Internet Engineering Task Force                            Juha Heinanen
INTERNET DRAFT                                       Telia Finland, Inc.
Expires May 1998                                           November 1997

       Use of the IPv4 TOS Octet to Support Differential Services

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
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   This document describes how the TOS octet in the IPv4 header can be
   used to support differential Internet services.  The proposal is
   based on the existing format of the TOS octet as defined in [RFC791]
   and [RFC1349].

1. Introduction

   In order to support differential services within the Internet, such
   as those proposed by [Clark] and [Kilkki], the routers must be able
   to distinguish, how they should treat IP packets in terms of Type of
   Service (TOS) and Drop Preference (DP).  The TOS usually indicates
   the desired delay characteristics of the packet, whereas the DP
   identifies how "important" the packet is if packets need to be
   discarded due to congestion.

   In a typical router implementation, each TOS has its own queue, which
   it serves with a policy that meets the characteristics of the TOS.
   The DPs, on the other hand, map to queue thresholds so that an

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   incoming packet with a DP value i may be discarded if the length of
   the corresponding queue has exceeded a threshold value th(i) of that

   The TOS of a packet is set by the application or, if the application
   is unaware of differential Internet services, by the operating system
   or a router.  In the latter two cases, the TOS may be determined from
   the TCP/UDP port numbers of the packet.

   The DP of a packet can also be set by the application, the OS, or a
   router, but the criteria based on which the DP is assigned can vary
   widely.  For example, a packet may be assigned a higher DP if it is
   consider to be "outside" of a given user profile.  For more
   information on policies to set the DP, see [Clark] and [Kilkki].

2. TOS and DP in the IPv4 Header

   Ideally, in order to support a wide range of TOSes and DPs, the IP
   packet header should have several bits available for this purpose.
   Luckily, the The IPv4 header has an 8 bit TOS octet, that can be used
   to carry the TOS and DP information.

   According to [RFC791], the IPv4 TOS octet is divided into a 3 bit
   Precedence field and a 3 bit TOS field.  The last two bits of the TOS
   octet are reserved for future use:

         Bits 0-2:  Precedence.
         Bit    3:  0 = Normal Delay,       1 = Low Delay.
         Bits   4:  0 = Normal Throughput,  1 = High Throughput.
         Bits   5:  0 = Normal Reliability, 1 = High Reliability.
         Bit  6-7:  Reserved for Future Use.

   The following two sections propose how the TOS and DP information
   needed to support the implementation of differential Internet
   services can be mapped to the IPv4 TOS octet.

3. Drop Preference (DP)

   The semantics of the 3 bit Precedence field as defined in [RFC791]
   doesn't contradict its use as the DP field:

       "Several networks offer service precedence, which somehow treats
       high precedence traffic as more important than other traffic
       (generally by accepting only traffic above a certain precedence
       at time of high load)."

   Further, [RFC791] assigns the following meaning to the eight possible
   values of Precedence field:

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       111 - Network Control
       110 - Internetwork Control
       101 - CRITIC/ECP
       100 - Flash Override
       011 - Flash
       010 - Immediate
       001 - Priority
       000 - Routine

   Consistently with the semantics of DP, the above values can be
   interpreted to describe the increasing importance of an IPv4 packet
   from the value 000 to value 111.

   It is proposes that differential Internet services use the Precedence
   field of the IPv4 header to indicate the DP of the packet.  In
   accordance with [RFC791], the DP of the packet can range from value
   000, indicating the highest DP, to value 111, indicating the lowest
   DP.  Note that if conformance with [RFC791] if not a requirement, it
   would be more natural to use the value 000 to indicate the lowest DP
   and 111 the highest DP, since value 000 is usually the default in
   IPv4 packets.

   At present it is still unclear how many levels of DP will be actually
   needed.  No matter what the conclusion will be, it is highly unlikely
   that every network would support all three Precedence bits and
   implement all eight levels of DP.  This documemnt therefore proposes
   that a network is allowed to support one, two, or three DP bits.  If
   it supports only one DP bit, it is the Bit 0, and if it supports two,
   they are the Bits 0 and 1.

4. Type of Service (TOS)

   The three bit TOS field can be used to indicate the Type of Service
   that an IPv4 packet expects to receive from a network.  As already
   shown above, [RFC791] specifies the following semantics for these 3

         Bit    3:  0 = Normal Delay,       1 = Low Delay.
         Bits   4:  0 = Normal Throughput,  1 = High Throughput.
         Bits   5:  0 = Normal Reliability, 1 = High Reliability.

   In [RFC1349], the TOS field has been extended by one of the reserved
   bits of [RFC791] resulting in the following semantics for the Bits
   3-6 of the IPv4 TOS octet:

         1000   --   minimize delay
         0100   --   maximize throughput
         0010   --   maximize reliability

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         0001   --   minimize monetary cost
         0000   --   normal service

   Ideally, the TOS field of IPv4 TOS octet should support a wide
   variety of delay characteristics.  However, it is not known yet, how
   many different levels of delay can be actually supported by
   implementations so that each level would have its own distinguised
   "feel".  It is therefore proposed that networks that support
   differential Internet services initially only use the Bit 3 of the
   TOS field to indicate the desired delay treatment of a packet.
   According to both [RFC791] and [RFC1349], by setting the Bit 3 to 0
   or 1, the packet requests a normal or low delay, respectively.

   This proposal doesn't exclude the possibility to later extend the
   delay "field" by another bit if it turns out that two levels of delay
   is not enough and that more levels can be supported by
   implementations.  It is proposed that the Bit 4 of the TOS octet is
   reserved for the possible extension of the delay "field" until the
   issue is resolved.  If two delay bits will be later defined, a
   network is still allowed to support only one of them, which is the
   Bit 3.

   The use of the Bits 5-7 of the TOS octet in connection with
   differential Internet services is left for further study.

5. Summary

   This document has proposed how the IPv4 TOS octet can be used to
   support differential services within the Internet.  The proposal does
   not include any syntactical changes to the IPv4 header and also the
   semantic changes are kept to the very minimum.

   The author hopes that this proposal could help to create a common
   convention on the usage of the TOS octet for the support of
   differential services within the Internet.  Such services will be or,
   more precisely, are being implemented anyhow and it is thus very
   important that the implementations and service offerings will be
   compatible with each other.

6. Security Considerations

   Security is not addressed in the current version of this document.


   [Clark]  Clark, D. and Wroclawski, J., "An Approach to Service
   Allocation in the Internet".  draft-clark-diff-svc-alloc-00.txt, July

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   [Kilkki]  Kilkki, K., "Simple Integrated Media Access (SIMA)".
   <draft-kalevi-simple-media-access-01.txt>, June 1997.

   [RFC791]  Information Sciences Institute, "Internet Protocol".  RFC
   791, September 1981.

   [RFC1349] Almquist, P., "Type of Service in the Internet Protocol
   Suite".  RFC 1349, July 1992.


   I would like to thank Kalevi Kilkki for his comments on earlier
   versions of this document.  The current version has been influenced
   by the discussions on the int-serv mailing lists.

Author Information

   Juha Heinanen
   Telia Finland, Inc.
   Myyrmaentie 2
   01600 VANTAA

   Phone +358 303 944 808

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