KEEPALIVE timing suggestions (rsocket#200)

* KEEPALIVE timing suggestions

- moved into SETUP where the value is
- include server-to-server and mobile-to-server commentary

* remove newline

* remove change

* soften language for 500ms

Author: benjchristensen

Protocol.md

@@ -282,7 +282,9 @@ Frame Contents
      * (__S__)trict: Adhere to strict interpretation of Data and Metadata.
 * __Major Version__: (16 bits = max value 65,535) Unsigned 16-bit integer of Major version number of the protocol.
 * __Minor Version__: (16 bits = max value 65,535) Unsigned 16-bit integer of Minor version number of the protocol.
-* __Time Between KEEPALIVE Frames__: (31 bits = max value 2^31-1 = 2,147,483,647) Unsigned 31-bit integer of Time (in milliseconds) between KEEPALIVE frames that the client will send. Value MUST be > 0.
+* __Time Between KEEPALIVE Frames__: (31 bits = max value 2^31-1 = 2,147,483,647) Unsigned 31-bit integer of Time (in milliseconds) between KEEPALIVE frames that the client will send. Value MUST be > 0. 
+   * For server-to-server connections, a reasonable time interval between client KEEPALIVE frames is 500ms.
+   * For mobile-to-server connections, the time interval between client KEEPALIVE frames is often > 30,000ms.
 * __Max Lifetime__: (31 bits = max value 2^31-1 = 2,147,483,647) Unsigned 31-bit integer of Time (in milliseconds) that a client will allow a server to not respond to a KEEPALIVE before it is assumed to be dead. Value MUST be > 0. 
 * __Resume Identification Token Length__: (16 bits = max value 65,535) Unsigned 16-bit integer of Resume Identification Token Length in bytes. (Not present if R flag is not set)
 * __Resume Identification Token__: Token used for client resume identification (Not present if R flag is not set)
@@ -398,7 +400,6 @@ When a LEASE expires due to time, the value of the __Number of Requests__ that a
 KEEPALIVE frames MUST always use Stream ID 0 as they pertain to the Connection.
 
 KEEPALIVE frames MUST be initiated by the client and sent periodically with the (__R__)espond flag set.
-A reasonable time interval between client KEEPALIVE frames SHOULD be 500ms.
 
 KEEPALIVE frames MAY be initiated by the server and sent upon application request with the (__R__)espond flag set.