User Datagram Protocol (UDP)

User Datagram Protocol (UDP) supports the network at the transport layer. User Datagram Protocol(UDP) is an unreliable connection-less protocol and is defined by RFC 768 and 1122. It is a datagramservice. There is no guarantee that the data will reach its destination. UDP is meant to provide serivcewith very little transmission overhead. It adds very little to IP datapackets except for some error checkingand port direction (Remember, UDP encapsulates IP packets). The following protocols or services useUDP:
DNS

1. DNS
2. SNMP
3. BOOTP
4. TFTP
5. NFS
6. RPC
7. RIP

UDP Message Format

The UDP header includes:

1. Source port number (16 bits) - An optional field

2. Destination port number (16 bits)

3. UDP length (16 bits)

4. UDP checksum (16 bits)

This is followed by data. The UDP checksum includes UDP data, not just the header as with IP messageformats. For UDP and TCP checksum calculation a 12 byte pseudo header is included which containssome fields form the IP message header. This header is not transmitted as part of UDP or TCP, but isonly used to help compute the checksum as a means of being sure that the data has arrived at the correctIP address. This is the TCP/UDP pseudo header:

1. Source IP address (32 bits)

2. Destination IP address (32 bits)

3. blank filler(0) (8 bits)

4. Protocol (8 bits)

5. UDP length (16 bits)

IPX/SPX

IPX/SPX is a routable protocol and can be used for small and large networks. The following protocolsare part of the IPX/SPX suite:

l SAP - Service Advertising Protocol packets are used by file and print servers to periodicallyadvertise the address of the server and the services available. It works at the application,presentation, and session levels.l

NCP - NetWare Core Protocol provides for client/server interactions such as file and printsharing. It works at the application, presentation, and session levels.l

SPX - Sequenced Packet Exchange operates at the transport layer providing connection orientedcommunication on top of IPX.l

IPX - Internetwork Packet Exchange supports the transport and network layers of the OSInetwork model. Provides for network addressing and routing. It provides fast, unreliable,communication with network nodes using a connection less datagram service.

RIP - Routing Information Protocol is the default routing protocol for IPX/SPX networks whichoperates at the network layer. A distance-vector algorithm is used to calculate the best route for apacket.

ODI - Open Data-link Interface operates at the data link layer allowing IPX to work with anynetwork interface card.

NetWare frame types

Novell NetWare 2.x and 3.x use Ethernet 802.3 as their default frame type. Novell NetWare 4.x networksuse Ethernet 802.2 as their default frame type. If communication does not occur between two NetWarecomputers it is a good idea to check the netware versions of the two computers to be sure their frametypes match. If the frame types do not match on an ethernet network, the computers cannot communicate.