COMPUTER NETWORKS Data link layer protocols

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1 Data link layer (layer 2) Gruppo Reti TLC Copyright Quest opera è protetta dalla licenza Creative Commons NoDerivs-NonCommercial. Per vedere una copia di questa licenza, consultare oppure inviare una lettera a: Creative Commons, 559 Nathan Abbott Way, Stanford, California 9435, USA. This work is licensed under the Creative Commons NoDerivs-NonCommercial License. To view a copy of this license, visit: or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 9435, USA. COMPUTER NETWORKS Data link layer protocols - COMPUTER NETWORKS Data link layer protocols - 2 Data link layer functions Frame delineation Explicit delimiters (flag) before and after packet transmission Lenght indicator Fixed length Silence between packets Multiplexing (of higher layer protocols) Addressing Error detecion Window protocol Flow control Sequence and error control through retransmissione Multiple access protocols for shared media Data link layer protocols Most derived from the SDLC (Syncronous Data Link Control) defined in the IBM netowrks architecture Ansi standardized SDLC as ADCCP (Advanced Data Communication Control Procedure) and ISO as HDLC (High-level Data Link Control) CCITT (later ITU-T) derived LAP (Link Access Procedure) and LAPB (Link Access Procedure Balanced) In Local Area Networks (LANs) a MAC sublayer solves the multiple access problem COMPUTER NETWORKS Data link layer protocols - 3 COMPUTER NETWORKS Data link layer protocols - 4 Data link layer protocols Several protocols belong to the HDLC family LAP-D (Link Access Procedure D-Channel) LAP-F (Link Access Procedure to Frame Mode Bearer Service) LLC 82.2 (Logical Link Control) PPP (Point-to-Point Protocol) LAPDm (LAP for the mobile D channel) Data link layer protocols Starting from HDLC.. we move to LAP-B, X.25 and ISDN B channel LLC, for LANs PPP, for point to point connections Used at home LAP-F, used in Frame Relay ATM, not derived from HDLC, used in B-ISDN COMPUTER NETWORKS Data link layer protocols - 5 COMPUTER NETWORKS Data link layer protocols - 6 Pag.

2 Example of public network : Data Terminal Equipment : Data Circuit-terminating Equipment Datsa link layer protocols NETWORK Data format (many layer 2 protocols) PDU format: address control data CRC 8 8 8/6 >= 6 8 Bit oriented protocol, with bit-stuffing to ensure data transparency (flag must not appear in other fields) Address field is used in multi-point configuration (master/slave) Control field differentiates the PDUs COMPUTER NETWORKS Data link layer protocols - 7 COMPUTER NETWORKS Data link layer protocols - 8 HDLC: Master/Slave Configuration Protocol used to manage the communication among master and slaves PDUs sent from the master are named command, PDUs send from slaves are named response slave HDLC: operational modes Normal Response Mode (NRM) Suited for point-to-point of unbalanced multipoint. One primary station (Master) and several slaves The Master sets the P/F bit to to enable slave transmission The slave sets the P/F bit to in the last PDU master COMPUTER NETWORKS Data link layer protocols - 9 COMPUTER NETWORKS Data link layer protocols - HDLC: operational modes Asynchronous Response Mode (ARM) Unbalanced configuration. The slave can send data without waiting to be polled by the master. Asynchronous Balanced Mode (ABM) Balanced point-to-point configurations. The P/F bit requires immediate response. Most widely used in data link layer protocols HDLC: three types of PDUs Control field Differentiates three types of PDUs P/F = poll/final bit Information N(S) P/F N(R) Supervision S S P/F N(R) Unnumbered M M P/F M M M COMPUTER NETWORKS Data link layer protocols - COMPUTER NETWORKS Data link layer protocols - 2 Pag. 2

3 HDLC: three types of PDUs Information Data sent after connection opening Supervision ACKs (positive and negative) Unnumbered Link management Data sent in connectionless mode HDLC: two numbering schemes Normal numbering (modulo 8) and extended numbering (modulo 28) Control field equal to either o 2 byte Information P/F Supervision N(S) N(R) S S x x x x P/F N(R) Unnumbered M M P/F M M M COMPUTER NETWORKS Data link layer protocols - 3 COMPUTER NETWORKS Data link layer protocols - 4 HDLC vs LAP-B LAP-B (ISDN B channel) uses only a subset of PDUs defined by HDLC We briefly describe the PDUs used by LAP-B LAP-B: information PDU Data transfer N(S) and N(R) fields needed by the window protocol to provide error and sequence control N(S) = transmitted PDU sequence number N(R) = acknowledge number, refers to the expected PDU at the receiver N(S) P/F N(R) COMPUTER NETWORKS Data link layer protocols - 5 COMPUTER NETWORKS Data link layer protocols - 6 LAP-B: supervision PDUs (S) ACK transfer RR (Receiver Ready - C/R) Positive ACK RNR (Receiver Not Ready - C/R) Positive ACK and flow control signal sent from the receiver which is unavailable (ON-OFF flow control) REJ (Reject - C/R) Request for retransmission of all PDU starting from N(R) S S P/F N(R) LAP-B: unnumbered PDUs (U) Mainly used to control connection management 5 M bits. A limited number of PDUs is used by LAP-B among the 32 available PDUs Command PDUs: SABM(E) (Set Asynchronous Balanced Mode), used to (re)open the connection E = Extended numbering scheme DISC (Disconnect): the connection is aborted M M P/F M M M COMPUTER NETWORKS Data link layer protocols - 7 COMPUTER NETWORKS Data link layer protocols - 8 Pag. 3

4 LAP-B: unnumbered PDUs (U) Response PDUs UA (Unnumbered Acknowledgment): ACK for PDUs opening the connection or to answer to DISC commands DM (Disconnect Mode) Connection was not set up correctly FRMR (FRaMe Reject) Answer to the reception of a correct but unknown PDU 24 additional bits to explain the reason why the PDU was rejected LAPB: command and response PDUs format command response code in control field Information I (Information) N(S) P N(R) Supervision RR (Receiver Ready) RR (Receiver Ready) P/F N(R) Unnumbered RNR (Rec. Not Ready) RNR (Rec. Not Ready) P/F N(R) REJ (Reject) REJ (Reject) P/F N(R) SABM (Set Asynchr. Balanced Mode) P DISC (Disconnect) P DM (Disconnect Mode) F UA (Unnumbered Acknowledgement) F FRMR (Frame Reject) F COMPUTER NETWORKS Data link layer protocols - 9 COMPUTER NETWORKS Data link layer protocols - 2 LAPB: Poll/Final Bit In command PDUs the P/F bit is used to poll stations (i.e. to require an answer) when set to In response PDUs the P/F bit is used to answer (final) to command PDUs with a P/F bit set to Poll bit set to by (or ) is a request to answer immediately for (or ). Final bit set to specifies the answer to the poll request and cannot send a command PDU with the P bit set to unless a response PDU with the F bit set to was received LAPB addresses has address (3) has address () 3 is the value of the address field in command PDUs and in response PDUs is the value of the address field in command PDUs and in response PDUs The address field permits to distinguish command PDUs from response PDUs and to understand whether the P/F bit is a poll or a final COMPUTER NETWORKS Data link layer protocols - 2 COMPUTER NETWORKS Data link layer protocols - 22 Layered architecture: X.25 and ISDN B channel LAPB: example of data transfer Q V Q V L3 L2 L X.25 packet layer LAP-B User terminal X.25 packet layer LAP-B LAP-B Switching node Error control X.25 packet layer LAP-B User terminal V,SABM,P V,I,P V,I V,I2 Q,RR2,F V,I32 V,I42 V,I52 V,I62 V,I72,P modo non connesso V,UA,F Q,I Q,I,P V,RR2,F V,RR3 V,RR4 Q,I25 Q,I36 V,I3 V,I4 V,I24 V,RR5,P Q,RNR6,F V,RNR6,P V,DISC,P modo non connesso Q,I47 V,RR2,F Q,I52,P V,RR3,F V,RR3,F V,UA,F COMPUTER NETWORKS Data link layer protocols - 23 COMPUTER NETWORKS Data link layer protocols - 24 Pag. 4

5 timeout timeout COMPUTER NETWORKS Data link layer protocols LAPB: error recovery via timeout LAPB: error recovery via REJ Q V Q V V,I V,I V,I2 V,I3 V,I4 V,I5 V,I6 V,RR,P V,I7 trasferim. dati V,RR7 V,RR7,F V,I V,I V,I2 V,I32 V,I23 V,I34 V,I45 trasferim. dati Q,I Q,I Q,I2 V,REJ2 Q,I32 Q,I42 Q,I53 COMPUTER NETWORKS Data link layer protocols - 25 COMPUTER NETWORKS Data link layer protocols - 26 LAPB: error recovery via P/F bit V,I,P V,I V,I22 V,I33,P V,I45 V,I5,P V,I26 Q trasferim. dati V Q,I Q,I V,I2,F Q,I3 Q,I4 V,RR,F Q,I5 V,RR2,F LAPB: error recovery via time-out and REJ V,I V,I V,I2 V,I3 V,I4 V,I5 V,I6 V,I V,I Q trasferim. dati V V,RR7 V,REJ7 V,I7 COMPUTER NETWORKS Data link layer protocols - 27 COMPUTER NETWORKS Data link layer protocols - 28 Data link layer in LANs IEEE 82.3 data link layer in LANs has two sub layers LLC: Logical Link Control MAC: Medium Access Control 3 2 network LLC MAC physical IEEE 82.2 Logical Link Control Derived from HDLC Standard ISO 882/2. Main characteristics Byte oriented protocol Does not use flag for packet delineation (done by the MAC) No erroro control (done by the MAC) PDU have source and destination addresses used for higher layer protocol multiplexing Variable packet lenghth COMPUTER NETWORKS Data link layer protocols - 29 COMPUTER NETWORKS Data link layer protocols - 3 Pag. 5

6 LLC PDU One unused bit in the source address is exploited to distinguish between command and response. Control field 8 bit in unnumbered PDUs 6 bit otherwise Maximun packet size depends on the MAC LLC PDU LLC addresses identify the higher layer protocols transported bt the LLC protocol Address field has two bit, unused for addressing, reserved for control purposes All s address identifies the LLC itself Up to 63 addresses available DSAP SSAP control information address address 8 bit 8 bit 8 o 6 bit 8 x M bit COMPUTER NETWORKS Data link layer protocols - 3 COMPUTER NETWORKS Data link layer protocols - 32 SNAP PDU Extended addressing scheme, named SNAP (SubNetwork Access Protocol) Used for connectionless transfer Identifies IP protocol among others AAH AAH 3H H 8H (3)-PDU DSAP SSAP controllo OUI protocol type (3) (2) informazione Point to Point Protocol (PPP) RFC 66 PPP (Point to Point Protocol) is mainly used to connect customers to Internet ISP over telephone or ADSL lines Also available over ISDN connections and even over SONET/SDH Point-to-point connection is easier to manage with respect to braodcast or radio channel PPP is simple (likely the simplest data link layer protocol) COMPUTER NETWORKS Data link layer protocols - 33 COMPUTER NETWORKS Data link layer protocols - 34 PPP functionalities PDU delineation Content transparency Error detection (but no correction) Higher layer protocols multiplexing Monitoring of link activity Negotiation of layer three (IP) addresses Useful for ISPs Functions not available in PPP Error correction Flow control Sequence control Point to point link Multipoint link management COMPUTER NETWORKS Data link layer protocols - 35 COMPUTER NETWORKS Data link layer protocols - 36 Pag. 6

7 PPP PDU Flag for framing or PDU delineation Address Not used and kept for compatibility with HDLC Control Not used and kept for compatibility with HDLC Protocol Higher layer protocol multiplexing Byte Stuffing Data transparency is obtained by defining a mechanism to send, in the data portion of the packet, the byte with value The sender inserts an escape byte before any byte or byte The first byte equal to is discarded by the receiver COMPUTER NETWORKS Data link layer protocols - 37 COMPUTER NETWORKS Data link layer protocols - 38 PPP Link Control Protocol PPP-LCP protocol opens (and close) the PPP connection Options are negotiated Max frame length; authentication protocol; skip address and control fields DD is the initiai state Once the PPP connection has been established and the authentication process was successful, for the IP protocol the IP Control Protocol (RFC332) is used to set up IP parameters (addresses, datagram compression, etc) PPP Data Control Protocol COMPUTER NETWORKS Data link layer protocols - 39 COMPUTER NETWORKS Data link layer protocols - 4 Frame Relay Standard to create packet networks based on virtual circuits (normally permanent VCs) on a wide area The standard was originally proposed within the ISDN framework Today used to create VPNs (Virtual Private Networks) for companies to interconnect LANs to build logical topologies to interconnect Internet routers for ISP Bit rate ranging from 64 kb/s to 2 Mb/s Variable size packets (well suited to data traffic) Maximum size 496 byte Frame Relay Operates on Permanent Virtual Circuit (although signaling protocols to deal with SVC are defined) Frame Relay network Virtual Circuits COMPUTER NETWORKS Data link layer protocols - 4 COMPUTER NETWORKS Data link layer protocols - 42 Pag. 7

8 LAPF Frame Relay defines the LAPF protocol (Link Access Procedure to Frame mode bearer services) LAPF is divided in two parts: DL-CORE (reccomendation I.233) Used in all network nodes DL-CONTROL Optionally used only by end users (today, mainly IP routers) In most applications, it is not used L>= 3 L2 L Higher layer protocols DL-CONTROL DL-CORE User terminal Core and edge approach DL-CORE Error control DL-CORE Frame Relay switching node Higher layer protocols DL-CONTROL DL-CORE User terminal COMPUTER NETWORKS Data link layer protocols - 43 COMPUTER NETWORKS Data link layer protocols - 44 LAPF packet Packet delineation through flag and bitstuffing to guarantee data transparency Flag Address Control Information CRC Flag DL-CORE DL-CONTROL (like HDLC with extended numbering) ADDRESS field contains the DLCI (Data Link Connection Identifier),the virtual circuit identifier some additional bits for congestion control and traffic policing LAPF packet DL-CORE DL-CONTROL FLAG ADDR (MSB) ADDR (LSB) control information CRC (MSB) CRC (LSB) FLAG COMPUTER NETWORKS Data link layer protocols - 45 COMPUTER NETWORKS Data link layer protocols - 46 DLCI: Data Link Connection Identifier FECN/BECN: forward/backward explicit congestion notification DE: discard eligibility C/R: command/response D/C: DLCI or DL- CORE : extension bit ADDRESS field lower DLCI DLCI upper DLCI upper DLCI lower DLCI or DL-CORE control C/R FECN BECN DE C/R FECN BECN DE D/C upper DLCI C/R DLCI FECN BECN DE DLCI lower DLCI or DL-CORE control D/C default format (2 byte) format 3 byte format 4 byte COMPUTER NETWORKS Data link layer protocols - 47 B-ISDN Private and public networks Integrated network Support all type of services, with different transmission speeds and quality of service requirements over the same network infrastructure Standardized by ITU-T and ATM Forum ISDN (re)evolution Exploit ATM as a transport, multiplexing and switching technique COMPUTER NETWORKS Data link layer protocols - 48 Pag. 8

9 Asynchronous Transfer Mode: ATM Defined as a data link layer protocol Wide area network exploiting packet switching networks with virtual circuit service Characteristics High bit rate (starting from 622 Mb/s) Low latencies for voice services Fixed size data units (cells) 53 byte (48 byte of data) B-ISDN: reference model Control plane Higher layers Management plane User plane Higher layer AAL (ATM Adaptation Layer) ATM layer layer Layer management Plane management COMPUTER NETWORKS Data link layer protocols - 49 COMPUTER NETWORKS Data link layer protocols - 5 L>=3 L2 L Higher layers protocols AAL ATM User terminal Core and edge approach in the user plane Error detection only on-demand for some AAL ATM ATM ATM switching node Higher layers protocols AAL ATM User terminal ATM: which OSI layer? Original idea End-to-end transport ATM from desktop to desktop Reality: IP router interconnection IP over ATM ATM is yet another data link technology COMPUTER NETWORKS Data link layer protocols - 5 COMPUTER NETWORKS Data link layer protocols - 52 Header (5 bytes) + payload (48 bytes) Fixed size cell To ease the switching task at high speed (synchronous switching) Small cell size Reduced latency (can be obtained by increasing transmission speed) Small packetization delay for interactive voice services Segmentation overhead Slightly different format at network edge and core GFC VPI VPI VPI VCI VPI VCI VCI VCI VCI PT CLP VCI PT CLP HEC HEC DATA DATA UNI CELL NNI CELL 5 BYTE 48 BYTE COMPUTER NETWORKS Data link layer protocols - 53 COMPUTER NETWORKS Data link layer protocols - 54 Pag. 9

10 ATM cell header (5 bytes = 4bit) GFC (4 bit): Generic Flow Control VPI (8-2 bit): Virtual Path Identifier VCI (6 bit): Virtual Circuit Identifier PT (3 bit): Payload Type CLP ( bit): Cell Loss Priority HEC (8 bit): Header Error Code GFC - Generic Flow Control Only at the UNI interface. The network issues information to user on the number of cells that can be sent Two control algorithms: ON-OFF Credit based COMPUTER NETWORKS Data link layer protocols - 55 COMPUTER NETWORKS Data link layer protocols - 56 ATM cell fromat VPI - Virtual Path Identifier VCI: Virtual Circuit Identifier Variable length: 8 bit at the UNI (256 VP s) 2 bit at the NNI (496 VP s) Some VPIs are reserved to network management functions and to signalling Identifies a single virtual circuit within a given VPI VC s are available for each VP. Example: link at 2,4 Gb/s, VP, all VCs with the same capacity 36Kb/s for each VC. COMPUTER NETWORKS Data link layer protocols - 57 COMPUTER NETWORKS Data link layer protocols - 58 PT field (Payload Type) PT - Payload Type Classifies the payload information type. It contains an identifier named Payload Type Identifier (PTI). Among the eight possible codes, four are reserved to network functions four to user function PT User cell User cell User cell User cell MNING EFCI No congestion AAL 5 indication= EFCI No congestion AAL 5 indication= EFCI Congestion AAL 5 indication= EFCI Congestion AAL 5 indication= COMPUTER NETWORKS Data link layer protocols - 59 COMPUTER NETWORKS Data link layer protocols - 6 Pag.

11 Campo PT (Payload Type) CLP - Cell Loss Priority PT SIGNIFICATO OAM cell (Operation and Maintenance) OAM cell (Operation and Maintenance) RM cell (Resource Management) Two priority levels at the ATM layer (within each VC) In ATM switches, it permits to selectively discard cells in case of buffer congestion CLP= indicates a high priority cell Not used Reserved for future use COMPUTER NETWORKS Data link layer protocols - 6 COMPUTER NETWORKS Data link layer protocols - 62 HEC - Header Error Code It permits to check the correctness of the ATM cell header only No error detection on paylod! Single errors are corrected Two errors are detected SEC/DED: Single errore correction/ Double Error Detection B-ISDN: reference model Control plane Higher layers Management plane User plane Higher layer AAL (ATM Adaptation Layer) ATM layer layer Layer management Plane management COMPUTER NETWORKS Data link layer protocols - 63 COMPUTER NETWORKS Data link layer protocols - 64 AAL: ATM Adaptation Layer Integrates ATM transport to offer service to users Servide dependent layer Examples of AAL functions: Transmission errors detection and managment Segmentation and reassembly Cell loss management Flow control Synchronization AAL: ATM Adaptation Layer It defines four classes of service (service classes) Through three main parametrs: Source transmission speed Type of connection (connection oriented/connectionless) Temporal relation between end user COMPUTER NETWORKS Data link layer protocols - 65 COMPUTER NETWORKS Data link layer protocols - 66 Pag.

12 AAL: 4 service classes A: CBR traffic, constant but rate, connection oriented, synchronism required AAL B: VBR traffic, connection oriented, synchronism required AAL 2 C: VBR traffic, connection oriented, synchronism not required AAL 3/4 D: VBR traffic, connectionless, synchronism not required AAL 5 Synchronism required between source and dest Speed Connection type AAL type Possible applications AAL service classes Class A Class B Class C Class D costant (CBR) required Connection oriented not required variable (VBR) AAL AAL 2 AAL 3/4-5 voice 64kbit/s video CBR video/audio VBR data connectionl ess data COMPUTER NETWORKS Data link layer protocols - 67 COMPUTER NETWORKS Data link layer protocols - 68 AAL layer: architecture The AAL layer is subdivided into two sublayers convergence sublayer (CS): Service and ATM traffic convergence Multiplexing Error detection Synchronism recovery segmentation and reassembly (): Segmentation in transmission, reassembly in reception of CS PDUs AAL AAL architecture CS convergence sublayer segmentation and reassembly SSCS service specific CS CPCS common part CS Some sub-layers can be empty CS SSCS CPCS COMPUTER NETWORKS Data link layer protocols - 69 COMPUTER NETWORKS Data link layer protocols - 7 AAL format AAL 3/4 ATM Cell Header SN SNP - SDU 47 byte AAL CPCS PDU B B 2B -3B B 2B 2B CPI B Tag BA size AAL payload pad AL E Lenght Tag ATM Cell Header SN IT - SDU LI CRC AAL 2 44 byte ATM Cell Header ST SN RES - SDU LI CRC AAL 3 44 byte ATM Cell Header ST SN MID - SDU LI CRC AAL 4 44 byte ATM Cell Header AAL 5 48 byte COMPUTER NETWORKS Data link layer protocols - 7 PDU 2 byte 44 byte 2 byte - PDU header trailer ST=BOM header ST=COM trailer header ST=EOM ST SN MID LI CRC 2 4 bit 6 bit bit trailer COMPUTER NETWORKS Data link layer protocols - 72 Pag. 2

13 Lenght CRC - 32 COMPUTER NETWORKS Data link layer protocols Layer PDU CS Layer PDU AAL 5 CS Layer Payload 48 bytes payload B -47B 2B 2B 4B 48 bytes payload 48 bytes payload End of segment = AAL 5 No CS layer layer exploit all 48 byte payload Last cell created by the segmentation process has the third bit in the PT field of the ATM header set to Layer separation principle violated! Error control over the full CS-PDU COMPUTER NETWORKS Data link layer protocols - 73 COMPUTER NETWORKS Data link layer protocols - 74 AAL 5 Advantages simplicity efficiency Improved reliability (CRC - 32) Disadvantages Use the third bit of the PT field in the ATM header Breaks layer separation principle Loss of the cell with the PT bit set = implies that two full CS-PDUs are lost Layer 2 protocols: a comparison Protocol Packet delineation Layer 3 protocol multiplexing Error detection LAPB Delimiter Higher layer YES YES LAPF core + LAPF control ATM (core)+ AAL (edge) Delimiter Done at the physical layer Via virtual circuits Via virtual circuits YES in LAPF core YES in AAL (edge) PPP Delimiter YES YES NO LLC Done in MAC IEEE 82.3 Error correction (window protocol) Optional in LAPF control (edge) NO YES Optional Optional Ethernet MAC Silence YES YES NO In gray info either not presented in this class or later descriebd COMPUTER NETWORKS Data link layer protocols - 75 COMPUTER NETWORKS Data link layer protocols - 76 Pag. 3