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AR46 Meets Resource Error because of TCP SYN FLOOD

Publication Date:  2012-07-27 Views:  69 Downloads:  0

Issue Description

A net-bar uses AR46 as egress device, and the CPU utilization is often very high.
The Resource Error occurs to the upstream interface, with impact on services.

Alarm Information

Resource Error occurs to upstream Ethernet port.

Handling Process

According to packets captured, there is a attack traffic at the upstream interface when the problem occurs, with AR46 as target, The attack is of TCP SYN FLOOD.
Deploy firewall to the network against attack. The CPU utilization is restored, and Resouce Error does not occur again.

Root Cause

Resouce Error is a unique statistics for Ethernet interface of MPU in AR46. It is defined as: once the Ethernet chip receives packets, it forwards them to CPU,
but the process needs to apply for cache; when the chip finds the cache invalid, it will report Resource Error to CPU.
If the Ethernet chip receive packets fast, and CPU cannot keep up with it, the cache queue will be congested with occupied cache. When the next packet arrives, it will generate Resouce Error, not Overrun error.
When the attack occurs, the instant traffic is very large. AR46 fails to handle TCP SYN packets in time. This results in high CPU utilization, and Resouce Error occurs to the interface.


AR46 can not forward the packets of attack in time, and it regards the error as Resouce Error, not Overrun.