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This document is not officialy released and has to be treated as confidential.

However, NAT can be configured to allow incoming connections. This is achieved by

sending packets destined for a specific port on the one, externally visible IP address

to a new machine within the private network. These rules can be added with a

configuration application. Packets arriving with different destination port numbers can

have their destination IP addresses re-written to different values; for example TCP

packets destined for port 80 can be redirected to a web server, and TCP packets

sent to port 25 can be sent to a separate mail server.

Application Level Gateway (ALG)

Some application level protocols embed IP address information in the payload of

TCP or UDP packets. As NAT itself only modifies information in the packet headers,

such protocols require an Application Level Gateway (ALG) in order to operate

transparently with NAT.

The NAT process is designed to allow additional Application Level Gateways (ALGs)

to be added easily, so that additional or new protocols can be supported. Although all

ALGs are an integral part of the NAT process, each ALG is modular, so that only

those which are required for a particular application need to be compiled.

Each ALG is required to register with NAT the TCP or UDP port numbers for which it

is interested in intercepting packets. It also supplies the addresses of functions which

NAT will call for each packet matching that gateway’s requirement.

NAT is transparent to the majority of common protocols, including but not limited to:

• DNS queries

• HTTP

• NNTP

• POP3

• SMTP

• SSH (most features)

• Telnet

• TFTP

• Windows drive sharing (most features)

2.8 Sending Dying Gasp message with the Copperjet.

The Copperjet has the ability to detect when the electrical power has been shut off.

After such detection of a near-end Loss-of-Power (LPR) condition, the Copperjet will

insert emergency priority eoc

messages into the ADSL upstream data to implement

a “dying gasp” as an LPR indicator.

At least six continuous dying gasp eoc messages will be inserted in the next

available ADSL upstream bytes available for eoc. The ATU-C

will not send a

response to a “dying gasp” message back to the ATU_R

(Copperjet).

Embedded Operations Channel

ADSL Transceiver Unit, Central office end

ADSL Transceiver Unit, Remote terminal end

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