Accept-Any is defined. It is critical [ I don't fully
see why but follow Accept here ], safe-to-foward and part of the
cache key. Its format is uint up to 2 long (indicating content
types), it is Class E in OSCORE, usable in requests only, and
repeatable
Repeatability is the only aspect in which it differs from Accept in
terms of option properties.
Its values indicate a list of acceptable representations in order of
decreasing preference. A server MUST answer with the first format it
can represent the requested state in, or 4.06 (Not Acceptable) if it
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could answer successfully but the response would not match any of the
option values.
The Accept-Any option MUST NOT be used exactly once; that request's
meaning would be identical to that of a single Accept option.
Instead, a single Accept option is used.
2.1. Proxy behavior
A proxy MAY ignore this option per its properties (and serve a cached
response if the cache key matches), but can implement additional
behavior to enhance its cache.
A proxy is allowed to serve a cached representation to a request with
a different sequence of Accept-Any options, provided the second
request has an Accept value of the cached representation, or all the
content formats that precede the available content format in the
second request's Accept-Any options also preceded the available
representation in any earlier (fresh) request's list.
When a request that carries Accept-Any is answered 4.06 (or with any
but the first format requested by Accept-Any in its Content-Format),
a proxy SHOULD [ we can't have a MUST here w/o making it non-safe-to-
forward, but I think it's sufficient ] invalidate all known
representations in any of the requested formats (or the formats
preceeding the returned one, respectively).
3. Update to RFC7641
3.1. Changed behavior
The requirement that subsequent notifications carry the same Content-
Format option as the original response ([RFC7641] Section 3.2) is
lifted.
3.2. Rationale
Observing resources whose available representations changed is a key
featuer of Accept-Any, and necessary to implement pub-sub topics that
have no initial value (but a "null" representation with a dedicated
content format) without losing content negotiation and direct
usability of the response.
The requirement was introduced initially before such content
negotiation was thought of, and is not a necessary part to the
remainder of the observation document.
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As long as the limitation is in place, the origin server has no clear
action guidance when its resource changes the available content
formats (see below).
3.3. Impact
Changes to the returned media type can either happen when
o Accept-Any was sent in the request - in which case both server and
client know the updated rules, or
o no Accept header was sent - in which case the server whose
representation changes to require a new content format has no
clear way of indicating that under [RFC7641] (ending with 4.06 Not
Acceptable would be close but isn't the expected response to a
repeated request); if the server changes the content format in a
notification to an unaware client, the client would catch it as a
bad response (probably similar to a response with a Content-Format
not matching the sent Accept). The client might regard the
observation as being aborted while the server does not, and will
terminate the observation with a RST on the next notification (or
close the connection in TCP).
Applications are still free to require constant content formats;
clients would treat what could previously be treated as a protocol
error would now treat it as an application error.
Impact on proxies: A proxy that enforces the previous rule on
Content-Format staying constant would close observations (probably
with something like 5.02 Bad Gateway), and the client would need to
re-establish. No proxy implementations are known that implement that
behavior.
4. References
4.1. Normative References
[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>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
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4.2. Informative References
[I-D.ietf-core-multipart-ct]
Fossati, T., Hartke, K., and C. Bormann, "Multipart
Content-Format for CoAP", draft-ietf-core-multipart-ct-03
(work in progress), March 2019.
4.3. URIs
[1] https://github.com/chrysn/accept-any
Appendix A. Open questions
o We could just as well make Accept repeatable with the same
semantics as Accept-Any.
o Is there any value in having an Accept-All option in parallel to
this option that asks for a multipart response that contains all
the representations?
Author's Address
Christian Amsuess
Hollandstr. 12/4
1020
Austria
Phone: +43-664-9790639
Email: christian@amsuess.com
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