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22. Historical Definitions for the IPv4 TOS Octet

22. Historical Definitions for the IPv4 TOS Octet​

The IPv4 TOS (Type of Service) byte has had multiple definitions and uses historically. This section briefly reviews these historical definitions to provide background on how ECN interacts with these fields.

Original Definition in RFC 791:

RFC 791 [RFC791] originally defined the TOS byte to contain the following fields:

  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
| Precedence    | D | T | R | C | 0 |
+---+---+---+---+---+---+---+---+

Where:

  • Bits 0-2: Precedence
  • Bit 3: Delay (D)
  • Bit 4: Throughput (T)
  • Bit 5: Reliability (R)
  • Bit 6: Cost (C)
  • Bit 7: Reserved (must be zero)

RFC 1349 Revision:

RFC 1349 [RFC1349] revised the TOS byte definition, but the basic structure remained similar.

RFC 2474 Differentiated Services Definition:

RFC 2474 [RFC2474] redefined this byte for Differentiated Services (DS):

  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
|        DSCP           | ECN   |
+---+---+---+---+---+---+---+---+

Where:

  • Bits 0-5: DSCP (Differentiated Services Codepoint)
  • Bits 6-7: ECN (Explicit Congestion Notification)

This document (RFC 3168) is consistent with RFC 2474, using the last two bits (bits 6-7) of the TOS byte for ECN.

It is important to note that the definition of the ECN field is coordinated with the Differentiated Services architecture to ensure that both can coexist and interoperate. The DSCP field and ECN field can be used independently without interfering with each other.

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