CCNA 2 Week 8 TCP/IP Suite Error Control Messages.

CCNA 2 Week 8

TCP/IP Suite Error Control Messages

Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton

Best Effort

May remember from CCNA 1 that IP (layer 3) is unreliable and operated on Best Effort basis

Data delivery may fail for many reasons– hardware failure, configuration/routing table errors

IP uses ICMP to notify sender of delivery problems


Error Notification

Nothing in the basic design of IP notifies the sender that a data transmission has failed

ICMP is the component of the TCP/IP protocol stack that addresses this basic limitation of IP

ICMP doesn't overcome the unreliability issues

Reliability is still provided by upper layer protocols


Error Reporting

When errors detected they are reported back to source

Reports sent to source IP address only

Does not notify routers along the way


ICMP Encapsulation

ICMP messages are encapsulated at layer 3 as packets and layer 2 frames

Sent by same mechanisms as regular IP packets

Subject to delivery failures too

To prevent escalating numbers of error messages, failure to delivery ICMP is not notified


Error Conditions

TCP/IP must be properly configured at each end– Including IP address and subnet mask.

A default gateway must be set if data will go outside the LAN

Properly configured routers must be in place

Failures may include destination device offline, intermediate links down

Router may send "Destination Unreachable" ICMP packet


Ping

Used to test whether a remote device is reachable

Sends ICMP echo request

Triggers ICMP echo reply

Packet includes a TTL value– Decremented each time it passes though a router– If it reaches zero, packet is dropped– Unreachable response may be sent


ICMP Message Types

Each ICMP packet type has a different format

All types contain– Type– Code– Checksum

Echo requests include Identifier and Sequence Number for example

0 Echo Reply

3 Destination Unreachable

4 Source Quench

5 Redirect/Change Request

8 Echo Request

9 Router Advertisement

10 Router Selection

11 Time Exceeded

12 Parameter Problem

13 Timestamp Request

14 Timestamp Reply

15 Information Request

16 Information Reply

17 Address Mask Request

18 Address Mask Response


Destination Unreachable


Parameter Problem

Errors in packet headers may prevent delivery

ICMP type 12 error generated

When code=0 pointer indicates problem octet in datagram


Control Messages

ICMP also allows devices to send control messages at layer 3

Can inform devices about congestion

Can identify alternative gateways


ICMP Redirect Request

All hosts configured with a default gateway (router) to forward packets to other networks

Where more than one router is on a segment, may be better to direct packets via other router for some destinations

Router will send ICMP redirect (5) message if outbound interface is same as source interface


Timestamps

The ICMP timestamp request (13) message allows a host to check the time at the remote host.

The remote host uses an ICMP timestamp reply (14) message to respond to the request

Times are expressed in milliseconds past midnight GMT


Information Request

Info Req (type 15) and response (type 16) can be used to configure network settings on a host

This protocol is obsolete

Alternatives:– DHCP– Bootp– RARP


Address Mask Request

Devices can learn their netmask from local router if not already configured

Request (Type 17) and Response (Type 18)

If address of router is unknown, request may be sent as broadcast


Router Solicitation

ICMP Type 9

Sent when default gateway not configured

Multicast to 224.0.0.2

Router responds with router advertisement which identifies default gateway (Type 10)


Source Quench

If a host is receiving too much traffic, can send ICMP source quench (4)

Asks sender to reduce sending rate

Cisco routers don't send by default because SQ packets can increase congestion

May be appropriate in small networks

CCNA 2 Week 8 TCP/IP Suite Error Control Messages.

Documents

Transcript of CCNA 2 Week 8 TCP/IP Suite Error Control Messages.