![]() AH is identified in the New IP header with an IP protocol ID of 51. The AH protects everything that does not change in transit. The AH does not protect all of the fields in the New IP Header because some change in transit, and the sender cannot predict how they might change. AH’s job is to protect the entire packet. The AH can be applied alone or together with the ESP, when IPSec is in tunnel mode. The packet diagram below illustrates IPSec Tunnel mode with AH header: The packet diagram below illustrates IPSec Tunnel mode with ESP header:ĮSP is identified in the New IP header with an IP protocol ID of 50. Between AH and ESP, ESP is most commonly used in IPSec VPN Tunnel configuration. In tunnel mode, an IPSec header ( AH or ESP header) is inserted between the IP header and the upper layer protocol. Once decrypted by the firewall appliance, the client’s original IP packet is sent to the local network. Traffic from the client is encrypted, encapsulated inside a new IP packet and sent to the other end. The client connects to the IPSec Gateway. In this example, each router acts as an IPSec Gateway for their LAN, providing secure connectivity to the remote network:Īnother example of tunnel mode is an IPSec tunnel between a Cisco VPN Client and an IPSec Gateway (e.g ASA5510 or PIX Firewall). Configuration and setup of this topology is extensively covered in our Site-to-Site IPSec VPN article. Tunnel mode is used to encrypt traffic between secure IPSec Gateways, for example two Cisco routers connected over the Internet via IPSec VPN. Tunnel mode is most commonly used between gateways (Cisco routers or ASA firewalls), or at an end-station to a gateway, the gateway acting as a proxy for the hosts behind it. This means IPSec wraps the original packet, encrypts it, adds a new IP header and sends it to the other side of the VPN tunnel (IPSec peer). With tunnel mode, the entire original IP packet is protected by IPSec. Use of each mode depends on the requirements and implementation of IPSec. IPSec can be configured to operate in two different modes, Tunnel and Transport mode. Understanding IPSec Modes –Tunnel Mode & Transport Mode Analysing the ESP and AH protocols is out of this article’s scope, however you can turn to our IPSec article where you’ll find an in-depth analysis and packet diagrams to help make the concept clear. IPSec’s protocol objective is to provide security services for IP packets such as encrypting sensitive data, authentication, protection against replay and data confidentiality.Īs outlined in our IPSec protocol article, Encapsulating Security Payload (ESP) and Authentication Header (AH) are the two IPSec security protocols used to provide these security services. ![]()
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