CS 640 Introduction to Computer Networks Aditya Akella Lecture 2 Layering Protocol Stacks and Standards 1 Today s Lecture Layers and Protocols Standards and standardization process Applications 2 Network Communication Lots of Functions Needed Links Multiplexing Routing Addressing naming locating peers Reliability Flow control Fragmentation How do you implement these functions Key Layering and protocols 3 What is Layering A way to deal with complexity Add multiple levels of abstraction Each level encapsulates some key functionality And exports an interface to other components Example Layering Modular approach to implementing network functionality by introducing abstractions Challenge how to come up with the right abstractions 4 Example of Layering Software and hardware for communication between two hosts Application semantics Application to application channels Host to host connectivity Link hardware Advantages Simplifies design and implementation Easy to modify evolve 5 What is a Protocol Could be multiple abstractions at a given level Build on the same lower level But provide diferent service to higher layers Protocol Abstract object or module in layered structure Application Request Reply Message stream Host to host connectivity Link hardware 6 Protocol Implements an agreement between parties on how communication should take place Friendly greeting Muttered reply Destination Madison Thank you 7 1 Protocols Offer Interfaces Each protocol offers interfaces One to higher level protocols on the same end hosts Expects one from the layers on which it builds Interface characteristics e g IP service model A peer interface to a counterpart on destinations Syntax and semantics of communications Assumptions about data formats Protocols build upon each other Adds value improves functionality overall E g a reliable protocol running on top of IP Reuse avoid re writing E g OS provides TCP so apps don t have to rewrite 8 2 Protocols Necessary for Interoperability Protocols are the key to interoperability Networks are very heterogenous Ethernet 3com etc Routers cisco juniper etc App Email AIM IE etc Hardware link Network Application The hardware software of communicating parties are often not built by the same vendor Yet they can communicate because they use the same protocol Actually implementations could be different But must adhere to same specification Protocols exist at many levels Application level protocols Protocols at the hardware level 9 How do protocols layers work One or more protocols implement the functionality in a layer Only horizontal among peers and vertical in a host communication Protocols layers can be implemented and modified in isolation Each layer offers a service to the higher layer using the services of the lower layer Peer layers on different systems communicate via a protocol higher level protocols e g TCP IP Appletalk can run on multiple lower layers multiple higher level protocols can share a single physical network 10 OSI Model One of the first standards for layering OSI Breaks up network functionality into seven layers This is a reference model For ease of thinking and implementation A different model TCP IP used in practice 11 The OSI Standard 7 Layers 1 Physical transmit bits link 2 Data link collect bits into frames and transmit frames adaptor device driver 3 Network route packets in a packet switched network 4 Transport send messages across processes end2end 5 Session tie related flows together 6 Presentation format of app data byte ordering video format 7 Application application protocols e g FTP OSI very successful at shaping thought TCP IP standard has been amazingly successful and it s not based on a rigid OSI model 12 OSI Layers and Locations Application Presentation Session Transport Network Data Link Physical Host Repeater Hub Bridge Switch Router Gateway Host Simply copy Full fledged Forward using packets out packet switch network layer addresses use dst addr to route 13 Internetworking Options 7 6 5 4 3 2 1 physical 1 7 6 5 4 3 2 1 Repeater or Hub 7 6 5 4 3 2 1 network 3 2 2 1 1 router 7 6 5 4 3 2 1 data link 2 1 1 7 6 5 4 3 2 1 bridge e g 802 MAC 7 6 5 4 3 2 1 7 6 5 4 3 2 1 3 2 1 3 2 1 7 6 5 4 3 2 1 gateway 14 The Reality TCP IP Model FTP HTTP NV TCP TFTP UDP NET2 Two transport protocols provide logical channels to apps Interconnection of n w technologies into a single logical n w IP NET1 App protocols NETn Network protocols implemented by a comb of hw and sw Note No strict layering App writers can define apps that run on any lower level protocols 15 The Thin Waist FTP HTTP NV TCP TFTP UDP TCP UDP Waist IP NET1 NET2 Applications NETn Data Link Physical The Hourglass Model The waist minimal carefully chosen functions Facilitates interoperability and rapid evolution 16 TCP IP vs OSI Application Presentation Application plus libraries Session Transport Network TCP UDP IP Data link Data link Physical Physical 17 TCP IP Layering Application Transport Network Link Physical Host Bridge Switch Router Gateway Host 18 Layers Encapsulation User A User B Get index html Connection ID Source Destination Link Address Header 19 Protocol Demultiplexing Multiple choices at each layer How to know which one to pick FTP HTTP NV TCP TFTP UDP Many Networks IP NET1 NET2 IP TCP UDP NETn 20 Multiplexing Demultiplexing Multiple implementations of each layer How does the receiver know what version module of a layer to use Packet header includes a demultiplexing field V HL TOS ID TTL Flags Offset Prot H Checksum Source IP address Destination IP address Used to identify the right module for next layer Filled in by the sender Used by the receiver Multiplexing occurs at multiple layers E g IP TCP Length Options TCP TCP IP IP 21 Layering vs Not Layer N may duplicate layer N 1 functionality E g error recovery Layers may need same info timestamp MTU Strict adherence to layering may hurt performance Some layers are not always cleanly separated Inter layer dependencies in implementations for performance reasons Many cross layer assumptions e g buffer management Layer interfaces are not really standardized It would be hard to mix and match layers from independent implementations e g windows network apps on unix w o compatibility library 22 History of IP The Early Days Early packet switching networks 61 72 Definition of packet switching Early ARPA net up to tens of nodes 4 at the end of 1969 15 at the end of 1972 single network Simple applications first email program 1972
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