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# diameter_codec (diameter v2.5.1)

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Decode and encode of Diameter messages.

Incoming Diameter messages are decoded from binary() before being communicated to `diameter_app` callbacks. Similarly, outgoing Diameter messages are encoded into binary() before being passed to the appropriate `diameter_transport` module for transmission. The functions documented here implement the default encode/decode.

#### Warning

The diameter user does not need to call functions here explicitly when sending and receiving messages using `diameter:call/4` and the callback interface documented in `diameter_app`: diameter itself provides encode/decode as a consequence of configuration passed to `diameter:start_service/2`, and the results may differ from those returned by the functions documented here, depending on configuration.

The header() and packet() records below are defined in diameter.hrl, which can be included as follows.

    -include_lib("diameter/include/diameter.hrl").

Application-specific records are defined in the hrl files resulting from dictionary file compilation.

## DATA TYPES

- `uint8() = 0..255`

- `uint24() = 0..16777215`

- `uint32() = 0..4294967295` - 8-bit, 24-bit and 32-bit integers occurring in Diameter and AVP headers.

- `avp() = #diameter_avp{}` - The application-neutral representation of an AVP. Primarily intended for use by relay applications that need to handle arbitrary Diameter applications. A service implementing a specific Diameter application (for which it configures a dictionary) can manipulate values of type message() instead.

  Fields have the following types.

  - `code = uint32()`

  - `is_mandatory = boolean()`

  - `need_encryption = boolean()`

  - `vendor_id = uint32() | undefined` - Values in the AVP header, corresponding to AVP Code, the M flag, P flags and Vendor-ID respectively. A Vendor-ID other than `undefined` implies a set V flag.

  - `data = iolist()` - The data bytes of the AVP.

  - `name = atom()` - The name of the AVP as defined in the dictionary file in question, or `undefined` if the AVP is unknown to the dictionary file in question.

  - `value = term()` - The decoded value of an AVP. Will be `undefined` on decode if the data bytes could not be decoded, the AVP is unknown, or if the decode format is `none`. The type of a decoded value is as document in diameter_dict(4).

  - `type = atom()` - The type of the AVP as specified in the dictionary file in question (or one it inherits). Possible types are `undefined` and the Diameter types: `OctetString`, `Integer32`, `Integer64`, `Unsigned32`, `Unsigned64`, `Float32`, `Float64`, `Grouped`, `Enumerated`, `Address`, `Time`, `UTF8String`, `DiameterIdentity`, `DiameterURI`, `IPFilterRule` and `QoSFilterRule`.

- `dictionary() = module()` - The name of a generated dictionary module as generated by diameterc(1) or `diameter_make:codec/2`. The interface provided by a dictionary module is an implementation detail that may change.

- `header() = #diameter_header{}` - The record representation of the Diameter header. Values in a packet() returned by `decode/2` are as extracted from the incoming message. Values set in an packet() passed to `encode/2` are preserved in the encoded binary(), with the exception of `length`, `cmd_code` and `application_id`, all of which are determined by the dictionary() in question.

  #### Note

  It is not necessary to set header fields explicitly in outgoing messages as diameter itself will set appropriate values. Setting inappropriate values can be useful for test purposes.

  Fields have the following types.

  - `version = uint8()`

  - `length = uint24()`

  - `cmd_code = uint24()`

  - `application_id = uint32()`

  - `hop_by_hop_id = uint32()`

  - `end_to_end_id = uint32()` - Values of the Version, Message Length, Command-Code, Application-ID, Hop-by-Hop Identifier and End-to-End Identifier fields of the Diameter header.

  - `is_request = boolean()`

  - `is_proxiable = boolean()`

  - `is_error = boolean()`

  - `is_retransmitted = boolean()` - Values corresponding to the R(equest), P(roxiable), E(rror) and T(Potentially re-transmitted message) flags of the Diameter header.

- `message() = record() | maybe_improper_list()` - The representation of a Diameter message as passed to `diameter:call/4` or returned from a handle_request/3 callback. The record representation is as outlined in diameter_dict(4): a message as defined in a dictionary file is encoded as a record with one field for each component AVP. Equivalently, a message can also be encoded as a list whose head is the atom-valued message name (as specified in the relevant dictionary file) and whose tail is either a list of AVP name/values pairs or a map with values keyed on AVP names. The format at decode is determined by diameter:service_opt() decode_format. Any of the formats is accepted at encode.

  Another list-valued representation allows a message to be specified as a list whose head is a header() and whose tail is an avp() list. This representation is used by diameter itself when relaying requests as directed by the return value of a handle_request/3 callback. It differs from the other two in that it bypasses the checks for messages that do not agree with their definitions in the dictionary in question: messages are sent exactly as specified.

- `packet() = #diameter_packet{}` - A container for incoming and outgoing Diameter messages. Fields have the following types.

  - `header =``header()``| undefined` - The Diameter header of the message. Can be (and typically should be) `undefined` for an outgoing message in a non-relay application, in which case diameter provides appropriate values.

  - `avps = [``avp()``] | undefined` - The AVPs of the message. Ignored for an outgoing message if the `msg` field is set to a value other than `undefined`.

  - `msg =``message()``| undefined` - The incoming/outgoing message. For an incoming message, a term corresponding to the configured decode format if the message can be decoded in a non-relay application, `undefined` otherwise. For an outgoing message, setting a `[``header()``|``avp()``]` list is equivalent to setting the `header` and `avps` fields to the corresponding values.

    #### Warning

    A value in the `msg` field does not imply an absence of decode errors. The `errors` field should also be examined.

  - `bin = binary()` - The incoming message prior to encode or the outgoing message after encode.

  - `errors = [5000..5999 | {5000..5999, avp()}]` - Errors detected at decode of an incoming message, as identified by a corresponding 5xxx series Result-Code (Permanent Failures). For an incoming request, these should be used to formulate an appropriate answer as documented for the handle_request/3 callback in `diameter_app`. For an incoming answer, the diameter:application_opt() `answer_errors` determines the behaviour.

  - `transport_data = term()` - An arbitrary term of meaning only to the transport process in question, as documented in `diameter_transport`.

## SEE ALSO

diameterc(1), `diameter_app`, diameter_dict(4), `diameter_make`

# Summary

## Types

dictionary()

The name of a generated dictionary module as generated by diameterc(1) or `diameter_make:codec/2`. The interface provided by a dictionary module is an implementation detail that may change.

message()

The representation of a Diameter message as passed to `diameter:call/4` or returned from a handle_request/3 callback.

packet()

A container for incoming and outgoing Diameter messages.

record()

## Functions

decode(Mod, Bin)

Decode a Diameter message.

encode(Mod, Msg)

Encode a Diameter message.

# Types

# dictionary()

(not exported) (since OTP R15B03)

    -type dictionary() :: module().

The name of a generated dictionary module as generated by diameterc(1) or `diameter_make:codec/2`. The interface provided by a dictionary module is an implementation detail that may change.

# message()

(since OTP R15B03)

    -type message() :: record() | maybe_improper_list().

The representation of a Diameter message as passed to `diameter:call/4` or returned from a handle_request/3 callback.

# packet()

(since OTP R15B03)

    -type packet() ::
              #diameter_packet{header :: term(),
                               avps :: term(),
                               msg :: term(),
                               bin :: term(),
                               errors :: term(),
                               transport_data :: term()}.

A container for incoming and outgoing Diameter messages.

# record()

(not exported) (since OTP R15B03)

    -type record() :: term().

# Functions

# decode(Mod, Bin)

(since OTP R15B03)

    -spec decode(Mod, Bin) -> Pkt when Mod :: dictionary(), Bin :: binary(), Pkt :: packet().

Decode a Diameter message.

# encode(Mod, Msg)

(since OTP R15B03)

    -spec encode(Mod, Msg) -> Pkt when Mod :: dictionary(), Msg :: message() | packet(), Pkt :: packet().

Encode a Diameter message.