Specifications

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2.6 Congestion Control
•• IP Precedence bits IP Precedence bits
•• MPLS MPLS CoS CoS bits bits
•• Incoming Physical InterfaceIncoming Physical Interface
•• Incoming Logical InterfaceIncoming Logical Interface
•• Source or Destination IP addressSource or Destination IP address
WW
RR
RR
REDRED
PLP=0
100%
100%
PLP=1
Stream
100%
Priority
Priority
QueuingQueuing
Traffic
Traffic
ClassificationClassification
Traffic Rate
Traffic Rate
PolicingPolicing
Congestion Congestion
AvoidanceAvoidance
WW
RR
RR
WW
RR
RR
REDREDREDRED
PLP=0
100%
PLP=0
100%
100%
PLP=1
100%
PLP=1PLP=1
Stream
100%
StreamStream
100%
Priority
Priority
QueuingQueuing
Traffic
Traffic
ClassificationClassification
Traffic Rate
Traffic Rate
PolicingPolicing
Congestion Congestion
AvoidanceAvoidance
2.6.1 Hardware Monitoring of Input Traffic for Congestion
The media-specific ASIC (DS3 or SONET) on each router PIC checks all input
traffic levels against the link bandwidth that is configured using the leaky bucket
algorithm. If the flow exceeds the bucket's threshold, the packets are either
dropped or tagged, depending on how the receive leaky bucket is configured. If it
is configured to tag packets, the PIC sets the packet-loss priority (PLP) bit in the
notification record associated with the packet to indicate that the packet
encountered congestion in the router. It also indicates that the packet should
have a greater probability of being dropped from the output transmission queue.
2.6.2 Monitoring of Output Queue Congestion and Dropping Packets
Random Early Detection (RED) is used for the Congestion control. While
weighted round-robin (WRR) transmits packets from the output queues, RED
attempts to manage transient and sustained congestion. RED tries to anticipate
incipient congestion and reacts by dropping a small percentage of packets from
the head of the queue to ensure that a queue never actually becomes congested.
RED examines the fullness of each output transmission queue to determine
whether it is congested. It uses two values to determine whether a output
transmission queue is congested:
§ Output queue fullness--RED calculates the fullness of the output
transmission queue by dividing the buffer being used by the total buffer
allocated for that queue.
§ Drop profile--You specify the drop probabilities for different levels of buffer
fullness.
RED uses two drop profiles to determine whether to drop a packet. One drop
profile is applied to the entire traffic stream passing out through a link. The
second profile is applied to individual output transmission queues depending on
whether the queue is congested. A packet must pass both the stream and packet
profile tests before being dropped by RED. For each output transmission queue,
there are two drop profiles for queue congestion: one queue for packets whose
PLP is set and one for packets whose PLP bit is not set.
Generally, you more aggressively drop packets in which the PLP bit is set
because they have experienced congestion, a likely indication that there is
congestion between the packet's source and the local router. When the sender
discovers that the packet has been dropped (because it does not receive an
acknowledgment from the destination), it throttles the rate at which it sends
packets, providing some relief to the congestion on the local router.
The figure below shows three drop profiles--mappings between queue fullness
and drop probabilities for a link's stream (on the left), packets in which the PLP bit
is not set (in the middle), and packets in which the PLP bit is set (on the right).
With these drop profiles, if the stream is 100 percent full and if the queue is 50