Error and Control
•An IP datagram travels from node to nodeon the way to its destination
•Each router operates autonomously
•Failures or problems may occur or developalong the way to a destination
Destinations may not be reachable
Referenced host may not have theappropriate application running
The Time to Live field may decrement to 0before reaching destination
Error and Control
•To assist with such situations a protocolwas included within the IP module
•Internet Control Message Protocol (ICMP)
•Routers and hosts can report certainconditions back to the source of theoriginal datagram
•This is strictly a reporting mechanism tothe source of the datagram that triggeredthe condition
ICMP
•The ICMP protocol is documented in RFC792(1981)
•ICMP messages are themselves encapsulated asIP datagrams
•ICMP messages may themselves encounterreportable conditions
•By convention, when ICMP datagrams encounternormally reportable conditions, these are not sent
•The reporting of conditions are not mandatory
ICMP
•No ICMP message is sent for a fragmentdatagram except for the first fragment
•No ICMP message is sent if source is amulticast address
•No ICMP message is sent to specialaddresses, such as 127.0.0.0
Position of ICMP in the network layer
Encapsulation of ICMP packet
ICMP messages
General format of ICMP messages
Error-reporting messages
Contents of data field for error messages
Destination-unreachable format
Common Codes
0 Network Unreachable (router)
1 Host Unreachable (router)
2 Protocol (IP) Unreachable (host)
3 Port Unreachable (host)
4 Fragmentation required, NOFRAG bit set (router)
5 Source Route failed (router)
Source-quench format
• Routers may become congested and traffic may backup
• If there is no buffer for an incoming datagram the router discards thedatagram and sends a Source Quench ICMP to the source host
• A source Quench ICMP is sent for every datagram discarded
• Source must reduce rate of traffic
• There is no indication sent when rate can again be increased
• Source host can later gradually increase traffic
Time-exceeded message format
Code 0 : Time to Live field has decremented to 0
A Time exceeded ICMP is sent to the source host and datagram discarded
This is sent by a router
Code 1: When all fragments of an original datagram do not arrive at a destination within a certain time, all fragments are discarded and an ICMP Time exceed message sent to source host (by host)
Parameter-problem message format
• Code 0: Problem exists in the basic IP header
• Code 1: Problem exits in an Options field
• Pointer points to the first byte where error is detected
• When a parameter problem is detected, datagram is discarded and thisICMP message is sent
• This message may be created by a router or a host
Host Routing Table
•Each host start with some minimal routing table
•In the simplest case, it contains the address of adefault router
•As time goes on, this table can be augmented asmore information becomes available
•For example, the defined default router may notalways be the best choice for a destination
•The occurs when more than one router isconnected to the same network
•When this occurs, a router may send a redirectionmessage to the host to notify it of the better choice
Redirection concept
Redirection message format
Code 0: Network specific
Code 1: Host specific
Code 2: Network specific (specified service)
Code 3: Host specific (specified service)
Note: This is sent by a router to a host on
the same local network
Query messages
Echo Request/Reply
•A host or a router may send an Echo Request
•When a host or router receives an EchoRequest it should send the same message tothe source of the request as a Reply
•This is essentially what the command PINGdoes
•This is a useful diagnostic to verifyreachability of a destination
Echo-request and echo-reply message format
Ping command can use ICMPRequest/Reply
Timestamp-request and
timestamp-reply message format
• Time values are in milliseconds since midnight UniversalTime (UT)
• UT is the same as Greenwich Mean Time (GMT)
Timestamp Request/Reply
•Originate timestamp set by the source of datagram, othervalues set to 0
•Receive timestamp set by destination when datagramreceived
•Transmit timestamp set when destination sends reply
•When reply arrives at source, time of reply noted
Sending time = Receive – Originate
Receive time = Reply – Transmit
Round trip = Sending time + Receive time
•The round trip time is accurate even if the two clocks are notsynchronized
Timestamp Request/Reply
•Synchronizing clocks using timestamp request/reply
One-way time = Round-trip/2
Time difference = Receive – (Originate + One-way)
•Example
Originate timestamp =46
Receive timestamp =59
Transmit timestamp =60
Reply timestamp = 67
Sending time = 59 – 46 = 13
Receive time = 67 – 60 = 7
Round-trip = 13 + 7 = 20
Time difference = 59 – (46 + 10) = 3
Mask-request and mask-reply message format
• Purpose is for a host to obtain the mask associated with its IP address
• Host sends this request to a router on the local network
• If host does not know the address of the router it broadcasts this request
• Router responds with the mask for this network
• Not really used today
• BOOTP and DHCP used instead
Router solicitation message format
• Purpose is to allow a host to discover routers on the local network
• Even if it has a default gateway configured, this is a way to discoverothers connected to the local network
• A host broadcasts this message
• Routers will reply with advertisement messages
Router advertisement message format
• May be sent even when not solicited by a request
• Advertisement is broadcast on the local network
ICMP Checksum
•An ICMP message contains a Checksummuch like an IP header
•The Checksum calculation is the same aspreviously described
•The Checksum covers the entire ICMPmessage, the header and the data
ICMP package
Traceroute
•Command that attempts to track the path adatagram takes to its destination
•Traceroute uses ICMP and the TTL field inthe IP header
•When TTL decrements to 0 ICMP TimeExceeded message sent to host
•ICMP message contains IP address ofrouter generating message
Traceroute
•Traceroute sends datagram with TTL = 1
•First router sends ICMP Time Exceeded
•Traceroute next sends datagram with TTL =2
•Second router sends ICMP Time Exceeded
•This continues until destination reached
•Each step logs a hop to the destination
•Notes:
Path of subsequent datagrams not guaranteed to be thesame each time
Some routers along the way may not respond