protocol/rtp: better comment for NTPTimestamp and renamed fields

protocol/rtcp/client.go

@@ -263,8 +263,8 @@ func (c *Client) jitter() uint32 {
 // setSenderTs allows us to safely set the current sender report timestamp.
 func (c *Client) setSenderTs(t NTPTimestamp) {
 	c.mu.Lock()
-	binary.BigEndian.PutUint32(c.senderTs[:], t.MSW)
+	binary.BigEndian.PutUint32(c.senderTs[:], t.Seconds)
-	binary.BigEndian.PutUint32(c.senderTs[4:], t.LSW)
+	binary.BigEndian.PutUint32(c.senderTs[4:], t.Fraction)
 	c.mu.Unlock()
 }
 
@@ -285,7 +285,7 @@ func (c *Client) delay() uint32 {
 	return uint32(time.Now().Sub(t).Seconds() / delayUnit)
 }
 
-// received is called when a sender report is received to mark the receive time.
+// markReceivedTime is called when a sender report is received to mark the receive time.
 func (c *Client) markReceivedTime() {
 	c.mu.Lock()
 	c.receiveTime = time.Now()

protocol/rtcp/parse.go

@@ -32,10 +32,20 @@ import (
 	"errors"
 )
 
-// NTPTimestamp describes the NTP timestamp format (http://www.beaglesoft.com/Manual/page53.htm)
+// NTPTimestamp describes an NTP timestamp.
+//
+// NTP timestamps are represented as a 64-bit unsigned fixed-
+// point number, in seconds relative to 0h on 1 January 1900. The integer
+// part is in the first 32 bits and the fraction part in the last 32 bits.
+// This format allows convenient multiple-precision arithmetic and
+// conversion to Time Protocol representation (seconds), but does
+// complicate the conversion to ICMP Timestamp message representation
+// (milliseconds). The precision of this representation is about 200
+// picoseconds, which should be adequate for even the most exotic
+// requirements.
 type NTPTimestamp struct {
-	MSW uint32
+	Seconds  uint32
-	LSW uint32
+	Fraction uint32
 }
 
 // Timestamp gets the timestamp from a receiver report and returns it as the most
@@ -54,7 +64,7 @@ func Timestamp(buf []byte) (NTPTimestamp, error) {
 	}
 
 	return NTPTimestamp{
-		MSW: binary.BigEndian.Uint32(buf[8:]),
+		Seconds:  binary.BigEndian.Uint32(buf[8:]),
-		LSW: binary.BigEndian.Uint32(buf[12:]),
+		Fraction: binary.BigEndian.Uint32(buf[12:]),
 	}, nil
 }

protocol/rtcp/parse_test.go

@@ -51,11 +51,11 @@ func TestTimestamp(t *testing.T) {
 		t.Fatalf("did not expect error: %v", err)
 	}
 
-	if ts.MSW != expectedMSW {
+	if ts.Seconds != expectedMSW {
-		t.Errorf("most significant word of timestamp is not what's expected. \nGot: %v\n Want: %v\n", ts.MSW, expectedMSW)
+		t.Errorf("most significant word of timestamp is not what's expected. \nGot: %v\n Want: %v\n", ts.Seconds, expectedMSW)
 	}
 
-	if ts.LSW != expectedLSW {
+	if ts.Fraction != expectedLSW {
-		t.Errorf("least significant word of timestamp is not what's expected. \nGot: %v\n Want: %v\n", ts.LSW, expectedLSW)
+		t.Errorf("least significant word of timestamp is not what's expected. \nGot: %v\n Want: %v\n", ts.Fraction, expectedLSW)
 	}
 }