Final Report
MC/159 Report on the Implications of Carrier Grade NATs
11
3 Technical Overview of Carrier Grade NAT
Network Address Translation (NAT) and Carrier Grade NAT are complex technologies. The
following sections provide an overview of their operation. There are many forms of NAT and
CGN, this section focuses on the main features rather than providing a comprehensive
description of every type of NAT and CGN.
This section also describes many of the associated technologies that have been developed to
mitigate the limitations of NAT and CGN. Many of these attempt to allow protocols, services and
applications that would not normally work behind NAT or CGN.
3.1 How Network Address Translation (NAT) Works
To enable communication between any two points on the public Internet, each host (or node) is
uniquely identified by an address: the IP address. Traffic between nodes is then routed through
the Internet based on the Destination IP address carried in the IP protocol header. The IP
header also contains the Source IP address so that nodes can respond to the source of any
traffic. Routing packets between nodes is the primary function of the network layer (layer 3 of
the OSI protocol stack).
12
It is intended to be simple, efficient and robust.
IP datagrams can carry many different transport layer protocols with different characteristics
and functionality. The most common transport layer protocols are the Internet Control Message
Protocol (ICMP), the User Datagram Protocol (UDP) and the Transmission Control Protocol
(TCP). Most applications use either TCP or UDP or a combination of both to communicate.
TCP and UDP include source and destination port numbers in their protocol headers. These
allow applications to uniquely identify individual sessions between them. The combination of IP
address and port number that identifies an individual session is called a “socket”. Some
applications may only use one session but others use many. Each session is uniquely identified
by a combination of source IP address, destination IP address, source port number, destination
port number and protocol number (usually TCP or UDP but it could be another protocol).
Figure 3.1 below illustrates the structure of the IPv4 Internet and access networks prior to the
introduction of NAT44. This is a routed network.
12
The OSI (Open Systems Interconnect) model is a seven-layer representation of how network
communications take place. Although it does not map directly to the Internet model, the IP layer in the
Internet hourglass model is equivalent to layer 3—the network layer—of the OSI model. For more
information, see Information technology – Open Systems Interconnection – Basic Reference Model: The
Basic Model, ISO/IEC 7498-1:1994,
http://standards.iso.org/ittf/PubliclyAvailableStandards/s020269_ISO_IEC_7498-1_1994(E).zip, 1994.
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