TDC 375 Network Protocols TDC 563 P T for Data Networks By example Goal browse to http www depaul edu TDC375 563 Spring 2013 14 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 563 Spring 2013 14 John Kristoff DePaul University 2 URL interpretation Parses request URI It is an HTTP GET For www depaul edu What is www depaul edu TDC375 563 Spring 2013 14 John Kristoff DePaul University 3 Domain name lookup Does the browser have the name cached Let us assume the answer is no Browser issues a gethostbyname or equivalent We embark on a resolution sub process TDC375 563 Spring 2013 14 John Kristoff DePaul University 4 What is involved in resolving Does hosts txt have www depaul edu in it Are we a stub forwarder or full resolver Assume we re a stub who do we talk to And how did we get that information That was probably derived from boot strap OK let s format an A query What about AAAA Maybe do both Getting pretty complicated isn t it TDC375 563 Spring 2013 14 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 datagram sending to DNS resolver easy From my IP address Uhm am I connected OK send it on the wire TDC375 563 Spring 2013 14 John Kristoff DePaul University 6 Finish your IP encap bub Fits in MTU checksum set TTL etc OK go go Wait what L2 destination Is it a local host Oh gotta talk to a router OK lets do that How do I know who that is Argh That was probably part of boot strap OK got it get this into Ethernet and off we go Done yet Not even close TDC375 563 Spring 2013 14 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 Presume it s non local router gets it now what Router has a decision to make Forwarding policy decision re encap ARP etc At least no DNS I think TDC375 563 Spring 2013 14 John Kristoff DePaul University 8 Skip ahead DNS server has query We haven t even gotten to HTTP request yet TDC375 563 Spring 2013 14 John Kristoff DePaul University 9 DNS server processing Process query Can we Do I know about this name Cache or auth How do I go about finding out If not auth and not cached how many more steps Quite a few maybe skip ahead skip ahead skip ahead TDC375 563 Spring 2013 14 John Kristoff DePaul University 10 Time warp Sending TCP packet UDP for DNS now TCP What gives Gotta setup a connection the 3 way handshake Connection Isn t IP connectionless WTF Exchange options sequence numbers Timers congestion control sliding window oh my Is it time for HTTP now TDC375 563 Spring 2013 14 John Kristoff DePaul University 11 This isn t a cake walk Its hard to learn how this all works even after year s of experience never mind a short networks 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 563 Spring 2013 14 John Kristoff DePaul University 12
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