Wireshark-users: Re: [Wireshark-users] Fwd: sequence number and packet id

From: Guy Harris <guy@xxxxxxxxxxxx>
Date: Sat, 12 Apr 2008 14:52:08 -0700
Fabiana moreno wrote:
im trying to calculate the end to end delay....

What are the "ends" in "end to end"? Do you mean the delay between the time at which the packet is sent from the host that's sending it to the time at which the packet is received by the host that's receiving it?

If so, that's what the RTP sequence number is used for.

if i have the id i can determine what packet is missing

If you have the RTP sequence number, you can determine what packets aren't in the capture (which could just mean that they weren't captured; it doesn't necessarily mean they weren't sent or weren't received) - to quote RFC 1889:

	sequence number: 16 bits
The sequence number increments by one for each RTP data packet sent, *and may be used by the receiver to detect packet loss* and to restore packet sequence. The initial value of the sequence number is
random (unpredictable) to make known-plaintext attacks on encryption
more difficult, even if the source itself does not encrypt, because the
packets may flow through a translator that does. Techniques for choosing
unpredictable numbers are discussed in [7].