CoAP（Constrained Application Protocol）は、インターネット技術の標準化を推進する任意団体のIETF（The Internet Engineering Task Force）で標準化策定が進む機器間通信向けのプロトコル。
CoAPは、ヘッダサイズが4バイトであり、代表的な通信プロトコルであるHTTPの140バイトより軽量であるため、HTTPと比較して約6割の通信量削減の効果が期待されている。
RFC7252 The Constrained Application Protocol (CoAP)
簡易HTTPなCoAP(Constrained Application Protocol)プロトコル
+----------------------+
| Application |
+----------------------+
+----------------------+ \
| Requests/Responses | |
|----------------------| | CoAP
| Messages | |
+----------------------+ /
+----------------------+
| UDP |
+----------------------+
Figure 1: Abstract Layering of CoAP
CoAPは、リクエスト/レスポンス及びメッセージのプロトコルを定義しています。
Client Server
| |
| CON [0x7d34] |
+----------------->|
| |
| ACK [0x7d34] |
|<-----------------+
| |
Figure 2: Reliable Message Transmission
CONとACKだけの転送。0x7d34はメッセージID。
Client Server
| |
| NON [0x01a0] |
+----------------->|
| |
Figure 3: Unreliable Message Transmission
NONだから返信が不要
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 Piggybacked Responses
温度センサーの交信例。CONのあと、HTTPのGETメソッドが使われ、ACKのあと、2.05や4.04などのステータスコードとコンテンツが帰ってくる。
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
非同期通信のイメージ。Token がリクエストの識別子。
Client Server
| |
| NON [0x7a11] |
| GET /temperature |
| (Token 0x74) |
+----------------->|
| |
| NON [0x23bc] |
| 2.05 Content |
| (Token 0x74) |
| "22.5 C" |
|<-----------------+
| |
Figure 6: A Request and a Response Carried in Non-confirmable
Messages
NONベースの通信。
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 | TKL | Code | Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Token (if any, TKL bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Message Format
+----------+--- --+--- --+--- --+--- --+
| | CON | NON | ACK | RST |
+----------+-----+-----+-----+-----+
| Request | X | X | - | - |
| Response | X | X | X | - |
| Empty | * | - | X | X |
+----------+--- --+--- --+-- ---+--- --+
Table 1: Usage of Message Types
+-------------------+---------------+
| name | default value |
+-------------------+---------------+
| ACK_TIMEOUT | 2 seconds |
| ACK_RANDOM_FACTOR | 1.5 |
| MAX_RETRANSMIT | 4 |
| NSTART | 1 |
| DEFAULT_LEISURE | 5 seconds |
| PROBING_RATE | 1 byte/second |
+-------------------+---------------+
Table 2: CoAP Protocol Parameters
+------+--------+-----------+
| Code | Name | Reference |
+------+--------+-----------+
| 0.01 | GET | [RFC7252] |
| 0.02 | POST | [RFC7252] |
| 0.03 | PUT | [RFC7252] |
| 0.04 | DELETE | [RFC7252] |
+------+--------+-----------+
Table 5: CoAP Method Codes
+------+------------------------------+-----------+
| Code | Description | Reference |
+------+------------------------------+-----------+
| 2.01 | Created | [RFC7252] |
| 2.02 | Deleted | [RFC7252] |
| 2.03 | Valid | [RFC7252] |
| 2.04 | Changed | [RFC7252] |
| 2.05 | Content | [RFC7252] |
| 4.00 | Bad Request | [RFC7252] |
| 4.01 | Unauthorized | [RFC7252] |
| 4.02 | Bad Option | [RFC7252] |
| 4.03 | Forbidden | [RFC7252] |
| 4.04 | Not Found | [RFC7252] |
| 4.05 | Method Not Allowed | [RFC7252] |
| 4.06 | Not Acceptable | [RFC7252] |
| 4.12 | Precondition Failed | [RFC7252] |
| 4.13 | Request Entity Too Large | [RFC7252] |
| 4.15 | Unsupported Content-Format | [RFC7252] |
| 5.00 | Internal Server Error | [RFC7252] |
| 5.01 | Not Implemented | [RFC7252] |
| 5.02 | Bad Gateway | [RFC7252] |
| 5.03 | Service Unavailable | [RFC7252] |
| 5.04 | Gateway Timeout | [RFC7252] |
| 5.05 | Proxying Not Supported | [RFC7252] |
+------+------------------------------+-----------+
Table 6: CoAP Response Codes
+--------+------------------+-----------+
| Number | Name | Reference |
+--------+------------------+-----------+
| 0 | (Reserved) | [RFC7252] |
| 1 | If-Match | [RFC7252] |
| 3 | Uri-Host | [RFC7252] |
| 4 | ETag | [RFC7252] |
| 5 | If-None-Match | [RFC7252] |
| 7 | Uri-Port | [RFC7252] |
| 8 | Location-Path | [RFC7252] |
| 11 | Uri-Path | [RFC7252] |
| 12 | Content-Format | [RFC7252] |
| 14 | Max-Age | [RFC7252] |
| 15 | Uri-Query | [RFC7252] |
| 17 | Accept | [RFC7252] |
| 20 | Location-Query | [RFC7252] |
| 35 | Proxy-Uri | [RFC7252] |
| 39 | Proxy-Scheme | [RFC7252] |
| 60 | Size1 | [RFC7252] |
| 128 | (Reserved) | [RFC7252] |
| 132 | (Reserved) | [RFC7252] |
| 136 | (Reserved) | [RFC7252] |
| 140 | (Reserved) | [RFC7252] |
+--------+------------------+-----------+
Table 7: CoAP Option Numbers
+--------------------------+----------+----+------------------------+ | Media type | Encoding | ID | Reference | +--------------------------+----------+----+------------------------+ | text/plain; | - | 0 | [RFC2046] [RFC3676] | | charset=utf-8 | | | [RFC5147] | | application/link-format | - | 40 | [RFC6690] | | application/xml | - | 41 | [RFC3023] | | application/octet-stream | - | 42 | [RFC2045] [RFC2046] | | application/exi | - | 47 | [REC-exi-20140211] | | application/json | - | 50 | [RFC7159] | +--------------------------+----------+----+------------------------+ Table 9: CoAP Content-Formats
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