WWW Programming Model
Wireless Datagram Protocol (WDP)
The Wireless Datagram Protocol (WDP)
operates on top of many different bearer services capable of carrying data. At
the T-SAP WDP offers a consistent datagram transport service independent of the
underlying bearer. To offer this consistent service, the adaptation needed in
the transport layer can differ depending on the services of the bearer. The
closer the bearer service is to IP, the smaller the adaptation can be. If the
bearer already offers IP services, UDP is used as WDP. WDP offers more or less
the same services as UDP.WDP offers source
and destination port numbers used
for multiplexing and demultiplexing of data respectively. The service primitive
to send a datagram is TDUnitdata.req
with the destination address (DA), destination port (DP), Source address (SA),
source port (SP), and user data (UD)
as mandatory parameters (see
Figure 10.11). Destination and source address are unique addresses for the
receiver and sender of the user data. These could be MSISDNs (i.e., a telephone
number), IP addresses, or any other unique identifiers. The T-DUnitdata.ind service primitive
indicates the reception of data. Here destination address and port are only
optional parameters.
If a
higher layer requests a service the WDP cannot fulfill, this error is indicated
with the T-DError.ind service primitive
as shown in Figure 10.11. An error code
(EC) is returned indicating the reason for the error to the higher layer.
WDP is not allowed to use this primitive to indicate problems with the bearer
service. It is only allowed to use the primitive to indicate local problems,
such as a user data size that is too large. If any errors happen when WDP
datagram‘s are sent from one WDP entity to another (e.g. the destination is
unreachable, no application is listening to the specified destination port
etc.), the wireless control message
protocol (WCMP) provides error handling mechanisms for WDP and should
therefore be implemented. WCMP contains control messages that resemble the
internet control message protocol messages and can also be used for diagnostic
and informational purposes.
WCMP can
be used by WDP nodes and gateways to report errors. However, WCMP error
messages must not be sent as response to other WCMP error messages. In IP-based
networks, ICMP will be used as WCMP (e.g., CDPD, GPRS). Typical WCMP messages
are destination unreachable (route,
port, address unreachable), parameter
problem (errors in the packet
header), message too big, reassembly
failure, or echo request/reply.
An additional
WDP management entity supports
WDP and provides information about changes in the environment, which may influence the correct operation of WDP.
Important information is the current configuration of the device, currently
available bearer services, processing and memory resources etc. Design and
implementation of this management component is considered vendor-specific and
is outside the scope of WAP.
If the
bearer already offers IP transmission, WDP (i.e., UDP in this case) relies on
the segmentation (called fragmentation in the IP context) and reassembly capabilities
of the IP layer as specified in (Postal, 1981a). Otherwise, WDP has to include
these capabilities, which is, e.g., necessary for the GSM SMS. The WAP
specification provides many more adaptations to almost all bearer services
currently available or planned for the future (WAP Forum, 2000q), (WAP Forum,
2000b).
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