RFC1903
Network Working Group SNMPv2 Working Group Request for Comments: 1903 J. Case Obsoletes: 1443 SNMP Research, Inc. Category: Standards Track K. McCloghrie
Cisco Systems, Inc.
M. Rose
Dover Beach Consulting, Inc.
S. Waldbusser
International Network Services
January 1996
Textual Conventions
for Version 2 of the
Simple Network Management Protocol (SNMPv2)
Status of this Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Contents
Introduction
A 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
authentication, authorization, access control, and privacy policies.
Management stations execute management applications which monitor and control managed elements. Managed elements are devices such as hosts, routers, terminal servers, etc., which are monitored and controlled via access to their management information.
Management information is viewed as a collection of managed objects, residing in a virtual information store, termed the Management Information Base (MIB). Collections of related objects are defined in MIB modules. These modules are written using a subset of OSI's Abstract Syntax Notation One (ASN.1) [1], termed the Structure of Management Information (SMI) [2].
When designing a MIB module, it is often useful to define new types similar to those defined in the SMI. In comparison to a type defined in the SMI, each of these new types has a different name, a similar syntax, but a more precise semantics. These newly defined types are termed textual conventions, and are used for the convenience of humans reading the MIB module. It is the purpose of this document to define the initial set of textual conventions available to all MIB modules.
Objects defined using a textual convention are always encoded by means of the rules that define their primitive type. However, textual conventions often have special semantics associated with them. As such, an ASN.1 macro, TEXTUAL-CONVENTION, is used to concisely convey the syntax and semantics of a textual convention.
For all textual conventions defined in an information module, the name shall be unique and mnemonic, and shall not exceed 64 characters in length. (However, names longer than 32 characters are not recommended.) All names used for the textual conventions defined in all "standard" information modules shall be unique.
A Note on Terminology
For the purpose of exposition, the original Internet-standard Network Management Framework, as described in RFCs 1155 (STD 16), 1157 (STD 15), and 1212 (STD 16), is termed the SNMP version 1 framework (SNMPv1). The current framework is termed the SNMP version 2 framework (SNMPv2).
Definitions
SNMPv2-TC DEFINITIONS ::= BEGIN
IMPORTS
ObjectSyntax, TimeTicks
FROM SNMPv2-SMI;
-- definition of textual conventions
TEXTUAL-CONVENTION MACRO ::= BEGIN
TYPE NOTATION ::=
DisplayPart
"STATUS" Status
"DESCRIPTION" Text
ReferPart
"SYNTAX" Syntax
VALUE NOTATION ::=
value(VALUE Syntax)
DisplayPart ::=
"DISPLAY-HINT" Text
| empty
Status ::=
"current"
| "deprecated"
| "obsolete"
ReferPart ::=
"REFERENCE" Text
| empty
-- uses the NVT ASCII character set Text ::= """" string """"
Syntax ::=
type(ObjectSyntax)
| "BITS" "{" Kibbles "}"
Kibbles ::=
Kibble
| Kibbles "," Kibble
Kibble ::=
identifier "(" nonNegativeNumber ")"
END
DisplayString ::= TEXTUAL-CONVENTION
DISPLAY-HINT "255a"
STATUS current
DESCRIPTION
"Represents textual information taken from the NVT ASCII
character set, as defined in pages 4, 10-11 of RFC 854.
To summarize RFC 854, the NVT ASCII repertoire specifies:
- the use of character codes 0-127 (decimal)
- the graphics characters (32-126) are interpreted as
US ASCII
- NUL, LF, CR, BEL, BS, HT, VT and FF have the special
meanings specified in RFC 854
- the other 25 codes have no standard interpretation
- the sequence 'CR LF' means newline
- the sequence 'CR NUL' means carriage-return
- an 'LF' not preceded by a 'CR' means moving to the
same column on the next line.
- the sequence 'CR x' for any x other than LF or NUL is
illegal. (Note that this also means that a string may
end with either 'CR LF' or 'CR NUL', but not with CR.)
Any object defined using this syntax may not exceed 255
characters in length."
SYNTAX OCTET STRING (SIZE (0..255))
PhysAddress ::= TEXTUAL-CONVENTION
DISPLAY-HINT "1x:"
STATUS current
DESCRIPTION
"Represents media- or physical-level addresses."
SYNTAX OCTET STRING
MacAddress ::= TEXTUAL-CONVENTION
DISPLAY-HINT "1x:"
STATUS current
DESCRIPTION
"Represents an 802 MAC address represented in the
`canonical' order defined by IEEE 802.1a, i.e., as if it
were transmitted least significant bit first, even though
802.5 (in contrast to other 802.x protocols) requires MAC
addresses to be transmitted most significant bit first."
SYNTAX OCTET STRING (SIZE (6))
TruthValue ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Represents a boolean value."
SYNTAX INTEGER { true(1), false(2) }
TestAndIncr ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Represents integer-valued information used for atomic
operations. When the management protocol is used to specify
that an object instance having this syntax is to be
modified, the new value supplied via the management protocol
must precisely match the value presently held by the
instance. If not, the management protocol set operation
fails with an error of `inconsistentValue'. Otherwise, if
the current value is the maximum value of 2^31-1 (2147483647
decimal), then the value held by the instance is wrapped to
zero; otherwise, the value held by the instance is
incremented by one. (Note that regardless of whether the
management protocol set operation succeeds, the variable-
binding in the request and response PDUs are identical.)
The value of the ACCESS clause for objects having this
syntax is either `read-write' or `read-create'. When an
instance of a columnar object having this syntax is created,
any value may be supplied via the management protocol.
When the network management portion of the system is re-
initialized, the value of every object instance having this
syntax must either be incremented from its value prior to
the re-initialization, or (if the value prior to the re-
initialization is unknown) be set to a pseudo-randomly
generated value."
SYNTAX INTEGER (0..2147483647)
AutonomousType ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Represents an independently extensible type identification
value. It may, for example, indicate a particular sub-tree
with further MIB definitions, or define a particular type of
protocol or hardware."
SYNTAX OBJECT IDENTIFIER
InstancePointer ::= TEXTUAL-CONVENTION
STATUS obsolete
DESCRIPTION
"A pointer to either a specific instance of a MIB object or
a conceptual row of a MIB table in the managed device. In
the latter case, by convention, it is the name of the
particular instance of the first accessible columnar object
in the conceptual row.
The two uses of this textual convention are replaced by
VariablePointer and RowPointer, respectively."
SYNTAX OBJECT IDENTIFIER
VariablePointer ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"A pointer to a specific object instance. For example,
sysContact.0 or ifInOctets.3."
SYNTAX OBJECT IDENTIFIER
RowPointer ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Represents a pointer to a conceptual row. The value is the
name of the instance of the first accessible columnar object
in the conceptual row.
For example, ifIndex.3 would point to the 3rd row in the
ifTable (note that if ifIndex were not-accessible, then
ifDescr.3 would be used instead)."
SYNTAX OBJECT IDENTIFIER
RowStatus ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"The RowStatus textual convention is used to manage the
creation and deletion of conceptual rows, and is used as the
value of the SYNTAX clause for the status column of a
conceptual row (as described in Section 7.7.1 of [2].)
The status column has six defined values:
- `active', which indicates that the conceptual row is
available for use by the managed device;
- `notInService', which indicates that the conceptual
row exists in the agent, but is unavailable for use by
the managed device (see NOTE below);
- `notReady', which indicates that the conceptual row
exists in the agent, but is missing information
necessary in order to be available for use by the
managed device;
- `createAndGo', which is supplied by a management
station wishing to create a new instance of a
conceptual row and to have its status automatically set
to active, making it available for use by the managed
device;
- `createAndWait', which is supplied by a management
station wishing to create a new instance of a
conceptual row (but not make it available for use by
the managed device); and,
- `destroy', which is supplied by a management station
wishing to delete all of the instances associated with
an existing conceptual row.
Whereas five of the six values (all except `notReady') may
be specified in a management protocol set operation, only
three values will be returned in response to a management
protocol retrieval operation: `notReady', `notInService' or
`active'. That is, when queried, an existing conceptual row
has only three states: it is either available for use by
the managed device (the status column has value `active');
it is not available for use by the managed device, though
the agent has sufficient information to make it so (the
status column has value `notInService'); or, it is not
available for use by the managed device, and an attempt to
make it so would fail because the agent has insufficient
information (the state column has value `notReady').
NOTE WELL
This textual convention may be used for a MIB table,
irrespective of whether the values of that table's
conceptual rows are able to be modified while it is
active, or whether its conceptual rows must be taken
out of service in order to be modified. That is, it is
the responsibility of the DESCRIPTION clause of the
status column to specify whether the status column must
not be `active' in order for the value of some other
column of the same conceptual row to be modified. If
such a specification is made, affected columns may be
changed by an SNMP set PDU if the RowStatus would not
be equal to `active' either immediately before or after
processing the PDU. In other words, if the PDU also
contained a varbind that would change the RowStatus
value, the column in question may be changed if the
RowStatus was not equal to `active' as the PDU was
received, or if the varbind sets the status to a value
other than 'active'.
Also note that whenever any elements of a row exist, the
RowStatus column must also exist.
To summarize the effect of having a conceptual row with a
status column having a SYNTAX clause value of RowStatus,
consider the following state diagram:
STATE
+--------------+-----------+-------------+-------------
| A | B | C | D
| |status col.|status column|
|status column | is | is |status column
ACTION |does not exist| notReady | notInService| is active
+--------------+-----------+-------------+-------------
set status |noError ->D|inconsist- |inconsistent-|inconsistent- column to | or | entValue| Value| Value createAndGo |inconsistent- | | |
| Value| | |
+--------------+-----------+-------------+-------------
set status |noError see 1|inconsist- |inconsistent-|inconsistent- column to | or | entValue| Value| Value createAndWait |wrongValue | | |
+--------------+-----------+-------------+-------------
set status |inconsistent- |inconsist- |noError |noError column to | Value| entValue| | active | | | |
| | or | |
| | | |
| |see 2 ->D| ->D| ->D
+--------------+-----------+-------------+-------------
set status |inconsistent- |inconsist- |noError |noError ->C column to | Value| entValue| | notInService | | | |
| | or | | or
| | | |
| |see 3 ->C| ->C|wrongValue
+--------------+-----------+-------------+-------------
set status |noError |noError |noError |noError column to | | | | destroy | ->A| ->A| ->A| ->A
+--------------+-----------+-------------+-------------
set any other |see 4 |noError |noError |see 5 column to some| | | | value | | see 1| ->C| ->D
+--------------+-----------+-------------+-------------
(1) goto B or C, depending on information available to the
agent.
(2) if other variable bindings included in the same PDU,
provide values for all columns which are missing but
required, then return noError and goto D.
(3) if other variable bindings included in the same PDU,
provide values for all columns which are missing but
required, then return noError and goto C.
(4) at the discretion of the agent, the return value may be
either:
inconsistentName: because the agent does not choose to
create such an instance when the corresponding
RowStatus instance does not exist, or
inconsistentValue: if the supplied value is
inconsistent with the state of some other MIB object's
value, or
noError: because the agent chooses to create the
instance.
If noError is returned, then the instance of the status
column must also be created, and the new state is B or C,
depending on the information available to the agent. If
inconsistentName or inconsistentValue is returned, the row
remains in state A.
(5) depending on the MIB definition for the column/table,
either noError or inconsistentValue may be returned.
NOTE: Other processing of the set request may result in a
response other than noError being returned, e.g.,
wrongValue, noCreation, etc.
Conceptual Row Creation
There are four potential interactions when creating a
conceptual row: selecting an instance-identifier which is
not in use; creating the conceptual row; initializing any
objects for which the agent does not supply a default; and,
making the conceptual row available for use by the managed
device.
Interaction 1: Selecting an Instance-Identifier
The algorithm used to select an instance-identifier varies
for each conceptual row. In some cases, the instance-
identifier is semantically significant, e.g., the
destination address of a route, and a management station
selects the instance-identifier according to the semantics.
In other cases, the instance-identifier is used solely to
distinguish conceptual rows, and a management station
without specific knowledge of the conceptual row might
examine the instances present in order to determine an
unused instance-identifier. (This approach may be used, but
it is often highly sub-optimal; however, it is also a
questionable practice for a naive management station to
attempt conceptual row creation.)
Alternately, the MIB module which defines the conceptual row
might provide one or more objects which provide assistance
in determining an unused instance-identifier. For example,
if the conceptual row is indexed by an integer-value, then
an object having an integer-valued SYNTAX clause might be
defined for such a purpose, allowing a management station to
issue a management protocol retrieval operation. In order
to avoid unnecessary collisions between competing management
stations, `adjacent' retrievals of this object should be
different.
Finally, the management station could select a pseudo-random
number to use as the index. In the event that this index
was already in use and an inconsistentValue was returned in
response to the management protocol set operation, the
management station should simply select a new pseudo-random
number and retry the operation.
A MIB designer should choose between the two latter
algorithms based on the size of the table (and therefore the
efficiency of each algorithm). For tables in which a large
number of entries are expected, it is recommended that a MIB
object be defined that returns an acceptable index for
creation. For tables with small numbers of entries, it is
recommended that the latter pseudo-random index mechanism be
used.
Interaction 2: Creating the Conceptual Row
Once an unused instance-identifier has been selected, the
management station determines if it wishes to create and
activate the conceptual row in one transaction or in a
negotiated set of interactions.
Interaction 2a: Creating and Activating the Conceptual Row
The management station must first determine the column
requirements, i.e., it must determine those columns for
which it must or must not provide values. Depending on the
complexity of the table and the management station's
knowledge of the agent's capabilities, this determination
can be made locally by the management station. Alternately,
the management station issues a management protocol get
operation to examine all columns in the conceptual row that
it wishes to create. In response, for each column, there
are three possible outcomes:
- a value is returned, indicating that some other
management station has already created this conceptual
row. We return to interaction 1.
- the exception `noSuchInstance' is returned,
indicating that the agent implements the object-type
associated with this column, and that this column in at
least one conceptual row would be accessible in the MIB
view used by the retrieval were it to exist. For those
columns to which the agent provides read-create access,
the `noSuchInstance' exception tells the management
station that it should supply a value for this column
when the conceptual row is to be created.
- the exception `noSuchObject' is returned, indicating
that the agent does not implement the object-type
associated with this column or that there is no
conceptual row for which this column would be
accessible in the MIB view used by the retrieval. As
such, the management station can not issue any
management protocol set operations to create an
instance of this column.
Once the column requirements have been determined, a
management protocol set operation is accordingly issued.
This operation also sets the new instance of the status
column to `createAndGo'.
When the agent processes the set operation, it verifies that
it has sufficient information to make the conceptual row
available for use by the managed device. The information
available to the agent is provided by two sources: the
management protocol set operation which creates the
conceptual row, and, implementation-specific defaults
supplied by the agent (note that an agent must provide
implementation-specific defaults for at least those objects
which it implements as read-only). If there is sufficient
information available, then the conceptual row is created, a
`noError' response is returned, the status column is set to
`active', and no further interactions are necessary (i.e.,
interactions 3 and 4 are skipped). If there is insufficient
information, then the conceptual row is not created, and the
set operation fails with an error of `inconsistentValue'.
On this error, the management station can issue a management
protocol retrieval operation to determine if this was
because it failed to specify a value for a required column,
or, because the selected instance of the status column
already existed. In the latter case, we return to
interaction 1. In the former case, the management station
can re-issue the set operation with the additional
information, or begin interaction 2 again using
`createAndWait' in order to negotiate creation of the
conceptual row.
NOTE WELL
Regardless of the method used to determine the column
requirements, it is possible that the management
station might deem a column necessary when, in fact,
the agent will not allow that particular columnar
instance to be created or written. In this case, the
management protocol set operation will fail with an
error such as `noCreation' or `notWritable'. In this
case, the management station decides whether it needs
to be able to set a value for that particular columnar
instance. If not, the management station re-issues the
management protocol set operation, but without setting
a value for that particular columnar instance;
otherwise, the management station aborts the row
creation algorithm.
Interaction 2b: Negotiating the Creation of the Conceptual
Row
The management station issues a management protocol set
operation which sets the desired instance of the status
column to `createAndWait'. If the agent is unwilling to
process a request of this sort, the set operation fails with
an error of `wrongValue'. (As a consequence, such an agent
must be prepared to accept a single management protocol set
operation, i.e., interaction 2a above, containing all of the
columns indicated by its column requirements.) Otherwise,
the conceptual row is created, a `noError' response is
returned, and the status column is immediately set to either
`notInService' or `notReady', depending on whether it has
sufficient information to make the conceptual row available
for use by the managed device. If there is sufficient
information available, then the status column is set to
`notInService'; otherwise, if there is insufficient
information, then the status column is set to `notReady'.
Regardless, we proceed to interaction 3.
Interaction 3: Initializing non-defaulted Objects
The management station must now determine the column
requirements. It issues a management protocol get operation
to examine all columns in the created conceptual row. In
the response, for each column, there are three possible
outcomes:
- a value is returned, indicating that the agent
implements the object-type associated with this column
and had sufficient information to provide a value. For
those columns to which the agent provides read-create
access (and for which the agent allows their values to
be changed after their creation), a value return tells
the management station that it may issue additional
management protocol set operations, if it desires, in
order to change the value associated with this column.
- the exception `noSuchInstance' is returned,
indicating that the agent implements the object-type
associated with this column, and that this column in at
least one conceptual row would be accessible in the MIB
view used by the retrieval were it to exist. However,
the agent does not have sufficient information to
provide a value, and until a value is provided, the
conceptual row may not be made available for use by the
managed device. For those columns to which the agent
provides read-create access, the `noSuchInstance'
exception tells the management station that it must
issue additional management protocol set operations, in
order to provide a value associated with this column.
- the exception `noSuchObject' is returned, indicating
that the agent does not implement the object-type
associated with this column or that there is no
conceptual row for which this column would be
accessible in the MIB view used by the retrieval. As
such, the management station can not issue any
management protocol set operations to create an
instance of this column.
If the value associated with the status column is
`notReady', then the management station must first deal with
all `noSuchInstance' columns, if any. Having done so, the
value of the status column becomes `notInService', and we
proceed to interaction 4.
Interaction 4: Making the Conceptual Row Available
Once the management station is satisfied with the values
associated with the columns of the conceptual row, it issues
a management protocol set operation to set the status column
to `active'. If the agent has sufficient information to
make the conceptual row available for use by the managed
device, the management protocol set operation succeeds (a
`noError' response is returned). Otherwise, the management
protocol set operation fails with an error of
`inconsistentValue'.
NOTE WELL
A conceptual row having a status column with value
`notInService' or `notReady' is unavailable to the
managed device. As such, it is possible for the
managed device to create its own instances during the
time between the management protocol set operation
which sets the status column to `createAndWait' and the
management protocol set operation which sets the status
column to `active'. In this case, when the management
protocol set operation is issued to set the status
column to `active', the values held in the agent
supersede those used by the managed device.
If the management station is prevented from setting the
status column to `active' (e.g., due to management station
or network failure) the conceptual row will be left in the
`notInService' or `notReady' state, consuming resources
indefinitely. The agent must detect conceptual rows that
have been in either state for an abnormally long period of
time and remove them. It is the responsibility of the
DESCRIPTION clause of the status column to indicate what an
abnormally long period of time would be. This period of
time should be long enough to allow for human response time
(including `think time') between the creation of the
conceptual row and the setting of the status to `active'.
In the absense of such information in the DESCRIPTION
clause, it is suggested that this period be approximately 5
minutes in length. This removal action applies not only to
newly-created rows, but also to previously active rows which
are set to, and left in, the notInService state for a
prolonged period exceeding that which is considered normal
for such a conceptual row.
Conceptual Row Suspension
When a conceptual row is `active', the management station
may issue a management protocol set operation which sets the
instance of the status column to `notInService'. If the
agent is unwilling to do so, the set operation fails with an
error of `wrongValue'. Otherwise, the conceptual row is
taken out of service, and a `noError' response is returned.
It is the responsibility of the DESCRIPTION clause of the
status column to indicate under what circumstances the
status column should be taken out of service (e.g., in order
for the value of some other column of the same conceptual
row to be modified).
Conceptual Row Deletion
For deletion of conceptual rows, a management protocol set
operation is issued which sets the instance of the status
column to `destroy'. This request may be made regardless of
the current value of the status column (e.g., it is possible
to delete conceptual rows which are either `notReady',
`notInService' or `active'.) If the operation succeeds,
then all instances associated with the conceptual row are
immediately removed."
SYNTAX INTEGER {
-- the following two values are states:
-- these values may be read or written
active(1),
notInService(2),
-- the following value is a state:
-- this value may be read, but not written
notReady(3),
-- the following three values are
-- actions: these values may be written,
-- but are never read
createAndGo(4),
createAndWait(5),
destroy(6)
}
TimeStamp ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"The value of the sysUpTime object at which a specific
occurrence happened. The specific occurrence must be
defined in the description of any object defined using this
type."
SYNTAX TimeTicks
TimeInterval ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"A period of time, measured in units of 0.01 seconds."
SYNTAX INTEGER (0..2147483647)
DateAndTime ::= TEXTUAL-CONVENTION
DISPLAY-HINT "2d-1d-1d,1d:1d:1d.1d,1a1d:1d"
STATUS current
DESCRIPTION
"A date-time specification.
field octets contents range
----- ------ -------- -----
1 1-2 year 0..65536
2 3 month 1..12
3 4 day 1..31
4 5 hour 0..23
5 6 minutes 0..59
6 7 seconds 0..60
(use 60 for leap-second)
7 8 deci-seconds 0..9
8 9 direction from UTC '+' / '-'
9 10 hours from UTC 0..11
10 11 minutes from UTC 0..59
For example, Tuesday May 26, 1992 at 1:30:15 PM EDT would be
displayed as:
1992-5-26,13:30:15.0,-4:0
Note that if only local time is known, then timezone
information (fields 8-10) is not present."
SYNTAX OCTET STRING (SIZE (8 | 11))
StorageType ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Describes the memory realization of a conceptual row. A
row which is volatile(2) is lost upon reboot. A row which
is either nonVolatile(3), permanent(4) or readOnly(5), is
backed up by stable storage. A row which is permanent(4)
can be changed but not deleted. A row which is readOnly(5)
cannot be changed nor deleted.
If the value of an object with this syntax is either
permanent(4) or readOnly(5), it cannot be modified.
Conversely, if the value is either other(1), volatile(2) or
nonVolatile(3), it cannot be modified to be permanent(4) or
readOnly(5).
Every usage of this textual convention is required to
specify the columnar objects which a permanent(4) row must
at a minimum allow to be writable."
SYNTAX INTEGER {
other(1), -- eh?
volatile(2), -- e.g., in RAM
nonVolatile(3), -- e.g., in NVRAM
permanent(4), -- e.g., partially in ROM
readOnly(5) -- e.g., completely in ROM
}
TDomain ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Denotes a kind of transport service.
Some possible values, such as snmpUDPDomain, are defined in
'Transport Mappings for Version 2 of the Simple Network
Management Protocol (SNMPv2)'."
SYNTAX OBJECT IDENTIFIER
TAddress ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Denotes a transport service address.
For snmpUDPDomain, a TAddress is 6 octets long, the initial 4
octets containing the IP-address in network-byte order and the
last 2 containing the UDP port in network-byte order. Consult
'Transport Mappings for Version 2 of the Simple Network
Management Protocol (SNMPv2)' for further information on
snmpUDPDomain."
SYNTAX OCTET STRING (SIZE (1..255))
END
Mapping of the TEXTUAL-CONVENTION macro
The TEXTUAL-CONVENTION macro is used to convey the syntax and semantics associated with a textual convention. It should be noted that the expansion of the TEXTUAL-CONVENTION macro is something which conceptually happens during implementation and not during run-time.
For all descriptors appearing in an information module, the descriptor shall be unique and mnemonic, and shall not exceed 64 characters in length. (However, descriptors longer than 32 characters are not recommended.) Further, the hyphen is not allowed as a character in the name of any textual convention.
Mapping of the DISPLAY-HINT clause
The DISPLAY-HINT clause, which need not be present, gives a hint as to how the value of an instance of an object with the syntax defined using this textual convention might be displayed. The DISPLAY-HINT clause may be present if and only if the syntax has an underlying primitive type of INTEGER or OCTET STRING. (Note, however, that the semantics defined for a particular syntax can cause the use of DISPLAY-HINT for that syntax to make no sense, e.g., for Counter32 [2].)
When the syntax has an underlying primitive type of INTEGER, the hint consists of an integer-format specification, containing two parts. The first part is a single character suggesting a display format, either: 'x' for hexadecimal, or 'd' for decimal, or 'o' for octal, or 'b' for binary. The second part is always omitted for 'x', 'o' and
'b', and need not be present for 'd'. If present, the second part starts with a hyphen and is followed by a decimal number, which defines the implied decimal point when rendering the value. For example:
Hundredths ::= TEXTUAL-CONVENTION
DISPLAY-HINT "d-2"
...
SYNTAX INTEGER (0..10000)
suggests that a Hundredths value of 1234 be rendered as "12.34"
When the syntax has an underlying primitive type of OCTET STRING, the hint consists of one or more octet-format specifications. Each specification consists of five parts, with each part using and removing zero or more of the next octets from the value and producing the next zero or more characters to be displayed. The octets within the value are processed in order of significance, most significant first.
The five parts of a octet-format specification are:
(1) the (optional) repeat indicator; if present, this part is a `*',
and indicates that the current octet of the value is to be used as the repeat count. The repeat count is an unsigned integer (which may be zero) which specifies how many times the remainder of this octet-format specification should be successively applied. If the repeat indicator is not present, the repeat count is one.
(2) the octet length: one or more decimal digits specifying the number
of octets of the value to be used and formatted by this octet- specification. Note that the octet length can be zero. If less than this number of octets remain in the value, then the lesser number of octets are used.
(3) the display format, either: `x' for hexadecimal, `d' for decimal,
`o' for octal, or `a' for ascii. If the octet length part is greater than one, and the display format part refers to a numeric format, then network-byte ordering (big-endian encoding) is used interpreting the octets in the value.
(4) the (optional) display separator character; if present, this part
is a single character which is produced for display after each application of this octet-specification; however, this character is not produced for display if it would be immediately followed by the display of the repeat terminator character for this octet- specification. This character can be any character other than a decimal digit and a `*'.
(5) the (optional) repeat terminator character, which can be present
only if the display separator character is present and this octet- specification begins with a repeat indicator; if present, this part is a single character which is produced after all the zero or more repeated applications (as given by the repeat count) of this octet-specification. This character can be any character other than a decimal digit and a `*'.
Output of a display separator character or a repeat terminator character is suppressed if it would occur as the last character of the display.
If the octets of the value are exhausted before all the octet-format specification have been used, then the excess specifications are ignored. If additional octets remain in the value after interpreting all the octet-format specifications, then the last octet-format specification is re-interpreted to process the additional octets, until no octets remain in the value.
Mapping of the STATUS clause
The STATUS clause, which must be present, indicates whether this definition is current or historic.
The values "current", and "obsolete" are self-explanatory. The "deprecated" value indicates that the definition is obsolete, but that an implementor may wish to support the use of this textual convention to foster interoperability with older implementations.
Mapping of the DESCRIPTION clause
The DESCRIPTION clause, which must be present, contains a textual definition of the textual convention, which provides all semantic definitions necessary for implementation, and should embody any information which would otherwise be communicated in any ASN.1 commentary annotations associated with the object.
Note that, in order to conform to the ASN.1 syntax, the entire value of this clause must be enclosed in double quotation marks, and therefore cannot itself contain double quotation marks, although the value may be multi-line.
Mapping of the REFERENCE clause
The REFERENCE clause, which need not be present, contains a textual cross-reference to a related item defined in some other published work.
Mapping of the SYNTAX clause
The SYNTAX clause, which must be present, defines abstract data structure corresponding to the textual convention. The data structure must be one of the alternatives defined in the ObjectSyntax CHOICE or the BITS construct (see section 7.1 in [2]).
Security Considerations
Security issues are not discussed in this memo.
Editor's Address
Keith McCloghrie Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134-1706 US
Phone: +1 408 526 5260 EMail: [email protected]
Acknowledgements
This document is the result of significant work by the four major contributors:
Jeffrey D. Case (SNMP Research, [email protected]) Keith McCloghrie (Cisco Systems, [email protected]) Marshall T. Rose (Dover Beach Consulting, [email protected]) Steven Waldbusser (International Network Services, [email protected])
In addition, the contributions of the SNMPv2 Working Group are acknowledged. In particular, a special thanks is extended for the contributions of:
Alexander I. Alten (Novell) Dave Arneson (Cabletron) Uri Blumenthal (IBM) Doug Book (Chipcom) Kim Curran (Bell-Northern Research) Jim Galvin (Trusted Information Systems) Maria Greene (Ascom Timeplex) Iain Hanson (Digital) Dave Harrington (Cabletron) Nguyen Hien (IBM) Jeff Johnson (Cisco Systems) Michael Kornegay (Object Quest)
Deirdre Kostick (AT&T Bell Labs) David Levi (SNMP Research) Daniel Mahoney (Cabletron) Bob Natale (ACE*COMM) Brian O'Keefe (Hewlett Packard) Andrew Pearson (SNMP Research) Dave Perkins (Peer Networks) Randy Presuhn (Peer Networks) Aleksey Romanov (Quality Quorum) Shawn Routhier (Epilogue) Jon Saperia (BGS Systems) Bob Stewart (Cisco Systems, [email protected]), chair Kaj Tesink (Bellcore) Glenn Waters (Bell-Northern Research) Bert Wijnen (IBM)
References
[1] Information processing systems - Open Systems Interconnection -
Specification of Abstract Syntax Notation One (ASN.1), International Organization for Standardization. International Standard 8824, (December, 1987).
[2] SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
S. Waldbusser, "Structure of Management Information for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1902, January 1996.
