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Ethernet

Ethernet

The term Ethernet refers to the family of local-areanetwork (LAN) products covered by the IEEE 802.3standard. Three data rates are currently defined for

operation over optical fiber and twisted-pair cables:10 Mbps 10Base-T Ethernet100 Mbps Fast Ethernet

1000 Mbps Gigabit Ethernet

10-Gigabit Ethernet is under development and will likelybe published as the IEEE 802.3ae supplement to theIEEE 802.3 base standard in late 2001 or early 2002

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Ethernet

The Ethernet MAC Sublayer

The MAC sublayer has two primaryresponsibilities:

Data encapsulation, including frameassembly before transmission, and frameparsing/error detection during and afterreception

Media access control, including initiation offrame transmission and recovery fromtransmission failure

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Ethernet

Frame FormatPreamble (PRE) Consists of 7 bytes. The PRE is an alternating patternof ones and zeros that tells receiving stations that a frame is coming, andthat provides a means to synchronize the frame-reception portions ofreceiving physical layers with the incoming bit stream.Start-of-frame delimiter (SOF) Consists of 1 byte. The SOF is analternating pattern of ones and zeros, ending with two consecutive 1-bitsindicating that the next bit is the left-most bit in the left-most byte of thedestination address.Destination address (DA ) Consists of 6 bytes. The DA field identifieswhich station(s) should receive the frame. The left-most bit in the DA fieldindicates whether the address is an individual address (indicated by a 0)or a group address (indicated by a 1). The second bit from the leftindicates whether the DA is globally administered (indicated by a 0) orlocally administered (indicated by a 1). The remaining 46 bits are auniquely assigned value that identifies a single station, a defined group ofstations, or all stations on the network.

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Ethernet

Frame Format

Source addresses (SA) Consists of 6 bytes. The SA fieldidentifies the sending station. The SA is always an individualaddress and the left-most bit in the SA field is always 0.

Length/Type Consists of 2 bytes (0600length-802.3).

# 0 - 1500 length field (IEEE 802.3 and/or 802.2)

# 0x0800 IP(v4), Internet Protocol version 4

# 0x0806 ARP, Address Resolution Protocol# 0x8137 IPX, Internet Packet eXchange (Novell)

# 0x86dd IPv6, Internet Protocol version 6

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Ethernet

Ethernet Frame Format

Data Is a sequence of n bytes of any value, where n is lessthan or equal to 1500. If the length of the Data field is lessthan 46, the Data field must be extended by adding a filler (apad) sufficient to bring the Data field length to 46 bytes.Frame check sequence (FCS) Consists of 4 bytes. Thissequence contains a 32-bit cyclic redundancy check (CRC)value, which is created by the sending MAC and isrecalculated by the receiving MAC to check for damagedframes. The FCS is generated over the DA, SA, Length/Type,and Data fields.

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ARP

and RARP

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Introduction

A computer connected to an IP/Ethernet hastwo addressesAddress of network card ( MAC address )

Globally unique and unchangeable address stored onthe network card.

Ethernet header contains the MAC address of thesource and the destination computer.

IP address Each computer on a network must have a unique IP

address to communicate. Virtual and assigned by software.

Fi 7 1

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Figure 7-1

ARP and RARP

Figure 7 2

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Figure 7-2

Position of ARP and RARPin TCP/IP protocol suite

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How ARP functions :1. Get IP address of target.

2. Create a request ARP message Fill sender physical address Fill sender IP address Fill target IP address

Target physical address is filled with 03. The message is passed to the data link layer

where it is encapsulated in a frame. Source address: physical address of the sender.

Destination address: broadcast address.

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1. Every host or router on the LAN receives theframe.

All stations pass it to ARP. All machines except the one targeted drop the

packet.

2. The target machine replies with an ARPmessage that contains its physical address.

A unicast message.

3. The sender receives the reply message andknows the physical address of the targetmachine.

Figure 7-3

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Figure 7 3ARP operation

Figure 7-4

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Figure 7 4

ARP packet

Figure 7-5

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g

Encapsulation of ARP packet

Figure 7-6:a

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g

Four cases using ARP

Figure 7-6:b

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Figure 7-6:c

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Figure 7-6:d

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An ARP request is broadcast ;

an ARP reply is unicast .

Figure 7-7: request

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Example 1

Figure 7-7: reply

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Example 1 (Continued)

Figure 7-8

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Proxy ARP

Figure 7-10

RARP

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RARPoperation

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The RARP request packets are

broadcast ; the RARP reply packets areunicast .

Figure 7-11

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RARP packet

Figure 7-12

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Encapsulation of RARP packet

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Networking Devices

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Hubs

Multiport repeater containing multiple ports tointerconnect multiple devices

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Bridges

Like a repeater, abridge has a singleinput and singleoutput port

Unlike a repeater,it can interpret thedata it retransmits

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Bridges

Filtering databaseCollection of data created and used by a bridge thatcorrelates the MAC addresses of connected workstationswith their locations

Also known as a forwarding table

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Bridges

Spanning tree algorithmRoutine that can detect circular traffic patterns and modifythe way multiple bridges work together, in order to avoidsuch patterns

Transparent bridgingMethod used on many Ethernet networks

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SwitchesSubdivide a network into smallerlogical piecesCreate multiple, smaller collisiondomainsIn a half duplex switched networkthere are only two devices in eachcollision domain.In a full-duplex switchedenvironment there is only 1 device

in each collision domain (0 %chance of collision)

C t Th gh M d d

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Cut-Through Mode andStore and Forward Mode

Cut-through modeSwitching mode in whichswitch reads a frames

header and decides whereto forward the data before it

receives the entire packetCut-through switches candetect runts , or packetfragments

Store and forward mode

Switching mode in whichswitch reads the entire dataframe into its memory andchecks it for accuracybefore transmitting theinformation

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Higher-Layer Switches

Switch capable of interpreting Layer 3 data iscalled a Layer 3 switchSwitch capable of interpreting Layer 4 data is

called a Layer 4 switchThese higher-layer switches may also be calledrouting switches or application switches

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Routers

Multiport connectivity deviceCan integrate LANs and WANs running atdifferent transmission speeds and using a

variety of protocolsRouters operate at the Network layer (Layer 3)of the OSI Model

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Agenda

Packet Tracer Exercise and Project QuestionsRouters and Routing protocolsRouting LABRemember to Bring the cable kit next week

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Router Features and Functions

Modular routerRouter withmultiple slots thatcan hold different

interface cards orother devices

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Filter out broadcast transmission toalleviate network congestionPrevent certain types of traffic from gettingto a networkSupport simultaneous local and remoteactivity

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Static routingTechnique in which a network administrator programs arouter to use a specified paths between nodes

Dynamic routingAutomatically calculates best path between nodes andaccumulates this information in a routing table

HopTerm used in networking to describe each trip data takefrom one connectivity device to another

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Routing Protocols

To determine the best path , routers communicatewith each other through routing protocolsIn addition to its ability to find the best path, a routingprotocol can be characterized according to its

convergence time and bandwidth overheadConvergence time

The time it takes for a router to recognize a best path in the event ofa change or outage

Bandwidth overheadBurden placed on an underlying network to support the routingprotocol

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Routing Protocols

The four most common routing protocols:RIP (Routing Information Protocol) for IP and IPXOSPF (Open Shortest Path First) for IP

EIGRP (Enhanced Interior Gateway RoutingProtocol) for IP, IPX, and AppleTalkBGP (Border Gateway Protocol) for IP

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Gateways

Combination of networking hardware and softwarethat connects two dissimilar kinds of networksPopular types of gateways include:

E-mail gatewaysIBM host gatewaysInternet gatewaysLAN gateways

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Ethernet Encapsulation

RFC 894

E h

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Ethernet

The term Ethernet generally refers to a standardpublished in 1982 by Digital Equipment Corp., IntelCorp., and Xerox Corp.It is the predominant form of Local Area Networks

technology used with TCP/IP today.It uses an access method called CSMA/CD, whichstands for Carrier Sense, Multiple Access withCollision Detection.

It operates at 10 Mbits /sec and uses 48-bitaddresses

E h E l i

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Ethernet Encapsulation

48-bit (6bytes) source and destination addresses. These arewhat we call hardware addresses .The Ethernet type field identifies the type of data that follows.The data field is the actual payload and must be at least 46bytesIf needed (frames with less than 46 bytes of payload), Padbytes are inserted to assure that the frame is long enough.The CRC field is a cyclic Redundancy check (a checksum)that detects errors in the frame. (This is also called FCS orFrame Check Sequence)

Eth t d MTU

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Ethernet and MTU

As you can see there is a limit on the size ofan Ethernet frame.This limits the number of bytes of data to 1500

bytes.This characteristics of the data link layer iscalled MTU, or Maximum Transmission Unite

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Eth t d MTU

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Ethernet and MTU

When two hosts on the same network arecommunicating with each other, it is the MTU of thenetwork that is important.But, when two hosts are communicating acrossmultiple networks, each link can have a differentMTU. The important numbers are the MTUs of thetwo networks to which the two hosts connect, butrather the smallest MTU of any data link that packets

traverse between the two hosts.

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Internet Protocol (IP)Provides information about how and where datashould be deliveredSubprotocol that enables TCP/IP to

internetworkTo internetwork is to traverse more than one LANsegment and more than one type of network througha router

In an internetwork, the individual networks that are joined together are called subnetworks

Internet Protocol (IP)

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( )

IP datagramIP portion of TCP/IP frame that acts as an envelope for dataContains information necessary for routers to transfer data betweensubnets

IP header format

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IP header format: Version

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4 bits. Indicates the version of IP

currently used. IPv4 : 0100 IPv6 : 0110

IP header format: Header length

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IP header format: Header length

4 bits. IP header length : Indicates the

datagram header length in 32 bit

words (4 bits), and thus points to thebeginning of the data.

IP header format: Service type

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8 bits. Specifies the level of importance that

has been assigned by a particularupper-layer protocol.

Precedence. Reliability. Speed.

IP header format: Total length

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16 bits. Specifies the length of the

entire IP packet, includingdata and header, in bytes.

IP header format: Identification

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16 bits. Identification contains an integer that

identifies the current datagram. Assigned by the sender to aid in

assembling the fragments of adatagram.

IP header format: Flags

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3 bits. The second bit specifying whether the packet

can be fragmented . The last bit specifying whether the packet is the

last fragment in a series of fragmented packets.

IP header format: Fragment offset

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13 bits. The field that is used to help piece together

datagram fragments. The fragment offset is measured in units of 8

octets (64 bits). The first fragment has offset zero.

IP header format: Time to Live

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8 bits.

Time-to-Live maintains a counter that graduallydecreases to zero, at which point the datagramis discarded, keeping the packets from loopingendlessly.

IP header format: Protocol

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8 bits. Indicates which upper-layer protocol receives

incoming packets after IP processing has beencompleted

06 : TCP 17 : UDP

IP header format: Header checksum

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16 bits. A checksum on the header only, helps

ensure IP header integrity.

IP header format: Addresses

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32 bits each. Source IP Address Destination IP Address

IP header format: Options

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Variable length. Allows IP to support various options, such

as security, route, error report ...

IP header format: Padding

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The header padding is used to ensure thatthe internet header ends on a 32 bitboundary.

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