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Saúl Ortega
Asked: 2020-04-08 17:11:10 +0800 CST 2020-04-08 17:11:10 +0800 CST

Understanding the specifications of network protocols

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I would like to understand the typical format table of a network communication message, in the specification of any protocol.

For example ( https://www.ietf.org/rfc/rfc3561.txt ):

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |R|A|    Reserved     |Prefix Sz|   Hop Count   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Destination IP address                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Destination Sequence Number                  |

What does each column mean? Is it a byte? a bit?

And, what puzzles me even more, I don't know how to interpret the rows. A message is supposed to be linear, one bit after another, so I don't understand how to interpret the rows.

I would also appreciate if you could show me an example of how to implement it in any programming language.

servidor

1 Answers

  1. Best Answer

    In the case that you show each column represents a bit. That's also obvious if you check what types of data the fields contain - a 32-bit IP makes a lot of sense, a 32-byte IP would be a bit overkill ;)

    The data is linear - the second row is bit 32-63, the third 64-95 etc.

    In the case of the RREP then it means:

    • type: 1 byte, value 2
    • flags: 2 byte of it 1 bit flag repair, 1 bit flag ACK, ultimpos 5 bit prefix
    • hops: 1 byte (the maximum number of hops to the peer
    • Destination IP: 4 byte
    • ...
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