[VOIPSEC] SRTP and ZRTP protocol measurements

Fri Apr 23 01:05:22 CDT 2010

See inline for some answers.

Regards,
Werner

-----Original Message-----
From: voipsec-bounces at voipsa.org [mailto:voipsec-bounces at voipsa.org] On
Behalf Of ext SERGE TUMBA
Sent: Friday, April 23, 2010 1:15 AM
To: VoIP Sec
Subject: [VOIPSEC] SRTP and ZRTP protocol measurements

I am requesting help with SRTP and ZRTP protocol measurements.

I have two configurations:

1. A Router connected to a switch and two switch ports used to connected
two IP Phones (Hardphones) with SRTP deployed so the media tream are
encrypted.

2. Using a VMware for workstations, I used 2 softphones in this second
scenario that I connected to a soft PBX and the two softphones (on
different operating systems, and the PBX on another one), the two
softphones have zfone on each ends for ZRTP support. 

Now is my problem:

I need to look for each protocol SRTP and ZRTP, I need to look at
RTP/SRTP streams (and ZRTP streams as well), the stream between the
phones in each scenario, and compare them:

1. How big the SRTP and ZRTP packets are.

Werner:
SRTP ist just an extension to RTP and RTP is a protocol that usually
uses UDP.
The size of an SRTP packet is usually just 4 bytes longer than a normal
RTP
packt, also the protocol id is the same as RTP. The size of an RTP
packet depends
on the audio/video codec you use. Normal PCM aLaw/uLaw packet are
usually ~340 bytes
if I recall correctly.
SRTP just encrypts the payload of an RTP packet and may add some bytes
for 
authentication (HMAC).

ZRTP packet vary in size but ZRTP is just at the very beginning of a
session, once
the keys are negotiated and exchanged you will see no more ZRTP packets.
During
negotiation ZRTP packet may become big (depending on the algorihms, ~800
bytes).

2. What is the size of their header (overhead).

Werner:
RTP overhead (thus also SRTP) is small, IIRC just 12 or 16 bytes
(included in
the numbers above)

3. What is the size of the payload.

Werner:
Depends on the codec, the frequency you send packets. RTP does not have
a fixed
packet size.

4. What is the delay for such packets.

Werner:
Depends on you application. Usually VoIP audio clients send a packet
every 20ms
(PCM aLaw/uLaw, 160 samples, each sample 2 bytes per packet)

5. What is the jitter for such packets.

Werner:
Depends on the quality of the transmissin. Usually VoIP clients have
jitter buffers
to handle this, some codecs also support packet loss concealment

I have been using the free sniffer program, the Wireshark. And in each
scenario:

1. The SRTP scenario: all the stream between the two IP Phones are "UDP"
and I do understand that this is because the media stream are encrypted,
hence the SRTP packets are encapsulated. Now I am wondering how to find
the above information.

Werner:
As said above: SRTP just encrypts the payload of RTP, does not
"encapsulate it", RTP usually uses UDP as
transport.

2. The ZRTP scenario: The packets between the phones are RTP and other
unknown RTP... Now, I don't see why these packets are not "UDP"? and How
to get the ZRTP information?

Werner:
ZRTP uses the same transport as RTP (UDP) - RTP, SRZP, ZRTP are
application level protocols,
UDP is the transport.

Also I strongly recommend to have a look into the RFCs for RTP, SRTP and
to the proposed
RFC for ZRTP (http://zfoneproject.com/zrtp_ietf.html)

Thank you!

Serge

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