Decoder une entete SAP (RFC 2974)

[Alex Martelli]

<posted & mailed>

Benoit BESSE wrote:
   ...
> Hello, I would like to decode heading SAP of a message which I receive on
> port 9875. Information are
> 
>>     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
>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>    | V=1 |A|R|T|E|C|   auth len    |         msg id hash           |
>>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>>    |                                                               |
>>    :                originating source (32 or 128 bits)            :
   ...
> My problem is to decode the version and information which follows. Is
> there someone to propose a solution because I maitrise not well the unpack

struct.unpack is suitable to take some binary bytes and make them into
Python values.  However, it does not deal with single bits within a byte,
which is what you must be dealing with here.  For that, you use bit-level
operations of 'masking' (&) and shifting (<< or >>).

I'm not sure if this schema is to be read as 'big-endian' (the 3 bits
of V are the top bits in the first byte) or 'little-endian' (they are
the bottom bits).  In the first case, you could transform the first
byte (seen as a Python value 0-255) into the V-A-R-T-E-C values as
follows, for example (warning, untested code):

def byte_to_VARTEC(byte):
    vartec = []
    for i in range(5):
        vartec.append(byte & 1)
        byte >>= 1
    vartec.append(byte & 7)
    vartec.reverse()
    return vartec

If you have the byte you need as the first byte in a plain string
object s, you can get the equivalent 0-255 value as ord(s[0]).

If the bits are the other way around (V takes the 3 least significant
ones, then A, and so on), then (again untested):

def byte_to_VARTEC(byte):
    vartec = [byte & 7]
    byte >>= 3
    for i in range(5):
        vartec.append(byte & 1)
        byte >>= 1
    return vartec


If you don't know either, get an actual example or two, try decoding
them in each way, and see which one makes sense;-).


Alex