[menog] Can Large Scale NAT Save IPv4?
aabuqayyas at citc.gov.sa
Tue Oct 5 11:46:23 GMT 2010
POINT VIEW !!!! What do you think???
Can Large Scale NAT Save IPv4?
LSNs are a Necessary but Imperfect Transitional Technology
By jdoyle <http://www.networkworld.com/community/user/2404> on Mon,
10/04/10 - 12:10pm.
I've written previously
<http://www.networkworld.com/community/node/42436> that as we make the
slow - and long overdue - transition from IPv4 to IPv6, we will soon be
stuck with an awkward interim period in which the only new globally
routable addresses we can get are IPv6, but most public content we want
to reach is still IPv4. Large Scale NAT
<http://www.networkworld.com/community/node/44989> (LSN, also known as
Carrier Grade NAT or CGN) is an essential tool for stretching a service
provider's public IPv4 address space during this transitional period.
I've yet to work an IPv6 project involving LSN in which someone does not
eventually, with great hope in his eyes, say, "If LSN extends the life
of our IPv4 space, why are we going to the pain and expense of deploying
IPv6? Can't we just deploy LSN and forget about IPv6 for now? Perhaps
until I retire?"
A first look LSN does indeed seem to promise an extended lifetime for
IPv4. Could it even mean that the Internet never has to transition to
This article looks beyond the mechanisms of LSN itself to examine the
implications of LSN in a practical network, and why this useful
technology should never be viewed as anything other than an interim
A Quick Review of LSN Architectures
A traditional broadband service provider network conserves IPv4
addresses by assigning a single public IPv4 address to the outside
interface of a NAT residing at the edge of each customer network. Behind
the NAT, all devices are assigned a private IPv4 address. The NAT works
by mapping each application flow - as identified by the combination of a
private IPv4 address and a TCP or UDP port - to the public IPv4 address
and one of its TCP or UDP ports. In other words, NAT multiplexes the
addresses of many inside devices to a single outside address by mapping
Ports are 16 bit numbers, so potentially 65,536 TCP flows and 65,536 UDP
flows could be mapped to a single IPv4 address. The average household or
small office does not generate nearly this many flows at one time,
making address translation at the edge of such small networks an
inefficient use of a public IPv4 address.
An LSN is a "centralized NAT" placed in the service provider's network.
Whether this is in addition tothe NAT at the customer edge, as with
NAT444 <http://www.networkworld.com/community/node/45776> , or instead
of the customer NAT, as with DS-Lite
<http://www.networkworld.com/community/node/46600> , the LSN concept is
the same: The public IPv4 addresses are pulled away from the customer
edge, where their multiplexing capacity is not efficiently exploited, to
the outside of the centralized LSN where many customer networks can
share a single public IPv4 address.
LSN architecture design, then, is mostly figuring out the strategic
placement of each LSN to best use the capacity of each public IPv4
address without oversubscribing the address or overtaxing the LSN
Although only a few studies
rand.pdf> of per-user port usage have been done, an LSN should be able
to support 3000 - 5000 users per public IPv4 address.
These numbers, coupled with the tens of thousands of public IPv4
addresses broadband service providers currently hold for customer
assignment, do appear to make LSN a practical alternative to near-term
IPv6 deployment, adding years to the life of IPv4.
Before coming to such a conclusion, the implications and practical
impact of LSN must be considered.
Who Are You?
A long practice in the networking industry is to identify a user by IP
address. This is especially the case when the user might not want to be
identified or when the identification of the machine is more important
than the identification of the individual using it.
By centralizing public IPv4 addresses, each address no longer represents
a single machine, a single household, or a single small office. The
address now represents thousands of machines, homes, and offices related
only in that they are behind the same LSN. Identification by IP address
becomes difficult or impossible.
Obfuscation of the network behind a NAT has long been considered
(incorrectly, in my opinion) a security benefit. Obfuscation of large
groups of networks, with nothing in common except the use of the same
broadband provider, creates an unprecedented set of challenges.
One of those challenges is not administrative or technical, but an
opening for an undesirable social behavior within certain Internet
I enjoy participating in a few political discussion groups on the
Internet, for the learning experience and for the fun of debating
political issues. As I was contemplating the ramifications of LSN one
evening I realized that LSN could introduce a new and unwelcome
phenomenon on such sites.
If you have ever participated in an open Internet discussion group,
particularly one that deals with contentious issues, you are probably
familiar with the concept of a "troll." A troll is someone who is not
really interested in the discussion at hand, but instead enjoys making
outrageous or inflammatory remarks just to upset the other participants.
They are a part of many websites where the general public is allowed to
register and leave comments, and they are particularly attracted to
political and religious websites. I remember the occasional troll even
on the old Cisco Usenet newsgroup, comp.dcom.sys.cisco, in the mid
Sometimes a troll will go too far, and the moderators of the discussion
group will "ban" him by deleting his user account. And sometimes a
banned participant will simply create a new Hotmail or Yahoo e-mail
address, register back to the site under a different user name, and
continue trolling until banned again.
To prevent this "repeat offender" behavior, some websites will ban a
misbehaving user by IP address rather than user name. This is assumed to
be more effective, by banning the user's machine rather than any account
he might create from that machine. If the IP address is on the outside
interface of a home or small office NAT, blacklisting it might restrict
others in the home or office from accessing the website but altogether
few "innocent bystanders" are affected.
What happens, though, if a website bans an IPv4 address on the outside
of an LSN? In the effort to restrict a single user, thousands of people
will be inadvertently restricted - generally all subscribers on a CMTS
or a group of DSLAMs behind the LSN.
A malicious user with a grudge against a particular website might, if he
knows his provider is using LSN, intentionally get himself blacklisted
by IP address on the site in order to simultaneously get a few thousand
of his neighbors banned - he will have performed a small-scale DoS
attack by causing the site administrators themselves to unwittingly
perform the denial of service.
Black and White
Remote sites are not the only ones occasionally needing to black-list a
user based on an IP address. The local provider also needs black-listing
capability. Some also use white-listing: the addition of some
preferential treatment or pre-approval. Generally white-listing and
black-listing are used in conjunction with spam and virus control, but
black-listing can also be applied to enforce use policies.
Black- or white-listing may need to be split in an LSN architecture.
Polices applying to incoming sources must be implemented on the outside
of the LSN; once the packets are translated, they cannot be easily
identified by IPv4 address without some correlation with the LSN's
mapping table. Policies applying to outgoing sources - that is, sources
within the customer networks - must be implemented on the
customer-facing side of the LSN for the same reason.
Centralized address and port translation within the provider network
presents serious challenges to the compliance with lawful intercept
requirements such as CALEA. DHCP assignments to traditional networks
with NATs at the customer edge change infrequently, making interception
easy. Lawful intercept might still be reasonably easy with NAT444
architectures, as long as the interception happens between the CPE NAT
and the LSN. The dependency here is whether both the inside and outside
addresses are of interest, or only the inside addresses.
Because of its IPv4-in-IPv6 tunneling, interception in DS-Lite
architectures must be performed on the LSN itself. Timestamped logging
of the address and port mappings at the LSN must be maintained, which in
turn can add a heavy resource burden to the LSN devices. Logging to a
storage device off the LSN may also contribute to network load.
Wiretapping of a single subject may mean statically mapping the user to
a certain range of ports on a single address, to remove the need to
follow dynamic port mappings. A single IPv4 address, or some range of
ports for each address, might be set aside for wiretapping purposes to
simplify such procedures. But any requirement that all users behind an
LSN be logged is going to mean logging not only traffic but all changes
to the mapping tables.
The timestamped logging of address and port mappings is essential not
only for lawful intercept but also for tracing back specific users when
a problem is identified from the outside of the LSN. Such a problem is
usually a misbehaving user - a spammer, a DoS source, or someone
violating a usage policy - and identification of the user might result
in black-listing, cancellation of service, or covert observation for
legal action. Without time-specific logs of the address and port
mappings, a misbehaving user stays well hidden behind the LSN.
But where lawful intercept might require logging of one or a few users,
logging for traceback purposes could mean logging all users, at least at
some sample rate, causing a massive consumption of device resources. A
compromise step might be to begin traceback logging only when a problem
is detected; while using far less resources, it assumes that the
undesirable action will continue long enough for all or most of the
traceback to be performed in real time.
A longstanding complaint about NAT44 is that it breaks some applications
that reference the IP address of its packets. In a perfect world - or at
least the conceptual world of IP networking - applications would be
agnostic to the network layer and thus immune to the address changes
through a NAT. But the reality is that many applications do reference
the IP address. For the ubiquitous user edge NAT, work-arounds have been
created for some applications.
The double-NAT structure of NAT444 can be expected to break some
applications that will work through a single NAT layer. A few MSOs are
currently conducting trials to determine what will be affected by
NAT444, and therefore what impact it might have on their customers.
DS-Lite avoids the double NAT problems of NAT444, and presently appears
to be the preferred solution for most broadband providers. But some LSN
vendors still have DS-Lite on their roadmaps rather than in their
products, and CPE with DS-Lite support is rare. This solution is
therefore not as immediately available as NAT444.
An Imperfect Necessity
There are a host of other concerns around LSN: Single points of failure,
potential address pool depletion attacks, performance and scalability,
effects on fragmented packets, effects on asymmetric traffic flows,
required modifications to provisioning systems, required modifications
to internal accounting systems.
Because we have waited far too long to begin implementing IPv6, Large
Scale NAT has become an unavoidable necessity for supporting dual
stacked broadband customers in the face of a depleted IPv4 address
supply. But the problems and complexities LSN introduce to a network
mean that it should never be viewed as anything but a transitional
technology. It is no substitute for IPv6.
Abdelfattah ABUQAYYAS, PhD
CITC-KSA Mobile: +966556642230
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