User manual

DIGITAL-LOGIC AG MSB900/L Detailed Technical Manual V1.0

13. PXE-BOOT AND PXE-SETUP IN THE BIOS

PXE Protocol

PXE is defined on a foundation of industry-standard Internet protocols and services that are widely deployed

in the industry, namely TCP/IP, DHCP, and TFTP. These standardize the form of the interactions between

clients and servers. To ensure that the meaning of the client-server interaction is standardized as well, cer-

tain vendor option fields in DHCP protocol are used, which are allowed by the DHCP standard. The opera-

tions of standard DHCP and/or BOOTP servers (that serve up IP addresses and/or NBPs) will not be dis-

rupted by the use of the extended protocol. Clients and servers that are aware of these extensions will rec-

ognize and use this information, and those that do not recognize the extensions will ignore them.

In brief, the PXE protocol operates as follows. The client initiates the protocol by broadcasting a

DHCPDISCOVER containing an extension that identifies the request as coming from a client that implements

the PXE protocol. Assuming that a DHCP server or a Proxy DHCP server implementing this extended proto-

col is available, after several intermediate steps, the server sends the client a list of appropriate Boot Serv-

ers. The client then discovers a Boot Server of the type selected and receives the name of an executable file

on the chosen Boot Server. The client uses TFTP to download the executable from the Boot Server. Finally,

the client initiates execution of the downloaded image. At this point, the client’s state must meet certain re-

quirements that provide a predictable execution environment for the image. Important aspects of this envi-

ronment include the availability of certain areas of the client’s main memory, and the availability of basic net-

work I/O services.

Deployment of servers

On the server end of the client-server interaction there must be available services that are responsible for

providing redirection of the client to an appropriate Boot Server. These redirection services may be deployed

in two ways:

1. Combined standard DHCP and redirection services.

The DHCP servers that are supplying IP addresses to clients are modified to become, or are replaced

by servers that serve up IP addresses for all clients and redirect PXE-enabled clients to Boot Servers

as requested.

2. Separate standard DHCP and redirection services.

PXE redirection servers (Proxy DHCP servers) are added to the existing network environment. They

respond only to PXE-enabled clients, and provide only redirection to Boot Servers. Each PXE Boot

Server must have one or more executables appropriate to the clients that it serves.

Preboot Execution Environment (PXE) Specification 11

Version 2.1 September 20, 1999

This diagram illustrates the relationship between the NBP (the remote boot program) and the PXE APIs.