media/cast/net/rtp/rtp_parser.cc

   1 // Copyright 2014 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "media/cast/net/rtp/rtp_parser.h"
   6
   7 #include "base/big_endian.h"
   8 #include "base/logging.h"
   9 #include "media/cast/cast_defines.h"
  10
  11 namespace media {
  12 namespace cast {
  13
  14 static const size_t kRtpHeaderLength = 12;
  15 static const size_t kCastHeaderLength = 7;
  16 static const uint8 kRtpExtensionBitMask = 0x10;
  17 static const uint8 kRtpMarkerBitMask = 0x80;
  18 static const uint8 kCastKeyFrameBitMask = 0x80;
  19 static const uint8 kCastReferenceFrameIdBitMask = 0x40;
  20
  21 RtpParser::RtpParser(uint32 expected_sender_ssrc, uint8 expected_payload_type)
  22     : expected_sender_ssrc_(expected_sender_ssrc),
  23       expected_payload_type_(expected_payload_type) {}
  24
  25 RtpParser::~RtpParser() {}
  26
  27 bool RtpParser::ParsePacket(const uint8* packet,
  28                             size_t length,
  29                             RtpCastHeader* header,
  30                             const uint8** payload_data,
  31                             size_t* payload_size) {
  32   DCHECK(packet);
  33   DCHECK(header);
  34   DCHECK(payload_data);
  35   DCHECK(payload_size);
  36
  37   if (length < (kRtpHeaderLength + kCastHeaderLength))
  38     return false;
  39
  40   base::BigEndianReader reader(reinterpret_cast<const char*>(packet), length);
  41
  42   // Parse the RTP header.  See
  43   // http://en.wikipedia.org/wiki/Real-time_Transport_Protocol for an
  44   // explanation of the standard RTP packet header.
  45   uint8 bits;
  46   if (!reader.ReadU8(&bits))
  47     return false;
  48   const uint8 version = bits >> 6;
  49   if (version != 2)
  50     return false;
  51   if (bits & kRtpExtensionBitMask)
  52     return false;  // We lack the implementation to skip over an extension.
  53   if (!reader.ReadU8(&bits))
  54     return false;
  55   header->marker = !!(bits & kRtpMarkerBitMask);
  56   header->payload_type = bits & ~kRtpMarkerBitMask;
  57   if (header->payload_type != expected_payload_type_)
  58     return false;  // Punt: Unexpected payload type.
  59   if (!reader.ReadU16(&header->sequence_number) ||
  60       !reader.ReadU32(&header->rtp_timestamp) ||
  61       !reader.ReadU32(&header->sender_ssrc)) {
  62     return false;
  63   }
  64   if (header->sender_ssrc != expected_sender_ssrc_)
  65     return false;  // Punt: Sender's SSRC does not match the expected one.
  66
  67   // Parse the Cast header.  Note that, from the RTP protocol's perspective, the
  68   // Cast header is part of the payload (and not meant to be an extension
  69   // header).
  70   if (!reader.ReadU8(&bits))
  71     return false;
  72   header->is_key_frame = !!(bits & kCastKeyFrameBitMask);
  73   const bool includes_specific_frame_reference =
  74       !!(bits & kCastReferenceFrameIdBitMask);
  75   uint8 truncated_frame_id;
  76   if (!reader.ReadU8(&truncated_frame_id) ||
  77       !reader.ReadU16(&header->packet_id) ||
  78       !reader.ReadU16(&header->max_packet_id)) {
  79     return false;
  80   }
  81   // Sanity-check: Do the packet ID values make sense w.r.t. each other?
  82   if (header->max_packet_id < header->packet_id)
  83     return false;
  84   uint8 truncated_reference_frame_id;
  85   if (!includes_specific_frame_reference) {
  86     // By default, a key frame only references itself; and non-key frames
  87     // reference their direct predecessor.
  88     truncated_reference_frame_id = truncated_frame_id;
  89     if (!header->is_key_frame)
  90       --truncated_reference_frame_id;
  91   } else if (!reader.ReadU8(&truncated_reference_frame_id)) {
  92     return false;
  93   }
  94
  95   // Only the lower 8 bits of the |frame_id| were serialized, so do some magic
  96   // to restore the upper 24 bits.
  97   //
  98   // Note: The call to |frame_id_wrap_helper_| has side effects, so we must not
  99   // call it until we know the entire deserialization will succeed.
 100   header->frame_id =
 101       frame_id_wrap_helper_.MapTo32bitsFrameId(truncated_frame_id);
 102   // When the upper 24 bits are restored to |reference_frame_id|, make sure
 103   // |reference_frame_id| will be strictly less than or equal to |frame_id|.
 104   if (truncated_reference_frame_id <= truncated_frame_id)
 105     header->reference_frame_id = header->frame_id & 0xffffff00;
 106   else
 107     header->reference_frame_id = (header->frame_id & 0xffffff00) - 0x00000100;
 108   header->reference_frame_id |= truncated_reference_frame_id;
 109
 110   // All remaining data in the packet is the payload.
 111   *payload_data = reinterpret_cast<const uint8*>(reader.ptr());
 112   *payload_size = reader.remaining();
 113
 114   return true;
 115 }
 116
 117 }  // namespace cast
 118 }  // namespace media