1-1 Foundation Objectives: 1.1 What’s the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and.
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1-1 Foundation Objectives: 1.1 What’s the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service model
1-2 What’s the Internet millions of connected computing devices: hosts = end systems running network apps communication links fiber, copper, radio, satellite transmission rate = bandwidth routers: forward packets (chunks of data) local ISP company network regional ISP router workstation server mobile
1-3 What’s the Internet protocols control sending, receiving of msgs e.g., TCP, IP, HTTP, FTP, PPP Internet: “network of networks” a collection of individual networks public Internet versus private intranet Internet standards RFC: Request for comments IETF: Internet Engineering Task Force local ISP company network regional ISP router workstation server mobile
1-4 What’s the Internet: a service view communication infrastructure enables distributed applications: Web, email, games, e- commerce, file sharing communication services provided to apps: Connectionless-oriented unreliable service connection-oriented reliable service
1-5 What’s a protocol? a human protocol and a computer network protocol: Hi Got the time? 2:00 TCP connection request TCP connection response Get http://www.kfupm.edu.sa/downloads time protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt
1-6 What’s a protocol? What does a protocol tell us? Syntax Semantics Actions A network that provides many services needs many protocols For example, consider a file transfer protocol Packet transfer is one step in the execution of a reliable file transfer protocol This form of dependency is called layering That is, reliable file transfer is layered above packet transfer protocol
1-7 Network structure: network edge: applications and hosts network core: routers network of networks access networks, physical media: communication links
1-8 The network edge: end systems (hosts): run application programs e.g. Web, email at “edge of network” client/server model client host requests, receives service from always-on server e.g. Web browser/server; email client/server peer-peer model: minimal (or no) use of dedicated servers e.g. Gnutella, KaZaA
1-9 Network edge: connection-oriented service Goal: data transfer between end systems A connection-oriented service requires Establishing a connection Maintaining the connection Releasing the connection TCP - Transmission Control Protocol Internet’s connection- oriented service TCP service [RFC 793] reliable, in-order byte- stream data transfer loss: acknowledgements and retransmissions flow control: sender won’t overwhelm receiver congestion control: senders “slow down sending rate” when network congested
1-10 Network edge: connectionless service Goal: data transfer between end systems same as before! UDP - User Datagram Protocol [RFC 768]: connectionless unreliable data transfer no flow control no congestion control App’s using TCP: HTTP (Web), FTP (file transfer), Telnet, SMTP App’s using UDP: streaming media, teleconferencing, DNS, Internet telephony
1-11 A Taxonomy of Communication Networks So far we have looked at only the topology of the Internet: a mesh of computers interconnected The fundamental question: how is data (the bits) transferred through a communication network?
1-12 Broadcast vs. Switched Communication Networks Broadcast networks Nodes share a common channel; information transmitted by a node is received by all other nodes in the network Examples: TV, radio Switched networks Information is transmitted to a small sub-set (usually only one) of the nodes communication networks switched networks broadcast networks
1-13 The Network Core mesh of interconnected routers In a switched network, how is data transferred through the net? circuit switching packet switching
1-14 Network Core: Circuit Switching dedicated circuit per call: telephone net End-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required
1-15 Network Core: Circuit Switching network resources (e.g., bandwidth) divided into “pieces” pieces allocated to calls resource piece idle if not used by owning call (no sharing) dividing link bandwidth into “pieces” frequency division time division
1-16 Circuit Switching: FDM and TDM FDM frequency time TDM frequency time 4 users Example:
1-17 Network Core: Packet Switching data sent thru net in discrete “chunks” each end-end data stream divided into packets user A, B packets share network resources each packet uses full link bandwidth resources used as needed resource contention: aggregate resource demand can exceed amount available Congestion can happen Packets queue, wait for link use store and forward Packets move one hop at a time Node receives complete packet before forwarding
1-18 Packet Switching: Statistical Multiplexing Sequence of A & B packets does not have fixed pattern, shared on demand statistical multiplexing. TDM: each host gets same slot in revolving TDM frame. A B C 10 Mb/s Ethernet 1.5 Mb/s D E statistical multiplexing queue of packets waiting for output link
1-19 Packet switching versus circuit switching Great for bursty data resource sharing simpler, no call setup Excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem Packet switching is Q: human analogies of reserved resources (circuit switching) versus on- demand allocation (packet-switching)?
1-20 Packet-switching: store-and-forward Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps Entire packet must arrive at router before it can be transmitted on next link: store and forward delay = 3L/R (assuming zero propagation delay) R R R L
1-21 How do loss and delay occur? packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turn A B packet being transmitted (delay) packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers
1-22 Four sources of packet delay 1. nodal processing: check bit errors determine output link A B propagation transmission nodal processing queueing 2. queueing time waiting at output link for transmission depends on congestion level of router
1-23 Delay in packet-switched networks 3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x10 8 m/sec) propagation delay = d/s A B propagation transmission nodal processing queueing
1-24 Nodal delay d proc = processing delay typically a few microsecs or less d queue = queuing delay depends on congestion d trans = transmission delay = L/R, significant for low-speed links d prop = propagation delay a few microsecs to hundreds of msecs
1-25 Packet loss queue (aka buffer) preceding link in buffer has finite capacity when packet arrives to full queue, packet is dropped (aka lost) lost packet may be retransmitted by previous node by source end system or not retransmitted at all
1-26 Queueing delay (revisited) R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate traffic intensity = La/R La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can be serviced, average delay infinite!
1-27 Internet structure: network of networks roughly hierarchical at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity, Sprint, AT&T), national/international coverage treat each other as equals Tier 1 ISP Tier-1 providers interconnect (peer) privately NAP Tier-1 providers also interconnect at public network access points (NAPs)
1-28 Internet structure: network of networks “Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs Tier 1 ISP NAP Tier-2 ISP Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer of tier-1 provider Tier-2 ISPs also peer privately with each other, interconnect at NAP
1-29 Internet structure: network of networks “Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems) Tier 1 ISP NAP Tier-2 ISP local ISP local ISP local ISP local ISP local ISP Tier 3 ISP local ISP local ISP local ISP Local and tier- 3 ISPs are customers of higher tier ISPs connecting them to rest of Internet
1-30 Internet structure: network of networks a packet passes through many networks! Tier 1 ISP NAP Tier-2 ISP local ISP local ISP local ISP local ISP local ISP Tier 3 ISP local ISP local ISP local ISP
1-31 Protocol “Layers” Networks are complex! many “pieces”: hosts routers links of various media applications protocols hardware, software
1-32 Why layering? Dealing with complex systems: explicit structure allows identification, relationship of complex system’s pieces layered reference model for discussion modularization eases maintenance, updating of system change of implementation of layer’s service transparent to rest of system e.g., change in gate procedure doesn’t affect rest of system
1-33 Internet protocol stack application: supporting network applications FTP, SMTP, STTP transport: host-host data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical: bits “on the wire” application transport network link physical
1-34 message segment datagram frame source application transport network link physical HtHt HnHn HlHl M HtHt HnHn M HtHt M M destination application transport network link physical HtHt HnHn HlHl M HtHt HnHn M HtHt M M network link physical link physical HtHt HnHn HlHl M HtHt HnHn M HtHt HnHn HlHl M HtHt HnHn M HtHt HnHn HlHl M HtHt HnHn HlHl M router switch Encapsulation
1-35 Foundation: Summary Material Covered Internet overview what’s a protocol? network edge, core, access network packet-switching versus circuit-switching Internet/ISP structure performance: loss, delay layering and service models history (which you will be reading on your own) You now have: context, overview, “feel” of networking more depth, detail to follow!