What we covered in the last 2 weeks!How to calculate the delay in packet delivery!! Queueing delay!" and congestion losses!! Transmission delay!! Propagation delay!! Round Trip Time!! Bandwidth-delay product!! Delay in delivering multiple packets across multiple hops!A few application protocols!! HTTP!" HTTP 1.0: w/, w/o parallel conn.!" HTTP 1.1: w/, w/o Pipelining!! Email and SMTP!! P2P and FTP in passing!Learned about app. protocols!! typical message exchange:!" client sends request!" server responds with status code and data!! Typical message formats:!" header: info about data!" data: info being communicated!CS118!1!Chapter 2: Application layer!! 2.1 Principles of network applications!! 2.2 Web and HTTP!! 2.3 FTP !! 2.4 Electronic Mail!" SMTP, POP3, IMAP!! 2.5 DNS!! 2.6 P2P file sharing!! 2.7 Socket programming with TCP!! 2.8 Socket programming with UDP!CS118!2!Domain Name System Why Internet needs a distributed name database!! People: many identifiers:!" SSN, name, passport #!! Internet hosts, routers:!" IP address (32 bit) - used by Internet to deliver data!" “name”, e.g., ww.yahoo.com - used by humans!! Domain Name System: Hostname # IP address translation!" and vice versa!Why not a centralize DNS?!! single point of failure!! traffic volume!! Maintenance!! Distant database services!! The most important factor: the need for distributed management!CS118!3!DNS: Domain Name System!4 Major Parts:!1. A hierarchical name space!2. A distributed database, realized through a hierarchy of authoritative servers!" Provided by individual domain owners!3. Local caching resolvers!" Provided by Internet service providers!4. An access protocol!$ Used to access the distributed DNS database!CS118!4!DNS: a hierarchical name space!! starting from the root, growing downward!" variable depth!! each non-leaf node in the tree is a domain!" Each domain belongs to an administrative authority!" any domain can set up its own sub-domains, no limit on the depth!! DNS name hierarchy is completely independent from the Internet's topological structure!CS118!5!edu com gov org . . . . . us . . . . uk fr . . . mit ucla xerox Dell nasa nsf acm ieee cs seas cad ... ..... ..... ..... root ccTLD Foo Bar TLD: top level domains gTLD in-addr.arpa in-addr.arpa domain!! Provides IP address to DNS name lookup!" address 1.2.3.4: ask for name for 4.3.2.1.in-addr.arpa!! Why it is useful!! Reverse DNS lookup is also commonly used today as a weak security checking (e.g. by email servers)!CS118!6!% traceroute mit.edu traceroute to mit.edu (18.7.22.69), 64 hops max, 40 byte packets 1 42.0.0.1 (42.0.0.1) 3.585 ms 1.638 ms 1.509 ms 2 cisco32-1 (131.179.32.3) 1.740 ms 5.856 ms 3.359 ms 3 border1 (131.179.12.3) 2.322 ms 3.008 ms 5.489 ms 4 compsci--mathsci.backbone.ucla.net (169.232.49.65) 2.070 ms 1.972 ms 3.855 ms 5 mathsci--core-1-10ge.backbone.ucla.net (169.232.4.20) 2.001 ms 2.048 ms 2.031 ms 6 core-1--border-1-10ge.backbone.ucla.net (169.232.4.101) 4.984 ms 2.055 ms 1.982 ms 7 lax-hpr1--ucla-ge.cenic.net (137.164.27.5) 6.371 ms 2.667 ms 3.373 ms 8 abilene-la--hpr-lax-gsr1-10ge.cenic.net (137.164.25.3) 10.095 ms 11.057 ms 3.664 ms 9 snvang-losang.abilene.ucaid.edu (198.32.8.95) 10.398 ms 11.527 ms 12.605 ms 10 dnvrng-snvang.abilene.ucaid.edu (198.32.8.2) 42.018 ms 35.360 ms 35.418 ms 11 ...... DNS: Root name servers!! 13 root name servers worldwide, operated by various parties on a coordinated, volunteering basis!" most of them have multiple instances via anycast! b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 36 other locations) i Autonomica, Stockholm (plus 28 other locations) k RIPE London (also 16 other locations) m WIDE Tokyo (also Seoul, Paris, SF) a Verisign, Dulles, VA c Cogent, Herndon, VA (also LA) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 21 locations) Application 2-7 DNS: Implemented as a distributed database!! each domain controlled by an organization, runs its own name servers (authoritative server)!! Each server of a domain knows!1. The name to value bindings for all the names in the zone!2. Where to redirect queries for any names in its children domains (sub-domains)!CS118!8!.com DNS servers .org DNS servers .edu DNS servers ucla.edu DNS servers umass.edu DNS servers yahoo.com DNS servers amazon.com DNS servers pbs.org DNS servers EE CS cs.ucla.edu netsec.cs.ucla.edu DNS root serversGlue together all pieces of the distributed DB!CS118!9!. net. edu. ucla.edu. edu. NS a.edu. edu. NS b.edu. ucla.edu. NS a.ucla.edu. ucla.edu. NS b.ucla.edu. net. NS a.net. net. NS b.net. root NAME TYPE TTL VALUE ucla.edu NS 24 a.ucla.edu a.ucla.edu A 600 1.1.1.1 www.ucla.edu A 600 2.2.2.2 mail.ucla.edu A 300 3.3.3.3 !. zones name servers A parent domain keeps the DNS server information for each of its child domain!DNS Look up: an example!CS118!10!Caching resolver ! Root server .edu server " # ucla.edu server $ % Other possibilities!• caching resolver may know the IP address for .edu server already!stub resolver Your browser needs IP address for www.ucla.edu:!Steps of Actions in Resolving a Name!CS118!11!. net. edu. ucla.edu. Caching Server Stub Resolvers Authoritative server Steps of Actions in Resolving a Name!CS118!12!. net. edu. ucla.edu. www.ucla.edu ? Cache edu NS a.edu edu NS b.edu a.edu A 1.1.1.1 b.edu A 1.1.1.2 ucla.edu NS a.ucla.edu ucla.edu NS b.ucla.edu a.ucla.edu A 2.2.2.1 b.ucla.edu A 2.2.2.2 www.ucla.edu A 2.2.2.3 www.ucla.edu ?Summary: How a DNS name gets resolve!! Your host sends a query for www.ucla.edu to a local DNS caching resolver!" provided by your ISP!! The caching resolver either finds a relevant answer in its cache, otherwise sends a query to one of the root servers!" “relevant”: any of the following!% An exact match: www.ucla.edu’s IP address!% DNS server for ucla.edu!% DNS server for .edu!! The root server replies with pointers to .edu servers !! The caching resolver queries .edu DNS server which replies
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