Internet Control Message Protocol (ICMP)

• IP provides unreliable and connectionless delivery– Provides unreliable delivery

– Make efficient use of network resources

• No error reporting, or correcting mechanism

• No management of queries– Network manager might need

information about a host/router

• What happens if – Router discards a datagram ?

– TTL expires ?

– Host didn’t receive all datagram’s fragments ?

Position of ICMP in the network layerAnd Encapsulation

• Special purpose message mechanism added to the TCP/IP protocols• Destination of an ICMP message is the ICMP software module• ICMP is a network layer protocol, but its messages are first encapsulated into IP

datagrams.

Error reporting vs. Error Correction

• ICMP can only report an error to the original source– Up to the source to deal with it.

• ICMP cannot be used to inform intermediate routers.– Source has no responsibility of routers problems.

• Why restrict ICMP messages to original source?– Except for record route option, datagrams only contains source +

destination @.

– No global knowledge of routes (i.e. routers establish and change their own routing tables)

ICMP messages

Type Message3 Destination Unreachable4 Source Quench

11 Time Exceeded12 Parameter problem5 Redirection

Type Message8 or 0 Echo request or reply13 or 14 Timestamp17 or 18 Address Mask10 or 9 Router Sollicitation/Adv

General format of ICMP messages

• Data section in– Error Messages carries information to find the original packet that had

the error• Rest of Header unused (all 0s), except for Redirection message format

– Query Messages carries extra information based on type of the query.• Rest of Header = Identifier (8 bits) + Sequence Number (8 bits)

Error-reporting messages

Important points about ICMP error messages:Important points about ICMP error messages:1. No ICMP error message for a datagram carrying an ICMP error message.2. No ICMP error message for a fragmented datagram that is not the first fragment.3. No ICMP error message for a datagram having a multicast address.4. No ICMP error message for a datagram with a special address such as 127.0.0.0 or 0.0.0.0

Contents of data field for error messages

• In ICMP error messages– The first 8 bytes of the Transport layer header is included

– Provides Information about the port numbers (TCP or UDP) and sequence number (TCP)

Destination-unreachable (Type 3)

• ICMP destination unreachable message for codes = 2 and 3 only created by a host

• All others are created by a routers

0 Network is unreachable 8 source host isolated1 Host is unreachable 9 dest Network administratively prohibited2 Protocol is unreachable 10 dest Host admin prohibited3 Port is unreachable 11 Network unreachable for TOS4 Fragmentation required 12 Host unreachable for TOS5 Source routing not feasible 13 Administrator put a filter on Host6 Network unknown 14 requested precedence not permitted7 Host is unknown 15 Precedence was cut-off

!!! Routers cannot detect all Problems that preventthe delivery of a packet.

Source-quench (type =4, code =0)

• IP do not provide a flow-control mechanism– Source never knows if routers of destination is congested

• A source-quench message informs the source that a datagram has been discarded due to congestion in a router or the destination host– Two purposes: (1) informs the source of dropped packet (2) inform of

congestion along the path– Source must slow down (quench) the sending of datagrams until the

congestion is relieved.

• One source-quench message should be sent to each datagram that has been discarded due to congestion– One-to-one congestion– Many-to-one congestion (congested router has no idea which source is

sending datagrams faster)

Time-exceeded message (Type 11)

• If router receives a datagram with TTL = 0– Discard the datagram

– Inform the source using a Time-exceeded message (code = 0)

• If a host does not receive all fragments of a datagram during within a certain time of receiving the first fragment– Discard all fragments

– Inform the source using a Time-exceeded message (code = 1)

Code 0: used only by routersCode 1: used only by Hosts

Parameter-problem message

• A parameter problem message is created by a router or destination host– If there is an error or ambiguity in the header field (code = 0), pointer

points to the byte with problem

– An option is missing or incorrect (code = 1) pointer not used

• Router discards the datagram and sends a Parameter-problem message

Redirection message format

Code 0: Network specificCode 1: Host specificCode 2: Network specific (specified service)Code 3: Host specific (specified service)

• Updates to routers’ routing tables are dynamic

• Updates to hosts’ routing tables are static– Starts with the small routing tables that gets updated one of the tools is

redirection message format

– Redirection always sent from a router to a host in the same network

Query messages

• Diagnose some network problems.• Information request/reply is now obsolete (replaced by

RARP/BOOTP)

Timestamp-request and timestamp-reply message format

• Can be used to determine RTT needed for an IP datagram to travel between two machines.

• Identifier and Sequence fields allows machine to associate request with replies

Timestamp-request and timestamp-reply message

• Can be used to synchronize two machines clocks.

• Example:– Orig = 83573336, recv =

83573330; trans = 83573330;

– With RTT = 2 ms, |diff| = 6 ms; implies that recv is 7 ms late.

– Orig = 83573336, recv = 83573000; trans = 83573000;

– With RTT = 2 ms; |diff|=336 ms Timestamp req/reply is useless

• In general: • SendingT = recv – orig

• ReceivingT = packet arrived – trans

• RTT = (sendingT + ReceivingT)

• If: One_way_time = RTT/2

• outOfSyncT = recv – (orig + RTT/2)

Router solicitation message format

• An important issue is how to accommodate routers in the same netwok:– Designers provided routers advertisement,– Default value 10 mn

• Compromise between rapid failure detection and low overhead.

• From a Host point of view this delay is very expensive– Example: when a host boots cannot wait 10 mn

• Designers provide Router solicitation message to request immediate advertisement.

– Host multicasts/broadcasts a router solicitation message– Identifier + Sequence number not used.

Router Advertisement

• Static routing works well for a network that has only one router. (no need to discover routes or change routes)

• For a network with many routers, and if a router crashes, then host uses router advertisement message.– ICMP route discovery helps in:

• Host can get the router address from the router itself, instead via a bootstrap protocol (i.e. static configuration)

• Use of Timers to update routes (Soft state technique)

Router advertisement message format

• Number of addresses (that follows usually 1)• Address size (IPv4 = 1)• Lifetime: time to use the specified address (a default 30 mn)• Address Preference: Shows the preference of the router’s address.

Mask-request and mask-reply message format

• In general to request a subnet mask from a router

• Diskless machine can use Mask-request to get its subnet mask.

Echo-request and echo-reply message format

• Designed for diagnosis purposes– Host or router can send a echo-request– Receivers echoes back the message with an echo-reply

• Echo-request/reply used by network admin to test the reachability of a specific host

• Identifier & Sequence Number are not formally used by the protocol, – Can be set to anything by the sender

• Example : Ping program : a statistical tool – Does not use Transport protocols (TCP or UDP)

Ping Program• Ping programs uses Echo-request/reply to test reachability of a host

• Identifiers : Process ID

– If many ping programs are running

• Sequence Number : increment for each echo-request

• RTT = received_reply_time – requested_time(stored in ICMP data packet)

• Other Options:

• Use ICMP request message encapsulated into an IP packet with record route option.– Example: Ping –R machine

– Limited number of IP addresses • IP header lenght = 4 bits

• Allows 15* 4 bytes (60 – 20 IP header– 3 bytes for option information ) Only 9 IP addresses

• Use ICMP request message encapsulated into IP packet with timestamp option– More severe limitation

ICMP package

Input Module (handles all types of received ICMP messages)

• Receive an ICMP packet from the IP layer

• 1- if (type = request type)– 1- Create a reply

– 2- Send the reply

• 2- if (type = router solicitation)– 1- if (station is a router)

• Create router advertisement

• Send the advertisement

• 3- if (type = one of the 3 reply messages or router advertisement)– 1- Extract information in the data section of the packet

– 2- Deliver extracted information to the process that requested it

• 4- if (type defines a redirection)– Modify the routing table

• 5- if (type = error messages other that redirection)– Inform the appropriate source protocol

• 6- return.

Output Module (responsible for: creating requests, solicitation, error messages requested

by higher level protocols or IP)

• Receive : a demand

• 1- if (demand = error messages)– If (demand is from IP)

• If (demand is forbidden i.e. 4 cases where ICMP does not error message)– Return

– If (type defines a redirection message)• If (station is not a router)

– Return

– Create the error message using type, code, and original IP packet

• 2- If (demand = request or solicitation)– Create a request or solicitation message

• 3- send the message

• 4- return

Ping Program (example)

ping source

C

Gateway

Router

BA

SLIP

X.Y.Z

X.Y.Z'

Ping destination

Ping -R C

X.Y.Z.B2

X.Y.Z'.R2

X.Y.Z'.C

X.Y.Z'.Gateway

X.Y.Z.R1

X.Y.Z.B1

X.Y.Z.A

.R1

.R2

B1

B2

• If a router crashes or connection between two routers is lost

• Can take time to re-route

• In this case TTL function is to avoid routing loops.

• If TTL =0/1 must not forward the datagram.

– Router sends ICMP “Exceeded Time” to sender.

• TRACEROUTE

• Create UDP datagram(“with unreachable port#”) to send to destination

• UDP datagram has 12bytes of data, (sequence#, TTL,time).

• TTL = 1;

• Send ‘n’ datagrams to destination

• while (!receive (ICMP “port_unreachable”) – (TTLth Router

• TTL--; Send ICMP “TimeExceeded”)

– Sender knows IP address of TTLth router in ICMP packet

– Print (IP address, 1st RTT, 2nd RTT, .., nth RTT)

– TTL++;

– Send ‘n’ datagrams to destination

• }

Traceroute Program (ICMP “Time Exceeded” revisited)

Traceroute Program ( IP Source route revisited)

S DR3R2R1

dest = D{#R1,R2,R3}

dest = R1{#R2,R3,D}

dest = R2{R1,#R3,D}

dest = R3{R1,R2,#D}

dest = D{R1,R2,R3#}

• Traceroute can specify a Strict Source routing– If failed, ICMP error source route unfeasible (type =3, code =5)

• Can also specify loose source routing– Traceroute [-LooseRoute Destination] Source– Traceroute Round Trips may be different

• How many source route IP addresses?

Traceroute or Ping RTT?

• Traceroute stores original time in UDP data. (12 bytes).

• Ping stores original time in ICMP packet, echoed by

receiver.