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【正文】 ThreeWay Handshake Application Transport Inter Network Data Application Transport Inter Network Data, ACK (+start byte) ACK TCP Segment Format User Datagram Protocol (UDP) ? Connectionless ? No session is established ? Does Not Guarantee Delivery ? No sequence numbers ? No acknowledgments ? Low overhead ? Reliability Is the Responsibility of the Application ? Doesn’t trust the lower layers and runs its own CRC ? Uses Port Numbers as Endpoints to Communicate UDP Segment Format ? Low overhead ? No sequence number ? No Acknowledgement number ? No windows size Transport Application Inter Network Ports Numbers (1) ? TCP and UDP must use port numbers to municate with the upper layers. ? Port numbers keep track of different conversations crossing the work simultaneously. TFTP Server Web Server 0 . . . 65535 0 . . . 65535 TCP Ports 20,21 UDP Port 69 TCP Port 80 Windows Sockets Interface TCP Protocol No. 6 UDP Protocol No. 17 IP FTP Server SMTP TCP Port 25 DNS UDP Port 53 POP3 UDP Port 110 SNMP UDP Port 161 Port Numbers (2) ? No. below 1024 ? Wellknown port no. defined in RFC 1700 ? Usually in destination port to tell the receiving host the purpose of the intended connection ? No. 1024 and above ? used by upper layers (randomly chosen) to set up sessions with other hosts ? Used by TCP to use as source and destination addresses in the TCP segment ? Usually in Source port to differentiate between sessions with different source hosts The Inter Layer Protocols ? For routing and providing a single work interface to the upper layer layers. ? All work paths through the model go through IP. ? Protocols works at the Inter Layer: ? Inter Protocol (IP) – essentially is the Inter Layer, and other protocols found here merely exist to support it. ? Inter Control Message Protocol (ICMP) ? Address Resolution Protocol (ARP) ? Reverse Address Resolution Protocol (RARP) Inter Protocol (IP) ? Addresses and Routes Packets according to the Routing Table ? Fragments and Reassembles Datagrams / Packets ? Connectionless ? No session is established ? Nonguaranteed “Best Effort” Delivery ? Reliability Is the Responsibility of HigherLayer Protocols and Applications IP header * Protocol number: 01 – ICMP, 06 – TCP, 17 – UDP, etc. Inter Control Message Protocol (ICMP) ? Management Protocol and messaging service provider for IP. ? In router solicitation, ICMP is used to send the following events and messages (in the Data area): ? Destination Unreachable ? Buffer Full ? Hops ? Ping ? Traceroute Address Resolution Protocol (ARP) ? Successful Mapping of an IP Address to a Hardware Address
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