Network Working Group J. Schoenwaelder, Editor Internet-Draft TU Braunschweig Expires December 1999 20 June 1999 SNMP Payload Compression Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Distribution of this document is unlimited. Please send comments to the Network Management Research Group . Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. Abstract This memo defines a mechanism for lossless compression of SNMP payloads. Compression is especially useful when retrieving large amounts of data or when SNMP encryption is used. J. Schoenwaelder [Page 1] Internet-Draft SNMP Payload Compression June 1999 Table of Contents 1 Introduction ................................................. 3 2 Requirements and Alternatives ................................ 3 2.1 Compression as an SNMPv3 Encryption Algorithm .............. 3 2.2 Indicating Compression in the msgFlags ..................... 4 2.3 Compression as a new PDU type .............................. 4 3 Acknowledgments .............................................. 6 4 References ................................................... 6 5 Editor's Address ............................................. 6 6 Full Copyright Statement ..................................... 7 J. Schoenwaelder [Page 2] Internet-Draft SNMP Payload Compression June 1999 1. Introduction This memo defines a mechanism for lossless compression of SNMP payloads. Compression is useful when retrieving large amounts of management data since the BER encoding used by SNMP is not very space efficient and the data tends to have a high degree of redundancy. SNMP payload compression is especially useful when SNMP encryption is used. Encrypting the SNMP payload causes the data to be random in nature, rendering compression at lower protocol layers (e.g., IP Payload Compression Protocol [2]) ineffective. If both compression and encryption are required, compression MUST be applied before encryption. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [1]. 2. Requirements and Alternatives A solution for SNMP payload compression has to satisfy the following requirements: - Compression must happen before encryption if compression is used together with encryption. Compression is most useful if there are regular pattern in the data. It is the nature of encryption algorithms to destroy any regular pattern and hence encrypted data does not compress very well. - SNMP payload compression should support multiple compression algorithms. This means that communicating SNMP engines must be able to find agreement on the compression algorithm they are using. Instead of carrying compression algorithm identifier in every protocol message, it seems more effective to indicate compression algorithms in a MIB module (similar to authentication or encryption algorithms in SNMPv3). 2.1. Compression as an SNMPv3 Encryption Algorithm The basic idea behind the first alternative is to treat compression as an SNMPv3 encryption algorithm. This has the following advantages / disadvantages: + No change required to the SNMPv3 specifications. - Support of N encryption algorithms and M compression algorithms leads to N*M possible combinations. J. Schoenwaelder [Page 3] Internet-Draft SNMP Payload Compression June 1999 - Compression requires authentication since there is no noAuthPriv security level. + Compression of the complete ScopedPDU. - Does not work with older versions of SNMP (SNMPv1, SNMPv2c). 2.2. Indicating Compression in the msgFlags To avoid some of the drawbacks of the previous approach, one can treat compression independent of encryption by allocating an unused bit in the msgFlags [3] to indicate whether compression is used or not. However, RFC 2572 [3] says in section 6.4: The remaining bits in msgFlags are reserved, and MUST be set to zero when sending a message and SHOULD be ignored when receiving a message. Similarly, RFC 2572 [3] specifies in section 7.1 step 7) and in section 7.2 step 5) that other bits msgFlags are set to zero or ignored. This means that this alternative can not be supported by an implementation which is compliant to RFC 2572 [3]. In summary, this approach has the following advantages / disadvantages: - Not strictly compliant to the current SNMPv3 specifications. + Combination of M compression and N encryption algorithms possible without having to define N*M algorithms. + Compression can be used with or without encryption or authentication. + Compression of the complete ScopedPDU. - Does not work with older versions of SNMP (SNMPv1, SNMPv2c). 2.3. Compression as a new PDU type The third alternative is to restrict compression to PDUs rather than ScopedPDUs and to introduce a new PDU type for compressed payloads. RFC 1157 [4] defines the SNMPv1 message header as follows: Message ::= SEQUENCE { version INTEGER { version-1(0) }, community OCTET STRING, J. Schoenwaelder [Page 4] Internet-Draft SNMP Payload Compression June 1999 data ANY -- e.g., PDUs if trivial authentication -- is being used } Similarly, RFC 2572 [3] defines the ScopedPDU as follows: ScopedPDU ::= SEQUENCE { contextEngineID OCTET STRING, contextName OCTET STRING, data ANY -- e.g., PDUs as defined in RFC 1905 } This means that a new PDU could be defined which holds the compressed version of a PDU: CompressedPDU ::= [42] IMPLICIT OCTET STRING -- contains a compressed PDU Its important to analyze how a compliant SNMP implementation behaves when it receives an unknown PDU type. From a formal point of view, any PDU which is a valid BER serialization of an ASN.1 type must be accepted since the data portion is of the ASN.1 type ANY. In practice, most SNMP implementations will only recognize the PDU types defined in the SNMP specifications. The SNMPv3 message processing model [3] defines in section 7.2 step 7) that parse errors while decoding the ScopedPDU cause the packet to be discarded after incrementing snmpInASNParseErrs. Even an implementation which is capable to decode arbitrary PDUs will have problems to determine the pduType as defined in section 7.2 step 9). This basically means that a compliant SNMPv3 engine will simply discard compressed PDUs. The SNMPv1 specification [4] defines in section 4.1 second step (4) that parse errors while decoding the PDU will cause the SNMP engine to drop the PDU. Hence, it can be expected that most implementations will simply drop a compressed PDU. In summary, this approach has the following advantages / disadvantages: - Not strictly compliant to the current SNMPv3 specifications. + Combination of M compression and N encryption algorithms possible without having to define N*M algorithms. + Compression can be used with or without encryption or authentication. J. Schoenwaelder [Page 5] Internet-Draft SNMP Payload Compression June 1999 - Compression of the PDU rather than the ScopedPDU. + Works with every version of SNMP. 3. Acknowledgments This document is the result of discussions of the Network Management Research Group (NMRG). Special thanks go to the following participants for their comments and contributions: Luca Deri, Jean-Philippe Martin-Flatin, Aiko Pras, Ron Sprenkels, Bert Wijnen. 4. References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Shacham, A., Monsour, R., Pereira, R. and M. Thomas, "IP Payload Compression Protocol (IPComp)", RFC 2393, December 1998. [3] Case, J., Harrington, D., Presuhn, R. and B. Wijnen, "Message Processing and Dispatching for the Simple Network Management Protocol (SNMP)", RFC 2572, April 1999. [4] Case, J., Fedor, M., Schoffstall, M. and J. Davin, "A Simple Network Management Protocol (SNMP)", RFC 1157, May 1990. 5. Editor's Address Juergen Schoenwaelder TU Braunschweig Bueltenweg 74/75 38106 Braunschweig Germany Phone: +49 531 391-3283 EMail: schoenw@ibr.cs.tu-bs.de J. Schoenwaelder [Page 6] Internet-Draft SNMP Payload Compression June 1999 6. Full Copyright Statement Copyright (C) The Internet Society (1999). 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 implementation 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. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. J. Schoenwaelder [Page 7]