How to enable SSH compression in PuTTY

Enabling SSH compression in PuTTY reduces the amount of data sent over the network, which can improve interactive terminal responsiveness on slow or high-latency links and lower bandwidth usage on constrained connections.

Compression is negotiated during the SSH-2 handshake: the client offers support for a compression algorithm (commonly zlib), the server chooses a compatible option, and each side compresses outbound data before encryption and decompresses inbound data after decryption.

Compression adds CPU overhead on both client and server, so it often provides little benefit on fast networks or when transferring already-compressed data (archives, media, many software packages). The setting must be applied before opening the session, and servers can disable compression entirely, making PuTTY’s Event Log the most direct way to confirm whether compression is actually in use.

1. Launch PuTTY.
2. Enter the server Host Name (or IP address) under Session.
3. Set the SSH Port value under Session.
4. Select Connection → SSH in the Category tree.
5. Enable the Enable compression option.

   Compression is only used if the server accepts it during negotiation.

6. Return to the Session category.
7. Type a profile name in Saved Sessions.

   Skip saving when compression is only needed for a one-off connection.

8. Click Save to store the updated settings.

   Saving with an existing name overwrites the stored settings for that profile.

9. Click Open to start the SSH session.
10. Open Event Log from the PuTTY window menu.
11. Confirm a negotiated compression method such as zlib or zlib@openssh.com is shown in the log.

    Compression is not active when the log indicates none, which typically means the server rejected or disabled compression.