Interview_TCP/UDP

📌 UDP

✅ What is UDP?

- User Datagram Protocol
- conectionless
- unreliable
- check just data error with checksum

✅ What are the benefits of UDP? and the disadvantages?

👍🏻 fast
👍🏻 less overhead
👍🏻 1:1, 1:N, N:N connection possible
👎🏻 packet loss, corruption
👎🏻 data can be recieved in different order

✅ What is UDP checksum?

- to check data error
- send data with checksum header

- divide segment into 16bits
- add all, add 1, create checksum
- send data with checksum

- reciever also creates checksum for recieved data
- if it matches, no error
- if doesnt match, error in segment!

✅ Where is UDP used?

- DNS
- SNMP
- RPC, NetBios, syslog, Kerberos, TFTP
- streaming, internet calls

- allow little data loss
- need to send fast

✅ How is UDP connection?

- server, client can be connected 1:1, 1:N, N:N

✅ What is overhead?

- excess usage of computing resource
- such as processing, memory, bandwidth

- extra memory required for supplemental information
- (ex) src to destination
- space eaten up by protocols to acheive certain goal

✅ What is payload?

- actual data to be transmitted

📌 신뢰적 데이터 전송

💊 How can we prevent data loss during transmission?

- ARQ
- countdown timer
- sliding window

✅ How long should be countdown timer?

- more than travel time
- more than time reciever needs to manage segment

✅ What is ARQ?

- Automatic Repeat Request/Query

- error checking
- reciever feedback
- sender retransmit

- ⭐️ sequence number
- ⭐️ countdown timer
- ⭐️ sliding window

⭐️ What is stop and wait?

- Send and wait until ACK response from reciever
- 👍🏻 reliable, 👎🏻 but slow

✅ What is pipeline protocol?

- Send packets without waiting for ACK response
- 👎🏻 cannot solve data packet loss
- 👎🏻 recieved order might be different
- from HTTP 1.1

⭐️ What is GBN ARQ?

- Go-back-N
- send n(window size) packets if not recieved
- sliding window protocol
- cumulative acknowledgement 내가 여기까진 받았다

✅ What are the benefits, disadvantages of GBN?

- 👍🏻 pipelining
- 👍🏻 faster than stop-and-wait
- 👎🏻 send packet multiple times(lost packet + following frames)

⭐️ What is SR?

- selective repeate
- only retransmit lost packet

✅ What is the benefit of SR?

- 👍🏻 pipelining
- 👍🏻 send only lost packet

📌 TCP

✅ What is TCP?

- Transmission control protocol
- Transport layer
- conection oriented
- reliable

✅ Where is TCP used?

- HTTP
- SMTP
- FTP
- SSH

✅ What is full duplex and point-to-point?

- full duplex: 양방향
- point-to-point: 2개의 종단점으로 연결

✅ How is TCP reliable?

- checksum
- ARQ
- timer
- sequence number

✅ What does it mean to be "reliable"?

- data integrity
- data is in the right order

✅ How is TCP connection-orinted?

- pre-connection process
- 3 way handshake

✅ Explain ACK lost scenario in TCP

- Segment send to reciever, but ACK from reciever to sender was lost
- Sender sends again same packet
- Reciever already recieved, throw away

✅ Explain premature timeout scenario in TCP

- Reciever recieves, sends ACK
- However, ACK arrvies later than timeout
- Sender thinks packets didnt arrive, so sends again

✅ Explain cumulative scenario in TCP

- Sender sent segment 1, 2
- Reciever recieved, sent ACK 1, 2
- ACK 1 was lost, ACK 2 arrived
- Sender thinks as ACK 2 arrived, ACK 1 has arrived successfully
- Retransmission of data does not occur

✅ How can TCP know which segment to send?

- sequence number

✅ What are the problems that can occur in TCP?

- packet loss
- packet corruption
- reciever overloaded

✅ How can re realize that a packet is lost?

- timeout
- 3 duplicate ACK

✅ What is 3-way-handshake?

- pre-connection-process
- **SYN**
* client: SYN-SENT - **ACK**
* sever: SYN_RCVD - **SYN** * client: ESTABLISHED, sever: ESTABLISHED

✅ Why not 2-way-handshake?

- server needs to check state of client

- 이럴 때는 연결되면 안된다.
- client request is retransmission of timeout
- TCP connection is over

✅ How can we confirm message was recieved by server?

- with ACK(acknowledgement) message
- server replies with ACK message to confirm that server recieved the message
- ACK message is control signal to (1) confirm it recieved message (2) is ready to recieve message

✅ What is 4-way-handshake?

- To finish connection

- FIN
* client: FIN_WAIT
- ACK
* server: CLOSE_WAIT
- FIN
* client: TIMED_WAIT
* server: LAST_ACK
- ACK
* client: CLOSED
* server:CLOSED

✅ What is fast retransmission in TCP?

- Sender does not wait until timeout
- When Sender recieves identical ACKs for three times, retransmits
- 3 duplicate ACK

- fast bc sender does not wait for timeout

✅ What is flow control?

- prevent reciever recieving too many segments before it could handle
- 💊 control speed

💊 configure sliding window size
💊 stop and wait

✅ How does "flow control" control speed?

- take reciever's buffer size into account

💊 configure sliding window size
💊 stop and wait

✅ What is congestion control?

- prevent too many packets on network
💊 AIMD(Additive Increase Multiplicative Decrease)
💊 slow start
💊 TCP Tahoe
💊 TCP RENO

✅ What happens when network is congested?

- latency
- packet loss

TCP 🆚 GBN

- 공통점: sliding window, sendbase, sequence number
- GBN: send n(sliding window size) number of segment
- TCP: send only the missing segment

TCP 🆚 SR

- 공통점: retransmit only lost segment
- SR: each segment has timer
- TCP: single timer