more than 1s takes to send packet during bulk transmission between source to sink

[JIBIN P JOHN]

Hello,

I am sending a large number of data (around 10 MB) from source to destination through the tcp socket in a traffic-free network.  In some simulations, it sends the packet to the destination without any problems. But during some bulk transfer, it takes a relatively huge amount of time than the normal. Then i have checked the pcap traces and found that, at some particular time sender waits morethan 1s to send next packet after the ACK is received.  I have attaached the screen shot of pcap trace, there you can see a sudden jump of 1s. Because of this my results looks very ugly. could someone suggest why it is happening ? is it tcp's expected behavior or is there some problem with ns3