The dual flow control problem of TCP over ATM ABR services

Yuan-Cheng Lai, Ying-Dar Lin, Hsiu-Fen Hung
1999 International Journal of Communication Systems  
In this paper, we investigate the dual control problem*TCP #ow control at the TCP layer and ABR #ow control at the ATM layer. First, we observe that TCP #ow control and ABR #ow control cannot co-operate well. The worst case is that the slow start after packet loss causes high but unused ACR (Allowed Cell Rate) which raises the potential of cell loss and an under#owed switch queue which reduces ABR throughput. We suggest to implement a use-it-or-lose-it policy for ABR and fast recovery for TCP
more » ... avoid these phenomena. Figure 5. Time-dependent behaviour in cell-loss-free case. (a) TCP window of SES 1. (b) ACR of SES 1. (c) Switch queue size In summary, we say that TCP #ow control and ABR #ow control cannot co-operate well because (1) the combined sending rate alternates between rate-based or window-based, i.e. the dual control cannot behave better than the single control, and (2) the adjustment frequency and the response to congestion are asynchronous. Phenomena and solutions 4.2.1. Unused high ACR and underyowed switch queue. We conducted two simulation experiments for cell-loss-free and cell-loss networks to investigate the dual control. Figures 5 and 6 show TCP window and ACR behaviour of one connection and switch queue behaviour. Since ABR #ow control is fair, one connection can represent other connections. Comparing ABR behaviour in Figures 5(b) and 6(b), in Figure 5 (b), ACR oscillates between 0)4;10 to 4;10 bytes/s, but in Figure 6 (b), there are some circumstances that cause ACR to be 316 Figure 6. Time-dependent behaviour in cell-loss case. (a) TCP window of SES 1. (b) ACR of SES 1. (c) Switch queue size very high. If any cells of a packet are lost, the destination cannot assemble the packet successfully. The lost packet is detected after receipt of three duplicate acknowledgements for fast retransmission and the TCP window is set to one packet. The drop of a TCP window and the succeeding slow start make the switch queue shrink. Hence ACR is increased to a much higher value. The high ACR is not fully used, but when the tra$c from the TCP layer to ATM grows later on, the high ACR will lead cells to swamp the switch bu!er. Cells may be lost and it is much worse in the con"gurations with a large number of connections. The high ACR should be reclaimed. The reclamation of unused bandwidth is the so-called use-it-or-lose-it policy in TM4.0. It is optionally implemented. Because a single cell-loss means an e!ective packet loss, TCP performs often a slow start, especially in congested networks. Therefore, it is important to implement the use-it-or-lose-it policy in ABR #ow control. Second, we compare the switch behaviour in Figures 5(c) and 6(c). When the packet gets lost and slow start is performed, the switch queue under#ows. Under#ow of the switch queue leads to 317 THE DUAL FLOW CONTROL PROBLEM
doi:10.1002/(sici)1099-1131(199907/08)12:4<309::aid-dac404>;2-7 fatcat:j4jdy7j6ufag5bxsjsrsq3y2ea