





Network Working Group                                        J. Heinanen
Request for Comments: 2697                                 Telia Finland
Category: Informational                                        R. Guerin
                                              University of Pennsylvania
                                                          September 1999


                    A Single Rate Three Color Marker

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   This document defines a Single Rate Three Color Marker (srTCM), which
   can be used as component in a Diffserv traffic conditioner [RFC2475,
   RFC2474].  The srTCM meters a traffic stream and marks its packets
   according to three traffic parameters, Committed Information Rate
   (CIR), Committed Burst Size (CBS), and Excess Burst Size (EBS), to be
   either green, yellow, or red.  A packet is marked green if it doesn't
   exceed the CBS, yellow if it does exceed the CBS, but not the EBS,
   and red otherwise.

1. Introduction

   The Single Rate Three Color Marker (srTCM) meters an IP packet stream
   and marks its packets either green, yellow, or red.  Marking is based
   on a Committed Information Rate (CIR) and two associated burst sizes,
   a Committed Burst Size (CBS) and an Excess Burst Size (EBS).  A
   packet is marked green if it doesn't exceed the CBS, yellow if it
   does exceed the CBS, but not the EBS, and red otherwise.  The srTCM
   is useful, for example, for ingress policing of a service, where only
   the length, not the peak rate, of the burst determines service
   eligibility.










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   The Meter meters each packet and passes the packet and the metering
   result to the Marker:

                     +------------+
                     |   Result   |
                     |            V
                 +-------+    +--------+
                 |       |    |        |
      Packet Stream ===>| Meter |===>| Marker |===> Marked Packet Stream
                 |       |    |        |
                 +-------+    +--------+

   The Meter operates in one of two modes.  In the Color-Blind mode, the
   Meter assumes that the packet stream is uncolored.  In the Color-
   Aware mode the Meter assumes that some preceding entity has pre-
   colored the incoming packet stream so that each packet is either
   green, yellow, or red.  The details of the pre-coloring process,
   including handling of error scenarios, and how the Meter determines
   the color of a pre-colored packet are DS domain specific and outside
   the scope of this document.

   The Marker (re)colors an IP packet according to the results of the
   Meter.  The color is coded in the DS field [RFC2474] of the packet in
   a PHB specific manner (see section 4 for an example).

   A companion document [RFC2698] describes another three color marker,
   called a Two Rate Three Color Maker (trTCM), where packets are marked
   based on two rates and two burst sizes.

2. Configuration

   The srTCM is configured by setting its mode and by assigning values
   to three traffic parameters: a Committed Information Rate (CIR), a
   Committed Burst Size (CBS), and an Excess Burst Size (EBS).

   The CIR is measured in bytes of IP packets per second, i.e., it
   includes the IP header, but not link specific headers.

   The CBS and the EBS and are measured in bytes.  The CBS and EBS must
   be configured so that at least one of them is larger than 0.  It is
   recommended that when the value of the CBS or the EBS is larger than
   0, it is larger than or equal to the size of the largest possible IP
   packet in the stream.








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3. Metering

   The behavior of the Meter is specified in terms of its mode and two
   token buckets, C and E, which both share the common rate CIR.  The
   maximum size of the token bucket C is CBS and the maximum size of the
   token bucket E is EBS.

   The token buckets C and E are initially (at time 0) full, i.e., the
   token count Tc(0) = CBS and the token count Te(0) = EBS.  Thereafter,
   the token counts Tc and Te are updated CIR times per second as
   follows:

     o If Tc is less than CBS, Tc is incremented by one, else

     o if Te is less then EBS, Te is incremented by one, else

     o neither Tc nor Te is incremented.

   When a packet of size B bytes arrives at time t, the following
   happens if the srTCM is configured to operate in the Color-Blind
   mode:

     o If Tc(t)-B >= 0, the packet is green and Tc is decremented by B
       down to the minimum value of 0, else

     o if Te(t)-B >= 0, the packets is yellow and Te is decremented by B
       down to the minimum value of 0, else

     o the packet is red and neither Tc nor Te is decremented.

   When a packet of size B bytes arrives at time t, the following
   happens if the srTCM is configured to operate in the Color-Aware
   mode:

     o If the packet has been precolored as green and Tc(t)-B >= 0, the
       packet is green and Tc is decremented by B down to the minimum
       value of 0, else

     o If the packet has been precolored as green or yellow and if
       Te(t)-B >= 0, the packets is yellow and Te is decremented by B
       down to the minimum value of 0, else

     o the packet is red and neither Tc nor Te is decremented.

   Note that according to the above rules, marking of a packet with a
   given color requires that there be enough tokens of that color to
   accommodate the entire packet.  Other marking policies are clearly
   possible. The above policy was chosen in order guarantee a



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   deterministic behavior where the volume of green packets is never
   smaller than what has been determined by the CIR and CBS, i.e.,
   tokens of a given color are always spent on packets of that color.

   The actual implementation of a Meter doesn't need to be modeled
   according to the above formal specification.

4. Marking

   The Marker reflects the metering result by setting the DS field of
   the packet to a particular codepoint.  In case of the AF PHB
   [RFC2597], the color can be coded as the drop precedence of the
   packet.

5. Service Example

   The srTCM can be used to mark a packet stream in a service, where
   different, decreasing levels of assurances (either absolute or
   relative) are given to packets which are green, yellow, or red.  For
   example, a service may discard all red packets, because they exceeded
   both the committed and excess burst sizes, forward yellow packets as
   best effort, and forward green packets with a low drop probability.

6. Security Considerations

   The srTCM has no known security concerns.

7. References

   [RFC2698] Heinanen, J. and R. Guerin, "A Two Rate Three Color
               Marker", RFC 2698, September 1999.

   [RFC2597] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski,
               "Assured Forwarding PHB Group", RFC 2597, June 1999.

   [RFC2474]   Nichols, K., Blake, S., Baker, F. and D. Black,
               "Definition of the Differentiated Services Field (DS
               Field) in the IPv4 and IPv6 Headers", RFC 2474, December
               1998.

   [RFC2475]   Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
               and W. Weiss, "An Architecture for Differentiated
               Services", RFC 2475, December 1998.








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8. Authors' Addresses

   Juha Heinanen
   Telia Finland, Inc.
   Myyrmaentie 2
   01600 Vantaa, Finland

   EMail: jh@telia.fi


   Roch Guerin
   University of Pennsylvania
   Department of Electrical Engineering, Rm 376 GRW
   200 South 33rd Street
   Philadelphia, PA 19104

   EMail: guerin@ee.upenn.edu


































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9.  Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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