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.It is friendly of a router to provide at least enoughmore than the network layer header so that the entire transport header is included.Thisenables the source to match the returned datagram with the user process.An error report packet has the segmentation permitted flag off.If an error report packetneeds to be forwarded but is too long, a router will truncate the packet.Note that this is trueonly of error report packets.An oversized CLNP data packet will be fragmented (if possible)or discarded, but never truncated.10.7.2 ICMP: IP Error MessagesIn IP the error reports are provided by the ICMP (Internet Control Message Protocol), RFC792.The ICMP packet is carried in the data portion of an ordinary IP packet.The protocolfield in the IP header indicates ICMP.The basic format of ICMP messages is shown in Figure10.28.Figure 10.28.ICMP Message formatThe ICMP message types are shown in Table 10.2.Table 10.2.ICMP Message TypesError Code Meaning0 Echo reply3 Destination unreachable4 Source quench5 Redirect8 Echo request11 Time exceeded12 Parameter problem13 Timestamp request 14 Timestamp reply15 Information request16 Information reply17 Address mask request18 Address mask replyTogether, the message types "destination unreachable," "time exceeded," and "parameterproblem" provide the functionality in CLNP error report packets.The general format of these ICMP error report messages is as follows.Type: 1 octet.12 parameter problem11 time exceeded3 destination unreachableCode: 1 octet.In time exceeded:0 died in transit1 died while being reassembled by destinationIn destination unreachable:0 net unreachable1 host unreachable2 protocol unreachable3 port unreachable4 fragmentation required but not allowed by DF (don't fragment flag)5 source route failedIn parameter problem: Code unused, set to 0.Pointer: 1 octet.Used only in parameter problem It is a pointer into theheader indicating where a problem was found.Unused: 3 octets.Internet header + at least 64 bits of doomed datagram, and preferably asmuch of it as possible without the ICMP packet exceeding 576 bytes.The other ICMP messages are as follows.Echo:An echo request can be used to decide whether some destination isreachable.Any IP machine receiving an echo request is supposed to respondwith an echo reply.An echo request is known as a ping.Ping can also be used as a verb.Toping node X means to send X an echo request.This is a method of testingwhether X is alive, well, and reachable. Type:1 octet.8 echo request message0 echo reply messageCode:1 octet, set to 0.Checksum:2 octets.16-bit 1's complement checksum of the ICMP message.Identifier:2 octets.A number that helps the echo requester match the reply to therequest.Sequence number:2 octets.Another number that helps the echo requester match the reply tothe request.Data:Variable.Anything echo requester wants to put in.(It will be copied into echoreply.)The identifier and sequence number fields can be thought of as a single 4-octet field that allows the echo requester to match the reply with the request.It was written as two fields because the suggested use was for the identifierto correspond to the TCP (transmission control protocol) or UDP (userdatagram protocol) port number and for the sequence number to beincremented on each request.Timestamp request or timestamp reply as follows." Type:1 octet.13 timestamp request message14 timestamp reply message" Code:1 octet, set to 0." Checksum:2 octets.16-bit 1's complement checksum of the ICMP message. " Identifier:2 octets.A number that helps the requester match the reply to the request." Sequence number:2 octets.Another number that helps the requester match the reply to the request." Originate timestamp:4 octets.Timestamp put in by the requester to indicate the most recent known timebefore transmission of the time-stamp request." Receive timestamp:4 octets.Timestamp put in by the replier to indicate the time that the request wasreceived." Transmit timestamp:4 octets.Timestamp put in by the replier to indicate the time that the reply wastransmitted.Information request or information reply as follows." Type:1 octet.15 information request message16 information reply message" Code:1 octet.Set to 0." Checksum:2 octets.16-bit 1's complement checksum of the ICMP message." Identifier:2 octets.A number that helps the requester match the reply to the request." Sequence number:2 octets.A number that helps the requester match the reply to the request.The information request message is used by a host that does not know the "network"number of the LAN on which it resides.The information request message is sent withthe network portion of the source and destination addresses in the IP header equal to0, and the reply has the addresses in the IP header fully specified. The information request ICMP message is largely obsolete.A much more practicalscheme for finding out the missing information is with the BOOTP and DHCPprotocols.Address mask request or address mask reply as follows." Type:1 octet.17 address mask request message18 address mask reply message" Code:1 octet = 0." Checksum:2 octets.16-bit 1's complement checksum of the ICMP message." Identifier:2 octets.A number that helps the requester match the reply to the request." Sequence number:2 octets.Another number that helps the requester match the reply to the request." Address mask:4 octets.The address mask request message is used by a host that does not know its addressmask.The reply contains the correct address mask.This message is described inRFC 950 [ Pobierz całość w formacie PDF ]