Homework $1$

Using the layer models in Figure $2\mathrm{.}15,$ describe the ordering and delivery of a pizza,

indicating the interactions at each level.

a$.$ The French and Chinese prime ministers need to come to an agreement by tele$-$

phone, but neither speaks the other's language. Further, neither has on hand a

translator that can translate to the language of the other. However, both prime

ministers have English translators on their staffs. Draw a diagram similar to

Figure $2\mathrm{.}15$ to depict the situation, and describe the interaction at each level.

b$.$ Now suppose that the Chinese prime minister's translator can translate only into

Japanese and that the French prime minister has a German translator available.

A translator between German and Japanese is available in Germany. Draw a

new diagram that reflects this arrangement and describe the hypothetical phone

conversation.

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 $(N-1)-$level segment con$-$

tain 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?

A TCP segment consisting of $1500$ bits of data and $160$ bits of header is sent to the IP

layer, which appends another $160$ bits of header. This is then transmitted through two

networks, each of which uses a $24-$bit packet header. The destination network has a

maximum packet size of $800$ bits. How many bits, including headers, are delivered to

the network layer protocol at the destination?

Why is UDP needed? Why can't a user program directly access IP$?$

IP$,$ TCP$,$ and UDP all discard a packet that arrives with a checksum error and do not

attempt to notify the source. Why?