Proxy Operations for CoAP Group Communication
draft-ietf-core-groupcomm-proxy-01
| Document | Type | Active Internet-Draft (core WG) | |
|---|---|---|---|
| Authors | Marco Tiloca , Esko Dijk | ||
| Last updated | 2024-03-04 | ||
| Replaces | draft-tiloca-core-groupcomm-proxy | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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draft-ietf-core-groupcomm-proxy-01
CoRE Working Group M. Tiloca
Internet-Draft RISE AB
Updates: 7252 (if approved) E. Dijk
Intended status: Standards Track IoTconsultancy.nl
Expires: 5 September 2024 4 March 2024
Proxy Operations for CoAP Group Communication
draft-ietf-core-groupcomm-proxy-01
Abstract
This document specifies the operations performed by a proxy, when
using the Constrained Application Protocol (CoAP) in group
communication scenarios. Such a proxy processes a single request
sent by a client over unicast, and distributes the request to a group
of servers, e.g., over UDP/IP multicast as the defined default
transport protocol. Then, the proxy collects the individual
responses from those servers and relays those responses back to the
client, in a way that allows the client to distinguish the responses
and their origin servers through embedded addressing information.
This document updates RFC7252 with respect to caching of response
messages at proxies.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Constrained RESTful
Environments Working Group mailing list (core@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/core/.
Source for this draft and an issue tracker can be found at
https://github.com/core-wg/groupcomm-proxy.
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
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/.
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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 5 September 2024.
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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
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. The Multicast-Timeout Option . . . . . . . . . . . . . . . . 6
3. The Reply-To Option . . . . . . . . . . . . . . . . . . . . . 7
4. Requirements and Objectives . . . . . . . . . . . . . . . . . 8
5. Protocol Description . . . . . . . . . . . . . . . . . . . . 9
5.1. Request Sending at the Client . . . . . . . . . . . . . . 9
5.1.1. Request Sending . . . . . . . . . . . . . . . . . . . 9
5.1.2. Supporting Observe . . . . . . . . . . . . . . . . . 11
5.2. Request Processing at the Proxy . . . . . . . . . . . . . 11
5.2.1. Request Processing . . . . . . . . . . . . . . . . . 11
5.2.2. Supporting Observe . . . . . . . . . . . . . . . . . 12
5.3. Request and Response Processing at the Server . . . . . . 12
5.3.1. Request and Response Processing . . . . . . . . . . . 12
5.3.2. Supporting Observe . . . . . . . . . . . . . . . . . 13
5.4. Response Processing at the Proxy . . . . . . . . . . . . 13
5.4.1. Response Processing . . . . . . . . . . . . . . . . . 13
5.4.2. Supporting Observe . . . . . . . . . . . . . . . . . 14
5.5. Response Processing at the Client . . . . . . . . . . . . 14
5.5.1. Response Processing . . . . . . . . . . . . . . . . . 15
5.5.2. Supporting Observe . . . . . . . . . . . . . . . . . 16
5.6. Example . . . . . . . . . . . . . . . . . . . . . . . . . 16
6. Reverse-Proxies . . . . . . . . . . . . . . . . . . . . . . . 18
6.1. Processing on the Proxy Side . . . . . . . . . . . . . . 18
6.2. Processing on the Client Side . . . . . . . . . . . . . . 20
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7. Caching . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1. Freshness Model . . . . . . . . . . . . . . . . . . . . . 22
7.2. Validation Model . . . . . . . . . . . . . . . . . . . . 23
7.2.1. Proxy-Servers Revalidation with Unicast Requests . . 23
7.2.2. Proxy-Servers Revalidation with Group Requests . . . 25
7.3. Client-Proxy Revalidation with Group Requests . . . . . . 26
7.4. Caching of End-To-End Protected Responses at Proxies . . 27
7.4.1. Deterministic Requests to Achieve Cacheability . . . 28
7.4.2. Validation of Responses . . . . . . . . . . . . . . . 28
8. Chain of Proxies . . . . . . . . . . . . . . . . . . . . . . 29
8.1. Request Processing at the Proxy . . . . . . . . . . . . . 30
8.1.1. Supporting Observe . . . . . . . . . . . . . . . . . 32
8.2. Response Processing at the Proxy . . . . . . . . . . . . 32
8.2.1. Supporting Observe . . . . . . . . . . . . . . . . . 33
9. HTTP-CoAP Proxies . . . . . . . . . . . . . . . . . . . . . . 34
9.1. The HTTP Multicast-Timeout Header Field . . . . . . . . . 34
9.2. The HTTP Reply-To Header Field . . . . . . . . . . . . . 35
9.3. The HTTP Group-ETag Header Field . . . . . . . . . . . . 35
9.4. Request Sending at the Client . . . . . . . . . . . . . . 36
9.5. Request Processing at the Proxy . . . . . . . . . . . . . 37
9.6. Response Processing at the Proxy . . . . . . . . . . . . 37
9.7. Response Processing at the Client . . . . . . . . . . . . 38
9.8. Example . . . . . . . . . . . . . . . . . . . . . . . . . 39
9.9. Streamed Delivery of Responses to the Client . . . . . . 40
9.10. Reverse-Proxies . . . . . . . . . . . . . . . . . . . . . 40
9.10.1. Processing on the Client Side . . . . . . . . . . . 41
9.10.2. Processing on the Proxy Side . . . . . . . . . . . . 41
10. Security Considerations . . . . . . . . . . . . . . . . . . . 41
10.1. Client Authentication . . . . . . . . . . . . . . . . . 41
10.2. Multicast-Timeout Option . . . . . . . . . . . . . . . . 42
10.3. Reply-To Option . . . . . . . . . . . . . . . . . . . . 43
10.4. Group-ETag Option . . . . . . . . . . . . . . . . . . . 44
10.5. HTTP-to-CoAP Proxies . . . . . . . . . . . . . . . . . . 45
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 45
11.1. CoAP Option Numbers Registry . . . . . . . . . . . . . . 45
11.2. Hypertext Transfer Protocol (HTTP) Field Name
Registry . . . . . . . . . . . . . . . . . . . . . . . . 46
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 46
12.1. Normative References . . . . . . . . . . . . . . . . . . 46
12.2. Informative References . . . . . . . . . . . . . . . . . 48
Appendix A. Examples with Reverse-Proxy . . . . . . . . . . . . 49
A.1. Example 1 . . . . . . . . . . . . . . . . . . . . . . . . 50
A.2. Example 2 . . . . . . . . . . . . . . . . . . . . . . . . 53
A.3. Example 3 . . . . . . . . . . . . . . . . . . . . . . . . 55
Appendix B. Document Updates . . . . . . . . . . . . . . . . . . 57
B.1. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 57
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 58
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 58
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1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] allows the
presence of proxies, as intermediary entities supporting clients by
performing requests on their behalf and relaying back responses.
CoAP supports also group communication over IP multicast
[I-D.ietf-core-groupcomm-bis], where a group request can be addressed
to multiple recipient servers, each of which may reply with an
individual unicast response. As discussed in Section 3.5 of
[I-D.ietf-core-groupcomm-bis], this group communication scenario
poses a number of issues and limitations to proxy operations.
In particular, the client sends to the proxy a single unicast
request, which the proxy forwards to a group of CoAP servers, e.g.,
using UDP/IP multicast as the defined default transport protocol for
CoAP group requests (see Section 1.1 of
[I-D.ietf-core-groupcomm-bis]). Later on, the proxy replies to the
client's original unicast request, by relaying back the responses
from the servers.
As per [RFC7252], a CoAP-to-CoAP proxy relays those responses to the
client as separate CoAP messages, all matching (by Token) with the
client's original unicast request. A possible alternative approach
for aggregating those responses into a single CoAP response sent to
the client would require a specific aggregation content-format, which
is not available yet. Both these approaches have open issues.
This document considers the former approach. That is, after
forwarding a CoAP group request from the client to the group of CoAP
servers, the proxy relays the individual responses back to the client
as separate CoAP messages. The described method addresses all the
related issues raised in Section 3.5 of
[I-D.ietf-core-groupcomm-bis]. To this end, this document defines a
dedicated signaling protocol based on two new CoAP options and used
by the client and the proxy.
Using this protocol, the client explicitly confirms its intent to
perform a proxied group request and its support for receiving
multiple responses as a result, i.e., one or more from each origin
server. Also, the client signals for how long it is willing to wait
for responses. When relaying to the client a response to the group
request, the proxy indicates the addressing information of the origin
server. This enables the client to distinguish multiple different
responses by origin and to possibly contact one or more of the
respective servers by sending individual unicast request(s) to the
indicated address(es). In doing these follow-up unicast requests,
the client may optionally bypass the proxy.
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Like [I-D.ietf-core-groupcomm-bis], this document refers to UDP/IP
multicast as the transport protocol that a proxy uses to forward a
CoAP group request to a group of servers. While other transport
protocols such as broadcast, non-IP multicast, and geocast can also
be possible to employ, their use is not considered in this document.
This document also defines how the proposed protocol is used between
an HTTP client and an HTTP-CoAP cross-proxy, in order to forward an
HTTP group request from the client to a group of CoAP servers, and
relay back the individual CoAP responses as HTTP responses.
Finally, this document defines a caching model for proxies and
specifies how they can serve a group request by using cached
responses. Therefore, this document updates [RFC7252].
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.
Readers are expected to be familiar with the terms and concepts from
the following specifications:
* CoAP [RFC7252] and Group Communication for CoAP
[I-D.ietf-core-groupcomm-bis].
* OSCORE [RFC8613] and Group OSCORE
[I-D.ietf-core-oscore-groupcomm].
* CDDL [RFC8610], CBOR [RFC8949], and CBOR sequences [RFC8742]
* Constrained Resource Identifiers (CRIs) [I-D.ietf-core-href].
Unless specified otherwise, the term "proxy" refers to a CoAP-to-CoAP
forward-proxy, as defined in Section 5.7.2 of [RFC7252].
This document also uses the following terminology.
* Individual request: a request that an origin client sends to a
single origin server within a group, either directly, or
indirectly via a proxy.
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2. The Multicast-Timeout Option
The Multicast-Timeout Option defined in this section has the
properties summarized in Figure 1, which extends Table 4 of
[RFC7252].
Since the option is not Safe-to-Forward, the column "N" indicates a
dash for "not applicable". The value of the Multicast-Timeout Option
specifies a timeout value in seconds, encoded as an unsigned integer
(see Section 3.2 of [RFC7252]).
+------+---+---+---+---+------------+--------+--------+---------+
| No. | C | U | N | R | Name | Format | Length | Default |
+------+---+---+---+---+------------+--------+--------+---------+
| | | | | | | | | |
| TBD1 | | x | - | | Multicast- | uint | 0-4 | (none) |
| | | | | | Timeout | | | |
| | | | | | | | | |
+------+---+---+---+---+------------+--------+--------+---------+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
Figure 1: The Multicast-Timeout Option.
This document specifically defines how this option is used by a
client in a CoAP request, to indicate to a proxy its support for and
interest in receiving multiple responses to a proxied CoAP group
request, i.e., one or more from each origin server, and for how long
it is willing to wait for receiving responses via that proxy (see
Section 5.1.1 and Section 5.2.1).
When sending a CoAP group request to a proxy via IP unicast, to be
forwarded by the proxy to a targeted group of servers, the client
includes the Multicast-Timeout Option into the request. The option
value indicates after how much time in seconds the client will stop
accepting responses matching its original unicast request, with the
exception of notifications if the CoAP Observe Option [RFC7641] is
used in the same request. This allows the proxy to stop relaying
responses back to the client, if those are received from servers
after the indicated amount of time has elapsed.
The Multicast-Timeout Option is of class U in terms of OSCORE
processing (see Section 4.1 of [RFC8613]).
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3. The Reply-To Option
The Reply-To Option defined in this section has the properties
summarized in Figure 2, which extends Table 4 of [RFC7252]. The
option is intended only for inclusion in CoAP responses, and builds
on the Base-Uri Option from Section 3 of [I-D.bormann-coap-misc].
Since the option is intended only for responses, the column "N"
indicates a dash for "not applicable".
+------+---+---+---+---+----------+--------+--------+---------+
| No. | C | U | N | R | Name | Format | Length | Default |
+------+---+---+---+---+----------+--------+--------+---------+
| | | | | | | | | |
| TBD2 | | | - | | Reply-To | (*) | 5-1034 | (none) |
| | | | | | | | | |
+------+---+---+---+---+----------+--------+--------+---------+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
(*) See below.
Figure 2: The Reply-To Option.
This document specifically defines how this option is used by a proxy
that can perform proxied CoAP group requests.
Upon receiving a response to such a request from an origin server,
the proxy includes the Reply-To Option into the response sent to the
origin client (see Section 5). The proxy uses the option to indicate
addressing information pertaining to that origin server, which the
client can use in order to send an individual request intended to
that server.
In particular, the client can use the addressing information
specified in the option in order to identify the response originator
and to possibly send it individual requests later on, either
directly, or indirectly via the proxy, as unicast requests.
When used as defined in this document, the option value is set to the
byte serialization of a CBOR sequence [RFC8742], which is composed of
at most two CBOR arrays.
* The first CBOR array is REQUIRED and specifies a CRI (see
[I-D.ietf-core-href]). In particular, both 'scheme' and
'authority' are given, while 'path', 'query', and 'fragment' are
not given.
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* The second CBOR array is OPTIONAL and specifies a CRI reference
(see [I-D.ietf-core-href]). In particular, 'scheme' is set to
null (0xf6), at least one of 'authority' and 'path' is given, and
both 'query' and 'fragment' are not given. This CRI reference is
relevant in some scenarios where the proxy is a reverse-proxy.
The detailed use of this option is specified in Section 5.4.1 and
Section 5.5.1 when the proxy is a forward-proxy, and in Section 6.2
and Section 6.1 when the proxy is a reverse-proxy.
The Reply-To Option is of class U in terms of OSCORE processing (see
Section 4.1 of [RFC8613]).
4. Requirements and Objectives
In this section, the word "proxy" is not limited to forward-proxies.
Instead, it comprises also reverse-proxies and HTTP-to-CoAP proxies.
This document assumes that the following requirements are fulfilled.
* REQ1. The proxy is explicitly configured (allow-list) to perform
proxied group requests on behalf of specific allowed client(s).
* REQ2. The proxy MUST identify a client sending a unicast group
request to be proxied, in order to verify whether the client is
allowed-listed to do so. For example, this can rely on one of the
following security associations.
- A TLS [RFC8446] or DTLS [RFC9147] channel between the client
and the proxy, where the client has been authenticated during
the secure channel establishment.
- A pairwise OSCORE [RFC8613] Security Context between the client
and the proxy, as defined in
[I-D.ietf-core-oscore-capable-proxies].
* REQ3. If secure, end-to-end communication is required between the
client and the servers in the CoAP group, exchanged messages MUST
be protected by using Group OSCORE
[I-D.ietf-core-oscore-groupcomm], as discussed in Section 5 of
[I-D.ietf-core-groupcomm-bis]. This requires the client and the
servers to have previously joined the correct OSCORE group, for
instance by using the approach described in
[I-D.ietf-ace-key-groupcomm-oscore]. The correct OSCORE group to
join can be pre-configured or alternatively discovered, for
instance by using the approach described in
[I-D.tiloca-core-oscore-discovery].
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This document defines how to achieve the following objectives.
* OBJ1. The proxy gets an indication from the client that the
client is in fact interested in and capable to handle multiple
responses to a proxied group request. With particular reference
to a unicast CoAP group request sent to the proxy, this means that
the client is capable to receive those responses as separate CoAP
responses, each matching with the original unicast request.
* OBJ2. The proxy learns for how long it should wait for responses
to a proxied group request, before starting to ignore following
responses to it (except for notifications, if a CoAP Observe
Option is used [RFC7641]).
* OBJ3. The proxy relays to the client any multiple responses to
the proxied group request. With particular reference to a
client's original CoAP unicast request sent to the proxy, those
responses are sent to the client as separate CoAP responses, each
matching with the original unicast request.
* OBJ4. The client is able to distinguish the different responses
to the proxied group request, as well as their corresponding
origin servers.
* OBJ5. The client is enabled to optionally contact one or more of
the responding origin servers in the future, either directly or
via the proxy.
5. Protocol Description
This section specifies the steps of the signaling protocol.
5.1. Request Sending at the Client
This section defines the operations performed by the client, for
sending a request targeting a group of servers via the proxy.
5.1.1. Request Sending
The client proceeds according to the following steps.
1. The client prepares a unicast CoAP group request addressed to the
proxy. The request specifies the group URI where the request has
to be forwarded to, as a string in the Proxi-URI option or by
using the Proxy-Scheme option with the group URI composed from
the URI-* options (see Section 3.5.1 of
[I-D.ietf-core-groupcomm-bis]).
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2. The client MUST retain the Token value used for this original
unicast request beyond the reception of a first CoAP response
matching with it. To this end, the client follows the same rules
for Token retention defined for multicast CoAP requests in
Section 3.1.5 of [I-D.ietf-core-groupcomm-bis].
In particular, the client picks an amount of time T that it is
fine to wait for before freeing up the Token value.
Specifically, the value of T MUST be such that:
* T < T_r , where T_r is the amount of time that the client is
fine to wait for before potentially reusing the Token value.
Note that T_r MUST NOT be less than MIN_TOKEN_REUSE_TIME
defined in Section 3.1.5 of [I-D.ietf-core-groupcomm-bis].
* T should be at least the expected worst-case time taken by the
request and response processing on the proxy and on the
servers in the addressed CoAP group.
* T should be at least the expected worst-case round-trip delay
between the client and the proxy plus the worst-case round-
trip delay between the proxy and any one of the origin
servers.
3. The client MUST include the Multicast-Timeout Option defined in
Section 2 into the unicast request to send to the proxy. The
option value specifies an amount of time T' < T. The difference
(T - T') should be at least the expected worst-case round-trip
time between the client and the proxy.
The client can specify T' = 0 as option value, thus indicating to
be not interested in receiving responses from the origin servers
through the proxy. In such a case, the client SHOULD also
include a No-Response Option [RFC7967] with value 26 (suppress
all response codes), if it supports the option.
Consistently, if the unicast request to send to the proxy already
included a No-Response Option with value 26, the client SHOULD
specify T' = 0 as value of the Multicast-Timeout Option.
4. The client processes the request as defined in
[I-D.ietf-core-groupcomm-bis], and also as in
[I-D.ietf-core-oscore-groupcomm] when secure group communication
is used between the client and the servers.
5. The client sends the request to the proxy as a unicast CoAP
message. When doing so, the client protects the request
according to the security association it has with the proxy.
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The exact method that the client uses to estimate the worst-case
processing times and round-trip delays mentioned above is out of the
scope of this document. However, such a method is expected to be
already used by the client when generally determining an appropriate
Token lifetime and reuse interval.
5.1.2. Supporting Observe
When using CoAP Observe [RFC7641], the client follows what is
specified in Section 3.7 of [I-D.ietf-core-groupcomm-bis], with the
difference that it sends a unicast request to the proxy, to be
forwarded to the group of servers, as defined in Section 5.1.1 of
this document.
Furthermore, the client especially follows what is specified in
Section 5 of [RFC7641], i.e., it registers its interest to be an
observer with the proxy, as if it was communicating with the servers.
5.2. Request Processing at the Proxy
This section defines the operations performed by the proxy, when
receiving a request to forward to a group of servers.
5.2.1. Request Processing
Upon receiving the request from the client, the proxy proceeds
according to the following steps.
1. The proxy decrypts and verifies the request, according to the
security association it has with the client.
2. The proxy identifies the client, and verifies that the client is
in fact allowed-listed to have its requests proxied to CoAP group
URIs.
3. The proxy verifies the presence of the Multicast-Timeout Option,
as a confirmation that the client is fine to receive multiple
CoAP responses matching with the same original request.
If the Multicast-Timeout Option is not present, the proxy MUST
stop processing the request and MUST reply to the client with a
4.00 (Bad Request) response. The response MUST include a
Multicast-Timeout Option with an empty (zero-length) value,
indicating that the Multicast-Timeout Option was missing and has
to be included in the request. As per Section 5.9.2 of [RFC7252]
The response SHOULD include a diagnostic payload.
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4. The proxy retrieves the value T' from the Multicast-Timeout
Option, and then removes the option from the client's request.
5. The proxy forwards the client's request to the group of servers.
In particular, the proxy sends it as a CoAP group request over IP
multicast, addressed to the group URI specified by the client.
6. The proxy sets a timeout with the value T' retrieved from the
Multicast-Timeout Option of the original unicast request.
In case T' > 0, the proxy will ignore responses to the forwarded
group request coming from servers, if received after the timeout
expiration, with the exception of Observe notifications (see
Section 5.4).
In case T' = 0, the proxy will ignore all responses to the
forwarded group request coming from servers.
If the proxy supports caching of responses, it can serve the original
unicast request also by using cached responses, as per Section 7.
5.2.2. Supporting Observe
When using CoAP Observe [RFC7641], the proxy takes the role of the
client and registers its own interest to observe the target resource
with the servers as per Section 5 of [RFC7641].
When doing so, the proxy especially follows what is specified for the
client in Section 3.7 of [I-D.ietf-core-groupcomm-bis], by forwarding
the group request to the servers over IP multicast as defined in
Section 5.2.1 of this document.
5.3. Request and Response Processing at the Server
This section defines the operations performed by the server, when
receiving a group request from the proxy.
5.3.1. Request and Response Processing
Upon receiving the request from the proxy, the server proceeds
according to the following steps.
1. The server processes the group request as defined in
[I-D.ietf-core-groupcomm-bis], and also as in
[I-D.ietf-core-oscore-groupcomm] when secure group communication
is used between the client and the server.
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2. The server processes the response to be relayed to the client as
defined in [I-D.ietf-core-groupcomm-bis], and also as in
[I-D.ietf-core-oscore-groupcomm] when secure group communication
is used between the client and the server.
5.3.2. Supporting Observe
When using CoAP Observe [RFC7641], the server especially follows what
is specified in Section 3.7 of [I-D.ietf-core-groupcomm-bis] and
Section 5 of [RFC7641].
5.4. Response Processing at the Proxy
This section defines the operations performed by the proxy, when
receiving a response matching with a forwarded group request.
5.4.1. Response Processing
Upon receiving a response matching with the group request before the
amount of time T' has elapsed, the proxy proceeds according to the
following steps.
1. The proxy MUST include the Reply-To Option defined in Section 3
into the response. The proxy sets the option value as follows.
The CRI present as first element of the CBOR sequence specifies
the addressing information of the server generating the response.
The second element of the CBOR sequence MUST NOT be present.
If the proxy supports caching of responses (see Section 7), the
proxy MUST include the Reply-To Option into the response before
caching it. This ensures that a response to a group request
conveys the addressing information of the origin server that
generated the response, also when the response is forwarded to a
client as retrieved from the proxy's cache.
2. The proxy forwards the response back to the client. When doing
so, the proxy protects the response according to the security
association it has with the client.
As discussed in Section 3.1.6 of [I-D.ietf-core-groupcomm-bis], it is
possible that a same server replies with multiple responses to the
same group request, i.e., with the same Token. As long as the proxy
forwards responses to a group request back to the origin client, the
proxy MUST follow the steps defined above and forward also such
multiple responses "as they come".
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Upon timeout expiration, i.e., T' seconds after having sent the group
request over IP multicast, the proxy frees up its local Token value
associated with that request. Thus, following late responses to the
same group request will be discarded and not forwarded back to the
client.
5.4.2. Supporting Observe
When using CoAP Observe [RFC7641], the proxy acts as a client
registered with the servers, as described earlier in Section 5.2.2.
Furthermore, the proxy takes the role of a server when forwarding
notifications from origin servers back to the client. To this end,
the proxy follows what is specified in Section 3.7 of
[I-D.ietf-core-groupcomm-bis] and Section 5 of [RFC7641], with the
following additions.
* At step 1 in Section 5.4, the proxy includes the Reply-To Option
in every notification, including non-2.xx notifications resulting
in removing the proxy from the list of observers of the origin
server.
* The proxy frees up its Token value used for a group observation
only if, after the timeout expiration, no 2.xx (Success) responses
matching with the group request and also including an Observe
option have been received from any origin server. Otherwise,
after the timeout expiration and as long as observations are
active with servers in the group for the target resource of the
group request, notifications from those servers are forwarded back
to the client, as defined in Section 5.4, and the Token value used
for the group observation is not freed during this time.
Finally, the proxy SHOULD regularly verify that the client is still
interested in receiving observe notifications for a group
observation. To this end, the proxy can rely on the same approach
discussed for servers in Section 3.7 of
[I-D.ietf-core-groupcomm-bis], with more details available in
Section 4.5 of [RFC7641].
5.5. Response Processing at the Client
This section defines the operations performed by the client, when
receiving a response matching with a request that targeted a group of
servers via the proxy.
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5.5.1. Response Processing
Upon receiving from the proxy a response matching with the original
unicast request before the amount of time T has elapsed, the client
proceeds according to the following steps.
1. The client processes the response as defined in
[I-D.ietf-core-groupcomm-bis]. When doing so, the client
decrypts and verifies the response according to the security
association it has with the proxy.
2. If secure group communication is used end-to-end between the
client and the servers, the client processes the response
resulting at the end of step 1, as defined in
[I-D.ietf-core-oscore-groupcomm].
3. The client retrieves the CRI from the value of the Reply-To
Option, and identifies the origin server whose addressing
information is specified by the CRI.
In particular, the client is able to distinguish different
responses as originated by different servers. Optionally, the
client may contact one or more of those servers individually,
i.e., directly (bypassing the proxy) or indirectly (via a proxied
unicast request).
In order to individually reach the origin server again through
the proxy, the client is not required to understand or support
the transport protocol indicated by 'scheme' in the CRI and used
between the proxy and the origin server, in case the protocol is
not CoAP over UDP (CRI scheme number: -1).
That is, by using the information specified in the retrieved CRI,
the client composes the correct URI for the individual request,
and specifies it by means of the Proxy-Uri Option or Proxy-Scheme
Option in the unicast request to the proxy. Alternatively, the
client can rely on the analogous Proxy-Cri or Proxy-Scheme-Number
Option defined in [I-D.ietf-core-href]. In either case, the
client uses the transport protocol that it knows, and has used
before, to send the request to the proxy.
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As discussed in Section 3.1.6 of [I-D.ietf-core-groupcomm-bis], it is
possible that the client receives multiple responses to the same
group request, i.e., with the same Token, from the same origin
server. The client normally processes at the CoAP layer each of
those responses from the same origin server, and decides at the
application layer how to exactly handle them depending on its
available context information (see Section 3.1.6 of
[I-D.ietf-core-groupcomm-bis]).
Upon the timeout expiration, i.e., T seconds after having sent the
original unicast request to the proxy, the client frees up its local
Token value associated with that request. Note that, upon this
timeout expiration, the Token value is not eligible for possible
reuse yet (see Section 5.1.1). Thus, until the actual amount of time
before enabling Token reusage has elapsed, following late responses
to the same request forwarded by the proxy will be discarded, as
these are not matching (by Token) with any active request from the
client.
5.5.2. Supporting Observe
When using CoAP Observe [RFC7641], the client frees up its Token
value only if, after the timeout T expiration, no 2.xx (Success)
responses matching with the original unicast request and also
including an Observe option have been received.
Instead, if at least one such response has been received, the client
continues receiving those notifications as forwarded by the proxy, as
long as the observation for the target resource of the original
unicast request is active.
5.6. Example
The example in this section refers to the following actors.
* One origin client C, with address C_ADDR and port number C_PORT.
* One proxy P, with address P_ADDR and port number P_PORT.
* Two origin servers S1 and S2, where the server Sx has address
Sx_ADDR and port number Sx_PORT.
The origin servers are members of a CoAP group with IP multicast
address G_ADDR and port number G_PORT. Also, the origin servers are
members of a same application group, and share the same resource /r.
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The communication between C and P is based on CoAP over UDP, as per
[RFC7252]. The communication between P and the origin servers is
based on CoAP over UDP and IP multicast, as per
[I-D.ietf-core-groupcomm-bis].
Finally, cri'X' denotes a CRI corresponding to the URI X.
C P S1 S2
| | | |
|------------------------------>| | |
| Src: C_ADDR:C_PORT | | |
| Dst: P_ADDR:P_PORT | | |
| Proxi-Uri: | | |
| "coap://G_ADDR:G_PORT/r" | | |
| Multicast-Timeout: 60 | | |
| | | |
| | | |
| | Src: P_ADDR:P_PORT | |
| | Dst: G_ADDR:G_PORT | |
| | Uri-Path: "r" | |
| |---------------+----->| |
| | \ | |
| | +----------------->|
| | | |
| | | |
| | / t = 0 : P starts | |
| | accepting responses | |
| | for this request / | |
| | | |
| | | |
| |<---------------------| |
| | Src: S1_ADDR:G_PORT | |
| | Dst: P_ADDR:P_PORT | |
| | | |
| | | |
|<------------------------------| | |
| Src: P_ADDR:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| Reply-To: | | |
| cri'coap://S1_ADDR:G_PORT' | | |
| | | |
| | | |
| |<-----------------------------------|
| | Src: S2_ADDR:S2_PORT |
| | Dst: P_ADDR:P_PORT |
| | | |
| | | |
|<------------------------------| | |
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| Src: P_ADDR:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| Reply-To: | | |
| cri'coap://S2_ADDR:S2_PORT' | | |
| | | |
| | | |
| / At t = 60, P stops accepting | |
| responses for this request / | |
| | | |
Figure 3: Workflow example with a forward-proxy
6. Reverse-Proxies
The use of reverse-proxies in group communication scenarios is
defined in Section 3.5.2 of [I-D.ietf-core-groupcomm-bis].
This section clarifies how the Multicast-Timeout Option is effective
also in such a context, in order for:
* The proxy to explicitly reveal itself as a reverse-proxy to the
client.
* The client to indicate to the proxy of being aware that it is
communicating with a reverse-proxy, and for how long it is willing
to receive responses to a proxied group request.
This practically addresses the additional issues compared to the case
with a forward-proxy, as compiled in Section 3.5.2 of
[I-D.ietf-core-groupcomm-bis]. A reverse-proxy may also operate
without support of the Multicast-Timeout Option, as defined in that
section.
Appendix A provides examples with a reverse-proxy.
6.1. Processing on the Proxy Side
If the proxy receives a CoAP request and determines that it should be
forwarded to a group of servers over IP multicast, then the proxy
performs the steps defined in Section 5.2.
In particular, when such a request does not include a Multicast-
Timeout Option, the proxy explicitly reveals itself as a reverse-
proxy, by sending a 4.00 (Bad Request) response including a
Multicast-Timeout Option with empty (zero-length) value.
The proxy processes the CoAP responses forwarded back to the client
as defined in Section 5.4, with the following additions.
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* As a first possible case, the proxy stands in both for the whole
group of servers and for the individual origin servers in the
group. That is, the origin client cannot reach the individual
servers directly, but only through the proxy.
In such a case, within a response forwarded back to the client,
the value of the Reply-To Option specifies an addressing
information TARGET that is directly associated with the proxy.
The addressing information is such that, when receiving a unicast
request that has been sent according to what is specified in
TARGET, the proxy forwards the request to the origin server that
originated the response. In particular, the proxy sets the option
value as follows.
- The CRI present as first element of the CBOR sequence specifies
an addressing information TARGET_1, such that a unicast request
reaches the proxy if it is sent according to TARGET_1.
- A CRI reference MUST be present as second element of the CBOR
sequence in case, upon receiving a unicast request that has
been sent according to TARGET_1, the proxy forwards the request
based on what is specified by the Uri-Host, Uri-Port, and Uri-
Path Options included in the request. The CRI reference
specifies the same information that the proxy expects to be
specified in the Uri-Host, Uri-Port, and Uri-Path Options of
such a unicast request.
Otherwise, the second element of the CBOR sequence MUST NOT be
present, in which case the proxy forwards the unicast request
solely based on the addressing information TARGET_1 according
to which the request has been sent to.
The client will be able to communicate individually with the
server that originated the response, by sending a follow-up
unicast request to the proxy at the specified addressing
information TARGET, according to which the proxy forwards the
request to that server. This is further specified in Section 6.2.
An example is provided in Appendix A.1 and Appendix A.2.
* As a second possible case, the proxy stands in only for the whole
group of servers, but not for the individual servers in the group.
That is, the origin client can reach the individual servers
directly, without recourse to the proxy.
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In such a case, within a response forwarded back to the client,
the value of the Reply-To Option specifies an addressing
information TARGET that is directly associated with the origin
server that originated the response. In particular, the proxy
sets the option value as follows.
- The CRI present as first element of the CBOR sequence specifies
the addressing information TARGET, such that a unicast request
reaches the origin server if sent according to TARGET. The
second element of the CBOR sequence MUST NOT be present.
The client will be able to use that information for sending a
follow-up unicast request directly to that server, i.e., bypassing
the proxy. This is further specified in Section 6.2. An example
is provided in Appendix A.3.
6.2. Processing on the Client Side
If a client sends a CoAP request intended to a group of servers and
is aware of actually communicating with a reverse-proxy, then the
client MUST perform the steps defined in Section 5.1.1. In
particular, this results in a request sent to the proxy including a
Multicast-Timeout Option.
The client processes the CoAP responses forwarded back by the proxy
as defined in Section 5.5, with the following differences at step 3.
* If the client wishes to send a follow-up unicast request intended
only to the server that originated the response, then the client
sends such a request according to the addressing information
specified by the CRI retrieved from the value of the Reply-To
Option. Effectively, the client sends the unicast request to the
proxy.
In case the value of the Reply-To Option specifies also a CRI
reference as second element of the CBOR sequence, then the client
includes the Uri-Host, Uri-Port, and Uri-Path Options in the
unicast request, according to what is specified by the
corresponding elements of the CRI reference. If the client wants
to specify additional path segments that identify a specific
resource at the origin server, then the corresponding Uri-Path
options are included in the request after the Uri-Path options
corresponding to the path component of the CRI reference.
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7. Caching
A proxy MAY cache responses to a group request, as defined in
Section 5.7.1 of [RFC7252]. In particular, the same rules apply to
determine the set of request options used as "Cache-Key", and to
determine the max-age values offered for responses served from the
cache.
A cache entry is associated with one server and stores one response
from that server, regardless whether it is a response to a unicast
request or to a group request. The following two types of requests
can produce a hit to a cache entry.
* A matching request intended to that server, i.e., to the
corresponding unicast URI.
When the stored response is a response to a unicast request to the
server, the unicast URI of the matching request is the same target
URI used for the original unicast request.
When the stored response is a response to a group request to the
CoAP group, the unicast URI of the matching request is the target
URI obtained by replacing the authority part of the group URI in
the original group request with the transport-layer source address
and port number of the response.
* A matching group request intended to the CoAP group, i.e., to the
corresponding group URI.
That is, a matching group request produces a hit to multiple cache
entries, each of which associated with one of the CoAP servers
currently member of the CoAP group.
Note that, as per the freshness model defined in Section 7.1, the
proxy might serve a group request exclusively from its cached
responses only when it knows all the CoAP servers that are current
members of the CoAP group and it has a valid cache entry for each
of them.
When forwarding a GET or FETCH group request to the servers in the
CoAP group, the proxy behaves like a CoAP client as defined in
Section 3.2 of [I-D.ietf-core-groupcomm-bis], with the following
additions.
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* As discussed in Section 5.4.1, the proxy can receive multiple
responses to the same group request from a same origin server, and
forwards them back to the origin client "as they come". When this
happens, each of such multiple responses is stored in the cache
entry associated with the server "as it comes", possibly replacing
an already stored response from that server.
* As discussed in Section 7.4, when communications in the group are
secured with Group OSCORE [I-D.ietf-core-oscore-groupcomm],
additional means are required to enable cacheability of responses
at the proxy.
The following subsections define the freshness model and validation
model that the proxy uses for cached responses.
7.1. Freshness Model
The proxy relies on the same freshness model defined in Section 3.2.1
of [I-D.ietf-core-groupcomm-bis], by taking the role of a CoAP client
with respect to the servers in the CoAP group.
In particular, when receiving a unicast group request from the
client, the proxy MAY serve it by using exclusively cached responses
without forwarding the group request to the servers in the CoAP
group, but only if both the following conditions hold.
* The proxy knows all the CoAP servers that are currently members of
the CoAP group for which the group request is intended to.
* The proxy's cache currently stores a fresh response for each of
those CoAP servers.
The specific way that the proxy uses to determine the CoAP servers
currently members of the target CoAP group is out of scope for this
document. As possible examples, the proxy can synchronize with a
group manager server; rely on well-known time patterns used by the
application or in the network for the addition of new CoAP group
members; observe group join requests or IGMP/MLD multicast group join
messages, e.g., if embedded in a multicast router.
When forwarding the group request to the servers, the proxy may have
fresh responses stored in its cache for (some of) those servers. In
such a case, the proxy uses (also) those cached responses to serve
the original unicast group request, as defined below.
* The request processing in Section 5.2.1 is extended as follows.
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After setting the timeout with value T' > 0 in step 6, the proxy
checks whether its cache currently stores fresh responses to the
group request. For each of such responses, the proxy compares the
residual lifetime L of the corresponding cache entry against the
value T'.
If a cached response X is such that L < T', then the proxy
forwards X back to the client at its earliest convenience.
Otherwise, the proxy does not forward X back to the client right
away, and rather waits for approaching the timeout expiration, as
discussed in the next point.
* The response processing in Section 5.4.1 is extended as follows.
Before the timeout with original value T' > 0 expires and the
proxy stops accepting responses to the group request, the proxy
checks whether it stores in its cache any fresh response X to the
group request such that both the following conditions hold.
- The cache entry E storing X was already existing when the proxy
forwarded the group request.
- The proxy has received no response to the forwarded group
request from the server associated with E.
Then, the proxy sends back to the client each response X stored in
its cache and selected as above, before the timeout expires.
Note that, from the forwarding of the group request until the
timeout expiration, the proxy still forwards responses to the
group request back to the client "as they come" (see
Section 5.4.1). Also, such responses possibly refresh older
responses from the same servers that the proxy has stored in its
cache, as defined earlier in Section 7.
7.2. Validation Model
This section defines the revalidation of responses, separately
between the proxy and the origin servers, as well as between the
origin client and the proxy.
7.2.1. Proxy-Servers Revalidation with Unicast Requests
The proxy MAY revalidate a cached response by making a GET or FETCH
request on the related unicast request URI, i.e., by taking the role
of a CoAP client with respect to a server in the CoAP group.
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As discussed in Section 7.4, this is however not possible for the
proxy if communications in the group are secured end-to-end between
origin client and origin servers by using Group OSCORE
[I-D.ietf-core-oscore-groupcomm].
[ TODO
It can be actually possible to enable revalidation of responses
between proxy and server, also in this case where Group OSCORE is
used end-to-end between client and origin servers.
Fundamentally, this requires to define the possible use of the ETag
option also as an outer option for OSCORE. Thus, in addition to the
normal inner ETag, a server can add also an outer ETag option
intended to the proxy.
Since validation of responses assumes that cacheability of responses
is possible in the first place, it would be convenient to define the
use of ETag as outer option in [I-D.amsuess-core-cachable-oscore].
In case OSCORE is also used between the proxy and an individual
origin server as per [I-D.ietf-core-oscore-capable-proxies], then the
outer ETag option would be seamlessly protected with the OSCORE
Security Context shared between the proxy and the origin server.
The following text can be used to replace the last paragraph above.
As discussed in Section 7.4, the following applies when Group OSCORE
[I-D.ietf-core-oscore-groupcomm] is used to secure communications
end-to-end between the origin client and the origin servers in the
group.
* Additional means are required to enable cacheability of responses
at the proxy (see Section 7.4.1).
* If a cached response included an outer ETag option intended to the
proxy, then the proxy can perform revalidation of the cached
response, by making a request to the unicast URI targeting the
server, and including outer ETag Option(s).
This is possible also in case the proxy and the origin server use
OSCORE to further protect the exchanged request and response, as
defined in [I-D.ietf-core-oscore-capable-proxies]. In such a
case, the originally outer ETag option is protected with the
OSCORE Security Context shared between the proxy and the origin
server, before transferring the message over the communication leg
between the proxy and origin server.
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]
7.2.2. Proxy-Servers Revalidation with Group Requests
When forwarding a group request to the servers in the CoAP group, the
proxy MAY revalidate one or more stored responses that it has cached.
To this end, the proxy relies on the same validation model defined in
Section 3.2.2 of [I-D.ietf-core-groupcomm-bis] and using the ETag
Option, by taking the role of a CoAP client with respect to the
servers in the CoAP group.
As discussed in Section 7.4, this is however not possible for the
proxy if communications in the group are secured end-to-end between
origin client and origin servers by using Group OSCORE
[I-D.ietf-core-oscore-groupcomm].
[ TODO
See the notes in Section 7.2.1.
The following text can be used to replace the last paragraph above.
As discussed in Section 7.4, the following applies when Group OSCORE
[I-D.ietf-core-oscore-groupcomm] is used to secure communications
end-to-end between the origin client and the origin servers in the
group.
* Additional means are required to enable cacheability of responses
at the proxy (see Section 7.4.1).
* If a cached response included an outer ETag option intended to the
proxy, then the proxy can perform revalidation of the cached
response, by making a request to the group URI targeting the CoAP
group, and including outer ETag Option(s).
This is possible also in case the proxy and the origin servers use
Group OSCORE to further protect the exchanged request and
response, as defined in [I-D.ietf-core-oscore-capable-proxies].
In such a case, the originally outer ETag option is protected with
the Group OSCORE Security Context shared between the proxy and the
origin server, before transferring the message over the
communication leg between the proxy and origin server.
]
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7.3. Client-Proxy Revalidation with Group Requests
A client MAY revalidate the full set of responses to a group request
by leveraging the corresponding cache entries at the proxy. To this
end, this document defines the new Group-ETag Option.
The Group-ETag Option has the properties summarized in Figure 4,
which extends Table 4 of [RFC7252]. The Group-ETag Option is
elective, safe to forward, part of the cache key, and repeatable.
The option is intended for group requests sent to a proxy to be
forwarded to the servers in a CoAP group, as well as for the
associated responses.
+------+---+---+---+---+------------+--------+--------+---------+
| No. | C | U | N | R | Name | Format | Length | Default |
+------+---+---+---+---+------------+--------+--------+---------+
| | | | | | | | | |
| TBD3 | | | | x | Group-ETag | opaque | 1-8 | (none) |
| | | | | | | | | |
+------+---+---+---+---+------------+--------+--------+---------+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
Figure 4: The Group-ETag Option.
The Group-ETag Option has the same properties of the ETag Option
defined in Section 5.10.6 of [RFC7252].
The Group-ETag Option is of class U in terms of OSCORE processing
(see Section 4.1 of [RFC8613]).
A proxy MUST NOT provide this form of validation if it is not in a
position to serve a group request by using exclusively cached
responses, i.e., without sending the group request to the servers in
the CoAP group (see Section 7.1).
If the proxy supports this form of response revalidation, the
following applies.
* The proxy defines J as a joint set including all the cache entries
currently storing fresh responses that satisfy a group request. A
set J is "complete" if it includes a valid cache entry for each of
the CoAP servers currently members of the CoAP group.
* When the set J becomes "complete", the proxy assigns it an entity-
tag value. The proxy MUST update the current entity-tag value,
when J is "complete" and one of its cache entry is updated.
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* When forwarding to the client a 2.05 (Content) response to a GET
or FETCH group request, the proxy MAY include one Group-ETag
Option, in case the set J is "complete". Such a response MUST NOT
include more than one Group-ETag Option. The option value
specifies the entity-tag value currently associated with the set
J.
When sending to the proxy a GET or FETCH request to be forwarded to
the servers in the CoAP group, the client MAY include one or more
Group-ETag Options. Each option specifies one entity-tag value,
applicable to the set J of cache entries that can be hit by the group
request.
The proxy MAY perform the following actions, in case the group
request produces a hit to the cache entry of each CoAP server
currently member of the CoAP group, i.e., in case the set J
associated with the group request is "complete".
* The proxy checks whether the current entity-tag value of the set J
matches with one of the entity-tag values specified in the Group-
ETag Options of the unicast group request from the client.
* In case of positive match, the proxy replies with a single 2.03
(Valid) response. This response has no payload and MUST include
one Group-ETag Option, specifying the current entity-tag value of
the set J.
That is, the 2.03 (Valid) response from the proxy indicates to the
client that the stored responses identified by the entity-tag given
in the response's Group-ETag Option can be reused, after updating
each of them as described in Section 5.9.1.3 of [RFC7252]. In
effect, the client can determine if any of the stored representations
from the respective cache entries at the proxy is current, without
needing to transfer any of them again.
7.4. Caching of End-To-End Protected Responses at Proxies
When using Group OSCORE [I-D.ietf-core-oscore-groupcomm] to protect
communications end-to-end between a client and multiple servers in
the group, it is normally not possible for an intermediary proxy to
effectively cache protected responses.
In fact, when starting from the same plain CoAP message, different
clients generate different protected requests to send on the wire.
This prevents different clients to generate potential cache hits, and
thus makes response caching at the proxy pointless.
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7.4.1. Deterministic Requests to Achieve Cacheability
For application scenarios that use secure group communication, it is
still possible to achieve cacheability of responses at proxies, by
using the approach defined in [I-D.amsuess-core-cachable-oscore]
which is based on Deterministic Requests protected with the pairwise
mode of Group OSCORE. This approach is limited to group requests
that are safe (in the RESTful sense) to process and do not yield side
effects at the server. As for any protected group request, it
requires the clients and all the servers in the CoAP group to have
already joined the correct OSCORE group.
Starting from the same plain CoAP request, this allows different
clients in the OSCORE group to deterministically generate a same
request protected with Group OSCORE, which is sent to the proxy for
being forwarded to the CoAP group. The proxy can now effectively
cache the resulting responses from the servers in the CoAP group,
since the same plain CoAP request will result again in the same
Deterministic Request and thus will produce a cache hit.
When caching of Group OSCORE secured responses is enabled at the
proxy, the same as defined in Section 7 applies, with respect to
cache entries and their lifetimes.
Note that different Deterministic Requests result in different cache
entries at the proxy. This includes the case where different plain
group requests differ only in their set of ETag Options, as defined
in Section 3.2.2 of [I-D.ietf-core-groupcomm-bis].
That is, even though the servers would produce the same plain CoAP
responses when replying to two different Deterministic Requests,
those will result in different protected responses to each respective
Deterministic Request, hence in different cache entries at the proxy.
Thus, given a plain group request, a client needs to reuse the same
set of ETag Options, in order to send that group request as a
Deterministic Request that can actually produce a cache hit at the
proxy. However, while this would prevent the caching at the proxy to
be inefficient and unnecessarily redundant, it would also limit the
flexibility of end-to-end response revalidation for a client.
7.4.2. Validation of Responses
Response revalidation remains possible end-to-end between the client
and the servers in the group, by means of including inner ETag
Option(s) as defined in Sections 3.2 and 3.2.2 of
[I-D.ietf-core-groupcomm-bis].
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Furthermore, it remains possible for a client to attempt revalidating
responses to a group request from a "complete" set of cache entries
at the proxy, by using the Group-ETag Option as defined in
Section 7.3.
When directly interacting with the servers in the CoAP group to
refresh its cache entries, the proxy cannot rely on response
revalidation anymore. This applies to both the case where the
request is addressed to a single server and sent to the related
unicast URI (see Section 7.2.1) or instead is a group request
addressed to the CoAP group and sent to the related group URI (see
Section 7.2.2).
[ TODO
See the notes in Section 7.2.1.
The following text can be used to replace the last paragraph above.
When directly interacting with the servers in the CoAP group to
refresh its cache entries, the proxy also remains able to perform
response revalidation. That is, if a cached response included an
outer ETag option intended to the proxy, then the proxy can perform
revalidation of the cached response, by making a request to the
unicast URI addressed to a single server and sent to the related
unicast URI (see Section 7.2.1) or a group request addressed to the
CoAP group and sent to the related group URI (see Section 7.2.2).
]
8. Chain of Proxies
A client may be interested to access a resource at a group of origin
servers which is reached through a chain of two or more proxies.
That is, these proxies are configured into a chain, where each non-
last proxy is configured to forward (group) requests to the next hop
towards the origin servers. Also, each non-first proxy is configured
to forward back responses to the previous hop proxy towards the
origin client.
This section specifies how the signaling protocol defined in
Section 5 is used in that setting. Except for the last proxy before
the origin servers, every other proxy in the chain takes the role of
client with respect to the next hop towards the origin servers.
Also, every proxy in the chain except the first takes the role of
server towards the previous proxy closer to the origin client.
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Accordingly, possible caching of responses at each proxy works as
defined in Section 7 and Section 7.4. Also, possible revalidation of
responses cached ad each proxy and based on the Group-ETag option
works as defined in Section 7.3 and Section 7.4.2.
The requirements REQ1 and REQ2 defined in Section 4 MUST be fulfilled
for each proxy in the chain. That is, every proxy in the chain has
to be explicitly configured (allow-list) to allow proxied group
requests from specific senders, and MUST identify those senders upon
receiving their group request. For the first proxy in the chain,
that sender is the origin client. For each other proxy in the chain,
that sender is the previous hop proxy closer to the origin client.
In either case, a proxy can identify the sender of a group request by
the same means mentioned in Section 4.
8.1. Request Processing at the Proxy
Upon receiving a group request to be forwarded to a CoAP group URIs,
a proxy proceed as follows.
If the proxy is the last one in the chain, i.e., it is the last hop
before the origin servers, the proxy performs the steps defined in
Section 5.2, with no modifications.
Otherwise, the proxy performs the steps defined in Section 5.2, with
the following differences.
* At steps 1-3, "client" refers to the origin client for the first
proxy in the chain; or to the previous hop proxy closer to the
origin client, otherwise.
* At step 4, the proxy rather performs the following actions.
1. The proxy retrieves the value T' from the Multicast-Timeout
Option, and does not remove the option.
2. In case T' > 0, the proxy picks an amount of time T it is fine
to wait for before freeing up its local Token value to use
with the next hop towards the origin servers. To this end,
the proxy MUST follow what is defined at step 2 of
Section 5.1.1 for the origin client, with the following
differences.
- T MUST be greater than the retrieved value T', i.e., T' <
T.
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- The worst-case message processing time takes into account
all the next hops towards the origin servers, as well as
the origin servers themselves.
- The worst-case round-trip delay takes into account all the
legs between the proxy and the origin servers.
3. In case T' > 0, the proxy replaces the value of the Multicast-
Timeout Option with a new value T'', such that:
- T'' < T. The difference (T - T'') should be at least the
expected worst-case round-trip time between the proxy and
the next hop towards the origin servers.
- T'' < T'. The difference (T' - T'') should be at least the
expected worst-case round-trip time between the proxy and
the (previous hop proxy closer to the) origin client.
If the proxy is not able to determine a value T'' that
fulfills both the requirements above, the proxy MUST stop
processing the request and MUST respond with a 5.05 (Proxying
Not Supported) error response to the previous hop proxy closer
to the origin client. The proxy SHOULD include a Multicast-
Timeout Option, set to the minimum value T' that would be
acceptable in the Multicast-Timeout Option of a group request
to forward.
Upon receiving such an error response, any proxy in the chain
MAY send an updated group request to the next hop towards the
origin servers, specifying in the Multicast-Timeout Option a
value T' greater than in the previous request. If this does
not happen, the proxy receiving the error response MUST also
send a 5.05 (Proxying Not Supported) error response to the
previous hop proxy closer to the origin client. Like the
received one, also this error response SHOULD include a
Multicast-Timeout Option, set to the minimum value T'
acceptable by the proxy sending the error response.
* At step 5, the proxy forwards the request to the next hop towards
the origin servers.
* At step 6, the proxy sets a timeout with the value T' retrieved
from the Multicast-Timeout Option of the request received from the
(previous hop proxy closer to the) origin client.
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In case T' > 0, the proxy will ignore responses to the forwarded
group request coming from the (next hop towards the) origin
servers, if received after the timeout expiration, with the
exception of Observe notifications (see Section 5.4).
In case T' = 0, the proxy will ignore all responses to the
forwarded group request coming from the (next hop towards the)
origin servers.
8.1.1. Supporting Observe
When using CoAP Observe [RFC7641], what is defined in Section 5.2.2
applies for the last proxy in the chain, i.e., the last hop before
the origin servers.
Any other proxy in the chain acts as a client and registers its own
interest to observe the target resource with the next hop towards the
origin servers, as per Section 5 of [RFC7641].
8.2. Response Processing at the Proxy
Upon receiving a response matching with the group request before the
amount of time T' has elapsed, the proxy proceeds as follows.
If the proxy is the last one in the chain, i.e., it is the last hop
before the origin servers, the proxy performs the steps defined in
Section 5.4 if it is a forward-proxy or in Section 6.1 if it is a
reverse-proxy, with no modifications.
Otherwise, the proxy performs the steps defined in Section 5.4, with
the following differences.
* In any of the two following cases, the proxy skips step 1, hence
the proxy MUST NOT remove, alter, or replace the Reply-To Option.
- The chain is composed of forward-proxies.
- The chain is composed of reverse-proxies, and the last reverse-
proxy (in fact, the whole chain) stands in only for the whole
group of servers, but not for the individual servers in the
group (see Section 6.1).
This ensures that, when receiving a response to a group request
and consuming the Reply-To Option, the origin client can retrieve
addressing information that is directly associated with the origin
server that originated the response.
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* At step 1, the following applies in case the chain is composed of
reverse-proxies, and the last reverse-proxy (in fact, the whole
chain) stands in both for the whole group of servers and for the
individual origin servers in the group (see Section 6.1).
In the Reply-To Option, the proxy MUST replace the old value
TARGET_OLD. The new value TARGET_NEW specifies addressing
information directly associated with the proxy. The new value is
such that, when receiving a unicast request that has been sent
according to what is specified in TARGET_NEW, the proxy forwards
the request according to what was specified in TARGET_OLD, i.e.,
to the (next hop towards the) origin server that originated the
response.
This ensures that, when receiving a response to a group request
and consuming the Reply-To Option, the origin client can retrieve
addressing information that is directly associated with the first
reverse-proxy in the chain, i.e., with the next hop towards the
origin server that originated the response.
* At step 2, "client" refers to the origin client for the first
proxy in the chain; or to the previous hop proxy closer to the
origin client, otherwise.
As to the possible reception of multiple responses to the same group
request from the same (next hop proxy towards the) origin server, the
same as defined in Section 5.4.1 applies. That is, as long as the
proxy forwards responses to a group request back to the (previous hop
proxy closer to the) origin client, the proxy MUST follow the steps
above and forward also such multiple responses "as they come".
Upon timeout expiration, i.e., T seconds after having forwarded the
group request to the next hop towards the origin servers, the proxy
frees up its local Token value associated with that request. Thus,
following late responses to the same group request will be discarded
and not forwarded back to the (previous hop proxy closer to the)
origin client.
8.2.1. Supporting Observe
When using CoAP Observe [RFC7641], what is defined in Section 5.4.2
applies for the last proxy in the chain, i.e., the last hop before
the origin servers.
As to any other proxy in the chain, the following applies.
* The proxy acts as a client registered with the next hop towards
the origin servers, as described earlier in Section 8.1.1.
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* The proxy takes the role of a server when forwarding notifications
from the next hop to the origin servers back to the (previous hop
proxy closer to the) origin client, as per Section 5 of [RFC7641].
* The proxy frees up its Token value used for a group observation
only if, after the timeout expiration, no 2.xx (Success) responses
matching with the group request and also including an Observe
option have been received from the next hop towards the origin
servers. Otherwise, after the timeout expiration and as long as
the observation for the target resource of the group request is
active with the next hop towards the origin servers in the group,
notifications from that hop are forwarded back to the (previous
hop proxy closer to the) origin client, as defined in Section 8.2.
* The proxy SHOULD regularly verify that the (previous hop proxy
closer to the) origin client is still interested in receiving
observe notifications for a group observation. To this end, the
proxy can rely on the same approach defined in Section 4.5 of
[RFC7641].
9. HTTP-CoAP Proxies
This section defines the components needed to use the signaling
protocol specified in this document, when an HTTP client wishes to
send a group request to the servers of a CoAP group, via an HTTP-CoAP
cross-proxy.
The following builds on the mapping of the CoAP request/response
model to HTTP and vice versa as defined in Section 10 of [RFC7252],
as well as on the additional details about the HTTP-CoAP mapping
defined in [RFC8075].
Furthermore, the components defined in Section 11 of [RFC8613] are
also used to map and transport OSCORE-protected messages over HTTP.
This allows an HTTP client to use Group OSCORE end-to-end with the
servers in the CoAP group.
9.1. The HTTP Multicast-Timeout Header Field
The HTTP Multicast-Timeout header field (see Section 11.2) is used
for carrying the content otherwise specified in the CoAP Multicast-
Timeout Option defined in Section 2.
Using the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] and
including the core ABNF syntax rule DIGIT (decimal digits) defined by
that specification, the HTTP Multicast-Timeout header field value is
as follows.
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Multicast-Timeout = *DIGIT
When translating a CoAP message into an HTTP message, the HTTP
Multicast-Timeout header field is set with the content of the CoAP
Multicast-Timeout Option, or is left empty in case the option is
empty.
The same applies in the opposite direction, when translating an HTTP
message into a CoAP message.
9.2. The HTTP Reply-To Header Field
The HTTP Reply-To header field (see Section 11.2) is used for
carrying the content otherwise specified in the CoAP Reply-To Option
defined in Section 3.
Using the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] and
including the following core ABNF syntax rules defined by that
specification: ALPHA (letters) and DIGIT (decimal digits), the HTTP
Reply-To header field value is as follows.
reply-to-char = ALPHA / DIGIT / "-" / "_"
Reply-To = 2*reply-to-char
When translating a CoAP message into an HTTP message, the HTTP Reply-
To header field is set to the value of the CoAP Reply-To Option in
base64url (see Section 5 of [RFC4648]) encoding without padding.
Implementation notes for this encoding are given in Appendix C of
[RFC7515].
When translating an HTTP message into a CoAP message, the CoAP Reply-
To Option is set to the value of the HTTP Reply-To header field
decoded from base64url (see Section 5 of [RFC4648]) without padding.
Implementation notes for this encoding are given in Appendix C of
[RFC7515].
9.3. The HTTP Group-ETag Header Field
The HTTP Group-ETag header field (see Section 11.2) is used for
carrying the content otherwise specified in the CoAP Group-ETag
Option defined in Section 7.3.
Using the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] and
including the following core ABNF syntax rules defined by that
specification: ALPHA (letters) and DIGIT (decimal digits), the HTTP
Group-ETag header field value is as follows.
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group-etag-char = ALPHA / DIGIT / "-" / "_"
Group-ETag = 2*group-etag-char
When translating a CoAP message into an HTTP message, the HTTP Group-
ETag header field is set to the value of the CoAP Group-ETag Option
in base64url (see Section 5 of [RFC4648]) encoding without padding.
Implementation notes for this encoding are given in Appendix C of
[RFC7515].
When translating an HTTP message into a CoAP message, the CoAP Group-
ETag Option is set to the value of the HTTP Group-ETag header field
decoded from base64url (see Section 5 of [RFC4648]) without padding.
Implementation notes for this encoding are given in Appendix C of
[RFC7515].
9.4. Request Sending at the Client
The client proceeds according to the following steps.
1. The client prepares an HTTP request to send to the proxy via IP
unicast, and to be forwarded by the proxy to the targeted group
of CoAP servers over IP multicast. With reference to Section 5
of [RFC8075], the request is addressed to a Hosting HTTP URI,
such that the proxy can extract the Target CoAP URI as the group
URI where to forward the request.
2. The client determines the amount of time T that it is fine to
wait for a response to the request from the proxy. Then, the
client determines the amount of time T' < T, where the difference
(T - T') should be at least the expected worst-case round-trip
time between the client and the proxy.
3. If Group OSCORE is used end-to-end between the client and the
servers, the client translates the HTTP request into a CoAP
request, as per [RFC8075]. Then, the client protects the
resulting CoAP request by using Group OSCORE, as defined in
[I-D.ietf-core-oscore-groupcomm]. Finally, the protected CoAP
request is mapped to HTTP as defined in Section 11.2 of
[RFC8613]. Later on, the resulting HTTP request MUST be sent in
compliance with the rules in Section 11.1 of [RFC8613].
4. The client includes the HTTP Multicast-Timeout header field in
the request, specifying T' as its value. The client can specify
T' = 0, thus indicating to be not interested in receiving
responses from the origin servers through the proxy.
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5. If the client wishes to revalidate responses to a previous group
request from the corresponding cache entries at the proxy (see
Section 7.3), the client includes one or multiple HTTP Group-ETag
header fields in the request (see Section 9.3), each specifying
an entity-tag value like they would in a corresponding CoAP Group
E-Tag option.
6. The client sends the request to the proxy, as a unicast HTTP
message. In particular, the client protects the request
according to the security association it has with the proxy.
9.5. Request Processing at the Proxy
The proxy translates the HTTP request to a CoAP request, as per
[RFC8075]. The additional rules for HTTP messages with the HTTP
Multicast-Timeout header field and HTTP Group-ETag header field are
defined in Section 9.1 and Section 9.3, respectively.
Once translated the HTTP request into a CoAP request, the proxy MUST
perform the steps defined in Section 5.2. If the proxy supports
caching of responses, it can serve the unicast request also by using
cached responses as per Section 7, considering the CoAP request above
as the potentially matching request.
In addition, in case the HTTP Multicast-Timeout header field had
value 0, the proxy replies to the client with an HTTP response with
status code 204 (No Content), right after forwarding the group
request to the group of servers.
9.6. Response Processing at the Proxy
Upon receiving a CoAP response matching with the group request before
the amount of time T' > 0 has elapsed, the proxy includes the Reply-
To Option in the response, as per step 1 of Section 5.4.1. Then, the
proxy translates the CoAP response to an HTTP response, as per
Section 10.1 of [RFC7252] and [RFC8075], as well as Section 11.2 of
[RFC8613] if Group OSCORE is used end-to-end between the client and
servers. The additional rules for CoAP messages specifying the
Reply-To Option are defined in Section 9.2.
After that, the proxy stores the resulting HTTP response until the
timeout with original value T' > 0 expires. If, before then, the
proxy receives another response to the same group request from the
same CoAP server, the proxy performs the steps above, and stores the
resulting HTTP response by superseding the currently stored one from
that server.
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When the timout expires, if no responses have been received from the
servers, the proxy replies to the client's original unicast group
request with an HTTP response with status code 204 (No Content).
Otherwise, the proxy relays to the client all the collected and
stored HTTP responses to the group request, according to the
following steps.
1. The proxy prepares a single HTTP batch response, which MUST have
200 (OK) status code and MUST have its HTTP Content-Type header
field with value multipart/mixed [RFC2046].
2. For each stored individual HTTP response RESP, the proxy prepares
a corresponding batch part to include in the HTTP batch response,
such that:
* The batch part has its own HTTP Content-Type header field with
value application/http [RFC9112].
* The body of the batch part is the individual HTTP response
RESP, including its status code, headers and body.
3. The proxy includes each batch part prepared at step 2 in the HTTP
batch response.
4. The proxy replies to the client's original unicast group request,
by sending the HTTP batch response. When doing so, the proxy
protects the response according to the security association it
has with the client.
9.7. Response Processing at the Client
When it receives an HTTP response as a reply to the original unicast
group request, the client proceeds as follows.
1. The client decrypts and verifies the response, according to the
security association it has with the proxy.
2. From the resulting HTTP batch response, the client extracts the
different batch parts.
3. From each of the extracted batch parts, the client extracts the
body as one of the individual HTTP response RESP.
4. For each individual HTTP response RESP, the client performs the
following steps.
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* If Group OSCORE is used end-to-end between the client and
servers, the client translates the HTTP response RESP into a
CoAP response, as per Section 11.3 of [RFC8613]. Then, the
client decrypts and verifies the resulting CoAP response by
using Group OSCORE, as defined in
[I-D.ietf-core-oscore-groupcomm]. Finally, the decrypted CoAP
response is mapped to HTTP as per Section 10.2 of [RFC7252] as
well as [RFC8075]. The additional rules for HTTP messages
with the HTTP Reply-To header field are defined in
Section 9.2.
* The client delivers to the application the individual HTTP
response.
Similarly to step 3 in Section 5.5.1, the client identifies the
origin server that originated the CoAP response corresponding to
the HTTP response RESP, by means of the addressing information
specified as value of the HTTP Reply-To header field. This
allows the client to distinguish different individual HTTP
responses as corresponding to different CoAP responses from the
servers in the CoAP group.
9.8. Example
The examples in this section build on Section 5.6, with the
difference that the origin client C is an HTTP client and the proxy P
is an HTTP-CoAP cross-proxy. The examples are simply illustrative
and are not to be intended as a test vector.
The following is an example of unicast group request sent by C to P.
The URI mapping and notation are based on the "Simple Form" defined
in Section 5.4.1 of [RFC8075].
POST https://proxy.url/hc/?target_uri=coap://G_ADDR:G_PORT/ HTTP/1.1
Content-Length: <REQUEST_TOTAL_CONTENT_LENGTH>
Content-Type: text/plain
Multicast-Timeout: 60
Body: Do that!
The following is an example of HTTP batch response sent by P to C, as
a reply to the client's original unicast group request
For readability, 'base64url(cri'X')' denotes the base64url encoding
of cri'X' without padding (see Section 5 of [RFC4648]), and cri'X'
denotes the byte serialization of a CRI corresponding to the URI X.
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HTTP/1.1 200 OK
Content-Length: <BATCH_RESPONSE_TOTAL_CONTENT_LENGTH>
Content-Type: multipart/mixed; boundary=batch_foo_bar
--batch_foo_bar
Content-Type: application/http
HTTP/1.1 200 OK
Content-Type: text/plain
Content-Length: <INDIVIDUAL_RESPONSE_1_CONTENT_LENGTH>
Reply-To: base64url(cri'coap://S1_ADDR:G_PORT')
Body: Done!
--batch_foo_bar
Content-Type: application/http
HTTP/1.1 200 OK
Content-Type: text/plain
Content-Length: <INDIVIDUAL_RESPONSE_2_CONTENT_LENGTH>
Reply-To: base64url(cri'coap://S2_ADDR:S2_PORT')
Body: More than done!
--batch_foo_bar--
9.9. Streamed Delivery of Responses to the Client
[ TODO
The proxy might still be able to forward back individual responses to
the client in a streamed fashion.
Individual responses can be forwarded back one by one as they come
(like a CoAP-to-CoAP proxy does), or as soon as a certain amount of
them have been received from the servers.
This can be achieved by combining the Content-Type multipart/mixed
used in the previous sections with the Transfer-Coding "chunked"
specified in RFC 9112.
The above applies to HTTP 1.1, while HTTP/2 has its own mechanisms
for data streaming.
]
9.10. Reverse-Proxies
In case an HTTP-to-CoAP proxy acts specifically as a reverse-proxy,
the same principles defined in Section 6 apply, as specified below.
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9.10.1. Processing on the Client Side
If an HTTP client sends a request intended to a group of servers and
is aware of actually communicating with a reverse-proxy, then the
client MUST perform the steps defined in Section 9.4. In particular,
this results in a request sent to the proxy including a Multicast-
Timeout header field.
The client processes the HTTP response forwarded back by the proxy as
defined in Section 9.7. If the client wishes to send a follow-up
unicast request intended only to one of the CoAP servers that
originated the response, the same concepts defined in Section 6.2
apply to the composition of HTTP requests.
9.10.2. Processing on the Proxy Side
If the proxy receives a request and determines that it should be
forwarded to a group of servers over IP multicast, then the same as
defined in Section 9.5 applies, with the following difference.
Once translated the HTTP request into a CoAP request, the proxy
performs what is defined in Section 6.1.
The proxy processes the HTTP response sent to the client as defined
in Section 9.6.
10. Security Considerations
The security considerations from [RFC7252][I-D.ietf-core-groupcomm-bi
s][RFC8613][I-D.ietf-core-oscore-groupcomm] hold for this document.
When a chain of proxies is used (see Section 8), the secure
communication between any two adjacent hops is independent.
When Group OSCORE is used for end-to-end secure group communication
between the origin client and the origin servers, this security
association is unaffected by the possible presence of a proxy or a
chain of proxies.
Furthermore, the following additional considerations hold.
10.1. Client Authentication
As per the requirement REQ2 (see Section 4), the client has to
authenticate to the proxy when sending a group request to forward.
This leverages an established security association between the client
and the proxy, that the client uses to protect the group request,
before sending it to the proxy.
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If the group request is (also) protected end-to-end between the
client and the servers using the group mode of Group OSCORE, the
proxy can act as external signature checker (see Section 8.5 of
[I-D.ietf-core-oscore-groupcomm]) and authenticate the client by
successfully verifying the signature embedded in the group request.
However, this requires the proxy to store, for each client to
authenticate, the authentication credential that the client uses in
the OSCORE group and the public key included therein, and to also
store the authentication credential of the Group Manager responsible
for the OSCORE group. This in turn would require a form of active
synchronization between the proxy and the Group Manager for that
group [I-D.ietf-core-oscore-groupcomm].
Nevertheless, the client and the proxy SHOULD still rely on a full-
fledged pairwise secure association. In addition to ensuring the
integrity of group requests sent to the proxy (see Section 10.2,
Section 10.3 and Section 10.4), this prevents the proxy from
forwarding replayed group requests with a valid signature, as
possibly injected by an active, on-path adversary.
The same considerations apply when a chain of proxies is used (see
Section 8), with each proxy but the last one in the chain acting as
client with the next hop towards the origin servers.
10.2. Multicast-Timeout Option
The Multicast-Timeout Option is of class U for OSCORE [RFC8613].
Hence, also when Group OSCORE is used between the client and the
servers [I-D.ietf-core-oscore-groupcomm], a proxy is able to access
the option value and retrieve the timeout value T', as well as to
remove the option altogether before forwarding the group request to
the servers. When a chain of proxies is used (see Section 8), this
also allows each proxy but the last one in the chain to update the
option value, as an indication for the next hop towards the origin
servers (see Section 8.1).
The security association between the client and the proxy MUST
provide message integrity, so that further intermediaries between the
two as well as on-path active adversaries are not able to remove the
option or alter its content, before the group request reaches the
proxy. Removing the option would otherwise result in not forwarding
the group request to the servers. Instead, altering the option
content would result in the proxy accepting and forwarding back
responses for an amount of time different than the one actually
indicated by the client.
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The security association between the client and the proxy SHOULD also
provide message confidentiality. Otherwise, any further
intermediaries between the two as well as any on-path passive
adversaries would be able to simply access the option content, and
thus learn for how long the client is willing to receive responses
from the servers in the group via the proxy. This may in turn be
used by an on-path active adversary to perform a more efficient,
selective suppression of responses from the servers.
When the client protects the unicast request sent to the proxy using
OSCORE (see [I-D.ietf-core-oscore-capable-proxies]) and/or (D)TLS,
both message integrity and message confidentiality are achieved in
the leg between the client and the proxy.
The same considerations above about security associations apply when
a chain of proxies is used (see Section 8), with each proxy but the
last one in the chain acting as client with the next hop towards the
origin servers.
10.3. Reply-To Option
The Reply-To Option is of class U for OSCORE [RFC8613]. Hence, also
when Group OSCORE is used between the client and the servers
[I-D.ietf-core-oscore-groupcomm], the proxy that has forwarded the
group request to the servers is able to include the option into a
server response, before forwarding this response back to the
(previous hop proxy closer to the) origin client.
Since the security association between the client and the proxy
provides message integrity, any further intermediaries between the
two as well as any on-path active adversaries are not able to
undetectably remove the Reply-To Option from a forwarded server
response. This ensures that the client can correctly distinguish the
different responses and identify their corresponding origin server.
When the proxy protects the response forwarded back to the client
using OSCORE (see [I-D.ietf-core-oscore-capable-proxies]) and/or
(D)TLS, message integrity is achieved in the leg between the client
and the proxy.
The same considerations above about security associations apply when
a chain of proxies is used (see Section 8), with each proxy but the
last one in the chain acting as client with the next hop towards the
origin servers.
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10.4. Group-ETag Option
The Group-ETag Option is of class U for OSCORE [RFC8613]. Hence,
also when Group OSCORE is used between the client and the servers
[I-D.ietf-core-oscore-groupcomm], a proxy is able to access the
option value and use it to possibly perform response revalidation at
its cache entries associated with the servers in the CoAP group, as
well as to remove the option altogether before forwarding the group
request to the servers. When a chain of proxies is used (see
Section 8), this also allows each proxy but the last one in the chain
to update the option value, to possibly ask the next hop towards the
origin servers to perform response revalidation at its cache entries.
The security association between the client and the proxy MUST
provide message integrity, so that further intermediaries between the
two as well as on-path active adversaries are not able to remove the
option or alter its content, before the group request reaches the
proxy. Removing the option would otherwise result in the proxy not
performing response revalidation at its cache entries associated with
the servers in the CoAP group, even though that was what the client
asked for.
Altering the option content in a group request would result in the
proxy replying with 2.05 (Content) responses conveying the full
resource representations from its cache entries, rather than with a
single 2.03 (Valid) response. Instead, altering the option content
in a 2.03 (Valid) or 2.05 (Content) response would result in the
client wrongly believing that the already stored or the just received
representation, respectively, is also the current one, as per the
entity value of the tampered Group-ETag Option.
The security association between the client and the proxy SHOULD also
provide message confidentiality. Otherwise, any further
intermediaries between the two as well as any on-path passive
adversaries would be able to simply access the option content, and
thus learn the rate and pattern according to which the group resource
in question changes over time, as inferable from the entity values
read over time.
When the client protects the unicast request sent to the proxy using
OSCORE (see [I-D.ietf-core-oscore-capable-proxies]) and/or (D)TLS,
both message integrity and message confidentiality are achieved in
the leg between the client and the proxy.
The same considerations above about security associations apply when
a chain of proxies is used (see Section 8), with each proxy but the
last one in the chain acting as client with the next hop towards the
origin servers.
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When caching of Group OSCORE secured responses is enabled at the
proxy, the same as defined in Section 7 applies, with respect to
cache entries and the way they are maintained.
10.5. HTTP-to-CoAP Proxies
Consistently with what is discussed in Section 10.1, an HTTP client
has to authenticate to the HTTP-to-CoAP proxy, and they SHOULD rely
on a full-fledged pairwise secure association. This can rely on a
TLS [RFC8446] channel as also recommended in Section 12.1 of
[RFC8613] for when OSCORE is used with HTTP, or on a pairwise OSCORE
[RFC8613] Security Context between the client and the proxy as
defined in [I-D.ietf-core-oscore-capable-proxies].
[ TODO
Revisit security considerations from [RFC8075]
]
11. IANA Considerations
This document has the following actions for IANA.
Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]"
with the RFC number of this specification and delete this paragraph.
11.1. CoAP Option Numbers Registry
IANA is asked to enter the following option numbers to the "CoAP
Option Numbers" registry within the "Constrained RESTful Environments
(CoRE) Parameters" registry group.
+========+===================+============+
| Number | Name | Reference |
+========+===================+============+
| TBD1 | Multicast-Timeout | [RFC-XXXX] |
+--------+-------------------+------------+
| TBD2 | Reply-To | [RFC-XXXX] |
+--------+-------------------+------------+
| TBD3 | Group-ETag | [RFC-XXXX] |
+--------+-------------------+------------+
Table 1: New CoAP Option Numbers
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11.2. Hypertext Transfer Protocol (HTTP) Field Name Registry
IANA is asked to enter the following HTTP header fields to the
"Hypertext Transfer Protocol (HTTP) Field Name" Registry registry.
+===================+===========+============+
| Field Name | Status | Reference |
+===================+===========+============+
| Multicast-Timeout | permanent | [RFC-XXXX] |
+-------------------+-----------+------------+
| Reply-To | permanent | [RFC-XXXX] |
+-------------------+-----------+------------+
| Group-ETag | permanent | [RFC-XXXX] |
+-------------------+-----------+------------+
Table 2: New HTTP Field Names
12. References
12.1. Normative 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-href]
Bormann, C. and H. Birkholz, "Constrained Resource
Identifiers", Work in Progress, Internet-Draft, draft-
ietf-core-href-14, 9 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
href-14>.
[I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
and J. Park, "Group Object Security for Constrained
RESTful Environments (Group OSCORE)", Work in Progress,
Internet-Draft, draft-ietf-core-oscore-groupcomm-20, 2
September 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-oscore-groupcomm-20>.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/rfc/rfc2046>.
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[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>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/rfc/rfc5234>.
[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>.
[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>.
[RFC8075] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
E. Dijk, "Guidelines for Mapping Implementations: HTTP to
the Constrained Application Protocol (CoAP)", RFC 8075,
DOI 10.17487/RFC8075, February 2017,
<https://www.rfc-editor.org/rfc/rfc8075>.
[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>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.
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[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>.
[RFC8742] Bormann, C., "Concise Binary Object Representation (CBOR)
Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
<https://www.rfc-editor.org/rfc/rfc8742>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/rfc/rfc8949>.
[RFC9112] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
June 2022, <https://www.rfc-editor.org/rfc/rfc9112>.
12.2. Informative References
[I-D.amsuess-core-cachable-oscore]
Amsüss, C. and M. Tiloca, "Cacheable OSCORE", Work in
Progress, Internet-Draft, draft-amsuess-core-cachable-
oscore-08, 10 January 2024,
<https://datatracker.ietf.org/doc/html/draft-amsuess-core-
cachable-oscore-08>.
[I-D.bormann-coap-misc]
Bormann, C. and K. Hartke, "Miscellaneous additions to
CoAP", Work in Progress, Internet-Draft, draft-bormann-
coap-misc-27, 14 November 2014,
<https://datatracker.ietf.org/doc/html/draft-bormann-coap-
misc-27>.
[I-D.ietf-ace-key-groupcomm-oscore]
Tiloca, M., Park, J., and F. Palombini, "Key Management
for OSCORE Groups in ACE", Work in Progress, Internet-
Draft, draft-ietf-ace-key-groupcomm-oscore-16, 6 March
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
ace-key-groupcomm-oscore-16>.
[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-00, 18 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
oscore-capable-proxies-00>.
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[I-D.tiloca-core-oscore-discovery]
Tiloca, M., Amsüss, C., and P. Van der Stok, "Discovery of
OSCORE Groups with the CoRE Resource Directory", Work in
Progress, Internet-Draft, draft-tiloca-core-oscore-
discovery-14, 8 September 2023,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
oscore-discovery-14>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/rfc/rfc7515>.
[RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for
No Server Response", RFC 7967, DOI 10.17487/RFC7967,
August 2016, <https://www.rfc-editor.org/rfc/rfc7967>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/rfc/rfc9147>.
Appendix A. Examples with Reverse-Proxy
The examples in this section refer to the following actors.
* One origin client C, with address C_ADDR and port number C_PORT.
* One proxy P, with address P_ADDR and server port number P_PORT.
* Two origin servers S1 and S2, where the server Sx has address
Sx_ADDR and port number Sx_PORT.
The origin servers are members of a CoAP group with IP multicast
address G_ADDR and port number G_PORT. Also, the origin servers are
members of a same application group, and share the same resource /r.
The communication between C and P is based on CoAP over TCP, as per
[RFC8323]. The group communication between P and the origin servers
is based on CoAP over UDP and IP multicast, as per
[I-D.ietf-core-groupcomm-bis].
Finally, cri'X' denotes a CRI or CRI reference corresponding to the
URI or URI reference X.
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A.1. Example 1
The example shown in Figure 5 considers a reverse-proxy P that
provides access to both the whole group of servers {S1,S2} and also
to each of those servers individually. The client C may not have a
way to reach the servers directly (e.g., P is acting as a firewall).
After the client C has received two responses to its group request
sent via the proxy, it selects one server (S1) and requests another
resource from it in unicast, again via the proxy.
In particular:
* The hostname 'p.example.com' resolves to the proxy's unicast IPv6
address P_ADDR.
* In its group request to P, the client C includes the Uri-Host
Option with value "group1.com" and the Uri-Path option with value
"r".
* The hostname 'group1.com' resolves to the IPv6 multicast address
G_ADDR. The proxy P performs this resolution upon receiving the
group request from C.
Since such a request does not include the Uri-Port option, P
infers G_PORT to be the default port number 5683 for the 'coap'
scheme.
Based on this information, P composes the group request and sends
it to the CoAP group at G_ADDR:G_PORT.
* Typically, S1_PORT and S2_PORT will be equal to G_PORT, but a
server Sx is allowed to reply to the multicast request from
another port number not equal to G_PORT. For this reason, the
notation Sx_PORT is used.
Note that this type of reverse-proxy only requires one unicast IP
address (P_ADDR) for the proxy, so it is well scalable to a large
number of servers Sx. Instead, the type of reverse-proxy in the
example in Appendix A.2 requires an additional IP address for each
server Sx and also for each CoAP group that it supports.
C P S1 S2
| | | |
|----------------------------------->| / C is not aware | |
| Src: C_ADDR:C_PORT | that P is in fact | |
| Dst: group1.com:P_PORT | a reverse-proxy / | |
| Uri-Path: "r" | | |
| | | |
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| | | |
|<-----------------------------------| | |
| Src: P_ADDR:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| 4.00 Bad Request | | |
| Multicast-Timeout: (empty) | | |
| Payload: "Please use | | |
| Multicast-Timeout" | | |
| | | |
| | | |
|----------------------------------->| | |
| Src: C_ADDR:C_PORT | | |
| Dst: p.example.com:P_PORT | | |
| Uri-Host: "group1.com" | | |
| Uri-Path: "r" | | |
| Multicast-Timeout: 60 | | |
| | | |
| | | |
| | Src: P_ADDR:P_PORT | |
| | Dst: G_ADDR:G_PORT | |
| | Uri-Path: "r" | |
| |---------------+----->| |
| | \ | |
| | +------------->|
| | | |
| | | |
| | / t = 0 : P starts | |
| | accepting responses | |
| | for this request / | |
| | | |
| | | |
| |<---------------------| |
| | Src: S1_ADDR:S1_PORT | |
| | Dst: P_ADDR:P_PORT | |
| | | |
| | | |
|<-----------------------------------| | |
| Src: P_ADDR:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| Reply-To: | | |
| cri'coap+tcp://P_ADDR:P_PORT', | | |
| cri'//S1_ADDR:S1_PORT' | | |
| | | |
| | | |
| |<-------------------------------|
| | Src: S2_ADDR:S2_PORT |
| | Dst: P_ADDR:P_PORT |
| | | |
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| | | |
|<-----------------------------------| | |
| Src: P_ADDR:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| Reply-To: | | |
| cri'coap+tcp://P_ADDR:P_PORT', | | |
| cri'//S2_ADDR:S2_PORT' | | |
| | | |
| | | |
| / At t = 60, P stops accepting | |
| responses for this request / | |
| | | |
| | | |
|----------------------------------->| / Request intended | |
| Src: C_ADDR:C_PORT | only to S1, via the | |
| Dst: P_ADDR:P_PORT | proxy P / | |
| Uri-Host: "S1_ADDR" | | |
| Uri-Port: S1_PORT | | |
| Uri-Path: "r1" | | |
| | | |
| | | |
| | Src: P_ADDR:P_PORT | |
| | Dst: S1_ADDR:S1_PORT | |
| | Uri-Path: "r1" | |
| |--------------------->| |
| | | |
| | | |
| |<---------------------| |
| | Src: S1_ADDR:S1_PORT | |
| | Dst: P_ADDR:P_PORT | |
| | | |
| | | |
|<-----------------------------------| | |
| Src: P_ADDR:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| | | |
Figure 5: Workflow example with reverse-proxy standing in for
both the whole group of servers and each individual server, and
requiring only one unicast IP address.
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A.2. Example 2
The example shown in Figure 6 considers a reverse-proxy that stands
in for both the whole group of servers {S1,S2} and for each of those
servers Sx. The client C may not have a way to reach the servers
directly (e.g., P is acting as a firewall). After the client C has
received two responses to its group request sent via the proxy, it
selects one server (S1) and requests at a later time the same
resource from it in unicast, again via the proxy.
In particular:
* The hostname 'group1.com' resolves to the unicast address P_ADDR.
The proxy forwards an incoming request to that address, for any
resource i.e., URI path, towards the CoAP group at G_ADDR:G_PORT
leaving the URI path unchanged.
* The address Dx_ADDR and port number Dx_PORT are used by the proxy,
which forwards an incoming request to that address towards the
server at Sx_ADDR:Sx_PORT. The different Dx_ADDR are effectively
'proxy IP addresses' used to provide access to the servers.
Note that this type of reverse-proxy implementation requires the
proxy to use (potentially) a large number of distinct IP addresses,
hence it is not very scalable. Instead, the type of reverse-proxy
shown in the example in Appendix A.1 uses only one IPv6 unicast
address to provide access to all servers and all CoAP groups.
C P S1 S2
| | | |
|---------------------------------->| / C is not aware | |
| Src: C_ADDR:C_PORT | that P is in fact | |
| Dst: group1.com:P_PORT | a reverse-proxy / | |
| Uri-Path: "r" | | |
| | | |
| | | |
|<----------------------------------| | |
| Src: P_ADDR:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| 4.00 Bad Request | | |
| Multicast-Timeout: (empty) | | |
| Payload: "Please use | | |
| Multicast-Timeout" | | |
| | | |
| | | |
|---------------------------------->| | |
| Src: C_ADDR:C_PORT | | |
| Dst: group1.com:P_PORT | | |
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| Multicast-Timeout: 60 | | |
| Uri-Path: "r" | | |
| | | |
| | | |
| | Src: P_ADDR:P_PORT | |
| | Dst: G_ADDR:G_PORT | |
| | Uri-Path: "r" | |
| |---------------+----->| |
| | \ | |
| | +-------------->|
| | | |
| | | |
| | / t = 0 : P starts | |
| | accepting responses | |
| | for this request / | |
| | | |
| | | |
| |<---------------------| |
| | Src: S1_ADDR:S1_PORT | |
| | Dst: P_ADDR:P_PORT | |
| | | |
| | | |
|<----------------------------------| | |
| Src: P_ADDR:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| Reply-To: | | |
| cri'coap+tcp://D1_ADDR:D1_PORT' | | |
| | | |
| | | |
| |<--------------------------------|
| | Src: S2_ADDR:S2_PORT |
| | Dst: P_ADDR:P_PORT |
| | | |
| | | |
|<----------------------------------| | |
| Src: P_ADDR:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| Reply-To: | | |
| cri'coap+tcp://D2_ADDR:D2_PORT' | | |
| | | |
| | | |
| / At t = 60, P stops accepting | |
| responses for this request / | |
| | | |
| | | |
... ... / Time passes / ... ...
| | | |
| | | |
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|---------------------------------->| / Request intended | |
| Src: C_ADDR:C_PORT | only to S1 for the | |
| Dst: D1_ADDR:D1_PORT | same resource /r / | |
| Uri-Path: "r" | | |
| | | |
| | Src: P_ADDR:P_PORT | |
| | Dst: S1_ADDR:S1_PORT | |
| | Uri-Path: "r" | |
| |--------------------->| |
| | | |
| | | |
| |<---------------------| |
| | Src: S1_ADDR:S1_PORT | |
| | Dst: P_ADDR:P_PORT | |
| | | |
| | | |
|<----------------------------------| | |
| Src: D1_ADDR:D1_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| | | |
Figure 6: Workflow example with reverse-proxy standing in for
both the whole group of servers and each individual server, and
requiring one pair (IP address, port number) for each origin
server.
A.3. Example 3
The example shown in Figure 7 builds on the example in Appendix A.2.
However, it considers a reverse-proxy that stands in for only the
whole group of servers, but not for each individual server Sx.
The final exchange between C and S1 occurs with CoAP over UDP.
C P S1 S2
| | | |
|------------------------------>| / C is not aware | |
| Src: C_ADDR:C_PORT | that P is in fact | |
| Dst: group1.com:P_PORT | a reverse-proxy / | |
| Uri-Path: "r" | | |
| | | |
|<------------------------------| | |
| Src: P_ADDR:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| 4.00 Bad Request | | |
| Multicast-Timeout: (empty) | | |
| Payload: "Please use | | |
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| Multicast-Timeout" | | |
| | | |
| | | |
|------------------------------>| | |
| Src: C_ADDR:C_PORT | | |
| Dst: group1.com:P_PORT | | |
| Multicast-Timeout: 60 | | |
| Uri-Path: "r" | | |
| | | |
| | | |
| | Src: P_ADDR:P_PORT | |
| | Dst: G_ADDR:G_PORT | |
| | Uri-Path: "r" | |
| |---------------+----->| |
| | \ | |
| | +----------------->|
| | | |
| | | |
| | / t = 0 : P starts | |
| | accepting responses | |
| | for this request / | |
| | | |
| | | |
| |<---------------------| |
| | Src: S1_ADDR:S1_PORT | |
| | Dst: P_ADDR:P_PORT | |
| | | |
| | | |
|<------------------------------| | |
| Dst: P_ADDR:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| Reply-To: | | |
| cri'coap://S1_ADDR:S1_PORT' | | |
| | | |
| | | |
| |<-----------------------------------|
| | Src: S2_ADDR:S2_PORT |
| | Dst: P_ADDR:P_PORT |
| | | |
| | | |
|<------------------------------| | |
| Dst: P_ADDR:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| Reply-To: | | |
| cri'coap://S2_ADDR:S2_PORT' | | |
| | | |
| | | |
| / At t = 60, P stops accepting | |
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| responses for this request / | |
| | | |
| | | |
... ... / Time passes / ... ...
| | | |
| | | |
|----------------------------------------------------->| |
| Src: C_ADDR:C_PORT | / Request intended | |
| Dst: S1.ADDR:S1_PORT | only to S1 for the | |
| Uri-Path: "r" | same resource /r / | |
| | | |
| | | |
|<-----------------------------------------------------| |
| | Src: S1.ADDR:S1_PORT | |
| | Dst: C_ADDR:C_PORT | |
| | | |
Figure 7: Workflow example with reverse-proxy standing in for
only the whole group of servers, but not for each individual
server
Appendix B. Document Updates
This section is to be removed before publishing as an RFC.
B.1. Version -00 to -01
* Definition of "individual request" in the terminology.
* UDP/IP multicast treated as the default transport protocol.
* Always use the Multicast-Timeout Option, also with reverse-
proxies.
* Response-Forwarding option:
- Renamed as "Reply-To".
- Revised encoding to use CRIs.
- Revised semantics to better address setups with reverse-
proxies.
- Added before possible response caching.
* Clarified response processing at reverse-proxies.
* Updated IANA considerations.
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* Editorial fixes and improvements.
Acknowledgments
The authors sincerely thank Christian Amsüss, Carsten Bormann, Rikard
Höglund, Jim Schaad, and Göran Selander for their comments and
feedback.
The work on this document has been partly supported by VINNOVA and
the Celtic-Next project CRITISEC; and by the H2020 project SIFIS-Home
(Grant agreement 952652).
Authors' Addresses
Marco Tiloca
RISE AB
Isafjordsgatan 22
SE-16440 Stockholm Kista
Sweden
Email: marco.tiloca@ri.se
Esko Dijk
IoTconsultancy.nl
Utrecht
Email: esko.dijk@iotconsultancy.nl
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