RFC 6733
Diameter Base Protocol
Pages: 152
Proposed Standard
→ Errata
Obsoletes: 3588 5719
Updated by: 7075 8553
Proposed Standard
→ Errata
Obsoletes: 3588 5719
Updated by: 7075 8553
Part 4 of 5 – Pages 87 to 122
7. Error Handling
There are two different types of errors in Diameter; protocol errors and application errors. A protocol error is one that occurs at the base protocol level and MAY require per-hop attention (e.g., a message routing error). Application errors, on the other hand, generally occur due to a problem with a function specified in a Diameter application (e.g., user authentication, missing AVP). Result-Code AVP values that are used to report protocol errors MUST only be present in answer messages whose 'E' bit is set. When a request message is received that causes a protocol error, an answer message is returned with the 'E' bit set, and the Result-Code AVP is set to the appropriate protocol error value. As the answer is sent back towards the originator of the request, each proxy or relay agent MAY take action on the message.
1. Request +---------+ Link Broken +-------------------------->|Diameter |----///----+ | +---------------------| | v +------+--+ | 2. answer + 'E' set | Relay 2 | +--------+ |Diameter |<-+ (Unable to Forward) +---------+ |Diameter| | | | Home | | Relay 1 |--+ +---------+ | Server | +---------+ | 3. Request |Diameter | +--------+ +-------------------->| | ^ | Relay 3 |-----------+ +---------+ Figure 7: Example of Protocol Error Causing Answer Message Figure 7 provides an example of a message forwarded upstream by a Diameter relay. When the message is received by Relay 2, and it detects that it cannot forward the request to the home server, an answer message is returned with the 'E' bit set and the Result-Code AVP set to DIAMETER_UNABLE_TO_DELIVER. Given that this error falls within the protocol error category, Relay 1 would take special action, and given the error, attempt to route the message through its alternate Relay 3. +---------+ 1. Request +---------+ 2. Request +---------+ | Access |------------>|Diameter |------------>|Diameter | | | | | | Home | | Device |<------------| Relay |<------------| Server | +---------+ 4. Answer +---------+ 3. Answer +---------+ (Missing AVP) (Missing AVP) Figure 8: Example of Application Error Answer Message Figure 8 provides an example of a Diameter message that caused an application error. When application errors occur, the Diameter entity reporting the error clears the 'R' bit in the Command Flags and adds the Result-Code AVP with the proper value. Application errors do not require any proxy or relay agent involvement; therefore, the message would be forwarded back to the originator of the request. In the case where the answer message itself contains errors, any related session SHOULD be terminated by sending an STR or ASR message. The Termination-Cause AVP in the STR MAY be filled with the appropriate value to indicate the cause of the error. An application MAY also send an application-specific request instead of an STR or ASR message to signal the error in the case where no state is maintained or to allow for some form of error recovery with the corresponding Diameter entity.
There are certain Result-Code AVP application errors that require
additional AVPs to be present in the answer. In these cases, the
Diameter node that sets the Result-Code AVP to indicate the error
MUST add the AVPs. Examples are as follows:
o A request with an unrecognized AVP is received with the 'M' bit
(Mandatory bit) set causes an answer to be sent with the Result-
Code AVP set to DIAMETER_AVP_UNSUPPORTED and the Failed-AVP AVP
containing the offending AVP.
o A request with an AVP that is received with an unrecognized value
causes an answer to be returned with the Result-Code AVP set to
DIAMETER_INVALID_AVP_VALUE, with the Failed-AVP AVP containing the
AVP causing the error.
o A received command that is missing AVPs that are defined as
required in the commands CCF; examples are AVPs indicated as
{AVP}. The receiver issues an answer with the Result-Code set to
DIAMETER_MISSING_AVP and creates an AVP with the AVP Code and
other fields set as expected in the missing AVP. The created AVP
is then added to the Failed-AVP AVP.
The Result-Code AVP describes the error that the Diameter node
encountered in its processing. In case there are multiple errors,
the Diameter node MUST report only the first error it encountered
(detected possibly in some implementation-dependent order). The
specific errors that can be described by this AVP are described in
the following section.
7.1. Result-Code AVP
The Result-Code AVP (AVP Code 268) is of type Unsigned32 and
indicates whether a particular request was completed successfully or
an error occurred. All Diameter answer messages in IETF-defined
Diameter application specifications MUST include one Result-Code AVP.
A non-successful Result-Code AVP (one containing a non-2xxx value
other than DIAMETER_REDIRECT_INDICATION) MUST include the Error-
Reporting-Host AVP if the host setting the Result-Code AVP is
different from the identity encoded in the Origin-Host AVP.
The Result-Code data field contains an IANA-managed 32-bit address
space representing errors (see Section 11.3.2). Diameter provides
the following classes of errors, all identified by the thousands
digit in the decimal notation:
o 1xxx (Informational) o 2xxx (Success) o 3xxx (Protocol Errors) o 4xxx (Transient Failures) o 5xxx (Permanent Failure) An unrecognized class (one whose first digit is not defined in this section) MUST be handled as a permanent failure.7.1.1. Informational
Errors that fall within this category are used to inform the requester that a request could not be satisfied, and additional action is required on its part before access is granted. DIAMETER_MULTI_ROUND_AUTH 1001 This informational error is returned by a Diameter server to inform the access device that the authentication mechanism being used requires multiple round trips, and a subsequent request needs to be issued in order for access to be granted.7.1.2. Success
Errors that fall within the Success category are used to inform a peer that a request has been successfully completed. DIAMETER_SUCCESS 2001 The request was successfully completed. DIAMETER_LIMITED_SUCCESS 2002 When returned, the request was successfully completed, but additional processing is required by the application in order to provide service to the user.7.1.3. Protocol Errors
Errors that fall within the Protocol Error category SHOULD be treated on a per-hop basis, and Diameter proxies MAY attempt to correct the error, if it is possible. Note that these errors MUST only be used in answer messages whose 'E' bit is set.
DIAMETER_COMMAND_UNSUPPORTED 3001
This error code is used when a Diameter entity receives a message
with a Command Code that it does not support.
DIAMETER_UNABLE_TO_DELIVER 3002
This error is given when Diameter cannot deliver the message to
the destination, either because no host within the realm
supporting the required application was available to process the
request or because the Destination-Host AVP was given without the
associated Destination-Realm AVP.
DIAMETER_REALM_NOT_SERVED 3003
The intended realm of the request is not recognized.
DIAMETER_TOO_BUSY 3004
When returned, a Diameter node SHOULD attempt to send the message
to an alternate peer. This error MUST only be used when a
specific server is requested, and it cannot provide the requested
service.
DIAMETER_LOOP_DETECTED 3005
An agent detected a loop while trying to get the message to the
intended recipient. The message MAY be sent to an alternate peer,
if one is available, but the peer reporting the error has
identified a configuration problem.
DIAMETER_REDIRECT_INDICATION 3006
A redirect agent has determined that the request could not be
satisfied locally, and the initiator of the request SHOULD direct
the request directly to the server, whose contact information has
been added to the response. When set, the Redirect-Host AVP MUST
be present.
DIAMETER_APPLICATION_UNSUPPORTED 3007
A request was sent for an application that is not supported.
DIAMETER_INVALID_HDR_BITS 3008
A request was received whose bits in the Diameter header were set
either to an invalid combination or to a value that is
inconsistent with the Command Code's definition.
DIAMETER_INVALID_AVP_BITS 3009
A request was received that included an AVP whose flag bits are
set to an unrecognized value or that is inconsistent with the
AVP's definition.
DIAMETER_UNKNOWN_PEER 3010
A CER was received from an unknown peer.
7.1.4. Transient Failures
Errors that fall within the transient failures category are used to
inform a peer that the request could not be satisfied at the time it
was received but MAY be able to satisfy the request in the future.
Note that these errors MUST be used in answer messages whose 'E' bit
is not set.
DIAMETER_AUTHENTICATION_REJECTED 4001
The authentication process for the user failed, most likely due to
an invalid password used by the user. Further attempts MUST only
be tried after prompting the user for a new password.
DIAMETER_OUT_OF_SPACE 4002
A Diameter node received the accounting request but was unable to
commit it to stable storage due to a temporary lack of space.
ELECTION_LOST 4003
The peer has determined that it has lost the election process and
has therefore disconnected the transport connection.
7.1.5. Permanent Failures
Errors that fall within the permanent failures category are used to
inform the peer that the request failed and should not be attempted
again. Note that these errors SHOULD be used in answer messages
whose 'E' bit is not set. In error conditions where it is not
possible or efficient to compose application-specific answer grammar,
answer messages with the 'E' bit set and which comply to the grammar
described in Section 7.2 MAY also be used for permanent errors.
DIAMETER_AVP_UNSUPPORTED 5001
The peer received a message that contained an AVP that is not
recognized or supported and was marked with the 'M' (Mandatory)
bit. A Diameter message with this error MUST contain one or more
Failed-AVP AVPs containing the AVPs that caused the failure.
DIAMETER_UNKNOWN_SESSION_ID 5002
The request contained an unknown Session-Id.
DIAMETER_AUTHORIZATION_REJECTED 5003
A request was received for which the user could not be authorized.
This error could occur if the service requested is not permitted
to the user.
DIAMETER_INVALID_AVP_VALUE 5004
The request contained an AVP with an invalid value in its data
portion. A Diameter message indicating this error MUST include
the offending AVPs within a Failed-AVP AVP.
DIAMETER_MISSING_AVP 5005
The request did not contain an AVP that is required by the Command
Code definition. If this value is sent in the Result-Code AVP, a
Failed-AVP AVP SHOULD be included in the message. The Failed-AVP
AVP MUST contain an example of the missing AVP complete with the
Vendor-Id if applicable. The value field of the missing AVP
should be of correct minimum length and contain zeroes.
DIAMETER_RESOURCES_EXCEEDED 5006
A request was received that cannot be authorized because the user
has already expended allowed resources. An example of this error
condition is when a user that is restricted to one dial-up PPP
port attempts to establish a second PPP connection.
DIAMETER_CONTRADICTING_AVPS 5007
The Home Diameter server has detected AVPs in the request that
contradicted each other, and it is not willing to provide service
to the user. The Failed-AVP AVP MUST be present, which contain
the AVPs that contradicted each other.
DIAMETER_AVP_NOT_ALLOWED 5008
A message was received with an AVP that MUST NOT be present. The
Failed-AVP AVP MUST be included and contain a copy of the
offending AVP.
DIAMETER_AVP_OCCURS_TOO_MANY_TIMES 5009
A message was received that included an AVP that appeared more
often than permitted in the message definition. The Failed-AVP
AVP MUST be included and contain a copy of the first instance of
the offending AVP that exceeded the maximum number of occurrences.
DIAMETER_NO_COMMON_APPLICATION 5010
This error is returned by a Diameter node that receives a CER
whereby no applications are common between the CER sending peer
and the CER receiving peer.
DIAMETER_UNSUPPORTED_VERSION 5011
This error is returned when a request was received, whose version
number is unsupported.
DIAMETER_UNABLE_TO_COMPLY 5012
This error is returned when a request is rejected for unspecified
reasons.
DIAMETER_INVALID_BIT_IN_HEADER 5013
This error is returned when a reserved bit in the Diameter header
is set to one (1) or the bits in the Diameter header are set
incorrectly.
DIAMETER_INVALID_AVP_LENGTH 5014
The request contained an AVP with an invalid length. A Diameter
message indicating this error MUST include the offending AVPs
within a Failed-AVP AVP. In cases where the erroneous AVP length
value exceeds the message length or is less than the minimum AVP
header length, it is sufficient to include the offending AVP
header and a zero filled payload of the minimum required length
for the payloads data type. If the AVP is a Grouped AVP, the
Grouped AVP header with an empty payload would be sufficient to
indicate the offending AVP. In the case where the offending AVP
header cannot be fully decoded when the AVP length is less than
the minimum AVP header length, it is sufficient to include an
offending AVP header that is formulated by padding the incomplete
AVP header with zero up to the minimum AVP header length.
DIAMETER_INVALID_MESSAGE_LENGTH 5015
This error is returned when a request is received with an invalid
message length.
DIAMETER_INVALID_AVP_BIT_COMBO 5016
The request contained an AVP with which is not allowed to have the
given value in the AVP Flags field. A Diameter message indicating
this error MUST include the offending AVPs within a Failed-AVP
AVP.
DIAMETER_NO_COMMON_SECURITY 5017
This error is returned when a CER message is received, and there
are no common security mechanisms supported between the peers. A
Capabilities-Exchange-Answer (CEA) message MUST be returned with
the Result-Code AVP set to DIAMETER_NO_COMMON_SECURITY.
7.2. Error Bit
The 'E' (Error Bit) in the Diameter header is set when the request
caused a protocol-related error (see Section 7.1.3). A message with
the 'E' bit MUST NOT be sent as a response to an answer message.
Note that a message with the 'E' bit set is still subjected to the
processing rules defined in Section 6.2. When set, the answer
message will not conform to the CCF specification for the command;
instead, it and will conform to the following CCF:
Message Format
<answer-message> ::= < Diameter Header: code, ERR [, PXY] >
0*1< Session-Id >
{ Origin-Host }
{ Origin-Realm }
{ Result-Code }
[ Origin-State-Id ]
[ Error-Message ]
[ Error-Reporting-Host ]
[ Failed-AVP ]
[ Experimental-Result ]
* [ Proxy-Info ]
* [ AVP ]
Note that the code used in the header is the same than the one found in the request message, but with the 'R' bit cleared and the 'E' bit set. The 'P' bit in the header is set to the same value as the one found in the request message.7.3. Error-Message AVP
The Error-Message AVP (AVP Code 281) is of type UTF8String. It MAY accompany a Result-Code AVP as a human-readable error message. The Error-Message AVP is not intended to be useful in an environment where error messages are processed automatically. It SHOULD NOT be expected that the content of this AVP be parsed by network entities.7.4. Error-Reporting-Host AVP
The Error-Reporting-Host AVP (AVP Code 294) is of type DiameterIdentity. This AVP contains the identity of the Diameter host that sent the Result-Code AVP to a value other than 2001 (Success), only if the host setting the Result-Code is different from the one encoded in the Origin-Host AVP. This AVP is intended to be used for troubleshooting purposes, and it MUST be set when the Result-Code AVP indicates a failure.7.5. Failed-AVP AVP
The Failed-AVP AVP (AVP Code 279) is of type Grouped and provides debugging information in cases where a request is rejected or not fully processed due to erroneous information in a specific AVP. The value of the Result-Code AVP will provide information on the reason for the Failed-AVP AVP. A Diameter answer message SHOULD contain an instance of the Failed-AVP AVP that corresponds to the error indicated by the Result-Code AVP. For practical purposes, this Failed-AVP would typically refer to the first AVP processing error that a Diameter node encounters. The possible reasons for this AVP are the presence of an improperly constructed AVP, an unsupported or unrecognized AVP, an invalid AVP value, the omission of a required AVP, the presence of an explicitly excluded AVP (see tables in Section 10) or the presence of two or more occurrences of an AVP that is restricted to 0, 1, or 0-1 occurrences. A Diameter message SHOULD contain one Failed-AVP AVP, containing the entire AVP that could not be processed successfully. If the failure reason is omission of a required AVP, an AVP with the missing AVP code, the missing Vendor-Id, and a zero-filled payload of the minimum required length for the omitted AVP will be added. If the failure reason is an invalid AVP length where the reported length is less
than the minimum AVP header length or greater than the reported
message length, a copy of the offending AVP header and a zero-filled
payload of the minimum required length SHOULD be added.
In the case where the offending AVP is embedded within a Grouped AVP,
the Failed-AVP MAY contain the grouped AVP, which in turn contains
the single offending AVP. The same method MAY be employed if the
grouped AVP itself is embedded in yet another grouped AVP and so on.
In this case, the Failed-AVP MAY contain the grouped AVP hierarchy up
to the single offending AVP. This enables the recipient to detect
the location of the offending AVP when embedded in a group.
AVP Format
<Failed-AVP> ::= < AVP Header: 279 >
1* {AVP}
7.6. Experimental-Result AVP
The Experimental-Result AVP (AVP Code 297) is of type Grouped, and
indicates whether a particular vendor-specific request was completed
successfully or whether an error occurred. This AVP has the
following structure:
AVP Format
Experimental-Result ::= < AVP Header: 297 >
{ Vendor-Id }
{ Experimental-Result-Code }
The Vendor-Id AVP (see Section 5.3.3) in this grouped AVP identifies
the vendor responsible for the assignment of the result code that
follows. All Diameter answer messages defined in vendor-specific
applications MUST include either one Result-Code AVP or one
Experimental-Result AVP.
7.7. Experimental-Result-Code AVP
The Experimental-Result-Code AVP (AVP Code 298) is of type Unsigned32
and contains a vendor-assigned value representing the result of
processing the request.
It is recommended that vendor-specific result codes follow the same
conventions given for the Result-Code AVP regarding the different
types of result codes and the handling of errors (for non-2xxx
values).
8. Diameter User Sessions
In general, Diameter can provide two different types of services to applications. The first involves authentication and authorization, and it can optionally make use of accounting. The second only makes use of accounting. When a service makes use of the authentication and/or authorization portion of an application, and a user requests access to the network, the Diameter client issues an auth request to its local server. The auth request is defined in a service-specific Diameter application (e.g., NASREQ). The request contains a Session-Id AVP, which is used in subsequent messages (e.g., subsequent authorization, accounting, etc.) relating to the user's session. The Session-Id AVP is a means for the client and servers to correlate a Diameter message with a user session. When a Diameter server authorizes a user to implement network resources for a finite amount of time, and it is willing to extend the authorization via a future request, it MUST add the Authorization- Lifetime AVP to the answer message. The Authorization-Lifetime AVP defines the maximum number of seconds a user MAY make use of the resources before another authorization request is expected by the server. The Auth-Grace-Period AVP contains the number of seconds following the expiration of the Authorization-Lifetime, after which the server will release all state information related to the user's session. Note that if payment for services is expected by the serving realm from the user's home realm, the Authorization-Lifetime AVP, combined with the Auth-Grace-Period AVP, implies the maximum length of the session for which the home realm is willing to be fiscally responsible. Services provided past the expiration of the Authorization-Lifetime and Auth-Grace-Period AVPs are the responsibility of the access device. Of course, the actual cost of services rendered is clearly outside the scope of the protocol. An access device that does not expect to send a re-authorization or a session termination request to the server MAY include the Auth- Session-State AVP with the value set to NO_STATE_MAINTAINED as a hint to the server. If the server accepts the hint, it agrees that since no session termination message will be received once service to the user is terminated, it cannot maintain state for the session. If the answer message from the server contains a different value in the Auth-Session-State AVP (or the default value if the AVP is absent), the access device MUST follow the server's directives. Note that the value NO_STATE_MAINTAINED MUST NOT be set in subsequent re- authorization requests and answers.
The base protocol does not include any authorization request messages, since these are largely application-specific and are defined in a Diameter application document. However, the base protocol does define a set of messages that are used to terminate user sessions. These are used to allow servers that maintain state information to free resources. When a service only makes use of the accounting portion of the Diameter protocol, even in combination with an application, the Session-Id is still used to identify user sessions. However, the session termination messages are not used, since a session is signaled as being terminated by issuing an accounting stop message. Diameter may also be used for services that cannot be easily categorized as authentication, authorization, or accounting (e.g., certain Third Generation Partnership Project Internet Multimedia System (3GPP IMS) interfaces). In such cases, the finite state machine defined in subsequent sections may not be applicable. Therefore, the application itself MAY need to define its own finite state machine. However, such application-specific state machines SHOULD follow the general state machine framework outlined in this document such as the use of Session-Id AVPs and the use of STR/STA, ASR/ASA messages for stateful sessions.8.1. Authorization Session State Machine
This section contains a set of finite state machines, which represent the life cycle of Diameter sessions and which MUST be observed by all Diameter implementations that make use of the authentication and/or authorization portion of a Diameter application. The term "Service- Specific" below refers to a message defined in a Diameter application (e.g., Mobile IPv4, NASREQ). There are four different authorization session state machines supported in the Diameter base protocol. The first two describe a session in which the server is maintaining session state, indicated by the value of the Auth-Session-State AVP (or its absence). One describes the session from a client perspective, the other from a server perspective. The second two state machines are used when the server does not maintain session state. Here again, one describes the session from a client perspective, the other from a server perspective. When a session is moved to the Idle state, any resources that were allocated for the particular session must be released. Any event not listed in the state machines MUST be considered an error condition, and an answer, if applicable, MUST be returned to the originator of the message.
In the case that an application does not support re-auth, the state
transitions related to server-initiated re-auth, when both client and
server sessions maintain state (e.g., Send RAR, Pending, Receive
RAA), MAY be ignored.
In the state table, the event "Failure to send X" means that the
Diameter agent is unable to send command X to the desired
destination. This could be due to the peer being down or due to the
peer sending back a transient failure or temporary protocol error
notification DIAMETER_TOO_BUSY or DIAMETER_LOOP_DETECTED in the
Result-Code AVP of the corresponding Answer command. The event 'X
successfully sent' is the complement of 'Failure to send X'.
The following state machine is observed by a client when state is
maintained on the server:
CLIENT, STATEFUL
State Event Action New State
---------------------------------------------------------------
Idle Client or device requests Send Pending
access service-
specific
auth req
Idle ASR Received Send ASA Idle
for unknown session with
Result-Code =
UNKNOWN_
SESSION_ID
Idle RAR Received Send RAA Idle
for unknown session with
Result-Code =
UNKNOWN_
SESSION_ID
Pending Successful service-specific Grant Open
authorization answer Access
received with default
Auth-Session-State value
Pending Successful service-specific Sent STR Discon
authorization answer received,
but service not provided
Pending Error processing successful Sent STR Discon
service-specific authorization
answer
Pending Failed service-specific Clean up Idle
authorization answer received
Open User or client device Send Open
requests access to service service-
specific
auth req
Open Successful service-specific Provide Open
authorization answer received service
Open Failed service-specific Discon. Idle
authorization answer user/device
received.
Open RAR received and client will Send RAA Open
perform subsequent re-auth with
Result-Code =
SUCCESS
Open RAR received and client will Send RAA Idle
not perform subsequent with
re-auth Result-Code !=
SUCCESS,
Discon.
user/device
Open Session-Timeout expires on Send STR Discon
access device
Open ASR received, Send ASA Discon
client will comply with
with request to end the Result-Code =
session = SUCCESS,
Send STR.
Open ASR Received, Send ASA Open
client will not comply with
with request to end the Result-Code !=
session != SUCCESS
Open Authorization-Lifetime + Send STR Discon
Auth-Grace-Period expires on
access device
Discon ASR received Send ASA Discon
Discon STA received Discon. Idle
user/device
The following state machine is observed by a server when it is
maintaining state for the session:
SERVER, STATEFUL
State Event Action New State
---------------------------------------------------------------
Idle Service-specific authorization Send Open
request received, and successful
user is authorized service-
specific
answer
Idle Service-specific authorization Send Idle
request received, and failed
user is not authorized service-
specific
answer
Open Service-specific authorization Send Open
request received, and user successful
is authorized service-
specific
answer
Open Service-specific authorization Send Idle
request received, and user failed
is not authorized service-
specific
answer,
Clean up
Open Home server wants to confirm Send RAR Pending
authentication and/or
authorization of the user
Pending Received RAA with a failed Clean up Idle
Result-Code
Pending Received RAA with Result-Code Update Open
= SUCCESS session
Open Home server wants to Send ASR Discon
terminate the service
Open Authorization-Lifetime (and Clean up Idle
Auth-Grace-Period) expires
on home server
Open Session-Timeout expires on Clean up Idle
home server
Discon Failure to send ASR Wait, Discon
resend ASR
Discon ASR successfully sent and Clean up Idle
ASA Received with Result-Code
Not ASA Received None No Change
Discon
Any STR Received Send STA, Idle
Clean up
The following state machine is observed by a client when state is not
maintained on the server:
CLIENT, STATELESS
State Event Action New State
---------------------------------------------------------------
Idle Client or device requests Send Pending
access service-
specific
auth req
Pending Successful service-specific Grant Open
authorization answer access
received with Auth-Session-
State set to
NO_STATE_MAINTAINED
Pending Failed service-specific Clean up Idle
authorization answer
received
Open Session-Timeout expires on Discon. Idle
access device user/device
Open Service to user is terminated Discon. Idle
user/device
The following state machine is observed by a server when it is not
maintaining state for the session:
SERVER, STATELESS
State Event Action New State
---------------------------------------------------------------
Idle Service-specific authorization Send Idle
request received, and service-
successfully processed specific
answer
8.2. Accounting Session State Machine
The following state machines MUST be supported for applications that
have an accounting portion or that require only accounting services.
The first state machine is to be observed by clients.
See Section 9.7 for Accounting Command Codes and Section 9.8 for
Accounting AVPs.
The server side in the accounting state machine depends in some cases
on the particular application. The Diameter base protocol defines a
default state machine that MUST be followed by all applications that
have not specified other state machines. This is the second state
machine in this section described below.
The default server side state machine requires the reception of
accounting records in any order and at any time, and it does not
place any standards requirement on the processing of these records.
Implementations of Diameter may perform checking, ordering,
correlation, fraud detection, and other tasks based on these records.
AVPs may need to be inspected as a part of these tasks. The tasks
can happen either immediately after record reception or in a post-
processing phase. However, as these tasks are typically application
or even policy dependent, they are not standardized by the Diameter
specifications. Applications MAY define requirements on when to
accept accounting records based on the used value of Accounting-
Realtime-Required AVP, credit-limit checks, and so on.
However, the Diameter base protocol defines one optional server side
state machine that MAY be followed by applications that require
keeping track of the session state at the accounting server. Note
that such tracking is incompatible with the ability to sustain long
duration connectivity problems. Therefore, the use of this state
machine is recommended only in applications where the value of the
Accounting-Realtime-Required AVP is DELIVER_AND_GRANT; hence,
accounting connectivity problems are required to cause the serviced
user to be disconnected. Otherwise, records produced by the client
may be lost by the server, which no longer accepts them after the
connectivity is re-established. This state machine is the third
state machine in this section. The state machine is supervised by a
supervision session timer Ts, whose value should be reasonably higher
than the Acct_Interim_Interval value. Ts MAY be set to two times the
value of the Acct_Interim_Interval so as to avoid the accounting
session in the Diameter server to change to Idle state in case of
short transient network failure.
Any event not listed in the state machines MUST be considered as an
error condition, and a corresponding answer, if applicable, MUST be
returned to the originator of the message.
In the state table, the event "Failure to send" means that the
Diameter client is unable to communicate with the desired
destination. This could be due to the peer being down, or due to the
peer sending back a transient failure or temporary protocol error
notification DIAMETER_OUT_OF_SPACE, DIAMETER_TOO_BUSY, or
DIAMETER_LOOP_DETECTED in the Result-Code AVP of the Accounting
Answer command.
The event "Failed answer" means that the Diameter client received a
non-transient failure notification in the Accounting Answer command.
Note that the action "Disconnect user/dev" MUST also have an effect
on the authorization session state table, e.g., cause the STR message
to be sent, if the given application has both authentication/
authorization and accounting portions.
The states PendingS, PendingI, PendingL, PendingE, and PendingB stand
for pending states to wait for an answer to an accounting request
related to a Start, Interim, Stop, Event, or buffered record,
respectively.
CLIENT, ACCOUNTING
State Event Action New State
---------------------------------------------------------------
Idle Client or device requests Send PendingS
access accounting
start req.
Idle Client or device requests Send PendingE
a one-time service accounting
event req
Idle Records in storage Send PendingB
record
PendingS Successful accounting Open
start answer received
PendingS Failure to send and buffer Store Open
space available and real time Start
not equal to DELIVER_AND_GRANT Record
PendingS Failure to send and no buffer Open
space available and real time
equal to GRANT_AND_LOSE
PendingS Failure to send and no Disconnect Idle
buffer space available and user/dev
real time not equal to
GRANT_AND_LOSE
PendingS Failed accounting start answer Open
received and real time equal
to GRANT_AND_LOSE
PendingS Failed accounting start answer Disconnect Idle
received and real time not user/dev
equal to GRANT_AND_LOSE
PendingS User service terminated Store PendingS
stop
record
Open Interim interval elapses Send PendingI
accounting
interim
record
Open User service terminated Send PendingL
accounting
stop req.
PendingI Successful accounting interim Open
answer received
PendingI Failure to send and (buffer Store Open
space available or old interim
record can be overwritten) record
and real time not equal to
DELIVER_AND_GRANT
PendingI Failure to send and no buffer Open
space available and real time
equal to GRANT_AND_LOSE
PendingI Failure to send and no Disconnect Idle
buffer space available and user/dev
real time not equal to
GRANT_AND_LOSE
PendingI Failed accounting interim Open
answer received and real time
equal to GRANT_AND_LOSE
PendingI Failed accounting interim Disconnect Idle
answer received and user/dev
real time not equal to
GRANT_AND_LOSE
PendingI User service terminated Store PendingI
stop
record
PendingE Successful accounting Idle
event answer received
PendingE Failure to send and buffer Store Idle
space available event
record
PendingE Failure to send and no buffer Idle
space available
PendingE Failed accounting event answer Idle
received
PendingB Successful accounting answer Delete Idle
received record
PendingB Failure to send Idle
PendingB Failed accounting answer Delete Idle
received record
PendingL Successful accounting Idle
stop answer received
PendingL Failure to send and buffer Store Idle
space available stop
record
PendingL Failure to send and no buffer Idle
space available
PendingL Failed accounting stop answer Idle
received
SERVER, STATELESS ACCOUNTING
State Event Action New State
---------------------------------------------------------------
Idle Accounting start request Send Idle
received and successfully accounting
processed. start
answer
Idle Accounting event request Send Idle
received and successfully accounting
processed. event
answer
Idle Interim record received Send Idle
and successfully processed. accounting
interim
answer
Idle Accounting stop request Send Idle
received and successfully accounting
processed stop answer
Idle Accounting request received; Send Idle
no space left to store accounting
records answer;
Result-Code =
OUT_OF_
SPACE
SERVER, STATEFUL ACCOUNTING
State Event Action New State
---------------------------------------------------------------
Idle Accounting start request Send Open
received and successfully accounting
processed. start
answer;
Start Ts
Idle Accounting event request Send Idle
received and successfully accounting
processed. event
answer
Idle Accounting request received; Send Idle
no space left to store accounting
records answer;
Result-Code =
OUT_OF_
SPACE
Open Interim record received Send Open
and successfully processed. accounting
interim
answer;
Restart Ts
Open Accounting stop request Send Idle
received and successfully accounting
processed stop answer;
Stop Ts
Open Accounting request received; Send Idle
no space left to store accounting
records answer;
Result-Code =
OUT_OF_
SPACE;
Stop Ts
Open Session supervision timer Ts Stop Ts Idle
expired
8.3. Server-Initiated Re-Auth
A Diameter server may initiate a re-authentication and/or re- authorization service for a particular session by issuing a Re-Auth- Request (RAR). For example, for prepaid services, the Diameter server that originally authorized a session may need some confirmation that the user is still using the services. An access device that receives an RAR message with the Session-Id equal to a currently active session MUST initiate a re-auth towards the user, if the service supports this particular feature. Each Diameter application MUST state whether server-initiated re-auth is supported, since some applications do not allow access devices to prompt the user for re-auth.8.3.1. Re-Auth-Request
The Re-Auth-Request (RAR), indicated by the Command Code set to 258 and the message flags' 'R' bit set, may be sent by any server to the access device that is providing session service, to request that the user be re-authenticated and/or re-authorized. Message Format <RAR> ::= < Diameter Header: 258, REQ, PXY > < Session-Id > { Origin-Host } { Origin-Realm } { Destination-Realm } { Destination-Host } { Auth-Application-Id } { Re-Auth-Request-Type } [ User-Name ] [ Origin-State-Id ] * [ Proxy-Info ] * [ Route-Record ] * [ AVP ]8.3.2. Re-Auth-Answer
The Re-Auth-Answer (RAA), indicated by the Command Code set to 258 and the message flags' 'R' bit clear, is sent in response to the RAR. The Result-Code AVP MUST be present, and it indicates the disposition of the request.
A successful RAA message MUST be followed by an application-specific
authentication and/or authorization message.
Message Format
<RAA> ::= < Diameter Header: 258, PXY >
< Session-Id >
{ Result-Code }
{ Origin-Host }
{ Origin-Realm }
[ User-Name ]
[ Origin-State-Id ]
[ Error-Message ]
[ Error-Reporting-Host ]
[ Failed-AVP ]
* [ Redirect-Host ]
[ Redirect-Host-Usage ]
[ Redirect-Max-Cache-Time ]
* [ Proxy-Info ]
* [ AVP ]
8.4. Session Termination
It is necessary for a Diameter server that authorized a session, for
which it is maintaining state, to be notified when that session is no
longer active, both for tracking purposes as well as to allow
stateful agents to release any resources that they may have provided
for the user's session. For sessions whose state is not being
maintained, this section is not used.
When a user session that required Diameter authorization terminates,
the access device that provided the service MUST issue a Session-
Termination-Request (STR) message to the Diameter server that
authorized the service, to notify it that the session is no longer
active. An STR MUST be issued when a user session terminates for any
reason, including user logoff, expiration of Session-Timeout,
administrative action, termination upon receipt of an Abort-Session-
Request (see below), orderly shutdown of the access device, etc.
The access device also MUST issue an STR for a session that was
authorized but never actually started. This could occur, for
example, due to a sudden resource shortage in the access device, or
because the access device is unwilling to provide the type of service
requested in the authorization, or because the access device does not
support a mandatory AVP returned in the authorization, etc.
It is also possible that a session that was authorized is never
actually started due to action of a proxy. For example, a proxy may
modify an authorization answer, converting the result from success to failure, prior to forwarding the message to the access device. If the answer did not contain an Auth-Session-State AVP with the value NO_STATE_MAINTAINED, a proxy that causes an authorized session not to be started MUST issue an STR to the Diameter server that authorized the session, since the access device has no way of knowing that the session had been authorized. A Diameter server that receives an STR message MUST clean up resources (e.g., session state) associated with the Session-Id specified in the STR and return a Session-Termination-Answer. A Diameter server also MUST clean up resources when the Session- Timeout expires, or when the Authorization-Lifetime and the Auth- Grace-Period AVPs expire without receipt of a re-authorization request, regardless of whether an STR for that session is received. The access device is not expected to provide service beyond the expiration of these timers; thus, expiration of either of these timers implies that the access device may have unexpectedly shut down.8.4.1. Session-Termination-Request
The Session-Termination-Request (STR), indicated by the Command Code set to 275 and the Command Flags' 'R' bit set, is sent by a Diameter client or by a Diameter proxy to inform the Diameter server that an authenticated and/or authorized session is being terminated. Message Format <STR> ::= < Diameter Header: 275, REQ, PXY > < Session-Id > { Origin-Host } { Origin-Realm } { Destination-Realm } { Auth-Application-Id } { Termination-Cause } [ User-Name ] [ Destination-Host ] * [ Class ] [ Origin-State-Id ] * [ Proxy-Info ] * [ Route-Record ] * [ AVP ]
8.4.2. Session-Termination-Answer
The Session-Termination-Answer (STA), indicated by the Command Code set to 275 and the message flags' 'R' bit clear, is sent by the Diameter server to acknowledge the notification that the session has been terminated. The Result-Code AVP MUST be present, and it MAY contain an indication that an error occurred while servicing the STR. Upon sending or receipt of the STA, the Diameter server MUST release all resources for the session indicated by the Session-Id AVP. Any intermediate server in the Proxy-Chain MAY also release any resources, if necessary. Message Format <STA> ::= < Diameter Header: 275, PXY > < Session-Id > { Result-Code } { Origin-Host } { Origin-Realm } [ User-Name ] * [ Class ] [ Error-Message ] [ Error-Reporting-Host ] [ Failed-AVP ] [ Origin-State-Id ] * [ Redirect-Host ] [ Redirect-Host-Usage ] [ Redirect-Max-Cache-Time ] * [ Proxy-Info ] * [ AVP ]8.5. Aborting a Session
A Diameter server may request that the access device stop providing service for a particular session by issuing an Abort-Session-Request (ASR). For example, the Diameter server that originally authorized the session may be required to cause that session to be stopped for lack of credit or other reasons that were not anticipated when the session was first authorized. An access device that receives an ASR with Session-ID equal to a currently active session MAY stop the session. Whether the access device stops the session or not is implementation and/or configuration dependent. For example, an access device may honor ASRs from certain agents only. In any case, the access device MUST
respond with an Abort-Session-Answer, including a Result-Code AVP to indicate what action it took.8.5.1. Abort-Session-Request
The Abort-Session-Request (ASR), indicated by the Command Code set to 274 and the message flags' 'R' bit set, may be sent by any Diameter server or any Diameter proxy to the access device that is providing session service, to request that the session identified by the Session-Id be stopped. Message Format <ASR> ::= < Diameter Header: 274, REQ, PXY > < Session-Id > { Origin-Host } { Origin-Realm } { Destination-Realm } { Destination-Host } { Auth-Application-Id } [ User-Name ] [ Origin-State-Id ] * [ Proxy-Info ] * [ Route-Record ] * [ AVP ]8.5.2. Abort-Session-Answer
The Abort-Session-Answer (ASA), indicated by the Command Code set to 274 and the message flags' 'R' bit clear, is sent in response to the ASR. The Result-Code AVP MUST be present and indicates the disposition of the request. If the session identified by Session-Id in the ASR was successfully terminated, the Result-Code is set to DIAMETER_SUCCESS. If the session is not currently active, the Result-Code is set to DIAMETER_UNKNOWN_SESSION_ID. If the access device does not stop the session for any other reason, the Result-Code is set to DIAMETER_UNABLE_TO_COMPLY.
Message Format
<ASA> ::= < Diameter Header: 274, PXY >
< Session-Id >
{ Result-Code }
{ Origin-Host }
{ Origin-Realm }
[ User-Name ]
[ Origin-State-Id ]
[ Error-Message ]
[ Error-Reporting-Host ]
[ Failed-AVP ]
* [ Redirect-Host ]
[ Redirect-Host-Usage ]
[ Redirect-Max-Cache-Time ]
* [ Proxy-Info ]
* [ AVP ]
8.6. Inferring Session Termination from Origin-State-Id
The Origin-State-Id is used to allow detection of terminated sessions
for which no STR would have been issued, due to unanticipated
shutdown of an access device.
A Diameter client or access device increments the value of the
Origin-State-Id every time it is started or powered up. The new
Origin-State-Id is then sent in the CER/CEA message immediately upon
connection to the server. The Diameter server receiving the new
Origin-State-Id can determine whether the sending Diameter client had
abruptly shut down by comparing the old value of the Origin-State-Id
it has kept for that specific client is less than the new value and
whether it has un-terminated sessions originating from that client.
An access device can also include the Origin-State-Id in request
messages other than the CER if there are relays or proxies in between
the access device and the server. In this case, however, the server
cannot discover that the access device has been restarted unless and
until it receives a new request from it. Therefore, this mechanism
is more opportunistic across proxies and relays.
The Diameter server may assume that all sessions that were active
prior to detection of a client restart have been terminated. The
Diameter server MAY clean up all session state associated with such
lost sessions, and it MAY also issue STRs for all such lost sessions
that were authorized on upstream servers, to allow session state to
be cleaned up globally.
8.7. Auth-Request-Type AVP
The Auth-Request-Type AVP (AVP Code 274) is of type Enumerated and is included in application-specific auth requests to inform the peers whether a user is to be authenticated only, authorized only, or both. Note any value other than both MAY cause RADIUS interoperability issues. The following values are defined: AUTHENTICATE_ONLY 1 The request being sent is for authentication only, and it MUST contain the relevant application-specific authentication AVPs that are needed by the Diameter server to authenticate the user. AUTHORIZE_ONLY 2 The request being sent is for authorization only, and it MUST contain the application-specific authorization AVPs that are necessary to identify the service being requested/offered. AUTHORIZE_AUTHENTICATE 3 The request contains a request for both authentication and authorization. The request MUST include both the relevant application-specific authentication information and authorization information necessary to identify the service being requested/ offered.8.8. Session-Id AVP
The Session-Id AVP (AVP Code 263) is of type UTF8String and is used to identify a specific session (see Section 8). All messages pertaining to a specific session MUST include only one Session-Id AVP, and the same value MUST be used throughout the life of a session. When present, the Session-Id SHOULD appear immediately following the Diameter header (see Section 3). The Session-Id MUST be globally and eternally unique, as it is meant to uniquely identify a user session without reference to any other information, and it may be needed to correlate historical authentication information with accounting information. The Session-Id includes a mandatory portion and an implementation-defined portion; a recommended format for the implementation-defined portion is outlined below. The Session-Id MUST begin with the sender's identity encoded in the DiameterIdentity type (see Section 4.3.1). The remainder of the Session-Id is delimited by a ";" character, and it MAY be any
sequence that the client can guarantee to be eternally unique;
however, the following format is recommended, (square brackets []
indicate an optional element):
<DiameterIdentity>;<high 32 bits>;<low 32 bits>[;<optional value>]
<high 32 bits> and <low 32 bits> are decimal representations of the
high and low 32 bits of a monotonically increasing 64-bit value. The
64-bit value is rendered in two part to simplify formatting by 32-bit
processors. At startup, the high 32 bits of the 64-bit value MAY be
initialized to the time in NTP format [RFC5905], and the low 32 bits
MAY be initialized to zero. This will for practical purposes
eliminate the possibility of overlapping Session-Ids after a reboot,
assuming the reboot process takes longer than a second.
Alternatively, an implementation MAY keep track of the increasing
value in non-volatile memory.
<optional value> is implementation specific, but it may include a
modem's device Id, a Layer 2 address, timestamp, etc.
Example, in which there is no optional value:
accesspoint7.example.com;1876543210;523
Example, in which there is an optional value:
accesspoint7.example.com;1876543210;523;mobile@200.1.1.88
The Session-Id is created by the Diameter application initiating the
session, which, in most cases, is done by the client. Note that a
Session-Id MAY be used for both the authentication, authorization,
and accounting commands of a given application.
8.9. Authorization-Lifetime AVP
The Authorization-Lifetime AVP (AVP Code 291) is of type Unsigned32
and contains the maximum number of seconds of service to be provided
to the user before the user is to be re-authenticated and/or re-
authorized. Care should be taken when the Authorization-Lifetime
value is determined, since a low, non-zero value could create
significant Diameter traffic, which could congest both the network
and the agents.
A value of zero (0) means that immediate re-auth is necessary by the
access device. The absence of this AVP, or a value of all ones
(meaning all bits in the 32-bit field are set to one) means no re-
auth is expected.
If both this AVP and the Session-Timeout AVP are present in a message, the value of the latter MUST NOT be smaller than the Authorization-Lifetime AVP. An Authorization-Lifetime AVP MAY be present in re-authorization messages, and it contains the number of seconds the user is authorized to receive service from the time the re-auth answer message is received by the access device. This AVP MAY be provided by the client as a hint of the maximum lifetime that it is willing to accept. The server MUST return a value that is equal to, or smaller than, the one provided by the client.8.10. Auth-Grace-Period AVP
The Auth-Grace-Period AVP (AVP Code 276) is of type Unsigned32 and contains the number of seconds the Diameter server will wait following the expiration of the Authorization-Lifetime AVP before cleaning up resources for the session.8.11. Auth-Session-State AVP
The Auth-Session-State AVP (AVP Code 277) is of type Enumerated and specifies whether state is maintained for a particular session. The client MAY include this AVP in requests as a hint to the server, but the value in the server's answer message is binding. The following values are supported: STATE_MAINTAINED 0 This value is used to specify that session state is being maintained, and the access device MUST issue a session termination message when service to the user is terminated. This is the default value. NO_STATE_MAINTAINED 1 This value is used to specify that no session termination messages will be sent by the access device upon expiration of the Authorization-Lifetime.8.12. Re-Auth-Request-Type AVP
The Re-Auth-Request-Type AVP (AVP Code 285) is of type Enumerated and is included in application-specific auth answers to inform the client of the action expected upon expiration of the Authorization-Lifetime.
If the answer message contains an Authorization-Lifetime AVP with a
positive value, the Re-Auth-Request-Type AVP MUST be present in an
answer message. The following values are defined:
AUTHORIZE_ONLY 0
An authorization only re-auth is expected upon expiration of the
Authorization-Lifetime. This is the default value if the AVP is
not present in answer messages that include the Authorization-
Lifetime.
AUTHORIZE_AUTHENTICATE 1
An authentication and authorization re-auth is expected upon
expiration of the Authorization-Lifetime.
8.13. Session-Timeout AVP
The Session-Timeout AVP (AVP Code 27) [RFC2865] is of type Unsigned32
and contains the maximum number of seconds of service to be provided
to the user before termination of the session. When both the
Session-Timeout and the Authorization-Lifetime AVPs are present in an
answer message, the former MUST be equal to or greater than the value
of the latter.
A session that terminates on an access device due to the expiration
of the Session-Timeout MUST cause an STR to be issued, unless both
the access device and the home server had previously agreed that no
session termination messages would be sent (see Section 8).
A Session-Timeout AVP MAY be present in a re-authorization answer
message, and it contains the remaining number of seconds from the
beginning of the re-auth.
A value of zero, or the absence of this AVP, means that this session
has an unlimited number of seconds before termination.
This AVP MAY be provided by the client as a hint of the maximum
timeout that it is willing to accept. However, the server MAY return
a value that is equal to, or smaller than, the one provided by the
client.
8.14. User-Name AVP
The User-Name AVP (AVP Code 1) [RFC2865] is of type UTF8String, which
contains the User-Name, in a format consistent with the NAI
specification [RFC4282].
8.15. Termination-Cause AVP
The Termination-Cause AVP (AVP Code 295) is of type Enumerated, and is used to indicate the reason why a session was terminated on the access device. The currently assigned values for this AVP can be found in the IANA registry for Termination-Cause AVP Values [IANATCV].8.16. Origin-State-Id AVP
The Origin-State-Id AVP (AVP Code 278), of type Unsigned32, is a monotonically increasing value that is advanced whenever a Diameter entity restarts with loss of previous state, for example, upon reboot. Origin-State-Id MAY be included in any Diameter message, including CER. A Diameter entity issuing this AVP MUST create a higher value for this AVP each time its state is reset. A Diameter entity MAY set Origin-State-Id to the time of startup, or it MAY use an incrementing counter retained in non-volatile memory across restarts. The Origin-State-Id, if present, MUST reflect the state of the entity indicated by Origin-Host. If a proxy modifies Origin-Host, it MUST either remove Origin-State-Id or modify it appropriately as well. Typically, Origin-State-Id is used by an access device that always starts up with no active sessions; that is, any session active prior to restart will have been lost. By including Origin-State-Id in a message, it allows other Diameter entities to infer that sessions associated with a lower Origin-State-Id are no longer active. If an access device does not intend for such inferences to be made, it MUST either not include Origin-State-Id in any message or set its value to 0.8.17. Session-Binding AVP
The Session-Binding AVP (AVP Code 270) is of type Unsigned32, and it MAY be present in application-specific authorization answer messages. If present, this AVP MAY inform the Diameter client that all future application-specific re-auth and Session-Termination-Request messages for this session MUST be sent to the same authorization server.
This field is a bit mask, and the following bits have been defined:
RE_AUTH 1
When set, future re-auth messages for this session MUST NOT
include the Destination-Host AVP. When cleared, the default
value, the Destination-Host AVP MUST be present in all re-auth
messages for this session.
STR 2
When set, the STR message for this session MUST NOT include the
Destination-Host AVP. When cleared, the default value, the
Destination-Host AVP MUST be present in the STR message for this
session.
ACCOUNTING 4
When set, all accounting messages for this session MUST NOT
include the Destination-Host AVP. When cleared, the default
value, the Destination-Host AVP, if known, MUST be present in all
accounting messages for this session.
8.18. Session-Server-Failover AVP
The Session-Server-Failover AVP (AVP Code 271) is of type Enumerated
and MAY be present in application-specific authorization answer
messages that either do not include the Session-Binding AVP or
include the Session-Binding AVP with any of the bits set to a zero
value. If present, this AVP MAY inform the Diameter client that if a
re-auth or STR message fails due to a delivery problem, the Diameter
client SHOULD issue a subsequent message without the Destination-Host
AVP. When absent, the default value is REFUSE_SERVICE.
The following values are supported:
REFUSE_SERVICE 0
If either the re-auth or the STR message delivery fails, terminate
service with the user and do not attempt any subsequent attempts.
TRY_AGAIN 1
If either the re-auth or the STR message delivery fails, resend
the failed message without the Destination-Host AVP present.
ALLOW_SERVICE 2
If re-auth message delivery fails, assume that re-authorization
succeeded. If STR message delivery fails, terminate the session.
TRY_AGAIN_ALLOW_SERVICE 3
If either the re-auth or the STR message delivery fails, resend
the failed message without the Destination-Host AVP present. If
the second delivery fails for re-auth, assume re-authorization
succeeded. If the second delivery fails for STR, terminate the
session.
8.19. Multi-Round-Time-Out AVP
The Multi-Round-Time-Out AVP (AVP Code 272) is of type Unsigned32 and
SHOULD be present in application-specific authorization answer
messages whose Result-Code AVP is set to DIAMETER_MULTI_ROUND_AUTH.
This AVP contains the maximum number of seconds that the access
device MUST provide the user in responding to an authentication
request.
8.20. Class AVP
The Class AVP (AVP Code 25) is of type OctetString and is used by
Diameter servers to return state information to the access device.
When one or more Class AVPs are present in application-specific
authorization answer messages, they MUST be present in subsequent re-
authorization, session termination and accounting messages. Class
AVPs found in a re-authorization answer message override the ones
found in any previous authorization answer message. Diameter server
implementations SHOULD NOT return Class AVPs that require more than
4096 bytes of storage on the Diameter client. A Diameter client that
receives Class AVPs whose size exceeds local available storage MUST
terminate the session.
8.21. Event-Timestamp AVP
The Event-Timestamp (AVP Code 55) is of type Time and MAY be included
in an Accounting-Request and Accounting-Answer messages to record the
time that the reported event occurred, in seconds since January 1,
1900 00:00 UTC.
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