Router internals

1. Router Internals
Dr. Ramana
I.I.T Rajasthan
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2. Outline of the Lectures
1 Functions of a Router
2 Router Components
3 Packet Flow
4 Further Reading
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3. Functions of a Router
Router functions
Basic forwarding functions
IP Header validation
Packet lifetime control
Checksum recalculation
Destination address lookup
Fragmentation
Handling IP options
Complex forwarding functions
Packet classification
Packet translation
Traffic prioritization
Routing processing functions
Handing routing updates
Sending routing updates
Router management (monitoring of various operations)
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4. Functions of a Router
(Cont.)
Forwarding table
Optimized for lookup a destination IP address against set of IP
prefixes
Could be implemented in hardware as lookup needs to be done for
every packet
Routing table
Optimized for storing/updating network topology and calculating
changes in network topology
Implemented in software
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5. Router Components
Router Components
Network interface - contains multiple ports which provide
connectivity to various physical links (implements L2/L3
processing)
Forwarding engine
does lookup and determine outgoing interface
packet classifying - to provide quality of services at the outgoing link
Queue manager
stores the packets in appropriate buffers at the outgoing port
implements policies for dropping packets due to buffer overflows
schedules packets depending on the packets priorities
Traffic manager
shapes outgoing traffic to meet service level agreements (SLAs)
traffic policing to check the voilation of SLAs by the received traffic
marking of traffic that has voilated the SLAs
Backplane - shared/switched circuit through which packets are
transfered between the incoming (ingress) and outgoing (egress)
ports
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6. Packet Flow
Packet flow in a router
Input
Network
Interface
Forwarding
Engine
Buffer Buffer
Backplane
Queue
Manager
Traffic
Manager
Output
Network
Interface
Router
Control
Processor
Forwarding
Table
Routing
Table
1
2
3 4
5
6
7
8
9 10 11
Control plane
Data plane
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7. Packet Flow
Packet processing: Fast path and slow path
Fast path (Data Path)
Packet processed and transfered from ingress to egress network
interfaces via backplane
To achieve high speeds, operations are implemented in customized
hardware
Time-critical operation
All basic and complex forwarding functions (except fragmentation
and handling IP options)
Slow path (Control path)
Packets partially processed at the ingress interface and forwarded
to CPU for further processing
CPU directly forwards the packets to egress interface once it
completes the processing
Non time-critical operation
Routing processing functions
Processing of data packets that lead to errors in the fast path and
generation of ICMP packets
Fragmentation and handling IP options and ARP packet processing
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8. Further Reading
Further Reading
IP Router Architectures: An Overview -
http://www.cs.virginia.edu/˜cs757/papers/awey99.pdf
Study of Internet Router Architectures -
http://home.iitj.ac.in/˜ramana/Study-of-Internet-Router-
Architectures.pdf
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