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5. Changes from RFC 793

This section summarizes the main changes made in this document relative to RFC 793 and its subsequent updates.

5.1. Protocol Specification Changes​

Header Format Updates:

  • Updated TCP header control bits according to RFC 3168, adding CWR (Congestion Window Reduced) and ECE (ECN-Echo) flags
  • Clarified handling requirements for reserved bits

Sequence Number Handling:

  • Clarified Initial Sequence Number (ISN) selection mechanism
  • Updated sequence number comparison and wrap-around handling

Connection Management:

  • Added clarification for Reset handling in SYN-RECEIVED state (updates RFC 5961)
  • Clarified simultaneous open handling
  • Updated TIME-WAIT state duration and handling

Option Processing:

  • Clarified the mandatory option set: End of Option List, No-Operation, Maximum Segment Size
  • Updated handling requirements for unknown options
  • Added references to experimental options

5.2. Obsoleted Content​

This document obsoletes the following RFCs:

  • RFC 793 - Original TCP specification (1981)
  • RFC 879 - TCP Maximum Segment Size and Related Topics
  • RFC 2873 - TCP Processing of the IPv4 Precedence Field
  • RFC 6093 - On the Implementation of the TCP Urgent Mechanism
  • RFC 6429 - TCP Sender Clarification for Persist Condition
  • RFC 6528 - Defending against Sequence Number Attacks
  • RFC 6691 - TCP Options and Maximum Segment Size (MSS)

5.3. Updated Content​

This document updates the following RFCs:

  • RFC 1011 - Official Internet Protocols (TCP section)
  • RFC 1122 - Requirements for Internet Hosts -- Communication Layers (TCP section)
  • RFC 5961 - Improving TCP's Robustness to Blind In-Window Attacks (SYN-RECEIVED state handling)

5.4. Security Enhancements​

  • Added protection mechanisms against blind attacks
  • Updated defenses against sequence number attacks
  • Clarified security implementation best practices
  • Added considerations for SYN flood attacks

5.5. Performance Improvements​

  • Referenced modern congestion control algorithms
  • Updated Retransmission Timeout (RTO) calculation
  • Added references to high-performance extensions (window scaling, timestamps, SACK)
  • Clarified Nagle algorithm implementation requirements

5.6. Interoperability Clarifications​

  • Clarified handling of malformed segments
  • Updated error condition handling
  • Clarified boundary case behavior
  • Unified terminology and definitions

5.7. Editorial Changes​

  • Modernized language and terminology
  • Reorganized content for improved readability
  • Added implementation guidance
  • Updated references
  • Standardized use of RFC 2119 keywords

5.8. Content Not Included​

The following content, while important parts of modern TCP implementations, is specified in separate documents:

  • Detailed congestion control algorithms (RFC 5681, etc.)
  • High-performance extensions (RFC 7323 - window scaling, timestamps)
  • Selective Acknowledgment SACK (RFC 2018, RFC 2883)
  • TCP Fast Open (RFC 7413)
  • Multipath TCP (RFC 8684)

This is a deliberate design choice to maintain clarity of the base specification while allowing these extensions to evolve independently.

Note: For a complete list of changes and errata fixes, see Section 5 of the official RFC 9293 document.

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