Spritz implements DLNA/UPnP AV directly instead of wrapping an existing library. This document walks through each protocol layer.
Spritz sends ssdp:alive announcements to 239.255.255.250:1900 (IPv4) and [FF02::C]:1900 (IPv6) on startup, responds to M-SEARCH requests (honoring the client's MX delay per UPnP 1.0 §1.2.3), and sends ssdp:byebye on exit. Announcements repeat every 3 minutes, and each NT is sent three times with small gaps to survive datagram loss on WiFi. LOCATION URLs use [ipv6]:port when answering an IPv6 search.
GET /upnp/description.xml returns a MediaServer:1 description advertising ContentDirectory, ConnectionManager, and Microsoft X_MS_MediaReceiverRegistrar (Xbox). The <dlna:X_DLNADOC>DMS-1.50</dlna:X_DLNADOC> tag marks it as a DLNA DMS, which strict clients (tvOS Infuse, SenPlayer) require.
POST /upnp/control/contentdirectory handles Browse, Search, GetSystemUpdateID, GetSearchCapabilities, and GetSortCapabilities. The root has three children: V (Videos, flat), A (Music, flat), and F (By folder, recursive). Empty containers are hidden. <res> tags include size=, duration= when the container header can be parsed, resolution= when width/height are known, a DLNA.ORG_PN only when the probed codec is a known DLNA profile (H.264 bands; HEVC/VP9/AV1 omit the PN rather than lie), and DLNA.ORG flags (OP=01 byte-seek plus standard streaming flags). Matching sidecar subtitles (.srt / .vtt / .ass) are extra <res> URLs. Sidecar covers (cover.jpg / same-stem .jpg) appear as <upnp:albumArtURI> pointing at /art/{index}. File responses set transferMode.dlna.org: Streaming and contentFeatures.dlna.org so Infuse will play them.
A SUBSCRIBE to an event URL is answered with a SID and an immediate HTTP NOTIFY carrying the current state variables (SystemUpdateID stays 1 because the library is scanned once at start).
GET /upnp/icon.png is a 48×48 PNG listed in iconList on the device description.
Each source directory is mounted at /m/{index}/ and served over HTTP with range support via tower-http's ServeFile. Requests that leave the tree, follow a symlink, or use an unknown extension return 404. Sidecar subtitles sharing a stem with an indexed file are reachable so clients can fetch the extra <res> URLs. Album art is served at /art/{index} from cover.jpg / folder.jpg / a same-stem image next to the file.