UDP Socket Coding Back

• UDP
  • does not ensure for the arrival of datagrams
    • solved by overtime or Resend
    • solved by Serial Number of datagrams
  • does not ensure for the order of datagrams
  • does not have Flow Control
    • solved by Programming
• Proerties of connected UDP
  • clients should call connect(), but there is not the process of three-way handshaking. connect() is used to mark down IP and port
  • do not need to set the address of the server when sending datagram
  • can only receive datagrams from the specifical server.
• Server of UDP:
  • do not accept, but bind to the port
  • loop for handling datagrams from different clients.
• Client of UDP:
  • do not connect, but choose a port when firstly calls sendto()
  • can receive any datagram from any host
  • will probably be blocked forever
• Model of UDP Socket Coding

1. recvfrom()

• recvfrom get one datagram from buffer each time, and will be blocked when there is no datagram
• empty datagram does not mean that the opposite end has stopped to send

method

int recvfrom(int sockfd, void* buf, int len, unsigned char flags, struct socketaddr* from, socklen_t* addrlen)

parameters

• 前四個參數與recv()相同
• from: 發送者socket地址, NULL表示不需要
• addrlen: socket地址長度, from為NULL時必須設置為NULL

return value

• >=0: success
• -1: failure

2. sendto()

• the socket address of receiver must be clear when calling sendto()
• there is no sending buffer for UDP
• allow to send empty datagram, and return 0 after calling sendto()

method

int sendto(int sockfd, const void* buf, int len, unsigned char flags, struct socketaddr* to, int tolen)

parameters

• 前四個參數與send()相同
• to: 接收者socket地址
• tolen: socket地址長度

return value

• >=0: success
• -1: failure

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