Network Protocols By example Goal browse to http www depaul edu TDC375 Autumn 2009 10 John Kristoff DePaul University 1 What things happen first What happens when you hit enter or click the link What does the browser do TDC375 Autumn 2009 10 John Kristoff DePaul University 2 URL interpretation Parses request URI Its an HTTP GET For www depaul edu What is www depaul edu TDC375 Autumn 2009 10 John Kristoff DePaul University 3 Domain name system Hold on maybe the browser has a name cache OK say it doesn t gethostbyname or equivalent The whole resolution process happens TDC375 Autumn 2009 10 John Kristoff DePaul University 4 Whats involved in resolving Are we a stub resolver Full resolver Does hosts txt have www depaul edu in it OK we re a stub who do we talk to And how did we get that information OK let s format an A Query what about AAAA Maybe oh man this is a PITA already TDC375 Autumn 2009 10 John Kristoff DePaul University 5 OK let s send the query Not so fast Put the DNS message in UDP Ya thats good OK IP packet sending to DNS resolver easy From my IP address Oh man am I connected OK send it on the wire TDC375 Autumn 2009 10 John Kristoff DePaul University 6 Finish your IP encap bub Fits in MTU checksum set TTL etc OK go go Who you talking to first Is it a local host Oh gotta talk to a router OK lets do that How do I know who that is Argh OK got it get this into Ethernet and awa OH darn gotta ARP TDC375 Autumn 2009 10 John Kristoff DePaul University 7 Here ya go router OK Ethernet daddr is to the router Wha is that right Yep unless your mask is broken What the is a mask Router s go itt what s it gonna do Re encap routing protocols now policy decisions ARP some more At least no DNS I think TDC375 Autumn 2009 10 John Kristoff DePaul University 8 Skip ahead DNS server has query We re not even close to done TDC375 Autumn 2009 10 John Kristoff DePaul University 9 Time warp Sending TCP packet UDP for DNS now TCP What gives Gotta setup a connection Connection I thought this was connectionless Exchange options sequence numbers Timers congestion control sliding window oh my TDC375 Autumn 2009 10 John Kristoff DePaul University 10 This ain t gonna be a cake walk Its hard to learn how this all works even after year s of experience never mind a short protocols course But we ll try our best I left out a whole bunch of stuff This slide deck could have been hundreds of pages long easily and that s without pictures TDC375 Autumn 2009 10 John Kristoff DePaul University 11
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