Subject: [AVT] Doubts in RTCP

From: Angai <[email-masked]>
Date: Thu, 06 Sep 2001 18:08:28 +0530

Hi  all ,

Doubts regarding implementation of rtcp :

1.In  algorithm  A.3  of rfc 1889
   extended_max = s->cycles + s->max_seq  ;
   expected = extended_max - s->base_seq + 1 ; --> Why this 1 is added
here ?

2.Regarding calculation of DLSR :
     I have 64 bits of NtpTime  format .When i find that difference the
value is 64 bits 32 bit secs and 32bit fractions .
      In Rfc it is given " expressed in units of  1/65536 seconds ".
      Whether i should take MidNtp( i.e 16 bit of secs and 16 bits of
frac as i do for last SR )

3.In RTP Data Header Validity check what does this statement means :
    "The length of the packet must be consistent with CC and payload
type "

Thanks in advance
Regards
Angai

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
http://www1.ietf.org/mailman/listinfo/avt

---

From: Magnus Westerlund <[email-masked]>
Date: Thu, 06 Sep 2001 18:04:22 +0200

Hi,

Have you also looked in
http://search.ietf.org/internet-drafts/draft-ietf-avt-rtp-new-10.txt
This clarifies a lot of things much better than RFC 1889

Anyway, the answers to your questions is:
1.
If you don't add the one, you will miss counting the first packet.
example
max_seq = 9
cycles = 0
base_seq = 4

You should have received all packets from seq nr 4 to 9  which is 6 packets

9-4 = 5, i.e. you need to add for the first packet.

2. It is in 1/65536 fractions, which looks similar to NTP 32 bits but DLSR
is an relative time
not an absolute one. Yes, the high 16 bits will be seconds, the low
fractions of a second.
See also figure 2 in the RTP-new draft.

3.  If CC has an value you know that CC*4 bytes are used for CSRC's in a
valid packet. Also the payload type can give at least minimum sizes
possible for it. Some audio codecs only have certain valid increments of
size. If the sum of the RTP header+CC*4+min payload is larger than the
received packet, than it can't be a valid packet.

Regards

Magnus Westerlund

--

Magnus Westerlund

Audio Technology, Ericsson Research
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