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Networking

  • UDP vs TCP
    • UDP is connectionless
      • Good for VPN tunneling / streaming / gaming / DNS / VoIP
    • TCP is connection-oriented
      • Provides error-checking, guarantees delivery & consistency of data
      • Flow control: send buffer, receive buffer
        • Receiver will drop packet if the buffer is full
        • Receiver will tell sender how much room is left in the receive buffer
      • Good for HTTP/HTTPS/SSH/FTP/SMTP/IMTP/POP protocols
      • Congestion control
  • TCP 在网络波动下的行为是什么样的？
  • Socket API
    • Socket pair
    • Each socket has two important attributes: a communication domain and a type. There are two main types, stream and datagram.
    • Server Lifecycle
      • create a new socket and bind to a port (externally visible)
      • listen on the socket -> mark it as passive for accepting incoming connection requests
      • accept an incoming connection (blocking if not any), and create a new socket for it.
      • read and write to the socket (blocking -- depending on available data/space in buffer)
      • close the connection after done
    • Client lifecycle
      • create a new socket
      • connect to a passive socket.
      • read and write to the socket ...
    • Socket type for IP socket
      • STREAM for TCP
      • DATAGRAM for UDP
    • Socket type for UDS: https://stackoverflow.com/questions/13953912/difference-between-unix-domain-stream-and-datagram-sockets
  • TCP/IP model
    • App -> Transport -> Internet -> Network access
      • Network access: Hardware addressing and physical transmission of data (WiFi? Wired? Ethernet vs. Infiniband?)
      • Internet layer
        • IP
        • Address Resolution Protocol -- ARP. Find MAC etc.
        • ICMP (providing hosts with information about the network problems)
      • Transport: TCP/UDP
      • Application: ...
  • Explain session?
  • HTTP/2
    • Problem of HTTP 1.0 -- connection can't be reused, head of line blocking
      • Each connection requires time for slow start (throughput) and handshake (latency)
      • HoL blocking: earlier request blocks later request
    • Reuse connection
      • Connection: Keep-Alive (normally within a minute for web, for mobile people typically use ad-hoc long connection)
      • WebSocket (long connection): support message
      • http long polling: establish connection before the data is prepared
    • HoL blocking
      • pipelining the requests
        • Only for GET/HEAD etc. ... similar to pipelining the CPU instructions
    • HTTP/2 solutions
      • Frame format is similar to TCP (binary payload)
        • So one connection can be used for sending multiple frames.
      • Avoid sending duplicates headers by caching
      • Server can send many things even if only HTML is requested
  • Explain HTTPS?
    • Encrypt traffic
      • 对称加密，每个 connection 对应唯一的密钥
    • Check integrity
      • Digital signature: From CA (digest algorithm)
    • Check ID (authenticity) of website
      • 非对称加密: encrypt request using pubkey From CA
      • pubkey
      • cert: CA generated cert for server from submitted & verified pubkey
        • Signature is part of the cert, cert includes many other info as well
        • cert is issued by server back to client
        • cert is checked by client using CA's pubkey' in order to trust the pubkey inside cert
    • Flow (TSL handshake)
      • client -> https request -> server
      • server turns cert
      • client checks cert
        • if good, generate symkey and send to server after encryping using pubkey in cert
          • server establishes symmetric encrypted connection
    • HTTPS is just HTTP over SSL
  • Networking code optimizing/tuning?
  • How does network look like?
  • How does a network packet look like?
    • The lower protocol header is on the outside
  • TCP connection stage
    • Three-way handshake for opening
    • Four-way handshake for terminating
      • Client -> FIN "Hi, I want to terminate" -> Server
      • Server -> ACK "okay, I will start cleaning up for it" -> Client
      • Server -> FIN "Hi, I'm finished, cya!" -> Client
      • Client -> ACK "Cya!" -> Server
      • https://stackoverflow.com/questions/46212623/why-tcp-connect-termination-need-4-way-handshake
        • Because setup doesn't take time in the server end, but closing does (sending remaining data)
      • What is TCP ACK seq?
        • The sequence number is the byte number of the first byte of data in the TCP packet sent (also called a TCP segment). The acknowledgement number is the sequence number of the next byte the receiver expects to receive.
      • Connection reset: RST：（Reset the connection）用于复位因某种原因引起出现的错误连接，也用来拒绝非法数据和请求。如果接收到 RST 位时候，通常发生了某些错误；发送 RST 包关闭连接时，不必等缓冲区的包都发出去，直接就丢弃缓冲区中的包，发送 RST；接收端收到 RST 包后，也不必发送 ACK 包来确认。
    • Fast open: ??
    • TCP Connection states
  • TCP 粘包?
  • QUIC, BBR?
  • Infiniband vs Ethernet
    • Infiniband supports RDMA
  • How to maximize the socket performance?
  • Imagine a user sitting at an Ethernet-connected PC. He has a browser open. He types "www.google.com" in the address bar and hits enter.
    • Now tell me what the first packet to appear on the Ethernet is.
    • It depends...
  • asynchronous and synchronous sockets?
  • Explained p2p protocol?
  • Nagle's algorithm and TCP optimization
    • https://www.extrahop.com/company/blog/2016/tcp-nodelay-nagle-quickack-best-practices/
    • Nagle: improving TCP efficiency by reducing the number of small packets sent over the network.
    • TCP delayed acknowledgment or Delayed ACK is another technique used by some implementations of the TCP in an effort to improve network performance and reduce congestion.
    • Nagle's algorithm and Delayed ACKs together create a problem because Delayed ACKs are waiting around to send the ACK while Nagle's is waiting around to receive the ACK!
    • Nagle's algorithm is undesirable in highly interactive environments.
    • TCP_NODELAY socket option allows your network to bypass Nagle Delays by disabling Nagle's algorithm
    • To disable Delayed ACKs, use the TCP_QUICKACK socket option.
    • Design a protocol for communicating between a client and a server for sending audio/video
      • https://juejin.im/post/6867043669682651143
        • Reliable UDP: QUIC, WebRTC, Aeron etc.
          • QUIC (Quick UDP Internet Connection):
            • QUIC 通过 DH 算法创建一个安全的连接后，客户端会缓存起来原始的连接信息等。在后续的过程中只要和同一个服务器建立链接都是直接发送数据，不需要再次协商秘钥，从而实现了后续的 0RTT。

            • QUIC focus on handshake optimization (very important!) QUIC does bring (at least) twice as fast connection establishment and reduces dramatically the impact of handover between different networks.

    • How does TCP windows work and what can I do to optimize performance for an application that performs lots of large reads across continents.
  • syn 洪泛攻击，如何应对？
  • DNS is using UDP than TCP
    • It is much faster
    • Fits in the UDP packet
    • Reliability can be added at app-layer
  • HTTP protocol
  • 缓存并发问题: https://www.cnblogs.com/dinglang/p/6133501.html