Mobility support on only lower layer is not enough to provide mobility support for applications.
As application is directly communicates with transport layer only such as TCP ( Transmission Control Protocol) or User datagram Protocol (UDP)
The main functions of transport layer in the internet are
Check summing over user data.
Multiplexing/Demultiplexing of data from/to applications.
TCP is the dominant support in the internet and supports many of the most popular internet applications, such as world wide web (WWW) file transfer and e-mail.
The network layer only addresses a host, ports in UDP or TCP allow dedicated applications to be addressed.
The Connectionless UDP does not offer much more than this addressing, so TCP will be used.
The UDP is connectionless and does not give certain guarantees about reliable data delivery.
TCP is much more complex and needs special mechanisms to be useful in mobile environments.
The main difference between UDP and TCP is that TCP offers connections between two applications.
Within a connection TCP can give certain guarantees, such as in-order delivery or Reliable data transmission using retransmission techniques.
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