When congestion builds up in a packet network, two general approaches are possible to cope with the shortage of buffer space. One approach is to drop incoming packets for the buffer that is not available and to rely on the end-to-end protocols for the recovery of lost packets. The alternative approach is to insist that no packets should be dropped inside a packet network, even when congestion builds up. One way to accomplish this goal is to have the congested nodes send backpressure feedback to neighboring nodes, informing them of unavailability of buffering capacity and in effect stopping them from forwarding packets until enough buffer becomes available. While there are potential advantages in backpressured networks that do not allow packet dropping, such networks are susceptible to a condition known as deadlock in which throughput of the network or part of the network goes to zero (i.e., no packets are transmitted). In this paper, we describe a simple, lossless method of preventing deadlocks and livelocks in backpressured packet networks. In contrast with prior approaches, our proposed technique does not introduce any packet losses, does not corrupt the packet sequence, and does not require any changes to packet headers. In addition to presenting the new congestion control protocol in a general context, we describe an important application of the technique to Gigabit Ethernet (IEEE 802.3z).
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