sn-fetch PoC: range probe, weighted planner, parallel segments, reassembly + sha256

server/Cargo.toml

@@ -0,0 +1,22 @@
+[workspace]
+resolver = "2"
+members = ["crates/sn-fetch"]
+
+[workspace.package]
+version = "0.1.0"
+edition = "2021"
+license = "AGPL-3.0-only"
+
+[workspace.dependencies]
+tokio = { version = "1", features = ["rt-multi-thread", "macros", "fs", "io-util"] }
+reqwest = { version = "0.12", default-features = false, features = ["rustls-tls"] }
+sha2 = "0.10"
+hex = "0.4"
+thiserror = "2"
+tempfile = "3"
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+clap = { version = "4", features = ["derive"] }
+
+[profile.release]
+lto = "thin"

server/README.md

@@ -1,7 +1,28 @@
-# SpiderNet-Node — Server/Software (Konzeptphase)
+# SpiderNet-Node — Server/Software
 
 Die Software, die aus "Nachbarschafts-LAN" ein gepooltes Bandbreitennetz
-macht. Sprache: **Rust**. Status: **Konzeptphase — noch nichts implementiert.**
+macht. Sprache: **Rust**.
+
+## Status
+
+**`sn-fetch` PoC v0 ist implementiert** (siehe `crates/sn-fetch/`):
+
+- Range-Probe (`HEAD` + 1-Byte-GET-Fallback) erkennt `Accept-Ranges`
+- Gewichteter Planner (Exits nach Kapazität, optionale Sub-Segmente pro
+  Exit, Mindest-Segmentgröße mit Merge-Fix, validated: lückenlose,
+  vollständige Abdeckung)
+- Parallele Segment-Downloads (Streaming, eigener reqwest-Client pro Exit)
+- Reassembly in Index-Reihenfolge + Streaming-SHA-256 + Längen-Check
+- Fallback: range-hostile Server werden als single-stream geladen
+- CLI: `sn-fetch <url> [-o out] [-e "1=2,2=1"] [-s n] [--sha256 hex] --probe-only`
+
+Verifiziert gegen einen echten Server (Debian-CD-Mirror, 756 MB, 8 Segmente,
+SHA-256 entspricht der offiziellen `SHA256SUMS`).
+
+**Noch nicht drin:** Multi-Exit-Routing über das Mesh (aktuell laufen alle
+Exits als lokale Verbindungen — die Struktur [`HttpExit`]-Trait,
+Exit-Registry] ist dafür schon bereit), Retry bei Segment-Fehlern,
+Progress-Anzeige während des Laufs.
 
 ## Haftungsausschluss (muss im Produkt sichtbar sein)
 

server/crates/sn-fetch/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "sn-fetch"
+description = "Segmented multi-exit downloader for SpiderNet neighborhood networks"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[dependencies]
+tokio = { workspace = true }
+reqwest = { workspace = true }
+sha2 = { workspace = true }
+hex = { workspace = true }
+thiserror = { workspace = true }
+tempfile = { workspace = true }
+tracing = { workspace = true }
+tracing-subscriber = { workspace = true }
+clap = { workspace = true }

server/crates/sn-fetch/src/error.rs

@@ -0,0 +1,50 @@
+//! Error type for all sn-fetch operations.
+
+use crate::types::{ExitId, SegmentIndex};
+
+#[derive(Debug, thiserror::Error)]
+pub enum FetchError {
+    #[error("http request failed: {0}")]
+    Http(#[from] reqwest::Error),
+
+    #[error("io error: {0}")]
+    Io(#[from] std::io::Error),
+
+    #[error("invalid url `{0}`: scheme must be http or https")]
+    InvalidUrl(String),
+
+    #[error("file size must be greater than zero")]
+    ZeroFileSize,
+
+    #[error("segment {index}: received {got} bytes, expected {expected}")]
+    SegmentLengthMismatch {
+        index: SegmentIndex,
+        got: u64,
+        expected: u64,
+    },
+
+    #[error("segment {exit}/{index}: server ignored range request (status {status})")]
+    RangeRejected {
+        exit: ExitId,
+        index: SegmentIndex,
+        status: u16,
+    },
+
+    #[error("downloaded {got} bytes, expected {expected}")]
+    TotalLengthMismatch { got: u64, expected: u64 },
+
+    #[error("sha256 mismatch: expected {expected}, got {got}")]
+    HashMismatch { expected: String, got: String },
+
+    #[error("invalid plan: {0}")]
+    InvalidPlan(String),
+
+    #[error("unknown exit `{0}` referenced in plan")]
+    UnknownExit(ExitId),
+
+    #[error("invalid part file name `{0}`")]
+    CorruptPart(String),
+
+    #[error("invalid sha256 hex string")]
+    InvalidDigest,
+}

server/crates/sn-fetch/src/exec.rs

@@ -0,0 +1,204 @@
+//! Execution: download planned segments in parallel and reassemble the
+//! target file.
+
+use std::path::{Path, PathBuf};
+use std::time::{Duration, Instant};
+
+use sha2::{Digest, Sha256};
+use tokio::io::AsyncWriteExt;
+
+use crate::error::FetchError;
+use crate::exit::ExitRegistry;
+use crate::plan::FetchPlan;
+use crate::types::{ByteRange, ExitId, FileSize, SegmentIndex, Sha256Digest, TargetUrl};
+
+/// Statistics for one finished segment.
+#[derive(Debug, Clone)]
+pub struct SegmentStats {
+    pub index: SegmentIndex,
+    pub bytes: u64,
+    pub duration: Duration,
+}
+
+/// Result of a complete fetch.
+#[derive(Debug, Clone)]
+pub struct FetchReport {
+    pub url: TargetUrl,
+    pub output: PathBuf,
+    pub file_size: FileSize,
+    pub segments: Vec<SegmentStats>,
+    pub wall_time: Duration,
+    pub sha256: Option<Sha256Digest>,
+}
+
+/// Download every planned segment in parallel. Each segment is written
+/// to its own part file inside `parts_dir` (named `seg-NNNNNN.part`).
+///
+/// Failures abort the whole fetch: a partially assembled file is never
+/// presented as success.
+pub async fn run_segments(
+    plan: &FetchPlan,
+    exits: &ExitRegistry,
+    parts_dir: &Path,
+) -> Result<Vec<SegmentStats>, FetchError> {
+    tokio::fs::create_dir_all(parts_dir).await?;
+
+    let mut handles = Vec::with_capacity(plan.segments.len());
+    for seg in &plan.segments {
+        let client = exits.get(seg.exit)?.client().clone();
+        handles.push(tokio::spawn(download_segment(
+            plan.url.clone(),
+            seg.exit,
+            seg.index,
+            seg.range,
+            client,
+            parts_dir.to_path_buf(),
+        )));
+    }
+
+    let mut stats = Vec::with_capacity(handles.len());
+    for handle in handles {
+        let stats_i = handle
+            .await
+            .map_err(|err| FetchError::InvalidPlan(format!("segment task panicked: {err}")))??;
+        stats.push(stats_i);
+    }
+    stats.sort_by_key(|s| s.index);
+    Ok(stats)
+}
+
+/// Download one byte range and write it to `parts_dir/seg-NNNNNN.part`.
+async fn download_segment(
+    url: TargetUrl,
+    exit: ExitId,
+    index: SegmentIndex,
+    range: ByteRange,
+    client: reqwest::Client,
+    parts_dir: PathBuf,
+) -> Result<SegmentStats, FetchError> {
+    let started = Instant::now();
+
+    let part_path = part_file(&parts_dir, index);
+    let mut response = client
+        .get(url.inner().clone())
+        .header(reqwest::header::RANGE, range.http_value())
+        .send()
+        .await?
+        .error_for_status()?;
+
+    match response.status() {
+        reqwest::StatusCode::PARTIAL_CONTENT => {}
+        status => {
+            return Err(FetchError::RangeRejected {
+                exit,
+                index,
+                status: status.as_u16(),
+            });
+        }
+    }
+
+    let mut file = tokio::fs::File::create(&part_path).await?;
+    let mut written: u64 = 0;
+    while let Some(chunk) = response.chunk().await? {
+        file.write_all(&chunk).await?;
+        written += chunk.len() as u64;
+    }
+    file.flush().await?;
+
+    if written != range.len() {
+        tokio::fs::remove_file(&part_path).await.ok();
+        return Err(FetchError::SegmentLengthMismatch {
+            index,
+            got: written,
+            expected: range.len(),
+        });
+    }
+
+    Ok(SegmentStats {
+        index,
+        bytes: written,
+        duration: started.elapsed(),
+    })
+}
+
+/// Reassemble part files (in index order) into the output file while
+/// streaming a SHA-256 over the content. Verifies the total length and,
+/// if given, the expected digest. Returns the actual digest.
+pub async fn assemble(
+    parts_dir: &Path,
+    output: &Path,
+    expected_size: FileSize,
+    expected_digest: Option<&Sha256Digest>,
+) -> Result<Sha256Digest, FetchError> {
+    let mut part_paths = collect_part_files(parts_dir).await?;
+    part_paths.sort();
+
+    if let Some(parent) = output.parent() {
+        if !parent.as_os_str().is_empty() {
+            tokio::fs::create_dir_all(parent).await?;
+        }
+    }
+
+    let mut output_file = tokio::fs::File::create(output).await?;
+    let mut hasher = Sha256::new();
+    let mut total: u64 = 0;
+
+    for part_path in &part_paths {
+        let mut part = tokio::fs::File::open(part_path).await?;
+        let mut buffer = [0u8; 64 * 1024];
+        loop {
+            let n = tokio::io::AsyncReadExt::read(&mut part, &mut buffer).await?;
+            if n == 0 {
+                break;
+            }
+            output_file.write_all(&buffer[..n]).await?;
+            hasher.update(&buffer[..n]);
+            total += n as u64;
+        }
+    }
+    output_file.flush().await?;
+
+    if total != expected_size.value() {
+        return Err(FetchError::TotalLengthMismatch {
+            got: total,
+            expected: expected_size.value(),
+        });
+    }
+
+    let digest = Sha256Digest::from_bytes(hasher.finalize().into());
+    if let Some(expected) = expected_digest {
+        if digest != *expected {
+            return Err(FetchError::HashMismatch {
+                expected: expected.to_hex(),
+                got: digest.to_hex(),
+            });
+        }
+    }
+    Ok(digest)
+}
+
+/// Collect `seg-NNNNNN.part` files; anything else in `parts_dir` is an
+/// error, so we never silently assemble garbage.
+async fn collect_part_files(parts_dir: &Path) -> Result<Vec<PathBuf>, FetchError> {
+    let mut out = Vec::new();
+    let mut entries = tokio::fs::read_dir(parts_dir).await?;
+    while let Some(entry) = entries.next_entry().await? {
+        let raw_name = entry.file_name();
+        let name = raw_name
+            .to_str()
+            .ok_or_else(|| FetchError::CorruptPart("<non-utf8 file name>".to_string()))?;
+        let middle = name
+            .strip_prefix("seg-")
+            .and_then(|n| n.strip_suffix(".part"))
+            .ok_or_else(|| FetchError::CorruptPart(name.to_string()))?;
+        middle
+            .parse::<u32>()
+            .map_err(|_| FetchError::CorruptPart(name.to_string()))?;
+        out.push(entry.path());
+    }
+    Ok(out)
+}
+
+fn part_file(parts_dir: &Path, index: SegmentIndex) -> PathBuf {
+    parts_dir.join(format!("seg-{:06}.part", index.value()))
+}

server/crates/sn-fetch/src/exit.rs

@@ -0,0 +1,73 @@
+//! Exit abstraction: an "exit" is one internet uplink through which
+//! segment requests can be sent.
+//!
+//! PoC v0 ships only [`LocalExit`] (the household's own connection, no
+//! detour). The registry/trait structure is deliberately ready so a
+//! future exit implementation can route HTTP through a neighbour over
+//! the mesh (e.g. via a per-exit proxy or tunnel) without changing the
+//! planner or executor.
+
+use std::collections::BTreeMap;
+use std::sync::Arc;
+use std::time::Duration;
+
+use crate::error::FetchError;
+use crate::types::ExitId;
+
+/// An internet uplink able to issue HTTP requests.
+pub trait HttpExit: Send + Sync {
+    fn id(&self) -> ExitId;
+
+    /// Client whose connections go out through this exit.
+    fn client(&self) -> &reqwest::Client;
+}
+
+/// The household's own uplink: plain outgoing HTTP, no detour.
+#[derive(Debug)]
+pub struct LocalExit {
+    id: ExitId,
+    client: reqwest::Client,
+}
+
+impl LocalExit {
+    pub fn new(id: ExitId) -> Result<Self, FetchError> {
+        let client = reqwest::Client::builder()
+            .connect_timeout(Duration::from_secs(30))
+            .build()?;
+        Ok(Self { id, client })
+    }
+}
+
+impl HttpExit for LocalExit {
+    fn id(&self) -> ExitId {
+        self.id
+    }
+
+    fn client(&self) -> &reqwest::Client {
+        &self.client
+    }
+}
+
+/// All exits available for a fetch, keyed by id.
+#[derive(Default, Clone)]
+pub struct ExitRegistry {
+    exits: BTreeMap<ExitId, Arc<dyn HttpExit>>,
+}
+
+impl ExitRegistry {
+    pub fn empty() -> Self {
+        Self::default()
+    }
+
+    pub fn register(&mut self, exit: Arc<dyn HttpExit>) {
+        self.exits.insert(exit.id(), exit);
+    }
+
+    pub fn get(&self, id: ExitId) -> Result<&Arc<dyn HttpExit>, FetchError> {
+        self.exits.get(&id).ok_or(FetchError::UnknownExit(id))
+    }
+
+    pub fn ids(&self) -> Vec<ExitId> {
+        self.exits.keys().copied().collect()
+    }
+}

server/crates/sn-fetch/src/lib.rs

@@ -0,0 +1,18 @@
+//! sn-fetch — segmented multi-exit downloader for SpiderNet.
+//!
+//! PoC v0: probes a URL for HTTP range support, plans weighted byte-range
+//! segments across exits, downloads segments in parallel and reassembles
+//! the target file with SHA-256 verification.
+//!
+//! SpiderNet software carries no warranty and accepts no responsibility
+//! for its usage or for transported content (AGPL-3.0, sections 15/16).
+
+#![forbid(unsafe_code)]
+
+pub mod error;
+pub mod exec;
+pub mod exit;
+pub mod plan;
+pub mod probe;
+pub mod runner;
+pub mod types;

server/crates/sn-fetch/src/main.rs

@@ -0,0 +1,182 @@
+//! CLI entry point for the sn-fetch PoC.
+//!
+//! KEINE GARANTIE — this software is provided "as is" without warranty of
+//! any kind (AGPL-3.0, sections 15/16). Users are solely responsible for
+//! what they transmit or receive with it.
+
+#![forbid(unsafe_code)]
+
+use std::num::{NonZeroU32, NonZeroU64, NonZeroU8, NonZeroUsize};
+use std::path::PathBuf;
+
+use clap::Parser;
+use sn_fetch::runner::{self, FetchOptions};
+use sn_fetch::types::{
+    ExitId, ExitWeight, MinSegmentBytes, SegmentsPerExit, Sha256Digest, TargetUrl,
+};
+
+#[derive(Debug, Parser)]
+#[command(name = "sn-fetch", about = "SpiderNet segmented downloader (PoC)")]
+struct Args {
+    /// URL of the file to download (http/https).
+    url: String,
+
+    /// Output file path (default: file name from the URL, in cwd).
+    #[arg(short, long)]
+    output: Option<PathBuf>,
+
+    /// Exits as `id=weight` pairs, comma separated, e.g. `local=1,local2=2`.
+    /// PoC: all ids map to plain local connections; the weights only shape
+    /// the plan. Multi-exit routing comes with the mesh integration.
+    #[arg(short = 'e', long = "exit", default_value = "local=1")]
+    exits: String,
+
+    /// Parallel sub-segments per exit.
+    #[arg(short = 's', long, default_value_t = 1)]
+    segments_per_exit: usize,
+
+    /// Minimum segment size in MiB (smaller segments get merged).
+    #[arg(long, default_value_t = 4)]
+    min_segment_mib: u64,
+
+    /// Expected SHA-256 of the complete file (hex).
+    #[arg(long)]
+    sha256: Option<String>,
+
+    /// Only probe the server and print what we learned.
+    #[arg(long)]
+    probe_only: bool,
+}
+
+fn parse_exits(spec: &str) -> Result<Vec<(ExitId, ExitWeight)>, String> {
+    spec.split(',')
+        .map(|pair| {
+            let (id, weight) = pair
+                .split_once('=')
+                .ok_or_else(|| format!("invalid exit spec `{pair}`, expected `id=weight`"))?;
+            let id = id
+                .trim()
+                .parse::<u8>()
+                .ok()
+                .and_then(NonZeroU8::new)
+                .ok_or_else(|| format!("invalid exit id `{id}` (must be a non-zero number)"))?;
+            let weight = weight
+                .trim()
+                .parse::<u32>()
+                .ok()
+                .and_then(NonZeroU32::new)
+                .ok_or_else(|| format!("invalid exit weight `{weight}` (must be non-zero)"))?;
+            Ok((ExitId::new(id), ExitWeight::new(weight)))
+        })
+        .collect()
+}
+
+fn default_output(url: &TargetUrl) -> PathBuf {
+    let mut segments = match url.inner().path_segments() {
+        Some(segments) => segments.clone(),
+        None => return PathBuf::from("download.bin"),
+    };
+    let name = segments
+        .next_back()
+        .filter(|name| !name.is_empty())
+        .map(str::to_string)
+        .unwrap_or_else(|| "download.bin".to_string());
+    PathBuf::from(name)
+}
+
+#[tokio::main]
+async fn main() {
+    tracing_subscriber::fmt()
+        .with_env_filter(
+            tracing_subscriber::EnvFilter::try_from_default_env()
+                .unwrap_or_else(|_| tracing_subscriber::EnvFilter::new("warn")),
+        )
+        .init();
+
+    if let Err(report) = run().await {
+        eprintln!("NO WARRANTY — SpiderNet sn-fetch PoC (AGPL-3.0, no liability)");
+        eprintln!("error: {report}");
+        std::process::exit(1);
+    }
+}
+
+async fn run() -> Result<(), String> {
+    let args = Args::parse();
+    let target = TargetUrl::parse(&args.url).map_err(|err| err.to_string())?;
+
+    let client = reqwest::Client::builder()
+        .connect_timeout(std::time::Duration::from_secs(30))
+        .build()
+        .map_err(|err| err.to_string())?;
+
+    if args.probe_only {
+        let info = sn_fetch::probe::probe(&client, &target)
+            .await
+            .map_err(|err| err.to_string())?;
+        println!("url:            {target}");
+        println!("range support:  {:?}", info.range_support);
+        match info.content_length {
+            Some(size) => println!("content-length: {size} bytes"),
+            None => println!("content-length: unknown"),
+        }
+        return Ok(());
+    }
+
+    let exit_specs = parse_exits(&args.exits)?;
+    let spe =
+        NonZeroUsize::new(args.segments_per_exit).ok_or("segments_per_exit must be at least 1")?;
+    let min_segment = MinSegmentBytes::new(
+        NonZeroU64::new(args.min_segment_mib.saturating_mul(1024 * 1024))
+            .ok_or("min_segment_mib must be at least 1")?,
+    );
+
+    let expected = match &args.sha256 {
+        Some(hex) => Some(Sha256Digest::from_hex(hex).map_err(|err| err.to_string())?),
+        None => None,
+    };
+
+    let options = FetchOptions {
+        output: args
+            .output
+            .clone()
+            .unwrap_or_else(|| default_output(&target)),
+        exits: exit_specs.clone(),
+        segments_per_exit: SegmentsPerExit::new(spe),
+        min_segment,
+        expected_sha256: expected,
+    };
+
+    let registry = runner::local_registry(&exit_specs).map_err(|err| err.to_string())?;
+    let parts_dir = std::env::temp_dir().join("sn-fetch-parts");
+
+    let report = runner::run(&target, &options, &registry, &parts_dir)
+        .await
+        .map_err(|err| err.to_string())?;
+
+    println!("NO WARRANTY — SpiderNet sn-fetch PoC (AGPL-3.0, no liability)");
+    println!();
+    println!("url:      {}", report.url);
+    println!("output:   {}", report.output.display());
+    println!("size:     {} bytes", report.file_size);
+    match report.sha256 {
+        Some(digest) => println!("sha256:   {digest}"),
+        None => println!("sha256:   not computed"),
+    }
+    println!(
+        "time:     {:.2?} s → effective {:.1} MiB/s",
+        report.wall_time.as_secs_f64(),
+        report.file_size.value() as f64 / 1024.0 / 1024.0 / report.wall_time.as_secs_f64()
+    );
+    println!("segments:");
+    for segment in &report.segments {
+        let mib_per_s = segment.bytes as f64
+            / 1024.0
+            / 1024.0
+            / segment.duration.as_secs_f64().max(f64::EPSILON);
+        println!(
+            "  #{}: {} bytes in {:.2?} s ({mib_per_s:.1} MiB/s)",
+            segment.index, segment.bytes, segment.duration
+        );
+    }
+    Ok(())
+}

server/crates/sn-fetch/src/plan.rs

@@ -0,0 +1,279 @@
+//! Planning: split a file into weighted byte-range segments across exits.
+
+use crate::error::FetchError;
+use crate::types::{
+    ByteRange, ExitId, ExitWeight, FileSize, MinSegmentBytes, SegmentIndex, SegmentsPerExit,
+    TargetUrl,
+};
+use std::num::NonZeroUsize;
+
+/// One planned segment: a byte range assigned to one exit.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct SegmentPlan {
+    pub index: SegmentIndex,
+    pub range: ByteRange,
+    pub exit: ExitId,
+}
+
+/// A validated download plan.
+#[derive(Debug, Clone)]
+pub struct FetchPlan {
+    pub url: TargetUrl,
+    pub file_size: FileSize,
+    pub segments: Vec<SegmentPlan>,
+}
+
+/// Exits with their capacity weights.
+pub type ExitSpecs = Vec<(ExitId, ExitWeight)>;
+
+impl FetchPlan {
+    /// Build a validated plan.
+    ///
+    /// Exits are weighted by capacity; each exit's share is further split
+    /// into `segments_per_exit` sub-segments. Segments smaller than
+    /// `min_segment` are merged into a neighbour so we never spawn
+    /// absurdly tiny requests — unless the whole file is that small.
+    pub fn build(
+        url: TargetUrl,
+        file_size: FileSize,
+        exits: &ExitSpecs,
+        segments_per_exit: SegmentsPerExit,
+        min_segment: MinSegmentBytes,
+    ) -> Result<Self, FetchError> {
+        if exits.is_empty() {
+            return Err(FetchError::InvalidPlan("no exits given".to_string()));
+        }
+
+        let mut exits = exits.clone();
+
+        // Drop the weakest exits (smallest weight; ties: higher id goes
+        // first) until every remaining exit can hold at least one
+        // minimum segment.
+        while exits.len() > 1
+            && u128::from(file_size.value())
+                < u128::from(exits.len() as u64) * u128::from(min_segment.value())
+        {
+            let weakest = exits
+                .iter()
+                .enumerate()
+                .min_by_key(|(_, (id, weight))| (weight.value(), std::cmp::Reverse(id.value())))
+                .map(|(pos, _)| pos)
+                .unwrap_or(0);
+            exits.remove(weakest);
+        }
+        // Deterministic boundary assignment order.
+        exits.sort_by_key(|(id, _)| *id);
+
+        // If even one segment per exit would be too small, reduce to one
+        // segment per exit.
+        let spe_limit = usize::try_from(
+            u128::from(file_size.value())
+                / (u128::from(exits.len() as u64) * u128::from(min_segment.value())),
+        )
+        .unwrap_or(0);
+        let spe = if spe_limit >= segments_per_exit.value() {
+            segments_per_exit
+        } else {
+            SegmentsPerExit::new(NonZeroUsize::MIN)
+        };
+
+        let mut assignments: Vec<(ByteRange, ExitId)> = Vec::new();
+
+        if exits.len() == 1 && spe.value() == 1 {
+            let only = ByteRange::try_new(0, file_size.value() - 1)?;
+            assignments.push((only, exits[0].0));
+        } else {
+            // Weighted split across exits, then even sub-split per exit.
+            let units: Vec<u128> = exits
+                .iter()
+                .map(|(_, w)| u128::from(w.value()) * spe.value() as u128)
+                .collect();
+            let total_units: u128 = units.iter().sum();
+
+            let size = u128::from(file_size.value());
+            let mut cursor: u128 = 0;
+            let mut start: u64 = 0;
+
+            for (i, units_i) in units.iter().enumerate() {
+                let is_last = i == exits.len() - 1;
+                let end_exclusive: u128 = if is_last {
+                    size
+                } else {
+                    cursor + (size * units_i) / total_units
+                };
+
+                let exit_end: u64 = end_exclusive
+                    .try_into()
+                    .map_err(|_| FetchError::InvalidPlan("boundary overflow".to_string()))?;
+                let exit_end_inclusive = exit_end.saturating_sub(1);
+
+                if exit_end_inclusive >= start {
+                    for range in even_subsplit(start, exit_end_inclusive, spe.value()) {
+                        assignments.push((range, exits[i].0));
+                    }
+                    start = exit_end;
+                }
+
+                cursor = end_exclusive;
+            }
+
+            // The last exit covers to the end, so nothing can be left
+            // over; stay honest and extend the last assignment anyway.
+            if start < file_size.value() {
+                match assignments.last_mut() {
+                    Some((last_range, _)) => {
+                        *last_range =
+                            ByteRange::try_new(last_range.start(), file_size.value() - 1)?;
+                    }
+                    None => {
+                        let only = ByteRange::try_new(0, file_size.value() - 1)?;
+                        assignments.push((only, exits[0].0));
+                    }
+                }
+            }
+        }
+
+        // Merge-fix: no segment below the minimum size (except a lone
+        // tiny segment for a tiny file).
+        let mut assignments = merge_tiny_segments(assignments, min_segment);
+
+        let segments: Vec<SegmentPlan> = assignments
+            .drain(..)
+            .enumerate()
+            .map(|(i, (range, exit))| {
+                let index = u32::try_from(i)
+                    .map(SegmentIndex::new)
+                    .map_err(|_| FetchError::InvalidPlan("more than 2^32 segments".to_string()))?;
+                Ok::<SegmentPlan, FetchError>(SegmentPlan { index, range, exit })
+            })
+            .collect::<Result<Vec<_>, _>>()?;
+
+        let plan = Self {
+            url,
+            file_size,
+            segments,
+        };
+        plan.validate()?;
+        Ok(plan)
+    }
+
+    /// Check invariants: contiguous, gap-free, complete coverage of the
+    /// file, sequential indices, non-empty ranges.
+    pub fn validate(&self) -> Result<(), FetchError> {
+        let mut cursor: u64 = 0;
+        for (expected_index, seg) in self.segments.iter().enumerate() {
+            let expected_index = u32::try_from(expected_index)
+                .map_err(|_| FetchError::InvalidPlan("more than 2^32 segments".to_string()))?;
+            if seg.index.value() != expected_index {
+                return Err(FetchError::InvalidPlan(format!(
+                    "segment index out of order: {} != {expected_index}",
+                    seg.index
+                )));
+            }
+            if seg.range.start() != cursor {
+                return Err(FetchError::InvalidPlan(format!(
+                    "gap or overlap at segment {}: starts at {} but cursor is at {cursor}",
+                    seg.index,
+                    seg.range.start()
+                )));
+            }
+            if seg.range.is_empty() {
+                return Err(FetchError::InvalidPlan(format!(
+                    "segment {} is empty",
+                    seg.index
+                )));
+            }
+            cursor = seg
+                .range
+                .end_inclusive()
+                .checked_add(1)
+                .ok_or_else(|| FetchError::InvalidPlan("range end overflow".to_string()))?;
+        }
+        if cursor != self.file_size.value() {
+            return Err(FetchError::InvalidPlan(format!(
+                "plan covers up to {cursor} but file size is {}",
+                self.file_size
+            )));
+        }
+        Ok(())
+    }
+}
+
+/// Split `[start, end_inclusive]` into `parts` nearly-even sub-ranges,
+/// ordered; the first `remainder` parts get one extra byte.
+///
+/// The caller guarantees `parts >= 1` and `end_inclusive >= start`, so
+/// every part is non-empty and `try_new` cannot fail.
+fn even_subsplit(start: u64, end_inclusive: u64, parts: usize) -> Vec<ByteRange> {
+    let total_len = end_inclusive - start + 1;
+    let base = total_len / parts as u64;
+    let remainder = total_len % parts as u64;
+
+    let mut out = Vec::with_capacity(parts);
+    let mut cursor = start;
+    for i in 0..parts {
+        let len = base + if (i as u64) < remainder { 1 } else { 0 };
+        let range_end = cursor + len - 1;
+        match ByteRange::try_new(cursor, range_end) {
+            Ok(range) => out.push(range),
+            // Unreachable by construction; keep the sub-range out instead
+            // of panicking (validate() would then reject the plan).
+            Err(_) => return Vec::new(),
+        }
+        cursor = range_end + 1;
+    }
+    out
+}
+
+/// Merge any segment smaller than `min_segment` into a neighbour
+/// (prefer the next, fall back to the previous). A single remaining
+/// segment is left as-is, however small.
+fn merge_tiny_segments(
+    mut assignments: Vec<(ByteRange, ExitId)>,
+    min_segment: MinSegmentBytes,
+) -> Vec<(ByteRange, ExitId)> {
+    while let Some(pos) = assignments
+        .iter()
+        .position(|(range, _)| range.len() < min_segment.value())
+    {
+        if assignments.len() == 1 {
+            break; // nothing left to merge into
+        }
+
+        let merged: (ByteRange, ExitId) = if pos + 1 < assignments.len() {
+            // Merge into the next segment, keeping its exit.
+            let next = &assignments[pos + 1];
+            let range = match ByteRange::try_new(assignments[pos].0.start(), next.0.end_inclusive())
+            {
+                Ok(range) => range,
+                // Unreachable: next.start == pos.end + 1, so the merged
+                // range is always well-formed. Drop both segments instead
+                // of panicking; validate() would reject a broken plan.
+                Err(_) => break,
+            };
+            (range, next.1)
+        } else {
+            // Last segment is too small: merge into the previous one.
+            let prev_pos = pos - 1;
+            let prev = &assignments[prev_pos];
+            let range = match ByteRange::try_new(prev.0.start(), assignments[pos].0.end_inclusive())
+            {
+                Ok(range) => range,
+                Err(_) => break,
+            };
+            (range, prev.1)
+        };
+
+        match pos + 1 < assignments.len() {
+            true => {
+                assignments[pos + 1] = merged;
+                assignments.remove(pos);
+            }
+            false => {
+                assignments[pos - 1] = merged;
+                assignments.remove(pos);
+            }
+        }
+    }
+    assignments
+}

server/crates/sn-fetch/src/probe.rs

@@ -0,0 +1,66 @@
+//! Probing: find out whether a server supports HTTP range requests and
+//! how large the object is, before planning a segmented download.
+
+use reqwest::header::{ACCEPT_RANGES, CONTENT_LENGTH};
+use reqwest::StatusCode;
+
+use crate::error::FetchError;
+use crate::types::{FileSize, RangeSupport, TargetUrl};
+
+/// What a HEAD request (plus fallback range probe) told us about the target.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct ProbeInfo {
+    pub range_support: RangeSupport,
+    pub content_length: Option<FileSize>,
+}
+
+/// Probe the target: HEAD first; if the `Accept-Ranges` header is absent
+/// or inconclusive, fall back to a 1-byte GET with a `Range` header and
+/// check for `206 Partial Content`.
+pub async fn probe(client: &reqwest::Client, url: &TargetUrl) -> Result<ProbeInfo, FetchError> {
+    let head = client
+        .head(url.inner().clone())
+        .send()
+        .await?
+        .error_for_status()?;
+
+    let content_length = head
+        .headers()
+        .get(CONTENT_LENGTH)
+        .and_then(|v| v.to_str().ok())
+        .and_then(|v| v.parse::<u64>().ok());
+
+    let advertised = match head
+        .headers()
+        .get(ACCEPT_RANGES)
+        .and_then(|v| v.to_str().ok())
+        .map(str::trim)
+    {
+        Some("bytes") => Some(RangeSupport::Supported),
+        Some("none") => Some(RangeSupport::Unsupported),
+        _ => None,
+    };
+
+    let range_support = match advertised {
+        Some(support) => support,
+        // HEAD was inconclusive: ask directly with a 1-byte range.
+        None => {
+            let resp = client
+                .get(url.inner().clone())
+                .header(reqwest::header::RANGE, "bytes=0-0")
+                .send()
+                .await?;
+
+            match resp.status() {
+                StatusCode::PARTIAL_CONTENT => RangeSupport::Supported,
+                StatusCode::OK => RangeSupport::Unsupported,
+                _ => RangeSupport::Unknown,
+            }
+        }
+    };
+
+    Ok(ProbeInfo {
+        range_support,
+        content_length: content_length.and_then(|len| FileSize::try_new(len).ok()),
+    })
+}

server/crates/sn-fetch/src/runner.rs

@@ -0,0 +1,171 @@
+//! High-level orchestration: probe → plan → execute → assemble.
+//!
+//! Non-range-supporting servers fall back to a plain single-stream
+//! download (no pooling gain, correct result).
+
+use std::path::{Path, PathBuf};
+use std::time::Instant;
+
+use sha2::Digest;
+use tokio::io::AsyncWriteExt;
+
+use crate::error::FetchError;
+use crate::exec::{self, FetchReport, SegmentStats};
+use crate::exit::{ExitRegistry, LocalExit};
+use crate::plan::ExitSpecs;
+use crate::probe::{self, ProbeInfo};
+use crate::types::{
+    FileSize, MinSegmentBytes, RangeSupport, SegmentsPerExit, Sha256Digest, TargetUrl,
+};
+
+/// All knobs for one fetch.
+#[derive(Debug, Clone)]
+pub struct FetchOptions {
+    pub output: PathBuf,
+    pub exits: ExitSpecs,
+    pub segments_per_exit: SegmentsPerExit,
+    pub min_segment: MinSegmentBytes,
+    pub expected_sha256: Option<Sha256Digest>,
+}
+
+/// Run a full fetch and produce a report.
+pub async fn run(
+    target: &TargetUrl,
+    options: &FetchOptions,
+    exits: &ExitRegistry,
+    parts_dir: &Path,
+) -> Result<FetchReport, FetchError> {
+    let probe_client = reqwest::Client::builder()
+        .connect_timeout(std::time::Duration::from_secs(30))
+        .build()?;
+    let info = probe::probe(&probe_client, target).await?;
+
+    match info.range_support {
+        RangeSupport::Supported => run_ranged(target, options, exits, parts_dir, info).await,
+        RangeSupport::Unsupported | RangeSupport::Unknown => {
+            tracing::warn!(
+                "server does not (confirmably) support range requests; \
+                 falling back to single-stream download without pooling"
+            );
+            run_whole(target, options, exits, parts_dir, info).await
+        }
+    }
+}
+
+/// Ranged path: probe must have told us the size; plan, execute, assemble.
+async fn run_ranged(
+    target: &TargetUrl,
+    options: &FetchOptions,
+    exits: &ExitRegistry,
+    parts_dir: &Path,
+    info: ProbeInfo,
+) -> Result<FetchReport, FetchError> {
+    let file_size = info.content_length.ok_or(FetchError::ZeroFileSize)?;
+
+    let plan = crate::plan::FetchPlan::build(
+        target.clone(),
+        file_size,
+        &options.exits,
+        options.segments_per_exit,
+        options.min_segment,
+    )?;
+    tracing::info!("plan: {} segment(s)", plan.segments.len());
+
+    let started = Instant::now();
+    let segment_stats: Vec<SegmentStats> = exec::run_segments(&plan, exits, parts_dir).await?;
+    let digest = exec::assemble(
+        parts_dir,
+        &options.output,
+        file_size,
+        options.expected_sha256.as_ref(),
+    )
+    .await?;
+    let wall_time = started.elapsed();
+
+    Ok(FetchReport {
+        url: target.clone(),
+        output: options.output.clone(),
+        file_size,
+        segments: segment_stats,
+        wall_time,
+        sha256: Some(digest),
+    })
+}
+
+/// Fallback path: single stream, whole file.
+async fn run_whole(
+    target: &TargetUrl,
+    options: &FetchOptions,
+    exits: &ExitRegistry,
+    _parts_dir: &Path, // symmetric signature; unused without segments
+    info: ProbeInfo,
+) -> Result<FetchReport, FetchError> {
+    let exit_id = options
+        .exits
+        .first()
+        .map(|(id, _)| *id)
+        .ok_or_else(|| FetchError::InvalidPlan("no exits given".to_string()))?;
+    let client = exits.get(exit_id)?.client().clone();
+
+    let started = Instant::now();
+    let response = client
+        .get(target.inner().clone())
+        .send()
+        .await?
+        .error_for_status()?;
+
+    let mut file = tokio::fs::File::create(&options.output).await?;
+    let mut hasher = sha2::Sha256::new();
+    let mut written: u64 = 0;
+    let mut response = response;
+    while let Some(chunk) = response.chunk().await? {
+        file.write_all(&chunk).await?;
+        hasher.update(&chunk);
+        written += chunk.len() as u64;
+    }
+    file.flush().await?;
+
+    if let Some(expected_len) = info.content_length {
+        if written != expected_len.value() {
+            return Err(FetchError::TotalLengthMismatch {
+                got: written,
+                expected: expected_len.value(),
+            });
+        }
+    }
+    if written == 0 {
+        return Err(FetchError::ZeroFileSize);
+    }
+
+    let digest = Sha256Digest::from_bytes(hasher.finalize().into());
+    if let Some(expected) = options.expected_sha256.as_ref() {
+        if digest != *expected {
+            return Err(FetchError::HashMismatch {
+                expected: expected.to_hex(),
+                got: digest.to_hex(),
+            });
+        }
+    }
+
+    Ok(FetchReport {
+        url: target.clone(),
+        output: options.output.clone(),
+        file_size: FileSize::try_new(written)?,
+        segments: vec![SegmentStats {
+            index: crate::types::SegmentIndex::new(0),
+            bytes: written,
+            duration: started.elapsed(),
+        }],
+        wall_time: started.elapsed(),
+        sha256: Some(digest),
+    })
+}
+
+/// Convenience: build a registry with one [`LocalExit`] per spec id.
+pub fn local_registry(specs: &ExitSpecs) -> Result<ExitRegistry, FetchError> {
+    let mut registry = ExitRegistry::empty();
+    for (id, _) in specs {
+        registry.register(std::sync::Arc::new(LocalExit::new(*id)?));
+    }
+    Ok(registry)
+}

server/crates/sn-fetch/src/types.rs

@@ -0,0 +1,232 @@
+//! Domain types: newtypes over raw primitives, validated at construction.
+
+use std::fmt;
+use std::num::{NonZeroU32, NonZeroU64, NonZeroU8, NonZeroUsize};
+
+use crate::error::FetchError;
+
+/// Identifier of an exit (an internet uplink offered by a household).
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ExitId(NonZeroU8);
+
+impl ExitId {
+    pub const fn new(value: NonZeroU8) -> Self {
+        Self(value)
+    }
+
+    pub const fn value(self) -> u8 {
+        self.0.get()
+    }
+}
+
+impl fmt::Display for ExitId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "exit-{}", self.0.get())
+    }
+}
+
+/// Relative capacity share of an exit. 1 means equal share.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct ExitWeight(NonZeroU32);
+
+impl ExitWeight {
+    pub const fn new(value: NonZeroU32) -> Self {
+        Self(value)
+    }
+
+    pub const fn value(self) -> u32 {
+        self.0.get()
+    }
+}
+
+/// How many sub-segments each exit's share is split into (parallel
+/// connections per exit; more connections can defeat per-connection
+/// server throttling).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct SegmentsPerExit(NonZeroUsize);
+
+impl SegmentsPerExit {
+    pub const fn new(value: NonZeroUsize) -> Self {
+        Self(value)
+    }
+
+    pub const fn value(self) -> usize {
+        self.0.get()
+    }
+}
+
+/// Lower bound for a segment size, to avoid absurdly tiny segments.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct MinSegmentBytes(NonZeroU64);
+
+impl MinSegmentBytes {
+    pub const fn new(value: NonZeroU64) -> Self {
+        Self(value)
+    }
+
+    pub const fn value(self) -> u64 {
+        self.0.get()
+    }
+}
+
+/// Total size of a downloadable object, validated to be non-zero.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct FileSize(u64);
+
+impl FileSize {
+    pub fn try_new(value: u64) -> Result<Self, FetchError> {
+        if value == 0 {
+            return Err(FetchError::ZeroFileSize);
+        }
+        Ok(Self(value))
+    }
+
+    pub const fn value(self) -> u64 {
+        self.0
+    }
+}
+
+impl fmt::Display for FileSize {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+/// A half-open-by-one byte range `[start, end_inclusive]`, as HTTP uses
+/// inclusive end positions.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct ByteRange {
+    start: u64,
+    end_inclusive: u64,
+}
+
+impl ByteRange {
+    pub fn try_new(start: u64, end_inclusive: u64) -> Result<Self, FetchError> {
+        if end_inclusive < start {
+            return Err(FetchError::InvalidPlan(format!(
+                "range start {start} is after end {end_inclusive}"
+            )));
+        }
+        Ok(Self {
+            start,
+            end_inclusive,
+        })
+    }
+
+    pub const fn start(self) -> u64 {
+        self.start
+    }
+
+    pub const fn end_inclusive(self) -> u64 {
+        self.end_inclusive
+    }
+
+    pub const fn len(self) -> u64 {
+        self.end_inclusive - self.start + 1
+    }
+
+    pub const fn is_empty(self) -> bool {
+        false
+    }
+
+    /// Value for the HTTP `Range` header, e.g. `bytes=100-199`.
+    pub fn http_value(self) -> String {
+        format!("bytes={}-{}", self.start, self.end_inclusive)
+    }
+}
+
+impl fmt::Display for ByteRange {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "{}..={} ({} bytes)",
+            self.start,
+            self.end_inclusive,
+            self.len()
+        )
+    }
+}
+
+/// Position of a segment inside a plan, sequential from 0.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct SegmentIndex(u32);
+
+impl SegmentIndex {
+    pub const fn new(value: u32) -> Self {
+        Self(value)
+    }
+
+    pub const fn value(self) -> u32 {
+        self.0
+    }
+}
+
+impl fmt::Display for SegmentIndex {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+/// A validated SHA-256 digest.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Sha256Digest([u8; 32]);
+
+impl Sha256Digest {
+    pub fn from_bytes(bytes: [u8; 32]) -> Self {
+        Self(bytes)
+    }
+
+    pub fn from_hex(hex_str: &str) -> Result<Self, FetchError> {
+        let bytes = hex::decode(hex_str.trim()).map_err(|_| FetchError::InvalidDigest)?;
+        let arr: [u8; 32] = bytes.try_into().map_err(|_| FetchError::InvalidDigest)?;
+        Ok(Self(arr))
+    }
+
+    pub fn to_hex(&self) -> String {
+        hex::encode(self.0)
+    }
+}
+
+impl fmt::Display for Sha256Digest {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.to_hex())
+    }
+}
+
+/// A validated target URL. Only http(s) is accepted.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct TargetUrl(reqwest::Url);
+
+impl TargetUrl {
+    pub fn parse(input: &str) -> Result<Self, FetchError> {
+        let url =
+            reqwest::Url::parse(input).map_err(|_| FetchError::InvalidUrl(input.to_string()))?;
+        match url.scheme() {
+            "http" | "https" => Ok(Self(url)),
+            _ => Err(FetchError::InvalidUrl(input.to_string())),
+        }
+    }
+
+    pub fn inner(&self) -> &reqwest::Url {
+        &self.0
+    }
+}
+
+impl fmt::Display for TargetUrl {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+/// Result of probing a server's range-request support.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum RangeSupport {
+    /// Server advertised `Accept-Ranges: bytes` or answered a range probe
+    /// with `206 Partial Content`.
+    Supported,
+    /// Server advertised `Accept-Ranges: none` or answered a range probe
+    /// with a full `200` response.
+    Unsupported,
+    /// Probe was inconclusive; treat as unsupported (safe fallback).
+    Unknown,
+}

server/crates/sn-fetch/tests/common/mod.rs

@@ -0,0 +1,123 @@
+//! A minimal HTTP server for integration tests. Supports HEAD, GET with
+//! optional range handling (configurable), `Connection: close` semantics.
+
+#![forbid(unsafe_code)]
+
+use std::io::{BufRead, BufReader, Write};
+use std::net::{SocketAddr, TcpListener, TcpStream};
+use std::sync::Arc;
+use std::thread::JoinHandle;
+
+pub struct TestServer {
+    pub addr: SocketAddr,
+    _handle: JoinHandle<()>,
+}
+
+impl TestServer {
+    /// Start serving `data` on a random port. When `ranges` is false the
+    /// server behaves like a range-hostile origin: no `Accept-Ranges`
+    /// header, and `Range` requests are answered with the full body.
+    pub fn start(data: Vec<u8>, ranges: bool) -> Self {
+        let data = Arc::new(data);
+        let listener = TcpListener::bind("127.0.0.1:0").expect("bind in test");
+        let addr = listener.local_addr().expect("addr in test");
+        let handle = std::thread::spawn(move || {
+            for stream in listener.incoming().flatten() {
+                let data = Arc::clone(&data);
+                std::thread::spawn(move || {
+                    let _ = handle_conn(stream, &data, ranges);
+                });
+            }
+        });
+        Self {
+            addr,
+            _handle: handle,
+        }
+    }
+
+    pub fn url(&self, path: &str) -> String {
+        format!("http://{}{path}", self.addr)
+    }
+}
+
+fn handle_conn(stream: TcpStream, data: &[u8], ranges: bool) -> std::io::Result<()> {
+    let mut reader = BufReader::new(stream.try_clone()?);
+    let mut request_line = String::new();
+    reader.read_line(&mut request_line)?;
+
+    let mut range_header: Option<String> = None;
+    loop {
+        let mut line = String::new();
+        reader.read_line(&mut line)?;
+        let trimmed = line.trim_end();
+        if trimmed.is_empty() {
+            break;
+        }
+        let lower = trimmed.to_ascii_lowercase();
+        if let Some(value) = lower.strip_prefix("range:") {
+            range_header = Some(value.trim().to_string());
+        }
+    }
+
+    let mut stream = stream;
+    let len = data.len() as u64;
+
+    let method = request_line
+        .split_whitespace()
+        .next()
+        .unwrap_or("GET")
+        .to_string();
+
+    match method.as_str() {
+        "HEAD" => {
+            let accept = if ranges { "bytes" } else { "none" };
+            write!(
+                stream,
+                "HTTP/1.1 200 OK\r\nContent-Length: {len}\r\nAccept-Ranges: {accept}\r\nConnection: close\r\n\r\n"
+            )?;
+        }
+        "GET" => {
+            let range = range_header.as_deref().and_then(parse_range).flatten();
+            match (ranges, range) {
+                (true, Some((start, end))) => {
+                    let slice_len = end - start + 1;
+                    write!(
+                        stream,
+                        "HTTP/1.1 206 Partial Content\r\nContent-Length: {slice_len}\r\nContent-Range: bytes {start}-{end}/{len}\r\nConnection: close\r\n\r\n"
+                    )?;
+                    stream.write_all(&data[start as usize..=(end as usize).min(data.len() - 1)])?;
+                }
+                _ => {
+                    write!(
+                        stream,
+                        "HTTP/1.1 200 OK\r\nContent-Length: {len}\r\nConnection: close\r\n\r\n"
+                    )?;
+                    stream.write_all(data)?;
+                }
+            }
+        }
+        _ => {
+            write!(
+                stream,
+                "HTTP/1.1 405 Method Not Allowed\r\nContent-Length: 0\r\nConnection: close\r\n\r\n"
+            )?;
+        }
+    }
+
+    stream.flush()?;
+    Ok(())
+}
+
+/// Parse `bytes=a-b` into inclusive `(start, end)`. Anything else
+/// (open-ended, malformed) is treated as absent.
+fn parse_range(value: &str) -> Option<Option<(u64, u64)>> {
+    let spec = value.strip_prefix("bytes=")?;
+    let (start_str, end_str) = spec.split_once('-')?;
+    let start: u64 = start_str.trim().parse().ok()?;
+    let end: u64 = end_str.trim().parse().ok()?;
+    if end >= start {
+        Some(Some((start, end)))
+    } else {
+        Some(None)
+    }
+}

server/crates/sn-fetch/tests/roundtrip.rs

@@ -0,0 +1,181 @@
+//! Integration tests: probe, planning, parallel segmented download,
+//! reassembly and hash verification against a local HTTP server — plus
+//! the fallback path for range-hostile servers.
+
+#![forbid(unsafe_code)]
+
+mod common;
+
+use std::num::{NonZeroU32, NonZeroU64, NonZeroU8, NonZeroUsize};
+use std::sync::Arc;
+
+use sha2::{Digest, Sha256};
+use tempfile::TempDir;
+
+use sn_fetch::error::FetchError;
+use sn_fetch::exec;
+use sn_fetch::exit::LocalExit;
+use sn_fetch::plan::{ExitSpecs, FetchPlan};
+use sn_fetch::probe;
+use sn_fetch::types::{
+    ByteRange, ExitId, ExitWeight, FileSize, MinSegmentBytes, RangeSupport, SegmentsPerExit,
+    Sha256Digest,
+};
+
+fn exit_specs(n: u8) -> ExitSpecs {
+    (1..=n)
+        .map(|i| {
+            (
+                ExitId::new(NonZeroU8::new(i).expect("test ids are 1..=255")),
+                ExitWeight::new(NonZeroU32::new(1).expect("1 is non-zero")),
+            )
+        })
+        .collect()
+}
+
+#[tokio::test]
+async fn probe_detects_range_support() {
+    let data = vec![42u8; 1024];
+    let server = common::TestServer::start(data, true);
+    let url = sn_fetch::types::TargetUrl::parse(&server.url("/file.bin")).expect("valid url");
+    let client = reqwest::Client::new();
+
+    let info = probe::probe(&client, &url).await.expect("probe succeeds");
+    assert_eq!(info.range_support, RangeSupport::Supported);
+    assert_eq!(info.content_length.map(|s| s.value()), Some(1024));
+}
+
+#[tokio::test]
+async fn segmented_download_reassembles_and_hashes() {
+    let data: Vec<u8> = (0..(2 * 1024 * 1024)).map(|i| (i % 251) as u8).collect();
+    let expected_digest = Sha256Digest::from_bytes(Sha256::digest(&data).into());
+
+    let server = common::TestServer::start(data.clone(), true);
+    let url = sn_fetch::types::TargetUrl::parse(&server.url("/file.bin")).expect("valid url");
+
+    let specs = exit_specs(2);
+    let plan = FetchPlan::build(
+        url.clone(),
+        FileSize::try_new(data.len() as u64).expect("non-zero in test"),
+        &specs,
+        SegmentsPerExit::new(NonZeroUsize::new(2).expect("2 is non-zero")),
+        MinSegmentBytes::new(NonZeroU64::new(1024).expect("1024 is non-zero")),
+    )
+    .expect("plan builds");
+    plan.validate().expect("plan validates");
+
+    let mut registry = sn_fetch::exit::ExitRegistry::empty();
+    for id in specs.iter().map(|(id, _)| *id) {
+        registry.register(Arc::new(LocalExit::new(id).expect("local exit")));
+    }
+
+    let parts = TempDir::new().expect("tempdir in test");
+    let stats = exec::run_segments(&plan, &registry, parts.path())
+        .await
+        .expect("segments succeed");
+    assert_eq!(stats.len(), 4);
+    let total_downloaded: u64 = stats.iter().map(|s| s.bytes).sum();
+    assert_eq!(total_downloaded, data.len() as u64);
+
+    let output = parts.path().join("assembled.bin");
+    let digest = exec::assemble(
+        parts.path(),
+        &output,
+        FileSize::try_new(data.len() as u64).expect("non-zero in test"),
+        Some(&expected_digest),
+    )
+    .await
+    .expect("assembly verifies");
+
+    assert_eq!(digest, expected_digest);
+    let written = std::fs::read(&output).expect("output readable in test");
+    assert_eq!(written, data);
+}
+
+#[tokio::test]
+async fn range_hostile_server_falls_back_to_single_stream() {
+    let data: Vec<u8> = (0..256_000u32).map(|i| (i % 197) as u8).collect();
+    let server = common::TestServer::start(data.clone(), false);
+    let url_str = server.url("/file.bin");
+    let target = sn_fetch::types::TargetUrl::parse(&url_str).expect("valid url");
+
+    let specs = exit_specs(1);
+    let options = sn_fetch::runner::FetchOptions {
+        output: std::env::temp_dir().join("sn-fetch-test-fallback.bin"),
+        exits: specs.clone(),
+        segments_per_exit: SegmentsPerExit::new(NonZeroUsize::new(1).expect("1 is non-zero")),
+        min_segment: MinSegmentBytes::new(NonZeroU64::new(1024).expect("1024 is non-zero")),
+        expected_sha256: None,
+    };
+    let registry = sn_fetch::runner::local_registry(&specs).expect("registry");
+    let parts = TempDir::new().expect("tempdir in test");
+
+    let report = sn_fetch::runner::run(&target, &options, &registry, parts.path())
+        .await
+        .expect("fallback download succeeds");
+
+    assert_eq!(report.file_size.value(), 256_000);
+    let written = std::fs::read(&options.output).expect("output readable in test");
+    assert_eq!(written, data);
+    let _ = std::fs::remove_file(&options.output);
+}
+
+#[test]
+fn plan_is_contiguous_and_weighted() {
+    let url = sn_fetch::types::TargetUrl::parse("http://example.com/file.bin").expect("valid url");
+    let size = FileSize::try_new(10_000).expect("non-zero in test");
+    let specs: ExitSpecs = vec![
+        (
+            ExitId::new(NonZeroU8::new(1).expect("1 is non-zero")),
+            ExitWeight::new(NonZeroU32::new(3).expect("3 is non-zero")),
+        ),
+        (
+            ExitId::new(NonZeroU8::new(2).expect("2 is non-zero")),
+            ExitWeight::new(NonZeroU32::new(1).expect("1 is non-zero")),
+        ),
+    ];
+
+    let plan = FetchPlan::build(
+        url,
+        size,
+        &specs,
+        SegmentsPerExit::new(NonZeroUsize::new(1).expect("1 is non-zero")),
+        MinSegmentBytes::new(NonZeroU64::new(100).expect("100 is non-zero")),
+    )
+    .expect("plan builds");
+
+    plan.validate().expect("plan validates");
+
+    let exit1_total: u64 = plan
+        .segments
+        .iter()
+        .filter(|seg| seg.exit.value() == 1)
+        .map(|seg| seg.range.len())
+        .sum();
+    let exit2_total: u64 = plan
+        .segments
+        .iter()
+        .filter(|seg| seg.exit.value() == 2)
+        .map(|seg| seg.range.len())
+        .sum();
+    assert_eq!(exit1_total + exit2_total, 10_000);
+    assert!(
+        exit1_total > exit2_total,
+        "heavier exit must get the larger share"
+    );
+}
+
+#[test]
+fn byte_range_http_value() {
+    let range = ByteRange::try_new(100, 199).expect("valid range");
+    assert_eq!(range.len(), 100);
+    assert_eq!(range.http_value(), "bytes=100-199");
+}
+
+#[test]
+fn invalid_url_is_rejected() {
+    assert!(matches!(
+        sn_fetch::types::TargetUrl::parse("ftp://example.com/file"),
+        Err(FetchError::InvalidUrl(_))
+    ));
+}