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rfc:rfc1681

Network Working Group S. Bellovin Request for Comments: 1681 AT&T Bell Laboratories Category: Informational August 1994

                     On Many Addresses per Host

Status of this Memo

 This memo provides information for the Internet community.  This memo
 does not specify an Internet standard of any kind.  Distribution of
 this memo is unlimited.

Abstract

 This document was submitted to the IETF IPng area in response to RFC
 1550.  Publication of this document does not imply acceptance by the
 IPng area of any ideas expressed within.  Comments should be
 submitted to the big-internet@munnari.oz.au mailing list.

Overview and Rational

 Currently, most hosts have only one address.  With comparatively rare
 exceptions, hosts as hosts -- as opposed to hosts acting as routers
 or PPP servers -- are single-homed.  Our address space calculations
 reflect this; we are assuming that we can estimate the size of the
 address space by counting hosts.  But this may be a serious error.  I
 suggest that that model may -- and should -- change.
 For the ideas outlined below, I do not claim that multiple addresses
 per host is the only or even necessarily the best way to accomplish
 the goal.  I do claim that my ideas are at the very least plausible,
 and that I expect that many of them will be tried.

Encoding Services

 More and more often, services are being encoded in the host name.
 One can fetch files from ftp.research.att.com, look up an IP address
 on ns.uu.net, synchronize clocks from ntp.udel.edu, etc.  Should this
 practice be generalized to the IP address domain?
 In some cases it would be a very good idea.  Certain services need to
 be configured by IP address; they are either used when the DNS is
 being bootstrapped (such as in glue records and root server cache
 records), or when its unavailable (i.e., when booting after a power
 hit, and the local name servers are slower to reboot than their
 diskless clients.

Bellovin [Page 1] RFC 1681 On Many Addresses per Host August 1994

 Security is another reason, in some cases.  Address-based
 authentication is bad enough; relying on the name service adds
 another layer of risk.  An attacker can go after the DNS, in that
 case.  A risk-averse system manager might prefer to avoid the extra
 exposure, instead granting privileges (i.e., rlogin or NFS) by
 address instead of name.  But that, of course, leads to all the usual
 headaches when the location of the service changes.  If the address
 for the service could be held constant, there would be much more
 freedom to move it to another machine.  One way to do that is by
 assigning the serving host a secondary address.
 A related notion comes from the need to offer different views of a
 service from a single host.  For example, research.att.com has long
 offered two distinct FTP archives, with slightly different access
 policies.  It would be nice if both could live on the same machine,
 without asking the user community to learn new protocols or custom
 port numbers.
 Archie is an even better example.  There are three principal ways to
 use Archie:  use a special protocol, and hence a special application
 program, on a dedicated port and host that is probably named
 archie.foo.bar; telnet to archie.foo.bar and go through an extra and
 gratuitous login as archie, or telnet to some special port on
 archie.foo.bar.  The latter two are examples of using a standard
 protocol (telnet) to offer a different service.  Neither alternative
 is very convenient.
 It would be better if archie.foo.bar provided the Archie service,
 while host.foo.bar provided a login prompt.  Again -- an easy way to
 do this is to assign the host a separate IP address for its extra
 service.
 Note that there are security advantages here, too.  A firewall could
 be configured to allow access to the address associated with the
 Archie server, but not the other addresses on that host.  That would
 provide a high degree of safety, assuming, of course, that the other
 servers on that host were bound to its primary addresses, and not the
 exposed address.
 Another way to implement this concept would be to extend the DNS, to
 return port number information as well as IP addresses.  Thus,
 netlib.att.com might return 192.20.225.3/221.  But that would
 necessitate changing every FTP client program, a daunting task.
 We could also look on this as the extension of the MX concept.  MX
 records are very valuable, but they apply only to mail, and they
 don't supply port numbers.  Again, changing this would require
 massive client program changes.

Bellovin [Page 2] RFC 1681 On Many Addresses per Host August 1994

Accounting and Billing

 For better or worse, some parts of the Internet are moving towards
 usage-sensitive charging.  At least four charging schemes seem
 possible; doubtless, the marketeers in charge of such things can and
 will come up with more.
 The first is the traditional "pay as you go" approach.  Each host is
 responsible for its own packets.  Of course, that means that in a
 typical conversation, both parties pay -- and the providers of free
 FTP archives will end up paying dearly for their beneficence.  That
 leads to our second model:  caller pays.  Other people might want to
 make collect calls, much as is done on the telephone today.  Finally,
 there might be the equivalent of American "900" numbers:  the caller
 pays a premium to the server.
 This is not at all far-fetched; UUNET already has a 900 number for
 anonymous uucp clients.  No need to register in advance; just dial
 in, and let the phone company act as your agent.
 Given all these schemes, it is vital that the caller and recipient
 know in advance who will pay.  It is not acceptable for users to
 learn, only after the fact, that they have incurred a cost.  We could
 envision use of IP options, but again, that would preclude use of
 today's standard clients.
 It is not sufficient to present a message at connection time warning
 of the charges.  Many interactions do not provide a hook for user
 interaction.  And there are security concerns -- suppose that someone
 puts up a gopher server that redirects a caller to some pay-to-play
 address, without displaying the required warning.  A scam?  Sure --
 but it's already happened with the phone network, and I see no reason
 to think that the Internet will be far behind.
 My suggestion, of course, is to encode the charge algorithm in the
 destination address (and perhaps in the DNS name space as well).  The
 bits themselves would determine who pays.  Organizational border
 routers could implement policies on pay services; the anonymous
 workstations in a dorm computer lab wouldn't be allowed to call
 collect.
 An extension of this scheme would use a comparatively large number of
 bits, letting the address act not just as a policy indicator, but
 also as an index to a charge algorithm table.

Bellovin [Page 3] RFC 1681 On Many Addresses per Host August 1994

Addresses per User

 It may be useful to assign each user on a host a separate IP address,
 for the duration of the login session.  This has a number of
 advantages.
 The first ties in with the charging scheme given above.  Usage-
 sensitive accounting today is done by routers, and they have no
 notion of who is using the hosts.  If each user had a separate IP
 address, we could continue to gather the accounting data at the
 router.  The host would simply have to record the address
 assignments; billing could be done offline.
 Similarly, different classes of users could have different forms of
 addresses.  Those with hard-money accounts might have some bits set
 in the address that would allow for access to costly services.  The
 border routers could make this sort of distinction, using today's
 technology.
 An IP address per user also fits in well with encryption.  There is a
 lot of attention today focused on network-layer encryption.  But that
 provides host-level granularity of protection, which is sometimes
 insufficient.  Transport-layer encryptors provide finer-grained
 protection, but does the Internet need two different low-level
 encryption schemes?  If each user had a separate IP address -- and
 perhaps had it only on hosts that cared about such matters -- we
 could provide user-level protection and accounability, with the same
 infrastructure used to support host-level accountability.

Low-Grade Mobility

 There are several schemes under discussion for mobile IP hosts.
 These are aimed at a fairly general model of hosts moving anywhere.
 While that is important, there is also some need for limited
 mobility, within a subnet.  This could be used for load-balancing.  A
 mail relay that had just been asked to send a large message to a huge
 mailing list could offload some of its IP addresses to its peers.
 That would divert future incoming messages without invalidating
 thousands of cached MX records and their associated IP addresses.
 Similarly, servers for low-speed X terminals could reside on
 different physical machines, all the while not disturbing sessions in
 progress.

Merging Subnets

 There has long been some need to merge subnets.  Sometimes this is
 due to organizational changes; other times, people have installed
 bridges when routers would have been a more appropriate choice.  Some

Bellovin [Page 4] RFC 1681 On Many Addresses per Host August 1994

 hosts need to live on both logical networks at once, to avoid an
 extra hop through a router.  It would be useful to be able to assign
 them such addresses.

How Many Addresses Do We Need?

 Assuming that some of these ideas bear fruit, how many addresses do
 we need, per host?
 Most of these schemes are fairly cheap.  Few people would offer more
 than a handful of distinct service views per system.  But the
 address-per-user notion could be quite costly.  We also have to
 account for address mask assignment policies.  In many of today's
 networks, enough bits of host address have to be allocated to allow
 for the largest subnet in an organization.  Even if we assume that
 IPng's routing protocols will be smarter about such things, foresight
 in address allocation will be needed to allow headroom for some
 networks to grow, while still maintaining a contiguous netmask.  This
 in turn will contribute to sparse utilization of the address space.
 Accordingly, I recommend that we allow for 2^6, and perhaps as many
 as 2^8, extra addresses per host, to leave room for the ideas
 presented here.
 I should note that the idea of encoding the service in the transport
 address bears some relation to OSI's model.  That similarity should
 not, of course, invalidate the idea.

Acknowledgements

 Some of these ideas were derived from conversations with Matt Blaze.

Security Considerations

 Security issues are discussed throughout this memo.

Author's Address

 Steven M. Bellovin
 Software Engineering Research Department
 AT&T Bell Laboratories
 600 Mountain Avenue
 Murray Hill, NJ  07974, USA
 Phone: +1 908-582-5886
 Fax: +1 908-582-3063
 EMail:  smb@research.att.com

Bellovin [Page 5]

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