Networking

The network stack is a from-scratch, RFC-shaped protocol tower over the virtio-net transport, built in bounded fixed-BSS staging.

The tower

Layer	Implementation	Notes
Transport	`virtio_net.zig`	DID 0x1041; queues 0/1; 12-byte `virtio_net_hdr` on every buffer
ARP	`arp.zig`	RFC 826; static IP; bounded 4-slot table; answer + learn
IPv4 / ICMP	`ipv4.zig`	RFC 791/792; RFC 1071 checksums; echo answer + ping; fragments dropped (counted)
UDP	`udp.zig`	RFC 768; pseudo-header checksum always computed; bounded listen table; loopback
DHCP	`dhcp.zig`	RFC 2131 client: DISCOVER → OFFER → REQUEST → ACK, plus renew/rebind/expiry
DNS	`dns.zig`	RFC 1035 client: A-record queries over UDP, parse + extract, `net dns <hostname>`
TCP	`tcp.zig`	RFC 793: client (handshake, one in-flight segment, bounded retransmission) + passive-open server
TCP seam	`syscall.zig` TCP slots	`sys_tcp_connect`/`sys_tcp_send`/`sys_tcp_recv`/`sys_tcp_close` for EL0
Sockets	`socket_core` + slot 80	B6 #1873: native generation-tagged handles — one passive listener with up to two simultaneous children; `AGENTD.ELF` and the SSH server run on it
HTTPD	`user/go/httpd` (`GOHTTPD.ELF`)	Go HTTP/1.1 web server over the passive-open listener (gate `live-httpd`; Zig `user/src/httpd.zig` retired in M71l)

What works end to end

Raw Ethernet — netsend transmits known frames the host captures byte-exactly; net recv prints received frames (MAC-filtered).
ARP — the guest answers requests for its IP and resolves peers.
ICMP — the guest answers echo requests and pings peers.
UDP — loopback (own-IP sends never touch the device), datagrams to/from peers, and the EL0 syscall seam (sys_udp_listen/send/recv).
DHCP — a bounded client that binds a lease, then renews, rebinds, and expires it on the lease timer.
TCP — a bounded client (connect, send, receive, close, plus fixed-RTO retransmission with an abort bound) and a passive-open server, exposed to EL0 through the syscall seam (TCP.BIN proves the client; GOHTTPD.ELF proves the server).
DNS — a bounded RFC 1035 resolver (net dns <hostname>) that queries port 53 and extracts A records, live-gated against a deterministic --net-dns-respond answer; the Go GODNS.ELF also resolves A records over the existing guest UDP syscall.
NAT — the launcher's --net-nat mode attaches a real VZNATNetworkDeviceAttachment; the guest pings the NAT gateway.
HTTP server — GOHTTPD.ELF accepts a connection on the passive-open listener and serves HTTP/1.1 over it; Zig HTTPD.BIN was retired to Go in M71l (gate live-httpd).
Userland network apps — TCP.BIN (echo client), GOFETCH.ELF (HTTP client, successor to retired FETCH.BIN), GOPING.ELF (Zig PING.BIN retired in M71n), GONETSTAT.ELF (the slot-62 network dashboard), GODNS.ELF (UDP A-record lookup), and GOTRACEROUTE.ELF (bounded direct-peer ICMP echo diagnostic) run the seam end to end. The Zig graphical chat demo (CHAT.BIN) was retired in M78c; the UDP seam remains live-tested. The Go diagnostics replace their Zig .BIN originals and are built with bash tools/go/build-netdiag.sh before their live gates.

Honest bounds

GOTRACEROUTE.ELF is a peer reachability probe, not route discovery: the current ICMP syscall has neither TTL selection nor time-exceeded responses. Its -m option bounds attempts; it does not set a packet TTL.
TCP is bounded: the legacy slots 30–33 seam is a singleton client, while the slot-80 native socket family allows one passive listener with at most two simultaneous children system-wide; no TCP loopback, no congestion control, fixed window 4096, payload ≤ 64 bytes in one segment.
RTO is fixed (3 ticks) with no Karn/exponential backoff; 10 retransmissions then an honest abort.
DHCP has no hostname/DNS options and no relay; the lease is enforced but the client is not a full DHCP implementation.
DNS is bounded: A records only, one query at a time, no caching, no other record types.
No routing beyond the NAT gateway; no IPv6 stack.