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Protocol specification

Basic protocol design

fastd uses UDP as the transport protocol for its packets. UDP has been chosen instead of raw IP packets (as they are used by IPIP and 6in4 tunnels or IPsec) to simplify the deployment of multiple fastd instances on the same host using different UDP ports and allow passing through common NAT routers without explicit configuration.

The first byte of the UDP payload is used to discern the different packet types used by fastd. For now only two values for the first byte have been defined: 0x01 indicates a handshake packet, and 0x02 a data packet. All other values are reserved for future use and must be ignored by current implementations.

Handshake format

The initial 0x01 byte together with the next three bytes form the 4-byte handshake header; the rest of the packet after the header consists of a list of TLV records. The second header byte is reserved and must always be 0x00; the following two header bytes contain the length of the following TLV records in bytes encoded as Big Endian.

The following TLV records start with a 2-byte type field, followed by a 2-byte length field and the arbitrary-length value. There is no special alignment defined for the TLV records. All integers that are part of the TLV format (in particular, the type and length fields) are encoded in little-endian byte order.

TLV record types

Record ID Value description Format Values
0x0000 Handshake type 1-byte unsigned integer {1, 2, 3}
0x0001 Reply code 1-byte unsigned integer {0 (success), 1 (mandatory record missing), 2 (unacceptable value)}
0x0002 Error detail 1/2-byte unsigned integer Record type which caused an error
0x0003 Flags (currently unused) variable-length bit field So far, no values are defined
0x0004 Mode 1-byte unsigned integer {0 (TAP mode), 1 (TUN mode)}

0x0005 0x0006 0x0007 0x0008 0x0009

Protocol name Sender key Recipient key Sender handshake key Recipient handshake key

variable-length string 32-byte public key 32-byte public key 32-byte public key 32-byte public key

"ec25519-fhmqvc"

0x000a 0x000b 0x000c 0x000d 0x000e 0x000f

Authentication tag (obsolete) MTU Method name Version name Method list TLV authentication tag

32-byte opaque value 2-byte unsigned integer variable-length string variable-length string zero-separated string list 32-byte opaque value

Not used anymore

Handshake protocol

The following specification describes the current handshake as it is performed by fastd versions since v11.

The handshake protocol consists of three packets. See also: /crypto/ec25519, /crypto/fhmqvc

The following fields are sent in all three packets as different fastd versions expect them in different parts of the handshake:

  • Mode (TUN/TAP)
  • MTU
  • fastd version (e.g. v15)
  • Protocol name (ec25519-fhmqvc)

Handshake request

The first packet of a handshake contains the following additional fields:

  • Handshake type (0x01)
  • FHMQV-C values:
    • Sender key Â
    • Recipient key B̂
    • Sender handshake key X

The recipient key may be omitted if the recipient identity is unknown because the handshake was triggered by an unexpected data packet.

Handshake reply

The second packet of a handshake contains the following additional fields:

  • Handshake type (0x02)
  • Reply code (0x00)
  • Method list (list of all supported methods)
  • FHMQV-C values:
    • Sender key B̂
    • Recipient key Â
    • Sender handshake key Y
    • Recipient handshake key X
    • TLV authentication tag MACB

Handshake finish

The second packet of a handshake contains the following additional fields:

  • Handshake type (0x03)
  • Reply code (0x00)
  • Method (the chosen encryption/authentication scheme)
  • FHMQV-C values:
    • Sender key Â
    • Recipient key B̂
    • Sender handshake key X
    • Recipient handshake key Y
    • TLV authentication tag MACA

Handshake error

When an unacceptable handshake is received, fastd will respond with an error packet. The error packet contains the following fields:

  • Handshake type (the type of the packet that is answered plus 1)
  • Reply code (0x01 when a record is missing from the handshake, 0x02 when a value is unacceptable)
  • Error detail (the record type ID which caused the error)

Payload packets

The payload packet structure is defined by the methods; at the moment most methods use the same format, starting with a 24 byte header, followed by the actual payload:

  • Byte 1: Packet type (0x02)
  • Byte 2: Flags (method-specific; unused, always 0x00)
  • Bytes 3-8: Packet sequence number/nonce (big endian; incremented by 2 for each packet; one side of a connection uses the even sequence numbers and the other side the odd ones)
  • Bytes 9-24: Authentication tag (method-specific)

The null method uses only a 1 byte header: The packet type is directly followed by the payload data.