School of Computing and Information Systems
COMP30023: Computer Systems
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Tutorial Week 10
Transport-Layer Services, Multiplexing & Demultiplexing
1. Suppose a process in Host C has a UDP socket with port number 6789. Suppose both Host
A and Host B each send a UDP segment to Host C with destination port number 6789. Will
both of these segments be directed to the same socket at Host C? If so, how will the process
at Host C know that these two segments originated from two different hosts?
2. Is it possible for an application to get reliable data transfer even when the application runs
over UDP? If so, how?
3. Indicate whether TCP or UDP (or both or neither) provide the following services to appli-
(a) Reliable data transfer between processes
(b) Minimum data transmission rate between processes
(c) Congestion-controlled data transfer between processes
(d) A guarantee that data will be delivered within a specified amount of time
(e) Preservation of application-level message boundaries. That is, when a sender sends a
group of bytes into a socket via a single send operation, that group of bytes will be
delivered as a group in a single receive operation at the receiving application
(f) Guaranteed in-order delivery of data to the receiver
4. Why does UDP exist? Would it not have been enough to just let the user processes send
raw IP packets?
5. Both UDP and TCP use port numbers to identify the destination entity when delivering
a message. Give two reasons for why these protocols invented a new abstract ID (port
numbers), instead of using process IDs, which already existed when these protocols were
6. What are the guarantees that a reliable data transfer must provide? (Look this up in the
text book or on the web.)
7. A process on host 1 has been assigned port p, and a process on host 2 has been assigned
(a) Is it possible for there to be two or more TCP connections between these two ports at
the same time?
(b) Is it possible for there to be more than one TCP connection on port p of host 1 at a
Weekly tutorial participation activity
To obtain a weekly tutorial mark (1% of your overall mark for the subject, totalling to 10% over
the semester), please answer the following questions in Canvas Quiz (called Week 10 Tutorial
Activity). You can have multiple attempts but need to submit the quiz by 11:59 pm AEST on
the day of your tutorial. Only answering all questions correctly will give you the weeks mark.
During the tutorial, your tutor will provide you with the access code that will unlock the quiz for
the corresponding week. The access code is valid only for students in this tutorial. As a result,
we expect you to attend your tutorial class as otherwise the access code you obtain in another
tutorial will not work for you.
If you have a valid reason for not being able to attend your tutorial, please fill in the form accessible
via Canvas by Friday 8pm AEDT of the corresponding week. You will be given an access code
during the next business day. The code will not be provided otherwise, hence, please do not email
the subject coordinator or other staff members including your tutor asking for the code. We will
monitor such requests and may limit the number of times an access code is given to the same
studentthroughout the semester to encourage participation and attendance of the tutorials.
1. For a TCP connection in which A sends data to B, how many ports in total are needed?
2. For a TCP connection in which A sends data to B and B sends data to A, how many ports
in total are needed?
3. Which of the following is a service that UDP provides that IP doesn’t?
(a) Reliable connectionless data transfer.
(b) Allow a pair of nodes (i.e., hosts) to exchange multiple separate streams of data.
(c) Route a packet from one location to any other in the network.
(d) Provide lower delay for video and voice packets than other packets
4. True or false? TCP could have been designed without port numbers, instead using the 16
bit process ID of the process that created the socket to distinguish between sockets.
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