Enterprise QoS Solution Reference Network Design Guide

Site-to-Site V3PN QoS Considerations
6-6
Figure 6-3 Anatomy of an IPSec-Encrypted G.711 Packet
G.711
200 Bytes
IP GRE
224 Bytes
IPSec ESP
Tunnel Mode
280 Bytes
IPSec
Hdr
ESP
Hdr
ESP
IV
GRE IP
Hdr
GRE IP
Hdr
Enterprise QoS Solution Reference Network Design Guide
20
GRE
4
GRE
IP
Hdr
20
IP
Hdr
20
IP
Hdr
UDP RTP Voice
Chapter 6 IPSec VPN QoS Design
The Layer 3 data rate for a G.729 call (at 50 pps) is 24 kbps. IP GRE tunnel overhead adds 24 bytes per
packet. IPSec ESP adds another 52 bytes. The combined additional overhead increases the rate from 24
kbps (clear voice) to just less than 56 kbps (IPSec ESP tunnel-mode encrypted voice).
The calculation is as follows:
60 bytes per packet (G.729 voice)
24 bytes per packet (IP GRE overhead)
+ 52 bytes per packet (IPSec ESP overhead)
136 bytes per packet
· 8 bits per byte
1088 bits per packet
· 50 packets per second
54,400 bits per second
The additional overhead represents a 227 percent increase in the bandwidth required for an encrypted
G.729 call.
The 136-byte packet’s header, data, and trailer fields for an IPSec tunnel-mode ESP encrypted G.729 call
are shown in Figure 6-4.
8
UDP
12
RTP
RTP
160
160
ESP
Pad/NH
ESP
Auth
20 8 8 20 4 20 8 12 160 2–257 12
8
UDP
Encrypted
12
Authenticated
Voice
Voice
Version 3.3