Data communication and networks
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Data Communication and Networks. Lecture 6 Networks: Part 1 Circuit Switching, Packet Switching, The Network Layer October 13, 2005. Switching Networks. Long distance transmission is typically done over a network of switched nodes Nodes not concerned with content of data

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Data communication and networks

Data Communication and Networks

Lecture 6

Networks: Part 1

Circuit Switching, Packet Switching, The Network Layer

October 13, 2005

Network Layer


Switching networks

Switching Networks

  • Long distance transmission is typically done over a network of switched nodes

  • Nodes not concerned with content of data

  • End devices are stations

    • Computer, terminal, phone, etc.

  • A collection of nodes and connections is a communications network

  • Data routed by being switched from node to node

Network Layer


Technology

Technology

  • Two different switching technologies

    • Circuit switching

    • Packet switching

Network Layer


Simple switched network

Simple Switched Network

Network Layer


Circuit switching

Circuit Switching

  • Dedicated communication path between two stations (during conversation)

  • Three phases

    • Establish

    • Transfer

    • Disconnect

  • Must have switching capacity and channel capacity to establish connection

  • Must have intelligence to work out routing

Network Layer


Circuit switching issues

Circuit Switching - Issues

  • Circuit switching is inefficient (designed for voice)

    • Resources dedicated to a particular call

    • Much of the time a data connection is idle

    • Data rate is fixed

      • Both ends must operate at the same rate

  • Set up (connection) takes time

  • Once connected, transfer is transparent

Network Layer


Packet switching

Packet Switching

Network Layer


Basic operation

Basic Operation

  • Data transmitted in small packets

    • Typically 1000 octets

    • Longer messages split into series of packets

    • Each packet contains a portion of user data plus some control info

  • Control info

    • Routing (addressing) info

  • Packets are received, stored briefly (buffered) and passed on to the next node

    • Store and forward

Network Layer


Use of packets

Use of Packets

Network Layer


Network layer

transport segment from sending to receiving host

on sending side encapsulates segments into datagrams

on rcving side, delivers segments to transport layer

network layer protocols in every host, router

Router examines header fields in all IP datagrams passing through it

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

application

transport

network

data link

physical

application

transport

network

data link

physical

Network layer

Network Layer


Key network layer functions

Key Network-Layer Functions

  • forwarding: move packets from router’s input to appropriate router output

  • routing: determine route taken by packets from source to dest.

    • Routing algorithms

analogy:

  • routing: process of planning trip from source to dest

  • forwarding: process of getting through single interchange

Network Layer


Data communication and networks

routing algorithm

local forwarding table

header value

output link

0100

0101

0111

1001

3

2

2

1

value in arriving

packet’s header

1

0111

2

3

Interplay between routing and forwarding

Network Layer


Connection setup

Connection setup

  • 3rd important function in some network architectures:

    • ATM, frame relay, X.25

  • Before datagrams flow, two hosts and intervening routers establish virtual connection

    • Routers get involved

  • Network and transport layer cnctn service:

    • Network: between two hosts

    • Transport: between two processes

Network Layer


Network service model

Example services for individual datagrams:

guaranteed delivery

Guaranteed delivery with less than 40 msec delay

Example services for a flow of datagrams:

In-order datagram delivery

Guaranteed minimum bandwidth to flow

Restrictions on changes in inter-packet spacing

Network service model

Q: What service model for “channel” transporting datagrams from sender to rcvr?

Network Layer


Network layer service models

Network layer service models:

Guarantees ?

Network

Architecture

Internet

ATM

ATM

ATM

ATM

Service

Model

best effort

CBR

VBR

ABR

UBR

Congestion

feedback

no (inferred

via loss)

no

congestion

no

congestion

yes

no

Bandwidth

none

constant

rate

guaranteed

rate

guaranteed

minimum

none

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Network Layer


Virtual circuit vs datagram networks

Virtual circuit vs. datagram networks

Network Layer


Network layer connection and connection less service

Network layer connection and connection-less service

  • Datagram network provides network-layer connectionless service

  • VC network provides network-layer connection service

  • Analogous to the transport-layer services, but:

    • Service: host-to-host

    • No choice: network provides one or the other

    • Implementation: in the core

Network Layer


Virtual circuits

call setup, teardown for each call before data can flow

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

“source-to-dest path behaves much like telephone circuit”

performance-wise

network actions along source-to-dest path

Virtual circuits

Network Layer


Vc implementation

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 a VC number.

  • VC number must be changed on each link.

    • New VC number comes from forwarding table

  • Network Layer


    Forwarding table

    VC number

    22

    32

    12

    3

    1

    2

    interface

    number

    Incoming interface Incoming VC # Outgoing interface Outgoing VC #

    1 12 2 22

    2 63 1 18

    3 7 2 17

    1 97 3 87

    … … … …

    Forwarding table

    Forwarding table in

    northwest router:

    Routers maintain connection state information!

    Network Layer


    Virtual circuits signaling protocols

    used to setup, maintain teardown VC

    used in ATM, frame-relay, X.25

    not used in today’s Internet

    application

    transport

    network

    data link

    physical

    application

    transport

    network

    data link

    physical

    Virtual circuits: signaling protocols

    6. Receive data

    5. Data flow begins

    4. Call connected

    3. Accept call

    1. Initiate call

    2. incoming call

    Network Layer


    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

    application

    transport

    network

    data link

    physical

    application

    transport

    network

    data link

    physical

    Datagram networks

    1. Send data

    2. Receive data

    Network Layer


    Forwarding table1

    Forwarding table

    4 billion

    possible entries

    Destination Address RangeLink Interface

    11001000 00010111 00010000 00000000

    through 0

    11001000 00010111 00010111 11111111

    11001000 00010111 00011000 00000000

    through 1

    11001000 00010111 00011000 11111111

    11001000 00010111 00011001 00000000

    through 2

    11001000 00010111 00011111 11111111

    otherwise 3

    Network Layer


    Longest prefix matching

    Longest prefix matching

    Prefix MatchLink Interface

    11001000 00010111 00010 0

    11001000 00010111 00011000 1

    11001000 00010111 00011 2

    otherwise 3

    Examples

    Which interface?

    DA: 11001000 00010111 00010110 10100001

    Which interface?

    DA: 11001000 00010111 00011000 10101010

    Network Layer


    Datagram or vc network why

    Internet

    data exchange among computers

    “elastic” service, no strict timing req.

    “smart” end systems (computers)

    can adapt, perform control, error recovery

    simple inside network, complexity at “edge”

    many link types

    different characteristics

    uniform service difficult

    ATM

    evolved from telephony

    human conversation:

    strict timing, reliability requirements

    need for guaranteed service

    “dumb” end systems

    telephones

    complexity inside network

    Datagram or VC network: why?

    Network Layer


    Ip internet protocol

    IP: Internet Protocol

    Network Layer


    The internet network layer

    Host, router network layer functions:

    • ICMP protocol

    • error reporting

    • router “signaling”

    • IP protocol

    • addressing conventions

    • datagram format

    • packet handling conventions

    • Routing protocols

    • path selection

    • RIP, OSPF, BGP

    forwarding

    table

    The Internet Network layer

    Transport layer: TCP, UDP

    Network

    layer

    Link layer

    physical layer

    Network Layer


    Ip addressing introduction

    IP address: 32-bit identifier for host, router interface

    interface: connection between host/router and physical link

    router’s typically have multiple interfaces

    host may have multiple interfaces

    IP addresses associated with each interface

    223.1.1.2

    223.1.2.2

    223.1.2.1

    223.1.3.2

    223.1.3.1

    223.1.3.27

    IP Addressing: introduction

    223.1.1.1

    223.1.2.9

    223.1.1.4

    223.1.1.3

    223.1.1.1 = 11011111 00000001 00000001 00000001

    223

    1

    1

    1

    Network Layer


    Subnets

    IP address:

    subnet part (high order bits)

    host part (low order bits)

    What’s a subnet ?

    device interfaces with same subnet part of IP address

    can physically reach each other without intervening router

    Subnets

    223.1.1.1

    223.1.2.1

    223.1.1.2

    223.1.2.9

    223.1.1.4

    223.1.2.2

    223.1.1.3

    223.1.3.27

    LAN

    223.1.3.2

    223.1.3.1

    network consisting of 3 subnets

    Network Layer


    Subnets1

    Recipe

    To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet.

    223.1.1.0/24

    223.1.2.0/24

    223.1.3.0/24

    Subnets

    Subnet mask: /24

    Network Layer


    Subnets2

    How many?

    Subnets

    223.1.1.2

    223.1.1.1

    223.1.1.4

    223.1.1.3

    223.1.7.0

    223.1.9.2

    223.1.9.1

    223.1.7.1

    223.1.8.1

    223.1.8.0

    223.1.2.6

    223.1.3.27

    223.1.2.1

    223.1.2.2

    223.1.3.1

    223.1.3.2

    Network Layer


    Ip addressing cidr

    host

    part

    subnet

    part

    11001000 0001011100010000 00000000

    200.23.16.0/23

    IP addressing: CIDR

    CIDR:Classless InterDomain Routing

    • subnet portion of address of arbitrary length

    • address format: a.b.c.d/x, where x is # bits in subnet portion of address

    Network Layer


    Ip datagram format

    IP protocol version

    number

    32 bits

    total datagram

    length (bytes)

    header length

    (bytes)

    type of

    service

    head.

    len

    ver

    length

    for

    fragmentation/

    reassembly

    fragment

    offset

    “type” of data

    flgs

    16-bit identifier

    max number

    remaining hops

    (decremented at

    each router)

    upper

    layer

    time to

    live

    Internet

    checksum

    32 bit source IP address

    32 bit destination IP address

    upper layer protocol

    to deliver payload to

    E.g. timestamp,

    record route

    taken, specify

    list of routers

    to visit.

    Options (if any)

    data

    (variable length,

    typically a TCP

    or UDP segment)

    IP datagram format

    how much overhead with TCP?

    • 20 bytes of TCP

    • 20 bytes of IP

    • = 40 bytes + app layer overhead

    Network Layer


    Ip fragmentation reassembly

    network links have MTU (max.transfer size) - largest possible link-level frame.

    different link types, different MTUs

    large IP datagram divided (“fragmented”) within net

    one datagram becomes several datagrams

    “reassembled” only at final destination

    IP header bits used to identify, order related fragments

    IP Fragmentation & Reassembly

    fragmentation:

    in: one large datagram

    out: 3 smaller datagrams

    reassembly

    Network Layer


    Ip fragmentation and reassembly

    length

    =1500

    length

    =1040

    length

    =1500

    length

    =4000

    ID

    =x

    ID

    =x

    ID

    =x

    ID

    =x

    fragflag

    =0

    fragflag

    =0

    fragflag

    =1

    fragflag

    =1

    offset

    =0

    offset

    =185

    offset

    =0

    offset

    =370

    One large datagram becomes

    several smaller datagrams

    IP Fragmentation and Reassembly

    Example

    • 4000 byte datagram

    • MTU = 1500 bytes

    1480 bytes in data field

    offset =

    1480/8

    Network Layer


    Nat network address translation

    NAT: Network Address Translation

    rest of

    Internet

    local network

    (e.g., home network)

    10.0.0/24

    10.0.0.1

    10.0.0.4

    10.0.0.2

    138.76.29.7

    10.0.0.3

    Datagrams with source or

    destination in this network

    have 10.0.0/24 address for

    source, destination (as usual)

    All datagrams leaving local

    network have same single source NAT IP address: 138.76.29.7,

    different source port numbers

    Network Layer


    Nat network address translation1

    NAT: Network Address Translation

    • Motivation: local network uses just one IP address as far as outside word is concerned:

      • no need to be allocated range of addresses from ISP: - just one IP address is used for all devices

      • can change addresses of devices in local network without notifying outside world

      • can change ISP without changing addresses of devices in local network

      • devices inside local net not explicitly addressable, visible by outside world (a security plus).

    Network Layer


    Nat network address translation2

    NAT: Network Address Translation

    Implementation: NAT router must:

    • outgoing datagrams:replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)

      . . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr.

    • remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair

    • incoming datagrams:replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

    Network Layer


    Nat network address translation3

    2

    4

    1

    3

    S: 138.76.29.7, 5001

    D: 128.119.40.186, 80

    S: 10.0.0.1, 3345

    D: 128.119.40.186, 80

    1: host 10.0.0.1

    sends datagram to

    128.119.40, 80

    2: NAT router

    changes datagram

    source addr from

    10.0.0.1, 3345 to

    138.76.29.7, 5001,

    updates table

    S: 128.119.40.186, 80

    D: 10.0.0.1, 3345

    S: 128.119.40.186, 80

    D: 138.76.29.7, 5001

    NAT: Network Address Translation

    NAT translation table

    WAN side addr LAN side addr

    138.76.29.7, 5001 10.0.0.1, 3345

    …… ……

    10.0.0.1

    10.0.0.4

    10.0.0.2

    138.76.29.7

    10.0.0.3

    4: NAT router

    changes datagram

    dest addr from

    138.76.29.7, 5001 to 10.0.0.1, 3345

    3: Reply arrives

    dest. address:

    138.76.29.7, 5001

    Network Layer


    Nat network address translation4

    NAT: Network Address Translation

    • 16-bit port-number field:

      • 60,000 simultaneous connections with a single LAN-side address!

    • NAT is controversial:

      • routers should only process up to layer 3

      • violates end-to-end argument

        • NAT possibility must be taken into account by app designers, eg, P2P applications

      • address shortage should instead be solved by IPv6

    Network Layer


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