CoRE Working Group Z. Shelby
Internet-Draft Sensinode
Intended status: Standards Track K. Hartke
Expires: January 9, 2012 C. Bormann
Universitaet Bremen TZI
B. Frank
SkyFoundry
July 8, 2011
Constrained Application Protocol (CoAP)
draft-ietf-core-coap-07
Abstract
This document specifies the Constrained Application Protocol (CoAP),
a specialized web transfer protocol for use with constrained networks
and nodes for machine-to-machine applications such as smart energy
and building automation. These constrained nodes often have 8-bit
microcontrollers with small amounts of ROM and RAM, while networks
such as 6LoWPAN often have high packet error rates and a typical
throughput of 10s of kbit/s. CoAP provides a method/response
interaction model between application end-points, supports built-in
resource discovery, and includes key web concepts such as URIs and
content-types. CoAP easily translates to HTTP for integration with
the web while meeting specialized requirements such as multicast
support, very low overhead and simplicity for constrained
environments.
Status of this Memo
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on January 9, 2012.
Copyright Notice
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Copyright (c) 2011 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Features . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. Constrained Application Protocol . . . . . . . . . . . . . . . 8
2.1. Messaging Model . . . . . . . . . . . . . . . . . . . . . 8
2.2. Request/Response Model . . . . . . . . . . . . . . . . . . 10
2.3. Intermediaries and Caching . . . . . . . . . . . . . . . . 12
2.4. Resource Discovery . . . . . . . . . . . . . . . . . . . . 12
3. Message Syntax . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1. Message Format . . . . . . . . . . . . . . . . . . . . . . 13
3.1.1. Message Size Implementation Considerations . . . . . 14
3.2. Option Format . . . . . . . . . . . . . . . . . . . . . . 15
4. Message Semantics . . . . . . . . . . . . . . . . . . . . . . 16
4.1. Reliable Messages . . . . . . . . . . . . . . . . . . . . 17
4.2. Unreliable Messages . . . . . . . . . . . . . . . . . . . 18
4.3. Message Types . . . . . . . . . . . . . . . . . . . . . . 19
4.3.1. Confirmable (CON) . . . . . . . . . . . . . . . . . . 19
4.3.2. Non-Confirmable (NON) . . . . . . . . . . . . . . . . 19
4.3.3. Acknowledgement (ACK) . . . . . . . . . . . . . . . . 20
4.3.4. Reset (RST) . . . . . . . . . . . . . . . . . . . . . 20
4.4. Multicast . . . . . . . . . . . . . . . . . . . . . . . . 20
4.5. Congestion Control . . . . . . . . . . . . . . . . . . . . 20
5. Request/Response Semantics . . . . . . . . . . . . . . . . . . 21
5.1. Requests . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.2. Responses . . . . . . . . . . . . . . . . . . . . . . . . 21
5.2.1. Piggy-backed . . . . . . . . . . . . . . . . . . . . 23
5.2.2. Separate . . . . . . . . . . . . . . . . . . . . . . 23
5.2.3. Non-Confirmable . . . . . . . . . . . . . . . . . . . 24
5.3. Request/Response Matching . . . . . . . . . . . . . . . . 24
5.4. Options . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.4.1. Critical/Elective . . . . . . . . . . . . . . . . . . 26
5.4.2. Length . . . . . . . . . . . . . . . . . . . . . . . 26
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5.4.3. Default Values . . . . . . . . . . . . . . . . . . . 26
5.4.4. Repeating Options . . . . . . . . . . . . . . . . . . 26
5.4.5. Option Numbers . . . . . . . . . . . . . . . . . . . 27
5.5. Payload . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.6. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 28
5.6.1. Freshness Model . . . . . . . . . . . . . . . . . . . 28
5.6.2. Validation Model . . . . . . . . . . . . . . . . . . 29
5.7. Proxying . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.8. Method Definitions . . . . . . . . . . . . . . . . . . . . 30
5.8.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 31
5.8.2. POST . . . . . . . . . . . . . . . . . . . . . . . . 31
5.8.3. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 31
5.8.4. DELETE . . . . . . . . . . . . . . . . . . . . . . . 32
5.9. Response Code Definitions . . . . . . . . . . . . . . . . 32
5.9.1. Success 2.xx . . . . . . . . . . . . . . . . . . . . 32
5.9.2. Client Error 4.xx . . . . . . . . . . . . . . . . . . 33
5.9.3. Server Error 5.xx . . . . . . . . . . . . . . . . . . 34
5.10. Option Definitions . . . . . . . . . . . . . . . . . . . . 35
5.10.1. Token . . . . . . . . . . . . . . . . . . . . . . . . 36
5.10.2. Uri-Host, Uri-Port, Uri-Path and Uri-Query . . . . . 36
5.10.3. Proxy-Uri . . . . . . . . . . . . . . . . . . . . . . 37
5.10.4. Content-Type . . . . . . . . . . . . . . . . . . . . 38
5.10.5. Accept . . . . . . . . . . . . . . . . . . . . . . . 38
5.10.6. Max-Age . . . . . . . . . . . . . . . . . . . . . . . 39
5.10.7. ETag . . . . . . . . . . . . . . . . . . . . . . . . 39
5.10.8. Location-Path and Location-Query . . . . . . . . . . 39
5.10.9. If-Match . . . . . . . . . . . . . . . . . . . . . . 40
5.10.10. If-None-Match . . . . . . . . . . . . . . . . . . . . 40
6. CoAP URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 41
6.1. coap URI Scheme . . . . . . . . . . . . . . . . . . . . . 41
6.2. coaps URI Scheme . . . . . . . . . . . . . . . . . . . . . 42
6.3. Normalization and Comparison Rules . . . . . . . . . . . . 42
6.4. Decomposing URIs into Options . . . . . . . . . . . . . . 43
6.5. Composing URIs from Options . . . . . . . . . . . . . . . 44
7. Finding and Addressing CoAP End-Points . . . . . . . . . . . . 45
7.1. Resource Discovery . . . . . . . . . . . . . . . . . . . . 45
7.1.1. Content-type code 'ct' attribute . . . . . . . . . . 45
7.2. Default Ports . . . . . . . . . . . . . . . . . . . . . . 46
8. HTTP Mapping . . . . . . . . . . . . . . . . . . . . . . . . . 46
8.1. CoAP-HTTP Mapping . . . . . . . . . . . . . . . . . . . . 47
8.1.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 47
8.1.2. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 48
8.1.3. DELETE . . . . . . . . . . . . . . . . . . . . . . . 48
8.1.4. POST . . . . . . . . . . . . . . . . . . . . . . . . 48
8.2. HTTP-CoAP Mapping . . . . . . . . . . . . . . . . . . . . 48
8.2.1. OPTIONS and TRACE . . . . . . . . . . . . . . . . . . 49
8.2.2. GET . . . . . . . . . . . . . . . . . . . . . . . . . 49
8.2.3. HEAD . . . . . . . . . . . . . . . . . . . . . . . . 50
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8.2.4. POST . . . . . . . . . . . . . . . . . . . . . . . . 50
8.2.5. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 50
8.2.6. DELETE . . . . . . . . . . . . . . . . . . . . . . . 50
8.2.7. CONNECT . . . . . . . . . . . . . . . . . . . . . . . 51
9. Protocol Constants . . . . . . . . . . . . . . . . . . . . . . 51
10. Security Considerations . . . . . . . . . . . . . . . . . . . 51
10.1. Securing CoAP with DTLS . . . . . . . . . . . . . . . . . 53
10.1.1. SharedKey and MultiKey Modes . . . . . . . . . . . . 53
10.1.2. Certificate Mode . . . . . . . . . . . . . . . . . . 54
10.2. Using CoAP with IPsec . . . . . . . . . . . . . . . . . . 54
10.3. Threat analysis and protocol limitations . . . . . . . . . 55
10.3.1. Protocol Parsing, Processing URIs . . . . . . . . . . 55
10.3.2. Proxying and Caching . . . . . . . . . . . . . . . . 56
10.3.3. Risk of amplification . . . . . . . . . . . . . . . . 56
10.3.4. Cross-Protocol Attacks . . . . . . . . . . . . . . . 57
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 59
11.1. CoAP Code Registry . . . . . . . . . . . . . . . . . . . . 59
11.1.1. Method Codes . . . . . . . . . . . . . . . . . . . . 59
11.1.2. Response Codes . . . . . . . . . . . . . . . . . . . 60
11.2. Option Number Registry . . . . . . . . . . . . . . . . . . 62
11.3. Media Type Registry . . . . . . . . . . . . . . . . . . . 63
11.4. URI Scheme Registration . . . . . . . . . . . . . . . . . 64
11.5. Secure URI Scheme Registration . . . . . . . . . . . . . . 65
11.6. Service Name and Port Number Registration . . . . . . . . 66
11.7. Secure Service Name and Port Number Registration . . . . . 67
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 67
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 68
13.1. Normative References . . . . . . . . . . . . . . . . . . . 68
13.2. Informative References . . . . . . . . . . . . . . . . . . 70
Appendix A. Integer Option Value Format . . . . . . . . . . . . . 71
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 72
Appendix C. URI Examples . . . . . . . . . . . . . . . . . . . . 78
Appendix D. Changelog . . . . . . . . . . . . . . . . . . . . . . 79
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 85
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1. Introduction
The use of web services on the Internet has become ubiquitous in most
applications, and depends on the fundamental Representational State
Transfer (REST) architecture of the web.
The Constrained RESTful Environments (CoRE) working group aims at
realizing the REST architecture in a suitable form for the most
constrained nodes (e.g. 8-bit microcontrollers with limited RAM and
ROM) and networks (e.g. 6LoWPAN). Constrained networks like 6LoWPAN
support the expensive fragmentation of IPv6 packets into small link-
layer frames. One design goal of CoAP has been to keep message
overhead small, thus limiting the use of fragmentation.
One of the main goals of CoAP is to design a generic web protocol for
the special requirements of this constrained environment, especially
considering energy, building automation and other M2M applications.
The goal of CoAP is not to blindly compress HTTP [RFC2616], but
rather to realize a subset of REST common with HTTP but optimized for
M2M applications. Although CoAP could be used for compressing simple
HTTP interfaces, it more importantly also offers features for M2M
such as built-in discovery, multicast support and asynchronous
message exchanges.
This document specifies the Constrained Application Protocol (CoAP),
which easily translates to HTTP for integration with the existing web
while meeting specialized requirements such as multicast support,
very low overhead and simplicity for constrained environments and M2M
applications.
1.1. Features
CoAP has the following main features:
o Constrained web protocol fulfilling M2M requirements.
o UDP binding with optional reliability supporting unicast and
multicast requests.
o Asynchronous message exchanges.
o Low header overhead and parsing complexity.
o URI and Content-type support.
o Simple proxy and caching capabilities.
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o A stateless HTTP mapping, allowing proxies to be built providing
access to CoAP resources via HTTP in a uniform way or for HTTP
simple interfaces to be realized alternatively over CoAP.
o Security binding to Datagram Transport Layer Security (DTLS).
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification requires readers to be familiar with all the terms
and concepts that are discussed in [RFC2616]. In addition, this
specification defines the following terminology:
Piggy-backed Response
A Piggy-backed Response is included right in a CoAP
Acknowledgement (ACK) message that is sent to acknowledge receipt
of the Request for this Response (Section 5.2.1).
Separate Response
When a Confirmable message carrying a Request is acknowledged with
an empty message (e.g., because the server doesn't have the answer
right away), a Separate Response is sent in a separate message
exchange (Section 5.2.2).
Critical Option
An option that would need to be understood by the end-point
receiving the message in order to properly process the message
(Section 5.4.1). Note that the implementation of critical options
is, as the name "Option" implies, generally optional: unsupported
critical options lead to rejection of the message.
Elective Option
An option that is intended be ignored by an end-point that does
not understand it, which nonetheless still can correctly process
the message (Section 5.4.1).
Resource Discovery
The process where a CoAP client queries a server for its list of
hosted resources (i.e., links, Section 7.1).
End-Point
An entity participating in the CoAP protocol. Colloquially, a
synonym is "Node", although "Host" would be more consistent with
Internet standards usage.
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Sender
The originating end-point of a message.
Recipient
The destination end-point of a message.
Client
The originating end-point of a request; the destination end-point
of a response.
Server
The destination end-point of a request; the originating end-point
of a response.
Origin Server
The server on which a given resource resides or is to be created.
Intermediary
A CoAP end-point that acts both as a server and as a client
towards (possibly via further intermediaries) an origin server.
There are two common forms of intermediary: proxy and reverse
proxy. In some cases, a single end-point might act as an origin
server, proxy, or reverse proxy, switching behavior based on the
nature of each request.
Proxy
A "proxy" is an end-point selected by a client, usually via local
configuration rules, to perform requests on behalf of the client,
doing any necessary translations. Some translations are minimal,
such as for proxy requests for "coap" URIs, whereas other requests
might require translation to and from entirely different
application-layer protocols.
Reverse Proxy
A "reverse proxy" is an end-point that acts as a layer above some
other server(s) and satisfies requests on behalf of these, doing
any necessary translations. Unlike a proxy, a reverse proxy
receives requests as if it was the origin server for the target
resource; the requesting client will not be aware that it is
communicating with a reverse proxy.
In this specification, the term "byte" is used in its now customary
sense as a synonym for "octet".
In this specification, the operator "^" stands for exponentiation.
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2. Constrained Application Protocol
The interaction model of CoAP is similar to the client/server model
of HTTP. However, machine-to-machine interactions typically result
in a CoAP implementation acting in both client and server roles
(called an end-point). A CoAP request is equivalent to that of HTTP,
and is sent by a client to request an action (using a method code) on
a resource (identified by a URI) on a server. The server then sends
a response with a response code; this response may include a resource
representation.
Unlike HTTP, CoAP deals with these interchanges asynchronously over a
datagram-oriented transport such as UDP. This is done logically
using a layer of messages that supports optional reliability (with
exponential back-off). CoAP defines four types of messages:
Confirmable, Non-Confirmable, Acknowledgement, Reset; method codes
and response codes included in some of these messages make them carry
requests or responses. The basic exchanges of the four types of
messages are transparent to the request/response interactions.
One could think of CoAP logically as using a two-layer approach, a
CoAP messaging layer used to deal with UDP and the asynchronous
nature of the interactions, and the request/response interactions
using Method and Response codes (see Figure 1). CoAP is however a
single protocol, with messaging and request/response just features of
the CoAP header.
+----------------------+
| Application |
+----------------------+
+----------------------+
| Requests/Responses |
|----------------------| CoAP
| Messages |
+----------------------+
+----------------------+
| UDP |
+----------------------+
Figure 1: Abstract layering of CoAP
2.1. Messaging Model
The CoAP messaging model is based on the exchange of messages over
UDP between end-points.
CoAP uses a short fixed-length binary header (4 bytes) that may be
followed by compact binary options and a payload. This message
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format is shared by requests and responses. The CoAP message format
is specified in Section 3. Each message contains a Message ID used
to detect duplicates and for optional reliability.
Reliability is provided by marking a message as Confirmable (CON). A
Confirmable message is retransmitted using a default timeout and
exponential back-off between retransmissions, until the recipient
sends an Acknowledgement message (ACK) with the same Message ID (for
example, 0x7d34) from the corresponding end-point; see Figure 2.
When a recipient is not able to process a Confirmable message, it
replies with a Reset message (RST) instead of an Acknowledgement
(ACK).
Client Server
| |
| CON [0x7d34] |
+----------------->|
| |
| ACK [0x7d34] |
|<-----------------+
| |
Figure 2: Reliable message delivery
A message that does not require reliable delivery, for example each
single measurement out of a stream of sensor data, can be sent as a
Non-confirmable message (NON). These are not acknowledged, but still
have a Message ID for duplicate detection; see Figure 3.
Client Server
| |
| NON [0x01a0] |
+----------------->|
| |
Figure 3: Unreliable message delivery
See Section 4 for details of CoAP messages.
As CoAP is based on UDP, it also supports the use of multicast IP
destination addresses, enabling multicast CoAP requests. Section 4.4
discusses the proper use of CoAP messages with multicast addresses
and precautions for avoiding response congestion.
Several security modes are defined for CoAP in Section 10 ranging
from no security to certificate-based security. The use of IPsec
along with a binding to DTLS are specified for securing the protocol.
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2.2. Request/Response Model
CoAP request and response semantics are carried in CoAP messages,
which include either a method code or response code, respectively.
Optional (or default) request and response information, such as the
URI and payload content-type are carried as CoAP options. A Token
Option is used to match responses to requests independently from the
underlying messages (Section 5.3).
A request is carried in a Confirmable (CON) or Non-confirmable (NON)
message, and if immediately available, the response to a request
carried in a Confirmable message is carried in the resulting
Acknowledgement (ACK) message. This is called a piggy-backed
response, detailed in Section 5.2.1. Two examples for a basic GET
request with piggy-backed response are shown in Figure 4.
Client Server Client Server
| | | |
| CON [0xbc90] | | CON [0xbc91] |
| GET /temperature | | GET /temperature |
| (Token 0x71) | | (Token 0x72) |
+----------------->| +----------------->|
| | | |
| ACK [0xbc90] | | ACK [0xbc91] |
| 2.05 Content | | 4.04 Not Found |
| (Token 0x71) | | (Token 0x72) |
| "22.5 C" | | "Not found" |
|<-----------------+ |<-----------------+
| | | |
Figure 4: Two GET requests with piggy-backed responses, one
successful, one not found
If the server is not able to respond immediately to a request carried
in a Confirmable message, it simply responds with an empty
Acknowledgement message so that the client can stop retransmitting
the request. When the response is ready, the server sends it in a
new Confirmable message (which then in turn needs to be acknowledged
by the client). This is called a separate response, as illustrated
in Figure 5 and described in more detail in Section 5.2.2.
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Client Server
| |
| CON [0x7a10] |
| GET /temperature |
| (Token 0x73) |
+----------------->|
| |
| ACK [0x7a10] |
|<-----------------+
| |
... Time Passes ...
| |
| CON [0x23bb] |
| 2.05 Content |
| (Token 0x73) |
| "22.5 C" |
|<-----------------+
| |
| ACK [0x23bb] |
+----------------->|
| |
Figure 5: A GET request with a separate response
Likewise, if a request is sent in a Non-Confirmable message, then the
response is usually sent using a new Non-Confirmable message,
although the server may send a Confirmable message. This type of
exchange is illustrated in Figure 6.
Client Server
| |
| NON [0x7a11] |
| GET /temperature |
| (Token 0x74) |
+----------------->|
| |
| NON [0x23bc] |
| 2.05 Content |
| (Token 0x74) |
| "22.5 C" |
|<-----------------+
| |
Figure 6: A NON request and response
CoAP makes use of GET, PUT, POST and DELETE methods in a similar
manner to HTTP, with the semantics specified in Section 5.8. (Note
that the detailed semantics of CoAP methods are "almost, but not
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entirely unlike" those of HTTP methods: Intuition taken from HTTP
experience generally does apply well, but there are enough
differences that make it worthwhile to actually read the present
specification.)
URI support in a server is simplified as the client already parses
the URI and splits it into host, port, path and query components,
making use of default values for efficiency. Response codes
correspond to a small subset of HTTP response codes with a few CoAP
specific codes added, as defined in Section 5.9.
2.3. Intermediaries and Caching
The protocol supports the caching of responses in order to
efficiently fulfill requests. Simple caching is enabled using
freshness and validity information carried with CoAP responses. A
cache could be located in an end-point or an intermediary. Caching
functionality is specified in Section 5.6.
Proxying is useful in constrained networks for several reasons,
including network traffic limiting, to improve performance, to access
resources of sleeping devices or for security reasons. The proxying
of requests on behalf of another CoAP end-point is supported in the
protocol. The URI of the resource to request is included in the
request, while the destination IP address is set to the proxy. See
Section 5.7 for more information on proxy functionality.
As CoAP was designed according to the REST architecture and thus
exhibits functionality similar to that of the HTTP protocol, it is
quite straightforward to map between HTTP-CoAP or CoAP-HTTP. Such a
mapping may be used to realize an HTTP REST interface using CoAP, or
for converting between HTTP and CoAP. This conversion can be carried
out by a proxy, which converts the method or response code, content-
type and options to the corresponding HTTP feature. Section 8
provides more detail about HTTP mapping.
2.4. Resource Discovery
Resource discovery is important for machine-to-machine interactions,
and is supported using the CoRE Link Format
[I-D.ietf-core-link-format] as discussed in Section 7.1.
3. Message Syntax
CoAP is based on the exchange of short messages which, by default,
are transported over UDP (i.e. each CoAP message occupies the data
section of one UDP datagram). CoAP may be used with Datagram
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Transport Layer Security (DTLS) (see Section 10.1). It could also be
used over other transports such as TCP or SCTP, the specification of
which is out of this document's scope.
3.1. Message Format
CoAP messages are encoded in a simple binary format. A message
consists of a fixed-sized CoAP Header followed by options in Type-
Length-Value (TLV) format and a payload. The number of options is
determined by the header. The payload is made up of the bytes after
the options, if any; its length is calculated from the datagram
length.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver| T | OC | Code | Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Message Format
The fields in the header are defined as follows:
Version (Ver): 2-bit unsigned integer. Indicates the CoAP version
number. Implementations of this specification MUST set this field
to 1. Other values are reserved for future versions.
Type (T): 2-bit unsigned integer. Indicates if this message is of
type Confirmable (0), Non-Confirmable (1), Acknowledgement (2) or
Reset (3). See Section 4 for the semantics of these message
types.
Option Count (OC): 4-bit unsigned integer. Indicates the number of
options after the header. If set to 0, there are no options and
the payload (if any) immediately follows the header. The format
of options is defined below.
Code: 8-bit unsigned integer. Indicates if the message carries a
request (1-31) or a response (64-191), or is empty (0). (All
other code values are reserved.) In case of a request, the Code
field indicates the Request Method; in case of a response a
Response Code. Possible values are maintained in the CoAP Code
Registry (Section 11.1). See Section 5 for the semantics of
requests and responses.
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Message ID: 16-bit unsigned integer. Used for the detection of
message duplication, and to match messages of type
Acknowledgement/Reset and messages of type Confirmable. See
Section 4 for Message ID generation rules and how messages are
matched.
While specific link layers make it beneficial to keep CoAP messages
small enough to fit into their link layer packets (see Section 1),
this is a matter of implementation quality. The CoAP specification
itself provides only an upper bound to the message size. Messages
larger than an IP fragment result in undesired packet fragmentation.
A CoAP message, appropriately encapsulated, SHOULD fit within a
single IP packet (i.e., avoid IP fragmentation) and MUST fit within a
single IP datagram. If the Path MTU is not known for a destination,
an IP MTU of 1280 bytes SHOULD be assumed; if nothing is known about
the size of the headers, good upper bounds are 1152 bytes for the
message size and 1024 bytes for the payload size.
3.1.1. Message Size Implementation Considerations
Note that CoAP's choice of message size parameters works well with
IPv6 and with most of today's IPv4 paths. (However, with IPv4, it is
harder to absolutely ensure that there is no IP fragmentation. If
IPv4 support on unusual networks is a consideration, implementations
may want to limit themselves to more conservative IPv4 datagram sizes
such as 576 bytes; worse, the absolute minimum value of the IP MTU
for IPv4 is as low as 68 bytes, which would leave only 40 bytes minus
security overhead for a UDP payload. Implementations extremely
focused on this problem set might also set the IPv4 DF bit and
perform some form of path MTU discovery; this should generally be
unnecessary in most realistic use cases for CoAP, however.) A more
important kind of fragmentation in many constrained networks is that
on the adaptation layer (e.g., 6LoWPAN L2 packets are limited to 127
bytes including various overheads); this may motivate implementations
to be frugal in their packet sizes and to move to block-wise
transfers [I-D.ietf-core-block] when approaching three-digit message
sizes.
Note that message sizes are also of considerable importance to
implementations on constrained nodes. Many implementations will need
to allocate a buffer for incoming messages. If an implementation is
too constrained to allow for allocating the above-mentioned upper
bound, it could apply the following implementation strategy:
Implementations receiving a datagram into a buffer that is too small
are usually able to determine if the trailing portion of a datagram
was discarded and to retrieve the initial portion. So, if not all of
the payload, at least the CoAP header and options are likely to fit
within the buffer. A server can thus fully interpret a request and
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return a 4.13 (Request Entity Too Large) response code if the payload
was truncated. A client sending an idempotent request and receiving
a response larger than would fit in the buffer can repeat the request
with a suitable value for the Block Option [I-D.ietf-core-block].
3.2. Option Format
Options MUST appear in order of their Option Number (see
Section 5.4.5). A delta encoding is used between options, with the
Option Number for each Option calculated as the sum of its Option
Delta field and the Option Number of the preceding Option in the
message, if any, or zero otherwise. Multiple options with the same
Option Number can be included by using an Option Delta of zero.
Following the Option Delta, each option has a Length field which
specifies the length of the Option Value, in bytes. The Length field
can be extended by one byte for options with values longer than 14
bytes. The Option Value immediately follows the Length field.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Option Delta | Length | for 0..14
+---+---+---+---+---+---+---+---+
| Option Value ...
+---+---+---+---+---+---+---+---+
for 15..270:
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| Option Delta | 1 1 1 1 | Length - 15 |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| Option Value ...
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 8: Option Format
The fields in an option are defined as follows:
Option Delta: 4-bit unsigned integer. Indicates the difference
between the Option Number of this option and the previous option
(or zero for the first option). In other words, the Option Number
is calculated by simply summing the Option Delta fields of this
and previous options before it. The Option Numbers 14, 28, 42,
... are reserved for no-op options when they are sent with an
empty value (they are ignored) and can be used as "fenceposts" if
deltas larger than 15 would otherwise be required.
Length: Indicates the length of the Option Value, in bytes.
Normally Length is a 4-bit unsigned integer allowing value lengths
of 0-14 bytes. When the Length field is set to 15, another byte
is added as an 8-bit unsigned integer whose value is added to the
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15, allowing option value lengths of 15-270 bytes.
The length and format of the Option Value depends on the respective
option, which MAY define variable length values. Options defined in
this document make use of the following formats for option values:
uint: A non-negative integer which is represented in network byte
order using a variable number of bytes (see Appendix A).
string: A Unicode string which is encoded using UTF-8 [RFC3629] in
Net-Unicode form [RFC5198]. Note that here and in all other
places where UTF-8 encoding is used in the CoAP protocol, the
intention is that the encoded strings can be directly used and
compared as opaque byte strings by CoAP protocol
implementations. There is no expectation and no need to
perform normalization within a CoAP implementation unless
Unicode strings that are not known to be normalized are
imported from sources outside the CoAP protocol. Note also
that ASCII strings (that do not make use of special control
characters) are always valid UTF-8 Net-Unicode strings.
opaque: An opaque sequence of bytes.
Option Numbers are maintained in the CoAP Option Number Registry
(Section 11.2). See Section 5.10 for the semantics of the options
defined in this document.
4. Message Semantics
CoAP messages are exchanged asynchronously between CoAP end-points.
They are used to transport CoAP requests and responses, the semantics
of which are defined in Section 5.
As CoAP is bound to non-reliable transports such as UDP, CoAP
messages may arrive out of order, appear duplicated, or go missing
without notice. For this reason, CoAP implements a lightweight
reliability mechanism, without trying to re-create the full feature
set of a transport like TCP. It has the following features:
o Simple stop-and-wait retransmission reliability with exponential
back-off for "confirmable" messages.
o Duplicate detection for both "confirmable" and "non-confirmable"
messages.
o Multicast support.
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4.1. Reliable Messages
The reliable transmission of a message is initiated by marking the
message as "confirmable" in the CoAP header. A recipient MUST
acknowledge such a message with an acknowledgement message (or, if it
lacks context to process the message properly, MUST reject it with a
reset message). The sender retransmits the confirmable message at
exponentially increasing intervals, until it receives an
acknowledgement (or reset message), or runs out of attempts.
Retransmission is controlled by two things that a CoAP end-point MUST
keep track of for each confirmable message it sends while waiting for
an acknowledgement (or reset): a timeout and a retransmission
counter. For a new confirmable message, the initial timeout is set
to a random number between RESPONSE_TIMEOUT and (RESPONSE_TIMEOUT *
RESPONSE_RANDOM_FACTOR), and the retransmission counter is set to 0.
When the timeout is triggered and the retransmission counter is less
than MAX_RETRANSMIT, the message is retransmitted, the retransmission
counter is incremented, and the timeout is doubled. If the
retransmission counter reaches MAX_RETRANSMIT on a timeout, or if the
end-point receives a reset message, then the attempt to transmit the
message is canceled and the application process informed of failure.
On the other hand, if the end-point receives an acknowledgement
message in time, transmission is considered successful.
An acknowledgement or reset message is related to a confirmable
message by means of a Message ID along with additional address
information of the corresponding end-point. The Message ID is a 16-
bit unsigned integer that is generated by the sender of a confirmable
message and included in the CoAP header. The Message ID MUST be
echoed in the acknowledgement or reset message by the recipient.
Several implementation strategies can be employed for generating
Message IDs. In the simplest case a CoAP end-point generates Message
IDs by keeping a single Message ID variable, which is changed each
time a new confirmable message is sent regardless of the destination
address or port. End-points dealing with large numbers of
transactions could keep multiple Message ID variables, for example
per prefix or destination address. The initial variable value SHOULD
be randomized. The same Message ID MUST NOT be re-used within the
potential retransmission window, calculated as RESPONSE_TIMEOUT *
RESPONSE_RANDOM_FACTOR * (2 ^ MAX_RETRANSMIT - 1) plus the expected
maximum round trip time.
A recipient MUST be prepared to receive the same confirmable message
(as indicated by the Message ID) multiple times, for example, when
its acknowledgement went missing or didn't reach the original sender
before the first timeout. The recipient SHOULD acknowledge each
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duplicate copy of a confirmable message using the same
acknowledgement or reset message, but SHOULD process any request or
response in the message only once. This rule MAY be relaxed in case
the confirmable message transports a request that is idempotent (see
Section 5.1). Examples for relaxed message deduplication:
o A server MAY relax the requirement to answer all retransmissions
of an idempotent request with the same response (Section 4.1), so
that it does not have to maintain state for Message IDs. For
example, an implementation might want to process duplicate
transmissions of a GET, PUT or DELETE request as separate requests
if the effort incurred by duplicate processing is less expensive
than keeping track of previous responses would be.
o (As an implementation consideration, a constrained server MAY even
want to relax this requirement for certain non-idempotent requests
if the application semantics make this trade-off favorable. For
example, if the result of a POST request is just the creation of
some short-lived state at the server, it may be less expensive to
incur this effort multiple times for a request than keeping track
of whether a previous transmission of the same request already was
processed.)
Implementation notes: Note that a CoAP end-point that sent a
confirmable message MAY give up in attempting to obtain an ACK even
before the MAX_RETRANSMIT counter value is reached: E.g., the
application has canceled the request as it no longer needs a
response, or there is some other indication that the CON message did
arrive. In particular, a CoAP request message may have elicited a
separate response, in which case it is clear to the requester that
only the ACK was lost and a retransmission of the request would serve
no purpose. However, a responder MUST NOT in turn rely on this
cross-layer behavior from a requester, i.e. it SHOULD retain the
state to create the ACK for the request, if needed, even if a
confirmable response was already acknowledged by the requester.
4.2. Unreliable Messages
As a more lightweight alternative, a message can be transmitted less
reliably by marking the message as "non-confirmable". A non-
confirmable message MUST NOT be acknowledged by the recipient. If a
recipient lacks context to process the message properly, it MAY
reject the message with a reset message or otherwise MUST silently
ignore it.
There is no way to detect if a non-confirmable message was received
or not at the CoAP-level. A sender MAY choose to transmit a non-
confirmable message multiple times which, for this purpose, specifies
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a Message ID as well. The same rules for generating the Message ID
apply.
A recipient MUST be prepared to receive the same non-confirmable
message (as indicated by the Message ID) multiple times. As a
general rule that may be relaxed based on the specific semantics of a
message, the recipient SHOULD silently ignore any duplicated non-
confirmable message, and SHOULD process any request or response in
the message only once.
4.3. Message Types
The different types of messages are summarized below. The type of a
message is specified by the T field of the CoAP header.
Separate from the message type, a message may carry a request, a
response, or be empty. This is signaled by the Code field in the
CoAP header and is relevant to the request/response model. Possible
values for the Code field are maintained by the CoAP Code Registry
(Section 11.1).
An empty message has the Code field set to 0. The OC field SHOULD be
set to 0 and no bytes SHOULD be present after the Message ID field.
The OC field and any bytes trailing the header MUST be ignored by any
recipient.
4.3.1. Confirmable (CON)
Some messages require an acknowledgement. These messages are called
"Confirmable". When no packets are lost, each confirmable message
elicits exactly one return message of type Acknowledgement or type
Reset.
A confirmable message always carries either a request or response and
MUST NOT be empty.
4.3.2. Non-Confirmable (NON)
Some other messages do not require an acknowledgement. This is
particularly true for messages that are repeated regularly for
application requirements, such as repeated readings from a sensor
where eventual arrival is sufficient.
A non-confirmable message always carries either a request or
response, as well, and MUST NOT be empty.
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4.3.3. Acknowledgement (ACK)
An Acknowledgement message acknowledges that a specific confirmable
message (identified by its Message ID) arrived. It does not indicate
success or failure of any encapsulated request.
The acknowledgement message MUST echo the Message ID of the
confirmable message, and MUST carry a response or be empty (see
Section 5.2.1 and Section 5.2.2).
4.3.4. Reset (RST)
A Reset message indicates that a specific confirmable message was
received, but some context is missing to properly process it. This
condition is usually caused when the receiving node has rebooted and
has forgotten some state that would be required to interpret the
message.
A reset message MUST echo the Message ID of the confirmable message,
and MUST be empty.
4.4. Multicast
CoAP supports sending messages to multicast destination addresses.
Such multicast messages MUST be Non-Confirmable. Some mechanisms for
avoiding congestion from multicast requests have been considered in
[I-D.eggert-core-congestion-control].
4.5. Congestion Control
Basic congestion control for CoAP is provided by the exponential
back-off mechanism in Section 4.1.
In order not to cause congestion, Clients (including proxies) SHOULD
strictly limit the number of simultaneous outstanding interactions
that they maintain to a given server (including proxies). An
outstanding interaction is either a CON for which an ACK has not yet
been received but is still expected (message layer) or a request for
which a response has not yet been received but is still expected
(which may both occur at the same time, counting as one outstanding
interaction). A good value for this limit is the number 1. (Note
that [RFC2616], in trying to achieve a similar objective, did specify
a specific number of simultaneous connections as a ceiling. While
revising [RFC2616], this was found to be impractical for many
applications [I-D.ietf-httpbis-p1-messaging]. For the same
considerations, this specification does not mandate a particular
maximum number of outstanding interactions, but instead encourages
clients to be conservative when initiating interactions.)
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Further congestion control optimizations and considerations are
expected in the future, which may for example provide automatic
initialization of the CoAP constants defined in Section 9.
5. Request/Response Semantics
CoAP operates under a similar request/response model as HTTP: a CoAP
end-point in the role of a "client" sends one or more CoAP requests
to a "server", which services the requests by sending CoAP responses.
Unlike HTTP, requests and responses are not sent over a previously
established connection, but exchanged asynchronously over CoAP
messages.
5.1. Requests
A CoAP request consists of the method to be applied to the resource,
the identifier of the resource, a payload and Internet media type (if
any), and optional meta-data about the request.
CoAP supports the basic methods of GET, POST, PUT, DELETE, which are
easily mapped to HTTP. They have the same properties of safe (only
retrieval) and idempotent (you can invoke it multiple times with the
same effects) as HTTP (see Section 9.1 of [RFC2616]). The GET method
is safe, therefore it MUST NOT take any other action on a resource
other than retrieval. The GET, PUT and DELETE methods MUST be
performed in such a way that they are idempotent. POST is not
idempotent, because its effect is determined by the origin server and
dependent on the target resource; it usually results in a new
resource being created or the target resource being updated.
A request is initiated by setting the Code field in the CoAP header
of a confirmable or a non-confirmable message to a Method Code and
including request information.
The methods used in requests are described in detail in Section 5.8.
5.2. Responses
After receiving and interpreting a request, a server responds with a
CoAP response, which is matched to the request by means of a client-
generated token.
A response is identified by the Code field in the CoAP header being
set to a Response Code. Similar to the HTTP Status Code, the CoAP
Response Code indicates the result of the attempt to understand and
satisfy the request. These codes are fully defined in Section 5.9.
The Response Code numbers to be set in the Code field of the CoAP
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header are maintained in the CoAP Response Code Registry
(Section 11.1.2).
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|class| detail |
+-+-+-+-+-+-+-+-+
Figure 9: Structure of a Response Code
The upper three bits of the 8-bit Response Code number define the
class of response. The lower five bits do not have any
categorization role; they give additional detail to the overall class
(Figure 9). There are 3 classes:
2 - Success: The request was successfully received, understood, and
accepted.
4 - Client Error: The request contains bad syntax or cannot be
fulfilled.
5 - Server Error: The server failed to fulfill an apparently valid
request.
The response codes are designed to be extensible: Response Codes in
the Client Error and Server Error class that are unrecognized by an
end-point MUST be treated as being equivalent to the generic Response
Code of that class. However, there is no generic Response Code
indicating success, so a Response Code in the Success class that is
unrecognized by an end-point can only be used to determine that the
request was successful without any further details.
As a human readable notation for specifications and protocol
diagnostics, the numeric value of a response code is indicated by
giving the upper three bits in decimal, followed by a dot and then
the lower five bits in a two-digit decimal. E.g., "Not Found" is
written as 4.04 -- indicating a value of hexadecimal 0x84 or decimal
132. In other words, the dot "." functions as a short-cut for
"*32+".
The possible response codes are described in detail in Section 5.9.
Responses can be sent in multiple ways, which are defined below.
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5.2.1. Piggy-backed
In the most basic case, the response is carried directly in the
acknowledgement message that acknowledges the request (which requires
that the request was carried in a confirmable message). This is
called a "Piggy-backed" Response.
The response is returned in the acknowledgement message independent
of whether the response indicates success or failure. In effect, the
response is piggy-backed on the acknowledgement message, so no
separate message is required to both acknowledge that the request was
received and return the response.
5.2.2. Separate
It may not be possible to return a piggy-backed response in all
cases. For example, a server might need longer to obtain the
representation of the resource requested than it can wait sending
back the acknowledgement message, without risking the client to
repeatedly retransmit the request message. Responses to requests
carried in a Non-Confirmable message are always sent separately (as
there is no acknowledgement message).
The server maybe initiates the attempt to obtain the resource
representation and times out an acknowledgement timer, or it
immediately sends an acknowledgement knowing in advance that there
will be no piggy-backed response. The acknowledgement effectively is
a promise that the request will be acted upon.
When the server finally has obtained the resource representation, it
sends the response. To ensure that this message is not lost, it is
again sent as a confirmable message and answered by the client with
an acknowledgement, echoing the new Message ID chosen by the server.
(Implementation notes: Note that, as the underlying datagram
transport may not be sequence-preserving, the confirmable message
carrying the response may actually arrive before or after the
acknowledgement message for the request. Note also that, while the
CoAP protocol itself does not make any specific demands here, there
is an expectation that the response will come within a time frame
that is reasonable from an application point of view; as there is no
underlying transport protocol that could be instructed to run a keep-
alive mechanism, the requester MAY want to set up a timeout that is
unrelated to CoAP's retransmission timers in case the server is
destroyed or otherwise unable to send the response.)
For a separate exchange, both the acknowledgement to the confirmable
request and the acknowledgement to the confirmable response MUST be
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an empty message, i.e. one that carries neither a request nor a
response.
5.2.3. Non-Confirmable
If the request message is non-confirmable, then the response SHOULD
be returned in a non-confirmable message as well. However, an end-
point MUST be prepared to receive a non-confirmable response
(preceded or followed an empty acknowledgement message) in reply to a
confirmable request, or a confirmable response in reply to a non-
confirmable request.
5.3. Request/Response Matching
Regardless of how a response is sent, it is matched to the request by
means of a token that is included by the client in the request as one
of the options along with additional address information of the
corresponding end-point. The token MUST be echoed by the server in
any resulting response without modification.
The exact rules for matching a response to a request are as follows:
1. For requests sent in a unicast message, the source of the
response MUST match the destination of the original request. How
this is determined depends on the security mode used (see
Section 10): With NoSec, the IP address and port number of the
request destination and response source must match. With other
security modes, in addition to the IP address and UDP port
matching, the request and response MUST have the same security
context.
2. In a piggy-backed response, both the Message ID of the
confirmable request and the acknowledgement, and the token of the
response and original request MUST match. In a separate
response, just the token of the response and original request
MUST match.
The client SHOULD generate tokens in a way that tokens currently in
use for a given source/destination pair are unique. (Note that a
client can use the same token for any request if it uses a different
source port number each time.)
An end-point receiving a token MUST treat it as opaque and make no
assumptions about its format. (Note that there is a default value
for the Token Option, so every message carries a token, even if it is
not explicitly expressed in a CoAP option.)
In case a confirmable message carrying a response is unexpected (i.e.
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the client is not waiting for a response with the specified address
and/or token), the confirmable response SHOULD be rejected with a
reset message and MUST NOT be acknowledged.
5.4. Options
Both requests and responses may include a list of one or more
options. For example, the URI in a request is transported in several
options, and meta-data that would be carried in an HTTP header in
HTTP is supplied as options as well.
CoAP defines a single set of options that are used in both requests
and responses:
o Content-Type
o ETag
o Location-Path
o Location-Query
o Max-Age
o Proxy-Uri
o Token
o Uri-Host
o Uri-Path
o Uri-Port
o Uri-Query
o Accept
o If-Match
o If-None-Match
The semantics of these options along with their properties are
defined in detail in Section 5.10.
Not all options have meaning with all methods and response codes.
The possible options for methods and response codes are defined in
Section 5.8 and Section 5.9 respectively. In case an option has no
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meaning, it SHOULD NOT be included by the sender and MUST be ignored
by the recipient.
5.4.1. Critical/Elective
Options fall into one of two classes: "critical" or "elective". The
difference between these is how an option unrecognized by an end-
point is handled:
o Upon reception, unrecognized options of class "elective" MUST be
silently ignored.
o Unrecognized options of class "critical" that occur in a
confirmable request MUST cause the return of a 4.02 (Bad Option)
response. This response SHOULD include a human-readable error
message describing the unrecognized option(s) (see Section 5.5).
o Unrecognized options of class "critical" that occur in a
confirmable response SHOULD cause the response to be rejected with
a reset message.
o Unrecognized options of class "critical" that occur in a non-
confirmable message MUST cause the message to be silently ignored.
Note that, whether critical or elective, an option is never
"mandatory" (it is always optional): These rules are defined in order
to enable implementations to reject options they do not understand or
implement.
5.4.2. Length
Option values are defined to have a specific length, often in the
form of an upper and lower bound. If the length of an option value
in a request is outside the defined range, that option MUST be
treated like an unrecognized option (see Section 5.4.1).
5.4.3. Default Values
Options may be defined to have a default value. If the value of
option is intended to be this default value, the option SHOULD NOT be
included in the message. If the option is not present, the default
value MUST be assumed.
5.4.4. Repeating Options
Each definition of an option specifies whether it is defined to occur
only at most once or whether it can occur multiple times. If a
message includes an option with more instances than the option is
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defined for, the additional option instances MUST be treated like an
unrecognized option (see Section 5.4.1).
5.4.5. Option Numbers
Options are identified by an option number. Odd numbers indicate a
critical option, while even numbers indicate an elective option.
(Note that this is not just a convention, it is a feature of the
protocol: Whether an option is elective or critical is entirely
determined by whether its option number is even or odd.)
The numbers 14, 28, 42, ... are reserved for "fenceposting", as
described in Section 3.2. As these option numbers are even, they
stand for elective options, and unless assigned a meaning, these MUST
be silently ignored.
The option numbers for the options defined in this document are
listed in the CoAP Option Number Registry (Section 11.2).
5.5. Payload
Both requests and responses may include payload, depending on the
method or response code respectively. Methods with payload are PUT
and POST, and the response codes with payload are 2.05 (Content) and
the error codes.
The payload of PUT, POST and 2.05 (Content) is typically a resource
representation. Its format is specified by the Internet media type
given by the Content-Type Option. No default value is assumed in the
absence of this option.
2.01 (Created), 2.02 (Deleted), 2.04 (Changed) MAY include payload
that is describing the result of the action. Again, the format of
this payload is specified by the Internet media type given by the
Content-Type Option; no default value is assumed in the absence of
this option.
A response with a code indicating a Client or Server Error SHOULD
include a brief human-readable diagnostic message as payload,
explaining the error situation. This diagnostic message MUST be
encoded using UTF-8 [RFC3629], more specifically using Net-Unicode
form [RFC5198]. The Content-Type Option has no meaning and SHOULD
NOT be included. (Similar to what one would find as a Reason-Phrase
on an HTTP status line, the message is not intended for end-users but
for software engineers that during debugging need to interpret it in
the context of the present, English-language specification; therefore
no language tagging is foreseen.)
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If a method or response code is not defined to have a payload, then
the sender SHOULD NOT include one, and the recipient MUST ignore it.
5.6. Caching
CoAP end-points MAY cache responses in order to reduce the response
time and network bandwidth consumption on future, equivalent
requests.
The goal of caching in CoAP is to reuse a prior response message to
satisfy a current request. In some cases, a stored response can be
reused without the need for a network request, reducing latency and
network round-trips; a "freshness" mechanism is used for this purpose
(see Section 5.6.1). Even when a new request is required, it is
often possible to reuse the payload of a prior response to satisfy
the request, thereby reducing network bandwidth usage; a "validation"
mechanism is used for this purpose (see Section 5.6.2).
Unlike HTTP, the cacheability of CoAP responses does not depend on
the request method, but the Response Code. The cacheability of each
Response Code is defined along the Response Code definitions in
Section 5.9. Response Codes that indicate success and are
unrecognized by an end-point MUST NOT be cached.
For a presented request, a CoAP end-point MUST NOT use a stored
response, unless:
o the presented request method and that used to obtain the stored
response match,
o all options match between those in the presented request and those
of the request used to obtain the stored response (which includes
the request URI), except that there is no need for a match of the
Token, Max-Age, or ETag request option(s), and
o the stored response is either fresh or successfully validated as
defined below.
5.6.1. Freshness Model
When a response is "fresh" in the cache, it can be used to satisfy
subsequent requests without contacting the origin server, thereby
improving efficiency.
The mechanism for determining freshness is for an origin server to
provide an explicit expiration time in the future, using the Max-Age
Option (see Section 5.10.6). The Max-Age Option indicates that the
response is to be considered not fresh after its age is greater than
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the specified number of seconds.
As the Max-Age Option defaults to a value of 60, if it is not present
in a cacheable response, then the response is considered not fresh
after its age is greater than 60 seconds. If an origin server wishes
to prevent caching, it MUST explicitly include a Max-Age Option with
a value of zero seconds.
5.6.2. Validation Model
When an end-point has one or more stored responses for a GET request,
but cannot use any of them (e.g., because they are not fresh), it can
use the ETag Option in the GET request to give the origin server an
opportunity to both select a stored response to be used, and to
update its freshness. This process is known as "validating" or
"revalidating" the stored response.
When sending such a request, the end-point SHOULD add an ETag Option
specifying the entity-tag of each stored response that is applicable.
A 2.03 (Valid) response indicates the stored response identified by
the entity-tag given in the response's ETag Option can be reused,
after updating its freshness with the value of the Max-Age Option
that is included with the response (see Section 5.9.1.3).
Any other response code indicates that none of the stored responses
nominated in the request is suitable. Instead, the response SHOULD
be used to satisfy the request and MAY replace the stored response.
5.7. Proxying
CoAP distinguishes between requests to an origin server and a request
made through a proxy. A proxy is a CoAP end-point that can be tasked
by CoAP clients to perform requests on their behalf. This may be
useful, for example, when the request could otherwise not be made, or
to service the response from a cache in order to reduce response time
and network bandwidth or energy consumption.
CoAP requests to a proxy are made as normal confirmable or non-
confirmable requests to the proxy end-point, but specify the request
URI in a different way: The request URI in a proxy request is
specified as a string in the Proxy-Uri Option (see Section 5.10.3),
while the request URI in a request to an origin server is split into
the Uri-Host, Uri-Port, Uri-Path and Uri-Query Options (see
Section 5.10.2).
When a proxy request is made to an end-point and the end-point is
unwilling or unable to act as proxy for the request URI, it MUST
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return a 5.05 (Proxying Not Supported) response. If the authority
(host and port) is recognized as identifying the proxy end-point,
then the request MUST be treated as a local request.
Unless a proxy is configured to forward the proxy request to another
proxy, it MUST translate the request as follows: The origin server's
IP address and port are determined by the authority component of the
request URI, and the request URI is decoded and split into the Uri-
Host, Uri-Port, Uri-Path and Uri-Query Options.
All options present in a proxy request MUST be processed at the
proxy. Critical options in a request that are not recognized by the
proxy MUST lead to a 4.02 (Bad Option) response being returned by the
proxy. Elective options not recognized by the proxy MUST NOT be
forwarded to the origin server. Similarly, critical options in a
response that are not recognized by the proxy server MUST lead to a
5.02 (Bad Gateway) response. Again, elective options that are not
recognized MUST NOT be forwarded.
If the proxy does not employ a cache, then it simply forwards the
translated request to the determined destination. Otherwise, if it
does employ a cache but does not have a stored response that matches
the translated request and is considered fresh, then it needs to
refresh its cache according to Section 5.6.
If the request to the destination times out, then a 5.04 (Gateway
Timeout) response MUST be returned. If the request to the
destination returns an response that cannot be processed by the
proxy, then a 5.02 (Bad Gateway) response MUST be returned.
Otherwise, the proxy returns the response to the client.
If a response is generated out of a cache, it MUST be generated with
a Max-Age Option that does not extend the max-age originally set by
the server, considering the time the resource representation spent in
the cache. E.g., the Max-Age Option could be adjusted by the proxy
for each response using the formula: proxy-max-age = original-max-age
- cache-age. For example if a request is made to a proxied resource
that was refreshed 20 seconds ago and had an original Max-Age of 60
seconds, then that resource's proxied max-age is now 40 seconds.
5.8. Method Definitions
In this section each method is defined along with its behavior. A
request with an unrecognized or unsupported Method Code MUST generate
a 4.05 (Method Not Allowed) response.
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5.8.1. GET
The GET method retrieves a representation for the information that
currently corresponds to the resource identified by the request URI.
If the request inlcudes one or more Accept Options, they indicate the
preferred content-type of a response. If the request includes an
ETag Option, the GET method requests that ETag be validated and that
the representation be transferred only if validation failed. Upon
success a 2.05 (Content) or 2.03 (Valid) response SHOULD be sent.
The GET method is safe and idempotent.
5.8.2. POST
The POST method requests that the representation enclosed in the
request be processed. The actual function performed by the POST
method is determined by the origin server and dependent on the target
resource. It usually results in a new resource being created or the
target resource being updated.
If a resource has been created on the server, a 2.01 (Created)
response that includes the URI of the new resource in a sequence of
one or more Location-Path Options and/or a Location-Query Option
SHOULD be returned. If the POST succeeds but does not result in a
new resource being created on the server, a 2.04 (Changed) response
SHOULD be returned. If the POST succeeds and results in the target
resource being deleted, a 2.02 (Deleted) response SHOULD be returned.
POST is neither safe nor idempotent.
5.8.3. PUT
The PUT method requests that the resource identified by the request
URI be updated or created with the enclosed representation. The
representation format is specified by the media type given in the
Content-Type Option.
If a resource exists at the request URI the enclosed representation
SHOULD be considered a modified version of that resource, and a 2.04
(Changed) response SHOULD be returned. If no resource exists then
the server MAY create a new resource with that URI, resulting in a
2.01 (Created) response. If the resource could not be created or
modified, then an appropriate error response code SHOULD be sent.
Further restrictions to a PUT can be made by including the If-Match
(see Section 5.10.9) or If-None-Match (see Section 5.10.10) options
in the request.
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PUT is not safe, but idempotent.
5.8.4. DELETE
The DELETE method requests that the resource identified by the
request URI be deleted. A 2.02 (Deleted) response SHOULD be sent on
success or in case the resource did not exist before the request.
DELETE is not safe, but idempotent.
5.9. Response Code Definitions
Each response code is described below, including any options required
in the response. Where appropriate, some of the codes will be
specified in regards to related response codes in HTTP [RFC2616];
this does not mean that any such relationship modifies the HTTP
mapping specified in Section 8.
5.9.1. Success 2.xx
This class of status code indicates that the clients request was
successfully received, understood, and accepted.
5.9.1.1. 2.01 Created
Like HTTP 201 "Created", but only used in response to POST and PUT
requests. The payload returned with the response, if any, is a
representation of the action result. The representation format is
specified by the media type given in the Content-Type Option.
If the response includes one or more Location-Path Options and/or a
Location-Query Option, the values of these options specify the
location at which the resource was created. Otherwise, the resource
was created at the request URI. A cache SHOULD mark any stored
response for the created resource as not fresh.
This response is not cacheable.
5.9.1.2. 2.02 Deleted
Like HTTP 204 "No Content", but only used in response to DELETE
requests. The payload returned with the response, if any, is a
representation of the action result. The representation format is
specified by the media type given in the Content-Type Option.
This response is not cacheable. However, a cache SHOULD mark any
stored response for the deleted resource as not fresh.
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5.9.1.3. 2.03 Valid
Related to HTTP 304 "Not Modified", but only used to indicate that
the response identified by the entity-tag identified by the included
ETag Option is valid. Accordingly, the response MUST include an ETag
Option.
When a cache receives a 2.03 (Valid) response, it needs to update the
stored response with the value of the Max-Age Option included in the
response (see Section 5.6.2).
5.9.1.4. 2.04 Changed
Like HTTP 204 "No Content", but only used in response to POST and PUT
requests. The payload returned with the response, if any, is a
representation of the action result. The representation format is
specified by the media type given in the Content-Type Option.
This response is not cacheable. However, a cache SHOULD mark any
stored response for the changed resource as not fresh.
5.9.1.5. 2.05 Content
Like HTTP 200 "OK", but only used in response to GET requests.
The payload returned with the response is a representation of the
target resource. The representation format is specified by the media
type given in the Content-Type Option.
This response is cacheable: Caches can use the Max-Age Option to
determine freshness (see Section 5.6.1) and (if present) the ETag
Option for validation (see Section 5.6.2).
5.9.2. Client Error 4.xx
This class of response code is intended for cases in which the client
seems to have erred. These response codes are applicable to any
request method.
The server SHOULD include a brief human-readable message as payload,
as detailed in Section 5.5.
Responses of this class are cacheable: Caches can use the Max-Age
Option to determine freshness (see Section 5.6.1). They cannot be
validated.
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5.9.2.1. 4.00 Bad Request
Like HTTP 400 "Bad Request".
5.9.2.2. 4.01 Unauthorized
The client is not authorized to perform the requested action. The
client SHOULD NOT repeat the request without previously improving its
authentication status to the server. Which specific mechanism can be
used for this is outside this document's scope; see also Section 10.
5.9.2.3. 4.02 Bad Option
The request could not be understood by the server due to one or more
unrecognized or malformed critical options. The client SHOULD NOT
repeat the request without modification.
5.9.2.4. 4.03 Forbidden
Like HTTP 403 "Forbidden".
5.9.2.5. 4.04 Not Found
Like HTTP 404 "Not Found".
5.9.2.6. 4.05 Method Not Allowed
Like HTTP 405 "Method Not Allowed", but with no parallel to the
"Allow" header field.
5.9.2.7. 4.12 Precondition Failed
Like HTTP 412 "Precondition Failed".
5.9.2.8. 4.13 Request Entity Too Large
Like HTTP 413 "Request Entity Too Large".
5.9.2.9. 4.15 Unsupported Media Type
Like HTTP 415 "Unsupported Media Type".
5.9.3. Server Error 5.xx
This class of response code indicates cases in which the server is
aware that it has erred or is incapable of performing the request.
These response codes are applicable to any request method.
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The server SHOULD include a human-readable message as payload, as
detailed in Section 5.5.
Responses of this class are cacheable: Caches can use the Max-Age
Option to determine freshness (see Section 5.6.1). They cannot be
validated.
5.9.3.1. 5.00 Internal Server Error
Like HTTP 500 "Internal Server Error".
5.9.3.2. 5.01 Not Implemented
Like HTTP 501 "Not Implemented".
5.9.3.3. 5.02 Bad Gateway
Like HTTP 502 "Bad Gateway".
5.9.3.4. 5.03 Service Unavailable
Like HTTP 503 "Service Unavailable", but using the Max-Age Option in
place of the "Retry-After" header field.
5.9.3.5. 5.04 Gateway Timeout
Like HTTP 504 "Gateway Timeout".
5.9.3.6. 5.05 Proxying Not Supported
The server is unable or unwilling to act as a proxy for the URI
specified in the Proxy-Uri Option (see Section 5.10.3).
5.10. Option Definitions
The individual CoAP options are summarized in Table 1 and explained
below.
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+-----+----------+----------------+--------+---------+-------------+
| No. | C/E | Name | Format | Length | Default |
+-----+----------+----------------+--------+---------+-------------+
| 1 | Critical | Content-Type | uint | 0-2 B | (none) |
| 2 | Elective | Max-Age | uint | 0-4 B | 60 |
| 3 | Critical | Proxy-Uri | string | 1-270 B | (none) |
| 4 | Elective | ETag | opaque | 1-8 B | (none) |
| 5 | Critical | Uri-Host | string | 1-270 B | (see below) |
| 6 | Elective | Location-Path | string | 1-270 B | (none) |
| 7 | Critical | Uri-Port | uint | 0-2 B | (see below) |
| 8 | Elective | Location-Query | string | 1-270 B | (none) |
| 9 | Critical | Uri-Path | string | 1-270 B | (none) |
| 11 | Critical | Token | opaque | 1-8 B | (empty) |
| 12 | Elective | Accept | uint | 0-2 B | (none) |
| 13 | Critical | If-Match | opaque | 0-8 B | (none) |
| 15 | Critical | Uri-Query | string | 1-270 B | (none) |
| 21 | Critical | If-None-Match | (none) | 0 B | (none) |
+-----+----------+----------------+--------+---------+-------------+
Table 1: Options
5.10.1. Token
The Token Option is used to match a response with a request. Every
request has a client-generated token which the server MUST echo in
any response. A default value of a zero-length token is assumed in
the absence of the option. Thus when the token value is empty, the
Token Option SHOULD be elided for efficiency.
A token is intended for use as a client-local identifier for
differentiating between concurrent requests (see Section 5.3). A
client SHOULD generate tokens in a way that tokens currently in use
for a given source/destination pair are unique. An empty token value
is appropriate e.g. when no other tokens are in use to a destination,
or when requests are made serially per destination. There are
however multiple possible implementation strategies to fulfill this.
An end-point receiving a token MUST treat it as opaque and make no
assumptions about its format.
This option is "critical". It MUST NOT occur more than once.
5.10.2. Uri-Host, Uri-Port, Uri-Path and Uri-Query
The Uri-Host, Uri-Port, Uri-Path and Uri-Query Options are used to
specify the target resource of a request to a CoAP origin server.
The options encode the different components of the request URI in a
way that no percent-encoding is visible in the option values and that
the full URI can be reconstructed at any involved end-point. The
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syntax of CoAP URIs is defined in Section 6.
The steps for parsing URIs into options is defined in Section 6.4.
These steps result in zero or more Uri-Host, Uri-Port, Uri-Path and
Uri-Query Options being included in a request, where each option
holds the following values:
o the Uri-Host Option specifies the Internet host of the resource
being requested,
o the Uri-Port Option specifies the port number of the resource,
o each Uri-Path Option specifies one segment of the absolute path to
the resource, and
o each Uri-Query Option specifies one argument parameterizing the
resource.
Note: Fragments ([RFC3986], Section 3.5) are not part of the request
URI and thus will not be transmitted in a CoAP request.
The default value of the Uri-Host Option is the IP literal
representing the destination IP address of the request message.
Likewise, the default value of the Uri-Port Option is the destination
UDP port. The default Uri-Host and Uri-Port options are sufficient
for requests to most servers, and are typically used when an end-
point hosts multiple virtual servers.
The Uri-Path and Uri-Query Option can contain any character sequence.
No percent-encoding is performed. The value of a Uri-Path Option
MUST NOT be "." or ".." (as the request URI must be resolved before
parsing it into options).
The steps for constructing the request URI from the options are
defined in Section 6.5. Note that an implementation does not
necessarily have to construct the URI; it can simply look up the
target resource by looking at the individual options.
Examples can be found in Appendix C.
All of the options are "critical". Uri-Host and Uri-Port MUST NOT
occur more than once; Uri-Path and Uri-Query MAY occur one or more
times.
5.10.3. Proxy-Uri
The Proxy-Uri Option is used to make a request to a proxy (see
Section 5.7). The proxy is requested to forward the request or
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service it from a valid cache, and return the response.
The option value is an absolute-URI ([RFC3986], Section 4.3). In
case the absolute-URI doesn't fit within a single option, the Proxy-
Uri Option MAY be included multiple times in a request such that the
concatenation of the values results in the single absolute-URI.
All but the last instance of the Proxy-Uri Option MUST have a value
with a length of 270 bytes, and the last instance MUST NOT be empty.
Note that the proxy MAY forward the request on to another proxy or
directly to the server specified by the absolute-URI. In order to
avoid request loops, a proxy MUST be able to recognize all of its
server names, including any aliases, local variations, and the
numeric IP addresses.
An end-point receiving a request with a Proxy-Uri Option that is
unable or unwilling to act as a proxy for the request MUST cause the
return of a 5.05 (Proxying Not Supported) response.
This option is "critical". It MAY occur one or more times and MUST
take precedence over any of the Uri-Host, Uri-Port, Uri-Path or Uri-
Query options (which MUST NOT be included at the same time).
5.10.4. Content-Type
The Content-Type Option indicates the representation format of the
message payload. The representation format is given as a numeric
media type identifier that is defined in the CoAP Media Type registry
(Section 11.3). No default value is assumed in the absence of the
option.
This option is "critical". It MUST NOT occur more than once.
5.10.5. Accept
The CoAP Accept option indicates when included one or more times in a
request, one or more media types, each of which is an acceptable
media type for the client, in the order of preference. The
representation format is given as a numeric media type identifier
that is defined in the CoAP Media Type registry (Section 11.3). If
no Accept options are given, the client does not express a preference
(thus no default value is assumed). The client prefers the
representation returned by the server to be in one of the media types
indicated. The server SHOULD return one of the preferred media types
if available. As a server might not support the Accept option (and
thus would ignore it as it is elective), or a server might not have a
preferred media type available, the client needs to be prepared to
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receive a representation in a different media type. The client can
simply discard a representation it can not make use of.
This option is "elective". It MAY occur more than once.
5.10.6. Max-Age
The Max-Age Option indicates the maximum time a response may be
cached before it MUST be considered not fresh (see Section 5.6.1).
The option value is an integer number of seconds between 0 and 2^32-1
inclusive (about 136.1 years). A default value of 60 seconds is
assumed in the absence of the option in a response.
This option is "elective". It MUST NOT occur more than once.
5.10.7. ETag
The ETag Option in a response provides the current value of the
entity-tag for the enclosed representation of the target resource.
An entity-tag is intended for use as a resource-local identifier for
differentiating between representations of the same resource that
vary over time. It may be generated in any number of ways including
a version, checksum, hash or time. An end-point receiving an entity-
tag MUST treat it as opaque and make no assumptions about its format.
(End-points generating an entity-tag are encouraged to use the most
compact representation possible, in particular in regards to clients
and intermediaries that may want to store multiple ETag values.)
An end-point that has one or more representations previously obtained
from the resource can specify the ETag Option in a request for each
stored response to determine if any of those representations is
current (see Section 5.6.2).
This option is "elective". It MUST NOT occur more than once in a
response, and MAY occur one or more times in a request.
5.10.8. Location-Path and Location-Query
The Location-Path and Location-Query Options indicates the location
of a resource as an absolute path URI. The Location-Path Option is
similar to the Uri-Path Option, and the Location-Query Option similar
to the Uri-Query Option.
The two options MAY be included in a response to indicate the
location of a new resource created with POST.
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If a response with a Location-Path and/or Location-Query Option
passes through a cache and the implied URI identifies one or more
currently stored responses, those entries SHOULD be marked as not
fresh.
Both options are "elective" and MAY occur one or more times.
5.10.9. If-Match
The If-Match Option MAY be used to make a request conditional on the
current existence or value of an ETag for one or more representations
of the target resource. If-Match is generally useful for resource
update requests, such as PUT requests, as a means for protecting
against accidental overwrites when multiple clients are acting in
parallel on the same resource (i.e., the "lost update" problem).
The value of an If-Match option is either an ETag or the empty
string. An empty string places the precondition on the existence of
any current representation for the target resource.
The If-Match Option can occur multiple times. If any of the ETags
given as an option value match the ETag of the selected
representation for the target resource, or if an If-Match Option with
an empty string as option value is given and any current
representation exists for the target resource, then the server MAY
perform the request method as if the If-Match Option was not present.
If none of the ETags match and, if an empty string is given, no
current representation exists at all, the server MUST NOT perform the
requested method. Instead, the server MUST respond with the 4.12
(Precondition Failed) response code.
If the request would, without the If-Match Options, result in
anything other than a 2.xx or 4.12 response code, then any If-Match
Options MUST be ignored.
This option is "critical". It MAY occur more than once.
5.10.10. If-None-Match
The If-None-Match Option MAY be used to make a request conditional on
the non-existance of the target resource. If-None-Match is useful
for resource creation requests, such as PUT requests, as a means for
protecting against accidental overwrites when multiple clients are
acting in parallel on the same resource. The If-None-Match Option
carries no value.
If the target resource does exist, then the server MUST NOT perform
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the requested method. Instead, the server MUST respond with the 4.12
(Precondition Failed) response code.
This option is "critical". It MAY NOT occur more than once.
6. CoAP URIs
CoAP uses the "coap" and "coaps" URI schemes for identifying CoAP
resources and providing a means of locating the resource. Resources
are organized hierarchically and governed by a potential CoAP origin
server listening for CoAP requests ("coap") or DTLS-secured CoAP
requests ("coaps") on a given UDP port. The CoAP server is
identified via the generic syntax's authority component, which
includes a host identifier and optional UDP port number. The
remainder of the URI is considered to be identifying a resource which
can be operated on by the methods defined by the CoAP protocol. The
"coap" and "coaps" URI schemes can thus be compared to the "http" and
"https" URI schemes respectively.
The syntax of the "coap" and "coaps" URI schemes is specified below
in Augmented Backus-Naur Form (ABNF) [RFC5234]. The definitions of
"host", "port", "path-abempty", "query", "segment", "IP-literal",
"IPv4address" and "reg-name" are adopted from [RFC3986].
6.1. coap URI Scheme
coap-URI = "coap:" "//" host [ ":" port ] path-abempty [ "?" query ]
If host is provided as an IP-literal or IPv4address, then the CoAP
server is located at that IP address. If host is a registered name,
then that name is considered an indirect identifier and the end-point
might use a name resolution service, such as DNS, to find the address
of that host. The host MUST NOT be empty. The port subcomponent
indicates the UDP port at which the CoAP server is located. If it is
empty or not given, then the default port 5683 is assumed.
The path identifies a resource within the scope of the host and port.
It consists of a sequence of path segments separated by a slash
character (U+002F SOLIDUS "/").
The query serves to further parameterize the resource. It consists
of a sequence of arguments separated by an ampersand character
(U+0026 AMPERSAND "&"). An argument is often in the form of a
"key=value" pair.
The "coap" URI scheme supports the path prefix "/.well-known/"
defined by [RFC5785] for "well-known locations" in the name-space of
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a host. This enables discovery of policy or other information about
a host ("site-wide metadata"), such as hosted resources (see
Section 7.1).
Application designers are encouraged to make use of short, but
descriptive URIs. As the environments that CoAP is used in are
usually constrained for bandwidth and energy, the trade-off between
these two qualities should lean towards the shortness, without
ignoring descriptiveness.
6.2. coaps URI Scheme
coaps-URI = "coaps:" "//" host [ ":" port ] path-abempty
[ "?" query ]
All of the requirements listed above for the "coap" scheme are also
requirements for the "coaps" scheme, except that a default UDP port
of [IANA_TBD_PORT] is assumed if the port subcomponent is empty or
not given, and the UDP datagrams MUST be secured for privacy through
the use of DTLS as described in Section 10.1.
Unlike the "coap" scheme, responses to "coaps" identified requests
are never "public" and thus MUST NOT be reused for shared caching.
They can, however, be reused in a private cache if the message is
cacheable by default in CoAP.
Resources made available via the "coaps" scheme have no shared
identity with the "coap" scheme even if their resource identifiers
indicate the same authority (the same host listening to the same UDP
port). They are distinct name spaces and are considered to be
distinct origin servers.
6.3. Normalization and Comparison Rules
Since the "coap" and "coaps" schemes conform to the URI generic
syntax, such URIs are normalized and compared according to the
algorithm defined in [RFC3986], Section 6, using the defaults
described above for each scheme.
If the port is equal to the default port for a scheme, the normal
form is to elide the port subcomponent. Likewise, an empty path
component is equivalent to an absolute path of "/", so the normal
form is to provide a path of "/" instead. The scheme and host are
case-insensitive and normally provided in lowercase; IP-literals are
in recommended form [RFC5952]; all other components are compared in a
case-sensitive manner. Characters other than those in the "reserved"
set are equivalent to their percent-encoded octets (see [RFC3986],
Section 2.1): the normal form is to not encode them.
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For example, the following three URIs are equivalent, and cause the
same options and option values to appear in the CoAP messages:
coap://example.com:5683/~sensors/temp.xml
coap://EXAMPLE.com/%7Esensors/temp.xml
coap://EXAMPLE.com:/%7esensors/temp.xml
6.4. Decomposing URIs into Options
The steps to parse a request's options from a string /url/ are as
follows. These steps either result in zero or more of the Uri-Host,
Uri-Port, Uri-Path and Uri-Query Options being included in the
request, or they fail.
1. If the /url/ string is not an absolute URI ([RFC3986]), then fail
this algorithm.
2. Resolve the /url/ string using the process of reference
resolution defined by [RFC3986], with the URL character encoding
set to UTF-8 [RFC3629].
NOTE: It doesn't matter what it is resolved relative to, since we
already know it is an absolute URL at this point.
3. If /url/ does not have a <scheme> component whose value, when
converted to ASCII lowercase, is "coap" or "coaps", then fail
this algorithm.
4. If /url/ has a <fragment> component, then fail this algorithm.
5. If the <host> component of /url/ does not represent the request's
destination IP address as an IP-literal or IPv4address, include a
Uri-Host Option and let that option's value be the value of the
<host> component of /url/, converted to ASCII lowercase, and then
converting all percent-encodings ("%" followed by two hexadecimal
digits) to the corresponding characters.
NOTE: In the usual case where the request's destination IP
address is derived from the host part, this ensures that Uri-Host
Options are only used for host parts of the form reg-name.
6. If /url/ has a <port> component, then let /port/ be that
component's value interpreted as a decimal integer; otherwise,
let /port/ be the default port for the scheme.
7. If /port/ does not equal the request's destination UDP port,
include a Uri-Port Option and let that option's value be /port/.
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8. If the value of the <path> component of /url/ is empty or
consists of a single slash character (U+002F SOLIDUS "/"), then
move to the next step.
Otherwise, for each segment in the <path> component, include a
Uri-Path Option and let that option's value be the segment (not
including the delimiting slash characters) after converting all
percent-encodings ("%" followed by two hexadecimal digits) to the
corresponding characters.
9. If /url/ has a <query> component, then, for each argument in the
<query> component, include a Uri-Query Option and let that
option's value be the argument (not including the question mark
and the delimiting ampersand characters) after converting all
percent-encodings to the corresponding characters.
Note that these rules completely resolve any percent-encoding.
6.5. Composing URIs from Options
The steps to construct a URI from a request's options are as follows.
These steps either result in a URI, or they fail. In these steps,
percent-encoding a character means replacing each of its (UTF-8
encoded) bytes by a "%" character followed by two hexadecimal digits
representing the byte, where the digits A-F are in upper case (as
defined in [RFC3986] Section 2.1; to reduce variability, the
hexadecimal notation in CoAP URIs MUST use uppercase letters).
1. If the request is secured using DTLS, let /url/ be the string
"coaps://". Otherwise, let /url/ be the string "coap://".
2. If the request includes a Uri-Host Option, let /host/ be that
option's value, where any non-ASCII characters are replaced by
their corresponding percent-encoding. If /host/ is not a valid
reg-name or IP-literal or IPv4address, fail the algorithm.
Otherwise, let /host/ be the IP-literal (making use of the
conventions of [RFC5952]) or IPv4address representing the
request's destination IP address.
3. Append /host/ to /url/.
4. If the request includes a Uri-Port Option, let /port/ be that
option's value. Otherwise, let /port/ be the request's
destination UDP port.
5. If /port/ is not the default port for the scheme, then append a
single U+003A COLON character (:) followed by the decimal
representation of /port/ to /url/.
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6. Let /resource name/ be the empty string. For each Uri-Path
Option in the request, append a single character U+002F SOLIDUS
(/) followed by the option's value to /resource name/, after
converting any character that is not either in the "unreserved"
set, "sub-delims" set, a U+003A COLON (:) or U+0040 COMMERCIAL
AT (@) character, to its percent-encoded form.
7. If /resource name/ is the empty string, set it to a single
character U+002F SOLIDUS (/).
8. For each Uri-Query Option in the request, append a single
character U+003F QUESTION MARK (?) (first option) or U+0026
AMPERSAND (&) (subsequent options) followed by the option's
value to /resource name/, after converting any character that is
not either in the "unreserved" set, "sub-delims" set (except
U+0026 AMPERSAND (&)), a U+003A COLON (:), U+0040 COMMERCIAL AT
(@), U+002F SOLIDUS (/) or U+003F QUESTION MARK (?) character,
to its percent-encoded form.
9. Append /resource name/ to /url/.
10. Return /url/.
Note that these steps have been designed to lead to a URI in normal
form (see Section 6.3).
7. Finding and Addressing CoAP End-Points
7.1. Resource Discovery
The discovery of resources offered by a CoAP end-point is extremely
important in machine-to-machine applications where there are no
humans in the loop and static interfaces result in fragility. A CoAP
end-point SHOULD support the CoRE Link Format of discoverable
resources as described in [I-D.ietf-core-link-format]. It is up to
the server which resources are made discoverable (if any).
7.1.1. Content-type code 'ct' attribute
This section defines a new Web Linking [RFC5988] attribute for use
with [I-D.ietf-core-link-format]. The Content-type code "ct"
attribute provides a hint about the Internet media type(s) this
resource returns. Note that this is only a hint, and does not
override the Content-type Option of a CoAP response obtained by
actually following the link. The value is in the CoAP identifier
code format as a decimal ASCII integer and MUST be in the range of
0-65535 (16-bit unsigned integer). For example application/xml would
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be indicated as "ct=41". If no Content-type code attribute is
present then nothing about the type can be assumed. The Content-type
code attribute MAY appear more than once in a link, indicating that
multiple content-types are available.
link-extension = <Defined in RFC5988>
link-extension = ( "ct" "=" cardinal ) ; Range of 0-65535
cardinal = "0" / %x31-39 *DIGIT
7.2. Default Ports
The CoAP default port number 5683 MUST be supported by a server for
resource discovery and SHOULD be supported for providing access to
other resources. The DTLS-secured CoAP default port number
[IANA_TBD_PORT] MAY be supported by a server for resource discovery
and for providing access to other resources. In addition other end-
points may be hosted in the dynamic port space.
When a CoAP server is hosted by a 6LoWPAN node, it SHOULD also
support a port in the 61616-61631 compressed UDP port space defined
in [RFC4944].
8. HTTP Mapping
CoAP supports a limited subset of HTTP functionality, and thus a
mapping to HTTP is straightforward. There might be several reasons
for mapping between CoAP and HTTP, for example when designing a web
interface for use over either protocol or when realizing a CoAP-HTTP
proxy. Likewise, CoAP could equally be mapped to other protocols
such as XMPP [RFC6120] or SIP [RFC3264]; the definition of these
mappings is out of scope of this specification.
There are two possible mappings via a forward proxy:
CoAP-HTTP Mapping: Enables CoAP clients to access resources on HTTP
servers through an intermediary. This is initiated by including
the Proxy-Uri Option with an "http" or "https" URI in a CoAP
request to a CoAP-HTTP proxy.
HTTP-CoAP Mapping: Enables HTTP clients to access resources on CoAP
servers through an intermediary. This is initiated by specifying
a "coap" or "coaps" URI in the Request-Line of an HTTP request to
an HTTP-CoAP proxy.
Either way, only the Request/Response model of CoAP is mapped to
HTTP. The underlying model of confirmable or non-confirmable
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messages, etc., is invisible and MUST have no effect on a proxy
function. The following sections describe the handling of requests
to a forward proxy. Reverse proxies are not specified as the proxy
function is transparent to the client with the proxy acting as if it
was the origin server.
8.1. CoAP-HTTP Mapping
If a request contains a Proxy-URI Option with an 'http' or 'https'
URI [RFC2616], then the receiving CoAP end-point (called "the proxy"
henceforth) is requested to perform the operation specified by the
request method on the indicated HTTP resource and return the result
to the client.
This section specifies for any CoAP request the CoAP response that
the proxy should return to the client. How the proxy actually
satisfies the request is an implementation detail, although the
typical case is expected to be the proxy translating and forwarding
the request to an HTTP origin server.
Since HTTP and CoAP share the basic set of request methods,
performing a CoAP request on an HTTP resource is not so different
from performing it on a CoAP resource. The meanings of the
individual CoAP methods when performed on HTTP resources are
explained below.
If the proxy is unable or unwilling to service a request with an HTTP
URI, a 5.05 (Proxying Not Supported) response SHOULD be returned to
the client. If the proxy services the request by interacting with a
third party (such as the HTTP origin server) and is unable to obtain
a result within a reasonable time frame, a 5.04 (Gateway Timeout)
response SHOULD be returned; if a result can be obtained but is not
understood, a 5.02 (Bad Gateway) response SHOULD be returned.
8.1.1. GET
The GET method requests the proxy to return a representation of the
HTTP resource identified by the request URI.
Upon success, a 2.05 (Content) response SHOULD be returned. The
payload of the response MUST be a representation of the target HTTP
resource, and the Content-Type Option be set accordingly. The
response MUST indicate a Max-Age value that is no greater than the
remaining time the representation can be considered fresh. If the
HTTP entity has an entity tag, the proxy SHOULD include an ETag
Option in the response and process ETag Options in requests as
described below.
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A client can influence the processing of a GET request by including
the following option:
Accept: The request MAY include one or more Accept Options,
identifying the preferred response content-type.
ETag: The request MAY include one or more ETag Options, identifying
responses that the client has stored. This requests the proxy to
send a 2.03 (Valid) response whenever it would send a 2.05
(Content) response with an entity tag in the requested set
otherwise.
8.1.2. PUT
The PUT method requests the proxy to update or create the HTTP
resource identified by the request URI with the enclosed
representation.
If a new resource is created at the request URI, a 2.01 (Created)
response MUST be returned to the client. If an existing resource is
modified, a 2.04 (Changed) response MUST be returned to indicate
successful completion of the request.
8.1.3. DELETE
The DELETE method requests the proxy to delete the HTTP resource
identified by the request URI at the HTTP origin server.
A 2.02 (Deleted) response MUST be returned to client upon success or
if the resource does not exist at the time of the request.
8.1.4. POST
The POST method requests the proxy to have the representation
enclosed in the request be processed by the HTTP origin server. The
actual function performed by the POST method is determined by the
origin server and dependent on the resource identified by the request
URI.
If the action performed by the POST method does not result in a
resource that can be identified by a URI, a 2.04 (Changed) response
MUST be returned to the client. If a resource has been created on
the origin server, a 2.01 (Created) response MUST be returned.
8.2. HTTP-CoAP Mapping
If an HTTP request contains a Request-URI with a 'coap' or 'coaps'
URI, then the receiving HTTP end-point (called "the proxy"
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henceforth) is requested to perform the operation specified by the
request method on the indicated CoAP resource and return the result
to the client.
This section specifies for any HTTP request the HTTP response that
the proxy should return to the client. How the proxy actually
satisfies the request is an implementation detail, although the
typical case is expected to be the proxy translating and forwarding
the request to a CoAP origin server. The meanings of the individual
HTTP methods when performed on CoAP resources are explained below.
If the proxy is unable or unwilling to service a request with a CoAP
URI, a 501 (Not Implemented) response SHOULD be returned to the
client. If the proxy services the request by interacting with a
third party (such as the CoAP origin server) and is unable to obtain
a result within a reasonable time frame, a 504 (Gateway Timeout)
response SHOULD be returned; if a result can be obtained but is not
understood, a 502 (Bad Gateway) response SHOULD be returned.
8.2.1. OPTIONS and TRACE
As the OPTIONS and TRACE methods are not supported in CoAP a 501 (Not
Implemented) error MUST be returned to the client.
8.2.2. GET
The GET method requests the proxy to return a representation of the
CoAP resource identified by the Request-URI.
Upon success, a 200 (OK) response SHOULD be returned. The payload of
the response MUST be a representation of the target CoAP resource,
and the Content-Type Option be set accordingly. The response MUST
indicate a Max-Age value that is no greater than the remaining time
the representation can be considered fresh. If the CoAP entity has
an entity tag, the proxy SHOULD include an ETag Option in the
response and process ETag Options in requests as described below.
A client can influence the processing of a GET request by including
the following option:
Accept: Each individual Media-type of the HTTP Accept header in a
request is mapped to a CoAP Accept option. HTTP Accept Media-type
ranges, parameters and extensions are not supported by the CoAP
Accept option. If the proxy cannot send a response which is
acceptable according to the combined Accept field value, then the
proxy SHOULD send a 406 (not acceptable) response.
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Conditional GETs: Conditional HTTP GET requests that include an "If-
Match" or "If-None-Match" request-header field can be mapped to a
corresponding CoAP request. The "If-Modified-Since" and "If-
Unmodified-Since" request-header fields are not directly supported
by CoAP, but SHOULD be implemented locally by a caching proxy.
8.2.3. HEAD
The HEAD method is identical to GET except that the server MUST NOT
return a message-body in the response.
Although there is no direct equivalent of HTTP's HEAD method in CoAP,
an HTTP-CoAP proxy responds to HEAD requests for CoAP resources, and
the HTTP headers are returned without a message-body.
8.2.4. POST
The POST method requests the proxy to have the representation
enclosed in the request be processed by the CoAP origin server. The
actual function performed by the POST method is determined by the
origin server and dependent on the resource identified by the request
URI.
If the action performed by the POST method does not result in a
resource that can be identified by a URI, a 200 (OK) or 204 (No
Content) response MUST be returned to the client. If a resource has
been created on the origin server, a 201 (Created) response MUST be
returned.
8.2.5. PUT
The PUT method requests the proxy to update or create the CoAP
resource identified by the Request-URI with the enclosed
representation.
If a new resource is created at the Request-URI, a 201 (Created)
response MUST be returned to the client. If an existing resource is
modified, either the 200 (OK) or 204 (No Content) response codes
SHOULD be sent to indicate successful completion of the request.
8.2.6. DELETE
The DELETE method requests the proxy to delete the CoAP resource
identified by the Request-URI at the CoAP origin server.
A successful response SHOULD be 200 (OK) if the response includes an
entity describing the status or 204 (No Content) if the action has
been enacted but the response does not include an entity.
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8.2.7. CONNECT
This method can not currently be satisfied by an HTTP-CoAP proxy
function as TLS to DTLS tunneling has not been specified. It is
however expected that such a tunneling mapping will be defined in the
future. A 501 (Not Implemented) error SHOULD be returned to the
client.
9. Protocol Constants
This section defines the relevant protocol constants defined in this
document:
RESPONSE_TIMEOUT 2 seconds
RESPONSE_RANDOM_FACTOR 1.5
MAX_RETRANSMIT 4
Future specifications are expected that will allow implementations to
use other sources for initializing RESPONSE_TIMEOUT. The
RESPONSE_TIMEOUT variable MAY be configured with a different value
for special environments that exhibit very short or very long RTTs.
10. Security Considerations
This section defines the DTLS binding for CoAP, the alternative use
of IPsec, and analyzes the possible threats to the protocol and its
limitations.
During the bootstrap and enrollment phases, a CoAP device is provided
with the security information that it needs, including keying
materials. How this is done is out of scope for this specification
but a couple of ways of doing this are described in
[I-D.oflynn-core-bootstrapping]. At the end of the enrollment and
bootstrap phase, the device will be in one of four security modes
with the following information for the given mode. The NoSec,
SharedKey and MultiKey modes are mandatory to implement for this
specification.
NoSec: There is no protocol level security (DTLS is disabled).
Alternative techniques to provide lower layer security SHOULD be
used when appropriate. The use of IPsec is discussed in
Section 10.2.
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SharedKey: DTLS is enabled and there is one shared key between all
the nodes that this CoAP node needs to communicate with.
MultiKey: DTLS is enabled and there is a list of shared keys and
each key includes a list of which nodes it can be used to
communicate with. At the extreme there may be one key for each
node this CoAP node needs to communicate with.
Certificate: DTLS is enabled and the device has an asymmetric key
pair with a X.509 [RFC5280] certificate that binds it to its
Authority Name and is signed by a some common trust root. The
device also has a list of root trust anchors that can be used for
validating a certificate. There may be an optional shared key
that all the nodes that communicate have access to.
The Authority Name in the certificate is the name that would be used
in the Authority part of a CoAP URI. It is worth noting that this
would typically not be either an IP address or DNS name but would
instead be a long term unique identifier for the device such as the
EUI-64 [EUI64]. The discovery process used in the system would build
up the mapping between IP addresses of the given devices and the
Authority Name for each device. Some devices could have more than
one Authority and would need more than a single certificate.
In the "NoSec" mode, the system simply sends the packets over normal
UDP over IP and is indicated by the "coap" scheme and CoAP default
port. The system is secured only by keeping attackers from being
able to send or receive packets from the network with the CoAP nodes;
see Section 10.3.4 for an additional complication with this approach.
The other three security modes are achieved using DTLS and are
indicated by the "coaps" scheme and DTLS-secured CoAP default port.
The result is a security association that can be used to authenticate
(within the limits of the security model) and, based on this
authentication, authorize the communication partner. CoAP itself
does not provide protocol primitives for authentication or
authorization; where this is required, it can either be provided by
communication security (i.e., IPsec or DTLS) or by object security
(within the payload). Devices that require authorization for certain
operations are expected to require one of these two forms of
security. Necessarily, where an intermediary is involved,
communication security only works when that intermediary is part of
the trust relationships; CoAP does not provide a way to forward
different levels of authorization that clients may have with an
intermediary to further intermediaries or origin servers -- it
therefore may be required to perform all authorization at the first
intermediary.
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10.1. Securing CoAP with DTLS
Just as HTTP is secured using Transport Layer Security (TLS) over
TCP, CoAP is secured using Datagram TLS (DTLS) [RFC4347] over UDP.
This section defines the CoAP binding to DTLS, along with the minimal
MUST implement configurations appropriate for constrained
environments. DTLS is in practice TLS with added features to deal
with the unreliable nature of the UDP transport.
In some constrained nodes (limited flash and/or RAM) and networks
(limited bandwidth or high scalability requirements), and depending
on the specific cipher suites in use, DTLS may not be applicable.
Some of DTLS' cipher suites can add significant implementation
complexity as well as some initial handshake overhead needed when
setting up the security association. Once the initial handshake is
completed, DTLS adds a limited per-datagram overhead of approximately
13 bytes, not including any initialization vectors (which are
generally implicitly derived with DTLS), integrity check values
(e.g., 8 bytes with TLS_PSK_WITH_AES_128_CCM_8
[I-D.mcgrew-tls-aes-ccm]) and padding required by the cipher suite.
Whether and which mode of using DTLS is applicable for a CoAP-based
application should be carefully weighed considering the specific
cipher suites that may be applicable, and whether the session
maintenance makes it compatible with application flows and sufficient
resources are available on the constrained nodes and for the added
network overhead. DTLS is not applicable to group keying (multicast
communication); however, it may be a component in a future group key
management protocol.
Devices SHOULD support the Server Name Indication (SNI) to indicate
their Authority Name in the SNI HostName field as defined in Section
3 of [RFC6066]. This is needed so that when a host that acts as a
virtual server for multiple Authorities receives a new DTLS
connection, it knows which keys to use for the DTLS session.
DTLS connections with certificates are set up using mutual
authentication so they can remain up and be reused for future message
exchanges in either direction. Devices can close a DTLS connection
when they need to recover resources but in general they should keep
the connection up for as long as possible. Closing the DTLS
connection after every CoAP message exchange is very inefficient.
10.1.1. SharedKey and MultiKey Modes
When forming a connection to a new node, the system selects an
appropriate key based on which nodes it is trying to reach then forms
a DTLS session using a PSK (Pre-Shared Key) mode of DTLS.
Implementations in these modes MUST support the mandatory to
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implement cipher suite TLS_PSK_WITH_AES_128_CCM_8 as specified in
[I-D.mcgrew-tls-aes-ccm].
The security considerations of [RFC4279] (Section 7) apply. In
particular, applications should carefully weigh whether they need
Perfect Forward Secrecy (PFS) or not and select an appropriate cipher
suite (7.1). The entropy of the PSK must be sufficient to mitigate
against brute-force and (where the PSK is not chosen randomly but by
a human) dictionary attacks (7.2). The cleartext communication of
client identities may leak data or compromise privacy (7.3).
10.1.2. Certificate Mode
Implementations in Certificate Mode MUST support the mandatory to
implement cipher suite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 as
specified in [RFC5246].
When a new connection is formed, the certificate from the remote
device needs to be verified. If the CoAP node has a source of
absolute time, then the node SHOULD check the validity dates are of
the certificate are within range. The certificate MUST also be
signed by an appropriate chain of trust. If the certificate contains
a SubjectAltName, then the Authority Name MUST match at least one of
the authority names of any CoAP URI found in a URI type fields in the
SubjectAltName set. If there is no SubjectAltName in the
certificate, then the Authoritative Name must match the CN found in
the certificate using the matching rules defined in [RFC2818] with
the exception that certificates with wildcards are not allowed.
If the system has a shared key in addition to the certificate, then a
cipher suite that includes the shared key such as
TLS_RSA_PSK_WITH_AES_128_CBC_SHA SHOULD be used.
10.2. Using CoAP with IPsec
One mechanism to secure CoAP in constrained environments is the IPsec
Encapsulating Security Payload (ESP) [RFC4303] when CoAP is used
without DTLS in NoSec Mode. Using IPsec ESP with the appropriate
configuration, it is possible for many constrained devices to support
encryption with built-in link-layer encryption hardware. For
example, some IEEE 802.15.4 radio chips are compatible with AES-CBC
(with 128-bit keys) [RFC3602] as defined for use with IPsec in
[RFC4835]. Alternatively, particularly on more common IEEE 802.15.4
hardware that supports AES encryption but not decryption, and to
avoid the need for padding, nodes could directly use the more widely
supported AES-CCM as defined for use with IPsec in [RFC4309], if the
security considerations in Section 9 of that specification can be
fulfilled.
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Necessarily for AES-CCM, but much preferably also for AES-CBC, static
keying should be avoided and the initial keying material be derived
into transient session keys, e.g. using a low-overhead mode of IKEv2
[RFC5996] as described in [I-D.kivinen-ipsecme-ikev2-minimal]; such a
protocol for managing keys and sequence numbers is also the only way
to achieve anti-replay capabilities. However, no recommendation can
be made at this point on how to manage group keys (i.e., for
multicast) in a constrained environment. Once any initial setup is
completed, IPsec ESP adds a limited overhead of approximately 10
bytes per packet, not including initialization vectors, integrity
check values and padding required by the cipher suite.
When using IPsec to secure CoAP, both authentication and
confidentiality SHOULD be applied as recommended in [RFC4303]. The
use of IPsec between CoAP end-points is transparent to the
application layer and does not require special consideration for a
CoAP implementation.
IPsec may not be appropriate for all environments. For example,
IPsec support is not available for many embedded IP stacks and even
in full PC operating systems or on back-end web servers, application
developers may not have sufficient access to configure or enable
IPsec or to add a security gateway to the infrastructure. Problems
with firewalls and NATs may furthermore limit the use of IPsec.
10.3. Threat analysis and protocol limitations
This section is meant to inform protocol and application developers
about the security limitations of CoAP as described in this document.
As CoAP realizes a subset of the features in HTTP/1.1, the security
considerations in Section 15 of [RFC2616] are also pertinent to CoAP.
This section concentrates on describing limitations specific to CoAP.
10.3.1. Protocol Parsing, Processing URIs
A network-facing application can exhibit vulnerabilities in its
processing logic for incoming packets. Complex parsers are well-
known as a likely source of such vulnerabilities, such as the ability
to remotely crash a node, or even remotely execute arbitrary code on
it. CoAP attempts to narrow the opportunities for introducing such
vulnerabilities by reducing parser complexity, by giving the entire
range of encodable values a meaning where possible, and by
aggressively reducing complexity that is often caused by unnecessary
choice between multiple representations that mean the same thing.
Much of the URI processing has been moved to the clients, further
reducing the opportunities for introducing vulnerabilities into the
servers. Even so, the URI processing code in CoAP implementations is
likely to be a large source of remaining vulnerabilities and should
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be implemented with special care. The most complex parser remaining
could be the one for the link-format, although this also has been
designed with a goal of reduced implementation complexity
[I-D.ietf-core-link-format]. (See also section 15.2 of [RFC2616].)
10.3.2. Proxying and Caching
As mentioned in 15.7 of [RFC2616], which see, proxies are by their
very nature men-in-the-middle, breaking any IPsec or DTLS protection
that a direct CoAP message exchange might have. They are therefore
interesting targets for breaking confidentiality or integrity of CoAP
message exchanges. As noted in [RFC2616], they are also interesting
targets for breaking availability.
The threat to confidentiality and integrity of request/response data
is amplified where proxies also cache. Note that CoAP does not
define any of the cache-suppressing Cache-Control options that
HTTP/1.1 provides to better protect sensitive data.
Finally, a proxy that fans out Separate Responses (as opposed to
Piggy-backed Responses) to multiple original requesters may provide
additional amplification (see below).
10.3.3. Risk of amplification
CoAP servers generally reply to a request packet with a response
packet. This response packet may be significantly larger than the
request packet. An attacker might use CoAP nodes to turn a small
attack packet into a larger attack packet, an approach known as
amplification. There is therefore a danger that CoAP nodes could
become implicated in denial of service (DoS) attacks by using the
amplifying properties of the protocol: An attacker that is attempting
to overload a victim but is limited in the amount of traffic it can
generate, can use amplification to generate a larger amount of
traffic.
This is particularly a problem in nodes that enable NoSec access,
that are accessible from an attacker and can access potential victims
(e.g. on the general Internet), as the UDP protocol provides no way
to verify the source address given in the request packet. An
attacker need only place the IP address of the victim in the source
address of a suitable request packet to generate a larger packet
directed at the victim.
As a mitigating factor, many constrained networks will only be able
to generate a small amount of traffic, which may make CoAP nodes less
attractive for this attack. However, the limited capacity of the
constrained network makes the network itself a likely victim of an
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amplification attack.
A CoAP server can reduce the amount of amplification it provides to
an attacker by using slicing/blocking modes of CoAP
[I-D.ietf-core-block] and offering large resource representations
only in relatively small slices. E.g., for a 1000 byte resource, a
10-byte request might result in an 80-byte response (with a 64-byte
block) instead of a 1016-byte response, considerably reducing the
amplification provided.
CoAP also supports the use of multicast IP addresses in requests, an
important requirement for M2M. Multicast CoAP requests may be the
source of accidental or deliberate denial of service attacks,
especially over constrained networks. This specification attempts to
reduce the amplification effects of multicast requests by limiting
when a response is returned. To limit the possibility of malicious
use, CoAP servers SHOULD NOT accept multicast requests that can not
be authenticated. If possible a CoAP server SHOULD limit the support
for multicast requests to specific resources where the feature is
required.
On some general purpose operating systems providing a Posix-style
API, it is not straightforward to find out whether a packet received
was addressed to a multicast address. While many implementations
will know whether they have joined a multicast group, this creates a
problem for packets addressed to multicast addresses of the form
FF0x::1, which are received by every IPv6 node. Implementations
SHOULD make use of modern APIs such as IPV6_RECVPKTINFO [RFC3542], if
available, to make this determination.
10.3.4. Cross-Protocol Attacks
The ability to incite a CoAP end-point to send packets to a fake
source address can be used not only for amplification, but also for
cross-protocol attacks:
o the attacker sends a message to a CoAP end-point with a fake
source address,
o the CoAP end-point replies with a message to the given source
address,
o the victim at the given source address receives a UDP packet that
it interprets according to the rules of a different protocol.
This may be used to circumvent firewall rules that prevent direct
communication from the attacker to the victim, but happen to allow
communication from the CoAP end-point (which may also host a valid
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role in the other protocol) to the victim.
Also, CoAP end-points may be the victim of a cross-protocol attack
generated through an end-point of another UDP-based protocol such as
DNS. In both cases, attacks are possible if the security properties
of the end-points rely on checking IP addresses (and firewalling off
direct attacks sent from outside using fake IP addresses). In
general, because of their lack of context, UDP-based protocols are
relatively easy targets for cross-protocol attacks.
Finally, CoAP URIs transported by other means could be used to incite
clients to send messages to end-points of other protocols.
One mitigation against cross-protocol attacks is strict checking of
the syntax of packets received, combined with sufficient difference
in syntax. As an example, it might help if it were difficult to
incite a DNS server to send a DNS response that would pass the checks
of a CoAP end-point. Unfortunately, the first two bytes of a DNS
reply are an ID that can be chosen by the attacker, which map into
the interesting part of the CoAP header, and the next two bytes are
then interpreted as CoAP's Message ID (i.e., any value is
acceptable). The DNS count words may be interpreted as multiple
instances of a (non-existent, but elective) CoAP option 0. The
echoed query finally may be manufactured by the attacker to achieve a
desired effect on the CoAP end-point; the response added by the
server (if any) might then just be interpreted as added payload.
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ID | T, OC, code
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| Opcode |AA|TC|RD|RA| Z | RCODE | message id
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QDCOUNT | (options 0)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ANCOUNT | (options 0)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| NSCOUNT | (options 0)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ARCOUNT | (options 0)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 10: DNS Header vs. CoAP Message
In general, for any pair of protocols, one of the protocols can very
well have been designed in a way that enables an attacker to cause
the generation of replies that look like messages of the other
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protocol. It is often much harder to ensure or prove the absence of
viable attacks than to generate examples that may not yet completely
enable an attack but might be further developed by more creative
minds. Cross-protocol attacks can therefore only be completely
mitigated if end-points don't authorize actions desired by an
attacker just based on trusting the source IP address of a packet.
Conversely, a NoSec environment that completely relies on a firewall
for CoAP security not only needs to firewall off the CoAP end-points
but also all other end-points that might be incited to send UDP
messages to CoAP end-points using some other UDP-based protocol.
In addition to the considerations above, the security considerations
for DTLS with respect to cross-protocol attacks apply. E.g., if the
same DTLS security association ("connection") is used to carry data
of multiple protocols, DTLS no longer provides protection against
cross-protocol attacks between these protocols.
11. IANA Considerations
11.1. CoAP Code Registry
This document defines a registry for the values of the Code field in
the CoAP header. The name of the registry is "CoAP Codes".
All values are assigned by sub-registries according to the following
ranges:
0 Indicates an empty message (see Section 4.3).
1-31 Indicates a request. Values in this range are assigned by
the "CoAP Method Codes" sub-registry (see Section 11.1.1).
32-63 Reserved
64-191 Indicates a response. Values in this range are assigned by
the "CoAP Response Codes" sub-registry (see
Section 11.1.2).
192-255 Reserved
11.1.1. Method Codes
The name of the sub-registry is "CoAP Method Codes".
Each entry in the sub-registry must include the Method Code in the
range 1-31, the name of the method, and a reference to the method's
documentation.
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Initial entries in this sub-registry are as follows:
+------+--------+-----------+
| Code | Name | Reference |
+------+--------+-----------+
| 1 | GET | [RFCXXXX] |
| 2 | POST | [RFCXXXX] |
| 3 | PUT | [RFCXXXX] |
| 4 | DELETE | [RFCXXXX] |
+------+--------+-----------+
Table 2: CoAP Method Codes
All other Method Codes are Unassigned.
The IANA policy for future additions to this registry is "IETF
Review" as described in [RFC5226].
The documentation of a method code should specify the semantics of a
request with that code, including the following properties:
o The response codes the method returns in the success case.
o Whether the method is idempotent, safe, or both.
o Whether the request causes a cache to mark responses stored for
the request URI as not fresh.
11.1.2. Response Codes
The name of the sub-registry is "CoAP Response Codes".
Each entry in the sub-registry must include the Response Code in the
range 64-191, a description of the Response Code, and a reference to
the Response Code's documentation.
Initial entries in this sub-registry are as follows:
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+------+-------------------------------+-----------+
| Code | Description | Reference |
+------+-------------------------------+-----------+
| 65 | 2.01 Created | [RFCXXXX] |
| 66 | 2.02 Deleted | [RFCXXXX] |
| 67 | 2.03 Valid | [RFCXXXX] |
| 68 | 2.04 Changed | [RFCXXXX] |
| 69 | 2.05 Content | [RFCXXXX] |
| 128 | 4.00 Bad Request | [RFCXXXX] |
| 129 | 4.01 Unauthorized | [RFCXXXX] |
| 130 | 4.02 Bad Option | [RFCXXXX] |
| 131 | 4.03 Forbidden | [RFCXXXX] |
| 132 | 4.04 Not Found | [RFCXXXX] |
| 133 | 4.05 Method Not Allowed | [RFCXXXX] |
| 140 | 4.12 Precondition Failed | [RFCXXXX] |
| 141 | 4.13 Request Entity Too Large | [RFCXXXX] |
| 143 | 4.15 Unsupported Media Type | [RFCXXXX] |
| 160 | 5.00 Internal Server Error | [RFCXXXX] |
| 161 | 5.01 Not Implemented | [RFCXXXX] |
| 162 | 5.02 Bad Gateway | [RFCXXXX] |
| 163 | 5.03 Service Unavailable | [RFCXXXX] |
| 164 | 5.04 Gateway Timeout | [RFCXXXX] |
| 165 | 5.05 Proxying Not Supported | [RFCXXXX] |
+------+-------------------------------+-----------+
Table 3: CoAP Response Codes
The Response Codes 96-127 are Reserved for future use. All other
Response Codes are Unassigned.
The IANA policy for future additions to this registry is "IETF
Review" as described in [RFC5226].
The documentation of a response code should specify the semantics of
a response with that code, including the following properties:
o The methods the response code applies to.
o Whether payload is required, optional or not allowed.
o The semantics of the payload. For example, the payload of a 2.05
(Content) response is a representation of the target resource; the
payload in an error response is a human-readable diagnostic
message.
o The format of the payload. For example, the format in a 2.05
(Content) response is indicated by the Content-Type Option; the
format of the payload in an error response is always Net-Unicode
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text.
o Whether the response is cacheable according to the freshness
model.
o Whether the response is validatable according to the validation
model.
o Whether the response causes a cache to mark responses stored for
the request URI as not fresh.
11.2. Option Number Registry
This document defines a registry for the Option Numbers used in CoAP
options. The name of the registry is "CoAP Option Numbers".
Each entry in the registry must include the Option Number, the name
of the option and a reference to the option's documentation.
Initial entries in this registry are as follows:
+--------+----------------+-----------+
| Number | Name | Reference |
+--------+----------------+-----------+
| 1 | Content-Type | [RFCXXXX] |
| 2 | Max-Age | [RFCXXXX] |
| 3 | Proxy-Uri | [RFCXXXX] |
| 4 | ETag | [RFCXXXX] |
| 5 | Uri-Host | [RFCXXXX] |
| 6 | Location-Path | [RFCXXXX] |
| 7 | Uri-Port | [RFCXXXX] |
| 8 | Location-Query | [RFCXXXX] |
| 9 | Uri-Path | [RFCXXXX] |
| 11 | Token | [RFCXXXX] |
| 12 | Accept | [RFCXXXX] |
| 13 | If-Match | [RFCXXXX] |
| 15 | Uri-Query | [RFCXXXX] |
| 21 | If-None-Match | [RFCXXXX] |
+--------+----------------+-----------+
Table 4: CoAP Option Numbers
The Option Number 0 is Reserved for future use. The Option Numbers
14, 28, 42, ... are Reserved for "fenceposting" (see Section 3.2).
All other Option Numbers are Unassigned.
The IANA policy for future additions to this registry is "IETF
Review" as described in [RFC5226].
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The documentation of an Option Number should specify the semantics of
an option with that number, including the following properties:
o The meaning of the option in a request.
o The meaning of the option in a response.
o Whether the option is critical of elective, as determined by the
Option Number.
o The format and length of the option's value.
o Whether the option must occur at most once or whether it can occur
multiple times.
o The default value, if any.
11.3. Media Type Registry
Media types are identified by a string, such as "application/xml"
[RFC2046]. In order to minimize the overhead of using these media
types to indicate the format of payloads, this document defines a
registry for a subset of Internet media types to be used in CoAP and
assigns each a numeric identifier. The name of the registry is "CoAP
Media Types".
Each entry in the registry must include the media type registered
with IANA, the numeric identifier in the range 0-65535 to be used for
that media type in CoAP, and a reference to a document describing
what payload with that media type means semantically.
Initial entries in this registry are as follows:
+---------------------------+-----+-----------------------------+
| Media type | Id. | Reference |
+---------------------------+-----+-----------------------------+
| text/plain; charset=utf-8 | 0 | [RFC2046][RFC3676][RFC5147] |
| application/link-format | 40 | [I-D.ietf-core-link-format] |
| application/xml | 41 | [RFC3023] |
| application/octet-stream | 42 | [RFC2045][RFC2046] |
| application/exi | 47 | [EXIMIME] |
| application/json | 50 | [RFC4627] |
+---------------------------+-----+-----------------------------+
Table 5: CoAP Media Types
The identifiers between 201 and 255 inclusive are reserved for
Private Use. All other identifiers are Unassigned.
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Because the name space of single-byte identifiers is so small, the
IANA policy for future additions in the range 0-200 inclusive to the
registry is "Expert Review" as described in [RFC5226]. The IANA
policy for additions in the range 256-65535 inclusive is "First Come
First Served" as described in [RFC5226].
In machine to machine applications, it is not expected that generic
Internet media types such as text/plain, application/xml or
application/octet-stream are useful for real applications in the long
term. It is recommended that M2M applications making use of CoAP
will request new Internet media types from IANA indicating semantic
information about how to create or parse a payload. For example, a
Smart Energy application payload carried as XML might request a more
specific type like application/se+xml or application/se+exi.
11.4. URI Scheme Registration
This document requests the registration of the Uniform Resource
Identifier (URI) scheme "coap". The registration request complies
with [RFC4395].
URI scheme name.
coap
Status.
Permanent.
URI scheme syntax.
Defined in Section 6.1 of [RFCXXXX].
URI scheme semantics.
The "coap" URI scheme provides a way to identify resources that
are potentially accessible over the Constrained Application
Protocol (CoAP). The resources can be located by contacting the
governing CoAP server and operated on by sending CoAP requests to
the server. This scheme can thus be compared to the "http" URI
scheme [RFC2616]. See Section 6 of [RFCXXXX] for the details of
operation.
Encoding considerations.
The scheme encoding conforms to the encoding rules established for
URIs in [RFC3986], i.e. internationalized and reserved characters
are expressed using UTF-8-based percent-encoding.
Applications/protocols that use this URI scheme name.
The scheme is used by CoAP end-points to access CoAP resources.
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Interoperability considerations.
None.
Security considerations.
See Section 10.3.1 of [RFCXXXX].
Contact.
IETF Chair <chair@ietf.org>
Author/Change controller.
IESG <iesg@ietf.org>
References.
[RFCXXXX]
11.5. Secure URI Scheme Registration
This document requests the registration of the Uniform Resource
Identifier (URI) scheme "coaps". The registration request complies
with [RFC4395].
URI scheme name.
coaps
Status.
Permanent.
URI scheme syntax.
Defined in Section 6.2 of [RFCXXXX].
URI scheme semantics.
The "coaps" URI scheme provides a way to identify resources that
are potentially accessible over the Constrained Application
Protocol (CoAP) using DTLS for session security. The resources
can be located by contacting the governing CoAP server and
operated on by sending CoAP requests to the server. This scheme
can thus be compared to the "https" URI scheme [RFC2616]. See
Section 6 of [RFCXXXX] for the details of operation.
Encoding considerations.
The scheme encoding conforms to the encoding rules established for
URIs in [RFC3986], i.e. internationalized and reserved characters
are expressed using UTF-8-based percent-encoding.
Applications/protocols that use this URI scheme name.
The scheme is used by CoAP end-points to access CoAP resources
using DTLS.
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Interoperability considerations.
None.
Security considerations.
See Section 10.3.1 of [RFCXXXX].
Contact.
IETF Chair <chair@ietf.org>
Author/Change controller.
IESG <iesg@ietf.org>
References.
[RFCXXXX]
11.6. Service Name and Port Number Registration
One of the functions of CoAP is resource discovery: a CoAP client can
ask a CoAP server about the resources offered by it (see
Section 7.1). To enable resource discovery just based on the
knowledge of an IP address, the CoAP port for resource discovery
needs to be standardized.
IANA has assigned the port number 5683 and the service name "coap",
in accordance with [I-D.ietf-tsvwg-iana-ports].
Besides unicast, CoAP can be used with both multicast and anycast.
Service Name.
coap
Transport Protocol.
UDP
Assignee.
IESG <iesg@ietf.org>
Contact.
IETF Chair <chair@ietf.org>
Description.
Constrained Application Protocol (CoAP)
Reference.
[RFCXXXX]
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Port Number.
5683
11.7. Secure Service Name and Port Number Registration
CoAP resource discovery may also be provided using the DTLS-secured
CoAP "coaps" scheme. Thus the CoAP port for secure resource
discovery needs to be standardized.
This document requests the assignment of the port number
[IANA_TBD_PORT] and the service name "coaps", in accordance with
[I-D.ietf-tsvwg-iana-ports].
Besides unicast, Secure CoAP can be used with anycast.
Service Name.
coaps
Transport Protocol.
UDP
Assignee.
IESG <iesg@ietf.org>
Contact.
IETF Chair <chair@ietf.org>
Description.
DTLS-secured CoAP
Reference.
[RFCXXXX]
Port Number.
[IANA_TBD_PORT]
12. Acknowledgements
Special thanks to Peter Bigot and Cullen Jennings for substantial
contributions to the ideas and text in the document, along with
countless detailed reviews and discussions.
Thanks to Michael Stuber, Richard Kelsey, Guido Moritz, Peter Van Der
Stok, Adriano Pezzuto, Lisa Dussealt, Alexey Melnikov, Gilbert Clark,
Salvatore Loreto, Petri Mutka, Szymon Sasin, Robert Quattlebaum,
Robert Cragie, Angelo Castellani, Tom Herbst, Ed Beroset, Gilman
Tolle, Robby Simpson, Colin O'Flynn, Eric Rescorla, Matthieu Vial,
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Linyi Tian, Kerry Lynn, Dale Seed, Akbar Rahman, Charles Palmer and
David Ryan for helpful comments and discussions that have shaped the
document.
Some of the text has been lifted from the working documents of the
IETF httpbis working group.
13. References
13.1. Normative References
[I-D.ietf-core-link-format]
Shelby, Z., "CoRE Link Format",
draft-ietf-core-link-format-06 (work in progress),
June 2011.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
Types", RFC 3023, January 2001.
[RFC3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
Algorithm and Its Use with IPsec", RFC 3602,
September 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3676] Gellens, R., "The Text/Plain Format and DelSp Parameters",
RFC 3676, February 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
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Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4309] Housley, R., "Using Advanced Encryption Standard (AES) CCM
Mode with IPsec Encapsulating Security Payload (ESP)",
RFC 4309, December 2005.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[RFC4395] Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
Registration Procedures for New URI Schemes", BCP 35,
RFC 4395, February 2006.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[RFC4835] Manral, V., "Cryptographic Algorithm Implementation
Requirements for Encapsulating Security Payload (ESP) and
Authentication Header (AH)", RFC 4835, April 2007.
[RFC5147] Wilde, E. and M. Duerst, "URI Fragment Identifiers for the
text/plain Media Type", RFC 5147, April 2008.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
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[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
April 2010.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, August 2010.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
13.2. Informative References
[EUI64] "GUIDELINES FOR 64-BIT GLOBAL IDENTIFIER (EUI-64)
REGISTRATION AUTHORITY", April 2010, <http://
standards.ieee.org/regauth/oui/tutorials/EUI64.html>.
[EXIMIME] "Efficient XML Interchange (EXI) Format 1.0",
December 2009, <http://www.w3.org/TR/2009/
CR-exi-20091208/#mediaTypeRegistration>.
[I-D.eggert-core-congestion-control]
Eggert, L., "Congestion Control for the Constrained
Application Protocol (CoAP)",
draft-eggert-core-congestion-control-01 (work in
progress), January 2011.
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
draft-ietf-core-block-03 (work in progress), May 2011.
[I-D.ietf-httpbis-p1-messaging]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., and J. Reschke,
"HTTP/1.1, part 1: URIs, Connections, and Message
Parsing", draft-ietf-httpbis-p1-messaging-14 (work in
progress), April 2011.
[I-D.ietf-tsvwg-iana-ports]
Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry",
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draft-ietf-tsvwg-iana-ports-10 (work in progress),
February 2011.
[I-D.kivinen-ipsecme-ikev2-minimal]
Kivinen, T., "Minimal IKEv2",
draft-kivinen-ipsecme-ikev2-minimal-00 (work in progress),
February 2011.
[I-D.mcgrew-tls-aes-ccm]
McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for TLS",
draft-mcgrew-tls-aes-ccm-01 (work in progress),
March 2011.
[I-D.oflynn-core-bootstrapping]
Sarikaya, B., Ohba, Y., Cao, Z., and R. Cragie, "Security
Bootstrapping of Resource-Constrained Devices",
draft-oflynn-core-bootstrapping-03 (work in progress),
November 2010.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, May 2003.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
Appendix A. Integer Option Value Format
Options of type uint contain a non-negative integer that is
represented in network byte order using a variable number of bytes,
as shown below.
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Length = 0 (implies value of 0)
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
Length = 1 | 0-255 |
+-+-+-+-+-+-+-+-+
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Length = 2 | 0-65535 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Length = 3 is 24 bits, Length = 4 is 32 bits etc.
Appendix B. Examples
This section gives a number of short examples with message flows for
GET requests. These examples demonstrate the basic operation, the
operation in the presence of retransmissions, and multicast.
Figure 11 shows a basic GET request causing a piggy-backed response:
The client sends a Confirmable GET request for the resource
coap://server/temperature to the server with a Message ID of 0x7d34.
The request includes one Uri-Path Option (Delta 0 + 9 = 9, Length 11,
Value "temperature"); the Token is left at its default value (empty).
This request is a total of 16 bytes long. A 2.05 (Content) response
is returned in the Acknowledgement message that acknowledges the
Confirmable request, echoing both the Message ID 0x7d34 and the
(implicitly empty) Token value. The response includes a Payload of
"22.3 C" and is 10 bytes long.
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Client Server
| |
| |
+----->| Header: GET (T=CON, Code=1, MID=0x7d34)
| GET | Uri-Path: "temperature"
| |
| |
|<-----+ Header: 2.05 Content (T=ACK, Code=69, MID=0x7d34)
| 2.05 | Payload: "22.3 C"
| |
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 0 | 1 | GET=1 | MID=0x7d34 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 9 | 11 | "temperature" (11 B) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 2 | 0 | 2.05=69 | MID=0x7d34 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "22.3 C" (6 B) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Confirmable request; piggy-backed response
Figure 12 shows a similar example, but with the inclusion of an
explicit Token Option (Delta 9 + 2 = 11, Length 1, Value 0x20) in the
request and (Delta 11 + 0 = 11) in the response, increasing the sizes
to 18 and 12 bytes, respectively.
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Client Server
| |
| |
+----->| Header: GET (T=CON, Code=1, MID=0x7d35)
| GET | Token: 0x20
| | Uri-Path: "temperature"
| |
| |
|<-----+ Header: 2.05 Content (T=ACK, Code=69, MID=0x7d35)
| 2.05 | Token: 0x20
| | Payload: "22.3 C"
| |
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 0 | 2 | GET=1 | MID=0x7d35 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 9 | 11 | "temperature" (11 B) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 1 | 0x20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 2 | 1 | 2.05=69 | MID=0x7d35 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 11 | 1 | 0x20 | "22.3 C" (6 B) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Confirmable request; piggy-backed response
In Figure 13, the Confirmable GET request is lost. After
RESPONSE_TIMEOUT seconds, the client retransmits the request,
resulting in a piggy-backed response as in the previous example.
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Client Server
| |
| |
+----X | Header: GET (T=CON, Code=1, MID=0x7d36)
| GET | Token: 0x31
| | Uri-Path: "temperature"
TIMEOUT |
| |
+----->| Header: GET (T=CON, Code=1, MID=0x7d36)
| GET | Token: 0x31
| | Uri-Path: "temperature"
| |
| |
|<-----+ Header: 2.05 Content (T=ACK, Code=69, MID=0x7d36)
| 2.05 | Token: 0x31
| | Payload: "22.3 C"
| |
Figure 13: Confirmable request (retransmitted); piggy-backed response
In Figure 14, the first Acknowledgement message from the server to
the client is lost. After RESPONSE_TIMEOUT seconds, the client
retransmits the request.
Client Server
| |
| |
+----->| Header: GET (T=CON, Code=1, MID=0x7d37)
| GET | Token: 0x42
| | Uri-Path: "temperature"
| |
| |
| X----+ Header: 2.05 Content (T=ACK, Code=69, MID=0x7d37)
| 2.05 | Token: 0x42
| | Payload: "22.3 C"
TIMEOUT |
| |
+----->| Header: GET (T=CON, Code=1, MID=0x7d37)
| GET | Token: 0x42
| | Uri-Path: "temperature"
| |
| |
|<-----+ Header: 2.05 Content (T=ACK, Code=69, MID=0x7d37)
| 2.05 | Token: 0x42
| | Payload: "22.3 C"
| |
Figure 14: Confirmable request; piggy-backed response (retransmitted)
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In Figure 15, the server acknowledges the Confirmable request and
sends a 2.05 (Content) response separately in a Confirmable message.
Note that the Acknowledgement message and the Confirmable response do
not necessarily arrive in the same order as they were sent. The
client acknowledges the Confirmable response.
Client Server
| |
| |
+----->| Header: GET (T=CON, Code=1, MID=0x7d38)
| GET | Token: 0x53
| | Uri-Path: "temperature"
| |
| |
|<- - -+ Header: (T=ACK, Code=0, MID=0x7d38)
| |
| |
|<-----+ Header: 2.05 Content (T=CON, Code=69, MID=0xad7b)
| 2.05 | Token: 0x53
| | Payload: "22.3 C"
| |
| |
+- - ->| Header: (T=ACK, Code=0, MID=0xad7b)
| |
Figure 15: Confirmable request; separate response
Figure 16 shows an example where the client loses its state (e.g.,
crashes and is rebooted) right after sending a Confirmable request,
so the separate response arriving some time later comes unexpected.
In this case, the client rejects the Confirmable response with a
Reset message. Note that the unexpected ACK is silently ignored.
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Client Server
| |
| |
+----->| Header: GET (T=CON, Code=1, MID=0x7d39)
| GET | Token: 0x64
| | Uri-Path: "temperature"
CRASH |
| |
|<- - -+ Header: (T=ACK, Code=0, MID=0x7d39)
| |
| |
|<-----+ Header: 2.05 Content (T=CON, Code=69, MID=0xad7c)
| 2.05 | Token: 0x64
| | Payload: "22.3 C"
| |
| |
+- - ->| Header: (T=RST, Code=0, MID=0xad7c)
| |
Figure 16: Confirmable request; separate response (unexpected)
Figure 17 shows a basic GET request where the request and the
response are non-confirmable, so both may be lost without notice.
Client Server
| |
| |
+----->| Header: GET (T=NON, Code=1, MID=0x7d40)
| GET | Token: 0x75
| | Uri-Path: "temperature"
| |
| |
|<-----+ Header: 2.05 Content (T=NON, Code=69, MID=0xad7d)
| 2.05 | Token: 0x75
| | Payload: "22.3 C"
| |
Figure 17: Non-confirmable request; Non-confirmable response
In Figure 18, the client sends a Non-confirmable GET request to a
multicast address: all nodes in link-local scope. There are 3
servers on the link: A, B and C. Servers A and B have a matching
resource, therefore they send back a Non-confirmable 2.05 (Content)
response. The response sent by B is lost. C does not have matching
response, therefore it sends a Non-confirmable 4.04 (Not Found)
response.
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Client ff02::1 A B C
| | | | |
| | | | |
+------>| | | | Header: GET (T=NON, Code=1, MID=0x7d41)
| GET | | | | Token: 0x86
| | | | Uri-Path: "temperature"
| | | |
| | | |
|<------------+ | | Header: 2.05 (T=NON, Code=69, MID=0x60b1)
| 2.05 | | | Token: 0x86
| | | | Payload: "22.3 C"
| | | |
| | | |
| X------------+ | Header: 2.05 (T=NON, Code=69, MID=0x01a0)
| 2.05 | | | Token: 0x86
| | | | Payload: "20.9 C"
| | | |
| | | |
|<------------------+ Header: 4.04 (T=NON, Code=132, MID=0x952a)
| 4.04 | | | Token: 0x86
| | | |
Figure 18: Non-confirmable request (multicast); Non-confirmable
response
Appendix C. URI Examples
The following examples demonstrate different sets of Uri options, and
the result after constructing an URI from them.
o coap://[2001:db8::2:1]/
Destination IP Address = [2001:db8::2:1]
Destination UDP Port = 5683
o coap://example.net/
Destination IP Address = [2001:db8::2:1]
Destination UDP Port = 5683
Uri-Host = "example.net"
o coap://example.net/.well-known/core
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Destination IP Address = [2001:db8::2:1]
Destination UDP Port = 5683
Uri-Host = "example.net"
Uri-Path = ".well-known"
Uri-Path = "core"
o coap://
xn--18j4d.example/%E3%81%93%E3%82%93%E3%81%AB%E3%81%A1%E3%81%AF
Destination IP Address = [2001:db8::2:1]
Destination UDP Port = 5683
Uri-Host = "xn--18j4d.example"
Uri-Path = the string composed of the Unicode characters U+3053
U+3093 U+306b U+3061 U+306f, usually represented in UTF-8 as
E38193E38293E381ABE381A1E381AF hexadecimal
o coap://198.51.100.1:61616//%2F//?%2F%2F&?%26
Destination IP Address = 198.51.100.1
Destination UDP Port = 61616
Uri-Path = ""
Uri-Path = "/"
Uri-Path = ""
Uri-Path = ""
Uri-Query = "//"
Uri-Query = "?&"
Appendix D. Changelog
Changed from ietf-06 to ietf-07:
o application/link-format added to Media types registration (#160)
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o Moved content-type attribute to the document from link-format.
o Added coaps scheme and DTLS-secured CoAP default port (#154)
o Allowed 0-length Content-type options (#150)
o Added congestion control recommendations (#153)
o Improved text on PUT/POST response payloads (#149)
o Added an Accept option for content-negotiation (#163)
o Added If-Match and If-None-Match options (#155)
o Improved Token Option explanation (#147)
o Clarified mandatory to implement security (#156)
o Added first come first server policy for 2-byte Media type codes
(#161)
o Clarify matching rules for messages and tokens (#151)
o Changed OPTIONS and TRACE to always return 501 in HTTP-CoAP
mapping (#164)
Changed from ietf-05 to ietf-06:
o HTTP mapping section improved with the minimal protocol standard
text for CoAP-HTTP and HTTP-CoAP forward proxying (#137).
o Eradicated percent-encoding by including one Uri-Query Option per
&-delimited argument in a query.
o Allowed RST message in reply to a NON message with unexpected
token (#134).
o Cache Invalidation only happens upon successful responses (#135).
o 50% jitter added to the initial retransmit timer (#142).
o DTLS cipher suites aligned with ZigBee IP, DTLS clarified as
default CoAP security mechanism (#138, #139)
o Added a minimal reference to draft-kivinen-ipsecme-ikev2-minimal
(#140).
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o Clarified the comparison of UTF-8s (#136).
o Minimized the initial media type registry (#101).
Changed from ietf-04 to ietf-05:
o Renamed Immediate into Piggy-backed and Deferred into Separate --
should finally end the confusion on what this is about.
o GET requests now return a 2.05 (Content) response instead of 2.00
(OK) response (#104).
o Added text to allow 2.02 (Deleted) responses in reply to POST
requests (#105).
o Improved message deduplication rules (#106).
o Section added on message size implementation considerations
(#103).
o Clarification made on human readable error payloads (#109).
o Definition of CoAP methods improved (#108).
o Max-Age removed from requests (#107).
o Clarified uniqueness of tokens (#112).
o Location-Query Option added (#113).
o ETag length set to 1-8 bytes (#123).
o Clarified relation between elective/critical and option numbers
(#110).
o Defined when to update Version header field (#111).
o URI scheme registration improved (#102).
o Added review guidelines for new CoAP codes and numbers.
Changes from ietf-03 to ietf-04:
o Major document reorganization (#51, #63, #71, #81).
o Max-age length set to 0-4 bytes (#30).
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o Added variable unsigned integer definition (#31).
o Clarification made on human readable error payloads (#50).
o Definition of POST improved (#52).
o Token length changed to 0-8 bytes (#53).
o Section added on multiplexing CoAP, DTLS and STUN (#56).
o Added cross-protocol attack considerations (#61).
o Used new Immediate/Deferred response definitions (#73).
o Improved request/response matching rules (#74).
o Removed unnecessary media types and added recommendations for
their use in M2M (#76).
o Response codes changed to base 32 coding, new Y.XX naming (#77).
o References updated as per AD review (#79).
o IANA section completed (#80).
o Proxy-Uri Option added to disambiguate between proxy and non-proxy
requests (#82).
o Added text on critical options in cached states (#83).
o HTTP mapping sections improved (#88).
o Added text on reverse proxies (#72).
o Some security text on multicast added (#54).
o Trust model text added to introduction (#58, #60).
o AES-CCM vs. AES-CCB text added (#55).
o Text added about device capabilities (#59).
o DTLS section improvements (#87).
o Caching semantics aligned with RFC2616 (#78).
o Uri-Path Option split into multiple path segments.
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o MAX_RETRANSMIT changed to 4 to adjust for RESPONSE_TIME = 2.
Changes from ietf-02 to ietf-03:
o Token Option and related use in asynchronous requests added (#25).
o CoAP specific error codes added (#26).
o Erroring out on unknown critical options changed to a MUST (#27).
o Uri-Query Option added.
o Terminology and definitions of URIs improved.
o Security section completed (#22).
Changes from ietf-01 to ietf-02:
o Sending an error on a critical option clarified (#18).
o Clarification on behavior of PUT and idempotent operations (#19).
o Use of Uri-Authority clarified along with server processing rules;
Uri-Scheme Option removed (#20, #23).
o Resource discovery section removed to a separate CoRE Link Format
draft (#21).
o Initial security section outline added.
Changes from ietf-00 to ietf-01:
o New cleaner transaction message model and header (#5).
o Removed subscription while being designed (#1).
o Section 2 re-written (#3).
o Text added about use of short URIs (#4).
o Improved header option scheme (#5, #14).
o Date option removed whiled being designed (#6).
o New text for CoAP default port (#7).
o Completed proxying section (#8).
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o Completed resource discovery section (#9).
o Completed HTTP mapping section (#10).
o Several new examples added (#11).
o URI split into 3 options (#12).
o MIME type defined for link-format (#13, #16).
o New text on maximum message size (#15).
o Location Option added.
Changes from shelby-01 to ietf-00:
o Removed the TCP binding section, left open for the future.
o Fixed a bug in the example.
o Marked current Sub/Notify as (Experimental) while under WG
discussion.
o Fixed maximum datagram size to 1280 for both IPv4 and IPv6 (for
CoAP-CoAP proxying to work).
o Temporarily removed the Magic Byte header as TCP is no longer
included as a binding.
o Removed the Uri-code Option as different URI encoding schemes are
being discussed.
o Changed the rel= field to desc= for resource discovery.
o Changed the maximum message size to 1024 bytes to allow for IP/UDP
headers.
o Made the URI slash optimization and method impotence MUSTs
o Minor editing and bug fixing.
Changes from shelby-00 to shelby-01:
o Unified the message header and added a notify message type.
o Renamed methods with HTTP names and removed the NOTIFY method.
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o Added a number of options field to the header.
o Combines the Option Type and Length into an 8-bit field.
o Added the magic byte header.
o Added new ETag Option.
o Added new Date Option.
o Added new Subscription Option.
o Completed the HTTP Code - CoAP Code mapping table appendix.
o Completed the Content-type Identifier appendix and tables.
o Added more simplifications for URI support.
o Initial subscription and discovery sections.
o A Flag requirements simplified.
Authors' Addresses
Zach Shelby
Sensinode
Kidekuja 2
Vuokatti 88600
Finland
Phone: +358407796297
Email: zach@sensinode.com
Klaus Hartke
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63905
Fax: +49-421-218-7000
Email: hartke@tzi.org
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Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Fax: +49-421-218-7000
Email: cabo@tzi.org
Brian Frank
SkyFoundry
Richmond, VA
USA
Phone:
Email: brian@skyfoundry.com
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