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.