According to RTSP documentation page 21 https://www.rfc-editor.org/rfc/rfc2326, an RTSP response is:
Request = Request-Line ; Section 6.1
*( general-header ; Section 5
| request-header ; Section 6.2
| entity-header ) ; Section 8.1
CRLF
[ message-body ] ; Section 4.3
The *, as far as I know and according to https://www.w3.org/Notation.html, means "1 or more of the thing after it". So I'm interpreting the thing above as
*(general-header|request-header|entity-header)CRLF
This would explain this example below, where the headers are of the type general-header \r\n, like this: Cseq: 2\r\nContent-Base: rtsp://example.com/media.mp4\r\nContent-Type: application/sdp\r\nContent-Length: 360\r\n:
S->C: RTSP/1.0 200 OK
CSeq: 2
Content-Base: rtsp://example.com/media.mp4
Content-Type: application/sdp
Content-Length: 460
m=video 0 RTP/AVP 96
a=control:streamid=0
a=range:npt=0-7.741000
a=length:npt=7.741000
a=rtpmap:96 MP4V-ES/5544
a=mimetype:string;"video/MP4V-ES"
a=AvgBitRate:integer;304018
a=StreamName:string;"hinted video track"
m=audio 0 RTP/AVP 97
a=control:streamid=1
a=range:npt=0-7.712000
a=length:npt=7.712000
a=rtpmap:97 mpeg4-generic/32000/2
a=mimetype:string;"audio/mpeg4-generic"
a=AvgBitRate:integer;65790
a=StreamName:string;"hinted audio track"
The headers are in the form general-header \r\n (where \r\n is CRLF). But what about that extra white line (a \r\n) before the message body? This is not explained by the repetition.
I think I'm interpreting something wrong.
This interpretation is correct:
Request = Request-Line *(general-header|request-header|entity-header) CRLF [message-body]
Although the example given is a response, which has this similar grammar:
Response = Status-Line *(general-header|response-header|entity-header) CRLF [message-body]
In either case, the set of headers is separated from the message-body by a CRLF (\r\n), which is NOT repeated. In the example:
RTSP/1.0 200 OK # This is the Status-Line
CSeq: 2 # general-header (see Section 12)
Content-Base: rtsp://example.com/media.mp4 # entity-header (see Section 8.1, 12.11)
Content-Type: application/sdp # entity-header (see Section 8.1, 12.16)
Content-Length: 460 # entity-header (see Section 8.1, 12.14)
# the CRLF marking the end of headers
m=video 0 RTP/AVP 96 # the message body follows
...
"The thing after the *" is the parenthesis block, without the CRLF. Each header ends with a CRLF because the grammar for message headers includes that. See Section 4.2, which refers to Section 4.2 of RFC 2068:
message-header = field-name ":" [ field-value ] CRLF