SOCKETS IN C - TCP Sequence Number

I need a help with sockets in C. My teacher spent a job in which I must execute the TCP communication between client and server where I should execute some rules ..

• 1 - The Client sends a message to the server. The message must contain the sequence number and the number of bytes loaded by the message.
• 2 - The server should receive the message and send an acknowledgment to the client. The acknowledgment message must contain the number of sequence of the acknowledgment message, the sequence number of the next segment to be sent by the client.

Can anyone explain me better how the sequence number works and if possible help me codify this? The part of TCP communication between client / server is attached and ready.

Client code snippet:

 sock = socket(AF_INET , SOCK_STREAM , 0);
    if (sock == -1)
    {
        printf("Could not create socket");
    }
    puts("Socket created");

    server.sin_addr.s_addr = inet_addr("127.0.0.1");
    server.sin_family = AF_INET;
    server.sin_port = htons( 8888 );

    //Connect to remote server
    if (connect(sock , (struct sockaddr *)&server , sizeof(server)) < 0)
    {
        perror("connect failed. Error");
        return 1;
    }

    puts("Connected\n");

    //keep communicating with server
    while(1)
    {
        printf("Enter message : ");
    gets(message);
    fflush(stdin);
        //scanf("%s" , message);

        //Send some data
        if( send(sock , message, strlen(message) , 0) < 0)
        {
            puts("Send failed");
            return 1;
        }

        //Receive a reply from the server
        if( recv(sock , server_reply , 2000 , 0) < 0)
        {
            puts("recv failed");
            break;
        }

        puts("Server reply :");
        //puts(server_reply);
    int vl = 0;
    for(int i =0; server_reply[i] != '
//Create socket
socket_desc = socket(AF_INET , SOCK_STREAM , 0);
if (socket_desc == -1)
{
    printf("Could not create socket");
}
puts("Socket created");

//Prepare the sockaddr_in structure
server.sin_family = AF_INET;
server.sin_addr.s_addr = INADDR_ANY;
server.sin_port = htons( 8888 );

//Bind
if( bind(socket_desc,(struct sockaddr *)&server , sizeof(server)) < 0)
{
    //print the error message
    perror("bind failed. Error");
    return 1;
}
puts("bind done");

//Listen
listen(socket_desc , 3);

//Accept and incoming connection
puts("Waiting for incoming connections...");
c = sizeof(struct sockaddr_in);

//accept connection from an incoming client
client_sock = accept(socket_desc, (struct sockaddr *)&client, (socklen_t*)&c);
if (client_sock < 0)
{
    perror("accept failed");
    return 1;
}
puts("Connection accepted");

//Receive a message from client
while( (read_size = recv(client_sock , client_message , 2000 , 0)) > 0 )
{
    //Send the message back to client
    write(client_sock , client_message , strlen(client_message));
}

if(read_size == 0)
{
    puts("Client disconnected");
    fflush(stdout);
}
else if(read_size == -1)
{
    perror("recv failed");
}
';i++)
    {
        if(server_reply[i] != ' ')
            vl += (int)server_reply[i];
    }
        printf("%d\n",vl);
}
    close(sock);

Server code snippet:

 sock = socket(AF_INET , SOCK_STREAM , 0);
    if (sock == -1)
    {
        printf("Could not create socket");
    }
    puts("Socket created");

    server.sin_addr.s_addr = inet_addr("127.0.0.1");
    server.sin_family = AF_INET;
    server.sin_port = htons( 8888 );

    //Connect to remote server
    if (connect(sock , (struct sockaddr *)&server , sizeof(server)) < 0)
    {
        perror("connect failed. Error");
        return 1;
    }

    puts("Connected\n");

    //keep communicating with server
    while(1)
    {
        printf("Enter message : ");
    gets(message);
    fflush(stdin);
        //scanf("%s" , message);

        //Send some data
        if( send(sock , message, strlen(message) , 0) < 0)
        {
            puts("Send failed");
            return 1;
        }

        //Receive a reply from the server
        if( recv(sock , server_reply , 2000 , 0) < 0)
        {
            puts("recv failed");
            break;
        }

        puts("Server reply :");
        //puts(server_reply);
    int vl = 0;
    for(int i =0; server_reply[i] != '
//Create socket
socket_desc = socket(AF_INET , SOCK_STREAM , 0);
if (socket_desc == -1)
{
    printf("Could not create socket");
}
puts("Socket created");

//Prepare the sockaddr_in structure
server.sin_family = AF_INET;
server.sin_addr.s_addr = INADDR_ANY;
server.sin_port = htons( 8888 );

//Bind
if( bind(socket_desc,(struct sockaddr *)&server , sizeof(server)) < 0)
{
    //print the error message
    perror("bind failed. Error");
    return 1;
}
puts("bind done");

//Listen
listen(socket_desc , 3);

//Accept and incoming connection
puts("Waiting for incoming connections...");
c = sizeof(struct sockaddr_in);

//accept connection from an incoming client
client_sock = accept(socket_desc, (struct sockaddr *)&client, (socklen_t*)&c);
if (client_sock < 0)
{
    perror("accept failed");
    return 1;
}
puts("Connection accepted");

//Receive a message from client
while( (read_size = recv(client_sock , client_message , 2000 , 0)) > 0 )
{
    //Send the message back to client
    write(client_sock , client_message , strlen(client_message));
}

if(read_size == 0)
{
    puts("Client disconnected");
    fflush(stdout);
}
else if(read_size == -1)
{
    perror("recv failed");
}
';i++)
    {
        if(server_reply[i] != ' ')
            vl += (int)server_reply[i];
    }
        printf("%d\n",vl);
}
    close(sock);