A Review of Evolving Network Technology

Ethernet & IP

J.J. EkstromMarch 2008

Who is winning? Ethernet has won the LAN wars Ethernet is winning the MAN wars

– IProvo, Utopia… 10X bandwidth same price. Ethernet is contending for part of the WAN… PPOE (Point to Point

over Ethernet) IP has won all best-effort wars wars…

– Most ATM traffic is IP– A large portion of Sonet Traffic is IP– MPLS is taking over the core to optimize IP

IETF and Vendors making IP transport of choice for future– Voice over IP – IP Multicast Streaming

Why?

Simple transports Work faster and cheaper Put the smarts where it can work for more

transports Not as much advantage to smarter

transports

Historical View: Ethernet Characteristics

Ethernet shared media cable Cable access method (CSMA/CD) Unreliable Packet Delivery Assumes higher layers do most of the work Simple and Relatively fast on whatever

physical transport with any generation of hardware.

Ethernet Shared Media Cable 1

Physics determined the maximum length of the Ethernet cable– signal strength– cable characteristics

Ethernet Shared Media Cable 2

All stations (nodes) hook to, and share a single cable

Ethernet Shared Media Cable 3

Each station “listens” as it transmits

Ethernet Shared Media Cable 4

Each station must transmit a minimum of 64 bytes to “fill” the cable before it stops listening

64 bytes min.

Ethernet Shared Media Cable 5

If a 2nd node transmits before the 1st node finishes, the two transmissions collide and they must retransmit

64 bytes min. 64 bytes min.

Ethernet Cable Access Method (CSMA/CD)

CSMA/CD is a media-access method used by Ethernet and 802.3 networks

CSMA/CD stands for Carrier Sense, Multiple Access / Collision Detection

How CSMA/CD Works - 1

A station wishing to transmit first listens for traffic on the cable indicated by a carrier signal (CSMA/CD-Carrier Sense)

Network Cable Carrier Signal

How CSMA/CD Works - 2

If the carrier signal is detected, the station waits a period of time and tries again

Network Cable Carrier Signal

How CSMA/CD Works - 3

If NO carrier signal is detected, the station starts transmitting its packet (min. of 64 bytes) and simultaneously listening

Network CableM

IN. O

F 6

4 B

YTE

S

How CSMA/CD Works - 4

TWO stations can start transmitting at the same time (CSMA/CD - Multiple Access)

Network Cable

MIN

. O

F 6

4 B

YTE

S

MIN

. O

F 6

4 B

YTE

S

How CSMA/CD Works - 5

If this happens, both stations hear garbage (CSMA/CD - Collision Detection)

Network Cable

MIN

. O

F 6

4 B

YTES

MIN

. O

F 6

4 B

YTES@&*!

How CSMA/CD Works - 6

When collisons are detected, both stations :– cancel transmissions by sending a jam signal– wait a random amount of time before trying to

transmit again

Network Cable

JAM

SIG

NA

L

JAM

SIG

NA

L

PROBLEM #1

Physics doesn’t allow you to have LAN wires as long as you would like.

SOLUTION #1

Repeater extended wire length, broadcast domain, and collision domain

Repeater

PROBLEM #2

Too many collisions. LAN wouldn’t carry enough traffic.

SOLUTION #2

Bridging segments extends broadcast domain without collisions: Bigger LANs

BRIDGE

PROBLEM #3 Broadcast storms - result from multi-port

bridges “flooding” all ports when packet destination is unknown and a loop exists.

BRIDGE 1

BRIDGE 3 BRIDGE 2

64 bytes min.

Packet returningto original bridge

PROBLEM #3– when the original packet returns to a previous

bridge, new packets are generated and a “storm” is generated.

BRIDGE

BRIDGE BRIDGE

Cycle Repeats

SOLUTION #3

3.1 - 802.1D (spanning tree) installed on bridges.

3.2 - Routers

SOLUTION #3.1

802.1D (Spanning Tree) added to bridges. – Spanning Tree is an algorithm that runs on

bridges to eliminate loops dynamically.

802.1DBRIDGE 1

802.1DBRIDGE 3

802.1DBRIDGE 2

64 bytes min.

802.1D (SpanningTree) determines thatthis link is redundant

and shuts it down

SOLUTION #3.2 Routers - make every segment another

network or subnet by refusing to pass through any packet whose address it does not recognize.

BRIDGE 1

BRIDGE 2

64 bytes min.

RouterBRIDGE 3

SOLUTION #3.2 NOTE:

– in XNS a single broadcast domain is called a “network.”

– in TCP a single broadcast domain is called a “subnet.”

– network personnel often call a collision domain a “segment.”

PROBLEM #4 Topology and failure characteristics -

problems with bus-oriented LANs (i.e., when the wire breaks NONE of the stations can communicate).

SOLUTION #4

Twisted pair LANs.– When any one wire segment fails, the whole

LAN does NOT go down.

Concentrator ConcentratorBridge

Concentrator

PROBLEM #5

Not enough Bandwidth– only 10 MBPS available on each collision

domain

BRIDGE

BRIDGE

BRIDGEConcentrator

Concentrator

Concentrator

SOLUTION #5

Switches (multiport Bridges) - allows more segments (bandwidth) at a lower cost per port.

Concentrator

Concentrator

SWITCH

PROBLEM #6

Controlling User Connectivity– keep groups separate– easily share resources between groups– do adds, moves, and changes without rewiring

SOLUTION #6 VLANs of various forms create isolated

broadcast domains (networks) Connection between Virtual LAN networks

requires a router. People do security in their routers and

firewalls at network boundaries anyway

Problem #7

During roughly the same 20-25 year period Token-Ring LANs, FDDI, ATM, and several other LAN and WAN technologies have been undergoing similar evolutionary tracks as ethernet.

It was not clear that there would be a clear winner. How do you hook them together and protect your

technology investments? Users don’t care how their bits get pushed around,

only that things work.

Solution #7

Internetworking…The real reason IP has won the protocol wars.– Works well on P2P links

– Works well on LANs

– Makes very few demands of participant networks

– “Rough consensus and working code” Motto of the IETF The way to get useful things quickly in a world of confusion…

what works best wins.

Internetworking

Outline Best Effort Service ModelGlobal Addressing Scheme

IP Internet

Concatenation of Networks

Protocol Stack

R2

R1

H4

H5

H3H2H1

Network 2 (Ethernet)

Network 1 (Ethernet)

H6

Network 3 (FDDI)

Network 4(point-to-point)

H7 R3 H8

R1

ETH FDDI

IPIP

ETH

TCP R2

FDDI PPP

IP

R3

PPP ETH

IP

H1

IP

ETH

TCP

H8

Service Model Connectionless (datagram-based) Best-effort delivery (unreliable service)

– packets are lost– packets are delivered out of order– duplicate copies of a packet are delivered– packets can be delayed for a long time– (Sound like Ethernet?)

Datagram format Version HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

Fragmentation and Reassembly

Each network has some MTU Strategy

– fragment when necessary (MTU < Datagram)– try to avoid fragmentation at source host– re-fragmentation is possible – fragments are self-contained datagrams– use CS-PDU (not cells) for ATM– delay reassembly until destination host– do not recover from lost fragments

Example

H1 R1 R2 R3 H8

ETH IP (1400) FDDI IP (1400) PPP IP (512)

PPP IP (376)

PPP IP (512)

ETH IP (512)

ETH IP (376)

ETH IP (512)

Ident = x Offset = 0

Start of header

0

Rest of header

1400 data bytes

Ident = x Offset = 0

Start of header

1

Rest of header

512 data bytes

Ident = x Offset = 512

Start of header

1

Rest of header

512 data bytes

Ident = x Offset = 1024

Start of header

0

Rest of header

376 data bytes

Global Addresses Properties

– globally unique– hierarchical: network + host

Dot Notation– 10.3.2.4– 128.96.33.81– 192.12.69.77

Network Host

7 24

0A:

Network Host

14 16

1 0B:

Network Host

21 8

1 1 0C:

Datagram Forwarding Strategy

– every datagram contains destination’s address– if directly connected to destination network, then forward to host– if not directly connected to destination network, then forward to

some router– forwarding table maps network number into next hop– each host has a default router– each router maintains a forwarding table

Example (R2) Network Number Next Hop 1 R3 2 R1 3 interface 1 4 interface 0

Address Translation Map IP addresses into physical addresses

– destination host– next hop router

Techniques– encode physical address in host part of IP address– table-based

ARP– table of IP to physical address bindings– broadcast request if IP address not in table– target machine responds with its physical address– table entries are discarded if not refreshed

ARP Details

Request Format– HardwareType: type of physical network (e.g., Ethernet)– ProtocolType: type of higher layer protocol (e.g., IP)– HLEN & PLEN: length of physical and protocol addresses– Operation: request or response – Source/Target-Physical/Protocol addresses

Notes– table entries timeout in about 10 minutes– update table with source when you are the target – update table if already have an entry– do not refresh table entries upon reference

ARP Packet Format

TargetHardwareAddr (bytes 2 – 5)

TargetProtocolAddr (bytes 0 – 3)

SourceProtocolAddr (bytes 2 – 3)

Hardware type = 1 ProtocolType = 0x0800

SourceHardwareAddr (bytes 4 – 5)

TargetHardwareAddr (bytes 0 – 1)

SourceProtocolAddr (bytes 0 – 1)

HLen = 48 PLen = 32 Operation

SourceHardwareAddr (bytes 0 – 3)

0 8 16 31

Internet Control Message Protocol (ICMP)

Echo (ping) Redirect (from router to source host) Destination unreachable (protocol, port, or host) TTL exceeded (so datagrams don’t cycle forever) Checksum failed Reassembly failed Cannot fragment

Summary