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LAN Addressing

LAN Addressing. Network Protocols and Standards Autumn 2004-2005. 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

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LAN Addressing

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  1. LAN Addressing Network Protocols and Standards Autumn 2004-2005 CS573: Network Protocols and Standards

  2. 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) CS573: Network Protocols and Standards

  3. 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 CS573: Network Protocols and Standards

  4. 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 LLC Type 1, 2, … Data Link MAC PHY CS573: Network Protocols and Standards

  5. 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) CS573: Network Protocols and Standards

  6. 48 bit LAN Addresses • Globally unique • Assigned by IEEE • Cost is $1250 for a “block” of addresses • A “block” includes 224 addresses 2nd octet 3rd octet 4th octet 5th octet 6th octet 1st octet Vendor code (OUI) Vendor-assigned values CS573: Network Protocols and Standards

  7. 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 CS573: Network Protocols and Standards

  8. 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) CS573: Network Protocols and Standards

  9. 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 CS573: Network Protocols and Standards

  10. 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 CS573: Network Protocols and Standards

  11. 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 XNS MAC Layer CS573: Network Protocols and Standards

  12. 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 2 octets variable 6 octets Destination Address Source Address Protocol Type Data CS573: Network Protocols and Standards

  13. 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 CS573: Network Protocols and Standards

  14. SAP Multiplexing • All 1’s  ALL SAPs • All 0’s (except G/L)  data link layer itself • 6-bit globally assigned SAP numbers (by IEEE) 6 octets 2 octets 2 octets variable 6 octets length DSAP SSAP Destination Address Source Address Protocol Type Data 10111101 G/I (group/individual) G/L (global/local) CS573: Network Protocols and Standards

  15. 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??? • G/I bit in LAN addresses was used to make hardware filtering convenient • Hardware filtering is meaningless in SAP multiplexing • Only 64 unique SAP protocols are supported • Strict rules for assigning a SAP number • Protocol must be designed by standard bodies CS573: Network Protocols and Standards

  16. SAP Multiplexing • Local SAP protocols can be used • Network/Protocol manager’s responsibility to ensure unique SAPs to protocols • Conversation startup is difficult CS573: Network Protocols and Standards

  17. 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 CS573: Network Protocols and Standards

  18. Addresses and Protocol Types • By using 5 octets to indicate protocol type, LAN address administration is tied to protocol type administration 2nd octet 3rd octet 4th octet 5th octet 6th octet 1st octet LAN Addresses Vendor code (IEEE-assigned) Vendor-assigned values Protocol Type 2nd octet 3rd octet 4th octet 5th octet 1st octet CS573: Network Protocols and Standards

  19. 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 CS573: Network Protocols and Standards

  20. Frame Formats • Ethernet • 802.3 Frame Format • Formats are compatible (Max length: 1536) • Protocols are assigned values > 0600 hex (=1536) 6 octets 2 octets 6 octets Destination Address Source Address Protocol Type Data 6 octets 2 octets 2 octets 6 octets length DSAP SSAP Destination Address Source Address Protocol Type Data CS573: Network Protocols and Standards

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