传输层服务和协议

• 为运行在不同主机的应用进程提供通信
• 传输协议运行在端系统
  • 发送方：将应用层的报文划分成报文段。然后传递给网络层
  • 接收方：将报文段重组成报文，然后传递给应用层

• 有多个传输层协议可供应选择

  • Internet: TCP/UDP

    传输层VS.网络层

• 网络层服务：主机之间的逻辑通信

• 传输层服务：进程之间的逻辑通信
  • 依赖于网络层的服务
    • 延时、带宽
  • 对网路层的服务机型加强
    • 数据丢失、顺序混乱、加密。就是说，传输层可以加强网络层的服务，保证数据的完整、有序

习题

1. Where is transport-layer functionality primarily implemented?

• Transport layer functions are implemented primarily at each end of a physical link connecting one host/router/switch to another one host/router/switch.
• Transport layer functions are implemented primarily at the routers and switches in the network.
• Transport layer functions are implemented primarily at the hosts at the “edge” of the network.

1. Check all of the services below that are provided by the TCP protocol.

• A message abstraction, that preserves boundaries between message data sent in different socket send calls at the sender.
• A flow-control service that ensures that a sender will not send at such a high rate so as to overflow receiving host buffers.
• In-order data delivery
• A byte stream abstraction, that does not preserve boundaries between message data sent in different socket send calls at the sender.
• Reliable data delivery.
• A congestion control service to ensure that multiple senders do not overload network links.
• A guarantee on the minimum amount of throughput that will be provided between sender and receiver.
• A guarantee on the maximum amount of time needed to deliver data from sender to receiver.

1. UDP protocal provide service

• A message abstraction, that preserves boundaries between message data sent in different socket send calls at the sender.