NETWORKING

- What is core router and Edge router?

OSI Layers

• Physical 
• Data 
• Network
•  Transport 
• Sesssion 
• Presentation 
• Application

What happens when you type an url?

This pretty much covers the basics of all major networking concepts.  Will go through layer by layer and get some insight



- DNS: get the server address resolved from the website address (This can be taken from browser cache, if not present, make a system call to the OS to get from OS cache, then to router and then to the ISP)


Application  layer 
Layer where the end user actually interacts with the software
Protocols:   HTTP , FTP

Quick things  to know about HTTP :
- Works on request reply model
- GET POST are the commonly used http requests
- HTTP cookies are used 
- Uses TCP protocol


Presentation Layer
Also called as syntax layer

• Translation: Representation of data might be different in end user system. One end it might be mac, other end linux
• Compression: To increase throughput
• Encryption: Security reasons. Protocol used SSL . Encryption done in lower levels of protocol stack as well called IPSec

Session Layer 

The name of this layer tells you much about what it is designed to do: to allow devices to establish and manage sessions. In general terms, a session is a persistent logical linking of two software application processes, to allow them to exchange data over a prolonged period of time. In some discussions, these sessions are called dialogs; they are roughly analogous to a telephone call made between two people.

eg: Socket interface APIs used by transport layer

Transport Layer

Transmission Control Protocol
- Connection oriented - 3 way handshake
- Reliable transmission - order transmission using the sequence number field.
- Flow control - sliding window protocol
- Congestion control - ack, timers
- MSS - set according to MTU so that fragmentation is avoided, path MTU discovery is used to set the MSS accordingly.



TCP Header

TCP Connection - 3 way handshake




TCP Connection close





Sliding Window Protocol

The TCP urgent pointer is used to push data out of band ahead of already sent data.

Textbook example for that would be a slow telnet session in which after sending many commands, you decide to abort the session or the command using a control character such as control-C.

If you do not use an urgent pointer, the control-C will have no effect since it will have to pend the processing of all of the previous commands.
Using urgent pointer, the control-C processing is done out of band BEFORE the other commands, and thus it can cancel them or the entire session.



UDP header

ICMP header

Network Layer

IPV4 header

IPV6 header

IPV4 and IPV6 header comparison

IPV4	IPV6
Header size – 20 bytes	Header size 40 bytes
Addresses 2^32	2^128
IP checksum	No IP checksum
32 bits aligned	64 bits aligned
Broadcast address	No broadcast (Use multicast)
NAT	No NAT
Network id/host id	Global routing prefix/subnet id/interface id

TCP Socket
UDP Socket

Endian Transformation

• Big Endian -  First Byte is MSB, Last byte, LSB Network byte order -ppc
• Little endian -  First Byte is LSB, Last byte is MSB - Intel processors
• Convert a given ipv4 address from one endian form to another 

Server	Desktop
No	Graphic intensive applications
Higher Cache size
More varied cache
Multiple cores
Serveral hard drives configured to appear as a single disk- RAID (Redundant Array of Inexpensive Disks)	Single hard drive
RPM of disk is more – faster
Hot Swappable drives
Faster RAM
ECC RAM
Redundant power supplies
No monitor, keyboard, mouse. Managed via network	Managed via monitor, keyboard, mouse
1GB n/w interface, multiple n/w interface

 



Trunk Port - Carries multiple VLAN frames

References
www.tcpipguide.com

http://beej.us/guide/


http://i.dell.com/sites/content/business/smb/sb360/en/Documents/What-Is-a-Server.pdf