Network Working Group W A Simpson Internet Draft Daydreamer expires in six months March 1993 PPP over X.25 Status of this Memo This memo is the product of the Point-to-Point Protocol Working Group of the Internet Engineering Task Force (IETF). Comments on this memo should be submitted to the ietf-ppp@ucdavis.edu mailing list. Distribution of this memo is unlimited. This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' Please check the 1id-abstracts.txt listing contained in the internet-drafts Shadow Directories on nic.ddn.mil, nnsc.nsf.net, nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au to learn the current status of any Internet Draft. Abstract The Point-to-Point Protocol (PPP) [1] provides a standard method of encapsulating Network Layer protocol information over point-to-point links. This document defines a method for using PPP to transport multi- protocol datagrams over X.25 circuits. Simpson expires in six months [Page i] DRAFT PPP over X.25 March 1993 1. Introduction PPP has three main components: 1. A method for encapsulating datagrams over serial links. 2. A Link Control Protocol (LCP) for establishing, configuring, and testing the data link connection. 3. A family of Network Control Protocols (NCPs) for establishing and configuring different network layer protocols. PPP was designed as a standard method of communicating over point- to-point links. Initial deployment has been over short local lines, leased lines, and plain-old-telephone-service (POTS) using modems. As new packet services and higher speed lines are introduced, PPP is easily deployed in these environments as well. One protocol to carry them all. One protocol to mind them. One protocol to link them all, and in the network bind them. PPP was intended to replace X.25. There are still ISO-lated pockets of existing X.25 links, and some interest in bringing the advantages of the PPP multi-protocol datagram service to this venue. When X.25 emulates a point-to-point circuit, PPP is well suited to use over X.25. 2. Encapsulation PPP provides an encapsulation protocol over both bit-oriented synchronous links and asynchronous links with 8 bits of data and no parity. These links MUST be full-duplex, but MAY be either dedicated or circuit-switched. This fits the X.25 model. PPP uses HDLC [2] as a basis for the default encapsulation. X.25 is also in the family of HDLC derivatives, and the X.25 header may be easily substituted for the smaller HDLC header. Simpson expires in six months [Page 1] DRAFT PPP over X.25 March 1993 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+ | Flag (0x7e) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address | Control |D|Q| SVC# (hi) | SVC# (lo) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |p(r) |M|p(s) |0| Pad (0) | PPP Protocol | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Unfortunately, the X.25 header is 5 octets in length. Therefore, a single octet of zero padding is used to align the header to a more convenient boundary. The use of this zero padding is conformant with both the ISO Network Layer Protocol Identifier (NLPID) Null Network layer, and the PPP protocol field extension mechanism. LCP negotiation MAY permit the Pad and Protocol fields to be compressed to a single octet. 3. In-Band Protocol Detection When Out-of-Band signaling is not used to configure call setup for the circuit, or the Null encapsulation is indicated, the PPP Protocol field may be easily distinguished from other NLPID values. Initial LCP packets will contain the sequence 00-c0-21 following the header. Older implementations [3] might contain the NLPID value CC hex. Other ISO conformant implementations might contain other NLPID values, such as 80 hex (SNAP), or 81 hex (CLNP). Such packets indicate that the link is not properly configured for PPP operation, and MUST generate a Protocol-Reject. 4. Out-of-Band signaling The first octet in the Call User Data (CUD) Field (the first data octet in the Call Request packet) is used for protocol demultiplexing, in accordance with the Subsequent Protocol Identifier (SPI) in ISO/IEC TR 9577. This field contains a one octet Network Layer Protocol Identifier (NLPID), which identifies the network layer protocol encapsulated over the X.25 virtual circuit. The CUD field MAY contain more than one octet of information, and receivers MUST ignore all extraneous octets in the field. The PPP encapsulation SHOULD be indicated by a value of C0 hex. When Simpson expires in six months [Page 2] DRAFT PPP over X.25 March 1993 specifically configured to do so, the implementation MAY also accept a value of 00. Other values of the CUD are beyond the scope of this specification. 5. Configuration Details The standard LCP configuration defaults apply to X.25 links. The following Configurations Options are recommended: Magic Number Link Quality Monitoring Address and Control Field Compression Protocol Field Compression A maximum PDU size of 1600 is commonly available. This translates to a PPP MRU size of 1598. Since the typical network feeding the link is unlikely to have a MRU of greater than 1500, it is not expected to be worth the trouble to negotiate a higher MRU. Simpson expires in six months [Page 3] DRAFT PPP over X.25 March 1993 Security Considerations Security issues are not discussed in this memo. References [1] Simpson, W. A., "The Point-to-Point Protocol", RFC 1331, May 1992. [2] International Organization For Standardization, ISO Standard 3309-1979, "Data communication - High-level data link control procedures - Frame structure", 1979. [3] Malis, A., Robinson, D., Ullman R., "Multiprotocol Interconnect on X.25 and ISDN in the Packet Mode", RFC 1356, August 1992. Acknowledgments Chair's Address The working group can be contacted via the current chair: Brian Lloyd B.P. Lloyd & Associates 3420 Sudbury Road Cameron Park, California 95682 Phone: (916) 676-1147 EMail: brian@lloyd.com Author's Address Questions about this memo can also be directed to: William Allen Simpson Daydreamer Computer Systems Consulting Services P O Box 6205 East Lansing, MI 48826-6205 EMail: Bill.Simpson@um.cc.umich.edu Simpson expires in six months [Page 4] DRAFT PPP over X.25 March 1993 Table of Contents 1. Introduction .......................................... 1 2. Encapsulation ......................................... 1 3. In-Band Protocol Detection ............................ 2 4. Out-of-Band signaling ................................. 2 5. Configuration Details ................................. 3 SECURITY CONSIDERATIONS ...................................... 4 REFERENCES ................................................... 4 ACKNOWLEDGEMENTS ............................................. 4 CHAIR'S ADDRESS .............................................. 4 AUTHOR'S ADDRESS ............................................. 4 Bill.Simpson@um.cc.umich.edu