Patton electronics 3120, Справочное руководство администратора

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16 • IP Model 3120 RAS Administrators’ Reference Guide
204 ICMP
Destinations Unreachable (IcmpInDestUnreachs, IcmpOutDestUnreachs)
The number of ICMP destination unreachable messages received/sent. For instance, if the information in a
gateway’s routing table determines that the network specified in a packet is unreachable, the gateway will send
back an ICMP message stating that the network is unreachable. The following conditions will send back an
unreachable message:
• The network is unreachable.
• The host is unreachable.
• The protocol is not available to the network.
• The port on the host is unavailable. A specified source route failed.
• A packet must be fragmented (that is, broken up into two or more packets) before being sent to the next
hop, but the packet was sent anyway with instructions not to be fragmented.
Times Exceeded (icmpInTimeExcds, icmpOutTimeExcds)
The number of ICMP time exceeded messages received/sent. Each time a packet passes through a gateway, that
gateway reduces the time-to-live (TTL) field by one. The default starting number is defined under the IP sec-
tion. If the gateway processing a packet finds that the TTL field is zero it will discard the packet and send the
ICMP time exceeded message. Time exceeded will also be incremented when a host which is reassembling a
fragmented packet cannot complete the reassembly due to missing packets within its time limit. In this case,
ICMP will discard the packet and send the time exceeded message.
Parameter Problems (icmpInParmProbs, icmpOutParmProbs)
The number of ICMP parameter problem messages received/sent. If while processing a packet, a gateway or
host finds a problem with one or more of the IP header parameters which prohibits further processing, the
gateway or host will discard the packet and return an ICMP parameter problem message. One potential source
of this problem may be with incorrect or invalid arguments in an option. ICMP sends the parameter problems
message if the gateway or host has discarded the whole packet.
Source Quenchs (icmpInSrcQuenchs, icmpOutSrcQuenchs)
The number of ICMP source quench messages received/sent. A gateway will discard packets if it cannot allo-
cate the resources, such as buffer space, to process the packet. If a gateway discards the packet, it will send an
ICMP source quench message back to the sending device. A host may send this messages if packets arrive too
fast to be processed or if there is network congestion. The source quench message is a request to reduce the rate
at which the source is sending traffic. If the access server receives a source quench, it will wait for acknowledg-
ment of all outstanding packets before sending more packets to the remote destination. Then it will begin
sending out packets at an increasing rate until the connection is restored to standard operating conditions.
Redirects (icmpInRedirects, icmpOutRedirects)
The number of ICMP redirect messages received/sent. A gateway sends a redirect message to a host if the net-
work gateways find a shorter route to the destination through another gateway.
Echos (icmpInEchos, icmpOutEchos)
The number of ICMP echo request messages received/send. The ICMP echo is used whenever one uses the diag-
nostic tool ping. Ping is used to test connectivity with a remote host by sending regular ICMP echo request pack-
ets and then waiting for a reply. Received echos (icmpInEchos) will increment when the access server is pinged.