[Ffmpeg-devel] rtp_write_header

Vermeir, Thijs thijs.vermeir
Wed Jul 26 14:00:42 CEST 2006


I found this part in the code in libavformat/rtp.c

static int rtp_write_header(AVFormatContext *s1)
// following 2 FIXMies could be set based on the current time, theres
normaly no info leak, as rtp will likely be transmitted immedeatly
    s->base_timestamp = 0; /* FIXME: was random(), what should this be?
    s->timestamp = s->base_timestamp;
    s->ssrc = 0; /* FIXME: was random(), what should this be? */
    s->first_packet = 1;

The 2 values base_timestamp and ssrc should be random. 
- base_timestamp : This should be random to gain a random offset, the
timestamp in rtp represent the time send, not the time playing that
packet of data. Randomising this value makes a plain text attacks more
difficult on an encrypted RTP stream more difficult. Using zero as
base_timestamp will work as good but leaves a very small hole for the
black hats....

- SSRC or Synchronization source is also random. It need to be unique on
an RTP network. If you use 1 stream there is no problem. but with more
streams this is not gonna work with the innitial value 0. An RTCP packet
let you know when you have choosen randomly the same ssrc. 

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