Appendix A. Examples
Appendix A. 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 16 shows a basic GET request causing a piggybacked 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 + 11 = 11, Length 11, Value "temperature"); the Token is left 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 empty Token value. The response includes a Payload of "22.3 C" and is 11 bytes long.
Client Server
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+----->| Header: GET (T=CON, Code=0.01, MID=0x7d34)
| GET | Uri-Path: "temperature"
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|<-----+ Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d34)
| 2.05 | Payload: "22.3 C"
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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 | 0 | GET=1 | MID=0x7d34 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 11 | 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| "22.3 C" (6 B) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16: Confirmable Request; Piggybacked Response
Figure 17 shows a similar example, but with the inclusion of an non-empty Token (Value 0x20) in the request and the response, increasing the sizes to 17 and 12 bytes, respectively.
Client Server
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+----->| Header: GET (T=CON, Code=0.01, MID=0x7d35)
| GET | Token: 0x20
| | Uri-Path: "temperature"
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|<-----+ Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d35)
| 2.05 | Token: 0x20
| | Payload: "22.3 C"
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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=0x7d35 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 11 | 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 | 1 | 2.05=69 | MID=0x7d35 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| "22.3 C" (6 B) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 17: Confirmable Request; Piggybacked Response
In Figure 18, the Confirmable GET request is lost. After ACK_TIMEOUT seconds, the client retransmits the request, resulting in a piggybacked response as in the previous example.
Client Server
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+----X | Header: GET (T=CON, Code=0.01, MID=0x7d36)
| GET | Token: 0x31
| | Uri-Path: "temperature"
TIMEOUT |
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+----->| Header: GET (T=CON, Code=0.01, MID=0x7d36)
| GET | Token: 0x31
| | Uri-Path: "temperature"
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|<-----+ Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d36)
| 2.05 | Token: 0x31
| | Payload: "22.3 C"
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Figure 18: Confirmable Request (Retransmitted); Piggybacked Response
In Figure 19, the first Acknowledgement message from the server to the client is lost. After ACK_TIMEOUT seconds, the client retransmits the request.
Client Server
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+----->| Header: GET (T=CON, Code=0.01, MID=0x7d37)
| GET | Token: 0x42
| | Uri-Path: "temperature"
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| X----+ Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d37)
| 2.05 | Token: 0x42
| | Payload: "22.3 C"
TIMEOUT |
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+----->| Header: GET (T=CON, Code=0.01, MID=0x7d37)
| GET | Token: 0x42
| | Uri-Path: "temperature"
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|<-----+ Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d37)
| 2.05 | Token: 0x42
| | Payload: "22.3 C"
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Figure 19: Confirmable Request; Piggybacked Response (Retransmitted)
In Figure 20, 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
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+----->| Header: GET (T=CON, Code=0.01, MID=0x7d38)
| GET | Token: 0x53
| | Uri-Path: "temperature"
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|<- - -+ Header: (T=ACK, Code=0.00, MID=0x7d38)
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|<-----+ Header: 2.05 Content (T=CON, Code=2.05, MID=0xad7b)
| 2.05 | Token: 0x53
| | Payload: "22.3 C"
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+- - ->| Header: (T=ACK, Code=0.00, MID=0xad7b)
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Figure 20: Confirmable Request; Separate Response
Figure 21 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.
Client Server
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+----->| Header: GET (T=CON, Code=0.01, MID=0x7d39)
| GET | Token: 0x64
| | Uri-Path: "temperature"
CRASH |
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|<- - -+ Header: (T=ACK, Code=0.00, MID=0x7d39)
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|<-----+ Header: 2.05 Content (T=CON, Code=2.05, MID=0xad7c)
| 2.05 | Token: 0x64
| | Payload: "22.3 C"
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+- - ->| Header: (T=RST, Code=0.00, MID=0xad7c)
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Figure 21: Confirmable Request; Separate Response (Unexpected)
Figure 22 shows a basic GET request where the request and the response are Non-confirmable, so both may be lost without notice.
Client Server
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+----->| Header: GET (T=NON, Code=0.01, MID=0x7d40)
| GET | Token: 0x75
| | Uri-Path: "temperature"
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|<-----+ Header: 2.05 Content (T=NON, Code=2.05, MID=0xad7d)
| 2.05 | Token: 0x75
| | Payload: "22.3 C"
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Figure 22: Non-confirmable Request; Non-confirmable Response
In Figure 23, 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.
Client ff02::1 A B C
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+------>| | | | Header: GET (T=NON, Code=0.01, MID=0x7d41)
| GET | | | | Token: 0x86
| | | | Uri-Path: "temperature"
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|<------------+ | | Header: 2.05 (T=NON, Code=2.05, MID=0x60b1)
| 2.05 | | | Token: 0x86
| | | | Payload: "22.3 C"
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| X------------+ | Header: 2.05 (T=NON, Code=2.05, MID=0x01a0)
| 2.05 | | | Token: 0x86
| | | | Payload: "20.9 C"
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|<------------------+ Header: 4.04 (T=NON, Code=4.04, MID=0x952a)
| 4.04 | | | Token: 0x86
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Figure 23: Non-confirmable Request (Multicast); Non-confirmable Response