z/VM: TCP/IP User's Guide - z/VM - IBM

IP provides the means to transmit blocks of data (or packets of bits) from sources
to destinations. Sources and destinations are hosts identified by fixed-length
internet addresses. Outgoing packets automatically have an IP header prefixed to
them, and incoming packets have their IP header removed before being sent to the
higher-level protocols. This protocol provides for the universal addressing of hosts
in an internet network.
IP does not ensure a reliable communication, because it does not require
acknowledgments from the sending host, receiving host, or intermediate hosts. IP
does not provide error control for data; it provides only a header checksum. IP
treats each packet as an independent entity unrelated to any other packet. IP does
not perform retransmissions or flow control. A higher-level protocol that uses IP
must implement its own reliability procedures.
For more information about IP, see RFC 791.
Internet Control Message Protocol (ICMP)
The Internet Control Message Protocol (ICMP) passes control messages between
hosts, gateways, and routers. For example, ICMP messages can be sent in any of
the following situations:
v When a host checks to see if another host is available (with a PING command)
v When a packet cannot reach its destination
v When a gateway or router can direct a host to send traffic on a shorter route
v When a host requests a netmask or a time stamp
v
When a gateway or router does not have the buffering capacity to forward a
packet
ICMP provides feedback about problems in the communication environment; it
does not make IP reliable. ICMP does not guarantee that an IP packet is delivered
reliably or that an ICMP message is returned to the source host when an IP packet
is not delivered or is incorrectly delivered.
For more information about ICMP, see RFC 792.
Address Resolution Protocol (ARP)
The Address Resolution Protocol (ARP) maps internet addresses to hardware
addresses. TCP/IP uses ARP to collect and distribute the information for mapping
tables.
ARP is not directly available to users or applications. When an application sends
an internet packet, IP requests the appropriate address mapping. If the mapping is
not in the mapping table, an ARP broadcast packet is sent to all the hosts on the
network requesting the physical hardware address for the host.
For more information about ARP, see RFC 826.
TCP/IP Protocols and Functions
Internet Group Management Protocol (IGMP)
The Internet Group Management Protocol (IGMP) is used to communicate
multicast group information. It lets all systems on a physical network know which
hosts belong to which multicast groups. Multicast routers use IGMP messages to
determine which multicast datagrams to forward onto which interfaces.
There are two types of IGMP messages — reports and queries. IGMP report
messages are sent out to notify others when a multicast group has been joined, and
Chapter 1. Introducing Computer Networks and Protocols 5