TURN TCP Rohan Mahy

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

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 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 –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. –Cons: 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