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Packet/Header size VoIP over IPSEC

Posted on 2006-06-29
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Hey all...

Anyone have an article describing packet size of IPSEC headers?

I am trying to calculate what the additional header size will be...
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Question by:shepimport
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grblades earned 2000 total points
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Have a look at http://www.cisco.com/warp/public/105/pmtud_ipfrag.pdf

The overhead can vary depending on the data, encoding method and a few other factors but generally good values to work with are:-

32 bytes when using transport mode where only the data in encrypted and the original IP headers are left intact. This is not commonly used because the source IP address is often on a private network.
52 bytes for tunnel mode where the whole original packet is encrypted and an additional IP header is added.

The ip/udp/rtp overhead is 20+8+12 = 40 bytes
See http://www.newport-networks.com/whitepapers/voip-bandwidth2.html

If you take the g711 codec for example the data size is typically 160 bytes (64kbps bandwidth).
So 200 bytes (80kbps) over IP and 252 bytes (101kbps) over IPSEC

Thats not too band for the high bandwidth ulaw codec but for lower bandwidth codecs the overheads become very significant.
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