i$.(2$ points$)P2\mathrm{.}16$

TFTP$,$ like most protocols will never send an error packet in response to an error packet it

receives. Why?

ii$.(2$ points$)$ P$2\mathrm{.}17$

We have seen that in order to deal with lost packets, TFTP implements a timeout$-$and$-$

retransmit scheme, by setting a retransmission timer when it transmits a packet to the remote

host. Most TFTP implementations set this timer to a fixed value of about $5$ seconds. Discuss

the advantages and the disadvantages of using a fixed value for the retransmission timer.

b$.(2$ points$)P2\mathrm{.}6$

Application

byte stream

TCP

segment

IP

datagram

Network$-$level

packet

Figure $2\mathrm{.}5$ Protocol Data Units $($PDUs$)$ in the TCP$/$IP Architecture

In Figure $2\mathrm{.}5,$ exactly one protocol data unit $($PDU$)$ in layer $N$ is encapsulated in a PDU at

layer $(N-1).$ It is also possible to break one $N-$level PDU into multiple $(N-1)-$level PDUs

$($segmentation$)$ or to group multiple $N-$level PDUs into one $(N-1-$level PDU $($blocking$).$

a$.$ In the case of segmentation, is it necessary that each $(N-L)-$level segment contain a copy

of the $N-$level header?

b$.$ In the case of blocking, is it necessary that each N$-$level PDU retain its own header, or can

the data be consolidated into a single $N-$level PDU with a single $N-$level header?