Fixed correct description for msgttl (Haivision#847)

* Fixed correct description for msgttl

docs/API-functions.md

@@ -558,7 +558,7 @@ The `SRT_MSGCTRL` structure:
 typedef struct SRT_MsgCtrl_
 {
    int flags;            // Left for future
-   int msgttl;           // TTL for a message, default -1 (delivered always)
+   int msgttl;           // TTL for a message, default -1 (no TTL limitation)
    int inorder;          // Whether a message is allowed to supersede partially lost one. Unused in stream and live mode.
    int boundary;         //0:mid pkt, 1(01b):end of frame, 2(11b):complete frame, 3(10b): start of frame
    uint64_t srctime;     // source timestamp (usec), 0: use internal time     
@@ -575,13 +575,13 @@ intended to specify some special options controlling the details of how the
 called function should work.
 
 * `msgttl`: [IN]. In **message** and **live mode** only, specifies the TTL for 
-sending messages (in `[ms]`). Not used for receiving messages. A packet is 
-scheduled for sending by this call and then waits in the sender buffer to be 
-picked up at the moment when all previously scheduled data are already sent, 
-which may be blocked when the data are scheduled faster than the network can 
-afford to send. Default -1 means to wait indefinitely. If specified, then the 
-packet waits for an opportunity to be sent over the network only up to this TTL, 
-and then, if still not sent, the packet is discarded.
+sending messages (in `[ms]`). Not used for receiving messages. If this value
+is not negative, it defines the maximum time up to which this message should
+stay scheduled for sending for the sake of later retransmission. A message
+is always sent for the first time, but the UDP packet carrying it may be
+(also partially) lost, and if so, lacking packets will be retransmitted. If
+the message is not successfully resent before TTL expires, further retransmission
+is given up and the message is discarded.
 
 * `inorder`: [IN]. In **message mode** only, specifies that sent messages should 
 be extracted by the receiver in the order of sending. This can be meaningful if 
@@ -636,7 +636,7 @@ Sends a payload to a remote party over a given socket.
 * `u`: Socket used to send. The socket must be connected for this operation.
 * `buf`: Points to the buffer containing the payload to send.
 * `len`: Size of the payload specified in `buf`.
-* `ttl`: Time (in `[ms]`) to wait for a possibility to send. See description of 
+* `ttl`: Time (in `[ms]`) to wait for a successful delivery. See description of 
 the [`SRT_MSGCTRL::msgttl`](#SRT_MSGCTRL) field.
 * `inorder`: Required to be received in the order of sending. See 
 [`SRT_MSGCTRL::inorder`](#SRT_MSGCTRL).

docs/API.md

@@ -193,7 +193,7 @@ following interpretation (except `flags` and `boundary` that are reserved for
 future use and should be 0):
 
 * `srt_sendmsg2`:
-    * msgttl: [IN] maximum time (in ms) to wait in sending buffer before being sent (-1 if unused)
+    * msgttl: [IN] maximum time (in ms) to wait for successful delivery (-1: indefinitely)
     * inorder: [IN] if false, the later sent message is allowed to be delivered earlier
     * srctime: [IN] timestamp to be used for sending (0 if current time)
     * pktseq: unused

srtcore/srt.h

@@ -599,7 +599,7 @@ SRT_API       int srt_setsockflag  (SRTSOCKET u, SRT_SOCKOPT opt, const void* op
 typedef struct SRT_MsgCtrl_
 {
    int flags;            // Left for future
-   int msgttl;           // TTL for a message, default -1 (delivered always)
+   int msgttl;           // TTL for a message, default -1 (no TTL limitation)
    int inorder;          // Whether a message is allowed to supersede partially lost one. Unused in stream and live mode.
    int boundary;         // 0:mid pkt, 1(01b):end of frame, 2(11b):complete frame, 3(10b): start of frame
    uint64_t srctime;     // source timestamp (usec), 0: use internal time