Rohan Mahy rohan@ekabal.com TURN TCP Rohan Mahy rohan@ekabal.com

Why do we care again? ICE-tcp and TURN-tcp very troubled documents – can we just drop them? Major customers: P2psip TOTE (new) Mediactrl (coming soon) Whiteboarding (eventually) So we need to work this!

Why split out TCP peers from TURN? Head of line blocking problem and fair sharing No consensus on best way to solve this problem. Need more consensus on requirements. Transport Area asked us to make the protocol more robust and more general. Lots of folks using STUN in ways we did not imagine. TURN likely to go same way. Could be used by HIP. Not much thought about TCP error cases, especially when using multiple peers. Most users are not using TCP peers anyway

More on blocking / fair sharing window full, connection stalled stop reading in case incoming data is for peer 1. peer 2 data waits Peer 1 TURN Server TURN Client empty pipe, blocked by peer 1 Peer 2 Peer 1 fast peer 1 takes more than its fair share of slow link fast link TURN Server TURN Client slow link slow link Peer 2

Some proposals design team discussed Ignore problem during setup, transition to single peer. Don’t send enough data to block the other peer. Ditch all but one peer before sending bulk data. Transport ADs did not like this because it is not general, and it is impossible to enforce this. Do full blown SSH-style per-channel windowing (TCP on top of application layer) Convey per-peer window quotas Similar to ssh per-channel sliding windows. Each channel has a per-direction “don’t send data past the <n>th octet” quota. Quota gets updated when window is at least half empty. Only allow one TCP peer per connection to client Don’t accept additional peer TCP connections. Send an indication that incoming request is waiting, client can open connection to TURN server, request the same allocation, and connect to the new peer. There could be many more.

Pros and Cons One-peer per client connection Per-peer window quotas Pro: conceptually easy to understand Cons: breaks one flow per allocation assumption adds an extra RTT of delay before exchanging data with an additional TCP peer. How often will this happen? Per-peer window quotas Pro: probably still simpler than SSH. unproven: complex and requires a lot of modeling or experimental data to see if it will work adds unknown amount of overhead Impossible to evaluate without a consensus on requirements

Next Steps Agree on what’s important about end result Fill out possible approaches more fully Get volunteers Compare the approaches that look promising