pc: ignore DTLS-decrypted packets in RtpTransport::OnReadPacket

RtpTransport subscribes to the underlying packet transport's read
callback, which fires for both SRTP-bypass packets (kSrtpEncrypted) and
DTLS-decrypted application data (kDtlsDecrypted, e.g. SCTP cleartext).
DcSctpTransport filters on decryption_info(); RtpTransport relied on
InferRtpPacketType to reject the cleartext.

That only works because the default SCTP common header starts with
0x13 0x88 (port 5000), which fails the V=2 check. Any payload whose
first byte has high bits 10 and whose second byte's low 7 bits land in
the RTCP-reserved PT range [64, 96) gets misrouted to SrtpTransport and
fails UnprotectRtcp with a noisy log.

Filter on decryption_info() up front.

Bug: webrtc:517079993
Change-Id: I769a07644eb1286fa6c685cef3e1ef5937545843
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/476180
Reviewed-by: Victor Boivie <boivie@webrtc.org>
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47830}

media/sctp/dcsctp_transport.cc

diff --git a/media/sctp/dcsctp_transport.cc b/media/sctp/dcsctp_transport.cc
index 96f02da..907140c 100644
--- a/media/sctp/dcsctp_transport.cc
+++ b/media/sctp/dcsctp_transport.cc
@@ -744,6 +744,7 @@
     PacketTransportInternal* /* transport */,
     const ReceivedIpPacket& packet) {
   RTC_DCHECK_RUN_ON(network_thread_);
+  // TODO: bugs.webrtc.org/517079993 - follow RFC 7983 design.
   if (packet.decryption_info() != ReceivedIpPacket::kDtlsDecrypted) {
     // We are only interested in SCTP packets.
     return;

pc/rtp_transport.cc

diff --git a/pc/rtp_transport.cc b/pc/rtp_transport.cc
index 8e9d1ea..9326f40 100644
--- a/pc/rtp_transport.cc
+++ b/pc/rtp_transport.cc
@@ -423,6 +423,12 @@
                                 const ReceivedIpPacket& received_packet) {
   TRACE_EVENT0("webrtc", "RtpTransport::OnReadPacket");
 
+  // DTLS-decrypted application data is not RTP/RTCP.
+  // TODO: bugs.webrtc.org/517079993 - follow RFC 7983 design.
+  if (received_packet.decryption_info() == ReceivedIpPacket::kDtlsDecrypted) {
+    return;
+  }
+
   // When using RTCP multiplexing we might get RTCP packets on the RTP
   // transport. We check the RTP payload type to determine if it is RTCP.
   RtpPacketType packet_type = InferRtpPacketType(received_packet.payload());