length) are inserted between the original IP header and ESP header. The total length, protocol, and checksum fields are changed to match this modification. Note
NAT keepalives can be used to keep the dynamic NAT mapping alive during a connection between two peers. NAT keepalives are UDP packets with an unencrypted payload of 1 byte. By default, no NAT keepalives are sent. Mode Configuration Option
Mode configuration is an option for pushing system parameters (for example, IP address and DNS attributes) to the peer, which is usually the client in a remote access VPN. The mode configuration option is used extensively for Easy VPN. Easy VPN allows remote clients to receive security policies from a Cisco Easy VPN Server, minimizing configuration requirements at the client. Extended Authentication
Xauth is based on the IKE protocol. Xauth allows authentication, authorization, and accounting (AAA) methods to perform user authentication in a separate phase after the IKE authentication Phase 1 exchange. Xauth does not replace IKE. IKE allows for device authentication while Xauth allows for user authentication, which occurs after IKE device authentication. A user authentication option can be a generic username and password, Challenge Handshake Authentication Protocol (CHAP), one-time passwords (OTPs), or Secure Key (S/Key).

Content 3.2 Understanding IPsec Components and IPsec VPN Features 3.2.6 ESP and AH Protocols, Transport, and Tunnel Modes These two IP protocols are used in the IPsec standard:

ESP: The ESP header (IP protocol 50) forms the core of the IPsec protocol. This protocol, in conjunction with an agreed-upon encryption method or transform set, protects data by rendering the data undecipherable. This protocol protects only the data portion of the packet. This protocol can optionally also provide for authentication of the protected data.
AH: The other part of IPsec is formed by the AH protocol (IP protocol 51). The AH does not protect data in the usual sense by hiding the data but by adding a tamper-evident seal to the data. This protocol also protects fields in the IP header carrying the data, including the address fields of the IP header. The AH protocol should not be used alone when data confidentiality is required.

IPsec has two methods of forwarding data across a network, tunnel mode and transport mode, which differ in their application and in the amount of overhead added to the passenger packet:

Tunnel mode: Tunnel mode works by encapsulating and protecting an entire IP packet. Because tunnel mode encapsulates or hides the IP header of the packet, a new IP header must be added for the packet to be successfully forwarded. The encrypting devices themselves own the IP addresses that are used in this new header. These addresses can be specified in the configuration of Cisco IOS routers. Tunnel mode can be used with either ESP or AH or with both. Tunnel mode results in an additional packet expansion of approximately 20 bytes because of the new IP header.
Transport mode: Because packet expansion can be a concern during the forwarding of small packets, a second forwarding method is also possible. IPsec transport mode works by inserting the ESP header between the IP header and the next protocol or the Transport layer of the packet. Both IP addresses of the two network nodes whose traffic is being protected by IPsec are visible. This mode of IPsec can sometimes be susceptible to traffic analysis. However, because there is no additional IP header added, the result is less packet expansion. Transport mode can be deployed with either ESP or AH or both. This mode works well with Generic Routing Encapsulation (GRE) because GRE already hides the addresses of the end stations by adding an IP header.

ESP and AH Header
You can achieve AH authentication by applying a keyed one-way hash function to the packet, creating a hash or message digest. The hash is combined with the text and then transmitted. Changes in any part of the packet that occur during transit are detected by the receiver when the receiver performs the same one-way hash function on the received packet and compares the value of the message digest that the sender has supplied. One-way hash also involves the use of a symmetric key between the two systems, meaning that authenticity is guaranteed. ESP provides confidentiality by encrypting the payload. The default algorithm for IPsec is 56-bit DES. Cisco products also support the use of 3DES for stronger encryption. The ESP encryption algorithms by themselves do not provide authentication or guarantee data integrity. ESP encryption with an authentication and data integrity service can be achieved in two ways:

Authenticated ESP format
Nested ESP within AH

With authenticated ESP, IPsec encrypts the payload using one symmetric key, then calculates an authentication value for the encrypted data using a second symmetric key and the HMAC-SHA1 or HMAC-MD5 algorithm. The ESP authentication value is appended to the end of the packet. The recipient computes its own authentication value for the encrypted data using the second symmetric key and the same algorithm. The recipient compares the result with the transmitted authentication value. If the values match, the recipient then decrypts the encrypted portion of the packet with the first symmetric key and extracts the original data. Figure depicts authenticated ESP in both transport and tunnel modes. An ESP packet can be nested within an AH packet. First, the payload is encrypted. Next, the encrypted payload is sent through a hash algorithm: MD5 or SHA-1. The hash provides origin authentication and data integrity for the data payload. Figure shows nested ESP in AH using transport mode.

Content 3.2 Understanding IPsec Components and IPsec VPN Features 3.2.7 AH Authentication and Integrity The AH function is applied to the entire datagram, except for any mutable IP header fields that change in transit, such as Time to Live (TTL) fields that are modified by the routers along the transmission path. AH works as follows and as shown in Figure : Step 1 The IP header and data payload are hashed. Step 2 The hash is used to build an AH header, which is appended to the original packet. Step 3 The new packet is transmitted to the IPsec peer router. Step 4 The peer router hashes the IP header and data payload. Step 5 The peer router extracts the transmitted hash from the AH header. Step 6 The peer router compares the two hashes. The hashes must exactly match. Even if one bit is changed in the transmitted packet, the hash output on the received packet will change and the AH header will not match. AH supports MD5 and SHA-1 algorithms. Figure shows an AH frame format in tunnel mode.

Content 3.2 Understanding IPsec Components and IPsec VPN Features 3.2.8 ESP Protocol Between two security gateways, the original payload is well protected because the entire original IP datagram is encrypted. An ESP header and trailer are added to the encrypted payload. With ESP authentication, the encrypted IP datagram and the ESP header or trailer are included in the hashing process. Last, a new IP header is appended to the front of the authenticated payload. The new IP address is used to route the packet through the Internet. When both ESP authentication and encryption are selected, encryption is performed first before authentication. The basis for this order of processing is to facilitate rapid detection and rejection of replayed or bogus packets by the receiving node. Prior to decrypting the packet, the receiver can authenticate inbound packets. By authenticating packets first, the receiver can detect problems and potentially reduce the impact of denial of service (DoS) attacks. Transport mode is the default mode for IPsec. Transport mode only protects the payload of the packet