Intermediate Traffic Management

Transcription

1 Intermediate Traffic Management

2 This presentation has been generated by the ATM Forum for the purpose of educating the public on ATM Technology and the ATM Forum s activities. This presentation is the property of the ATM Forum and can only be given to external audiences by an authorized ATM Forum Ambassador. ATM Forum Ambassador s regularly attend ATM Forum meetings, so they can relate first hand about Forum activities. There are no restrictions on how this material is used for educational purposes to internal audiences. To request an ATM Forum Ambassador to present this material, please visit our web site, atmforum.com, contact us via e at info@atmforum atmforum.com. A

3 Topics ATM Network Objectives Service Categories Traffic Contract Traffic Management Mechanisms ABR - Available Bit Rate Current Work in TM Group Summary A

4 Topics ATM Network Objectives Service Categories Traffic Contract Traffic Management Mechanisms ABR - Available Bit Rate Current Work in TM Group Summary A

5 ATM Applications and Network Objectives Voice IPX Compressed Video Video Circuit emulation Distance learning ATM Network Ethernet Large file transfer TCP/IP Video on Demand Frame Relay Teleconferencing Super computer communication Low cost data transfer A

6 ATM Applications and Network Objectives All traffic carried via the same network elements (potentially multi-vendor) Optimize use of network resources Meet stringent QoS requirements specific to each application Protects networks and users to achieve network performance objectives

7 Topics ATM Network Objectives Service Categories Traffic Contract Traffic Management Mechanisms Available Bit Rate - ABR Current Work in TM Group Summary A

8 ATM Service Architecture Five Service Categories CBR rt-vbr nrt-vbr ABR UBR Constant Bit Rate Real-Time Variable Bit Rate Non-Real Time Variable Bit Rate Available Bit Rate Unspecified Bit Rate Example -voice -video -frame relay -data -data

9 ATM Forum Service Categories Attribute CBR rt-vbr ATM Layer Services Categories nrt-vbr ABR UBR CLR specified network specific unspecified CTD and CDV specified unspecified unspecified unspecified PCR and CDVT 1 specified specified specified SCR and BT n/a specified n/a MCR n/a specified n/a Feedback unspecified specified unspecified Notes: 1. CVDT is not signalled,, it is network specific. B

10 Topics ATM Network Objectives Service Categories Traffic Contract Traffic Management Mechanisms Available Bit Rate - ABR Current Work in TM Group Summary A

11 Traffic Contract Traffic Traffic contract of a connection includes Service Category QoS Requirement Traffic Descriptors Conformance Definition Definition of a Compliant Connection A

12 Traffic Contract Quality of Service (QoS) Rate Guarantees On PCR, SCR, MCR, and ACR Delay Guarantees On Cell Transfer Delay - CTD On Cell Delay Variation - CDV Loss Guarantees On Cell Loss Ratio - CLR (Lost cells/total cells)

13 ATM Service Architecture Rate Guarantees For: CBR Peak Cell Rate (PCR) VBR Sustained Cell Rate (SCR) ABR Minimum Cell Rate (MCR) & Allowed Cell Rate (ACR) - Dynamically Network Controlled UBR No Guarantees A

14 Quality of Service Cell Transfer Delay (CTD) Cell Delay Variation (CDV) Cell arrival pattern Queuing point (e.g. mux,, switch) Cell departure pattern without CDV Cell departure pattern with CDV Switch transit delay Negative CDV Positive CDV A

15 ATM Service Architecture Delay Guarantees For: CBR Cell Delay (CTD) & Variation (CDV) rt-vbr Cell Delay (CTD) & Variation (CDV) nrt-vbr No Guarantees ABR No Guarantees UBR No Guarantees

16 ATM Service Architecture Cell Loss Guarantees For: CBR Cell Loss Rate (CLR) VBR Cell Loss Rate (CLR) ABR Cell Loss Rate (CLR) - Network Specific* UBR No Guarantees * CLR for ABR is not specified in signalling,, it is specific to the network A

17 Traffic Contracts - Conformance Guarantees are only valid if the traffic conforms to the negotiated Traffic Contract. For CBR, VBR and UBR, conformance is defined by the Generic Cell Rate Algorithm (GCRA). For ABR, conformance is defined by the source and destination behavior, but a dynamic GCRA is still a useful example of how to test conformance. Example causes of non-conformance: Excessive rate Excessive clumping Excessive burst Non-Conforming cells may be discarded or, when permitted, tagged with CLP=1 for lower priority. A

18 Generic Cell Rate Algorithm GCRA is a continuous Leaky Bucket Algorithm. As each cell arrives, GCRA checks its conformance to an agreed rate. The GCRA has two parameters: I is the Increment per time unit L is the Limit or capacity When checking a rate with GCRA: I is the reciprocal of the rate L is the tolerance

19 L + I Generic Cell Rate Algorithm I for each cell arrival One unit leak per unit of time For a sequence of cell arrival times, {t k }, GCRA determines which cells conform to the traffic contract A counter scheme based on two parameters denoted GCRA(I,L) Increment parameter: I - Defines minimum cell spacing Limit parameter: L - Allows cell bursts and CDV Leaky bucket A cell that would cause the bucket to overflow is non-conforming A

20 Bursty Traffic Cell Cell GCRA(4.5, 7) Cell No Cell No Cell t- t+ t- t+ t- t+ t- t+ Bucket fill just before and just after cell transmit time t- t+

21 If a GCRA Were Really a Leaky Bucket I P I S CDVT + I BT + CDVT + I P S CDVT +I P BT + CDVT + I S CA CA CA CA CA CA CA=Cell Arrival A

22 Example Conformance Definition CLP0+1 PCR 0+1 GCRA (1/PCR, CDVT) CLP1 SCR CLP0 0 GCRA (1/SCR, BT + CDVT) Conforming Non-conforming

23 Traffic Contracts - GCRA Conformance Tests Typically Policed by the Network CBR - Must conform to GCRA (1/PCR, CDVT) VBR - Must conform to both: GCRA (1/PCR, CDVT) GCRA (1/SCR, CDVT+BT) ABR - Must conform to DGCRA (1/ACR, t) where: ACR is the network specified rate (PCR>ACR>MCR) t is a dynamically variable burst tolerance UBR - Whether UBR must conform to GCRA (1/PCR, CDVT) is network specific CDVT is the Cell Variance Delay Tolerance and is specific to the network, not signalled BT is the Burst Tolerance for SCR B

24 Traffic Contract - Compliance Conformance is an attribute of the cell Compliance is an attribute of the connection QoS is guaranteed for all conforming cells of a compliant connection A connection is defined as compliant when the number of non-conforming cells is below a threshold set by the network provider The threshold for compliance is specified in the traffic contract by the network provider

25 Topics ATM Network Objectives Service Categories Traffic Contract Traffic Management Mechanisms Available Bit Rate - ABR Current Work in TM Group Summary A

26 Traffic Management Components TM Mechanisms Connection Admission Control (CAC) Usage Parameter Control (UPC) Cell Loss Priority Control (CLP) Traffic Shaping (TS) ABR Flow Control ATM Layer Traffic Parameters Applications Traffic Contract Traffic Management Mechanisms Quality of Service

27 Connection Admission Control Connection Admission Control uses Connection Traffic Descriptors Requested QoS to determine whether a PVC or SVC connection can be accepted while meeting the requested QoS and WITHOUT affecting the QoS of connections already established. The ATM Forum does not specify any particular algorithm for CAC.

28 Usage Parameter Control The Usage Parameter Control (UPC) is used to Check the validity of VPI/VCI. Monitor cells of a connection to determine whether they conform to the traffic descriptors. Discard or tag (to CLP=1) the non-conforming cells. The UPC objectives Operate in a timely manner without affecting the cell flow. Are not more stringent than the theoretical conformance definition (GCRA). That is, the number of cells found conforming by the UPC should be at least as large as the number conforming to the conformance definition. B

29 CLP - Cell Loss Priority Control CLP=0 cells are guaranteed a CLR no higher than that of CLP=1 cells. Priority control (also known as selective discard) is a mechanism by which network elements selectively discard CLP=1 cells in order to guarantee a lower CLR to the CLP=0 cells. Tagging is when the network marks nonconformant cells as CLP=1 rather than immediately discarding them.

30 Cell Loss Priority (CLP) and Tagging There are two basic modes of operation supported: 1. Network Ignores CLP. Does not mark it or look at it. Available for CBR, VBR, and UBR GCRA conformance on all cells, CLP 0 and Network may selectively discard CLP = 1 cells. Available for CBR*, VBR, ABR, and UBR Network Tagging permitted for VBR and UBR * For UNI 3.1 compatibility, a CBR call may be signalled where CLP=1 cells may be selectively discarded. This capability is specified in the UNI 3.1 specification. B

31 Traffic Shaping Traffic shaping is a mechanism used by the terminal equipment to schedule the entry of cells in the network so they conform to the traffic descriptors at the UPC. Traffic shaping is optional in TM 4.0 Traffic shaping allows To increase the efficiency of the resource allocation by introducing more deterministic traffic pattern and reduce the burstiness. To control the CDV at the ingress of the network. If applied at the egress of the network traffic shaping cancels the accumulated CDV. A

32 Frame Discard For AAL-5 traffic where large blocks of data (frames) are being sent under checksum control with retransmission, it would be necessary to retransmit the whole block or many cells if one cell were discarded. Improved efficiency can be achieved if the network discards total frames rather than individual cells. When AAL-5 is being used the user may signal that he would prefer frame discard if supported by the network. To implement frame discard, the network watches for the end of AAL-5 frames and then, if congested, discards the whole next frame rather than individual cells. B

33 Topics ATM Network Objectives Service Categories Traffic Contract Traffic Management Mechanisms Available Bit Rate - ABR Current Work in TM Group Summary A

34 Why ABR? While many data applications are well suited to the original non-real-time service categories, VBR and UBR, some are not. They may need CLR QoS, which is not part of the UBR model. Also, their traffic may be dynamic and unpredictable, and not fit well into the VBR model, (SCR, BT, PCR). The bandwidth requirement is as much as available on an as needed basis. They may also need a minimum bandwidth to stay alive. ABR replaces the static VBR traffic constraint with a dynamic allowed cell rate (ACR), based on network feedback. ABR sources that follow the feedback rules get low cell loss. B

35 ABR - Flow Control Requirement Flow control plays an important role in avoiding congestion collapse in data networks. The Internet, router networks, and pre- TM 4.0 ATM networks have no internal flow control; they depend on end- to- end flow control (e.g. TCP) at a higher layer. How does ABR Flow Control compare with Higher Layer Flow Control? B

36 ABR - Flow Control Global View Higher Layer Flow Control Loop Large Delay - Implicit Binary Feedback Retransmissions and Low Utilization ABR Flow Control Loop, Minimal Delay -ExplicitExplicit Feedback Near Zero Retransmission and High Utilization LAN LAN WAN TCP flow control is the current basis of the Internet. ABR provides a vastly improved basis for future networks. B

37 ABR - Basic Concept Create End-to-End Flow Control At the Source End-Station a set of rules called the Source Behavior determine the sending rate, based on control information from the network. At each network element, a set of rules called the Switch Behavior define how feedback may be provided to control the source rate. The decision process for deciding the content of the feedback is unspecified. At the Destination End-Station a set of rules called the Destination Behavior combine two functions: 1) they define how a destination that receives forward congestion indicators from the network should reflect these back to the source, and 2) they allow a destination to provide its own feedback. D

38 ABR - Operation Source and Destination Behavior are implemented at the end-system, for example in the NIC card. End System NIC Card ATM Switch ATM Switch NIC Card End System Switch Behavior is implemented in the network switches. A

39 ABR - RM Cells In order to provide a mechanism for the network to provide congestion and rate change feedback to the source, the ABR source is required to insert Resource Management (RM) cells periodically within the data flow. The RM cells are turned around by the destination and returned to the source along the return path. Each RM cell contains an Explicit Rate (ER) at which the source wishes to operate, and this rate may be reduced by any network element to the rate it can currently support. Each RM cell also includes binary fields that the source can use to request permission to increase its rate, and that network elements may modify to prevent an increase or force a decrease. C

40 ABR - RM Cell Flow RM cells are sent from source to destination and back every Nrm cells, typically a 3% overhead per direction. The switches and destination modify them to indicate their congestion and the maximum rate they can support. SWITCH SWITCH SWITCH SWITCH FORWARD Source BACKWARD RM Destination Switches adjust ER, CI and NI to control source rate STANDARD ATM HEADER with PTI = 6 ID DIR BN CI NI ER CCR MCR CRC10 BYTE # RM Cell Format D

41 ABR - Source Behavior (Part 1) What to do with a returning RM cell: If CI = 1, decrease rate proportionally: - ACR = (1 - RDF)ACR else if NI = 0, allow linear rate increase - ACR = ACR + RIF * PCR Also look at explicit rate feedback - ACR = Min(ACR, ER) Also keep ACR between MCR and PCR. B

42 ABR - Modes of Switch Operation EFCI Binary Rate Operation - Mark EFCI in data cells Relative Rate Binary Operation - Mark CI, NI bits in RM cell Explicit Rate Operation - Mark ER field in RM cell Explicit Rate Operation Performance Relative Rate Binary Operation EFCI Binary Rate Operation Complexity C

43 ABR - Transient Exposure When a new or idle source becomes active, network feedback will not take effect for one round trip time (RTT) ACR ACR ICR ICR RTT ER Feedback Time RTT CI Feedback BECN can be used to shorten feedback delay Time D

44 ABR - Signalled Parameters PCR - Peak Cell Rate. TBE - Transient Buffer Exposure. FRTT - Fixed Round Trip Delay. Accumulated. ICR - Initial Cell Rate. Or computed. MCR - Minimum Cell Rate. MCRmin - Minimum Cell Rate Acceptable. RIF - Rate Increase Factor. RDF - Rate Decrease Factor. (RIF / RDF ratio) = Negotiated Down along signalling path. B

45 ABR - Typical Signalled Parameters End-Stations need only specify PCR and MCR Switches need only: Negotiate PCR & MCR based on conditions. Negotiate TBE, RDF, and RIF down to preset values. ICR will be then be computed as TBE/FRTT. A

46 ABR - Binary Rate Operation Source Cell Rate PCR - Peak Cell Rate ACR - Allowed Cell Rate RDF*ACR Rate Decrease Factor Network Capacity ICR - Initial Cell Rate Area=TBE Transient Buffer Exposure Average Source Rate MCR Minimum Cell Rate FRTT Fixed Round Trip Time Time RIF*PCR Rate Increase Factor Network Not Congested - CI=0 Network Congested CI=1 Actual simulation of EFCI binary rate operation B

47 ABR - Explicit Rate Operation Source Cell Rate PCR - Peak Cell Rate ACR - Allowed Cell Rate Network Capacity RIF*PCR Rate Increase Factor ICR - Initial Cell Rate MCR - Minimum Cell Rate Time Actual simulation of explicit rate operation A

48 ABR Virtual Source / Virtual Destination It is permissible to insert Virtual Source / Virtual Destination,, VS / VD, modules into a network so as to protect one segment of the network from another, and to shorten the control loops. Source VD/ VS VD/ VS Destination Destination Control Loop Control Loop Control Loop Rate-based segmented control loop A

49 ABR - Source Behavior (Part 2) Send a forward RM cell once per Nrm cells, or more frequently if ACR is low When sending a forward RM cell, do two implicit tests 1) if source is coming out of idle, reduce ACR to ICR 2) if feedback pipe is broken, reduce ACR: - ACR = (1 - CDF)ACR B

50 ABR - Destination Behavior Data Cell Received Save EFCI Indicator Data Cell RM Cell Receive F-RM F cell Return RM cell as B-RM B cell If saved EFCI=1 then CI=1, saved EFCI=0 If congested then adjust ER or CI RM Cell

51 ABR - Switch Behavior Data Cell Received If A and congested then set EFCI Send Cell Modes of Operation A: EFCI Binary Rate Operation B: Relative Rate Binary Operation C: Explicit Rate Operation Data Cell RM Cell RM Cell Received If B and congested then may set CI=1 (probably on B-RM) B If C then reduce ER as needed (probably on B-RM) B Send RM Cell RM Cell A

52 Topics ATM Network Objectives Service Categories Traffic Contract Traffic Management Mechanisms Available Bit Rate - ABR Current Work in TM Group Summary A

53 Work Plan The Traffic Management Working Group has agreed to produce an updated TM specification Scheduled release: First half of 1999 New work in this specification: Guaranteed Frame Rate (GFR) - A new service QoS Interoperability - Clarifications improve alignment with ITU Other minor additions, clarifications and advisory text This is work in progress and subject to change. A

54 Guaranteed Frame Rate (GFR) Objective: a non-real-time service that provides some QoS commitment while placing minimal constraints on the user. User is free to send any rate up to a PCR all the time. With high probability traffic will be delivered at least at a Minimum Cell Rate (MCR). Excess traffic is best effort - shaping to MCR is not expected. GFR service is frame-based. Example of a frame: AAL5 PDU (may be many cells long). In most cases, either all cells of a frame are delivered or all cells are discarded. GFR is not available for virtual paths Formal definition introduces a frame-based GCRA: F-GCRA. Possible Application: Carry aggregate TCP/IP traffic between two routers. This is work in progress and subject to change. A

55 Topics ATM Network Objectives Service Categories Traffic Contract Traffic Management Mechanisms Available Bit Rate - ABR Current Work in TM Group Summary A

56 Summary ATM carries a wide variety of applications which translates to a complex mix of traffic characteristics and QoS requirements. QoS guarantees supported include: Rate Guarantees Delay Guarantees Cell Loss Guarantees ABR flow control provides a nearly lossless, high efficiency service for bursty data applications. ATM Layer Traffic Parameters Applications Traffic Contract Traffic Management Mechanisms Quality of Service A

57 This concludes the presentation provided by The ATM Forum

58 ABR - Mix and Match A given ABR connection might traverse switches of all three types ABR Source ABR Destination EFCI Switch Explicit Rate Switch Relative Rate Switch Source looks at all the feedback in the B-RM (i.e. CI, NI, ER). Source reacts to congestion, however indicated. ATMTMmod A