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6. Security Considerations

As a core transport protocol of the Internet, TCP faces multiple security threats. This section outlines the main security considerations and mitigation measures.

6.1. Sequence Number Attacks​

Threat: Attackers may attempt to guess or predict TCP sequence numbers to inject malicious data or hijack connections.

Mitigation Measures:

  • Use cryptographically strong random number generators to select Initial Sequence Numbers (ISN)
  • Implement sequence number protection mechanisms from RFC 6528
  • Strictly validate out-of-window segments (RFC 5961)

6.2. SYN Flood Attacks​

Threat: Attackers send numerous SYN segments without completing the handshake, exhausting server resources.

Mitigation Measures:

  • Implement SYN Cookie mechanisms
  • Limit the number of half-open connections
  • Use SYN proxies or firewall protection
  • Configure appropriate timeout values

6.3. Connection Hijacking​

Threat: Attackers attempt to take over established TCP connections.

Mitigation Measures:

  • Use encryption protocols like TLS/SSL (e.g., HTTPS)
  • Verify sequence and acknowledgment numbers
  • Implement strict state checking
  • Use TCP MD5 Signature Option (RFC 2385) or TCP-AO (RFC 5925)

6.4. RST Attacks​

Threat: Attackers send forged RST segments to terminate legitimate connections.

Mitigation Measures:

  • Strictly validate sequence numbers of RST segments
  • Implement RST handling improvements from RFC 5961
  • Use encrypted connection protection

6.5. Data Injection Attacks​

Threat: Attackers attempt to inject malicious data into connections.

Mitigation Measures:

  • Strict sequence number validation
  • Use encryption and authentication mechanisms
  • Verify data integrity (checksums)

6.6. Denial of Service (DoS) Attacks​

Threat Types:

  • Resource Exhaustion: Consuming server CPU, memory, or connection resources
  • Bandwidth Consumption: Sending large amounts of data to saturate the network
  • State Exhaustion: Creating numerous connections to exhaust server state

Mitigation Measures:

  • Implement connection rate limiting
  • Use firewalls and intrusion detection systems
  • Configure appropriate resource limits
  • Implement fast path optimizations to reduce CPU usage

6.7. Man-in-the-Middle (MITM) Attacks​

Threat: Attackers intercept and possibly modify communications.

Mitigation Measures:

  • Use end-to-end encryption like TLS
  • Implement certificate validation
  • Use strong authentication mechanisms

6.8. Blind Attacks​

Threat: Attackers attempt to attack connections without seeing the traffic.

Mitigation Measures:

  • Improvements from RFC 5961, including:
    • Challenge ACKs for out-of-window segments
    • Strict RST and SYN validation
    • Rate limiting challenge ACKs

6.9. Timing Attacks​

Threat: Inferring information by analyzing timing characteristics of TCP behavior.

Mitigation Measures:

  • Implement timestamp randomization
  • Avoid predictable timeout behavior
  • Use application-layer encryption

6.10. Implementation Recommendations​

MUST:

  • Use cryptographically strong random number generators for ISN generation
  • Properly implement and validate TCP checksums
  • Implement RFC 5961 security enhancements

SHOULD:

  • Support TLS for applications requiring security
  • Implement connection rate limiting
  • Log suspicious activity

RECOMMENDED:

  • Use firewalls and intrusion detection systems
  • Regularly update TCP implementations to fix known vulnerabilities
  • Monitor abnormal traffic patterns

6.11. Encryption and Authentication​

TCP itself does not provide encryption or authentication. Applications requiring security should use:

  • TLS/SSL (Transport Layer Security) - Most commonly used solution
  • IPsec - Network layer security
  • TCP-AO (RFC 5925) - TCP Authentication Option
  • Application-layer encryption - Such as SSH, HTTPS

6.12. Security Configuration​

Recommended System Configuration:

# Enable SYN Cookies (Linux)
net.ipv4.tcp_syncookies = 1

# Limit SYN retry attempts
net.ipv4.tcp_syn_retries = 3
net.ipv4.tcp_synack_retries = 3

# Enable RFC 1337 TIME-WAIT protection
net.ipv4.tcp_rfc1337 = 1

# Enable reverse path filtering
net.ipv4.conf.all.rp_filter = 1
  • RFC 4953 - Defending TCP Against Spoofing Attacks
  • RFC 5925 - The TCP Authentication Option (TCP-AO)
  • RFC 5961 - Improving TCP's Robustness to Blind In-Window Attacks
  • RFC 6528 - Defending against Sequence Number Attacks
  • RFC 7414 - A Roadmap for TCP Specification Documents (security sections)

Important Note: Security is a continuously evolving field. Implementers should stay informed about the latest security best practices and vulnerability announcements.

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