Network Working Group J. Galvin Request for Comments: 1446 Trusted Information Systems K. McCloghrie Hughes LAN Systems April 1993
Status of this Memo
This RFC specifes an IAB standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "IAB Official Protocol Standards" for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Table of Contents
1 Introduction
1.1 A Note on Terminology
1.2 Threats
1.3 Goals and Constraints
1.4 Security Services
1.5 Mechanisms
1.5.1 Message Digest Algorithm
1.5.2 Symmetric Encryption Algorithm
2 SNMPv2 Party
3 Digest Authentication Protocol
3.1 Generating a Message
3.2 Receiving a Message
4 Symmetric Privacy Protocol
4.1 Generating a Message
4.2 Receiving a Message
5 Clock and Secret Distribution
5.1 Initial Configuration
5.2 Clock Distribution
5.3 Clock Synchronization
5.4 Secret Distribution
5.5 Crash Recovery
6 Security Considerations
6.1 Recommended Practices
6.2 Conformance
6.3 Protocol Correctness
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A network management system contains: several (potentially many) nodes, each with a processing entity, termed an agent, which has access to management instrumentation; at least one management station; and, a management protocol, used to convey management information between the agents and management stations. Operations of the protocol are carried out under an administrative framework which defines both authentication and authorization policies.
Network management stations execute management applications which monitor and control network elements. Network elements are devices such as hosts, routers, terminal servers, etc., which are monitored and controlled through access to their management information.
In the Administrative Model for SNMPv2 document [1], each SNMPv2 party is, by definition, associated with a single authentication protocol and a single privacy protocol. It is the purpose of this document, Security Protocols for SNMPv2, to define one such authentication and one such privacy protocol.
The authentication protocol provides a mechanism by which SNMPv2 management communications transmitted by the party may be reliably identified as having originated from that party. The authentication protocol defined in this memo also reliably determines that the message received is the message that was sent.
The privacy protocol provides a mechanism by which SNMPv2 management communications transmitted to said party are protected from disclosure. The privacy protocol in this memo specifies that only authenticated messages may be protected from disclosure.
These protocols are secure alternatives to the so-called "trivial" protocol defined in [2].
USE OF THE TRIVIAL PROTOCOL ALONE DOES NOT CONSTITUTE SECURE NETWORK MANAGEMENT. THEREFORE, A NETWORK MANAGEMENT SYSTEM THAT IMPLEMENTS ONLY THE TRIVIAL PROTOCOL IS NOT CONFORMANT TO THIS SPECIFICATION.
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The Symmetric Privacy Protocol is described in Section 4. It protects messages from disclosure by encrypting their contents according to a secret cryptographic key known only to the originator and recipient. The additional functionality afforded by this protocol is assumed to justify its additional computational cost.
The Digest Authentication Protocol depends on the existence of loosely synchronized clocks between the originator and recipient of a message. The protocol specification makes no assumptions about the strategy by which such clocks are synchronized. Section 5.3 presents one strategy that is particularly suited to the demands of SNMP network management.
Both protocols described here require the sharing of secret information between the originator of a message and its recipient. The protocol specifications assume the existence of the necessary secrets. The selection of such secrets and their secure distribution to appropriate parties may be accomplished by a variety of strategies. Section 5.4 presents one such strategy that is particularly suited to the demands of SNMP network management.
For the purpose of exposition, the original Internet-standard Network Management Framework, as described in RFCs 1155, 1157, and 1212, is termed the SNMP version 1 framework (SNMPv1). The current framework is termed the SNMP version 2 framework (SNMPv2).
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Several of the classical threats to network protocols are applicable to the network management problem and therefore would be applicable to any SNMPv2 security protocol. Other threats are not applicable to the network management problem. This section discusses principal threats, secondary threats, and threats which are of lesser importance.
The principal threats against which any SNMPv2 security protocol should provide protection are:
Modification of Information
The SNMPv2 protocol provides the means for management
stations to interrogate and to manipulate the value of
objects in a managed agent. The modification threat is
the danger that some party may alter in-transit messages
generated by an authorized party in such a way as to
effect unauthorized management operations, including
falsifying the value of an object.
Masquerade
The SNMPv2 administrative model includes an access
control model. Access control necessarily depends on
knowledge of the origin of a message. The masquerade
threat is the danger that management operations not
authorized for some party may be attempted by that party
by assuming the identity of another party that has the
appropriate authorizations.
Two secondary threats are also identified. The security protocols defined in this memo do provide protection against:
Message Stream Modification
The SNMPv2 protocol is based upon a connectionless
transport service which may operate over any subnetwork
service. The re-ordering, delay or replay of messages
can and does occur through the natural operation of many
such subnetwork services. The message stream
modification threat is the danger that messages may be
maliciously re-ordered, delayed or replayed to an extent
which is greater than can occur through the natural
operation of a subnetwork service, in order to effect
unauthorized management operations.
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There are at least two threats that a SNMPv2 security protocol need not protect against. The security protocols defined in this memo do not provide protection against:
Denial of Service
A SNMPv2 security protocol need not attempt to address
the broad range of attacks by which service to authorized
parties is denied. Indeed, such denial-of-service
attacks are in many cases indistinguishable from the type
of network failures with which any viable network
management protocol must cope as a matter of course.
Traffic Analysis
In addition, a SNMPv2 security protocol need not attempt
to address traffic analysis attacks. Indeed, many
traffic patterns are predictable - agents may be managed
on a regular basis by a relatively small number of
management stations - and therefore there is no
significant advantage afforded by protecting against
traffic analysis.
Based on the foregoing account of threats in the SNMP network management environment, the goals of a SNMPv2 security protocol are enumerated below.
(1) The protocol should provide for verification that each received SNMPv2 message has not been modified during its transmission through the network in such a way that an unauthorized management operation might result.
(2) The protocol should provide for verification of the identity of the originator of each received SNMPv2 message.
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(4) The protocol should provide, when necessary, that the contents of each received SNMPv2 message are protected from disclosure.
In addition to the principal goal of supporting secure network management, the design of any SNMPv2 security protocol is also influenced by the following constraints:
(1) When the requirements of effective management in times of network stress are inconsistent with those of security, the former are preferred.
(2) Neither the security protocol nor its underlying security mechanisms should depend upon the ready availability of other network services (e.g., Network Time Protocol (NTP) or secret/key management protocols).
(3) A security mechanism should entail no changes to the basic SNMP network management philosophy.
The security services necessary to support the goals of a SNMPv2 security protocol are as follows.
Data Integrity
is the provision of the property that data has not been
altered or destroyed in an unauthorized manner, nor have
data sequences been altered to an extent greater than can
occur non-maliciously.
Data Origin Authentication
is the provision of the property that the claimed origin
of received data is corroborated.
Data Confidentiality
is the provision of the property that information is not
made available or disclosed to unauthorized individuals,
entities, or processes.
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Further, there is no provision for data confidentiality without both data integrity and data origin authentication.
The security protocols defined in this memo employ several types of mechanisms in order to realize the goals and security services described above:
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The security protocols in this memo are defined independently of the particular choice of a message digest and encryption algorithm - owing principally to the lack of a suitable metric by which to evaluate the security of particular algorithm choices. However, in the interests of completeness and in order to guarantee interoperability, Sections 1.5.1 and 1.5.2 specify particular choices, which are considered acceptably secure as of this writing. In the future, this memo may be updated by the publication of a memo specifying substitute or alternate choices of algorithms, i.e., a replacement for or addition to the sections below.
In support of data integrity, the use of the MD5 [3] message digest algorithm is chosen. A 128-bit digest is calculated over the designated portion of a SNMPv2 message and included as part of the message sent to the recipient.
An appendix of [3] contains a C Programming Language implementation of the algorithm. This code was written with portability being the principal objective. Implementors may wish to optimize the implementation with respect to the characteristics of their hardware and software platforms.
The use of this algorithm in conjunction with the Digest Authentication Protocol (see Section 3) is identified by the ASN.1 object identifier value v2md5AuthProtocol, defined in [4]. (Note that this protocol is a modified version of the md5AuthProtocol protocol defined in RFC 1352.)
For any SNMPv2 party for which the authentication protocol is v2md5AuthProtocol, the size of its private authentication key is 16 octets.
Within an authenticated management communication generated by such a party, the size of the authDigest component of that communication (see Section 3) is 16 octets.
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In support of data confidentiality, the use of the Data Encryption Standard (DES) in the Cipher Block Chaining mode of operation is chosen. The designated portion of a SNMPv2 message is encrypted and included as part of the message sent to the recipient.
Two organizations have published specifications defining the DES: the National Institute of Standards and Technology (NIST) [5] and the American National Standards Institute [6]. There is a companion Modes of Operation specification for each definition (see [7] and [8], respectively).
The NIST has published three additional documents that implementors may find useful.
The use of this algorithm in conjunction with the Symmetric Privacy Protocol (see Section 4) is identified by the ASN.1 object identifier value desPrivProtocol, defined in [4].
For any SNMPv2 party for which the privacy protocol is desPrivProtocol, the size of the private privacy key is 16 octets, of which the first 8 octets are a DES key and the second 8 octets are a DES Initialization Vector. The 64-bit DES key in the first 8 octets of the private key is a 56 bit quantity used directly by the algorithm plus 8 parity bits - arranged so that one parity bit is the least significant bit of each octet. The setting of the parity bits is ignored.
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If the length of the octet sequence to be decrypted is not an integral multiple of 8 octets, the processing of the octet sequence should be halted and an appropriate exception noted. Upon decrypting, the padding should be ignored.
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Recall from [1] that a SNMPv2 party is a conceptual, virtual execution context whose operation is restricted (for security or other purposes) to an administratively defined subset of all possible operations of a particular SNMPv2 entity. A SNMPv2 entity is an actual process which performs network management operations by generating and/or responding to SNMPv2 protocol messages in the manner specified in [12]. Architecturally, every SNMPv2 entity maintains a local database that represents all SNMPv2 parties known to it.
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SnmpParty ::= SEQUENCE { partyIdentity OBJECT IDENTIFIER, partyTDomain OBJECT IDENTIFIER, partyTAddress OCTET STRING, partyMaxMessageSize INTEGER, partyAuthProtocol OBJECT IDENTIFIER, partyAuthClock INTEGER, partyAuthPrivate OCTET STRING, partyAuthPublic OCTET STRING, partyAuthLifetime INTEGER, partyPrivProtocol OBJECT IDENTIFIER, partyPrivPrivate OCTET STRING, partyPrivPublic OCTET STRING }
For each SnmpParty value that represents a SNMPv2 party, the generic significance of each of its components is defined in [1]. For each SNMPv2 party that supports the generation of messages using the Digest Authentication Protocol, additional, special significance is attributed to certain components of that party's representation:
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For each SNMPv2 party that supports the receipt of messages via the Symmetric Privacy Protocol, additional, special significance is attributed to certain components of that party's representation:
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This section describes the Digest Authentication Protocol. It provides both for verifying the integrity of a received message (i.e., the message received is the message sent) and for verifying the origin of a message (i.e., the reliable identification of the originator). The integrity of the message is protected by computing a digest over an appropriate portion of a message. The digest is computed by the originator of the message, transmitted with the message, and verified by the recipient of the message.
A secret value known only to the originator and recipient of the message is prefixed to the message prior to the digest computation. Thus, the origin of the message is known implicitly with the verification of the digest.
A requirement on parties using this Digest Authentication Protocol is that they shall not originate messages for transmission to any destination party which does not also use this Digest Authentication Protocol. This restriction excludes undesirable side effects of communication between a party which uses these security protocols and a party which does not.
Recall from [1] that a SNMPv2 management communication is represented by an ASN.1 value with the following syntax:
SnmpMgmtCom ::= [2] IMPLICIT SEQUENCE { dstParty OBJECT IDENTIFIER, srcParty OBJECT IDENTIFIER, context OBJECT IDENTIFIER, pdu PDUs }
For each SnmpMgmtCom value that represents a SNMPv2 management communication, the following statements are true:
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Recall from [1] that a SNMPv2 authenticated management communication is represented by an ASN.1 value with the following syntax:
SnmpAuthMsg ::= [1] IMPLICIT SEQUENCE { authInfo ANY, - defined by authentication protocol authData SnmpMgmtCom }
For each SnmpAuthMsg value that represents a SNMPv2 authenticated management communication, the following statements are true:
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In support of the Digest Authentication Protocol, an authInfo component is of type AuthInformation:
AuthInformation ::= [2] IMPLICIT SEQUENCE { authDigest OCTET STRING, authDstTimestamp UInteger32, authSrcTimestamp UInteger32 }
For each AuthInformation value that represents authentication information, the following statements are true:
This section describes the behavior of a SNMPv2 entity when it acts as a SNMPv2 party for which the authentication protocol is administratively specified as the Digest Authentication Protocol. Insofar as the behavior of a SNMPv2 entity when transmitting protocol messages is defined generically in [1], only those aspects of that behavior that are specific to the Digest Authentication Protocol are described below. In
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According to Section 3.1 of [1], a SnmpAuthMsg value is constructed during Step 3 of generic processing. In particular, it states the authInfo component is constructed according to the authentication protocol identified for the SNMPv2 party originating the message. When the relevant authentication protocol is the Digest Authentication Protocol, the procedure performed by a SNMPv2 entity whenever a management communication is to be transmitted by a SNMPv2 party is as follows.
(1) The local database is consulted to determine the authentication clock and private authentication key (extracted, for example, according to the conventions defined in Section 1.5.1) of the SNMPv2 party originating the message. The local database is also consulted to determine the authentication clock of the receiving SNMPv2 party.
(2) The authSrcTimestamp component is set to the retrieved authentication clock value of the message's source. The authDstTimestamp component is set to the retrieved authentication clock value of the message's intended recipient.
(3) The authentication digest is temporarily set to the private authentication key of the SNMPv2 party originating the message. The SnmpAuthMsg value is serialized according to the conventions of [13] and [12]. A digest is computed over the octet sequence representing that serialized value using, for example, the algorithm specified in Section 1.5.1. The authDigest component is set to the computed digest value.
As set forth in [1], the SnmpAuthMsg value is then encapsulated according to the appropriate privacy protocol into a SnmpPrivMsg value. This latter value is then serialized and transmitted to the receiving SNMPv2 party.
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This section describes the behavior of a SNMPv2 entity upon receipt of a protocol message from a SNMPv2 party for which the authentication protocol is administratively specified as the Digest Authentication Protocol. Insofar as the behavior of a SNMPv2 entity when receiving protocol messages is defined generically in [1], only those aspects of that behavior that are specific to the Digest Authentication Protocol are described below.
According to Section 3.2 of [1], a SnmpAuthMsg value is evaluated during Step 9 of generic processing. In particular, it states the SnmpAuthMsg value is evaluated according to the authentication protocol identified for the SNMPv2 party that originated the message. When the relevant authentication protocol is the Digest Authentication Protocol, the procedure performed by a SNMPv2 entity whenever a management communication is received by a SNMPv2 party is as follows.
(1) If the ASN.1 type of the authInfo component is not AuthInformation, the message is evaluated as unauthentic, and the snmpStatsBadAuths counter [14] is incremented. Otherwise, the authSrcTimestamp, authDstTimestamp, and authDigest components are extracted from the SnmpAuthMsg value.
(2) The local database is consulted to determine the authentication clock, private authentication key (extracted, for example, according to the conventions defined in Section 1.5.1), and lifetime of the SNMPv2 party that originated the message.
(3) If the authSrcTimestamp component plus the lifetime is less than the authentication clock, the message is evaluated as unauthentic, and the snmpStatsNotInLifetimes counter [14] is incremented.
(4) The authDigest component is extracted and temporarily recorded.
(5) A new SnmpAuthMsg value is constructed such that its authDigest component is set to the private authentication key and its other components are set to the value of the corresponding components in the received SnmpAuthMsg
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NOTE
Because serialization rules are unambiguous but may
not be unique, great care must be taken in
reconstructing the serialized value prior to
computing the digest. Implementations may find it
useful to keep a copy of the original serialized
value and then simply modify the octets which
directly correspond to the placement of the
authDigest component, rather than re-applying the
serialization algorithm to the new SnmpAuthMsg
value.
(6) If the computed digest value is not equal to the digest
value temporarily recorded in step 4 above, the message
is evaluated as unauthentic, and the
snmpStatsWrongDigestValues counter [14] is incremented.
(7) The message is evaluated as authentic.
(8) The local database is consulted for access privileges permitted by the local access policy to the originating SNMPv2 party with respect to the receiving SNMPv2 party. If any level of access is permitted, then:
the authentication clock value locally recorded for the originating SNMPv2 party is advanced to the authSrcTimestamp value if this latter exceeds the recorded value; and,
the authentication clock value locally recorded for the
receiving SNMPv2 party is advanced to the
authDstTimestamp value if this latter exceeds the
recorded value.
(Note that this step is conceptually independent from Steps 15-17 of Section 3.2 in [1]).
If the SnmpAuthMsg value is evaluated as unauthentic, an authentication failure is noted and the received message is
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This section describes the Symmetric Privacy Protocol. It provides for protection from disclosure of a received message. An appropriate portion of the message is encrypted according to a secret key known only to the originator and recipient of the message.
This protocol assumes the underlying mechanism is a symmetric encryption algorithm. In addition, the message to be encrypted must be protected according to the conventions of the Digest Authentication Protocol.
Recall from [1] that a SNMPv2 private management communication is represented by an ASN.1 value with the following syntax:
SnmpPrivMsg ::= [1] IMPLICIT SEQUENCE { privDst OBJECT IDENTIFIER, privData [1] IMPLICIT OCTET STRING }
For each SnmpPrivMsg value that represents a SNMPv2 private management communication, the following statements are true:
This section describes the behavior of a SNMPv2 entity when it communicates with a SNMPv2 party for which the privacy protocol is administratively specified as the Symmetric Privacy Protocol. Insofar as the behavior of a SNMPv2 entity when transmitting a protocol message is defined generically in [1], only those aspects of that behavior that are specific to the Symmetric Privacy Protocol are described below. In
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According to Section 3.1 of [1], a SnmpPrivMsg value is constructed during Step 5 of generic processing. In particular, it states the privData component is constructed according to the privacy protocol identified for the SNMPv2 party receiving the message. When the relevant privacy protocol is the Symmetric Privacy Protocol, the procedure performed by a SNMPv2 entity whenever a management communication is to be transmitted by a SNMPv2 party is as follows.
(1) If the SnmpAuthMsg value is not authenticated according to the conventions of the Digest Authentication Protocol, the generation of the private management communication fails according to a local procedure, without further processing.
(2) The local database is consulted to determine the private privacy key of the SNMPv2 party receiving the message (represented, for example, according to the conventions defined in Section 1.5.2).
(3) The SnmpAuthMsg value is serialized according to the conventions of [13] and [12].
(4) The octet sequence representing the serialized SnmpAuthMsg value is encrypted using, for example, the algorithm specified in Section 1.5.2 and the extracted private privacy key.
(5) The privData component is set to the encrypted value.
As set forth in [1], the SnmpPrivMsg value is then serialized and transmitted to the receiving SNMPv2 party.
This section describes the behavior of a SNMPv2 entity when it acts as a SNMPv2 party for which the privacy protocol is administratively specified as the Symmetric Privacy Protocol. Insofar as the behavior of a SNMPv2 entity when receiving a
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According to Section 3.2 of [1], the privData component of a received SnmpPrivMsg value is evaluated during Step 4 of generic processing. In particular, it states the privData component is evaluated according to the privacy protocol identified for the SNMPv2 party receiving the message. When the relevant privacy protocol is the Symmetric Privacy Protocol, the procedure performed by a SNMPv2 entity whenever a management communication is received by a SNMPv2 party is as follows.
(1) The local database is consulted to determine the private privacy key of the SNMPv2 party receiving the message (represented, for example, according to the conventions defined in Section 1.5.2).
(2) The contents octets of the privData component are decrypted using, for example, the algorithm specified in Section 1.5.2 and the extracted private privacy key.
Processing of the received message continues as specified in [1].
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The protocols described in Sections 3 and 4 assume the existence of loosely synchronized clocks and shared secret values. Three requirements constrain the strategy by which clock values and secrets are distributed.
When the value of an authentication clock is decreased,
messages that have been sent with a timestamp value
between the value of the authentication clock and its new
value may be replayed. Changing the private
authentication key obviates this threat.
Protecting the secrets from disclosure is critical to the security of the protocols. Knowledge of the secrets must be as restricted as possible within an implementation. In particular, although the secrets may be known to one or more persons during the initial configuration of a device, the secrets should be changed immediately after configuration such that their actual value is known only to the software. A management station has the additional responsibility of recovering the state of all parties whenever it boots, and it may address this responsibility by recording the secrets on a long-term storage device. Access to information on this device must be as restricted as is practically possible.
This management station is responsible for ensuring that
all authentication clocks are synchronized and for
changing the secret values when necessary. Although more
than one management station may share this
responsibility, their coordination is essential to the
secure management of the network. The mechanism by which
multiple management stations ensure that no more than one
of them attempts to synchronize the clocks or update the
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A responsible management station may either support clock synchronization and secret distribution as separate functions, or combine them into a single functional unit.
The first section below specifies the procedures by which a SNMPv2 entity is initially configured. The next two sections describe one strategy for distributing clock values and one for determining a synchronized clock value among SNMPv2 parties supporting the Digest Authentication Protocol. For SNMPv2 parties supporting the Symmetric Privacy Protocol, the next section describes a strategy for distributing secret values. The last section specifies the procedures by which a SNMPv2 entity recovers from a "crash."
This section describes the initial configuration of a SNMPv2 entity that supports the Digest Authentication Protocol or both the Digest Authentication Protocol and the Symmetric Privacy Protocol.
When a network device is first installed, its initial, secure configuration must be done manually, i.e., a person must physically visit the device and enter the initial secret values for at least its first secure SNMPv2 party. This requirement suggests that the person will have knowledge of the initial secret values.
In general, the security of a system is enhanced as the number of entities that know a secret is reduced. Requiring a person to physically visit a device every time a SNMPv2 party is configured not only exposes the secrets unnecessarily but is administratively prohibitive. In particular, when MD5 is used, the initial authentication secret is 128 bits long and when DES is used an additional 128 bits are needed - 64 bits each for the key and initialization vector. Clearly, these values will need to be recorded on a medium in order to be transported between a responsible management station and a managed agent. The recommended procedure is to configure a small set of initial SNMPv2 parties for each SNMPv2 entity, one pair of which may be used initially to configure all other SNMPv2 parties.
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The last of these SNMPv2 parties in both the responsible management station and the managed agent could be used to create all other SNMPv2 parties.
Configuring one pair of SNMPv2 parties to be used to configure all other parties has the advantage of exposing only one pair of secrets - the secrets used to configure the minimal, useful set identified above. To limit this exposure, the responsible management station should change these values as its first operation upon completion of the initial configuration. In this way, secrets are known only to the peers requiring knowledge of them in order to communicate.
The Management Information Base (MIB) document [4] supporting these security protocols specifies 6 initial party identities and initial values, which, by convention, are assigned to the parties and their associated parameters.
These 6 initial parties are required to exist as part of the configuration of implementations when first installed, with the exception that implementations not providing support for a privacy protocol only need the 4 initial parties for which the privacy protocol is noPriv. When installing a managed agent, these parties need to be configured with their initial secrets, etc., both in the responsible management station and in the new agent.
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(1) Determine the initial values for each of the attributes of the SNMPv2 party to be configured. Some of these values may be computed by the responsible management station, some may be specified in the MIB document, and some may be administratively determined.
(2) Configure the parties in the responsible management station, according to the set of initial values. If the management station is computing some initial values to be entered into the agent, an appropriate medium must be present to record the values.
(3) Configure the parties in the managed agent, according to the set of initial values.
(4) The responsible management station must synchronize the authentication clock values for each party it shares with each managed agent. Section 5.3 specifies one strategy by which this could be accomplished.
(5) The responsible management station should change the secret values manually configured to ensure the actual values are known only to the peers requiring knowledge of them in order to communicate. To do this, the management station generates new secrets for each party to be reconfigured and distributes the updates using any strategy which protects the new values from disclosure; use of a SNMPv2 set operation acting on the managed objects defined in [4] is such a strategy. Upon receiving positive acknowledgement that the new values have been distributed, the management station should update its local database with the new values.
If the managed agent does not support a protocol that protects messages from disclosure, e.g., the Symmetric Privacy Protocol (see section 5.4), then the distribution of new secrets, after the compromise of existing secrets, is not possible. In this case, the new secrets can only be distributed by a physical
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If there are other SNMPv2 protocol entities requiring knowledge of the secrets, the responsible management station must distribute the information upon completion of the initial configuration. The considerations, mentioned above, concerning the protection of secrets from disclosure, also apply in this case.
A responsible management station must ensure that the authentication clock value for each SNMPv2 party for which it is responsible
The skew among the clock values must be accounted for in the lifetime value, in addition to the expected communication delivery delay.
A skewed authentication clock may be detected by a number of strategies, including knowledge of the accuracy of the system clock, unauthenticated queries of the party database, and recognition of authentication failures originated by the party.
Whenever clock skew is detected, and whenever the SNMPv2 entities at both the responsible management station and the relevant managed agent support an appropriate privacy protocol (e.g., the Symmetric Privacy Protocol), a straightforward strategy for the correction of clock skew is simultaneous alteration of authentication clock and private key for the relevant SNMPv2 party. If the request to alter the key and clock for a particular party originates from that same party, then, prior to transmitting that request, the local notion of the authentication clock is artificially advanced to assure acceptance of the request as authentic.
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In addition to correcting skewed notions of authentication clocks, every SNMPv2 entity must react correctly as an authentication clock approaches its maximal value. If the authentication clock for a particular SNMPv2 party ever reaches the maximal time value, the clock must halt at that value. (The value of interest may be the maximum less lifetime. When authenticating a message, its authentication timestamp is added to lifetime and compared to the authentication clock. A SNMPv2 entity must guarantee that the sum is never greater than the maximal time value.) In this state, the only authenticated request a management station should generate for this party is one that alters the value of at least its authentication clock and private authentication key. In order to reset these values, the responsible management station may set the authentication timestamp in the message to the maximal time value.
The value of the authentication clock for a particular SNMPv2 party must never be altered such that its new value is less than its old value, unless its private authentication key is also altered at the same time.
Unless the secrets are changed at the same time, the correct way to synchronize clocks is to advance the slower clock to be equal to the faster clock. Suppose that party agentParty is realized by the SNMPv2 entity in a managed agent; suppose that party mgrParty is realized by the SNMPv2 entity in the corresponding responsible management station. For any pair of parties, there are four possible conditions of the authentication clocks that could require correction:
(1) The management station's notion of the value of the authentication clock for agentParty exceeds the agent's notion.
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(3) The agent's notion of the value of the authentication clock for agentParty exceeds the management station's notion.
(4) The agent's notion of the value of the authentication clock for mgrParty exceeds the management station's notion.
The selective clock acceleration mechanism intrinsic to the protocol corrects conditions 1, 2 and 3 as part of the normal processing of an authentic message. Therefore, the clock adjustment procedure below does not provide for any adjustments in those cases. Rather, the following sequence of steps specifies how the clocks may be synchronized when condition 4 is manifest.
(1) The responsible management station saves its existing notion of the authentication clock for the party mgrParty.
(2) The responsible management station retrieves the authentication clock value for mgrParty from the agent. This retrieval must be an unauthenticated request, since the management station does not know if the clocks are synchronized. If the request fails, the clocks cannot be synchronized, and the clock adjustment procedure is aborted without further processing.
(3) If the notion of the authentication clock for mgrParty just retrieved from the agent exceeds the management station's notion, then condition 4 is manifest, and the responsible management station advances its notion of the authentication clock for mgrParty to match the agent's notion.
(4) The responsible management station retrieves the authentication clock value for mgrParty from the agent. This retrieval must be an authenticated request, in order that the management station may verify that the clock value is properly synchronized. If this authenticated query fails, then the management station restores its
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Administrative advancement of a clock as described above does not introduce any new vulnerabilities, since the value of the clock is intended to increase with the passage of time. A potential operational problem is the rejection of authentic management operations that were authenticated using a previous value of the relevant party clock. This possibility may be avoided if a management station suppresses generation of management traffic between relevant parties while this clock adjustment procedure is in progress.
This section describes one strategy by which a SNMPv2 entity that supports both the Digest Authentication Protocol and the Symmetric Privacy Protocol can change the secrets for a particular SNMPv2 party.
The frequency with which the secrets of a SNMPv2 party should
be changed is a local administrative issue. However, the more
frequently a secret is used, the more frequently it should be
changed. At a minimum, the secrets must be changed whenever
the associated authentication clock approaches its maximal
value (see Section 6). Note that, owing to both
administrative and automatic advances of the authentication
clock described in this memo, the authentication clock for a
SNMPv2 party may well approach its maximal value sooner than
might otherwise be expected.
The following sequence of steps specifies how a responsible management station alters a secret value (i.e., the private authentication key or the private privacy key) for a particular SNMPv2 party. There are two cases.
First, setting the initial secret for a new party:
(1) The responsible management station generates a new secret value.
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Its source supports the Digest Authentication Protocol and the Symmetric Privacy Protocol.
Its destination supports the Symmetric Privacy Protocol and the Digest Authentication Protocol.
(3) The SNMPv2 private management communication is transmitted to its destination.
(4) Upon receiving the request, the recipient processes the message according to [12] and [1].
(5) The recipient encapsulates a SNMPv2 response in a SNMPv2 private management communication with at least the following properties.
Its source supports the Digest Authentication Protocol and the Symmetric Privacy Protocol.
Its destination supports the Symmetric Privacy Protocol and the Digest Authentication Protocol.
(6) The SNMPv2 private management communication is transmitted to its destination.
(7) Upon receiving the response, the responsible management station updates its local database with the new value.
Second, modifying the current secret of an existing party:
(1) The responsible management station generates a new secret value.
(2) The responsible management station encapsulates a SNMPv2 setRequest in a SNMPv2 management communication with at least the following properties.
Its source and destination supports the Digest Authentication Protocol.
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(4) Upon receiving the request, the recipient processes the message according to [12] and [1].
(5) The recipient encapsulates a SNMPv2 response in a SNMPv2 management communication with at least the following properties.
Its source and destination supports the Digest Authentication Protocol.
(6) The SNMPv2 management communication is transmitted to its destination.
(7) Upon receiving the response, the responsible management station updates its local database with the new value.
If the responsible management station does not receive a response to its request, there are two possible causes.
In order to distinguish the two possible error conditions, a responsible management station could check the destination to see if the change has occurred. Unfortunately, since the secret values are unreadable, this is not directly possible.
The recommended strategy for verifying key changes is to set the public value corresponding to the secret being changed to a recognizable, novel value: that is, alter the public authentication key value for the relevant party when changing its private authentication key, or alter its public privacy key value when changing its private privacy key. In this way, the responsible management station may retrieve the public value when a response is not received, and verify whether or not the change has taken place. (This strategy is available since the public values are not used by the protocols defined in this memo. If this strategy is employed, then the public values are significant in this context. Of course, protocols
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One other scenario worthy of mention is using a SNMPv2 party to change its own secrets. In this case, the destination will change its local database prior to generating a response. Thus, the response will be constructed according to the new value. However, the responsible management station will not update its local database until after the response is received. This suggests the responsible management station may receive a response which will be evaluated as unauthentic, unless the correct secret is used. The responsible management station may either account for this scenario as a special case, or use an alteration of the relevant public values (as described above) to verify the key change.
Note, during the period of time after the request has been sent and before the response is received, the management station must keep track of both the old and new secret values. Since the delay may be the result of a network failure, the management station must be prepared to retain both values for an extended period of time, including across reboots.
This section describes the requirements for SNMPv2 protocol entities in connection with recovery from system crashes or other service interruptions.
For each SNMPv2 party in the local database for a particular SNMPv2 entity, its identity, authentication clock, private authentication key, and private privacy key must enjoy non- volatile, incorruptible representations. If possible, lifetime should also enjoy a non-volatile, incorruptible representation. If said SNMPv2 entity supports other security protocols or algorithms in addition to the two defined in this memo, then the authentication protocol and the privacy protocol for each party also require non-volatile, incorruptible representation.
The authentication clock of a SNMPv2 party is a critical component of the overall security of the protocols. The inclusion of a reliable representation of a clock in a SNMPv2 entity is required for overall security. A reliable clock
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If a managed agent crashes and does not reboot in time for its responsible management station to prevent its authentication clock from reaching its maximal value, upon reboot the clock must be halted at its maximal value. The procedures specified in Section 5.3 would then apply.
Upon recovery, those attributes of each SNMPv2 party that do not enjoy non-volatile or reliable representation are initialized as follows.
Upon detecting that a managed agent has rebooted, a responsible management station must reset all other party attributes, including the lifetime if it was not retained. In order to reset the lifetime, the responsible management station should set the authentication timestamp in the message to the sum of the authentication clock and desired lifetime. This is an artificial advancement of the authentication timestamp in order to guarantee the message will be authentic
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This section highlights security considerations relevant to the protocols and procedures defined in this memo. Practices that contribute to secure, effective operation of the mechanisms defined here are described first. Constraints on implementation behavior that are necessary to the security of the system are presented next. Finally, an informal account of the contribution of each mechanism of the protocols to the required goals is presented.
This section describes practices that contribute to the secure, effective operation of the mechanisms defined in this memo.
Although it would be typical for a management station to do this as a matter of course, in the context of these security protocols it is significant owing to the possibility of message duplication (malicious or otherwise).
It is possible for authentication failure traps to be lost or suppressed as a result of authentication clock skew or inconsistent notions of shared secrets. In order either to facilitate administration of such SNMPv2 parties or to provide for continued management in times of network stress, a management station implementation may provide for arbitrary, artificial advancement of the timestamp or selection of shared secrets on locally generated messages.
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A large lifetime increases the vulnerability to malicious delays of SNMPv2 messages. The implementation of a management station may accommodate changing network conditions during periods of network stress by effectively increasing the lifetimes of the source and destination parties. The management station accomplishes this by artificially advancing its notion of the source party's clock on messages it sends, and by artificially increasing its notion of the source party`s lifetime on messages it receives.
No message ordering is imposed by the SNMPv2. Messages may be received in any order relative to their time of generation and each will be processed in the ordered received. Note that when an authenticated message is sent to a managed agent, it will be valid for a period of time that does not exceed lifetime under normal circumstances, and is subject to replay during this period.
Indeed, a management station must cope with the loss and re-ordering of messages resulting from anomalies in the network as a matter of course.
However, a managed object, snmpSetSerialNo [14], is specifically defined for use with SNMPv2 set operations in order to provide a mechanism to ensure the processing of SNMPv2 messages occurs in a specific order.
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Changing a secret after each use is generally regarded as the most secure practice, but a significant amount of overhead may be associated with that approach.
Note, too, in a local environment the threat of disclosure may be insignificant, and as such the changing of secrets may be less frequent. However, when public data networks are the communication paths, more caution is prudent.
Owing to the use of symmetric cryptography in the protocols defined here, the secrets associated with a particular SNMPv2 party must be known to all other SNMPv2 parties with which that party may wish to communicate. As the number of locations at which secrets are known and used increases, the likelihood of their disclosure also increases, as does the potential impact of that disclosure. Moreover, if the set of SNMPv2 protocol entities with knowledge of a particular secret numbers more than two, data origin cannot be reliably authenticated because it is impossible to determine with any assurance which entity of that set may be the originator of a particular SNMPv2 message. Thus, the greatest degree of security is afforded by configurations in which the secrets for each SNMPv2 party are known to at most two protocol entities.
A SNMPv2 entity implementation that claims conformance to this memo must satisfy the following requirements:
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noAuth This protocol signifies that messages generated by a party using it are not protected as to origin or integrity. It is required to ensure that a party's authentication clock is always accessible.
noPriv This protocol signifies that messages received by a party using it are not protected from disclosure. It is required to ensure that a party's authentication clock is always accessible.
(2) It must implement the Digest Authentication Protocol in conjunction with the algorithm defined in Section 1.5.1.
(3) It must include in its local database at least one SNMPv2 party with the following parameters set as follows:
partyAuthProtocol is set to noAuth and
partyPrivProtocol is set to noPriv.
This party must have a MIB view [1] specified that includes at least the authentication clock of all other parties. Alternatively, the authentication clocks of the other parties may be partitioned among several similarly configured parties according to a local implementation convention.
(4) For each SNMPv2 party about which it maintains information in a local database, an implementation must satisfy the following requirements:
(a) It must not allow a party's parameters to be set to a value inconsistent with its expected syntax. In particular, Section 1.4 specifies constraints for the chosen mechanisms.
(b) It must, to the maximal extent possible, prohibit read-access to the private authentication key and private encryption key under all circumstances except as required to generate and/or validate SNMPv2 messages with respect to that party.
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(c) It must allow the party's authentication clock to be publicly accessible. The correct operation of the Digest Authentication Protocol requires that it be possible to determine this value at all times in order to guarantee that skewed authentication clocks can be resynchronized.
(d) It must prohibit alterations to its record of
the authentication clock for that party
independently of alterations to its record of the
private authentication key (unless the clock
alteration is an advancement).
(e) It must never allow its record of the authentication clock for that party to be incremented beyond the maximal time value and so "roll-over" to zero.
(f) It must never increase its record of the lifetime for that party except as may be explicitly authorized (via imperative command or securely represented configuration information) by the responsible network administrator.
(g) In the event that the non-volatile,
incorruptible representations of a party's
parameters (in particular, either the private
authentication key or private encryption key) are
lost or destroyed, it must alter its record of these
quantities to random values so subsequent
interaction with that party requires manual
redistribution of new secrets and other parameters.
(5) If it selects new value(s) for a party's secret(s), it must avoid bad or obvious choices for said secret(s). Choices to be avoided are boundary values (such as all- zeros) and predictable values (such as the same value as previously or selecting from a predetermined set).
(6) It must ensure that a received message for which the originating party uses the Digest Authentication Protocol but the receiving party does not, is always declared to
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The correctness of these SNMPv2 security protocols with respect to the stated goals depends on the following assumptions:
(1) The chosen message digest algorithm satisfies its design criteria. In particular, it must be computationally infeasible to discover two messages that share the same digest value.
(2) It is computationally infeasible to determine the secret used in calculating a digest on the concatenation of the secret and a message when both the digest and the message are known.
(3) The chosen symmetric encryption algorithm satisfies its
design criteria. In particular, it must be
computationally infeasible to determine the cleartext
message from the ciphertext message without knowledge of
the key used in the transformation.
(4) Local notions of a party's authentication clock while it
is associated with a specific private key value are
monotonically non-decreasing (i.e., they never run
backwards) in the absence of administrative
manipulations.
(5) The secrets for a particular SNMPv2 party are known only to authorized SNMPv2 protocol entities.
(6) Local notions of the authentication clock for a particular SNMPv2 party are never altered such that the authentication clock's new value is less than the current value without also altering the private authentication key.
For each mechanism of the protocol, an informal account of its contribution to the required goals is presented below.
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By pairing each sequence of a clock's values with a unique
key, the protocols partially realize goal 3, and the
conjunction of this property with assumption 6 above is
sufficient for the claim that, with respect to a specific
private key value, all local notions of a party's
authentication clock are, in general, non-decreasing with
time.
The protocols require computation of a message digest computed over the SNMPv2 message prepended by the secret for the relevant party. By virtue of this mechanism and assumptions 1 and 2, the protocols realize goal 1.
Normally, the inclusion of the message digest value with the digested message would not be sufficient to guarantee data integrity, since the digest value can be modified in addition to the message while it is enroute. However, since not all of the digested message is included in the transmission to the destination, it is not possible to substitute both a message and a digest value while enroute to a destination.
Strictly speaking, the specified strategy for data integrity does not detect a SNMPv2 message modification which appends extraneous material to the end of such messages. However, owing to the representation of SNMPv2 messages as ASN.1 values, such modifications cannot - consistent with goal 1 - result in unauthorized management operations.
The data integrity mechanism specified in this memo protects only against unauthorized modification of individual SNMPv2 messages. A more general data integrity service that affords protection against the threat of message stream modification is not realized by this mechanism, although limited protection against reordering, delay, and duplication of messages within a message stream are provided by other mechanisms of the
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The data integrity mechanism requires the use of a secret value known only to communicating parties. By virtue of this mechanism and assumptions 1 and 2, the protocols explicitly prevent unauthorized modification of messages. Data origin authentication is implicit if the message digest value can be verified. That is, the protocols realize goal 2.
This memo requires that implementations preclude
administrative alterations of the authentication clock for a
particular party independently from its private authentication
key (unless that clock alteration is an advancement). An
example of an efficient implementation of this restriction is
provided in a pseudocode fragment below. This pseudocode
fragment meets the requirements of assumption 6. Observe that
the requirement is not for simultaneous alteration but to
preclude independent alteration. This latter requirement is
fairly easily realized in a way that is consistent with the
defined semantics of the SNMPv2 set operation.
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{ if (party->clockAltered) { party->clockAltered = FALSE; party->keyAltered = FALSE; party->keyInUse = newKeyValue; party->clockInUse = party->clockCache; } else { party->keyAltered = TRUE; party->keyCache = newKeyValue; } }
Void partySetClock (party, newClockValue)
{ if (party->keyAltered) { party->keyAltered = FALSE; party->clockAltered = FALSE; party->clockInUse = newClockValue; party->keyInUse = party->keyCache; } else { party->clockAltered = TRUE; party->clockCache = newClockValue; } }
The definition of the SNMPv2 security protocols requires that, if the authentication timestamp value on a received message - augmented by an administratively chosen lifetime value - is less than the local notion of the clock for the originating SNMPv2 party, the message is not delivered.
if (timestampOfReceivedMsg +
party->administrativeLifetime <=
party->localNotionOfClock) {
msgIsValidated = FALSE;
}
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The definition of the SNMPv2 security protocols requires that, if either of the timestamp values for the originating or receiving parties on a received, validated message exceeds the corresponding local notion of the clock for that party, then the local notion of the clock for that party is adjusted forward to correspond to said timestamp value. This mechanism is neither strictly necessary nor sufficient to the security of the protocol; rather, it fosters the clock synchronization on which valid message delivery depends - thereby enhancing the effectiveness of the protocol in a management context.
if (msgIsValidated) {
if (timestampOfReceivedMsg >
party->localNotionOfClock) {
party->localNotionOfClock =
timestampOfReceivedMsg;
}
}
The effect of this mechanism is to synchronize local notions of a party clock more closely in the case where a sender's notion is more advanced than a receiver's. In the opposite case, this mechanism has no effect on local notions of a party clock and either the received message is validly delivered or not according to other mechanisms of the protocol.
Operation of this mechanism does not, in general, improve the probability of validated delivery for messages generated by party participants whose local notion of the party clock is relatively less advanced. In this case, queries from a management station may not be validly delivered and the
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The protocols require the use of a symmetric encryption algorithm when the data confidentiality service is required. By virtue of this mechanism and assumption 3, the protocols realize goal 4.
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This document is based, almost entirely, on RFC 1352.
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[1] Galvin, J., and McCloghrie, K., "Administrative Model for version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1445, Trusted Information Systems, Hughes LAN Systems, April 1993.
[2] Case, J., Fedor, M., Schoffstall, M., Davin, J., "Simple Network Management Protocol", STD 15, RFC 1157, SNMP Research, Performance Systems International, MIT Laboratory for Computer Science, May 1990.
[3] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, MIT Laboratory for Computer Science, April 1992.
[4] McCloghrie, K., and Galvin, J., "Party MIB for version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1447, Hughes LAN Systems, Trusted Information Systems, April 1993.
[5] Data Encryption Standard, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 46-1. Supersedes FIPS Publication 46, (January, 1977; reaffirmed January, 1988).
[6] Data Encryption Algorithm, American National Standards Institute. ANSI X3.92-1981, (December, 1980).
[7] DES Modes of Operation, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 81, (December, 1980).
[8] Data Encryption Algorithm - Modes of Operation, American National Standards Institute. ANSI X3.106-1983, (May 1983).
[9] Guidelines for Implementing and Using the NBS Data Encryption Standard, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 74, (April, 1981).
[10] Validating the Correctness of Hardware Implementations of the NBS Data Encryption Standard, National Institute of Standards and Technology. Special Publication 500-20.
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[12] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S., "Protocol Operations for version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1448, SNMP Research, Inc., Hughes LAN Systems, Dover Beach Consulting, Inc., Carnegie Mellon University, April 1993.
[13] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S., "Transport Mappings for version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1449, SNMP Research, Inc., Hughes LAN Systems, Dover Beach Consulting, Inc., Carnegie Mellon University, April 1993.
[14] Case, J., McCloghrie, K., Rose, M., and Waldbusser, S., "Management Information Base for version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1450, SNMP Research, Inc., Hughes LAN Systems, Dover Beach Consulting, Inc., Carnegie Mellon University, April 1993.
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James M. Galvin
Trusted Information Systems, Inc.
3060 Washington Road, Route 97
Glenwood, MD 21738
Phone: +1 301 854-6889
EMail: galvin@tis.com
Keith McCloghrie
Hughes LAN Systems
1225 Charleston Road
Mountain View, CA 94043
US
Phone: +1 415 966 7934
Email: kzm@hls.com
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