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Do We Still Need ALGs
in CGNAT?

NFWARE BLOG

13/09/2023

As Carrier-Grade NAT (CGNAT) has become more widely used, there has been a debate about whether Application Layer Gateways (ALGs) are still necessary.

ALGs or Application Layer Gateways are software components of NAT (Network Address Translation) that manage specific application protocols. It allows those applications to work correctly over a network. There are several types of ALGs, such as FTP (File Transfer Protocol) ALGs, SIP (Session Initiation Protocol) ALGs, RTP (Real-time Transport Protocol) ALGs, PPTP (The Point-to-Point Tunneling Protocol) ALGand others. Each type of ALG is designed to handle a specific type of application traffic and protocol.

Application Layer Gateways (ALGs) were originally developed to solve a problem that arose with the widespread adoption of Network Address Translation (NAT). NAT was designed to allow multiple devices to share a single public IP address, which helped to conserve IPv4 address space. However, NAT also brought with it a number of challenges, particularly when it came to application-specific protocols that used dynamically assigned ports.

In order to maintain the end-to-end nature of the communication, ALGs were developed to work with NAT. ALGs essentially act as intermediaries between the application and the NAT device, translating the application-specific protocol and making sure that packets are forwarded correctly. This allows applications to work correctly even when they are running behind a NAT device.

ALGs were particularly important in the early days of NAT, when many applications were not designed to work with NAT. However, as NAT has become more prevalent, many applications have been updated to work correctly with NAT without the need for ALGs. This has led to a debate about whether ALGs are still necessary in modern networks, particularly in the context of Carrier-Grade NAT (CGNAT).

What are some of the arguments against ALGs?

One argument against using ALGs in CGNAT is that many modern applications are now designed to work with NAT without the need for any additional support. This means that ALGs may not be necessary for a majority of the applications that are used today. As a result, network operators may be able to turn off ALGs without experiencing any negative impact.

Also, some applications may not function correctly when ALGs are enabled, leading to issues with performance and reliability. In some cases, enabling ALGs can actually cause more problems than it solves. This is particularly true for applications that use non-standard protocols or exhibit poor design, such as cases where clients' applications do not adhere to RFCs (Request for Comments).

Among the most frequently questioned ALGs as to whether they are still necessary in CGNAT are SIP, ESP, and GRE ALGs. Why is that? SIP might not be needed because all modern SIP clients support NAT traversal techniques like STUN. ESP and GRE work only with static IP-IP rules, when an internal address is strictly mapped to an external address. Nowadays, IPSec inside UDP covers ESP, and PPTP kind of covers GRE, making ESP and GRE ALGs useless and unnecessary.

What are the arguments for it?

Despite the arguments against using ALGs, they still offer several benefits in the context of CGNAT.

Firstly, ALGs can help to improve security by enforcing granular access controls and preventing attacks on specific application protocols. For example, a DNS ALG can prevent DNS cache poisoning attacks by only allowing traffic from trusted sources.

Secondly, ALGs can help to simplify network management by reducing the need for complex port forwarding rules and other NAT configurations. This can help to reduce the workload on network administrators and improve overall network efficiency.

In general, while ALGs may not be necessary for all applications, they do offer several benefits within the context of CGNAT. The ultimate decision regarding ALG usage depends on the specific needs and requirements of the network. Some applications may function perfectly without ALGs, while others may require them for proper operation. A general recommendation would be to use only the critical ALGs that are essential for certain applications, such as FTP, PPTP, and TFTP.

NFWare CGNAT provides support for multiple types of ALGs, including FTP, TFTP, PPTP, SIP, RTSP, and DNS. However, it also allows users to enable or disable them as needed. Our experienced engineers and consultants can collaborate with ISPs to determine the optimal configuration for their network, ensuring reliable and efficient connectivity for their customers.

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