HDLC PROTOCOL. Michel GARDIE

Size: px

Start display at page:

Download "HDLC PROTOCOL. Michel GARDIE"

Robert Marshall
5 years ago
Views:

1 HDLC PROTOCOL Michel GARDIE INT/LOR/RIP October 15, 2007

2 The version of this document is temporary. There are still several mistakes. I'm sorry for that FRANCE TELECOM INT/DIT/TI 1992 INT/DSR/TI 1994 INT/LOR/AIGRI 2000 GET/INT/LOR/RIP 2001 GET/INT/LOR/RIP 2005 GET/INT/LOR/RIP 2006 GET/INT/LOR/RIP 2007 GET/INT/LOR/RIP Generated with OpenOffice.org 2.3 under Fedora 5 Tous droits réservés. Aucune partie de ce document ne peut être reproduite ou transmise sous quelque forme ou par quelque procédé que ce soit (machine électronique ou mécanique, photocopie, enregistrement, photographie ou tout autre) sans le consentement écrit préalable de l auteur ou de ses ayant-droits. GET/INT/LOR/RIP 2 of 33 October 15, 2007

3 HDLC ISO 3309 Data communication High-level data link control procedures Frame structure. ISO 4335 Data communication High-level data link control procedures Elements of procedure. ISO 6256 Data communication HDLC unbalanced classes of procedures. ISO 8885 Information processing systems Data communication -- Highlevel data link control procedures General purpose XID frame information field content and format. ISO/IEC Information technology Telecommunications and information exchange between systems High-level data link control (HDLC) procedures (Revises 3309, 4335, 6256, 8885) HDLC is used in X.25 networks, has been adapted to create the PPP protocol (Internet), or the LAPDm protocol (data link layer on the GSM radio interface). GET/INT/LOR/RIP 3 of 33 October 15, 2007

4 HIGH-LEVEL DATA LINK CONTROL HDLC Overview Synchronous Bit-oriented Full-duplex Transparent Point-to-point or multipoint links Features One frame structure Several elements of procedure GET/INT/LOR/RIP 4 of 33 October 15, 2007

5 FRAME STRUCTURE Definition Data and control information are transmitted within a single structure: the frame Frame structure Flag Address 8 bits Control 8 bits Information 0 n < bits FCS 16 bits Flag flag: address: control: information: FCS: sequence for synchronization and delimitation address of the secondary station controls the protocol behavior user data field frame check sequence GET/INT/LOR/RIP 5 of 33 October 15, 2007

6 FRAME FIELDS Flag delimitation start of frame end of frame synchronization inter-frame filling Address Address of the secondary station a secondary station receives command frames Control identifies the different frames numbers the information frames acknowledges the information frames supervises the data link GET/INT/LOR/RIP 6 of 33 October 15, 2007

7 FRAME FIELDS Information field optional sequence of n bits user defined FCS allows detection of transmission errors cyclic redundancy check polynomial: x 16 + x 12 + x GET/INT/LOR/RIP 7 of 33 October 15, 2007

8 FRAME CHARACTERISTICS Transparency Allows to transmit any bit pattern Principle (between two [non included] flags) Emission: inserts 1 "zero" after 5 consecutive "ones" Reception: suppresses every "zero" following 5 consecutive "ones" Frame inter-filling Maintains the synchronization Continuous emission of flags Frame abandon Forces the receiver to ignore a frame Sequence of M bits: M 7 GET/INT/LOR/RIP 8 of 33 October 15, 2007

9 TRANSPARENCY The aim of transparency is to transmit any bit pattern between two flags. HDLC uses a special bit pattern ( ) as a delimiter. This special pattern is called a flag. A receiver enters the reception state (and remains in this state) when it receives any bit pattern different from the delimitation pattern. It quits the reception state when it detects a new flag. However, the delimitation pattern can be found anywhere between two real flags: it can be an ASCII character (tilde; "~") whose binary coding is it can be the association of two adjacent bytes giving the illusion of a flag. For instance, let us consider the two following bytes: and When these byte are put together, we obtain the following binary stream: where a "flag" seems to appear. We absolutely must remove these pseudo-flags when sending a frame. On the reception side, we will have to restore the original bit stream. The principle to do that is simple, and is applied only between two flags. On the sender side, we systematically add one "0" after five consecutive "1"s. On the receiver side, each time we receive five consecutive "1"s, we apply the following algorithm: 1. if the next bit is a "0", this bit is immediately and systematically removed 2. if the next bit is a "1", there are two possibilities: a) the next bit after this "1" is a "0": we are detecting a real flag. This is the end of the frame; b) the next bit after this "1" is still a "1": we are detecting an abandon sequence. The frame must be discarded. GET/INT/LOR/RIP 9 of 33 October 15, 2007

10 FRAME CHARACTERISTICS Invalid frames Frame too short: less than 32 bits Frame too Long 1 Extensions Address field (2 or 3 bytes) Control field (2 bytes) 1 Example: the final flag has not been detected by the receiver, and its reception buffer is full. GET/INT/LOR/RIP 10 of 33 October 15, 2007

11 ELEMENTS OF PROCEDURE The elements of procedure define how to use the basic element of the protocol: the frame. The rest of this document presents the following points: the different types of frame information supervision unnumbered the rules of restart the transmission controls the "command" and "response" frames GET/INT/LOR/RIP 11 of 33 October 15, 2007

12 FRAME TYPES F A C Info FCS F N(R) P/F N(S) 0 N(R) P/F S S 0 1 U U U P/F U U 1 1 N(S) N(R) SS UUUUU P/F number of the sent frame number of the next expected frame bits defining the supervision functions bits defining the extra functions P (poll) = request of an explicit response F (final) = indication of an explicit response GET/INT/LOR/RIP 12 of 33 October 15, 2007

13 FRAME TYPES Information The information field is not empty and its content is defined by the user. The frame numbers are modulo 8 2 N(S) number of the sent frame N(R) number of the next expected frame Supervision No information field Number transmitted by the frame: N(R) This type of frame is used to: acknowledge information frames reject information frames specify the state of the receiving station 2 It is possible to use a modulus of 128. This is negotiated when establishing the connection. GET/INT/LOR/RIP 13 of 33 October 15, 2007

14 FRAME TYPES Unnumbered No information field No frame number Basic control of the data link initialization (connection) connection reject disconnection indication of protocol errors GET/INT/LOR/RIP 14 of 33 October 15, 2007

15 SUPERVISION FUNCTIONS F A C Info FCS F N(R) P/F S S 0 1 S 4, S 3 = 0,0 RR Receiver Ready S 4, S 3 = 1,0 REJ Reject S 4, S 3 = 0,1 RNR Receiver Not Ready S 4, S 3 = 1,1 SREJ Selective Reject GET/INT/LOR/RIP 15 of 33 October 15, 2007

16 SUPERVISION FRAMES All the frames below acknowledge information frames whose number is less or equal to N(R) 1 (modulo n). RR RECEIVER READY The receiver is ready to process new information frames. The previous received information frames have been correctly processed and their content has been delivered to the user. REJ REJECT The receiver requests the retransmission of all information frames starting from the number N(R). GET/INT/LOR/RIP 16 of 33 October 15, 2007

17 SUPERVISION FRAMES RNR RECEIVER NOT READY The receiver temporarily cannot receive extra information frames. The previous frames (up to N(R) - 1) were correctly received but their content could not be delivered to the user. SREJ SELECTIVE REJECT The receiver requests the retransmission of the information frame whose number is N(R). The next received frames (after N(R)) have been correctly received, and have been stored; the content of these frames has not been delivered to the user. The use of this supervision frame is optional. GET/INT/LOR/RIP 17 of 33 October 15, 2007

18 EXTRA FUNCTIONS FORMAT OF THE CONTROL FIELD OF THE UNNUMBERED FRAMES The table below only gives the most common frames. Table 1: Bits of the control field Meaning P/F SARM Set Asynchronous Response Mode P/F SABM Set Asynchronous Balanced Mode P/F DISC Disconnect P/F UA Unnumbered Acknowledgment P/F DM Disconnected Mode P/F CMDR / FRMR Command Reject / Frame Reject The CMDR / FRMR frames contain an information field explaining the reason of the reject. GET/INT/LOR/RIP 18 of 33 October 15, 2007

19 EXTRA FUNCTIONS LAPB (LINK ACCESS PROTOCOL BALANCED) SABM Frame nature: command. Select the balanced mode (LAPB). The stations can both manage the data link. The acceptation of the connection is done with a UA frame. The rejection of the connection is done with a DM frame. DISC Frame nature: command. Generates the logical disconnection of the data link between two stations. The receiving station must answer with a UA. GET/INT/LOR/RIP 19 of 33 October 15, 2007

20 EXTRA FUNCTIONS UA Frame nature: response. Accepts a command. FRMR Frame nature: response. Rejects a command: after an error; if the requested command is not implemented. DM Frame nature: response. Two functions: Indicates that a station is not logically connected. Rejects a command GET/INT/LOR/RIP 20 of 33 October 15, 2007

21 RULES OF RESTART Some error conditions or abnormal behavior may occur. The rules of restart resolve these problems. OVERFLOW A station, which cannot receive any more information frames, sends a RNR frame with N(R) indicating the first unaccepted frame. The station sends a RR frame as soon as it is ready again to receive new information frames. TRANSMISSION ERROR Any frame, whose FCS computation indicates a transmission error, must be discarded. No other specific action should be undertaken by the receiver. WRONG N(S) (REJ VERSION) Any information frame whose N(S) does not contain the expected value must be discarded; the next frames must be ignored too. The station receiving such a frame must then send a REJ frame with N(R) indicating the expected frame number. GET/INT/LOR/RIP 21 of 33 October 15, 2007

22 RULES OF RESTART WRONG N(S) (SREJ VERSION) Any information frame whose N(S) does not contain the expected value must be discarded. The eventual next information frames should be stored. A station receiving such a frame must then send a SREJ frame with N(R) indicating the expected frame number. COMMAND REJECT After receiving a CMDR / FRMR frame, the data link is supposed to be disconnected. RESET Clears a data link. The SABM and UA frames are exchanged without prior disconnection. The N(S) and N(R) counters are reset; the pending information frames are discarded. GET/INT/LOR/RIP 22 of 33 October 15, 2007

23 RULES OF RESTART RESTART AFTER TIME-OUT A station which does not receive any acknowledgment before T1 seconds, must retransmit the first unacknowledged information frame. The P/F bit of this information frame must be set. The station can then resume the transmission after the reception of an acknowledgment with the P/F bit set. BIT P/F A station which receives a command frame with the P/F 3 bit set, must send a response frame with the P/F 4 bit also set. 3 The P/F bit is called P (Poll) in a command frame (e.g. Information frames) 4 The P/F bit is called F (Final) in a response frame (e.g. RR, RNR or REJ frames) GET/INT/LOR/RIP 23 of 33 October 15, 2007

24 TRANSMISSION CONTROL OVERVIEW Any transmission occurs between a PRIMARY and a SECONDARY. The primary station controls the data link. The secondary station is tied to the primary station. STATION A station may be composed of: a primary a secondary a primary and a secondary GET/INT/LOR/RIP 24 of 33 October 15, 2007

25 ADDRESS FIELD The address field is used to make a distinction between the command and the response frames. CODING OF THE ADDRESS FIELD IN THE X.25 PROTOCOL There are only two values with the following coding: Address Bit number hexadecimal A B The use of these values is explained below: Sender Frame nature Network DTE Command A B Response B A GET/INT/LOR/RIP 25 of 33 October 15, 2007

26 FLOW CONTROL The flow control is used to regulate the information flow between stations. The mechanism used is a window. A station can send up to W information frames before receiving any acknowledgment. The window is closed if the station has sent W frames and did not receive any acknowledgment. A station cannot send other information frames if its window is closed. The receiver can acknowledge one, several, or all the information frames previously received. When a station acknowledges some informations frames, it opens more or less the window of its peer. GET/INT/LOR/RIP 26 of 33 October 15, 2007

27 FLOW CONTROL DTE W = 3 NETWORK DLDTreq I 0,0 DLDTreq DLDTind I 0,0 DLDTreq I 0,0 DLDTind DLDTreq RR 2 DLDTind I 0,0 DLDTind RR 2 W = 3 the window size is 3. DLDTreq : DLDTind : I,0,2 : RR,2 : primitive of service sent by the user of the level 2 (DTE side): DLDTreq = Data Link Data request. primitive of service sent by the user of the level 2 (network side): DLDTind = Data Link Data indication. information frame, N(R)=0, N(S)=2 supervision frame RR, N(R)=2 GET/INT/LOR/RIP 27 of 33 October 15, 2007

28 LIST OF SOME PARAMETERS USED AT THE FRAME LEVEL TIMER T1 On timeout of this timer, the first unacknowledged information frame must be sent again. Standard values: T1 = 100, 200, 400, 800, 1600, 2500 ms TIMER T2 Maximum time that a receiver can wait to acknowledge an information frame. N2 Maximum number of retransmission of the same information frame. If this value is reached, the link is considered to be out-of-order. Standard value: 10 N1 Maximum size of an information frame. GET/INT/LOR/RIP 28 of 33 October 15, 2007

29 EXCHANGE MODES DEFINITIONS INFORMATION SOURCE ACKNOWLEDGMENT DESTINATION COMMAND PRIMARY RESPONSE SECONDARY PRIMARY SOURCE SELECTING ACCEPTANCE INFORMATION ACKNOWLEDGMENT SECONDARY DESTINATION PRIMARY DESTINATION POLLING ACCEPTANCE INFORMATION ACKNOWLEDGMENT SECONDARY SOURCE GET/INT/LOR/RIP 29 of 33 October 15, 2007

30 EXCHANGE MODES ASYMMETRIC POINT-TO-POINT Station A Station B PRIMARY SECONDARY SOURCE I SEL POLL I ACK I DEST. ACK DEST. I I I SOURCE This mode is typically intended for multi-point links. Asymmetric point-to-point link is an adaptation of multi-point links. The information frames can be either "command" frames, or "response" frames. This mode is selected with the command frame "SNRM (Set Normal Response Mode)". GET/INT/LOR/RIP 30 of 33 October 15, 2007

31 EXCHANGE MODES SYMMETRIC POINT-TO-POINT Station A Station B PRIMARY I I I SECONDARY SOURCE ACK DEST. SECONDARY ACK PRIMARY DEST. I I I SOURCE This mode is selected with the command frame "SARM (Set Asynchronous Response Mode)". The information frames are always "command" frames. The supervisions frames (RR, RNR, REJ...) are generally "response" frames. GET/INT/LOR/RIP 31 of 33 October 15, 2007

32 EXCHANGE MODE BALANCED POINT-TO-POINT Station A Station B SOURCE DEST. Combined station Combined station DEST. SOURCE In this mode, the concepts of "primary" and "secondary" are replaced by the concept of "combined station". The command frames (INFO, SABM, DISC...) always carry the address of the remote station. The response frames (RR, RNR, UA, DM...) always carry the address of the local station. This mode is selected with the command frame "SABM (Set Asynchronous Balanced Mode)". GET/INT/LOR/RIP 32 of 33 October 15, 2007

33 CREDITS This document was written with OpenOffice.org 2.3 ( Different fonts were used: Font name Web site Sample (12pt) Calisto MT The quick brown fox jumps over the lazy dog Daniel The quick brown fox jumps over the lazy dog Goodfish The quick brown fox jumps over the lazy dog Morpheus The quick brown fox jumps over the lazy dog Times New Roman (not free) The quick brown fox jumps over the lazy dog GET/INT/LOR/RIP 33 of 33 October 15, 2007

3. Data Link Layer 3-2

3. Data Link Layer 3-2 3. Data Link Layer 3.1 Transmission Errors 3.2 Error Detecting and Error Correcting Codes 3.3 Bit Stuffing 3.4 Acknowledgments and Sequence Numbers 3.5 Flow Control 3.6 Examples: HDLC, PPP 3. Data Link