Fixing off-by-one error with max SCTP id.

webrtc/media/sctp/sctpdataengine.h

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 19 matching lines @@
 #include "webrtc/media/base/mediachannel.h"
 #include "webrtc/media/base/mediaengine.h"
 
 // Defined by "usrsctplib/usrsctp.h"
 struct sockaddr_conn;
 struct sctp_assoc_change;
 struct sctp_stream_reset_event;
 // Defined by <sys/socket.h>
 struct socket;
 namespace cricket {
-// The highest stream ID (Sid) that SCTP allows, and the number of streams we
-// tell SCTP we're going to use.
-const uint32_t kMaxSctpSid = 1023;
+// The number of outgoing streams that we'll negotiate. Since stream IDs (SIDs)
+// are 0-based, the highest usable SID is 1023.
+//
+// It's recommended to use the maximum of 65535 in:
+// https://tools.ietf.org/html/draft-ietf-rtcweb-data-channel-13#section-6.2
+// However, we use 1024 in order to save memory. usrsctp allocates 104 bytes
+// for each pair of incoming/outgoing streams (on a 64-bit system), so 65535
+// streams would waste ~6MB.
+const uint16_t kMaxSctpStreams = 1024;

[pthatcher1, 2016/08/22 23:09:30]

Why not have *both* kMaxSctpStreams and kMaxSctpSid.   Then the places where it
makes sense for one in the code can use it, and where it makes sense for other
other can use the other.

I think const expr can handle "kMaxSctpSid = kMaxSctpStreams - 1" to avoid them
getting out of sync.

[Taylor Brandstetter, 2016/08/23 00:53:14]

Ok. I thought "max streams" made it more clear that it was an inclusive maximum,
but on second thought maybe it doesn't. I'll just have both and leave a comment
to reduce future confusion. And add a "min" too for good measure.

 
 // This is the default SCTP port to use. It is passed along the wire and the
 // connectee and connector must be using the same port. It is not related to the
 // ports at the IP level. (Corresponds to: sockaddr_conn.sconn_port in
 // usrsctp.h)
 const int kSctpDefaultPort = 5000;
 
 class SctpDataMediaChannel;
 
 // A DataEngine that interacts with usrsctp.
@@ skipping 176 matching lines @@
   StreamSet queued_reset_streams_;
   StreamSet sent_reset_streams_;
 
   // A static human-readable name for debugging messages.
   const char* debug_name_;
 };
 
 }  // namespace cricket
 
 #endif  // WEBRTC_MEDIA_SCTP_SCTPDATAENGINE_H_