One document matched: draft-silverman-diffserv-mlefphb-00.txt
Internet Engineering Task Force Steve Silverman
INTERNET DRAFT Houston Associates
Expires: Aug. 2003 February 2003
Dan Sullivan
Houston Associates
Mike Pierce
Artel
Don Choi
Defense Information Systems Agency
draft-silverman-diffserv-mlefphb-00.txt
Multi-Level Expedited Forwarding Per Hop Behavior (MLEF
PHB)
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026 [ ].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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Abstract
Some networks require certain connections to have greater
priority than others. This draft defines a new PHB (Per Hop
Behavior), the Multi-Level Expedited Forwarding (MLEF) (patent
pending)
PHB. The standard Expedited Forwarding PHB (RFC3246) defines a PHB
for
applications requiring low latency. This document extends that
concept and defines a PHB with multiple priority levels for
applications requiring low latency.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
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"OPTIONAL" in this document are to be interpreted as described in
RFC-2119 [ ].
Table of Contents
1 INTRODUCTION 2
1.1 Background 3
2 Applicability 3
2.1 Scope 3
3 Overview 3
4 Packet Processing 4
5 Security Considerations 4
6 References 4
7 Author's Addresses 5
8 ISSUES 5
8.1 Name 5
8.2 Byte vs. Packet count 5
8.3 Maximum Packet Size 6
9 Appendix - Examples 6
9.1 Sample Procedure 6
9.2 Sample Configuration for Emergency Services 7
9.3 Sample Configuration for MLPP 7
1 INTRODUCTION
This draft defines an experimental Per Hop Behavior (PHB) to
support the Multi-Level Precedence & Preemption function (MLPP)
which is required by the U.S. Department of Defense and various
other government organizations in both the US and other countries.
This draft is an extension of RFC 3246. RFC 3246 requires that
packets be dropped if in excess of the "negotiated rate". This
draft extends the EF PHB and describes a dropping algorithm based
on multiple Differentiated Services Code Points (DSCPs) values by
providing different amounts of buffering for different DSCPs.
1.1 Background
Military networks are often unable to provision all of the
bandwidth that their users need. The widespread use of mobile
platforms (limiting the use of fiber optic trunks), the need to
avoid detection, and the exposure to unexpected loss of resources
aggravate this problem. A partial solution to this problem is
the Multi-Level Precedence & Preemption function (MLPP). This
assigns priority to certain users. If there is congestion in the
voice network, higher priority calls get precedence for various
resources relative to lower priority users. In certain private
networks, some users may be preempted by higher priority users.
This function is intended for use in those private networks that
require this function. THIS DOES NOT INCLUDE PREEMPTION IN THE
PUBLIC NETWORK. In the traditional Circuit-Switched telephone
networks, this function has been supported for forty years.
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2 Applicability
This PHB will be experimental and optional and is expected to be
deployed only on certain private networks.
2.1 Scope
This draft defines the Per Hop Behavior (PHB) to support multiple
priority levels in Expedited Forwarding. It does not define the
signaling required to establish the priority connections, the
accounting that might be required, or security issues that should
be addressed in conjunction with the use of this PHB.
3 Overview
Expedited Forwarding [RFC3246] limited the buffer queue of an
output port to a size that would not introduce significant delay
into a hop by monitoring the queue occupancy and admitting new
packets to the queue only if the buffer occupancy was below a
configured threshold. This resulted in dropping of packets that
were in excess of the configured capacity. MLEF extends this by
making the thresholds for dropping packets a function of class,
which is based on priority level. The buffer size, the
Differentiated Services Code Points (DSCPs) for each class, and
the per class thresholds may be configured for each router
supporting this option.
Draft-pierce-ieprep-pref-treat-examples-00 describes how this
Multi-level packet dropping procedure replaces the need for
actual preemption.
4 Packet Processing
A configuration/initialization procedure would calculate the
maximum count in the buffer for each traffic class.
(MaxClassCnt(i) ). This is the maximum buffer percentage times
the buffer size.
A maximum packet size should be specified as part of
configuration.
As each packet is received, any packet larger than that maximum
should be dropped. The current Buffer Occupancy Count (BOC),
expressed as a number of packets, is then checked against the
threshold limit for the Class (MaxClassCnt) and, if the BOC is
less than the limit, the packet is enqueued on the MLEF output
queue. If the BOC is greater than or equal to this limit, the
packet is discarded.
All processing for this could be done at the byte level rather
than the packet level. The process has been described dealing
with packets because at this time we believe that this will
simplify the processing and minimize the calculation load. If
subsequent analysis shows that working with byte counts is more
efficient, that approach would be equivalent to the packet
approach. Such an implementation would have no impact on
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interoperability since this process affects which packets are
forwarded but does not change any packets.
5 Security Considerations
This document addresses a way to provide multiple priority levels
to sessions that require Expedited Forwarding. Since the network
can not provide special service to all users, some form of
security is required so that only authorized users can invoke
these services.
6 References
1 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
2 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
3 Davie, B., "An Expedited Forwarding PHB (Per-Hop Behavior)"
RFC3246
4 Draft-pierce-ieprep-pref-treat-examples-00
7 Author's Addresses
Steve Silverman
Houston Associates Inc.
4601 N. Fairfax Drive, Arlington, VA 22203>
Phone: 540 631-0711
Email: <steves@shentel.net>
Dan Sullivan
Houston Associates Inc.
4601 N. Fairfax Drive, Arlington, VA 22203>
Phone:703 284-8837
Email: dsullivan@hai.com
Michael Pierce
Artel
1893 Preston White Drive
Reston, VA 20191
Phone: +1 410.817.4795
Email: pierce1m@ncr.disa.mil
Don Choi
DISA
5600 Columbia Pike
Falls Church, VA 22041-2717
Phone: +1 703.681.2312
Email: choid@ncr.disa.mil
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8 ISSUES
Questions to be addressed before this is submitted.
8.1 Name
MLPP or Assured Services ??
8.2 Byte vs. Packet count
Byte work would be much more cumbersome to execute or code.
That's why we originally did it as packet count.
8.3 Maximum Packet Size
This is a sensitive issue but we think it is key to making this
work. Large packets will trample voice.
If we want to make something usable, we have to explain the
realities and let whoever is in charge
decide what they want to do. It is not a minor tweak of the
protocol. Fragmentation and limited
packet size are necessary to support voice over low bandwidth (E1
and under) lines. It should be noted
that this is not an issue particular to MLEF. It is fundamental to
voice traffic over any protocol that
does not fragment large packets.
9 Appendix - Examples
9.1 Sample Procedure
The following is one possible way to implement the function
described above.
Variables
N = number of classes in the system. One DSCP per class
i is the index for the traffic classes. It may take the value from
1
to N.
MaxPacketSize: the maximum packet size for a packet using this
behavior
MaxPacketCnt: Number of packets buffered in this router for a port
for the MLEF PHB. Output bandwidth rate times .75 (leaving
something for router control) * the maximum time that can be
added to a packet and still have it useful for voice (50 ms) /
bits/pkt. This assumes several routers are in the call path.
BP(i) For each class, the buffer percentage that can be used
(range [0 - 1])
MaxClassCnt(i) For each class, = MaxPacketCnt * BP(i) rounded
up to an integer
BOC Buffer Occupancy Count The number of MLEF packets in the
buffer.
Procedures:
Initial procedure:
Calculate PC(i)
For each incoming packet:
If pkt size > MaxPacketSize either discard packet or move to
best effort queue
If BOC >= PC(i) discard packet
If BOC < PC(i) then enqueue packet and increment BOC
End
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Outgoing Packet
For each packet to be transmitted, dequeue packet, transmit,
decrement BOC
End
9.2 Sample Configuration for MLPP
This is an example of how the PHB could be used to support the DOD
requirement for MLPP. It
defines 5 classes of traffic. The calculation is for a T1, 1.54
Mb/s. The 200 byte packet size assumes
G.711, and 20 ms samples.
Number levels 5
Max Size pkt 200
Number pkts 36
DSCP Name Buffer % pkts Rounded
42 Flash O 100 36 36
43 Flash 0.85 30.6 31
44 Immediate 0.7 25.2 26
45 Priority 0.6 21.6 22
46 Routine 0.5 18 18
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