Internet Architecture I EECS 122: Lecture 2

Transcription

1 Back to a simple Internet rchitecture I EECS 122: Lecture 2 C E epartment of Electrical Engineering and Computer Sciences University of California Berkeley B eview: Hosts connect to specialized devices called outers. outers are built (HW and Software) to discover the topology, select good paths and forward messages quickly. They also respond to changing conditions. January 19, 2006 EECS122 Lecture 2 (KP) 4 Today s Outline Last lecture was a lean back overview of s Today, let s begin leaning forward How the functions of the internet are organized into layers How these functions lead to a protocol stack The End-to-End Principle Packet Sniffing Hey, avid! igitize the message January 19, 2006 EECS122 Lecture 2 (KP) 2 January 19, 2006 EECS122 Lecture 2 (KP) 5 What s a protocol? a human protocol and a computer protocol: Hey, avid! Hi, Over protocols define format, order of msgs Hello, sent Over and received among entities, and actions taken on msg Let s hang up, Over transmission, receipt Sure! Over and out time TCP connection request TCP connection response Get <file> Need a host address! Packetize the message ssume equal packet sizes for simplicity although this isn t true in IP January 19, 2006 EECS122 Lecture 2 (KP) 3 January 19, 2006 EECS122 Lecture 2 (KP) 6 1

2 Hey, avid! Hey, avid! Unreliable Packets are mostly lost over the unreliable Fix address width Need an address field! Now there are two data recovery protocols running simultaneously dd a destination label etransmitting all the way from adds load to the protocol between and which recovers packets lost over the could get the message to much faster than having all retransmissions sent from January 19, 2006 EECS122 Lecture 2 (KP) 7 January 19, 2006 EECS122 Lecture 2 (KP) 10 Hey, avid! Hey, avid! What if packets arrive out of order? What if packets are lost? Unreliable Packets are mostly lost over the unreliable. No need to retransmit all the way from dd Sequence Numbers! dd field for sequence numbers Now the routers can look for a fixed width address How many bits wide should it be if our can be at most 1000 nodes? In IP the address is 32 bits wide Why not just use the host sequence numbers? January 19, 2006 EECS122 Lecture 2 (KP) 8 January 19, 2006 EECS122 Lecture 2 (KP) 11 Hey, avid! Hey, avid! What if packets arrive out of order? What if packets are lost? dd Sequence Numbers! Unreliable What if packet 2 is lost before reaching router,? If the device at is using the purple seq numbers it will keep asking for 2 Node can reorder packets by sequence number Can run a protocol between the hosts that will cause to retransmit lost packets The sequence number field is filled out/rewritten at each hop. January 19, 2006 EECS122 Lecture 2 (KP) 9 January 19, 2006 EECS122 Lecture 2 (KP) 12 2

3 What s an pplication Protocol? Examples The I protocol HTTP ail Bittorrent What about a protocol that routers use to learn the topology? message segment H t datagram H n H t frame H l H n H t H t H l destination source H l H n H t H l H n H t H l Encapsulation H l switch router January 19, 2006 EECS122 Lecture 2 (KP) 13 January 19, 2006 EECS122 Lecture 2 (KP) 16 What did we learn? The IP Packet Format Need separate headers for: Link level reliable transmission Network routing Host-Host eliable transmission: Transport pplication protocols ifferent devices look at different headers Link Level: ata Link Control (LC) Network Level: outer Transport Level: Hosts Two devices that look at the same header are peers Each set of peers runs its own protocols Protocols at one level can be improved/changed without affecting the correctness of protocols at another level January 19, 2006 EECS122 Lecture 2 (KP) 14 January 19, 2006 EECS122 Lecture 2 (KP) 17 What about a broadcast system? Layering of airline functionality Example: Satellite, ethernet, wireless LNs Individual transmissions can interfere and destroy many frames multiaccess protocol is required to try and minimize/avoid these collisions or the will be too unreliable Example: T, CS LC C LC C ultiaccess edium LC C Need its own set of ddresses! Burned into NICs ticket (purchase) baggage (check) gates (load) runway (takeoff) departure airport intermediate air-traffic control centers ticket (complain) baggage (claim gates (unload) runway (land) arrival airport Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below ticket baggage gate takeoff/landing January 19, 2006 EECS122 Lecture 2 (KP) 15 January 19, 2006 EECS122 Lecture 2 (KP) 18 3

4 Internet protocol stack : supporting s FTP, STP, HTTP : host-host data transfer TCP, UP : routing of datagrams from source to destination IP, routing protocols : data transfer between neighboring elements PPP, Ethernet : bits on the wire Internet End-to-End rgument (Saltzer, eed and Clark 1984) Implement a function at the end hosts unless it cannot be implemented correctly in this manner. O on t implement a function at the lower levels of the system unless it can be completely implemented at this level (Peterson and avie) This principle was a fundamental guiding principle for the first phase of the internet January 19, 2006 EECS122 Lecture 2 (KP) 19 January 19, 2006 EECS122 Lecture 2 (KP) 22 Question E2E rgument pplied Can multiple protocols co-exist on the internet? 1. Transport 2. outing 3. Link Yes to all three! outing Just doing it on the hosts would eliminate routers and the internet would not scale Error ecovery Can t just implement it at the lower layers since packets can be lost within the host as well ust implement at the host so don t implement it at the level Security Similar argument as for error recovery E2E argument is a great principle, but it is violated a lot January 19, 2006 EECS122 Lecture 2 (KP) 20 January 19, 2006 EECS122 Lecture 2 (KP) 23 n dvanced View of Internet Layering The downside of layering pplication TCP UP IP Network BGP HTTP TP TFTP TCP UP IP Ethernet FI Token Etc. lmost ny kind of can write directly on IP Including new protocols IP cannot be avoided s long as the routers speak IP, any that can make do with datagram service can be written and implemented on the end devices. No co-ordination, standards activity etc. is required!! Efficiency Suboptimal behavior Transport layer can t get information from the routers dded Overhead Fragmentation and reassembly Confusion in actual s What layer does the function security belong to? Is routing just a layer function? Layer 2, Layer 4 and Layer 4-7 switches! January 19, 2006 EECS122 Lecture 2 (KP) 21 January 19, 2006 EECS122 Lecture 2 (KP) 24 4

5 Example: Layer 4-7 switch Be a protocol detective For What if we want many servers to appear as one? Load balancing for server performance Number of TCP connections Server capacity Load balance over different geographies ll requests for that url go to a device called a Layer 4-7 switch ownload a free packet sniffer, Ethereal from packet sniffer allows you to examine the packets sent from and received at your host. Classified by protocol. This allows you to see how your host is communicating with other devices of the internet January 19, 2006 EECS122 Lecture 2 (KP) 25 January 19, 2006 EECS122 Lecture 2 (KP) 28 Layer 4-7 Switch Summary/Questions Internet/ Intranet Can buy a content switch from Cisco, Nortel, F5, Foundry etc. What content is requested? Which server is least loaded? Who is making Which the server request? is best? m I the closest router? Is request special? January 19, 2006 EECS122 Lecture 2 (KP) 26 How the functions of the internet are organized into layers How these functions lead to a protocol stack The End-to-End Principle Packet Sniffing Next time we will study elay odels and how to think about performance January 19, 2006 EECS122 Lecture 2 (KP) 29 Where are internet protocols standardized? The Internet Engineering Task Force: IB reas Working Groups rafts, FC, Proposed Standards Being forced to evolve to deal with the total commercialization of the internet Good: Output has immediate global impact Bad: any more demands of the Weaknesses in the architecture have immediate global impact January 19, 2006 EECS122 Lecture 2 (KP) 27 5