Windows® Internals, Sixth Edition, Part 1

Compound TCP (CTCP). Network congestion occurs when a node or link reaches its carrying capacity. CTCP implements a congestion-avoidance algorithm that monitors network bandwidth, latency, and packet losses. It aggressively increases the amount of data that can be sent by a machine when the network will support it, and it backs off when the network is congested. Using CTCP on a high-bandwidth, low-latency network can significantly improve transfer speeds

Explicit Congestion Notification (ECN). Whenever a TCP packet is lost (unacknowledged), the TCP protocol assumes that the data was dropped because of router congestion and enforces congestion control, which dramatically lowers the sender’s transmission rate. ECN allows routers to explicitly mark packets as being forwarded during congestion, which is read by the Windows TCP/IP stack as a sign that transmission rates should be lowered. Lowering rates in this manner results in better performance than relying on loss-based congestion control. ECN is disabled by default, because many outdated routers might drop packets with the ECN bit set instead of ignoring the bit. To determine whether your network supports ECN, you can use the Microsoft Internet Connectivity Evaluation Tool (http://www.microsoft.com/windows/using/tools/igd/default.mspx). You can examine and modify the ECN capability using the network shell (from an Admin command window), as shown in Figure 7-34.

Figure 7-34. Using the network shell to examine and configure TCP parameters

High-loss throughput improvements, including the NewReno Fast Recovery Algorithm, Enhanced Selective Acknowledgment (SACK), Forward RTO-Recovery (F-RTO), and Limited Transit. These algorithms reduce the overall retransmission of acknowledgments or TCP segments during high-loss scenarios while still maintaining the integrity of the TCP stream. This allows for greater bandwidth in these environments and preserves TCP’s reliable transport semantics.

The Next Generation TCP/IP Stack (%SystemRoot%\System32\Drivers\Tcpip.sys), shown in Figure 7-35, implements TCP, UDP, IP, ARP, ICMP, and IGMP. To support legacy protocols such as NetBIOS, which make use of the deprecated TDI interface, the network stack also includes a component called TDX (TDI translation), which creates device objects that represent legacy protocols so that clients can obtain a file object representing a protocol and issue network I/O to the protocol using TDI IRPs. The TDX component creates several device objects that represent various TDI client–accessible protocols: \Device\Tcp6, \Device\Tcp, \Device\Udp6, \Device\Udp, \Device\Rawip, and \Device\Tdx.

Figure 7-35. Windows Next Generation TCP/IP Stack

EXPERIMENT: Looking at TCP/IP’s Device Objects