Matter Specification

A Universal Group ID (UGID) is one that resides in the 16-bit subrange of Group ID that is reserved for groups that are common across this standard. These special multicast, groupcast, or anycast destinations are constant and known to all Nodes on any Fabric. The Universal Group ID space is allocated as described in Table 3, “Universal Group ID Allocations”:

The Commissioner SHALL configure one or more shared keys for these groups on all Nodes within the Fabric. Because the keys and IPv6 multicast prefixes are different across Fabrics, Universal Groups only enable communication within a specific Fabric.

An Operational Node ID is a 64-bit number that uniquely identifies an individual Node on a Fabric. All messages must have an Operational Node ID as the source address. All unicast messages must have an Operational Node ID as the destination address.

While source or destination address MAY be elided from a message, it MUST remain unambiguously derivable from the Session ID.

A Group Node ID is a 64-bit Node ID that contains a particular Group ID in the lower half of the Node ID.

A Temporary Local Node ID is a 64-bit Node ID that contains an implementation-dependent value in its lower 32 bits. This allows implementations to keep track of connections or transport-layer links and similar housekeeping internal usage purposes in contexts where an Operational Node ID is unavailable.

This subrange of Node ID is used to assign an access control subject to a particular PAKE key as specified in Section 6.6.2.1.1, “PASE and Group Subjects”. An example usage would be to create an ACL entry to provide administrative access to any commissioner communicating via a PASE session established with a particular pincode.

This subrange of Node ID is used to assign an access control subject to a group of peer nodes that share a single CASE session as specified in Section 6.6.2.1.2, “Subjects identified by CASE Authenticated Tag”.

The Unspecified Node ID (0x0000_0000_0000_0000) is a reserved value that never appears in messages or protocol usage. It exists to mark or detect the presence of uninitialized, missing, or invalid Node IDs.

An IPv6 Unicast Address is one that uniquely identifies and addresses a single Node on an IPv6 network. A primary design goal for this standard is to allow Nodes to leverage native IPv6 technologies. As such, an operational IPv6 Unicast address that provides connectivity and routability between Nodes SHALL be used. This includes a global unicast address (GUA), a link-local address (LLA), or a unique local address (ULA).

An IPv6 Multicast Address is formed using Unicast-Prefix-based IPv6 Multicast Addresses [RFC 3306]:

The network prefix portion of the Multicast Address is the 64-bit bitstring formed by concatenating:

The 32-bit group identifier portion of the Multicast Address is the 32-bits formed by:

An example of the site local scoped multicast address for a given <Fabric ID> and <Group ID>:

The Multicast Address formation ensures a low probability of a node receiving a multicast message it is not interested in. If a collision does occur on the multicast address (which requires two identical 64-bit Fabric IDs and two identical 16-bit Group IDs), processing of the message disambiguates which fabric and group is relevant by checking which operational group key leads to the message’s 64-bit MIC.

The IANA assigned port number is 5540.

Matter devices SHALL be tolerant of IPv4 addresses and MAY ignore those addresses for the purposes of Matter operations.

The Section 10.5.2.1, “EnableTagCompression” mechanism for encoding the interaction model is provisional.

Nodes SHALL maintain a stored Last Known Good UTC Time. This time is used as a fallback for cryptographic credentials expiry enforcement, if all other available time synchronization mechanisms fail.

The last known good UTC time SHALL be updated at commissioning and MAY be updated after a successful time synchronization, or by an embedded time in an OTA. Nodes SHOULD store a Last Known Good UTC Time value to persistent storage at least once a month. A Node’s initial out-of-box Last Known Good UTC time SHALL be the compile-time of the firmware.

A Node MAY adjust the Last Known Good UTC Time backwards if it believes the current Last Known Good UTC Time is incorrect and it has a good time value from a trusted source. The Node SHOULD NOT adjust the Last Known Good UTC to a time before the later of:

If a Node has used the Last Known Good UTC Time, it SHOULD recheck its security materials and existing connections if it later achieves time synchronization.

Messages are defined as either a control message or data message. Most messages are data messages. Control messages are reserved for internal protocols such as MCSP to initialize security. Both message types are identical in format, but use separate message counter domains so they can operate securely over the same security key.

Messages are of finite size and are sent as individual packets over the supported transports:

BTP is provided as a transport protocol for commissioning. TCP and MRP (UDP with added reliability) are provided as transport protocols for operational messaging.

Messages provide an Exchange Layer to track related messages that make up small, discrete transactions. The Exchange Layer provides this transaction tracking facility to the Interaction Model Layer above, providing a means to multiplex multiple such concurrent transactions over a given underlying session. The Exchange Layer also integrates the Message Reliability Protocol (MRP) as a service for use over UDP transports. This Message Layer architecture is shown below in Figure 6, “Message Layer Stack”:

For DNS‑SD a target host name is required, in addition to the instance name. The target host name SHALL be constructed using one of the available link-layer addresses, such as a 48-bit device MAC address (for Ethernet and Wi‑Fi) or a 64-bit MAC Extended Address (for Thread) expressed as a fixed-length twelve-character (or sixteen-character) hexadecimal string, encoded as ASCII (UTF-8) text using capital letters, e.g., B75AFB458ECD.<domain>. In the event that a device performs MAC address randomization for privacy, then the target host name SHALL use the privacy-preserving randomized version and the hostname SHALL be updated in the record every time the underlying link-layer address rotates. Note that it is legal to reuse the same hostname on more than one interface, even if the underlying link-layer address does not match the hostname on that interface, since the goal of using a link-layer address is to ensure local uniqueness of the generated hostname. If future link layers are supported by Matter that do not use 48-bit MAC addresses or 64-bit MAC Extended Address identifiers, then a similar rule will be defined for those technologies.

A Matter Commissionee that advertises Commissionable Node Discovery service records is not necessarily in a state that will allow Commissioning (this state is referred to below as "in Commissioning Mode"). While Section 5.4.2.3, “Announcement Duration” is limited for some forms of device advertisement, a Matter device MAY advertise Matter Commissionable Node Discovery service records for longer periods, possibly permanently. Advertising Commissionable Node Discovery when not in Commissioning Mode is referred to here as Extended Discovery. Extended Discovery is allowed only for DNS-SD advertisements and not for the other forms of Device Discovery such as BLE Commissioning Discovery and Wi-Fi Temporary Access Point Commissioning Discovery.

To protect customer privacy on public networks, a Matter Commissionee SHALL provide a way for the customer to set a timeout on Extended Discovery, or otherwise disable Extended Discovery. The default behavior for Commissionable Node Discovery SHOULD default to limiting announcement as defined in Section 5.4.2.3, “Announcement Duration” unless the Manufacturer wishes to enable longer periods for specific use cases.

The following subtypes for Matter Commissionable Node Discovery are defined:

The long discriminator subtype (e.g., _L840) enables filtering of results to find only Commissionees that match the full discriminator code, as provided in the onboarding payload.

The short discriminator subtype (e.g., _S3) enables filtering of results to find only Commissionees that match the upper 4 bits of the discriminator code, as provided in the manual pairing code.

The optional Vendor ID subtype (e.g., _V123) enables a vendor-specific app to achieve filtering of results to find only Nodes that match that Vendor ID.

The Commissioning Mode subtype (e.g., _CM) enables filtering of results to find only Nodes that are currently in Commissioning Mode. Note that the subtype is _CM regardless of whether the TXT record for commissioning mode is set to 1 (CM=1) or 2 (CM=2). A Commissionee that is not in commissioning mode (CM=0) SHALL NOT publish this subtype.

The optional device type subtype (e.g., _T10) enables filtering of results to find only Nodes that match the device type, generally used for the User-Initiated Beacon Detection, Not Yet Commissioned Device and the User-Initiated Beacon Detection, Already Commissioned Device use cases.

In the event that a vendor-specific app wishes to show the user only some of that vendor’s Commissionees awaiting commissioning but not all of them, any desired filtering logic (based upon arbitrary criteria, not only Product ID) MAY be implemented within that vendor’s proprietary commissioning app.

After discovery, IPv6 addresses are returned in the AAAA records and key/value pairs are returned in the DNS‑SD TXT record.

Nodes SHALL publish AAAA records for all available IPv6 addresses upon which they are willing to accept Matter commissioning messages.

TXT records available for Commissionable Node Discovery include the common TXT record key/value pairs defined in Section 4.3.4, “Common TXT Key/Value Pairs”.

Commissioners SHALL silently ignore TXT record keys that they do not recognize. This is to facilitate future evolution of this specification without breaking backwards compatibility with existing Commissioners that do not implement the new functionality.

The following subsections describe key/value pairs that are defined specifically for Commissionable Node discovery.

The key D SHALL provide the full 12-bit discriminator for the Commissionable Node and SHALL be present in the DNS-SD TXT record.

The discriminator value SHALL be encoded as a variable-length decimal number in ASCII text, with up to four digits, omitting any leading zeroes.

Any key D with a value mismatching the aforementioned format SHALL be silently ignored.

As an example, value D=840 would indicate that this Commissionable Node has decimal long discriminator 840. When needed, the upper 4 bits of the discriminator provided by the manual pairing code can be algorithmically derived from the full discriminator.

The optional key VP, if present, MAY provide Vendor ID and Product ID information of the device.

A vendor MAY choose not to include it at all, for privacy reasons.

If the VP key is present then it MAY take two forms:

The Vendor ID and Product ID SHALL both be expressed as variable-length decimal numbers, encoded as ASCII text, omitting any leading zeroes, and of maximum length of 5 characters each to fit their 16-bit numerical range.

If the Product ID is present, it SHALL be separated from the Vendor ID using a ‘+’ character.

If the VP key is present without a Product ID, the value SHALL contain only the Vendor ID alone, with no ‘+’ character.

If the VP key is present, the value SHALL contain at least the Vendor ID.

If the VP key is present, it SHALL NOT have a missing or empty value.

The key CM (Commissioning Mode) SHALL indicate whether or not the publisher of the record is currently in Commissioning Mode and available for immediate commissioning. When in commissioning mode, the value associated with the CM key indicates the source of the passcode.

Four situations are legal:

A key value of 2 MAY be used to disambiguate collisions of discriminators between uncommissioned Nodes and commissioned Nodes announcing after a commissioning window was opened. A key value of 2 serves as a hint to Commissioners to possibly expect multiple Nodes with the same discriminator (see Commissioning Discriminator), and to instruct the user to enter the Onboarding Payload presented by another Administrator rather than a code provided by the Commissionee.

Since Extended Discovery can be disabled by the customer, a key value of 0 may not ever be returned by a publisher. When Extended Discovery is disabled and the publisher is not in commissioning mode, then the publisher will not respond to Commissionable Node Discovery.

The optional key DT MAY provide the publisher’s primary device type (see Section 11.22.5.3, “DeviceTypeID”). In case the device combines multiple device types, the manufacturer SHOULD choose the device type identifier of the primary function of the device for which the device wishes to be discoverable. If present, it SHALL be encoded as a variable-length decimal number in ASCII text, omitting any leading zeroes.

For example, the DT=10 key/value pair would indicate that the primary device type is 10 (0x000A), which is the device type identifier for a Door Lock.

The optional key DN MAY provide a device advertisement name. If present, it SHALL be encoded as a valid UTF-8 string with a maximum length of 32 bytes (matching the maximum length of the NodeLabel string in the Basic Information Cluster).

When provided, the source of this value SHALL be editable by the user with use clearly designated as being for on-network advertising and MAY be the value stored in the NodeLabel attribute of the Basic Information Cluster) of the Node.

To protect customer privacy on public networks, if a Commissionee supports this key/value pair, then the Commissionee SHALL provide a way for the customer to disable its inclusion.

A Commissionee SHOULD NOT include this field unless doing so for specific use cases which call for it.

For example, the DN=Living Room key/value pair indicates that the advertisement name specified by the user is 'Living Room'.

The optional key RI MAY provide a Rotating Device Identifier.

If present, the value associated with the RI key SHALL contain the octets of the Rotating Device Identifier octet string encoded as the concatenation of each octet’s value as a 2-digit uppercase hexadecimal number.

The resulting ASCII string SHALL NOT be longer than 100 characters, which implies a Rotating Device Identifier of at most 50 octets.

The optional key PH MAY provide a pairing hint.

If present, it SHALL be encoded as a variable-length decimal number in ASCII text, omitting any leading zeroes.

The pairing hint represents a base-10 numeric value for a bitmap of methods supported by the Commissionee in its current state for putting it in Commissioning Mode.

For example, the PH=5 key/value pair represents a hint value with bits 0 and 2 are set.

This value MAY change during the lifecycle of the device.

For example, a device may have a value with bit 0 (Power Cycle) set and bit 2 (Administrator app) unset when in a factory reset state, and then have a value with bit 0 unset and bit 2 set after it has been Commissioned.

The bitmap of methods is defined in Table 5, “Pairing Hint Values”.

If the Commissionee has enabled Extended Discovery, then it SHALL include the key/value pair for PH in the DNS‑SD TXT record when not in Commissioning Mode (CM=0).

This key/value pair MAY be returned when in Commissioning Mode (CM=1).

If the Commissioner does not recognize this value, for example, if the value indicates bit indices defined in a newer version of this specification than the version which the Commissioner implements, then the Commissioner MAY utilize the bits that it does understand and MAY utilize additional data sets available for assisting the user. For example, when a Vendor ID and Product ID are available to the Commissioner, the Section 11.22, “Distributed Compliance Ledger” may also provide a URL for the Device User Guide which can contain additional information to help in Commissioning this Commissionee.

Some of the pairing hints MAY require additional information to be encoded for proper expression of their meaning. This is accomplished with the PI TXT key, described in a following section. Dependency on usage of the PI key is expressed by the PI Dependency column in the table below.

The following fields in the bitmap are currently defined for values of the PH key:

Note 1: When the PH key indicates a light to blink (one or more of bits 9, 11, 16 or 18 is set), information on color of light MAY be made available via PI key. When using such color indication in PI key, only basic primary and secondary colors that could unambiguously be decoded by a commissioner and understood by an end-user, but without worry of localization, SHOULD be used, e.g. white, red, green, blue, orange, yellow, purple.

Note 2: Any undefined values are reserved for future use.

Note 3: A Commissionee can indicate multiple ways of being put into Commissioning Mode by setting multiple bits in the bitmap at the same time. However, only one method can be specified which has a dependency on the PI key (PI Dependency=True) at a time.

For example:

When the PH key is provided, at least one bit in the above bitmap SHALL be set. That is, a PH value of 0 is undefined and illegal.

When the PH key is provided, the Commissioner SHOULD take its value into account when providing guidance to the user regarding steps required to put the Commissionee into Commissioning Mode.

The optional key PI MAY give the pairing instruction.

If present, the value SHALL be encoded as a valid UTF-8 string with a maximum length of 128 bytes.

The meaning of this key is dependent upon the PH key value, see Table 5, “Pairing Hint Values”.

For example, given PH=256, bit 8 is set which indicates "Press Reset Button for N seconds". Therefore, a value of PI=10 would indicate that N is 10 in that context.

When bit 4 of the value expressed by the PH key is set, indicating presence of a custom string, the Commissionee SHALL be responsible for localization (translation to user’s preferred language) as required using the Device’s currently configured locale. The Custom Instruction option is NOT recommended for use by a Commissionee that does not have knowledge of the user’s language preference.

It is RECOMMENDED to keep the length of PI field small and adhere to the guidance given in section 6.2 of [RFC 6763].

This key/value pair SHALL only be returned in the DNS‑SD TXT record if the PH bitmap value has a bit set which has PI Dependency = True, see Table 5, “Pairing Hint Values”. The PH key SHALL NOT not have more than one bit set which has a dependency on the PI key (PI Dependency = True) to avoid ambiguity in PI key usage.

The examples below simulate a Node in commissioning mode advertising its availability for commissioning.

Examples are shown using both the dns-sd command-line test tool and the avahi command-line test tool. The dns-sd command-line test tool is included in all versions of macOS. It is installed as a DOS command with Bonjour for Windows, and is available on Linux by installing the mDNSResponder package. The Avahi package of command line tools is available from the Avahi project for most Linux distributions.

These examples are given for illustrative purposes only. Real Matter Commissionees and Commissioners would not use a command-line test tool for advertising and discovery. Real Matter Commissionees and Commissioners would use the appropriate DNS‑SD APIs in their respective chosen programming languages.

Consider a device on Wi-Fi using the 48-bit device MAC address of B75AFB458ECD as its host name and a value of DD200C20D25AE5F7 as its commissionable service instance name. DNS-SD records for it can be set up as follows:

or

Avahi only sends a single AAAA record. To force the link-local address to be used, use avahi-publish-address. For example:

The DNS‑SD service registration commands shown above results in the creation of the following Multicast DNS records:

Consider a device on Wi-Fi using the 48-bit device MAC address of B75AFB458ECD as its host name. DNS-SD records for it can be set up as follows, when it is in Commissionable Node Discovery.

or

Avahi only sends a single AAAA record. To force the link-local address to be used, use avahi-publish-address. For example:

The DNS‑SD service registration commands shown above results in the creation of the following Multicast DNS records:

The port number 11111 is given here purely as an example. One of the benefits of using DNS‑SD is that services are not constrained to use a single predetermined well-known port. The port, along with the IPv6 address, is discovered by Commissioners at run time.

A Commissioner can discover all available Commissionees awaiting commissioning:

or

A Commissioner can discover Commissionees awaiting commissioning with short discriminator 3:

or

A Commissioner can discover Commissionees awaiting commissioning with long discriminator 840:

or

A Commissioner can discover Commissionees awaiting commissioning with Vendor ID 123:

or

A Commissioner can discover all Commissionees in commissioning mode:

or

A commissioner can discover Matter Nodes with Device Type 81:

or

A Commissioner can discover Nodes that are currently in Commissioning Mode as a result of a commissioning window opened by a current Administrator as a result of invoking either the Open Commissioning Window command or the Open Basic Commissioning Window command, using the presence of the _CM subtype as a browsing filter:

or

Discovering and using an offered service on the network typically involves several steps:

Although the first three steps are exposed in some APIs as separate steps, at a protocol level they usually require only a single network round-trip. When a PTR query is issued to discover service instances, the usual DNS Additional Record mechanism, where packet space permits, automatically places the related SRV, TXT, and address records into the Additional Record section of the reply. These additional records are stored by the client, to enable subsequent steps in the sequence to be performed without additional redundant network operations to learn the same information a second time.

DNS‑SD over Multicast DNS works by receiving replies from other Nodes attached to the same local link, Nodes that may have been previously completely unknown to the requester. Because of this, Multicast DNS, like IPv6 Neighbor Discovery, does not have any easy way to distinguish genuine replies from malicious or fraudulent replies. Consequently, application-layer end-to-end security is essential. Should a malicious device on the same local link give deliberately malicious or fraudulent replies, the misbehavior will be detected when the device is unable to establish a cryptographically secure application-layer communication channel. This reduces the threat to a Denial-of-Service attack, which can be remedied by physically removing the offending device.

For Matter operational discovery the DNS‑SD instance name is constructed from a 64-bit compressed Fabric identifier, and a 64-bit Node identifier, as assigned by the commissioner, each expressed as a fixed-length sixteen-character hexadecimal string, encoded as ASCII (UTF-8) text using capital letters, separated by a hyphen. For example, a Matter Node with Matter compressed fabric identifier 2906-C908-D115-D362 and Matter Node identifier 8FC7-7724-01CD-0696 has Matter operational discovery DNS‑SD instance name 2906C908D115D362-8FC7772401CD0696.

The Matter operational discovery DNS‑SD instance name needs to be unique within the namespace of the local network (the .local link-local namespace of the Ethernet and Wi‑Fi links [RFC 6762], or the unicast domain selected by the Thread Border Router for devices on the Thread mesh). This uniqueness is assumed to be guaranteed by appropriate selection of a unique Matter fabric identifier and unique Node identifier within that Matter fabric.

In order to reduce the very large size of a full Fabric Reference which would need to be used as the scoping construct in the instance name, a 64-bit compressed version of the full Fabric Reference SHALL be used. The computation of the Compressed Fabric Identifier SHALL be as follows:

Where:

For example, if the root public key for a given Operational Certificate chain containing the identity to be advertised were the following:

Then the value for TargetOperationalRootPublicKey to use in the derivation of the compressed Fabric Identifier would be without the leading 04:

If using the above TargetOperationalRootPublicKey and a TargetOperationalFabricID value of 0x2906_C908_D115_D362 (octet string 29:06:c9:08:d1:15:d3:62 in big-endian), then the CompressedFabricIdentifier to use in advertising would be 87E1B004E235A130 (octet string 87:e1:b0:04:e2:35:a1:30).

The DNS‑SD service type [RFC 6335] for Matter Operational Discovery is _matter._tcp. Note that the string _tcp is boilerplate text inherited from the original DNS SRV specification [RFC 2782], and doesn’t necessarily mean that the advertised application-layer protocol runs only over TCP. It is merely mnemonic text which is there to help human readers, and in no way affects software advertising or using the application-layer protocol identified by that unique IANA-recorded service type string.

The following subtype is defined:

For link-local Multicast DNS the service domain SHALL be local. For Unicast DNS such as used on Thread the service domain SHALL be as configured automatically by the Thread Border Router.

For DNS‑SD a target host name is required, in addition to the instance name. The target host name SHALL be constructed using one of the available link-layer addresses, such as a 48-bit device MAC address (for Ethernet and Wi‑Fi) or a 64-bit MAC Extended Address (for Thread) expressed as a fixed-length twelve-character (or sixteen-character) hexadecimal string, encoded as ASCII (UTF-8) text using capital letters, e.g., B75AFB458ECD.<domain>. In the event that a device performs MAC address randomization for privacy, then the target host name SHALL use the privacy-preserving randomized version and the hostname SHALL be updated in the record every time the underlying link-layer address rotates. Note that it is legal to reuse the same hostname on more than one interface, even if the underlying link-layer address does not match the hostname on that interface, since the goal of using a link-layer address is to ensure local uniqueness of the generated hostname. If future link layers are supported by Matter that do not use 48-bit MAC addresses or 64-bit MAC Extended Address identifiers, then a similar rule will be defined for those technologies.

After discovery, IPv6 addresses are returned in the AAAA records and key/value pairs are returned in the DNS-SD TXT record. The TXT record MAY be omitted if no keys are defined.

Nodes SHALL publish AAAA records for all available IPv6 addresses upon which they are willing to accept operational messages.

Only the common TXT record key/value pairs defined in Section 4.3.4, “Common TXT Key/Value Pairs” are defined for use in Operational Discovery.

Nodes SHALL silently ignore TXT record keys that they do not recognize.

To improve overall performance of operational discovery, especially in large installations, the following recommendations SHOULD be taken in account:

The example below simulates a commissioned Matter Node advertising its availability for control via the Matter protocol.

Examples are shown using both the dns-sd command-line test tool and the avahi command-line test tool. The dns-sd command-line test tool is included in all versions of macOS. It is installed as a DOS command with Bonjour for Windows, and is available on Linux by installing the mDNSResponder package. The avahi command line-test tool is available from the Avahi project for most Linux distributions.

This example is given for illustrative purposes only. Real Matter Nodes and controllers would not use a command-line test tool for advertising and discovery. Real Matter Nodes and controllers would use the appropriate DNS‑SD APIs in their respective chosen programming languages.

Consider a device on Wi-Fi using the 48-bit device MAC address of B75AFB458ECD as its host name. DNS-SD records for can be set up as follows:

or

The port number 22222 is given here purely as an example. One of the benefits of using DNS‑SD is that services are not constrained to use a single predetermined well-known port. This means that multiple instances of the Matter Node control service can run on the same device at the same time, listening on different ports [RFC 6760]. The port, along with the IPv6 address, is discovered by the Matter controller at run time.

Avahi only sends a single AAAA record. To force the link-local address to be used, use avahi-publish-address. For example:

A Matter controller can discover the current IPv6 address and port for a known commissioned Matter Node:

or

The examples below simulate a Matter Commissioner advertising that it is present on the network.

Examples are shown using both the dns-sd command-line test tool and the avahi command-line test tool. The dns-sd command-line test tool is included in all versions of macOS. It is installed as a DOS command with Bonjour for Windows, and is available on Linux by installing the mDNSResponder package. The avahi command line-test tool is available from the Avahi project for most Linux distributions.

These examples are given for illustrative purposes only.

Consider a device on Wi-Fi using the 48-bit device MAC address of B75AFB458ECD as its host name. DNS-SD records for can be set up as follows:

or

This produces DNS-SD messages with the following characteristics:

Avahi only sends a single AAAA record. To force the link-local address to be used, use avahi-publish-address. For example:

The DNS‑SD service registration command shown above results in the creation of the following Multicast DNS records:

The port number 33333 is given here purely as an example.

A Commissionee can discover all Commissioners:

or

A Commissionee can discover Commissioners with device type 35:

or

A Commissionee can discover Commissioners with Vendor ID 123:

or

An optional, unsigned integer value specifying the overall length of the message in bytes, not including the size of the Message Length field itself. This field SHALL only be present when the message is being transmitted over a stream-oriented channel such as TCP. When transmitted over a message-oriented channel, the message length is conveyed by the underlying channel. For example, when transmitted over UDP, the message length is equal to the payload length of the UDP packet.

An unsigned integer bit field containing the following subfields: