Dec 14, 2006CS573: Network Protocols and Standards1 LAN Addressing Network Protocols and Standards Autumn

Dec 14, 2006CS573: Network Protocols and Standards2 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)

Dec 14, 2006CS573: Network Protocols and Standards3 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

Dec 14, 2006CS573: Network Protocols and Standards4 IEEE 802 Subcommittees common issues LLC Does not deal with PHY and MAC CSMA/CD Token Bus Token Ring Type 1, 2, … LLC MAC PHY Data Link

Dec 14, 2006CS573: Network Protocols and Standards5 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)

Dec 14, 2006CS573: Network Protocols and Standards6 48 bit LAN Addresses Globally unique Assigned by IEEE Cost is $1250 for a “block” of addresses A “block” includes 2 24 addresses 1st octet 2nd octet3rd octet4th octet5th octet6th octet Vendor code (OUI)Vendor-assigned values

Dec 14, 2006CS573: Network Protocols and Standards7 48 bit LAN Addresses OUI = Organizationally unique identifier Fixed value assigned by IEEE 2 24 different possibilities Not all of them are used!!! Vendor-assigned Values A total of 2 24 unique addresses are available by purchasing one block A block may be shared A vendor can buy more blocks with different OUIs

Dec 14, 2006CS573: Network Protocols and Standards8 Group/Individual bit in OUI In fact, One block  2 25 addresses 2 24 of the addresses are unicast 2 24 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 G/I (group/individual) --- first bit on the wire G/L (global/local)

Dec 14, 2006CS573: Network Protocols and Standards9 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 2 22 unique OUIs

Dec 14, 2006CS573: Network Protocols and Standards10 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

Dec 14, 2006CS573: Network Protocols and Standards11 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! IPIPXARP MAC Layer XNS

Dec 14, 2006CS573: Network Protocols and Standards12 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 6 octets 2 octetsvariable Destination Address Source Address Protocol Type Data

Dec 14, 2006CS573: Network Protocols and Standards13 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

Dec 14, 2006CS573: Network Protocols and Standards14 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 octetsvariable Destination Address Source Address Protocol Type Data G/I (group/individual) G/L (global/local) DSAP SSAPlength 2 octets CTL

Dec 14, 2006CS573: Network Protocols and Standards15 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

Dec 14, 2006CS573: Network Protocols and Standards16 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!

Dec 14, 2006CS573: Network Protocols and Standards17 SNAP SAP Subnetwork Access Protocol Single globally assigned SAP value AA hex ( ) --- 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!)

Dec 14, 2006CS573: Network Protocols and Standards18 Addresses and Protocol Types By using 5 octets to indicate protocol type, LAN address administration is tied to protocol type administration 1st octet 2nd octet3rd octet4th octet5th octet6th octet Vendor code (IEEE-assigned)Vendor-assigned values 1st octet 2nd octet3rd octet4th octet5th octet LAN Addresses Protocol Type

Dec 14, 2006CS573: Network Protocols and Standards19 Transmission Bit Order defines a canonical format for LAN addresses D A and LSB is transmitted first and FDDI MSB is transmitted first Internetworking different topologies Bit order should be shuffled if forwarding frames between incompatible LAN topologies

Dec 14, 2006CS573: Network Protocols and Standards20 Frame Formats Ethernet Frame Format Formats are compatible (Max length: 1536) Protocols are assigned values > 0600 hex (=1536) 6 octets 2 octets Destination Address Source Address Protocol Type Data 6 octets 2 octets Destination Address Source Address Protocol Type Data DSAP SSAPlength 2 octets CTL