qcp: Faster File Transfers Over High-Latency Networks Using QUIC

If you've ever tried to scp a large file across continents, you know the pain. TCP's congestion control wasn't designed for high-latency links, and throughput often crawls even when bandwidth is available. Enter qcp, a drop-in replacement for scp that uses the QUIC protocol to maximize throughput on challenging network paths.

What Is qcp?

qcp (QUIC Copier) is a file transfer utility written in Rust that tunnels file data over QUIC instead of TCP. It uses SSH for authentication (just like scp), then establishes a QUIC session for the actual data transfer.

Key features:

• Drop-in replacement for scp (same syntax)
• Uses SSH for authentication (no new credentials needed)
• QUIC-based transfer for better performance on congested/high-RTT links
• Supports multiple files and recursive directory copies
• Cross-platform: Linux, macOS, Windows, FreeBSD

Why QUIC Beats TCP for Long-Distance Transfers

TCP was designed in an era of low-latency, low-loss networks. On modern internet paths with 100ms+ round-trip times, TCP's congestion control becomes overly conservative:

Factor	TCP (scp)	QUIC (qcp)
Connection setup	3-way handshake + TLS	0-RTT possible
Loss recovery	Per-connection	Per-stream
Congestion control	Conservative	Modern algorithms (BBR, CUBIC)
Head-of-line blocking	Yes	No

QUIC was built by Google specifically for high-latency, lossy networks. It handles packet loss more gracefully and can push more data through the pipe.

Installation

Package Manager

# Debian/Ubuntu
sudo dpkg -i qcp_*.deb

# Arch Linux (AUR)
yay -S qcp

# macOS/Linux via Cargo
cargo install --locked qcp

# Nix
nix shell 'github:crazyscot/qcp?dir=nix'

From Release Binaries

Download from GitHub Releases:

• .deb packages for Debian/Ubuntu
• Static musl binaries for other Linux
• macOS and Windows archives

Important: Install qcp on both the local and remote machines. The remote binary must be in your PATH.

Basic Usage

The syntax mirrors scp:

# Copy local file to remote
qcp myfile.tar.gz server:/tmp/

# Copy remote file to local
qcp server:/var/log/app.log ./

# Copy directory recursively
qcp -r ./data/ server:/backup/

# Multiple sources
qcp file1.txt file2.txt server:/destination/

Example output:

$ qcp my-server:/tmp/testfile /tmp/
⠂ Transferring data                              2.1MB/s (last 1s)
testfile ████████████████████████░░░░░░░░░░░░░ 1s @ 6.71 MB/s [10.49 MB]

How It Works

1. qcp connects to the remote via SSH and launches qcp --server
2. Both sides generate ephemeral TLS certificates and exchange them over SSH
3. A QUIC session is established using these certificates
4. Files transfer over QUIC with progress reporting
5. SSH ensures authentication; QUIC handles the heavy lifting

This hybrid approach gives you SSH's security model with QUIC's performance.

Tuning for Your Network

The default configuration assumes 100Mbit symmetric bandwidth with 300ms RTT. For better performance, tune these values:

# Specify your actual bandwidth
qcp server:/file /tmp/ --rx 100M --tx 50M

# Also specify RTT if known
qcp server:/file /tmp/ --rx 100M --tx 50M --rtt 150

Persistent Configuration

Create ~/.qcp.conf:

Host *
Rx 100M
Tx 50M

Host us-west-server
Rx 500M
Tx 100M
Rtt 80

UDP Buffer Tuning

For best performance, increase UDP buffer sizes:

# Linux - add to /etc/sysctl.conf
net.core.rmem_max = 4194304
net.core.wmem_max = 4194304

# Apply
sudo sysctl -p

Firewall Considerations

qcp needs UDP connectivity to the remote host:

• If you're behind NAT (most home/office networks), it usually just works
• If the remote is firewalled, open a UDP port range and configure qcp:

qcp --remote-port 50000-50100 server:/file /tmp/

Or in config:

Host firewalled-server
RemotePort 50000-50100

Performance Comparison

Real-world results vary, but qcp typically shines on:

Scenario	scp	qcp	Improvement
Same datacenter	110 MB/s	112 MB/s	~same
Cross-continent (200ms RTT)	15 MB/s	45 MB/s	3x
Lossy connection (1% loss)	5 MB/s	35 MB/s	7x

The bigger the latency or packet loss, the more qcp helps.

Comparison with Alternatives

Tool	Protocol	Auth	Best For
scp	TCP/SSH	SSH keys	Local/low-latency
rsync	TCP/SSH	SSH keys	Incremental sync
qcp	QUIC/SSH	SSH keys	High-latency transfers
bbcp	TCP (parallel)	SSH	Bulk transfers (complex setup)
Aspera	FASP (UDP)	License	Enterprise (expensive)

qcp fills the gap between scp (simple but slow) and enterprise solutions (fast but complex/costly).

Tips for SRE Teams

1. Cross-region backups: Use qcp for copying database dumps between regions
2. Log collection: Pull large log archives from remote DCs faster
3. Deployment artifacts: Ship build artifacts to edge locations
4. Disaster recovery: Speed up restore operations from remote backups

FAQ

Does qcp work with my existing SSH keys?

Yes. qcp uses your system's SSH binary and respects ~/.ssh/config, so existing keys and host aliases work.

Can I use qcp through a jump host?

Not directly yet, but you can use SSH's ProxyJump in your SSH config, and qcp will follow it for the initial connection.

Is qcp secure?

Yes. Authentication happens over SSH (same as scp). The QUIC session uses TLS 1.3 with ephemeral keys exchanged over the SSH channel.

Why not just use rsync?

rsync is great for incremental syncs, but it still uses TCP. For bulk transfers of new data over high-latency links, qcp will be faster.

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