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, , 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 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,  at “edge of network”  client/server model  client host requests, receives service from always-on server  e.g. Web browser/server; 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!