Describe IP operations [ ICMP Unreachable and Redirects ] CCNP
The Internet Control Message Protocol (ICMP) is a supporting protocol in
the Internet protocol suite. It is used by network devices, like routers, to
send error messages and operational information.
ICMP TYPE NUMBERS
The Internet Control Message Protocol (ICMP) has many messages that are identified by a "type" field.
Type Name Reference
---- ------------------------- ---------
0 Echo
Reply
[RFC792]
1 Unassigned [JBP]
2 Unassigned [JBP]
3 Destination
Unreachable [RFC792]
4 Source
Quench [RFC792]
5 Redirect [RFC792]
6 Alternate
Host Address [JBP]
7 Unassigned [JBP]
8 Echo [RFC792]
9 Router
Advertisement [RFC1256]
10 Router
Selection [RFC1256]
11 Time
Exceeded [RFC792]
12 Parameter
Problem [RFC792]
13 Timestamp [RFC792]
14 Timestamp
Reply [RFC792]
15 Information
Request [RFC792]
16 Information
Reply [RFC792]
17 Address
Mask Request [RFC950]
18 Address
Mask Reply [RFC950]
19 Reserved
(for Security) [Solo]
20-29 Reserved (for Robustness Experiment) [ZSu]
30 Traceroute [RFC1393]
31 Datagram
Conversion Error [RFC1475]
32 Mobile Host Redirect [David Johnson]
33 IPv6 Where-Are-You [Bill Simpson]
34 IPv6 I-Am-Here [Bill Simpson]
35 Mobile Registration Request [Bill Simpson]
36 Mobile Registration Reply [Bill Simpson]
37 Domain Name Request [Simpson]
38 Domain Name Reply [Simpson]
39 SKIP [Markson]
40 Photuris [Simpson]
41-255 Reserved [JBP]
ICMP type 3, Destination unreachable message:
ICMP type 3, Destination unreachable message:
The ICMP destination unreachable message is generated by a router to inform the
source host that the destination unicast address is unreachable.
The IP header plus the first 8 bytes of the original datagram’s data is
returned to the sender. This data is used by the host to match the message to
the appropriate process. If a higher level protocol uses port numbers, they are
assumed to be in the first 64 data bits of the original datagram’s data.
If the Cisco IOS receives a nonbroadcast packet destined for itself that
uses an unknown protocol, it sends an ICMP protocol unreachable message back to
the source. If the software receives a packet that it is unable to deliver to
the ultimate destination because it knows of no route to the destination
address, it sends an ICMP host unreachable message to the source. (enabled by
default).
3 Destination Unreachable [RFC792]
Codes
0 Net Unreachable
1 Host Unreachable
2 Protocol Unreachable
3 Port Unreachable
4 Fragmentation Needed and Don't Fragment was
Set
5 Source Route Failed
6 Destination Network Unknown
7 Destination Host Unknown
8 Source Host Isolated
9 Communication with Destination Network
is Administratively Prohibited
10 Communication with Destination Host is Administratively Prohibited
11 Destination Network Unreachable for Type of
Service
12 Destination Host Unreachable for Type of
Service
13 Communication Administratively
Prohibited [RFC1812]
14 Host Precedence Violation [RFC1812]
15 Precedence cutoff in effect [RFC1812]
Explaination -
1. Network unreachable- it has a code value of 0. In this the datagram
cannot be delivered to the network that is specified in the network ID portion
of the IP address. This means that there was a problem with the routing and was
caused by a bad address.
2. The second is the host unreachable. This has a value code of 1. In
this the datagram was delivered to the network that is specified but it could
not be sent to the host who was mentioned in the address. This also implies a
routing issue.
3. The protocol unreachable is the next. The value code of this is 2. In
this the destination port that was specified as UDP or TCP header was not
valid.
4. Fragmentation needed and DP set - the MTU is always smaller than the
packet size. The router is not allowed to fragment the packet. This message is
very often used in a very clever way. This is done by intentionally sending
messages of increasing size to understand the maximum transmission size that
the link can take. This process is also called the MTU path discovery.
5. Source route failed - this is generated if the source route was
specified for the datagram in the option but the router could not forward the
datagram to the next step in the route.
6. Destination network unknown - this is not used any more.
7. Destination unknown host - in this the host that is specified is not
known. This is generated by a router call made to the destination host. This
usually means a bad address.
8. Source host isolated - this is again not used anymore.
9. Communication with the destination network is prohibited
administratively -in this the source device will not be allowed to send the
network where the destination device is located.
10. Communication with the destination host is administratively prohibited
- in this the source device is allowed to send network where the destination
device is located but not to the particular device.
11. Destination network is unreachable for the type of service - in this
the network that is specified in the IP address cannot be reached due to the
inability to provide the service that is clearly specified in the "type of
service" field of the datagram header.
12. Destination not reachable for the type of service - in this the
destination host that is specified in the IP address cannot be reached due to
the inability to provide the "type of service" mentioned in the
datagram header.
13. Communication administratively prohibited - in this the datagram
cannot be forwarded as the filtering blocks the message based on the contents.
14. Host precedence violation - this is sent by the first hop router when
the precedence value in the "type of service field" is one that
cannot be permitted.
15. Precedence cutoff in effect - this is sent by the datagram that is in the
receiving end. The precedence value should be lower than the minimum allowed
for the network at that time.
ICMP type 5, Redirects message:
ICMP type 5, Redirects message:
This mechanism is used by the host to
notify that a better next hop is available to exit from the network. The CISCO
routers will send the ICMP redirect message when the following conditions are
met:
5 Redirect [RFC792]
Codes
0 Redirect Datagram for the Network (or subnet)
1 Redirect Datagram for the Host
2 Redirect Datagram for the Type of Service and
Network
3 Redirect Datagram for the Type of Service and
Host
Explaination –
- The subnet or the network of the source IP
address is same as that of the subnet or network of the next hop IP
address.
- The interface where the packet comes into
the router are same as the interface on which the packets are routed out.
- The datagram is not source routed.
- The kernel is such that it is configured to
send redirects.
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