The Point-to-Point Protocol: LLC over PPP INTERNET-DRAFT Abstract The Point-to-Point Protocol (PPP) provides a method for transmitting datagrams over serial point-to-point links. PPP is composed of three parts: 1. A method for encapsulating datagrams over serial links. 2. An extensible Link Control Protocol (LCP). 3. A family of Network Control Protocols (NCP) for establishing and configuring different network layer protocols. The IEEE 802.2 Logical Link Control (LLC) protocol provides additional services beyond those available directly from the various IEEE 802 Medium Access Control (MAC) data link protocols. This document defines the operation of the LLC protocol over PPP. Status of this memo This draft document is for review by the IETF. Distribution of this memo is unlimited. Please send comments to the author. INTERNET-DRAFT LLC over PPP Page 2 Introduction 19 December 1990 1 Introduction 1.1 Overview Of PPP PPP is a point-to-point serial data link protocol providing a multiplexing, datagram delivery service. It has three main components: 1. A method for encapsulating datagrams of multiple data link users over serial links. PPP uses HDLC as a basis for encapsulating datagrams over point-to-point links. 2. An extensible Link Control Protocol (LCP) to establish, configure, and test the data link connection. 3. A family of Network Control Protocols (NCP) for establishing and configuring different network layer protocols. PPP is designed to allow the simultaneous use of multiple network layer protocols. To establish communications over a point-to-point link, the originating PPP sends LCP packets to configure and test the data link. After the link has been established and optional facilities have been negotiated as needed by the LCP, the originating PPP may send NCP packets to choose and configure one or more network layer protocols. Once each of the chosen network layer protocols has been configured, datagrams from each network layer protocol can be sent over the link. The link will remain configured for communications until explicit LCP or NCP packets close the link down, or until some external event occurs (e.g. inactivity timer expires or user intervention). | | The PPP encapsulating scheme, the basic LCP, and an NCP for | controlling and establishing the Internet Protocol (IP) (called the IP | Control Protocol, IPCP) are defined in The Point-to-Point Protocol | (PPP) [1]. Configuration Options for PPP are defined in [2]. 1.2 Overview Of LLC The IEEE 802.2 LLC protocol provides a set of services which are common across all the 802 data links [3]. Some of the services provided are: the multiplexing of data link users over a single MAC; a connectionless Protocol Data Unit (PDU) delivery service providing no error recovery; a reliable PDU delivery service; information exchange and test functions. The PPP protocol type used by LLC is . The availability of uniform services and PDU formats for all data links should simplify implementation of Network Layer protocols. Operating LLC over PPP extends the set of data link protocols capable INTERNET-DRAFT LLC over PPP Page 3 Introduction 19 December 1990 of providing this uniformity. 2 A PPP NCP For LLC The LLC Network Control Protocol is responsible for configuring, enabling, and disabling the LLC modules on both ends of the point-to-- point link. As with the Link Control Protocol, this is accomplished through an exchange of packets. These NCP packets may not be exchanged until LCP has reached the network layer Protocol Configuration Negotiation phase. NCP packets received before this phase is reached should be silently discarded. Likewise, LLC PDUs may not be exchanged until the NCP has first opened the connection (reached the Open state). The LLC Network Control Protocol is exactly the same as the Link Control Protocol [1] with the following exceptions: Data Link Layer Protocol Field Exactly one LLC NCP packet is encapsulated in the Information field of PPP data link layer frames where the Protocol field indicates type hex (LLC Network Control Protocol). Code field Only Codes 1 through 7 (Configure-Request, Configure-Ack, Configure-Nak, Configure-Reject, Terminate-Request, Terminate-Ack and Code-Reject) are used. Other Codes should be treated as unrecognized and should result in Code-Rejects. Timeouts LLC NCP packets may not be exchanged until the Link Control Protocol has reached the Network Layer Protocol Configuration Negotiation phase. An implementation should be prepared to wait for Link Quality testing to finish before timing out waiting for a Configure-Ack or other response. It is suggested that an implementation give up only after user intervention or a configurable amount of time. Configuration Option Types Currently, the LLC NCP has no Configuration Options defined. INTERNET-DRAFT LLC over PPP Page 4 A PPP NCP For LLC 19 December 1990 2.1 Sending LLC PDUs Before any LLC PDUs may be communicated, both the Link Control Protocol and the LLC Network Control Protocol must reach the Open state. Exactly one LLC PDU is encapsulated in the Information field of PPP data link layer frames where the Protocol field indicates type hex (LLC). The maximum length of an LLC PDU transmitted over a PPP link is two octets less than the maximum length of the Information field of a PPP data link layer frame. The first two octets of the PPP Information field contain the size of the LLC PDU. 2.1.1 Format - The format of a PPP frame containing an LLC PDU is +----------+---------+---------+----------+---------+--------+--------+--- | Flag | Address | Control | Protocol | Length | DSAP | SSAP | | 01111110 | 1111111 | 0000011 | 16 bits | 16 bits | 8 bits | 8 bits | +----------+---------+---------+----------+---------+--------+--------+--- -------------------+---------------+---------+----------+ LLC Control | LLC user data | FCS | Flag | 8 bits or 16 bits | N*8 bits | 16 bits | 01111110 | -------------------+---------------+---------+----------+ The fields Flag, Address, Control, Protocol, and FCS are the same as described in the PPP specification. Length LLC Protocol Data Unit length. This indicates the number of octets in the LLC PDU and is encoded as a 16-bit integer. Its value is the sum of the number of octets in the DSAP, SSAP, LLC Control and LLC user data fields. DSAP LLC Destination Service Access Point address. This identifies the LLC user for which the LLC user data field is intended. SSAP LLC Source Service Access Point address. This identifies the LLC user from which the LLC user data field was initiated. LLC Control This indicates the LLC PDU type and function LLC user data This is the data passed to the LLC entity by a user of the LLC. INTERNET-DRAFT LLC over PPP Page 5 A PPP NCP For LLC 19 December 1990 References | [1] Perkins, D., "The Point-to-Point Protocol for the Transmission of | Multi-Protocol Datagrams Over Point-to-Point Links", RFC 1171. | [2] Perkins, D., "The Point-to-Point Protocol (PPP) Initial | Configuration Options", RFC 1172. [3] IEEE, "IEEE 802.2: Logical Link Control", IEEE Std. 802.2-1985 (ISO 8802/2). Chairman's Address This proposal is within the purview of the Point-to-Point Protocol Working Group of the Internet Engineering Task Force (IETF). The working group can be contacted via the chair: Stev Knowles FTP Software 26 Princess Street Wakefield, MA 01880-3004 Phone: (617) 246-0900 x270 EMail: Stev@FTP.com Author's Address Questions about this memo can also be directed to the author: Arthur Harvey Digital Equipment Corporation 550 King Street, LKG1-2/A19 Littleton, MA 01460-1289 Phone: (508) 486-7647 EMail: Harvey@gah.enet.dec.com