BROCADE IP PRIMER

Chapter 2: TCP/IPUDP adds a Transport Layer header to create its segment, similar to TCP. Let'slook at UDP's header:SourcePort16 bitsDestinationPort16 bitsLength16 bitsChecksum16 bitsDataSource Port. This is a 16-bit number. You wouldn't think this would be necessarywith a connectionless protocol, and you're right. It's not...always. If noreply is expected, this number can be zero. If a reply is expected, it should bethe port number on which to expect the reply. It is very common for UDP protocolsto use the same source and destination ports for their communication.Destination Port. This is a 16-bit number. This can help distinguish whichApplication Layer protocol you're communicating with.Length. This is a 16-bit number. This number is the length, in bytes, of theentire segment (including the header). The minimum length is 8 bytes, as thiswould be the minimum size of the header with no data.Checksum. This is a 16-bit number. Believe it or not, this is a checksum of theentire segment. Wait a minute! Isn't UDP supposed to be unreliable? Yes, but itdoes provide a checksum to each of its segments. The checksum will verifywhether or not data has been altered or damaged in transport. What is doneabout that really depends on the Application Layer. There are some ApplicationLayer protocols that use UDP, but implement some reliability at Layer 7. Likewise,there are some Application Layer protocols that will even ignore thechecksum.Data. This is the data received by the Session Layer.Dynamic Host Configuration Protocol (DHCP)It's hard to have a conversation about UDP without talking a little bit aboutDHCP. DHCP is a protocol that is used to automatically configure a host with anIP address, subnet mask, default gateway, DNS configuration, and other network-relatedconfigurations. It uses UDP to do its job.For DHCP to work, you need to have a DHCP server. This can be an actualserver, a router, or any other network device that has DHCP-serving capability.The idea is that the DHCP server keeps a bank of usable IP addresses withinits subnet. When it receives a request, it leases out an address to the client. Itkeeps track of the life of the lease (which is configurable; it could be a week; itcould be five minutes). When the lease is at its half-way point, the serversends a packet to the client asking if it would still like to keep using theaddress. If it does, the lease timer is reset.Let's say you have a client that wants to receive an IP address. It needs to createa message to ask for one. DHCP uses UDP, so we have the Transport Layerfigured out. It sends a segment with a destination port of 67 (well-known portfor DHCP request).60 Brocade IP Primer