CoAP: Non-traditional response forms
draft-bormann-core-responses-02
| Document | Type | Active Internet-Draft (individual) | |
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| Authors | Carsten Bormann , Christian Amsüss | ||
| Last updated | 2024-03-03 | ||
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draft-bormann-core-responses-02
Network Working Group C. Bormann
Internet-Draft Universität Bremen TZI
Intended status: Informational C. Amsüss
Expires: 4 September 2024 3 March 2024
CoAP: Non-traditional response forms
draft-bormann-core-responses-02
Abstract
In CoAP as defined by RFC 7252, responses are always unicast back to
a client that posed a request. The present memo describes two forms
of responses that go beyond that model. These descriptions are not
intended as advocacy for adopting these approaches immediately, they
are provided to point out potential avenues for development that
would have to be carefully evaluated.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-bormann-core-responses/.
Discussion of this document takes place on the Constrained RESTful
Environments (CoRE) Working Group mailing list
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Source for this draft and an issue tracker can be found at
https://github.com/core-wg/core-responses.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 4 September 2024.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Sending non-traditional responses . . . . . . . . . . . . . . 4
2.1. Preconditions to sending non-traditional responses . . . 4
2.2. Responses without request . . . . . . . . . . . . . . . . 5
3. OSCORE processing for non-traditional responses . . . . . . . 5
4. Response with embedded request . . . . . . . . . . . . . . . 7
5. Response for configured request . . . . . . . . . . . . . . . 7
5.1. Examples for configured requests . . . . . . . . . . . . 7
5.1.1. Example: Periodic request . . . . . . . . . . . . . . 7
5.1.2. Example: Event driven request . . . . . . . . . . . . 7
5.1.3. Example: Configured observe . . . . . . . . . . . . . 8
5.2. Multicast responses . . . . . . . . . . . . . . . . . . . 8
5.3. Respond-To option . . . . . . . . . . . . . . . . . . . . 8
5.4. Leisure-For-Responses Option . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. CoAP extensions explained by non-traditional
responses . . . . . . . . . . . . . . . . . . . . . . . . 12
A.1. Observation . . . . . . . . . . . . . . . . . . . . . . . 12
A.2. Responses to multicast requests . . . . . . . . . . . . . 12
A.3. Triangular responses (Response-To) . . . . . . . . . . . 13
A.4. Other current documents . . . . . . . . . . . . . . . . . 13
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
In CoAP as defined by RFC 7252, responses are always unicast back to
a client that posed a request. A server may want to send a response
to a request that it did not receive, may want to multicast a
response, or both.
The descriptions in this specification are not intended as advocacy
for adopting these approaches immediately, they are provided to point
out potential avenues for development that would have to be carefully
evaluated.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The term "byte" is used in its now customary sense as a synonym for
"octet".
Terms used in this draft:
Non-traditional response: A response that is not the single response
generated for a request received on the same transport.
Non-matching response: A response that has properties (typically
options) that make it incompatible with the original request, and
thus in particular unsuitable as a cached response to that request
(but possibly suitable to populate the cache for a similar
request). Options that make a response non-matching need to be
proxy unsafe.
For example, a Block2 response with a different value of block
number × block size than indicated in the request is non-matching.
Configured request: A request that reaches the server in another way
than by transmitting a usual CoAP request on the same
communication channel a response is expected on.
Embedded request: A request that is provided by the server to the
recipient of its response by embedding it into the response.
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2. Sending non-traditional responses
Non-traditional responses are sets of responses produced for a single
request, or responses sent without a transmitted request.
Where tokens are involved, all non-traditional responses use the
request's token; in any case, they are bound to the original request
(e.g. by using the same request_kid/request_piv pair in OSCORE
[RFC8613]). Where message IDs are involved, one of the non-
traditional response (the first sent, not necessarily the first
received as generally the network might reorder messages) can be sent
as a piggybacked response in an ACK (thus sharing the request's
message ID), the others are CON or NON responses.
Some established responses (observations defined in [RFC7641], and
responses to multicast requests in [I-D.ietf-core-groupcomm-bis])
match this definition and already follow the guidance set out here
for non-traditional responses; Appendix A gives details for them.
A second response differing from the first that can be sent by a non-
deduplicating server responding to a retransmission of a request is
not non-traditional because there is a second request -- that is
probably the last corner case at the line separating traditional from
non-traditional responses.
2.1. Preconditions to sending non-traditional responses
A server may send multiple responses to a request if there is any
property in the request that indicates the client's intention to
receive them. This is typically indicated by a request option, and
rarely in external properties of the message (in the multicast case,
the destination address).
A mechanism for eliciting multiple responses must specify the
conditions under which a token gets freed, as the traditional arrival
of the response is insufficient. It may also specify for which
requests the token can be reused immediately in follow-up requests.
On unordered transports, or when it's a client's follow-up request
and not a response that terminates the token, the client needs to
wait with reuse until no reordered non-traditional responses can be
expected anymore.
If a non-traditional response answers the original request, no
further action is required (this is the case of observation: ordering
is added on top of that to ensure that only the latest response is
used). If the response does not answer the original request, it must
be non-matching, either by an option introduced with the eliciting
option or by a generic option like Response-For.
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2.2. Responses without request
Endpoints may agree out of band on a token (or other request-matching
details). One way to do that is to exchange a "phantom request",
which is a request that client and server will agree to have sent and
received, respectively, without it actually being sent between those
endpoints.
As tokens are managed by the client, that request needs to be
generated by the client, or in close collaboration with the client
(for example by the client allowing a third party to use a subset of
its token values in order to set up non-traditional responses).
3. OSCORE processing for non-traditional responses
OSCORE [RFC8613] is built with the general assumption that requests
are processed into exactly one response. The specification contains
explicit provisions for Observe requests, and a whole protocol
extension for multicast requests.
OSCORE's binding between requests and responses remains unmodified:
Each response is cryptographically bound to an OSCORE request.
Therefore, any phantom request needs to be an OSCORE request as well,
and the parties need to agree on the sender and sequence number of
the phantom request. An easy way to do that securely is to deliver
the phantom request in a way that the server can do the full OSCORE
request processing on it. The server may process the OSCORE request
into internal data structures at reception time, or may process it
whenever a response is to be sent. In the latter case, it may need
to relax the requirements of Section 8.2 (Verifying the Request) of
[RFC8613] item 3.
To avoid reinventing the same rules as for Observe requests for any
other non-traditional response, this document defines a set of
processing instructions which can be referenced when specifying their
options. These rules generalize Sections 8.3 (Protecting the
Response) and 8.4 (Verifying the Response) of [RFC8613]:
* In 8.3 step 3, "use the AEAD nonce from the request" is only an
option once, i.e., after the sequence number expressed in that
request was removed from the replay window. This option is
usually taken in the first response, necessitating the use of
encoded Sender Sequence Numbers in later responses. (Non-
traditional responses such as Observe that rely on message
ordering may require that the request's nonce is used either in
the first response or not at all.)
// CA: We could also just mandate the "either the first or never"
// behavior.
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As a convenient effect, this generalized rule also implies that
when a server performs Appendix B.1.2 (Replay Window) of
[RFC8613], it needs to use its own Partial IV for the nonce (which
without this generalized rule necessitated a "MUST" statement in
the appendix).
It is unclear why one would delay sending the one response that has
the least overhead, but that may be lack of imagination. An approach
where instances can not generally be duplicated and are used at most
once (as in an affine type system) can make this doable in a safe
way. In the end it's a tradeoff between implementer flexibility and
specification simplicity.
* In 8.4 between steps 5 and 6, the Sender Sequence Number of the
response establishes an order in the received messages, which
users of non-traditional responses may rely on. If an option
specified that only the first response may use the request's
nonce, then the one response that uses it is ordered before all
other responses to the same request.
* If the handling of multiple responses is not idempotent, then at
8.4 step 5:
- For responses that use a Sender Sequence Number from the
server, the client consults the replay window before
decryption, and removes its number from the replay window after
successful decryption.
- For responses that use the request's Sender Sequence Number,
duplication is tracked for each request.
As a simplification, applications that only process the latest
response may track the latest sequence number for deduplication.
* In 8.4 step 8, the Option establishing the non-traditional
responses may specify that error conditions processing a response
are not fatal for the whole request. This should be done when an
Option allows immediate follow-up requests. This is the case for
the Observe option: When an observation is refreshed, a response
encrypted with the earlier request's request_kid may still be in
flight. That in-flight response will fail decryption, but
responses generated after the server has received the refresh will
be decryptable again.
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4. Response with embedded request
A server can send a response to a request that it did not actually
receive by embedding the request which the response answers in the
response.
The option "Response-For" contains a request packaged as in
Section 5.3 of [RFC8613]. The response is then intended to serve as
a response to this request.
+=====+===+===+===+===+==============+========+========+=========+
| No. | C | U | N | R | Name | Format | Length | Default |
+=====+===+===+===+===+==============+========+========+=========+
| TBD | C | - | - | - | Response-For | opaque | 0-1023 | (none) |
+-----+---+---+---+---+--------------+--------+--------+---------+
Table 1: The Response-For Option
The CoAP Token becomes meaningless for this form of response;
responses with embedded requests are therefore sent with a zero-
length Token. (In essence, the "Response-For" option takes the place
of the request the Token usually stands for.)
The congestion control considerations for confirmable and non-
confirmable messages apply unchanged.
5. Response for configured request
A request may reach the server using a different means than that used
for the response. For instance, the request may be configured in the
server. Without limiting generality, we speak about _configured
requests_.
The client MUST be cognizant of that configuration as the request
uses a token from the token name space it controls.
5.1. Examples for configured requests
5.1.1. Example: Periodic request
A server may be configured to act on a configured request every day
at 12:00.
5.1.2. Example: Event driven request
A server may be configured to act on a configured request each time
it reboots.
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5.1.3. Example: Configured observe
A server may be configured with a GET request from a client that
includes an Observe option with value 0. This means that the server
will send updates to the state of the resource addressed by the GET
request to the configured address of the client.
The considerations of Section 4.5 of [RFC7641] apply. How losing
interest reflects back into to configuration and whether there is
some form of error notification to the source of the configuration is
out of scope of the present specification.
5.2. Multicast responses
A server MAY send a response to a multicast address. (This needs to
be a response to a configured request as a normal request cannot be
sent _from_ a multicast address.)
Note that, as the originator of a multicast response is a unicast
address, the relaxation of matching rules described in Section 8.2 of
[RFC7252] does not apply.
The token space in CoAP is owned by the client, which is identified
by a transport endpoint (address/port). Here, the address is a
multicast address, so the token name space is shared by all nodes
joined to that multicast address. The assumption for multicast
responses is that, for each multicast group, there is some form of
management for the token space (and the port number) that everyone
can participate that needs to join that multicast group; the specific
form of management is out of the scope of this specification. Note
that this means that multicast responses MUST NOT be sent to
unmanaged multicast addresses such as All CoAP Nodes (Section 12.8 of
[RFC7252]).
Multicast responses are always non-confirmable. The congestion
control considerations for non-confirmable multicast messages apply
unchanged.
5.3. Respond-To option
What has been called "configured request" here may also be triggered
by a usual CoAP request that carries the Respond-To option. (The
term "configured request" is still appropriate as the server ought to
be configured to accept this option; see Section 7.)
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If a single client wants to request a server to send the response to
a specific multicast address, it can include the "Respond-To" option.
This contains an opaque string with the port number as a 16-bit
number (in network byte order), followed by the IP address (4-byte
IPv4 or 16-byte IPv6).
+=====+===+===+===+===+============+========+========+=========+
| No. | C | U | N | R | Name | Format | Length | Default |
+=====+===+===+===+===+============+========+========+=========+
| TBD | C | U | - | - | Respond-To | opaque | 6-18 | (none) |
+-----+---+---+---+---+------------+--------+--------+---------+
Table 2: The Respond-To Option
5.4. Leisure-For-Responses Option
This new option indicates a number expressed as a uint. It allows
the server to send that number of non-traditional response messages
in addition to the requested response. They are to be sent without
undue delay after the original response.
+=====+=+=+=+=+=======================+========+========+=========+
| No. |C|U|N|R| Name | Format | Length | Default |
+=====+=+=+=+=+=======================+========+========+=========+
| TBD | |U|-| | Leisure-For-Responses | uint | 1-4 | 0 |
+-----+-+-+-+-+-----------------------+--------+--------+---------+
Table 3: The Leisure-For-Responses Option
The option is elective, but unsafe for proxies (as the option would
otherwise cause multiple responses to a proxy that expects only one
and that needs to be a matching response). A proxy that chooses not
to implement it may forward the request with the Leisure-For-
Responses option removed.
On its own, the option does not indicate which kind of additional
responses the client would expect (though further elective proxy-safe
no-cache-key options can be added on top of that to give better
guidance), and the server may choose not to send any at all.
Intermediaries may add or remove the option, and use incoming
responses to populate their cache. They may serve additional
responses from their cache, but in most cases the sensible course of
action is to forward the additional responses the origin server
sends.
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Use cases for Leisure-For-Responses include sending further blocks in
a Block2 transfer (which are obviously non-matching and thus don't
need a Response-For), or serving follow-up documents (a response
containing a single link can be followed by a representation of the
linked resource, which needs a Request-For header that indicates the
URI).
6. IANA Considerations
This draft adds the following option numbers to the CoAP Option
Numbers registry of [RFC7252]:
+========+=======================+===========+
| Number | Name | Reference |
+========+=======================+===========+
| TBD | Response-For | RFCthis |
+--------+-----------------------+-----------+
| TBD | Respond-To | RFCthis |
+--------+-----------------------+-----------+
| TBD | Leisure-For-Responses | RFCthis |
+--------+-----------------------+-----------+
Table 4: CoAP Option Numbers
7. Security Considerations
TBD
(Clearly, multicast responses pose a potential for amplification, in
particular if unverified sources can cause them via Respond-To.
Discuss how to mitigate.)
A Respond-To option can be used to incite a server to send data to a
third party. This ought not be done blindly, i.e., only with
considered application assent.
The CoAP request/response mechanism allows the client to ascertain a
level of authentication (not resistant though to on-path attackers
unless the communication is protected) and freshness of the response:
The Token echoed in the response shows that the responder had
knowledge of the (fresh) request (Section 5.3.1 of [RFC7252]).
Responses with embedded requests can not be authenticated or checked
for freshness this way. Their content therefore is less trustworthy
than normal responses unless authenticated in another way (e.g., via
[RFC8613]).
8. References
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8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[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/rfc/rfc7252>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/rfc/rfc8613>.
8.2. Informative References
[I-D.ietf-core-groupcomm-bis]
Dijk, E., Wang, C., and M. Tiloca, "Group Communication
for the Constrained Application Protocol (CoAP)", Work in
Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
10, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
groupcomm-bis-10>.
[I-D.ietf-core-observe-multicast-notifications]
Tiloca, M., Höglund, R., Amsüss, C., and F. Palombini,
"Observe Notifications as CoAP Multicast Responses", Work
in Progress, Internet-Draft, draft-ietf-core-observe-
multicast-notifications-07, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
observe-multicast-notifications-07>.
[I-D.tiloca-core-groupcomm-proxy]
Tiloca, M. and E. Dijk, "Proxy Operations for CoAP Group
Communication", Work in Progress, Internet-Draft, draft-
tiloca-core-groupcomm-proxy-09, 31 August 2023,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
groupcomm-proxy-09>.
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[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/rfc/rfc7641>.
Appendix A. CoAP extensions explained by non-traditional responses
A.1. Observation
This section describes the Observe option [RFC7641] in the terms of
this document, [ so nothing in here should contradict that document
].
When Observe:0 is present in a request, this sets up non-traditional
responses until either of the following conditions is met:
* A follow-up request on the same token carries an Observe:1 option.
(This is primarily in here because; Observe:1 and No-Response:any
could be combined; otherwise, the other conditions suffice).
* Any response does not carry an Observe option.
* Any response has a non-successful status.
Follow-up requests are limited to extending the request ETag set.
Responses are obviously non-matching by their Observe option; each
hop discards the Observe option for the purpose of caching and
refreshes its cache with the most recent one as per the Observe
value.
A.2. Responses to multicast requests
As with observe, this just phrases the existing mechanism in the
context of this generalization.
When the destination address of a CoAP request is a multicast
address, that token is valid for any member of that group (which, for
the purpose of the client, is any server at all) on any port.
(Except for that the implications of having received a multicast
request still need to be followed, it might be seen as a template for
creating a phantom request to any endpoint, if that suits the
reader's mental model.)
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Responses can only be sent for up to the deployment's Leisure time
(see Section 8.2 of [RFC7252]) plus the application's timeout (in
proxy situations, this needs to be communicated explicitly in the
Multicast-Signaling option of [I-D.tiloca-core-groupcomm-proxy]).
A.3. Triangular responses (Response-To)
The Response-To option can be viewed as a shorthand notation for
"Consider this a No-Response:any request, but take a copy of it, make
it into a CoAP-over-UDP request with that particular address as a
source and any address of yours as a response, and treat that as a
phantom request".
[ It may make sense to add an explicit return token, and include a
No-Response option; that might allow it to be used even across
proxies. ]
A.4. Other current documents
[I-D.ietf-core-observe-multicast-notifications] is a straightforward
application of the phantom requests (the concept was developed
there); Leisure-For-Responses could help it around the topic of
joining a multicast group securely through a proxy.
[I-D.tiloca-core-groupcomm-proxy] seems to fit well with the concepts
here as well, and might be simplified by it both in terminology and
by replacing Response-Forwarding with Response-For(Proxy-Scheme, Uri-
Host).
Acknowledgements
TBD
Authors' Addresses
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Christian Amsüss
Email: christian@amsuess.com
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