Advanced Computer Networks
Oct 21, 2014 CS524: Advanced Computer Networks 1

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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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First part of the course
IEEE 802 Committee
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LAN Standardization
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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)
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LLC (allows sharing data link resources)
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Several LANs were standardized
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802.1 --- common issues
802.2 --- LLC
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Does not deal with PHY and MAC
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Data Link
802.3 --- CSMA/CD
802.4 --- Token Bus
802.5 --- Token Ring
Type 1, 2, …
LLC
MAC
PHY
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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?
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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
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48 bit (unique globally --- plug and play)
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Globally unique
Assigned by IEEE
Cost is $1250 for a “block” of addresses
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A “block” includes 2 24 addresses
1st octet 2nd octet 3rd octet 4th octet 5th octet 6th octet
Vendor code (OUI) Vendor-assigned values
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OUI = Organizationally unique identifier
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Fixed value assigned by IEEE
2 24 different possibilities
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Not all of them are used!!!
Vendor-assigned Values
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A total of 2 24 unique addresses are available by purchasing one block
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A block may be shared
A vendor can buy more blocks with different OUIs
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In fact, One block  2 25 addresses
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2 24 of the addresses are unicast
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2 24 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
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G/I = 1 means a (LAN) multicast address
10111101
G/L (global/local)
G/I (group/individual) --- first bit on the wire
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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 2 22 unique OUIs
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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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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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Original Ethernet design
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2 octet long field included in LAN header
6 octets 6 octets 2 octets variable
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Destination
Address
Source
Address
Protocol
Type
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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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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6 octets 6 octets 2 octets 2 octets variable
Destination
Address
Source
Address length DSAP SSAP CTL
Protocol
Type
Data
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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/L (global/local)
G/I (group/individual)
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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
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Protocol must be designed by standard bodies
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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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Subnetwork Access Protocol
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Single globally assigned SAP value
AA hex (10101010) --- SNAP SAP
When DSAP = SSAP = SNAP SAP
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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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802.1 defines a canonical format for LAN addresses
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00-60-1D-23-20-A9
802.3 and 802.4
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LSB is transmitted first
802.5 and FDDI
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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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Ethernet
6 octets 6 octets 2 octets
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Destination
Address
Source
Address
Protocol
Type
802.3 Frame Format
6 octets 6 octets 2 octets 2 octets length DSAP SSAP
Data
CTL
Destination
Address
Source
Address
Protocol
Type
Data
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Formats are compatible (Max length: 1500B – 802.3)
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Protocols are assigned values > 1500
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