What is a LAN? Multiple systems attached to a shared medium “High” bandwidth “Low” delay “low” error rate Broadcast (multicast) capability Limited geography (several kilometers) Limited stations (hundreds) Peer (equivalent) relationships (rather than master/slave) Confined to private property Can be viewed as a cloud (like a WAN)

Sharing LAN Bandwidth Each station gets a fair share Each station gains access in reasonable time Bandwidth waste is minimized

Popular Arbitration Mechanisms Token schemes –Token ring –Token bus Contention –If two stations transmit at same time – collision –Probabilistically fair –Starvation is possible –Worrisome, but we rely on probability every day

IEEE 802 LANs 802 is a IEEE committee whose purpose is to standardize several LANs LAN protocols reside at the bottom two layers of the OSI Reference Model

Data Link Layer 802 devides the data link layer into two sublayers –Medium Access Control (MAC) addresses issues specific to a particular type of LAN, i.e., token passing and collision detection, priorities, error detection, and framing Logical Link Control (LLC) –LLC type 1 datagram protocol (packet is delivered with “best-effort” service –LLC type 2 reliable connection-oriented data link protocol High-Level Data Link Control (HDLC)

Other Types of LLC – Issues common across all 802 LANs – Defines LLC, MAC and physical layers – Deals with Carrier Sense Multiple Access with Collision Detection (CSMA/CD) LAN used with Ethernet – Deals with Token Bus LAN – Deals with Token Ring LAN

Names, Addresses, Routes Name: what something is (identifier (ID)) –Location-independent –Unchanged even if the destination moves Address: where it is –Valid regardless of the location –If a destination moves, it is assigned a new address Route: how to get there –Dependent on the location of both the source and destination

LAN Addresses Every station on LAN hears every packet, so it is necessary to include a destination field in each packet Source field is also included LAN adaptors can filter out packets not addressed to a particular station 48-bit addresses (6 octets) enable stations to be provided with a globally unique identifier at the time of manufacture Global authority is responsible for handing out blocks of addresses Global authority used to be XEROX, now it’s IEEE

Octets First three octets form vendor code or Organizationally Unique Identifier (OUI) The vendor allocates the remaining three octets

Fixed-Value Octets Group/Individual –If 0, address refers to a particular station –If 1, address refers to a logical group of stations (multicast) Global/Local –If 0 (local), vendors purchase a block of addresses from the global authority –If 1 (global), vendors are free to use any address

IEEE LAN Address 1 st octet2 nd octet3 rd octet4 th octet5 th octet6 th octet Global/Local (G/L) Bit Group/Individual (G/I) Bit

Multicast Addresses Most common use of multicast addresses is for discovering appropriate neighbors by: –Solicitation: Clients multicast message to group address (i.e., Zservers) in order to find a server –Advertisement: Z servers periodically transmit packets to Zclients address. When client receives packet, it now knows address of Z servers

Multicast vs Unicast Each all Z servers must listen for multicast address as well as their own If any unicast address can multicast, each Z server must listen for all packets – promiscuous listening Normally, promiscuity is not desirable If the packet is of interest, the hardware should pass it up to the software, if not, the hardware should filter it

If Software Receives Addresses Chip designer would have to specify max addresses to receive If max is too large it would be expensive If max is too small, chip would be useless for station that required more addresses than provided by the chip

Compromise Solution Station informs chip of the single individual address it wants to receive A bit in the address designates address as individual or group Chip partitions set of group addresses into a number of groups (hash buckets) Station wishing to receive a particular group address requests bucket into which that address hashes After bucket is selected, chip will send all packets whose destination addresses hash to that bucket

Theory A station will only be interested in its own address and some small number of group addresses

Alternative Chip Design Software can receive some fixed number of exact addresses (group or individual) Station can request any subset of some number of hash buckets for addresses –With this design, the chip gets perfect filtering (it does not get software interrupts for addresses it isn’t interested in –If it needs more addresses, it can fall back on the hashing scheme

Broadcast Address One particular group address is named broadcast address Broadcast address consists of all 1’s All stations should receive any packet transmitted to the broadcast address – but really it is only all stations that have implemented this particular protocol

Protocol Type Multiplexing 662 destinationsourceprotocol typedata

802 Standard Separate fields for source and destination These fields are known as Service Access Points (SAPs) 802 header contains Source Service Access Point (SSAP) and Destination Service Access point (DSAP) SAP gives flexibility in assigning numbers to protocols differently in each machine SAP fields are 8 bits (1 octet) long

SAP Multiplexing 6611data destinationsourceDSAPSSAPdata

Problems with SAP With global, SSAP and DSAP are equal (the same as the SAP value assigned that protocol) Protocols with locally assigned SAP can have unique numbers, but it is hard to send a protocol message to another machine when the SAP numbering of a foreign machine is unknown

Structure of SAP Global – 802 committee has assigned the number Local – Owner of the network or system manages the number Group/Individual – Allows you to send a packet that can be received by multiple higher-layer protocols, which provides little real benefit Six bits does not allow the 802 committee to grant numbers to every organization that wants to design its own protocol Global/Local (G/L) Bit Group/Individual (G/I) Bit

SNAP assigned by IEEE vendor assigned address block assigned by IEEE vendorassigned protocol type

SNAP Addressing When an organization purchases a block of addresses it receives a 3-octet quantity such as XYZ and addresses have the form XYZ*** Organizations can assign station addresses of XYZ*** They can assign multicast addresses X’YZ*** where X’ is X with G/I set to group It can assign protocol types having the form XYZ**

Address Representation A serious problem with addresses is how they are represented and transmitted committee defined a canonical format for writing addresses –6 octets separated by hyphens –Each octet is two hex digits –Example: a

Bit ordering and – Least significant bit is transmitted first and FDDI – Most significant bit is transmitted first

Transmission Problems Group/Individual bit is not defined as least or most significant, but rather the first bit on the wire Thus, group address on would not necessarily be group on because adifferent bit would be transmitted

Canonical Format Assumes least-significant-bit-first order Therefore a is not a group address because least significant bit of a2 ( ) is 0

Canonical Least-Significant Bit First Address – a st octet2 nd octet3 rd octet4 th octet5 th octet6 th octet

Most-Significant Bit First 1 st octet2 nd octet3 rd octet4 th octet5 th octet6 th octet Address – a

Address Shuffling Bridges must shuffle address fields when forwarding packets between (or FDDI) and any other LANs If LAN addresses in higher-layer protocol messages such as Address Resolution Protocol (ARP) are not converted to canonical format before placing it in a protocol message, the destination cannot interpret the field that contains the LAN address without knowing the type of LAN the message originated from This results in confusion and interoperability problems

More Shuffling Problems At least one protocol reported failure if the bits at the data link layer did not match the bits in the higher-layer header If a bridge did the appropriate bit shuffling, when forwarding between and 802.5, the protocol failed Vendors were forced to specifically check for this protocol type and not shuffle Bridged packets will have different addresses than they should –Station address could appear to be multicase –Station address with flipped order could be address of another station

Logical Link Control (LLC) LLC is described in specs as if it were a sublayer separate from the MAC sublayer If people had agreed on datagram model, 802 committee might not have felt a need to divide data link layer into MAC and LLC Only data link layer fields DSAP, SSAP, and CTl are within LLC Source and destination are considered part of MAC This means LANs can define addresses as they choose even though they end up pretty much the same

LLC Type 1 CTL Field Type 1 implies datagrams CTL field is always 1 byte long and has one of following values: –UI (unnumbered information) – datagram –XID (exchange identification) command – informs recipient of identity of transmitter of XID command message and LLC types the transmitter supports response – required reply to XID command message –TEST command – sends a test message response – responds to test message XID and TEST are distinguished by group/individual bit in SSAP field – no reason to send message from SAP group

LLC Type 2 CTL Field Type 2 implies connection-oriented CTL field is 1 or 2 bytes long depending on what type of packet it is Packet types with CTL of 2 bytes contain a sequence number

2-byte LCL Type 2 CTL Field I (Information) – is a data packet with 7 bit sequence number RR (Receive Ready) – is an acknowledgement with sequence number. Indicates all packets with lower sequence numbers have been received RNR (Receive Not Ready) – Like RR but also indicates receiver is temporarily busy. No further packets should transmitted until RR is received REJ (Reject) – Receiver requests retransmission of packets starting with number in sequence field

1-Byte LCL Type 2 CTL Field SABME (Set Asynchronous Balanced Mode Extended) – Start connection DISC (Disconnect) – Terminate connection DM (Disconnected Mode) – Transmitted in response to DISC indicating DISC has been received FRMR (Frame Reject) – Indicates receipt of invalid packet such as out of order sequence number UA (Unnumbered Acknowledgement) – Acknowledges a DISC or SABME message

CSMA/CD CS (Carrier Sense) –A station wishing to transmit listens first –If another station is transmitting, first station waits until medium is idle MA (Multiple Access) Many stations share the same medium CD (Collision Detect) – Stations monitor the medium even while they are transmitting –Detect occurrence of a collision (another station transmitting)

Collision Detection Time t 0 S1 starts transmission Just before Time t 0 + X Bits have not quite reached S2. S2 begins transmission Time t 0 + X S2 detects that collision has occurred Time t 0 + 3/2 X Half of the stations detect a collision has occurred Time t X S1 detects that collision has occurred S2 S1

Ethernet Original CSMA/CD LAN was designed at Xerox and was called Ethernet To guarantee collision detection, length of wire had to be limited to minimum size determined by packet lengths If station 2 initiates transmission just before bits from station 1 arrive, station 1 will not detect collision until twice the length of time it takes for signal to go from one end of wire to the other (slot time)

Slot time If 512 bits can be transmitted before a collision can be detected, then packets should be at least 512 bits long If station finishes transmitting a packet and collision is not detected, it will assume no collision occurred Protocols wishing to send shorter packets must pad them to the required minimum length

802.3 Modifies Ethernet Format – 1,5004 destinationsourceprotocoldataFCS ethernet – 1,4974 destinationsourcelengthDSAPSSAPCTLdataFCS 802.3

Distinguishing Ethernet from Luckily formats can be distinguished by 2-byte protocol and length fields Max packet length in is 1,500 bytes Xerox made no protocol types smaller than 1,500 2-byte field after source < 1500 implies byte field after source > 1500 implies Ethernet

Another Difference Another difference between and Ethernet is protocol type and SAP fields. Currently (except for XID and TEST), SAP uses either: an 802 approved protocol in which DSAP = SSAP and they equal the globally assigned SAP value (not the SNAP value) or DSAP = SSAP and they equal the globally assigned SNAP SAP value. Following the CTL field is a 5-byte protocol type field

Technology Limitations Limited number of stations – For example each station attached to a ring increases delay Limited size – In 802.3, the cable must be sufficiently short so that transmitter will detect a collision during transmission Limited amount of traffic – Available bandwidth must be shared among stations. The more stations, the smaller the share of bandwidth

Reason for Bridges Single LAN is often insufficient to meet requirements of organization If all packets are forwarded to LANs by routers, all stations must implement network layer protocol However, if stations are designed assuming entire world is a single LAN, they will not have implemented layer 3 and thus cannot use the router For stations without layer 3, the bridge allows LANs to be glued together Packets can be forwarded from one LAN to another without any cooperation from the stations Bridges can also support any layer 3 protocol

Bridge Advantages Bridges are popular because they: Are simple Attain high performance Allow IP nodes to move within the bridged topology and maintain their layer 3 (IP) addresses