Dell POWEREDGE M1000E - Manual (Page 219)

Web Tools Administrator’s Guide 191
53-1002756-01
IPsec concepts
15
Encapsulating Security Payload
ESP provides authentication, and also provides privacy by encrypting the IP datagram. The use of
an ESP header is similar to the use of the AH header. A hash algorithm is used to calculate an
authentication value, the authentication value is sent in an IP datagram, and the same hash
algorithm is used by the receiver to verify the authentication value. ESP can be used in either
transport mode or tunnel mode, as shown in
Figure 43 .
FIGURE 43 ESP header in transport mode and tunnel mode
Basic IPsec configurations
There are three basic configurations for IPsec use:
• Endpoint to Endpoint
• Gateway to Gateway
• Endpoint to Gateway
Endpoint to Endpoint
In an endpoint to endpoint configuration, both endpoints implement IPsec. Transport mode is
commonly used in endpoint to endpoint configurations, and only a single pair of addresses is used.
Typically, this kind of configuration would be used for direct communication between hosts. There
are two drawbacks to consider:
• If network address translation (NAT) is used on the connection, one or both endpoints may be
behind a NAT node. If that is the case, UDP must be used to encapsulate the tunneled packets.
Port numbers in the UDP headers can then be used to identify the endpoint behind the NAT
node.
• Packets cannot be inspected or modified in transit. This means that QoS, traffic shaping, and
firewall applications cannot access the packets, and does not work.