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7. Error Handling

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:

  • 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.

  • 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.

  • 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.

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