Let Kolkata Amaze You

Where the mind is without fear, 

and the head is held high

Where knowledge is free, 

into that heaven of freedom

My Father! Let my country awake.

With these beautiful lines by Gurudev Rabindranath Tagore, let me open a view of place called "city of joy". My dear friends I am talking about Kolkata the place I belong to.

A city is defined by its heritage, its culture and most important by its people.
Kolkata was capital of British India and the architecture is inspired by the colonial era.

Today I am not going to present you the travelogue  instead would be telling you the story of kolkata's social fabric which I am proud of.  

Before going there let me give you the reason behind this amazing social fabric.
Kolkata was leading light of India during the 18th and the 19th century be it arts , science , literature, music and politics.

In the field of literature and music  Kolkata had Rabindranath Tagore, a noble laureates  He is the only person in the history to have authored national anthem for two countries India and Bangladesh  he gave a new music genre called rabindra sangeet which is learned and loved by every kolkatan.

Kolkata is a place of philosophers and thinkers. who can  forget Swami Vivekanand who amazed people in  in world religious summit held in Chicago 1893 by his views on religion harmony and brotherhood.

In politics kolkata gave Netaji Subhash Chandra Bose, only person to build army from scratch in 5 different countries to fight for Inida's Independence.

Stayajit ray pioneered Indian cinema and was awarded honorary Academy award.
famous scientist like JC Bose, Meghnad Sahah led the scientific advancement in early 19th century. The social reformers like Vidyasagar and Rajaram Mohan Roy brought equality for weaker section of the society.

Kolkata is also famous for its love of sports. 1st football club in India was established in kolkata. During 1900 start it was said that All world plays bootball where as India play football. That time Indians used to play football bare foot.


Why am I telling you all this?

This rich heritage has assimilated into people of kolkata. Go to kolkata to see the love between people  and care for each other. No caste, religion everyone is united  by the sense of being a Bengali. Bengali  the native language of kolkata. 
experience the joy and calm in the life of people of this city. if you are lucky and land into kolkata during Durga Puja a festive season you will experience the vibrancy and warmth of the city. You can dance on the road like you belong to this place, no nationality , no language barrier everyone is connected by notion of "being human".

Come to kolkata and let kolkata amaze you..

Congestion Control in IP network

Oxford dictionary defines Congestion as:
Congestion (Noun) - the state of being crowded and full of traffic

We all experience Congestion in our day to day life for example  traffic, food store, stadium, fair ground and tourist spot etc

The truth is that no one likes congestion. We have developed many  rules to avoid congestion,  get out of congestion and control the congestion.

If everyone wants to get rid of the congestion then why congestion happen. Answer is very simple It can be caused by two sets of people
1. Who don't care for others.
2. Who don't know about others.

Set 1: They will not be influenced  by congestion avoiding mechanism. they heed to no principle.
Set 2: situation is tricky, this set of people are considerate but they dont have information about other people. They just keep on estimating based on their experience.


Solution designer, the Architects or system designer has to design a mechanism to assist people being more considerate towards each other.

 
Let me move from human world to world of IP network.

Network is a graph defined by links and vertices's. Vertices's can any be network element ( host switch router etc.) A link connects two vertices's.

So where is the congestion link or vertices's ?
A link can never be congested as it can be either 100% utilized and 0% utilized. When there is a packet forwarded over the link it goes at link speed when there is no packet to forward the link utilization is zero. So if a packet keeps on coming at line  rate and leaves at line rate the link utilization is 100% . Is this a congested environment-- No. If  incoming traffic rate is less than  the line rate  condition is not congested.

So where is the congestion?
It is at the buffer of the vertices's.  If the incoming traffic rate is more than the line rate the packet gets queued in the buffer. Delay increases  due to queuing.  If queue keeps on increasing the problem can become further severe.  Remember,  that  when there is a packet in the buffer, Link functions at the 100% rate.
Is there any problem? We are utilizing the link at 100%  and we should not expect to do more.

Yes there is a problem and i.e Size of buffer. If a node keeps on getting traffic at higher rate than the output port line rate, the buffer will be full and any more packet to it would be dropped. Excessive congestion may overflow the buffer. So who will keep check that buffer is not overflown and many innocent packets are not dropped?

Whose responsibility is to check that buffer is not overflown  ?
Traditionally, when the responsibility is big it comes to bigger shoulder. TCP being one of the main pillar of TCP/IP stack ( Most common stack in IP network) takes the responsibility.  How?

How TCP does the congestion control?
 It keeps on monitoring end to end delay ( RTT Round Trip time). If the queues are bigger then a packet will experience more delay ( i.e more RTT) . If innocent packets are dropped TCP will not hear about their ACKs in few RTT time and realizes that somewhere down the path there is congestion and reduces the rate ( window size).  

Poor TCP it can only rely on end to end information. For TCP it seems a big pipe is directly connecting the source and destination . If there is more e2e delay or packet drops in big pipe it relies on its congestion control mechanism.  


So the crux is we want to utilize all the link to 100 % provided we have that much traffic. To tackle  imbalance in traffic  we would like to queue the packets in the buffer and keep link utilization to 100%. We can keep lot of packets in buffer because of its limited size. Somebody has to take care of it, either TCP or IP layer or link layer.



So finally an account of the efforts of different layer of TCP/ip stack
Transport-- TCP has many variants to improve the congestion control mechanism
Network ---Lots of routing protocol take bandwidth* delay product as the metric while making the forwarding plane
Link-- Layer 2 congestion control are upcoming a looks promising.

-------- More on TCP awesomeness and L2 promise on Congestion Control