Oct 21, 2014CS524: Advanced Computer Networks1 LAN Addressing Advanced Computer Networks

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)

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

4 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

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)

6 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

7 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

8 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)

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 2 22 unique OUIs

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 One station will be interested in one unicast address and multiple multicast addresses Unicast address is hardwired Multicast addresses fall into hardwired hash buckets

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! IPIPXARP MAC Layer XNS

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 6 octets 2 octetsvariable Destination Address Source Address Protocol Type Data

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

14 SAP Multiplexing 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) 6 octets 2 octetsvariable Destination Address Source Address Protocol Type Data G/I (group/individual) G/L (global/local) DSAP SSAPlength 2 octets CTL

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??? Only 64 unique SAP protocols are supported Strict rules for assigning a SAP number Protocol must be designed by standard bodies

16 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!

17 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!)

18 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

19 Frame Formats Ethernet Frame Format Formats are compatible (Max length: 1500B – 802.3) Protocols are assigned values > 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