1. Introduction

   Paths through the Internet vary widely in the quality of service they
   provide.  Some paths are more reliable than others.  Some impose high
   call setup or per-packet charges, while others do not do usage-based
   charging.  Throughput and delay also vary widely.  Often there are
   tradeoffs: the path that provides the highest throughput may well not
   be the one that provides the lowest delay or the lowest monetary
   cost.  Therefore, the "optimal" path for a packet to follow through
   the Internet may depend on the needs of the application and its user.

   Because the Internet itself has no direct knowledge of how to
   optimize the path for a particular application or user, the IP
   protocol [11] provides a (rather limited) facility for upper layer
   protocols to convey hints to the Internet Layer about how the
   tradeoffs should be made for the particular packet.  This facility is
   the "Type of Service" facility, abbreviated as the "TOS facility" in
   this memo.

   Although the TOS facility has been a part of the IP specification
   since the beginning, it has been little used in the past.  However,
   the Internet host specification [1, 2] now mandates that hosts use the
   TOS facility.  Additionally, routing protocols (including OSPF [10]
   and Integrated IS-IS [7]) have been developed which can compute
   routes separately for each type of service.  These new routing
   protocols make it practical for routers to consider the requested
   type of service when making routing decisions.

   This specification defines in detail how hosts and routers use the
   TOS facility.  Section 2 introduces the primary considerations that
   motivated the design choices in this specification.  Sections 3 and 4
   describe the Type of Service octet in the IP header and the values
   which the TOS field of that octet may contain.  Section 5 describes
   how a host (or router) chooses appropriate values to insert into the
   TOS fields of the IP datagrams it originates.  Sections 6 and 7
   describe the ICMP Destination Unreachable and Redirect messages and
   how TOS affects path choice by both hosts and routers.  Section 8
   describes some additional ways in which TOS may optionally affect
   packet processing.  Appendix A describes how this specification
   updates a number of existing specifications.  Appendices B and C
   expand on the discussion in Section 2.