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Versions: 00 01

Network Working Group                                   J. Schoenwaelder
Internet-Draft                                           TU Braunschweig
Expires: October 10, 2001                                 April 11, 2001


                        SNMP Payload Compression
                draft-irtf-nmrg-snmp-compression-01.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

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   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
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   This Internet-Draft will expire on October 10, 2001.

Copyright Notice

   Copyright (C) The Internet Society (2001).  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 being used over a
   transport which provides data compression.











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Table of Contents

   1.      Introduction . . . . . . . . . . . . . . . . . . . . . . .  3
   2.      Requirements . . . . . . . . . . . . . . . . . . . . . . .  3
   3.      Identifying Compressed Payload . . . . . . . . . . . . . .  4
   3.1     Compression as an SNMPv3 Encryption Algorithm  . . . . . .  4
   3.2     Indicating Compression in the msgFlags . . . . . . . . . .  4
   3.3     Compression as a new PDU Type  . . . . . . . . . . . . . .  5
   4.      Negotiating Compression Algorithms . . . . . . . . . . . .  6
   5.      Compression Algorithms . . . . . . . . . . . . . . . . . .  6
   5.1     DEFLATE  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.2     OID Delta Compression (ODC)  . . . . . . . . . . . . . . .  7
   5.2.1   ODC Algorithm  . . . . . . . . . . . . . . . . . . . . . .  7
   5.2.2   ODC Examples . . . . . . . . . . . . . . . . . . . . . . . 10
   5.2.2.1 Simple Substitutions . . . . . . . . . . . . . . . . . . . 10
   5.2.2.2 Range Substitutions  . . . . . . . . . . . . . . . . . . . 11
   5.2.2.3 Substitutions and Truncations  . . . . . . . . . . . . . . 12
   6.      Acknowledgments  . . . . . . . . . . . . . . . . . . . . . 14
           References . . . . . . . . . . . . . . . . . . . . . . . . 14
           Author's Address . . . . . . . . . . . . . . . . . . . . . 14
           Full Copyright Statement . . . . . . . . . . . . . . . . . 15






























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1. Introduction

   This memo defines mechanisms 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 payload tends to have a high degree of redundancy.

   SNMP payload compression is especially useful when retrieving large
   amounts of data.  In particular, compression allows command
   responders to put more data into responses to bulk retrieval
   requests, which can significantly reduce the overall number of
   transactions needed to retrieve a certain amount of data [5].

   SNMP payload compression is also 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, then compression should be applied before
   encryption.

2. Requirements

   A solution for SNMP payload compression has to satisfy the following
   requirements:

   1.  Compression must happen before encryption if compression is used
       together with encryption.  Compression is most useful if there
       are regular patterns in the data.  It is the nature of encryption
       algorithms to destroy any regular pattern and hence encrypted
       data does not compress very well.

   2.  SNMP payload compression should be able to support multiple
       compression algorithms.  This implies that SNMP engines must be
       able to negotiate the compression algorithm they are using.
       Instead of carrying a 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).

   3.  Each SNMP message is compressed and decompressed by itself
       without any relation to other SNMP messages ("stateless
       compression"), as SNMP messages may arrive out of order or not
       arrive at all.

   4.  The size of a compressed SNMP message must never exceed the size
       of the uncompressed SNMP message ("non-expansion policy").  A
       sender may send an uncompressed SNMP message if an attempt to
       compress the message produces a result which is longer than the



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       uncompressed SNMP message.  An SNMP engine configured to receive
       compressed SNMP messages must thus also accept uncompressed SNMP
       messages.

   5.  The abstract syntax of compressed SNMP messages must be defined
       using ASN.1.  This ensures that a compressed SNMP message has a
       valid ASN.1/BER encoding which simplifies the integration into
       existing SNMP toolkits.

   6.  It is desirable to define common compression algorithms in order
       to achieve interoperability.  The compression algorithms should
       be openly available and they should represent different trade-
       offs between compression efficiency and CPU efficiency.


3. Identifying Compressed Payload

   It is necessary to distinguish between compressed and uncompressed
   SNMP payload.  There are several ways to implement this.  The
   following sections discuss alternatives that have been considered.

3.1 Compression as an SNMPv3 Encryption Algorithm

   The idea behind the first approach is to treat compression as an
   additional SNMPv3 encryption algorithm.  In fact, compression as well
   as encryption can both be treated as a loss-less data transformation.
   This approach has the following advantages / disadvantages:

   +  No change required to the SNMPv3 specifications.

   +  Compression of the complete ScopedPDU.

   -  Support of N encryption algorithms and M compression algorithms
      leads to N*M possible combinations.

   -  Compression requires authentication since there is no noAuthPriv
      security level.

   -  Does not work with older and widely deployed versions of SNMP
      (SNMPv1, SNMPv2c).


3.2 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:



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   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 in the msgFlags are set to zero
   or ignored.  This means that this alternative can not be supported by
   an implementation which is strictly compliant to RFC 2572 [3].

   In summary, this approach has the following advantages /
   disadvantages:

   +  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.

   -  Not strictly compliant to the current SNMPv3 specifications.

   -  Does not work with older widely deployed versions of SNMP (SNMPv1,
      SNMPv2c).


3.3 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,
           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



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   version of a PDU:

       CompressedPDU ::= [42] IMPLICIT OCTET STRING
                       -- contains a compressed PDU

   It is 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:

   +  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.

   +  Works with every version of SNMP.

   -  Not strictly compliant to the current SNMPv3 specifications.

   -  Compression of the PDU rather than the ScopedPDU.


4. Negotiating Compression Algorithms

   [TBD]

5. Compression Algorithms





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5.1 DEFLATE

   The DEFLATE algorithm is a well-know compression algorithm which
   achieves good compression ratios and which is used for example in RFC
   2394 for IP payload compression.  It is also used on the World Wide
   Web to compress documents before transmission over HTTP.

   Using DEFLATE for SNMP payload compression however shows some
   undesirable aspects.  First, DEFLATE compression is relatively CPU
   intensive.  Furthermore, the DEFLATE algorithm requires to transmit a
   dictionary, which reduces the compression efficiency for small
   messages.  On the other hand, DEFLATE compresses both names and
   values and may achieve particularly good compression if the encoded
   values show a similar structure.

   Experiments with DEFLATE show that it should not be used on
   relatively short SNMP messages.  Furthermore, DEFLATE introduces
   significant delays due to the compression computations.  Finally,
   estimating message sizes is hard with DEFLATE since there is no upper
   bound for the message length addition if one adds another varbind.
   This has impacts in particular on the getbulk PDU implementation.  It
   is therefore not recommended to adopt DEFLATE compression.

5.2 OID Delta Compression (ODC)

   SNMP payloads use OIDs to represent the names of SNMP variables.  The
   amount of space used for encoding these OIDs frequently exceeds the
   space needed to represent the values identified by the OIDs.
   Furthermore, subsequent OIDs usually have many sub-identifier in
   common.

   The OID Delta Compression (ODC) algorithm has been designed to reduce
   the OID overhead inherent in SNMP messages.  The general idea is to
   encode an OID as a delta to the previous OID.  The ODC algorithm only
   achieves a reduction in the SNMP naming information.  It does not
   compress the data of MIB variables, even if the value itself is an
   OID.  (The reason for not compressing OID values is that they are (a)
   relatively infrequent and (b) compressing value OIDs has negative
   impact on the compression achieved for the following variable name.)
   In many cases (e.g., when retrieving large tables which basically
   contain numbers), the achieved compression ratio is significant while
   the CPU cycles spent on the compression algorithm itself are very
   small.

5.2.1 ODC Algorithm

   The ODC algorithm encodes OID deltas using three mechansisms:




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   1.  Substitution of a single sub-identifier values at a certain
       position.  A sub-identifier substitution is encoded as follows:

        0             7 8
       +---------------+--------------------------//-+
       |    s-offset   | BER encoded sub-identifier  |
       +---------------+--------------------------//-+

       s-offset      Defines the offset of the sub-identifier to
                     substitute. The offset value is in the range
                     0-0x7f. The value 0 refers to the first OID
                     sub-identifier.

   2.  Substitution of ranges of sub-identifiers at a given starting
       position.  A substitution of a range of sub-identifiers is
       encoded as follows:

        0             7 8            15 16
       +---------------+---------------+--------------------------//-+
       |    r-offset   |   r-length    | BER encoded sub-identifiers |
       +---------------+---------------+--------------------------//-+

       r-offset      Defines the offset of the sub-identifier range
                     to substitute. The offset value has the most
                     significant bit set and is in the range
                     0x80-0xff. The value 0x80 refers to the first
                     OID sub-identifier.
       r-length      Defines the number of BER encoded sub-identifiers
                     that follow the r-length field and which will
                     be substituted. The range of the r-length field
                     is 0x01-0x7f.

   3.  Truncation of OIDs (which may shorten or enlarge OIDs).  A
       truncation, which may only appear at the end, is encoded as
       follows:

        0             7
       +---------------+
       |    t-length   |
       +---------------+

       t-length      Defines the new length of the OID in the range
                     0x01-0x7f. The t-length value specifies the number
                     if sub-identifiers minus 1. Hence, the value 0x01
                     identifies an OID which consists of two
                     sub-identifiers. Truncation may be used to shorten
                     or enlarge an OID. New sub-identifiers will have
                     the value 0 if an OID is enlarged.



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   An ODC compressed OID is simply the combination of several sub-
   identifier or sub-identifier range substitutions followed by an
   optional truncation.  Note that substitutions can increase the size
   of the OID if the offset or range specifies sub-identifier positions
   outside of the previous OID.  New sub-identifiers without an explicit
   value assignement will have the value 0.  An ODC compressed OID is
   distinguished from a normal OID by introducing the following new
   ASN.1 type:

                   CompOID ::= [42] IMPLICIT OCTET STRING

   A high-level description of the compression algorithm is as follows:

   1.  Loop through the SNMP PDU until you find an OID name value pair
       (varbind).

   2.  If it is the first varbind, make a copy of the OID, pass it to
       the output buffer and continue with the next varbind.

   3.  Otherwise, compute the delta to the last OID and BER encode it
       into the CompOID value.

   4.  If the CompOID representation is larger than the OID, pass the
       OID to the output buffer, else pass the CompOID to the output
       buffer.

   5.  Update the last OID and goto step 2 if there are additional
       varbinds.

   Some SNMP implementations use a reverse encoding scheme where PDUs
   are encoded from the end to the beginning.  The ODC algorithm can
   also be used efficiently in this case by using an OID look-ahead of 1
   varbind.

   A high-level description of the decompression algorithm is as
   follows:

   1.  Loop through the SNMP PDU until you find an OID name value pair
       (varbind).

   2.  If the varbind name contains an uncompressed OID, pass it to the
       output buffer and continue with the next varbind.

   3.  Otherwise, if the varbind name contains a compressed OID, loop
       through the compressed OID value doing the following:

       1.  If the first byte is in the range 0-0x7f and there are more
           bytes, then decode the following byte(s) as a BER encoded



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           sub-identifier and perform a sub-identifier substitution.

       2.  If the first byte is in the range 0x80-0xff, then read the
           following byte as the r-length value.  Decode the following
           r-length BER encoded sub-identifier and perform a range
           substitution.

       3.  If the first byte is in the range 0x01-0x7f and there are no
           more bytes, then perform a truncation.

   4.  Pass the decoded OID to the output buffer.

   5.  Update the last OID and goto step 2 if there are additional
       varbinds.


5.2.2 ODC Examples

   This section shows some example ODC encodings.  It is provided to
   better understand how ODC encodings work.  It is not intended to give
   a formal analysis of the compression ratios that can be achieved on
   arbitrary SNMP payload.

5.2.2.1 Simple Substitutions

   Lets assume a command generator uses getbulk operations to retrieve
   the tcpConnTable of the TCP-MIB.  A good manager will do that by
   retrieving the tcpConnState column.  The command responder will
   return a sequence of tcpConnState (1.3.6.1.2.1.6.13.1.1) values:

   tcpConnState.0.0.0.0.21.0.0.0.0.0 = listen(2)
   tcpConnState.0.0.0.0.22.0.0.0.0.0 = listen(2)
   tcpConnState.0.0.0.0.23.0.0.0.0.0 = listen(2)
   tcpConnState.0.0.0.0.98.0.0.0.0.0 = listen(2)

   The BER encoding of this varbind list is the following sequence of
   bytes:

   30:18                                   // sequence
      06:13:2B:06:01:02:01:06:0D:01:01:    // tcpConnState
            00:00:00:00:15:00:00:00:00:00  // instance identifier
      02:01:02                             // value
   30:18                                   // sequence
      06:13:2B:06:01:02:01:06:0D:01:01:    // tcpConnState
            00:00:00:00:16:00:00:00:00:00  // instance identifier
      02:01:02                             // value
   30:18                                   // sequence
      06:13:2B:06:01:02:01:06:0D:01:01:    // tcpConnState



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            00:00:00:00:17:00:00:00:00:00  // instance identifier
      02:01:02                             // value
   30:18                                   // sequence
      06:13:2B:06:01:02:01:06:0D:01:01:    // tcpConnState
            00:00:00:00:62:00:00:00:00:00  // instance identifier
      02:01:02                             // value

   Using ODC compression, the following sequence of bytes would be used:

   30:18                                   // sequence
      06:13:2B:06:01:02:01:06:0D:01:01:    // tcpConnState
            00:00:00:00:15:00:00:00:00:00  // instance identifier
      02:01:02                             // value
   30:07                                   // sequence
      2a:02:0E:16                          // substitution
      02:02:02                             // value
   30:07                                   // sequence
      2a:02:0E:17                          // substitution
      02:01:02                             // value
   30:07                                   // sequence
      2a:02:0E:62                          // substitution
      02:01:02                             // value

   In this particular example, the total size of the encoded varbind
   list drops from 104 bytes to 53 bytes.

5.2.2.2 Range Substitutions

   This example expands the previous example by showing how range
   substitutions work.  In this example, we assume that the command
   responder will return a sequence of tcpConnState values with
   different IP addresses in the instance identifier:

   tcpConnState.134.169.34.190.50054.130.240.64.53.80 = closeWait(8)
   tcpConnState.134.169.34.190.50366.212.185.76.85.80 = closeWait(8)
   tcpConnState.134.169.34.190.53370.134.169.34.117.6000 = established(5)
   tcpConnState.134.169.34.190.56398.134.169.34.190.120 = closeWait(8)

   The BER encoding of this varbind list is the following sequence of
   bytes:

   30:1F                                   // sequence
      06:1A:2B:06:01:02:01:06:0D:01:01:    // tcpConnState
         81:06:81:29:22:81:3E:83:87:06:    // instance
         81:02:81:70:40:35:50              // identifier
      02:01:08                             // value
   30:1F                                   // sequence
      06:1A:2B:06:01:02:01:06:0D:01:01:    // tcpConnState



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         81:06:81:29:22:81:3E:83:89:3E:    // instance
         81:54:81:39:4C:55:50              // identifier
      02:01:08                             // value
   30:20                                   // sequence
      06:1B:2B:06:01:02:01:06:0D:01:01:    // tcpConnState
         81:06:81:29:22:81:3E:83:A0:7A:    // instance
         81:06:81:29:22:75:AE:70           // identifier
      02:01:05                             // value
   30:20                                   // sequence
      06:1B:2B:06:01:02:01:06:0D:01:01:    // tcpConnState
         81:06:81:29:22:81:3E:83:B8:4E:    // instance
         81:06:81:29:22:81:3E:78           // identifier
      02:01:08                             // value

   Using ODC compression, the following sequence of bytes would be used:

   30:1F                                   // sequence
      06:1A:2B:06:01:02:01:06:0D:01:01:    // tcpConnState
         81:06:81:29:22:81:3E:83:87:06:    // instance
         81:02:81:70:40:35:50              // identifier
      02:01:08                             // value
   30:10                                   // sequence
      2A:0B:8E:05:                         // range
      83:89:3E:81:54:81:39:4C:55           // substitution
      02:01:08                             // value
   30:12                                   // sequence
      2A:0D:8E:06:                         // range
      83:A0:7A:81:06:81:29:22:75:AE:70     // substitution
      02:01:05                             // value
   30:0E                                   // sequence
      2A:09:0E:83:A0:7A                    // substitution
      92:02:81:3E:78                       // range substitution
      02:01:08                             // value

   In this particular example, the total size of the encoded varbind
   list drops from 134 bytes to 87 bytes.

5.2.2.3 Substitutions and Truncations

   This example assumes that a command generator retrieves the
   ipNetToMediaTable defined in the IP-MIB using a getbulk on
   ipNetToMediaPhysAddress (1.3.6.1.2.1.4.22.1.2) and ipNetToMediaType
   (1.3.6.1.2.1.4.22.1.4) pairs.  The following values might be returned
   for a system which only has one entry in the cache:

   ipNetToMediaPhysAddress.2.224.8.8.0 = 01:00:5E:08:08:00
   ipNetToMediaType.2.224.8.8.0 = dynamic(3)
   ipNetToMediaNetAddress.2.224.8.8.0 = 224.8.8.0



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   ipRoutingDiscards.0 = 0

   Note that the getbulk overshoots and retrieves the following
   instances in lexicographic order, which is an ipNetToMediaNetAddress
   (1.3.6.1.2.1.4.22.1.3) and an ipRoutingDiscards (1.3.6.1.2.1.4.23)
   instance.

   The BER encoding of this varbind list is the following sequence of
   bytes:

   30:19                                   // sequence
      06:0F:2B:06:01:02:01:04:16:01:02:    // ipNetToMediaPhysAddress
         02:81:60:08:08:00                 // instance identifier
      04:06:01:00:5E:08:08:00              // value
   30:14                                   // sequence
      06:0F:2B:06:01:02:01:04:16:01:04:    // ipNetToMediaType
         02:81:60:08:08:00                 // instance identifier
      02:01:03                             // value
   30:17                                   // sequence
      06:10:2B:06:01:02:01:04:16:01:03:    // ipNetToMediaNetAddress
         02:81:60:08:08:00                 // instance identifier
      40:04:E0:08:08:00                    // value
   30:0D                                   // sequence
      06:08:2B:06:01:02:01:04:17           // ipRoutingDiscards
      00                                   // instance identifier
      41:01:00                             // value

   Using ODC compression, the following sequence of bytes would be used:

   30:19                                   // sequence
      06:0F:2B:06:01:02:01:04:16:01:02:    // ipNetToMediaPhysAddress
         02:81:60:08:08:00                 // instance identifier
      04:06:01:00:5E:08:08:00              // value
   30:07                                   // sequence
      2A:02:09:04                          // substitution
      02:01:03                             // value
   30:0A                                   // sequence
      2A:02:09:04                          // substitutions
      40:04:E0:08:08:00                    // value
   30:0A                                   // sequence
      2A:05:87:02:23:00:                   // range substitution
         08                                // truncation
      41:01:00                             // value

   In this particular example, the total size of the encoded varbind
   list drops from 89 bytes to 60 bytes.





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6. Acknowledgments

   This document is the result of discussions within the Network
   Management Research Group (NMRG).  of the Internet Research Task
   Force[6] (IRTF).  Special thanks go to Luca Deri, Wes Hardacker,
   Jean-Philippe Martin-Flatin, Joe Marzot, Aiko Pras, Ron Sprenkels,
   Frank Strauss, and Bert Wijnen for their comments and suggestions.

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)", STD 15, RFC 1157, May 1990.

   [5]  Sprenkels, R. and J. Martin-Flatin, "Bulk Transfers of MIB
        Data", Simple Times 7(1), March 1999.

   [6]  <http://www.irtf.org/>


Author's Address

   Juergen Schoenwaelder
   TU Braunschweig
   Bueltenweg 74/75
   38106 Braunschweig
   Germany

   Phone: +49 531 391-3289
   EMail: schoenw@ibr.cs.tu-bs.de












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