School of Computing Science Simon Fraser University

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School of Computing Science Simon Fraser
University

• CMPT 371 Data Communications and Networking
• Review

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Course Objectives

• Understand principles of designing and operating
  computer networks,
• Understand the structure and protocols of the
  largest network of networks (Internet),
• Know how to implement network protocols and
  networked applications, and
• Have fun!

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A snapshot of the Internet in 1999 showing major
ISPs
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Internet structure packet journey

• a packet passes through many networks!

Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
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Layering of airline functionality

• Layers each layer implements a service
• via its own internal-layer actions
• relying on services provided by layer below

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Internet protocol stack

• application supporting network applications
• FTP, SMTP, HTTP
• 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

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Encapsulation
datagram
frame
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What is a network app?

• Programs that
• run on different end systems and
• communicate over a network.
• e.g., Web Web server software communicates with
  browser software
• little software written for devices in network
  core
• network core devices do not run user application
  code
• application on end systems allows for rapid app
  development, propagation

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How to create a network app?

• Design application architecture
• how to organize the app over end systems
• Choose network transport service(s)
• which service to use (TCP, UDP)
• depends on app requirements (delay, loss, bw, )
• Design app protocol
• message types, format, actions,
• Write code
• implement the protocol

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Socket Programming

• process sends/receives messages to/from its
  socket
• socket analogous to door
• sending process shoves message out door
• sending process relies on transport
  infrastructure on other side of door which brings
  message to socket at receiving process

controlled by app developer
Internet
controlled by OS

• socket is the interface (API) between application
  and transport layer

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Sample app-level protocols

• Web and HTTP
• web caching
• FTP
• Domain Name System (DNS)

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Transport layer

• provide logical communication between app
  processes
• transport protocols run in end systems
• send side breaks app messages into segments,
  passes to network layer
• rcv side reassembles segments into messages,
  passes to app layer
• more than one transport protocol available to
  apps
• Internet TCP and UDP

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Reliable data transfer principles
send side
receive side
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Reliable data transfer Go-Back-N

• Sender
• k-bit seq in pkt header
• window of up to N, consecutive unacked pkts
  allowed

• ACK(n) ACKs all pkts up to, including seq n -
  cumulative ACK
• may receive duplicate ACKs (see receiver)
• timer for each in-flight pkt
• timeout(n) retransmit pkt n and all higher seq
  pkts in window
• i.e., go back to n

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Reliable data transfer Selective repeat
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TCP Overview RFCs 793, 1122, 1323, 2018, 2581

• point-to-point
• one sender, one receiver
• reliable, in-order byte steam
• no message boundaries
• congestion controlled
• will not overwhelm network
• send receive buffers

• full duplex data
• bi-directional data flow in same connection
• MSS maximum segment size
• connection-oriented
• handshaking (exchange of control msgs) inits
  sender, receiver state before data exchange
• flow controlled
• sender will not overwhelm receiver

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TCP Congestion Control Summary

• Initially
• Threshold is set to large value (65 Kbytes), has
  not effect
• CongWin 1 MSS
• Slow Start (SS) CongWin grows exponentially
• till a loss event occurs (timeout or 3 dup ack)
  or reaches Threshold
• Congestion Avoidance (CA) CongWin grows
  linearly
• 3 duplicate ACK occurs
• Threshold CongWin/2 CongWin Threshold CA
• Timeout occurs
• Threshold CongWin/2 CongWin 1 MSS SS
  till Threshold

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Network layer

• transport segment from sending to receiving host
• on sending side encapsulates segments into
  datagrams
• on receiving side, delivers segments to transport
  layer
• network layer protocols in every host, router
• Router examines header fields in all IP datagrams
  passing through it

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Network Taxonomy
Telecommunication networks

• Datagram network is neither connection-oriented
• nor connectionless.
• Internet provides both connection-oriented (TCP)
  and
• connectionless services (UDP) to apps.

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Interplay between routing and forwarding
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Router Architecture Overview

• Two key router functions
• run routing algorithms/protocol (RIP, OSPF, BGP)
• forward datagrams from incoming to outgoing link

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Addressing, Subnets

• Subnet is
• a group of devices that can reach each other
  without intervening router
• identified by high order bits of IP addresses

11011111 00000001 00000001 00000001
Host ID
Subnet ID
223.1.1.0/24
/24 bits in subnet portion of address, subnet
mask
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Hierarchical addressing route aggregation
Hierarchical addressing allows efficient
advertisement of routing information
Organization 0
Organization 1
Send me anything with addresses beginning
200.23.16.0/20
Organization 2
Fly-By-Night-ISP
Internet
Organization 7
Send me anything with addresses beginning
199.31.0.0/16
ISPs-R-Us
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Routing algorithms Graph abstraction

• cost of link (x1, x2)
• Metric value, e.g., c(w,z) 5
• could be
• 1, or
• inversely related to bandwidth, or
• inversely related to congestion
• Cost of path (x1, x2, x3,, xp)
• c(x1,x2) c(x2,x3) c(xp-1,xp)

Routing algorithm algorithm that finds
least-cost path
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Classification of Routing Algorithms

• Global or local information?
• Global
• all routers have complete topology, link cost
  info
• link state algorithms
• local
• router knows physically-connected neighbors, link
  costs to neighbors
• iterative process of computation, exchange of
  info with neighbors
• distance vector algorithms

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Hierarchical Routing

• aggregate routers into regions, autonomous
  systems (AS)
• routers in same AS run same routing protocol
• intra-AS routing protocol
• routers in different AS can run different
  intra-AS routing protocol
• Gateway router
• Direct link to router in another AS

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Hierarchical Routing

• Forwarding table is configured by both intra- and
  inter-AS routing algorithm
• Intra-AS sets entries for internal dests
• Inter-AS Intra-As sets entries for external
  dests

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BGP reachability and policy routing

• A,B,C are provider networks
• X,W,Y are customer (of provider networks)
• X is dual-homed attached to two provider
  networks
• X does not want to route traffic from B via X to
  C
• .. so X will not advertise to B a route to C

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Unicast, multicast, broadcast

• Unicast one source, one destination
• E.g., web session
• Multicast one source, multiple destinations
• Subset of all possible destinations
• E.g., streaming a hockey game to interested fans
• Broadcast one source, all destinations
• E.g., broadcasting link state info to ALL routers
  in a domain in OSPF protocol
• Anycast multiple possible sources, one
  destination
• Sources have same (anycast) address
• Request is forwarded to appropriate source
• (Still in research phases)

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Link Layer

• Some terminology
• hosts and routers are nodes
• communication channels that connect adjacent
  nodes along communication path are links
• wired links
• wireless links
• LANs
• layer-2 packet is a frame, encapsulates datagram

data-link layer has responsibility of
transferring datagram from one node to adjacent
node over a link
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Adaptors Communicating
datagram
rcving node
link layer protocol
sending node
adapter
adapter

• receiving side
• looks for errors, rdt, flow control, etc
• extracts datagram, passes to rcving node
• adapter is semi-autonomous
• link physical layers

• link layer implemented in adaptor (aka NIC)
• Ethernet card, PCMCI card, 802.11 card
• sending side
• encapsulates datagram in a frame
• adds error checking bits, rdt, flow control, etc.

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CRC basic idea

• Sender and receiver agree on a divisor polynomial
  G(x) of degree r
• Sender transmits T(x), which consists of d1
  data bits AND r redundant bits such that
  G(x)T(x),
• i.e., the remainder of dividing T(x) by G(x) is 0
• Receiver gets T(x) which may have corrupted
  bits
• If G(x) T(x) then no errors occurred

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MAC Protocols a taxonomy

• Three broad classes
• Channel Partitioning
• divide channel into smaller pieces (time slots,
  frequency, code)
• allocate piece to node for exclusive use
• Random Access
• channel not divided, allow collisions
• recover from collisions
• Taking turns
• Nodes take turns, but nodes with more to send can
  take longer turns

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MAC and IP addresses

• Why do we have TWO addresses (IP,MAC)? Do we have
  to have MAC addresses?
• Yes, we must have both
• To allow different network-layer protocols over
  same card (e.g., IP, Novell IPX, DECnet)
• Enable flexibility, mobility of cards
• Efficiency imagine that nodes have only IP
  addresses ? ALL packets sent over LAN will be
  forwarded by NIC to the IP layer ? too many
  useless interrupts

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Ethernet CSMA/CD algorithm

• 1. Adaptor receives datagram from net layer
  creates frame
• 2. If adapter senses channel idle, it starts to
  transmit frame. If it senses channel busy, waits
  until channel idle and then transmits
• 3. If adapter transmits entire frame without
  detecting another transmission, the adapter is
  done with frame !

• 4. If adapter detects another transmission while
  transmitting, aborts and sends jam signal
• 5. After aborting, adapter enters exponential
  backoff after the mth collision, adapter chooses
  a K at random from 0,1,2,,2m-1. Adapter waits
  K?512 bit times and returns to Step 2

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Institutional network
mail server
to external network
web server
router
switch
IP subnet
hub
hub
hub
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Point to Point Data Link Control

• one sender, one receiver, one link easier than
  broadcast link
• no Media Access Control
• no need for explicit MAC addressing
• e.g., dialup link, ISDN line
• popular point-to-point DLC protocols
• PPP (point-to-point protocol)
• HDLC High level data link control

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The Internet virtualizing networks

• Gateway
• embed internetwork packets in local packet
  format or extract them
• route (at internetwork level) to next gateway

gateway
satellite net
ARPAnet
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What is next?

• If you have passion for networking
• More networking
• CMPT 471 (Systems)
• CMPT 408 (Theory)
• Some theory
• Computer Simulation and Modelling CMPT 305
• Probability and Statistics
• Algorithms and graph theory
• Some systems
• C/C coding and Unix
• OS CMPT 300, CMPT 401

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• That is all!
• Good luck on your final