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1 ENTS689L: Packet Processing and Switching Traffic Manager Traffic Manager Vahid Tabatabaee Fall 2007.

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Presentation on theme: "1 ENTS689L: Packet Processing and Switching Traffic Manager Traffic Manager Vahid Tabatabaee Fall 2007."— Presentation transcript:

1 1 ENTS689L: Packet Processing and Switching Traffic Manager Traffic Manager Vahid Tabatabaee Fall 2007

2 2 ENTS689L: Packet Processing and Switching Traffic Manager References  Title: Network Processors Architectures, Protocols, and Platforms Author: Panos C. Lekkas Publisher: McGraw-Hill  Tam-Anh Chu, “WAN Multiprotocol Traffic Management Theory & Practice,” Communications Design Conference San Jose, 9/23- 26/2002

3 3 ENTS689L: Packet Processing and Switching Traffic Manager Why do we need Traffic Management?  Until recently traffic was treated under a best effort paradigm.  Internet protocol has ended up to be the common network protocol for multi-service networks and applications.  By emergence of new applications, specially those provided by new generation of wireless protocols, the situation is getting more complex in the future.  These applications have different performance requirements.  We have to use network resources efficiently to have a profitable network.

4 4 ENTS689L: Packet Processing and Switching Traffic Manager Traffic Management for Best Effort Traffic  Best effort should not be interpreted as no effort !!  In reality edge routers are frequently over-subscribed.  In the best effort paradigm we want to treat all users equally.  Connection to the core network is usually over-subscribed.  Users are distributed non-uniformly across the access ports.  A simple Round-Robin schedulers treats all PORTS equally.  We want to treat all users equally. Core Networ k DSL Modem DSLAM 1 DSLAM 2 DSLAM K Ethernet Switch 1 Ethernet Switch 30 Edge Router DSL Modem OC-48 access port  Situation gets more complex when:  Number of active users change dynamically.  Each user has different applications, requirements and service level agreements.

5 5 ENTS689L: Packet Processing and Switching Traffic Manager Traffic Management Objective  To unequally share the network resources (bandwidth and memory) between the users and applications.  Traffic flows should be identified and classified in multiple queues to be able to control QoS.  Network protocols and architectures such as IntServ, DiffServ and MPLS help us to provide QoS in network.  QoS seeks to specify and control five fundamental network variables:  Bandwidth or throughput  Latency  Jitter  Packet loss  Link availability

6 6 ENTS689L: Packet Processing and Switching Traffic Manager Traffic Management vs. Traffic Engineering  Traffic management is performed on the data plane over the packets:  Resource allocation:  Scheduling  Shaping  Congestion Control  Packet discard  Traffic Engineering is performed on the control plane to set up the routes and paths:  Load balancing  Failure recovery  Link Utilization Control

7 7 ENTS689L: Packet Processing and Switching Traffic Manager Traffic Management Obstacles  We have enough knowledge about algorithms and their properties:  Bounds on delay and memory requirement  Unresolved Challenges  Is there a systematic way to set the parameters?  To some extent the answer is yes, but the theoretical bounds are very loose.  What if we set the parameters wrong? Is there a systematic way to pin point the problem?  As far as I know the answer is no.

8 8 ENTS689L: Packet Processing and Switching Traffic Manager Major Tasks and Algorithms  Statistics gathering  Traffic policing  Traffic Shaping  Scheduling  Queueing and Buffer Management  Congestion avoidance and packet dropping

9 9 ENTS689L: Packet Processing and Switching Traffic Manager Statistics  We need to gather statistics  Number of packet arrivals for each flow  Number of discarded packets for each flow  Number of non-conforming packets  Usually they use on-chip counters to gather this information  Only TM has information related to the network congestion level  Packet marking should be done based on the congestion level.

10 10 ENTS689L: Packet Processing and Switching Traffic Manager Packet Marking  It is important to make sure that the packets are conforming to the SLA.  In the DiffServ AF PHB a marking algorithm such as two-rate three-color marker (trTCM) or single-rate three-color marker (srTCM) established the packet-discarding precedence:  In trTCM we have two rate and three colors for the packets:  Useful when peak rate should be enforced  In srTCM we have one rate and three colors for the packets  Useful when only burst size matters  Green maps to AFx1, Yellow to AFx2 and Red to AFx3 Source:

11 11 ENTS689L: Packet Processing and Switching Traffic Manager Two Rate TCM  Parameters:  Peak Information Rate (PIR) and Peak Burst Size (PBS)  Committed Information Rate (CIR) and Committed Burst Size (CBS).  PIR > CIR Source:

12 12 ENTS689L: Packet Processing and Switching Traffic Manager Single Rate TCM  Parameters:  Committed Information Rate (CIR)  Committed Burst Size (CBS).  Excess Burst Size (EBS) Source:

13 13 ENTS689L: Packet Processing and Switching Traffic Manager Traffic Shaping  Traffic shaping is usually done in the egress line card to shape and smooth the outgoing traffic.  Token rate regulates transfer of packets  If sufficient tokens available, packets enter network without delay  B determines how much burstiness allowed into the network

14 14 ENTS689L: Packet Processing and Switching Traffic Manager Congestion Management  We discard packets to avoid congestion.  Simple tail dropping results in TCP global synchroniztion.  RED starts to randomly drop packets when buffers are more than Tmin.  In WRED different queues have different buffer occupation thresholds.

15 15 ENTS689L: Packet Processing and Switching Traffic Manager Dropping Policy in RED Floyd, S., and Jacobson, V., Random Early Detection gateways for Congestion Avoidance V.1 N.4, August 1993, p

16 16 ENTS689L: Packet Processing and Switching Traffic Manager Scheduling  Scheduler decides which queue to be served next?  Round Robin Scheduler: Every queue is served in a round-robin fashion.  Weighted Round Robin (WRR): Queue i is served Ni times in a round robin fashion.  Priority Queueing: A lower priority queue is only served when there is no higher priority backlogged traffic.  Weighted Fair Queuing (WFQ) provides minimum bandwidth guarantees for different queues (their fair shares)  Excess bandwidth (if any) distributed equally among flows  Proven to provide delay bounds for well-behaved traffic flows  Deficit Round Robin: Good approximation of the WFQ.

17 17 ENTS689L: Packet Processing and Switching Traffic Manager GPS and WFQ  One problem with WRR is penalization of short packets.  Genralized Processor Sharing (GPS) to take care of this problem.  In GPS each flow i is assigned a weight  The service rate for any non-empty queue is  Using GPS we can bound delay of packets.  If a flow is limited by a token bucket specification, where Bi and Ri are the bucket size and token rate and

18 18 ENTS689L: Packet Processing and Switching Traffic Manager GPS and WFQ  Implementing GPS explicitly is only possible if we can send and serve flows at the bit granularity.  It is said that GPS is a fluid policy, because it needs to serve fraction of packets.  WFQ is a packetized policy that tracks output of GPS.  The idea is to calculate the finishing time of every packet if we were able to implement GPS.  WFQ always serve the packet with smallest finishing time.  WFQ has a bounded delay too:

19 19 ENTS689L: Packet Processing and Switching Traffic Manager Bounded Delay for WFQ Pay for the burst once

20 20 ENTS689L: Packet Processing and Switching Traffic Manager Arrival and Service Curves  Backlog bound Source: Patrick Maillé, “An introduction to Network Calculus”.

21 21 ENTS689L: Packet Processing and Switching Traffic Manager DRR  Each queue has a deficit counter.  At the beginning of each round deficit counter of each queue is incremented by its quantum value.  Quantum value determines how many bytes from that queue we want to schedule in each round.  A round is one round-robin iteration over backlogged queues.  In a round every queue that its packet length is less than its deficit counter.  If a queue is served its deficit counter is reduced by the packet length.  In each round each backlogged queues deficit is incremented by its quantum value.

22 22 ENTS689L: Packet Processing and Switching Traffic Manager Comparison of Scheduling

23 23 ENTS689L: Packet Processing and Switching Traffic Manager RECAP


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