rfc3133.txt

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Network Working Group                                            J. Dunn
Request for Comments: 3133                                     C. Martin
Category: Informational                                        ANC, Inc.
                                                               June 2001


                Terminology for Frame Relay Benchmarking

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 (2001).  All Rights Reserved.

Abstract

   This memo discusses and defines terms associated with performance
   benchmarking tests and the results of these tests in the context of
   frame relay switching devices.

I. Background

1. Introduction

   This document provides terminology for Frame Relay switching devices.
   It extends terminology already defined for benchmarking network
   interconnect devices in RFCs 1242, 1944 and 2285.  Although some of
   the definitions in this memo may be applicable to a broader group of
   network interconnect devices, the primary focus of the terminology in
   this memo is on Frame Relay Signaling.

   This memo contains two major sections: Background and Definitions.
   The background section provides the reader with an overview of the
   technology and IETF formalisms.  The definitions section is split
   into two sub-sections.  The formal definitions sub-section is
   provided as a courtesy to the reader.  The measurement definitions
   sub-section contains performance metrics with inherent units.

   The BMWG produces two major classes of documents: Benchmarking
   Terminology documents and Benchmarking Methodology documents.  The
   Terminology documents present the benchmarks and other related terms.
   The Methodology documents define the procedures required to collect
   the benchmarks cited in the corresponding Terminology documents.




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   For the purposes of computing several of the metrics, certain textual
   conventions are required.  Specifically:

   1) The notation sum {i=1 to N} A_i denotes: the summation of N
   instances of the observable A.  For example, the set of observations
   {1,2,3,4,5} would yield the result 15.

   2) The notation max {I=1 to N} A_i and min {I=1 to N} A_i denotes:
   the maximum or minimum of the observable A over N instances.  For
   example, given the set of observations {1,2,3,4,5}, max {i=1 to 5} =
   5 and min {I=1 to 5} = 1.

   The terms defined in this memo will be used in addition to terms
   defined in RFCs 1242, 1944 and 2285.  This memo is a product of the
   Benchmarking Methodology Working Group (BMWG) of the Internet
   Engineering Task Force(IETF).

2. Existing Definitions

   RFC 1242, "Benchmarking Terminology for Network Interconnect
   Devices", should be consulted before attempting to make use of this
   document.  RFC 1944, "Benchmarking Methodology for Network
   Interconnect Devices", contains discussions of a number of terms
   relevant to the benchmarking of switching devices and should also be
   consulted.  RFC 2285, "Benchmarking Terminology for LAN Switching
   Devices", contains a number of terms pertaining to traffic
   distributions and datagram interarrival.  For the sake of clarity and
   continuity this RFC adopts the template for definitions set out in
   Section 2 of RFC 1242.

II. Definitions

   The definitions presented in this section have been divided into two
   groups.  The first group is formal definitions, which are required in
   the definitions of the performance metrics but are not themselves
   strictly metrics.  These definitions are subsumed from other work
   done in other working groups both inside and outside the IETF.  They
   are provided as a courtesy to the reader.













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1. Formal Definitions

1.1. Definition Format (from RFC1242)

   Term to be defined.

   Definition: The specific definition for the term.

   Discussion: A brief discussion of the term, its application and any
   restrictions on measurement procedures.

   Specification:  The working group and document in which the term is
   specified.  Listed in the references.

1.2. Frame Relay Related Definitions

1.2.1. Access Channel

   Definition: Access channel refers to the user access channel across
   which frame relay data travels.  Within a given DS-3, T1 or E1
   physical line, a channel can be one of the following, depending of
   how the line is configured.  Possible line configurations are:

   A. Unchannelized: The entire DS-3/T1/E1 line is considered a channel,
   where:

   The DS-3 line operates at speeds of 45 Mbps and is a single channel.
   The T1 line operates at speeds of 1.536 Mbps and is a single channel
   consisting of 24 T1 time slots.  The E1 line operates at speeds of
   1.984 Mbps and is a single channel consisting of 30 DS0 time slots.

   B. Channelized: The channel is any one of N time slots within a given
   line, where:

   The T1 line consists of any one or more channels.  Each channel is
   any one of 24 time slots.  The T1 line operates at speeds in
   multiples of 56/64 Kbps to 1.536 Mbps, with aggregate speed not
   exceeding 1.536 Mbps.  The E1 line consists of one or more channels.
   Each channel is any one of 31 time slots.  The E1 line operates at
   speeds in multiples of 64 Kbps to 1.984 Mbps, with aggregate speed
   not exceeding 1.984 Mbps.

   C. Fractional: The T1/E1 channel is one of the following groupings of
   consecutively or non-consecutively assigned time slots:

   N DS0 time slots (NX56/64Kbps where N = 1 to 24 DS0 time slots per
   FT1 channel).




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   N E1 time slots (NX64Kbps, where N = 1 to 30 DS0 time slots per E1
   channel).

   Discussion: Access channels specify the physical layer interface
   speed of a DTE or DCE.  In the case of a DTE, this may not correspond
   to either the CIR or EIR.  Specifically, based on the service level
   agreement in place, the user may not be able to access the entire
   bandwidth of the access channel.

   Specification: FRF

1.2.2. Access Rate (AR)

   Definition: The data rate of the user access channel.  The speed of
   the access channel determines how rapidly (maximum rate) the end user
   can inject data into a frame relay network.

   Discussion: See Access Channel.

   Specification: FRF

1.2.3. Backward Explicit Congestion Notification (BECN)

   Definition: BECN is a bit in the frame relay header.  The bit is set
   by a congested network node in any frame that is traveling in the
   reverse direction of the congestion.

   Discussion: When a DTE receives frames with the BECN bit asserted, it
   should begin congestion avoidance procedures.  Since the BECN frames
   are traveling in the opposite direction as the congested traffic, the
   DTE will be the sender.  The frame relay layer may communicate the
   possibility of congestion to higher layers, which have inherent
   congestion avoidance procedures, such as TCP.  See Frame Relay Frame.

   Specification: FRF

1.2.4. Burst Excess(Be)

   Definition: The maximum amount of uncommitted data (in bits) in
   excess of Committed Burst Size (Bc) that a frame relay network can
   attempt to deliver during a Committed Rate Measurement Interval (Tc).
   This data (Be) generally is delivered with a lower probability than
   Bc.  The network treats Be data as discard eligible.

   Discussion: See also Committed burst Size (Bc), Committed Rate
   Measurement Interval (Tc) and Discard Eligible (De).

   Specification: FRF



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1.2.5. Committed Burst Size (Bc)

   Definition: The maximum amount of data (in bits) that the network
   agrees to transfer, under normal conditions, during a time interval
   Tc.

   Discussion: See also Excess Burst Size (Be) and Committed Rate
   Measurement Interval (Tc).

   Specification: FRF

1.2.6. Committed Information Rate (CIR)

   Definition: CIR is the transport speed the frame relay network will
   maintain between service locations when data is presented.

   Discussion: CIR specifies the guaranteed data rate between two frame
   relay terminal connected by a frame relay network.  Data presented to
   the network in excess of this data rate and below the Excess
   Information Rate (EIR) will be marked as Discard Eligible and may be
   dropped.

   Specification: FRF

1.2.7. Committed Rate Measurement Interval (Tc)

   Definition: The time interval during which the user can send only
   Bc-committed amount of data and Be excess amount of data.  In
   general, the duration of Tc is proportional to the "burstiness" of
   the traffic.  Tc is computed (from the subscription parameters of CIR
   and Bc) as Tc = Bc/CIR.  Tc is not a periodic time interval.
   Instead, it is used only to measure incoming data, during which it
   acts like a sliding window.  Incoming data triggers the Tc interval,
   which continues until it completes its computed duration.

   Discussion: See also Committed Information Rate (CIR) and committed
   Burst Size (Bc).

   Specification: FRF

1.2.8. Cyclic Redundancy Check (CRC)

   Definition: A computational means to ensure the accuracy of frames
   transmitted between devices in a frame relay network.  The
   mathematical function is computed, before the frame is transmitted,






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   at the originating device.  Its numerical value is computed based on
   the content of the frame.  This value is compared with a recomputed
   value of the function at the destination device.  See also Frame
   Check Sequence (FCS).

   Discussion: CRC is not a measurement, but it is possible to measure
   the amount of time to perform a CRC on a string of bits.  This
   measurement will not be addressed in this document.

   Specification: FRF

1.2.9. Data Communications Equipment (DCE)

   Definition: Term defined by both frame relay and X.25 committees,
   that applies to switching equipment and is distinguished from the
   devices that attach to the network (DTE).

   Discussion: Also see DTE.

   Specification: FRF

1.2.10. Data Link Connection Identifier (DLCI)

   Definition: A unique number assigned to a PVC end point in a frame
   relay network.  Identifies a particular PVC endpoint within a user's
   access channel in a frame relay network and has local significance
   only to that channel.

   Discussion: None.

   Specification: FRF

1.2.11. Data Terminal Equipment (DTE)

   Definition: Any network equipment terminating a network connection
   and is attached to the network.  This is distinguished from Data
   Communications Equipment (DCE), which provides switching and
   connectivity within the network.

   Discussion: See also DCE.

   Specification: FRF









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1.2.12. Discard Eligible (DE)

   Definition: This is a bit in the frame relay header that provides a
   two level priority indicator, used to bias discard frames in the
   event of congestion toward lower priority frames.  Similar to the CLP
   bit in ATM.

   Discussion: See Frame Relay Frame.

   Specification: FRF

1.2.13. Discardable frames

   Definition: Frames identified as being eligible to be dropped in the
   event of congestion.

   Discussion: The discard eligible field in the frame relay header is
   the correct -- and by far the most common -- means of indicating
   which frames may be dropped in the event of congestion.  However, DE
   is not the only means of identifying which frames may be dropped.
   There are at least three other cases that apply.

   In the first case, network devices may prioritize frame relay traffic
   by non-DE means.  For example, many service providers prioritize
   traffic on a per-PVC basis.  In this instance, any traffic from a
   given DLCI (data link channel identifier) may be dropped during
   congestion, regardless of whether DE is set.

   In the second case, some implementations use upper-layer criteria,
   such as IP addresses or TCP or UDP port numbers, to prioritize
   traffic within a single PVC.  In this instance, the network device
   may evaluate discard eligibility based on upper-layer criteria rather
   than the presence or absence of a DE bit.

   In the third case, the frame is discarded because of an error in the
   frame.  Specifically, frames that are too long or too short, frames
   that are not a multiple of 8 bits in length, frames with an invalid
   or unrecognized DLCI, frames with an abort sequence, frames with
   improper flag delimitation, and frames that fail FCS.

   Specification: FRMIB

1.2.14. Discarded frames

   Definition: Those frames dropped by a network device.






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   Discussion: Discardable and discarded frames are not synonymous.
   Some implementations may ignore DE bits or other criteria, even
   though they supposedly use such criteria to determine which frames to
   drop in the event of congestion.

   In other cases, a frame with its DE bit set may not be dropped.  One
   example of this is in cases where congestion clears before the frame
   can be evaluated.

   Specification: DN

1.2.15. Forward Explicit Congestion Notification (FECN)

   Definition:  FECN is a bit in the frame relay header.  The bit is set
   by a congested network node in any frame that is traveling in the
   same direction of the congestion.

   Discussion: When a DTE receives frames with the FECN bit asserted, it
   should begin congestion avoidance procedures.  Since the FECN frames
   are traveling in the same direction as the congested traffic, the DTE
   will be the receiver.  The frame relay layer may communicate the
   possibility of congestion to higher layers, which have inherent
   congestion avoidance procedures, such as TCP.  See Frame Relay Frame.

   Specification: FRF

1.2.16. Frame Check Sequence (FCS)

   Definition: The standard 16-bit cyclic redundancy check used for HDLC
   and frame relay frames.  The FCS detects bit errors occurring in the
   bits of the frame between the opening flag and the FCS, and is only
   effective in detecting errors in frames no larger than 4096 octets.
   See also Cyclic Redundancy Check (CRC).

   Discussion: FCS is not a measurement, but it is possible to measure
   the amount of time to perform a FCS on a string of bits.  This
   measurement will not be addressed in this document.

   Specification: FRF

1.2.17. Frame Entry Event

   Definition: Frame enters a network section or end system.  The event
   occurs when the last bit of the closing flag of the frame crosses the
   boundary.

   Discussion: None.




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