Oct 21, 2014 CS524: Advanced Computer Networks 1

LAN Addressing

Advanced Computer Networks

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An Internet Connection End stations are connected to LANs LANs are connected through Bridges to

form extended LANs Extended LANs are connected through

gateways/routers/switches Layered architecture

Connection is between “peers” Service Models (Fig. 1.3 of Perlman)

PDUs (between peers) and SDUs(from up layers)

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Local Area Networks First part of the course IEEE 802 Committee

LAN Standardization Physical and Data Link Layers of OSI

Model Data Link layer subdivided by them:

MAC (Dependent on the type of LAN) LLC (allows sharing data link resources)

Several LANs were standardized

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IEEE 802 Subcommittees 802.1 --- common issues 802.2 --- LLC

Does not deal with PHY and MAC

802.3 --- CSMA/CD 802.4 --- Token Bus 802.5 --- Token Ring

Type 1, 2, … LLC

MAC

PHY

Data Link

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LAN Addresses Most LANs are “broadcast” type LAN addresses solve two problems on

shared (or broadcast) LANs Who is the sender? Who is the receiver?

IEEE 802 standardized the address length

Two different lengths were chosen 16 bit (unique on the network) --- obsolete 48 bit (unique globally --- plug and play)

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48 bit LAN Addresses Globally unique Assigned by IEEE Cost is $1250 for a “block” of

addresses A “block” includes 224 addresses1st octet 2nd octet 3rd octet 4th octet 5th octet 6th octet

Vendor code (OUI) Vendor-assigned values

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48 bit LAN Addresses OUI = Organizationally unique identifier

Fixed value assigned by IEEE 224 different possibilities

Not all of them are used!!!

Vendor-assigned Values A total of 224 unique addresses are available

by purchasing one block A block may be shared A vendor can buy more blocks with different

OUIs

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Group/Individual bit in OUI In fact, One block 225 addresses

224 of the addresses are unicast 224 of the addresses are multicast G/I bit decides if the address is

multicast G/I = 0 means unicast or individual

station G/I = 1 means a (LAN) multicast address

10111101

G/I (group/individual) --- first bit on the wire

G/L (global/local)

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Global/Local bit in OUI Another bit in the OUI is designated by

the IEEE as G/L bit IEEE sets G/L = 0 when giving out the

blocks of addresses Addresses with G/L = 1 can be used

without paying IEEE but the network administrator is responsible to assign addresses such that there is no collision

This leaves with 222 unique OUIs

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Why multicast addresses? In most LANs (e.g., CSMA/CD LANs), every

entity receives all the data on the LAN segment it is connected to

Hardware filtering is desirable because promiscuous listening is expensive

Some entities (e.g., bridges and LAN monitors) have to listen promiscuously

One station will be interested in one unicast address and multiple multicast addresses

Unicast address is hardwired Multicast addresses fall into hardwired hash buckets

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Protocol Type Multiplexing One station, many higher layer protocols

Which protocol is the desired recipient? Which protocol constructed the packet?

This information is also included in the LAN header --- just like LAN addresses are!

IP IPX ARP

MAC Layer

XNS

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Protocol Type Multiplexing Original Ethernet design

2 octet long field included in LAN header

Previously administered by Xerox, currently by IEEE

Protocol vendors need to negotiate for getting a protocol type added

http://standards.ieee.org/regauth/ethertype/index.html

6 octets 6 octets 2 octets variable

DestinationAddress

SourceAddress

ProtocolType

Data

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SAP Multiplexing More flexible to have separate source

and destination protocol type fields Can assign different numbers to the

same protocol on different machines Service Access Points (SAPs)

Included in 802 LAN header SSAP and DSAP

1 octet each but only 6 bits are used

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SAP Multiplexing

All 1’s ALL SAPs – like broadcast All 0’s (except G/L) data link layer itself 6-bit globally assigned SAP numbers (by

IEEE)

6 octets 6 octets 2 octets variable

DestinationAddress

SourceAddress

ProtocolType

Data

10111101

G/I (group/individual)

G/L (global/local)

DSAP SSAPlength

2 octets

CTL

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SAP Multiplexing G/L bit is similar to the one used in LAN

addresses G/I bit --- perhaps to keep compatibility

with the LAN addresses??? Only 64 unique SAP protocols are

supported Strict rules for assigning a SAP number Protocol must be designed by standard

bodies

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SAP Multiplexing Local SAP protocols can be used

Network/Protocol manager’s responsibility to ensure unique SAPs to protocols

Conversation startup is difficult SAP number at the destination machine is

not known at the source machine!

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SNAP SAP Subnetwork Access Protocol

Single globally assigned SAP value AA hex (10101010) --- SNAP SAP

When DSAP = SSAP = SNAP SAP Header is expanded to include a “protocol

type” field A “longer” protocol type field can then be

used Standardized to 5 octets (see book for reason!)

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Transmission Bit Order 802.1 defines a canonical format for LAN

addresses 00-60-1D-23-20-A9

802.3 and 802.4 LSB is transmitted first

802.5 and FDDI MSB is transmitted first

Internetworking different topologies Bit order should be shuffled if forwarding

frames between incompatible LAN topologies

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Frame Formats Ethernet

802.3 Frame Format

Formats are compatible (Max length: 1500B – 802.3)

Protocols are assigned values > 1500

6 octets 6 octets 2 octets

DestinationAddress

SourceAddress

ProtocolType

Data

6 octets 6 octets 2 octets

DestinationAddress

SourceAddress

ProtocolType

Data

DSAP SSAPlength

2 octets

CTL