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Hdlc
- 1. HDLC – High-Level Data Link Control The Data Link Layer in the Internet
- 2. End data field: mayon start with techniques we can the any length, is used CRCcontrol field begin with themultipleflag havehave which field although The address field: obtained with information. Itacknowledgements and stuffed if All HDLC frames:usedlinessequence numbers,01111110,address is byte other tothe of frame (same contain of frame) – checksum as for any HDLC terminals, described. efficiency purposes. of rest of the frame. identify one the checksum falls down with increasing frame length, due to the occurs in theof the terminals probability of occurrence of multiple burst errors. Frame format for bit-oriented protocols
- 3. Control field of (a) An information frame. (b) A supervisory frame. (c) An unnumbered frame.
- 4. SEQ – 3 bits sequence numbers are used NEXT – used for piggybacking acknowledge ◦ All of the HDLC protocols adhere to the convention that instead of sending the sequence number for the frame that has been received correctly, the acknowledgement contains the sequence number of the next frame that is expected (not received yet). The P/F bit stands for Pool/Final and it is used when a computer is polling a group of terminals. ◦ When used as P, the computer is inviting the terminals to send data. All the frames from the terminal, except the final one, have the P/F flag set to P. The final one is F. ◦ The F flag is sometimes used to force the other machine to send a supervisory frame immediately, without waiting for reverse traffic where to piggyback the window information.
- 5. Various kinds of SUPERVISORY frames are distinguished by the type filed: ◦ Type 0 is an acknowledgement frame (called RECEIVE READY), used to indicate the next frame expected ◦ Type 1 is a negative acknowledgement frame (called REJECT), used to indicate an error. The NEXT field indicates the first frame in sequence that has not been received correctly. The sender is requested to send all frames beginning at NEXT ◦ Type 2 is RECEIVE NOT READY. It acknowledges all frames up to but not including the NEXT one. It also tells the sender to stop sending. When the receiver is ready again, it sends a RECEIVE READY. ◦ Type 3 is a SELECTIVE REJECT. It calls for retransmission of only the frame specified. Thus, if a receiver wants to buffer out of sequence frames, it can force the retransmission of a specific frame by using a selective reject.
- 6. UNNUMBERED FRAME is used for control and to carry data when unreliable connectionless service is required. Using this frame, a number of commands can be sent: ◦ DISConect – allows a machine to announce its intention to bring down the connection ◦ SABM (Set Asynchronous Balanced Mode) – resets the line ◦ FRMR (Frame Reject) – indicates that a frame with correct checksum but incorrect semantics has arrived More control frames are actually available for initialization, polling and status reporting.
- 7. A home personal computer acting as an internet host.
- 8. Provides three features ◦ A framing method; the frame format also handles error detection ◦ A link control protocol called LCP (Link Control Protocol) ◦ A way to negotiate network specific options in a way that is independent of the network layer protocol to be used. The method chosen is to have a different NCP (Network Control Protocol) for each network layer supported
- 9. Checksum field is default data, is but of packetindicate that stations are to Payload – variableis with standard11111111 which indicates4an is byteCodes if The Addressfield default value up00000011 be negotiated toallbytes. negotiated All PPP frames start always bytes,kind it can ,to 01111110, which unnumbered are Protocol field length 2 set to HDLC negotiated maximum. field. Control field specifies what to someflag is in the payload If no stuffed maximum LCP, Using this value solves transmission using sequence numbers defined forvalue, of the frame. frame in frame. NCP, IP, IPX, AppleTalk and accept thethe rest a default 1500 value is used. other protocols. Protocols starting occurs(doesn’t provide by default reliablethe issue of assigning data link layer with zero and acknowledgements). addresses.are network layer protocols (IP, IPX, etc). Those starting with 1 are used to negotiate other protocols. These include LCP and a different NCP for each network layer protocol supported. The PPP full frame format for unnumbered mode operation.
- 10. A simplified phase diagram for bring a line up and down.
- 11. The LCP frame types.
Editor's Notes
- The frame begins with a frame sequence: 01111110 The address field : on lines with multiple terminals, the address field is used to identify one of the terminals The control field: used for sequence numbers, acknowledgements and other purposes as shown on the next slide The data field: may contain any information. It can have any length, although the efficiency of the checksum falls down with increasing frame length, due to the probability of occurrence of multiple burst errors. The Checksum is the Cyclic Redundancy Code obtained with the technique described earlier The frame is ended with another frame sequence: 01111110
- There are three types of frames: information, supervisory and unnumbered.
- The protocol is using a sliding window with 3 bits sequence numbers. Up to 7 unacknowledged frames can be in the traffic at any time. The SEQ field is the frame sequence number, while the NEXT field is the piggyback acknowledgement. All of the HDLC protocols adhere to the convention that instead of sending the sequence number for the frame that has been received correctly, the acknowledgement contains the sequence number of the next frame that is expected (not received yet). The P/F bit stands for Pool/Final and it is used when a computer is polling a group of terminals. When used as P, the computer is inviting the terminals to send data. All the frames from the terminal, except the final one, have the P/F flag set to P. The final one is F. The F flag is sometimes used to force the other machine to send a supervisory frame immediately, without waiting for reverse traffic where to piggyback the window information.
- Various kinds of SUPERVISORY frames are distinguished by the type filed: Type 0 is an acknowledgement frame (called RECEIVE READY), used to indicate the next frame expected Type 1 is a negative acknowledgement frame (called REJECT), used to indicate an error. The NEXT field indicates the first frame in sequence that has not been received correctly. The sender is requested to send all frames beginning at NEXT Type 2 is RECEIVE NOT READY. It acknowledges all frames up to but not including the NEXT one. It also tells the sender to stop sending. When the receiver is ready again, it sends a RECEIVE READY. Type 3 is a SELECTIVE REJECT. It calls for retransmission of only the frame specified. Thus, if a receiver wants to buffer out of sequence frames, it can force the retransmission of a specific frame by using a selective reject.
- Internet consists of individual machines (hosts and routers) and the communication infrastructure that connects them. Some of the machines are interconnected using LANs (we will study this case latter) and some are interconnected using point to point lines (especially the ones that are far apart).
- PPP handles error detection, supports multiple protocols, allows IP addresses to be negotiated at connection time, permits authentication (using two methods – PAP and CHAP) PPP format was chosen to closely resemble HDLC frame format. The major difference is that PPP is character oriented, while HDLC is bit oriented protocol. In particular, PPP is using byte stuffing on dialup modem lines, so all frames are integral number of bytes.
- All PPP frames start with standard HDLC flag 01111110, which is byte stuffed if occurs in the rest of the frame. The Address field is always set to 11111111 to indicate that all stations are to accept the frame. Using this value solves the issue of assigning data link layer addresses. The control field default value is 00000011 , which indicates an unnumbered frame (doesn’t provide by default reliable transmission using sequence numbers and acknowledgements). Since the address field and control field are constant, LCP can negotiate and option that allows them to be omitted and to save two bytes per frame. The Protocol field specifies what kind of packet is in the payload field. Codes are defined for LCP, NCP, IP, IPX, AppleTalk and other protocols. Protocols starting with zero are network layer protocols (IP, IPX, etc). Those starting with 1 are used to negotiate other protocols. These include LCP and a different NCP for each network layer protocol supported. The default size of the Protocol field is 2 bytes, but it can be negotiated down to one byte by LCP. Payload – variable length data, up to some negotiated maximum. If no negotiated maximum value, a default 1500 value is used. Checksum field is default 2 bytes, but it can be negotiated to 4 bytes.
- Line is in DEAD state. After physical connection is established, the line moves into ESTABLISHED phase. At that point, LCP option negotiation begins, and if successful leads to AUTHENTICATE. If authentication successful, the NETWORK state is entered. During this state, the appropriate NCP protocol is invoked, to configure the network layer. If the configuration is successful, the OPEN state is entered, where data transfer can take place. When the data transport is finished, the line moves into TERMINATE phase, and from there back to DEAD when the carrier is dropped.