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 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 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 specificCode 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 – OriginateReceive 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/2Time difference = Receive – (Originate + One-way)

• Example

Originate timestamp =46Receive timestamp = 59Transmit timestamp = 60Reply 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 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