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EEC484584 Computer Networks

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AB. 2. 6,A. 2,A. A. 1. D. H. F. C. E. G. B. Permanently labeled. Step. Spring Semester 2007 ... Dijkstra's Algorithm: Exercise ... – PowerPoint PPT presentation

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Title: EEC484584 Computer Networks


1
EEC-484/584Computer Networks
  • Lecture 11
  • Wenbing Zhao
  • wenbing_at_ieee.org
  • (Part of the slides are based on Drs. Kurose
    Rosss slides for their Computer Networking book,
    and on materials supplied by Dr. Louise Moser at
    UCSB and Prentice-Hall)

2
Outline
  • Quiz2 results
  • Network layer design issues
  • Router architecture
  • Routing algorithm
  • Link state routing
  • Distance vector routing

3
EEC 484/584 Quiz2 Results
  • High 97, low 49, average 78
  • Q1 20/30, Q2 23/30, Q3 9/10, Q4 9/10, Q5
    18/20

4
Network Layer
  • Main concern end-to-end transmission
  • Perhaps over many hops at intermediate nodes
  • Services provided to the transport layer
  • Routing congestion control
  • Internetworking connection of multiple networks
  • Goals services should
  • Be independent of subnet technologies
  • Shield transport layer from number, type,
    topology of subnets
  • Uniform network addresses across LAN/WAN

5
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

6
Two Key Network-Layer Functions
  • Analogy
  • Routing process of planning trip from source to
    dest
  • Forwarding process of getting through single
    intersection
  • Forwarding move packets from routers input to
    appropriate router output
  • Routing determine route taken by packets from
    source to dest.
  • Routing algorithms

7
Interplay between Routing Forwarding
Forwarding table is also referred to as routing
table
8
Network Service Model
Q What service model for channel transporting
datagrams from sender to receiver?
  • Example services for a flow of datagrams
  • In-order datagram delivery
  • Guaranteed minimum bandwidth to flow
  • Restrictions on changes in inter-packet spacing
  • No guarantee whatsoever
  • Example services for individual datagrams
  • Guaranteed delivery
  • Guaranteed delivery with less than 40 msec delay
  • Best effort

9
Network Layer Connection and Connection-less
Service
  • Datagram network provides network-layer
    connectionless service
  • Virtual Circuit network provides network-layer
    connection-oriented service

10
Datagram Networks
  • No call setup at network layer
  • Routers no state about end-to-end connections
  • no network-level concept of connection
  • Packets forwarded using destination host address
  • packets between same source-dest pair may take
    different paths

1. Send data
2. Receive data
11
Routing within a Datagram Subnet
  • Router has forwarding table telling which
    outgoing line to use for each possible
    destination router
  • Each datagram has full destination address
  • When packet arrives, router looks up outgoing
    line to use and transmits packet

12
Virtual Circuits
  • source-to-dest path behaves much like telephone
    circuit
  • performance-wise
  • network actions along source-to-dest path
  • Call setup for each call before data can flow
    (teardown afterwards)
  • Each packet carries VC identifier (not
    destination host address)
  • Every router on source-dest path maintains
    state for each passing connection
  • Link, router resources (bandwidth, buffers) may
    be allocated to VC (dedicated resources
    predictable service)

13
VC Implementation
  • A VC consists of
  • Path from source to destination
  • VC numbers, one number for each link along path
  • Entries in forwarding tables in routers along
    path
  • Packet belonging to VC carries VC number (rather
    than destination address)
  • VC number can be changed on each link
  • New VC number comes from forwarding table

14
Virtual Circuit Network
Routers maintain connection state information!
15
Virtual Circuits Signaling Protocols
  • Used to setup, maintain teardown VC
  • Used in ATM, frame-relay, X.25
  • Not used in todays Internet

6. Receive data
5. Data flow begins
4. Call connected
3. Accept call
1. Initiate call
2. incoming call
16
Datagram or VC Network Why?
  • ATM (VC)
  • evolved from telephony
  • human conversation
  • strict timing, reliability requirements
  • need for guaranteed service
  • dumb end systems
  • telephones
  • complexity inside network
  • Internet (datagram)
  • data exchange among computers
  • elastic service, no strict timing requirement
  • smart end systems (computers)
  • can adapt, perform control, error recovery
  • simple inside network, complexity at edge

17
Whats in a Router?
  • Run routing algorithms/protocol (RIP, OSPF, BGP)
  • Forwarding datagrams from incoming to outgoing
    link

18
Input Port Functions
Physical layer bit-level reception
  • Decentralized switching
  • given datagram dest., lookup output port using
    forwarding table in input port memory
  • queuing newly arrived datagrams might be queued
    before processing

Data link layer e.g., Ethernet
19
Types of Switching Fabrics
20
Output Ports
  • Buffering required when datagrams arrive from
    fabric faster than the transmission rate
  • Scheduling discipline chooses among queued
    datagrams for transmission

21
Routing Algorithms
Routing algorithm algorithm that finds
least-cost path
  • Least-cost in what sense?
  • Number of hops, geographical distance, least
    queueing and transmission delay
  • Desirable properties
  • Correctness, simplicity
  • Robustness to faults
  • Stability converge to equilibrium

22
Routing Algorithm Classification
  • Static or dynamic?
  • Non-adaptive (static) - Route computed in
    advance, off-line, downloaded to routers
  • Adaptive (dynamic) - Route based on measurements
    or estimates of current traffic and topology

23
Routing Algorithm Classification
  • Global or decentralized information?
  • Global
  • all routers have complete topology link cost
    info
  • link state algorithms
  • Decentralized
  • router knows physically-connected neighbors, link
    costs to neighbors
  • iterative process of computation, exchange of
    info with neighbors
  • distance vector algorithms

24
Link State Routing
  • Basic idea
  • Assumes net topology, link costs known to all
    nodes
  • Accomplished via link state broadcast
  • All nodes have same info
  • Computes least cost paths from one node
    (source) to all other nodes, using Dijkstras
    Algorithm
  • Gives forwarding table for that node

25
Dijkstras Algorithm
  • Each node labeled with distance from source node
    along best known path
  • Initially, no paths known so all nodes labeled
    with infinity
  • As algorithm proceeds, labels may change
    reflecting shortest path
  • Label may be tentative or permanent, initially,
    all tentative
  • When label represents shortest path from source
    to node, label becomes permanent

26
Compute Shortest Path from A to D
  • Start with node A as the initial working node
  • Examine each of the nodes adjacent to A, i.e., B
    and G, relabeling them with the distance to A
  • Examine all the tentatively labeled nodes in the
    whole graph and make the one with the smallest
    label permanent, i.e., B. B is the new working
    node

27
Compute Shortest Path from A to D
28
(No Transcript)
29
Computation Results
Destination
link
Routing Table in A
(A,B) (A,B) (A,B) (A,B) (A,B) (A,B) (A,B)
B C D E F G H
30
Dijkstras Algorithm Exercise
  • Given the subnet shown below, using the
    Dijkstras Algorithm, determine the shortest path
    tree from node u and its routing table
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