Traffic Manager
Vahid Tabatabaee
Fall 2007
ENTS689L: Packet Processing and Switching
Traffic Manager
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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, 9/2326/2002
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Traffic Manager
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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.
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Traffic Manager
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Traffic Management for Best Effort Traffic
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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.
Situation gets more
DSLAM 1
Edge Router
complex when:
 Number of active users
change dynamically.
 Each user has different
applications, requirements
and service level
agreements.
DSL Modem
DSLAM 2
OC-48
Ethernet Switch 1
DSL Modem
access port
Core
Networ
k
DSLAM K
DSL Modem
ENTS689L: Packet Processing and Switching
Traffic Manager
access port
Ethernet Switch 30
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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
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Traffic Manager
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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
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Traffic Manager
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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.
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Traffic Manager
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Major Tasks and Algorithms
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Statistics gathering
Traffic policing
Traffic Shaping
Scheduling
Queueing and Buffer Management
Congestion avoidance and packet dropping
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Statistics
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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.
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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: http://www3.ietf.org/proceedings/99mar/slides/diffserv-tcm-99mar/
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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: http://www3.ietf.org/proceedings/99mar/slides/diffserv-tcm-99mar/
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Single Rate TCM
 Parameters:
 Committed Information Rate (CIR)
 Committed Burst Size (CBS).
 Excess Burst Size (EBS)
Source: http://www3.ietf.org/proceedings/99mar/slides/diffserv-tcm-99mar/
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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
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Congestion Management
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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.
http://www.cisco.com/warp/public/473/187.html#topic5
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Dropping Policy in RED
avg  TH min
F
TH max  TH min
Pb  F  Pmax
count  number of consecutiv e packets not discarded
Pa 
1
1
 count
Pb
Floyd, S., and Jacobson, V., Random Early Detection gateways for Congestion Avoidance V.1 N.4, August 1993, p. 397-413
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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.
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GPS and WFQ
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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 
i
The service rate for any non-empty queue is
gi 
i

C
j
jbusy queues
 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 i  Ri
Bi
Di 
Ri
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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:
Bi ( K i  1) Li
Lmax
Di  

Ri
Ri
m 1 Cm
Ki
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Bounded Delay for WFQ
Bi ( K i  1) Li K i Lmax
Di  

Ri
Ri
m 1 Cm
Di : maximum delay of flow i
Bi : token bucket size of flow i
Ri : token rate of flow i
K i : number of nodes in the path flow i
Li : maximum packet size of flow i
Lmax : maximum packet length for all flows through t he nodes in the path
Cm : outgoing link capacity at node m
Pay for the burst once
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Arrival and Service Curves
 Backlog bound
Source: Patrick Maillé, “An introduction to Network Calculus”.
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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.
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Comparison of Scheduling
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RECAP
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Traffic Manager
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