IPSec Modes: Transport and Tunnel
(Page 2 of 4)
As its name suggests, in transport mode, the protocol protects the message passed down to IP from the transport layer. The message is processed by AH/ESP and the appropriate header(s) added in front of the transport (UDP or TCP) header. The IP header is then added in front of that by IP.
Another way of looking at this is as follows. Normally the transport layer packages data for transmission and sends it to IP. From IP's perspective, this transport layer message is the payload of the IP datagram. When IPSec is used in transport mode, the IPSec header is applied only over this IP payload, not the IP header. The AH and/or ESP headers appears between the original, single IP header and the IP payload. This is illustrated in Figure 119.
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