1. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University1 Taj Mahal, Agra, India

2. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University2 Opera House, Sydney, Australia

3. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University3 Parthenon, Athens, Greece

4. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University4 Burj al Arab Hotel, Dubai, UAE

5. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University5 Fallingwater, Mill Run, USA

6. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University6 Notre Dame Cathedral, Paris, France

7. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University7 Stata Hall, MIT, Cambridge, USA

8. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University8 COMP/ELEC 429/556 Introduction to Computer Networks Internet architecture Some slides used with permissions from Edward W. Knightly, T. S. Eugene Ng, Ion Stoica, Hui Zhang

9. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University9 Architecture: Organizing Network Functionality Goals: Functional, flexible, elegant/beautiful Many kinds of networking functionality –e.g., encoding, framing, routing, addressing, reliability, etc. Many different network styles and technologies –circuit-switched vs packet-switched, etc. –wireless vs wired, electrical vs optical, etc. Many different applications –dropbox, web, voice, video, etc. A network has many nodes (routers, switches, hosts) Network architecture –On which node(s) should each functionality be placed? –How should functionalities on a node be organized?

10. On which node(s) should each f u n c t i o n a l i t y b e p l a c e d ? T. S. Eugene Ngeugeneng at cs.rice.edu Rice University10

11. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University11 Example: Addressing Which node(s) should be able to interpret network addresses in packets?

12. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University12 Example: Routing Which node(s) should make routing decisions?

13. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University13 Example: Routing Which node(s) should make routing decisions?

14. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University14 Example: Reliable File Transfer Idea 1: make network reliable Idea 2: implement correctness check at application and retry if failed OS App OS App Host AHost B

15. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University15 Example (cont’d) Idea 1 not complete –Bugs can exist in OS code! –Data on disk can be corrupted during write –What happens if any network element misbehaves? –The receiver has to do the check anyway! Idea 2 is complete –Full functionality can be entirely implemented at application with no need for guaranteed reliability from other components Is there any remaining reason to implement reliability features at other components?

16. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University16 Placing Functionality The most influential paper about placing functionality is “End-to-End Arguments in System Design” by Saltzer, Reed, and Clark

17. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University17 Basic Observations Some applications have end-to-end performance requirements –reliability, security, etc. Implementing these in the network is very hard –every step along the way must be fail-proof The applications: –can satisfy the requirement –can’t depend on the network

18. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University18 Basic Observations Implementing such requirements in the network: Doesn’t reduce application implementation complexity Does increase network complexity Probably imposes delay and overhead on all applications, even if they don’t have such requirements However, implementing in network can enhance performance in some cases –e.g. in-network retransmission over very lossy link

19. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University19 Conservative Interpretation of the End-to-End Argument There are many levels in a distributed system: application, OS, network, etc. Don’t implement a function in a low level unless it can be completely implemented in that level –Unless you can relieve the burden from applications, then don’t bother

20. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University20 Radical Interpretation Don’t implement anything in the network that can be implemented correctly by the hosts Make the network absolutely minimal –ignore performance issues

21. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University21 Moderate Interpretation Think twice before implementing functionality in the network If hosts can implement functionality correctly, implement it at a lower level only as a performance enhancement But do so only if it does not impose burden on applications that do not require that functionality

22. How to Organize Functionalities on Each Node? T. S. Eugene Ngeugeneng at cs.rice.edu Rice University22

23. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University23 Software Modularity Break system into modules: Well-defined interfaces gives flexibility –can change implementation of modules –can extend functionality of system by adding new modules Interfaces hide information –allows for flexibility –but can hurt performance

24. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University24 Network Modularity Like software modularity, but with a twist: Implementation distributed across routers and hosts Must decide both: –how to break system into modules –where modules are implemented

25. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University25 Layering Layering is a particular form of modularization The system is broken into a vertical hierarchy of logically distinct entities (layers) The service provided by one layer is based solely on the service provided by layer below Rigid structure: easy reuse, performance may suffer

26. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University26 Terminology Service – says what a layer does –Ethernet: unreliable subnet unicast/multicast/broadcast datagram service –IP: unreliable end-to-end unicast datagram service –TCP: reliable end-to-end bi-directional byte stream service Service Interface – says how to access the service –E.g. UNIX socket interface Protocol – says how the service is implemented –a set of rules and formats that govern the communication between two peers

27. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University27 A Naïve Architecture new application has to interface to all existing media –adding new application requires O(m) work, m = number of media new media requires all existing applications be modified –adding new media requires O(a) work, a = number of applications total work in system O(ma)  eventually too much work to add apps/media Application end points may not be on the same media! SMTP SSHFTP Packet radio Coaxial cable Fiber optic Application Transmission Media HTTP

28. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University28 Solution: Indirection Solution: introduce an intermediate layer that provides a single abstraction for various network technologies –O(1) work to add app/media SMTP SSHNFS 802.11 LAN Coaxial cable Fiber optic Application Transmission Media HTTP Intermediate layer

29. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University29 Take home point: The Internet Hourglass

30. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University30 Implications of Hourglass A single Internet layer module: Allows all networks to interoperate –all networks technologies that support IP can exchange packets Allows all applications to function on all networks –all applications that can run on IP can use any network Simultaneous developments above and below IP

31. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University31 Internet Protocol Architecture The TCP/IP protocol suite is the basis for the networks that we call the Internet. The TCP/IP suite has four layers: Application, Transport, Network, and (Data) Link Layer. Physical Layer Dropbox, Skype, Web

32. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University32 Physical Layer (1) Service: move information between two systems connected by a physical link Interface: specifies how to send a bit Protocol: coding scheme used to represent a bit, voltage levels, duration of a bit Examples: coaxial cable, optical fiber links

33. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University33 Datalink Layer (2) Service: –framing (attach frame separators) –send data frames between peers –others: arbitrate the access to common physical media per-hop reliable transmission per-hop flow control Interface: send a data unit (packet) to a machine connected to the same physical media Protocol: layer addresses, implement Medium Access Control (MAC) (e.g., IEEE802.3, IEEE802.11, CSMA/CD)…

34. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University34 Network Layer (3) Service: –deliver a packet to specified network destination –perform segmentation/reassemble –others: packet scheduling buffer management Interface: send a packet to a specified destination Protocol: define global unique addresses; construct routing tables (e.g IPv4, IPv6)

35. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University35 Transport Layer (4) Service: –Multiplexing/demultiplexing –optional: error-free and flow-controlled delivery Interface: send message to specific destination Protocol: implements reliability and flow control Examples: TCP and UDP

36. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University36 Application Layer (7) Service: any service provided to the end user Interface: depends on the application Protocol: depends on the application Examples: Dropbox, Skype, Web

37. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University37 Internet Protocol Architecture IP protocol Ethernet protocol ATM protocol

38. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University38 Encapsulation As data is moving down the protocol stack, each protocol is adding layer-specific control information.

39. T. S. Eugene Ngeugeneng at cs.rice.edu Rice University39 Reality Layering and E2E Principle regularly violated: –Firewalls –Transparent caches –Other middleboxes Battle between architectural purity and commercial pressures