1

Congestion Control Algorithm

• Preallocation of Buffers e.g.,– Allocate Buffer to Each Virtual Circuit in Each

IMP.

Note: This May be Expensive, and Only Used Where Low Delay & High Bandwidth are Essential (e.g., Digitized Voice)

2

Packet Discarding

• Datagram Service: Packet Discard at Will• Virtual Circuit Service: A Copy of Packet Must be

Kept

Note: If Congestion is to be Avoided by Discarding Packets, A Rule is Needed

InputLines

OutputLines Free Buffer

Congestion can be reduced by putting an upper bound on the number of buffers queued on an output file

3

Packet Discarding (cont.)

(Irland) Discovered Simple Rule of Thumb

(Not Optimal) for Determining Max Q Length, m, for an IMP With k Buffers

m = k / s, Where s # of Output Lines

4

Internetworking

• Many Different Networks Exist • Different Networks Have Radically Different

Technology • Still Desirable to Connect These Networks

Examples of This Follow:1. LAN-LAN: A Computer Scientist Downloading a File

to Engineering 2. LAN-WAN: A Computer Scientist Sending Mail to a

Distant Physicist3. WAN-WAN: Two poets Exchanging Sonnets 4. LAN-WAN-LAN: Engineers at Different Universities

Communicating

5

Four Common Types of Relays

• Layer 1: Repeaters Copy Individual Bits Between Cable Segments

• Layer 2: Bridges Store and Forward Frames Between LANs

• Layer 3: Gateways Store and Forward Packets Between Dissimilar Networks

• Layer 4: Protocol Converters Provide Interfacing in Higher Layers

6

Network interconnection

The boxes marked B are bridges. Those marked G are gateways.

7

Gateways

Operate at The Network LevelTwo Styles: • Connection-Oriented

e.g., Virtual Circuit• Connectionless-Oriented

e.g., Datagram

8

Internetworking(A Full Gateway)

Net 1 toInternet

Internet to Net 1

Net 2 toInternet

Internet to Net 2

BUFFER

1 2

Network 1 Network 2

Machine owned jointlyby both network

9

Internetworking(Two half-Gateways)

Net 1 toInternet

Internet to Net 1

Net 2 toInternet

Internet to Net 2

1 2

Network 1 Network 2

Machine owned byNetwork 1

Communication Line

Machine owned byNetwork 2

10

The Network Layer

DestinationHost

SourceHost

NetworksGateway

(a) Internetworking using concatenated virtual circuits

11

The Network Layer (cont)

SourceHost

DestinationHost

(b) Internetworking using datagrams

12

A datagram moving fromnetwork to network

13

A datagram moving fromnetwork to network (cont.)

Frame 1 DH1 IP TH DT1

Frame 2 DH2 IP TH DT2

Frame 3 DH3 IP TH DT3

Internetpacket

DHX: Data link Header for network XDTX: Data link Trailer for network XIP : Internet Protocol headerTH : Transport Header

14

Transparent Fragmentation

PacketNetwork 1

Network 2

G1 fragments

a largepacket

G2 reassembles

thefragment

G3 fragments

again

G4 reassembles

again

15

Non-transparent Fragmentation

Packet

G1 fragmentsa large packet

The fragments are notreassembled until thefinal destination (a host)is reached

Firewall

•A dedicated gateway machine with special

security precautions on it.

•Out-going/in-coming packets may be blocked.

•IP address and a port number may be used to

block the packets.

17

The IP Protocol

Version IHL Type of Service Total Length

Identification Fragment OffsetDF

MF

Header ChecksumTime to Live Protocol

Source Address

Destination Address

Options (0 or more words)

32 Bits

The IP (Internet Protocol) header

18

The IP Addresses32 Bits

0

10

110

1110

11110

Network

Network

Network

Host

Host

Host

Multicast Address

Reserved for Future use

Range of host address

1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to239.255.255.255

240.0.0.0 to247.255.255.255

IP address formats

Class

A

B

C

D

E

The IP Addresses

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Special IP addresses

This host

0 0 … 0 0 A host on this network

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1Broadcast on thelocal network

1 1 1 1 … 1 1 1 1

127 Loopback

Host

NetworkBroadcast on thedistant network

(Anything)

The IP Addresses

32 Bits

1 0 Network Host

One of the ways to subnet a class B network

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0

SubnetSubnetmask

ARP

Provides a mapping between the two forms of addresses: 32-bit IP addresses and data link addresses (e.g., 48-bit Ethernet addr.)

32-bit Internet Addr

48-bit Ethernet Addr

ARP RARP

V W X Y Z

V W X Y Z

V W X Y Z

ARP Request/Reply

ARP (cont’d)

Fund. Concept: The network interface has a hardware address,

and frames exchanged at the hardware level must be addressed to

the correct interface. TCP/IP works with its own addresses (i.e.,

32-bit IP addresses). Knowing a host’s IP addresses does not let

the kernel (i.e., Ethernet driver) must know the hardware address

to send the data.

Summary

Protocol addresses cannot be used when transmitting frames

across physical network hardware, because the hardware does

not understand IP addressing.

So, a frame sent across a given physical network must use hard

ware’s frame format, and all addresses in the frame must

be hardware address

Three interconnected class C networks: two Ethernets and an FDDI ring

1 2 3 4F2F1 F3

E4 E5 E6E1 E2 E3

192.31.65.7 192.31.65.5 192.31.63.8

192.31.60.4192.31.65.1

Router has2IP addresses

To

WA

N

192.31.60.7192.31.63.3

Router has2IP addresses

CS Ethernet192.31.65.0

EE Ethernet192.31.63.0

Ethernetaddresses

CampusFDDI ring

192.31.60.0

26

Internet Control Protocols

• ICMP (Internet Control Message Protocol)• ARP (Address Resolution Protocol)• RARP (Reverse Address Resolution Protocol)• BOOTP (Bootstrap Protocol)

BOOTP (cont’d)

BOOTP uses UDP and is intended as an alternative to RARP For bootstrapping a diskless system to find its IP address.Bootp can also return additional information such as the IP address of a router, the client’s subnet mask, and the IP address of a name server.

BOOTP (cont’d)

BOOTP DHCP (Dynamic Host Configuration Protocol)Unlike BOOTP, DHCP does not require an administrator to add an entry for each computer to the database that a server uses. Instead, DHCP provides a mechanism that allows a computer to join a new network and obtain an IP address without manual intervention.Note: An administrator can configure a DHCP server to have 2 types of addresses: Permanent addresses and a pool of addresses to be allocated on demand.

29

Operation of ARP

FTP TCP IP

Resolver

HostName

IPaddr

(2)

establish connectionwith IP address

(3)

send IP datagramto IP address

ARP

(1)

(4)

(5)

Ethernet Driver

Ethernet Driver Ethernet Driver

ARP IP

TCP

ARP

ARP request (Ethernet broadcast)

(6)

(7)

(8) (9)

Operation of ARP when user types “ftp hostname”

Goals: IPv6

1. Support billions of hosts, even with inefficient address space allocation

2. Reduce the size of the routing tables

3. Simplify the protocol, to allow routers to process packets faster

4. Provide better security (authentication and privacy) than current IP

5. Pay more attention to type of service, particularly for real-time data

IPv6 (cont’d)

6. Aid multicasting by allowing scopes to be specified

7. Make it possible for a host to roam without changing its address

8. Allow the protocol to evolve in the future

9. Permit the old and new protocols to coexist for years

32 Bits

Version Priority Flow label

Payload length Next header Hop limit

Source address(16 bytes)

Destination address(16 bytes)

The IPv6 fixed header (required)

33

Examples of The Network Layer

7

6

5

4

3

2

1

7

6

5

4

3

2

1

3

2

1

3

2

1

3

2

1

X.25 layer 3

X.25 layer 2

X.25 layer 1

X.25 layer 3

X.25 layer 2

X.25 layer 1

Internal protocols are not defined by X.25

Application protocol (not defined by X.25)

Presentation protocol (not defined by X.25)

Session protocol (not defined by X.25)

Transport protocol (not defined by X.25)

The place of X.25 in the protocol hierarchy

DTE DTE

34

Two Forms of Connections

• Virtual Calls --- A Connection is Established, Data Are Transferred, & Then The Connection is Released

• Permanent Virtual Calls --- Like A Leased Line, DTE at Either End LAN Just Send Data Whenever It Wants, Without Any Setup

• Note: The Choice of Circuit # on Outgoing Calls is Determined by The DTE, and on Incoming Calls by The DCE, May Lead to A Call Collision.

35

The ThreePhasesof anX.25

connection