Internet Engineering Task Force Dave Thaler INTERNET-DRAFT Microsoft Expires January 1999 27 July 1998 IP Tunnel MIB Status of this Memo 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 and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet- Drafts Shadow Directories on ftp.ietf.org (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). Copyright Notice Copyright (C) The Internet Society (1998). All Rights Reserved. 1. Introduction This memo defines an experimental portion of the Management Information Base (MIB) for use with network management protocols in the Internet community. In particular, it describes managed objects used for managing tunnels of any type in IP networks, including GRE [16,17], IP- in-IP [18], Minimal Encapsulation [19], L2TP [20], L2F [25], and PPTP [21] tunnels. Extension MIBs (e.g., [22]) may be designed for managing protocol-specific objects. Likewise, extension MIBs may be designed for Expires January 1999 [Page 1] Draft IP Tunnel MIB July 1998 managing security-specific objects (e.g., IPSEC [24]). 2. Revision History A record of changes which will be removed before publication. 27 July 1998 (1) Added tunnel config table to improve support for dynamic tunnel creation. (2) Added L2F as an encapsulation method. (3) Added Security Considerations and copyright notice, and updated SNMP Framework text and references. 18 April 1997 (1) initial version. 3. The SNMPv2 Network Management Framework The SNMP Management Framework presently consists of five major components: o An overall architecture, described in RFC 2271 [1]. o Mechanisms for describing and naming objects and events for the purpose of management. The first version of this Structure of Management Information (SMI) is called SMIv1 and described in RFC 1155 [2], RFC 1212 [3] and RFC 1215 [4]. The second version, called SMIv2, is described in RFC 1902 [5], RFC 1903 [6] and RFC 1904 [7]. o Message protocols for transferring management information. The first version of the SNMP message protocol is called SNMPv1 and described in RFC 1157 [8]. A second version of the SNMP message protocol, which is not an Internet standards track protocol, is called SNMPv2c and described in RFC 1901 [9] and RFC 1906 [10]. The third version of the message protocol is called SNMPv3 and described in RFC 1906 [10], RFC 2272 [11] and RFC 2274 [12]. o Protocol operations for accessing management information. The first set of protocol operations and associated PDU formats is described Expires January 1999 [Page 2] Draft IP Tunnel MIB July 1998 in RFC 1157 [8]. A second set of protocol operations and associated PDU formats is described in RFC 1905 [13]. o A set of fundamental applications described in RFC 2273 [14] and the view-based access control mechanism described in RFC 2275 [15]. Managed objects are accessed via a virtual information store, termed the Management Information Base or MIB. Objects in the MIB are defined using the mechanisms defined in the SMI. This memo specifies a MIB module that is compliant to the SMIv2. A MIB conforming to the SMIv1 can be produced through the appropriate translations. The resulting translated MIB must be semantically equivalent, except where objects or events are omitted because no translation is possible (use of Counter64). Some machine readable information in SMIv2 will be converted into textual descriptions in SMIv1 during the translation process. However, this loss of machine readable information is not considered to change the semantics of the MIB. 3.1. Object Definitions Managed objects are accessed via a virtual information store, termed the Management Information Base or MIB. Objects in the MIB are defined using the subset of Abstract Syntax Notation One (ASN.1) defined in the SMI. In particular, each object type is named by an OBJECT IDENTIFIER, an administratively assigned name. The object type together with an object instance serves to uniquely identify a specific instantiation of the object. For human convenience, we often use a textual string, termed the descriptor, to refer to the object type. 4. Overview This MIB module contains two tables: o the Tunnel Interface Table, containing information on the tunnels known to a router; and o the Tunnel Config Table, which is used for dynamic creation of tunnels. Expires January 1999 [Page 3] Draft IP Tunnel MIB July 1998 4.1. Relationship to the Interfaces MIB This section clarifies the relationship of this MIB to the Interfaces MIB [23]. Several areas of correlation are addressed in the following subsections. The implementor is referred to the Interfaces MIB document in order to understand the general intent of these areas. 4.1.1. Layering Model Each logical interface (physical or virtual) has an ifEntry in the Interfaces MIB [23]. Tunnels are handled by creating a logical interface (ifEntry) for each tunnel. These are then correlated to physical interfaces using the ifStack table of the Interfaces MIB. The basic model, therefore, looks something like this (for example): | | | | | | +--+ +---+ +--+ +---+ | | |IP-in-IP| | GRE | | | | tunnel | | tunnel | | | +--+ +---+ +--+ +---+ | | | | | | | | <== attachment to physical +--+ +---------+ +----------+ +--+ interfaces, to be provided | Physical interface | by ifStack table +--------------------------------+ 4.1.2. ifTestTable The ifTestTable usage is defined in the MIBs defining the encapsulation below the network layer. For example, if IP-in-IP encapsulation is being used, the ifTestTable is defined by IP-in-IP. 4.1.3. ifRcvAddressTable The ifRcvAddressTable usage is defined in the MIBs defining the encapsulation below the network layer. For example, if IP-in-IP encapsulation is being used, the ifRcvAddressTable is defined by IP- in-IP. Expires January 1999 [Page 4] Draft IP Tunnel MIB July 1998 4.1.4. ifEntry IfEntries are defined in the MIBs defining the encapsulation below the network layer. For example, if IP-in-IP encapsulation [20] is being used, the ifEntry is defined by IP-in-IP. The ifType of a tunnel should be set to "tunnel" (131). An entry in the IP Tunnel MIB will exist for every ifEntry with this ifType. An implementation of the IP Tunnel MIB MAY allow ifEntries to be created via the tunnelConfigTable. Creating a tunnel will also add an entry in the ifTable and in the tunnelIfTable, and deleting a tunnel will likewise delete the entry in the ifTable and the tunnelIfTable. The use of two different tables in this MIB was an important design decision. Traditionally, ifIndex values are chosen by agents, and are permitted to change across restarts. Allowing row creation directly in the Tunnel Interface Table, indexed by ifIndex, would complicate row creation and/or cause interoperability problems (if each agent had special restrictions on ifIndex). Instead, a separate table is used which is indexed only by objects over which the manager has control. Namely, these are the addresses of the tunnel endpoints and the encapsulation protocol. Finally, an additional manager- chosen ID is used in the index to support protocols such as L2F which allow multiple tunnels between the same endpoints. Expires January 1999 [Page 5] Draft IP Tunnel MIB July 1998 5. Definitions TUNNEL-MIB DEFINITIONS ::= BEGIN IMPORTS MODULE-IDENTITY, OBJECT-TYPE, transmission, Integer32, IpAddress FROM SNMPv2-SMI RowStatus FROM SNMPv2-TC MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF ifIndex FROM IF-MIB; tunnelMIB MODULE-IDENTITY LAST-UPDATED "9807271200Z" ORGANIZATION "Microsoft Corporation" CONTACT-INFO " Dave Thaler Microsoft Corporation One Microsoft Way Redmond, WA 98052-6399 EMail: dthalerd@microsoft.com" DESCRIPTION "The MIB module for management of IP Tunnels, independent of the specific encapsulation scheme in use." ::= { transmission 131 } tunnelMIBObjects OBJECT IDENTIFIER ::= { tunnelMIB 1 } tunnel OBJECT IDENTIFIER ::= { tunnelMIBObjects 1 } Expires January 1999 [Page 6] Draft IP Tunnel MIB July 1998 -- the IP Tunnel MIB-Group -- -- a collection of objects providing information about -- IP Tunnels tunnelIfTable OBJECT-TYPE SYNTAX SEQUENCE OF TunnelIfEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "The (conceptual) table containing information on configured tunnels." ::= { tunnel 1 } tunnelIfEntry OBJECT-TYPE SYNTAX TunnelIfEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "An entry (conceptual row) containing the information on a particular configured tunnel." INDEX { ifIndex } ::= { tunnelIfTable 1 } TunnelIfEntry ::= SEQUENCE { tunnelIfLocalAddress IpAddress, tunnelIfRemoteAddress IpAddress, tunnelIfEncapsMethod INTEGER, tunnelIfHopLimit Integer32, tunnelIfPriority Integer32, tunnelIfEncapsLimit Integer32, tunnelIfSecurity INTEGER } tunnelIfLocalAddress OBJECT-TYPE SYNTAX IpAddress MAX-ACCESS read-only STATUS current DESCRIPTION "The address of the local endpoint of the tunnel, or 0.0.0.0 if unknown." ::= { tunnelIfEntry 1 } tunnelIfRemoteAddress OBJECT-TYPE Expires January 1999 [Page 7] Draft IP Tunnel MIB July 1998 SYNTAX IpAddress MAX-ACCESS read-only STATUS current DESCRIPTION "The address of the remote endpoint of the tunnel, or 0.0.0.0 if unknown." ::= { tunnelIfEntry 2 } tunnelIfEncapsMethod OBJECT-TYPE SYNTAX INTEGER { other(1), -- none of the following native(2), -- no intermediate header gre(3), -- GRE encapsulation minimal(4), -- Minimal encapsulation l2tp(5), -- L2TP encapsulation pptp(6), -- PPTP encapsulation l2f(7) -- L2F encapsulation } MAX-ACCESS read-only STATUS current DESCRIPTION "The encapsulation method used by the tunnel. The value native indicates that the packet is encapsulated inside a normal IPv4 header and unicast to the remote tunnel endpoint. The value gre indicates that a GRE header is inserted between the outer header and the payload header, and minimal indicates that a Minimal Forwarding Header (RFC 2004) is inserted between the outer header and the payload data. The value pptp indicates that an enhanced GRE header and a PPP header are inserted." ::= { tunnelIfEntry 3 } tunnelIfHopLimit OBJECT-TYPE SYNTAX Integer32 (0..255) MAX-ACCESS read-create STATUS current DESCRIPTION "The IPv4 TTL (or IPv6 hop limit) to use in the outer IP header. A value of 0 indicates that the value is copied from the payload's header." ::= { tunnelIfEntry 4 } tunnelIfPriority OBJECT-TYPE SYNTAX Integer32 (-1..15) MAX-ACCESS read-create Expires January 1999 [Page 8] Draft IP Tunnel MIB July 1998 STATUS current DESCRIPTION "The IPv4 Preference or IPv6 Priority to use in the outer IP header. A value of -1 indicates that the value is copied from the payload's header." ::= { tunnelIfEntry 6 } tunnelIfEncapsLimit OBJECT-TYPE SYNTAX Integer32 MAX-ACCESS read-create STATUS current DESCRIPTION "The maximum number of encapsulations permitted for packets undergoing encapsulation at this node. A value of 0 indicates that no limit is present (except as a result of the packet size)." ::= { tunnelIfEntry 7 } tunnelIfSecurity OBJECT-TYPE SYNTAX INTEGER { none(1), -- no security ipsec(2), -- IPSEC security other(3) } MAX-ACCESS read-only STATUS current DESCRIPTION "The method used by the tunnel to secure the outer IP header." ::= { tunnelIfEntry 8 } tunnelConfigTable OBJECT-TYPE SYNTAX SEQUENCE OF TunnelConfigEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "The (conceptual) table containing information on configured tunnels. This table can be used to map a set of tunnel endpoints to the associated ifIndex value. It can also be used for row creation." ::= { tunnel 2 } tunnelConfigEntry OBJECT-TYPE SYNTAX TunnelConfigEntry MAX-ACCESS not-accessible Expires January 1999 [Page 9] Draft IP Tunnel MIB July 1998 STATUS current DESCRIPTION "An entry (conceptual row) containing the information on a particular configured tunnel." INDEX { tunnelConfigLocalAddress, tunnelConfigRemoteAddress, tunnelConfigEncapsMethod, tunnelConfigID } ::= { tunnelConfigTable 1 } TunnelConfigEntry ::= SEQUENCE { tunnelConfigLocalAddress IpAddress, tunnelConfigRemoteAddress IpAddress, tunnelConfigEncapsMethod INTEGER, tunnelConfigID Integer32, tunnelConfigIfIndex Integer32, tunnelConfigStatus RowStatus } tunnelConfigLocalAddress OBJECT-TYPE SYNTAX IpAddress MAX-ACCESS not-accessible STATUS current DESCRIPTION "The address of the local endpoint of the tunnel, or 0.0.0.0 if unknown." ::= { tunnelConfigEntry 1 } tunnelConfigRemoteAddress OBJECT-TYPE SYNTAX IpAddress MAX-ACCESS not-accessible STATUS current DESCRIPTION "The address of the remote endpoint of the tunnel, or 0.0.0.0 if unknown." ::= { tunnelConfigEntry 2 } tunnelConfigEncapsMethod OBJECT-TYPE SYNTAX INTEGER { other(1), -- none of the following native(2), -- no intermediate header gre(3), -- GRE encapsulation minimal(4), -- Minimal encapsulation l2tp(5), -- L2TP encapsulation pptp(6), -- PPTP encapsulation Expires January 1999 [Page 10] Draft IP Tunnel MIB July 1998 l2f(7) -- L2F encapsulation } MAX-ACCESS not-accessible STATUS current DESCRIPTION "The encapsulation method used by the tunnel." ::= { tunnelConfigEntry 3 } tunnelConfigID OBJECT-TYPE SYNTAX Integer32 MAX-ACCESS not-accessible STATUS current DESCRIPTION "An identifier used to distinguish between multiple tunnels of the same encapsulation method, with the same endpoints. If the encapsulation protocol only allows one tunnel per set of endpoint addresses (such as for GRE or IP-in-IP), the value of this object is 1. For encapsulation methods (such as L2F) which allow multiple parallel tunnels, the manager is responsible for choosing any ID which does not conflict with an existing row, such as choosing a random number." ::= { tunnelConfigEntry 4 } tunnelConfigIfIndex OBJECT-TYPE SYNTAX Integer32 MAX-ACCESS read-only STATUS current DESCRIPTION "The value of ifIndex corresponding to the tunnel interface." ::= { tunnelConfigEntry 5 } tunnelConfigStatus OBJECT-TYPE SYNTAX RowStatus MAX-ACCESS read-create STATUS current DESCRIPTION "The status of this row, by which new entries may be created, or old entries deleted from this table. Creating a row in this table will create a corresponding row in the ifTable and in the tunnelIfTable. Deleting a row in this table will likewise delete the corresponding row in the ifTable and in the tunnelIfTable." ::= { tunnelConfigEntry 6 } Expires January 1999 [Page 11] Draft IP Tunnel MIB July 1998 -- conformance information tunnelMIBConformance OBJECT IDENTIFIER ::= { tunnelMIB 2 } tunnelMIBCompliances OBJECT IDENTIFIER ::= { tunnelMIBConformance 1 } tunnelMIBGroups OBJECT IDENTIFIER ::= { tunnelMIBConformance 2 } -- compliance statements tunnelMIBCompliance MODULE-COMPLIANCE STATUS current DESCRIPTION "The compliance statement for the IP Tunnel MIB." MODULE -- this module MANDATORY-GROUPS { tunnelMIBBasicGroup } OBJECT tunnelIfHopLimit MIN-ACCESS read-only DESCRIPTION "Write access is not required." OBJECT tunnelIfPriority MIN-ACCESS read-only DESCRIPTION "Write access is not required." OBJECT tunnelIfEncapsLimit MIN-ACCESS read-only DESCRIPTION "Write access is not required." OBJECT tunnelConfigStatus MIN-ACCESS read-only DESCRIPTION "Write access is not required." ::= { tunnelMIBCompliances 1 } -- units of conformance tunnelMIBBasicGroup OBJECT-GROUP OBJECTS { tunnelIfLocalAddress, tunnelIfRemoteAddress, tunnelIfEncapsMethod, tunnelIfHopLimit, tunnelIfPriority, tunnelIfEncapsLimit, tunnelIfSecurity, Expires January 1999 [Page 12] Draft IP Tunnel MIB July 1998 tunnelConfigIfIndex, tunnelConfigStatus } STATUS current DESCRIPTION "A collection of objects to support basic management of IP Tunnels." ::= { tunnelMIBGroups 1 } END Expires January 1999 [Page 13] Draft IP Tunnel MIB July 1998 6. Security Considerations This MIB contains readable objects whose values provide information related to IP tunnel interfaces. There are also a number of objects that have a MAX-ACCESS clause of read-write and/or read-create, such as those which allow an administrator to dynamically configure tunnels. While unauthorized access to the readable objects is relatively innocuous, unauthorized access to the write-able objects could cause a denial of service, or could cause unauthorized creation and/or manipulation of tunnels. Hence, the support for SET operations in a non-secure environment without proper protection can have a negative effect on network operations. SNMPv1 by itself is such an insecure environment. Even if the network itself is secure (for example by using IPSec [24]), even then, there is no control as to who on the secure network is allowed to access and SET (change/create/delete) the objects in this MIB. It is recommended that the implementers consider the security features as provided by the SNMPv3 framework. Specifically, the use of the User-based Security Model RFC 2274 [12] and the View-based Access Control Model RFC 2275 [15] is recommended. It is then a customer/user responsibility to ensure that the SNMP entity giving access to this MIB, is properly configured to give access to those objects only to those principals (users) that have legitimate rights to access them. 7. Acknowledgements This MIB module was updated based on feedback from the IETF's Interfaces MIB (IF-MIB) and Point-to-Point Protocol Extensions (PPPEXT) Working Groups. 8. Author's Address Dave Thaler Microsoft Corporation One Microsoft Way Redmond, WA 48105-6399 Phone: +1 425 703 8835 EMail: dthaler@microsoft.com Expires January 1999 [Page 14] Draft IP Tunnel MIB July 1998 9. References [1] Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture for Describing SNMP Management Frameworks", RFC 2271, Cabletron Systems, Inc., BMC Software, Inc., IBM T. J. Watson Research, January 1998. [2] Rose, M., and K. McCloghrie, "Structure and Identification of Management Information for TCP/IP-based Internets", RFC 1155, Performance Systems International, Hughes LAN Systems, May 1990. [3] Rose, M., and K. McCloghrie, "Concise MIB Definitions", RFC 1212, Performance Systems International, Hughes LAN Systems, March 1991. [4] M. Rose, "A Convention for Defining Traps for use with the SNMP", RFC 1215, Performance Systems International, March 1991. [5] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Structure of Management Information for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1902, SNMP Research,Inc., Cisco Systems, Inc., Dover Beach Consulting, Inc., International Network Services, January 1996. [6] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Textual Conventions for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1903, SNMP Research, Inc., Cisco Systems, Inc., Dover Beach Consulting, Inc., International Network Services, January 1996. [7] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Conformance Statements for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1904, SNMP Research, Inc., Cisco Systems, Inc., Dover Beach Consulting, Inc., International Network Services, January 1996. [8] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple Network Management Protocol", RFC 1157, SNMP Research, Performance Systems International, Performance Systems International, MIT Laboratory for Computer Science, May 1990. [9] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Introduction to Community-based SNMPv2", RFC 1901, SNMP Research, Inc., Cisco Systems, Inc., Dover Beach Consulting, Inc., International Network Services, January 1996. Expires January 1999 [Page 15] Draft IP Tunnel MIB July 1998 [10] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Transport Mappings for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1906, SNMP Research, Inc., Cisco Systems, Inc., Dover Beach Consulting, Inc., International Network Services, January 1996. [11] Case, J., Harrington D., Presuhn R., and B. Wijnen, "Message Processing and Dispatching for the Simple Network Management Protocol (SNMP)", RFC 2272, SNMP Research, Inc., Cabletron Systems, Inc., BMC Software, Inc., IBM T. J. Watson Research, January 1998. [12] Blumenthal, U., and B. Wijnen, "User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)", RFC 2274, IBM T. J. Watson Research, January 1998. [13] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Protocol Operations for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1905, SNMP Research, Inc., Cisco Systems, Inc., Dover Beach Consulting, Inc., International Network Services, January 1996. [14] Levi, D., Meyer, P., and B. Stewart, "MPv3 Applications", RFC 2273, SNMP Research, Inc., Secure Computing Corporation, Cisco Systems, January 1998. [15] Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP)", RFC 2275, IBM T. J. Watson Research, BMC Software, Inc., Cisco Systems, Inc., January 1998. [16] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 1701, October 1994. [17] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic Routing Encapsulation over IPv4 networks", RFC 1702, October 1994. [18] Perkins, C., "IP Encapsulation within IP", RFC 2003, October 1996. [19] Perkins, C., "Minimal Encapsulation within IP", RFC 2004, October 1996. [20] Hamzeh, Kolar, Littlewood, Pall, Taarud, Valencia, and Verthein, "Layer Two Tunneling Protocol (L2TP)", draft-ietf-pppext-l2tp- 04.txt, June 1997. Expires January 1999 [Page 16] Draft IP Tunnel MIB July 1998 [21] Hamzeh, Pall, Verthein, Taarud, and Little, "Point-to-Point Tunneling Protocol--PPTP", draft-ietf-pppext-pptp-02.txt, July 1997. [22] Calhoun, Reddy, Vroman, and Wheeler. "Layer Two Tunneling Protocol "L2TP" Management Information Base", draft-ietf-pppext-l2tp-mib- 00.txt, October 1997. [23] McCloghrie, K., and F. Kastenholz. "The Interfaces Group MIB using SMIv2", RFC 2233, November 1997. [24] R. Atkinson. "Security architecture for the internet protocol", RFC 1825, August 1995. [25] Valencia, A., Littlewood, M., and T. Kolar. "Cisco Layer Two Forwarding (Protocol) "L2F"", RFC 2341, May 1998. 10. Full Copyright Statement Copyright (C) The Internet Society (1998). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implmentation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. 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Expires January 1999 [Page 17] Draft IP Tunnel MIB July 1998 Table of Contents 1 Introduction .................................................... 1 2 Revision History ................................................ 2 3 The SNMPv2 Network Management Framework ......................... 2 3.1 Object Definitions ............................................ 3 4 Overview ........................................................ 3 4.1 Relationship to the Interfaces MIB ............................ 4 4.1.1 Layering Model .............................................. 4 4.1.2 ifTestTable ................................................. 4 4.1.3 ifRcvAddressTable ........................................... 4 4.1.4 ifEntry ..................................................... 5 5 Definitions ..................................................... 6 6 Security Considerations ......................................... 14 7 Acknowledgements ................................................ 14 8 Author's Address ................................................ 14 9 References ...................................................... 15 10 Full Copyright Statement ....................................... 17 Expires January 1999 [Page 18]