Optional ETags in response from server?

[JosefWN]

From my understanding of the RFC ETags are optional: https://datatracker.ietf.org/doc/html/rfc7252#section-5.10.6

However, in go-coap they are always included as there is a fallback, potentially increasing the response message size needlessly: https://github.com/plgd-dev/go-coap/blob/4d466801917705aec7cd97b6b53c7502d4c1f7c5/udp/client/responsewriter.go#L47-L53

For the endpoints that I use ETags with, I calculate them using the hashstructure library (before encoding and compressing the payload). For all other endpoints I don't use ETags, and I have other means of verifying the integrity of the payload (gzip's CRC32 checksum).

Would it make sense to add a boolean option to the server deciding whether to automatically calculate checksums for ETags, or better yet, include an option to set an ETag generator as an option (nullable/pointer or with a "no-op" generator for those that don't want it). This way, the library users can choose not to auto-generate ETags, and if they auto-generate them, they can choose how to do so (everyone might not want to use CRC64)?

[Danielius1922]

Sounds reasonable to me. I think allowing the user to optionally add a custom ETag generator is a useful extension of functionality. @jkralik what do you think?

[jkralik]

@Danielius1922 I agree. We can create option to the server that allows to set function - etag generator. And nil will be means that etag option will be not set by server/client.

[MariusMagiera]

I have a related question (less advanced):

I implemented a go server with endpoints that are written similar to the handleA endpoint in: https://github.com/plgd-dev/go-coap/tree/master/examples/simple/server

With w.SetResponse(codes.Content, message.TextPlain, bytes.NewReader([]byte("hello world"))) it automatically sets options which look like: "options": [ "ETag(etag=00 00 90 62 7E 81 4E 6E", "Content-Format: text/plain; charset=utf-8" ]

Since I don't need any Options to be sent, I worked with w.SetResponse(codes.Content, message.TextPlain, bytes.NewReader([]byte("hello world")), message.Option{make([]byte,0), 4}) it sets e.g. only the Etag to 0. But there is still an Etag in the options.

Is it possible not to send any options at all?

[jkralik]

Is it possible not to send any options at all?

Yes. Just follow the code in example: https://github.com/plgd-dev/go-coap/blob/e59fa916285a183e0adbcbf6d0c2e1329ddc448e/examples/simple/server/main.go#L28

[jkralik]

This is addressed in github.com/plgd-dev/go-coap/v3. ETag could be set via options:

https://github.com/plgd-dev/go-coap/blob/master/net/responsewriter/responseWriter.go#L41

[jkralik]

I will close it. Feel free to reopen it. Thx