Network Working Group J. Schoenwaelder, Editor
Internet-Draft TU Braunschweig
Expires December 1999 20 June 1999
SNMP Payload Compression
<draft-irtf-nmrg-snmp-compression-00.txt>
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 <nmrg@ibr.cs.tu-bs.de>.
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]
