Transcript Addressing - Punjab University College of Information

LAN Addressing
Advanced Computer Networks
Oct 21, 2014
CS524: Advanced Computer Networks
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An Internet Connection
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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
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Connection is between “peers”
Service Models (Fig. 1.3 of Perlman)
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PDUs (between peers) and SDUs(from up layers)
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Local Area Networks
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First part of the course
IEEE 802 Committee
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LAN Standardization
Physical and Data Link Layers of OSI Model
Data Link layer subdivided by them:
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MAC (Dependent on the type of LAN)
LLC (allows sharing data link resources)
Several LANs were standardized
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IEEE 802 Subcommittees
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802.1 --- common issues
802.2 --- LLC
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Does not deal with PHY and MAC
Data Link
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802.3 --- CSMA/CD
802.4 --- Token Bus
802.5 --- Token Ring
Type 1, 2, …
LLC
MAC
PHY
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LAN Addresses
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Most LANs are “broadcast” type
LAN addresses solve two problems on shared
(or broadcast) LANs
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Who is the sender?
Who is the receiver?
IEEE 802 standardized the address length
Two different lengths were chosen
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16 bit (unique on the network) --- obsolete
48 bit (unique globally --- plug and play)
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48 bit LAN Addresses
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Globally unique
Assigned by IEEE
Cost is $1250 for a “block” of addresses
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A “block” includes 224 addresses
1st octet
2nd octet
3rd octet
Vendor code (OUI)
4th octet
5th octet
6th octet
Vendor-assigned values
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48 bit LAN Addresses
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OUI = Organizationally unique identifier
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Fixed value assigned by IEEE
224 different possibilities
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Not all of them are used!!!
Vendor-assigned Values
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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
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In fact, One block  225 addresses
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224 of the addresses are unicast
224 of the addresses are multicast
G/I bit decides if the address is multicast
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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
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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?
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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
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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
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Unicast address is hardwired
Multicast addresses fall into hardwired hash buckets
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Protocol Type Multiplexing
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One station, many higher layer protocols
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Which protocol is the desired recipient?
Which protocol constructed the packet?
IP
IPX
ARP
XNS
MAC Layer
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This information is also included in the LAN
header --- just like LAN addresses are!
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Protocol Type Multiplexing
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Original Ethernet design
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2 octet long field included in LAN header
6 octets
6 octets
Destination
Address
Source
Address
2 octets
Protocol
Type
variable
Data
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
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SAP Multiplexing
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More flexible to have separate source
and destination protocol type fields
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Can assign different numbers to the same
protocol on different machines
Service Access Points (SAPs)
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Included in 802 LAN header
SSAP and DSAP
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1 octet each but only 6 bits are used
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SAP Multiplexing
6 octets
6 octets
2 octets
length
Destination
Address
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Source
Address
2 octets
variable
DSAP SSAP CTL
Protocol
Type
Data
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)
10111101
G/I (group/individual)
G/L (global/local)
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SAP Multiplexing
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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
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Strict rules for assigning a SAP number
Protocol must be designed by standard bodies
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SAP Multiplexing
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Local SAP protocols can be used
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Network/Protocol manager’s responsibility
to ensure unique SAPs to protocols
Conversation startup is difficult
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SAP number at the destination machine is not
known at the source machine!
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SNAP SAP
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Subnetwork Access Protocol
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When DSAP = SSAP = SNAP SAP
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Single globally assigned SAP value
AA hex (10101010) --- SNAP SAP
Header is expanded to include a “protocol type”
field
A “longer” protocol type field can then be
used
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Standardized to 5 octets (see book for reason!)
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Transmission Bit Order
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802.1 defines a canonical format for LAN
addresses
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802.3 and 802.4
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LSB is transmitted first
802.5 and FDDI
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00-60-1D-23-20-A9
MSB is transmitted first
Internetworking different topologies
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Bit order should be shuffled if forwarding frames
between incompatible LAN topologies
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Frame Formats
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Ethernet
6 octets
6 octets
2 octets
Destination
Address
Source
Address
Protocol
Type
802.3 Frame Format
6 octets
6 octets
2 octets
length
Destination
Address
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Data
Source
Address
2 octets
DSAP SSAP CTL
Protocol
Type
Data
Formats are compatible (Max length: 1500B – 802.3)
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Protocols are assigned values > 1500
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