# Transport selection `Transport::Unspec` selects an eligible payload transport before admission. `EngineConfig::selector` optionally applies ordered first-match policies to every request, including explicit memory and file transport hints. Sync and async submissions use the same selection in `TransferEngine::prepare`; the selected transport stays fixed through queueing, execution and drain. ## Configuration ```rust use orbite::{EngineConfig, TransportSelectorConfig}; let selector = TransportSelectorConfig::from_json( r#"{ "policy": [ {"name": "small_host", "segment_type": "memory", "local_memory": "cpu", "remote_memory": "cpu", "max_size": 4096, "priority": "high", "transports": ["tcp"]}, {"name": "bulk", "segment_type": "memory", "transports": ["nvlink", "rdma", "tcp"]} ] }"#, )?; let config = { let mut config = EngineConfig::default(); config.selector = Some(selector); config }; # Ok::<(), orbite::Error>(()) ``` Pass `Transport::Unspec` to the request constructor to use candidate order. ```rust use orbite::{IntentType, TransferRequest, Transport}; let request = TransferRequest::new( Transport::Unspec, local.base_address(), remote.base_address(), length, ).with_intent(IntentType::ForegroundGet); // Bind to one known policy even when another rule would match first. let named = request.with_policy_name("small_host")?; ``` An explicit transport must appear in the matching policy's candidate list and be eligible. A rejected or unavailable hint fails the request. With `selector: None`, explicit routing keeps its existing behavior and `Unspec` tries the configured MNNVL or NVLink profile, RDMA, SHM, then the configured socket transport for memory targets. A file target tries eligible GDS, then io_uring. An unnamed request uses the first matching rule. Within that rule, `Unspec` skips ineligible candidates. An exhausted rule fails before admission. An empty candidate list uses the segment default. A **named request** matches the exact name and segment type, bypassing intent, size, priority, placement and memory filters. Its transport still has to be authorized and eligible. Unknown names, wrong segment types and unavailable hints fail before payload I/O or task publication. `Error::TransportPolicy` carries the bound rule and reason. When no custom rules exist the default names are `memory_default` and `file_storage`. | Policy field | Consumed behavior | | --- | --- | | `name` | Unique label of 1–256 UTF-8 bytes. The builder retains a bounded shared name. Preparation resolves the compiled rule index once | | `segment_type` | `memory` or local `file`. A file target is on the same machine with CPU remote memory and requires a Linux file provider | | `local_memory`, `remote_memory` | `cpu`, `cuda`, or `*`. Pinned host allocations are CPU | | `same_machine` | Boolean. A memory peer is local only when both descriptors identify the same host boot. An absent identity gives no locality | | `min_size`, `max_size` | Inclusive byte bounds | | `priority` | Exact effective QoS or request priority. Accepts high/medium/low or 0/1/2 | | `intent_type` | Case-insensitive `unspec`, `foreground_get`, `background_prefetch`, `migration`, `checkpoint`, `weight_loading`, `staging_internal`, or the corresponding integer 0–6. Omission matches any intent | | `transports` | Ordered unique candidates. Memory accepts `tcp`, `hp_tcp`, `shm`, `rdma`, `nvlink` and `mnnvl`. Local files accept `gds` and `io_uring` | | `devices` | RDMA NIC names compiled to a 64-bit mask using stable discovered IDs and the custom topology's verified mapping | | `qp_pool` | Selects a contiguous endpoint QP range. An omitted, empty or unknown name permits every index | | `service_level`, `traffic_class` | Policy values are diagnostic. Only the endpoint pool changes native SL/TC | `transports.force_tcp` restricts memory requests to TCP. File selection keeps its own rules. Automatic selection uses the compiled rules when present, otherwise the default owner order. Explicit name binding remains strict. JSON input is bounded to 1 MiB and 256 policies. Unknown fields and invalid intent reject the configuration; frozen TENT skips a rule with an invalid intent. The removed `fabric` candidate and configuration key are rejected as unknown. CUDA fabric-backed MNNVL remains a separate supported owner. ## RDMA device and QP selection For example, these settings allow checkpoint writes on one NIC and the second segment of the endpoint's QP array. ```json { "transports": {"rdma": {"endpoint": {"qp_pools": [ {"name": "interactive", "num_qp": 2}, {"name": "bulk", "num_qp": 1, "service_level": 3, "traffic_class": 40} ]}}}, "policy": [{"name": "save", "segment_type": "memory", "intent_type": "checkpoint", "devices": ["mlx5_1"], "qp_pool": "bulk", "transports": ["rdma"]}] } ``` An unresolved or out-of-width NIC increments `EngineStats::unresolved_policy_devices` when a topology is available. Partial resolution warns and keeps the resolved subset. If none resolve, an error log identifies the names and the filter allows all NICs. Without a topology the mask also allows all NICs. Both single grants and multirail slice plans intersect this mask with the engine's device mask, location restrictions, healthy contexts and compatible peer paths. A mask never overrides any of those checks. Named pools have unique names and partition the data QPs in configuration order. Counts are positive and their sum fits the endpoint limit of 64. With empty pools the flat width is `qp_mul_factor`, defaulting to `RdmaConfig::num_lanes`. With pools the sum sets the endpoint width independently of the worker/CQ lane count. Each selected index inherits its pool's SL (0–15) and TC (0–255), or the RDMA defaults when omitted or set to the TENT sentinel `-1`. The requester folds its least-loaded worker's index into the selected range. The QP uses the CQ at that index modulo the lane count. Unbound traffic can use a named pool's QP and inherit its settings too. Both ends use the selected index for fresh connections and cache reuse. The selector runs on each original public range before merging. Two adjacent requests can fold only if their resolved results agree. A merged length cannot reroute them through a size rule or a different pool. See [physical owners](async-batches.md#physical-owners-and-public-tasks). ## Owner-based eligibility `open_segment` receives source-owned memory type and IPC export consent, plus configured segment capabilities. These are immutable descriptor snapshots, not grants or health guarantees. Refresh explicitly after registration changes. Every execution still checks epoch, permission, retained owner and provider readiness. Closing or failing a provider does not authorize replay elsewhere. | Transport | Required eligibility | | --- | --- | | TCP | Both ends' normal TCP handlers; CUDA endpoints use their existing local host staging. A CPU-only requester can READ and WRITE a peer CUDA range. Only the peer needs a device runtime for its staged copy | | HP-TCP | HP-TCP enabled at both ends, matching IPv4 rail counts, and an eligible host registration. The request must fit both endpoint frame limits | | SHM | SHM enabled at both ends, matching host boot identity and backing namespace, local host memory and a source-published shared range | | RDMA | Both engines configure RDMA. Each side's existing host/CUDA Direct or Staged policy remains authoritative; this capability does not promise GPUDirect or current RNIC health | | MNNVL | Matching handle kinds and fabric domains, a source-owned VMM export and a CUDA-capable local registration. POSIX fd also requires matching host boot identity. Both peers enable EGM for a remote host allocation | | NVLink | Both engines opt in; equal host boot identity and different process nonces; remote `allocate_ipc` export consent; local owned CUDA or owned CUDA-pinned host backing | Priority selects matching rules but does not make a transport ineligible. When the matching rule permits it, a Low prefetch can use TCP when RDMA is unavailable, or NVLink when both endpoints qualify for IPC. TCP and NVLink treat priority as advisory. RDMA workers use it to dispatch ready payloads. `remote_staging: true` requires a remote CUDA range and source Pin capability, and restricts selection to TCP/RDMA. It does not silently turn an ordinary transfer into Pin/Delegate/Unpin. Policy memory filters always inspect the original registered ranges, including when staging is selected. CUDA IPC capability is captured from allocation construction. Describe and selection do not take payload locks, wait for producers, create exports or query CUDA. Same-host placement never follows from an IP address or equal CUDA ordinal. Process identity reuses the existing IPC handshake owner. ## Correspondence with TENT The reference is frozen TENT commit `e1ea5d31bb5e46ff2df9f90e7c2fbaf52fcdee8c`, specifically `runtime/transport_selector.h` and `runtime/transport_selector.cpp`. | TENT owner/behavior | OrbiTE implementation | | --- | --- | | Ordered and named policies, request intent | `runtime/transport_selector/config.rs` and `TransferRequest`. An invalid intent rejects the config. A named request keeps its binding in legacy mode | | `SelectionContext`, owner capability checks | Original memory types and both segment descriptors, including boot identity and IPC consent | | `select`, hint and host staging | One prepared route carried into TCP, HP-TCP, SHM, RDMA, NVLink, local file I/O and the TCP/RDMA CROSS leg | | `resolveDeviceMasks`, RDMA sub-batch grouping | One resolution per engine topology. Each prepared request retains its own mask and QP range through every child grant | | Endpoint `qp_pools`, `selectQpInPool`, `setupOneQP` | Contiguous index segments and pool SL/TC on the physical RC QP. An individual grant retains its connection, range and native owner | | `canMergeRequests` policy boundary | Physical merging compares the complete preselected route, including its policy, intent, mask and QP pool, along with hint, priority, QoS and registration authority. A larger physical span keeps its members’ selected route | TENT's automatic memory order follows the buffer's registered transport list. OrbiTE registrations advertise capabilities without an insertion order, so automatic memory selection tries MNNVL, NVLink, RDMA, SHM, then the configured socket payload (TCP or HP-TCP). A policy can reverse that order. TENT documents GDS → io_uring → RDMA for files. Its frozen `getDefaultPolicies` lists GDS and io_uring. OrbiTE supports these two local owners. RDMA-to-file needs a remote file segment. SHM merges respect the physical registration and segment transfer limits. Overlapping allocation offsets retain their public order. The smaller HP-TCP endpoint frame limit caps merging. The selected rule retains its bounded candidate order. Only the engine’s physical retry owner advances that index. A rejected initial hint still fails selection. An eligible hint moves to the front of the rule and its later candidates can run only after a safe failure. Caller-supplied fallback indices, local recompute and compression remain unsupported. See [transport attempts](async-batches.md#transport-attempts). The selector scans at most 256 compiled rules and seven candidates, once per public request. The compiled rule scan allocates no strings, takes no selector lock and calls no driver; no selector work is added per slice, WR or CQ completion. Existing registration resolution, admission and transport owners keep their own costs. This is an implementation property, not a measured throughput comparison. ## Protocol and qualification Describe replies carry a four-byte region count, 44 bytes of segment capabilities, then 32 bytes per region. Bits 0–2 of capability byte zero identify RDMA, remote staging and NVLink. Bit 3 denotes a boot-only identity when NVLink is disabled. Bit 4 disables ordinary TCP payloads and bit 5 announces the SHM trailer. Bit 6 announces MNNVL fabric capabilities. Bytes 4–19 hold the boot identity when known and 20–35 hold the process nonce only for NVLink. Bytes 36–43 give the maximum accepted transfer length as a big-endian unsigned integer. An unknown limit prevents folding two requests. Bytes 1–3 are reserved and must be zero. Region bytes 25–27 carry memory type, IPC export consent and registration flags (HP-TCP plus observed CUDA RDMA routes; see [Describe layout](cpu-tcp.md#development-wire-contract)). The [Describe layout](cpu-tcp.md#development-wire-contract) gives the trailer order and SHM fields. The RDMA path reply carries the advertised endpoint width. The rendezvous hello carries the agreed reusable width and an optional pool index on both cached and transient QPs. Deploy the same build and endpoint layout on both peers. The header cannot detect a mismatch in pool definitions. See the [wire policy](tent-parity.md#wire-and-consumer-boundaries). CPU controls check TCP sync/async payloads, the RDMA verbs table's per-NIC bytes and connection SL/TC, NVLink group boundaries, staging CROSS and local files. Physical NIC selection and link-layer QoS await the installed gate. See [qualification](qualification.md#transport-policy-selection).