Charge staged reservations to each tunnel instead of dividing a static share
of the window by routines. Several readers sending on one tunnel then share
its credit: the limit is half the peer replay window, assumed from
ReplayWindow, which must be a power of two of at least 8192 (checked at
compile time). The other half allows finite unreserved traffic (inline and
control sends, relay forwarding through SendVia) and network reordering;
unlimited bypass traffic or delay can still exceed it.
An outstanding-count cap alone would let a fast reader repeatedly send and
reacquire credit, overtaking a paused reader without bound. So once readers
overlap, spent credit is only recycled when every reservation has drained. A
sole ordered reader gets each completion refunded, since its later batches
cannot overtake its earlier ones. A reader that fails to acquire ends sole
ownership, so a busy owner cannot keep a larger waiter blocked forever. This
restores progress, not FIFO fairness. All transitions are a single CAS.
Waiting readers flush their own oldest batch before retrying and never hold a
partial acquisition: a failed multi-tunnel (relay) acquisition rolls back.
Credit is released only after the flush or an explicit drop. A read that alone
exceeds a tunnel's credit, such as a tiny-MSS superpacket, is sent segment by
segment, synchronously. Acquisition is per read, so pressure on one tunnel
also delays the other tunnels in the same read.
The combined staged transmit, receive and shared credit measured 15 to 25
Gbit/s over one tunnel with eight TCP streams between two 100 GbE hosts.
Tests model overtaking, ordered reuse, a blocked reader behind a busy owner,
and relay rollback with both window sizes, and run eight competing readers
plus unreserved control traffic on one tunnel, checking every packet is
accepted by the receiver's replay window in wire order.
Set tun.staged.workers to a positive count on Linux, with tun.use_offloads
enabled, to move encryption off each tun reader. The reader filters and
reserves counters for a whole read, hands the batch to that many encrypt
workers, and an ordered sender flushes the batches in read order. Eight slots
per routine bound the batches in flight; a full pipeline blocks the reader.
Zero workers, the default, keeps today's inline path, as do tun queues that
cannot hand off their read buffer and builds whose AES-GCM needs nonce-ordered
encryption (BoringCrypto, FIPS).
IMPORTANT: staged senders need peers with the 8192-entry replay window of
master. Released v1.11.x peers use 1024 and will drop reordered batches. The
window is not negotiated.
Readers keep today's pinning. The sender is only pinned with an explicit
tun.cpu_affinity: sender i uses entry i+1, unless that is its reader's CPU,
so a one-entry list leaves the sender unpinned rather than sharing it: two
busy transmit threads on one CPU serialise (eight TCP streams over 100 GbE
fell from 25.9 to 22.8 Gbit/s). Without a list the sender is not pinned. Placing it by queue index on a
hybrid CPU put it on an efficiency core, and one TCP stream over 100 GbE
reached 18.6-19.6 Gbit/s pinned against 20.2-20.6 unpinned.
New API, each needed across a package boundary:
- tio.Offload.SwapRxBuf hands the buffer of the last Read to the reader and
reads into the one given. It is a method on the concrete queue, found through
an unexported interface in nebula, so tio.Queue and its other
implementations are unchanged.
- batch.SendBatch.Reset drops queued packets without sending, for a sender
whose sockets are already closed; Flush now uses it.
Metrics, registered only for active pipelines: flow.tx.batches counts reads
handed to the sender, flow.stage.{tx_read,tx_encrypt,tx_send}.q<N>.busy_ns the
elapsed busy time of each stage, to find the limiting one.
Tests cover ordering and counters under backpressure with both ciphers (AES
skipped where nonce order is required), read errors with batches in flight,
shutdown, exhaustion, relays, fallback, firewall parity, and the actual reader
and send thread affinity for every placement, with pinning on and off: the
sender is pinned only to an explicit entry other than its reader's CPU.
Build on the inside-packet filtering split with shared preparation and encryption helpers for inline and reserved sends. Reserve each batch’s per-tunnel counter range before encryption, including relay counters, without enabling staged processing yet. Test reserved encryption against a real receiver and check segmentation counts.
Ryan Huber