SSH Sends 100 Packets Per Keystroke: Why?

Developers working with Secure Shell (SSH) connections often notice unusual network behavior that can seem counterintuitive. A recent technical analysis has uncovered a startling fact: under specific conditions, a single keystroke can generate as many as 100 separate network packets.

This discovery, which emerged from a detailed examination of SSH traffic patterns, challenges common assumptions about how data travels between a client and a remote server. For a protocol relied upon by millions of developers and system administrators worldwide, this level of packet overhead raises critical questions about efficiency and performance.

The investigation delves into the intricate dance between terminal emulation, encryption protocols, and network transport layers. It reveals that the seemingly simple act of typing a character triggers a complex cascade of network events, each contributing to the final packet count.

The root of this phenomenon lies in the layered architecture of a remote terminal session. When a user types a key, the action initiates a multi-stage process before any data crosses the network.

First, the local terminal emulator processes the keystroke. It often generates an immediate local echo—the character appearing on the user's screen—before the data is even sent to the SSH client. This local echo is a fundamental part of the user experience, providing instant feedback.

Next, the SSH client encrypts the keystroke data. This encrypted payload is then handed off to the operating system's TCP/IP stack. Here, the data is broken down into segments based on the Maximum Transmission Unit (MTU) of the network interface. If the data payload is small (like a single character), and if the Nagle algorithm or similar TCP optimizations are not perfectly tuned, each small piece of data can be sent in its own packet.

The server-side process mirrors this flow. The server receives the packet, decrypts it, passes it to the remote shell, which then generates its own echo. This echo is sent back to the client, re-encrypted, and transmitted. This entire round-trip for a single character, combined with protocol overheads like acknowledgments and window updates, is what inflates the packet count to such high numbers.

A significant contributor to the packet storm is the echo mechanism. In a standard SSH session, every character typed is echoed back by the remote server to ensure the user sees what they are typing. This means a single keystroke effectively generates two data transmissions: the character sent to the server, and the echoed character returned from the server.

Each of these transmissions is subject to the same encryption and packetization process. When combined with protocol acknowledgments and the potential for TCP corking or Nagle's algorithm interactions, the result is a chatty conversation between client and server.

The interaction between terminal echo and TCP packetization creates a multiplicative effect on packet counts.

For users on high-latency or lossy networks, this translates directly to perceptible lag. Each packet represents a unit of work for the network stack on both ends, and a hundred packets for a single keystroke represents significant overhead that can degrade the interactive experience.

While modern networks are fast, the sheer volume of packets can still cause issues. The primary bottleneck is not bandwidth, but latency and CPU load. Each packet requires processing by the kernel's network stack on both the client and server, involving tasks like encryption, decryption, and routing decisions.

For interactive sessions, this overhead contributes to a feeling of sluggishness, where typing feels delayed. For automated scripts or tools that rely on SSH for data transfer, this inefficiency can significantly slow down operations that involve many small, sequential commands.

• Increased CPU usage on both client and server
• Higher network latency for interactive input
• Potential for packet loss affecting responsiveness
• Greater power consumption on mobile devices

Understanding this behavior is the first step toward mitigation. It explains why certain optimizations, like disabling terminal echo or using control master connections, can have a surprisingly large impact on perceived performance.

While the default behavior is a byproduct of ensuring a reliable and responsive terminal, there are ways to reduce the packet overhead. Techniques such as TCP_NODELAY can disable Nagle's algorithm, which often delays sending small packets in hopes of coalescing them, though this can sometimes have the opposite effect depending on the application.

Another approach is to use SSH connection multiplexing, or ControlMaster, which allows multiple SSH sessions to share a single underlying TCP connection. This reduces the overhead of establishing new connections and their associated handshakes for each new terminal window or file transfer.

Ultimately, the discovery that SSH can generate 100 packets per keystroke is not an indictment of the protocol, but a window into the complexity of modern networked systems. It underscores the importance of looking beyond simple byte counts to understand the true cost of data transmission.

The investigation into SSH's packet generation reveals a hidden layer of complexity in everyday tools. It demonstrates that the user experience of a command-line interface is built on a sophisticated and sometimes 'chatty' network protocol.

For developers and system architects, this knowledge is power. It allows for more informed decisions when debugging network performance, designing remote applications, and optimizing infrastructure. The next time you type a command and see a slight delay, you'll know that dozens of packets are likely traversing the network to make that simple character appear on your screen.