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.

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RFC 2761            Terminology for ATM Benchmarking       February 2000


   Discussion: CAC is based on the ATM traffic descriptor (see ATM
   traffic descriptor) associated with the call as well as the presented
   and existing load.  It may also be based on administrative policies
   such as calling party number required or access limitations. The
   effect on performance of these policies is beyond the scope of this
   document and will be handled in the BMWG document: Benchmarking
   Terminology for Firewall Performance.

   Specification: AF-UNI3.1

1.2.18. Constant Bit Rate (CBR)

   Definition: An ATM service category which supports a constant and
   guaranteed rate to transport services such as video or voice as well
   as circuit emulation which requires rigorous timing control and
   performance parameters.  CBR requires the specification of PCR and
   QoS (see PCR and QoS).

   Discussion: Because CBR provides minimal cell delay variation (see
   CDV), it should improve TCP throughput by stabilizing the RTT
   calculation.  Further, as CBR generally provides a high priority
   service, meaning that cells with a CBR traffic contract usually take
   priority over other cells during congestion, TCP segment and IP
   packet loss should be minimized.  The cost associated with using CBR
   is the loss of statistical multiplexing.  Since CBR guarantees both
   throughput and CDV control, the connections must be subscribed at
   PCR.  This is extremely wasteful as most protocols, e.g., TCP, only
   utilize full bandwidth on one half of a bi-directional connection.

   Specification: AF-UNI3.1

1.2.19. Cyclic Redundancy Check (CRC)

   Definition: A mathematical algorithm that computes a numerical value
   based on the bits in a block of data. This number is transmitted with
   the data, the receiver uses this information and the same algorithm
   to insure the accurate delivery of data by comparing the results of
   algorithm, and the number received.  If a mismatch occurs, an error
   in transmission is presumed.

   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. See discussion
   under AAL5.

   Specification: AF-UNI3.1





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RFC 2761            Terminology for ATM Benchmarking       February 2000


1.2.20. End System (ES)

   Definition: A system where an ATM connection is terminated or
   initiated.  An originating end system initiates the ATM connection,
   and terminating end system terminates the ATM connection.  OAM cells
   may be generated and received.

   Discussion: An ES can be the user side of a UNI signaling interface.

   Specification: AF-TEST-0022

1.2.21. Explicit Forward Congestion Indication (EFCI)

   Definition: EFCI is an indication in the PTI field of the ATM cell
   header.  A network element in an impending-congested state or a
   congested state may set EFCI so that this indication may be examined
   by the destination end-system. For example, the end-system may use
   this indication to implement a protocol that adaptively lowers the
   cell rate of the connection during congestion or impending
   congestion.  A network element that is not in a congestion state or
   an impending congestion state will not modify the value of this
   indication.  Impending congestion is the state when network equipment
   is operating around its engineered capacity level.

   Discussion: EFCI may be used to prevent congestion by alerting a
   positive acknowledgement protocol and causing action to be taken.  In
   the case of TCP, when EFCI cells are received the driver software
   could alert the TCP software of impending congestion.  The TCP
   receiver would then acknowledge the current segment and set the
   window size to some very small number.

   Specification: AF-TM4.0

1.2.22. Generic Cell Rate Algorithm (GCRA)

   Definition: The GCRA is used to define conformance with respect to
   the traffic contract of the connection.  For each cell arrival, the
   GCRA determines whether the cell conforms to the traffic contract.
   The UPC function may implement the GCRA, or one or more equivalent
   algorithms to enforce conformance. The GCRA is defined with two
   parameters: the Increment (I) and the Limit (L).

   Discussion: The GCRA increment and limit parameters are mapped to CBR
   and VBR in the following fashion.  For CBR, I=1/PCR and L=CDVT (CDV
   tolerance).  For VBR, there are two GCRA algorithms running (dual
   leaky bucket).  The first functions in the same fashion .bp as CBR,
   I=1/PCR and L=CDVT.  The second, which polices cells which are in
   conformance with the first GCRA uses I=1/SCR and L=BT (see BT, CDV,



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RFC 2761            Terminology for ATM Benchmarking       February 2000


   MBS, PCR and SCR).

   Specification: AF-TM4.0

1.2.23. Generic Flow Control (GFC)

   Definition: GFC is a field in the ATM header, which can be used to
   provide local functions (e.g., flow control). It has local
   significance only and the value encoded in the field is not carried
   end-to-end.

   Discussion: none.

   Specification: AF-UNI3.1

1.2.24. Guaranteed Frame Rate (GFR)

   Definition: The GFR service provides the user with a Minimum Cell
   Rate (MCR) guarantee under the assumption of a given maximum frame
   size (MFS) and a given Maximum Burst Size (MBS).  The MFS and MBS are
   both expressed in units of cells.  GFR only applies to virtual
   channel connections (VCCs).

   Discussion: GFR is intended for users who are either not able to
   specify the range of traffic parameters needed to request most ATM
   services, or are not equipped to comply with the (source) behavior
   rules required by existing ATM services.  Specifically, GFR provides
   the user with the following minimum service guarantee: When the
   network is congested, all frames whose length is less than MFS and
   presented to the ATM interface in bursts less than MBS and at a rate
   less than PCR will be handled with minimum frame loss.  When the
   network is not congested, the user can burst at higher rates.

   The effect of GFR on performance is somewhat problematic as the
   policing algorithm associated with GFR depends on the network load;
   however, under congested condition and assuming a user who is
   following the GFR service agreement, it should improve performance.

   Specification: AF-TM4.1












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RFC 2761            Terminology for ATM Benchmarking       February 2000


1.2.25. Header Error Control (HEC)

   Definition: A check character calculated using an 8 bit CRC computed
   over the first 4 octets of the ATM cell header. This allows for
   single bit error correction or multiple bit error detection.

   Discussion: none.

   Specification: AF-UNI3.1

1.2.26. Integrated Local Management Interface

   Definition: A management protocol which uses SNMPv1 carried on AAL5
   to provide ATM network devices with status and configuration
   information concerning VPCs, VCCs, registered ATM addresses and the
   capabilities of ATM interfaces.

   Discussion: ILMI is a conditionally required portion of UNI3.1;
   however, ILMI 4.0 has been issued as a separate specification.  This
   document will refer to ILMI 4.0.

   Specification: AF-ILMI4.0

1.2.27. Intermediate System (IS)

   Definition: A system that provides forwarding functions or relaying
   functions or both for a specific ATM connection. OAM cells may be
   generated and received.

   Discussion: An IS can be either the user or network side of a UNI
   signaling interface, or the network side of a PNNI signaling
   interface.

   Specification: AF-TEST-0022

1.2.28. Leaky Bucket (LB)

   Definition: Leaky Bucket is the term used as an analogous description
   of the algorithm used for conformance checking of cell flows from a
   user or network. See GCRA and UPC.  The "leaking hole in the bucket"
   applies to the sustained rate at which cells can be accommodated,
   while the "bucket depth" applies to the tolerance to cell bursting
   over a given time period.

   Discussion: There are two types of LB algorithms - single and dual.
   Single LB is used in CBR; dual LB is used in VBR (see CBR and VBR).

   Specification: AF-TM4.0



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RFC 2761            Terminology for ATM Benchmarking       February 2000


1.2.29. Maximum Burst Size (MBS)

   Definition: In the signaling message, the Burst Tolerance (BT) is
   conveyed through the MBS that is coded as a number of cells. The BT
   together with the SCR and the PCR determine the MBS that may be
   transmitted at the peak rate and still is in conformance with the
   GCRA.

   Discussion: See the discussion under BT.

   Specification: AF-TM4.0

1.2.30. Maximum Frame Size (MFS)

   Definition: The MFS is the maximum length of a frame, expressed in
   units of cells, which in interface implementing GFR will accept
   during congested conditions (see GFR).

   Discussion: During congestion, frames whose size is in excess of the
   MFS may be dropped or tagged.  Assuming that the user is adhering to
   the MFS limit, this behavior should improve performance by improving
   congestion.

   Specification: AF-TM4.1

1.2.31. Operations, Administration, and Maintenance (OAM)

   Definition: A group of network management functions that provide
   network fault indication, performance information, and data and
   diagnosis functions.

   Discussion: There are four types of ATM OAM flows: segment or end-
   to-end VP termination management (i.e. F4 segment, F4 E2E) and
   segment or end-to-end VC termination management (i.e. F5 segment, F5
   E2E). These OAM cells can be used to identify fault management,
   connection verification, and loop back measurements.

   Specification: AF-UNI3.1













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RFC 2761            Terminology for ATM Benchmarking       February 2000


1.2.32. Payload Type Indicator (PTI)

   Definition: Payload Type Indicator is the Payload Type field value
   distinguishing the various management cells and user cells as well as
   conveying explicit forward congestion indication (see EFCI).
   Example:  Resource Management cell is indicated as PTI=110, End-to-
   end OAM F5 Flow cell is indicated as PTI=101.

   Discussion: none.

   Specification: AF-UNI3.1

1.2.33. Peak Cell Rate (PCR)

   Definition: A traffic parameter, which specifies the upper bound on
   the rate at which ATM cells can be submitted to an ATM connection.
   This parameter is used by the GCRA.

   Discussion: PCR directly limits the maximum data rate on an ATM
   connection.  If a user violates the PCR, 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

1.2.34. Permanent Virtual Circuit (PVC)

   Definition: This is a link with static route(s) defined in advance,
   usually by manual setup.

   Discussion: none.

   Specification: AF-UNI3.1

1.2.35. Permanent Virtual Channel Connection (PVCC)

   Definition: A Virtual Channel Connection (VCC) is an ATM connection
   where switching is performed on the VPI/VCI fields of each cell. A
   permanent VCC is one that is provisioned through some network
   management function and left up indefinitely.

   Discussion: none.

   Specification: AF-UNI3.1






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RFC 2761            Terminology for ATM Benchmarking       February 2000


1.2.36. Permanent Virtual Path Connection: (PVPC)

   Definition: A Virtual Path Connection (VPC) is an ATM connection
   where switching is performed on the VPI field only of each cell. A
   permanent VPC is one that is provisioned through some network
   management function and left up indefinitely.

   Discussion: none.

   Specification: AF-UNI3.1

1.2.37. Private Network-Network Interface (PNNI)

   Definition: A routing information protocol that enables extremely,
   scalable, full function, dynamic multi-vendor ATM switches to be
   integrated in the same network.

   Discussion: PNNI consists of signaling and routing between ATM
   network devices.  PNNI signaling is based on UNI 4.0 signaling
   between two network side interfaces, while PNNI routing provides a
   mechanism to route ATM cells between two separate, autonomous ATM
   networks.

   Specification: AF-PNNI1.0

1.2.38. Protocol Data Unit (PDU)

   Definition: A PDU is a message of a given protocol comprising payload
   and protocol-specific control information, typically contained in a
   header.  PDUs pass over the protocol interfaces that exist between
   the layers of protocols (per OSI model).

   Discussion: In ATM networks, a PDU can refer to an ATM cell, multiple
   ATM cells, an AAL segment, an IP datagram and others.

   Specification: Common Usage

1.2.39. Segmentation and Reassembly (SAR)

   Definition: The process used by the AAL in the B-ISDN reference model
   (see B-ISDN) which fragments higher layer PDUs into ATM cells.

   Discussion: SAR is not a measurement, but the speed in which SAR can
   be completed on a bit stream can be measured. Although this
   measurement is not included in this document, it should be noted that
   the manner in which SAR is performed will greatly effect performance.
   SAR can be performed either on the interface card (on board) or in
   machine central memory (in core).  On-board computation should



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RFC 2761            Terminology for ATM Benchmarking       February 2000


   produce only a small, constant delay; however, in core computation
   will produce variable delay, which will negatively effect TCP RTT
   computations.  This situation is further complicated by the location
   of the CRC-32 calculation.  Given an in core CRC-32 calculation, bus
   contention may cause on board SAR to be slower than in core SAR.
   Clearly, on board CRC-32 calculation and SAR will produce the most
   favorable performance results.

   SAR performance will also be effected by ATM layer impairments.  Cell
   error (CE), cell loss(CL), cell mis-insertion (CM) and cell delay
   variation (CDV) will all negatively effect SAR.  CE will cause an
   AAL5 PDU to fail the CRC-32 check and be discarded, thus discarding
   the packet which the PDU contained.  CL and CM will both cause an
   AAL5 PDU to fail the length check and be discarded.  CL can have
   other effects depending on whether the cell which was lost is the
   final cell (PTI=1) of the AAL5 PDU.  The following discussion
   enumerates the possibilities.

   1. PTI=0 cell is lost.  In this case, re-assembly registers a length
   discrepancy and discards the PDU.

   2. PTI=1 cell is lost.

   2.  A.  The AAL5 re-assembly timer expires before the first cell,
   PTI=0, of the next AAL5 PDU arrives.  The AAL5 PDU with the missing
   PTI=1 cell is discarded due to re-assembly timeout and one packet is
   lost.

   2.  B.  The first cell of the next AAL5 PDU arrives before the re-
   assembly timer expires.  The AAL5 with the missing PTI=1 cell is
   prepended to the next AAL5 PDU in the SAR engine.  This yields two
   possibilities:

   2. B. i. The AAL5 re-assembly timer expires before the last cell,
   PTI=1, of the next AAL5 PDU arrives.  The AAL5 PDU with the missing
   PTI=1 cell and the next AAL5 PDU are discarded due to re-assembly
   timeout and two packets are lost.

   2. B. ii. The last cell of the next AAL5 PDU arrives before the re-
   assembly timer expires.  In this case, AAL5 registers a length
   discrepancy and discards the PDU; therefore, the AAL5 PDU with the
   missing PTI=1 cell and the next AAL5 PDU are discarded due to their
   concatenation and two packets are lost.








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