IP Precedence and ToS (Type of Service) RFCs 791 and 1349*

• Early ARPANET (20+ years ago) & RFC 791 defined this byte

  • Inconsistently used, often ignored completely
  • Includes “Precedence” (three bits), defines eight priority levels
  • RFC 791 also defined original ToS field (next three bits)
    • If D=1, Minimize Delay
    • If T=1, Maximize Throughput
    • If R=1, Maximize Reliability

• RFC 1349 added fourth bit to ToS and other rules

  • If C=1, Minimize Monetary Cost
  • Not allowed to have more than one bit of ToS field = 1
  • If all four of D,T,R & C are zero then “normal service”

* RFC 1349 superceded RFC 791 (the basic IP spec) with respect to this 1 byte field. RFC 1349 now itself being superceded by “DiffServ” (RFC 2474 and 2475)

D

T

R

C

0

Three bits of IP Precedence

1 Byte

called “ToS”

in IP V4 header

The four bits of IP ToS

Unused

Unrelated to IP Precedence.

Not used in forwarding decisions.

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