Example Data Link Protocols

Transcription

1 Example Data Link Protocols Savera Tanwir Bit-Oriented & Character-Oriented Protocols Bit Oriented Protocol Basic unit: Bit Frame length is not a multiple of character size Example: HDLC (High-level Data Link Control) Character Oriented Protocol Basic unit: character Frame length is a multiple of character size Example: PPP

2 HDLC High Level Data-link Control ISO Bit-oriented data link layer protocol Provides both connectionless and connection-oriented services to Network Layer Can be used for point-point as well as point-multipoint (usually for wireless) connections Framing HDLC can operate on both synchronous & asynchronous links Each frame begins and ends with a frame delimiter bitpattern When no frames are being transmitted on a synchronous link, the frame delimiter is continuously transmitted It generates a continuous bit pattern used by modems to train and synchronize their clocks Bit-stuffing is used to avoid the flag pattern from occurring in the data

3 Primary terminal Types of Stations Responsible for operation control over the link It issues the frames which are called commands Secondary terminal operates under the control of the primary Frames issues, are responses only Primary is linked with secondaries by multiple logical links Combined terminal Has the features of both primary and secondary terminals It issues both commands and responses High-Level Data Link Control Address Field Contains address of the Slave/Secondary station Control Field Used for Seq #, ACKs and other control features etc. Data Field Variable length, but has an upper limit depending on the network Checksum Field CRC-16 of Address, Control and Data fields (x 16 + x 12 + x 5 + 1) Minimum frame contains three fields and total 32 bits, excluding the flags on either side

4 Frame Types Three kinds of frames in HDLC Information Frames To transport user data (both normal data & piggybacked control data) Supervisory Frames To transport control data only used whenever piggybacking is impossible or inappropriate Unnumbered Frames To transport link management and control information High-Level Data Link Control (2) Control field of (a) An information frame. (b) A supervisory frame. (c) An unnumbered frame.

5 I-Frame Format Seq Field Three bits to denote the seq # of the frame Poll/Final Flag It is used to signal which side is talking When a primary station has finished transmitting a series of frames, it sets the Poll bit to obtain a response from a secondary station, thus giving control to the secondary station At this time the secondary station may reply to the primary station When the secondary station finishes transmitting its frames, it sets the Final bit and control returns to the primary station Next Field Three bits to denote the seq # of the piggybacked +ve ACK S-Frame Format Type Field Used to distinguish the 4 types of S-Frames Receive Ready (RR) 00 +ve ACK and cancels a previous RNR Receive Not Ready (RNR) 10 +ve ACK plus receiver cannot receive more I-frames Reject (REJ) 01 Signals NAK for Go-Back-N protocol Selective Reject (SREJ) 11 Signals NAK for Selective Repeat protocol Next Field Three bits to denote the Seq # of the ACK or NAK

6 U-Frame Format First 2 bits are for identification The 5 bits of Type and Modifier fields are used in combination to indicate the U-Frame type Command/response SNRM SNRME SABM SABME UI UA DISC FRMR Meaning Set normal response mode Set normal response mode (extended) Set asynchronous balanced mode Set asynchronous balanced mode (extended) Unnumbered information Unnumbered acknowledgment Disconnect Frame reject HDLC Modes of Operation Three modes of operation according to the strength of Primary/Slave relationship 1. Normal Response Mode (NRM) Primary station initiates the session and full polling is used for all frame transmissions The P/F Flag is used in this mode only Signified by the SNRM(E) frame 2. Asynchronous Response Mode (ARM) Slave terminals may transmit without permission from the Primary terminal However, the primary terminal still retains responsibility for line initialization, error recovery, and logical disconnect 3. Asynchronous Balanced Mode (ABM) Either station may initiate the transmission and send frames at any time Signified by the SABM(E) frame

7 The Data Link Layer in the Internet A home personal computer acting as an internet host. PPP Point to Point Protocol RFCs 1661, 1662, 1663 More of a protocol suite than a particular protocol PPP is a connection-oriented protocol Character-oriented with byte-stuffing Can operate in half-duplex or full-duplex mode Designed to carry IP traffic but is general enough to allow any type of network layer datagram to be sent over a PPP connection Frames are sent and received in the same order Doesn t provide reliable data transfer using sequence numbers and acknowledgments as the default Reliable data transfer can be requested as an option One of the most popular Layer 2 WAN technologies in the networking world, esp. for dial-up access An encapsulated form of PPP, called PPP over Ethernet (PPPoE), is commonly used with DSL Internet service

8 PPP Advantages and Benefits 1. Specification of the encapsulated protocol, to allow multiple Layer 3 protocols to be multiplexed on a single link 2. Error detection for each transmitted frame through the use of a CRC code in each frame header 3. A robust mechanism for negotiating link parameters, including the maximum frame size permitted PPP Advantages and Benefits 4. A method for testing links before datagram transmission takes place, and monitoring link quality 5. Support for authentication of the connection using multiple authentication protocols 6. Support for additional optional features, including compression, encryption and link aggregation (allowing two devices to use multiple physical links as if they were a single, higher-performance link)

9 PPP Components The PPP standard itself describes three main components of PPP: 1. PPP Encapsulation Method 2. Link Control Protocol (LCP) 3. Network Control Protocols (NCPs) PPP Encapsulation The primary job of PPP is to take higher-layer messages such as IP datagrams and encapsulate them for transmission over the underlying physical layer link PPP defines a special frame format for encapsulating data for transmission, based on the framing used in the HDLC protocol PPP frame has been specially designed to be small in size and contain only simple fields, to maximize bandwidth efficiency and speed in processing

10 Link Control Protocol (LCP) LCP is responsible for setting up, maintaining, testing, negotiating and terminating the data-link connection between devices Optional support protocols Authentication Protocols PAP, CHAP etc. PPP Compression Control Protocol (CCP) allows compression of PPP data PPP Encryption Control Protocol (ECP) enables datagrams to be encrypted for security PPP Multilink Protocol (PPP MP) allows a single PPP link to be operated over multiple physical links Network Control Protocols (NCPs) PPP supports the encapsulation of many different Layer 3 protocol packets After the general link setup is completed with LCP, control is passed to the Network Control Protocol (NCP) specific to the Layer 3 protocol being carried on the PPP link For example, when IP is carried over PPP the NCP used is the PPP IP Control Protocol (IPCP) Other NCPs are defined for supporting the IPX protocol, the NetBIOS Frames (NBF) protocol, and so forth

11 Connection Establishment A host machine (PC) calling ISP for internet connectivity 1. PC calls ISP s Access Server/Router via modem 2. AS s modem answers the phone and a physical connection is established 3. PC sends a series of LCP packets in one or more PPP frame payload 4. Responses to establish PPP parameters Time Connection Establishment & Data Transfer Once PPP parameters have been agreed upon 5. A series of NCP packets sent to configure network layer (e.g., IP address request in TCP/IP suite) 6. ISP dynamically assigns IP address 7. PC is now a proper host on the Internet and can send and receive IP packets Time 8. Transmission and reception of IP packets (encapsulated in PPP frame) takes place

12 Connection Release When data transfer is over 1. NCP tears down internet connectivity and frees IP address 2. LCP shuts down data link layer connection 3. Modem hangs up the phone to release physical line Time Line Up & Down phases

13 Frame Format of PPP Control field Controls services offered to the network layer Protocol field Kind of packet in the payload e.g. LCP, NCP, IP, IPX, AppleTalk etc. 0 Refers to Network layer protocols like IP, IPX etc. 1 Refers to negotiation protocols e.g., LCP, NCP Payload field Variable length, up to some max. size negotiated by LCP at setup time Default is 1500 bytes CRC Checksum

14 LCP Overview LCP is used to negotiate data link protocol options during ESTABLISH phase Deals with the way for the initialization process to make a proposal and for the responding process to accept or reject it Allows testing of line quality Network Layer protocol configuration negotiation (NCP comes in here) Handles connection termination Encapsulated LCP frame For LCP, value of Protocol field is or 0xC021 Code Defines type of LCP frame ID To match a request with a corresponding reply Length Defines the length of entire frame Information Extra information needed for some frames

15 LCP frame types 11 types defined in RFC 1661 LCP Information field Contains options that are to be negotiated 1. Maximum-Receive-Unit (MRU) The maximum size datagram it wants the link to be able to carry 2. Authentication-Protocol Indicate the type of authentication protocol it wishes to use (if any) 3. Quality-Protocol To enable quality monitoring on the link and what protocol to use (if any) 4. Magic-Number Used to detect looped back links or other anomalies in the connection 5. Protocol-Field-Compression To specify use of compressed (8 bit) Protocol fields in PPP data frames instead of the normal 16 bit Protocol field 6. Address-and-Control-Field-Compression (ACFC) Used to compress the Address and Control fields, again for bandwidth savings

16 Authentication Protocols Password Authentication Protocol (PAP) A simple authentication protocol with a two-step process

17 Challenge Handshake Authentication Protocol (CHAP) It is a three-way hand-shaking authentication protocol that provides more security than PAP Password is kept secret and never sent online Network Control Protocol (NCP) Is a set of control protocols for negotiation at the network layer Each is specific to some network layer protocol and allows protocol-specific configuration requests to be made For example dynamic address assignment of IP address Each of the common network layer technologies has a PPP NCP defined for it in a separate RFC The most common ones are:- The PPP Internet Protocol Control Protocol (IPCP) The PPP Internetworking Packet Exchange Control Protocol (IPXCP) The PPP NetBIOS Frames Control Protocol (NBFCP) Te PPP IP Version 6 Control Protocol (IPv6CP)

18 Acknowledgements Ali Sajjad s lecture slides