P4-enabled Network-assisted Congestion Feedback:

A Case for NACKs

Anja FeldmannBala Chandrasekaran, Emilia Weyulu, Seif Fathalli

Motivation – some observations

• Buffers‣ Are getting shallow (relative to link speed)‣ Should be shallow to ensure short, deterministic delays

• Traffic ‣ Mice vs. Elephants‣ Elephants contribute most traffic – React to congestion signal‣ Mice contribute most flows – Not subject to congestion control‣ Separating mice from elephants can be beneficial

• Today’s switches

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Background – Buffer Occupancy

• NS2 simulations results• Mice can fill any queue as they

are not subject to congestion control

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Server

Clients

Used to limit capacity1.5 Mbits to 3 Mbits

Used to vary delay640 ms or 40 ms

40-100 Kbits/second

Used to measure before bottleneck

Infinite sources Web like sources

Motivation – some observations

• Buffers‣ Are getting shallow (relative to link speed)‣ Should be shallow to ensure short, deterministic delays

• Traffic ‣ Mice vs. Elephants‣ Elephants contribute most traffic – React to congestion signal‣ Mice contribute most flows – Not subject to congestion control‣ Separating mice from elephants can be beneficial

• Today’s switches ‣ Enable gathering of fine-grain statistics – The P4 advantage‣ Enable use of multiple queues – The QoS leftover

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Today‘s Congestion Inference

• Left to the end-systems

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SourceDestination

Implicit(delay, discard)

Explicit(binary, rate)

Policing

Choke Packet

Back Pressure

Today

Today

Archaic

Archaic

Tomorrow‘s Congestion Inference

• Network-assisted• Network-assisted Congestion Notification (NCF)

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SourceDestinationNCF

NCF – Insights

• Network (switches/routers) can initiate NACKs with P4 ‣ Trim packets in data plane‣ Reflect packet as NACK back to source

• Address elephants and mice‣ Detect elephants in data plane‣ Use separate queues

• Three queues suffice‣ Dynamically allocate memory (to keep buffers where needed)‣ Congestion signals independent of queue usage

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Network-assisted Congestion Feedback (NCF)

• Rolling Sketches (Counters)‣ How

- Rolling: Multiple time windows- Counters: Per flow per time window- Mice: Sum of counters < threshold- Elephant: Sum of counters > threshold

‣ Advantages- Small memory foot-print- Efficient and flexible- Feasible: Due to skewed flow distributions

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Advantages NCF

• Fast direct feedback directly to source(includes lost packet info)

• Separation of Mice and Elephants• Flexible allocation of buffer memory

• Design‣ Is work-conserving‣ Is scalable‣ Shares resources fairly‣ Ensures short queues‣ Can handle TCP Incast challenge‣ Incrementally deployable

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SourceDestinationNCF

Status

• Prototype implementations‣ P4‣ NS3

• First results look promising

• Needed: NCF enabled congestion control algorithm

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NCF – Insights

• Network (switches/routers) can initiate NACKs with P4 ‣ Trim packets in data plane‣ Reflect packet as NACK back to source => Fast congestion feedback

• Separate elephants and mice‣ Detect elephants in data plane‣ Use separate queues

• Three queues suffice‣ Dynamically allocate memory (to keep buffers where needed)‣ Congestion signals independent of queue usage

11MPI INF | Anja Feldmann