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Minutes interim-2021-core-07: Wed 16:00

Meeting Minutes Constrained RESTful Environments (core) WG
Title Minutes interim-2021-core-07: Wed 16:00
State Active
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Last updated 2021-06-09

# CoRE Virtual interim - 2021-06-09 - 14:00 UTC


* Marco Tiloca, RISE
* Jaime Jiménez, Ericsson

## Remote instructions


Minute takers: Christian Amsüss
Jabber scribes:

## Participants

1. Marco Tiloca, RISE
2. Christian Amsüss
3. Rikard Höglund, RISE
4. Peter Yee, AKAYLA
5. Göran Selander, Ericsson
6. Esko Dijk,

*[MT]: Marco Tiloca
*[JJ]: Jaime Jiménez
*[FP]: Francesca Palombini
*[CB]: Carsten Bormann
*[CA]: Christian Amsüss
*[KH]: Klaus Hartke
*[RH]: Rikard Höglund
*[TF]: Thomas Fossati
*[DN]: David Navarro
*[GS]: Göran Selander
*[BS]: Bilhanan Silverajan
*[AS]: Alan Soloway
*[MR]: Michael Richardson
*[AK]: Ari Keränen
*[MJK]: Michael Koster
*[JM]: John Mattsson
*[NW]: Niklas Widell
*[ED]: Esko Dijk

## Agenda

### Note Well

Remember that the [note well](
applies, for [IPR]( but also for [WG
processes]( and [code of
conduct]( Try to be nice.

### Bluesheets / Jabber & Minutes / Agenda bashing

Fill the "Participants" list and other info above.

MT: CA, updates on ERT?
CA: Working on it; GS please look at point-to-point responses growing as

### Groupcomm-bis


Presented slides:

ED: going through slides: caching model for the proxy and validation
Client-Proxy moved to groupcomm-proxy. Going through John's review/comments.

Q&A up to p4:

GS: What's written today about amplification and spoofing attacks?
ED: Now only mentioned, pointing to group OSCORE for mitigation. But request
was for more detail to that, and it doesn't address all the problems. GS: There
certainly *are* threat analyses around that we can use, but don't have any off
my head. Esp. on multicast -- then see what is applicable. (So basically "what
you said"). MT: John wanted to cover something like this also in coap-attacks,
AFAIK not done now but intended. Maybe can connect. For groupcomm-bis doesn't
want to give false hopes that group-OSCORE and Echo alone solve this. ED: Some
text in the document is already covering the problem (see 6.2.3 "Countering
Attacks"); the idea was to make that more explicit and detailed (e.g. to what
extent that applies). Anyway, good to clarify.

ED: continuing on slides (p5..=7)
GS on slide 7 top: This is about the CoAP group, right?
ED: Yes.
GS: Is anything different when it's about the OSCORE group?
MT: Not in this respect, you need to know about the servers possibly in the
OSCORE group, as they are the one sending responses to possibly cache. ED: The
new caching model at the client is limited to a client that knows of *all* the
servers in the group and has a fresh response for each of those. ED: For the
proxy case, it's similar when thinking of the "Aggregated" cache entry of the
proxy. Eg. if proxy is on BR it can see when someone joins the group and
quickly have an up-to-date view of the group membership. Unless you have that
kind of knowledge, can't do anything and it's rather better to not have this
aggregated cache entry (which risks to not reflect the current group
membership). Can also be other out-of-band knowledge related to the
application/network context, eg. if you know that group members only join at
midnight. Still, have to be sure you have fresh responses from all known

ED: continuing on slides (p7...10). Different ways to do response revalidation,
good to converge to a single one. Alt 1: Multi-ETag option from v-03 of the
draft; Alt 2: adapted use of the original ETag; Alt 3 and 4 proposed by CA. ED:
Plan for this document is to follow alternative 2, i.e. adapted use of ETag.
CA: Alternative 3 was a form of compressing away the addresses from Alternative
1, and lossily compressing the ETags to allow some chance of misbehavior with
still compact requests. Alternative 4 is most likely to be useful. Alternatives
1 and 3 would need very strong use cases especially to justify the overhead on
the wire.

ED: continuing on slides (p8..), \[ clarifying on GS and CA's questions \]

GS: Thinking about John's input on DoS aspects ... we have NoSec mode, also
here. If we designed CoAP today, would we really allow that? (Would also ask
that if we were thinking about a version 2 of CoAP). MT: E.g. early discovery.
GS: But just that can be exploitable. CA: Not all applications of multicast do
result in an exploitable number of responses. Say, discovery of group manager
or resource directory -- there's only one or two of them active on the network
joined in the group. It's just that the client doesn't know their address and
thus has to send it out to the group, but in these deployments it's not an
amplification vector. ED: Agree, this use of discovery doesn't result in
amplification. If request is for "anyone with any CoAP resources", that will
result in large sets. GS: Should be clear at design time what can and can not
be used. Shouldn't be able to trigger multiplication attacks.

GS: Maybe it's a security consideration to add: Try to restrict the cases where
NoSec multicast works and is acceptable in the face of possible amplification.
MT: Scope and type of traffic should be narrow, ensuring there's only a few
possible places targeted and possible to exploit for amplification. CA: Might
be worth pointing out that you don't just always respond to MC request if you
can, but only if explicitly configured to join the group and respond.

### Groupcomm-proxy


Presented slides:

MT: presenting based on slides.
CA on p5: isn't the aggregate state obsolete now that "fresh only if group
known out of band" model is there in groupcomm-bis? MT: coming back to that,
continuing. GS: So what is the suggestion?

CA: Thinking of proxy as server tacked to caching client, so rules should be
the same, and if we need more than what can be done on a client, move that
there and not make this special.

MT: So build more on the requirements for the client recently added to
groupcomm-bis. Like for the client covered in the previous presentation, the
proxy has to know of all the other group members and have fresh responses for
all of them. This may be possible by sitting on a (multicast) BR or through
other out-of-band knowledge related to the application/network context, eg. if
you know that group members only join at midnight.

ED: Client can have an aggregate entry?
MT: Sounds like overkill.

CA: With client-has-to-know-which-servers-are-around (explicitly or
implicitly), there is nothing to store any more in the aggregate entry. GS:
What is conclusion? MT: The functionality is good to have. Given the hypothesis
above fulfilled, it's sufficient to rely on individual cache entries only. For
phrasing, better to not consider explicit aggregated cache entry, but more in
terms of "client request may hit all the individual cache entries", because
client has to know (for caching) all servers anyway.

MT: Continuing on p7 / handover to ED, about more examples/additions on
Reverse-Proxies. ED: Example to be added with URI templates, based on RFC 8075.

CA, ED: Discussion about whether to allow a reverse-proxy to operate on a group
request, or not, also in case the client didn't include the Multicast-Signaling
option. Christian thinks it should error out in this case. So only a client
prepared for receiving multiple responses will get those responses, while an
unsuspecting client would not. The current examples are in fact aligned with
this thinking.

CA: The URI (see example) doesn't necessarily tell that it would resolve to a
group request. \[ added only to minutes: but the client may have side
information about the URI, like "it was entered into the field for
where-to-send-multicast-requests, and if that resolves to a unicast address,
then better try sending a Multicast-Signaling in case the actual multicast
happens *behind* that" \].

CA: on Response-Forwarding and sending Response-Forwarding: needs further
coordination w/ protocol-indication.

MT: Resuming at p8, heads up on updates in the queue, to process with different
priorities. * p8: Multicast-Signaling will still express server addressing
information like in
but both documents will likely consider the more compact encoding possible with
CRIs, of which details are being nailed down in
[HREF]( * p9: OSCORE
between client and proxy is convenient, especially if Group OSCORE is also used
between client and server; this is now in Appendix A. Agreed at IETF 110 that:
there are more use cases than this document; this requires proper design and
analysis, better handled in a dedicated document. Ongoing writing of the new
document; plan to remove the current Appendix A. * p10: revalidation between
proxy and servers can work in the same way it would work in groupcomm-bis
(which is converging to the adapted use of ETag); though it keeps moving back
to the end of the queue, still aiming to define how this all works with a
HTTP-to-CoAP proxy, which would enable a HTTP client to talk to a group of CoAP
servers. There's a rough idea of how it can work.

### AOB

CA: Plugging the FAQ on the occasion of having updated them for lockstep
protocols: <> MT: Is this perhaps
useful seminal material for [corrclarr](,
whenever it happens? CA: Likely so.