Transport and Tunnel Modes in IPsec
The IPsec standards define two distinct modes of IPsec operation, transport mode and
tunnel mode. The modes do not affect the encoding of packets. The packets
are protected by AH, ESP, or both in each mode. The modes differ
in policy application when the inner packet is an IP packet.
In transport mode, the outer header determines the IPsec policy that protects the inner IP packet.
In tunnel mode, the inner IP packet determines the IPsec policy that protects its contents.
In transport mode, the outer header, the next header, and any ports that
the next header supports, can be used to determine IPsec policy. In effect,
IPsec can enforce different transport mode policies between two IP addresses to the
granularity of a single port. For example, if the next header is TCP,
which supports ports, IPsec policy can be set for a TCP port of
the outer IP address. Similarly, if the next header is an IP
header, the outer header and the inner IP header can be used to
determine IPsec policy.
Tunnel mode works only on IP-in-IP datagrams. Tunneling in tunnel mode can be
useful when computer workers at home are connecting to a central computer location.
In tunnel mode, IPsec policy is enforced on the contents of the inner
IP datagram. Different IPsec policies can be enforced for different inner IP addresses.
That is, the inner IP header, its next header, and the ports that
the next header supports, can enforce a policy. Unlike transport mode, in tunnel
mode the outer IP header does not dictate the policy of its inner
IP datagram.
Therefore, in tunnel mode, IPsec policy can be specified for subnets of a
LAN behind a router and can be specified for ports on those subnets.
IPsec policy can also be specified for particular IP addresses, that is, hosts,
on those subnets. The ports of those hosts can also have a specific
IPsec policy. However, if a dynamic routing protocol is run over a tunnel,
neither subnet selection nor address selection should be used, because the view of
the network topology on the peer network could change. Changes would invalidate
the static IPsec policy. For examples of tunneling procedures that include configuring
static routes, see Protecting a VPN With IPsec.
In Solaris, tunnel mode can be enforced only on an IP tunneling network
interface. The ipsecconf command provides a tunnel keyword to select an IP
tunneling network interface. When the tunnel keyword is present in a rule,
all selectors that are specified in that rule apply to the inner packet.
In transport mode, ESP, AH, or both ESP and AH, can protect the
datagram.
Figure 19-3 shows an IP header with an unprotected TCP packet.
Figure 19-3 Unprotected IP Packet Carrying TCP Information
In transport mode, ESP protects the data as shown in Figure 19-4.
Figure 19-4 Protected IP Packet Carrying TCP Information
In transport mode, AH protects the data as shown in Figure 19-5.
Figure 19-5 Packet Protected by an Authentication Header
AH actually covers the data before the data appears in the datagram. Consequently,
the protection that is provided by AH, even in transport mode, covers some
of the IP header.
In tunnel mode, the entire datagram is inside the protection of an IPsec header.
The datagram in Figure 19-3 is protected in tunnel mode by an outer
IPsec header, and in this case ESP, as is shown in Figure 19-6.
Figure 19-6 IPsec Packet Protected in Tunnel Mode
The ipsecconf command includes keywords to set tunnels in tunnel mode or in
transport mode.
