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4.4BSD-Lite2 TCP/IP stack implementation - This is the implementation that arguably most modern implementations are based on. Even though one of FreeBSD, OpenBSD, NetBSD, or BSD/OS would be used today rather than actual 4.4BSD, this code remains to a very large extent the reference implementation of TCP.
A User's Guide to TCP Windows - A User's Guide to TCP Windows
Automatic TCP Buffer Tuning Research - at PSC
BCP28 - Enhancing TCP Over Satellite Channels using Standard Mechanisms - Best current practices document describing ways to improve TCP performance over satellite links.
CMX-MicroNet TCP/IP Networking - A commercial TCP/IP stack (plus HTTP and FTP server, etc.) implementation for 8-bit and 16-bit embedded processors.
CPC/IP - CPC/IP is an implementation of the PPP, SLIP, IP, ICMP, UDP, TCP, DNS, TFTP, HTTP, ping, finger and telnet protocols for Amstrad CPC computers with an Amstrad, Pace or CPC Amstrad International serial interface. The code occupies about 14K, excluding the serial, filing system and IP buffers.
ECN - Sally Floyd's ECN (Explicit Congestion Notification) links
eCos - Free embedded OS (Linux-based); includes a tiny TCP/IP stack.
Enabling High Performance Data Transfers on Hosts - TCP performance tuning notes from PSC
Equation-Based Congestion Control for Unicast Applications - Research papers describing experience getting TCP-compatible throughput from rate-based (rather than window-based) approaches. Since resulting traffic stream is less bursty, this approach is suitable for streaming multimedia applications that need congestion-avoidance properties.
Experience with TCP Vegas from SunOS 4.1.3 to NetBSD 1.0 - Including source code, descriptions of experiments, and links to other TCP Vegas resources.
FreeBSD's TCP implementation - Based on TCP in 4.4BSD.
Global Optimization with End-to-End Congestion Control: Pointers to the Literature - links from Sally Floyd
Increasing TCP's Initial Window - including links to simulations, experiments, talks, and the RFC
LiveDevices - Commercial embedded TCP/IP stack.
lwIP - A Lightweight TCP/IP stack - lwIP is a lightweight implementation of the TCP/IP protocol stack. The focus of the lwIP TCP/IP implementation is to reduce RAM usage.
Mentat TCP - Mentat TCP is a commercially available TCP/IP implementation.
NASA TCP Research - including SACK/FACK, larger initial windows, and TCP over Satellite
NetBSD's TCP implementation - Based on TCP in 4.4BSD.
OpenBSD's TCP implementation - Based on TCP in 4.4BSD.
Proposed Changes to TCP - a set of pointers from Sally Floyd
RED - Sally Floyd's RED (Random Early Detection) links
Research Projects - Reports of experience with TCP SACK at UCLA
RFC1323 - TCP Extensions for High Performance - Defines TCP Window Scale option, timestamps, and protection against wrapped sequence numbers (PAWS).
RFC2018 - TCP Selective Acknowledgement Options - Defines SACK (a technique to better recover from multiple packet losses per round-trip time interval).
RFC2481 - A Proposal to add Explicit Congestion Notification (ECN) to IP - Experimental RFC.
RFC2581 - TCP Congestion Control - Proposed standard set of congestion avoidance features in a TCP implementation.
RFC2582 - The NewReno Modification to TCP's Fast Recovery Algorithm - Experimental.
RFC2883 - An Extension to the Selective Acknowledgement (SACK) Option for TCP - Proposed standard.
RFC2988 - Computing TCP's Retransmission Timer - Proposed standard.
RFC793 - Transmission Control Protocol - The original TCP definition. Also STD7.
SACK TCP Implementations - Both shipping and experimental.
SCPS-TP - A suite of protocols aimed at space users. Includes an implementation of a protocol extending TCP.
T/TCP Home Page - Historic description of T/TCP. Includes links to RFCs and source code. Not being updated since Richard W. Stevens has passed away.
TCP Implementation WG Mail Archive - The IETF tcpimpl working group was shut down in Sep 2000, but the mailing list still exists. Historic archive may also be of interest.
TCP Research Papers - pointers to dozens of TCP papers
TCP Santa Cruz - An implementation aimed at improving TCP congestion control over internets with heterogeneous transmission media.
TCP Selective Acknowledgment Options - Links from Sally Floyd.
TCP Smooth Start - A refinement to slow start algorithm.
TCP Vegas - Princeton University TCP Vegas work.
TCP-BFA - Implementation from Stanford; TCP Buffer Fill Avoidance.
TCP/IP Resources List - A meta-FAQ about TCP/IP regularly posted to Usenet.
The TCP-Friendly Website - TCP-friendly algorithms and TCP performance models
The Web 100 Project - The goal is to make 100Mbps TCP streams over wide-area networks possible for general users.
TinyTCP/TinyFTP - A simple implementation of TCP & FTP, suitable for burning into ROM. It is, in effect, a hack put together in two or three days written in C--in 1986.
uIP - A Free Small TCP/IP Stack for 8- and 16-bit Microcontrollers - uIP is an implementation of the TCP/IP protocol stack that uses very little resources. Code size is on the order of a few kilobytes and MMMMMM usage is on the order of a few hundreds of bytes.
Winsock - A third-pary commercial stack than can be used with Windows 95/98 and NT; supports IPv6.
Xinu TCP implementation - The implementation of TCP used in Comer's Xinu Unix-like operating system.