Unit 7 Network Architecture Acknowledgments: These slides were originally developed by Prof. Jean Walrand for EE122. The past and current EE122 instructors including Kevin Fall, Abhay Parekh, Shyam Parekh, and Adam Wolisz have contributed to their evolution.
Important concepts so far
Properties of the Communication Channel Throughput Bits can be inserted in the channel with some speed. If bits arrive with higher speed than the channel can adopt than they have to be stored and delayed before transmission!! Delay Bits arrive at the receiver with a delay relative to their transmission. The minimal delay is defined by the signal propagation, further delay might occure if the channel is not a direct one. Errors in transmission Bits might be corrupted, but also lost.. less frequently duplicated..
Transmission & Propagation Time [Walrand]
Frequency Multiplex Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum (in the synchronous case for the whole time!) Note: guard zones in frequency are needed!! Special case: Wave division Mux
Time Multiplex The whole bandwidth is used all the time, but alternatively by different channels!
Statistical Multiplexing Gain [From N.Mc Keown, Stanford] Rate A+B 2C R < 2C C A B R time Statistical multiplexing gain = 2C/R Other definitions of SMG: The ratio of rates that give rise to a particular queue occupancy, or particular loss probability.
Circuit Switching [Fairhurst] Duplex connectivity might be set up... The disconnect might be Issued by either side!!!
Store and forward packet switching [Walrand]
The basic queuing system
The Little s Theorem [EE122, Walrand]
Other concepts Physical media Encoding methods Framing Error control
Network Architecture Introduction Layering Example Internet Layers First Look Layering Step by Step Downside of Layering Interconnecting Networks The Internet TOC Architecture
Introduction Issues: Inter-operability Extensibility Applications & Technologies Scalability Internet Solution: Layered Architecture End-to-End Principle Hierachical Addressing & Naming TOC Architecture Introduction
Layer: Example Please fax letter to Prof. B, Urgent Prof. A Here is my review of paper 554. I believe its main result is wrong. Here is a counterexample. Prof. B Here is a letter from Prof. A SECRETARY John COVER To: Prof. B Fr: Prof. A Urgent Here is my review of paper 554. I believe its main result is wrong. Here is a counterexample. SECRETARY Mary FAX SYSTEM TOC Architecture Layer Example
Layer: Example Prof. A Prof. B Please fax letter to Prof. B, Urgent INTERFACE Here is my review of paper 554. I believe its main result is wrong. Here is a counterexample. Here is a letter from Prof. A FUNCTIONS SECRETARY John SERVICE - SECY COVER To: Prof. B Fr: Prof. A Urgent HEADER Here is my review of paper 554. I believe its main result is wrong. Here is a counterexample. SECRETARY Mary SERVICE - FAX FAX SYSTEM TOC Architecture Layer Example
Layer: Example Prof. A SECRETARY John Here is my review of paper 554. I believe its main result is wrong. Here is a counterexample. COVER To: Prof. B Fr: Prof. A Urgent Here is my review of paper 554. I believe its main result is wrong. Here is a counterexample. Prof. B SECRETARY Mary Secretaries implement functions that transform a simple service into a more complex one (E.g., add MULTIPLEXING) They follow rules of communication: PROTOCOL They use the header as control information Note: Encapsulation (adding header) and Decapsulation (removing header) FAX SYSTEM TOC Architecture Layer Example
Internet Layers - Intro Browsing Services Names Examples Specifications Encapsulation Functions TOC Architecture Internet Layers: Intro
Browsing Name IP Address Connect; Get; Close Supervise Connection Forward Packets Across Many Links Transmit Packets on Each Link Transmit Bits on Each Medium DNS Servers End hosts 5 End hosts 4 Routers 3 Link/LAN 2 Transceivers 1 5 4 3 2 1 H R R H TOC Architecture Internet Layers: Intro Browsing
Services 5 4 Application Connection Packets: End to End 3 2 1 Packets Bits Signals TOC Architecture Internet Layers: Intro Services
Names 5 4 Application Connection Packets: End to End Application Transport 3 2 1 Packets Bits Signals Network Link Physical TOC Architecture Internet Layers: Intro Names
Examples Application 5 Connection Application DNS; HTTP; TFTP; RTP 4 Packets: End to End Transport TCP; UDP; 3 Packets Network IP; ATM; 2 Bits Link Ethernet; ADSL; 1 Signals Physical Fiber-1Gbps; Cat5-100Mbps; Wireless; SONET TOC Architecture Internet Layers: Intro Examples
Specifications N + 1 N N - 1 Interfaces: Formats, Functions: Service Provided Typically: State Machine TOC Architecture Internet Layers: Intro Specification
Encapsulation N + 1 Data Unit that N Delivers N N - 1 H Header: Control Info of N Examples of H: Addresses Error Control Codes Framing Delimiters TOC Architecture Internet Layers: Intro Encapsulation
Functions APP TRAN NET LINK PHY Set Up Connections; Presentation; Multiplexing; Flow Control; Congestion Control Global Addressing; Routing; Forwarding Framing; Error Coding; Local Addressing; Switching Modulation; Demodulation TOC Architecture Internet Layers: Intro Functions
Layers Step By Step Physical Data Link Network Transport Application DNS TOC Architecture Layers Step By Step
Physical In the beginning were two computers The hosts want to communicate over a physical medium (e.g. twisted pair, CAT5 etc.) TOC Architecture Layers Step By Step Physical
Physical Two hosts, want to communicate over a bidirectional link Example: Manchester Encoding Physical Medium Virtual Bit Pipe 1 0 1 1 0 1 1 Synchronous unreliable bit pipe RS232-C Interface Modem Modem RS232-C Interface Interface wires Physical Medium Interface wires TOC Architecture Layers Step By Step Physical
Data Link Control Layer 1. Frame the data 2. Number frames 3. Send CRC 4. Retransmit if required Data Link Protocol Two hosts, want to communicate asynchronously and reliably over a bidirectional link Data Link Control Physical Interface Asynchronous reliable bit pipe FH Data Synchronous unreliable bit pipe Bit Stream Data Link Control Physical Interface Example: SONET Physical Link Step 4 is omitted in reliable links or if reliability is not required TOC Architecture Layers Step By Step Link
Data Link Control Layer What about a broadcast system? Example: Satellite, ethernet, 802.11 Individual transmissions can interfere and destroy many frames A multiaccess protocol is required to try and avoid these collisions or the link will be too unreliable Example: TDM, CSMA Makes the bit-pipe provided by the physical layer look intermittent to the DLC Protocol must interface with Physical and DLC layers By convention the Multiaccess Control Layer (MAC) is considered a sub-layer of the DLC TOC Architecture Layers Step By Step Link
Data Link Control Layer What about a broadcast system? DLC Need Addresses Burned into NICs DLC MAC DLC MAC Physical Interface Physical Interface Multiaccess Medium MAC Physical Interface TOC Architecture Layers Step By Step Link
Network Layer: Routing C D A B E Nodes assume subnet functions The nodes must contain routing tables Routing information must be contained in a message unless it is part of a circuit Network addressing and protocol required to accomplish delivery over multiple hops TOC Architecture Layers Step By Step Network
Network Layer: Complexity C D A E B Topology discovery Link State monitoring Forwarding Buffer management Choosing the best path involves running a set of potentially complex protocols in which all nodes are involved The forwarding function also becomes a performance bottleneck TOC Architecture Layers Step By Step Network
Network Layer: Routers/Switches C D A R S B E R and S are specialized network routers Optimized to discover the topology, Select good paths and forward messages quickly. They respond to changing network conditions. TOC Architecture Layers Step By Step Network
Network Layer Network Asynchronous path Asynchronous path Network PH Data PH Data Network Data Link Control Asynchronous reliable bit pipe FH Data Data Link Control Asynchronous reliable bit pipe FH Data Data Link Control Physical Interface Synchronous unreliable bit pipe Physical Link Physical Interface Synchronous unreliable bit pipe Physical Link Physical Interface Examples: IP, ATM TOC Architecture Layers Step By Step Network
Transport Layer APP C D a b c A B x y z E APP The application views the network as providing a channel connecting (a,y). In a datagram network this is the lowest layer at which a connection is set up. TOC Architecture Layers Step By Step Transport
Transport Layer Transport TH Data Transport Network Asynchronous routed path Asynchronous routed path Network PH Data PH Data Network Data Link Control Asynchronous reliable bit pipe FH Data Data Link Control Asynchronous reliable bit pipe FH Data Data Link Control Physical Interface Synchronous unreliable bit pipe Synchronous unreliable bit pipe Physical Interface Physical Interface Physical Link Physical Link End Node Subnet Node End Node Examples: TCP, UDP TOC Architecture Layers Step By Step Transport
Application Layer Application Data Application Transport TH Data Transport Network Asynchronous routed path Asynchronous routed path Network PH Data PH Data Network Data Link Control Asynchronous reliable bit pipe FH Data Data Link Control Asynchronous reliable bit pipe FH Data Data Link Control Physical Interface Synchronous unreliable bit pipe Physical Interface Synchronous unreliable bit pipe Physical Interface Physical Link Physical Link End Node Subnet Node End Node Examples: Web Browser, ftp, http TOC Architecture Layers Step By Step Application
Network Directory Servers C D X A R S B E X is a directory server that helps an application at node figure out which network address(es) it should send its messages to. In the internet, this is DNS TOC Architecture Layers Step By Step DNS
Encapsulation Application Application Data Transport Transport TH Data Network Asynchronous routed path PH TH Data Network Asynchronous routed path PH TH Data Network Data Link Control Asynchronous reliable bit pipe Asynchronous reliable bit pipe Data Link Control FH PH TH Data FH PH TH Data Data Link Control Physical Interface Synchronous unreliable bit pipe Physical Interface Synchronous unreliable bit pipe Physical Interface Physical Link Physical Link End Node Subnet Node End Node Each layer just looks at its own header TOC Architecture Layers Step By Step Encapsulation
The downside of layering Efficiency Suboptimal network behavior TCP and wireless links Redundant Implementation Fragmentation and reassembly Multiple address spaces Confusion in actual networks Layer 2, Layer 4 and Layer 4-7 switches What layer does the function security belong to? Network devices (such as routers) may run application protocols TOC Architecture Downside
Inter-Connecting Networks Global Address IP1 E1 IP1 IP4 Data A = E1 E2 PCKT PCKT IP2 E2 R IP3 E3 B - Addresses IP4 E4 E3 E4 PCKT A - Addresses Destination Next Hop IP4 E4 B Router R sees network B as a direct link TOC Architecture Interconnecting
The Internet Overview Minimal Router State Layering End-to-End Argument Success Caveats TOC Architecture - Internet
Overview Interconnect networks with different Speeds, Reliability, Cost Go across multiple networks Internet Service = Best Effort datagram service: Try hard to deliver each packet Anything more, e.g. voice grade service, would Preclude many networks from joining the internet Require great amounts of co-ordination and compliance monitoring Create an endless clamoring for even other kinds of guarantees such as video grade service TOC Architecture - Internet Overview
Case for minimal router state Adding connection state to routers creates problems in the presence of failures How to clean up the bad state? Scaling router state with number of transport connections would be very expensive Adding connection state in only some routers might not be good enough What if many congested routers did not implement state So: The internet provides datagram service. Under this constraint, the internet is designed to support as many different types of applications as possible. TOC Architecture - Internet Minimal State
Layering Application TCP UDP IP Network BGP HTTP RTP TFTP TCP UDP IP Ethernet FDDI Token Etc. Almost Any kind of application can write directly on IP Including new transport protocols IP cannot be avoided As long as the routers speak IP, any application that can make do with datagram service can be written and implemented on the end devices. No co-ordination, standards activity etc. is required!! TOC Architecture - Internet Layering
End-to-End Argument (Saltzer, Reed and Clark 1984) Implement a network function at the end hosts unless it cannot be implemented correctly in this manner. OR Don t implement a function at the lower levels of the system unless it can be completely implemented at this level (Peterson and Davie) TOC Architecture - Internet E2E Argument
Success TOC Architecture - Internet - Success
Success 4/1-4/16 2002 1,224,733 IP addresses, 2,093,194 IP links, 932,000 destinations, 70% of globally routable network prefixes; 10,999 ASes (84% of ASes), 34,209 peering sessions TOC Architecture - Internet Success
Caveats Internet is actually a lot more complicated and messy than today s lecture would suggest The end-to-end argument is being subverted and under attack The internet does a poor job of supporting high performance traffic such as voice and video The phone network is hardly going away We will deal with these issues in much more detail towards the end of the course TOC Architecture Internet - Caveats