Weighted Fair Queuing (cont.)

Weighted Fair Queuing (cont.)

• WFQ uses per-flow FIFO queues.

• Weighted Fair Queuing System

• Hardware
• Queuing System

• Flow 1?

• Queue 1

• WFQ
• Scheduler

• Interface

• Forwarded Packets

• Hardware Q

• Flow 2?

• Queue 2

• Flow N?

• Queue N

• WFQ drop

• WFQ drop

• WFQ drop

Weighted Fair Queuing Implementations

• Implementation parameters:
• Queuing platform: central CPU or VIP
• Classification mechanism
• Weighted fairness
• Modified tail drop within each queue

WFQ Classification

• IP TCP Payload

• Src.
• Addr.

• Dest.
• Addr.

• Protocol

• ToS

• Src.
• Port

• Dest.
• Port

• Hash Algorithm

• #queue (index of the queue)

• Packets of the same flow end up in the same queue.
• The ToS field is the only parameter that might change, causing packets of the same flow to end up in different queues.

• WFQ classification uses these parameters:
• Source IP address
• Destination IP address
• Source TCP or UDP port
• Destination TCP or UDP port
• Transport protocol
• Type of service (ToS) field

• A hash algorithm is used to produce the index of the queue where the packet is enqueued.

WFQ Classification Details

• A fixed number of per-flow queues is configured.
• A hash function is used to translate flow parameters into queue number.
• System packets (eight queues) and RSVP flows (if configured) are mapped into separate queues.
• Two or more flows could map into the same queue, resulting in lower per-flow bandwidth.
• Important: The number of queues configured has to be larger than the expected number of flows.

WFQ Insertion and Drop Policy

• WFQ has two modes of dropping:
• Early dropping when the congestive discard threshold (CDT) is reached
• Aggressive dropping when the hold-queue out limit (HQO) is reached
• WFQ always drops packets of the most aggressive flow.

WFQ Insertion and Drop Policy (cont.)

• HQO (hold-queue out limit) is the maximum. number of packets that the WFQ system can hold.
• CDT (congestive discard threshold) is the threshold when WFQ starts dropping packets of the most aggressive flow.
• N is the number of packets in the WFQ system when the Nth packet arrives.

• N>CDT?

• N>HQO?

• Worst
• Finish
• Time?

• Worst
• Finish
• Time?

• Enqueue
• Packet

• Nth Packet

• Drop the packet with the worst finish time (old) and enqueue the Nth packet (new).

• No

• No

• Yes

• Yes

• Yes

• No

• No

• Yes

• New

• Old