gns3-server/docs/features/dynamips-kernel-datapath.md

Dynamips kernel datapath

Dynamips routers emulate every port inside the Dynamips process; on the relay datapath a link reaches a router through a UDP tunnel (a Dynamips nio create_udp, a loopback port pair, the node's uBridge relay, and the link's UDP NIOs). The kernel datapath replaces that segment with the same anchor model Docker, QEMU and IOU use: every Ethernet slot/port owns a persistent TAP (gd{node_id[:8]}e{slot}p{port}), created by the server through uBridge's tap module, and links attach to the anchor — a per-link Linux bridge enslaves it, tc impairments run on it, capture and markers anchor on it. The Dynamips hypervisor opens the TAP with nio create_tap (QEMU's move: an external process holds the fd), unprivileged because the server handed the device's ownership to its own user.

Architecture

The diagram shows the e2e shape — two c7200 routers on one link (slot 0 port 0 each), the topology tests/e2e/test_dynamips_kernel_datapath.py drives. A Dynamips↔Docker/QEMU/IOU link is the same shape with the peer's own anchor on the other side; a routed transit is two such links with IOS forwarding between them. Interface names follow the deterministic rules gd{node_id[:8]}e{slot}p{port} for the anchor TAPs and gns3{link_id[:11]} for the bridge; the concrete names in the diagrams use node ids 7c1d4e2f / 9e8b6a3f and link id prefix 5d3c2b1a4e6 (gdA/gdB shorthand in the sequence below). Terms the diagrams use:

Term	Meaning
hypervisor	the Dynamips process emulating the router; it holds every anchor TAP's fd
anchor	the host-side interface a link attaches to — for Dynamips the gd persistent TAP, for Docker the gv veth host end
persistent TAP	the TAP created per Ethernet slot/port at node start (tap create + tap set_owner, ownership handed to the server user so the unprivileged hypervisor can open it); born DOWN, it outlives vm stop and hypervisor crashes
per-link bridge	the Linux bridge gns3{link_id[:11]} that stands in for the cable; the two anchors are its only ports
tap fd	a TAP's file descriptor: reads return frames the kernel received on the device, writes inject frames into it — the hypervisor opens it with nio create_tap and holds it for the port's lifetime
FDB	the bridge's forwarding database (learned MAC → port); unknown, broadcast and multicast destinations are flooded
01:80:c2 / group_fwd_mask	the IEEE 802.1D reserved multicast range (STP, LACP, LLDP, 802.1X, PAUSE); the bridge forwards it only per the port's group_fwd_mask — 0xfffd passes everything except PAUSE/PFC
clsact	the tc classifier carrier on the anchor; its egress chain is where the drop classifiers run
eBPF classifier	the stateful drop modes (every-Nth, quota, time window) at clsact egress prio 1
bpf_drop	match-drop BPF expressions at clsact egress prio 10–99
netem	the impairment qdisc behind clsact: delay, loss, rate, corrupt, duplicate, …
AF_PACKET tap	uBridge's packet socket on the anchor — how capture and markers observe frames off the forwarding path
carrier	the anchor's admin state (link set up/down); suspend is carrier down — traffic dead both ways
nio_bridge	the NIO record the controller posts to each endpoint node: bridge name, filters, markers, suspend flag
uBridge	the per-node helper process — on a kernel link it created the anchor, enslaves it and taps it, but never carries frames

flowchart LR
    subgraph H1["Dynamips hypervisor R1"]
        ios1["IOS router 1"]
    end
    subgraph RootNS["root namespace"]
        gdA["gd7c1d4e2fe0p0"]
        BR["gns35d3c2b1a4e6"]
        gdB["gd9e8b6a3fe0p0"]
        ub1["uBridge R1"]
        ub2["uBridge R2"]
    end
    subgraph H2["Dynamips hypervisor R2"]
        ios2["IOS router 2"]
    end
    ios1 <-->|"TAP fd"| gdA
    gdA <-->|"bridge port"| BR
    BR <-->|"bridge port"| gdB
    gdB <-->|"TAP fd"| ios2
    gdA -.->|"AF_PACKET tap"| ub1
    ub1 -.->|"tc qdisc"| gdA
    gdB -.->|"AF_PACKET tap"| ub2
    ub2 -.->|"tc qdisc"| gdB

The only userspace crossings left are the Dynamips processes themselves — the routers are the emulation, and the hypervisor reads and writes the TAP directly (QEMU's external-fd-holder shape, no extra relay hop on the node leg unlike IOU's fabric). What the kernel datapath buys is the link segment: anchor → per-link bridge → peer anchor crosses entirely in the kernel, impairments run as kernel tc instead of userspace filters, and suspend maps to anchor admin-down. Each side's uBridge does its own tap-and-qdisc work on its node's anchor — but neither dashed edge is a steady fixture: the qdisc exists only while the link carries filters (deletion tc resets it), the tap only while capture or markers run, and by convention only on the capture node's end, so an unfiltered, uncaptured link shows neither. The anchors themselves were created through uBridge at node start (Anchor lifecycle below).

The legacy relay is untouched: a port without a kernel link keeps its Dynamips UDP NIO and the node's uBridge relay tunnel exactly as before, and a kernel link binds the same port to its anchor instead — per port, no node-wide switch. The same R1 ↔ R2 link on the relay — one direction drawn, the reverse is the mirror image; this is the exact shape the negative control keeps when enable_kernel_datapath = false (Verified below). Each relay bridge is named DYNAMIPS-{lport}-{rport} after the loopback port pair its NIO allocates at link time:

flowchart LR
    subgraph H1["Dynamips hypervisor R1"]
        ios1["IOS router 1"]
        nio1["UDP NIO"]
    end
    subgraph ub1["uBridge R1"]
        rl1["relay bridge"]
    end
    subgraph KERN["kernel"]
        lp1["loopback UDP"]
        tun["UDP tunnel"]
        lp2["loopback UDP"]
    end
    subgraph ub2["uBridge R2"]
        rl2["relay bridge"]
    end
    subgraph H2["Dynamips hypervisor R2"]
        nio2["UDP NIO"]
        ios2["IOS router 2"]
    end
    ios1 --> nio1
    nio1 -->|"copy 1: user→kernel"| lp1
    lp1 -->|"copy 2: kernel→user"| rl1
    rl1 -->|"copy 3: user→kernel"| tun
    tun -->|"copy 4: kernel→user"| rl2
    rl2 -->|"copy 5: user→kernel"| lp2
    lp2 -->|"copy 6: kernel→user"| nio2
    nio2 --> ios2

Six kernel/user crossings each way, against the kernel datapath's two — and those two are the TAP-fd legs that are the emulation itself, so the relay's four extra copies are pure overhead. Userspace filters run inside the relay bridges, between copies 2-3 and 4-5. On a same-compute link the UDP tunnel rides loopback as drawn; cross-compute it rides the real network with the copy count unchanged.

Frame path for one R1 → R2 frame. The impairment chain fires on the egress of the destination end's anchor, in order clsact → netem (a classifier drop means netem never sees the frame); each direction is impaired exactly once and R2 → R1 is symmetric on gdA:

flowchart LR
    subgraph H1["Dynamips hypervisor R1"]
        S1["IOS port sends"]
    end
    subgraph ROOT["root namespace"]
        subgraph GA["gd7c1d4e2fe0p0 — ingress (egress chain serves R2→R1)"]
            S3["ingress"]
        end
        subgraph BRP["per-link bridge"]
            G1{"reserved 01:80:c2?"}
            D["dropped"]
            S4["FDB lookup / flood"]
        end
        subgraph GB["gd9e8b6a3fe0p0 — egress chain (clsact, then netem)"]
            S5["eBPF prio 1"]
            S6["bpf_drop prio 10-99"]
            S7["netem qdisc"]
        end
        T["capture / markers"]
    end
    subgraph H2["Dynamips hypervisor R2"]
        S9["IOS port receives"]
    end
    S1 -->|"tap fd, copy 1: user→kernel"| S3
    S3 --> G1
    G1 -->|"PAUSE 0x01"| D
    G1 -->|"0xfffd passes"| S4
    S4 --> S5
    S5 --> S6 --> S7
    S7 -->|"tap fd, copy 2: kernel→user"| S9
    S3 -. "AF_PACKET tap" .-> T
    S7 -. "AF_PACKET tap" .-> T

What each stage drops: the reserved-range gate passes LACP, LLDP, 802.1X and STP per the port mask but never PAUSE/PFC (Link-local frames below); the clsact/netem stages drop or impair per the installed filters (Capture, markers, filters below). The R2 → R1 reply is symmetric — its impairment points are gd9e8b6a3fe0p0's egress (entering R2) and gd7c1d4e2fe0p0's egress (entering R1) — so every direction is impaired exactly once, and delay 100 measures ≈200 ms round trip (the e2e-verified number). Copies 1-2 are the whole kernel/user bill — both are the TAP-fd legs of the emulation itself; the same frame on the relay datapath crosses six times (diagram above). The dashed sideband is uBridge's observation: capture and markers run as AF_PACKET taps on whichever end hosts them, cloning this frame at that anchor's stage — gdA's rx or gdB's tx — and a clsact-dropped frame never reaches the tap.

Datapath selection

UDPLink._kernel_datapath_eligible extends the common rules with:

• the compute must report ubridge_tap — the anchors are uBridge-created TAPs the hypervisor merely opens, the same capability QEMU gates on; there is no Dynamips-specific one;
• both ports must be Ethernet. The controller's port matrix decides per port; serial, ATM and POS ports keep the relay whatever the capabilities (the adapter registry in compute/dynamips/adapters/adapter.py names the Ethernet models — ETHERNET_ADAPTERS / ETHERNET_WICS, WIC-1ENET included, at its Dynamips port number 16 * (wic_slot + 1)).

Link creation sequence

Both routers sit on the same compute (a kernel-link precondition) and each drives its own uBridge process; the hypervisor is one process per router. The two node handlers attach concurrently and compute the same bridge name independently (a pure function of the link id), so either may win the brctl create race — the loser verifies the bridge exists (brctl show) and continues:

sequenceDiagram
    participant API as REST API
    participant C as Controller
    participant R1 as Router R1
    participant HY1 as Hypervisor R1
    participant UB1 as uBridge R1
    participant R2 as Router R2
    participant HY2 as Hypervisor R2
    participant UB2 as uBridge R2

    API->>C: POST /links
    C->>C: eligibility check
    C->>C: bridge = gns3 + link_id[:11]
    C--)R1: nio_bridge
    C--)R2: nio_bridge (same bridge)
    par Router R1
        R1->>HY1: nio create_tap
        R1->>HY1: vm slot_add_nio_binding
        R1->>HY1: vm slot_enable_nio
        R1->>UB1: brctl create
        R1->>UB1: link set gns35d3c2b1a4e6 up
        R1->>UB1: brctl addif gns35d3c2b1a4e6 gdA
        R1->>UB1: capture start_kernel gdA
        R1->>UB1: marker add_kernel <name> gdA
        R1->>UB1: tc netem set gdA
        R1->>UB1: link set gdA up
    and Router R2
        R2->>HY2: nio create_tap
        R2->>UB2: same on gdB
    end
    Note over HY1,UB2: steady state — kernel forwards between the anchors, the hypervisors hold the tap fds

nio create_tap, vm slot_add_nio_binding and vm slot_enable_nio are the Dynamips-specific steps: the hypervisor opens the anchor and binds it to the slot/port before any bridge work, so a half-wired port never appears on the link. Capture, markers and tc netem set run only when the NIO carries them — a marker's real form is marker add_kernel <name> <if> "<bpf>", bpf expressions arrive as tc bpf_drop add, the stateful modes as tc nth/quota/window_drop — and brctl addif is itself what opens the port's group_fwd_mask to 0xfffd (no separate command exists). The anchor's carrier stays down while the link is suspended. A link created while the router is stopped is stored and wired by the next start — the NIO simply waits in the slot adapter (deferred wiring, Anchor lifecycle below).

Anchor lifecycle

• Birth (node start) — _prepare_tap_datapath probes the tap module (one create/delete cycle) and creates one persistent TAP per Ethernet slot/port, ownership handed to the server user, born DOWN. A stale leftover from a previous run is swept first; an anchor that already exists (restart) is kept — an anchor is never recreated under a live link. Serial/ATM/POS ports simply have no anchor.
• Life — a kernel link opens the anchor in the hypervisor (nio create_tap), binds it to the slot/port and enslaves it into the per-link bridge; removing the link unbinds the port, deletes the hypervisor's TAP NIO (releasing the fd) and tears the anchor out of the bridge. A link bound while the router is stopped is stored and wired by the next start (deferred wiring — the NIO simply waits in the slot adapter).
• Death (node close) — _stop_ubridge runs in QEMU's order, not IOU's: here the Dynamips hypervisor holds the anchor fds, so its TAP NIOs go first (nio delete closes the fd), then tap delete per anchor, then the per-link kernel bridges, then uBridge itself.

A stop is not a close. stop only halts the emulated router: the hypervisor process — and with it the TAP fds, the port bindings and the per-link bridges — survives vm stop (the same reason a relay link's tunnel always survived a stop). Links therefore keep working through a stop/start with no re-wiring at all; the restart's attach pass skips every port that still has its TAP NIO. If the hypervisor died mid-flight (crash or kill), the persistent TAPs remain and the next start re-attaches from the stored NIOs.

Link operations

Operation	Kernel-datapath action
Create / attach	nio create_tap (hypervisor holds the fd) + vm slot_add_nio_binding + brctl create + brctl addif + carrier up + markers + tc netem set
Delete / detach	marker teardown → tc reset → carrier off → brctl delif/brctl delete → unbind + nio delete (release the fd)
Update (filters/markers/suspend changed)	reconcile on the anchor only — no re-binding, no re-enslaving
Suspend	anchor admin-down (traffic dead both ways) + resume restores
Adapter hot-add (OIR)	anchors for the new adapter's Ethernet ports created at once, so a kernel link can attach immediately

Capture, markers, filters

Capture on a kernel link is capture start_kernel <anchor>; markers ride the base reconcile with the anchor as the location (marker add_kernel — the shared KernelDatapathMixin machinery, no Dynamips-specific code); filters are one tc netem qdisc per anchor plus cls_bpf/eBPF drops. A relay port keeps its Dynamips NIO and the node's uBridge relay bridge (DYNAMIPS-*) with userspace filters, exactly as before.

Link-local frames (LACP, LLDP, 802.1X, STP)

The bridge stands in for a cable, but the kernel's br_handle_frame() does not forward the IEEE 802.1D reserved range (01:80:c2:00:00:00-0f) by default — LACP, LLDP/DCBX and 802.1X would never cross a kernel link while the UDP relay carries them fine. uBridge therefore opens every port's per-port group_fwd_mask (IFLA_BRPORT_GROUP_FWD_MASK) to 0xfffd at brctl addif time (Linux 4.15+, best-effort on older kernels). The bridge-level knob cannot do this: BR_GROUPFWD_RESTRICTED rejects bits 0-2, so LACP is reachable per port only.

Result on kernel links: ordinary multicast, STP/RSTP, LACP, 802.1X and LLDP/DCBX all cross; 802.3x PAUSE / PFC does not — case 0x01 in br_handle_frame() drops it unconditionally and no mask can enable it. That is a documented limit, not a regression (PFC's hardware semantics are out of emulation's reach anyway); a lab that needs PAUSE frames on the wire must use a relay link. Verified live by tests/e2e/test_docker_link_local_frames.py (per-MAC guest-to-guest matrix); the contract and kernel references live in docs/design/ubridge-link-local-frame-forwarding-spec.md.

Relay fallback

A uBridge without the tap module keeps the node relay-only: _prepare_tap_datapath warns, no anchors are created, the warning says the node cannot carry kernel links, and every link keeps its nio create_udp tunnel. The controller never offers such a node a kernel link (capability unreported).

Verified

Unit level (tests/compute/dynamips/test_dynamips_kernel_datapath.py): anchor topology (Ethernet models and WIC-1ENET port numbering only), the lifecycle (create/sweep/skip-existing), deferred wiring of NIOs bound while stopped, attach/update/remove command shapes on both datapaths, the stop order (hypervisor NIO delete before tap delete before brctl delete), capture and marker shapes, hot-added adapters, and the manager's nio_bridge construction. Plus the controller-eligibility cases (mixable with docker/qemu/iou, tap capability required, serial excluded) in tests/controller/test_kernel_datapath_link.py.

End-to-end (tests/e2e/test_dynamips_kernel_datapath.py, pytest marker e2e) on a live server with two real c7200 routers on a real IOS image, driven through the REST API and the IOS consoles with real ICMP:

• a link created while both routers are stopped selects the kernel datapath and is wired by node start (deferred wiring);
• the per-link kernel bridge on the host has exactly the two anchor TAPs enslaved; show ip int brief is up/up and ping R1→R2 succeeds;
• delay 100 ⇒ netem qdisc visible on both anchors and the ping RTT grows by ~200 ms (one-way 100 per direction, each anchor impairing its own ingress side); clearing resets the qdiscs and the RTT;
• suspend ⇒ anchor administratively down, ping 0%; resume restores both;
• capture writes a pcap full of ICMP-over-Ethernet records;
• a serial link between the same two routers stays on the relay (per-port exclusion) and pings over it — the relay regression net;
• link delete/re-create: the bridge goes and comes back, the anchors survive, traffic resumes;
• router stop/start keeps the wiring intact (bridge membership unchanged, no duplicate NIOs) and traffic resumes after the reboot;
• deleting the project removes every anchor and bridge.

test_dynamips_relay_control is the negative control: the same topology on an isolated relay-configured instance (enable_kernel_datapath = false) has no per-link bridge and nothing enslaved, and still pings — so the kernel objects above can only come from the kernel datapath.

Configuration

[Server]
# Wire eligible links (same compute, both endpoints Ethernet and able to
# anchor) through kernel interfaces instead of the uBridge UDP relay.
enable_kernel_datapath = True

Roadmap

The Ethernet switch has landed (see ethernet-switch-kernel-datapath.md — it absorbs the peer's anchor into its own kernel bridge; switch-to-switch cascades and the IOL runner container have landed too — see ethernet-switch-kernel-datapath.md and iol-docker-kernel-datapath.md). Cross-compute kernel links need VXLAN/GENEVE encapsulation — deferred until every node type is kernelized.