Proxy Operations for CoAP Group Communication
draft-tiloca-core-groupcomm-proxy-05
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| Authors | Marco Tiloca , Esko Dijk | ||
| Last updated | 2021-10-25 | ||
| Replaced by | draft-ietf-core-groupcomm-proxy | ||
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draft-tiloca-core-groupcomm-proxy-05
CoRE Working Group M. Tiloca
Internet-Draft RISE AB
Updates: 7252 (if approved) E. Dijk
Intended status: Standards Track IoTconsultancy.nl
Expires: 28 April 2022 25 October 2021
Proxy Operations for CoAP Group Communication
draft-tiloca-core-groupcomm-proxy-05
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 over IP
multicast to a group of servers. 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://gitlab.com/crimson84/draft-tiloca-core-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/.
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."
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This Internet-Draft will expire on 28 April 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. The Multicast-Signaling Option . . . . . . . . . . . . . . . 5
3. The Response-Forwarding Option . . . . . . . . . . . . . . . 6
3.1. Encoding of Server Address . . . . . . . . . . . . . . . 8
3.2. Default Values of the Server Port Number . . . . . . . . 9
4. Requirements and Objectives . . . . . . . . . . . . . . . . . 9
5. Protocol Description . . . . . . . . . . . . . . . . . . . . 11
5.1. Request Sending at the Client . . . . . . . . . . . . . . 11
5.1.1. Request Sending . . . . . . . . . . . . . . . . . . . 11
5.1.2. Supporting Observe . . . . . . . . . . . . . . . . . 12
5.2. Request Processing at the Proxy . . . . . . . . . . . . . 13
5.2.1. Request Processing . . . . . . . . . . . . . . . . . 13
5.2.2. Supporting Observe . . . . . . . . . . . . . . . . . 14
5.3. Request and Response Processing at the Server . . . . . . 14
5.3.1. Request and Response Processing . . . . . . . . . . . 14
5.3.2. Supporting Observe . . . . . . . . . . . . . . . . . 14
5.4. Response Processing at the Proxy . . . . . . . . . . . . 14
5.4.1. Response Processing . . . . . . . . . . . . . . . . . 15
5.4.2. Supporting Observe . . . . . . . . . . . . . . . . . 15
5.5. Response Processing at the Client . . . . . . . . . . . . 16
5.5.1. Response Processing . . . . . . . . . . . . . . . . . 16
5.5.2. Supporting Observe . . . . . . . . . . . . . . . . . 18
5.6. Example . . . . . . . . . . . . . . . . . . . . . . . . . 18
6. Reverse-Proxies . . . . . . . . . . . . . . . . . . . . . . . 20
6.1. Processing on the Client Side . . . . . . . . . . . . . . 20
6.2. Processing on the Proxy Side . . . . . . . . . . . . . . 20
7. Caching . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1. Freshness Model . . . . . . . . . . . . . . . . . . . . . 22
7.2. Validation Model . . . . . . . . . . . . . . . . . . . . 23
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7.2.1. Proxy-Servers Revalidation with Unicast Requests . . 23
7.2.2. Proxy-Servers Revalidation with Group Requests . . . 24
7.3. Client-Proxy Revalidation with Group Requests . . . . . . 25
7.4. Caching of End-To-End Protected Responses at Proxies . . 27
7.4.1. Deterministic Requests to Achieve Cacheability . . . 27
7.4.2. Validation of Responses . . . . . . . . . . . . . . . 28
8. Chain of Proxies . . . . . . . . . . . . . . . . . . . . . . 29
8.1. Request Processing at the Proxy . . . . . . . . . . . . . 29
8.1.1. Supporting Observe . . . . . . . . . . . . . . . . . 31
8.2. Response Processing at the Proxy . . . . . . . . . . . . 31
8.2.1. Supporting Observe . . . . . . . . . . . . . . . . . 32
9. HTTP-CoAP Proxies . . . . . . . . . . . . . . . . . . . . . . 33
9.1. The HTTP Multicast-Signaling Header Field . . . . . . . . 33
9.2. The HTTP Response-Forwarding Header Field . . . . . . . . 34
9.3. The HTTP Group-ETag Header Field . . . . . . . . . . . . 34
9.4. Request Sending at the Client . . . . . . . . . . . . . . 35
9.5. Request Processing at the Proxy . . . . . . . . . . . . . 36
9.6. Response Processing at the Proxy . . . . . . . . . . . . 36
9.7. Response Processing at the Client . . . . . . . . . . . . 37
9.8. Example . . . . . . . . . . . . . . . . . . . . . . . . . 38
10. Security Considerations . . . . . . . . . . . . . . . . . . . 39
10.1. Client Authentication . . . . . . . . . . . . . . . . . 39
10.2. Multicast-Signaling Option . . . . . . . . . . . . . . . 40
10.3. Response-Forwarding Option . . . . . . . . . . . . . . . 41
10.4. Group-ETag Option . . . . . . . . . . . . . . . . . . . 42
10.5. HTTP-CoAP Proxies . . . . . . . . . . . . . . . . . . . 43
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 43
11.1. CoAP Option Numbers Registry . . . . . . . . . . . . . . 43
11.2. CoAP Transport Information Registry . . . . . . . . . . 43
11.3. Header Field Registrations . . . . . . . . . . . . . . . 44
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 44
12.1. Normative References . . . . . . . . . . . . . . . . . . 45
12.2. Informative References . . . . . . . . . . . . . . . . . 46
Appendix A. Examples with Reverse-Proxy . . . . . . . . . . . . 48
A.1. Example 1 . . . . . . . . . . . . . . . . . . . . . . . . 48
A.2. Example 2 . . . . . . . . . . . . . . . . . . . . . . . . 51
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 53
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 53
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
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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 servers over IP
multicast. 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, a dedicated signaling
protocol is defined, using two new CoAP options.
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 diffent
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.
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].
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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 terms and concepts defined
in CoAP [RFC7252], Group Communication for CoAP
[I-D.ietf-core-groupcomm-bis], CBOR [RFC8949], OSCORE [RFC8613] and
Group OSCORE [I-D.ietf-core-oscore-groupcomm].
Unless specified otherwise, the term "proxy" refers to a CoAP-to-CoAP
forward-proxy, as defined in Section 5.7.2 of [RFC7252].
2. The Multicast-Signaling Option
The Multicast-Signaling 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-Signaling
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 | 0-5 | (none) |
| | | | | | Signaling | | | |
| | | | | | | | | |
+------+---+---+---+---+------------+--------+--------+---------+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
Figure 1: The Multicast-Signaling 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).
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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 Signaling 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-Signaling Option is of class U in terms of OSCORE
processing (see Section 4.1 of [RFC8613]).
3. The Response-Forwarding Option
The Response-Forwarding 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 | | | - | | Response- | (*) | 10-25 | (none) |
| | | | | | Forwarding | | | |
| | | | | | | | | |
+------+---+---+---+---+------------+--------+--------+---------+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
(*) See below.
Figure 2: The Response-Forwarding 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 request from a server, the proxy
includes the Response-Forwarding Option into the response sent to the
origin client (see Section 5). The proxy uses the option to indicate
the addressing information where the client can send an individual
request intended to that origin server.
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In particular, the client can use the addressing information
specified in the option to identify the response originator and
possibly send it individual requests later on, either directly, or
indirectly via the proxy, as unicast requests.
The option value is set to the byte serialization of the CBOR array
'tp_info' defined in Section 2.2.1 of
[I-D.ietf-core-observe-multicast-notifications], including only the
set of elements 'srv_addr'. In turn, the set includes the integer
'tp_id' identifying the used transport protocol, and further elements
whose number, format and encoding depend on the value of 'tp_id'.
The value of 'tp_id' MUST be taken from the "Value" column of the
"CoAP Transport Information" registry defined in Section 14.5 of
[I-D.ietf-core-observe-multicast-notifications]. The elements of
'srv_addr' following 'tp_id' are specified in the corresponding entry
of the Registry, under the "Server Addr" column.
If the server is reachable through CoAP transported over UDP, the
'tp_info' array includes the following elements, encoded as defined
in Section 2.2.1.1 of
[I-D.ietf-core-observe-multicast-notifications].
* 'tp_id': the CBOR integer with value 1. This element MUST be
present.
* 'srv_host': a CBOR byte string, encoding the unicast IP address of
the server. This element is tagged and identified by the CBOR tag
260 "Network Address (IPv4 or IPv6 or MAC Address)". This element
MUST be present.
* 'srv_port': a CBOR unsigned integer or the CBOR simple value Null.
This element MAY be present.
If present as a CBOR unsigned integer, it has as value the
destination UDP port number to use for individual requests to the
server.
If present as the CBOR simple value Null, the client MUST assume
that the default port number 5683 defined in [RFC7252] can be used
as the destination UDP port number for individual requests to the
server.
If not present, the client MUST assume that the same port number
specified in the group URI of the original unicast CoAP group
request sent to the proxy (see Section 5.1.1) can be used for
individual requests to the server.
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The CDDL notation [RFC8610] provided below describes the 'tp_info'
CBOR array using the format defined above.
tp_info = [
tp_id : 1, ; UDP as transport protocol
srv_host : #6.260(bstr), ; IP address where to reach the server
? srv_port : uint / null ; Port number where to reach the server
]
At present, 'tp_id' is expected to take only value 1 (UDP) when using
forward proxies, UDP being the only currently available transport for
CoAP to work over IP multicast. While additional multicast-friendly
transports may be defined in the future, other current tranport
protocols can still be useful in applications relying on a reverse-
proxy (see Section 6).
The rest of this section considers the new values of 'tp_id'
registered by this document (see Section 11.2), and specifies:
* The encoding for the elements of 'tp_info' following 'tp_id' (see
Section 3.1).
* The port number assumed by the client if 'srv_port' in 'tp_info'
specifies the CBOR simple value Null (see Section 3.2).
The Response-Forwarding Option is of class U in terms of OSCORE
processing (see Section 4.1 of [RFC8613]).
3.1. Encoding of Server Address
This document defines some values used as transport protocol
identifiers, whose respective new entries are included in the "CoAP
Transport Information" registry defined in Section 14.5 of
[I-D.ietf-core-observe-multicast-notifications].
For each of these values, the following table summarizes the elements
specified under the "Srv Addr" and "Req Info" columns of the
registry, together with their CBOR encoding and short description.
While not listed here for brevity, the element 'tp_id' is always
present as a CBOR integer in the element set "Srv Addr".
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+----------+-------------+----------+--------------+---------------+
| 'tp_id' | Element Set | Element | CBOR Type | Description |
| Values | | | | |
+----------+-------------+----------+--------------+---------------+
| 2, 3, 4, | Srv Addr | srv_host | #6.260(bstr) | Address of |
| 5, 6 | | | (*) | the server |
| | +----------+--------------+---------------+
| | | srv_port | uint / null | Port number |
| | | | | of the server |
| +-------------+----------+--------------+---------------+
| | Req Info | cli_host | #6.260(bstr) | Address of |
| | | | (*) | the client |
| | +----------+--------------+---------------+
| | | cli_port | uint | Port number |
| | | | | of the client |
+----------+-------------+----------+--------------+---------------+
* The CBOR byte string is tagged and identified by the
CBOR tag 260 "Network Address (IPv4 or IPv6 or MAC Address)".
3.2. Default Values of the Server Port Number
If the 'srv_port' element in the 'tp_info' array specifies the CBOR
simple value Null, the client MUST assume the following value as port
number where to send individual requests intended to the server,
based on the value of 'tp_id'.
* If 'tp_id' is equal to 2, i.e., CoAP over UDP secured with DTLS,
the default port number 5684 as defined in [RFC7252].
* If 'tp_id' is equal to 3, i.e., CoAP over TCP, the default port
number 5683 as defined in [RFC8323].
* If 'tp_id' is equal to 4, i.e., CoAP over TCP secured with TLS,
the default port number 5684 as defined in [RFC8323].
* If 'tp_id' is equal to 5, i.e., CoAP over WebSockets, the default
port number 80 as defined in [RFC8323].
* If 'tp_id' is equal to 6, i.e., CoAP over WebSockets secured with
TLS, the default port number 443 as defined in [RFC8323].
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.
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* 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 [RFC6347][I-D.ietf-tls-dtls13] 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.tiloca-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].
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.
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* 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 constructed from
the URI-* options (see Section 3.5.1 of
[I-D.ietf-core-groupcomm-bis]).
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.
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* 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-Signaling 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-Signaling 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.
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.
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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 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-Signaling
Option, as a confirmation that the client is fine to receive
multiple CoAP responses matching with the same original request.
If the Multicast-Signaling 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-Signaling Option with an empty (zero-length) value,
indicating that the Multicast-Signaling 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.
4. The proxy retrieves the value T' from the Multicast-Signaling
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-Signaling 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.
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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.
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.
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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 Response-Forwarding Option defined in
Section 3 into the response. The proxy specifies as option value
the addressing information of the server generating the response,
encoded as defined in Section 3. In particular:
* The 'srv_addr' element of the 'srv_info' array MUST specify
the server IPv6 address if the multicast request was destined
for an IPv6 multicast address, and MUST specify the server
IPv4 address if the multicast request was destined for an IPv4
multicast address.
* If present, the 'srv_port' element of the 'srv_info' array
MUST specify the port number of the server as the source port
number of the response. This element MUST be present if the
source port number of the response differs from the port
number specified in the group URI of the original unicast CoAP
group request (see Section 5.1.1). Otherwise, the 'srv_port'
element MAY be omitted.
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".
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 to 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.
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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 Response-
Forwarding 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. After that, 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.
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 the response according to the security association it
has with the proxy.
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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 identifies the origin server, whose addressing
information is specified as value of the Response-Forwarding
Option. If the port number is omitted in the value of the
Response-Forwarding Option, the client MUST assume that the port
number where to send unicast requests to the server -- in case
this is needed -- is the same port number specified in the group
URI of the original unicast group request sent to the proxy (see
Section 5.1.1).
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 an origin server again through the
proxy, the client is not required to understand or support the
transport protocol indicated in the Response-Forwarding Option,
as used between the proxy and the origin server, in case it
differs from "UDP" (1). That is, using the IPv4/IPv6 address
value and optional port value from the Response-Forwarding
Option, the client simply creates the correct URI for the
individual request, by means of the Proxy-Uri or Uri-Scheme
Option in the unicast request to the proxy. The client uses the
transport protocol it knows, and has used before, to send the
request to the proxy.
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 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 to 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, any 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.
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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.
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, 'bstr(X)' denotes a CBOR byte string with value the byte
serialization of X.
C P S1 S2
| | | |
|------------------------->| | |
| Src: C_ADDR:C_PORT | | |
| Dst: P_ADDR:P_PORT | | |
| Proxi-URI { | | |
| coap://G_ADDR:G_PORT/r | | |
| } | | |
| Multicast-Signaling: 60 | | |
| | | |
| | | |
| | | |
| | | |
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| | | |
| | 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 | | |
| Response-Forwarding { | | |
| [1, /*CoAP over UDP*/ | | |
| #6.260(bstr(S1_ADDR)) | | |
| ] | | |
| } | | |
| |<-----------------------------------|
| | Src: S2_ADDR:S2_PORT |
| | Dst: P_ADDR:P_PORT |
| | | |
| | | |
| | | |
|<-------------------------| | |
| Src: P_ADDR:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| Response-Forwarding { | | |
| [1, /*CoAP over UDP*/ | | |
| #6.260(bstr(S2_ADDR)), | | |
| S2_PORT | | |
| ] | | |
| } | | |
| /* At t = 60, P stops accepting | |
| responses for this request */ | |
| | | |
Figure 3: Workflow example with a forward-proxy
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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-Signaling Option is
effective also in such a context, in order for:
* The proxy to explictly 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 addional 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-Signaling Option, as defined in that
section.
Appendix A provides examples with a reverse-proxy.
6.1. Processing on the Client Side
If a 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 5.1.1. In particular, this
results in a request sent to the proxy including a Multicast-
Signaling Option.
The client processes the responses forwarded back by the proxy as
defined in Section 5.5.
6.2. Processing on the Proxy Side
If the proxy receives a request and determines that it should forward
it to a group of servers over IP multicast, then the proxy MUST
perform the steps defined in Section 5.2.
In particular, when such request does not include a Multicast-
Signaling Option, the proxy explicitly reveals itself as a reverse-
proxy, by sending a 4.00 (Bad Request) response including a
Multicast-Signaling Option with empty (zero-length) value.
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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 to 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 to 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 to 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 in 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 to 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
This last limitation requires more thinking, but it can be actually
possible to have revalidation between proxy and server also in this
case, if OSCORE is used in the leg between proxy and server. Then,
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 the proxy wants to revalidate a cached response by making a GET
or FETCH request on the related unicast URI, then the proxy and
the origin server have to use OSCORE to (further) protect the
exchanged request and response, as defined in
[I-D.tiloca-core-oscore-capable-proxies].
]
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
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This last limitation requires more thinking, but it can be actually
possible to have revalidation between proxy and servers also in this
case, if Group OSCORE is used in the leg between proxy and servers.
Then, 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 the proxy wants to revalidate a cached response by making a GET
or FETCH group request on the related group request URI, then the
proxy and the origin servers have to use Group OSCORE to (further)
protect the exchanged request and responses, as defined in
[I-D.tiloca-core-oscore-capable-proxies].
]
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.
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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.
* 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 to 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., the set J associated to 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.
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That is, the 2.03 (Valid) response from the proxy indicates to the
client that the stored responses idenfied 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
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.
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].
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That is, even though the servers would produce the same plain CoAP
responses in reply 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].
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
This last limitation requires more thinking, but it can be actually
possible to have revalidation between proxy and servers also in this
case, if (Group) OSCORE is used in the leg between proxy and servers.
Then, the following text can be used to replace the last paragraph
above.
If the proxy wants to revalidate a cached response by making a GET or
FETCH request on the related unicast URI, then the proxy and the
origin server have to use OSCORE to further protect the exchanged
request and response, as defined in
[I-D.tiloca-core-oscore-capable-proxies].
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If the proxy wants to revalidate a cached response by making a GET or
FETCH group request on the related group request URI, then the proxy
and the origin servers have to use Group OSCORE to further protect
the exchanged request and responses, as defined in
[I-D.tiloca-core-oscore-capable-proxies].
]
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.
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.
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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-Signaling
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.
- 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-
Signaling 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
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to the origin client. The proxy SHOULD include a Multicast-
Signaling Option, set to the minimum value T' that would be
acceptable in the Multicast-Signaling 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-Signaling 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-Signaling 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-Signaling Option of the request received from
the (previous hop proxy closer to the) origin client.
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.
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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, with no modifications.
Otherwise, the proxy performs the steps defined in Section 5.4, with
the following differences.
* The proxy skips step 1. In particular, the proxy MUST NOT remove,
alter or replace the Response-Forwarding Option.
* 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 to 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.
* 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. After that, as long as the observation for the target
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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-Signaling Header Field
The HTTP Multicast-Signaling header field (see Section 11.3) is used
for carrying the content otherwise specified in the CoAP Multicast-
Signaling 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-Signaling header field value
is as follows.
Multicast-Signaling = *DIGIT
When translating a CoAP message into an HTTP message, the HTTP
Multicast-Signaling header field is set with the content of the CoAP
Multicast-Signaling 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.
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9.2. The HTTP Response-Forwarding Header Field
The HTTP Response-Forwarding header field (see Section 11.3) is used
for carrying the content otherwise specified in the CoAP Response-
Forwarding Option defined in Section 3.
Using the Uniform Resource Identifier (URI) syntax components defined
in [RFC3986], the HTTP Response-Forwarding header field value is as
follows.
scheme = <scheme, see Section 3.1 of [RFC3986]>
authority = <authority, see Section 3.2 of [RFC3986]>
Response-Forwarding = scheme "://" authority
In particular:
* The scheme component indicates the URI scheme otherwise specified
in the CoAP Response-Forwarding Option, as per the 'tp_id' element
of the 'tp_info' array (see Section 3). That is, the 'tp_id'
element with integer value 1 results in the scheme "coap".
* The authority component indicates the URI authority otherwise
specified in the CoAP Response-Forwarding Option, as per the
'srv_host' and 'srv_port' elements of the 'tp_info' array (see
Section 3).
When translating a CoAP message into an HTTP message, the HTTP
Response-Forwarding header field is set to the URI specified in the
CoAP Response-Forwarding Option, as per the rules defined above. If
the 'srv_port' element of the 'tp_info' array is the CBOR simple
value Null, the URI authority of the header field includes a default
port number depending on the value of the 'tp_id' element of the
'tp_info' array, as per Section 3.2.
When translating an HTTP message into a CoAP message, the CoAP
Response-Forwarding Option is set to the URI specified by the HTTP
Response-Forwarding header field. In particular, the URI is encoded
according to the format specified in Section 3.
9.3. The HTTP Group-ETag Header Field
The HTTP Group-ETag header field (see Section 11.3) is used for
carrying the content otherwise specified in the CoAP Group-ETag
Option defined in Section 7.3.
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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.
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].
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4. The client includes the HTTP Multicast-Signaling 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.
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-Signaling 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-Signaling 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
Response-Forwarding 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 Response-Forwarding Option are defined in
Section 9.2.
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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.
When the timout expires, 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
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 [RFC7230].
* 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 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.
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4. For each individual HTTP response RESP, the client performs the
following steps.
* 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 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 Response-
Forwarding 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 correspoding to
the HTTP response RESP, by means of its addressing information
specified as value of the HTTP Response-Forwarding 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-Signaling: 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.
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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>
Response-Forwarding: coap://S1_ADDR
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>
Response-Forwarding: coap://S2_ADDR:S2_PORT
Body: More than done!
--batch_foo_bar
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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Note that, if the group request is (also) protected with Group
OSCORE, i.e., end-to-end between the client and the servers, the
proxy can authenticate the client by successfully verifying the
signature embedded in the group request. However, this requires
that, for each client to authenticate, the proxy stores the public
key used by that client in the OSCORE group, which 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-Signaling Option
The Multicast-Signaling 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 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.tiloca-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. Response-Forwarding Option
The Response-Forwarding 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 Response-Forwarding 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.tiloca-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 to 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 to
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.tiloca-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-CoAP Proxies
[ TODO
Revisit security considerations from [RFC8075]
]
11. IANA Considerations
This document has the following actions for IANA.
11.1. CoAP Option Numbers Registry
IANA is asked to enter the following option numbers to the "CoAP
Option Numbers" registry within the "CoRE Parameters" registry group.
+--------+---------------------+-------------------+
| Number | Name | Reference |
+--------+---------------------+-------------------+
| TBD1 | Multicast-Signaling | [[this document]] |
+--------+---------------------+-------------------+
| TBD2 | Response-Forwarding | [[this document]] |
+--------+---------------------+-------------------+
| TBD3 | Group-ETag | [[this document]] |
+--------+---------------------+-------------------+
11.2. CoAP Transport Information Registry
IANA is asked to add the following entries to the "CoAP Transport
Information" registry defined in Section 14.5 of
[I-D.ietf-core-observe-multicast-notifications].
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+------------+-------------+-------+----------+-----------+-----------+
| Transport | Description | Value | Srv Addr | Req Info | Reference |
| Protocol | | | | | |
+------------+-------------+-------+----------+-----------+-----------+
| UDP | UDP with | 2 | tp_id | token | [This |
| secured | DTLS is | | srv_host | cli_host | document] |
| with DTLS | used as per | | srv_port | ?cli_port | |
| | RFC8323 | | | | |
+------------+-------------+-------+----------+-----------+-----------+
| TCP | TCP is used | 3 | tp_id | token | [This |
| | as per | | srv_host | cli_host | document] |
| | RFC8323 | | srv_port | ?cli_port | |
+------------+-------------+-------+----------+-----------+-----------+
| TCP | TCP with | 4 | tp_id | token | [This |
| secured | TLS is | | srv_host | cli_host | document] |
| with TLS | used as per | | srv_port | ?cli_port | |
| | RFC8323 | | | | |
+------------+-------------+-------+----------+-----------+-----------+
| WebSockets | WebSockets | 5 | tp_id | token | [This |
| | are used as | | srv_host | cli_host | document] |
| | per RFC8323 | | srv_port | ?cli_port | |
+------------+-------------+-------+----------+-----------+-----------+
| WebSockets | WebSockets | 6 | tp_id | token | [This |
| secured | with TLS | | srv_host | cli_host | document] |
| with TLS | are used as | | srv_port | ?cli_port | |
| | per RFC8323 | | | | |
+------------+-------------+-------+----------+-----------+-----------+
11.3. Header Field Registrations
IANA is asked to enter the following HTTP header fields to the
"Message Headers" registry.
+---------------------+----------+----------+-----------+
| Header Field Name | Protocol | Status | Reference |
+---------------------+----------+----------+-----------+
| Multicast-Signaling | http | standard | [This |
| | | | document] |
+---------------------+----------+----------+-----------+
| Response-Forwarding | http | standard | [This |
| | | | document] |
+---------------------+----------+----------+-----------+
| Group-ETag | http | standard | [This |
| | | | document] |
+---------------------+----------+----------+-----------+
12. References
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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-
05, 25 October 2021, <https://www.ietf.org/archive/id/
draft-ietf-core-groupcomm-bis-05.txt>.
[I-D.ietf-core-observe-multicast-notifications]
Tiloca, M., Höglund, R., Amsüss, C., and F. Palombini,
"Observe Notifications as CoAP Multicast Responses", Work
in Progress, Internet-Draft, draft-ietf-core-observe-
multicast-notifications-02, 25 October 2021,
<https://www.ietf.org/archive/id/draft-ietf-core-observe-
multicast-notifications-02.txt>.
[I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
and J. Park, "Group OSCORE - Secure Group Communication
for CoAP", Work in Progress, Internet-Draft, draft-ietf-
core-oscore-groupcomm-13, 25 October 2021,
<https://www.ietf.org/archive/id/draft-ietf-core-oscore-
groupcomm-13.txt>.
[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/info/rfc2046>.
[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/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/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/info/rfc5234>.
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[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[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/info/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/info/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/info/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/info/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/info/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/info/rfc8610>.
[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/info/rfc8613>.
[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/info/rfc8949>.
12.2. Informative References
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[I-D.amsuess-core-cachable-oscore]
Amsüss, C. and M. Tiloca, "Cacheable OSCORE", Work in
Progress, Internet-Draft, draft-amsuess-core-cachable-
oscore-02, 12 July 2021, <https://www.ietf.org/archive/id/
draft-amsuess-core-cachable-oscore-02.txt>.
[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://www.ietf.org/archive/id/draft-bormann-coap-misc-
27.txt>.
[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-12, 25 October
2021, <https://www.ietf.org/archive/id/draft-ietf-ace-key-
groupcomm-oscore-12.txt>.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
dtls13-43, 30 April 2021, <https://www.ietf.org/internet-
drafts/draft-ietf-tls-dtls13-43.txt>.
[I-D.tiloca-core-oscore-capable-proxies]
Tiloca, M. and R. Hoeglund, "OSCORE-capable Proxies", Work
in Progress, Internet-Draft, draft-tiloca-core-oscore-
capable-proxies-01, 25 October 2021,
<https://www.ietf.org/archive/id/draft-tiloca-core-oscore-
capable-proxies-01.txt>.
[I-D.tiloca-core-oscore-discovery]
Tiloca, M., Amsuess, C., and P. V. D. Stok, "Discovery of
OSCORE Groups with the CoRE Resource Directory", Work in
Progress, Internet-Draft, draft-tiloca-core-oscore-
discovery-10, 25 October 2021,
<https://www.ietf.org/archive/id/draft-tiloca-core-oscore-
discovery-10.txt>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
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[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/info/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/info/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/info/rfc8446>.
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 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 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, 'bstr(X)' denotes a CBOR byte string with value the byte
serialization of X.
A.1. Example 1
The example shown in Figure 5 considers a reverse-proxy that stands
in for both the whole group of servers and for each of those servers
(e.g., acting as a firewall).
In particular:
* The address 'group1.com' resolves to 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.
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* 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.
Note that this type of reverse-proxy implementation requires the
proxy to use (potentially) a large number of distinct IP addresses,
so it is not very scalable.
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: group1.com:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| 4.00 Bad Request | | |
| Multicast-Signaling: (empty) | | |
| Payload: "Please use | | |
| Multicast-Signaling" | | |
| | | |
|----------------------------->| | |
| Src: C_ADDR:C_PORT | | |
| Dst: group1.com:P_PORT | | |
| Multicast-Signaling: 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 | |
| | | |
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| | | |
|<-----------------------------| | |
| Src: group1.com:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| Response-Forwarding { | | |
| [3, /*CoAP over TCP*/ | | |
| #6.260(bstr(D1_ADDR)), | | |
| D1_PORT | | |
| ] | | |
| } | | |
| | | |
| |<-----------------------------------|
| | Src: S2_ADDR:S2_PORT |
| | Dst: P_ADDR:P_PORT |
| | | |
|<-----------------------------| | |
| Src: group1.com:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| Response-Forwarding { | | |
| [3, /*CoAP over TCP*/ | | |
| #6.260(bstr(D2_ADDR)), | | |
| D2_PORT | | |
| ] | | |
| } | | |
| | | |
| /* At t = 60, P stops accepting | |
| responses for this request */ | |
| | | |
| | | |
|----------------------------->| /* Request intended | |
| Src: C_ADDR:C_PORT | only to S1 */ | |
| Dst: D1_ADDR:D1_PORT | | |
| 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 | | |
| | | |
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Figure 5: Workflow example with reverse-proxy standing in for
both the whole group of servers and each individual server
A.2. Example 2
The example shown in Figure 6 builds on the example in Appendix A.1.
However, it considers a reverse-proxy that stands in for only the
whole group of servers, but not for each individual server.
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: group1.com:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| 4.00 Bad Request | | |
| Multicast-Signaling: (empty) | | |
| Payload: "Please use | | |
| Multicast-Signaling" | | |
| | | |
| | | |
|----------------------------->| | |
| Src: C_ADDR:C_PORT | | |
| Dst: group1.com:P_PORT | | |
| Multicast-Signaling: 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 */ | |
| | | |
| | | |
| |<---------------------| |
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| | Src: S1_ADDR:S1_PORT | |
| | Dst: P_ADDR:P_PORT | |
| | | |
|<-----------------------------| | |
| Dst: group1.com:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| Response-Forwarding { | | |
| [1, /*CoAP over UDP*/ | | |
| #6.260(bstr(S1_ADDR)), | | |
| S1_PORT | | |
| ] | | |
| } | | |
| | | |
| |<-----------------------------------|
| | Src: S2_ADDR:S2_PORT |
| | Dst: P_ADDR:P_PORT |
| | | |
|<-----------------------------| | |
| Dst: group1.com:P_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| Response-Forwarding { | | |
| [1, /*CoAP over UDP*/ | | |
| #6.260(bstr(S2_ADDR)), | | |
| S2_PORT | | |
| ] | | |
| } | | |
| | | |
| | | |
| /* At t = 60, P stops accepting | |
| responses for this request */ | |
| | | |
| | | |
| | | |
|---------------------------------------------------->| |
| Src: C_ADDR:C_PORT | /* Request intended | |
| Dst: S1.ADDR:S1_PORT | only to S1 */ | |
| Uri-Path: /r | | |
| | | |
| | | |
|<----------------------------------------------------| |
| Src: S1.ADDR:S1_PORT | | |
| Dst: C_ADDR:C_PORT | | |
| | | |
Figure 6: Workflow example with reverse-proxy standing in for
only the whole group of servers, but not for each individual
server
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Acknowledgments
The authors sincerely thank Christian Amsuess, Jim Schaad and Goeran
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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