DSS Framing (Data Stream Structure)

Every DRDA message on the wire is wrapped in a DSS envelope. This is the lowest protocol layer.

Header Format

+----------------------------------------------+
|  DSS Header (6 bytes)                        |
|  +--------+--------+--------+--------------+ |
|  | Length  | Magic  | Format | Correlation  | |
|  | 2 bytes| 0xD0   | 1 byte | ID (2 bytes) | |
|  | (BE)   |        |        |              | |
|  +--------+--------+--------+--------------+ |
|  Payload: DDM Object(s)                      |
+----------------------------------------------+

Fields

Offset	Size	Field	Description
0	2 bytes	Length	Total DSS length including header (big-endian u16)
2	1 byte	Magic	Always 0xD0
3	1 byte	Format	Type and flags (see below)
4	2 bytes	Correlation ID	Links requests to replies (big-endian u16)

Format Byte

The format byte encodes both the DSS type and control flags:

Bit	Mask	Meaning
0	0x01	Chained — more DSS frames follow in this request
1	0x02	Continue on error — process next DSS even if this one fails
2	0x04	Same correlation — continuation of a previous DSS
3-4	0x38	DSS type (see below)

DSS Types

Value	Type	Description
1	Request	Client-to-server command
2	Reply	Server-to-client response
3	Object	Data object (e.g., SQLSTT carrying SQL text)
4	Communication	Protocol-level message

Rust Representation

pub struct DssHeader {
    pub length: u16,
    pub magic: u8,         // always 0xD0
    pub format: DssFormat,
    pub correlation_id: u16,
}

pub struct DssFormat {
    pub chained: bool,
    pub continue_on_error: bool,
    pub same_correlation: bool,
    pub dss_type: DssType,
}

pub enum DssType {
    Request = 1,
    Reply = 2,
    Object = 3,
    Communication = 4,
}

Chaining

DSS chaining is a critical optimization. By setting the chained bit, multiple commands can be sent in a single TCP write, reducing round trips.

For example, the connection handshake chains EXCSAT + ACCSEC in one write:

[DSS(EXCSAT, chained=true)] [DSS(ACCSEC, chained=false)]

The server responds with both replies in one TCP read:

[DSS(EXSATRD)] [DSS(ACCSECRD)]

Maximum DSS Size

A single DSS can be up to 32,767 bytes (max u16 value). For larger payloads (e.g., SQL text > 32KB), use DSS continuation — set the same_correlation flag on the continuation DSS.

Validation Rules

When parsing a DSS:

1. Verify the magic byte is 0xD0
2. Verify length is at least 6 (header size)
3. Verify the DSS type is valid (1-4)
4. Track correlation IDs to match requests with replies