rfc2761.txt

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   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).






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   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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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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   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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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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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