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EECS 122: Introduction to Computer Networks Multicast

EECS 122: Introduction to Computer Networks Multicast. Computer Science Division Department of Electrical Engineering and Computer Sciences University of California, Berkeley Berkeley, CA 94720-1776. Barriers to Multicast. Hard to change IP multicast means change to IP

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EECS 122: Introduction to Computer Networks Multicast

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  1. EECS 122: Introduction to Computer Networks Multicast Computer Science Division Department of Electrical Engineering and Computer Sciences University of California, Berkeley Berkeley, CA 94720-1776

  2. Barriers to Multicast • Hard to change IP • multicast means change to IP • details of multicast were very hard to get right • Not always consistent with ISP economic model • charging done at edge, but single packet from edge can explode into millions of packets within network • Troublesome security model • Anyone can send to a group • Denial-of-service attacks on known groups

  3. Application Layer Multicast (ALM) • Let the hosts do all the “special” work • only require unicast from infrastructure • Basic idea: • hosts do the copying of packets • set up tree between hosts • Example: Narada [Yang-hua et al, 2000] • Small group sizes <= hundreds of nodes • Typical application: chat

  4. Narada: End System Multicast Gatech Stanford Stan1 Stan2 CMU Berk1 Berk2 Berkeley “Overlay” Tree Stan1 Gatech Stan2 CMU Berk1 Berk2

  5. Algorithmic Challenge • Choosing replication/forwarding points among hosts • how do the hosts know about each other • and know which hosts should forward to other hosts

  6. Advantages of ALM • No need for changes to IP or routers • No need for ISP cooperation • End hosts can prevent other hosts from sending • Easy to implement reliability • use hop-by-hop retransmissions

  7. Performance Concerns • Stretch • ratio of latency in the overlay to latency in the underlying network • Stress • number of duplicate packets sent over the same physical link

  8. Performance Concerns Delay from CMU to Berk1 increases Stan1 Gatech Stan2 CMU Berk2 Berk1 Duplicate Packets: Bandwidth Wastage Gatech Stanford Stan1 Stan2 CMU Berk1 Berk2 Berkeley

  9. Single Sender Multicast • Many problems with IP multicast disappear if each group is associated with a single source • Hosts joining multicast group can send join messages to source • this sets up delivery tree • no worry about “root” being in wrong place • This solves several problems: • better security and charging model • simple algorithm

  10. Example • Group members: M1, M2, M3 source M1 M2 M3 control (join) messages data

  11. What’s Wrong with SSM? • Multiple sources? • can set up group per source, or... • source can serve as relay for other senders • Algorithm? • trivial • So, why isn’t SSM the answer? • multicast no longer serves as “rendezvous” • ok for “broadcast” apps, not good for “meeting” apps

  12. What Do You Need to Know? • DVRMP • CBT • SSM • How they compare

  13. EECS 122: Introduction to Computer Networks DNS and WWW Computer Science Division Department of Electrical Engineering and Computer Sciences University of California, Berkeley Berkeley, CA 94720-1776

  14. Internet Names & Addresses • Names: e.g., ariachne.berkeley.edu • human-usable labels for machines • conforms to “organizational” structure • Addresses: e.g., 169.229.131.109 • router-usable labels for machines • conforms to “network” structure • How do you map from one to another? • Domain Name System (DNS)

  15. DNS: History • Initially all host-addess mappings were in a file called hosts.txt (in /etc/hosts) • Changes were submitted to SRI by email • New versions of hosts.txt ftp’d periodically from SRI • An administrator could pick names at their discretion • As the internet grew this system broke down because: • SRI couldn’t handled the load • Names were not unique • Many hosts had inaccurate copies of hosts.txt • Internet growth was threatened! • Domain Name System (DNS) was born

  16. Basic DNS Features • Hierarchical namespace • As opposed to original flat namespace • Distributed storage architecture • As opposed to centralized storage (plus replication) • Client--server interaction on UDP Port 53 • But can use TCP if desired

  17. Naming Hierarchy root • “Top Level Domains” are at the top • Depth of tree is arbitrary (limit 128) • Domains are subtrees • E.g: .edu, berkeley.edu, eecs.berkeley.edu • Name collisions avoided • E.g. berkeley.edu and berkeley.com can coexist, but uniqueness is job of domain edu gov mil net uk fr etc. com org mit berkeley eecs sims argus

  18. Host names are administered hierarchically root root A zone corresponds to an administrative authority that is responsible for that portion of the hierarchy eecs controls names: x.eecs.berkeley.edu berkeley controls names: x.berkeley.edu and y.sims.berkeley.edu edu edu gov gov mil mil net net uk uk fr fr com com org org mit berkeley berkeley eecs eecs sims sims argus

  19. Server Hierarchy • Each server has authority over a portion of the hierarchy • A server maintains only a subset of all names • Each server contains all the records for the hosts in its zone • might be replicated for robustness • Each server needs to know other servers that are responsible for the other portions of the hierarchy • Every server knows the root • Root server knows about all top-level domains

  20. DNS Name Servers • Local name servers: • Each ISP (company) has local default name server • Host DNS query first goes to local name server • Authoritative name servers: • For a host: stores that host’s (name, IP address) • Can perform name/address translation for that host’s name • Can also do IP to name translation, but won’t discuss

  21. Contacted by local name server that can not resolve name Root name server: Contacts authoritative name server if name mapping not known Gets mapping Returns mapping to local name server ~ Dozen root name servers worldwide DNS: Root Name Servers

  22. Host whistler.cs.cmu.edu wants IP address of www.berkeley.edu 1. Contacts its local DNS server, mango.srv.cs.cmu.edu 2. mango.srv.cs.cmu.edu contacts root name server, if necessary 3. Root name server contacts authoritative name server,ns1.berkeley.edu, if necessary local name server mango.srv.cs.cmu.edu Simple DNS Example root name server 2 4 3 5 authorititive name server ns1.berkeley.edu 1 6 requesting host whistler.cs.cmu.edu www.berkeley.edu

  23. Root name server: May not know authoritative name server May know intermediate name server: who to contact to find authoritative name server? Recursive query: Puts burden of name resolution on contacted name server Heavy load? local name server mango.srv.cs.cmu.edu intermediate name server (edu server) Example of Recursive DNS Query root name server 6 2 7 3 5 4 1 8 authoritative name server ns1.berkeley.edu requesting host whistler.cs.cmu.edu www.berkeley.edu

  24. Iterated query: Contacted server replies with name of server to contact “I don’t know this name, but ask this server” local name server mango.srv.cs.cmu.edu intermediate name server (edu server) Example of Iterated DNS Query root name server iterated query 2 3 4 5 7 6 1 8 authoritative name server ns1.berkeley.edu requesting host whistler.cs.cmu.edu www.berkeley.edu

  25. DNS Records • Four fields: (name, value, type, TTL) • Type = A: • name = hostname • value = IP address • Type = NS: • name = domain • value = name of dns server for domain

  26. DNS Records (cont’d) • Type = CNAME: • name = hostname • value = canonical name • Type = MX: • name = domain in email address • value = canonical name of mail server

  27. DNS as Indirection Service • Can refer to machines by name, not address • not only easier for humans • also allows machines to change IP addresses without having to change way you refer to machine • Can refer to machines by alias • www.berkeley.edu can be generic web server • but DNS can point this to particular machine that can change over time • But, this flexibility applies only within domain!

  28. Special Topics • DNS caching • Improve performance by saving results of previous lookups • DNS “hacks” • Return records based on requesting IP address • dynamic DNS • Allows remote updating of IP address for mobile hosts • DNS politics (ICANN) and branding battles

  29. Important Properties of DNS Administrative delegation and distributed server architecture results in: • Easy unique naming • Fate sharing for network failures • Reasonable trust model

  30. The Web • A distributed database of “pages” • Core components: • Servers: store files and execute remote commands • Browsers: retrieve and display “pages” • URLs: way to refer to pages • Need a protocol to transfer information between clients and servers • HTTP

  31. Uniform Record Locator • protocol://host-name:port/directory-path/resource • Extend the idea of hierarchical namespaces to include anything in a file system • ftp://www.eecs.berkeley.edu/122/Lecture6/presentation.ppt • Extend to program executions as well… • http://us.f413.mail.yahoo.com/ym/ShowLetter?box=%40B%40Bulk&MsgId=2604_1744106_29699_1123_1261_0_28917_3552_1289957100&Search=&Nhead=f&YY=31454&order=down&sort=date&pos=0&view=a&head=b • Server side processing can be incorporated in the name

  32. Web and DNS • URLs use hostnames • Thus, content names are tied to specific hosts • This is bad! • URNs are one proposal to achieve persistence

  33. Hyper Text Transfer Protocol • Client-server architecture • Synchronous request/reply protocol • Runs over TCP, Port 80 • Stateless • Uses unicast

  34. . . . Big Picture Client Server TCP Syn Establish connection TCP syn + ack Client request TCP ack + HTTP GET Request response Close connection

  35. Hyper Text Transfer Protocol Commands • GET – transfer resource from given URL • HEAD – GET resource metadata (headers) only • PUT – store/modify resource under given URL • DELETE – remove resource • POST – provide input for a process identified by the given URL (usually used to post CGI parameters)

  36. Response Codes • 1x informational • 2x success • 3x redirection • 4x client error in request • 5x server error; can’t satisfy the request

  37. Client Request • Steps to get the resource: http://www.eecs.berkeley.edu/index.html • Use DNS to obtain the IP address of www.eecs.berkeley.edu • Send to an HTTP request: GET /index.html HTTP/1.0

  38. Server Response HTTP/1.0 200 OK Content-Type: text/html Content-Length: 1234 Last-Modified: Mon, 19 Nov 2001 15:31:20 GMT <HTML> <HEAD> <TITLE>EECS Home Page</TITLE> </HEAD> … </BODY> </HTML>

  39. Example (from Kurose and Ross) • http://www.mylife.org/mypictures.htm • After finding out the IP address of the host… • http client initiates a TCP connection on :80 • Client sends the get request via socket established in 1 • Server sends the html file, which is encapsulated in its response • http server tells tcp to terminate connection • http client receives the file and the browser parses it…contains ten jpeg images • Client repeats steps 1-4

  40. HTTP/1.0 Example Server Client Request image 1 Transfer image 1 Request image 2 Transfer image 2 Request text Transfer text Finish display page

  41. HHTP/1.0 Performance • Create a new TCP connection for each resource • Large number of embedded objects in a web page • Many short lived connections • TCP transfer • Too slow for small object • May never exit slow-start phase • Connections may be set up in parallel (5 is default in most browsers)

  42. HTTP/1.0 Caching • Exploit locality of reference • A modifier to the GET request: • If-modified-since – return a “not modified” response if resource was not modified since specified time • A response header: • Expires – specify to the client for how long it is safe to cache the resource • A request directive: • No-cache – ignore all caches and get resource directly from server • These features can be best taken advantage of with HTTP proxies • Locality of reference increases if many clients share a proxy

  43. . . . Web Proxies • Intermediaries between client and server Client 1 Client 2 Proxy Proxy Server Client N

  44. HTTP/1.1 (1996) • Performance: • Persistent connections • Pipelined requests/responses • … • Support for virtual hosting • Efficient caching support • Network Cache assumed more explicitly in the design • Gives more control to the server on how it wants data cached

  45. Persistent Connections • Allow multiple transfers over one connection • Avoid multiple TCP connection setups • Avoid multiple TCP slow starts

  46. Pipelined Requests/Responses Server Client • Buffer requests and responses to reduce the number of packets • Multiple requests can be contained in one TCP segment • Note: order of responses has to be maintained Request 1 Request 2 Request 3 Transfer 1 Transfer 2 Transfer 3

  47. What You Need to Know • DNS: record types, and how they are used • HTTP basics (and essential differences between 1.0 and 1.1)

  48. What’s the Moral of this Story? • QoS and IP Multicast: • interesting algorithmic and architectural issues • thousands of academic papers • ubiquitous in routers, but not deployed by ISPs • little or no impact on end users • DNS and the Web: • no research papers on topic before deployment • really boring designs • they changed the world....

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