Decode and encode of Diameter messages.

Incoming Diameter messages are decoded from binary() before being communicated to diameter_app(3) callbacks. Similarly, outgoing Diameter messages are encoded into binary() before being passed to the appropriate diameter_transport(3) module for transmission. The functions documented here implement the default encode/decode.


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(3): 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.


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


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 or the AVP is unknown. 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 file in question.


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() | 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 a list of {AvpName, AvpValue} pairs.

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 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 record 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.


A record-valued 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(3). 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(3).


decode(Mod, Bin) -> Pkt

Decode a Diameter message.

encode(Mod, Msg) -> Pkt

Encode a Diameter message.