Router and Routing Algorithm

Router

-A router acts as a junction between two or more
networks

-Routers use IP addresses to identify packet
recipients (commonly a layer 3 item)

-Routers communicate with other routers to
build/identify complex data paths

(=> routing)

Router

layers in Router

layers in Router

Routing

-A router acts as a junction between two or more networks
-Routers use IP addresses to identify packet
recipients (commonly a layer 3 item)
-Routers communicate with other routers to
build/identify complex data paths

routing Overview

Types of routing
- static routing
- dynamic routing

Dynamic mechanisms
- distance vector
- link-state algorithm

Routing basics

-Routing takes place on IP level

network layer (3)

-Routing is needed

- when the packages‘ destination is beyond the own network

- to manage traffic between different networks

-Routes are determined by routers

- Therefore routers uses so called routing-tables

Basic Routing algorithms using by Router

Static routing

— Predefined static route between two hosts in

different networks

— Static routes are defined in so called static

routing tables

— Advantages:

- Routes are transparent and comprehensible

- Routes are trusted .

(at least the direct neighbors & default gateway)

- Simple tracing of errors (who to blame)

— Disadvantages:

- High administration effort and very large routing tables

static routing tables

static routing tables

Static Routing in LAN

— In local area networks (LANs)

static routes_are used to

connectthe Internal hosts

to superior networks

 

— Therefore each hosts has an

entry_ In Its routing table, to

specn‘y the router, that has

to be used as default gateway

(at the border of the LAN)

Dynamic routing

— Router is able to select packages' routes dynamically

— This selection is determined by different routing
protocols / Implementations

— Advantages:
- Less care necessarily
- Short reaction time on failures or resources bottlenecks

— Disadvantages:
- Exchange of control information necessarily
- Error tracing is more difficult
- Installation required



— Used where static routing is nearly impossible
- the number of networks / subnets is to large
- high dynamic network structures / metrics

— Used to interconnect a huge number of larger networks

Dynamic Routing

(so called autonomous systems)
- distributed enterprise networks
- distributed educational networks

-> the ”parts”, which build the internet


— Autonomous System (AS)

is an IP network that is administrated
as one single entity

e.g. universities, lSPs, international
companies

— The connection between all public
ASs is called the internet

— AS's external routers use
Exterior Gateway Protocols (EGP)
to exchange the routing information

Autonomous Systems

— The worldwide standard EGP is the Border Gateway Protocol


— Autonomous Systems (AS)
are IP networks that are
administrated as one
single entity

— Within an AS the router uses
Interior Gateway Protocols

— IGPs route between the subnets
of an AS

— At the ed e of the AS the routers
speak EG and IGP

— Both, EGP and IGP are dynamic routing
protocols

The two kinds of dynamic protocols

Main principles of the two kind of dynamic protocols:

- Distance vector protocols:

tell your neighbours, how the world looks for you

e.g. RIP (routing information protocol)
- Link-state protocols:

tell the world, who your neighbours are

e.g. OSPF (open shortest path first)
based on Dijkstra‘s algorithm

- Overview: RIP

— Routing Information Protocol

— Defined in RFC 1058

— Uses UDP protocol to exchange routing information
- via port 520

— Distributed computing of the routes

— Maximum of 15 hops

— No authentication of routers (less secure)

— Distributed computing of the routes

After initialization each router possesses a matrix
that stores the distances to all possible destination
networks (called routing table)

The used metric here is the hop count and gets increased
for each router, the package has to passes hrough

Hop count 16 means destination network is unreachable
Update time is every 30 sec

— if router is not responding for 180 sec, it gets tagged as
unreachable

Overview: RIP

Overview: OSPF

— Open Shortest Path First

— Defined in the RFC 2328

— Uses IP protocol 89

— Decentralized computing of the shortest way

between two routers

(Dijkstra algorithm - shortest path first)

— no hop-count-Iimit

(—> suitable for large networks)

— Authentication via MD-5 checksum

7 After initia/izat/on each router possesses a map of

the whole network topology

* Used metric is mostly the accumulated bandwidth

7 Using “hello protocol” to ensure, that the neighbours

are still available (hello package every 10 - 30 sec)

7 Actualization of the network information via the flooding

protocol (broadcasting to whole network)

r If there is no respond for a certain number of hello

packages all routers get informed / updated by using

flooding packages

OSPF - Algorithms

Sequence in principles of OSPF - Algorithms

Sequence in principles of OSPF - Algorithms

IP Subnetting

Subnetting

With the rapid growth of the Internet & the ever-increasing

demand for new addresses, the standard address class structure

has been expanded by borrowing bits from the Host portion to

allow for more Networks.

Subnetting reduces the size of the routing tables stored

in routers.

Subnetting extends the existing IP address base &

restructures the IP address.

As a result, routers must have a way to extract from a IP

address both the Network address & the Host address.

Subnetting Networks ID

A 3-step example of how the default Class A subnet

mask is applied to a Class A address:

subnetting networks ID

Subnetting, Subnet & Subnet Mask

-Subnetting, a subnet & a subnet mask are all

different.

-In fact, the 1 st creates the 2 nd & is identified by the 3 rd .

-Subnetting is the process of dividing a network & its

IP addresses into segments, each of which is called a

subnetwork or subnet.

Subnetting

-A network has its own unique address, such as a

Class B network with the address 172.20.0.0 which

has all zeroes in the host portion of the address.

-From the basic definitions of a Class B network &

the default Class B subnet mask, you know that this

network can be created as a single network that

contains 65,534 individual hosts.

Benefits of Subnetting

-Fewer IP addresses are needed to provide

addressing to a network & subnetting.

-Subnetting usually results in smaller routing tables

in routers

Example of Subnetting

-when the network administrator divides the

172.20.0.0 network into 5 smaller networks: –

172.20.1.0, 172.20.2.0, 172.20.3.0, 172.20.4.0 &

172.20.5.0 –

-the outside world still knows the network as

172.20.0.0, but the internal routers now break the

network addressing into the 5 smaller subnetworks.

Subnetting Example 

Subnetmask Function

-The function of a subnet mask is to determine whether an IP address

exists on the local network or whether it must be routed outside the

local network.

-It is applied to a message’s destination address to extract the network

address.

-If the extracted network address matches the local network ID, the

destination is located on the local network.

-However, if they don’t match, the message must be routed outside the

local network.

Subnetting Concept

-The key concept in subnetting is borrowing bits from

the host portion of the network to create a subnetwork.

-Rules govern this borrowing, ensuring that some bits

are left for a Host ID.

-The rules require that two bits remain available to use

for the Host ID & that all of the subnet bits cannot be

all 1s or 0s at the same time ( -2 ).

Fixed Subnet (Classful)

Fixed Subnet (Classful)

CIDR: Classless InterDomain Routing

- Subnet portion of address of arbitrary length

- Address format: a.b.c.d/x, where x is # bits in subnet portion

of address subnet part host part:

Example:

11001000 00010111 00010000 00000000

200.23.16.0/23

Knowing How to Calculate Subnets

To determine the number of subnets & hosts per

subnet available for any of the available subnet

masks, 2 simple formulas to calculate these numbers:

Knowing How to Calculate Subnets

Class A Subnetting Options

Class A Subnetting Options

Class B Subnetting Options

Class B Subnetting Options

Class C Subnetting Options

SubnetMask                      SubnetMask                   # Hosts

255.255.255.0/24                                                    256 (254)

255.255.255.128 /25                                              128 (126)

255.255.255.192 /26                                                  64 (62)

255.255.255.224 /27                                                  32 (30)

255.255.255.240 /28                                                  16 (14)

255.255.255.248 /29                                                      8 (6)

255.255.255.252 /30                                                      4 (2)

Subnetting

Example:

In network 192.168.10.0

255.255.255.0

We have here ONE Class C network, with 253 usable IPs

for Client-PCs.

The usable IP range of this network is

192.168.10.1 - 192.168.10.254

The very last IP of each Subnet is called:

Broadcast-Address

In this example 192.168.10.255 and it is NOT! usable for host

PCs

If we want to divide this network in two parts,we must use

subnetting

With subnetmask 255.255.255.128 we divide the network in two

Parts :

    192.168.10.1 – 192.168.10.127

    192.168.10.128 – 192.168.10.255

So in this example BEFORE, we had one big network but

With the change of the subnetmask we divided it in two smaller

networks

First with Subnetmask 255.255.255.0 we had this network:

192.168.10.0

The range:

192.168.10.1, 192.168.10.2

...

192.168.10.253, 192.168.10.254, 192.168.10.255

Now with Subnetmask 255.255.255.128 we have these two networks:

1st Subnet:

{ 192.168.10.0 (!NOT usable for Host PCs)

192.168.10.1, 192.168.10.2,192.168.10.3,

...

192.168.10.125, 192.168.10.126, 192.168.10.127 }

2nd Subnet:

192.168.10.128

192.168.10.129, 192.168.10.130,192.168.10.131,

...

192.168.10.253, 192.168.10.254,192.168.10.255 (is NOT! usable for Host PCs)

Used Bits for Network

Examples:

/16 = 255.255.0.0 =

11111111.11111111.00000000.00000000

/20 = 255.255.240.0 =

11111111.11111111.11110000.00000000

Calculation of a subnetmask for a specified number of hosts

Example:

You get the following order:

"Create a subnet with minimum 10 host Ips".

1st : 

Calculate a power of two, that is minimum 10:

2^3 = 8 Is it enough?

2^4 = 16

It is higher than 10 AND WORKS!

2nd :

Now put the LAST 4 Bits of your subnetmask to 0:

11111111.11111111.11111111.11110000

That is in decimal 255.255.255.240

With this subnetmask, you have at least 10 Host IPs in the

subnet, without wasting to much IP Addresses !

Assignment

A: You get this order as system administrator:

A. The company has a network with 150 computers. Create a Subnet from

“ 172.31.0.0 ” so that it is smallest possible subnet to provide

enough IPs.

B. Convert the second IP of your solution to Hexadecimal.

Calculating the the Broadcast IP of a Subnet

Example:

There is a subnet 172.16.64.0/20

Question:

What is the BROADCAST of that subnet?

1st , /20 means 255.255.240.0

2nd , Analyze the subnet octet to find out the "network-jump"

240 means in binary 11110000

The last of the 1's is equal to decimal 16

That is our "network-jump"

(128/64/32/16/8/4/2/1)

The last network started at 172.16.64.0

16 is the "network-jump". That means our next

network starts from 172.16.80.0 – 172.16.95.255

and the next one?

+16    172.16.96.0 - 172.16.111.255

+16    172.16.112.0 – 172.16.127.255

...

Because the next subnet in the example starts from

172.16.80.0, the broadcast must be 172.16.79.255

Because that IP before the next subnet starts is the

Broadcast Address !

OSI and TCP/IP Models

OSI and TCP/IP Models

The OSI Reference Model

- Planned to be a protocol stack for use

- Reference & communication model

- Used for troubleshooting

- It divides a complex process into small and

realizable units

The OSI Reference Model

Data Encapsulation:

The basic action of OSI

– Each layer responds services from superior layer

and issue services to the subordinate

– data moves into this seven layers to get the control

information

- Control informations are added in header & footer

- Headers and footer are fields that contain control

information

- Like post message

Each layer has its own PDU (Protocol Data Unit) which

contains different information

The PDU contains of a header and data field

Horizontal Connection

Horizontal Connection

Encapsulation Words

Packet

– Naming data unit in each step

- The data traveling through the media

Frame

- In data link layer

Datagram

– In network layer

Segment

– In transport layer

– Sequence

- Collection of segments

Message

– In Application layer

Encapsulation

Encapsulation

The difference between OSI and TCP/IP Models

OSI and TCP/IP Models

TCP Packet Structure

TCP Source Port - Port of sending host

Destination Port – Port of End Point Destination

Sequence # – Sequence of Bytes transmitted

in a segment, required to verify that all bytes are

received

Acknowledgment Number – The sequence number

of the byte the local host expects to receive next

Data Length – Length of the TCP Segment

Flags – Specified what content is in the segment

Window – How much space is currently available in

the TCP window

Checksum – Verify that the Header is not corrupted

TCP Port

A TCP port provides a specific location for delivery of

TCP Segments. Port Numbers below 1024 are well-

known, and are assigned by:

Internet Assigned Numbers Authority (IANA)

TCP Port

In Ubuntu to see list of ports: less /etc/services

Well-Known ports

– For network applications

– Range

1-1023

Registered ports

– Range

1024 – 49151

– Can be either source or destination

– Used by organizations to register specific applications

such as IM application

Private ports

– Range

49152 through 65535

– Used as source ports, these ports can be used by any applications

Domain Name System (DNS)

Domain Name System (DNS)

Stands for Domain Name System (or Service or Server), an Internet

service that translates domain name into IP addresses. Because

domain names are alphabetic, they're easier to remember.

The Internet however, is really based on IP addresses. Every time

You use a domain name, therefore, a DNS service must translate the

name into the corresponding IP address.

More about DNS

For example, the domain name www.itch.hu.edu.af might be

translated to 182.50.190.26.

Two Zomes in DNS Server!

Forward Lookup:Name to IP

Reverse Lookup:IP to Name

Requests and responses are normally sent in UDP packets, port 53

Occasionally uses TCP, port 53

DNS is Hierarchical

DNS is Hierarchical

Domain name space hierarchy

domain name hierarchy

DNS Hierarchy

There are several high level domain each group allow to

choose between geographical or organization .

Com = Commercial organizations

Mil = Military groups

Net = Major network support centres

Int = International organizations

Arpa = Temporary ARPANET domain

DNS Hierarchy

Every person or every organization can register a second level

domain on that high level domain by referring to the responsible

of that high level domain and with less price.

For Example:

The edu and gov are educational and governmental Every one can

register a second level domain in these high level domains.

When registering a domain the responsible can register any number

of sub domains or hosts on that domain without any limitation.

Name server

A name server translates domain name into IP addresses.

This makes it possible for a user to access a website by

typing in the domain name instead of the websites actual IP.

name server is big and active database system.

DNS server

The Domain Name System

DNS is a distributed database for holding name to IP address (and

other) information

Distributed:

– Shares the Administration

– Shares the Load

Robustness and improved performance achieved through

– replication

– and caching

Uses a client-server architecture

And is the critical piece of the Internet's infrastructure

Types of Queries

recursive query

Host at cis.poly.edu wants IP

address for gaia.cs.umass.edu

recursive query:

Ask for name resolution from

nearby name server.

heavy load! Why?

Iterated query:

Iterated querry

contacted server replies with

name of server to contact

“I don’t know this name, but

ask this server”

There are three roles involved in DNS

Three roles involved in DNS

RESOLVER

– Takes request from application, formats it into UDP packet, sends to cache

CACHING NAMESERVER

– Returns the answer if already known

– Otherwise searches for an authoritative server which has the information

– Caches the result for future queries

– Also known as RECURSIVE nameserver

AUTHORITATIVE NAMESERVER

– Contains the actual information put into the DNS by the domain owner

ROLE 1: THE RESOLVER

A piece of software which formats a DNS request into a UDP

packet, sends it to a cache, and decodes the answer

Usually a shared library (e.g. libresolv.so under Unix) because so

many applications need it

EVERY host needs a resolver

- e.g. every Windows workstation has one

How does the resolver find a caching nameserver?

It has to be explicitly configured (statically, or via DHCP, etc)

Must be configured with the IP ADDRESS of a cache

why not name?

(As#3: Part A)

Good idea to configure more than one cache

(As#3: Part B)

How do you choose which cache(s) to configure?

Must have PERMISSION to use it

– e.g. cache at your ISP, or your own

Prefer a nearby cache

– Minimises round-trip time and packet loss

– Can reduce traffic on your external link, since often the

cache can answer without contacting other servers

Prefer a reliable cache

– Perhaps our own!?

Example: Unix/Linux resolver configuration

/etc/resolv.conf

domain itch.hu.edu.af

nameserver 172.16.1.236

nameserver 172.16.0.252

That's all you need to configure a resolver

The old solution: HOSTS.TXT

A centrally-maintained file, distributed to all hosts on the Internet

SPARKY                       128.4.13.9

UCB-MAILGATE        4.98.133.7

FTPHOST                     200.10.194.33

... etc

This feature still exists:

 /etc/hosts (UNIX)

 c:\windows\hosts

hosts.txt does not scale

- Huge file (traffic and load)

- Name collisions (name uniqueness)

- Consistency

- Always out of date

- Single point of Administration

- Did not scale well

Testing DNS with "dig"

"dig" is a program which just makes DNS queries and displays the results

dig itch.hu.edu.af.

-- defaults to query type "A"

dig itch.hu.edu.af. mx

-- specified query type

dig @8.8.8.8 itch.hu.edu.af. mx

-- send to particular cache (overrides

/etc/resolv.conf)

Commonly seen Resource Records (RRs)

A (address): map hostname to IPv4 address

AAAA (quad A): map a hostname to IPv6 address

PTR (pointer): map IP address to hostname

MX (mail exchanger): where to deliver mail for user@domain

CNAME (canonical name): map alternative hostname to real

hostname

TXT (text): any descriptive text

NS (name server)

SOA (start of authority): used for delegation and management of the

DNS itself

A Simple Query Example

● Query:              www.itch.hu.edu.af.

● Query type:     A

● Result:

www.itch.hu.edu.af.   22725    IN   A       182.50.190.26

In this case a single RR is found, but in general, multiple RRs

may be returned.

(IN is the "class" for INTERNET use of the DNS)

A Simple Query Example

Understanding output from dig

Answer section (RRs requested)

– Each record has a Time To Live (TTL)

– Says how long the cache will keep it

Authority section

– Which nameservers are authoritative for this domain

Additional section

– More RRs (typically IP addresses for the authoritative nameservers)

Total query time

Check which server gave the response!

– If you make a typing error, the query may go to a default server

DNS records

DNS: distributed db storing resource records (RR)

DNS records

DNS protocol, messages

DNS protocol : query and reply messages, both with same message

format

msg header

DNS protocol, messages

identification: 16 bit # for

query, reply to query uses

same # of bit

flags:

 query or reply

 recursion desired

 recursion available

 reply is authoritative

DNS protocol, messages

Hostname, Host, and Nslookup

Hostname utility

– Provides client’s host name

• Administrator may change the name if needed

Nslookup

– Query DNS database from any network computer

• Find the device host name by specifying its IP address

– Verify host configured correctly

(troubleshoot DNS resolution problems)

Whois

Query DNS registration database

– Obtain domain information

Troubleshoot network problems

Syntax on Linux or Unix

– whois xxx.yy

• xxx.yy is second-level domain name

Windows system

– Requires additional utilities

Web sites provide simple, Web-based interfaces

Dynamic Host Configuration Protocol (DHCP)

DHCP

Stands for “Dynamic Host Configuration Protocol”

Allows host/client to dynamically obtain its IP address from network server

when it joins network

– Client can renew its lease on address in use

– Allows reuse of addresses (only hold address while connected and “on”)

– Support for mobile users who want to join network

– Automated and centralized configuration of network

– Ports:

UDP 67 (request)

UDP 68 (response)

When we want to join/connect to a network we might:

- Request the network administrator to set/give us a static IP address

- Simply connect to the network and wait for DHCP server to do it!

DHCP

DHCP overview:

A host/client in order to get an IP from a dhcp server,

Should pass the following dialogue:

– Host/client broadcasts “DHCP discover” message

– DHCP server responds with “DHCP offer” message

– host requests IP address: “DHCP request” message

– DHCP server sends address: “DHCP ack” message

IP address leasing

IP address leasing 

– DHCPDISCOVER

● Client broadcasts to discover dhcp

server in the network

– DHCPOFFER

● Server sends uni-cast to DHCP client

(suggest. IP, subnet, gateway, etc.)

– DHCPREQUEST

● Client sends broadcast to all DHCP

servers ! Why? and includes

server identifier to choose from offers

– DHCPACK

● Server sends uni-cast to client

(IP, subnet, gateway, etc.)

DHCP DORA

DHCP DORA 

DHCP Services

DHCP server assigns (at least) the following information:

- Client IP address and subnet mask

- Default gateway (default route)

- Name server (for name resolution)

- NTP server (for synchrony internal time setting)

Allocation Modes

DHCP server can assign IP addresses to the requested clients in

different ways:

Manual mode:

- Static mapping table for MAC and IP address

- Only hosts with listed MAC address receives IP address

- Allocation on undefined time

- Mostly used for servers ( static mapping, port forwarding)

Automatic static mode:

- Defined range of IP address for allocation

- Automated mapping of MAC and IP addresses

- Allocation on undefined time

Automatic dynamic mode:

- Defined range of IP address for allocation

- Automated mapping of MAC addresses and IP addresses

- Provides re-use of IP address

DHCP server leases IP address for a defined time period

Client renews lease time or release its IP address

DHCP commands

DHCPDISCOVER: Client broadcasts for DHCP server discovery

DHCPOFFER: DHCP servers answer on DHCPDISCOVER including their

specific values and parameters

DHCPREQUEST: Client broadcasts DHCP request including server

identifier to choose one of the DHCP server that has respond on

DHCPDISCOVER

DHCPACK: related DHCP server gives acknowledgment to the client's

related DHCPREQUEST

DHCPNAK: related DHCP server gives negative acknowledgment to the

client's related DHCPREQUEST (because of concurrent requests, etc.)

DHCPDECLINE: Client declines offer because of the IP is already used

in the network (checked with ARP)

DHCPRELEASE: Client releases it's actual configuration (for example if

network interface is set down) – that configuration than can be used

by other clients

 DHCPINFORM: only request for information / parameter excluding the

IP address

(for example if IP is configured in a static way for this client)

DHCP commands Analysis

DHCP Discover

DHCP Discover 

DHCP Offer

DHCP offer

DHCP Request

DHCP Request

DHCP Ack

DHCP Ack

DHCP Decline

DHCP Decline

Sample DHCP Configuration

#this is test DHCP server

ddns-update-style none;

option domain-name-servers 192.168.2.1;

default-lease-time 86400;

max-lease-time 604800;

authoritative;

subnet 192.168.1.0 netmask 255.255.255.0 {

range 192.168.1.100 192.168.1.155;

option subnet-mask 255.255.255.0;

option broadcast-address 192.168.1.255;

option routers 192.168.1.1;

DHCP Security

What happens if an unauthorized DHCP server connects

to a network?

DHCP Security

Scenario: Client requests for a bank website

CCNA tutorials 

But before that, client has obtained IP address with offer of

a fake DHCP server !

DHCP CCNA tutorials

As DHCP provides DNS server to the client who requested the IP,

Client will be cheated with fake and wrong DNS server as well !

fake client on DHCP 

Download Windows 8 ISO file with serial number

Download Windows 8 ISO file with serial number

To download Windows 8 ISO simply click on the link below and download the file

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And to learn how to install it on your PC watch the video:

 

Have fun

Principles of Application Layer Protocols (FTP)

FTP

FTP (File Transfer Protocol) is a protocol for transferring a file

from one host to another host.

ftp

- Allows a user to copy files to/from remote hosts

- Client program connects to FTP server

- Provides a login id and password

- Allows the user to explore the directories and download and

upload files with the server

HTTP and FTP are both file transfer protocols and have many common characteristics.

Example:

Both run on top of TCP, the Internet's connection-oriented, transport-layer,

reliable data transfer protocol.

But FTP uses two parallel TCP connections to transfer a file, a control connection

and a data connection.

The control connection is used for sending control information between the two hosts.

information such as user identification, password, commands to change remote

directory, and commands to "put" and "get" files.

The data connection is used to actually send a file.

As FTP uses a separate control connection, FTP is said to send its control

information out-of-band which avoids requiring the user to log in again ! Why?

TCP control connection 

When a user starts an FTP session with a remote host, FTP first sets up

a control TCP connection on server port number 21.

The client side of FTP sends the user identification and password over

this control connection.

The client side of FTP also sends, over the control connection,

commands to change the remote directory.

When the user requests a file transfer (either to, or from, the remote

host), FTP opens a TCP data connection on server port number 20.

FTP sends exactly one file over the data connection and then closes

the data connection.

If, during the same session, the user wants to transfer another file, FTP opens

another data TCP connection.

Thus, with FTP, the control connection remains open throughout the duration

of the user session, but a new data connection is created for each file

transferred within a session.

- FTP maintains state and therefor is stateful.

FTP Commands

The commands, from client to server, and replies, from server to client,

are sent across the control TCP connection in 7-bit ASCII format.

Common cmds:

Authentication

– USER: specify the user name to log in as

– PASS: specify the user’s password

Exploring the files

– LIST: list the files for the given file specification

– CWD: change to the given directory

Downloading and uploading files

– TYPE: set type to ASCII (A) or binary image (I)

– RETR: retrieve the given file

– STOR: upload the given file

Closing the connection

– QUIT: close the FTP connection

FTP Respond

There is typically a one-to-one correspondence between the command that the user

issues and the FTP command sent across the control connection.

Each command is followed by a reply, sent from server to client. The replies are threedigit

numbers, with an optional message following the number.

Eg:

331 Username OK, password required

125 Data connection already open; transfer starting

425 Can't open data connection

Server Response Codes

1xx: positive preliminary reply

– The action is being started, but expect another reply before sending the next

command.

2xx: positive completion reply

– The action succeeded and a new command can be sent.

3xx: positive intermediate reply

– The command was accepted but another command is now required.

4xx: transient negative completion reply

– The command failed and should be retried later.

5xx: permanent negative completion reply

– The command failed and should not be retried.

HTTPS

HTTPS

HTTPS stands for Hypertext Transfer Protocol over Secure Socket

Layer, or HTTP over SSL.

- SSL acts like a sub layer under regular HTTP application

- HTTPS encrypts an HTTP message prior to transmission and

decrypts a message upon arrival.

What are certificates?

For secure communication over untrusted networks we need to encrypt the

traffic. That is often done with SSL/TLS.

Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer

(SSL), are cryptographic protocols that provide secure communications on

the Internet for such things as web browsing, e-mail, Internet faxing, instant

messaging and other data transfers.

What is a Certification Authority?

For organizations and servers a hierarchical key system is invented,

where a "Certification Authority" (CA) can "sign" key-pairs for multiple

servers or users.

It means the CA confirms the authenticity of the keys and its holders.

If a computer / user trust this CA, they automatically trust all keys that were

signed by it.

In such a scenario we call the public keys "certificates".

HTTPS Transaction

HTTPS Transaction

SSL as HTTP Security Concerns

Secure Sockets Layer technology protects Web site and makes it easy for Web site

visitors to trust in three essential ways:

Privacy

An SSL Certificate enables encryption of sensitive information during online

transactions.

Integrity

A Certificate Authority verifies the identity of the certificate owner when it is issued.

Authentication

Each SSL Certificate contains unique, authenticated information about the

certificate owner.

Web Server (www)

Is to serve www. (web sites)

Try them !

- Apache Web Server

very large and powerful, a lot of extensions

- Lighttpd

small but also provides some dynamic content

- Apache Tomcat

web server that provides servlet container (java servlets, jsp)

Principles of Application Layer Protocols (HTTP_2)

Principles of Application Layer Protocols (HTTP_2)

HTTP Performance

What effects?

Different kinds of requests:

– Lots of small requests (loading a web page)

– Big request (fetching a download)

Requires different solutions!

Small requests

Latency matters:

Governed by RTT between hosts.

Two major causes of delay:

– Opening a TCP connection

– Data response-request

Solutions:

– Persistent connections (Why?)

– Pre-fetching !

– Others?

Big requests

Eg: When doing a big request (big file to download).

Problem is throughput on bottleneck links (usually edge links)

A solution:

Use an HTTP proxy cache or mirror

– Can also improve latency!

big requests in HTTP

Old Cached Data

Cache needs a way to conditionally ask for a document as Items in the cache

can get staled ( eg: We don’t want to read stored of weeks ago)

- Cache can issue a conditional GET (with an “If-modified-since”

header)

- Server can reply with a “304 Not Modified”

Web caching

Cache acts as both client and server.

- Typically cache is installed by ISP (university, company,

residential ISP)

- Reduce response time for client request

- Reduce traffic on an institution’s access link

HTTP Transaction

HTTP Transaction

Principles of Application Layer Protocols (HTTP)

Principles of Application Layer Protocols (HTTP)

The World Wide Web: HTTP

The Hypertext Transfer Protocol (HTTP), the Web's application-layer protocol,

is at the heart of the Web.

HTTP is implemented in two programs: a client program and server program.

The client program and server programs, executing on different end systems,

talk to each other by exchanging HTTP messages.

HTTP server maintains no information about the clients, HTTP is said to be a

stateless protocol. It does not have to keep track of any user state.

Three components:

- File transfer protocol:HTTP (hyper text transfer protocol); uses TCP

- Format for documents with links (“hyperdocuments”): HTML (hyper text

markup language)

- URLs (universal resource locators)

HTTP

- Web page consists of objects

- Object can be HTML file, JPEG image, Java applet, audio file,…

- Web page consists of base HTML-file which includes several

referenced objects

- Each object is addressable by a URL (Uniform Resource Locator)

URL

- Identify documents to be transferred and application layer protocol

to use

for example:
http://www.ccnatutorials.com/2016/02/ethernet.html

HTTP and URL

http overview

HTTP overview

HTTP: hypertext transfer protocol            

Web’s application layer protocol

client/server model

– client: browser that requests,

receives, “displays” Web objects

– server: Web server sends objects

in response to requests

HTTP 1.0: RFC 1945

HTTP 1.1: RFC 2068

HTTP uses TCP

Open TCP connection

Uses TCP:

- Client initiates TCP connection

(creates socket) to server, port 80

- Server accepts TCP connection

from client

- When HTTP messages (applicationlayer

protocol messages) exchanged

between browser (HTTP client) and

Web server (HTTP server)

TCP connection will be closed

HTTP is “stateless”

server maintains no information

about past client requests !?

Protocols that maintain “state”

are complex!

- past history (state) must

be maintained

- if server/client crashes,

their views of “state” may

be inconsistent, must be

reconciled

HTTP connections

Nonpersistent HTTP

At most one object is sent over

a TCP connection.

HTTP/1.0 uses nonpersistent

HTTP

Persistent HTTP

Multiple objects can be sent

over single TCP connection

between client and server.

HTTP/1.1 uses persistent

connections in default mode

Nonpersistent HTTP

Suppose user enters URL:

http://www.ccnatutorials.com/2016/02/ethernet.html

and it contains text,

references to 10

jpeg images)

1a. HTTP client initiates TCP

connection to HTTP server

(process) at

www.ccnatutorial.com on port 80

1b. HTTP server at host

www.ccnatutorial.com waiting for TCP

connection at port 80.

“accepts” connection, notifying

client

2. HTTP client sends HTTP request

message (containing URL) into TCP

connection socket. Message

indicates that client wants object

2016/02/ethernet.html

3. HTTP server receives request

message, forms response

message containing requested

object, and sends message

into its socket

4. HTTP server closes TCP

connection.

5. HTTP client receives response

message containing html file,

displays html. Parsing html

file, finds 10 referenced jpeg

objects

6. Steps 1-5 repeated for each of

10 jpeg objects

Problems with Nonpersistent HTTP

- A brand new connection must be established and maintained for

each requested object.

- For each of these connections, TCP buffers must be allocated and TCP

variables must be kept in both the client and server.

(Load on server for simultaneous connections!)

- Each object suffers two RTTs – one RTT to establish the TCP connection

and one RTT to request and receive an object.

Response time modeling

Response time modeling

Definition of RTT:

time to send a small packet to

travel from client to server and

Back.

Response time:

- One RTT to initiate TCP connection

- One RTT for HTTP request and

first few bytes of HTTP response to

return

- File transmission time

total = 2RTT+transmit time

Nonpersistent HTTP issues:

- requires 2 RTTs per object

- OS must work and allocate host

resources for each TCP

connection but browsers often

open parallel TCP connections

to fetch referenced objects

Persistent HTTP

- server leaves connection open

after sending response

- subsequent HTTP messages

between same client/server are

sent over connection

Persistent without pipelining:

- client issues new request only

when previous response has

been received

- one RTT for each referenced

object

Persistent with pipelining:

- default in HTTP/1.1

- client sends requests as soon

as it encounters a referenced

object

- as little as one RTT for all the

referenced objects

HTTP Request Format

HTTP Request Format

HTTP Request Format: GET, HEAD, PUT, POST, DELETE

A small browser request: http://localhost

user@host:~$ telnet localhost 80

Trying ::1...

Connected to localhost.localdomain.

Escape character is '^]'.

GET / HTTP/1.0

HTTP/1.1 200 OK

Date: Thu, 18 Aug 2016 14:46:28 GMT

Server: Apache/2.2.16 (Ubuntu)

Last-Modified: Mon, 08 Aug 2016 10:14:21 GMT

...

Connection: close

Content-Type: text/html

<html><body><h1>It works!</h1>

<p>This is the default web page for this server.</p>

<p>The web server software is running but no content has been added, yet.</p>

</body></html>

Connection closed by foreign host.

Conditional GET: client-side caching

Conditional GET: client-side caching

HTTP Respond Format

HTTP Respond Format

1xx codes: Informational 

2xx codes: Successes 

3xx codes: Redirection

4xx codes: Client error 

5xx codes: Server error

HTTP response status codes

A few sample codes:

200 OK

– request succeeded, requested object later in this message

301 Moved Permanently

– requested object moved, new location specified later in this

message (Location:)

400 Bad Request

– request message not understood by server

404 Not Found

– requested document not found on this server

505 HTTP Version Not Supported

Cookies: keeping “state”

Cookies are an alternative mechanism for sites to keep track of users.

A: The server's response will include a Set-cookie: header.

Often this header line contains an identification number generated by the

Web server.

For example, the header line might be:

Set-cookie: 1678453

B: When the the HTTP client receives the response message, it sees the

Set-cookie: header and identification number.

C: Client appends a line to a special cookie file that is stored in the client

machine and includes the host name of the server and user's associated

identification number.

D: In subsequent requests to the same server, say one week later, the

client includes a Cookie: request header, and this header line specifies the

identification number for that server.

In the current example, the request message includes the header line:

Cookie: 1678453.

E: In this manner, the server does not know the username of the user, but

the server does know that this user is the same user

that made a specific request one week ago!

Many major Web sites use cookies                             Example:

Four components:                                                       – Ahmad.m access Internet

                                                                                        always from same PC

1) cookie header line in the HTTP                                            

response message                                                           – He visits a specific ecommerce

2) cookie header line in HTTP                                          site for first time

request message                                                                                                                         

3) cookie file kept on user’s host and                                – When initial HTTP requests arrives at site,

managed by user’s browser                                                   site creates a unique ID

4) back-end database at Web site                                           and creates an entry in backend database for ID

Cookies: keeping “state”

Cookies and privacy:

- Cookies permit sites to learn a lot about you

- You may supply name and e-mail to sites (Maybe Credentials!)

- Search engines use redirection & cookies to learn yet more

- Advertising companies obtain info across sites