2 - Communication and Sockets

Question 1

What is unicast and multicast?

Answer 1

Unicast - message to 1 receiver
Multicast - message to 1+ receiver

Question 2

Write down the flow diagram of sockets

Answer 2

Create socket (CS)
bind to socket (S)
connect to server (C)
listen on socket (S)
accept connection (S)
send/receive loop (CS)

Question 3

What happens when you fetch a webpage on the socket level?

Answer 3

1) Open socket to port 80 on a server
2) Send it an HTTP request
3) Wait for response
4) Server closes connection

Question 4

What encoding do sockets send?

Answer 4

Binary (used instead of text when bandwidth is limited, or battery life needs to be saved)

Question 5

What are things in binary that we can’t encode (at least for free) and how do we fix it?

Answer 5

Circular linked list, file-like objects (socket/etc)

To fix, have an “encode” function to flatten object

Question 6

Describe when multicast is useful, and its limitations

Answer 6

Useful for: Needing to talk to many servers (Twitch, video game multiplayer servers)

Limitations: Sent out of order, no acks, data loss

Question 7

What are the pros and cons of polling for a server?

Answer 7

Pros:
can still doing some things while loading/waiting, can get whatever is there (even if just a little bit) e.g. low quality YouTube video

Cons:
- possible infinite loop, high CPU usage maybe

Question 8

What’s the third option that’s not blocking or non-blocking?

Answer 8

Block to sync, if something weird happens, timeout socket to close

Question 9

In a very general and lazy sense, when to use TCP and UDP?

Answer 9

TCP does things for us on the protocol level that we don’t have to worry about

Use UDP when TCP is not suitable

Question 10

How to get message back with UDP?

Answer 10

Along with your message, send your return address (also could be anything, DOSing)

• TCP does this already

Question 11

What’s the function for TCP to keep a connection?

Answer 11

Select! Allows to keep sending messages as they wish. Select unblocks when something happens

Question 12

What are the TCP advantages?

Answer 12

1) Connection-oriented
2) Reliable, delivery and order is guaranteed

• Generally used for most applications

Question 13

What are the TCP disadvantages?

Answer 13

• more bandwidth (lots of protocol overhead)
• no broadcast/multicast support
• slower than UDP

Question 14

What are the UDP advantages?

Answer 14

• connectionless (no 3-way handshake)
• support multicast
• fast + lighter than TCP
• support live/real-time data transmission

Question 15

What are the UDP disadvantages?

Answer 15

• messages can be out of order
• unreliable data transfer
• usually treated as less important by router than TCP