Wireshark Packet Capturing and Analysis of RTMP Protocol: A Comprehensive Guide

 

This article primarily explains the RTMP protocol and demonstrates Wireshark packet capturing and analysis of RTMP.

1. Introduction to RTMP

RTMP is the acronym for Real Time Messaging Protocol. This protocol is based on TCP and encompasses a family of protocols, including the basic RTMP protocol as well as its various variants such as RTMPT, RTMPS, and RTMPE.

RTMP is a network protocol designed for real-time data communication, primarily used for audio, video, and data communication between the Flash/AIR platform and streaming/interactive servers that support the RTMP protocol. Software that supports this protocol includes Adobe Media Server/Ultrant Media Server/red5, among others. Like HTTP, RTMP falls under the application layer of the TCP/IP four-layer model.

RTMP (Real Time Messaging Protocol) is an open protocol developed by Adobe Systems for the transmission of audio, video, and data between Flash players and servers.

The RTMP protocol formats data during transmission. To better achieve multiplexing, packetization, and fairness of information, the sender divides the Message into Chunks with a Message ID. Each Chunk may be an individual Message or a part of a Message. At the receiving end, the Chunk is reassembled into a complete Message based on the data length, message ID, and message length contained in the Chunk, thereby enabling the sending and receiving of information. (Message, Chunk)

Variants

It has multiple variants:

• RTMP operates over TCP, using port 1935 by default;
• RTMPE enhances RTMP by adding encryption functionality; encrypt
• RTMPT is encapsulated over HTTP requests, allowing it to penetrate firewalls; http–rtmp
• RTMPS is similar to RTMPT, adding the security features of TLS/SSL;

2. Capturing RTMP Packets with Wireshark

RTMP Server: Nginx+RTMP (Windows)
Streaming: FFmpeg
Player: VLC (Virtual Machine Linux)
Packet Capture: Wireshark

1. Set Up an RTMP Server

RTMP Server: Setting up the Environment with Nginx+RTMP (Windows) Available for Download if Needed:

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2. Run the RTMP Server

Double-click nginx8080.exe

In Task Manager, you can see that nginx has started working.

3. Open Wireshark

The virtual machine uses the VMware Network Adapter VMnet8 virtual network card (NAT virtual network) to communicate with the host (Windows). Therefore, we can directly capture traffic using the VMware Network Adapter VMnet8 network card.

4. ffmpeg Stream Pushing

Stream command:

ffmpeg -re -i SampleVideo_1280x720_20mb.mp4 -vcodec libx264 -acodec aac -r 30 -g 150 -f flv -y rtmp://192.168.36.176:1935/live/test1

This command uses the FFmpeg tool to convert the input video file SampleVideo_1280x720_20mb.mp4 to FLV format and stream it via the RTMP protocol to the specified URL address rtmp://192.168.36.176:1935/live/test1;

• -re: Read input files in real-time mode, simulating the speed of live streaming.
• -i SampleVideo_1280x720_20mb.mp4Please specify the input file name as SampleVideo_1280x720_20mb.mp4.
• -vcodec libx264: Select the H.264 encoder as the video encoder;
• -acodec aacChoose AAC encoder as the audio encoder;
• -r 30: Set the output video frame rate to 30 frames per second;
• -g 150: Set the keyframe interval to 150 frames. Keyframes are the starting points for video decoding, and a shorter keyframe interval can improve fast forward/rewind performance;
• -f flv: Specify output format as FLV (Flash Video);
• -y: Automatically overwrite the output file if a file with the same name exists; it will be replaced.
• rtmp://192.168.36.176:1935/live/test1: Specify the output URL to stream via the RTMP protocol to the `test1` stream in the `live` application on the 192.168.36.176 server at port 1935.

Among them:

rtmp://ip:port/application/channelnameipLocal IP address

• portRTMP operates over TCP and uses port 1935 by default.
• Application: Refer to nginx.conf, this is live.

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Streaming Process:

5. VLC Stream Pulling

1. Open the VLC client on the virtual machine, Media -> Open Network Stream, enter rtmp://192.168.36.176:1935/live/test1

②. Click play, and you can see that the stream has started successfully.

3. Check Wireshark on the Windows side to see the RTMP packets we need.

among 192.168.36.176 For the host ip Address 192.168.137.128 For virtual machine endpoint ip Address, we only need to look at the RTMP packets.

I’ve captured the RTMP packets, storing them here to conveniently use for future analysis:RTMP Message

Detailed Explanation of the RTMP Protocol

We study the RTMP protocol based on the packets we captured using Wireshark above.

1. Introduction

Live Stream: After Video and Audio are encoded by the encoder, they are interleaved Timestamp: Timestamps Video (Video Stream): ———————————– (each “-” represents a video packet) Audio (Audio Stream): ++++++++++++++++++++ (each “+” represents an audio packet) MP4 Format: —-++——+++——, a single file, with only one header (if the file header is lost during network transmission, the middle audio and video streams cannot be decoded) TS Format: [—-+] [—-+] [—-+], each packet includes a header (suitable for network transmission) RTMP: Message: [—-+] [—-+] [—-+]; Chunk (if the message is too long, it’s divided into chunks). (RTMP uses an FLV transport format) FLV: [—-+] [—-+] [—-+];

2. General Overview.

The RTMP protocol is an application layer protocol, which relies on the underlying reliable transport layer protocol (usually TCP) to ensure the reliability of information transmission. After the link based on the transport layer protocol is established, the RTMP protocol also requires the client and server to establish an RTMP Connection link based on the transport layer link through the “RTMP handshake”. Some control information will be transmitted on the Connection link, such as SetChunkSize and SetACKWindowSize.

The CreateStream command will create a Stream link for transmitting specific audio and video data and command information to control the transmission of these information.

When the RTMP protocol is transmitted, it will format the data (live streaming, pushing local video files) in its own way (Message/Chunk). We call this format of message RTMP Message. In actual transmission, in order to better achieve multiplexing, packetization and information fairness, the sender will divide the message into Chunks with Message IDs. Each Chunk may be a separate Message or part of a Message. The receiving end will restore the chunk to a complete Message based on the length of the data contained in the chunk, the message id and the length of the message, thereby realizing the sending and receiving of information.

3. Handshake

To establish an effective RTMP connection, the first step is the “RTMP handshake.”

• The client needs to send three chunks, C0, C1, C2, in sequence, to the server.
• The server sends three chunks S0, S1, S2 (in order) to the client, and only then can effective data transmission occur.

The RTMP protocol itself does not specify the exact transmission order of these 6 Messages, but implementers of the RTMP protocol need to ensure the following points:

• The client must wait to receive S1 before it can send C2.
• The client must wait to receive S2 before it can send any other information (such as control information and actual audio-video data).
• The server needs to wait until it receives C0 before sending S1.
• The server must wait until it receives C1 before it can send S2.
• The server must wait until it receives the C2 before sending other information (control information and actual audio-video data).

Theoretically speaking, as long as the above conditions are met, the order in which the 6 Messages are arranged is flexible. However, in actual implementation, to minimize the number of communications while ensuring the identity authentication function of the handshake, the typical sending sequence is as follows. This can be verified by capturing ffmpeg streaming packets with Wireshark.

4、RTMP Chunk Stream

Chunk Stream is a logical abstraction of the stream that transmits RTMP Chunks. All RTMP-related information between the client and the server is communicated on this stream. The operation on this stream is also the focus of our attention on the RTMP protocol.

[Control information, audio and video stream information]

4.1. message

Here, Message refers to a message that meets the protocol format and can be divided into Chunks for sending. The fields contained in the message are as follows:

Timestamp: The timestamp of the message (but not necessarily the current time), 4 bytes;

Length: refers to the length of the Message Payload, i.e., the data length of audio and video information, 3 bytes;

TypeId: The type ID of the message, 1 byte;

Message Stream ID: The unique identifier of each message. When dividing into Chunks and restoring Chunks to Messages, this ID is used to identify whether it is the same message Chunk, 4 bytes, and stored in little-endian format.

4.2. Chunking (Message Segmentation)

RTMP does not send and receive data in units of Message, but splits Message into Chunks and sends them. And the next Chunk can only be sent after a Chunk is sent. Each Chunk contains a MessageID to indicate which Message it belongs to, and the receiving end will assemble the chunk into a Message according to this ID.

Q: Why does RTMP split Message into different Chunks?
A: By splitting, Messages with large data volumes can be split into smaller “Messages”, so that low-priority messages can be prevented from continuously blocking high-priority data. For example, during the transmission of a video, it includes video frames, audio frames, and RTMP control information. If audio data or control data is continuously sent, it may cause video frames to be blocked, which will cause the most annoying lag when watching videos. At the same time, for messages with smaller data volumes, the information can be compressed by the Chunk Header field, thereby reducing the amount of information transmitted.

The default size of a Chunk is 128 bytes. During the transmission process, the maximum value of the Chunk data volume can be set through a control message called Set Chunk Size. The sender and receiver will each maintain a Chunk Size, and this value can be set to change the maximum size of the Chunk sent by their own side.

A larger Chunk reduces the time to calculate each chunk and thus reduces the CPU occupancy rate, but it will take up more time to send, especially in low-bandwidth networks, and may block the transmission of more important information later. A smaller Chunk can reduce this blocking problem, but a small Chunk will introduce too much additional information (Header in the Chunk), and a small amount of multiple transmissions may also cause interruptions in transmission, resulting in failure to fully utilize the advantages of high bandwidth, so it is not suitable for transmission in high-bitrate streams.

When actually sending the data, you should try different chunk sizes for the data to be sent, and determine the appropriate chunk size through packet capture analysis and other means. In addition, during the transmission process, you can dynamically adjust the chunk size based on the current bandwidth information and the actual size of the information, so as to maximize CPU utilization and reduce the probability of information blocking.

③. Chunk Format (Block Format)

1) Basic Header (Fundamental Header Information)

Containschunk stream ID(Channel ID) and chunk type，chunk stream id is typically abbreviated as CSID, and it is used to uniquely identify a specific stream channel; the chunk type determines the format of the subsequent Message Header. The Basic Header can vary in length from 1, 2, or 3 bytes, with the chunk type having a fixed length (occupying 2 bits; note that the unit is bits). The length of the Basic Header depends on the size of the CSID, aiming to use as few bytes as possible to store these two fields sufficiently, thereby minimizing the additional data volume introduced by the Header.

The RTMP protocol allows users to customize CSID within the range of [3, 65599], with 0, 1, and 2 reserved by the protocol to denote special information. 0 indicates that the Basic Header occupies a total of 2 bytes, with CSID ranging from [64, 319]. 1 signifies that it occupies 3 bytes, with CSID ranging from [64, 65599]. 2 signifies that the chunk contains control information and certain command information.

The length of the chunk type is fixed at 2 digits, so the length of the CSID is one of the following: (6=8-2), (14=16-2), (22=24-2).When the Basic Header is 1 byteThe CSID occupies 6 bits, allowing a maximum representation of 64 numbers. Therefore, in this case, the CSID ranges from [0, 63], with a user-definable range of [3, 63].When Basic Header is 2 bytes, the CSID is 14 bits long. At this point, the protocol will set the remaining bits of the byte where the chunk type resides to 0, leaving one byte to represent CSID-64. This uses 8 binary bits to store the CSID, which can represent 256 numbers in the range [0, 255]. Therefore, in this case, CSID falls within the range [64, 319], where 319 = 255 + 64;When the Basic Header is 3 bytesThe CSID occupies 22 bits; at this time, the protocol sets the bytes [2,8] to 1, with the remaining 16 bytes representing CSID-64. This gives us 16 bits to store the CSID, which can represent numbers from [0, 65535] totaling 65,536 numbers. In this case, the CSID ranges from [64, 65599], where 65599=65535+64. It is important to note that the Basic Header uses little-endian storage, where higher-value bytes come later. Therefore, when calculating the CSID using each bit of these 3 bytes, it should be: x 256 + + 64.

The CSIDs that can be represented by the 2-byte and 3-byte Basic Headers overlap in the range of [64, 319]. However, in actual implementation, the principle of using the minimum number of bytes should be adhered to, so the 2-byte representation should be used for the CSIDs within [64, 319].

2) Message Header (Message Header Message)

Description information containing the actual data to be sent (which may be complete or partial).The format and length of the Message Header depend on the chunk type of the Basic Header and there are four different formats.is controlled by the `fmt` field in the Basic Header mentioned above. Below are the four formats of the Message Header introduced in descending order of byte size.

• Type=0
  
  
  • type=0The Message Header occupies 11 bytes. It can represent the data indicated by the other three but must utilize this format at the start of a chunk stream’s first chunk and when the timestamp in the header moves backward (i.e., the value decreases compared to the previous chunk, typically seen during playback rewind).
  
  
  • timestamp(Timestamp): Occupies 3 bytes; hence, it can represent up to 16777215=0xFFFFFF=
  
  

  2^{24} -1

  When its value exceeds this maximum, all three bytes are set to 1, and the actual timestamp is stored in the Extended Timestamp field. When the receiving end detects that all 24 bits of the timestamp field are set to 1, it then parses the actual timestamp from the Extended Timestamp.

  • message length(Message Data Length): Occupies 3 bytes and signifies the length of the actual data being sent, such as audio frames, video frames, etc., measured in bytes. Note that this refers to the length of the Message, which is the total data length of the Message to which the chunk belongs, not the length of the Data within the chunk itself.
  • message type id（Type of message id): Occupies 1 byte, indicating the type of data actually sent, such as 8 representing audio data, 9 representing video data.
  • msg stream id(Stream ID of the message): Occupies 4 bytes, representing the ID of the stream to which this chunk belongs. Note: I’m still not entirely clear about what this ID means in terms of stream ID. Since there’s already a chunk stream ID in the basic header, why is there another msg stream ID mentioned? It’s noted below that “4 bytes representing the msg stream ID are omitted, indicating this chunk belongs to the same stream as the last sent chunk,” which pertains to stream linkage. So, does it mean that the msg stream ID and chunk stream ID refer to the same thing? Similar to the Basic Header’s CSID, it utilizes little-endian format.
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  • type=1The Message Header occupies 7 bytes, eliminating the need for 4 bytes that represent the msg stream id, indicating that this chunk belongs to the same stream as the previous chunk sent. When there is only one stream connection between the sending end and the remote end, it’s preferable to use this format whenever possible.
  • timestamp delta: Occupies 3 bytes, and it’s important to note the difference here compared to when type=0; what is stored is the time difference from the previous chunk. Similar to the aforementioned timestamp, when its value exceeds the maximum represented by 3 bytes, all three bytes are set to 1. The actual time difference is then transferred to the Extended Timestamp field. When the receiving end detects that all 24 bits of the timestamp delta field are set to 1, it will parse the difference from the previous timestamp in the Extended Timestamp field.
• Type=2
  • type=2The Message Header occupies 3 bytes. Compared to the type=1 format, it also saves 3 bytes used to indicate the message length and 1 byte used to represent the message type, indicating that the chunk is from the same stream as the previously sent chunk and that the message length and type are the same. The remaining three bytes signify the timestamp delta, functioning the same as in type=1.
• It's important to note that "Type＝3" seems to be a parameter or identifier rather than plain text that requires translation. If this content is part of the post text
  • 0 bytes! Well, this indicates that the Message Header of this chunk is exactly the same as the previous one, so there’s no need to transmit it again. When it follows a Type=0 chunk, it means the timestamp is identical to the previous chunk. When are the timestamps the same? This occurs when a single Message is divided into multiple chunks, and this chunk and the previous chunk belong to the same Message. However, when it follows a Type=1 or Type=2 chunk, it indicates that the timestamp difference from the previous chunk is the same. For instance, if the first chunk is Type=0 with a timestamp of 100, the second chunk is Type=2 with a timestamp delta of 20, meaning the timestamp is 100+20=120. The third chunk being Type=3 implies the timestamp delta is 20, resulting in a timestamp of 120+20=140.

3) Extended Timestamp (Extended Timestamp)

In the section above, we mentioned that within a chunk, there will be a timestamp and a timestamp delta, and they will not both exist simultaneously. Only when either of these exceeds the maximum value that can be represented by 3 bytes, 0xFFFFFF = 16777215, will this field be used to denote the actual timestamp; otherwise, this field is set to 0. The extended timestamp occupies 4 bytes, allowing it to represent a maximum value of 0xFFFFFFFF = 4294967295.

When extended timestamps are enabled, the timestamp field or timestamp delta should be set entirely to 1, indicating that the extended timestamp field should be used to extract the actual timestamp or timestamp difference.Note that the extended timestamp stores the complete value, rather than subtracting the timestamp or storing the timestamp difference.。

4) Chunk data (Chunk data)

the length of the actual user-level data intended to be transmitted, independent of the protocol, is in(0,chunkSize]Between.

5) Chunk Table

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  • First, the type of the chunk containing the first message is 0, as there is no preceding chunk to reference. The timestamp is 1000, indicating the time stamp. A type 0 header, which is the message header, occupies 11 bytes. Assuming the chunkStreamId is less than 127, the Basic Header occupies 1 byte. Including the 32 bytes of Data, the first chunk totals 44 bytes, equating to 11+1+32 bytes.
  • The second chunk and the first chunk have the same CSID, TypeId, and Data length, so we use Chunk Type = 2, timestamp delta = 1020 – 1000 = 20. Therefore, the second chunk occupies 36 = 3 + 1 + 32 bytes.
  • The CSID, TypeId, Data length, and timestamp differences in the third chunk and the second chunk are identical, so Chunk Type = 3 is utilized to save all Message Header information, occupying 33 = 1 + 32 bytes.
  • The situation with the fourth chunk is the same as with the third chunk, also occupying 33=1+32 bytes.
• Example Two
  • Notice that the Data Length=307 > 128, therefore this Message needs to be split into several chunks for transmission. The first chunk has a Type=0 and Timestamp=1000, handling 128 bytes of Data (Note: The default Chunk size is 128 bytes), hence it occupies a total of 140=11+1+128 bytes.
  • The second chunk also has to send 128 bytes. The other fields are the same as the first chunk, so we use Chunk Type = 3. In this case, the timestamp is also 1000, occupying a total of 129 = 1 + 128 bytes.
  • The length of the Data to be sent in the third chunk is 307-128-128=51 bytes, and it still uses Type＝3, totaling 1+51＝52 bytes.

4. Protocol Control Message

In RTMP, certain special values are used in the chunk streams to represent protocol control messages. Their Message Stream ID must be 0 (indicating control stream information), CSID must be 2, and the Message Type ID (note: the message type ID is in the message header) can be 1, 2, 3, 5, or 6. The specific messages they represent will be explained below. The recipient of the control messages will disregard the timestamp in the chunk, and the messages take effect immediately upon receipt. (Note: As I understand it, protocol control messages should be placed within the chunk data.)

• Set Chunk Size(Message Type ID=1): Set the maximum number of bytes that the Data field in a chunk can carry; the default is 128B. During communication, this message can be sent to adjust the chunk size (which must not be less than 128B). Both communicating parties maintain their own chunkSizes, and the chunkSizes on each end are independent. For example, if A wants to send a 200B Message to B, but the default chunkSize is 128B, the message needs to be split into two chunks, one with 128B of Data and the other with 72B. If A sends a message to adjust the chunkSize to 256B and then sends a chunk with Data of 200B, the message is no longer divided locally. When B receives the Set Chunk Size protocol control message, it adjusts the size of the chunk’s Data it accepts and does not need to reassemble the chunks into a single message.
  
  
  
  • The following represents the data for the Set Chunk Size message chunk: The first bit must be 0, the chunk size occupies 31 bits, and the maximum value it can represent is 2,147,483,647 = 0x7FFFFFFF =
  
  
  
  

  2^{31}-1

  
  

  However, in reality, any value greater than 16777215=0xFFFFFF is not usable because the chunk size cannot exceed the length of the Message. This means that the length of the Message is represented using 3 bytes, with a maximum value of 0xFFFFFF.

  
  
• Abort Message(Message Type ID=2): When a message is segmented into multiple chunks, and the receiver only receives a portion of these chunks, (note: the subject is missing here, but I understand it as “the sender sends this control message”) the sender sends this control message to indicate that it will no longer transmit chunks of the same message. Upon receiving this message, the receiver should discard these incomplete chunks. In the data, only one CSID is required to indicate the discarding of all chunks received for that CSID.
• Acknowledgement(Message Type ID=3)When the message size from the peer equals the window size, the receiving end should send an ACK to the sending end to inform that they can continue sending data. The window size refers to the maximum number of bytes that can be sent before receiving an ACK from the receiver. The returned ACK will include the number of bytes received since the last ACK from the sender. (Note: I didn’t fully understand this part. It seems to be a confirmation of the window size, but I won’t delve into it further for now.)
• Window Acknowledgement Size(Message Type ID=5): The maximum number of bytes that the sender can transmit between receiving two ACKs from the receiver.
• Set Peer Bandwidth(Message Type ID=6): Limit the output bandwidth of the sender. Upon receiving this message, the receiver will restrict the size of messages sent but not yet acknowledged by setting the Window ACK Size in the message. If the Window ACK Size in the message differs from the size previously sent to the sender, a control message for Window Acknowledgement Size needs to be fed back.
• Hard (Limit Type = 0): The receiving end should set the Window Ack Size to the value specified in the message.
• Soft(Limit Type=1): The receiving end can set the Window Acknowledgment Size to the value in the message or keep the original value (provided that the original size is smaller than the Window Acknowledgment Size in the control message).
• Dynamic(Limit Type=2): If the Limit Type in the last Set Peer Bandwidth message was 0, treat this one as Hard as well. Otherwise, ignore this message and do not set the Window Ack Size.

5、RTMP message

Different Types of RTMP Message

• Command Message (Message Type ID = 17 or 20): Represents command messages transmitted between the client and server to execute certain operations on the remote end (note: simply put, it’s a command notifying the other party to start doing something), such as `connect`, which indicates connecting to the remote end. If the remote end agrees to the connection, it will log the sender’s information and return a connection success message. `publish` indicates beginning to stream data to the remote end, and upon receiving the command, the receiving end will prepare to accept streaming information from the remote end. Specific details about common Command Messages will be introduced later. When information is encoded using AMF0, the Message Type ID is 20, and when using AMF3 encoding, the Message Type ID is 17.
• Data Message (Message Type ID = 15 or 18): Transmit some metadata (such as video name, resolution, etc.) or some user-defined messages. When the information is encoded using AMF0, the Message Type ID is 18; when encoded with AMF3, the Message Type ID is 15.
• Shared Object Message (Message Type ID = 16 or 19): Represents a Flash object type, consisting of a collection of key-value pairs, used in multi-client and multi-instance scenarios. When the information is encoded using AMF0, the Message Type ID equals 19, and when encoded using AMF3, the Message Type ID equals 16.
• Audio Message (Message Type ID=8)I’m sorry for any confusion, but could you please provide the WordPress post content you need assistance with? I’ll be happy to help you translate the text while preserving the HTML formatting.
• Video Message (Message Type ID = 9)It seems the text is in Chinese and translates to “video data” in English. If you have a WordPress post that contains text requiring translation, please provide the text section, and I’ll assist you in translating it while preserving the HTML structure.
• Aggregate Message (Aggregated Information, Message Type ID=22)Collection of Multiple RTMP Submessages
• User Control Message Events (Message Type ID=4): Inform the other party to execute user control events contained within the message, such as the Stream Begin event, which notifies the other party that stream information is beginning to transmit. Unlike the previously mentioned Protocol Control Messages, this is at the RTMP protocol layer, not the RTMP chunk stream protocol layer, which can easily be confused. When this message is sent within a chunk stream, the Message Stream ID=0, Chunk Stream ID=2, and Message TypeID=4.

1. Command Message (Message Type ID = 17 or 20)

When sending, the sender includes the name of the command, such as connect; the TransactionID indicates the identifier for this command, and the Command Object specifies relevant parameters. After the receiver gets the command, it will return one of the following three types of messages:

1. _result message indicates that the command is accepted and the peer can proceed with the process.
2. _error message signifies the rejection of the operation intended by the command.
3. method name message represents the function name to be executed on the sender of the previous command.

All three response messages must include the TransactionId from the received command message to signify which command the response pertains to.

It can be understood that there are two types of objects that send command messages: one is NetConnection, which represents a higher-level connection between endpoints, and the other is NetStream, which serves as the transmission channel for stream information, controlling the state of the stream, such as Play to play a stream and Pause to pause it.

1) NetConnection Commands (Commands of the Connection Layer)

Used to manage the connection state between two ends, it also provides asynchronous remote procedure calls (RPC) to execute a method on the remote side. Here are common commands for the connection layer.

• connect: Used for the client to send a connection request to the server, the structure of the message is as follows

There are two types of responses to the message, _result indicates a successful connection, and _error indicates a connection failure.

• Call: Used to execute a certain function on the remote side, commonly known as RPC: Remote Procedure Call. The structure of the message is as follows:

If the TransactionID in the message is not 0, the peer is required to respond to the command. The structure of the response message is as follows:

• Create Stream: Create a channel for conveying specific information, allowing specific information to be transmitted within this stream. The transmission unit is a Chunk.

2) NetStream Commands (Commands on the Stream Connection)

Netstream is built on top of NetConnection and is created through the NetConnection’s createStream command. It is used to transmit specific information such as audio and video. Above the transport layer protocol, only one NetConnection can be connected, but a single NetConnection can establish multiple NetStreams to create different stream channels for data transmission. Below are some commonly used NetStream Commands; upon receiving a command, the server will respond to the client through an onStatus command, indicating the current status of the NetStream.

• The message structure of the onStatus command is as follows:Hello! Could you please provide the text content from the WordPress post that you’d like me to translate?

• play (playback): A client initiates a request to the server to receive data from the server side (if the transmitted information is video, this means requesting to start streaming). This can be invoked multiple times, thereby creating a group of data stream receivers locally. Note the presence of a reset field, which indicates whether to override the previous stream (set to true) or to start a new stream (set to false).

• play2 (Play): Unlike the play command mentioned above, the play2 command allows you to switch the currently playing stream to one with the same data but at a different bitrate. The server maintains multiple bitrate versions of the file for the client to switch using the play2 command.

• deleteStream (Delete Stream): Used by the client to inform the server that a local stream object has been deleted and does not need to be transmitted anymore.

• receiveAudio (Receive Audio): Notify the server whether the client intends to send audio, the structure of the `receiveAudio` command is as follows:

• receiveVideo (Receive Video): Notify the server whether the client wants to send video. The command structure for `receiveVideo` is as follows:

• "Publish (Push Data)": Initiate a request from the client to stream to the server. The structure of the publish command is as follows:

• The command `seek` is used to find the position within the stream.: Locate a specific position in a video or audio file in milliseconds. The structure of the seek command is as follows:

• pause: The client tells the server to stop or resume playback. The structure of the pause command is as follows:

If `Pause` is true, indicating that the client has requested a pause, the server will suspend the corresponding stream and return the `NetStream.Pause.Notify` onStatus command to inform the client that the stream is currently paused. When `Pause` is false, the server will return the `NetStream.Unpause.Notify` command to inform the client that the stream has resumed. If the server fails to respond to this command, it will return an `_error` message.

RTMP Streaming Playback Process

1. Introduction

Below is an analysis of the entire process from opening an RTMP stream to starting the playback of audio and video data.

The RTMP protocol specifies that there are two prerequisite steps to streaming media:

• First, establish a network connection (NetConnection);
• Step 2: Establish a network stream (NetStream).

In this context, a network connection represents the foundational link between the server-side application and the client. A network stream signifies the channel for transmitting multimedia data. Only one network connection can be established between the server and the client, but multiple network streams can be created based on that connection. Their relationship is illustrated in the diagram:

To stream media using the RTMP protocol involves several steps: handshake, establishing a connection, establishing a stream, and playback. RTMP connections always begin with a handshake. The connection phase is used to establish the “network connection” between the client and the server. The stream establishment phase sets up the “network stream” between the client and the server. The playback phase is for transmitting audio and video data.

We analyze the packets captured above with Wireshark.

2. Handshake

The RTMP connection begins with a handshake. It consists of three fixed-size chunks. The three chunks sent by the client are named C0, C1, and C2; the three chunks sent by the server are named S0, S1, and S2.

Handshake sequence as follows:

• The handshake begins when the client sends the C0 and C1 packets. The server, upon receiving C0 or C1, responds by sending S0 and S1.
• When the client has fully received S0 and S1, it begins sending C2. When the server has completely received C0 and C1, it starts sending S2.
• Once the client and server respectively receive S2 and C2, the handshake is complete.

Handshake illustration shown below:

Combine the captured packets with the above theory for comparison:

①. The client sends a handshake C0+C1 to the server.

2. The server responds to the client handshake with S0+S1+S2.

③. The client sends handshake C2 to the server.

Packet capture summary:

• Step 1: The client sends C0+C1

• Step 2: The server sends S0+S1+S2

• Step 3: The client sends C2.

Question: Between C2 and S2, which one is sent first? ? ? [The protocol does not specify]

3. Establishing a Network Connection (NetConnection)

Include the following steps:

• 1. The client initiates a connection request.
• 2. Set the response window size for the server client.
• 3、Configure the client transmission bandwidth size on the server.
• 4. Server Configures Client Receive Block Size
• 5. Server response connection results
• 6. Setting the server’s receive block size by the client

1. The client initiates a connection request using Wireshark packet capturing.

The client sends a command message containing “connect” to the server, requesting to establish a connection with a service application instance.

2. Configure the server to set the client’s response window size, transmission bandwidth, and receive block size with Wireshark packet capturing.

After the server receives the connect command message, it sends a Window Acknowledgement Size protocol message to the client to confirm the window size, send bandwidth size, and receive chunk size. At the same time, it connects to the application mentioned in the connect command.

3. Server Response to Stream Creation Result

The “result” (_result) in the server’s command message informs the client about the connection status.

④. Client-side configuration of server receive block size.

The client sends a Window Acknowledgement Size protocol message to the server.

4. Establishing a Network Stream (NetStream)

Include the following steps:

• The client sends a “createStream” command within the command message to the server.
• Upon the server-side receipt of the “create stream” command, it sends the command message containing the “result” (_result) to inform the client about the stream’s status.
• The client requests the server to obtain the length of the specified stream.

The client initiates a stream creation request with Wireshark packet capturing.

②. Server Response for Stream Creation Results

3. The client requests the server to obtain the length of the specified stream using Wireshark packet capturing.

5. Play

• The client sends the “Play” command in the command message to the server.
• The server sends a user control message “stream begin” to inform the client of the stream ID.
• The server sends the audio and video data that the client needs to play.

①. The client sends a playback request.

②. Server sends stream begin.

3. The server sends audio and video data to the client using Wireshark packet capturing.