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Lecture 12

IST 233 Lecture 12: IST 233 M001 Molta 2:9

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Syracuse University
Information Studies
IST 233
David Molta

IST 233 M001 Molta 292017 What a packet? o A packet is a structured sequence of bits How many bits? o A few hundred to a thousand Packets include: o Header, payload (data) and trailer Header are broken into fields whose starting and ending bits are define by the standard Packets have different names, depending on their layer (frames, segments, datagrams) Packets and Packet switching o Semantics the meaning of field contents, varies between packets, meaning of the combination of 1s and 0s Addresses, data encoding, flags, etc. o Syntax Organization Like english grammar, but more rigid Every field has a precise location (offset) o Order Taking turns in a fair manner (eg. WiFi) Proper message sequencing (eg. TCP 3 way handshake < know how to explain this) Postcard or Registered Letter? o Reliable Network Protocols A reliable protocol detects AND corrects errors during transmission, at the cost of additional overhead Reliability is not always necessary or even desirable. Voice over IP or facetime, real time communication it doesnt make any sense to retransmit any packets with errors, doing so creates overhead, thats why we dont use TCP with real time communication o Unreliable Network Protocol Besteffort protocol, may perform error detection but not necessarily error correction Unreliable protocols are in fact quite reliable. Your postcard usually gets delivered whether you pay for verification Ethernet doesnt have reliable network but Wifi does, because the medium is different, ethernet= as long as you put the wires and other devices together perfectly there is nothing that can interfere, while the medium for wifi is air so reliability is important. If theres an error ethernet discards the packet Because the medium of wifi is so much more inherently unreliable the designers knew there were going to be interference which means more packet errors so we should build error correction protocols to fix the errors EndtoEnd Reliability o TCP is the workhorse of internet apps, insuring that app information flows smoothly and is received in the exact same format it was transmitted o Reliability requires a virtual connection between application ports. The TCP 3 way handshake sets up these endtoend connections o When two hosts establish a TCP connection, they negotiate a starting sequence number (used for error correction) and a set of port numbers that define which specific application are using this connection Packet Syntax and DIagrams
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