Re: NAT natural example, Re: [midcom] WG scope/deliverables
Steve Deering wrote: > At 8:12 AM -0800 2/16/01, Ed Gerck wrote: > >1. there is a natural need for heterogeneous address systems and, > > Agreed. > > >2. therefore, there is a natural need for address translation. > > Only if there's some need to interconnect them, and even then only as > a temporary measure, if at all, because there is an alternative and > preferable way to deal with heterogeneous address systems -- and the > only long-term successful way if history is any guide -- which is to > layer a homogenous address system on top of them, which is the basic > idea behind IP. The other way, which can be theoretically justified as well, is to implictly define a "third system" that defines an internal reference for a set of relationships between the two address spaces. This third system can take the form of a NAT. Note that this third system is not an address space, much less a homogeneous one. And, as "The Tulip" discussion thread showed, such a NAT can take various forms that could be defined in an RFC with interoperation in mind. In particular, the capability of including the outside origin address:port as well as the global destination address:port in the translated packet which has the usual NAT-defined local destination address:port and the local origin address:port. Cheers, Ed Gerck
Re: NAT natural example, Re: [midcom] WG scope/deliverables
> 1. there is a natural need for heterogeneous address systems and, okay > 2. therefore, there is a natural need for address translation. no. it doesn't follow, at least not in the sense of address translation as done by NAT. there is a natural need for *routing* or *mapping* between higher and lower layer addresses, but this isn't the same thing as NAT. Keith
Re: NAT natural example, Re: [midcom] WG scope/deliverables
At 8:12 AM -0800 2/16/01, Ed Gerck wrote: >1. there is a natural need for heterogeneous address systems and, Agreed. >2. therefore, there is a natural need for address translation. Only if there's some need to interconnect them, and even then only as a temporary measure, if at all, because there is an alternative and preferable way to deal with heterogeneous address systems -- and the only long-term successful way if history is any guide -- which is to layer a homogenous address system on top of them, which is the basic idea behind IP. Yes, the first attempt to join networks using different address systems is often to install translators, which is the way "interworking" was done before IP and Pup were invented, the way email systems were interconnected before universal adoption of the [EMAIL PROTECTED] name space, and the way people are gluing together the phone network and IP phones, not to mention the IPv4 and IPv6 Internets, today. Such approaches have always turned out to be so complex, fragile, unmanageable, unscalable, and function-limiting that they are sooner or later abandoned in favor of the one-global-namespace approach. If people understood that they didn't "need" to do translation, they just might take that step sooner and save everyone a lot of grief. Steve
Re: NAT natural example, Re: [midcom] WG scope/deliverables
List: My example of the UK postal system, with addresses that behave as names, was NOT an attempt to make a parallel between the postal system and the full glory of the Internet. BTW, I don't believe in such parallels. Sorry to disapoint those that thought so! ;-) My sole puprose with that example was to show that there is a natural need for heterogeneous address systems and, therefore, for address translation. Many features found in Internet NAT are also IMO found in the UK postal scheme. The analogy is not perfect (as I said myself) but, what analogy ever is? So, rather than trying to find where the analogy is wrong, or claiming that I am ignoring the difference between identification, location, and routing, this dialogue was based on illustrating those two points, to wit: 1. there is a natural need for heterogeneous address systems and, 2. therefore, there is a natural need for address translation. Nothing else, and nothing more, was claimed. Cheers, Ed Gerck
Re: NAT natural example, Re: [midcom] WG scope/deliverables
Ed, you seem to be ignoring the difference between identification, location, and routing. What the post office does is routing, not NAT. The NAT problem is a problem because IP addresses mix the concepts of identification and location in a single bit string. There's nothing natural about it, at least nothing more natural than shooting oneself in the foot. Brian Ed Gerck wrote: > > "Steven M. Bellovin" wrote: > > > In message <[EMAIL PROTECTED]>, Ed Gerck writes: > > > > > > > >Actually, in the UK you can do just what you wish ;-) > > >You give a name to your house (say, "The Tulip") and > > >the post office knows where The Tulip is. If you move, > > >you can do the same at your new location, provided > > >there is no conflict. This seems to be more similar to the > > >notion of using an IP number as a name -- but isn't this > > >why we need DNS? ;-) > > > > > > > And if you move from London to Belfast, this will still work? > > In the UK, as I said. I would think that other countries may have > a similar system. Note that this is a natural example of NAT, > in which the post office is doing the address translation to a local > address that only that post office knows, but which is globally > reachable through that post office. And the post office does so > without changing the global addresses or the local addresses. > > I don't want to be philosophical about this, but IMO this example > actually supports the view that NATs are naturally occuring solutions > to provide for local flexibility without decreasing global connectivity. > The Internet NAT is perhaps less an "invention" than a translation of > an age old mechanism that we see everywhere. We use the same > principle for nicknames in a school for example. > > IMO, it is thus artificial to try to block Internet NATs. Far better would be > to define their interoperation with other network components that we also > need to use, in each case. > > Cheers, > > Ed Gerck
Re: NAT natural example, Re: [midcom] WG scope/deliverables
> The original example, of a single house with the global address of > "The Tulip, UK" is a naturally occurring example of something like ARP > or something like tunneling, not something like NAT. The distinction > is betweeen doing a mapping/encapsulation and doing an address > substitution. NATs are all about doing address substitution; the > post office does mapping/encapsulation to deliver to The Tulip. Number portability is probably a better example - in the US, at least, the called party's address is swapped out at the ingress "router" and then swapped back in at the last hope "router." Melinda
Re: NAT natural example, Re: [midcom] WG scope/deliverables
[I've taken the bulk of my response to Ed's last reply to private mail, since I assume few here are interested in tedious arguments about exactly how the Internet is analogous to the postal system, but I'll just make his one public observation:] At 9:45 PM -0800 2/15/01, Ed Gerck wrote: >I agree that you can define many different analogies, from that example. >But, as above, if you consider the way that information is received then >a NAT box is IMO one valid analogy for reception because it satisfies >the functionality observed in a NAT box when receiving packets. Your postal example doesn't entail the modification of an address on the received package, which is the defining characteristic of a NAT. Your postal analogy does show how you can get nice properties of address portability and location-hiding within a local network *without* resorting to address modification, i.e., it shows that you can have the flexibility you so prize without doing NAT. Maybe that's the lesson you should draw from this "naturally occurring" analog to packet networks. Steve
Re: NAT natural example, Re: [midcom] WG scope/deliverables
Steve Deering wrote:
> At 6:21 PM -0800 2/15/01, Ed Gerck wrote:
> ...
> >In Internet NAT terms, "The Tulip" is the globally routable IP number for
> >my DSL, the post office is my NAT box and the physical address
> >"545 Abbey St." is the local, non-routable IP number of my host A.
>
> That would be analogous to having "The Tulip, UK" be the address of
> a post office, with all houses served by that post office sharing
> the same global address of "The Tulip, UK". That indeed is like a
> NAT, but is not the same as the original example.
To be precise and still with the original example, the analogy is that
"The Tulip, CMZ 62N, UK " is the full global address (which was described
in the context of my email as <"The Tulip" at that post office>).
The full designation "The Tulip, CMZ 62N, UK" is thus similar to a globally
routable address (Internet IP) that is available at the post office
"CMZ 62N, UK" (NAT box) and which may at times correspond
to a house at "545 Abbey St" (host A) or to a house at "636 North Av"
(host B), which mapping that post office knows at each time and uses
to direct correspondence to the proper house without revealing to the
outside world what that local address might be -- ie, either "545 Abbey St."
(host A) or "636 North Av" (host B), or any other.
All houses served by that post office share "CMZ 62N, UK" while the house
name is similar to a port number in NAT (different for each house being served).
Note also that my NAT analogy only dealt with receiving mail, not sending mail.
Mr. Tulip may send mail any way he wishes, with a global return address as
"The Tulip, UK", with a local address as "545 Abbey St", with a fake return
address or even with no return address.
Let me now address your objection that "A host behind a NAT, on the other hand,
doesn't know its own global address and, in most cases, doesn't even have a
global address (or one port's share of a global address), except temporarily as a
side-effect of sending a packet to the outside world". We may agree that
we are dealing here with two different processes -- sending information and
receiving information. An UK post office was presented as a NAT analogy
for receiving information, not to send information. In receiving information,
Mr. X (a host behind the NAT) does not need to know how the house
he just moved in is named at the post office -- and, nonetheless, he will get
any letters addressed to "The Tulip, CMZ 62N, UK" if that is the house's name at
the post office "CMZ 62N, UK". The temporary property of the global address is
also present in the UK post office example for receiving information -- just that
the time scale may be hundreds of years, not milliseconds.
Your other objection was that "In the case of NAT, on the other hand,
the destination address used across the public part of the Internet is no longer
present in the packet finally delivered to the destination host -- it has been
been replaced by (i.e., translated to) a different address". My reply is
that this does not occur in NATs if the destination address is also included
in the packet payload, which is the case here -- the envelope is part of
the message's payload in the post office case. Pls see also my last comment,
below.
> >In other words, this is a natural NAT example...
>
> The original example, of a single house with the global address of
> "The Tulip, UK" is a naturally occurring example of something like ARP
> or something like tunneling, not something like NAT.
I agree that you can define many different analogies, from that example. But,
as above, if you consider the way that information is received then a NAT box
is IMO one valid analogy for reception because it satisfies the functionality
observed in a NAT box when receiving packets. Yes, the UK post office does
not erase the global address on the envelope but a NAT will also keep that
information in the translated packet if it is in the packet's payload (which
is the case for the letter's envelope), and without any impact in its functionality
as a NAT.
> The distinction is betweeen doing a mapping/encapsulation and doing an
> address substitution. NATs are all about doing address substitution; the
> post office does mapping/encapsulation to deliver to The Tulip.
At the post office routing level, letters that enter a common input bin are moved to
different output bins at the post office. The common input bin is a globally
routable address such as "The Tulip, CMZ 62N, UK", "The Raven, CMZ 62N, UK",
etc. -- where the only part that is globally meaningful is "CMZ 62N, UK". Each
output bin corresponds to a local address mapped from the local qualifier
"The Tulip", "The Raven", etc. Each output bin, however, has no marking for
any local qualifier ("The Tulip"), just for a local address ("545 Abbey St").
Thus, there is no encapsulation at the post office routing level -- anyone
looking just at the bin "545 Abbey St" could not tell which local qualifier
was used for
Re: NAT natural example, Re: [midcom] WG scope/deliverables
> In the UK, as I said. I would think that other countries may have > a similar system. Note that this is a natural example of NAT, > in which the post office is doing the address translation to a local > address that only that post office knows, but which is globally > reachable through that post office. And the post office does so > without changing the global addresses or the local addresses. I think the example you give is more like ARP or VLAN than NAT. If the postal service were NATted, you'd send your mail to the post office, the mail clerk would decide that you really intended it to go somewhere else, and would erase your original destination and return addresses and fill them in with something different. Any address that you actually put in the text of the message would be useless to the recipient. Similarly, business cards, telephone directories, or any other means used to look up addresses outside of the postal service's control, would be useless. Each post office would need to have its own telephone directory for every telephone with which that you might want to call, so that you could look up a telephone number using your local post office's spelling of the address. If you moved from one place to another, such that you were now using a different post office than before, you wouldn't be able to continue using snail-mail to correspond with anyone with whom you'd previously been corresponding, because you would no longer have a usable address for that person. Keith
Re: NAT natural example, Re: [midcom] WG scope/deliverables
At 6:21 PM -0800 2/15/01, Ed Gerck wrote: >Steve Deering wrote: > > They also do it without removing the original destination address and > > replacing it with another one -- the original envelope arrives at the > > house with the destination address still saying "The Tulip", i.e., it > > has not been translated, and thus is not analogous to NAT. > >I think you got the example addresses reversed. In the case I mention, >"The Tulip" is the global address and (for the sake of example) suppose >now that "545 Abbey St." is the local physical address known to the post office. Yes, I understood that. >Thus, when the mailman delivers an envelope addressed to "The Tulip" at >"545 Abbey St.", that mailman is doing address translation -- and he may >even have written "545 Abbey St." on the envelope as a reminder. No, he's doing address mapping, similar to the the mapping that is done from an IP address to an Ethernet address to accomplish last-hop delivery. The original, globally unique name (The Tulip, UK) is still present on the letter. The local address may or may not also be present; depending on whether or not "encapsulation" (i.e., adding on the local address) was required to accomplish the delivery. In the case of NAT, on the other hand, the destination address used across the public part of the Internet is no longer present in the packet finally delivered to the destination host -- it has been been replaced by (i.e., translated to) a different address. > So, when the original envelope arrives at the destination address it >did so not because it had "The Tulip" written on it but because the post >office was able to do address translation to the *current* location which >is "545 Abbey St." No, it was because they were able to do the mapping to the current location. Translation, (i.e., replacing the address on the envelope with another address) is not necessary and not done. The envelope may well be *augmented* with an additional address, but the original address is not removed. >Note that the local address which only the post office (and Mr. Tulip) knows is "545 >Abbey St." while the global address is "The Tulip". The important point is that Mr. Tulip knows *both* addresses, and can tell his international correspondents what his globally-unique address is. A host behind a NAT, on the other hand, doesn't know its own global address and, in most cases, doesn't even have a global address (or one port's share of a global address), except temporarily as a side-effect of sending a packet to the outside world. >In Internet NAT terms, "The Tulip" is the globally routable IP number for >my DSL, the post office is my NAT box and the physical address >"545 Abbey St." is the local, non-routable IP number of my host A. That would be analogous to having "The Tulip, UK" be the address of a post office, with all houses served by that post office sharing the same global address of "The Tulip, UK". That indeed is like a NAT, but is not the same as the original example. >In other words, this is a natural NAT example... The original example, of a single house with the global address of "The Tulip, UK" is a naturally occurring example of something like ARP or something like tunneling, not something like NAT. The distinction is betweeen doing a mapping/encapsulation and doing an address substitution. NATs are all about doing address substitution; the post office does mapping/encapsulation to deliver to The Tulip. Steve
Re: NAT natural example, Re: [midcom] WG scope/deliverables
"Steven M. Bellovin" wrote: > In message <[EMAIL PROTECTED]>, Ed Gerck writes: > > > > > >"Steven M. Bellovin" wrote: > > > >> In message <[EMAIL PROTECTED]>, Ed Gerck writes: > >> > >> > > >> >Actually, in the UK you can do just what you wish ;-) > >> >You give a name to your house (say, "The Tulip") and > >> >the post office knows where The Tulip is. If you move, > >> >you can do the same at your new location, provided > >> >there is no conflict. This seems to be more similar to the > >> >notion of using an IP number as a name -- but isn't this > >> >why we need DNS? ;-) > >> > > >> > >> And if you move from London to Belfast, this will still work? > > > >In the UK, as I said. I would think that other countries may have > >a similar system. Note that this is a natural example of NAT, > >in which the post office is doing the address translation to a local > >address that only that post office knows, but which is globally > >reachable through that post office. And the post office does so > >without changing the global addresses or the local addresses. > > Last I checked, Belfast was in the UK, though I realize that some folks > wish it were not so. It will work in the UK was my reply. > But you missed my point -- as you note above, the > house name is known to "that post office". In other words, there is > hierarchy in the routing algorithm; it's not globablly known, or even > known throughout the UK. I disagreed with your point, not missed it. "The Tulip" together with *that* post office's postcode (for example CM22 6SX, which they assign on a geographical basis) is globally routable. Even from Belfast ;-) > The same is true of the Internet, and it's why IP addresses aren't portable. IP addresses are not portable simply due to a design choice. If IP numbers were designed the way the UK designed their postal service long ago, then IP numbers would be portable indeed. > >IMO, it is thus artificial to try to block Internet NATs. Far better would be > >to define their interoperation with other network components that we also > >need to use, in each case. > > Block them? Not at all; I have no desire to do that. But we need to > recognize that *with the current Internet architecture*, there are some > inherent limitations. To use your analogy, suppose that senders > sometimes wrote their house name on the letter enclosed in the envelope > -- but they didn't include the post office name, so the recipient > couldn't reply. I see that we are in agreement with my post office example. "The Tulip" together with the postal code (ie, the post office's "name") is globally routable. > Or imagine that the Post Office only kept track of > house names when there was a recent outgoing letter. These are security choices -- the time to live in a NAT could be unlimited, with fixed port numbers. The address:port numbers could also be pre-registered, before any message is sent. This is the current UK post-office model. Likewise, the UK post-office model could only kept track of house names when there was a recent outgoing letter, with "recent" defined by policy. > That's the reality of NAT today. IMO, this is simply a security choice -- NATs could work with the current UK post-office model as well. But if the house owner only wants to allow the post office to kept track of his house's name when there was a recent outgoing letter, then who is going to say otherwise? After all, he may refuse to receive any letter and just send them One way or another, the house (network) owner is sovereign over his house (network). My network is my castle. > Please pay careful attention to two things I did *not* say. I did > *not* say that NATs were an irrational engineering choice in today's > environment. In fact, they clearly are rational in some circumstances, > despite their disadvantages. I would say characteristics, not disadvantages. An apple is a bad orange. > Second, I didn't say that one couldn't > have designed an Internet architecture with nested addresses. Quite > obviously, that could have been done. In my view, this is already done. It works this way, although not engineered this way. The Internet has its own dynamics is the lesson I see in this. It routes around blocks ;-) > But it wasn't, and we have an > Internet that likes single, fixed-length addresses. NATs are at best > an ugly add-on in such a world. An alternative view is that we have an Internet that likes so much to work with heterogeneous networks that it now supports NATs even though NATs were not originally designed into it. > (My personal techo-religion preaches > that *all* successful systems run out of address space ;-) agreed, but only systems with finitary address space. > , and that you're > better off planning for it up front. I (among others) argued strongly > for IPv6 addresses of 8, 16, 24, or 32 bytes, precisely to plan ahead. > In fact, the penultimate design called for fixed-length, 8-byte > addresses. The swit
Re: NAT natural example, Re: [midcom] WG scope/deliverables
Steve Deering wrote:
> At 3:41 PM -0800 2/15/01, Ed Gerck wrote:
>
> > > >You give a name to your house (say, "The Tulip") and
> > > >the post office knows where The Tulip is. If you move,
> > > >you can do the same at your new location, provided
> > > >there is no conflict.> >
> >
> >...Note that this is a natural example of NAT,
> >in which the post office is doing the address translation to a local
> >address that only that post office knows, but which is globally
> >reachable through that post office. And the post office does so
> >without changing the global addresses or the local addresses.
>
> They also do it without removing the original destination address and
> replacing it with another one -- the original envelope arrives at the
> house with the destination address still saying "The Tulip", i.e., it
> has not been translated, and thus is not analogous to NAT.
I think you got the example addresses reversed. In the case I mention,
"The Tulip" is the global address and (for the sake of example) suppose
now that "545 Abbey St." is the local physical address known to the post office.
Thus, when the mailman delivers an envelope addressed to "The Tulip" at
"545 Abbey St.", that mailman is doing address translation -- and he may
even have written "545 Abbey St." on the envelope as a reminder. So,
when the original envelope arrives at the destination address it did so not
because it had "The Tulip" written on it but because the post office was
able to do address translation to the *current* location which is "545 Abbey St."
If another location is assigned to "The Tulip" (for example, because the owner
Mr. Tulip moved), the post office will deliver the original envelope there and
not at "545 Abbey St."
Note that the local address which only the post office (and Mr. Tulip) knows is
"545 Abbey St." while the global address is "The Tulip".
In Internet NAT terms, "The Tulip" is the globally routable IP number for my DSL,
the post office is my NAT box and the physical address "545 Abbey St." is the
local, non-routable IP number of my host A. For my other hosts, I simply tell
the NAT box (post office) what is the local IP number that will receive the next
packet for "The Tulip" -- my single global name. If now you add a mailbox number to
"The Tulip" you have the same functionality of port translation as well, where
different local addresses (for private mail, for example) will correspond to different
"n" in "The Tulip, PO Box n".
In other words, this is a natural NAT example and clearly supports the view that
NATs are naturally occuring solutions to provide for local flexibility (Mr. Tulip
can change residence at will and can have more than one recipient for private mail)
without decreasing global connectivity ("The Tulip" is always responsive).
Cheers,
Ed Gerck
Re: NAT natural example, Re: [midcom] WG scope/deliverables
In message <[EMAIL PROTECTED]>, Ed Gerck writes: > > >"Steven M. Bellovin" wrote: > >> In message <[EMAIL PROTECTED]>, Ed Gerck writes: >> >> > >> >Actually, in the UK you can do just what you wish ;-) >> >You give a name to your house (say, "The Tulip") and >> >the post office knows where The Tulip is. If you move, >> >you can do the same at your new location, provided >> >there is no conflict. This seems to be more similar to the >> >notion of using an IP number as a name -- but isn't this >> >why we need DNS? ;-) >> > >> >> And if you move from London to Belfast, this will still work? > >In the UK, as I said. I would think that other countries may have >a similar system. Note that this is a natural example of NAT, >in which the post office is doing the address translation to a local >address that only that post office knows, but which is globally >reachable through that post office. And the post office does so >without changing the global addresses or the local addresses. Last I checked, Belfast was in the UK, though I realize that some folks wish it were not so. But you missed my point -- as you note above, the house name is known to "that post office". In other words, there is hierarchy in the routing algorithm; it's not globablly known, or even known throughout the UK. The same is true of the Internet, and it's why IP addresses aren't portable. > >I don't want to be philosophical about this, but IMO this example >actually supports the view that NATs are naturally occuring solutions >to provide for local flexibility without decreasing global connectivity. >The Internet NAT is perhaps less an "invention" than a translation of >an age old mechanism that we see everywhere. We use the same >principle for nicknames in a school for example. > >IMO, it is thus artificial to try to block Internet NATs. Far better would be >to define their interoperation with other network components that we also >need to use, in each case. Block them? Not at all; I have no desire to do that. But we need to recognize that *with the current Internet architecture*, there are some inherent limitations. To use your analogy, suppose that senders sometimes wrote their house name on the letter enclosed in the envelope -- but they didn't include the post office name, so the recipient couldn't reply. Or imagine that the Post Office only kept track of house names when there was a recent outgoing letter. That's the reality of NAT today. Please pay careful attention to two things I did *not* say. I did *not* say that NATs were an irrational engineering choice in today's environment. In fact, they clearly are rational in some circumstances, despite their disadvantages. Second, I didn't say that one couldn't have designed an Internet architecture with nested addresses. Quite obviously, that could have been done. But it wasn't, and we have an Internet that likes single, fixed-length addresses. NATs are at best an ugly add-on in such a world. (My personal techo-religion preaches that *all* successful systems run out of address space, and that you're better off planning for it up front. I (among others) argued strongly for IPv6 addresses of 8, 16, 24, or 32 bytes, precisely to plan ahead. In fact, the penultimate design called for fixed-length, 8-byte addresses. The switch to 16 bytes was done to satisfy those of us who feared that that was not nearly enough.) --Steve Bellovin, http://www.research.att.com/~smb
Re: NAT natural example, Re: [midcom] WG scope/deliverables
At 3:41 PM -0800 2/15/01, Ed Gerck wrote: >"Steven M. Bellovin" wrote: > > >You give a name to your house (say, "The Tulip") and > > >the post office knows where The Tulip is. If you move, > > >you can do the same at your new location, provided > > >there is no conflict.> > > >...Note that this is a natural example of NAT, >in which the post office is doing the address translation to a local >address that only that post office knows, but which is globally >reachable through that post office. And the post office does so >without changing the global addresses or the local addresses. They also do it without removing the original destination address and replacing it with another one -- the original envelope arrives at the house with the destination address still saying "The Tulip", i.e., it has not been translated, and thus is not analogous to NAT. If delivery is accomplished by having all the necessary the UK post offices and postpersons remember a routing from "The Tulip" to its current street address, then its IP analog is having the routers within a site maintain a host route for a specific IP address. If, on the other hand, only the UK-entry post office maintains the mapping and sticks the original envelope inside another envelope (or puts a yellow sticky note over the original address), addressed to The Tulip's current street address, then its IP analog is having the border router maintain a tunnel to an individual interior host, encapsulating the original packet with another header. A closer postal analog to the typical port-and-address-mapping NAT is a system in which postal envelopes only have room for a street address or a town name, but not both. If I send a letter to someone outside my town, the letter starts off with a return address of: Steve Deering 123 Main Street and the town's post office overwrites that return address, changing it to: Priscilla Presley San Jose, CA, USA and they remember for a while that they did that, so that if my correspondent decides to reply to that return address, the town post office knows who it should be delivered to. (They replaced my name because someone else named Steve Deering recently sent mail from another street address in my town, and the only way to keep the replies separate is to change the name that I will be [temporarily] known by in the outside world.) At some point, they discard the remembered mapping, to free up some names. Perhaps they do that based on a time-out, in which case the mapping may disappear before we are finished corresponding, and thus cause our communication to fail. Or maybe they open up our letters and look at the contents to try to identify the final letter of our correspondence, to guess when we might be done. Of course that latter approach doesn't help if they don't understand what language our letters are written in, so maybe they decide to limit us to only a small choice of languages, and just discard anything they don't understand. Furthermore, no one outside my town can initiate a correspondence with me, unless I work out some arrangement with the post office to get long term external use of someone's (preferably my own) name. Or else I have to go and get a town name for myself. >I don't want to be philosophical about this, but IMO this example >actually supports the view that NATs are naturally occuring solutions >to provide for local flexibility without decreasing global connectivity. Since the example was not an example of a NAT, I don't think it supports any such view. However, I suppose a postal system like the one I described might "naturally occur" as a response to having envelopes that were no longer big enough to contain full addresses. But I think it much more likely that post offices and people would somehow arrange to just use bigger envelopes, rather than incurring all the extra complexity, cost, fragility, and loss of functionality of the translating approach, except as a temporary stop-gap. Unless, that is, we were talked out of it by folks claiming that changing the size of envelopes would be an impossibly large task, and that we're better off anyway with the translating system, because our personal names and street addresses can be kept secret within our town, and we can change the name of our town any time we like without bothering anybody in it. Steve
