Fixes to PGM wire format description

doc/zmq_pgm.txt

@@ -75,48 +75,55 @@ boundaries and a single 0MQ message may span several PGM datagrams. This stream
 of data consists of 0MQ messages encapsulated in 'frames' as described in
 linkzmq:zmq_tcp[7].
 
+
+PGM datagram payload
+~~~~~~~~~~~~~~~~~~~~
+The following ABNF grammar represents the payload of a single PGM datagram as
+used by 0MQ:
+
+....
+datagram               = (offset data)
+offset                 = 2OCTET
+data                   = *OCTET
+....
+
 In order for late joining consumers to be able to identify message boundaries,
 each PGM datagram payload starts with a 16-bit unsigned integer in network byte
 order specifying either the offset of the first message 'frame' in the datagram
 or containing the value 0xFFFF if the datagram contains solely an intermediate
 part of a larger message.
 
-The payload of a single PGM datagram as used by 0MQ can thus be defined by the
-following ABNF grammar:
+The following diagram illustrates the layout of a single PGM datagram payload:
 
 ....
-datagram               = (offset data)
-offset                 = 2OCTET
-data                   = *OCTET
++------------------+----------------------+
+| offset (16 bits) |         data         |
++------------------+----------------------+
 ....
 
-As already explained above, the data from consecutive datagrams are interpreted
-as a bytestream that's parsed according to the grammar described in
-linkzmq:zmq_tcp[7].
+The following diagram further illustrates how three example 0MQ frames are laid
+out in consecutive PGM datagram payloads:
+
+....
+First datagram payload
++--------------+-------------+---------------------+
+| Frame offset |   Frame 1   |   Frame 2, part 1   |
+|    0x0000    | (Message 1) | (Message 2, part 1) |
++--------------+-------------+---------------------+
+
+Second datagram payload
++--------------+---------------------+
+| Frame offset |   Frame 2, part 2   |
+| 0xFFFF       | (Message 2, part 2) |
++--------------+---------------------+
+
+Third datagram payload
++--------------+----------------------------+-------------+
+| Frame offset |   Frame 2, final 8 bytes   |   Frame 3   |
+| 0x0008       | (Message 2, final 8 bytes) | (Message 3) |
++--------------+----------------------------+-------------+
+....
 
-Each packet thus looks like this:
- 
-----
-+-----------+------------+------------------+----------------------+
-| IP header | PGM header | offset (16 bits) |        data          |
-+-----------+------------+------------------+----------------------+
-----
- 
-Following example shows how messages are arranged in subsequent packets:
- 
-----
-+---------------+--------+-----------+-----------------------------+
-| PGM/IPheaders | 0x0000 | message 1 |      message 2 (part 1)     |
-+---------------+--------+-----------+-----------------------------+
- 
-+---------------+--------+-----------------------------------------+
-| PGM/IPheaders | 0xFFFF |            message 2 (part 2)           |
-+---------------+--------+-----------------------------------------+
- 
-+---------------+--------+--------------------------+-----------+
-| PGM/IPheaders | 0x0008 | message 2 (last 8 bytes) | message 3 |
-+---------------+--------+--------------------------+-----------+
-----
 
 EXAMPLE
 -------