Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 17 | Networking
17.5 What are the differences between TCP and UDP? Explain how TCP handles reliable
delivery (explain ACK mechanism), flow control (explain TCP sender’s / receiver’s window)
and congestion control.
SOLUTION
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TCP (Transmission Control Protocol): TCP is a connection-oriented protocol. A connection can
be made from client to server, and from then on any data can be sent along that connection.
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Reliable - when you send a message along a TCP socket, you know it will get there unless
the connection fails completely. If it gets lost along the way, the server will re-request
the lost part. This means complete integrity; data will not get corrupted.
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Ordered - if you send two messages along a connection, one after the other, you know
the first message will get there first. You don’t have to worry about data arriving in the
wrong order.
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Heavyweight - when the low level parts of the TCP “stream” arrive in the wrong order, resend
requests have to be sent. All the out of sequence parts must be put back together,
which requires a bit of work.
UDP(User Datagram Protocol): UDP is connectionless protocol. With UDP you send messages
(packets) across the network in chunks.
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Unreliable - When you send a message, you don’t know if it’ll get there; it could get lost
on the way.
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Not ordered - If you send two messages out, you don’t know what order they’ll arrive in.
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Lightweight - No ordering of messages, no tracking connections, etc. It’s just fire and
forget! This means it’s a lot quicker, and the network card / OS have to do very little work
to translate the data back from the packets.
Explain how TCP handles reliable delivery (explain ACK mechanism), flow control (explain TCP
sender’s/receiver’s window).
For each TCP packet, the receiver of a packet must acknowledge that the packet is received.
If there is no acknowledgement, the packet is sent again. These guarantee that every single
packet is delivered. ACK is a packet used in TCP to acknowledge receipt of a packet. A TCP
window is the amount of outstanding (unacknowledged by the recipient) data a sender can
send on a particular connection before it gets an acknowledgment back from the receiver
that it has gotten some of it.
For example, if a pair of hosts are talking over a TCP connection that has a TCP window with
a size of 64 KB, the sender can only send 64 KB of data and then it must wait for an acknowledgment
from the receiver that some or all of the data has been received. If the receiver
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