Internet control message protocol

Internet Control Message Protocol ICMP The Internet Control Message Protocol (ICMP) is a control protocol that is considered to be an integral part of IP, although it is architecturally layered upon IP - it uses IP to carry its data end-to-end. ICMP provides error reporting, congestion reporting, and first-hop router redirection. 1

Introduction IP provides best-effort delivery Delivery problems can be ignored; datagrams can be "dropped on the floor" Internet Control Message Protocol (ICMP) provides error-reporting mechanism 2

ICMP Features ICMP uses IP as if ICMP were a higher- level protocol (that is, ICMP messages are encapsulated in IP datagrams). However, ICMP is an integral part of IP and must be implemented by every IP module. ICMP is used to report some errors, not to make IP reliable. Datagrams may still be undelivered without any report on their loss. Reliability must be implemented by the higher-level protocols that use IP. 4

ICMP Features ICMP can report errors on any IP datagram with the exception of ICMP messages, to avoid infinite repetitions. For fragmented IP datagrams, ICMP messages are only sent about errors on fragment zero. That is, ICMP messages never refer to an IP datagram with a non- zero fragment offset field. 5

ICMP Features ICMP has rules regarding error message generation to prevent broadcast storms ICMP messages are never sent in response to datagrams with a destination IP address that is a broadcast or a multicast address. ICMP messages are never sent in response to a datagram which does not have a source IP address which represents a unique host. That is, the source address cannot be zero, a loopback address, a broadcast address or a multicast address. 6

Error Message Generation Rules ICMP errors messages are not generated in response to an ICMP error message datagrams destined to an IP broadcast address datagrams sent as a link-layer broadcast a fragment other than the first a datagram whose source address does not define a single host 7

ICMP Features ICMP messages are never sent in response to ICMP error messages. They may be sent in response to ICMP query messages (ICMP types 0, 8, 9, 10 and 13 through 18). RFC 792 states that ICMP messages “may” be generated to report IP datagram processing errors, not “must”. In practice, routers will almost always generate ICMP messages for errors, but for destination hosts, the number of ICMP messages generated is implementation dependent. 8

ICMP Message Format ICMP messages are described in RFC 792 and RFC 950, belong to STD 5 and are mandatory. ICMP messages are sent in IP datagrams. The IP header will always have a Protocol number of 1, indicating ICMP and a type of service of zero (routine). The IP data field will contain the actual ICMP message in the format shown in the figure below: 9

ICMP Message Transport ICMP encapsulated in IP But ... how can that work? ICMP messages sent in response to incoming datagrams with problems ICMP message not sent for ICMP message 10

Error Detection Internet layer can detect a variety of errors: Checksum (header only!) TTL expires No route to destination network Can't deliver to destination host (e.g., no ARP reply) Internet layer discards datagrams with problems Some - e.g., checksum error - can't trigger error messages 11

Types of Messages ICMP defines two types of messages: error and informational messages Error messages: Source quench Time exceeded Destination unreachable Redirect Fragmentation required Informational messages: Echo request/reply Address mask request/reply Router discovery 12

ICMP: Message Types Type Message 0 Echo reply 3 Destination unreachable 4 Source quench 5 Redirect 8 Echo request 11 Time exceeded 12 Parameter unintelligible 13 Time-stamp request 14 Time-stamp reply 15 Information request 16 Information reply 17 Address mask request 18 Address mask reply 13

ICMP Message Types Type Code Description Query Error Type Code Description Query Error 0 0 Echo reply • 5 Redirect 3 Destination unreachable: 0 Redirect for network • 0 Network unreachable • 1 Redirect for host • 1 Host unreachable • 2 Redirect for TOS and Net • 2 Protocol unreachable 3 Redirect for TOS and Host • • 3 Port unreachable • 8 0 Echo request • 4 Fragmentation needed • 9 0 Router advertisement • 5 Source route failed • 10 0 Router solicitation • 6 Destination network unknown 11 Time exceeded • 7 Destination host unknown 0 TTL equals 0 during transit • • 8 Source host isolated 1 TTL equals 0 during reassembly • 9 Destination net prohibited • 12 Parameter problem 10 Destination host prohibited • 0 IP header bad • 11 Network unreachable for TOS • 1 Required option missing • 12 Host unreachable for TOS • 13 0 Timestamp request • 13 Communication prohibited • 14 0 Timestamp reply • 14 Host precedence violation • 15 0 Information request • 15 Precedence cutoff in effect • 16 0 Information reply • 4 0 Source quench • 17 0 Address mask request • 18 0 Address mask reply • 14

ICMP and Reachability An internet host, A, is reachable from another host, B, if datagrams can be delivered from A to B ping program tests reachability - sends datagram from B to A that A echoes back to B Uses ICMP echo request and echo reply messages Internet layer includes code to reply to incoming ICMP echo request messages 15

Destination Unreachable Codes Code Meaning 0 Network unreachable 1 Host unreachable 2 Protocol unreachable 3 Port unreachable 4 Fragmentation need and don’t fragment bit set 5 Source route failed 6 Destination network unknown 7 Destination host unknown 8 Source host isolated 9 Communication with dest net administratively prohibited 10 Communication with dest host administratively prohibited 11 Network unreachable for type of service 12 Host unreachable for type of service 16

ICMP Unreachable Error Unreachable errors are generate for a number of reasons network unreachable host unreachable type (3) code (0-15) 16-bit checksum unused (must be 0) IP header (including options) + first 8 bytes of IP datagram data 17

Handling of ICMP Messages Type Code Description Handled by 0 0 Echo reply User process 3 Destination unreachable: 0 Network unreachable “No route to host” 1 Host unreachable “No route to host” 2 Protocol unreachable “Connection refused” 3 Port unreachable “Connection refused” 4 Fragmentation needed “Message too long” 5 Source route failed “No route to host” 6 Destination network unknown “Network is unreachable” 7 Destination host unknown “No route to host” 8 Source host isolated “No route to host” 9 Destination net prohibited “Network is unreachable” 10 Destination host prohibited “No route to host” 11 Network unreachable for TOS “Network is unreachable” 12 Host unreachable for TOS “No route to host” 13 Communication prohibited (ignored) 14 Host precedence violation (ignored) 15 Precedence cutoff in effect (ignored) 4 0 Source quench Kernel for TCP; ignored by UDP 18

Handling of ICMP Messages Type Code Description Handled by 5 Redirect 0 Redirect for network Kernel updates routing table 1 Redirect for host Kernel updates routing table 2 Redirect for TOS and Net Kernel updates routing table 3 Redirect for TOS and Host Kernel updates routing table 8 0 Echo request Kernel generates reply 9 0 Router advertisement User process 10 0 Router solicitation User process 11 Time exceeded 0 TTL equals 0 during transit User process 1 TTL equals 0 during reassembly User process 12 Parameter problem 0 IP header bad “Protocol not available” 1 Required option missing “Protocol not available” 13 0 Timestamp request Kernel generates reply 14 0 Timestamp reply User process 15 0 Information request Kernel generates reply 16 0 Information reply User process 17 0 Address mask request Kernel generates reply 18 0 Address mask reply User process 19

Ping Program Ping stands for “Packet InterNet Groper” The ping program tests whether another host is reachable The program works by sending an ICMP echo request to a host, expecting an ICMP echo reply to be returned Normally ping is used as a diagnostic tool to test network connectivity 20

Client/Server Programs Ping is is an example of a client/server program the client sends the request to a server the server returns the reply Most TCP/IP implementations support the ping server directly in the kernel 21

Ping Implementation Unix implementations set the identifier field to the process ID of the sender The sequence number starts at 0 and is incremented every time a new echo request is sent Ping operates in one of two modes send a single request, if a response is received the host is alive send one request every second Ping in Action IP Record Route Option 22

ICMP and Internet Routes List of all routers on path from A to B is called the route from A to B traceroute uses UDP to non-existent port and TTL field to find route via expanding ring search Sends ICMP echo messages with increasing TTL Router that decrements TTL to 0 sends ICMP time exceeded message, with router's address as source address First, with TTL 1, gets to first router, which discards and sends time exceeded message Next, with TTL 2, gets through first router to second router Continue until message from destination received traceroute must accommodate varying network delays Must also accommodate dynamically changing routes 23

ICMP and Path MTU Discovery Fragmentation should be avoided How can source configure outgoing datagrams to avoid fragmentation? Source determines path MTU - smallest network MTU on path from source to destination Source probes path using IP datagrams with don't fragment flag Router responds with ICMP fragmentation required message Source sends smaller probes until destination reached 24

Information Request/Reply: This request is intended for a diskless system to obtain its subnet mask Set source and destination addresses to 0 in the request and broadcast Server replies back with your IP address (Not used. Replaced by RARP and BOOTP) Address Mask Request/Reply: What is the subnet mask on this net? Replied by “Address mask agent” type (17 or 18) code (0) 16-bit checksum identifier (can be set to anything) sequence (can be set to anything) 32-bit subnet mask 25

ICMP Messages: Fragmentation Required Fragmentation Required: Datagram was longer than MTU and “No Fragment bit” was set. 26

ICMP Messages: Time Exceeded Time Exceeded: Time to live field in one of your packets became zero.” or “Reassembly timer expired at the destination. 27

ICMP Messages: Redirect Default route may cause extra hop Router that forwards datagram on same interface sends ICMP redirect Host installs new route with correct router as next hop Host Y Router A Host Y is reachable in one hop through Router A IP Packet Redirection: “Next time, use Router A Host X for this destination host” Router B 28

ICMP Messages: Redirect Redirect: Please send to router X instead of me. 0 = Redirect datagrams for the network 1 = Redirect datagrams for the host 2 = Redirect datagrams for the type of service and net 3 = Redirect datagrams for the type of service and host 29

ICMP Timestamp Request & Reply Used to return the current time from another host Could be used for (primitive) time synchronization protocol, but NTP and XNTP do a much better job type (13 or 14) code (0) 16-bit checksum identifier (can be set to anything) sequence (can be set to anything) 32-bit originate timestamp 32-bit receive timestamp 32-bit transmit timestamp 30

ICMP Timestamp Request & Reply The recommended value to be returned is the number of milliseconds since midnight, Coordinated Universal Time (UTC). A drawback is that only the time since midnight is returned. The caller must know the date form some other means 31

ICMP Summary Internet layer provides best-effort delivery service May choose to report errors for some problems ICMP provides error message service ICMP is the control sibling of IP ICMP is used by IP and uses IP as network layer protocol - Encapsulated in IP datagram - Not reliable Feedback about problems e.g. time to live expired ICMP is used for ping, traceroute, and path MTU discovery Transfer of (control) messages from routers and hosts to hosts 32

Internet control message protocol

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Internet control message protocol