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March 13, 2024 07:04
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SRS: how to read RTMP basic header?
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| /* | |
| * https://github.com/ossrs/srs/blob/fa8096ad0117a085515729e12a3758ca26036552/trunk/src/protocol/srs_protocol_rtmp_stack.cpp#L843-L924 | |
| */ | |
| /** | |
| * 6.1.1. Chunk Basic Header | |
| * The Chunk Basic Header encodes the chunk stream ID and the chunk | |
| * type(represented by fmt field in the figure below). Chunk type | |
| * determines the format of the encoded message header. Chunk Basic | |
| * Header field may be 1, 2, or 3 bytes, depending on the chunk stream | |
| * ID. | |
| * | |
| * The bits 0-5 (least significant) in the chunk basic header represent | |
| * the chunk stream ID. | |
| * | |
| * Chunk stream IDs 2-63 can be encoded in the 1-byte version of this | |
| * field. | |
| * 0 1 2 3 4 5 6 7 | |
| * +-+-+-+-+-+-+-+-+ | |
| * |fmt| cs id | | |
| * +-+-+-+-+-+-+-+-+ | |
| * Figure 6 Chunk basic header 1 | |
| * | |
| * Chunk stream IDs 64-319 can be encoded in the 2-byte version of this | |
| * field. ID is computed as (the second byte + 64). | |
| * 0 1 | |
| * 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 | |
| * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| * |fmt| 0 | cs id - 64 | | |
| * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| * Figure 7 Chunk basic header 2 | |
| * | |
| * Chunk stream IDs 64-65599 can be encoded in the 3-byte version of | |
| * this field. ID is computed as ((the third byte)*256 + the second byte | |
| * + 64). | |
| * 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 | |
| * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| * |fmt| 1 | cs id - 64 | | |
| * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| * Figure 8 Chunk basic header 3 | |
| * | |
| * cs id: 6 bits | |
| * fmt: 2 bits | |
| * cs id - 64: 8 or 16 bits | |
| * | |
| * Chunk stream IDs with values 64-319 could be represented by both 2- | |
| * byte version and 3-byte version of this field. | |
| */ | |
| srs_error_t SrsProtocol::read_basic_header(char& fmt, int& cid) | |
| { | |
| srs_error_t err = srs_success; | |
| if ((err = in_buffer->grow(skt, 1)) != srs_success) { | |
| return srs_error_wrap(err, "basic header requires 1 bytes"); | |
| } | |
| fmt = in_buffer->read_1byte(); | |
| cid = fmt & 0x3f; | |
| fmt = (fmt >> 6) & 0x03; | |
| // 2-63, 1B chunk header | |
| if (cid > 1) { | |
| return err; | |
| // 64-319, 2B chunk header | |
| } else if (cid == 0) { | |
| if ((err = in_buffer->grow(skt, 1)) != srs_success) { | |
| return srs_error_wrap(err, "basic header requires 2 bytes"); | |
| } | |
| cid = 64; | |
| cid += (uint8_t)in_buffer->read_1byte(); | |
| // 64-65599, 3B chunk header | |
| } else { | |
| srs_assert(cid == 1); | |
| if ((err = in_buffer->grow(skt, 2)) != srs_success) { | |
| return srs_error_wrap(err, "basic header requires 3 bytes"); | |
| } | |
| cid = 64; | |
| cid += (uint8_t)in_buffer->read_1byte(); | |
| cid += ((uint8_t)in_buffer->read_1byte()) * 256; | |
| } | |
| return err; | |
| } |
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