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23.1 The ECN Field in IP

23.1 The ECN Field in IP​

This document specifies the use of the two least significant bits (bits 6-7) of the IPv4 TOS byte and the IPv6 Traffic Class octet for ECN. These two bits form the ECN field as defined in Section 5.

The four codepoints defined for the ECN field are:

  • 00: Not-ECT (Not ECN-Capable Transport)
  • 01: ECT(1) (ECN-Capable Transport(1))
  • 10: ECT(0) (ECN-Capable Transport(0))
  • 11: CE (Congestion Experienced)

This specification updates RFC 2474 [RFC2474], which previously defined these bits as part of the DS field. With this document, the DS field is reduced from 8 bits to 6 bits (the DSCP), and the ECN field occupies the remaining 2 bits of the TOS byte or Traffic Class octet.

IANA has registered these ECN codepoint definitions in the appropriate registries for IPv4 and IPv6.

This specification is consistent with the architecture defined in [RFC2474] for the Differentiated Services field, allowing ECN and Differentiated Services to be used simultaneously and independently in IP packets.

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