rfc2761.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                           J. Dunn
Request for Comments: 2761                                    C. Martin
Category: Informational                                       ANC, Inc.
                                                          February 2000


                    Terminology for ATM 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 (2000).  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
   Asynchronous Transfer Mode (ATM) based switching devices. The terms
   defined in this memo will be used in addition to terms defined in
   RFCs 1242, 2285, and 2544. This memo is a product of the Benchmarking
   Methodology Working Group (BMWG) of the Internet Engineering Task
   Force (IETF).

Introduction

   This document provides terminology for benchmarking ATM based
   switching devices. It extends terminology already defined for
   benchmarking network interconnect devices in RFCs 1242, 2285, and
   2544. 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 ATM cell relay and signaling.

   This memo contains two major sections: Background and Definitions.
   Within the definitions section is a formal definitions subsection,
   provided as a courtesy to the reader, and a measurement definitions
   sub-section, that contains performance metrics with inherent units.
   The divisions of the measurement sub-section follow the BISDN model.









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   The BISDN model comprises four layers and two planes. This document
   addresses the interactions between these layers and how they effect
   IP and TCP throughput. A schematic of the B-ISDN model follows:

   ---------|--------------------------|-------------------------------
            |     User Plane           |       Control Plane
   ---------|--------------------------|--------------------------------
   Services |          IP              |   ILMI   |       UNI, PNNI
   ---------|--------------------------|----------|---------------------
   AAL      | AAL1, AAL2, AAL3/4, AAL5 |   AAL5   |        SAAL
   ---------|--------------------------|----------|---------------------
   ATM      |      Cell Relay          |        OAM, RM
   ---------|--------------------------|--------------------------------
            |       Convergence        |
   Physical |--------------------------|--------------------------------
            |         Media            |
   ---------|--------------------------|--------------------------------

   This document assumes that necessary services are available and
   active.  For example, IP connectivity requires SSCOP connectivity
   between signaling entities.  Further, it is assumed that the SUT has
   the ability to configure ATM addresses (via hard coded addresses,
   ILMI or PNNI neighbor discovery), has the ability to run SSCOP, and
   has the ability to perform signaled call setups (via UNI or PNNI
   signaling).  This document covers only CBR, VBR and UBR traffic
   types.  ABR will be handled in a separate document.  Finally, this
   document presents only the terminology associated with benchmarking
   IP performance over ATM; therefore, it does not represent a total
   compilation of ATM test terminology.

   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.

Existing Definitions

   RFC 1242, "Benchmarking Terminology for Network Interconnect Devices"
   should be consulted before attempting to make use of this document.
   RFC 2544, "Benchmarking Methodology for Network Interconnect Devices"
   contains discussions of a number of terms relevant to the
   benchmarking of switching devices and should 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.
   Definitions are indexed and grouped together in sections for ease of



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   reference.  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
   "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
   "OPTIONAL" go in this document are to be interpreted as described in
   RFC 2119.

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.

1. Formal Definitions

1.1. Definition Format (from RFC 1242)

   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.  These discussions pertain
   solely to the impact of a particular ATM parameter on IP or TCP;
   therefore, definitions which contain no configurable components or
   whose components will have the discussion: None.

   Specification: The working group and document in which the terms are
   specified and are listed in the references section.

1.2.  Related Definitions

1.2.1. ATM Adaptation Layer (AAL)

   Definition: The layer in the B-ISDN reference model (see B-ISDN)
   which adapts higher layer PDUs into the ATM layer.

   Discussion: There are four types of adaptation layers: AAL 1: used
   for circuit qemulation, voice over ATM AAL2: used for sub-rated voice
   over ATM AAL3/4: used for data over noisy ATM lines AAL5: used for
   data over ATM, most widely used AAL type









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   These AAL types are not measurements, but it is possible to measure
   the time required for Segmentation and Reassembly (SAR).

   Specification: I.363

1.2.2. ATM Adaptation Layer Type 5 (AAL5)

   Definition: AAL5 adapts multi-cell higher layer PDUs into ATM with
   minimal error checking and no error detection.  The AAL5 CPCS (Common
   Paer Convergence Sub-layer) PDU is defined as follows:

|---------------------------|---------------------------|--------------|
|    Higher Layer PDU       |   Padding (If needed)     |    Trailer   |
|---------------------------|---------------------------|--------------|

   Where the padding is used to ensure that the trailer occupies the
   final 8 octets of the last cell.

   The trailer is defined as follows:

   |--------------|--------------|--------------|--------------|
   |   CPCS-UU    |     CPI      |    Length    |   CRC-32     |
   |--------------|--------------|--------------|--------------|

   where:

   CPCS-UU is the 1 octet Common Part Convergence Sub-layer User to User
   Indication and may be used to communicate between two AAL5 entities.

   CPI is the 1 octet Common Part Indicator and must be set to 0.

   Length is the 2 octet length of the higher layer PDU.

   CRC-32 is a 32 bit (4 octet) cyclic redundancy check over the entire
   PDU.

   Discussion: AAL5 is the adaptation layer for UNI signaling, ILMI,
   PNNI signaling, and for IP PDUs. It is the most widely used AAL type
   to date.  AAL5 requires two distinct processes.  The first is the
   encapsulation, on the transmit side, and de-encapsulation, on the
   receive side, of the higher layer PDU into the AAL5 CPCS PDU which
   requires the computation of the length and the CRC-32.  The time
   required for this process depends on whether the CRC-32 computation
   is done on the interface (on-board) or in machine central memory (in
   core).  On-board computation should produce only a small, constant
   delay; however, in core computation will produce variable delay,
   which will negatively effect TCP RTT computations. The second process
   is segmentation and re-assembly (SAR) which is defined below (see



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   SAR).

   Specification: I.363.5

1.2.3. Asynchronous Transfer Mode (ATM)

   Definition: A transfer mode in which the information is organized
   into 53 octet PDUs called cells. It is asynchronous in the sense that
   the recurrence of cells containing information from an individual
   user is not necessarily periodic.

   Discussion: ATM is based on the ISDN model; however, unlike ISDN, ATM
   uses fixed length (53 octet) cells. Because of the fixed length of
   ATM PDUs, higher layer PDUs must be adapted into ATM using one of the
   four ATM adaptation layers (see AAL).

   Specification: AF-UNI3.1

1.2.4. ATM Link

   Definition: A virtual path link (VPL) or a virtual channel link
   (VCL).

   Discussion: none.

   Specification: AF-UNI3.1

1.2.5. ATM Peer-to-Peer Connection

   Definition: A virtual channel connection (VCC) or a virtual path
   connection (VPC).

   Discussion: none.

   Specification: AF-UNI3.1

1.2.6. ATM Traffic Descriptor

   Definition: A generic list of traffic parameters, which specify the
   intrinsic traffic characteristics of a requested ATM connection (see
   GCRA), which must include PCR and QoS and may include BT, SCR and
   best effort (UBR) indicator.









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   Discussion: The effects of each traffic parameter will be discussed
   individually.

   Specification: AF-UNI3.1

1.2.7. ATM User-User Connection

   Definition: An association established by the ATM Layer to support
   communication between two or more ATM service users (i.e., between
   two or more next higher entities or between two or more ATM-
   entities).  The communications over an ATM Layer connection may be
   either bi-directional or unidirectional. The same Virtual Channel
   Identifier (VCI) is issued for both directions of a connection at an
   interface.

   Discussion: Because ATM is connection oriented, certain features of
   IP (i.e. those which require multicast) are not available.

   Specification: AF-UNI3.1

1.2.8. Broadband ISDN (B-ISDN) Model

   Definition: A layered service model that specifies the mapping of
   higher layer protocols onto ATM and its underlying physical layer.
   The model is composed of four layers: Physical, ATM, AAL and Service.

   Discussion: See discussion above.

   Specification: I.321

1.2.9. Burst Tolerance (BT)

   Definition: A traffic parameter, which, along with the Sustainable
   Cell Rate (SCR), specifies the maximum number of cells which will be
   accepted at the Peak Cell Rate (PCR) on an ATM connection.

   Discussion: BT applies to ATM connections supporting VBR services and
   is the limit parameter of the GCRA. BT will effect TCP and IP PDU
   loss in that cells presented to an interface which violate the BT may
   be dropped, which will cause AAL5 PDU corruption. BT will also effect
   TCP RTT calculation.  BT=(MBS-1)*(1/SCR 1/PCR) (see MBS, PCR, SCR).

   Specification: AF-TM4.0








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1.2.10. Call

   Definition: A call is an association between two or more users or
   between a user and a network entity that is established by the use of
   network capabilities. This association may have zero or more
   connections.

   Discussion: none.

   Specification: AF-UNI3.1

1.2.11. Cell

   Definition: A unit of transmission in ATM. A fixed-size frame
   consisting of a 5-octet header and a 48-octet payload.

   Discussion: none.

   Specification: AF-UNI3.1

1.2.12. Call-based

   Definition: A transport requiring call setups - see CALL definition.

   Discussion: none.

   Specification: AF-UNI3.1

1.2.13. Cell Delay Variation Tolerance (CDVT)

   Definition: ATM layer functions may alter the traffic characteristics
   of ATM connections by introducing Cell Delay Variation. When cells
   from two or more ATM connections are multiplexed, cells of a given
   ATM connection may be delayed while cells of another ATM connection
   are being inserted at the output of the multiplexer.  Similarly, some
   cells may be delayed while physical layer overhead or OAM cells are
   inserted.  Consequently, some randomness may affect the inter-arrival
   time between consecutive cells of a connection as monitored at the
   UNI.  The upper bound on the "clumping" measure is the CDVT.

   Discussion: CDVT effects TCP round trip time calculations.  Large
   values of CDVT will adversely effect TCP throughput and cause SAR
   timeout.  See discussion under SAR.

   Specification: AF-TM4.0






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1.2.14. Cell Header

   Definition: ATM Layer protocol control information.

   Discussion: The ATM cell header is a 5-byte header that contains the
   following fields: Generic Flow Control (GFC) 4 bits Virtual Path
   Identifier (VPI) 8 bits Virtual Channel Identifier (VCI) 16 bits
   Payload Type (PT) 3 bits Cell Loss Priority (CLP) 1 bit Header Error
   Check (HEC) 8 bit CRC computed over the previous four octets

   Each field is discussed in this document.

   Specification: AF-UNI3.1

1.2.15. Cell Loss Priority (CLP)

   Definition: This bit in the ATM cell header indicates two levels of
   priority for ATM cells. CLP=0 cells are higher priority than CLP=1
   cells.  CLP=1 cells may be discarded during periods of congestion to
   preserve the CLR of CLP=0 cells.

   Discussion: The CLP bit is used to determine GCRA contract
   compliance.  Specifically, two traffic contracts may apply to a
   single connection:  CLP=0, meaning only cells with CLP=0, and
   CLP=0+1, meaning cells with CLP=0 or CLP=1.

   Specification: AF-UNI3.1

1.2.16. Connection

   Definition: An ATM connection consists of concatenation of ATM Layer
   links in order to provide an end-to-end information transfer
   capability to access points.

   Discussion: none.

   Specification: AF-UNI3.1

1.2.17. Connection Admission Control (CAC)

   Definition: Connection Admission Control is defined as the set of
   actions taken by the network during the call set-up phase (or during
   call re-negotiation phase) in order to determine whether a connection
   request can be accepted or should be rejected (or whether a request
   for re-allocation can be accommodated).