# 序列号的工作方式
# 初始序列号 (ISN) 是随机生成的

# 发送数据时，序列号递增
# 假设 ISN = 1000，发送 100 字节数据：

# 发送第1个段：Seq=1000, 数据=100字节
# 发送第2个段：Seq=1100, 数据=100字节
# 发送第3个段：Seq=1200, 数据=100字节

def calculate_next_seq(current_seq, data_len):
    """计算下一个序列号"""
    return current_seq + data_len

# 示例
seq = 1000
for i in range(3):
    next_seq = calculate_next_seq(seq, 100)
    print(f"发送: Seq={seq}, 下一个Seq={next_seq}")
    seq = next_seq

# 窗口大小用于流量控制
# 接收方通过窗口大小告诉发送方：我还能接收多少数据

def flow_control_example():
    """窗口大小变化示例"""
    
    receiver_buffer = 10000  # 接收缓冲区大小
    received_data = 0
    
    print(f"初始窗口大小: {receiver_buffer}")
    
    # 模拟数据接收
    for i in range(5):
        incoming_data = 2000
        received_data += incoming_data
        window = max(0, receiver_buffer - received_data)
        
        print(f"接收 {incoming_data} 字节后，窗口大小: {window}")
        
        # 模拟应用层读取数据
        if received_data > 5000:
            read_size = 3000
            received_data -= read_size
            print(f"  应用层读取 {read_size} 字节，窗口恢复: {receiver_buffer - received_data}")

flow_control_example()

def tcp_checksum(src_ip, dst_ip, tcp_segment):
    """计算 TCP 校验和（含伪头部）"""
    import struct
    
    # 伪头部
    src = socket.inet_aton(src_ip)
    dst = socket.inet_aton(dst_ip)
    placeholder = 0
    protocol = 6  # TCP
    tcp_len = len(tcp_segment)
    
    pseudo_header = struct.pack('!4s4sBBH', src, dst, placeholder, protocol, tcp_len)
    
    # 计算校验和
    data = pseudo_header + tcp_segment
    
    if len(data) % 2:
        data += b'\x00'
    
    checksum = 0
    for i in range(0, len(data), 2):
        word = (data[i] << 8) + data[i+1]
        checksum += word
    
    checksum = (checksum >> 16) + (checksum & 0xFFFF)
    checksum = ~checksum & 0xFFFF
    
    return checksum

import struct
import socket

def parse_tcp_header(data):
    """解析 TCP 头部"""
    if len(data) < 20:
        return None
    
    # 解析固定头部（20字节）
    fields = struct.unpack('!HHIIBBHHH', data[:20])
    
    src_port = fields[0]
    dst_port = fields[1]
    seq_num = fields[2]
    ack_num = fields[3]
    data_offset = (fields[4] >> 4) * 4
    flags = fields[5]
    window = fields[6]
    checksum = fields[7]
    urgent_ptr = fields[8]
    
    # 解析标志位
    flag_names = []
    if flags & 0x01: flag_names.append('FIN')
    if flags & 0x02: flag_names.append('SYN')
    if flags & 0x04: flag_names.append('RST')
    if flags & 0x08: flag_names.append('PSH')
    if flags & 0x10: flag_names.append('ACK')
    if flags & 0x20: flag_names.append('URG')
    
    return {
        'src_port': src_port,
        'dst_port': dst_port,
        'seq_num': seq_num,
        'ack_num': ack_num,
        'data_offset': data_offset,
        'flags': flag_names,
        'window': window,
        'checksum': f'0x{checksum:04x}',
        'urgent_pointer': urgent_ptr,
        'options': data[20:data_offset]
    }

# 使用示例
header = parse_tcp_header(tcp_data)
if header:
    print(f"{header['src_port']} → {header['dst_port']}")
    print(f"Seq={header['seq_num']}, Ack={header['ack_num']}")
    print(f"Flags: {', '.join(header['flags'])}")
    print(f"Window: {header['window']}")