Skip to main content

14. New STUN Attributes

This STUN extension defines the following new attributes:

  • 0x000C: CHANNEL-NUMBER
  • 0x000D: LIFETIME
  • 0x0010: Reserved (was BANDWIDTH)
  • 0x0012: XOR-PEER-ADDRESS
  • 0x0013: DATA
  • 0x0016: XOR-RELAYED-ADDRESS
  • 0x0018: EVEN-PORT
  • 0x0019: REQUESTED-TRANSPORT
  • 0x001A: DONT-FRAGMENT
  • 0x0021: Reserved (was TIMER-VAL)
  • 0x0022: RESERVATION-TOKEN

Some of these attributes have lengths that are not multiples of 4. By the rules of STUN, any attribute whose length is not a multiple of 4 bytes MUST be immediately followed by 1 to 3 padding bytes to ensure the next attribute (if any) would start on a 4-byte boundary (see [RFC5389]).

14.1. CHANNEL-NUMBER​

The CHANNEL-NUMBER attribute contains the channel number. The value portion of this attribute is 4 bytes long and consists of a 16-bit unsigned integer followed by a two-octet RFFU (Reserved For Future Use) field, which MUST be set to 0 on transmission and MUST be ignored on reception.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        Channel Number         |         RFFU = 0              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

14.2. LIFETIME​

The LIFETIME attribute represents the duration for which the server will maintain an allocation in the absence of a refresh. The value portion of this attribute is 4 bytes long and consists of a 32-bit unsigned integral value representing the number of seconds remaining until expiration.

14.3. XOR-PEER-ADDRESS​

The XOR-PEER-ADDRESS specifies the address and port of the peer as seen from the TURN server. (For example, the peer's server-reflexive transport address if the peer is behind a NAT.) It is encoded in the same way as XOR-MAPPED-ADDRESS [RFC5389].

14.4. DATA​

The DATA attribute is present in all Send and Data indications. The value portion of this attribute is variable length and consists of the application data (that is, the data that would immediately follow the UDP header if the data was sent directly between the client and the peer). If the length of this attribute is not a multiple of 4, then padding must be added after this attribute.

14.5. XOR-RELAYED-ADDRESS​

The XOR-RELAYED-ADDRESS is present in Allocate responses. It specifies the address and port that the server allocated to the client. It is encoded in the same way as XOR-MAPPED-ADDRESS [RFC5389].

14.6. EVEN-PORT​

This attribute allows the client to request that the port in the relayed transport address be even, and (optionally) that the server reserve the next-higher port number. The value portion of this attribute is 1 byte long. Its format is:

     0
     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |R|    RFFU     |
    +-+-+-+-+-+-+-+-+

The value contains a single 1-bit flag:

  • R: If 1, the server is requested to reserve the next-higher port number (on the same IP address) for a subsequent allocation. If 0, no such reservation is requested.

The other 7 bits of the attribute's value must be set to zero on transmission and ignored on reception.

Since the length of this attribute is not a multiple of 4, padding must immediately follow this attribute.

14.7. REQUESTED-TRANSPORT​

This attribute is used by the client to request a specific transport protocol for the allocated transport address. The value of this attribute is 4 bytes with the following format:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    Protocol   |                    RFFU                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Protocol field specifies the desired protocol. The codepoints used in this field are taken from those allowed in the Protocol field in the IPv4 header and the NextHeader field in the IPv6 header [Protocol-Numbers]. This specification only allows the use of codepoint 17 (User Datagram Protocol).

The RFFU field MUST be set to zero on transmission and MUST be ignored on reception. It is reserved for future use.

14.8. DONT-FRAGMENT​

This attribute is used by the client to request that the server set the DF (Don't Fragment) bit in the IP header when relaying the application data to the peer. This attribute has no value part and thus the attribute length field is 0.

14.9. RESERVATION-TOKEN​

The RESERVATION-TOKEN attribute contains a token that uniquely identifies a relayed transport address being held in reserve by the server. The server includes this attribute in a success response to tell the client about the token, and the client includes this attribute in a subsequent Allocate request to request the server use that relayed transport address for the allocation.

The attribute value is 8 bytes and contains the token value.

Last updated on