Issues and Trends in Router Design

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Issues and Trends in Router Design

• Presented by
• Lilian Atieno

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Outline

• Introduction
• Components of a Router
• Design Issues
• Backbone Routers
• Enterprise Routers
• Access Routers
• Advances and Trends in Router Design
• Problems in Router Design
• Conclusion

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Introduction

• Router is layer 3 packet forwarder
• Links global internet
• Transfer packets from input to output links
• Issues in router design
• Diverse link technologies
• Scheduling support for differential services
• Wide range of routing protocols and packet
  formats e.t.c.
• Router classification
• Backbone routers
• Enterprise routers
• Access routers

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Router Components

• Main components
• Input port
• Output port
• Switching fabric
• Routing processor

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Input Port

• Functions
• Point of entry for incoming packets
• Route lookup
• Classify packets into predefined service classes
  to guarantee QoS
• May need to run data-link layer protocols e.g
  Point-to-Point Protocol
• Participate in arbitration protocols

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Switching Fabric

• Interconnects input and output ports
• Switch fabric technologies
• Buses
• Crossbars
• Shared memories

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Shared bus switch fabrics
Shared bus
1
3
N
1
2
M
Input ports
Output ports

• Issues
• Speed limited by arbitration overhead and bus
  capacitance

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Crossbar Switch Fabric
Inactive connection
controller
Input ports
switching fabric
1
2
Active connection
N
-simultaneous data paths -scheduler turns off/on
crossbars -speed limited by scheduler
1
2
M
Output ports
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Shared Memory Switch Fabric
-Incoming packets stored in memory -Controller
informs o/p port that packet is ready -O/p port
retrieves packet Bottleneck - Speed
limited by memory access time
controller
Input ports
shared memory switching fabric
1
2
N
1
2
M
Output ports
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Output Ports

• Function
• Stores packets before transmission to output link
• Need to support data-link layer encapsulation and
  decapsulation

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Routing Processor

• Functions
• Computes forwarding table
• Implement routing protocols
• Run software to configure and manage router
• Handles packets whose destination address are not
  in the forwarding table

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Design Issues of Backbone Router

• Backbone routers interconnect enterprise networks
• High link cost shared among large customer base
• Main challenge
• Maintaining high speed
• Reliability
• Techniques of achieving reliability
• Dual Power supplies
• Hot spares
• Duplicate data paths through routers

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High speed routing in Backbone Routers

• Major performance bottleneck
• Route lookup in the forwarding table
• Routing table may contain thousand of entries
• Finding the longest matching prefix
• Small packets increases cost of lookup
• Large number of destinations increases cost
• Output-queued routers
• Switch fabric runs faster than sum speed of
  incoming links
• Problem
• Limitation on speed of router
• Rate of accessing output buffers limited by SRAM
  or DRAM access time

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Performance bottleneck

• Input queued routers
• Problem
• Contention for switching fabric and output queue
• Difficult to design high speed arbiters that will
  fairly schedule switch fabric and output lines
• Stability and reliability of routing protocol
  implementation
• Routers running different protocols
• Router misconfigurations may cause routing
  oscillations

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Design Issues of Enterprise Routers

• Enterprise/campus routers interconnect end
  systems
• Main objectives
• Provide connectivity to large number of end
  points as cheaply as possible
• Provide different service qualities
• Main challenges
• Routers have low cost/port
• Routers have large number of ports
• Easy to configure
• Support QoS
• Carry multicast traffic efficiently
• Support multiple protocols eg. IP, IPX
• Support feature like traffic filters, firewalls,
  virtual LANS etc.

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Design Issues of Access Routers

• Acess routers link customers at home or in small
  businesses with an ISP
• Main objective
• Support heterogeneous high speed ports
• Support variety of protocols at each port

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Advances and Trends in Router Design

• High speed route lookup
• Speed of algorithm determined by
• number of memory accesses
• Speed of memory
• Techniques to improve performance of route lookup
  algorithms
• Hardware oriented techniques
• Based on CAMs and caches
• Table compaction techniques
• Build compact data structures for the forwarding
  table and store in L1 cache
• Hash-based solutions
• Use markers on the hash tables

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Cont

• Advances in switching Fabric
• ATM switch fabric cores
• Allows router to support different QoS streams
• Once destination port is determined, IP packet is
  fragmented into ATM cells and switched
• ATM cells are reassembled at the output port
  before transmission
• Enterprise-Level mgt and centralization
• Single administrator controlling all routers in
  the enterprise
• Centralize some router functions
• Central route server computes loop-free routes
  for entire enterprise and loads them on all
  routers forwarding table

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Cont

• Avoid route lookups
• Backbones can provide virtual circuit interface
  (VCI) by carrying IP over ATM over SONET
• Require edge networks to translate destination
  address to a VCI
• Speeding up output queues
• Bottleneck of output queues is access speed of
  the buffer
• Solutions
• Build very wide memories that can load entire
  cell in a single memory cycle
• Integrate port controller and the associated
  queues on a single chip eg. 77v400 chipset from
  IDT Inc
• Access entire memory row at a time

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Cont

• Input queued switches
• Problems
• Head-of-line blocking
• Arbitrating access to switch fabric at high speed
• Difficult to implement scheduling algorithm that
  simultaneously schedules both fabric and output
  queues
• Output port connected to different link
  technologies eg. FDDI, Fast-Ethernet etc.
• Varying speeds and transmission policies
• Solution
• Multiple queues, one per output port

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Cont

• Scheduling
• Fair Queuing
• Each packet source sharing a link is allocated a
  a weight at bottleneck link
• Protects well-behaved sources from lossing
  packets due to misbehavior of other sources
• Provide fair bandwidth allocation
• Reducing port cost
• Cost depend on
• Amount and kind of memory used SRAMs vs DRAMs
• Processing power ASICs vs General purpose
  processors
• Complexity of protocol used for communication
  between port and routing processor

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Open Problems in Router Design

• Flow identification
• Need efficient and fast flow classifier
• Ease of configuration
• Configuring routers is hard
• Detecting mistakes in configuration files is
  difficult
• Misconfigured routers causes performance problems
• Software stability of large systems is difficult
  to achieve
• Interaction between bugs from different vendors
  can lead to persistent network instability

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Conclusion

• While advances in router design have solved some
  difficult routing problems, some important issues
  still remain unresolved
• Trade-off between cost, speed, flexibility and
  ease of configuration will still be a challenge
  for router design