WDP Datagram Format and Transport Contract

Status: ACTIVE Date: 2026-03-04

1) What WDP provides

In this project WDP is a constrained, testable datagram abstraction for upper protocol layers. It maps to one transport datagram in memory and to one UDP datagram in the MVP transport.

For the selected WAP 1.2.1 Class C path, WAP-200 plus RFC 768 and RFC 791 define the active behavior:

WDP is the standard datagram interface between bearer services and upper layers.
CDPD is selected as an IP-capable bearer and WDP maps directly to UDP/IPv4.
WSP/WTP/WTLS session identity is by address/port quadruplet (client/server address + ports).
the selected AMPS/CDPD/IPv4 bearer assignment is 0x0D.
bearer fragmentation/reassembly terminates below WDP; no second WDP segmentation header is added to this IP path.

2) MVP datagram model

pub struct WdpDatagram {
    pub src_addr: WdpAddress,
    pub dst_addr: WdpAddress,
    pub src_port: u16,
    pub dst_port: u16,
    pub payload: Vec<u8>,
}

pub struct WdpAddress {
    pub addr_type: AddressType, // IPv4, IPv6, or future GSM/USSD forms
    pub value: Vec<u8>,
}

The source-shaped service primitive types are:

pub struct TDUnitdataRequest {
    pub source_address: WdpAddress,
    pub source_port: u16,
    pub destination_address: WdpAddress,
    pub destination_port: u16,
    pub user_data: Vec<u8>,
}

pub struct TDUnitdataIndication {
    pub source_address: WdpAddress,
    pub source_port: u16,
    pub destination_address: Option<WdpAddress>,
    pub destination_port: Option<u16>,
    pub user_data: Vec<u8>,
}

3) WAP service ports in profile

The complete WAP-200 Appendix B registry is recognized:

WTA secure connectionless/session: 2805, 2923.
Push connectionless/secure connectionless: 2948, 2949.
WSP connectionless/session/secure connectionless/secure session: 9200..9203.
vCard/vCalendar datagram and secure datagram: 9204..9207.

Transport policy:

server entities MUST bind well-known service ports.
client entities may use ephemeral source ports.
the profile must log/validate source/destination quadruplets for each session.
recognizing a registered port does not activate that optional application or security profile. The selected non-secure connectionless WSP service uses destination port 9200.

4) Bearer implementation contracts

4.1 CDPD/UDP/IPv4 — selected strict path

addresses are exactly four octets and UDP is IPv4 protocol 17;
map WDP user data 1:1 to UDP data with 16-bit source and destination ports;
encode and validate IPv4 and UDP lengths and checksums, including odd-octet checksum padding, computed-zero encoding as 0xFFFF, and permitted all-zero UDP checksum omission;
accept IPv4 datagrams through the 576-octet baseline and require explicit destination assurance for larger sends;
treat 65,535 octets as the IPv4 protocol maximum and 65,507 octets as the corresponding no-options UDP payload maximum;
discard TTL-zero, bad-header-checksum, malformed-length, bad non-zero UDP checksum, unsupported-protocol, and DF/path-MTU failures deterministically;
keep the stateless WDP/UDP decoder restricted to complete IPv4 datagrams;
collect fragments in the explicit destination-IP boundary below WDP, keyed by identification, source, destination, and protocol.

4.2 SMS/USSD/other GSM bearers — deferred

Implementation is deferred but reserved in model:

optional UDH-style fragmentation and reassembly,
bearer-specific maximum payload sizes,
optional sequence numbers/retry semantics.

5) Segmentation boundary

the selected CDPD/IP path adds no WDP segmentation header;
IPv4 gateways may fragment and the destination IP module reassembles before delivery to WDP;
the stateless codec still returns Ipv4FragmentRequiresReassembly when it is called directly with a fragment, preserving the TRN-701 boundary;
Ipv4Reassembler owns bounded fragment collection at the destination-IP boundary, outside the WDP datagram service;
the selected strict receive policy accepts complete/reassembled datagrams through 576 octets, rejects larger units without truncating WDP user data, and can be widened only by an explicit destination capability;
pending assemblies are bounded by datagram count, buffered bytes, total datagram size, and deterministic simulated-time expiry;
duplicate fragments are idempotent, while conflicting overlaps, malformed flags/lengths, oversize extents, and incomplete expiry are explicit outcomes.

The optional generic WdpSarPolicy::Enabled path remains an inactive future bearer capability. It is not used to manufacture a WDP segmentation layer for CDPD/IPv4.

6) Error taxonomy

Minimum errors expected by upper layers:

WdpError::TransportUnavailable
WdpError::AddressTypeUnsupported
WdpError::AddressUnresolvable
WdpError::PayloadOversize
WdpError::Ipv4FragmentMalformed
WdpError::Ipv4ReassemblyOversize
WdpError::Ipv4ReassemblyCapacityExceeded
WdpError::Ipv4ReassemblyBufferExceeded
WdpError::Timeout
WdpError::CorruptOrMalformed

7) Implementation notes for this repo

Keep WDP free of HTTP/WSP semantics.
expose only transport-visible fields and typed errors.
ensure all parse/codec methods stay stateless to keep replayability in tests.

8) Evidence

The direct fixture transport-rust/tests/fixtures/transport/wdp_cdpd_ipv4_mapped/wdp_fixture.json links the nine selected SCR rows and all 49 mapped TRN-701 clauses to exact profile constants, registered ports, positive wire packets, and deterministic malformed outcomes.

The additive TRN-702 fixture transport-rust/tests/fixtures/transport/wdp_constrained_payload_mapped/reassembly_fixture.json maps the narrow nine-clause payload/fragment subset to whole, out-of-order, duplicate, malformed, overlap, oversize, and incomplete-expiry replays. This does not claim broader TRN-706 replay-corpus closure. Validate both with:

cargo test --manifest-path transport-rust/Cargo.toml --lib network::wdp
cargo test --manifest-path transport-rust/Cargo.toml --test wdp_constrained_replay
node scripts/check-wap-transport-conformance-ledgers.mjs
node scripts/check-wap-selected-normative-clauses.mjs
node scripts/wap-context-pack.mjs TRN-702