Internet Command Message Protocol (ICMP)

1. Internet Command Message Protocol(ICMP) CS-431 Dick Steflik

2. ICMP • Internet Command Message Protocol (ICMP) • RFC 792 • Used to communicate IP status and error messages between host and routers

3. ICMP • Used to communicate IP status and error messages between hosts and routers • Uses IP to route its messages between hosts • Must be implemented with IP • remember, IP is just a packet delivery system • transmits and routes datagrams from sources to destinations through a series of interconnected networks • it has a checksum in the IP header to detect lost bits • no error detection on the datagram payload though • but has no native mechanism for source host notification • This is where ICMP comes in • its used to report IP errors to the source host • ICMP data is carried as the payload of an IP datagram • specifies additional message formats within this area

4. Basic ICMP Header • Headers are 32 bits in length; all contain same three fields • type - 8 bit message type code • thirteen message type are defined • code - 8 bit; indicating why message is being sent • checksum - standard internet checksum • 16 bit 1’s complement sum of the payload and header • for purpose of calculation the checksum field is set to zero

5. ICMP Message types • 0 - Echo Reply • 3 - Destination Unreachable • 4 - Source Quench • 5 - Redirect • 8 - Echo • 11 - Time Exceeded • 12 - Parameter Problem • 13 - Timestamp • 14 - Timestamp Reply • 15 - Information Request • 16 - Information Reply • 17 - Address Mask Request • 18 - Address Mask Reply

6. Destination Unreachable (3) • ICMP header (4 bytes) + unused 32 bits (4 bytes) + IP header (24 bytes) + first 64 bits of data (8 bytes) = 40 bytes • Codes: • 0 - net unreachable ; 1 - host unreachable • 2 - protocol unreachable ; 3 - port unreachable • sent by destination host IP module • 4 - fragmentation needed DF set ; 5 source route failed • 6 - destination network unknown ; 7 destination host unknown • 8 - source host isolated ; 9 - comm. with destn network prohibited • 10 - comm. With dest host prohibited ; 11 - network unreachable for service • 12 - host unreachable for service • Sent to originating host because destination is unreachable • may be determined by a router • destination IP may find the indicated protocol unavailable • Don’t Fragment (DF) bit in the IP header is set but fragmentation is required to continue forwarding

7. Source Quench (4) • Same message format as type 3 • Code : 0 • Sent to a host when an intermediate router or the destination host with the source host’s transmission rate • may be sent to a source when a router is saturated • may be sent by a receiving host if it receive buffers are filling up • Upon receipt the source host should throttle back on its transmission rate until the Source Quench goes away. • Can then increase its transmission rate

8. Redirect (5) • Same format as type 3 • Code: • 0 - redirect datagrams for the network • 1 - redirect datagrams for the host • 2 - redirect datagrams for the type of service and the network • 3 - redirect datagrams for the type of service and host • a router sends a message to a host when it determines a datagram that originated from the host must be forwarded to router that can be directly reached • allows the host to sent future datagrams to the optimal first-hop router increasing network efficiency • not used for datagrams that have source routing options

9. Echo (8)/Echo Reply (0) • ICMP header (4 bytes) + identifier (2 bytes) + sequence number (2 bytes) + data (4 bytes) • identifier - used to match Echoes and Echo Replies • sequence - used to match Echoes with Echo Replies • Used to determine if a host is reachable • a host receiving an echo message • reverses the IP source and destination addresses • sets the ICMP type field to zero (echo reply) • recomputes the ICMP checksum • identifier, sequence and data are sent back unchanged

10. Time Exceeded (11) • Same format as type 3 • Code: • 0 - time to live exceeded in transit • 1 - fragment reassembly time exceeded • Time exceeded message is sent if: • a router finds a datagram with TTL set to zero • router discards the datagram and sends message with code field set to 0 • a host does not receive all of the fragments of a datagram before its local reassemble timer expires • host discards all fragments and return message with code field set to 1

11. Parameter Problem (12) • ICMP Header (4 bytes) + pointer (1 byte) + unused (3 bytes) + IP header (24 bytes) + first 64 bits of data (8 bytes) • pointer - identifies octet where error occurred • Code: • 0 - misc parameter problem • 2 - required option missing • sent to a host when a router or host processing a datagram finds a problem with the information in the datagram. • Only sent if the datagram had to be discarded • pointer field is zero based • ex: 1 indicates problem with type of service; 20 indicates first option

12. Timestamp(13)/Timstamp Reply(14) • ICMP Header (4 bytes) + identifier (2 bytes) + seq num (2 bytes) + Originate Timestamp (4 bytes) + Receive Timestamp (4 bytes) + Transmit Timestamp (4 bytes) • Timestamps are number of msec past midnight UTC • used to determine the latency between the sender and receiver • receiver forms a message by • reversing the originate and destination addr in the IP header • setting ICMP type code to 14 • updating the timestamp fields • recomput the ICMP checksum

13. Info Request (15)/Info Reply(16) • Same as type 8, but no data • Code: 0 • used by a host to determine the network number the host resides on • receiver • reverses the source and destination addresses in the IP header • set the correct network number • set ICMP type to 16 • recompute the ICMP checksum • Obsolete; shouldn’t be used replaced by BOOTP and RARP

14. Addr Mask Rqst (17)/Addr Mask Reply(18) • ICMP Header (4 bytes) + identifier (2 bytes) + seq.num. (2 bytes) + addr mask (4 bytes) • identifier - used to match requests with replies. • seq.num. - used to match requests with replies. • Hosts and routers can request the subnet address mask for the network they reside on at boot time. • Host or router broadcasts it on the local network • a receiving router should return it in a reply message • This message is defined in RFC 950

15. ICMPv6 • New version of ICMP to go along with IPv6 • Absorbs many IGMP and ARP functions • Used for: • Reporting errors in IP packet processing • Performing diagnostics • performing Neighbor Discovery and reporting IPv6 multicast memberships • Two types of messages: • Error messages • Information messages

16. Error Message Categories • Destination Unreachable • Packet too big • Time exceeded • Parameter problem

17. Informational Messages • Diagnostic messages • Multicast group management messages • Neighbor discovery messages

18. Every ICMPv6 message is preceeded by an IPv6 header and 0 or more IPv6 extension headers. • A next header field of 58 nidentifies the ICMPv6 message (different than IPv4)

19. Message Format 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | Type | Code | Checksum | |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | | | Message Body | | | | +-------------------------------------------------------------+