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.


RFC 2761            Terminology for ATM Benchmarking       February 2000


   2. C. Coupled with re-assembly, there exists some mechanism for
   identifying the start of a higher layer PDU, e.g., IP, and the cells
   associated with the first incomplete AAL5 PDU are discarded,
   resulting in the loss of one packet.

   Specification: AF-UNI3.1

1.2.40. Sustainable Cell Rate (SCR)

   Definition: The SCR is an upper bound on the conforming average rate
   of an ATM connection over time scales which are long relative to
   those for which the PCR is defined. Enforcement of this bound by the
   UPC could allow the network to allocate sufficient resources, but
   less than those based on the PCR, and still ensure that the
   performance objectives (e.g., for Cell Loss Ratio) can be achieved.

   Discussion: SCR limits the average data rate on an ATM connection.
   If a user violates the SCR, cells may be dropped resulting in Cell
   Loss.  This in turn will negatively impact AAL5 PDUs, which may be
   carrying IP datagrams.  See the discussion under SAR.

   Specification: AF-TM4.0

1.2.41. Switched Connection

   Definition: A connection established via signaling.

   Discussion: none.

   Specification: AF-UNI3.1

1.2.42. Switched Virtual Channel Connection (SVCC)

   Definition: A Switched VCC is one that is established and taken down
   dynamically through control signaling. A Virtual Channel Connection
   (VCC) is an ATM connection where switching is performed on the
   VPI/VCI fields of each cell.

   Discussion: none.

   Specification: AF-UNI3.1










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1.2.43. Switched Virtual Circuit (SVC)

   Definition: A connection established via signaling. The user defines
   the endpoints when the call is initiated.

   Discussion: SVCs are established using either UNI signaling or PNNI
   signaling.  The signaling state machine implements several timers,
   which can effect the time required for call establishment.  This will
   effect TCP round trip time calculation, effecting TCP throughput.
   Specifically, there are two possibilities.  In the case where Call
   Proceeding is not implemented, there is only one timer, T310, with a
   value of 10 seconds.  In the case where Call Proceeding is
   implemented, there are two timers, T303 and T310, with the values 4
   and 10 seconds, respectively.  In either case, if a timer, either
   T303 or T310, expires after a Setup message is send, the calling
   party has the option of re-transmitting the Setup.  In the T303 case,
   this yields a maximum setup time of 18 seconds and, In the T310 case,
   a maximum setup time of 20 seconds.  Thus, the initial TCP RTT
   calculation will be on he order of 20 seconds.

   Specification: AF-UNI3.1, AF-UNI4.0, AF-PNNI1.0

1.2.44. Switched Virtual Path Connection (SVPC)

   Definition: A Switched Virtual Path Connection is one that is
   established and taken down dynamically through control signaling.  A
   Virtual Path Connection (VPC) is an ATM connection where switching is
   performed on the VPI field only of each cell.

   Discussion: none.

   Specification: AF-UNI3.1

1.2.45. Traffic Contract

   Definition: A specification of the negotiated traffic characteristics
   of an ATM connection.

   Discussion: See discussions under BT, CAC, CDV, GCRA, PCR and SCR.

   Specification: AF-TM4.0

1.2.46. Traffic Management (TM)

   Definition: Traffic Management is the aspect of the traffic control
   and congestion control procedures for ATM. ATM layer traffic control
   refers to the set of actions taken by the network to avoid congestion
   conditions.  ATM layer congestion control refers to the set of



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RFC 2761            Terminology for ATM Benchmarking       February 2000


   actions taken by the network to minimize the intensity, spread and
   duration of congestion. The following functions form a framework for
   managing and controlling traffic and congestion in ATM networks and
   may be used in appropriate combinations.

      Connection Admission Control
      Feedback Control
      Usage Parameter Control
      Priority Control
      Traffic Shaping
      Network Resource Management
      Frame Discard
      ABR Flow Control

   Discussion: See CAC and traffic shaping.

   Specification: AF-TM4.0

1.2.47. Traffic Shaping (TS)

   Definition: Traffic Shaping is a mechanism that alters the traffic
   characteristics of a stream of cells on a connection to achieve
   better network efficiency, while meeting the QoS objectives, or to
   ensure conformance at a subsequent interface. Traffic shaping must
   maintain cell sequence integrity on a connection.  Shaping modifies
   traffic characteristics of a cell flow with the consequence of
   increasing the mean Cell Transfer Delay.

   Discussion: TS should improve TCP throughput by reducing RTT
   variations.  As a result, TCP RTT calculations should be more stable.

   Specification: AF-UNI3.1

1.2.48. Transmission Convergence (TC)

   Definition: A sub-layer of the physical layer of the B-ISDN model
   transforms the flow of cells into a steady flow of bits and bytes for
   transmission over the physical medium. On transmit the TC sublayer
   maps the cells to the frame format, generates the Header Error Check
   (HEC), and sends idle cells when the ATM layer has none. to send. On
   reception, the TC sublayer delineates individual cells in the
   received bit stream, and uses the HEC to detect and correct received
   errors.








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   Discussion: TC is not a measurement, but the speed in which TC can
   occur on a bit stream can be measured. This measurement will not be
   discussed in this document; however, its value should be constant and
   small with respect to cell inter-arrival at the maximum data rate.

   Specification: AF-UNI3.1

1.2.49. Unspecified Bit Rate (UBR)

   Definition: UBR is an ATM service category, which does not specify
   traffic related service guarantees. Specifically, UBR does not
   include the notion of a per-connection-negotiated bandwidth.  No
   commitments are made with respect to the cell loss ratio experienced
   by a UBR connection, or as to the cell transfer delay experienced by
   cells on the connection.

   Discussion: RFC 2331 specifies UBR service class for IP over ATM.
   UBR service models the "best effort" service type specified in RFC
   791; however, UBR has specific drawbacks with respect to TCP service.
   Since UBR makes no guarantee with respect to cell loss (CL), cell
   delay variation (CDV) or cell mis-insertion(CM), TCP RTT estimates
   will be highly variable.  Further, all negatively impact AAL5 re-
   assembly, which in turn may cause packet loss.  See discussions under
   CDV and SAR.

   Specification: AF-TM4.0

1.2.50. Usage Parameter Control (UPC)

   Definition: Usage Parameter Control is defined as the set of actions
   taken by the network to monitor and control traffic, in terms of
   traffic offered and validity of the ATM connection, at the end-system
   access.  Its main purpose is to protect network resources from
   malicious as well as unintentional misbehavior, which can affect the
   QoS of established connections, by detecting violations of negotiated
   parameters and taking appropriate actions.

   Discussion: See discussions under BT, CAC, CDV, GCRA, PCR and SCR.

   Specification: AF-TM4.0

1.2.51. User-Network Interface (UNI)

   Definition: An interface point between ATM end users and a private
   ATM switch, or between a private ATM switch and the public carrier
   ATM network; defined by physical and protocol specifications per ATM
   Forum UNI documents.  The standard adopted by the ATM Forum to define
   connections between users or end stations and a local switch.



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   Discussion: none.

   Specification: AF-UNI3.1

1.2.52. Variable Bit Rate (VBR)

   Definition: An ATM Forum defined service category which supports
   variable bit rate data traffic with average and peak traffic
   parameters.

   Discussion: VBR may potentially adversely effect TCP throughput due
   to large RTT variations.  This in turn will cause the TCP RTT
   estimates to be unstable.

   Specification: AF-TM4.0

1.2.53.  Virtual  Channel  (VC)

   Definition: A communications channel that provides for the sequential
   unidirectional transport of ATM cells.

   Discussion: none.

   Specification: AF-TM3.1

1.2.54. Virtual Channel Connection (VCC)

   Definition: A concatenation of VCIs that extends between the points
   where the ATM service users access the ATM layer. The points at which
   the ATM cell payload is passed to, or received from, the users of the
   ATM Layer (i.e., a higher layer or ATM-entity) for processing signify
   the endpoints of a VCC. VCCs are unidirectional.

   Discussion: none.

   Specification: AF-TM3.1















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1.2.55. Virtual Channel Identifier (VCI)

   Definition: A unique numerical tag as defined by a 16 bit field in
   the ATM cell header that identifies a virtual channel, over which the
   cell is to travel.

   Discussion: none.

   Specification: AF-UNI3.1

1.2.56. Virtual Path (VP)

   Definition: A unidirectional logical association or bundle of VCs.

   Discussion: none.

   Specification: AF-UNI3.1

1.2.57. Virtual Path Connection (VPC)

   Definition: A concatenation of VPIs between Virtual Path Terminators
   (VPTs). VPCs are unidirectional

   Discussion: none.

   Specification: AF-TM3.1

1.2.58. Virtual Path Identifier (VPI)

   Definition: An eight-bit field in the ATM cell header that indicates
   the virtual path over which the cell should be routed.

   Discussion: none.

   Specification: AF-UNI3.1

2. Performance Metrics

2.1. Definition Format (from RFC 1242)

   Metric to be defined.

   Definition: The specific definition for the metric.

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





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   Measurement units: Intrinsic units used to quantify this metric. This
   includes subsidiary units; e.g., microseconds are acceptable if the
   intrinsic unit is seconds.

2.2. Definitions

2.2.1. Physical Layer - SONET

2.2.1.1. Pointer Movements

   Definition: Pointer Movements is the number of changes in a SONET
   pointer due to clock synchronization slips.

   Discussion: SONET Pointer Movements can cause loss of information in
   the SONET payload envelop (SPE) which contains IP datagrams, either
   in the form of ATM cells or as PPP delimited PDUs.

   Measurement Units: Per second.

2.2.1.2. Transport Overhead Error Count

   Definition: SONET Transport Overhead Error Count is the number of
   SONET transport overhead errors detected.

   Discussion: SONET Transport Overhead Errors SONET Transport Overhead
   Errors cause SONET frames to be lost.  These frames may contain IP
   datagrams; either in the form of cells or as PPP delimited PDUs.

   Measurement Units: Positive integer

2.2.1.3. Path Overhead Error Count

   Definition: SONET Path Overhead Error Count is the number of SONET
   path overhead errors detected.

   Discussion: SONET Path Overhead Errors cause SONET frames to be lost.
   These frames may contain IP datagrams; either in the form of cells or
   as PPP delimited PDUs.

   Measurement Units: Positive integer











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RFC 2761            Terminology for ATM Benchmarking       February 2000


2.2.2. ATM Layer

2.2.2.1. Cell Delay Variation (CDV)

   Definition: The variation in cell transfer delay (CTD) associated
   with a given traffic load, orientation and distribution, as well as
   an integration period. CDV = max (CTD) - min (CTD) where max and min
   indicate the maximum and minimum over the integration period,
   respectively.

   Discussion: CDV is a component of cell transfer delay, induced by
   buffering and cell scheduling. Peak-to-peak CDV is a QoS delay
   parameter associated with CBR and VBR services. The peak-to-peak CDV
   is the ((1-a) quantile of the CTD) minus the fixed CTD that could be
   experienced by any delivered cell on a connection during the entire
   connection holding time.  The parameter "a" is the probability of a
   cell arriving late.

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

   Measurement Units: seconds

2.2.2.2. Cell Error Ratio (CER)

   Definition: The ratio of cells with payload errors in a transmission
   in relation to the total number of cells sent in a transmission
   associated with a given traffic load, orientation and distribution,
   as well as an integration period.  Note that errors occurring in the
   cell header will cause cell loss at the ATM layer.  Note further that
   multiple errors in a payload will only be counted as one cell payload
   error.

        CER = Cells with payload errors / Total Cells Transmitted.

   Discussion: The measurement is taken over a time interval and is
   desirable to be measured on an in-service circuit.  CER is closely
   related to the number of corrupted AAL5 PDUs; however, there is not a
   direct numerical correlation between the number of errored cells and
   the number of corrupted AAL5 PDUs.  There are two cases described
   below.

   1. Only one cell in an AAL5 PDU contains payload errors.  In this
   case, there is a one-to-one correspondence between cell payload
   errors and the number of corrupted AAL5 PDUs.





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