Subject: [SIP] SIP, RTP, NTP (SNTP) and End Device Synchronization

From: "Mark J. Yusko" <[email-masked]>
Date: Tue, 29 May 2001 11:05:28 -0400

Dr. Rosenberg -

I have questions concerning required timing synchronization within a
SIP-based voice network:

a. Which network elements within an SIP-based, IP network should be
synchronized?
Should IP Phones?  Mixers (Conference Bridges)?
Is NTP (SNTP) recommended for all devices?

b. Is end-point (IP phone-to-IP phone) synchronization required for RTP to
function properly?

The "old world" PSTN is dependent upon accurate timing and synchronization.
With packetized
voice, variable packet delay and loss can cause havoc with synchronization
and window processing.

c. Is a delayed, RTP encapsulated UDP packet ever "ignored" at the
destination device due to extraneous
packet delay or packet ordering?

Regards,

Mark J. Yusko
Product Manager
Vonage
+1 (732) 528-2617

_______________________________________________
This list is for continuing development of the SIP protocol.
The sip-implementor's list is the place to discuss implementation,
and to receive advice on understanding existing sip.
To subscribe to it, send mail to 
[email-masked] with "subscribe" in the body.

---

From: Colin Perkins <[email-masked]>
Date: Tue, 29 May 2001 13:14:34 -0400

Questions about RTP should be addressed to the [email-masked] mailing
list, rather than SIP...

>Is NTP (SNTP) recommended for all devices?

None of them are required to be synchronised (expect if you are sending a
multimedia presentation from multiple machines and want to lip-sync the
result, in which case the sending systems need to be sync'd).

>b. Is end-point (IP phone-to-IP phone) synchronization required for RTP to

No.

>voice, variable packet delay and loss can cause havoc with synchronization

The receiver needs to be smart enough to cope with variable delay, and to
reconstruct the timing. 

>packet delay or packet ordering?

Possibly, yes. It's application dependent, the standard does not specify
the behaviour.

Colin