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)

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. 

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. 

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.

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. 

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.

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

Answer: 

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.