7
IPsec VPN
133
However,
the
IPsec
tunnel
itself
does
not
need
any
IP
rules
defined.
These
rules
are
hidden,
and
created
automatically
by
the
SEG
so
that
IKE
negotiations
and
tunnel
establishment
can
take
place.
Dead Peer Detection
After
the
establishment
of
an
IPsec
tunnel
to
a
client,
the
SEG
uses
an
Informational
message
sent
through
the
tunnel
to
the
client
as
a
Dead
Peer
Detection
(DPD)
mechanism.
This
message
contains
an
empty
encrypted
payload.
If
the
client
doesn’t
respond,
six
more
DPD
messages
are
sent
by
the
SEG
at
progressively
longer
intervals
before
DPD
is
triggered,
the
client
is
considered
unreachable,
and
the
tunnel’s
SAs
are
torn
down.
DPD
is
unidirectional,
concerned
only
with
incoming
tunnel
traffic.
The
basic
interval
for
DPD
message
sending
is
determined
by
the
IPsecTunnel
property
IKEDPDInterval
.
This
has
a
default
value
of
90
seconds,
which
is
recommended
for
most
I
‐
WLAN
scenarios.
If
it
is
set
to
zero,
DPD
is
disabled.
IPsec connection using two IPsec tunnel objects
As
discussed
in
IPsec
troubleshooting
on
page
143
,
a
potential
problem
can
occur
when
a
single
IPsec
VPN
tunnel
is
set
up
referencing
two
IPsec
tunnel
objects.
Consider
the
situation
of
an
IPsec
client
sending
a
request
to
open
a
tunnel
to
the
SEG.
The
client
begins
by
sending
an
IKE_INIT
request
that
includes
a
proposal
list,
key
information
and
a
notification
payload.
The
SEG
then
scans
the
list
of
IPsec
tunnel
objects
looking
for
the
closest
match
and
uses
the
tunnel
object
it
finds.
However,
the
tunnel
chosen
may
not
be
the
“right”
tunnel
since
not
all
information
needed
for
a
correct
match
is
available
yet
to
the
SEG.
In
the
next
IKE_AUTH
stage
of
the
setup
process,
the
client
sends
the
remainder
of
the
required
information
and
this
may
result
in
the
SEG
needing
to
find
another
IPsec
tunnel
object
for
the
connection.
An
example
of
this
is
two
tunnels
A
and
B
with
the
following
characteristics:
•
Tunnel
A
has:
•
AES
‐
128
in
its
proposal
list.
•
Authentication
with
PSK
using
ID
•
A
remote
endpoint
of
all
‐
nets
.
•
Tunnel
B
has:
•
3DES
in
its
proposal
list.
•
Authentication
with
certificates
using
ID
•
A
remote
endpoint
of
177.22.0.47
.