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Introduction to Computer Networks

University of Ilam

By: Dr. Mozafar Bag-Mohammadi

Internetworking

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Internetworking Communication between networks. Problems:

Different Networking technologies (Heterogeneity). So many Networks (Scaling).

Some terminologies: “internetworking” refer to an arbitrary collection of

connected networks. “Internet” the global internetwork. “Network” either directly connected or switched

network using any LAN technology such as Ethernet, Token ring, ATM, etc.

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IP Internet

Concatenation of Networks or “networks of Networks”.

“R” is routers and “H” is hosts.

R2

R1

H4

H5

H3H2H1

Network 2 (Ethernet)

Network 1 (Ethernet)

H6

Network 3 (FDDI)

Network 4(point-to-point)

H7 R3 H8

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IP Internet (cont)

Protocol Stack

Everything is running on top IP

R1

ETH FDDI

IPIP

ETH

TCP R2

FDDI PPP

IP

R3

PPP ETH

IP

H1

IP

ETH

TCP

H8

5

Source Routing

0

13

2

0

1 3

2

0

13

2

0

13

2

3 0 1 3 01

30 1

Switch 3

Host B

Switch 2

Host A

Switch 1

• All routing information is provided by the source.•The address can be implemented by a linked list in the packet header.

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Networking Technologies

Circuit Based Packet Based

Virtual Circuits Connectionless

TDM TelephonySONET/SDH

Frame RelayATM

IP

X.25

CLNP (ISO)SNA (IBM)Appletalk IPX (Novell)

DWDM

Connection Oriented

(variable rate, store-and-forward)(constant rate)

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Virtual Circuit Switching Problems with source routing:

The source must know the whole topology of network.

The number of switches (header) is variable. 2nd solution: use the telephone model or virtual

circuits. Explicit connection setup (and tear-down) phase. This

is called signaling. Each flow is identified by a Virtual Circuits Identifier

(VCI). Switch needs to maintains a VC table.

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Virtual Circuit Switching (cont) Subsequence packets follow the same circuit

Sometimes called connection-oriented model. VCIs is swapped in the switches. Example: Lookup table.

0

13

2

01 3

2

0

13

25 11

4

7

Switch 3

Host B

Switch 2

Host A

Switch 1

In-port In-VCI Out-port Out-VCI

2 5 1 11

3 11 0 7

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Virtual Circuit Model Typically wait full RTT for connection setup

before sending first data packet.

While the connection request contains the full address for destination, each data packet contains only a small identifier, making the per-packet header overhead small.

If a switch or a link on the path fails, the connection is broken and a new one needs to be established.

Connection setup provides an opportunity to reserve resources.

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Datagram Switching No connection setup phase since it is costly. Each packet forwarded independently Sometimes called connectionless model

0

132

01 3

2

013

2

Switch 3 Host B

Switch 2

Host A

Switch 1

Host C

Host D

Host EHost F

Host G

Host H

Analogy: postal system

Each switch maintains a forwarding (routing) table

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Datagram Model There is no round trip time delay waiting for connection

setup; a host can send data as soon as it is ready.

Source host has no way of knowing if the network is capable of delivering a packet or if the destination host is even up.

Since packets are treated independently, it is possible to route around link and node failures.

Since every packet must carry the full address of the destination, the overhead per packet is higher.

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Connection Oriented vs. Connectionless

Connection Oriented

Connectionless•Best-effort delivery (Send and Pray)

•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

• Connection set up. Signaling reserves resources along the end-to-end path

• Traffic flows • Connection torn down and resources freed

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Encapsulation Example

Ethernet Header

IP Header

TCP Header

HTTP Header

….

HTTP Data ….

An Ethernet segment transmitting HTTP data.

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IP 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.

Version HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

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Packet Headers

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.

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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

PrecedenceType of service

D T R C

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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

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Fragmentation and Reassembly0 7|0 7|0 4 7|0 7

Identification Flags Fragment Offset

0 1 2

0 DF MF

Flags DF: Don’t Fragment MF: More Fragment coming

In fragmentation, IP copy the original header and only modifyThe length, which is the new length, and offset. Offset is used for reassembly. Note: Fragmentation may degrade the network performance.

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.

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Fragmentation and Reassembly (cont)

Each network has a Maximum Transfer Unit size, MTU Strategy

fragment when necessary (MTU < Datagram) try to avoid fragmentation at source host re-fragmentation is possible fragments are self-contained datagrams delay reassembly until destination host do not recover from lost fragments

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Example

• Packet delivery from host H1 to host H8

H1 R1 R2 R3 H8

ETH IP (1400) FDDI IP (1400) PPP IP (512)

PPP IP (376)

PPP IP (512)

ETH IP (512)

ETH IP (376)

ETH IP (512)

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Example (cont)

Ident = x Offset = 0

Start of header

0

Rest of header

1400 data bytes

Ident = x Offset = 0

Start of header

1

Rest of header

512 data bytes

Ident = x Offset = 512

Start of header

1

Rest of header

512 data bytes

Ident = x Offset = 1024

Start of header

0

Rest of header

376 data bytes

The packets are fragmented as:

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Addressing

Each host in the network is identified by an address having the following property. globally unique hierarchical: network + host

11111111 00010001 10000111 00000000

Network Number Host Number

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IPv4 Implementation of Addresses

Thirty Two Bits: 0 8 1

624

11111111 00010001 10000111 00000000

255 013517

255.17.135.0

Dotted Quad notation for “human readability”

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Classful Addresses

0nnnnnnn

10nnnnnn nnnnnnnn

nnnnnnnn nnnnnnnn110nnnnn

hhhhhhhh hhhhhhhh hhhhhhhh

hhhhhhhh hhhhhhhh

hhhhhhhh

n = network address bit h = host identifier bit

Class A

Class C

Class B

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The Classful Address Space

Class Networks Hosts Share of IPaddress space

A 127 16,777,214 1/2

B 16,384 65,534 1/4

C 2,097,152 254 1/8

Leads to very inefficient allocation of addresses …

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IP Addresses

Example:

Class “A” address www.mit.edu18.181.0.31

(18<128 => Class A)

Class “B” address mekong.stanford.edu171.64.74.155

(128<171<128+64 => Class B)

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Classless AddressingCIDR

A B C D0 232-1

0 232-1

128.9/16

128.9.0.0

216

142.12/19

65/8

Classless:

Class-based:

128.9.16.14

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Classless AddressingCIDR

0 232-1

128.9/16

128.9.16.14

128.9.16/20128.9.176/20

128.9.19/24

128.9.25/24

Most specific route = “longest matching prefix”

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Forwarding Datagrams

“Network ID” uniquely identifies a physical network. All hosts and routers sharing a Network ID share

same physical network. Every datagram contains a destination

address. Is the datagram for a host on directly

attached network? If no, consult forwarding table to find next-hop. If only one next-hop, can use default routing.

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Forwarding Datagrams

128.9/16128.9.16/20

128.9.176/20

128.9.19/24128.9.25/24

142.12/19

65/8

Prefix Port

3227213

128.17.14.1128.17.14.1

128.17.20.1

128.17.10.1128.17.14.1

128.17.16.1

128.17.16.1

Next-hop

R1

R2

R3

R4

12

3

128.17.20.1

128.17.16.1

e.g. 128.9.16.14 => Port 2

128.17.14.1

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Default Routing

R1

R2 R3 R4 R5

DefaultRouting

RequiresRoutingTable

DefaultRouting

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Inside a Router

ForwardingDecision

ForwardingDecision

ForwardingDecision

Forwarding Table

Forwarding

Table

ForwardingTable

Interconnect

OutputScheduling

1.

2.

3.

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IP Forwarding Process

Forwarding Process

IP Forwarding Table Router

1. Remove a packet from an input queue

3. Match packet’s destination to a table entry

2. Check for sanity, decrement TTL field

4. Place packet on correct output queue

If queuesget full, just

drop packets!

If queuesget full, just

drop packets!

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Address Translation

Map IP addresses into physical addresses destination host next hop router

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

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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

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ARP Packet Format

TargetHardwareAddr (bytes 2 – 5)

TargetProtocolAddr (bytes 0 – 3)

SourceProtocolAddr (bytes 2 – 3)

Hardware type = 1 ProtocolT ype = 0x0800

SourceHardwareAddr (bytes 4 – 5)

TargetHardwareAddr (bytes 0 – 1)

SourceProtocolAddr (bytes 0 – 1)

HLen = 48 PLen = 32 Operation

SourceHardwareAddr (bytes 0 – 3)

0 8 16 31

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Internet Control Message Protocol (ICMP) Echo (ping) Redirect (from router to source host) Destination unreachable (protocol, port, or

host) TTL exceeded (so datagrams don’t cycle

forever) Checksum failed Reassembly failed Cannot fragment