update protocol specification

protocol.txt

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+KCP PROTOCOL SPECIFICATION
+
+
+1. Packet (aka. segment) Structure
+
+KCP has only one kind of segment: both the data and control messages are 
+encoded into the same structure and share the same header.
+
+The KCP packet (aka. segment) structure is as following:
+
+0               4   5   6       8 (BYTE)
++---------------+---+---+-------+
+|     conv      |cmd|frg|  wnd  |
++---------------+---+---+-------+   8
+|     ts        |     sn        |
++---------------+---------------+  16
+|     una       |     len       |
++---------------+---------------+  24
+|                               |
+|        DATA (optional)        |
+|                               |
++-------------------------------+
+
+
+- conv: conversation id (32 bits integer)
+
+The conversation id is used to identify each connection, which will not change
+during the connection life-time.
+
+It is represented by a 32 bits integer which is given at the moment the KCP
+control block (aka. struct ikcpcb, or kcp object) has been created. Each
+packet sent out will carry the conversation id in the first 4 bytes and a
+packet from remote endpoint will not be accepted if it has a different
+conversation id.
+
+The value can be any random number, but in practice, both side between a
+connection will have many KCP objects (or control block) storing in the
+containers like a map or an array. A index is used as the key to look up one
+KCP object from the container. 
+
+So, the higher 16 bits of conversation id can be used as caller's index while
+the lower 16 bits can be used as callee's index. KCP will not handle
+handshake, and the index in both side can be decided and exchanged after 
+connection establish.
+
+When you receive and accept a remote packet, the local index can be extracted
+from the conversation id and the kcp object which is in charge of this
+connection can be find out from your map or array.
+
+
+- cmd: command
+
+- frg: fragment count
+
+- wnd: window size
+
+- ts: timestamp
+
+- sn: serial number
+
+- una: un-acknowledged serial number
+
+
+# vim: set ts=4 sw=4 tw=0 noet cc=78 wrap textwidth=78 :
+