Network Architectures

Week 3 Part 2

Comparing The Internet & OSI

Internet “layers”

Application – layer 5 Message Transport – layer 4 Segment Network – layer 3 Datagram Data Link – layer 2 Frame Physical – layer 1 Bit

PDU (Protocol Data Unit)Layers as per Kurose & Rose

PDU

Application – layer 5 Similar to OSI application layer Examples of application services

E-mail – SMTP Simple Mail Transfer Protocol

FTP – File Transfer Protocol Telnet – Remote login protocol POP3 Post Office Protocol 3 HTTP – Hyper Text Transfer Protocol

The Web is not synonymous with the Internet – it is an application service.

Transport – layer 4 Has two primary protocols

UDP – a connectionless “unreliable” datagram service. Does not provide any re-transmission or congestion control

TCP – provides a connection oriented reliable data service with end-to-end error detection and control

We will cover both of these in more detail later

Network – layer 3 This is the IP layer Transfers packets from source to

destination via a connectionless datagram service

Uses IP addresses as a road map to locate a host within the Internet

Relies on routers to pass on the datagram via the correct link

Data Link – layer 2 Moves data in frames from one

node to the next Protocol may be different on each

link

Physical – layer 1

Moves bit from one node to the next

Protocols relate to the different media on each link

It is worth noting that the Data link and Physical layers are notSpecifically Internet layers – e.g. PPP, Frame Relay & ATM

Addressing within the Internet We have talked about applications passing

messages through the network We know it is more accurate to say that

Processes on each host pass messages Thus for two processes to communicate we

need: Source & destination host addresses specified by the

IP address A processes on the hosts specified by a port number.

Common processes have specific post numbers HTTP is port 80 SMTP is port 25

Domain Name Service - DNS

We prefer to use host names such as “usyd.edu.au”

But the Internet uses a hierarchical IP address

In IPv4 this is a 32 bit address shown as a four byte address eg 193.92.216.9, where each byte has a range 0-255 i.e. 9=00001001

In IPv6 this will become a 128 bit address, providing around 3 devices per square metre of the globe

Network Address Translation (NAT) where many numbers internal to an organisation are not externally visible has reduced the move to 128 bit addresses

DNS functions DNS uses three levels of server to translate the

domain name into IP address –see Kurose for details

Hosts may have alias – DNS will find the real name

Load distribution – a domain may have several replicated sites – DNS will provide the different IP addresses to each successive request.

Application level protocol Runs over UDP using port 53

Defining a Network

A Network within the Internet

A group of hosts isolated on one side of a router

The path between two routers

Network IP addresses Was defined in terms of A, B, & C classes

each with different numbers of devices This was an inefficient method and was

replaced in 1993 with IETF’s Classless Interdomain Routing (CIDR)

In this structure the network can be any number of bits. It is expressed in the form a.b.c.d/x where x is the number of bits defining the network and 32-x is the number of devices

Allocating Host addresses

Manually by the system administrator – usually used for servers

Dynamically at boot time following the Dynamic Host Configuration Protocol (DHCP). This is normal for largish organisations for workstations

Address Management

Domain names and IP Network addresses are managed under the auspices of ICANN (The Internet Corporation for Assigned Numbers & Names)

Three registries: US, Europe, Asia Pacific They resolve Domain name disputes

and maintain the DNS root servers

IPv6 is not universally accepted

Transport layer

Provides logical communication between two processes on different host. It provides two protocols: UDP – User Datagram Service TCP – Transmission Control Protocol

Transport layer The transport layer is implemented

on each end host onlyApplication

Transport

Network

Link

Physical

Network

Link

Physical

Network

Link

Physical

Application

Transport

Network

Link

Physical

UDP At the Transport layer it does very little.

Passes the message straight to the IP packet service

It is connectionless No overhead in handshaking No connection state in end systems

It has a smaller packet overhead No congestion control Trades off risk of data loss against higher

throughput Used in multimedia – telephony, video Some apps. provide own error control

TCP – Reliable transmission service

To provide a reliable data transmission service it needs:

An error detection mechanism. This is based on a checksum calculation

A receiver feedback mechanism. The receiver sends an ACK(nowledge) when an error free packet is received, and sends a NAK when the packet has errors.

A retransmission service is a packet is damaged or not received

Three problems

How can you send a NAK if the packet never arrives?

But if the second packet is not sent on its way until an ACK is received back, won’t it be slow? The “stop & wait” problem

How do you prevent congestion in the network?

Packets that do not arrive

Packets are given a sequence number and this number is returned on the ACK or NAK. Thus the sending host knows which packets have not been acknowledged and must be presumed lost – it is an implied NAK

It is possible for packets to circulate forever and this problem is prevented by giving the packet a life expressed in numbers of links. This number is decremented at each router and the packet discarded when the count is zero

The last problem is duplicate packets. The sequence number resolves this one too.

Stop & wait problem

An example in Kurose shows an effective transfer rate of 267kbps on a 1gbps line

The answer is pipelining or a sliding window approach. This allows multiple packets to be sent without waiting for the acknowledgements. It can best be shown in a diagram

Sliding Window Protocol

End-to-End flow control It is relatively easily to send data too quickly, such

that the receiver or a router cannot handle the flow. TCP provides congestion control

Host B establishes a connection buffer Host B tells A how much room is available in the

connection buffer on each segment it sends back Host A then has to ensure that the amount of data in the

pipeline, i.e. data that has not been acknowledged does not exceed the connection buffer

It does attempt to dynamically increase flow rate and chokes backs as congestion increases

This is a simplistic explanation – see Kurose page 246

Network layer The network layer is implemented

on each host and routerApplication

Transport

Network

Link

Physical

Network

Link

Physical

Network

Link

Physical

Application

Transport

Network

Link

Physical

IP

Provides a connectionless datagram service

Source to destination Relies on routers to route the datagram

through the network Routers use the IP addresses together

with their internal routing tables to direct datagram down the appropriate link

IP Datagram header

IP protocol i.e. 4 & Transport protocol Header length & header checksum Datagram length – theoretically up to 64k

but rarely > 1,500 and often limited to 576

Message fragmentation information Time to live Source & destination IP addresses Timing & routing parameters