Chapter 2 – Data Link Layer Issues Computer Networks Chapter 2 – Data Link Layer Issues CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman LAN Properties Shared medium High data rate Low delay Low error rate Native broadcast support Limited physical extent (a few kms) Limited number of stations (100’s) STAs are peers Local management (not under PTT regulation) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Medium Access Access allocation so that One STA at a time accesses medium Each STA gets a fair share Delays are reasonable Overhead and waste are minimized Approaches Tokens Contention CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

Token-based Approaches Token Ring STAs linked (simplex) to two neighbors Token circulates physical ring Add STA by insertion into ring Token Bus STAs attached to bus Token circulates logical ring Add STA to bus and insert into logical ring CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

Contention Approaches ALOHA Can’t sense medium, so just talk! CSMA Listen before talk CSMA/CD Add collision detection (need sensitive PCS) CSMA/CA Use collision avoidance (when VCS used) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman IEEE 802 802.1 – common issues Addressing, management, bridging, security 802.2 – LLC Type 1 – best effort / Type 2 - reliable 802.3 – CSMA/CD LAN From Xerox Ethernet 802.4 – Token Bus 802.5 – Token Ring 802.11 – Wireless LAN 802.16 – Wimax Note – FDDI standardized by ANSI CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

Names, Addresses, Routes Name – what Location-independent identifier May be human-friendly or not Address – where Independent of source location, but will change if destination moves Route – how to get there Depends on both source and destination CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman LAN Addresses LANs are broadcast medium – need both Source address (for return messages) Destination address (to filter) IEEE 802 addresses 16 and 48 bits (also 60 for 802.6 DQDB) 48 bit addresses managed by IEEE Pay to get 224 address block, Vendor Code (OUI) G/L bit is 0 if global, 1 if locally managed OUI 2nd Octet 3rd Octet 4th Octet 5th Octet 6th Octet G/L bit (global/local) G/I bit (group/individual) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Multicast Addresses Service Discovery Solicitation (client mcast to Z-Servers address) Advertisement (Z-Servers mcast to Z-Client addr) Why Multicast (group) addresses? Reduce interrupt handling by hosts Hardware filter Why G/I bit? Allow filtering by hash buckets in HW SW filters all hits in relevant hash buckets CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Broadcast The all 1’s address (0xffffffffffff) is bcast Means that all STAs must receive Really, though, only those implementing the protocol used in the broadcast packet have to… Broadcast address interrupts everyone anyway CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Multiplexing Field Multiple higher layer protocols Format and data alone cannot distinguish Multiplexing field selects which one Protocol type in Ethernet DSAP and SSAP in IEEE 802 Service Access Point (SAP) Structure G/L and G/I bits also, hence 6 bits All 1’s = all SAPs (!!!) Others assigned by IEEE – too few! SNAP (Subnetwork Access Protocol) When DSAP = SSAP = 0xaa extra protocol type field (5 octets) 3 OUI octets, 3 vendor-assigned octets CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Bit Order Bit order is order in which bits are put onto the medium Not standard () LSB first canonical and for most LANs MSB first for 802.5, FDDI Bridges must convert Shuffle bit order within octets Impact on ARP and higher layer protocols CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman LLC 802.2 LLC has two significant types Type 1 – datagrams (best effort) Type 2 – reliable (connection oriented) Type 1 Control – 1 octet UI – unnumbered info (datagram) XID – Exchange ID (command/response) ID of transmitter LLC types supported Test – (Cmd/Rsp) – Rsp echo data in Cmd CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman LLC Type 2 Control is 1 or 2 octets, per type 2-octet control fields contain 7-bit seq #s I = Information (data) – SN plus ACK SN RR = Receive Ready (ACK) – ACK SN RNR = Rcv Not Ready (Busy) – ACK SN REJ = Reject – ACK SN CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman LLC Type 2 Control is 1 or 2 octets, per type 1-octet control field types SABME = Set Asynchronous Balanced Mode Extended (start connection) DISC = Disconnect (end connection) DM = Disconnected Mode (confirm DISC) FRMR = Frame Reject (receipt of invalid pkt) UA = Unnumbered ACK (for DISC/SABME) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 802.3 Issues CSMA/CD Listen before talk Detect collisions Binary exponential backoff Minimum frame length So all STAs detect collision Slot time = 2t (512 bits at 2.5km and 10Mbps) Start Tx End Tx Start Tx Start Rx Detect Collision End Tx End Rx CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 802.5 Issues Each packet traverses every station in physical ring Each STA has transceiver buffer with special HW to recognize token, addresses Two bits at end of each frame for ACK: A bit (address recognized) C bit (frame copied) Each STA may modify bits Sender sees A/C bits when frame returns CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 802.5 Issues A/C Bits on return: A=0: Address not recognized (give up) A=1, C=0: Address recognized, but STA busy (try again) A=1, C=1: Address recognized and frame copied (success!) What does a bridge do with these? Clear both? Leave unmodified Set A and C if bridge forwards Clear A and set C if bridge forwards A/C used for other purposes: Ring order (bcast frame with A bit clear indicates predecessor) Only 31 functional addresses for multicast Mapping Oversubscription CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Packet Bursts Station processing rate may be slower than the LAN data rate While OK on average, packets may be sent in a burst Early packets received, later ones lost Problem if naïve protocol retransmits whole burst every time CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Why Bridges? Limit number of stations in LAN Packet lengths Delay Size limitations 802.3 collision detection Traffic Capacity is shared Simple, high performance, allow limited location transparency (keep IP address) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Point-to-Point Links flag address control data checksum flag HDLC format Flag Special pattern to delimit frames In HDLC, 01111110 In DDCMP, DLE-SOF & DLE-EOF Bit-stuffing/character-stuffing for data transparency In HDLC, 011111… -> 0111110… on Transmit In DDCMP, … DLE … -> … DLE DLE … on Transmit Addressing Needed if multiple stations on medium Traditionally assume master/slave Control – Like LLC Type 2 Checksum – 16 bit CRC CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman Point-to-Point Links flag Addr=0xff Ctl=0x03 protocol data checksum flag PPP format Multiplexing Protocol field in PPP (16 bits – see RFC 1700) Service If links not reliable, need reliable transfer per hop (HDLC, DDCMP, LLC Type 1) What is probability of success for k hops with FER P? What is cost per attempt for k hops with FER P? What is overall cost for success for k hops with FER P? If links reliable, then datagram service OK (PPP, LLC Type 2) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

Link Reliability Issues CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

Link Reliability Issues CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

Link Reliability Issues CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

Link Reliability Issues CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

Per Hop vs. End-to-End BEC Pkt Sent Pkt Sent Pkt Recv Pkt Recv ACK Recv ACK Recv Per Hop End-to-End CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman