[rtcweb] SCTP issue: Datachannel MTU seems to be problematic.

Harald Alvestrand <harald@alvestrand.no> Mon, 01 March 2021 19:18 UTC

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From: Harald Alvestrand <harald@alvestrand.no>
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Date: Mon, 1 Mar 2021 20:17:54 +0100
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Subject: [rtcweb] SCTP issue: Datachannel MTU seems to be problematic.
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This for info:

We have recently discovered some problems with SCTP on small-MTU networks.

Turns out we underestimated the max size of the encapsulating headers.

Current calculation:

// The biggest SCTP packet. Starting from a 'safe' wire MTU value of 1280,
// take off 85 bytes for DTLS/TURN/TCP/IP and ciphertext overhead.
// Additionally, it's possible that TURN adds an additional 4 bytes of 
// after a channel has been established, so we subtract an additional 4 
// 1280 IPV6 MTU
//  -40 IPV6 header
//   -8 UDP
//  -24 GCM Cipher
//  -13 DTLS record header
//   -4 TURN ChannelData
// = 1191 bytes.

Which is somewhat smaller than the number we assumed in the RFC of 1200 

We didn't discover the issue until we encountered a situation with large 
packets being sent on IPv6-only networks.