





Network Working Group                                      O. Aboul-Magd
Request for Comments: 4115                                      S. Rabie
Category: Informational                                  Nortel Networks
                                                               July 2005


       A Differentiated Service Two-Rate, Three-Color Marker with
               Efficient Handling of in-Profile Traffic

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 (2005).

IESG Note

   This RFC is not a candidate for any level of Internet Standard.  The
   IETF disclaims any knowledge of the fitness of this RFC for any
   purpose and in particular notes that the decision to publish is not
   based on IETF review for such things as security, congestion control,
   or inappropriate interaction with deployed protocols.  The RFC Editor
   has chosen to publish this document at its discretion.  Readers of
   this document should exercise caution in evaluating its value for
   implementation and deployment.  See RFC 3932 for more information.

Abstract

   This document describes a two-rate, three-color marker that has been
   in use for data services including Frame Relay services.  This marker
   can be used for metering per-flow traffic in the emerging IP and L2
   VPN services.  The marker defined here is different from previously
   defined markers in the handling of the in-profile traffic.
   Furthermore, this marker doesn't impose peak-rate shaping
   requirements on customer edge (CE) devices.

1.  Introduction

   The differentiated service defines a quality-of-service (QoS)
   architecture for the Internet [RFC2475].  Two integral components of
   this architecture are traffic metering and marking.  This document
   describes a two-rate, three-color metering/marker algorithm that is





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   suitable for the differentiated service applications such as IP and
   L2 VPNs.  This algorithm has been in use for data services including
   Frame Relay Service.

   The metering/marker defined here is different from those in [RFC2697]
   and [RFC2698].  It is different from [RFC2697] in that it is a two-
   rate, three-color marker.  In contrast, [RFC2697] is a single-rate
   marker.  It is different from [RFC2698] in the way its parameters are
   defined, which allows a better handling of in-profile traffic for
   predominant service scenarios over a wider range of traffic
   parameters.

   Furthermore, the algorithm described here eliminates the need for the
   CE to shape its traffic to a certain peak information rate (PIR), as
   might be the case for the marker defined in [RFC2698] when the value
   for the peak burst size (PBS) is smaller than that for the committed
   burst size (CBS).

   The marker described here operates for both color-blind and color-
   aware modes, as defined in [RFC2698].

2.  Configuration

   The operation of the marker is described by two rate values.  The
   committed information rate (CIR) and the excess information rate
   (EIR).  CIR and EIR define the token generation rate of a token
   bucket with size that is equal to committed burst size (CBS) and
   excess burst size (EBS), respectively.

   The CBS and EBS are measured in bytes and must configure to be
   greater than the expected maximum length of the incoming PDU.  The
   CIR and EIR are both measured in bits/s.  The CIR and EIR can be set
   independently of each other.  Alternatively, the CIR and EIR can be
   linked together by defining a burst duration parameter, T, where
   T=CBS/CIR=EBS/EIR.

3.  Metering and Marking

   The behavior of the meter is defined in terms of its mode and two
   token buckets, C and E, with the rates, CIR and EIR, respectively,
   and maximum sizes CBS and EBS.

   The token buckets C and E are initially (at time 0) full; i.e., the
   token count Tc(0) = CBS and Te(0) = EBS.  Thereafter, the token count
   Tc is incremented by one CIR times per second (up to CBS), and the
   token count Te is incremented by one EIR times per second (up to
   EBS).




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   In the color-aware operation, it is assumed that the algorithm can
   recognize the color of the incoming packet (green, yellow, or red).
   The color-aware operation of the metering is described below.

   When a green packet of size B arrives at time t, then

      o  if Tc(t)- B > 0, the packet is green, and Tc(t) is decremented
         by B; else

      o  if Te(t)- B > 0, the packet is yellow, and Te(t) is decremented
         by B; else

      o  the packet is red.

   When a yellow packet of size B arrives at time t, then

      o  if Te(t)- B > 0, the packet is yellow, and Te(t) is decremented
         by B; else

      o  the packet is red.

   Incoming red packets are not tested against any of the two token
   buckets and remain red.

   In the color-blind operation, the meter assumes that all incoming
   packets are green.  The operation of the meter is similar to that in
   the color-aware operation for green packets.

   The salient feature of the algorithm described above is that traffic
   within the defined CIR is colored green directly, without the need to
   pass additional conformance tests.  This feature is the main
   differentiator of this algorithm from that described in [RFC2698],
   where traffic is marked green after it passes two conformance tests
   (those for PIR and CIR).  In either color-blind or color-aware mode,
   the need to pass two conformance tests could result in packets being
   dropped at the PIR token bucket even though they are perfectly within
   their CIR (in-profile traffic).  Furthermore, in the color-aware mode
   of operation, the need to pass two conformance tests could make
   yellow traffic starve incoming in-profile green packets.












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   The operation of the algorithm is illustrated in the flow chart
   below:

                   +---------------------------------+
                   |periodically every T sec.        |
                   | Tc(t+)=MIN(CBS, Tc(t-)+CIR*T)   |
                   | Te(t+)=MIN(EBS, Te(t-)+EIR*T)   |
                   +---------------------------------+

          Packet of size
              B arrives   /----------------\
         ---------------->|color-blind mode|
                          |       OR       |YES  +---------------+
                          |  green packet  |---->|packet is green|
                          |      AND       |     |Tc(t+)=Tc(t-)-B|
                          |    B <= Tc(t-) |     +---------------+
                          \----------------/
                                  |
                                  | NO
                                  v
                          /----------------\
                          |color-blind mode|
                          |       OR       |YES  +----------------+
                          | NOT red packet |---->|packet is yellow|
                          |      AND       |     |Te(t+)=Te(t-)-B |
                          |    B <= Te(t-) |     +----------------+
                          \----------------/
                                  |
                                  | NO
                                  v
                          +---------------+
                          |packet is red  |
                          +---------------+

              Figure 1: Traffic Metering/Marking Algorithm

   In Figure 1, we have X(t-) and X(t+) to indicate the value of a
   parameter X right before and right after time t.













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4.  Service Scenario

   The described marker can be used to mark an IP packet stream in a
   service where different, decreasing levels of assurances (either
   absolute or relative) are given to packets that are green, yellow, or
   red.  For example, a service may discard all red packets because they
   exceeded the service rates, forward yellow packets as best effort,
   and forward green packets with low drop probability.  The marker
   could also be used for metering L2 VPN services such as the emerging
   Ethernet transport over IP networks.

5.  Security Considerations

   Security issues resulting from this document are similar to those
   mentioned in [RFC2697] and [RFC2698].

6.  Informative References

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

   [RFC2697] Heinanen, J. and R. Guerin, "A Single Rate Three Color
             Marker", RFC 2697, September 1999.

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

   [RFC3932] Alvestrand, H., "The IESG and RFC Editor Documents:
             Procedures", BCP 92, RFC 3932, October 2004.

Authors' Addresses

   Osama Aboul-Magd
   Nortel Networks
   3500 Carling Ave
   Ottawa, ON K2H8E9
   EMail: osama@nortel.com

   Sameh Rabie
   Nortel Networks
   3500 Carling Ave
   Ottawa, ON K2H8E9
   EMail: rabie@nortel.com







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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

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Acknowledgement

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







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