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Versions: 00                                                            
Internet Engineering Task Force                            Wu-chang Feng
INTERNET-DRAFT                                    University of Michigan
draft-feng-dp-services-00.txt                                  June 1998
                                                   Expires December 1998

                        Drop Preference Services

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its
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   Drop preference has been proposed as a possible means for providing
   differentiated services in the Internet.  Used as a parameter in a
   PHB group, drop preference can provide weighted bandwidth sharing
   amongst flows and flow aggregates of a PHB group.  This memo
   documents a number of services which can be implemented using either
   a single drop preference PHB setting or multiple drop preference

1. Introduction:

   Drop preference (DP) has been proposed in both connection-oriented
   [ATM94,ATM96] and connectionless networks [Diffserv] as a means to
   provide service differentiation between indivitual flows and flow
   aggregates.  The idea behind drop preference is to mark packets with
   different drop priorities and to selectively drop lower priority
   packets when network resources are congested.  In all of its
   proposed forms, drop preference is used to provide some form of
   weighted bandwidth sharing between flows in the network.  Depending
   on how packets are marked, a variety of services can be implemented
   with this functionality.  This memo documents a number of proposed
   services which can be implemented using either a single or multiple
   drop preference settings/parameters within a PHB group.  Services
   implementable using the expedited forwarding (EF) PHB [Nichols] are
   beyond the scope of this document.

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2. Differential Services Architecture

   Drop preference marking, in its current diffserv context, is done at
   the network edges using traffic conditioners (markers) which set the
   appropriate PHB fields.  In the differential services framework,
   drop preference can used as a parameter within a PHB group in order
   to provide service differentiation between flows of a particular PHB
   group.  Within the network, the PHB setting specifies a local
   forwarding treatment or mechanism for transmitting the packet.
   While the proposed mechanisms and algorithms for providing drop
   preference functionality differ, most of them share the same basic
   characteristics [Clark95,Clark97,Feng97,Feng97a,Kilkki]. In
   particular, the mechanisms rely on queueing decisions at the
   bottleneck link in times of congestion.  When such situations occur
   and queues build up, packets with lower priorities are dropped
   preferentially in an attempt to reduce the offered load at the
   bottleneck.  In most cases, it is assumed that the packets of
   individual flows, regardless of their DP setting, are not re-ordered
   by the router.  The use of drop preference gives the network a
   simple, low-overhead, fine-grained means to control bandwidth usage.
   By controlling the amount of priority marking done, the network can
   effectively control the bandwidth usage over time for individual
   flows and flow aggregates.

3. Two-level Drop Preference Services

   There have been several proposed services which use a two-level drop
   preference setting as a building block in their implementations.

3.1 Assured and Controlled-load Services

   In both assured and controlled-load service, the marker/policer
   determines the drop preference setting based on a fixed profile.  In
   assured service [Clark95,Clark97] the profile is based on the
   expected capacity of a flow/aggregate in times of congestion.  The
   profile itself uses a fairly long sliding window to measure
   bandwidth usage.  Based on this profile, packets within their
   expected capacity profile are marked as "In" (conformant) and given
   a higher priority.  When congestion occurs and queues build up, low
   priority packets are dropped first while high priority packets are
   still allowed in the queue.  In controlled-load service
   [Wroclawski], and in particular its diffserv implementation
   [Feng97], the profile is based on a token bucket specification of
   the flow or flow aggregate.  Packets which are within the traffic
   envelope, as specified by the token bucket, are sent with higher
   priority while excess traffic is sent along with normal traffic with
   lower priority.  In both cases, because of the burstiness of the
   traffic being policed and the dynamics of TCP congestion control, a
   large profile must be used in order to maintain predictable quality
   of service.

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   The marking behavior of assured and controlled-load service affords
   flows a constant amount of marking regardless of their sending rate
   at all times.  That is, when a flow/aggregate sends an arbitrary
   amount over its profile, the marker still marks packets at the rate
   indicated in the profile.  This marking differentiates the two
   services from SIMA and CBR as described below.

3.2 Two-level SIMA (Simple Integrated Media Access)

   The original SIMA service [Kilkki], as described in Section 4, uses
   multiple drop preference levels to provide service differentiation
   between flows in times of congestion.  In SIMA, sources contract a
   sending rate with the network referred to as a nominal bit rate
   (NBR).  This rate serves as a profile from which drop preference
   marking is done.  Depending on the actual rate the source sends, the
   network marks each packet of a flow with a particular drop
   preference setting which is calculated as a function of the profile
   and the actual sending rate.  When the source sends at rates below
   its profile, its packets are marked with increasingly higher
   priority.  In contrast, when the source sends at rates above its
   profile, its packets are marked with increasingly lower priority.
   The effect of this type of SIMA marking is to ensure that flows
   within their contracted rate receive a low amount of loss when
   congestion occurs.  Flows which increasingly send above their
   contracted rate see increasingly higher packet loss rates in times
   of congestion as the SIMA marker changes lowers their priorities.
   While SIMA relies on using multiple drop priorities, it is possible
   to build a SIMA-like service using a two-level drop preference
   scheme.  Instead of marking all of a flow's packets with a
   particular drop priority, a variable fraction of a flow's packets
   can be marked with either high or low priority.  Flows sending below
   the profile have most of their packets marked with high priority.
   As flows send increasingly above their profile, the amount of high
   priority marking given steadily decreases to zero.

   The marking behavior of this implementation of SIMA is different
   than both assured service and controlled-load service in that the
   amount of marking steadily decreases as the flow/aggregate exceeds
   its profile.  As described above, the marking remains constant in
   assured and controlled-load service regardless of the amount of data
   the sources are sending above the profile.

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3.3 CBR

   Drop preference can also be used to implement a constant bit rate
   service [Feng97a].  As in assured service, the marker/policer
   observes the bandwidth usage of a connection using a sliding window
   and uses the contracted bit rate as a profile for determining its
   marking behavior.  The marker implementing CBR service attempts to
   mark a minimal amount of packets in order for the flow/aggregate to
   meet its contracted profile.  Thus, for a given profile, the CBR
   marker marks packets at a rate which is less than or equal to that
   of the assured and controlled-load services.  The marker does this
   by observing the sending rate of the flow/aggregate over the sliding
   window.  As long as the flow/aggregate is sending above its profile,
   the marker slowly reduces its marking.  As long as the
   flow/aggregate is sending below its profile, the marker slowly
   increases its marking.  Over a period of time, such adaptive marking
   delivers the flow/aggregate a constant amount bandwidth.  By
   minimizing the amount of priority packets in the network, the CBR
   marker can help improve the performance of priority-based queueing
   algorithms in the network.

   The marking behavior of CBR is different than the above services in
   that marking is reduced to zero when the flow/aggregate exceeds its
   profile.  This is in contrast to the assured and controlled-load
   services which maintain a constant rate of marking regardless of the
   flow/aggregate's actual sending rate and in contrast to SIMA which
   slowly reduces the marking as the flow/aggregate increases its rate.

4. Multi-level Drop Preference Services

   Several services which use more than two drop preference levels have
   been proposed to provide weighted bandwidth sharing amongst flows
   and flow aggregates.

4.1 SIMA

   As described earlier, the original SIMA service uses multiple drop
   priorities to provide service differentiation between flows.  In
   this service, marking is done strictly based on the sending rate of
   the flow/aggregate and its negotiated rate (NBR).  The SIMA marker
   marks all of the packets of a particular flow/aggregate with the
   same priority according to how much it is sending with regard to its
   profile.  As the source(s) increasingly sends above its profile, the
   marker steadily decreases the drop priority.

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4.2 Proportional Sharing

   Cooperative Dropping [Weiss] is a framework for flexibly providing a
   number of bandwidth services (including proportional sharing) using
   multiple drop priorities.  In this scheme, the packets of a
   flow/aggregate are striped across the multiple priority levels.  At
   each intermediate router, preferential dropping is done depending on
   the priority level of the packets and the current level of
   congestion.  Given an incoming multi-priority traffic stream, the
   congested router adaptively selects a drop priority threshold in
   which all packets below the threshold are dropped.  Depending on how
   the packets of a flow/aggregate are striped across the multiple
   priorities, proportional sharing and a variety of other services can
   potentially be implemented.  For example, using the preferential
   dropping mechanism in conjunction with a marker which stripes a
   flow/aggregate's packets evenly across all priorities, sharing in
   direct proportion to the flow/aggregate's sending rate can be
   achieved.  Note that the granularity and accuracy of the bandwidth
   sharing is dependent on the number of priority levels used.  Using
   more priority levels results in a finer degree of bandwidth sharing.

   The marking behavior of proportional sharing, given a profile per
   priority level, remains fixed.  Additional marking policies in the
   cooperative dropping framework can be deployed, however, in order to
   implement a larger range of services.

5. References

   [ATM94]      ATM User-Network Interface (UNI) Signalling
                Specification Version 3.1 (af-uni-0010.002),
                September 1994.

   [ATM96]      ATM Traffic Management Specification Version 4.0
                (af-tm-0056.000), April 1996.

   [Clark95]    D. Clark, "Adding Service Discrimination to the
                September 1995.

   [Clark97]    D. Clark and J. Wroclawski, "An Approach to Service
                Allocation in the Internet", Internet Draft
                <draft-clark-diff-svc-alloc-00.txt>, July 1997.

   [Feng97]     W. Feng, D. Kandlur, D. Saha, and K. Shin,
                "Understanding TCP Dynamics in an Integrated Services
                Internet", Proc. of NOSSDAV '97,
                http://www.eecs.umich.edu/~wuchang/ered/ ,
                July 1997.

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   [Feng97a]    W. Feng, D. Kandlur, D. Saha, and K. Shin, "Adaptive
                Packet Marking for Providing Differentiated Services
                in the Internet", UM CSE-TR-347-97,
                http://www.eecs.umich.edu/~wuchang/pmg/ ,
                September 1997.

   [Ferguson]   P. Ferguson, "Simple Differential Services: IP TOS and
                Precedence, Delay Indication, and Drop Preference,
                Internet Draft <draft-ferguson-delay-drop-00.txt>,
                November 1997.

   [Diffserv]   Differentiated Services Framework Draft, May 1998.

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

   [Nichols]    K. Nichols and S. Blake, "Definition of the
                Differentiated Services Field (DS Byte) in the IPv4
                and IPv6 Headers", Internet Draft
                <draft-ietf-diffserv-headers-00.txt>, May 1998.

   [Weiss]      W. Weiss, "Providing Differentiated Services through
                Cooperative Dropping and Delay Indication", Internet
                Draft <draft-weiss-cooperative-drop-00.txt>, March 1998.

   [Wroclawski] J. Wroclawski, "Specification of the Controlled-Load
                Network Element Service." RFC 2211, September 1997.

Author's Address

   Wu-chang Feng
   University of Michigan
   Dept. of EECS
   Real-time Computing Laboratory
   1301 Beal Ave./2228 EECS Building
   Ann Arbor, MI  48109-2122
   Phone: +1-734-763-5363
   Fax:   +1-734-763-4617
   E-mail: wuchang@eecs.umich.edu

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