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Carrier-Grade NAT

The NAT444 Model

So far we have discussed traditional NAT (mainly NAPT/NAT overload) which is starting to be called NAT44 because it translates one IPv4 address for another (4 to 4). Now let’s explore what I (still) call NAT444, which was also called Carrier Grade NAT (CGN) at one point and is currently called Large Scale NAT (LSN) by all the cool geeks. I like NAT444 because it explains what is really going to occur in most places when LSN is implemented; a triple NAT (IPv4 to IPv4 to IPv4). Sounds like a nightmare already, doesn’t it? We just doubled the NAT, which means doubling the interference with network traffic and further impeding the end to end principle. Let’s see what that looks like.

This diagram illustrates the NAT444 model as envisioned in the IETF NAT444 draft and as it will most likely be deployed (unless we can figure out a way around it altogether). In this diagram you should notice two things right off the bat:

  1. Large Scale NAT for IPv4 will be deployed dual-stacked with global (“public”) IPv6.
  2. Large Scale NAT adds a second layer of NAT and thus a second area of “private”/inside addressing.

As you can guess from point two, NAT444 exacerbates all the problems that traditional NAT44 introduced. What may not be obvious at first is that LSN/NAT444 aggravates those issues in new ways as well.

In addition to adding a second layer of NAT that creates major problems with law enforcement and abuse logging as well as geolocation and others (all because many distinct customers are behind one provider address), we have to deal with the fact that the second layer of NAT is not going to participate in UPnP, NAT-PMP or other LAN-based NAT traversal protocols. No ISP (Internet Service Provider) in their right mind is going to open up their own routers (or other network devices) to customer control – which is exactly what these protocols require. They are simply not secure and the risk of one customer being able to impact other customers’ service is too great. So where does that leave us?


What NAT444 Breaks

We are left with a number of applications (and application types) that currently break when Large Scale NAT is introduced. To avoid the doom and gloom feeling that is sure to follow a list of just the broken stuff, let’s start with a list of what isn’t broken by NAT444/LSN:

  • Web browsing
  • Email
  • FTP download
    • Small files
  • BitTorrent and Limewire
    • Leeching (download)
  • Skype video and voice calls
  • Instant messaging
  • Facebook and Twitter chat

Not too shabby really, all things considered. That is quite a bit of functionality for being behind a fairly large kludge. If that were the end of the story I wouldn’t have written this article though. So, without further adieu, here is the list you’ve been waiting for; what NAT444 breaks:

  • FTP download
    • Large files
  • BitTorrent and Limewire
    • Seeding (upload)
  • On-line gaming
    • Xbox
    • PlayStation
    • Etc.
  • Video streaming
    • Hulu
    • Netflix
    • Slingcatcher
    • Etc.
  • Webcam
    • Remote viewing
  • Tunneling
    • 6to4
    • Teredo
    • Etc.
  • VPN & Encryption
    • IPSec
    • SSL
  • VoIP
    • Limited ALG/SIP support
  • All custom applications with the IP embedded
    • Lack of ALGs

Wow, is it just me or is that list a bit longer? There’s that doom and gloom feeling creeping up.

For our purposes here, “breaks” means that the service was degraded or completely failed. The data behind this list primarily comes from Assessing the Impact of NAT444 on Network Applications, an IETF draft which documents testing that was done by CableLabs, Time Warner Cable, and Rogers Communication on “many popular Internet services using a variety of test scenarios, network topologies, and vendor equipment.” If this kind of thing interests you at all, I highly recommend checking out the full draft, it’s a quick and informative read. I also have a bit of experience dealing with NAT444 myself, but that’s a story for another day.

Port Control Protocol

I would be remiss if I didn’t at least mention Port Control Protocol (PCP) in this discussion. The basic goal of PCP is to create a new, more advanced, technique to control port forwarding on NAT devices so that the brokenness fixed with protocols like UPnP-IGD and NAT-PMP in the LAN can be solved in a NAT444 (or other LSN) environment. I have not, as of yet, dug very deep into the work being done but I do see some challenges:

  1. New protocols require new equipment (or at least new code).
  2. Will providers sign on to allow customer application control of their network devices?
  3. Time. Is there enough? We will hit RIR exhaustion in at least 3 out of 5 regions before the proposed standards are published.

The Solution

You knew I was going to say it eventually: The only true solution is to deploy IPv6. This is why the NAT444 model includes global IPv6; the only way to get around the brokenness introduced by NAT is to eliminate it. Luckily we have the means to do so; Internet Protocol version 6 (IPv6). So, dual-stack today, with NAT444 if you must, and then do everything you can to get everyone you do business with to do the same.

Author Chris Grundemann

Assessing the Impact of NAT444 on Network Applications

Cisco Solution        Juniper     A10         F5



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