Technical Goals of Flow Control for Supporting ATM ABR Services

Flow control mechanisms designed to support ATM ABR services should meet a variety of technical goals, including the following:

  • Data should rarely, if ever, be discarded due to exhaustion of node buffer memory. As mentioned above, such data may have to be retransmitted after a possibly lengthy time-out period, further contributing to network congestion and the delay experienced by the user.
  • Network links should be used at full capacity whenever possible. For instance, if one connection sharing a link reduces the rate at which it sends, the others should increase their rates as soon as possible. In particular, as illustrated in Figure 5, the flow control mechanism should allow ABR traffic to fill in, instantly, unused bandwidth left on the link after guaranteed traffic is served.
  • All the connections that are constrained by a bottleneck link should get fair shares of that link.
  • The flow control mechanism should be robust. Loss or delay of control messages, and admission of additional connections while maintaining the total traffic load, for instance, should not cause increased congestion.
  • The network administrator should not have to adjust any complex parameters to achieve high performance.
  • The flow control mechanism should have a cost commensurate with the benefits it provides.

Generally speaking, some existing LANs such as Ethernets have satisfied these goals. This explains at least partially why they have been used widely for data applications.

New high-speed networks, such as ATM networks and switched Ethernets, use switches to achieve high performance. They are unlike conventional Ethernets, which use shared media. End systems on switch-based networks cannot monitor network congestion as easily as can end systems on shared-medium networks. Designing flow control schemes to satisfy the above technical goals for these new switch-based networks—especially for wide area networks (WANs)—is a significant challenge.

Figure 5

Available bit rate (ABR traffic filling in bandwidth slack left by guaranteed traffic, to maximize network utilization).

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