Post on 01-Jan-2016
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