End-host Perspectives on Congestion Management

Murari Sridharanmuraris@microsoft.comCONEX BOF, IETF 76, Hiroshima

Its all about the apps

• Apps have diverse needs– P2P, VoIP, TV, online apps, games

• Filling the pipe while being TCP fair• High-speed congestion control, Autotuning the receive

window, host OS (including app) bottlenecks

• Low priority scavenger service• Protocols changes to reduce latency• Multipath transport to maximize link capacity,

improve resiliency

End-host view of the network

• Mostly works as expected• Establishing connectivity is sometimes

problematic• Largely remains a black box– Congestion, bottleneck capacity is implicitly

inferred• Network seems change/risk averse• Imposes complex usage requirements– Volume caps, pay-per-use, metered data plans …

Network’s view of the end-hosts

• End-hosts cannot be trusted, need to be explicitly controlled

• Application performance needs can be inferred and improved upon by inspecting packets on the wire

• End-hosts typically establish connectivity to well-known servers

• And the reality is …

Splitting at the seams • Devices, Devices, Devices …

– IGDs/NATs/Firewalls/DPI devices/Load balancers/Soft switches• Layering violations, RFC non-conformance, no future proofing

– Backwards compatibility• Thanks to deep packet inspection you can’t even touch padding bits!

– Hampered ECN deployment• New protocols hard or impossible to deploy

– No changes to protocol on the wire– Takes time for apps to move to using new APIs

• Not all apps need TCP, but only TCP passes reliably E2E– Tunneling, feedback loops & TCP over TCP issues

• Network traffic management is a nightmare due to increased appetite for large volumes

CONEX: Things to consider• Let’s please stop inferring each other. – How can the end-host learn about network policy? – Explicit feedback to/from the network – OS has tons of context than the small amount reflected in

the packets• Wireless/Cellular network operators are moving

towards usage based models– Tactically, policy based routing/interface selection is the

high-order bit• How does the end-host learn about the network

policy?

CONEX: Things to consider

• Incremental deployment & incentives• Bottleneck is largely within the home and/or last mile– Congestion exposure doesn’t add any benefit but

shouldn’t make it worse• Primal-dual congestion control– Performance is a function of source rate controller, AQM,

marking thresholds, congestion measure– Measure of congestion, feedback of congestion measure,

and use of congestion measure by the source controller are all important.

• Beyond TCP – UDP(DCCP) + ECN