IPSec’s protocol objective is to provide
security services for IP packets such as encrypting sensitive data,
authentication, protection against replay and data confidentiality.
As 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. 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.
Understanding IPSec Modes –Tunnel Mode & Transport Mode
IPSec can be configured to operate in
two different modes, Tunnel and Transport mode. Use of each mode depends
on the requirements and implementation of IPSec.
IPSec Tunnel Mode
IPSec tunnel mode is the default mode.
With tunnel mode, the entire original IP packet is protected by IPSec.
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).
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.
Tunnel mode is used to encrypt traffic between secure IPSec Gateways,
for example two Cisco routers connected over the Internet via IPSec
VPN. Configuration and setup of this topology is extensively covered in
our Site-to-Site IPSec VPN article. In this example, each router acts as an IPSec Gateway for their LAN, providing secure connectivity to the remote network:
example of tunnel mode is an IPSec tunnel between a Cisco VPN Client
and an IPSec Gateway (e.g ASA5510 or PIX Firewall). The client connects
to the IPSec Gateway. Traffic from the client is encrypted, encapsulated
inside a new IP packet and sent to the other end. Once decrypted by the
firewall appliance, the client’s original IP packet is sent to the
In tunnel mode, an IPSec header (AH or ESP header)
is inserted between the IP header and the upper layer protocol. Between
AH and ESP, ESP is most commonly used in IPSec VPN Tunnel
The packet diagram below illustrates IPSec Tunnel mode with ESP header:
ESP is identified in the New IP header with an IP protocol ID of 50.
The packet diagram below illustrates IPSec Tunnel mode with AH header:
The AH can be applied alone or together
with the ESP, when IPSec is in tunnel mode. AH’s job is to protect the
entire packet. 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. The AH protects everything that does not change in
transit. AH is identified in the New IP header with an IP protocol ID of 51.
IPSec Transport Mode
IPSec Transport mode is used for
end-to-end communications, for example, for communication between a
client and a server or between a workstation and a gateway (if the
gateway is being treated as a host). A good example would be an
encrypted Telnet or Remote Desktop session from a workstation to a
Transport mode provides the protection
of our data, also known as IP Payload, and consists of TCP/UDP header +
Data, through an AH or ESP header. The payload is encapsulated by the
IPSec headers and trailers. The original IP headers remain intact,
except that the IP protocol field is changed to ESP (50) or AH (51), and
the original protocol value is saved in the IPsec trailer to be
restored when the packet is decrypted.
IPSec transport mode is usually used
when another tunneling protocol (like GRE) is used to first encapsulate
the IP data packet, then IPSec is used to protect the GRE tunnel
packets. IPSec protects the GRE tunnel traffic in transport mode.
The packet diagram below illustrates IPSec Transport mode with ESP header:
Notice that the original IP Header is moved
to the front. Placing the sender’s IP header at the front (with minor
changes to the protocol ID), proves that transport mode does not provide
protection or encryption to the original IP header and ESP is
identified in the New IP header with an IP protocol ID of 50.
The packet diagram below illustrates IPSec Transport mode with AH header:
The AH can be applied alone or together with the ESP when IPSec is in transport mode. AH’s job is to protect
the entire packet, however, IPSec in transport mode does not create a
new IP header in front of the packet but places a copy of the original
with some minor changes to the protocol ID therefore not providing
essential protection to the details contained in the IP header (Source
IP, destination IP etc). AH is identified in the New IP header with an IP protocol ID of 51.
In both ESP and AH cases with IPSec Transport mode, the IP header is exposed.