Internet DRAFT - draft-feng-dp-services
draft-feng-dp-services
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
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Abstract
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
settings.
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
Internet",
http://ana-www.lcs.mit.edu/anaweb/ps-papers/TPRC2-0.ps,
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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