关于Linux 2.4/2.6内核TCP窗口大小为5840的技术咨询

Great question! Let's unpack this about the 5840 TCP window size in Linux 2.4/2.6 kernels.

The 5840 value isn't arbitrary—it's tied directly to how TCP and Ethernet work together:

• It's calculated around the Maximum Segment Size (MSS) for standard Ethernet. With an MTU (Maximum Transmission Unit) of 1500 bytes, subtract 20 bytes for the IP header and 20 bytes for the TCP header, leaving 1460 bytes of usable payload per segment.
• 5840 is exactly 4 times that 1460-byte MSS. This multiple was chosen to balance two key factors:
  1. Transmission efficiency: A window size that fits 4 full segments reduces the number of ACK packets sent back, cutting overhead for both sender and receiver.
  2. Memory constraints: Early Linux kernels (2.4/2.6) targeted a wide range of hardware, including systems with limited memory. A 5840-byte window didn't consume excessive RAM while still providing good throughput for typical networks of that era.
• It also aligns with the bandwidth-delay product (BDP) assumptions common at the time—enough to keep the pipe full for most local and short-haul connections without overprovisioning.

Yes, there are official places to verify this:

• Linux Kernel Source Documentation: Look for Documentation/networking/ip-sysctl.txt in the 2.4 or 2.6 kernel source trees. This file outlines TCP sysctl parameters, including default window sizes, and sometimes includes context about their design.
• Kernel Source Code: Check files like net/ipv4/tcp.c or related header files (e.g., tcp.h) in the 2.4/2.6 source. You'll find the TCP_WINDOW_DEFAULT definition set to 5840, often with comments that reference the MSS-based calculation.
• TCP RFCs: While RFCs don't dictate Linux's specific default, RFC 1323 (TCP Window Scale Option) provides the foundational logic behind window size tuning, which informed the kernel's choice of 5840 as a baseline that worked well without needing immediate scaling for most use cases.

内容的提问来源于stack exchange，提问作者wizzardm