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IP Router Architectures
許凱銘
Outline Basic IP Router Functionalities IP Router Architectures
Generic architecture of a router
Basic architecture Routing components
Basic IP Router Functionalities
Route Processing Routing table construction and maintenance using routing
protocols (such as RIP or OSPF)
Packet Forwardinga) IP Packet Validationb) Destination IP Address Parsing and Table Lookupc) Packet Lifetime Controld) Checksum Calculation
Special Services
Route Table Lookup Major performance bottleneck Longest prefix matching problem(CIDR) Cache IP destination-to-next-hop
association in a separate database the front-end database:hash table (problem:may hash to the same value)
Need efficient lookup method software , hardware
IP Router Architectures Bus-based Router Architectures with
Single Processor Bus-based Router Architectures with
Multiple Processors Architectures with Multiple Parallel
Forwarding Engines Switch-based Router Architectures with
Multiple Processors Switch-based Router Architectures with
Fully Distributed Processors
Bus-based Router Architectures with Single Processor
Traditional bus-based router architecture
Problem Share bus Bottleneck:data cross the bus twice Everything are implemented on the
central processor No Cache in Line Card
Bus-based Router Architectures with Multiple Processors
Architectures with Route Caching Architectures with Multiple Parallel
Forwarding Engines
Architectures with Route Caching
Reducing bus copies using a route cache
Problem:Architectures with Route Caching
Shared bus is the bottleneck Traffic dependent throughput Enhance each of network interface
cards with larger memories and complete forwarding tables can improve performance
Architecture with Multiple Parallel Forwarding Engines
head data
headtagRound-robin
Round-robin
Problem:Architecture with Multiple Parallel Forwarding Engines
Forward engine can work on different header in parallel
Decrease the packet payload transfer over the bus
Switch-based Router Architectures with Multiple Processors
Three stages of forwarding process
1. The following are done in parallel
• Confirm the header is IPv4 datagram• Confirm packet and header lengths are reasonable • Confirm IPv4 header has no option• Computes the hash offset into the route cache and
loads the route• Start loading in the next header
(cont)2. Forwarding engine start if cache hit -> ok else ->
• lookup the extended forward table• update the route cache• checks TTL compute updated TTL and IP
checksum3.
• Put the updated TTL and checksum in IP header• Updated IP header is written out along with link-
layer information from forwarding table
Limitation of IP Packet Forwarding based on Route Caching
how big the cache is how the cache is maintained (the three most
popular cache maintenance strategies are random replacement, (FIFO), (LRU)
what the performance of the slow path is, since at least some percentage of the traffic will take the slow path in any application
Switch-based Router Architectures with Fully Distributed Processors
Three bottleneck in a router:processing power,memory bandwidth,internal bus bandwidth , these can be avoided by using a distributed switch based architecture
Critical Data Path(Fast Path)Non-Critical Data Path(Slow Path)
(cont)
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