Struct SessionAck 

pub struct SessionAck {
    pub src_coords: TreeCoordinate,
    pub dest_coords: TreeCoordinate,
    pub flags: u8,
    pub handshake_payload: Vec<u8>,
}

Session acknowledgement.

Carried inside a SessionDatagram envelope which provides src_addr and dest_addr. The SessionAck payload contains both the acknowledger’s and initiator’s coordinates for route cache warming (ensuring return-path transit nodes can route independently of the forward path) and the Noise XK handshake response.

SessionSetup, SessionAck, and SessionMsg3 are identified by the phase field in the FSP common prefix (0x1, 0x2, 0x3), not by a message-type byte.

Wire Format

Encoded with FSP common prefix: [ver_phase:1][flags:1][payload_len:2 LE][body], where ver_phase = 0x02 (version 0, phase MSG2) and flags = 0 for handshake.

Body (after 4-byte FSP prefix):

Offset	Field	Size	Description
0	flags	1 byte	Reserved
1	src_coords_count	2 bytes LE	Number of acknowledger coordinate entries
3	src_coords	16 × n	Acknowledger’s ancestry (for cache warming)
…	dest_coords_count	2 bytes LE	Number of initiator coordinate entries
…	dest_coords	16 × m	Initiator’s ancestry (for return-path cache warming)
…	handshake_len	2 bytes LE	Noise payload length
…	handshake_payload	variable	Noise XK msg2 (57 bytes — ephemeral key + encrypted epoch)

Fields

src_coords: TreeCoordinate

Acknowledger’s coordinates.

dest_coords: TreeCoordinate

Initiator’s coordinates (for return-path cache warming).

flags: u8

Reserved flags byte (for forward compatibility).

handshake_payload: Vec<u8>

Noise IK handshake message 2.

Implementations

impl SessionAck

pub fn new(src_coords: TreeCoordinate, dest_coords: TreeCoordinate) -> Self

Create a new session acknowledgement.

pub fn with_handshake(self, payload: Vec<u8>) -> Self

Set the Noise handshake payload.

pub fn encode(&self) -> Vec<u8>

Encode as wire format (4-byte FSP prefix + flags + coords + handshake).

The 4-byte prefix: [ver_phase:1][flags:1][payload_len:2 LE] where ver_phase = 0x02 (version 0, phase MSG2).

pub fn decode(payload: &[u8]) -> Result<Self, ProtocolError>

Decode from wire format (after 4-byte FSP prefix has been consumed).

Trait Implementations

impl Clone for SessionAck

fn clone(&self) -> SessionAck

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for SessionAck

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.