| CoRE Working Group | K. Hartke |
| Internet-Draft | Universitaet Bremen TZI |
| Intended status: Standards Track | July 15, 2013 |
| Expires: January 16, 2014 |
Observing Resources in CoAP
draft-ietf-core-observe-09
CoAP is a RESTful application protocol for constrained nodes and networks. The state of a resource on a CoAP server can change over time. This document specifies a simple protocol extension for CoAP that enables CoAP clients to “observe” resources, i.e., to retrieve a representation of a resource and keep this representation updated by the server over a period of time. The protocol follows a best-effort approach for sending new representations to clients, and provides eventual consistency between the state observed by each client and the actual resource state at the server.
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This Internet-Draft will expire on January 16, 2014.
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CoAP [I-D.ietf-core-coap] is an application protocol for constrained nodes and networks. It is intended to provide RESTful services [REST] not unlike HTTP [RFC2616] while reducing the complexity of implementation as well as the size of packets exchanged in order to make these services useful in a highly constrained network of themselves highly constrained nodes.
The model of REST is that of a client exchanging representations of resources with a server. A representation captures the current or intended state of a resource. The server is the definitive source for representations of the resources in its namespace. A client interested in the state of a resource initiates a request to the server; the server then returns a response with a representation of the resource that is current at the time of the request.
This model does not work well when a client is interested in having a current representation of a resource over a period of time. Existing approaches from HTTP, such as repeated polling or HTTP long polling [RFC6202], generate significant complexity and/or overhead and thus are less applicable in a constrained environment.
The protocol specified in this document extends the CoAP core protocol with a mechanism for a CoAP client to “observe” a resource on a CoAP server: the client can retrieve a representation of the resource and keep this representation updated by the server over a period of time.
The protocol keeps the architectural properties of REST. It enables high scalability and efficiency through the support of caches and proxies. There is no intention for it, though, to solve the full set of problems that the existing HTTP solutions solve, or to replace publish/subscribe networks that solve a much more general problem [RFC5989].
The protocol is based on the well-known observer design pattern [GOF]. In this design pattern, components called “observers” register at a specific, known provider called the “subject” that they are interested in being notified whenever the subject undergoes a change in state. The subject is responsible for administering its list of registered observers. If multiple subjects are of interest to an observer, it must register separately for all of them.
Observer Subject
| |
| Registration |
+----------------->|
| |
| Notification |
|<-----------------+
| |
| Notification |
|<-----------------+
| |
| Notification |
|<-----------------+
| |
Figure 1: The Observer Design Pattern
The observer design pattern is realized in CoAP as follows:
Figure 2 below shows an example of a CoAP client registering its interest in a resource and receiving three notifications: the first upon registration with the current state, and then two upon changes to the resource state. Both the registration request and the notifications are identified as such by the presence of the Observe Option defined in this document. In notifications, the Observe Option provides a sequence number for reordering detection. All notifications carry the token specified by the client in the request, so the client can easily correlate them to the request.
Client Server
| |
| GET /temperature |
| Token: 0x4a | Registration
| Observe: (empty) |
+----------------->|
| |
| 2.05 Content |
| Token: 0x4a | Notification of
| Observe: 12 | the current state
| Payload: 22.9 C |
|<-----------------+
| |
| 2.05 Content |
| Token: 0x4a | Notification upon
| Observe: 44 | a state change
| Payload: 22.8 C |
|<-----------------+
| |
| 2.05 Content |
| Token: 0x4a | Notification upon
| Observe: 60 | a state change
| Payload: 23.1 C |
|<-----------------+
| |
Figure 2: Observing a Resource in CoAP
The server is the authority for determining under what conditions resources change their state and how often observers are notified. The protocol does not offer explicit means for setting up triggers, thresholds or other conditions; it is up to the server to expose observable resources that change their state in a way that is useful in the application context. Resources can be parameterized to achieve similar effects, though; see Appendix B for examples.
A client's entry remains on the list of observers as long as the server can determine the client's continued interest in the resource. The interest is determined from the client's acknowledgement of notifications sent in confirmable messages by the server: If the client actively rejects a notification or if the transmission of a notification times out after several transmission attempts, then the client is assumed to be no longer interested and its entry is removed from the list of observers.
While a client is in the list of observers of a resource, the goal of the protocol is to keep the resource state observed by the client as closely in sync with the actual state at the server as possible.
Becoming out of sync at times cannot be avoided: First, there is always some latency between the change of the resource state and the receipt of the notification. Second, messages with notifications can get lost, which will cause the client assume an old state until it receives a new notification. And third, the server may erroneously come to the conclusion that the client is no longer interested in the resource, which will cause the server to stop sending notifications and the client to assume an old state until it registers its interest again.
The protocol addresses this issue as follows:
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 RFC 2119 [RFC2119].
| No. | C | U | N | R | Name | Format | Length | Default |
|---|---|---|---|---|---|---|---|---|
| 6 | x | - | Observe | empty/uint | 0 B/0-3 B | (none) |
C=Critical, U=Unsafe, N=No-Cache-Key, R=Repeatable
The Observe Option, when present in a request, extends the GET method so it does not only retrieve a current representation of the target resource, but also requests the server to add a new entry to the list of observers of the resource. The list entry consists of the client endpoint and the token specified by the client in the request. The value of the option in a request MUST be empty on transmission and MUST be ignored on reception.
The Observe Option is not critical for processing the request. If the server is unwilling or unable to add the client to the list of observers of the target resource, then the request falls back to a normal GET request.
In a response, the Observe Option identifies the message as a notification. This implies that the server has added the client to the list of observers and that it will notify the client of changes to the resource state. The value of the option is a 24-bit sequence number for reordering detection (see Section 3.5 and Section 4.4). The sequence number is encoded in network byte order using a variable number of bytes ('uint' format; see Section 3.2 of RFC XXXX [I-D.ietf-core-coap]).
The Observe Option is not part of the cache-key: a cacheable response obtained with an Observe Option in the request can be used to satisfy a request without an Observe Option, and vice versa. When a stored response with an Observe Option is used to satisfy a normal GET request, the option MUST be removed before the response is returned to the client.
A client can register its interest in a resource by issuing a GET request that includes an empty Observe Option. If the server returns a 2.xx response that includes an Observe Option as well, the server has added the client successfully to the list of observers of the target resource and the client will be notified of changes to the resource state.
Notifications are additional responses sent by the server in reply to the GET request. Each notification includes the token specified by the client in the GET request, an Observe Option with a sequence number for reordering detection (see Section 3.5) and a payload in the same Content-Format as the initial response.
Notifications have a 2.05 (Content) response code, or a 2.03 (Valid) response code if the client included one or more ETag Options in the request (see Section 3.3). In the event that the resource changes in a way that would cause a normal GET request at that time to return a non-2.xx response (for example, when the resource is deleted), the server sends a notification with an appropriate response code (such as 4.04 Not Found) and removes the client from the list of observers.
As notifications are just additional responses to a GET request, notifications partake in caching as defined in Section 5.6 of RFC XXXX [I-D.ietf-core-coap]. Both the freshness model and the validation model are supported.
A client MAY store a notification like a response in its cache and use a stored notification that is fresh without contacting the server. Like a response, a notification is considered fresh while its age is not greater than the value indicated by the Max-Age Option and no newer notification/response has been received.
The server will do its best to keep the resource state observed by the client as closely in sync with the actual state as possible. However, a client cannot rely on observing every single state that a resource might go through. For example, if the network is congested or the state changes more frequently than the network can handle, the server can skip notifications for any number of intermediate states.
The server uses the Max-Age Option to indicate an age up to which it is acceptable that the observed state and the actual state are inconsistent. If the age of the latest notification becomes greater than its indicated Max-Age, then the client MUST NOT use the enclosed representation unless it is validated.
When a client has one or more notifications stored in its cache for a resource, it can use the ETag Option in the GET request to give the server an opportunity to select a stored notification to be used.
The client MAY include an ETag Option for each stored response that is applicable in the GET request. Whenever the observed resource changes to a representation identified by one of the ETag Options, the server can select a stored response by sending a 2.03 (Valid) notification with an appropriate ETag Option instead of a 2.05 (Content) notification.
A client implementation needs to keep all candidate responses in its cache until it is no longer interested in the target resource or it issues a GET request with a new set of entity-tags.
Every successful GET request with an Observe Option yields a new, independent stream of notifications. Like a fresh response can be used to satisfy a request without contacting the server, the stream of notifications resulting from one request can be used to satisfy another request if the target resource is the same.
A client MUST aggregate GET requests with an Observe Option for the same target resource. The target resource SHALL be identified for this purpose by the request URI and all options in the request that are part of the cache-key (such as the Accept Option).
To make sure it has a current representation, it MAY issue a GET request without an Observe Option at any time. It is RECOMMENDED that the client does not issue a GET request (with or without Observe Option) for a resource while it still has a fresh notification/response in its cache. Additionally, the client SHOULD wait for a random amount of time between 5 and 15 seconds before issuing the request to avoid synchronicity with other clients.
Messages with notifications can arrive in a different order than they were sent. Since the goal is to keep the observed state as closely in sync with the actual state as possible, a client MUST NOT update the observed state with a notification that arrives later than a newer notification.
For reordering detection, the server sets the value of the Observe Option in each notification to the 24 least-significant bits of a strictly increasing sequence number. An incoming notification is newer than the newest notification received so far when one of the following conditions is met:
(V1 < V2 and V2 - V1 < 2^23) or (V1 > V2 and V1 - V2 > 2^23) or (T2 > T1 + 128 seconds)
where V1 is the value of the Observe Option of the newest notification received so far, V2 the value of the Observe Option of the incoming notification, T1 a client-local timestamp of the newest notification received so far, and T2 a client-local timestamp of the incoming notification.
The client MUST specify a token in its GET request that is currently not in use for the client/server pair, as the sequence numbers provide an order only among the notifications resulting from the same request.
A notification can be confirmable or non-confirmable, i.e., be sent in a confirmable or a non-confirmable message. The message type used is independent from the type used for the request or for any previous notification.
If a client does not recognize the token in a confirmable notification, it MUST NOT acknowledge the message and SHOULD reject it with a Reset message; otherwise, the client MUST acknowledge the message as usual. In the case of a non-confirmable notification, rejecting the message with a Reset message is OPTIONAL.
An acknowledgement message signals to the server that the client is alive and interested in receiving further notifications; if the server does not receive an acknowledgement in reply to a confirmable notification, it will assume that the client is no longer interested and will eventually remove the associated entry from the list of observers.
A client that is no longer interested in receiving notifications for a resource can simply reject the next notification with a Reset message. In the case of a confirmable notification, the server will then remove the associated entry from the list of observers of this resource. In the case of a non-confirmable notification, the server may (but is not required to) remove the list entry. So the client may have to wait for a confirmable notification if the servers seems to ignore the Reset messages that the client sends to reject non-confirmable notifications.
A GET request that includes an Observe Option requests the server not only to return a current representation of the target resource, but also to add a new entry to the list of observers of that resource. The list entry consists of the client endpoint and the token specified by the client in the request. If a client sends multiple requests, each request creates a new entry in the list.
Upon success, the server MUST return a current representation of the resource and MUST notify the client of subsequent changes to the resource state for each entry of the client in the list of observers.
A server that is unable or unwilling to add the client to the list of observers of the target resource MAY silently ignore the Observe Option and process the GET request as usual. The resulting response MUST NOT include an Observe Option, the absence of which signals to the client that it will not be notified of changes to the resource and, e.g., needs to poll the resource for its state instead.
A client is notified of changes to the resource state by additional responses sent by the server in reply to the GET request. Each such notification response (including the initial response) MUST include an Observe Option and MUST echo the token specified by the client in the GET request. If there are multiple entries in the list of observers, the order in which the clients are notified is not defined; the server is free to use any method to determine the order.
A notification SHOULD have a 2.05 (Content) or 2.03 (Valid) response code. However, in the event that the state of a resource changes in a way that would cause a normal GET request at that time to return a non-2.xx response (for example, when the resource is deleted), the server SHOULD notify the client by sending a notification with an appropriate response code (such as 4.04 Not Found) and MUST remove the client from the list of observers of the resource.
The Content-Format used in a notification MUST be the same as the one used in the initial response to the GET request. If the server is unable to continue sending notifications in this Content-Format, it SHOULD send a notification with a 4.06 (Not Acceptable) response code and MUST remove the client from the list of observers of the resource.
A non-2.xx notification MUST NOT include an Observe Option.
As notifications are just additional responses sent by the server, they are subject to caching as defined in Section 5.6 of RFC XXXX [I-D.ietf-core-coap].
After returning the initial response, the server MUST try to keep the returned representation current, i.e., keep the resource state observed by the client as closely in sync with the actual resource state as possible.
Since becoming out of sync at times cannot be avoided, the server MUST indicate for each representation an age up to which it is acceptable that the observed state and the actual state are inconsistent. This age is application-dependent and MUST be specified in notifications using the Max-Age Option.
When the resource does not change and the client has a current representation, the server does not need to send a notification. However, if the client does not receive a notification, it cannot tell if the observed state and the actual state are still in sync. Thus, when the the age of the latest notification becomes greater than its indicated Max-Age, the client must assume that the states have become inconsistent. The server MAY wish to prevent that by sending a notification with the unchanged representation just before Max-Age expires.
A client can include a set of entity-tags in its request using the ETag Option. When a observed resource changes its state and the origin server is about to send a 2.05 (Content) notification, then, whenever that notification has an entity-tag in the set of entity-tags specified by the client, the server MAY send a 2.03 (Valid) response with an appropriate ETag Option instead.
Because messages can get reordered, the client needs a way to determine if a notification arrived later than a newer notification. For this purpose, the server MUST set the value of the Observe Option of each notification it sends to the 24 least-significant bits of a strictly increasing sequence number. The sequence number MAY start at any value and MUST NOT increase so fast that it increases by more than 2^24 within less than 256 seconds.
The sequence number selected for a notification MUST be greater than that of any preceding notification sent to the same client for the same resource with the same token. The value of the Observe Option MUST be current at the time of transmission; if a notification is retransmitted, the server MUST update value of option to the sequence number that is current at that time before sending the message.
A notification can be sent in a confirmable or a non-confirmable message. The message type used is typically application-dependent and MAY be determined by the server for each notification individually. For example, for resources that change in a somewhat predictable or regular fashion, notifications can be sent in non-confirmable messages; for resources that change infrequently, notifications can be sent in confirmable messages. The server can combine these two approaches depending on the frequency of state changes and the importance of individual notifications.
A server MAY choose to skip sending a notification if it knows that it will send another notification soon, for example, when the state is changing frequently. Similarly, it MAY choose to send a notification more than once. However, above all, the server MUST ensure that a client in the list of observers of a resource eventually observes the latest state if the resource does not undergo a new change in state. For example, when state changes occur in bursts, the server can skip some notifications, send the notifications in non-confirmable messages, and make sure that the client observes the latest state change by repeating the last notification in a confirmable message when the burst is over.
The client's acknowledgement of a confirmable notification signals to the server that the client is interested in receiving further notifications. If a client rejects a confirmable notification with a Reset message, the client is no longer interested and the server MUST remove the associated entry from the list of observers. If the client rejects a non-confirmable notification, the server MAY remove the entry from the list of observers as well. (It is expected that the server does remove the entry if it has the information available that is needed to match the Reset message to the non-confirmable notification, but the server is not required to keep this information.)
At a minimum, the server MUST send a notification in a confirmable message instead of a non-confirmable message at least every 24 hours, so a client that went away or is no longer interested does not remain forever in the list of observers.
The server MUST limit the number of confirmable notifications for which an acknowledgement has not been received yet to NSTART (1 by default; see Section 4.7 of RFC XXXX [I-D.ietf-core-coap]); and it SHOULD NOT send more than one non-confirmable notification every 3 seconds on average.
When the state of an observed resource changes while the server is still waiting for a confirmable notification to be acknowledged or the 3 seconds for a non-confirmable notification to elapse, then the server MUST proceed as follows:
A client may be interested in a resource in the namespace of an origin server that is reached through a chain of one or more CoAP intermediaries. In this case, the client registers its interest with the first intermediary towards the origin server, acting as if it was communicating with the origin server itself as specified in Section 3. It is the task of this intermediary to provide the client with a current representation of the target resource and send notifications upon changes to the target resource state, much like an origin server as specified in Section 4.
To perform this task, the intermediary SHOULD make use of the protocol specified in this document, taking the role of the client and registering its own interest in the target resource with the next hop towards the origin server. If the next hop does not return a response with an Observe Option, the intermediary MAY resort to polling the next hop or MAY itself return a response without an Observe Option.
The communication between each pair of hops is independent; each hop in the server role MUST determine individually how many notifications to send, of which message type, and so on. Each hop MUST generate its own values for the Observe Option, and MUST set the value of the Max-Age Option according to the age of the local current representation.
If two or more clients have registered their interest in a resource with an intermediary, the intermediary MUST register itself only once with the next hop and fan out the notifications it receives to all registered clients. This relieves the next hop from sending the same notifications multiple times and thus enables scalability.
An intermediary is not required to act on behalf of a client to observe a resource; an intermediary MAY observe a resource, for example, just to keep its own cache up to date.
See Appendix A.1 for examples.
A web link [RFC5988] to a resource accessible over CoAP (for example, in a link-format document [RFC6690]) MAY include the target attribute "obs".
The "obs" attribute, when present, is a hint indicating that the destination of a link is useful for observation and thus, for example, should have a suitable graphical representation in a user interface. Note that this is only a hint; it is not a promise that the Observe Option can actually be used to perform the observation. A client may need to resort to polling the resource if the Observe Option is not returned in the response to the GET request.
A value MUST NOT be given for the "obs" attribute; any present value MUST be ignored by parsers. The "obs" attribute MUST NOT appear more than once in a given link-value; occurrences after the first MUST be ignored by parsers.
The security considerations of RFC XXXX [I-D.ietf-core-coap] apply.
The considerations about amplification attacks are somewhat amplified when observing resources. Without client authentication, a server MUST therefore strictly limit the number of notifications that it sends between receiving acknowledgements that confirm the actual interest of the client in the data; i.e., any notifications sent in non-confirmable messages MUST be interspersed with confirmable messages. (An attacker may still spoof the acknowledgements if the confirmable messages are sufficiently predictable.)
As with any protocol that creates state, attackers may attempt to exhaust the resources that the server has available for maintaining the list of observers for each resource. Servers may want to access-control this creation of state. As degraded behavior, the server can always fall back to processing the request as a normal GET request (without an Observe Option) if it is unwilling or unable to add a client to the list of observers of a resource, including if system resources are exhausted or nearing exhaustion.
Intermediaries must be careful to ensure that notifications cannot be employed to create a loop. A simple way to break any loops is to employ caches for forwarding notifications in intermediaries.
The following entry is added to the CoAP Option Numbers registry:
| Number | Name | Reference |
|---|---|---|
| 6 | Observe | [RFCXXXX] |
Carsten Bormann was an original author of this draft and is acknowledged for significant contribution to this document.
Thanks to Daniele Alessandrelli, Jari Arkko, Peter Bigot, Angelo P. Castellani, Gilbert Clark, Esko Dijk, Thomas Fossati, Brian Frank, Bert Greevenbosch, Jeroen Hoebeke, Cullen Jennings, Matthias Kovatsch, Salvatore Loreto, Charles Palmer, Zach Shelby, and Floris Van den Abeele for helpful comments and discussions that have shaped the document.
This work was supported in part by Klaus Tschira Foundation, Intel, Cisco, and Nokia.
| [RFC1982] | Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982, August 1996. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC5988] | Nottingham, M., "Web Linking", RFC 5988, October 2010. |
| [I-D.ietf-core-coap] | Shelby, Z., Hartke, K. and C. Bormann, "Constrained Application Protocol (CoAP)", Internet-Draft draft-ietf-core-coap-14, March 2013. |
| [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. |
| [RFC5989] | Roach, A.B., "A SIP Event Package for Subscribing to Changes to an HTTP Resource", RFC 5989, October 2010. |
| [RFC6202] | Loreto, S., Saint-Andre, P., Salsano, S. and G. Wilkins, "Known Issues and Best Practices for the Use of Long Polling and Streaming in Bidirectional HTTP", RFC 6202, April 2011. |
| [RFC6690] | Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, August 2012. |
| [REST] | Fielding, R., "Architectural Styles and the Design of Network-based Software Architectures", Ph.D. Dissertation, University of California, Irvine, 2000. |
| [GOF] | Gamma, E., Helm, R., Johnson, R. and J. Vlissides, "Design Patterns: Elements of Reusable Object-Oriented Software", Addison-Wesley, Reading, MA, USA, November 1994. |
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
1 | |
2 unknown | | 18.5 C
3 +----->| Header: GET 0x41011633
4 | GET | Token: 0x4a
5 | | Uri-Path: temperature
6 | | Observe: (empty)
7 | |
8 | |
9 ____________ |<-----+ Header: 2.05 0x61451633
10 | 2.05 | Token: 0x4a
11 18.5 C | | Observe: 9
12 | | Max-Age: 15
13 | | Payload: "18.5 C"
14 | |
15 | | ____________
16 ____________ |<-----+ Header: 2.05 0x51457b50
17 | 2.05 | 19.2 C Token: 0x4a
18 19.2 C | | Observe: 16
29 | | Max-Age: 15
20 | | Payload: "19.2 C"
21 | |
Figure 3: A client registers and receives one notification of the current state and one of a new state upon a state change
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
22 | |
23 19.2 C | | 19.2 C
24 | | ____________
25 | X----+ Header: 2.05 0x51457b51
26 | 2.05 | 19.7 C Token: 0x4a
27 | | Observe: 25
28 | | Max-Age: 15
29 | | Payload: "19.7 C"
30 | |
31 ____________ | |
32 | |
33 19.2 C | |
34 (stale) | |
35 | |
36 | |
37 | |
38 +----->| Header: GET 0x41011634
39 | GET | Token: 0xb2
40 | | Uri-Path: temperature
41 | | Observe: (empty)
42 | |
43 | |
44 ____________ |<-----+ Header: 2.05 0x61451634
45 | 2.05 | Token: 0xb2
46 19.7 C | | Observe: 44
47 | | Max-Age: 15
48 | | ETag: 0x78797a7a79
49 | | Payload: "19.7 C"
50 | |
Figure 4: The client re-registers after Max-Age ends
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
51 | |
52 19.7 C | | 19.7 C
53 | |
54 | | ____________
55 | crash
56 |
57 |
58 |
59 ____________ |
60 |
61 19.7 C |
62 (stale) |
63 | reboot____________
64 | |
65 | | 20.0 C
66 | |
67 +----->| Header: GET 0x41011635
68 | GET | Token: 0xf9
69 | | Uri-Path: temperature
70 | | Observe: (empty)
71 | | ETag: 0x78797a7a79
72 | |
73 | |
74 ____________ |<-----+ Header: 2.05 0x61451635
75 | 2.05 | Token: 0xf9
76 20.0 C | | Observe: 74
77 | | Max-Age: 15
78 | | Payload: "20.0 C"
79 | |
80 | | ____________
81 ____________ |<-----+ Header: 2.03 0x5143aa0c
82 | 2.03 | 19.7 C Token: 0xf9
83 19.7 C | | Observe: 81
84 | | ETag: 0x78797a7a79
85 | | Max-Age: 15
86 | |
Figure 5: The client re-registers and gives the server the opportunity to select a stored response
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
87 | |
88 19.7 C | | 19.7 C
89 | |
90 | | ____________
91 ____________ |<-----+ Header: 2.05 0x4145aa0f
92 | 2.05 | 19.3 C Token: 0xf9
93 19.3 C | | Observe: 91
94 | | Max-Age: 15
95 | | Payload: "19.3 C"
96 | |
97 | |
98 +- - ->| Header: 0x7000aa0f
99 | |
100 | |
101 | |
102 | | ____________
103 | |
104 | | 19.0 C
105 | |
106 ____________ | |
107 | |
108 19.3 C | |
109 (stale) | |
110 | |
Figure 6: The client rejects a notification and thereby cancels the observation
CLIENT PROXY SERVER | | | | +----->| Header: GET 0x41015fb8 | | GET | Token: 0x1a | | | Uri-Host: sensor.example | | | Uri-Path: status | | | Observe: (empty) | | | | |<-----+ Header: 2.05 0x61455fb8 | | 2.05 | Token: 0x1a | | | Observe: 42 | | | Max-Age: 60 | | | Payload: "ready" | | | +----->| | Header: GET 0x41011633 | GET | | Token: 0x9a | | | Proxy-Uri: coap://sensor.example/status | | | |<-----+ | Header: 2.05 0x61451633 | 2.05 | | Token: 0x9a | | | Max-Age: 53 | | | Payload: "ready" | | | | |<-----+ Header: 2.05 0x514505fc0 | | 2.05 | Token: 0x1a | | | Observe: 135 | | | Max-Age: 60 | | | Payload: "busy" | | | +----->| | Header: GET 0x41011634 | GET | | Token: 0x9b | | | Proxy-Uri: coap://sensor.example/status | | | |<-----+ | Header: 2.05 0x61451634 | 2.05 | | Token: 0x9b | | | Max-Age: 49 | | | Payload: "busy" | | |
Figure 7: A proxy observes a resource to keep its cache up to date
CLIENT PROXY SERVER | | | +----->| | Header: GET 0x41011635 | GET | | Token: 0x6a | | | Proxy-Uri: coap://sensor.example/status | | | Observe: (empty) | | | |<- - -+ | Header: 0x60001635 | | | | +----->| Header: GET 0x4101af90 | | GET | Token: 0xaa | | | Uri-Host: sensor.example | | | Uri-Path: status | | | Observe: (empty) | | | | |<-----+ Header: 2.05 0x6145af90 | | 2.05 | Token: 0xaa | | | Observe: 67 | | | Max-Age: 60 | | | Payload: "ready" | | | |<-----+ | Header: 2.05 0x4145af94 | 2.05 | | Token: 0x6a | | | Observe: 17346 | | | Max-Age: 60 | | | Payload: "ready" | | | +- - ->| | Header: 0x6000af94 | | | | |<-----+ Header: 2.05 0x51455a20 | | 2.05 | Token: 0xaa | | | Observe: 157 | | | Max-Age: 60 | | | Payload: "busy" | | | |<-----+ | Header: 2.05 0x5145af9b | 2.05 | | Token: 0x6a | | | Observe: 17436 | | | Max-Age: 60 | | | Payload: "busy" | | |
Figure 8: A client observes a resource through a proxy
A server may want to provide notifications that respond to very specific conditions on some state. This is best done by modeling the resources that the server exposes according to these needs.
For example, for a CoAP server with an attached temperature sensor,
In any case, the client is notified about the current state of the resource whenever the state of the appropriately modeled resource changes. By designing resources that change their state on certain conditions, it is possible to notify the client only when these conditions occur instead of continuously supplying it with information it doesn't need.
By parameterizing resources, this is not limited to conditions defined by the server, but can be extended to arbitrarily complex conditions defined by the client. Thus, the server designer can choose exactly the right level of complexity for the application envisioned and devices used, and is not constrained to a "one size fits all" mechanism built into the protocol.
(To be removed by RFC editor before publication.)
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