Friday, January 30, 2009

Expensive NEPAL

Today, I found a product named Nepal EVO GTX in Korean ebusiness site. I surprised with the price 785,000, even a discount price is 700,000 Korean won (around 500 USD or NRs 40,000). According to the manufacturer LA Sportiva, it’s the best leather mountaineering boot on the market. The Nepal last offers an excellent fit for any foot and is easily adjustable with the removable/adjustable tongue. It’s around 1 KG.

Because of few reasons I am particularly interested in it. One is I like adventure and recently I am going to skiing. Another is, the name of the shoes is Nepal, the name of my country.Finally, the shoes is looks like very durable and comfortable.

New Google

Thesedays, Google is a synonyms for search engine. People say googling for search in the internet. Many new innovative (not sure how innovative they are though) search engines are showing their existence in the market. 

Today, I received an email from SONSIK yahoo group member mentioning that the search engines that we are mostly using Google and Yahoo, there is a new searching engine promoting by Nepalese. The mail requested to try the search engine. I tried it and searched some stuffs too. I read the About Us column. It seems that basically the search engine is operating from US. It has offices in both country US and India. The promoters are seems to be Nepalese and Indian origin. The official blog is written in January 28, 2009, looks pretty new. 

The name of the search engine is given Sagoon. Although the GOO of Sagoon looks like taken from Google, it’s not true. It’s derived from Sanskrit which means Auspicious or ushering good results. In News section there are two language changing options – Nepali and Hindi. It can be assumed that this search engine is targeted to the Nepali and Indian users. History shows that Google and Yahoo are not big success in non-english speaking countries like China and Korea. So this search engine can be very popular as a regional search engine. 

The founder of the company Sagoon Inc. Govinda mentions in the blog that it’s the beginning of their journey, a journey with bigger challenge. Of course, as there are many Big Bosses in search engine industries occupying large market shares already, it might be challenging for Sagoon. 

Anyway, finger crossed to Sagoon team for their success.

Thursday, January 29, 2009

Mata Saileshwori, Silgadhi, Doti

Photo taken by two different nonprofessional cameras. 

Saturday, January 17, 2009

Why TCP modeling?

There are certain things we can achieve with TCP modeling that cannot be achieved with simulation. Some of them are as follows. 

  • We can model some uncertainties using stochastic process that cannot be determined by simply simulation. For example, number of current TCP connections, how much bandwidth is occupied by the non TCP transport protocols and what is the effect of those in TCP etc. 
  • TCP operating scale is very large in terms of geographical scale and the number of inputs/outputs. To simulate the large number of inputs/outputs and geographical scale is not easy due to hardware constraints (some times budget constraints as well).
  • Mathematical models are required to quantify metrics that define system performance and develop control strategies that result in the optimum being achieved.
  • It also helps to determine how close it is to achieving a system optimum, as defined by a specific cost function, and subsequently identify issues that need to be addressed in future control protocol. 
  • TCP is already mostly used transport layer protocol in the world. To test some optimization in it is not easy to implement in real world scenario and observe its performances. So, before implementation any optimization patches it is very much necessary to model it. Some examples of those patches are Fast Retransmission and Fast Recovery. These were not in the very first version of TCP.

Monday, January 12, 2009

The role of the "gateway" option of tcpdump

‘Gateway’ in tcpdump specifies a name of gate that transfers the packet. We can use gateway to monitor traffic from specific TCP/IP applications at strategic locations of a given TCP/IP network. For example to print all ftp traffic through snup gateway the command is as follows:
#tcpdump 'gateway snup and (port ftp or ftp-data)'

To print IP packets longer than 576 bytes sent through gateway snup the command is:
#tcpdump 'gateway snup and ip[2:2] > 576'

Saturday, January 10, 2009

Disadvantages of IP fragmentation?

Some disadvantages of IP fragmentation are: 
A. Once packet is fragmented it can be reassembled only at another end. If there is larger bandwidth on the way, it cannot take an advantage of that. 
B. The overhead is large when IP fragmentation is used. IPv4 does not restrict fragmentation and can be performed at any link along the path. This can result in heavy processing demands on the routers in the network. (Not in IPv6).
C. There are some security issues with fragmentation. (e.g. Tiny Fragment Attack and Overlapping Fragment Attack)

Friday, January 9, 2009

Ma Bagalamukhi - A new photograph I have found in the net.

Out of all the 10 Mahavidyas, Ma Bagalamukhi is associated with a huge amount of occult powers. 

Thursday, January 8, 2009

Situations when RTO occurs even though packets reach receiver

When the ACK sent by the receiver lost and sender could not get the ACK then the RTO occurs. If there is a delay in arriving ACK to the sender due to some reasons (as described below) than RTO occurs even though packets reach receiver. For example, on a low-bandwidth link the arrival of competing traffic (possibly with higher priority), or some other change in available bandwidth, can cause a sudden increase of the RTT. This may cause a RTO. If the communication takes place from a low latency path to a high latency path, it may suddenly increase the RTT beyond the current RTO value. 

Wednesday, January 7, 2009

How cumulative acknowledgement can compensate for a lost ACK?

Cumulative ACK can compensate for a lost ACK with the subsequent ACKs of higher number. For example, A is a sender and B is a receiver. A sends packets to B from sequence number 101 to 201. After receiving all bytes from A, B sends a cumulative ACK of 202. Here, ACK Number 202 means, B received all the bytes up to 201. Let’s assume another case that there is no cumulative acknowledgement and all the packets are received by the receiver but one ACK is lost. In that case even if the packet is successfully received by the receiver, sender retransmits the packets, which it didn’t receive the ACK. In this case if there is cumulative acknowledgement, even one ACK is lost, if sender receives higher ACK number, sender determines that packets are received. The ACK with higher value compensate the previously lost ACK.

Tuesday, January 6, 2009

Why is the ISN (initial sequence number) of TCP selected randomly?

TCP layer on the TCP devices exchange Initial Sequence Number (ISN) that is assigned to any device during the startup of the TCP connection. The ISN is always random. There are several reasons to select ISN randomly. One of the reasons is that TCP segments may get mixed up with the different connections if the sequence number is started from the same number by all the devices. For example, a TCP connection is started with ISN 1. Let’s say there are 1 to 50 bytes. Suppose that there is some problem and the TCP connection is terminated. Now a new connection with the same ISN (i.e. 1) is started for the same number of bytes (i.e. 1 to 50 bytes). The device may think that these segments are the part of new connection. So, the data may get collapsed. 

The another reason to select ISN number randomly is any malicious person could write code to predict the ISN of subsequent TCP connection based on the ISNs used in earlier ones, which causes serious security risk. 

Monday, January 5, 2009

How an efficient TCP/IP implementation can extend the battery life of a mobile device?

Efficient TCP/IP implementation can reduce packet loss rate, data transfer time and resource consumption. Retransmissions of the packet eat up most of the battery power unnecessarily. If the packet loss can be reduced by the efficient flow control and congestion control mechanism then the packet retransmissions reduced significantly that conserve battery energy of a mobile device.

Sunday, January 4, 2009

Which of the following applications can run satisfactorily ?

One-way delay measurement of a corporate TCP/IP network spanning two distant sites reveals that the mean one way delay is 500 ms. Which of the following applications can run satisfactorily over this network?
(a) Voice-over IP
(b) File Transfer
(c) Web Browsing
(d) E-mail
(e) Video on demand


Following applications can sun satisfactorily over given network: (a) File Transfer (b) Web Browsing (c) e-mail. 
The reason VOIP and VOD may not run satisfactorily is that the tolerable delay of these applications are several milliseconds only. e.g. <= 400 ms for VOIP.