TCP Sticky Packet Problem Explained

TCP sticky packet is a common problem in network programming, referring to multiple packets being merged into one packet when received, or one packet being split into multiple packets when received.

Causes of Sticky Packets

1. TCP is a Byte-Stream Oriented Protocol

• TCP does not preserve message boundaries, treats data as a continuous byte stream
• Multiple packets sent by the sender may be merged or split at the receiver
• This is a design feature of TCP, not an error

2. Nagle Algorithm

• Purpose: Reduce the number of small packets in the network, improve transmission efficiency
• Mechanism: Merge multiple small packets into one large packet for transmission
• Trigger Condition: Packet size is less than MSS and no ACK received
• Impact: May cause multiple small packets to be merged into one packet

3. Receiver Buffer

• When the receiver reads data from the buffer, it may read multiple packets at once
• Application layer reads data less frequently than data arrives
• Accumulated packets in the buffer are read at once

Manifestations of Sticky Packets

1. Sticky Packets

• Sender sends two packets A and B
• Receiver receives A+B merged data at once

2. Packet Splitting

• Sender sends one large packet A
• Receiver receives A1 and A2 in two separate reads

3. Mixed Sticky Packets and Splitting

• Sender sends packets A, B, C
• Receiver receives A+B, C1, C2

Solutions

1. Fixed Length

• Method: Each packet has a fixed length, pad if insufficient
• Advantage: Simple to implement
• Disadvantage: Wastes bandwidth, poor flexibility

2. Special Delimiter

• Method: Add special delimiter between packets (such as \n, \r\n)
• Advantage: Simple to implement, suitable for text protocols
• Disadvantage: Need to escape delimiters, lower efficiency

3. Length Field

• Method: Add length field at the beginning of packet to identify packet length
• Advantage: Efficient, suitable for binary protocols
• Disadvantage: Need to parse protocol header

4. Message Delimiter

• Method: Use special start and end markers
• Advantage: Clear and explicit
• Disadvantage: Need to escape marker characters

Code Example

Length Field Solution

python
def send_data(sock, data):
    length = len(data)
    sock.send(struct.pack('!I', length) + data)

def recv_data(sock):
    length_bytes = sock.recv(4)
    length = struct.unpack('!I', length_bytes)[0]
    data = b''
    while len(data) < length:
        data += sock.recv(length - len(data))
    return data

Related Questions

• Does UDP have sticky packet problems?
• When should Nagle algorithm be disabled?
• How to design efficient binary protocols?