DECbit

DECbit is a technique implemented in routers to avoid congestion. Its utility is to predict possible congestion and prevent it. This protocol works with TCP.

When a router wants to signal congestion to the sender, it adds a bit in the header of packets sent. When a packet arrives at the router, the router calculates the average queue length for the last (busy + idle) period plus the current busy period. (The router is busy when it is transmitting packets, and idle otherwise). When the average queue length exceeds 1, then the router sets the congestion indication bit in the packet header of arriving packets.

When the destination replies, the corresponding ACK includes a bit of congestion. The sender receives the ACK and calculates how many packets it received with the congestion indication bit set to one. If less than half of the packets in the last window had the congestion indication bit set, then the window is increased linearly. Otherwise, the window is decreased exponentially.

This technique provides distinct advantages:

• Dynamically manages the window to avoid congestion and increasing freight if it detects congestion.
• Try to balance bandwidth with respect to the delay.

Note that this technique does not allow for effective use of the line, because it fails to take advantage of the available bandwidth. Besides, the fact that the tail has increased in size from one cycle to another does not always mean there is congestion.

See also

Other congestion avoidance techniques for TCP

• RED
• TCP Vegas

References

1. A Binary Feedback scheme for Congestion Avoidance