The Data Link Layer Johan Lukkien Computer Networks 2002/2003 The Data Link Layer Johan Lukkien 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

The OSI model 13-Jul-19 PSDU SSDU TSDU NSDU DSDU PhSDU Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Data Link Layer services Provide packet oriented service (“frames”) send/receive frames between machines deal with transmission errors support addressing, in the broadcast domain support sharing by several network layer protocols Services provided by physical layer put bits (signals) on (p2p or broadcast) wires diverse technologies Note: quality of service differs per technology e.g. reliability provided service itself as well e.g. connection orientation 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Data Link Layer issues Address LAN and point-2-point issues define DPDU and DSDU More or less “stack-independent, see picture Part of the link layer is technology dependent: the medium access (MAC) sublayer structure transmissions into frames (DPDU) policy for shared medium access e.g. dealing with collisions error detection flow control addressing 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Some stacks and the OSI model 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Data Link Layer issues (cnt’d) The other part (Logical Link Control, LLC) concerns the connection to multiple network layers additional functionality to improve quality of provided services reliability, connection orientation Note: these quality issues re- occur in network and transport layer e.g., flow control, error checking Rule of thumb: don’t make a low level service unnecessarily complicated e.g. is reliability useful at this level? When? Why? 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Framing methods Byte count Flags include length in header not used very much: difficult to re-sync Flags dedicated start and stop byte (sequence) often chosen to be identical need to be escaped when it occurs as part of payload byte stuffing: add an escape byte in front used the escape to escape itself as well generalize to bit sequence, HDLC: special pattern for start/stop, e.g. 01111110 after each occurrence of 5 1’s a 0 is stuffed Use special, unused bit patterns in physical layer 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Byte count A stream encoded with byte count without errors with errors Note: difficult to resynchronize 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Stuffing 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Flow control policies Feed-back based Rate-based communicate receiver state to sender so sender can take decision explicit start/stop commands ask for another fixed maximum amount of data more smoothly Rate-based internal mechanism to adjust the rate Data-Link: mainly feed-back based methods 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Example data link protocols Point-to-point (tradionally, reliable service) High level data link control protocol (HDLC) ISO 3309-1979 Point-to-point protocol RFC 1661 (1662,1663) The IEEE 802 LANs 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

High-Level Data Link Control Protocol Background master and tributaries sharing a multi-access link controller for several terminals “concentrator” Developed by IBM: SDLC adapted by ISO: HDLC adapted by CCITT to support X.25: LAP, LAPB ....collection of bit-oriented link-layer protocols 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

High-Level Data Link Control Protocol Frame format Point to many-point and vice versa use flags for separation of frames Control: sequence numbers, acks, .... Data: payload, any length Checksum: CRC Address: the involved tributary 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Frame types Three types, encoded in Control P/F: poll/final Information frame. A supervisory frame info concerning the communication status An unnumbered frame no numbering P/F: poll/final Next: expected frame Seq: serial number (sliding window) 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

PPP: Point to Point Protocol The Internet choice: derived from HDLC 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

PPP (cnt’d) Byte oriented – byte stuffing Derived from HDLC Protocol so it works on both bit- and byte oriented ‘wires’ Derived from HDLC address = 11111111 (broadcast...) mostly, control = 00000011 (unnumbered frame, no reliability) but may use HDLC mechanism Protocol type of packet in payload e.g. LCP, NCP, AUTHENTICATE, IP, IPX, AppleTalk, .... 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

PPP (cnt’d) Link Control Protocol: Authenticate setup line and bring it down evaluate line performance negotiate communication options in dependent of network protocol, e.g. header compression Authenticate optional Network Control Protocol negotiate details of supported network layer e.g. IP address more than one may be supported 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

PPP: LCP frame types 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Dial-up scenario Physical connection Line negotiation Authenticate PC user initiates connection to ISP router modems synchronize and negotiate Line negotiation LCP packets are sent via PPP frames to router finalize with this phase with connection-ack Authenticate using an authentication protocol, e.g. challenge/response; passwd Network IP-NCP packets are sent to negotiate IP as network protocol dynamically assigned IP address is returned from router PC operates as a part of the Internet User is done with connection NCP tears down the network connection (freeing the assigned IP address for re-use) LCP tears down the data link connection Modem is told to hangup 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Local area networks: IEEE 8-2 Communication based on broadcasting multi-access links or just packet-level broadcasting LAN technologies define both the physical and the MAC layer Particular issue addressing including multi-cast 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Multiple Access Protocols ALOHA Carrier Sense Multiple Access Protocols Collision-Free Protocols Limited-Contention Protocols Wavelength Division Multiple Access Protocols Wireless LAN Protocols 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Name, address, location, route.... Context a source sends a message to a destination; both connect to the network via an access point at a certain location (“identifier”) Access point a fixed physical location to connect to the network Name an identification that is independent of access point and communicating partner Address the name of an access point Route a path from source to destination dependent on the access points of both 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

IEEE 802 MAC addresses In fact: identifiers Attached to a Network Interface Card 48 bit numbers, with some structure globally unique – global authority (first Xerox, then IEEE), $1250/224 3 bytes fixed: Organizational Unique Identifier G/L bit: global unique-ness G/I bit: multicast? all 1’s: broadcast – not really used 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Logical Link Control (802.2) Standardizes towards network layer Adds quality to the service type 1 datagram service: best effort packet delivery type 2 reliable connection oriented service HDLC-like type ... achieve some reliability without the complete overhead of type 2 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

802.2 Format DSAP: destination access point (1byte) SSAP: source access point (1 byte) Control: 1 or 2 bytes (Data, i.e. the packet) Access points: protocols used at both sides usually identical two bits reserved group/individual (not really useful to address multiple protocols though....) global/local only 802-approved numbers are admitted and those have both SAP fields equal 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Using your own SAP Use locally assigned SAP’s Use header extension but that’s difficult to synchronize dynamically Use header extension both SAP’s: 10101010 (aa) a.k.a. SNAP, subnetwork access protocol indicates a header of 5 extra bytes to make headers have an even length to use the OUI of MAC addresses to standardize buying a block of addresses also brings a block of protocols.... 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking