css432: switching and bridging 1 switching and bridging textbook ch3.1 and 3.4 instructor: joe...

CSS432: Switching and Bridging 1

Switching and Bridging Textbook Ch3.1 and 3.4

Instructor: Joe McCarthy(based on Prof. Fukuda’s slides)

Network Layer

CSS432: Switching and Fowarding 2

3

Scalable Networks Why switches?

Connects two or more network segments (>2500m in Ethernet) Support large numbers of hosts (>1024 hosts in Ethernet) Maintain performance (> two packets through a switch)

Network Switches

Packet Switch Circuit Switch

Connectionless Connection Oriented Dynamic Static

IP datagrams ATM X25 DTM SONET

virtualTDMFDM

Dynamically reallocate bandwidthFrame sent over optics in synchronizationwith atomic/satellite clock

CSS432: Switching and Bridging

4

Three Approaches

Datagram/connectionless switching IP

Virtual circuit/connection-oriented switchingX.25ATM (Asynchronous Transfer Mode)

Source routingConnection requests in virtual circuit


5

Datagram Switching No connection setup phase Each packet forwarded (routed) independently

0

13

2

0

1 3

2

0

13

2

Switch 3 Host B

Switch 2

Host A

Switch 1

Host C

Host D

Host EHost F

Host G

Host H

Analogy: postal system Each switch maintains a

forwarding (routing) table

Dest Port

A 3

B 0

C 3

D 3

E 2

F 1

G 0

H 0

Table at Switch 2


6

Datagram Switching (Cont’d)

No connection setup Pros 1: A source can send data as soon as it is ready.

No way of knowing if a packet is delivered. Cons 1: A source must estimate network congestion or

disconnection

Each packet may take a different route. Pros 2: No single point of failure Cons 2: May arrive in different order


7

Virtual Circuit Switching Explicit connection setup (and tear-down) phase Subsequence packets follow same circuit

Analogy: phone call Each switch maintains

a VC table0

13

2

01 3

2

0

13

25 11

4

7

Switch 3

Host B

Switch 2

Host A

Switch 1

VCI = 5

VCI = 11

VCI = 7

VCI = 4

Port (in) VCI Port (out) VCI

2 5 1 11


3 11 0 7


0 7 3 4

Switch 1

Switch 2

Switch 3

Question:It seems like the table needs only incoming & outgoing port pairs…. Why do we need a VCI for each?


8

Virtual Circuit Switching (Cont’d)

0

13

2

0

1 3

2

0

13

2

Switch 3 Host B

Switch 2

Host A

Switch 1

Host C

Host D

Host EHost F

Host G

Host H

VCI=5

VCI=11

VCI=7

VCI=4

VCI=6 VCI=12

VCI=8

VCI=4


2 5 1 11

2 6 1 12


3 11 0 7

3 12 0 8


0 7 3 4

0 8 2 4

Global address information is replaced with local VCI. Each switch has local but not global

information. To set up connection:

Each switch still needs a global view of network configuration to forward a connection request message to destination

Switch 1

Switch 2

Switch 3


9

Virtual Circuit Model (Cont’d)

Connection setup required Pros 1: An opportunity to reserve resources (QoS) Cons 1: Wait for a full RTT before sending first data packet. Cons 2: Full address for destination still required for connection.

Packets sent along the same route Pros 2: Each data packet contains only a VCI. Pros 3: Flow control possible along the entire connection Cons 3: If the connection is broken, a new one needs to be

established.


10

Source Routing

0

13

2

0

1 3

2

0

13

2

0

13

2

3 0 1 3 01

30 1

Switch 3

Host B

Switch 2

Host A

Switch 1

Each packet has complete switching information from source to destination

Three ways to maintain switching information: Rotation Striping Use of a pointer

Useful to send a connection setup request in VC


11

Bridges and Extended LANs Connecting two or more LANs

Repeater / Hub L1: Physical Layer Forwards everything everywhere Limitations: <= 2500m and <= 1024 nodes

Bridge (or LAN switch) L2: Datalink Layer Forwards frames using MAC address Static configuration + partial dynamic

configuration (Spanning Tree Protocol)

Router L3: Network Layer Forwards packets using IP address Dynamic configuration

RQuickTime? Ç?

TIFFÅiîÒà?èkÅj êLí£ÉvÉçÉOÉâÉÄÇ™Ç±ÇÃÉsÉNÉ`ÉÉÇ?å©ÇÈÇ?Ç?Ç…ÇÕïKóvÇ-Ç?ÅB

destination

R

R

R

BQuickTime? Ç?


destination

B

B

B

RQuickTime? Ç?


destination

R

R

R


12

Learning Bridges Do not forward frames when unnecessary

Ex. A frame sent from A to B Maintain forwarding table

Host Port

A 1 B 1 C 1 X 2 Y 2 Z 2

Learn table entries based on source address E.g. An entry for A is registered upon receiving a frame from A E.g. When forwarding a frame to B, don’t forward to Port 2

Table is an optimization; need not be complete Entries are expired after a specific period of time

Linux brctl command: makes a logical bridge with max age = 4sec

A

Bridge

B C

X Y Z

Port 1

Port 2

Based on datagram switching


13

STP: Spanning Tree Protocol Loops

Example: B1 receives a frame from Node X on

LAN H to Node Y on LAN C. B1 registers an entry for Node X but not

yet Node Y. B1 forwards frame to all ports except to

LAN H. B7 receives frame 7 & forwards to LAN B B5 forwards frame to LAN A & D. B1 receives this frame (again) and

registers an entry for X. B1 forwards it to all ports except to LAN

H & D. Node Y eventually receives a frame.

Problem: Duplicated frames are forwarded along

loops. Spanning Tree Algorithm

Inactivate bridge ports so that no cycle exists in extended LAN

Radia Perlman, “Mother of the Internet” IEEE 802.1 Specification

B3

A

C

E

DB2

B5

B

B7 K

F

H

B4

J

B1

B6

G

I

X

Y


14

STP Details

Initially, each bridge believes it is the root When a bridge learns it is not the root, it

stops generating configuration messages When a bridge learns it is not a

designated bridge for a LAN, it stops forwarding configuration messages

In steady state: The network (tree) has exactly 1 root

(only bridge generating config msgs) Each LAN has exactly 1 designated bridge

(only bridges forwarding config msgs) If any bridge does not receive a

configuration message after a period of time, it starts generating configuration messages claiming to be the root.

B3

A

C

E

DB2

B5

B

B7 K

F

H

B4

J

B1

B6

G

I

Bridges exchange configuration messages (Y, d, X) Y: the id of reigning root d: #hops from X to Y X: the sending bridge id

(1, 0, 1)

(1, 1, 2)

(1, 1, 5)

(1, 0, 1)

(1, 0, 1)

root

1 hop

1 hopB5 < B7

1 hopB4 < B6

1 hop

1 hop

2 hops


Another STP illustration

Determine RPs (root ports) DPs (designated

ports) BPs (blocked ports)


http://en.wikipedia.org/wiki/Spanning_Tree_Protocol

One more STP illustration


http://www.cisco.com/warp/public/473/spanning_tree1.swf

18

X.25 Source Routing and Virtual Circuit Switching

0 0 0 1 group

channel

type 1

caller address length callee address length

caller address (up to 14bits)

callee address (up to 14bits)Data

Higher Layer Protocol

receive sequence send sequence

channel

0 0 modulo group

Call request

Call accept

Source VCI

Destination VCI

DTE/DCE

Call request/Idle mode/call clearing Data packet

4K channelsCall request

control

3 bits: country code1 bit: network code10 bits: address

Why sequence#?1. Sliding window2. Error recovery


19

Cell Switching (ATM) Connection-oriented packet-switched network Used in both WAN and LAN settings Packets are called cells

5-byte header + 48-byte payload (easier to switch, high frame utilization for small data)

Commonly transmitted over SONET Error correction

End-to-end but not at each switch (i.e., at data link layer) Congestion control

Admission control Leaky packet transfer


20

Switch Implementation

Using a workstationFlexible controlPerformance problem

Using a custom hardwareShared/share memory-based switchCrossbar switchSelf-routing switch (Batch Banyan switch)


21

Workstation Used as a Switch

Advantage: flexible because a workstation has a CPU. Example

33MHz 32bit I/O bus 1Gbps for one way from NIC to main memory 500Mbps for a round trip between NIC and main memory Enough to support five 100Mbps Ethenet

What if a packet is very small like 64byes The workstation has 500,000 packets per second (pps). Throughput: 500,000 x 64 x 8 = 256Mbps

NIC

NIC

NIC

I/Octlr

CPU

Main memory I/O Bus

LAN A

LAN B

LAN C

Workstation


22

Shared Bus/Memory-Based Switch

A simple design Shared bus or memory becomes a bottleneck. (Max. 16 bus masters)

Output Port

Output Port

Output Port

Input Port

Input Port

Input Port

Shared memory

Sha

red

bus

Controlprocessor

DMA from port to port


23

Crossbar Switch

Without a collision, all inputs delivered to each output All inputs may go to the same output which causes a

collision in the output buffer.


24

Crossbar + Knockout Switch

Knockout switch: buffers only L out of N packets, where L < N. Ex. L =8 is sufficient. 1 packet loss per million for a large N. (logarithmically in the loss rate)

Detailed explanation:http://www.laynetworks.com/Knockout%20Switch.htm

N = 4

L = 3

Knockout Switch

Round-robin


25

Banyan Switch

2 x 2 switching elements interconnected in regular patterns. Collisions occur if packets are not presented in ascending order

001

011

110

111

111

110

011

001

001

000

110

111

000

001

111

110

0 0

1

0

1 1


26

Batcher Network + Banyan Switch

Sort packets in ascending order through Batcher network

Then switch them through Banyan switch

001

000

110

111

∞

∞

∞

∞

000

001

111

110

000

001

111

110


27

Reviews Datagram switching Virtual Circuit switching Source routing Bridges: STP and limitations Switches: workstation-based, shared bus/memory-based,

crossbar + (knockout), and (batcher network) + banyan

Exercises in Chapter 3 Ex. 1 (vc sw) Ex. 4 (datagram sw) Ex. 13 (STP) Ex. 26,32 (Switch implementation)


css432: switching and bridging 1 switching and bridging textbook ch3.1 and 3.4 instructor: joe...

Documents

complete switching information

switch 2css432

switch 3css432

connection setup request

data packet

phone calleach switch

source routingeach packet

connection request message