Network.txt

What happens when u try to connect from your home to internet
DHCP
Process:
1.request encapsulated in UDP,then in IP, then in 802.1 Ethernet
2.Ethernet frame broadcast to (dest: FFFFFFFFFFFF ) on LAN,
3.received at router running DHCP server
4.DHCP server formulates DHCP ACK with <Output>
encapsulation at DHCP server, frame forwarded (switch learning) through
LAN, demultiplexing at client.
Output:
1.IP address for self client
2.name & addr of DNS server
3.IP address of its first-hop router

ARP
Process:
ARP query :
broadcast,received by router.
Output:
ARP reply:
MAC address of router interface.

DNS
Process :
IP datagram routed in intranet (RIP, OSPF, IS-IS and/or BGP routing protocols)and internet
Output :
IP address of destination server

TCP :
Process:
3 way hand shake to server
Ouput : 
connect to destination server 

HTTP (req and reply)

How to send data from end to end: two switching
methods 
Circuit switching (eg.ISDN)
Packet switching (e.g. ATM, X.25, Frame Relay, Internet)

Sharing a link : Multiplexing
To combine multiple signals (analog or digital) for
transmission over a single line or medium.

E2E (End to End) design principle
Application-specific functions ought to reside in the end hosts of a network rather than in intermediary nodes – provided they can be implemented "completely and correctly" in the end hosts.

Mux Technologies :
1.Frequency Division Multiplexing (FDM) : each signal is
assigned a different frequency range (e.g. FM radio).
2.Time Division Multiplexing (TDM) : each signal is
assigned a fixed time slot in a “fixed” rotation.
3.Statistical Time Division Multiplexing (STDM): time
slots are assigned to signals dynamically to make better
use of bandwidth.
4.Wavelength Division Multiplexing (WDM) : each signal
is assigned a particular wavelength; used in optical fiber.

Service
The set of operations provided by the lower layer to
the upper layer to perform a service
Example: 
Connection-oriented services:
connect, send, receive, disconnect
Connectionless services:
send, receive

Protocol
algorithms to implement functions provided by service

Diff Types of protocols
1.Public protocol
HTTP for web server and client, SMTP for email clients and server
BitTorrent,P2P
2.Proprietary protocol
Real,Kazza,Skype

Service Access Point
How do two processes on two computers identify
themselves to each other ?
Answer: use a triple
(protocol, ip_address, port_number) = socket
e.g. (TCP, 192.168.0.1, 80)
(Actually a socket is a quintuple, more later)
In general: each entity of a layer access lower layer’s
services via Service Access Points

Application(HTTP,telent,DNS,SMTP)
Supports Network Applications
Transports applications’ messages

Transport (TCP,UDP,ATM AAL)
TCP: connection-oriented, reliable
UDP: connectionless, unreliable

Network(IP,ATM layer)
Routes data packets from hosts to hosts
IP: Internet Protocol, and many routing protocols

Data link(CSMA/CD,ATM physical,PPP)
Deals with algorithms to achieve reliable, efficient
communication between two adjacent machines

Physical(raw bits)
Moves raw bits (0/1) between adjacent nodes
depending on the physical medium used

TCP 
1.Establish connection: 3-way handshake
2.Data transmission
  Byte-stream service
  Reliable (retransmission with timer)
  In-order delivery (reorder packets if necessary)
  Support flow control (fast sender vs slow receiver)
  Full-duplex (data transferred both ways)
3.Close connection

connect : service
3-way handshake : Protcol

Read/Write : service
Send Data realiablly and in-order: protcol

how to calculate time required to establish tcp connection ?
how to determine to which remote process I am communicating ?
Ans : Socket pair , quadruple (local_IP,
local_port, remote_IP, remote_port) must be unique
The quadruple is also called a socket pair

At a time of connection establishment
Allowing each side to know the other exists

Negotiation of optional parameters 
#Max segment size
#Initial Sequence Numbers (ISN)

Triggering allocation of transport entity resources
#Buffer
#Timers (if any)

Problems
HAlf-open Connection
Symptoms : 
- One end quits w/o the knowledge of the other
 - For example, you turn off your PC while a telnet is
still on, or your OS freezes and you have to reboot

Solution
Timing out alone does not work (look at a web browser)
TCP has the keep alive option
RST is used to
Reply to connection requests to some port no-one is
listening on
Reply to connection requests within 2MSL after crashing
In UDP, an ICMP port unreachable is generated
instead

TCP/IP use big-endian for all its integers 
Serious run-time performance penalties occur when
using TCP/IP on a little endian processor.
Thus, it may be unwise to select a little endian
processor for use in a device, such as a router or
gateway, with an abundance of network
functionality.


DNS 
Domain Name System
Distributed database system
information about names in the domain
name space
Maintaining a many-to-many mapping between
domain name space and IP address space
Realized by name servers

Domain and Domain name
domain is a subtree
domain name is a node in the tree, may point to
Network addresses (IP address)
Hardware information
Mail routing information
Information about the domain rooted at that node

Name servers are programs storing info. about the
domain name space, answering queries on it

To query the DNS USE 
dig buffalo.edu a
dig buffalo.edu cname
dig buffalo.edu mx
dig buffalo.edu ns

It gives Resource record with key and value
Type = A
Key : hostname
value : IP address

Type = cname
Key : domain name
value : authoritative name server


TCP State Diagram
Channel bit errors require
Error detecting codes
Receiver feedback (ACK/NAK)
Retransmissions (when NAK received or ACK/NAK corrupted)

Retransmissions introduce duplicates
Need sequence numbers


Packet loss requires
Timeout + retransmission (again introduce duplicates)
Estimating the “right” timeout is a fundamental problem!

Pipelining improves utilization + throughput
Needs to enlarge sequence number space
Needs more buffer space at both sender & receiver
ACK + retransmission strategies: Go-Back-N & Selective Repeat
Window size & sequence number range strongly related


EXtensions to TCP
Selective acknowledgements: TCP SACK
Explicit congestion notification: ECN
Delay-based congestion avoidance: TCP Vegas
Discriminating between congestion losses and other losses: cross-layer signaling and guesses
Randomized drops (RED) and other router-based mechanisms

TCP
1.Multiplexing and Demultiplexing
2.Byte-stream service
Stream of bytes sent and received, not stream of packets
3.Reliable data transfer
A combination of go-back-N and selective repeat, and
performance tuning heuristics
4.Connection management
Connection establishment and tear down
5.Flow control
Prevent sender from overflowing receiver
6.Congestion control
General principles & How TCP does it

AODV:
Distance vector : count to infinity
Link state : only link information sharing ,scalable
AODV : Avoids count to infinity problem : 
Count to infinity : takes infinte ammount of time to converge when topology chages or link breaks
solution : use AODV that uses PREQ an RREP messges and PERROR when link breaks or topology changes
Runs periodically 
Unique seq number to avoid flodding of packets in network.