1. EE 122: Lecture 7
Ion Stoica
September 18, 2001

2. Routers A router consists Router implements two main functions. .
A router consists
A set of input interfaces at which packets arrive
A se of output interfaces from which packets depart
Router implements two main functions
Forward packet to corresponding output interface
Manage bandwidth and buffer space resources

3. Generic Router Architecture
Input and output interfaces are connected through a backplane
A backplane can be implemented by
Shared memory
low capacity routers (e.g., PC-based routers)
Shared bus
Medium capacity routers
Point-to-point (switched) bus
High capacity routers
input interface
output interface
Inter-
connection
Medium
(Backplane)

4. Speedup C – input/output link capacity
RI – maximum rate at which an input interface can send data into backplane
RO – maximum rate at which an output can read data from backplane
B – maximum aggregate backplane transfer rate
Back-plane speedup: B/C
Input speedup: RI/C
Output speedup: RO/C
input interface
output interface
Inter-
connection
Medium
(Backplane) C RI B RO C

5. Functional division Input interfaces: Output interfaces:
Must perform packet forwarding – need to know to which output interface to send packets
May enqueue packets and perform scheduling
Output interfaces:
input interface
output interface
Inter-
connection
Medium
(Backplane) C RI B RO C

6. Three Router Architectures
Output queued
Input queued
Combined Input-Output queued

7. Output Queued (OQ) Routers
Only output interfaces store packets
Advantages
Easy to design algorithms: only one congestion point
Disadvantages
Requires an output speedup of N, where N is the number of interfaces  not feasible
input interface
output interface
Backplane RO C

8. Input Queueing (IQ) Routers
Only input interfaces store packets
Advantages
Easy to built
Store packets at inputs if contention at outputs
Relatively easy to design algorithms
Only one congestion point, but not output…
need to implement backpressure
Disadvantages
Hard to achieve utilization  1 (due to output contention, head-of-line blocking)
However, theoretical and simulation results show that for realistic traffic an input/output speedup of 2 is enough to achieve utilizations close to 1
input interface
output interface
Backplane RO C

9. Combined Input-Output Queueing (CIOQ) Routers
Both input and output interfaces store packets
Advantages
Easy to built
Utilization 1 can be achieved with limited input/output speedup (<= 2)
Disadvantages
Harder to design algorithms
Two congestion points
Need to design flow control
input interface
output interface
Backplane RO C

10. Generic Architecture of a High Speed Router Today
Combined Input-Output Queued Architecture
Input/output speedup <= 2
Input interface
Perform packet forwarding (and classification)
Output interface
Perform packet (classification and) scheduling
Backplane
Point-to-point (switched) bus; speedup N
Schedule packet transfer from input to output

11. Backplane
Point-to-point switch allows to simultaneously transfer a packet between any two disjoint pairs of input-output interfaces
Goal: come-up with a schedule that
Provide Quality of Service
Maximize router throughput
Challenges:
Address head-of-line blocking at inputs (see next slide)
Resolve input/output speedups contention
Avoid packet dropping at output if possible
Note: packets are fragmented in fix sized cells at inputs and reassembled at outputs

12. Head-of-line Blocking
The cell at the head of an input queue cannot be transferred, thus blocking the following cells
Cannot be transferred because
is blocked by red cell
Input 1
Output 1
Cannot be
transferred
because output
buffer overflow
Input 2
Output 2
Input 3
Output 3

13. Solution to Avoid Head-of-line Blocking
Maintain at each input N virtual queues, i.e., one per output
Input 1
Output 1
Output 2
Input 2
Output 3
Input 3

14. Typical Functions Performed by Input Interface on Data Path
Packet forwarding: decide to which output interface to forward each packet based on the information in packet header

15. Datagram (Connectionless) Forwarding
Each IP router maintains a routing table
A routing entry: (address prefix, output port)
Upon packet arrival
input port forwards the packet to the output port whose address prefix matches packet’s destination address in the most number of bits (longest prefix matching)
xxx 1
12.82.xxx.xxx 2 … … 1 2

16. Datagram Forwarding (cont’d)
Each packet is forwarded independently of previous packets
Packets between two hosts can follow different paths
Link and router failure may be transparent for end-hosts
In case of failure a new alternative route is usually found
No service guarantees
When a source sends a packet, it does not have precise information about the available resources along the path

17. Virtual Circuit (VC) Forwarding
Each router maintains a routing table
A routing entry: (input port, input VCI, output port, output VCI);
VCI – Virtual Circuit Identifier
Each packet carries a VCI in its header
Upon a packet arrival at interface i
Input port uses i and the packet’s VCI v to find the routing entry (i, v, i’, v’)
Replaces v with v’ in the packet header
Forwards packet to output port I’

18. VC Forwarding: Example
in-VCI
out
out-VCI … … … … in
in-VCI
out
out-VCI 1 7 4 1 … … … … … … … …
destination
source 3 5 4 11 … … … … 1 1 2 2 1 1 1 3 3 2 2 4 4 3 3 11 5 4 4 1 1 7 2 2 3 3 4 4 in
in-VCI
out
out-VCI … … … … 2 11 3 7 … … … …

19. VC Forwarding (cont’d)
A signaling protocol is required to set up the state for each VC in the routing table
A source needs to wait for one RTT (round trip time) before sending the first data packet
Can provide per-VC QoS
When we set the VC, we can also allocate bandwidth and buffer resources along the path
If a link or router fails, the connection is broken and a new connection needs to be established

20. Source Routing
Each packet specifies the sequence of routers, or alternatively the sequence of output ports, from source to destination
source 1 1 2 2 1 1 3 3 2 2 4 4 3 3 4 3 4 4 4 1 1 2 2 4 3 4 3 3 4 3 4 4 4

21. Source Routing (cont’d)
Give the source control of the path
Doesn’t require the source to wait for an RTT before sending the first packet
Not scalable
Packet overhead proportional to the number of routers
Typically, require variable header length which is harder to implement
Hard for source to have complete information
IP implements lose source routing  sender specifies only a subset of routers along the path

22. Typical Functions Performed by Output Interface
Buffer management: decide when and which packet to drop
Scheduler: decide when and which packet to transmit
Buffer
Scheduler 1 2

23. Typical Functions Performed by Output Interface (cont’d)
Packet classification: map each packet to a predefined flow/connection (for datagram forwarding)
use to implement more sophisticated services (e.g., QoS)
Flow: a subset of packets between any two endpoints in the network
flow 1
Classifier
flow 2
Scheduler 1 2
flow n
Buffer management

24. Summary Routing architecture Router functionalities
Input and output ports interconnected by a high-speed fabric
Goals: high utilization  solve input and output contentions
Router functionalities
Packet forwarding
Datagram
Virtual circuit
Source routing
Manage router resources
Packet classification
Buffer management