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View Code? Open in Web Editor NEWNew CoAP Block Options
New CoAP Block Options
Hello Med, hello Jon,
here's a few concrete points I'd like to suggest for new-block:
Applicability: I'd have expected to read something like this in the
introduction:
The block-wise transfer of [RFC7959] covers the general case, but
falls short in situations where packet loss is highly asymmetrical.The new options introduced here provide roughly similar features to
the Block1/Block2 options. They provide additional properties that
are tailored towards the intended use case, and leave out mechanisms
like non-atomic access and proxy support that would complicate their
description.They are not intended for general CoAP usage, and any use outside
the DOTS use case should be carefully weighed against the loss of
interoperability with generic CoAP applications. It is hoped that
experience gained with these options can go into future extensions
of the block-wise mechanism that are both generally applicable and
serve this particular use case.
Separation of layers, sub-topic "congestion control": It should be
possible to phrase all congestion control parts of this in a dedicated
section (which may also make sense as an appendix).
I'd avoid all other mentions of things like PROBING_RATE: For example,
Section 3.4 right in the middle of a paragraph on re-requesting
missing blocks states that
The rate of requests for missing blocks is subject to PROBING_RATE.
Yes it is, but so is every single CoAP request until the client hears
back from the server. Repeating these things in places where are not a
new requirement may make it easier for some implementers that try to
build the whole stack in a single go, but even for them is prone to
the authors missing a spot -- and for everyone else this will each
time trigger the question of "is this new here or isn't this already
handled by a component two layers down the stack?".
I haven't tracked every single retransmission and rate limiting
statement throughout the document, but I expect that this section
could boil down to something like
For the DOTS use case, the following default CoAP parameters are
updated to better reflect the typical deployments:
- PROBING_RATE: 10 kilobyte/second
The averaging process mentioned in RFC7252 Section 4.7 after which
PROBING_RATE applies is not fully defined there. For the above
applications, a new parameter PROBING_RATE_WINDOW is introduced.
Messages up to a total size of PROBING_RATE_WINDOW may be sent
before PROBING_RATE is considered, and the probing rate may
generally be exceeded by that amount of data in short term.
- PROBING_RATE_WINDOW: 15 kilobyte
(The latter replaces MAX_PAYLOADS, and I'm not sure whether it is
better to express this in terms of package count or bytes on the wire,
just giving an example here. All numbers in the above were pulled out
of uninformed air.)
Separation of layers, sub-topic "Tokens": Same thing, next layer.
Whenever you're tempted to talk of a token, consider some of these
replacement phrases as a first step:
Naming: It's technically a minor thing, but people have already
complained to me about how hard the Block[12] names are to comprehend,
and introducing these options as Block[34] would not help here.
I'd like to suggest QuickBlock-Block[12].
(I know that this part of designing a new option is annoying. In the
interest of winding up with something that can be learned easily, it
is unfortunately essential).
In connection with that, it may make sense to briefly think about a
draft name before it is submitted as a WG draft. new-block sounds like
a 7959bis, which in my understanding this does not aim to be.
4.TBA3 Missing Payloads: I don't quite see why this needs a distinct
code. 4.08 Request Entity Incomplete sounds perfectly suitable to me.
If you think that sending an existing 4.xx code in the middle of a
block-wise transfer would cancel the transfer, there's still two ways
out of this:
You're just defining what a block-wise transfer is; just allow it.
(In a RFC7959 block-wise transfer that runs over a proxy, the proxy
may be currently incapable of doing any forwarding and return 5.03
Service Unavailable Max-Age:5. The client would then just pause 5
seconds and send the latest block again.)
For all but the last block, it may also be an option to send it as a
payload to a 2.31 code -- I don't think that would be outright
forbidden (and at any rate, all involved clients speak the protocol
anyway).
The draft describes this all as "just working through a proxy" --
original block-wise does not, and I'd like to hear how this is hoping
to get away without the options being proxy-unsafe.
Is there any point at all in using block-by-block proxying for these
use cases? Unless a proxy is sitting right inside the link under
attack, and unless the bodies get huge, it may be way easier for this
to be purely hop-by-hop. Proxies would reassemble the full bodies and,
for the next hop, just decide what works best there (be it regular
block-wise, Block34 or BERT).
(If there's a point in allowing block-by-block forwarding, we can talk
it through and work through the initial ideas that will come up like
"We use large enough Request-Tag values so they are unique", "how
large does that need to be", "why do we end up having a Request-Tag in
every request now" and "why do we not want that", but my gut feeling
is we won't need to go there.)
Kind regards
Christian
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