Latency In DOCSIS Networks - Internet Society

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Latency in DOCSIS NetworksGreg WhiteSept. 26, 2013 2013 CableLabs . All rights reserved.

The various DOCSIS versionsFrom a latency perspective DOCSIS 1.0 – ca. 1996, deployments 1998– Fundamental request-grant upstream MAC layer definitionDOCSIS 1.1 – ca. 1999, deployments 2001– Additions for configured Quality of Service Packet classifiersFlow independenceQoS configuration per flowDOCSIS 2.0 – ca. 2001, deployments 2003– TCP ACK suppression/prioritizationDOCSIS 3.0 – ca. 2006, deployments 2007– Buffer Control feature added in 2011DOCSIS 3.1 – ca. 2013, deployments 2015– Light Sleep Mode– AQM MandatorySlide 2 2013 CableLabs . All rights reserved.

DOCSIS Request-Grant Upstream MAC Upstream channel scheduling is driven by “MAP” Intervals (typ. 2ms) Packet(s) arrive at the cable modem (CM)CM waits* for the next contention request opportunity– *typically less than 2msCM sends request message (subject to rate shaping)CMTS** scheduler collects requests, then schedules and communicatesfuture transmit opportunities (grants)Due to serialization, propagation and interleaver delays, as well as CMTS/CMprocessing delays, grant occurs 2 MAP Intervals after the request was sent Without congestion, typically 4-8ms access latency**Cable Modem Termination SystemSlide 3 2013 CableLabs . All rights reserved.

Quality of Service (D1.1 and above) For known applications with known QoS requirements Operator configures packet classifiers and service flow QoSparameters– Token bucket rate shaping, priority, guaranteed rate, lowlatency scheduling, etc. Service Flows queue traffic and access channelindependently Modems today support 16 or 32 service flows, each with anindependent hardware queueSlide 4 2013 CableLabs . All rights reserved.

TCP ACK Suppression/PrioritizationDealing with buffer bloat before “Bufferbloat” Queue build-up from upstream TCP sessions delaysupstream TCP ACKs – downstream throughput suffers. All modem vendors implement proprietary mechanisms inD2.0 and above to move ACKs to the head of the queue anddiscard superfluous ACKs. TCP RTT depends on which side you measure fromSlide 5 2013 CableLabs . All rights reserved.

Buffer Control (D3.0)Bufferbloat is everywhere! Amended specification in 2011 to allow operator to set perservice flow buffer sizes. Requires configuration by operator Interest was high, adoption has been slow.Slide 6 2013 CableLabs . All rights reserved.

Light Sleep Mode (D3.1)Reducing Energy Consumption during “Idle” Response to Political Pressures such as:– More Efficient Modems, Routers Could Save Consumers 330 MillionAnnually – NRDC ––––“These small, innocuous black boxes that never sleep consume enough electricity each year topower all 1.2 million homes in the Silicon Valley area, the hi-tech capital of the world,” said NRDCsenior scientist Noah Horowitz. “Small network devices suck roughly the same amount of energyaround the clock, whether or not you are sending or receiving any data. But there are steps thatmanufacturers can – and should – take to make sure these devices are no longer energy vampires.”88 million Internet consumers in US:– More Efficient Modems, Routers Could Save Each Consumer 0.31 MonthlyEPA Energy Star - Small Network Equipment SpecCalifornia Energy Commission – Consumer Electronics Efficiency Pre-RulemakingEU Lot 26 & Networked Standby RegulationEU Broadband Code of ConductSlide 7 2013 CableLabs . All rights reserved.

Light Sleep Mode (D3.1) Modems in Light Sleep Mode will shut down receiver for periods ofup to 200ms.– Interval set by CMTS, could be less.– Downstream packets queued at CMTS until wake interval– Baseline latency measured during network idle conditions maynot give you the results you expect.– Latency under load might actually be better than “baseline”Slide 8 2013 CableLabs . All rights reserved.

Active Queue Management (D3.1) AQM will be mandatory for both CM and CMTS in D3.1On by default, can be disabled on a per service flow basisCMTS can implement an algo. of the vendor’s choosingCM MUST implement single-queue PIE, but can alsoimplement other algorithms– PIE chosen over CoDel, CoDel-DT, SFQ-CoDel, SFQ-PIE Currently investigating if existing D3.0 equipment can beupgraded to support AQMSlide 9 2013 CableLabs . All rights reserved.

Why no *FQ? Hardware complexity of 32 Service Flows x 32 queues– Or, operational complexity of Service Flows sharing a pool of NqueuesTight deadlines between MAP & grant– *any* additional processing at dequeue time is hardLimited additional benefit compared to single queue AQM at 100Mbps Concerns about VPN trafficHash collisions – not feasible to have 1024 queues (see above)Slide 10 2013 CableLabs . All rights reserved.

The various DOCSIS versions DOCSIS 1.0 – ca. 1996, deployments 1998 – Fundamental request-grant upstream MAC layer definition DOCSIS 1.1 – ca. 1999, deployments 2001 – Additions for configured Quality of Service Packet cla

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