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LANs (addresses and standardization)

LANs (addresses and standardization). Digital Switching By Kashif Hesham Khan. 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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LANs (addresses and standardization)

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  1. LANs (addresses and standardization) Digital Switching By Kashif Hesham Khan Digital Switching

  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) Digital Switching

  3. Local Area Networks • 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 Digital Switching

  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 Digital Switching

  5. IEEE 802 Subcommittees • 802.11 – Wireless LAN • 802.15 – WPAN (PHY and MAC layer) • 802.16 – fixed broadband wireless access systems • 802.17 – resilient packet ring • 802.20 – air interface for mobile broadband wireless access systems • 802.21 – media independent handover for MAN and LAN Digital Switching

  6. IETF - RFCs • STD 64 (RFC3550) RTP: A Transport Protocol for Real-Time Applications • STD 56 (RFC2453) RIP Version 2 • STD 54 (RFC2328) OSPF Version 2 J. • STD 53 (RFC1939) Post Office Protocol - Version 3 • STD 51 (RFC1661) Point-to-Point Protocol (PPP) • STD 44 (RFC0891) DCN Local-Network Protocols • STD 38 (RFC0903) A Reverse Address Resolution Protocol • STD 37 (RFC0826) Ethernet Address Resolution Protocol • STD 26 (RFC0868) Time Protocol • STD 23 (RFC0865) Quote of the Day Protocol • STD 22 (RFC0864) Character Generator Protocol • STD 9 (RFC0959) File Transfer Protocol • STD 7 (RFC0793) Transmission Control Protocol • STD 6 (RFC0768) User Datagram Protocol • STD 5 (RFC0792) Internet Control Message Protocol • STD 5 (RFC0791) Internet Protocol Digital Switching

  7. 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) Digital Switching

  8. 48 bit LAN Addresses • Globally unique • Assigned by IEEE • Cost is $1650 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 Digital Switching

  9. 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 Digital Switching

  10. 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) Digital Switching

  11. 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 Digital Switching

  12. 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 • Looking for appropriate neighbors • 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 Digital Switching

  13. 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 Digital Switching

  14. 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 Digital Switching

  15. 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 Digital Switching

  16. 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 CTL Destination Address Source Address Protocol Type Data 10111101 G/I (group/individual) G/L (global/local) Digital Switching

  17. 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 Digital Switching

  18. 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! Digital Switching

  19. 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!) Digital Switching

  20. 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 Digital Switching

  21. 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 Digital Switching

  22. Frame Formats • Ethernet (Ethernet II) • 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 CTL Destination Address Source Address Protocol Type Data Digital Switching

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