Fix raw socket maximum length limit (#508)

* Fix raw socket maximum length limit
* Introduce 25th raw socket payload length bit
* Tweak raw socket payload length example text

rfc/text/advanced/ap_38_transport_rawsocket.md

@@ -63,6 +63,7 @@ The possible values for `LENGTH` are:
          0: 2**9 octets
          1: 2**10 octets
         ...
+        14: 2**23 octets
         15: 2**24 octets
 
 This means a *Client* can choose the maximum message length between **512** and **16M** octets.
@@ -221,16 +222,32 @@ The serialized octets for a message to be sent are prefixed with exactly 4 octet
 {align="left"}
         MSB                                 LSB
         31                                    0
-        RRRR RTTT LLLL LLLL LLLL LLLL LLLL LLLL
+        RRRR XTTT LLLL LLLL LLLL LLLL LLLL LLLL
 
-The *first octet* has the following structure
+The *three least significant octets* constitute an unsigned 24 bit integer that provides the length of transport message payload following, excluding the 4 octets that constitute the prefix.
+
+The *most significant octet* has the following structure
 
 {align="left"}
         MSB   LSB
         7       0
-        RRRR RTTT
+        RRRR XTTT
+
+The four bits `RRRR` are reserved for future use and MUST be all zeros for now.
+
+`X` is an extra (25th) bit used to encode the message payload length. This bit is only set when the payload length is exactly 16M octets, with the 24 remaining `L` bits cleared:
+
+{align="left"}
+        MSB                                 LSB
+        31                                    0
+        RRRR 1TTT 0000 0000 0000 0000 0000 0000
+
+When the payload length is 16M-1 octets, for example, the prefix would be:
 
-The five bits `RRRRR` are reserved for future use and MUST be all zeros for now.
+{align="left"}
+        MSB                                 LSB
+        31                                    0
+        RRRR 0TTT 1111 1111 1111 1111 1111 1111
 
 The three bits `TTT` encode the type of the transport message:
 
@@ -240,8 +257,6 @@ The three bits `TTT` encode the type of the transport message:
         2: PONG
         3-7: reserved
 
-The *three remaining octets* constitute an unsigned 24 bit integer that provides the length of transport message payload following, excluding the 4 octets that constitute the prefix.
-
 For a regular WAMP message (`TTT == 0`), the length is the length of the serialized WAMP message: the number of octets after serialization (excluding the 4 octets of the prefix).
 
 For a `PING` message (`TTT == 1`), the length is the length of the arbitrary payload that follows. A *Peer* MUST reply to each `PING` by sending exactly one `PONG` immediately, and the `PONG` MUST echo back the payload of the `PING` exactly.