Computer Networks. College of Computing. Copyleft 2003~2018

Transcription

1 Computer Networks Prof. Lin Weiguo Coege of Computing Copyeft 2003~

2 Internet Contro Message Protoco (ICMP), RFC 792 Reates to Lab ICMP: A short modue on the ICMP. Reference: 2

3 Overview The IP (Internet Protoco) reies on severa other protocos to perform necessary contro and routing functions: Contro functions (ICMP) Muticast signaing (IGMP) Setting up routing tabes (RIP, OSPF, BGP, PIM, ) RIP OSPF BGP PIM Routing ICMP IGMP Contro 3

4 Purpose of ICMP The Internet Contro Message Protoco (ICMP) is a heper protoco that supports IP with faciity for Error reporting Simpe queries 2018/5/17 inwei@cuc.edu.cn 4

5 Layering view From a ayering point of view, ICMP is a separate protoco that sits above IP and uses IP to transport messages. In practice, ICMP is an integra part of IP and a IP modues must support the ICMP protoco. ICMP datagrams are encapsuated within IP datagrams and processed by IP in the same way as TCP and UDP datagrams; Transport TCP/UDP ICMP IP 2018/5/17 inwei@cuc.edu.cn 5

6 Message Encapuation ICMP messages are encapsuated as IP datagrams: IP header ICMP message IP payoad 6

7 ICMP message format bit # type code checksum additiona information or 0x byte header: Type (1 byte): type of ICMP message Code (1 byte): subtype of ICMP message Checksum (2 bytes): simiar to IP header checksum. Checksum is cacuated over entire ICMP message If there is no additiona data, there are 4 bytes set to zero. à each ICMP messages is at east 8 bytes ong 7

8 Types of ICMP Msges two genera types of ICMP messages: Information messages, where a sender sends a query to another machine (either host or gateway) and expects an answer. For exampe, a host might want to know if a gateway is aive. Error indication messages, where the IP software on a host or gateway has encountered a probem processing an IP datagram. For exampe, it may be unabe to route a datagram to its destination, or it may have had to drop a frame. 2018/5/17 inwei@cuc.edu.cn 8

9 Exampe ICMP messages Type Code description 0 0 echo repy (ping) 3 0 dest. network unreachabe 3 1 dest host unreachabe 3 2 dest protoco unreachabe 3 3 dest port unreachabe 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion contro choke packet) 8 0 echo request (ping) 9 0 route advertisement 10 0 router soicitation 11 0 TTL expired 12 0 bad IP header 2018/5/17 inwei@cuc.edu.cn 9

10 ICMP Query message ICMP query: Request sent by host to a router or host Repy sent back to querying host 10

11 Exampe of ICMP Queries Type/Code: 8/0 Echo Request 0/0 Echo Repy Description The ping command uses Echo Request/ Echo Repy 13/0 Timestamp Request 14/0 Timestamp Repy 10/0 Router Soicitation 9/0 Router Advertisement 11

12 Purpose of Req/Repy The ICMP echo request and echo repy messages are usefu for network debugging. If machine A sends an echo request message to machine B, machine B is required to respond with an ICMP echo repy. Most systems suppy an appication program that sends and receives ICMP echo messages. In UNIX, the program ping aows a user to check whether a machine is reachabe and functioning. Because ICMP messages are handed just ike other IP datagrams, ICMP echo messages test the reachabiity of any host. Aso, because ICMP is an integra part of IP, a hosts and gateways must impement ICMP. 2018/5/17 inwei@cuc.edu.cn 12

13 Exampe of a Query: Echo Request and Repy Ping s are handed directy by the kerne Each Ping is transated into an ICMP Echo Request The Ping ed host responds with an ICMP Echo Repy Host or Router ICMP ECHO REQUEST ICMP ECHO REPLY Host or router 13

14 Timestamp Messages ICMP timestamp messages are used to estimate the transmission deays between machines and to synchronize cocks: Incuding both the receive and transmit timestamp aows the sending host to determine the fraction of time spent transmitting vs. processing the request. By averaging the measurements of severa messages, the sender can estimate the offset between its oca cock and that on the remote machine. Note: it is quite feasibe to synchronize the cocks of a machines on a LAN to within severa miiseconds of each other. 2018/5/17 inwei@cuc.edu.cn 14

15 Exampe of a Query: ICMP Timestamp A system (host or router) asks another system for the current time. Timestamp Request Time is measured in miiseconds after midnight UTC (Universa Coordinated Time) of the current day Sender sends a request, receiver responds with repy Sender Timestamp Repy Receiver Type (= 17 or 18) Code (=0) Checksum identifier sequence number 32-bit sender timestamp 32-bit receive timestamp 32-bit transmit timestamp 15

16 ICMP Error message ICMP error messages report error conditions Typicay sent when a datagram is discarded Error message is often passed from ICMP to the appication program 16

17 ICMP Error message ICMP Message from IP datagram that triggered the error IP header ICMP header IP header 8 bytes of payoad type code checksum Unused (0x ) ICMP error messages incude the compete IP header and the first 8 bytes of the payoad (typicay: UDP, TCP) 17

18 Frequent ICMP Error message Type Code Description Destination unreachabe Notification that an IP datagram coud not be forwarded and was dropped. The code fied contains an expanation Redirect Informs about an aternative route for the datagram and shoud resut in a routing tabe update. The code fied expains the reason for the route change. 11 0, 1 Time exceeded 12 0, 1 Parameter probem Sent when the TTL fied has reached zero (Code 0) or when there is a timeout for the reassemby of segments (Code 1) Sent when the IP header is invaid (Code 0) or when an IP header option is missing (Code 1) 18

19 Some subtypes of the Destination Unreachabe Code Description Reason for Sending 0 Network Unreachabe 1 Host Unreachabe 2 Protoco Unreachabe 3 Port Unreachabe 4 Fragmentation Needed and DF Bit Set No routing tabe entry is avaiabe for the destination network. Destination host shoud be directy reachabe, but does not respond to ARP Requests. The protoco in the protoco fied of the IP header is not supported at the destination. The transport protoco at the destination host cannot pass the datagram to an appication. IP datagram must be fragmented, but the DF bit in the IP header is set. (used for Path MTU Discovery) 19

20 Exampe: ICMP Port Unreachabe RFC 792: If, in the destination host, the IP modue cannot deiver the datagram because the indicated protoco modue or process port is not active, the destination host may send a destination unreachabe message to the source host. Scenario: Cient Request a service at a port 80 Server No process is waiting at port 80 Port Unreachabe 20

21 END OF ICMP 2018/5/17 21