CS计算机代考程序代写 database DHCP assembly Slide 1

Slide 1

Networking

ICMP
© Janice Regan, 2012-2017

© Janice Regan, 2012-2017
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Errors in Packet Switching Networks
Possible causes of errors include
Hardware failure
Network congestion
Inability to fragment (DF set)
Routing loops
Unavailable host (disconnected or failed)
Queue overrun on routers
IP offers best effort delivery, it needs a mechanism to inform the source of packets dropped because of errors (except transmission errors). In the remainder of this lecture errors will mean errors not cause by transmission impairments

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Dealing with errors
IP datagram delivery (network level) has a header checksum to detect transmission errors in the IP header
TCP has a checksum which covers the TCP header, pseudo header and data
Higher level protocols (for example TCP) also handle more types of errors
Higher level protocols may deal with any errors not caught at lower levels. It may be more efficient to catch errors in the IP layer than in the TCP layer
Within IP need an error reporting mechanism to report such errors, one such mechanism is the ICMP protocol.

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ICMP
Internet Control Message Protocol is an integral part of IP
ICMP messages are carried as the data portion of an IP datagram
Communicates between the IP software on different hosts
Used (primarily by routers) to report errors back to the original source.
Used to enquire about the state and configuration of routers and hosts
Reports errors and in some cases suggests corrective action

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ICMP message format
There are several types of ICMP messages designed to report different types of errors
Each ICMP message has its own format, but all start with the same three fields
A type field (1 octet) indicating which type of ICMP message follows the first three flelds
A 1 octet code following the type that further defines the message (see text for list)
For example type specifies destination unreachable, code specifies router or host
The 3rd common field is a 2 octet checksum. The checksum is calculated the same way as the ARP checksum, including the entire ICPM PDU

Comer 2000:

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IPv4
Destination Unreachable Codes
0 Net Unreachable
Host Unreachable
Protocol Unreachable
Port Unreachable
Fragmentation Needed and Don’t Fragment was Set
Source Route Failed
Destination Network Unknown
Destination Host Unknown
Source Host Isolated
Communication with Destination Network is Administratively Prohibited 1
Communication with Destination Host is Administratively Prohibited
Destination Network Unreachable for Type of Service
Destination Host Unreachable for Type of Service

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ICMP Echo Request/Reply
Echo request is sent by the ping command to test for reachability
Echo reply is sent in response to a received echo reply to confirm reachability
Type: request 8, reply 0, Code 0 : no additional qualifying codes
Identifier and sequence number are optional, they can be used to match replies with requests
The optional data in a echo request must be returned in the resulting echo reply
Linux ping has a record route and a timestamp option

Comer 2000: fig 9.2

Example:
ping may from august

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ICMP destination unreachable
Sent when a router or host cannot deliver a datagram due to an identified failure (not all failures are identified)
Can be disabled, not all hosts or routers will send ICMP messages
The codes indicate what destination could not be reached and why (see table in text)
The header and datagram information is provided to identify the packet needing retransmission (port numbers and sequence number for TCP UDP)

Comer 2000: fig 9.3

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ICMP Source Quench Message
Used to help control congestion
When a packet must be dropped due to congestion a source quench packet may be sent
When the source receives a source quench message it may reduce the rate at which it transmits to the network
1 quench message per round trip travel time should cause change

Comer 2000: fig 9.4

ICMP redirect message
Temporary of permanent changes to network routing on one host may cause other routes on other hosts to become incorrect
When a router detects a host on a directly connected network using non optimal routing it can send an ICMP redirect message
This mechanism allows dynamic calculation of routes by routers (not hosts) to interact with host routing tables
Allowing redirct messages to prompt routing changes can be a security issue and redirect messages are often disabled to protect from the security risk
Comer 2000: fig 9.6

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ICMP time exceeded message
Possible causes for timeout
To much time in transit (TTL reaches 0). Code 0
Missing fragments in reassembly of fragmented packets. Code 1
Similar format to source quench message

Comer 2000: fig 9.7

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ICMP parameter problem
When a router finds problems not handled by previous messages that are severe enough to require the packet to be discarded
Can deal with problems such as incorrect parameters for IP options

Comer 2000: fig 9.8

© Janice Regan, 2012-2017
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ICMP timestamp request/reply
Used for synchronization and estimation of travel times
Originate timestamp added just before transmission
Receive timestamp added immediately after receipt
Transmit timestamp added immediately before the reply is transmitted
This means that transit time (not queuing time) is measured
In practice, because clocks on different systems may show different times, travel time and synchronization is complicated
Comer 2000: fig 9.9

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ICMP address mask request/reply
To determine the subnetwork address mask for the local subnetwork a host may send an address mask reply to a directly connected router
When the router replies the address mask field will contain the subnet mask
Identifier and sequence number are used to associate the request with the reply
Comer 2000: fig 9.10

© Janice Regan, 2012-2017
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ICMP Router discovery
Used to dynamically determine the address of directly connected routers
An alternative to BOOTP and DHCP (both based on manually configured databases)
This is a router advertisement packet that is sent periodically (default every 10 minutes, with lifetime of 30 minutes)
If an advertisment is not received by a host during the lifetime, the host will discard the routing entry for that router (until it hears again)
Comer 2000: fig 9.11

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ICMP Router Solicitation
A host that has just booted will not wish to wait 10 minutes to discover the routers on the network
A host can send a solicitation requesting an additional immediate router discovery message
Comer 2000: fig 9.12

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ICMPv6 (RFC 2463)
Retains functionality of IPv4
Adds functionality of ARP and IGMP
Adds new functionality, autoconfiguration and unreachability detection
Includes neighbor discovery (RFC 2461) a sub protocol that replaces ARP and router discovery in ICMP. Can determine when a neighbor has become unreachable
Includes Multicast Listener Discoverer (replacing IGMP)

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