Introduction to Routers and Switches

What do they look like?

Access routerse.g. ISDN, ADSL

Core routere.g. OC48c POS

Core ATM switch

Basic Architectural Componentsof an IP Router

Control Plane

Datapath”per-packet processingSwitching

ForwardingTable

Routing Table

Routing Protocols

Per-packet processing in an IP Router

1. Accept packet arriving on an incoming link.

2. Lookup packet destination address in the forwarding table, to identify outgoing port(s).

3. Manipulate packet header: e.g., decrement TTL, update header checksum.

4. Send packet to the outgoing port(s).

5. Buffer packet in the queue.

6. Transmit packet onto outgoing link.

General Switch Model

Cross-bar

InputBuffer

Control

OutputPorts

Input Receiver Transmiter

Ports

Routing, Scheduling

OutputBuffer

Interconnect

IP Switch Model

2. Interconnect 3. EgressForwardin

gTable

ForwardingDecision

1. Ingress

Forwarding

Table

ForwardingDecision

Forwarding

Table

ForwardingDecision

Forwarding Engine

header

payload

Packet

Router

Destination Address

Outgoing Port

Dest-network PortForwarding Table

Routing Lookup Data Structure

65.0.0.0/8

128.9.0.0/16

149.12.0.0/19

3

1

7

The Search Operation is not a Direct Lookup

(Incoming port, label)

Ad

dre

ss

MemoryD

ata

(Outgoing port, label)

IP addresses: 32 bits long 4G entries

The Search Operation is also not an Exact Match Search

• Hashing • Balanced binary search trees

Exact match search: search for a key in a collection of keys of the same length.

Relatively well studied data structures:

0 224 232-1

128.9.0.0/16

65.0.0.0

142.12.0.0/19

65.0.0.0/8

65.255.255.255

Example Forwarding Table

Destination IP Prefix Outgoing Port

65.0.0.0/8 3

128.9.0.0/16 1

142.12.0.0/19 7

IP prefix: 0-32 bits

Prefix length

128.9.16.14

Prefixes can Overlap

128.9.16.0/21 128.9.172.0/21

128.9.176.0/24

Routing lookup: Find the longest matching prefix (the most specific route) among all prefixes that match the destination address.

0 232-1

128.9.0.0/16142.12.0.0/1965.0.0.0/8

128.9.16.14

Longest matching prefix

8

32

24

Prefixes

Pre

fix L

en

gth

128.9.0.0/16142.12.0.0/19

65.0.0.0/8

Difficulty of Longest Prefix Match

128.9.16.14

128.9.172.0/21

128.9.176.0/24

128.9.16.0/21

Lookup Rate Required

12540.0OC768c2002-03

31.2510.0OC192c2000-01

7.812.5OC48c1999-00

1.940.622OC12c1998-99

40B packets (Mpps)

Line-rate (Gbps)

LineYear

DRAM: 50-80 ns, SRAM: 5-10 ns

31.25 Mpps 33 ns

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

Size of the Forwarding Table

Source: http://www.telstra.net/ops/bgptable.html

95 96 97 98 99 00Year

Num

ber

of

Pre

fixes

10,000/year

Internal Interconnects

Io

I1

I2

I3

Io I1 I2 I3

O0

Oi

O2

O3

RAMphase

O0

Oi

O2

O3

DoutDin

Io

I1

I2

I3

addr

1. Multiplexers 2. Tri-State Devices

3. Shared Memory

InterconnectsTwo basic techniques

Input Queueing Output Queueing

Usually a non-blockingswitch fabric (e.g. crossbar)

Usually a fast bus

Shared Memory Bandwidth

SharedMemory

200 byte bus

5ns SRAM

1

2

N

• 5ns per memory operation• Two memory operations per packet• Therefore, up to 160Gb/s• In practice, closer to 80Gb/s

Input buffered swtich

• Independent routing logic per input– FSM

• Scheduler logic arbitrates each output– priority, FIFO, random

• Head-of-line blocking problem

Cross-bar

OutputPorts

Input Ports

Scheduling

R0

R1

R2

R3

Internconnect

Input QueueingHead of Line Blocking

Dela

y

Load58.6% 100%

Head of Line Blocking

(Virtual) Output Buffered Switch

• How would you build a shared pool?

Control

OutputPorts

Input Ports

OutputPorts

OutputPorts

OutputPorts

R0

R1

R2

R3

N buffers per input

Solving HOL with Input QueueingVirtual output queues

Input QueueingVirtual Output Queues

Dela

y

Load100%

Output scheduling

• n independent arbitration problems?– static priority, random, round-robin

• simplifications due to routing algorithm?• general case is max bipartite matching

Cross-bar

OutputPorts

R0

R1

R2

R3

O0

O1

O2

InputBuffers

Finding a maximum size match

• How do we find the maximum size (weight) match?

A

B

C

D

E

F

1

2

3

4

5

6

Inputs Outputs

Requests