390
CHAPTER
12 / l\!lOBILE IP
ANDWHULESS
APPLICATION
PROTOCOL
Content type ,
riC
I
S L
I
Sequence
number
Record length
Plaintext
(optionally
compressed)
MAC
(0, 16,
or
20 bytes)
A.A.A.A.·A.AA.A.AAAA.~
~
'V\./\.
r
=
reserved
C
=
cipher spec indicator
S
=
sequence
number
field indicator
L
=
record length field indicator
MAC
=
message authentication code
Figure 12.18 WTLS Record Format
• Sequence
Number
(16
bits): A sequence
number
associated with this PDU.
This provides reliability over an unreliable transport service.
• Record Length (32 bits): The length in bytes of the plaintext data (or compressed
data
if
compression
is
used).
Change
Cipher
Spec
Protocol
Associated with the current transaction
is
a
cipher spec, which specifies the encryption algorithm, the hash algorithm used
as
part
of HMAC, and cryptographic attributes, such
as
MAC code size. There are actually
a two states associated with each session. Once a session
is
established, there
is
a cur-
rent operating state for both read and write (i.e., receive and send). In addition, during
the Handshake Protocol, pending read and write states are created.
The Change Cipher Spec Protocol is one
of
the
three
WTLS-specific protocols
that
use
the
WTLS
Record
Protocol, and it is
the
simplest. This protocol consists of
a single message, which consists
of
a single byte with the value
1.
The sole purpose
of this message is
to
cause the pending state to
be
copied into the current state,
which updates
the
cipher suite to be used
on
this connection.
Alert
Protocol
The Alert Protocol
is
used to convey WTLS-related alerts to the
peer
entity. As with
other
applications
that
use WTLS, alert messages are compressed
and encrypted, as specified by the current state.
Each
message in this protocol consists
of
2 bytes. The first byte takes the
value
warning(l),
critical(2),
or
fatal(3)
to
convey the severity
of
the message.
If
the
level is fatal, WTLS immediately terminates the connection.
Other
connections
on
the
same session may continue,
but
no
new
connections
on
this session may be
