Error and Control An IP datagram travels from node to node on the way to its destination Each router operates autonomously Failures or problems may occur or develop along the way to a destination  Destinations may not be reachable  Referenced host may not have the appropriate application running  The Time to Live field may decrement to 0 before reaching destination

Error and Control To assist with such situations a protocol was included within the IP module Internet Control Message Protocol (ICMP) Routers and hosts can report certain conditions back to the source of the original datagram This is strictly a reporting mechanism to the source of the datagram that triggered the condition

ICMP The ICMP protocol is documented in RFC792 (1981) ICMP messages are themselves encapsulated as IP datagrams ICMP messages may themselves encounter reportable conditions By convention, when ICMP datagrams encounter normally reportable conditions, these are not sent The reporting of conditions are not mandatory

ICMP No ICMP message is sent for a fragment datagram except for the first fragment No ICMP message is sent if source is a multicast address No ICMP message is sent to special addresses, such as

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 the datagram 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 this ICMP 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 a default router As time goes on, this table can be augmented as more information becomes available For example, the defined default router may not always be the best choice for a destination The occurs when more than one router is connected to the same network When this occurs, a router may send a redirection message 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 Echo Request it should send the same message to the source of the request as a Reply This is essentially what the command PING does This is a useful diagnostic to verify reachability of a destination

Echo-request and echo-reply message format Ping command can use ICMP Request/Reply

Timestamp-request and timestamp-reply message format Time values are in milliseconds since midnight Universal Time (UT) UT is the same as Greenwich Mean Time (GMT)

Timestamp Request/Reply Originate timestamp set by the source of datagram, other values set to 0 Receive timestamp set by destination when datagram received 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 not synchronized

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 = = 20 Time difference = 59 – ( ) = 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 discover others 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 Checksum much like an IP header The Checksum calculation is the same as previously described The Checksum covers the entire ICMP message, the header and the data

ICMP package

Traceroute Command that attempts to track the path a datagram takes to its destination Traceroute uses ICMP and the TTL field in the IP header When TTL decrements to 0 ICMP Time Exceeded message sent to host ICMP message contains IP address of router 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 the same each time  Some routers along the way may not respond