References allow you to refer to arbitrary positions inside your document.

Power over TCP

  • Power is transmitted as payload in the TCP packet.
  • During the TCP handshake, the requested current and voltage are encoded in optional header options.
  • To power on, the device opens a connection to a Power Provider and specifies the parameters for the required power in the optional TCP header.


  • Digital Power: The power is transmitted digitally. There is no loss due to resistance of the cable.
  • Discovery: A device can discover a Power Provider on the local network using the Dynamic Power Discovery Protocol (DPDP).
  • Routing: The inherent routing capabilities of the Internet Protocol are utilized and power is routed to the endpoint wherever it is needed.
  • Multiple Endpoints: Due to the utilization of TCP, an endpoint has access to several connections carrying different configurations of current and voltage.
  • Wireless LAN: Mobile devices are enabled to power on and operate without any connection to the power grid. A battery is not required any more. Internet Service Providers can enhance their hot spots to provide power and network connectivity at the same time.
  • VPPN: Corporate users can rely on their employer to provide power via Virtual Private Power Networks (VPPNs).
  • Firewalls: Unauthorized power transmission can be intercepted and the power can be reused by the device, either to power the infrastructure or to offer power to internal devices.
  • Encryption: By encrypting power, the power provider can prevent unauthorized usage of the transmitted payload.


  • In data centers, the distribution of power plugs is not necessary any longer because a single power provider can transmit power via the existing networking infrastructure.
  • A firewall attacked by a malicious power provider can reuse the received power.
  • The first law of thermodynamics (conservation of energy) does not apply to Power-over-TCP because a broadcast is able to distribute (the same) power to multiple endpoints at the same time.

Perl Module Errno

This module provides a variable containing the results for all known error constants:

PDF Bookmarks

Sometimes it is desirable to add custom lines to the list of bookmarks inside a PDF file:

PDF Annotations

PDF annotations provide a nice way to add notes to your document (actually they are much more powerful, but i have merely used text annotations so far):

Numbering of Equations

The following commands change the numbering of equations to include the current section:

My Setup 2: Public Key Authentication

In addition to my setup, this code assumes that your SSH keys are stored on a hot-swappable mass storage device (USB stick, flash card etc.). It first asks you to connect the storage device and then adds your key(s) into the SSH agent.

Line Spacing

This command will alter the spacing of the lines of text by the specified factor:


This note introduces commands to add an index to your document:


Not much to say about that: