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Extension protocols and network applications 64
DHCP option 82 is a DHCP Relay Agent Information Option. This option has been developed so
that a DHCP Relay Agent network can add specific information to a message that it forwards
to a DHCP server. Here the option uses two associated pieces of data: Circuit ID and Remote
ID. The DHSP server must obtain this information about the location of the host that is sending
the request. This information is heavily dependent on the DHSP Relay Agent and for Ethernetbased
networks this consists of the MAC addresses of the ports on the Relay Agent that form
the link with the end host. This information may be used to indicate where an assigned IP
address is physically located in the network. This information can also be used by the DHSP
server when deciding on the assignment of a specific IP address.
4.3 ICMP
4.3.1 Introduction
Data packets can get lost in the IP communication service, the delivery of these can be
greatly delayed or delivered in the wrong order. IP is not a reliable communication service
but tries to avoid errors and reports any problems when they occur. A primary example of
error detection is the header checksum. Every time a data packet is received, the checksum
is checked to make sure that the header is not damaged. If a checksum error is observed,
then this message is straight away discarded. No message is produced in this case as the
source address is deleted together with the message. Other less serious problems may be
reported though.
4.3.2 Internet Control Message Protocol
The TCP/IP protocol suite contains a protocol (Internet Control Message Protocol (ICMP)) to
send error messages. This way it can be reported if a certain network feature is not available
or if a certain host or router is not available. Sometimes, a computer user gets in touch
directly with the ICMP protocol, especially when using the network diagnosis commands -
ping and traceroute.
ICMP has five error messages and four informative messages. The five ICMP error messages
are:
• Source quench: this is sent by a router if it is temporarily short of buffer space and
therefore has to discard incoming IP packets. This message is sent to the host that has
created the IP packet. The sending host will have to adapt the transmission speed.
• Time exceeded, this is sent by a router after the Time to live field has been reduced to
zero.
• Destination unreachable: this is sent by a router if it notices that an IP packet cannot
reach its destination. The error message distinguishes between a situation, whereby
a complete network is temporarily delinked from the Internet (if a certain router is not
functioning correctly) and the case where a certain host is temporarily offline.
• Redirect: this is sent by a router if it notices that the IP packet actually has to be sent
to another router to reach the final destination.