to find common agreement between the local and the remote security gateway on
how to protect that specific data flow.
A transform set includes encapsulation protocols and transforms; for example,
encryption/decryption/authentication algorithms. These parameters are grouped to
specify the acceptable protection for a given data flow. Many transform sets are
supported, since different traffic requires distinct security levels.
A secure IP tunnel is associated with one transform set. Multiple secure IP tunnels
can refer to the same transform set.
Changing existing transform sets affects only future user SA negotiations. User SAs
that are already established remain valid and do not use the changed transform set
until they are renegotiated.
For manually configured secure IP tunnels, the associated transform set must contain
a single transform option.
Encapsulation Protocols
Both the AH and ESP protocols are supported. See supported transforms in Table 11
on page 136.
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AH provides authentication.
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ESP provides data confidentiality and antireplay functions. ESP can also provide
data authentication; although, in this implementation, ESP does not cover the
outer IP header.
Encapsulation Modes
IPSec supports two encapsulation modes—tunnel mode and transport mode. Tunnel
mode creates a second IP header in the packet and uses both the local and remote
security gateway addresses as source and destination IP addresses. Also, tunnel
mode allows an IP interface to be created and stacked right above it.
Transport mode does not add a second IP header and does not allow an IP interface
to be created and stacked right above it. Instead, transport mode allows other
tunneling applications, such as an L2TP tunnel, to be created and stacked on top of
an IPSec transport mode connection. See “Securing L2TP and IP Tunnels with IPSec”
on page 287 for a description of L2TP transport mode.
Supported Transforms
Table 11 on page 136 describes the supported transforms.
Table 11: Supported Transforms
Description
Transform
IPSec performs AH protocol encapsulation using the MD5 hash function with
HMAC message authentication.
AH-MD5
136
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IPSec Concepts
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