Windows® Internals, Sixth Edition, Part 1

If an application wants to use NetBIOS over the TCP/IP protocol, the NetBIOS emulator requires the presence of the NetBT driver (%SystemRoot%\System32\Drivers\Netbt.sys). NetBT is known as the NetBIOS over TCP/IP driver and is responsible for supporting NetBIOS semantics that are inherent to the NetBIOS Extended User Interface (NetBEUI) protocol (included in previous versions of Windows) but not the TCP/IP protocol. For example, NetBIOS relies on NetBEUI’s message-mode transmission and NetBIOS name-resolution facilities, so the NetBT driver implements them on top of the TCP/IP protocol.

Other Networking APIs

Windows includes other networking APIs that are used less frequently or are layered on the APIs already described (and outside the scope of this book). Five of these, however—Background Intelligent Transfer Service (BITS), Distributed Component Object Model (DCOM), Message Queuing (MSMQ), Peer-to-Peer Infrastructure (P2P), and Universal Plug and Play (UPnP) with Plug and Play Extensions (PnP-X)—are important enough to the operation of a Windows system and many applications to merit brief descriptions.

Background Intelligent Transfer Service

BITS is a service and an API that provides reliable asynchronous transfer of files between systems, using either the SMB, HTTP, or HTTPS protocol. BITS normally runs in the background, making use of unutilized network bandwidth by monitoring network utilization and throttling itself so that it consumes only resources that would otherwise be unused; however, BITS transfers might also take place in the foreground and compete for resources with other processes running on the system.

BITS keeps track of ongoing, or scheduled, transfers in what are known as transfer jobs (not to be confused with jobs and job objects as described in Chapter 5) for each user. Each job is an entry in a queue and describes the files to transfer, the security context (access tokens) to run under, and the priority of the job. BITS version 4.0 is integrated into BranchCache (described later in this chapter) to further reduce network bandwidth.

BITS is used by many other components in Windows, such as Microsoft Update, Windows Update, Internet Explorer (version 9 and later, for downloading files), Microsoft Outlook (for downloading address books), Microsoft Security Essentials (for downloading daily virus signature updates), and others, making BITS the most widely used network file-transfer system in use today.

BITS provides the following capabilities:

Seamless data transfer. Components create BITS transfer jobs that will then run until the files are transferred. When a user logs out, the system restarts, or the system loses network connectivity, BITS pauses the transfer. The transfer resumes from where it left off once the user logs in again or network connectivity is restored. The application that created a transfer job does not need to remain running, but the user must remain logged in, while the transfer is taking place. Transfer jobs created under service accounts (such as Windows Update) are always considered to be logged on, allowing those jobs to run continuously.

Multiple transfer types. BITS supports three transfer types: download (server to client), upload (client to server), and upload-reply (client to server, with a notification receipt from the server).

Prioritization of transfers. When a transfer job is created, the priority is specified (either Foreground, Background High, Background Normal, or Background Low). All background priority jobs make use only of unutilized network resources, while jobs with foreground priority compete with applications for network resources. If there are multiple jobs, BITS processes them in priority order, using a round-robin scheduling system within a particular priority so that all jobs make progress on their transfers.

Secure data transfer. BITS normally runs the transfer job using the security context of the job’s creator, but you can also use the BITS API to specify the credentials to use for impersonating a user. For privacy across the network, you should use the HTTPS protocol.

Management. The BITS API consists of methods for creating, starting, stopping, monitoring, enumerating, modifying, or requesting notification of transfer-job status changes. Tools include BITSAdmin (which is deprecated and will be removed in a future version of Windows), and Windows PowerShell cmdlets (the preferred management mechanism).