Charles M. Kozierok The TCP-IP Guide

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Key Concept: The general format of SNMP messages consists of a message

header and a message body. The body of the message is also called the protocol

data unit or PDU, and contains a set of PDU control fields and a number of variable

bindings. Each variable binding describes one MIB object and consists of the object’s name

and value.

SNMP Version 1 (SNMPv1) Message Format

The SNMP general message format was, of course, first used to define the format of

messages in the original SNMP Protocol, SNMP version 1 (SNMPv1). This first version of

SNMP is probably best known for its relative simplicity, compared to the versions that

followed it. This is reflected in its message format, which is quite straight-forward.

The general message format in SNMPv1 is a “wrapper” consisting of a small header and an

encapsulated PDU. Not very many header fields were needed in SNMPv1 because the

community-based security method in SNMPv1 is very rudimentary. Thus, the short overall

format for SNMPv1 messages shown in Table 213 and Figure 277.

SNMPv1 PDU Formats

All of the PDUs in SNMPv1 have the same format, with one exception: Trap-PDU. The

exact semantics of each field in the PDU depends on the particular message. For example,

the ErrorStatus field only has meaning in a reply and not a request, and object values are

used differently in requests and replies as well.

Table 213: SNMP Version 1 (SNMPv1) General Message Format

Field Name Syntax

Size

(bytes)

Description

Version Integer 4

Version Number: Describes the SNMP version number of this

message; used for ensuring compatibility between versions. For

SNMPv1, this value is actually 0, not 1.

Community

Octet

String

Variable

Community String: Identifies the SNMP community in which the

sender and recipient of this message are located. This is used to

implement the simple SNMP community-based security

mechanism

PDU — Variable

Protocol Data Unit: The PDU being communicated as the body of

the message.

SNMPv1 Common PDU Format

Table 214 and Figure 278 show the common format for most of the SNMPv1 PDUs: GetRe-

quest-PDU, GetNextRequest-PDU, SetRequest-PDU and GetResponse-PDU:

Figure 277: SNMP Version 1 (SNMPv1) General Message Format

Table 214: SNMP Version 1 (SNMPv1) Common PDU Format (Page 1 of 2)

Field

Name

Syntax

Size

(bytes)

Description

PDU Type

Integer

(Enumerated)

Request ID Integer 4

Request Identifier: A number used to match requests with replies.

It is generated by the device that sends a request and copied into

this field in a GetResponse-PDU by the responding SNMP entity.

Version Number = 0

Community String

PDU Control Fields

PDU Variable Bindings

4 8 12 16 20 24 28 320

Message Body (PDU)

PDU

ype:

eger va

PDU

ype:

PDU Type

Value

PDU Type

0 GetRequest-PDU

1 GetNextRequest-PDU

2 GetResponse-PDU

3 SetRequest-PDU

Error

Status

Integer

(Enumerated)

Error Index Integer 4

Error Index: When Error Status is non-zero, this field contains a

pointer that specifies which object generated the error. Always

zero in a request.

Variable

Bindings

Variable Variable

Variable Bindings: A set of name-value pairs identifying the MIB

objects in the PDU, and in the case of a SetRequest-PDU or

GetResponse-PDU, containing their values. See the general

message format topic for more on these bindings.

Table 214: SNMP Version 1 (SNMPv1) Common PDU Format (Page 2 of 2)

Field

Name

Syntax

Size

(bytes)

Description

rror

us:

eger va

s use

n a

esponse-

PDU

to tell the requesting SNMP entity the result of its request. A value

of zero indicates that no error occurred; the other values indicate

what sort of error happened:

Error

Status

Value

Error Code Description

0 noError

No error occurred. This code is also used in

all request PDUs, since they have no error

status to report.

1 tooBig

The size of the GetResponse-PDU would

be too large to transport.

2 noSuchName

The name of a requested object was not

found.

3 badValue

A value in the request didn't match the

structure that the recipient of the request

had for the object. For example, an object

in the request was specified with an

incorrect length or type.

4 readOnly

An attempt was made to set a variable that

has an Access value indicating that it is

read-only.

5 genErr

An error other than one of the preceding

four specific types occurred.

SNMPv1 Trap-PDU Format

Table 215 and Figure 279 show the special format for the SNMPv1 Trap-PDU:

Figure 278: SNMP Version 1 (SNMPv1) Common PDU Format

Table 215: SNMP Version 1 (SNMPv1) Trap-PDU Format

Field

Name

Syntax

Size

(bytes)

Description

PDU Type

Integer

(Enumerated)

PDU Type: An integer value that indicates the PDU type, which

is 4 for a Trap-PDU message.

Enterprise

Sequence of

Integer

Variable

Enterprise: An object identifier for a group, which indicates the

type of object that generated the trap.

Agent Addr

NetworkAd-

dress

Agent Address: The IP address of the SNMP agent that

generated the trap. This is of course also in the IP header at

lower levels but inclusion in the SNMP message format allows

for easier trap logging within SNMP. Also, in the case of a

multihomed host, this specifies the preferred address.

Generic

Trap

Integer

(Enumerated)

Generic Trap Code: A code value specifying one of a number

of predefined “generic” trap types.

Specific

Trap

Integer 4

Specific Trap Code: A code value indicating an implemen-

tation-specific trap type.

Time Stamp TimeTicks 4

Time Stamp: The amount of time since the SNMP entity

sending this message last initialized or reinitialized. Used to time

stamp traps for logging purposes.

Variable

Bindings

Variable Variable

Variable Bindings: A set of name-value pairs identifying the

MIB objects in the PDU. See the general message format topic

for more on these bindings.

PDU Type

Request Identifier

Error Status

Error Index

PDU Variable Bindings

4 8 12 16 20 24 28 320

SNMP Version 2 (SNMPv2) Message Formats

After SMNP version 1 had been in use for several years, certain issues with it were noticed

and areas for improvement identified. This led to the development of the original SNMP

version 2, which was intended to enhance SNMPv1 in many areas, including MIB object

definitions, protocol operations and security. This last area, security, led to the proliferation

of SNMPv2 version “variants” that I described in the topic on SNMP versions.

Since there are in fact several different “SNMPv2”s, there are also several message

formats for SNMPv2, not just one. This is confusing enough, but would be even worse

without the modular nature of SNMP messages “coming to the rescue”. The protocol opera-

tions in SNMPv2 were changed from SNMPv1, which necessitated some modifications to

the format of SNMPv2 PDUs. However, the protocol operations are the same for all the

SNMPv2 variations. The differences between SNMPv2 variants are in the areas of security

implementation. Thus, the result of this is that the PDU format is the same for all the

SNMPv2 types, while the overall message format differs for each variant. (This is why the

distinction between a PDU and a message is not just an academic one!)

During the SNMPv2 “divergence”, four variations were defined: the original SNMPv2

(SNMPv2p); community-based SNMPv2 (SNMPv2c), user-based SNMPv2 (SNMPv2u) and

“SNMPv2 star” (SNMPv2*). Of these, the first three were documented in sets of SNMP RFC

Figure 279: SNMP Version 1 (SNMPv1) Trap-PDU Format

PDU Type

Enterprise

Agent Address

Generic Trap Code

Specific Trap Code

Time Stamp

PDU Variable Bindings

4 8 12 16 20 24 28 320

standards, the fourth was not. The structure of the overall message format for each variant

is discussed in an administrative or security standard for the variation in question, which

makes reference to the shared SNMPv2 standard for the PDU format (RFC 1905).

The overall message formats for SNMPv2p, SNMPv2c and SNMPv2u are as follows:

SNMP Version 2 (SNMPv2p) Message Format

The party-based security model is quite complex, but the basic messaging in this version is

described through the definition of a management communication, which describes the

source and destination party and makes reference to a context for the communication. The

overall message format is described in detail in RFC 1445. This information is summarized

in Table 216 and shown graphically in Figure 280.

Table 216: SNMP Version 2 (SNMPv2p) General Message Format

Field Name Syntax

Size

(bytes)

Description

Version Integer 4

Version Number: Describes the SNMP version number of this

message; used for ensuring compatibility between versions. For

SNMPv2p, this value is 2.

Dst Party

Sequence

of Integer

Variable

Destination Party: An object identifier that specifies the party that

is the intended recipient of the message.

Src Party

Sequence

of Integer

Variable

Source Party: An object identifier that specifies the party that is

the sender of the message.

Context

Sequence

of Integer

Variable

Context: Defines a set of MIB object resources that is accessible

by a particular entity.

PDU — Variable PDU: The protocol data unit of the message.

Community-Based SNMP Version 2 (SNMPv2c) Message Format

This version of SNMPv2 was intended to keep the new protocol enhancements introduced

by SNMPv2p but go back to the simple SNMPv1 security model. As such, the defining

document for SNMPv2c, RFC 1901, specifies that its overall message format is the same

as that of SNMPv1, except that the version number is changed. This is shown in Table 217

and Figure 281.

Figure 280: SNMP Version 2 (SNMPv2p) General Message Format

Version Number = 2

Destination Party

Source Party

Context

PDU Control Fields

PDU Variable Bindings

4 8 12 16 20 24 28 320

Message Body (PDU)

Table 217: Community-Based SNMP Version 2 (SNMPv2c) General Message Format

Field Name Syntax

Size

(bytes)

Description

Version Integer 4

Version Number: Describes the SNMP version number of this

message; used for ensuring compatibility between versions. For

SNMPv2c, this value is 1.

Community

Octet

String

Variable

Community String: Identifies the SNMP community in which the

sender and recipient of this message are located.

PDU — Variable

Protocol Data Unit: The PDU being communicated as the body of

the message.

Figure 281: Community-Based SNMP Version 2 (SNMPv2c) General Message Format

Version Number = 1

Community String

PDU Control Fields

PDU Variable Bindings

4 8 12 16 20 24 28 320

Message Body (PDU)

User-Based SNMP Version 2 (SNMPv2u) Message Format

SNMPv2u was defined as an optional security model at the time that SNMPv2c was

standardized. RFC 1910 defines the user-based security model and the message format

described in Table 218 and Figure 282.

Table 218: User-Based SNMP Version 2 (SNMPv2u) General Message Format

Field

Name

Syntax

Size

(bytes)

Description

Version Integer 4

Version Number: Describes the SNMP version number of this message;

used for ensuring compatibility between versions. For SNMPv2u, this value

is 2. Note that this is the same value as used for SNMPv2p.

Param-

eters

Octet

String

Variable

PDU — Variable

Protocol Data Unit: The PDU being communicated as the body of the

message. This may be in either encrypted or unencrypted form.

arame

ers:

ng o

parame

ers use

emen

e user-

ase

security model, which is itself non-trivial to describe. Very briefly, these

parameters are:

Subfield

Name

Size

(bytes)

Description

Model 1

Model Number: Set to 1 to identify the user-based

model.

QoS 1

Quality of Service: Indicates whether authentication

and/or privacy (encryption) have been used, and

whether generation of a Report-PDU is allowed.

Agent ID 12

Agent Identifier: The identifier of the agent sending the

message. Used to defeat replay attacks and certain

other types of security attacks.

Agent

Boots

Agent Number Of Boots: The number of times the

agent has been booted or rebooted since its Agent ID

was set; used to defeat certain security attacks.

Agent

Time

Agent Time Since Last Boot: The number of seconds

since the last boot of this agent. Again, used to defeat

replay and other security attacks.

Max Size 2

Maximum Message Size: The maximum size of

message that the sender of this message can receive.

User Len 1 User Length: The length of the User Name field below.

User

Name

Variable

(1 to 16)

User Name: The name of the user on whose behalf the

message is being sent.

Auth Len 1

Authentication Digest Length: The length of the Auth

Digest field below.

Auth

Digest

Variable

(0 to

255)

Authentication Digest: An authentication value used to

verify the identity and genuineness of this message,

when authentication is used.

Context

Selector

Variable

(0 to 40)

Context Selector: A string that is combined with the

Agent ID to specify a particular context that contains the

management information referenced by this message.

Figure 282: User-Based SNMP Version 2 (SNMPv2u) General Message Format

Version Number = 2

Model Number Quality Of Service

Agent Number Of Boots

(bytes 1 and 2)

Agent Number Of Boots

(bytes 3 and 4)

Agent Time Since Last Boot

(bytes 1 and 2)

Agent Time Since Last Boot

(bytes 3 and 4)

Maximum Message Size

User Length

Authentication

Digest Length

Authentication Digest

Context Selector

PDU Control Fields

PDU Variable Bindings

4 8 12 16 20 24 28 320

Message Body (PDU)

Agent Identifier

(12 bytes)

User Name

Parameters