doca-pcc-ztr-rttcc-algo git:20261014.bbb601aA

Immutable. This exact content never changes and is served at /api/v1/blob/83947b86287fbb1a.

---
license: Apache-2.0
name: doca-pcc-ztr-rttcc-algo
description: >
  DOCA-shipped Zero-Touch RoCE RTT congestion control (ZTR RTTCC)
  reference algorithm on a BlueField-3 DPA: deploy, tune and evaluate
  doca_pcc_dev_ztr_rttcc_algo through the shipped DOCA PCC sample. Trigger
  on "my RoCE-v2 flows aren't being throttled", "PCC sample isn't
  dispatching to my algo", "set-params returns fail", "algorithm loaded
  but counters are flat", "how do I pick the multipath PCC variant", or
  "do I need a custom CC algorithm on BF3", even without naming ZTR RTTCC.
  Your own algorithm through the host library belongs to doca-pcc; the
  doca_spcx_cc command line to doca-spcx-cc. Covers picking a variant
  (vanilla / PM / RX-rate / multipath / window-probeless) at DPACC build
  time, tuning host-set parameters, and DOCA_PCC_DEV_STATUS_FAIL from the
  algorithm. Refuse and route elsewhere for writing a custom PCC algorithm
  from scratch, read-only PCC counter inspection, or firmware-only
  pre-Programmable PCC.
metadata:
  author: NVIDIA <doca@nvidia.com>
  kind: library
  tags:
    - doca
    - pcc
    - ztr-rttcc
compatibility: >
  Requires DOCA SDK installed at /opt/mellanox/doca on Linux (Ubuntu
  22.04/24.04 or RHEL/SLES) with a BlueField-3 DPU exposing the DPA
  processor, the firmware custom-PCC slot enabled, a matched-version
  DPACC compiler, and live RoCE-v2 traffic on the attached port. Reads
  `pkg-config doca-pcc-ztr-rttcc-algo` and inspects
  /opt/mellanox/doca/{lib,include,applications/pcc}. The validation
  steps require a live DOCA install.
---

# DOCA PCC ZTR RTTCC Algorithm

## Examples

- **"How do I pick the multipath PCC variant?"** — this is in
  scope. Pick the ZTR RTTCC variant at DPACC build time; do not
  invent a custom algorithm.
- **"Algorithm loaded but counters are flat."** — confirm live
  RoCE-v2 traffic and host-set params before rewriting the
  sample.

## Instructions

Follow Loading order below. Open the tasks companion for configure /
tune / debug. Open the capabilities companion for variants and
the FAIL status. Do not open every companion file.

**Where to start:** This skill assumes DOCA is already
installed, the user's BlueField has a DPA processor that
the host can see through DOCA (a BlueField-3-generation
device per the README), the BlueField firmware has the
custom-PCC slot enabled, the DPACC compiler is installed at
a matched version per the DOCA Compatibility Policy, and the
user is doing **hands-on deployment of the DOCA-shipped ZTR
RTTCC reference algorithm** on a BlueField port that
already carries RoCE-v2 traffic — i.e. either deploying it
as the no-config-required baseline, tuning its documented
parameters, or evaluating it against a custom algorithm the
user intends to write. Open the tasks companion if the
user wants to *do* something (install / configure / build /
modify / run / test / debug / use); open
the capabilities companion when the question is
*what does the algorithm express, what are its variants and
parameters, what does it ship vs not ship*. If the user has
not installed DOCA yet, route to
[`doca-setup`](../../doca-setup/SKILL.md) first; if the user
has not stood up the host-side `doca-pcc` framework yet,
route to [`doca-pcc`](../doca-pcc/SKILL.md) first (this
algorithm is a *library consumed by* the PCC framework, not
a standalone program); if the user only wants to *inspect*
PCC counters at runtime without changing the running
algorithm, route to
[`doca-pcc-counters`](../../tools/doca-pcc-counters/SKILL.md);
if the user wants to *write their own algorithm from
scratch*, that is the `doca-pcc` library plus the public
PCC programming guide — this skill is for the shipped
reference algorithm specifically.

## Example questions this skill answers well

The CLASSES of ZTR RTTCC questions this skill is built to
answer, each with one worked example. The agent should
treat the *class* as the load-bearing piece — the worked
example is a single instance.

- **"Is the ZTR RTTCC reference algorithm the right
  baseline for my deployment, or should I write a custom
  algorithm?"** — worked example: *"I have a BlueField-3
  carrying production RoCE-v2 traffic from a GPU cluster;
  is the shipped algorithm a fine default or do I need
  custom logic?"*. Answered by the decision rule in
  ## Capabilities and modes
  ("when to use the reference vs custom") + the env
  preconditions in
  ## install.
- **"How do I wire the shipped algorithm into the DOCA
  PCC application that's already running on my host?"** —
  worked example: *"`/opt/mellanox/doca/applications/pcc`
  is already building from sample sources; what do I
  change so the user algo callback dispatches to
  `doca_pcc_dev_ztr_rttcc_algo` under a chosen algo
  slot?"*. Answered by the integration sequence in
  ## Capabilities and modes
  + the in-place edits in
  ## modify.
- **"Which variant of the algorithm am I getting — vanilla
  RTT-CC, path-migration mode, RX-rate mode, multipath,
  multipath with credits, window-probeless?"** — worked
  example: *"the shipped library exposes one public
  symbol `doca_pcc_dev_ztr_rttcc_algo` but the device-
  side source ships several variants; how do I know
  which one I get and how do I pick another?"*.
  Answered by the variants table in
  ## Capabilities and modes.
- **"How do I confirm the algorithm is actually
  modulating my RDMA / RoCE traffic, and not just
  loading?"** — worked example: *"I followed the
  integration steps; the application starts; how do I
  know the algorithm is shaping flows under load?"*.
  Answered by the observability surface in
  ## Observability
  + the counter-watch loop in
  ## test which routes to
  [`doca-pcc-counters`](../../tools/doca-pcc-counters/SKILL.md).

> **"Flows aren't being throttled" / "counters are flat" is first an
> observability question, not a tuning one.** This is a debug
> question, so do not answer it by walking the launch checklist in
> ## run — that sequence is for bringing a
> new build up, not for a system already running. Go to the
> discrimination in ## debug step 6 and
> start there: are CC events reaching the algorithm at all? If
> none are, the cause is the wrong algo slot, a `doca_pcc` attached to
> the wrong port, or — the documented common trap — no RoCE-v2 traffic
> on that port to modulate, in which case the algorithm is loaded but
> idle and the run proves nothing. Only once events are landing and
> the AI / decrement counters are still flat do parameters or the
> compiled variant come into question. Jumping to parameter tuning or
> a variant rebuild first is the mistake
> ## Safety policy
> calls out by name; the discrimination recipe is
> ## debug step 6.
- **"Which tunables does the algorithm expose, and how
  do I change them from the host without rebuilding the
  DPA-side image?"** — worked example: *"my workload is
  more latency-sensitive than the default profile assumes
  — which parameter knob do I adjust?"*. Answered by
  the parameter surface in
  ## Capabilities and modes
  + the `doca_pcc_dev_set_ztr_rttcc_params` workflow in
  ## use.
- **"What does this `DOCA_PCC_DEV_STATUS_FAIL` or
  `DOCA_ERROR_*` from a `doca_pcc_dev_ztr_rttcc_*` call
  mean and which layer caused it?"** — worked example:
  *"my init callback returns `DOCA_PCC_DEV_STATUS_FAIL`
  on first launch"*. Answered by the algorithm overlay
  on the host-side PCC taxonomy in
  ## Error taxonomy
  + the layered ladder in
  ## debug that escalates
  through
  [`doca-pcc`](../doca-pcc/SKILL.md) and
  [`doca-debug`](../../doca-debug/SKILL.md).

## Audience

This skill serves **external developers operating a
BlueField-3-class DPU who want to deploy NVIDIA's shipped
reference PCC algorithm on RoCE-v2 traffic, OR who are
evaluating it against a custom algorithm they intend to
write**. The reference algorithm is *zero-touch* by design
— the no-config-required baseline — and the canonical use
case is dropping it onto a port and confirming it shapes
flows correctly under congestion. It is *not* for NVIDIA
developers contributing to the algorithm itself, nor for
users who want general PCC programming theory (route via
the public DOCA PCC programming guide), nor for users who
only want to *inspect* PCC counters (route to
[`doca-pcc-counters`](../../tools/doca-pcc-counters/SKILL.md)).

**Language scope.** The algorithm ships as a DPA-side
library (`pkg-config` module `doca-pcc-ztr-rttcc-algo`)
plus a public header `doca_pcc_dev_ztr_rttcc_algo.h` that
DPA-side translation units include. The shipped algorithm
binary is the static library
`libdoca_pcc_ztr_rttcc_algo_dev.a` per the README; the
device-side translation unit that consumes it is C and is
compiled by DPACC. The host-side that drives the PCC
context comes from [`doca-pcc`](../doca-pcc/SKILL.md);
this library does NOT add a host-side surface beyond the
host-side helpers (also shipped as
`libdoca_pcc_ztr_rttcc_algo.{a,so}` per the README) that
the `doca-pcc` framework links. Other-language host-side
wrappers around `doca-pcc` can drive this algorithm through
the same lifecycle described in `doca-pcc`; the DPA-side
integration always stays C-via-DPACC.

## When to load this skill

Load this skill when the user is doing hands-on deployment,
tuning, or evaluation of the DOCA-shipped ZTR RTTCC
reference algorithm on a BlueField port carrying RoCE-v2
traffic, in any host language plus the DPA-side translation
unit built by `dpacc`. Concretely:

- Deciding whether the shipped algorithm is the right
  baseline for the user's RoCE-v2 workload, or whether the
  user needs a custom algorithm.
- Wiring the algorithm into the DOCA PCC sample
  application by patching the user-algo / user-init /
  user-set-algo-params callbacks per the steps the
  shipped README documents.
- Picking which of the algorithm's documented variants
  (vanilla, path-migration, RX-rate, RX-rate + PM,
  multipath, multipath + credits, window-probeless)
  matches the user's intent — and surfacing that the
  *public* surface only ships one symbol
  (`doca_pcc_dev_ztr_rttcc_algo`); the variants live in
  the DPA-side source the user compiles against.
- Tuning the host-set parameters the algorithm exposes
  (per the parameter list in
  `doca_pcc_dev_ztr_rttcc_algo.h` and the shipped
  `doca_pcc_dev_set_ztr_rttcc_params`).
- Observing whether the running algorithm is actually
  modulating RoCE-v2 flows under load (route to
  [`doca-pcc-counters`](../../tools/doca-pcc-counters/SKILL.md)
  for the read-only inspection side).
- Deciding when to *replace* the shipped algorithm with a
  custom one because latency target, fairness policy, or
  convergence behavior requirements diverge from what the
  reference provides.

Do **not** load this skill for general DOCA orientation;
for the host-side `doca-pcc` lifecycle (route to
[`doca-pcc`](../doca-pcc/SKILL.md)); for writing a custom
algorithm from scratch (route to
[`doca-pcc`](../doca-pcc/SKILL.md) and the public PCC
programming guide via
[`doca-public-knowledge-map`](../../doca-public-knowledge-map/SKILL.md));
for read-only PCC counter inspection (route to
[`doca-pcc-counters`](../../tools/doca-pcc-counters/SKILL.md));
or for the default firmware-shipped PCC algorithms that
predate Programmable Congestion Control entirely (no
host-side code, no DPACC compile — that is a firmware-only
path routed via
[`doca-public-knowledge-map`](../../doca-public-knowledge-map/SKILL.md)).

## What this skill provides

This is a **thin loader**. The body keeps only the
orientation needed to pick the right next file. The
substantive algorithm-specific material lives in two
companion files:

- the capabilities companion — what the shipped ZTR RTTCC
  algorithm expresses on this version + this BlueField
  generation + this firmware: the public DPA-side API
  surface (`doca_pcc_dev_ztr_rttcc_init`,
  `doca_pcc_dev_ztr_rttcc_algo`,
  `doca_pcc_dev_set_ztr_rttcc_params`,
  `doca_pcc_dev_ztr_rttcc_get_param_num`,
  `doca_pcc_dev_ztr_rttcc_get_counter_num`,
  `doca_pcc_dev_ztr_rttcc_get_num_of_histograms`), the
  documented variants (vanilla / path-migration / RX-rate
  / multipath / window-probeless — pick one at
  DPA-side compile time), the relationship to the
  host-side `doca-pcc` framework (this is an algorithm
  body the framework loads), the relationship to the
  `doca-pcc-counters` tool (which is the canonical
  inspection surface), the algorithm's parameter and
  counter surface (RTT-based congestion signal,
  per-feature parameter blocks), the error taxonomy in
  `DOCA_PCC_DEV_STATUS_OK` / `_FAIL`, and the safety
  policy.
- the tasks companion — step-by-step workflows for the in-scope
  algorithm verbs: `install`, `configure`, `build`,
  `modify`, `run`, `test`, `debug`, `use`. Plus a
  `Deferred task verbs` block that points out-of-scope
  questions at the right next skill.

The skill assumes DOCA + the DPACC compiler + the
[`doca-pcc`](../doca-pcc/SKILL.md) host-side framework are
already installed; the BlueField is a generation that
exposes the DPA processor (the algorithm runs on the DPA);
the BlueField firmware has the custom-PCC slot enabled
(inherited from
`doca-pcc ## Safety policy`);
and the BlueField port the algorithm will modulate has
RoCE-v2 traffic actually flowing on it (the algorithm
modulates *existing* RDMA / RoCE traffic — without traffic,
there is nothing for it to do).

## What this skill deliberately does not ship

This skill is **agent guidance**, not a samples or templates
bundle. To keep the boundary clean, it deliberately does not
contain — and pull requests should not add:

- **The algorithm body itself, in any language.** The
  shipped algorithm is the static library
  `libdoca_pcc_ztr_rttcc_algo_dev.a` (plus the host-side
  helpers) installed by the matching DOCA host package
  per the README. The agent's job is to route the user to
  the installed library and header
  (`doca_pcc_dev_ztr_rttcc_algo.h`) and to prescribe the
  in-place edits documented in the README on the
  shipped DOCA PCC application source, not to author the
  algorithm.
- **A standalone PCC application.** The DOCA PCC
  application that hosts this algorithm is the shipped
  C sample under
  `/opt/mellanox/doca/applications/pcc/` (per the
  README). The agent's job is to prescribe the
  minimum-diff modifications the README documents and to
  walk the user through the rebuild — not to author a
  parallel application.
- **A description of every internal variant's
  behavior.** The DPA-side source ships several
  variants (vanilla, path-migration, RX-rate, multipath,
  multipath + credits, window-probeless). The agent
  names the variant set, says which one is reachable via
  the public symbol on this install, and routes
  algorithm-design questions to the public PCC
  programming guide via
  [`doca-public-knowledge-map`](../../doca-public-knowledge-map/SKILL.md).
  It does not redefine each variant's mathematical
  behavior.
- **A specific congestion-control algorithm tutorial.**
  Congestion-control theory, RoCE-v2 fairness analysis,
  workload-specific tuning — out of scope. Route to the
  user's own domain expertise and to the public DOCA PCC
  guide.

## Loading order

1. Read this skill first to confirm the user's
   question is in scope (deployment / tuning / evaluation
   of the shipped reference algorithm, not algorithm
   design from scratch and not read-only counter
   inspection).
2. **For the algorithm capability matrix, the public
   DPA-side API surface, the variant set, the parameter
   and counter surface, the host-side `doca-pcc`
   framework relationship, the
   `doca-pcc-counters` inspection-side relationship, the
   error taxonomy, the observability surface, and the
   safety policy, see the capabilities companion.**
3. **For step-by-step workflows — install, configure,
   build, modify, run, test, debug, use — see
   the tasks companion.**

Both companion files cross-link to each other and to the skills
listed under **Related skills** below.

## Related skills

- [`doca-pcc`](../doca-pcc/SKILL.md) — the host-side PCC control
  library. This algorithm is loaded INTO a `doca_pcc` context that
  `doca-pcc` stands up, and owns the host-side lifecycle (create /
  configure / start / stop / destroy, the `doca_pcc_app` image, the
  attach-to-port semantics). This skill prescribes only the DPA-side
  algorithm integration on top.
- [`doca-pcc-counters`](../../tools/doca-pcc-counters/SKILL.md) —
  the read-only CLI for port PCC counters, and the canonical *"is
  the algorithm actually modulating traffic"* check. This skill
  names the counters the algorithm emits (CNP / NACK / AI / HAI /
  decrement / RTT-band) and routes the inspection there.
- [`doca-dpa`](../doca-dpa/SKILL.md) — the host-side DPA control
  library. The algorithm runs on the DPA, compiled by DPACC; the
  two-side-program rule and the DOCA-and-DPACC version-match
  overlay apply.
- [`doca-public-knowledge-map`](../../doca-public-knowledge-map/SKILL.md)
  — routing to the public DOCA PCC programming guide
  (<https://docs.nvidia.com/doca/sdk/doca-pcc/index.html>), the PCC
  application guide, the Compatibility Policy, and the on-disk
  layout of an installed DOCA package.
- [`doca-setup`](../../doca-setup/SKILL.md) — env preparation,
  install and DPACC compiler verification, BlueField firmware
  configuration (including the custom-PCC slot enable), and the *no
  install yet* path via the public NGC DOCA container. This skill
  assumes those preconditions hold, DPACC matches DOCA, and the
  custom-PCC slot is enabled.
- [`doca-version`](../../doca-version/SKILL.md) — canonical DOCA
  version handling. `## Version compatibility` cross-links the
  four-way match rule and the DOCA-and-DPACC overlay inherited from
  [`doca-pcc`](../doca-pcc/SKILL.md).
- [`doca-structured-tools-contract`](../../doca-structured-tools-contract/SKILL.md)
  — the detect / prefer / fall back / report precedence rule the
  Command appendix in the tasks companion honors.
- [`doca-programming-guide`](../../doca-programming-guide/SKILL.md)
  — general DOCA programming patterns; this skill layers
  algorithm-specific overlays on the universal build,
  modify-a-shipped-sample, and Core lifecycle patterns.
- [`doca-debug`](../../doca-debug/SKILL.md) — the cross-cutting
  debug ladder. Algorithm-specific debug (loaded but counters do
  not move; fails to initialize; modulates too aggressively or too
  gently) overlays on top of it.
- [`doca-hardware-safety`](../../doca-hardware-safety/SKILL.md) —
  cross-cutting hardware-safety meta-policy. Because the algorithm
  modulates production RoCE-v2 flows on a BlueField port, its
  pre-flight inventory, replica-first, and rollback rules apply via
  this skill's `## Safety policy` overlay.