{links} response content-type header

[Bonnarel]

From Markus Demleitner today :+1: there's one skeleton in the closet that recently came up again, and perhaps we can still somehow bury it before RFC.

The problem is the following text:

Unless the incoming request included a RESPONSEFORMAT parameter requesting a different format, the content-type header of the response MUST be application/x-votable+xml'' with thecontent'' parameter set to ``datalink'', with the canonical form given in \ref{sec:mime} strongly recommended.

The purpose of this language is that clients can (relatively) easily work out that they are dealing with a Datalink document regardless of where they get it from (as long as it's http). I think that's a good idea, although I'm not aware of a client that actually looks at content-type when retrieving things that could be datalink documents.

But at the same time this is blocking an important use case: Displaying datalink documents in the browser (Background: http://mail.ivoa.net/pipermail/dal/2021-April/008426.html and https://github.com/msdemlei/datalink-xslt). When I wrote the XSLT for that in ~2016, I planned it as a temporary hack until there are good datalink clients, but now I think letting people open datalinks with the browser and getting something actually usable is a major use case in itself.

The trouble with this: Web browsers will not apply the XSLT to documents with a media type of application/x-votable+xml;content=datalink. I have to give them text/xml to start the whole magic.

I hence at the moment have the choice of violating the standard or breaking a use case important to me. I weaseled around that first by inspecting user agent strings and only returning text/xml if the user agent looked as if I was dealing a web browser, praying nobody would notice. But that broke rather quickly (I forget the details), and I switched to inspecting the accept header. If I find a text/html in there, I return text/xml (yeah, it's that twisted), otherwise I'm compliant with the datalink spec.

But it's still a violation of the standard. I had hoped programmatic use would not be impacted, but it turns out that, for instance, the JVMs earlier than 11 actually indicate acceptance of text/html, too. Sigh.

So... it's trouble, and I have not found any solution that doesn't make me cringe. But I increasingly have the impression that ignoring the problem will only make matters worse.

The least horrible proposal I have would be to replace the text quoted above:

When a datalink service returns a datalink VOTable (i.e., absent a RESPONSEFORMAT parameter requesting something else), it MUST indicate that in the response's content-type header. When the request's accept header includes application/x-votable+xml'', then it MUST beapplication/x-votable+xml'' with the content'' parameter set todatalink'', with the canonical form given in \ref{sec:mime} strongly recommended. Otherwise, any legal VOTable media type, including text/xml, is allowed.

That is: clients wishing to do dispatch based on the datalink media type must indicate that they accept VOTable. It's a pretty safe bet that major browsers won't do that (and potential future VO-enabled browsers wouldn't need the XSLT, I'm sure). And although HTTP content negotiation isn't as popular as it should be, I think it's implementationally not very intrusive.

The only alternative I could come up with would be to codify what I'm currently doing:

Unless the incoming request included a RESPONSEFORMAT parameter requesting a different format, and unless the user agent indicates it will accept text/html, the content-type header of the response MUST be application/x-votable+xml'' with thecontent'' parameter set to ``datalink'', with the canonical form given in \ref{sec:mime} strongly recommended.

We could then have a footnote explaining what the text/html exception is supposed to do. The downside here is that it's really an ugly hack to return text/xml when accept has text/html, and there's too much library code that wantonly sticks text/html into accept behind the programmers' backs.

I think given the media type hasn't seen too much use so far anyway and when a client wants to use it, it would be new code anyway, I'd go for option one.

But if anyone had a less painful idea, that'd be even better. Does anyone?

[Bonnarel]

Answer by Mark Taylor :

Given that the proposed text changes are rather convoluted, and that nobody is, as far as I know, actually using the currently mandated content-type behaviour (evidence: Markus has been violating it and nobody beside validation nerds have complained/noticed), another possibility would just be to downgrade that MUST to a SHOULD:

Unless the incoming request included a RESPONSEFORMAT parameter requesting a different format, the content-type header of the response SHOULD be application/x-votable+xml'' with thecontent'' parameter set to ``datalink'', with the canonical form given in \ref{sec:mime} strongly recommended.

So: use the datalink content-type unless you've got a good reason not to (as does Markus, and other service providers that use the same tricks to render links tables in browsers, at least as long as browsers won't apply XSLT to content-types marked with +xml).

This would technically be a breaking change from DL 1.0 to 1.1 (as would Markus's other proposed changes). But given the likely impact in practice of the change (none?) I think we could turn a blind eye.

Mark

From Markus Demleitner today +1 there's one skeleton in the closet that recently came up again, and perhaps we can still somehow bury it before RFC.

The problem is the following text:

Unless the incoming request included a RESPONSEFORMAT parameter requesting a different format, the content-type header of the response MUST be application/x-votable+xml'' with thecontent'' parameter set to ``datalink'', with the canonical form given in \ref{sec:mime} strongly recommended.

The purpose of this language is that clients can (relatively) easily work out that they are dealing with a Datalink document regardless of where they get it from (as long as it's http). I think that's a good idea, although I'm not aware of a client that actually looks at content-type when retrieving things that could be datalink documents.

But at the same time this is blocking an important use case: Displaying datalink documents in the browser (Background: http://mail.ivoa.net/pipermail/dal/2021-April/008426.html and https://github.com/msdemlei/datalink-xslt). When I wrote the XSLT for that in ~2016, I planned it as a temporary hack until there are good datalink clients, but now I think letting people open datalinks with the browser and getting something actually usable is a major use case in itself.

The trouble with this: Web browsers will not apply the XSLT to documents with a media type of application/x-votable+xml;content=datalink. I have to give them text/xml to start the whole magic.

I hence at the moment have the choice of violating the standard or breaking a use case important to me. I weaseled around that first by inspecting user agent strings and only returning text/xml if the user agent looked as if I was dealing a web browser, praying nobody would notice. But that broke rather quickly (I forget the details), and I switched to inspecting the accept header. If I find a text/html in there, I return text/xml (yeah, it's that twisted), otherwise I'm compliant with the datalink spec.

But it's still a violation of the standard. I had hoped programmatic use would not be impacted, but it turns out that, for instance, the JVMs earlier than 11 actually indicate acceptance of text/html, too. Sigh.

So... it's trouble, and I have not found any solution that doesn't make me cringe. But I increasingly have the impression that ignoring the problem will only make matters worse.

The least horrible proposal I have would be to replace the text quoted above:

When a datalink service returns a datalink VOTable (i.e., absent a RESPONSEFORMAT parameter requesting something else), it MUST indicate that in the response's content-type header. When the request's accept header includes application/x-votable+xml'', then it MUST beapplication/x-votable+xml'' with the content'' parameter set todatalink'', with the canonical form given in \ref{sec:mime} strongly recommended. Otherwise, any legal VOTable media type, including text/xml, is allowed.

That is: clients wishing to do dispatch based on the datalink media type must indicate that they accept VOTable. It's a pretty safe bet that major browsers won't do that (and potential future VO-enabled browsers wouldn't need the XSLT, I'm sure). And although HTTP content negotiation isn't as popular as it should be, I think it's implementationally not very intrusive.

The only alternative I could come up with would be to codify what I'm currently doing:

Unless the incoming request included a RESPONSEFORMAT parameter requesting a different format, and unless the user agent indicates it will accept text/html, the content-type header of the response MUST be application/x-votable+xml'' with thecontent'' parameter set to ``datalink'', with the canonical form given in \ref{sec:mime} strongly recommended.

We could then have a footnote explaining what the text/html exception is supposed to do. The downside here is that it's really an ugly hack to return text/xml when accept has text/html, and there's too much library code that wantonly sticks text/html into accept behind the programmers' backs.

I think given the media type hasn't seen too much use so far anyway and when a client wants to use it, it would be new code anyway, I'd go for option one.

But if anyone had a less painful idea, that'd be even better. Does anyone?

[Bonnarel]

Hum, Thinking about it, I tend to agree with Markus option one because depends of VO tool developpers in that case. Anyway I create a new github issue with this discussion

François

Answer by Mark Taylor :

Given that the proposed text changes are rather convoluted, and that nobody is, as far as I know, actually using the currently mandated content-type behaviour (evidence: Markus has been violating it and nobody beside validation nerds have complained/noticed), another possibility would just be to downgrade that MUST to a SHOULD:

Unless the incoming request included a RESPONSEFORMAT parameter requesting a different format, the content-type header of the response SHOULD be application/x-votable+xml'' with thecontent'' parameter set to ``datalink'', with the canonical form given in \ref{sec:mime} strongly recommended.

So: use the datalink content-type unless you've got a good reason not to (as does Markus, and other service providers that use the same tricks to render links tables in browsers, at least as long as browsers won't apply XSLT to content-types marked with +xml).

This would technically be a breaking change from DL 1.0 to 1.1 (as would Markus's other proposed changes). But given the likely impact in practice of the change (none?) I think we could turn a blind eye.

Mark

From Markus Demleitner today +1 there's one skeleton in the closet that recently came up again, and perhaps we can still somehow bury it before RFC. The problem is the following text: Unless the incoming request included a RESPONSEFORMAT parameter requesting a different format, the content-type header of the response MUST be application/x-votable+xml'' with thecontent'' parameter set to datalink'', with the canonical form given in \ref{sec:mime} strongly recommended. The purpose of this language is that clients can (relatively) easily work out that they are dealing with a Datalink document regardless of where they get it from (as long as it's http). I think that's a good idea, although I'm not aware of a client that actually looks at content-type when retrieving things that could be datalink documents. But at the same time this is blocking an important use case: Displaying datalink documents in the browser (Background: http://mail.ivoa.net/pipermail/dal/2021-April/008426.html and https://github.com/msdemlei/datalink-xslt). When I wrote the XSLT for that in ~2016, I planned it as a temporary hack until there are good datalink clients, but now I think letting people open datalinks with the browser and getting something actually usable is a major use case in itself. The trouble with this: Web browsers will not apply the XSLT to documents with a media type of application/x-votable+xml;content=datalink. I have to give them text/xml to start the whole magic. I hence at the moment have the choice of violating the standard or breaking a use case important to me. I weaseled around that first by inspecting user agent strings and only returning text/xml if the user agent looked as if I was dealing a web browser, praying nobody would notice. But that broke rather quickly (I forget the details), and I switched to inspecting the accept header. If I find a text/html in there, I return text/xml (yeah, it's that twisted), otherwise I'm compliant with the datalink spec. But it's still a violation of the standard. I had hoped programmatic use would not be impacted, but it turns out that, for instance, the JVMs earlier than 11 actually indicate acceptance of text/html, too. Sigh. So... it's trouble, and I have not found any solution that doesn't make me cringe. But I increasingly have the impression that ignoring the problem will only make matters worse. The least horrible proposal I have would be to replace the text quoted above: When a datalink service returns a datalink VOTable (i.e., absent a RESPONSEFORMAT parameter requesting something else), it MUST indicate that in the response's content-type header. When the request's accept header includes `application/x-votable+xml'', then it MUST be `application/x-votable+xml'' with the `content'' parameter set to `datalink'', with the canonical form given in \ref{sec:mime} strongly recommended. Otherwise, any legal VOTable media type, including text/xml, is allowed. That is: clients wishing to do dispatch based on the datalink media type must indicate that they accept VOTable. It's a pretty safe bet that major browsers won't do that (and potential future VO-enabled browsers wouldn't need the XSLT, I'm sure). And although HTTP content negotiation isn't as popular as it should be, I think it's implementationally not very intrusive. The only alternative I could come up with would be to codify what I'm currently doing: Unless the incoming request included a RESPONSEFORMAT parameter requesting a different format, and unless the user agent indicates it will accept text/html, the content-type header of the response MUST be `application/x-votable+xml'' with the `content'' parameter set todatalink'', with the canonical form given in \ref{sec:mime} strongly recommended. We could then have a footnote explaining what the text/html exception is supposed to do. The downside here is that it's really an ugly hack to return text/xml when accept has text/html, and there's too much library code that wantonly sticks text/html into accept behind the programmers' backs. I think given the media type hasn't seen too much use so far anyway and when a client wants to use it, it would be new code anyway, I'd go for option one. But if anyone had a less painful idea, that'd be even better. Does anyone?

[pdowler]

I'm inclined to agree with this change, although it's a shame to make changes because browsers don't actually support the standards (VOTable section 8 lists the RFCs that justify the application/x-votable+xml mime type).

Given the content in the above section of VOTable, DataLink should say the minimum necessary. Specifying that the output content-type header has to match a specified RESPONSEFORMAT (or fail) should be sufficient and that also means one can make a links URL intended to be opened in a browser... the section on RESPONSEFORMAT in DALI already says that quite explicitly and for this very reason:

"If a client requests a format by specifying the media type (as opposed to one of the short forms), the response that delivers that content must set that media type in the Content-Type header. This is only an issue when a format has multiple acceptable media types (e.g., VOTable). This allows the client to control the Content-Type so that it can reliably cause specific applications to handle the response (e.g., a browser rendering a VOTable generally requires the text/xml media type)."

The only "change" needed in DataLink is to relax and say that any valid VOTable mimetype can be used.

For completeness: there is also https://github.com/ivoa-std/VOTable/issues/15 which is about the content param itself.

So, generally +1 from me but it's mainly clarifying/relaxing the DataLink spec because details are in DALI and VOTable.

Patrick Dowler Canadian Astronomy Data Centre Victoria, BC, Canada

On Mon, 9 Jan 2023 at 08:42, François Bonnarel @.***> wrote:

From Markus Demleitner today 👍 there's one skeleton in the closet that recently came up again, and perhaps we can still somehow bury it before RFC.

The problem is the following text:

Unless the incoming request included a RESPONSEFORMAT parameter requesting a different format, the content-type header of the response MUST be application/x-votable+xml'' with the content'' parameter set to ``datalink'', with the canonical form given in \ref{sec:mime} strongly recommended.

The purpose of this language is that clients can (relatively) easily work out that they are dealing with a Datalink document regardless of where they get it from (as long as it's http). I think that's a good idea, although I'm not aware of a client that actually looks at content-type when retrieving things that could be datalink documents.

But at the same time this is blocking an important use case: Displaying datalink documents in the browser (Background: http://mail.ivoa.net/pipermail/dal/2021-April/008426.html and https://github.com/msdemlei/datalink-xslt). When I wrote the XSLT for that in ~2016, I planned it as a temporary hack until there are good datalink clients, but now I think letting people open datalinks with the browser and getting something actually usable is a major use case in itself.

The trouble with this: Web browsers will not apply the XSLT to documents with a media type of application/x-votable+xml;content=datalink. I have to give them text/xml to start the whole magic.

I hence at the moment have the choice of violating the standard or breaking a use case important to me. I weaseled around that first by inspecting user agent strings and only returning text/xml if the user agent looked as if I was dealing a web browser, praying nobody would notice. But that broke rather quickly (I forget the details), and I switched to inspecting the accept header. If I find a text/html in there, I return text/xml (yeah, it's that twisted), otherwise I'm compliant with the datalink spec.

But it's still a violation of the standard. I had hoped programmatic use would not be impacted, but it turns out that, for instance, the JVMs earlier than 11 actually indicate acceptance of text/html, too. Sigh.

So... it's trouble, and I have not found any solution that doesn't make me cringe. But I increasingly have the impression that ignoring the problem will only make matters worse.

The least horrible proposal I have would be to replace the text quoted above:

When a datalink service returns a datalink VOTable (i.e., absent a RESPONSEFORMAT parameter requesting something else), it MUST indicate that in the response's content-type header. When the request's accept header includes application/x-votable+xml'', then it MUST be application/x-votable+xml'' with the content'' parameter set to datalink'', with the canonical form given in \ref{sec:mime} strongly recommended. Otherwise, any legal VOTable media type, including text/xml, is allowed.

That is: clients wishing to do dispatch based on the datalink media type must indicate that they accept VOTable. It's a pretty safe bet that major browsers won't do that (and potential future VO-enabled browsers wouldn't need the XSLT, I'm sure). And although HTTP content negotiation isn't as popular as it should be, I think it's implementationally not very intrusive.

The only alternative I could come up with would be to codify what I'm currently doing:

Unless the incoming request included a RESPONSEFORMAT parameter requesting a different format, and unless the user agent indicates it will accept text/html, the content-type header of the response MUST be application/x-votable+xml'' with the content'' parameter set to ``datalink'', with the canonical form given in \ref{sec:mime} strongly recommended.

We could then have a footnote explaining what the text/html exception is supposed to do. The downside here is that it's really an ugly hack to return text/xml when accept has text/html, and there's too much library code that wantonly sticks text/html into accept behind the programmers' backs.

I think given the media type hasn't seen too much use so far anyway and when a client wants to use it, it would be new code anyway, I'd go for option one.

But if anyone had a less painful idea, that'd be even better. Does anyone?

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[msdemlei]

On Mon, Jan 09, 2023 at 12:46:50PM -0800, Patrick Dowler wrote:

The only "change" needed in DataLink is to relax and say that any valid VOTable mimetype can be used.

Hm, no, I don't think I'd like that. You see, with the current regulation a client can know that it gets a datalink document (as opposed to an SDM-compliant spectrum, say, or a TAP result) as soon as it parses the HTTP headers. Without it, it has a very hard time figuring this out even when it has the full document; DALI circulates the idea to have a standardID info for that purpose, but not even DaCHS does this yet (it will in the future).

Side note: If we relax the media type thing in Datalink 1.1, we really must make the standardID INFO mandatory at the same time.

So, I think we need to spell out conditions under which the client can reliably expect to get the content=datalink when something is a datalink document. Use case: Splat gets a link sent via SAMP and retrieves it. This could be an SDM VOTable or a Datalink document. How will it tell the two situations (requiring entirely different handling) apart?

In the end, we somehow need to reconcile the requirements from the non-aware browsers (which need text/xml) and the aware clients (which need a/x-v;content=datalink), which means that the server needs some halfway reliable way to tell the two apart. And that's how I came up with my two proposals:

(1) We either say "detect unaware clients by looking for text/html in accept", (2) or we say "detect aware clients by looking for a/x-v;content=datalink in accept".

(1) would be less intrusive, except that clients may send text/html without even knowing given the sometimes slightly sorry state of content negotiation even in widely-used libraries; (2) is another thing to think about when retrieving VO resources, which isn't nice.

Of course, if someone has a good idea on how to reconcile the competing requirements without (ab-) using the dreaded HTTP content negotiation, I'd be most interested to learn about them.

[mbtaylor]

In practice, as author of a client that does have to figure out when something is a links table and when it's a catalogue or whatever, I generally do that by duck typing - if it's a VOTable with most or all of the columns required by DataLink sec 3.2 then I can treat it as a links document. I'm likely to carry on doing that in preference to looking at the content-type for practical reasons - content-types are not always present and correct, they can be fiddly to obtain and parse, and you might be acquiring one of these tables in some way other than HTTP.

When I would like some signal about whether something is or is not a links table is before I've acquired it, to give the user a hint about whether they will want to download it. In principle the HTTP content-type in conjunction with a HEAD request could help there, but really I want that information without needing any HTTP interaction.

So from my point of view some relaxation of constraints on the HTTP content-type header (like Pat's: any valid content-type is OK, or mine: use SHOULD not MUST) is not likely to present practical problems. Other resource consumers may have different views of course.

[pdowler]

I could change my "any valid VOTable content-type" to "SHOULD use application/x-votable+xml;content=datalink unless the client asks for something else via the RESPONSEFORMAT param" so prefer the current approach but allow client to control it and then the "should" allows services to detect/help web browsers if they want to.

set hat=implementor; If our site had a link to a a links doc that people could click on, I'd tell the page author to use RESPONSEFORMAT and I'd resist writing code to detect browsers. Yeah, that doesn't help some other web site that links to a {links} result at CADC, but it would be really hard to convince me to write and maintain that browser detection code.

Patrick Dowler Canadian Astronomy Data Centre Victoria, BC, Canada

On Tue, 10 Jan 2023 at 03:37, Mark Taylor @.***> wrote:

In practice, as author of a client that does have to figure out when something is a links table and when it's a catalogue or whatever, I generally do that by duck typing - if it's a VOTable with most or all of the columns required by DataLink sec 3.2 then I can treat it as a links document. I'm likely to carry on doing that in preference to looking at the content-type for practical reasons - content-types are not always present and correct, they can be fiddly to obtain and parse, and you might be acquiring one of these tables in some way other than HTTP.

When I would like some signal about whether something is or is not a links table is before I've acquired it, to give the user a hint about whether they will want to download it. In principle the HTTP content-type in conjunction with a HEAD request could help there, but really I want that information without needing any HTTP interaction.

So from my point of view some relaxation of constraints on the HTTP content-type header (like Pat's: any valid content-type is OK, or mine: use SHOULD not MUST) is not likely to present practical problems. Other resource consumers may have different views of course.

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[msdemlei]

On Tue, Jan 10, 2023 at 03:08:34PM -0800, Patrick Dowler wrote:

I could change my "any valid VOTable content-type" to "SHOULD use application/x-votable+xml;content=datalink unless the client asks for something else via the RESPONSEFORMAT param" so prefer the current approach but allow client to control it and then the "should" allows services to detect/help web browsers if they want to.

Hm... while the SHOULD of course fixes my immediate problem, it will still keep SPLAT and friends from being able to rely on the content=datalink (it's just a SHOULD, after all), and that makes the whole media type rather pointless (in this situation; it's of course still fine for the content_type column).

set hat=implementor; If our site had a link to a a links doc that people could click on, I'd tell the page author to use RESPONSEFORMAT and I'd resist writing code to

Ah, that's not my situation. My situation is that people get a datalink URL from somewhere and paste it into the browser. And blindly adding RESPONSEFORMAT=text/xml to all of my datalinks would not only be ugly, it would again break the "make SPLAT recognise a datalink up front" case.

detect browsers. Yeah, that doesn't help some other web site that links to a {links} result at CADC, but it would be really hard to convince me to write and maintain that browser detection code.

Well, you don't need to if you don't want to serve XSLT (though, personally, I think that's a shame). An implementation stubbornly returning x-v;content=datalink still is correct whatever we do, and in particular we don't require any sort of support for HTTP content negotiation.

So, it seems to me the tableau of possible solutions now looks like this:

(1) Open up ("SHOULD") the content type and rely on duck typing for datalink detection as per Mark's suggestion (my take: possible, but if we go there, I'd say we need to find a spot to recommend that somehwere in the document, as it feels somewhat unorthodox as the recommended way to do this kind of thing).

(1a) Open up the content type and require a standardID info for Datalink 1.1.

(2) Keep the MUST unless there's an accept of text/html

(3) Keep the MUST if there's an accept of x-v;content=datalink

Unless we find something sucking less than any of these, I suppose it's time for a pain level poll (https://blog.g-vo.org/building-consensus.html#scale). Here's my list: (1) - 6; (1a) - 4; (2) - 4; (3) - 5

[Bonnarel]

Hi there Le 11/01/2023 à 09:45, msdemlei a écrit :

On Tue, Jan 10, 2023 at 03:08:34PM -0800, Patrick Dowler wrote:

I could change my "any valid VOTable content-type" to "SHOULD use application/x-votable+xml;content=datalink unless the client asks for something else via the RESPONSEFORMAT param" so prefer the current approach but allow client to control it and then the "should" allows services to detect/help web browsers if they want to.

Hm... while the SHOULD of course fixes my immediate problem, it will still keep SPLAT and friends from being able to rely on the content=datalink (it's just a SHOULD, after all), and that makes the whole media type rather pointless (in this situation; it's of course still fine for the content_type column).

set hat=implementor; If our site had a link to a a links doc that people could click on, I'd tell the page author to use RESPONSEFORMAT and I'd resist writing code to

Ah, that's not my situation. My situation is that people get a datalink URL from somewhere and paste it into the browser. And blindly adding RESPONSEFORMAT=text/xml to all of my datalinks would not only be ugly, it would again break the "make SPLAT recognise a datalink up front" case.

detect browsers. Yeah, that doesn't help some other web site that links to a {links} result at CADC, but it would be really hard to convince me to write and maintain that browser detection code.

Well, you don't need to if you don't want to serve XSLT (though, personally, I think that's a shame). An implementation stubbornly returning x-v;content=datalink still is correct whatever we do, and in particular we don't require any sort of support for HTTP content negotiation.

So, it seems to me the tableau of possible solutions now looks like this:

(1) Open up ("SHOULD") the content type and rely on duck typing for datalink detection as per Mark's suggestion (my take: possible, but if we go there, I'd say we need to find a spot to recommend that somehwere in the document, as it feels somewhat unorthodox as the recommended way to do this kind of thing).

(1a) Open up the content type and require a standardID info for Datalink 1.1.

We already made a path in this direction in the 1.1 text wich reads

The DALI specification states that a standardID INFO element with name “standardID” and the actual standardID string as a value SHOULD be provided. It is recommended to include such an element to help users and applications to identify VOTables as results of DataLink services this way: where DataLink 1.0 didn't say anything

We can replace It is recommended by "it is required" if this solve the problem.

(2) Keep the MUST unless there's an accept of text/html

(3) Keep the MUST if there's an accept of x-v;content=datalink

Unless we find something sucking less than any of these, I suppose it's time for a pain level poll (https://blog.g-vo.org/building-consensus.html#scale). Here's my list: (1) - 6; (1a) - 4; (2) - 4; (3) - 5

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[jd-au]

The CASDA implementation is also non-compliant with Datalink 1.0 as it always has a content-type of text/xml to support XSLT rendering. Adding a standardID INFO would be fine for us.

However, §3.3 of Datalink 1.1 still has a MUST for the content-type header being application/x-votable+xml which precludes rendering with XSLT. Markus' option 1a seems like the best compromise here.

[pdowler]

I am preparing some editorial changes that include MUST -> SHOULD and allows any valid VOTable mime type.

I am open to making the standardID INFO mandatory in 1.1 so that clients have a clear way to detect that a links table is in there... that's somewhat better then the http header anyway since it gets saved in the xml file for later use. I will make that change as well and see how it looks at review.