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An Introduction to Computer Networks

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An Introduction to Computer Networks Lecture 8: Internetworking University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani – PowerPoint PPT presentation

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Title: An Introduction to Computer Networks


1
An Introduction to Computer Networks
Lecture 8 Internetworking
  • University of Tehran
  • Dept. of EE and Computer Engineering
  • By
  • Dr. Nasser Yazdani

2
Outline
  • Internetworking
  • Best Effort Service Model
  • Segmentation and Reassembly (SAR)
  • Global Addressing Scheme
  • Packet forwarding.

3
Internetworking
  • Communication between networks, either directly
    connected or switched
  • Problems
  • Different Networking technologies
    (Heterogeneity).
  • So many Networks (Scaling).
  • Surviving in case of failure.
  • Different services
  • etc.,
  • Some terminologies
  • internetworking refer to an arbitrary
    collection of connected networks.
  • Internet the global internetwork.

4
Goals of the DARPA project
  • Connect existing networks
  • initially ARPANET and ARPA packet radio network
  • Survivability
  • ensure communication service even in the presence
    of network and router failures
  • Support multiple types of services
  • Must accommodate a variety of networks
  • Allow distributed management
  • Allow host attachment with a low level of effort
  • Allow resource accountability

5
IP Internet
  • Concatenation of Networks or networks of
    Networks.
  • R is routers and H is hosts.

Network 1 (Ethernet)
H7
R3
H8
H2
H3
H1
Network 4
Network 2 (Ethernet)
(point-to-point)
R1
R2
H4
Network 3 (FDDI)
H5
H6
6
IP Internet (cont)
  • Protocol Stack
  • Everything is running on top of IP, IP over
    everythings

H1
H8
TCP
TCP
R2
R3
R1
IP
IP
IP
IP
IP
FDDI
PPP
ETH
ETH
ETH
FDDI
PPP
ETH
7
Service Model
  • Connectionless (datagram-based)
  • Best-effort delivery (unreliable service)
  • packets are lost. No recover from lost.
  • packets are delivered out of order
  • duplicate copies of a packet are delivered
  • packets can be delayed for a long time
  • Datagram format

0
4
8
16
19
31
Version
HLen
TOS
Length
Ident
Flags
Offset
TTL
Protocol
Checksum
  • Contains all information
  • for routing of a packet.

SourceAddr
DestinationAddr
Pad
Options (variable)
(variable)
Data
8
Packet Headers
  • The current Version is 4 or IPv4.
  • HLen- the Header Length from 5-15 in 32-bit
    words.
  • Length- the total length of the packet including
    headers. Max length is 64K.
  • TTL Time To Live is expressed in second. It is
    to prevent packet from permanently circulating in
    a loop.
  • Protocol specify the packet application ex. 1
    for ICMP. It is for demultiplexing to higher
    layer protocols.
  • Checksum is a 1-complement error checksum for
    the header only.

9
Packet Headers (Cont)
  • TOS type of Service
  • Precedence
  • Specify the priority
  • Type of Services
  • Specify routing, for instance cheapest, fastest
    and more reliable
  • D for Delay
  • T for Throughput
  • R for Reliability
  • C for low cost.
  • Note Precedence is only for inside channel
    queuing.

0 2 3 7 0 2 3 7 0 2 3 7 0 2 3 7 0 2 3 7
Precedence Type of service Type of service Type of service Type of service
Precedence D T R C
10
Packet Headers (Cont)
  • Options
  • If C set, the option will copied to all
    fragments. Otherwise, only to the first one.
  • Class 0 for control
  • Class 2 for debugging and measurement.
  • Options are rarely used in today except for
    loose and strict source routing parameters.
  • loose and strict source option sometimes, is
    used for IP encapsulation in another IP or
    Tunneling

C Class Number
11
Fragmentation and Reassembly
0 70 70 4 70 7 0 70 70 4 70 7 0 70 70 4 70 7
Identification Flags Fragment Offset
  • Flags
  • DF Dont Fragment
  • MF More Fragment coming
  • In fragmentation, IP copy the original header and
    only modify
  • The length, which is the new length, and offset.
  • Offset is used for reassembly.
  • Note Fragmentation may degrade the network
    performance.
  • That is why the IP packet should be the same of
    TCP packets
  • Modern TCP implement Path MTU discovery.
  • It start with large packet and with DF set flag,
    if it passed
  • TCP keeps the same packet size, otherwise, it
    reduces it.

0 1 2
0 DF MF
12
Fragmentation and Reassembly (cont)
  • Each network has a Maximum Transfer Unit size,
    MTU
  • Strategy
  • fragment when necessary (MTU lt Datagram)
  • try to avoid fragmentation at source host
  • re-fragmentation is possible
  • fragments are self-contained datagrams
  • use CS-PDU (not cells) for ATM
  • delay reassembly until destination host
  • do not recover from lost fragments

13
Example
  • Packet delivery from host H1 to host H8

14
Example (cont)
The packets are fragmented as
15
Addressing
  • Each host in the network is identified by an
    address having the following property.
  • globally unique
  • hierarchical network host
  • Address Classes
  • Class D for Multicasting
  • Class E for experiments
  • Address Notation
  • 10.3.2.4
  • 128.96.33.81
  • 192.12.69.77

7
24
Network
A
Host
0
14
16
1
0
B
Network
Host
21
8
C
Network
Host
1
1
0
16
IP Addresses
Example Class A address www.mit.edu 18.18
1.0.31
(18lt128 gt Class A) Class B
address mekong.stanford.edu 171.64.74.155

(128lt171lt12864 gt Class C) www.ece.ut.ac.ir 1
94.225.
17
Addressing in IP
  • IP addresses are names of interfaces
  • Domain Name System (DNS) names are names of hosts
  • DNS binds host names to interfaces
  • Routing binds interface names to paths

18
How to assign IP Addresses?
  • Manually
  • Uniqueness
  • Too much and tedious job
  • Dynamically use DHCP Dynamic Host Configuration
    Protocol.

19
Making a Forwarding Decision
IP Address Space
Class A
Class B
Class C
D
Class A
Routing Table
Class B
212.17.9.4
Exact

Match
Class C
212.17.9.0
Port 4
212.17.9.0
20
Forwarding Datagrams
  • Every datagram contains a destination address.
  • Network ID uniquely identifies a physical
    network.
  • All hosts and routers sharing a Network ID share
    same physical network.

21
Forwarding an IP Router
  • Lookup packet DA in forwarding table.
  • If known, forward to correct port.
  • If unknown, drop packet.
  • Decrement TTL, update header Checksum.
  • Forward packet to the outgoing interface.
  • Transmit packet onto link.

22
Address Translation
  • Map IP addresses into physical addresses
  • destination host
  • next hop router
  • Techniques
  • encode physical address in host part of IP
    address
  • table-based
  • ARP
  • table of IP to physical address bindings
  • broadcast request if IP address not in table
  • target machine responds with its physical address
  • table entries are discarded if not refreshed

23
ARP Details
  • Request Format
  • HardwareType type of physical network (e.g.,
    Ethernet)
  • ProtocolType type of higher layer protocol
    (e.g., IP)
  • HLEN PLEN length of physical and protocol
    addresses
  • Operation request or response
  • Source/Target-Physical/Protocol addresses
  • Notes
  • table entries timeout in about 10 minutes
  • update table with source when you are the target
  • update table if already have an entry
  • do not refresh table entries upon reference

24
ARP Packet Format
  • HLen Hardware Address Length
  • PLen Protocol Address Length

25
Internet Control Message Protocol (ICMP)
  • Echo (ping)
  • Redirect (from router to source host)
  • Destination unreachable (protocol, port, or host)
  • TTL exceeded (so datagrams dont cycle forever)
  • Checksum failed
  • Reassembly failed
  • Cannot fragment

26
Tunneling
  • Big companies having different networks want to
    connect them together.
  • Virtual Private Network (VPN)
  • Use leased line to connect networks.
  • Use Internet (Shared line).
  • Sloution
  • Encapsulate packets in R1 in IP packets for
    destination R2.

Internet
Network 2
R2
Network 1
R1
Virtual line
27
IP Address Problem (1991)
  • Address space depletion
  • In danger of running out of classes A and B
  • Why?
  • Class C too small for most domains
  • Very few class A IANA (Internet Assigned
    Numbers Authority) very careful about giving
  • Class B greatest problem

28
IP Address Utilization (98)
http//www.caida.org/outreach/resources/learn/ipv4
space/
29
Classless AddressingCIDR
Class-based
A
B
C
D
0
232-1
Classless
128.9.0.0
65/8
128.9/16
0
232-1
216
128.9.16.14
30
Classless AddressingCIDR
128.9/16
0
232-1
128.9.16.14
31
Forwarding Datagrams
128.17.20.1
e.g. 128.9.16.14 gt Port 1
R2
Prefix
Port
Next-hop
3
65/8
128.17.16.1
R1
R3
1
128.9/16
2
128.17.14.1
2
128.9.16/20
1
128.17.14.1
3
128.9.19/24
7
128.17.10.1
128.9.25/24
2
128.17.14.1
R4
128.9.176/20
1
128.17.20.1
142.12/19
3
128.17.16.1
128.17.16.1
32
Default Routing
R1
R2
R3
R4
R5
33
Inside a Router
3.
1.
Output Scheduling
2.
Forwarding Table
Interconnect
Forwarding Decision
Forwarding Table
Forwarding Decision
Forwarding Table
Forwarding Decision
34
IP Version 6
  • Features
  • 128-bit addresses (classless)
  • multicast
  • real-time service
  • authentication and security
  • autoconfiguration
  • Any cast address
  • protocol extensions
  • Header
  • 40-byte base header
  • extension headers (fixed order, mostly fixed
    length)
  • No fragmentation
  • source routing
  • authentication and security
  • other options

35
IPV6 Packet format
0
4
16
24
31
12
Version
TrafficClass
FlowLabel
PayloadLen
NextHeader
HopLimit
SourceAddr (16 bytes)

DestinationAddr (16 bytes)

Next header/ Data
  • Next header IP option and protocol fields in
    IPv4. If options (i.e. fragmentation) indicated
    by this field, otherwise, it is protocol fields.
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