Nokia Voyager Reference Manual page 585

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In transport mode the original IP header remains the outer header. The

security header is placed between the IP header and the IP payload. This mode

offers some light bandwidth savings, at the expense of exposing the original

IP header to third party elements in the packet path. It is generally used by

hosts—communication end-points. It can be used by routers if they are acting

as communication end-points.

In tunnel mode, the original IP datagram is placed inside a new datagram, and

AH and/or ESP are inserted between the IP header of the new packet and the

original IP datagram. The new header points to the tunnel endpoint, and the

original header points to the final destination of the datagram. Tunnel mode

offers the advantage of complete protection of the encapsulated datagram and

the possibility to use private/public address space. Tunnel mode is meant to be

used by routers—gateways. Hosts can operate in tunnel mode too.

With IPsec transport mode:

Voyager Reference Guide

If AH is used, selected portions of the original IP header and the data

payload are authenticated.

If ESP is used, no protection is offered to the IP header, but data payload

is authenticated and can be encrypted.

Authenticated