1. 1 Katz, Stoica F04 EECS 122: Introduction to Computer Networks Network Service and Applications Computer Science Division Department of Electrical Engineering and Computer Sciences University of California, Berkeley Berkeley, CA 94720-1776

2. 2 Katz, Stoica F04 Overview Taxonomy of Communication Networks Services and Applications

3. 3 Katz, Stoica F04  Communication networks can be classified based on the way in which the nodes exchange information: Taxonomy of Communication Networks Communication Network Switched Communication Network Broadcast Communication Network Circuit-Switched Communication Network Packet-Switched Communication Network Datagram Network Virtual Circuit Network

4. 4 Katz, Stoica F04  Broadcast Communication Networks -Information transmitted by any node is received by every other node in the network Examples: usually in LANs (Ethernet, WiFi) -Problem: coordinate the access of all nodes to the shared communication medium (Multiple Access Problem)  Switched Communication Networks -Information transmitted to a sub-set of designated nodes Examples: WANs (Telephony Network, Internet) -Problem: how to forward information to intended node(s)? Done by special nodes (e.g., routers, switches) executing routing protocols Broadcast vs. Switched Communication Networks

5. 5 Katz, Stoica F04  Communication networks can be classified based on the way in which the nodes exchange information: Taxonomy of Communication Networks Communication Network Switched Communication Network Broadcast Communication Network Circuit-Switched Communication Network Packet-Switched Communication Network Datagram Network Virtual Circuit Network

6. 6 Katz, Stoica F04 Circuit Switching  Three phases 1.circuit establishment 2.data transfer 3.circuit termination  If circuit not available: “Busy signal”  Examples -Telephone networks -ISDN (Integrated Services Digital Networks)

7. 7 Katz, Stoica F04 Telegraph Network  Alexander Graham Bell -1876: Demonstrates the telephone at US Centenary Exhibition in Philadelphia

8. 8 Katz, Stoica F04 Telephone Network  Almon Brown Strowger (1839 - 1902) -1889: Invents the “girl-less, cuss-less” telephone system, aka the mechanical switching system

9. 9 Katz, Stoica F04 Timing in Circuit Switching DATA Circuit Establishment Data Transmission Circuit Termination Host 1Host 2 Node 1Node 2 propagation delay between Host 1 and Node 1 propagation delay between Host 2 and Node 1 processing delay at Node 1

10. 10 Katz, Stoica F04 Circuit Switching  Node (switch) in a circuit switching network incoming linksoutgoing links Node

11. 11 Katz, Stoica F04 Circuit Switching: Multiplexing/Demultiplexing  Time divided in frames and frames into slots  Relative slot position inside a frame determines to which conversation data belongs -E.g., slot 0 belongs to red conversation  Requires synchronization between sender and receiver— surprisingly difficult!  In case of non-permanent conversations -Needs to dynamically bind a slot to a conservation -How to do this?  If a conversation does not use its circuit the capacity is lost! Frames 012 3 45012 3 45 Slots =

12. 12 Katz, Stoica F04  Communication networks can be classified based on the way in which the nodes exchange information: Taxonomy of Communication Networks Communication Network Switched Communication Network Broadcast Communication Network Circuit-Switched Communication Network Packet-Switched Communication Network Datagram Network Virtual Circuit Network

13. 13 Katz, Stoica F04 Packet Switching  Data sent as formatted bit-sequences (Packets)  Packets have following structure: Header and Trailer carry control information (e.g., destination address, check sum)  Each packet traverses the network from node to node along some path (Routing)  At each node the entire packet is received, stored briefly, and then forwarded to the next node (Store-and-Forward Networks)  Typically no capacity is allocated for packets HeaderData Trailer

14. 14 Katz, Stoica F04 Packet Switching  Node in a packet switching network incoming linksoutgoing links Node Memory

15. 15 Katz, Stoica F04 Packet Switching: Multiplexing/Demultiplexing  Data from any conversation can be transmitted at any given time -Single conversation can use the entire link capacity if it is alone  How to tell them apart? -Use meta-data (header) to describe data

16. 16 Katz, Stoica F04  Communication networks can be classified based on the way in which the nodes exchange information: Taxonomy of Communication Networks Communication Network Switched Communication Network Broadcast Communication Network Circuit-Switched Communication Network Packet-Switched Communication Network Datagram Network Virtual Circuit Network

17. 17 Katz, Stoica F04 Datagram Packet Switching  Each packet is independently switched -Each packet header contains destination address  No resources are pre-allocated (reserved) in advance  Example: IP networks

18. 18 Katz, Stoica F04 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Timing of Datagram Packet Switching Packet 1 Packet 2 Packet 3 processing delay of Packet 1 at Node 2 Host 1Host 2 Node 1Node 2 propagation delay between Host 1 and Node 2 transmission time of Packet 1 at Host 1

19. 19 Katz, Stoica F04 Datagram Packet Switching Host A Host B Host E Host D Host C Node 1 Node 2 Node 3 Node 4 Node 5 Node 6 Node 7

20. 20 Katz, Stoica F04  Communication networks can be classified based on the way in which the nodes exchange information: Taxonomy of Communication Networks Communication Network Switched Communication Network Broadcast Communication Network Circuit-Switched Communication Network Packet-Switched Communication Network Datagram Network Virtual Circuit Network

21. 21 Katz, Stoica F04 Virtual-Circuit Packet Switching  Hybrid of circuit switching and packet switching -Data is transmitted as packets -All packets from one packet stream are sent along a pre-established path (=virtual circuit)  Guarantees in-sequence delivery of packets  However, packets from different virtual circuits may be interleaved  Example: ATM networks

22. 22 Katz, Stoica F04 Virtual-Circuit Packet Switching  Communication with virtual circuits takes place in three phases 1.VC establishment 2.data transfer 3.VC disconnect  Note: packet headers don’t need to contain the full destination address of the packet

23. 23 Katz, Stoica F04 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Timing of Virtual-Circuit Packet Switching Packet 1 Packet 2 Packet 3 Host 1Host 2 Node 1Node 2 propagation delay between Host 1 and Node 1 VC establishment VC termination Data transfer

24. 24 Katz, Stoica F04 Datagram Packet Switching Host A Host B Host E Host D Host C Node 1 Node 2 Node 3 Node 4 Node 5 Node 6 Node 7

25. 25 Katz, Stoica F04 Packet-Switching vs. Circuit-Switching  Most important advantage of packet-switching over circuit switching: Exploitation of statistical multiplexing: -Efficient bandwidth usage; ratio between peek and average rate is 3:1 for audio, and 15:1 for data traffic  However, packet-switching must handle congestion: -More complex routers -Harder to provide good network services (e.g., delay and bandwidth guarantees)  In practice they are combined: -IP over SONET, IP over Frame Relay

26. 26 Katz, Stoica F04 Overview Taxonomy of Communication Networks  Services and Applications

27. 27 Katz, Stoica F04 The Internet Protocol (IP)  Problem: -Many different network technologies -e.g., Ethernet, Token Ring, ATM, Frame Relay, etc. -How can you hook them together? n x n translations?  IP was invented to glue them together -n translations -Minimal requirements (datagram)  The Internet is founded on IP -“IP over everything”

28. 28 Katz, Stoica F04 Addressing  Every Internet host has an IP address -e.g., 67.114.133.15  Packets include destination address -Network is responsible for routing packet to address  Host-view: Sourc e Destination Network

29. 29 Katz, Stoica F04 IP-centric View Host A Host B Host E Host D Host C Router 1 Router 2 Router 3 Router 4 Router 5 Router 6 Router 7

30. 30 Katz, Stoica F04 Physical View  A big mess!  Every “link” could be a whole network of ATM, frame relay, ethernet, DSL, etc.  Beauty of IP: you can ignore these different network technologies  In many networks, IP is used only at the edge

31. 31 Katz, Stoica F04 Back to IP Host A Host B Host E Host D Host C Router 1 Router 2 Router 3 Router 4 Router 5 Router 6 Router 7

32. 32 Katz, Stoica F04 Routing  Routers have “routing tables” -Tables mapping each destination with an outgoing link -Requires that routing table is highly compressible! -Implications for address assignment, mobility, etc.  Routing decisions made packet-by-packet -Routers keep no connection state  Question: Why have the network do routing? -Why not the hosts? -Compare delivery-by-hand to FedEx

33. 33 Katz, Stoica F04 Internet Service “Best-Effort” service -No guarantees about packet delivery -Hosts must cope with loss and delay  Why this service model? -Why not guarantee no loss and low delay?

34. 34 Katz, Stoica F04 Domain Name Service (DNS)  Humans/applications use machine names -e.g., www.cs.berkeley.edu  Network (IP) uses IP addresses -e.g., 67.114.112.23  DNS translates between the two -An overlay service in its own right -Global distribution of name-to-IP address mappings—a kind of content distribution system as well -Unsung hero of the Internet

35. 35 Katz, Stoica F04 File Transfer (FTP, SCP, etc.) Get file from soup.cs.berkeley.edu DNS soup.cs.berkeley.edu Get address for soup.cs.berkeley.edu 67.132.22.5 Get file file Your PC

36. 36 Katz, Stoica F04 Question  Why isn’t the network in this picture? -Network just delivers (or not) packets to their destination -It plays no other role in application  Obvious concept now, but novel at the time -Compare to phone switches  Makes it both harder and easier for applications -Hosts more complex, applications less efficient -Long-term flexibility

37. 37 Katz, Stoica F04 Email Email message exchange is similar to previous example, except  Exchange is between mail servers  DNS gives name of mail server for domain

38. 38 Katz, Stoica F04 Web Get www.icir.org/file.html DNS www.icir.org Get address for www.icir.org 67.132.22.5 Get file.html file.html Your PC Proxy

39. 39 Katz, Stoica F04 Caching  Caches can be visible or transparent  Visible: -Client is configured to ask cache  Transparent: -Cache intercepts packet on its way to web server -Example of a “application-aware middlebox” -Violates purity of architecture, but are prevalent...

40. 40 Katz, Stoica F04 Content Distribution Network (CDN) How to get closest copy of replicated content?  CDNs have mirror servers distributed globally  CDN customers allow CDN to run their DNS  “Smart” DNS server returns results based on requester’s IP address

41. 41 Katz, Stoica F04 Gnutella (P2P) Overlay Network Dashed lines are “virtual links”

42. 42 Katz, Stoica F04 Gnutella (cont’d)  User asks for file (by metadata)  Each host sends request to its “neighbors” in overlay network  Responses sent back to original requesting node  Many variations on P2P file sharing.....

43. 43 Katz, Stoica F04 Overlay Networks  Create a set of “virtual links” between hosts  Communication between neighbors on overlay is done by IP  But the overlay can use different routing, or application-specific processing, at overlays nodes  IP is often overlay on circuit-switched network - App-specific networks increasingly overlaid on IP

44. 44 Katz, Stoica F04 Architecture  The assignment of tasks and knowledge  Who does what, and where is the state kept?  How they do it: algorithms and implementation

45. 45 Katz, Stoica F04 Internet Architecture  Routers do routing, and almost nothing else -No application-specific functions  Hosts do all application-specific processing  Allowed wide variety of applications to flourish on Internet

46. 46 Katz, Stoica F04 Different Application Requirements  Delay -Interactive audio/video: low per-packet delay -Large file transfer: per-packet delay unimportant  Bandwidth -High-quality video needs lots of bandwidth -Low-quality audio needs little  Loss -Applications respond to loss very differently

47. 47 Katz, Stoica F04 Requirements for File Transfer  Size of transfers differ wildly -Vast majority of transfers are small -But most bytes are in the large transfers -“Mice vs elephants”  Small transfers: -Care about per-packet delay, not about bandwidth  Large transfers: -Care about bandwidth, not per-packet delay

48. 48 Katz, Stoica F04 Host-centric vs Data-centric  Most early Internet applications were host-centric -FTP: file exchange between hosts -Email: exchange between email servers -Audio/Video: stream between hosts -Users knew the host they wanted to contact, and used DNS to figure out the IP address of the relevant host.  Some modern applications are data-centric -Web: I want www.cnn.com, but I don’t care which host it comes fromwww.cnn.com -Gnutella: I know the song title, but not who has it  What is the right data-centric architecture?