1. Tiktok

1.1. TCP/IP 5 layers protocol

physical layer: ethernet pysical signal

Data Link layer: Ethernet, MAC, ARP, VLAN

Nework Layer: IP, ICMP (ping, traceroute, ), BGP, IPsec

Transport Layer: TCP / UDP

Applicaiton Layer: HTTP, HTPPs, FTP, DNS, SSH, DNS

1.2. layer 2 CRC

In Ethernet: a 32-bit CRC (Frame Check Sequence, FCS) is appended to each frame. The receiver recalculates CRC and compares with the received CRC: If mismatch → frame discarded (hardware drop, not passed to IP layer).

This part is ususally done by the NIC.

1.3. TCP/IP packet 校验和计算 checksum

sender side: header + data, using 16 bits (2 bytes) as a group, sum them up and do negation we can get the check sum receiver side: also 16 bits as a group, add the header, check sum and data, the final result should be FFFF

1.4. what if the IP packets is too large to fit into the ethernet frame?

Layer 4 TCP segment and Layer 3 IP fragmentation.

1.5. Does TCP segmentation cause later segments to contain the TCP header?

yes, each TCP segment is independent from each other, and each contains different seq number, which ensures ack and segment replay.

1.6. TCP 3 way handshake connection

It is the connection establishment process in TCP (Layer 4) ensuring: Both client and server agree on starting sequence numbers (propsed by each side). Both sides are ready to transmit and receive data reliably. 

1. SYN (Client → Server)
Client sends a SYN packet to initiate the connection. Chooses an Client side Initial Sequence Number (ex: ISN_C = 10). No payload (only header). Header Flag (SYC = 1)

Purpose: Client requests a connection. Informs the server of the client's starting sequence number for the session.

2. SYN-ACK (Server → Client)
Client sends back an ACK packet to the server. Propose Server side Initial Sequence Number. Ack back client side sequence number, Flags (SYC = 1, ACK = 1). No

3. ACK  (Client → Server)
Client sends back an ACK packet to the server. Ack back Server side sequence number

Both client side and server side maintains its own sequence number, so that's why the thrid step is still need for client to ack back to server.

1.7. TCP 4 way termination

TCP 4-way termination is the connection closing procedure in TCP, ensuring graceful, reliable, bidirectional shutdown of a TCP connection.

Why 4-Way? Each direction must be closed independently. Termination requires: Each side to send a FIN (to indicate no more data to send). Each side to ACK the peer’s FIN. 4 control packets (FIN, ACK, FIN, ACK) → “4-way” termination. 

1. Active Close (Client sends FIN)
The client (or the actively closing side) sends: FIN = 1 Sequence Number = X

2. Passive Close Acknowledgment (Server sends ACK)
The server (or passively closing side) replies: ACK = 1 Acknowledgment Number = X + 1

At this point, Client → Server direction is closed for sending (half-close).
Server can still send data to the client.

3. Passive Close (Server sends FIN)
When the server finishes sending remaining data, it sends: FIN = 1 Sequence Number = Y

4. Active Close Acknowledgment (Client sends ACK) The client replies: ACK = 1 Acknowledgment Number = Y + 1

Why 3 way handshke only needs 3 compared with 4 way handshake? 

In the second SYN-ACK, the server do propse new seq number and accept client sequence number. two actions in one step simoutinuously.

For 4 way handshake, the server on the third step side may need to finsih the data flush after close connection with client

1.8. Why is TimeWAIT needed in TCP?

TIMEWAIT happens only on the side that actively closes the connection. (The one First send the FIN or the one sends the final ACK) (In the above example: it’s the client) On the passive closer’s side: after sending its FIN and receiving the ACK, it immediately transitions to the CLOSED state.

Avoid delayed packets interfere with the next new connections: Allows retransmission of the final ACK if the peer’s FIN is retransmitted.

1.9. What are the 4 primary timers used by TCP

1. retransmission timer (RTO)
Detects lost segments. Triggers retransmission if an ACK is not received within the calculated RTO (Retransmission Timeout).

2. Persist Timer
TCP uses flow control with the Window Size (rwnd) to indicate how much data the receiver can accept.
If the receiver’s window = 0, it tells the sender: "I cannot receive any more data now."
The persist timer ensures that the sender periodically probes the receiver to check if the window has opened


3. Keepalive Timer
Detects dead TCP connections when there is no activity for a long period. Used to ensure the peer is still alive.

4. TIME_WAIT Timer
Ensures the connection remains in TIME_WAIT state after connection termination.

1.10. how does IP longest

Trie, Masks which can perform parallel matching