Information Technology Reference
In-Depth Information
d. If the packet is normal priority, the packet is dropped, and the input drop counter
is incremented.
e. If the input hold queue is not full, the packet is kept, and the input hold queue
counter is incremented.
The packet is processed.
3
The input hold queue is decremented.
4
The three major components of the packet-reception process are each a possible discard
location. The queue optimization involves tuning each of these components to minimize
dropping legitimate packets.
Hold Queue Optimization
The input hold queue has a default value of 75. In an environment with a larger number of
BGP sessions, this is insufficient to hold a rush of incoming TCP ACKs. The following
interface configuration command is used to change the size of the input hold queue:
hold-queue
value
in
The size change is
nonimpacting
, meaning that it can be configured without causing nega-
tive impact on a running router. The size of the input hold queue is shown in the output of
the
show interfaces
command.
In determining the value to use when configuring the input hold queue, use the worst-case
scenario to ensure that the hold queue size is sufficient. You can determine the maximum
possible number of TCP ACKs that will be waiting to be processed by checking the
following TCP parameters for the BGP sessions:
•
TCP window size and TCP MSS
—TCP window size is a static 16 KB. The TCP
window is the amount of information a TCP session can transmit before it must
receive an ACK. TCP MSS indicates how much data is sent in each packet.
The TCP MSS and the TCP window size together determine the maximum number of
outstanding TCP packets requiring acknowledgment.
The TCP window size of 16,000 and a default TCP MSS of 536 indicate that 29 packets
can be sent before an acknowledgment is required. A TCP MSS of 1460 allows 10
unacknowledged packets, and a TCP MSS of 4430 allows three packets.
To optimize TCP ACK generation, only every other TCP packet generates a TCP
ACK. The TCP ACK acknowledges all the TCP packets received up to the sequence
number in the TCP ACK. This reduces the TCP ACK count to 50% of the TCP data
packet count.
