[Merge M89] - Change the safe SCTP MTU size to 1191
TBR=tommi@webrtc.org
(cherry picked from commit c50097798342271431bac0ee1abf0b5ce7d61dfb)
Bug: webrtc:12495, chromium:1182634
Change-Id: Ie149391a5a9f61095cf3f31db141c9bbc8be8bee
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208642
Reviewed-by: Taylor <deadbeef@webrtc.org>
Reviewed-by: Philipp Hancke <philipp.hancke@googlemail.com>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Original-Commit-Position: refs/heads/master@{#33338}
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/211350
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/branch-heads/4389@{#7}
Cr-Branched-From: 7acc2d9fe3a6e3c4d8881d2bdfc9b8968a724cd5-refs/heads/master@{#32986}
diff --git a/media/sctp/sctp_transport.cc b/media/sctp/sctp_transport.cc
index 6bb4a8f..7ffb9e4 100644
--- a/media/sctp/sctp_transport.cc
+++ b/media/sctp/sctp_transport.cc
@@ -54,8 +54,19 @@
namespace {
// The biggest SCTP packet. Starting from a 'safe' wire MTU value of 1280,
-// take off 80 bytes for DTLS/TURN/TCP/IP overhead.
-static constexpr size_t kSctpMtu = 1200;
+// take off 85 bytes for DTLS/TURN/TCP/IP and ciphertext overhead.
+//
+// Additionally, it's possible that TURN adds an additional 4 bytes of overhead
+// after a channel has been established, so we subtract an additional 4 bytes.
+//
+// 1280 IPV6 MTU
+// -40 IPV6 header
+// -8 UDP
+// -24 GCM Cipher
+// -13 DTLS record header
+// -4 TURN ChannelData
+// = 1191 bytes.
+static constexpr size_t kSctpMtu = 1191;
// Set the initial value of the static SCTP Data Engines reference count.
ABSL_CONST_INIT int g_usrsctp_usage_count = 0;
