Constrained Application Protocol (CoAP): Corrections and Clarifications
draft-ietf-core-corr-clar-00
The information below is for an old version of the document.
| Document | Type |
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|---|---|---|---|
| Author | Carsten Bormann | ||
| Last updated | 2024-10-14 | ||
| Replaces | draft-bormann-core-corr-clar | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
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draft-ietf-core-corr-clar-00
Network Working Group C. Bormann
Internet-Draft Universität Bremen TZI
Updates: 6690, 7252, 7641, 7959, 8132, 8323 (if 14 October 2024
approved)
Intended status: Standards Track
Expires: 17 April 2025
Constrained Application Protocol (CoAP): Corrections and Clarifications
draft-ietf-core-corr-clar-00
Abstract
RFC 7252 defines the Constrained Application Protocol (CoAP), along
with a number of additional specifications, including RFC 7641, RFC
7959, RFC 8132, and RFC 8323. RFC 6690 defines the link format that
is used in CoAP self-description documents.
Some parts of the specification may be unclear or even contain errors
that may lead to misinterpretations that may impair interoperability
between different implementations. The present document provides
corrections, additions, and clarifications to the RFCs cited; this
document thus updates these RFCs. In addition, other clarifications
related to the use of CoAP in other specifications, including RFC
7390 and RFC 8075, are also provided.
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-ietf-core-corr-clar/.
Discussion of this document takes place on the core Working Group
mailing list (mailto:core@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/core/. Subscribe at
https://www.ietf.org/mailman/listinfo/core/.
Source for this draft and an issue tracker can be found at
https://github.com/core-wg/corrclar.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 17 April 2025.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Process . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1. Original text . . . . . . . . . . . . . . . . . . . . 3
1.1.2. Proposed text based on IETF 117 and 2023-08-30 CoRE WG
interim discussion . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. RFC 7252 . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. RFC7252-1.2: Terminology (Request-URI) . . . . . . . . . 5
2.2. RFC7252-5.10.5: Max-Age . . . . . . . . . . . . . . . . . 6
2.3. RFC7252-6.4: Decomposing URIs into Options . . . . . . . 7
2.4. RFC7252-7.2.1: "ct" Attribute (content-format code) . . . 8
2.5. RFC7252-9.1.1/9.1.2: (match boxing) . . . . . . . . . . . 9
2.5.1. DTLS with Connection ID . . . . . . . . . . . . . . . 9
2.5.2. OSCORE, KUDOS, and Group OSCORE . . . . . . . . . . . 10
2.5.3. Eclipse/Californium . . . . . . . . . . . . . . . . . 14
2.6. RFC 7252-12.3: Content-Format Registry . . . . . . . . . 15
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
4. Security Considerations . . . . . . . . . . . . . . . . . . . 16
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.1. Normative References . . . . . . . . . . . . . . . . . . 16
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5.2. Informative References . . . . . . . . . . . . . . . . . 17
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
[RFC7252] defines the Constrained Application Protocol (CoAP), along
with a number of additional specifications, including [RFC7641],
[RFC7959], [RFC8132], and [RFC8323]. [RFC6690] defines the link
format that is used in CoAP self-description documents.
During implementation and interoperability testing of these RFCs, and
in their practical use, some ambiguities and common
misinterpretations have been identified, as well as a few errors.
The present document summarizes identified issues and provides
corrections needed for implementations of CoAP to interoperate, i.e.,
it constitutes an update to the RFCs referenced. This document also
provides other clarifications related to common misinterpretations of
the specification. References to CoAP should, therefore, also
include this document.
In addition, some clarifications and corrections are also provided
for documents that are related to CoAP, including RFC 7390 and RFC
8075.
1.1. Process
1.1.1. Original text
The present document is an Internet-Draft, which is not intended to
be published as an RFC quickly. Instead, it will be maintained as a
running document of the CoRE WG, probably for a number of years,
until the need for new entries tails off and the document can finally
be published as an RFC. (This paragraph to be rephrased when that
happens.)
The status of this document as a running document of the WG implies a
consensus process that is applied in making updates to it. The rest
of this subsection provides more details about this consensus
process. (This is the intended status; currently, the document is an
individual submission only.)
(Consensus process TBD, but it will likely be based on an editor's
version in a publicly accessible git repository, as well as periodic
calls for consensus that lead to a new published Internet-Draft.)
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1.1.2. Proposed text based on IETF 117 and 2023-08-30 CoRE WG interim
discussion
This section describes the process that will be used for developing
the present document (called "-corr-clar" colloquially).
This process might be revised as its execution progresses.
0. (Done as of this a draft): include the present process proposal.
The document can then already be considered for WG adoption.
1. Go through the following available material and revise/create
Github issues at ISSUES (https://github.com/core-wg/corrclar/
issues) as needed:
* Existing issues at ISSUES (https://github.com/core-
wg/corrclar/issues)
- More to be opened by Jon Shallow regarding Block-wise, see
JON-ISSUES (https://datatracker.ietf.org/doc/minutes-
interim-2023-core-11-202307051400/#attacks-on-the-
constrained-application-protocol-coap-christian-amsuss-jon-
shallow)
* CoAP FAQ at the CoRE WIKI WIKI-FAQ (https://github.com/core-
wg/wiki/wiki/CoAP-FAQ)
- Each point likely to become a new, short issue
2. Categorize the Github issues at ISSUES (https://github.com/core-
wg/corrclar/issues) as to the topics they relate to, by tagging
them.
Completing a first round of this will be a task for a dedicated
team.
3. For each issue or set of issues at ISSUES (https://github.com/
core-wg/corrclar/issues), confirm with the CoRE WG and gather
feedback from affected protocol designers/implementors if the
issue is best to be:
* Included and covered in -corr-clar, as is or revised
* Simply omitted in -corr-clar
* Omitted in -corr-clar and left for a possible -bis document.
(For example, this might be the case for some specific points
related to RFC 7959.)
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4. Reshape the -corr-clar document in order to reflect a sequence of
pairs (Diagnosis, Therapy), where:
* Diagnosis is the gist of a set of Github issues to cover, and
* Therapy is the correction or clarification to address those.
Even though at a high-level, the scope should be already clear by
looking at the table of contents. That is, at this stage, there
is no need to necessarily elaborate the Therapy in detail, but it
is necessary to make a reader understand "what we are dealing
with and taking which direction".
5. WG document work can then focus on improving the therapy parts,
until all points are satisfactorily addressed and documented.
1.2. 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.
When a section of this document makes formal corrections, additions
or invalidations to text in a referenced RFC, this is clearly
summarized. The text from the RFC that is being addressed is given
and labeled "INCOMPLETE", "INCORRECT", or "INCORRECT AND
INVALIDATED", followed by the correct text labeled "CORRECTED", where
applicable. When text is added that does not simply correct text in
previous specifications, it is given with the label "FORMAL
ADDITION".
Where a resolution has not yet been agreed, the resolution is marked
PENDING.
In this document, a reference to a section in RFC nnnn is written as
RFC nnnn-<number>, where <number> is the section number.
2. RFC 7252
2.1. RFC7252-1.2: Terminology (Request-URI)
[RFC7252] uses the term "request URI" repeatedly, but only provides a
vague definition in Section 5.7.2 of [RFC7252]. Text should be added
to the definitions in Section 1.2 (Terminology) of [RFC7252].
INCOMPLETE; FORMAL ADDITION (Section 1.2):
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Request URI: The URI that identifies a resource on a server
intended to be addressed by a request; constructed from the
context of the request combined with Options present in the
request using the process defined in Section 6.5 (Composing
URIs from Options) of [RFC7252], see Section 5.7.2
(Forward-Proxies) of [RFC7252] for further details. Related to
the HTTP concept of "target URI"; see Section 7.1 (Determining
the Target Resource) of [RFC9110]; previously called "effective
request URI" in Section 5.5 (Effective Request URI) of
[RFC7230].
PENDING.
Note that a similar, but distinct concept is the "base URI", relative
to which relative URIs are resolved. This is more complex in CoAP
than in HTTP because CoAP can multicast requests (Section 8 of
[RFC7252]), expecting unicast responses; these need to be interpreted
relative to the unicast source address from which the responses come.
Section 8.2 of [RFC7252] has:
| For the purposes of interpreting the Location-* options and any
| links embedded in the representation, the request URI (i.e., the
| base URI relative to which the response is interpreted) is formed
| by replacing the multicast address in the Host component of the
| original request URI by the literal IP address of the endpoint
| actually responding.
Similarly, Section 8.2.1 of [RFC7252] (Caching) says:
| A response received in reply to a GET request to a multicast group
| MAY be used to satisfy a subsequent request on the related unicast
| request URI. The unicast request URI is obtained by replacing the
| authority part of the request URI with the transport-layer source
| address of the response message.
Further discussion of a more generalized response concept can be
found in [I-D.bormann-core-responses].
2.2. RFC7252-5.10.5: Max-Age
In the discussion of [RFC8516], a comment was made that it would be
needed to define the point in time relative to which Max-Age is
defined. A sender might reference it to the time it actually sends
the message containing the option (and paragraph 3 of RFC7252-5.10.5
indeed requests that Max-Age be updated each time a message is
retransmitted). The receiver of the message does not have reliable
information about the time of sending, though. It may instead
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reference the Max-Age to the time of reception. This in effect
extends the time of Max-Age by the latency of the packet. This
extension was deemed acceptable for the purposes of [RFC8516], but
may be suboptimal when Max-Age is about the lifetime of a response
object.
INCOMPLETE:
The value is intended to be current at the time of transmission.
PENDING.
2.3. RFC7252-6.4: Decomposing URIs into Options
[Err4895] notes (text updated with newer link):
| The current specification for decomposing a URI into CoAP Options
| (Section 6.4) is correct; however the text may still be unclear to
| implementers who may think that the phrase "not including the
| delimiting slash characters" means simply omitting a trailing
| slash character in the URI path. This is incorrect. See the
| discussion outcome in email thread
| https://mailarchive.ietf.org/arch/msg/core/
| vqOiUreodGXqWZGeGOTREChCsKY/
| (https://mailarchive.ietf.org/arch/msg/core/
| vqOiUreodGXqWZGeGOTREChCsKY/).
[Err4895] proceeds to propose adding another note at the end of
Section 6.4 of [RFC7252]. A slightly updated version of the proposed
note might be:
FORMAL ADDITION at the end of Section 6.4 of [RFC7252]:
Also note that a trailing slash character in the <path> component
represents a separate, zero-character path segment (see the ABNF
in Section 3.3 of [RFC3986]). This is encoded using a Uri-Path
Option of zero length. The exception at the start of step 8 means
that no such zero-character path segment is added for a trailing
slash that immediately follows the authority (host and port).
| This exception means that, for a CoAP server, no difference
| is visible between a request that was generated from the URI
| coap://authority/ and one that was generated from the URI
| coap://authority — in both cases, there is no Uri-Path
| Option in the request. (The URI continues to be parsed as
| defined: e.g., for the URIs coap://authority/? and
| coap://authority?, there is no Uri-Path Option but a single
| Uri-Query Option that carries an empty query component.)
|
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| Note that the exception at the start of step 8 does not mean
| that a client cannot create a request with a single empty
| Uri-Path Option; such a request just cannot be generated
| from a URI because of the algorithm given here. It also
| does not mean that a server is compelled to treat a request
| with such a single empty Uri-Path Option in the same way as
| one without any Uri-Path Option — the exception at the start
| of step 8 is only about the process of generating a sequence
| of CoAP Options from a URI.
|
| The exception only applies to initial Uri-Path Options. So
| for coap://authority/foo, a single Uri-Path Option with the
| value foo is generated, while for coap://authority/foo/ that
| Uri-Path Option is followed by an empty Uri-Path Option (an
| established idiom for a collection resource).
|
| Similarly, there is a difference between requests generated
| from coap://authority/, coap://authority//, and
| coap://authority///, respectively: The first has no Uri-Path
| Options (due to the special exception); the second, two
| (empty ones); the third, three (empty ones). No server is
| compelled to offer resources under URIs with multiple empty
| path name components, which would generally be considered
| weird.
2.4. RFC7252-7.2.1: "ct" Attribute (content-format code)
As has been noted in [Err5078], there is no information in [RFC7252]
about whether the "ct" target attribute can be present multiply or
not.
The text does indicate that the value of the attribute MAY be "a
space-separated sequence of Content-Format codes, indicating that
multiple content-formats are available", but it does not repeat the
prohibition of multiple instances that the analogously structured
"rt" and "if" attributes in Sections 3.1 and 3.2 of [RFC6690] have.
This appears to be an oversight. Published examples in Section 4.1
of [RFC9148] and Section 4.3 of [RFC9176] further illustrate that the
space-separated approach is generally accepted to be the one to be
used. There is no gain to be had from allowing both variants, and it
would be likely to cause interoperability problems.
At the 2022-11-23 CoRE WG interim meeting, there was agreement that
[Err5078] should be marked "VERIFIED", which was done on 2023-01-18.
INCOMPLETE; FORMAL ADDITION:
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The Content-Format code attribute MUST NOT appear more than once
in a link.
2.5. RFC7252-9.1.1/9.1.2: (match boxing)
Sections 9.1.1 and 9.1.2 of [RFC7252] provide details about using
CoAP over DTLS connections; in particular they restrict both message-
id matching and request/response matching to within a single
combination of DTLS session/DTLS epoch.
At the time, this was a decision to be very conservative based on the
wide variety of security implications a new DTLS epoch might have
(which also were not widely understood, e.g., for a re-
authentication). The normally short time between a request and a
response made this rather strict boxing appear acceptable.
However, with the Observe functionality [RFC7641], it is quite likely
that significant time elapses between a request and the arrival of a
notification that is sent back as a response, causing a need for the
latter to use a different box from the original request.
Also, additions to CoAP such as CoAP over connection-oriented
transports [RFC8323] and OSCORE [RFC8613] create similar matching
boxes that also do not fit certain likely use cases of these
additions (e.g., with short-lived TCP connections as discussed in
Section 4.3 of [RFC9006]).
The match boxing semantics of the current protocol are clearly
defined, but can be unsatisfactory given the current requirements.
This calls for careful design choices and enhancements when
developing extensions for CoAP or protocols and methods applicable to
CoAP, such as in the cases overviewed in the following Section 2.5.2
and Section 2.5.3.
2.5.1. DTLS with Connection ID
PENDING:
| Protocol mechanisms that have been defined for stitching
| together connections or phases of an underlying connection,
| such as Connection Identifiers for DTLS 1.2 [RFC9146], may
| enable keeping the session/epoch unchanged and even to change
| the transport address (ip-address/port), once appropriately
| modified match boxing rules are specified for the stitching
| mechanism. (These rules either need to be defined to be
| implicitly active for any use of the mechanism or they may
| require negotiation.)
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2.5.2. OSCORE, KUDOS, and Group OSCORE
The security protocol Object Security for Constrained RESTful
Environments (OSCORE) defined in [RFC8613] provides end-to-end
security for CoAP messages at the application level, by using CBOR
Object Signing and Encryption (COSE) [RFC9052]. In order to protect
their communications, two peers need to have already established an
OSCORE Security Context.
Appendix B.2 of [RFC8613] provides an example for a key update
procedure, which two OSCORE peers can run for establishing a new
shared OSCORE Security Context that replaces their old one. The
recent key update protocol KUDOS [I-D.ietf-core-oscore-key-update]
specifies how two OSCORE peers can establish a new shared OSCORE
Security Context that replaces their old one, with a number of
advantages compared to the method defined in Appendix B.2 of
[RFC8613].
The security protocol Group Object Security for Constrained RESTful
Environments (Group OSCORE) defined in
[I-D.ietf-core-oscore-groupcomm] builds on OSCORE and protects group
communication for CoAP [I-D.ietf-core-groupcomm-bis]. The management
of the group keying material is entrusted to a Group Manager (see
Section 3 of [I-D.ietf-core-oscore-groupcomm]), which provides the
latest group keying material to a new group member upon its group
joining, and can update the group keying material by performing a
group rekeying.
The following discusses how OSCORE, KUDOS, and Group OSCORE position
themselves with respect to the match boxing, the transport used
underlying CoAP, and the renewal of the keying material.
2.5.2.1. Match Boxing
The security processing of (Group) OSCORE is agnostic of the value
assumed by the CoAP Token and Message ID. Also, (Group) OSCORE can
be seamlessly used in the presence of (cross-)proxies that will
change the value of the CoAP Token and Message ID on the different
communication legs. This does not affect the security processing at
the (Group) OSCORE endpoints.
Before any security processing is performed, the only use that
(Group) OSCORE makes of the Token value is on the CoAP client upon
receiving a response, in order to retrieve the right Security Context
to use for decrypting and verifying the response.
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Even in case the Token value in a CoAP response is manipulated to
induce a Request-Response matching at the client, there is no risk
for the client to successfully decrypt the response instead of
failing as expected. This is because, per Section 12.3 of [RFC8613],
the OSCORE Master Secret of each OSCORE Security Context is required
not only to be secret, but also to have a good amount of randomness.
Building on that, an HKDF is used to derive the actual encryption
keys from the Master Secret and, optionally, from an additional
Master Salt. Furthermore, for each OSCORE Security Context, the
quartet (Master Secret, Master Salt, ID Context, Sender ID) must be
unique. As per Section 3.3 of [RFC8613], this is a hard requirement
and guarantees unique (key, nonce) pairs for the AEAD algorithm used.
In Group OSCORE, the Security Context extends that of OSCORE, and the
same as above holds (see Sections 2, 2.2, and 13.11 of
[I-D.ietf-core-oscore-groupcomm]).
Finally, (Group) OSCORE performs a separate secure match boxing under
its own control, by cryptographically binding each protected request
with all the corresponding protected responses. This is achieved by
means of the COSE external_aad involved during the security
processing of protected messages (see Section 5.4 of [RFC8613] and
Section 4.4 of [I-D.ietf-core-oscore-groupcomm]).
2.5.2.2. Underlying Transport
The security protocol (Group) OSCORE does not have any requirement on
binding the Security Context in use to specific addressing
information used by the transport protocol underlying CoAP. What
occurs below (Group) OSCORE with transport-specific addressing
information is transparent to (Group) OSCORE, but it needs to work
well enough to ensure that received data is delivered to (Group)
OSCORE for security processing.
Consistent with the above, (Group) OSCORE does not interfere with any
low-layer, transport specific information. Instead, it entrusts data
to a Request-Response exchange mechanism that can rely on different
means to enforce the Request-Response matching at the transport level
(e.g., the 5-tuple, the CoAP Message ID, a file handle). Also,
(Group) OSCORE does not alter the fact that a CoAP response needs to
come from where the corresponding CoAP request was sent, which simply
follows from using transports where that is a requirement.
Furthermore, two peers can seamlessly use (Group) OSCORE also in the
presence of cross-proxies that change transport across different
communication legs. This does not affect the security processing at
the (Group) OSCORE endpoints.
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Practically, (Group) OSCORE relies on the underlying CoAP
implementation for obtaining received CoAP messages on which to
perform the expected security processing.
Upon receiving a protected message, the recipient endpoint retrieves
the OSCORE Security Context to use for decryption based on key
identifier information specified in the CoAP OSCORE Option of
protected requests, and on the value of the CoAP Token of protected
responses.
In OSCORE, the key identifier information in request messages is
typically limited to a "kid", with a value the OSCORE Sender ID
associated with the message sender. In case Sender IDs are not
unique among different OSCORE Security Contexts stored by the same
peer, it is possible to disambiguate by additionally using a "kid
context" identifying the OSCORE Security Context as a whole.
Instead, response messages are not required to convey key identifier
information, as the client can rely on the Token conveyed in the
response for retrieving the Security Context to use (see above).
In Group OSCORE, the key identifier information in request messages
always includes also a "kid context", whose value is used as
identifier of the OSCORE group associated with the Security Context
to use for security processing of the exchanged message. Response
messages are also required to convey a "kid" as key identifier
information (i.e., the OSCORE Sender ID associated with the message
sender), if the corresponding request was protected with the group
mode of Group OSCORE (see Section 8 of
[I-D.ietf-core-oscore-groupcomm]) .
Some particular uses of (Group) OSCORE enable to build OSCORE-based
tunneling [I-D.ietf-core-oscore-capable-proxies]. In such a case, a
CoAP server might have to enforce that some OSCORE Security Contexts
are not just looked up by a "kid" and similar key identifier
information from the CoAP OSCORE Option in the incoming request to
decrypt. Such a lookup should also rely on the alleged client's
address, or on an alternative identifier of the tunnel from which the
request came from.
2.5.2.3. Key Update
Updating an OSCORE Security Context does not change or interfere with
the values of the Token or Message ID used in the exchanged CoAP
messages. However, if long-term exchanges are involved (e.g., CoAP
Observations [RFC7641]), one has to be careful to ensure that
updating the Security Context does not impair the security properties
of (Group) OSCORE or result in other security vulnerabilities.
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The following provides more details about key update, separately for
OSCORE, KUDOS, and Group OSCORE.
* OSCORE: [RFC8613] tacitly assumes that two peers terminate any
ongoing CoAP Observation that they still have ongoing, upon
installing a new OSCORE Security Context, irrespective of the
method used to perform the key update.
On these premises, a belated response protected with the old
OSCORE Security Context will fail decryption, as that Security
Context is not available anymore on the receiving client.
* KUDOS: The key update protocol KUDOS allows the two OSCORE peers
to negotiate about preserving their ongoing CoAP Observations
across the performed key update. If and only if both peers agree
to do that during an execution of KUDOS, their Observations will
remain active after installing the new OSCORE Security Context,
which the two peers will use from then on to protect their
exchanged Observe notifications.
Furthermore, irrespective of the method used to perform a key
update, Section 3 of [I-D.ietf-core-oscore-key-update] updates the
security protocol OSCORE [RFC8613] in order to prevent security
issues that can arise from misbinding a request and a response,
when those are protected with two different OSCORE Security
Contexts.
Such an update to the OSCORE protocol requires a server to include
its own Sender Sequence Number as Partial IV of an outgoing
response, when protecting it with a Security Context different
from the one used to protect the corresponding request. An
exception safely applies to the response messages that are sent
when running the key update procedure defined in Appendix B.2 of
[RFC8613].
* Group OSCORE: The Group Manager can distribute new group keying
material to the members of an OSCORE group, by performing a group
rekeying. When receiving updated group keying material from the
Group Manager, either upon joining the group or by participating
in a group rekeying, a group member uses that material to install
a new, commonly shared Group OSCORE Security Context, which
replaces the old one (if any is stored).
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Also, Group OSCORE makes it possible for group members to safely
preserve their ongoing active requests (e.g., CoAP Observations),
also across the establishment of new Group OSCORE Security
Contexts. This is achieved by virtue of how the Group Manager
assigns and maintains the identifiers of OSCORE groups (see
Section 3.2.1.1 of [I-D.ietf-core-oscore-groupcomm]).
Furthermore, analogous to the update that
[I-D.ietf-core-oscore-key-update] makes on the OSCORE protocol
with respect to protecting responses, Group OSCORE prevents
security issues that can arise from misbinding a request and a
response, when those are protected with two different Group OSCORE
Security Contexts.
In the same way specified in Section 3 of
[I-D.ietf-core-oscore-key-update] for OSCORE, Group OSCORE
requires a server to include its own Sender Sequence Number as
Partial IV of an outgoing response, when protecting it with a
Security Context different from the one used to protect the
corresponding request (see Sections 8.3 and 9.5 of
[I-D.ietf-core-oscore-groupcomm]).
2.5.3. Eclipse/Californium
Enhancements may be called for optimizations such as Eclipse/
Californium EndpointContextMatcher [CF-MATCHER] might not work
properly unless both sides of the communication agree on the extent
of the matching boxes. Mechanisms to indicate capabilities and
choices selected may need to be defined; also, a way to define the
progression of matching boxes needs to be defined that is compatible
with the security properties of the underlying protocols. This may
require new efforts, to be initiated based on some formative
contributions.
PENDING.
Relevant starting points for retrieving existing discussion of this
issue include:
* https://mailarchive.ietf.org/arch/msg/core/
biDJ8n4w0kBQATzyh9xHlKnGy1o/
(https://mailarchive.ietf.org/arch/msg/core/
biDJ8n4w0kBQATzyh9xHlKnGy1o/)
("DTLS and Epochs")
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* https://mailarchive.ietf.org/arch/msg/core/
TsyyKIHQ1FJtAvAo0ODNEt2Ileo/
(https://mailarchive.ietf.org/arch/msg/core/
TsyyKIHQ1FJtAvAo0ODNEt2Ileo/)
("Connection ID")
* https://github.com/core-wg/corrclar/issues/9 (https://github.com/
core-wg/corrclar/issues/9)
("Clarify/revise language around epochs in section 9.1.1 #9")
2.6. RFC 7252-12.3: Content-Format Registry
Section 12.3 of [RFC7252] established the CoAP Content-Formats
Registry, which maps a combination of an Internet Media Type with an
HTTP Content Coding, collectively called a Content-Format, to a
concise numeric identifier for that Content-Format. The "Media Type"
is more than a Media-Type-Name (see [RFC9193] for an extensive
discussion), i.e., it may contain parameters beyond the mere
combination of a type-name and a subtype-name registered in
[IANA.media-types], as per [RFC6838], conventionally identified by
the two names separated by a slash. This construct is often called a
Content Type to reduce the confusion with a Media-Type-Name (e.g., in
Section 8.3 of [RFC9110], which then however also opts to use the
term Media Type for the same information set).
The second column of the Content-Format registry is the Content
Coding, which is defined in Section 8.4.1 of [RFC9110]. For
historical reasons, the HTTP header field that actually carries the
content coding is called Content-Encoding; this often leads to the
misnaming of Content Coding as "content encoding".
As has been noted in [Err4954], the text in Section 12.3 of [RFC7252]
incorrectly uses these terms in the context of content types and
content coding:
1. The field that describes the Content Type is called "Media Type".
This can lead to the misunderstanding that this column just
carries a Media-Type-Name (such as "text/plain"), while it
actually also can carry media type parameters (as in "text/plain;
charset=UTF-8").
2. The field that describes the Content Coding uses the incorrect
name "Content Encoding".
INCORRECT, CORRECTED:
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The VERIFIED Errata Report [Err4954] corrects the usage of
"Content-Encoding" in the text and changes the name of the first
column of the Content-Format registry to "Content Type" and the
name of the second field to "Content Coding". This change has
been carried out by IANA.
[Err4954] also has some notes on what would be valid or invalid
Content-Format registrations. These are not repeated here; they are
however quite useful as a reference when preparing additional
Content-Format registrations.
3. IANA Considerations
This document makes no new requests to IANA.
Individual clarifications may contain IANA considerations; as for
example in Section 2.6.
4. Security Considerations
This document provides a number of corrections and clarifications to
existing RFCs, but it does not make any changes with regard to the
security aspects of the protocol. As a consequence, the security
considerations of the referenced RFCs apply without additions.
(To be changed when that is no longer true; probably the security
considerations will then be on the individual clarifications.)
5. References
5.1. 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>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/rfc/rfc6690>.
[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>.
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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>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/rfc/rfc7959>.
[RFC8132] van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
FETCH Methods for the Constrained Application Protocol
(CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
<https://www.rfc-editor.org/rfc/rfc8132>.
[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>.
[RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
RFC 8323, DOI 10.17487/RFC8323, February 2018,
<https://www.rfc-editor.org/rfc/rfc8323>.
[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>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/rfc/rfc9052>.
5.2. Informative References
[CF-MATCHER]
"EndpointContextMatcher.java", <https://github.com/
eclipse-californium/californium/blob/main/element-
connector/src/main/java/org/eclipse/californium/elements/
EndpointContextMatcher.java>.
[Err4895] RFC Errata Report 4895, RFC 7252,
<https://www.rfc-editor.org/errata/eid4895>.
[Err4954] RFC Errata Report 4954, RFC 7252,
<https://www.rfc-editor.org/errata/eid4954>.
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[Err5078] RFC Errata Report 5078, RFC 7252,
<https://www.rfc-editor.org/errata/eid5078>.
[I-D.bormann-core-responses]
Bormann, C. and C. Amsüss, "CoAP: Non-traditional response
forms", Work in Progress, Internet-Draft, draft-bormann-
core-responses-03, 1 September 2024,
<https://datatracker.ietf.org/doc/html/draft-bormann-core-
responses-03>.
[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-
11, 24 April 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-groupcomm-bis-11>.
[I-D.ietf-core-oscore-capable-proxies]
Tiloca, M. and R. Höglund, "OSCORE-capable Proxies", Work
in Progress, Internet-Draft, draft-ietf-core-oscore-
capable-proxies-02, 8 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
oscore-capable-proxies-02>.
[I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
and R. Höglund, "Group Object Security for Constrained
RESTful Environments (Group OSCORE)", Work in Progress,
Internet-Draft, draft-ietf-core-oscore-groupcomm-23, 26
September 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-oscore-groupcomm-23>.
[I-D.ietf-core-oscore-key-update]
Höglund, R. and M. Tiloca, "Key Update for OSCORE
(KUDOS)", Work in Progress, Internet-Draft, draft-ietf-
core-oscore-key-update-08, 8 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
oscore-key-update-08>.
[IANA.media-types]
IANA, "Media Types",
<https://www.iana.org/assignments/media-types>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
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[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/rfc/rfc6838>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/rfc/rfc7230>.
[RFC8516] Keranen, A., ""Too Many Requests" Response Code for the
Constrained Application Protocol", RFC 8516,
DOI 10.17487/RFC8516, January 2019,
<https://www.rfc-editor.org/rfc/rfc8516>.
[RFC9006] Gomez, C., Crowcroft, J., and M. Scharf, "TCP Usage
Guidance in the Internet of Things (IoT)", RFC 9006,
DOI 10.17487/RFC9006, March 2021,
<https://www.rfc-editor.org/rfc/rfc9006>.
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/rfc/rfc9110>.
[RFC9146] Rescorla, E., Ed., Tschofenig, H., Ed., Fossati, T., and
A. Kraus, "Connection Identifier for DTLS 1.2", RFC 9146,
DOI 10.17487/RFC9146, March 2022,
<https://www.rfc-editor.org/rfc/rfc9146>.
[RFC9148] van der Stok, P., Kampanakis, P., Richardson, M., and S.
Raza, "EST-coaps: Enrollment over Secure Transport with
the Secure Constrained Application Protocol", RFC 9148,
DOI 10.17487/RFC9148, April 2022,
<https://www.rfc-editor.org/rfc/rfc9148>.
[RFC9176] Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
P. van der Stok, "Constrained RESTful Environments (CoRE)
Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
2022, <https://www.rfc-editor.org/rfc/rfc9176>.
[RFC9193] Keränen, A. and C. Bormann, "Sensor Measurement Lists
(SenML) Fields for Indicating Data Value Content-Format",
RFC 9193, DOI 10.17487/RFC9193, June 2022,
<https://www.rfc-editor.org/rfc/rfc9193>.
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Acknowledgements
The present document is modeled after RFC 4815 and the Internet-
Drafts of the ROHC WG that led to it. Many thanks to the co-chairs
of the ROHC WG and WG members that made this a worthwhile and
successful experiment at the time.
Author's Address
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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