15Nov/1219

COAP

In the following exercise, we are going to study the CoAP protocol (Constraint Application Protocol ) used in Wireless Sensor Network (WSN), to simplify implementation in Sensor Nodes which have very limited resources in term of memory, computing power and energy. The standard is generic and does not make any assumption on Layer 2, but in our case we will suppose that layer 1 and 2 are IEEE 802.15.4. At layer 3, the 6LoWPAN protocol compresses the 40 bytes of the IPv6 header into 20 bytes. Another characteristic of WSN, is the high error rate on the radio link which leads to frame losses.

Basic CoAP protocol

CoAP standard states 1 :


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.

Question 1 Draw the protocol stack of a Sensor Node


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).
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.

                        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

Question 2 (1 point) What happens if the first CON frame is lost ?

Question 3 (1 point) Suppose that the request is “Add temperature by 1 degree” what happens if the acknowledgment of this request is lost ?

Question 4 (2 points) Define a Finite State Machine for this protocol for the sender.

The draft gives the CoAP PDU 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).

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.

Code: 8-bit unsigned integer. Indicates if the message carries a request (1-31) or a response (64-191), or is empty (0).

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.

3.2. Option Format

Options MUST appear in order of their Option Number. 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.

    0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | Option Delta  |    Length     | for 0..14
   +---+---+---+---+---+---+---+---+
   |   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.

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. 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 .
-string: A Unicode string which is encoded using UTF-8 [RFC3629] in Net-Unicode form [RFC5198].

Question 5 (1 point) What is the advantage of using a TLV structure ?


The individual CoAP options are summarized in Table 1 and explained below.

   +-----+----------+----------------+--------+---------+-------------+
   | 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
`

The annex of the document gives an example of how CoAP messages are coded.


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.


   Client  Server
      |      |
      |      |
      ------>|     ader f:2| C/-------+er f:2| C/
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   |   Opt3
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   | +-+-+-+-+
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   | +-+-+-+-+
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   | +-+-+-+-+
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