srTCM: A Single Rate Three Color Marker
A two-bucket marker that colors IP packets according to a committed rate and burst sizes.
Heinanen, J. and R. Guerin, "A Single Rate Three Color Marker", RFC 2697, DOI 10.17487/RFC2697, September 1999, <https://www.rfc-editor.org/info/rfc2697>.
@misc{rfc2697,
series = {Request for Comments},
number = 2697,
howpublished = {RFC 2697},
publisher = {RFC Editor},
doi = {10.17487/RFC2697},
url = {https://www.rfc-editor.org/info/rfc2697},
author = {J. Heinanen and R. Guerin},
title = {{A Single Rate Three Color Marker}},
pagetotal = 6,
year = 1999,
month = sep,
} RFC text
Contents
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.
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.
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
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.
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
9. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
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included on all such copies and derivative works. However, this
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
ISO Plain Language
On this page
§1What the Marker Does
The Single Rate Three Color Marker (srTCM) is a component of a Differentiated Services traffic conditioner. Its Meter evaluates each IP packet and passes the result to a Marker, which records the color in the packet's DS field.
The three colors represent how a packet fits the Committed Burst Size (CBS) and Excess Burst Size (EBS): green packets fit the committed allowance, yellow packets exceed CBS but fit EBS, and red packets exceed both. The RFC gives ingress policing as an example use where burst length, rather than peak rate, determines service eligibility.
The meter checks each packet against the configured rate and burst allowances. The marker writes the result as green, yellow or red in the packet's DS field.
§2Configuration Parameters
An srTCM is configured with a mode, a Committed Information Rate (CIR), a CBS and an EBS. CIR is measured in bytes of IP packets per second, including the IP header but not link-specific headers. CBS and EBS are measured in bytes.
At least one of CBS or EBS must be greater than zero. The RFC recommends that each nonzero burst size be at least as large as the largest possible IP packet in the stream.
The CIR sets the rate at which tokens become available. CBS and EBS set the capacity of the two token buckets in bytes.
§3How the Two Token Buckets Work
The Meter has two token buckets, C and E, with maximum sizes CBS and EBS. Both begin full and share the CIR. As tokens are added, bucket C fills first; bucket E receives a token only when C is full and E is not.
In Color-Blind mode, a packet uses tokens from C if it fits, otherwise from E if it fits, and is red if neither bucket has enough tokens for the whole packet. In Color-Aware mode, a packet's existing color also limits whether it can be marked green or yellow. An implementation need not use this exact internal model if its behavior is equivalent.
A packet uses tokens from the committed bucket when enough are available. Otherwise it may use excess-bucket tokens; if neither bucket can cover the packet, it is red.
§4Writing the Color and Using It
The Marker sets the DS field to a codepoint that represents the Meter's result. With the Assured Forwarding per-hop behavior, color can be represented by the packet's drop precedence.
The RFC's example service discards red packets, forwards yellow packets as best effort and forwards green packets with a low drop probability. The document says the srTCM has no known security concerns.
A network can use the color to decide how to treat a packet. The example discards red packets, gives yellow packets best-effort service and gives green packets a lower drop probability.