CMPE 150 Winter 2009 Lecture 18 March 10 2009 P E Mantey CMPE 150 Introduction to Computer Networks Instructor Patrick Mantey mantey soe ucsc edu http www soe ucsc edu mantey Office Engr 2 Room 595J Office hours Tues 3 5 PM Mon 5 6 PM TA Anselm Kia akia soe ucsc edu Web site http www soe ucsc edu classes cmpe150 Winter09 Text Tannenbaum Computer Networks 4th edition available in bookstore etc Syllabus Reading Assignment Chapter 7 Applications Layer Internet Layering Level 5 Application Layer rlogin ftp SMTP POP3 IMAP HTTP Level 4 Transport Layer a k a Host to Host TCP UDP Level 3 Network Layer a k a Internet IP ICMP ARP Level 2 Data Link Layer MAC sub layer a k a Network Interface or Network Access Layer Level 1 Physical Layer Today s Agenda Transport Layer Performance Applications Layer DNS dig nslookup Akami Transitional TCP a RPC using normal TPC b RPC using T TCP Performance Issues Performance Problems in Computer Networks Network Performance Measurement System Design for Better Performance Fast TPDU Processing Protocols for Gigabit Networks Performance Problems in Computer Networks The state of transmitting one megabit from San Diego to Boston a At t 0 b After 500 sec c After 20 msec d after 40 msec Network Performance Measurement The basic loop for improving network performance Measure relevant network parameters performance Try to understand what is going on Change one parameter pathchar Jacobsen System Design for Better Performance 2 Response as a function of load System Design for Better Performance 3 Four context switches to handle one packet with a user space network manager Fast TPDU Processing The fast path from sender to receiver is shown with a heavy line The processing steps on this path are shaded Fast TPDU Processing 2 a TCP header b IP header In both cases the shaded fields are taken from the prototype without change Fast TPDU Processing 3 A timing wheel Protocols for Gigabit Networks Time to transfer and acknowledge a 1 megabit file over a 4000 km line DNS Hierarchical name space Distributed database RFCs 1034 and 1035 History Original approach ARPANET 1970 s File hosts txt listed all hosts and their IP addresses Every night every host fetches file from central repository OK for a few hundred hosts Scalability File size Centrally managed How is it used Client server model Client DNS running on client hosts or resolver Application calls resolver with name Resolver contacts local DNS server using UDP passing the name Server returns corresponding IP address DNS Name Space int com ibm eng sales cs edu gov mil org usc ee Tree based hierarchy net us ca Name Space Structure Top level domains Generic Countries Leaf domains no sub domains In practice all US organizations are under a generic domain while everything outside the US is under the corresponding country domain DNS Names Domain names Concatenation of all domain names starting from its own all the way to the root separated by Refers to a tree node and all names under it Case insensitive Components up to 63 characters Full name less than 255 characters Name Space Management Domains are autonomous Organizational boundaries Each domain manages its own name space independently of other domains Delegation When creating new domain register with parent domain For name uniqueness For name resolution Resource Records Entry in the DNS database Several types of entries or RRs Example RR A contains IP address Name several resource records RR format five tuple Name TTL in seconds Class usually IN for Internet info Type type of RR Value RR Types 1 SOA start of authority Marks beginning of zone s database Provides general info about the zone e mail address of admin default TTL etc A address Contains 32 bit IP address Single name several A RRs MX mail exchange Name of mail server for this domain RR Types 2 NS name server Name of name server for this domain CNAME canonical name Alias HINFO host description Provides information about host e g CPU type OS etc TXT arbitrary string of characters Generic description of the domain where it is located etc Name Servers Entire database in a single name server Practical Why DNS database is partitioned into zones Each zone contains part of the DNS tree Zone name server Each zone may be served by more than 1 server A server may serve multiple zones Primary and secondary name servers Name Resolution 1 Application wants to resolve name Resolver sends query to local name server Resolver configured with list of local name servers Select servers in round robin fashion If name is local local name server returns matching authoritative RRs Authoritative RR comes from authority managing the RR and is always correct Cached RRs may be out of date Name Resolution 2 If information not available locally not even cached local NS will have to ask someone else It asks the server of the top level domain of the name requested Recursive Resolution Recursive query Each server that doesn t have info forwards it to someone else Response finds its way back Alternative Name server not able to resolve query sends back the name of the next server to try Some servers use this method More control for clients Example Suppose resolver on flits cs vu nl wants to resolve linda cs yale edu Local NS cs vu nl gets queried but cannot resolve it It then contacts edu server edu server forwards query to yale edu server yale edu contacts cs yale edu which has the authoritative RR Response finds its way back to originator cs vu nl caches this info Not authoritative since may be out of date RR TTL determines how long RR should be cached Resource Records The principal DNS resource records types Resource Records 2 A portion of a possible DNS database for cs vu nl DNS The Domain Name System The DNS Name Space Resource Records Name Servers The DNS Name Space A portion of the Internet domain name space Name Servers Part of the DNS name space showing the division into zones Name Servers 2 How a resolver looks up a remote name in eight steps http www zoneedit com lookup html
View Full Document
Unlocking...