network protocols handbook.pdf

68Protocols GuideTCP/IP - Network Layer ProtocolsProtocol NameICMP & ICMPv6: Internet MessageControl Protocol andICMP version 6Protocol DescriptionInternet Control Message Protocol (ICMP) is an integrated partof the IP suite. ICMP messages, delivered in IP packets, areused for out-of-band messages related to network operation ormis-operation. ICMP packet delivery is unreliable, so hosts can’tcount on receiving ICMP packets for any network problems. Thekey ICMP functions are:• Announce network errors, such as a host or entire portionof the network being unreachable, due to some typeof failure. A TCP or UDP packet directed at a port numberwith no receiver attached is also reported via ICMP.• Announce network congestion. When a router beginsbuffering too many packets, due to an inability totransmit them as fast as they are being received, it willgenerate ICMP Source Quench messages. Directed atthe sender, these messages should cause the rate ofpacket transmission to be slowed. Of course, generatingtoo many Source Quench messages would cause evenmore network congestion, so they are used sparingly.• Assist Troubleshooting. ICMP supports an Echo function,which just sends a packet on a round--trip betweentwo hosts. Ping, a common network management tool,is based on this feature. Ping will transmit a series ofpackets, measuring average round--trip times and computingloss percentages.• Announce Timeouts. If an IP packet’s TTL field drops tozero, the router discarding the packet will often generatean ICMP packet announcing this fact. TraceRoute isa tool which maps network routes by sending packetswith small TTL values and watching the ICMP timeoutannouncements.The Internet Control Message Protocol (ICMP) was revised duringthe definition of IPv6. In addition, the multicast control functionsof the IPv4 Group Membership Protocol (IGMP) are nowincorporated in the ICMPv6.• Type -- Messages can be error or informational messages.Error messages can be Destination unreachable,Packet too big, Time exceed, Parameter problem.The possible informational messages are, EchoRequest, Echo Reply, Group Membership Query,Group Membership Report, Group Membership Reduction.• Code -- For each type of message several differentcodes are defined. An example of this is the DestinationUnreachable message, where possible messagesare: no route to destination, communicationwith destination administratively prohibited, not aneighbor, address unreachable, port unreachable.For further details, refer to the standard.• Checksum -- The 16-bit one’s complement of theone’s complement sum of the ICMP message startingwith the ICMP Type. For computing the checksum,the checksum field should be zero.• Identifier -- An identifier to aid in matching requests/replies; may be zero.• Sequence number -- Sequence number to aid inmatching requests/replies; may be zero.• Address mask -- A 32-bit mask.Related protocolsIP, TCP, IGMP, SNMP, DNS, TFTP and NFSSponsor SourceICMP is defined by IETF (http://www.ietf.org) RFC792 and 950;ICMPv6 is defined by RFC 2461, 2463.Referencehttp://www.javvin.com/protocol/rfc792.pdfInternet Control Message Protocolhttp://www.javvin.com/protocol/rfc950.pdfInternet Standard Subnetting Procedurehttp://www.javvin.com/protocol/rfc2461.pdfNeighbor Discovery for IP Version 6 (IPv6).http://www.javvin.com/protocol/rfc2463.pdfICMPv6 for the Internet Protocol Version 6 (IPv6) SpecificationProtocol Structure8 16 32bitType Code ChecksumIdentifierSequence numberAddress mask