Network Working Group                                      B. Carpenter
Request for Comments: 2775                                          IBM
Category: Informational                                   February 2000

                         Internet Transparency

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.


   This document describes the current state of the Internet from the
   architectural viewpoint, concentrating on issues of end-to-end
   connectivity and transparency. It concludes with a summary of some
   major architectural alternatives facing the Internet network layer.

   This document was used as input to the IAB workshop on the future of
   the network layer held in July 1999. For this reason, it does not
   claim to be complete and definitive, and it refrains from making

Table of Contents

   1. Introduction.................................................2
   2. Aspects of end-to-end connectivity...........................3
   2.1 The end-to-end argument.....................................3
   2.2 End-to-end performance......................................4
   2.3 End-to-end address transparency.............................4
   3. Multiple causes of loss of transparency......................5
   3.1 The Intranet model..........................................6
   3.2 Dynamic address allocation..................................6
   3.2.1 SLIP and PPP..............................................6
   3.2.2 DHCP......................................................6
   3.3 Firewalls...................................................6
   3.3.1 Basic firewalls...........................................6
   3.3.2 SOCKS.....................................................7
   3.4 Private addresses...........................................7
   3.5 Network address translators.................................7
   3.6 Application level gateways, relays, proxies, and caches.....8
   3.7 Voluntary isolation and peer networks.......................8

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   3.8 Split DNS...................................................9
   3.9 Various load-sharing tricks.................................9
   4. Summary of current status and impact.........................9
   5. Possible future directions..................................11
   5.1 Successful migration to IPv6...............................11
   5.2 Complete failure of IPv6...................................12
   5.2.1 Re-allocating the IPv4 address space.....................12
   5.2.2 Exhaustion...............................................13
   5.3 Partial deployment of IPv6.................................13
   6. Conclusion..................................................13
   7. Security Considerations.....................................13
   Author's Address...............................................17
   Full Copyright Statement.......................................18

1. Introduction

      "There's a freedom about the Internet: As long as we accept the
       rules of sending packets around, we can send packets containing
       anything to anywhere." [Berners-Lee]

   The Internet is experiencing growing pains which are often referred
   to as "the end-to-end problem". This document attempts to analyse
   those growing pains by reviewing the current state of the network
   layer, especially its progressive loss of transparency. For the
   purposes of this document, "transparency" refers to the original
   Internet concept of a single universal logical addressing scheme, and
   the mechanisms by which packets may flow from source to destination
   essentially unaltered.

   The causes of this loss of transparency are partly artefacts of
   parsimonious allocation of the limited address space available to
   IPv4, and partly the result of broader issues resulting from the
   widespread use of TCP/IP technology by businesses and consumers. For
   example, network address translation is an artefact, but Intranets
   are not.

   Thus the way forward must recognise the fundamental changes in the
   usage of TCP/IP that are driving current Internet growth. In one
   scenario, a complete migration to IPv6 potentially allows the
   restoration of global address transparency, but without removing
   firewalls and proxies from the picture. At the other extreme, a total
   failure of IPv6 leads to complete fragmentation of the network layer,
   with global connectivity depending on endless patchwork.

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   This document does not discuss the routing implications of address
   space, nor the implications of quality of service management on
   router state, although both these matters interact with transparency
   to some extent. It also does not substantively discuss namespace

2. Aspects of end-to-end connectivity

   The phrase "end to end", often abbreviated as "e2e", is widely used
   in architectural discussions of the Internet. For the purposes of
   this paper, we first present three distinct aspects of end-to-

2.1 The end-to-end argument

   This is an argument first described in [Saltzer] and reviewed in [RFC
   1958], from which an extended quotation follows:

      "The basic argument is that, as a first principle, certain
      required end-to-end functions can only be performed correctly by
      the end-systems themselves. A specific case is that any network,
      however carefully designed, will be subject to failures of
      transmission at some statistically determined rate. The best way
      to cope with this is to accept it, and give responsibility for the
      integrity of communication to the end systems. Another specific
      case is end-to-end security.

      "To quote from [Saltzer], 'The function in question can completely
      and correctly be implemented only with the knowledge and help of
      the application standing at the endpoints of the communication
      system.  Therefore, providing that questioned function as a
      feature of the communication system itself is not possible.
      (Sometimes an incomplete version of the function provided by the
      communication system may be useful as a performance enhancement.)'

      "This principle has important consequences if we require
      applications to survive partial network failures. An end-to-end
      protocol design should not rely on the maintenance of state (i.e.
      information about the state of the end-to-end communication)
      inside the network. Such state should be maintained only in the
      endpoints, in such a way that the state can only be destroyed when
      the endpoint itself breaks (known as fate-sharing). An immediate
      consequence of this is that datagrams are better than classical
      virtual circuits.  The network's job is to transmit datagrams as
      efficiently and flexibly as possible.  Everything else should be
      done at the fringes."

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   Thus this first aspect of end-to-endness limits what the network is
   expected to do, and clarifies what the end-system is expected to do.
   The end-to-end argument underlies the rest of this document.

2.2 End-to-end performance

   Another aspect, in which the behaviour of the network and that of the
   end-systems interact in a complex way, is performance, in a
   generalised sense. This is not a primary focus of the present
   document, but it is mentioned briefly since it is often referred to
   when discussing end-to-end issues.

   Much work has been done over many years to improve and optimise the
   performance of TCP. Interestingly, this has led to comparatively
   minor changes to TCP itself; [STD 7] is still valid apart from minor
   additions [RFC 1323, RFC 2581, RFC 2018]. However a great deal of
   knowledge about good practice in TCP implementations has built up,
   and the queuing and discard mechanisms in routers have been fine-
   tuned to improve system performance in congested conditions.

   Unfortunately all this experience in TCP performance does not help
   with transport protocols that do not exhibit TCP-like response to
   congestion [RFC 2309]. Also, the requirement for specified quality of
   service for different applications and/or customers has led to much
   new development, especially the Integrated Services [RFC 1633, RFC
   2210] and Differentiated Services [RFC 2475] models. At the same time
   new transport-related protocols have appeared [RFC 1889, RFC 2326] or
   are in discussion in the IETF. It should also be noted that since the
   speed of light is not set by an IETF standard, our current notions of
   end-to-end performance will be largely irrelevant to interplanetary

   Thus, despite the fact that performance and congestion issues for TCP
   are now quite well understood, the arrival of QOS mechanisms and of
   new transport protocols raise new questions about end-to-end
   performance, but these are not further discussed here.

2.3 End-to-end address transparency

   When the catenet concept (a network of networks) was first described
   by Cerf in 1978 [IEN 48] following an earlier suggestion by Pouzin in
   1974 [CATENET], a clear assumption was that a single logical address
   space would cover the whole catenet (or Internet as we now know it).
   This applied not only to the early TCP/IP Internet, but also to the
   Xerox PUP design, the OSI connectionless network design, XNS, and
   numerous other proprietary network architectures.

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   This concept had two clear consequences - packets could flow
   essentially unaltered throughout the network, and their source and
   destination addresses could be used as unique labels for the end

   The first of these consequences is not absolute.  In practice changes
   can be made to packets in transit. Some of these are reversible at
   the destination (such as fragmentation and compression). Others may
   be irreversible (such as changing type of service bits or
   decrementing a hop limit), but do not seriously obstruct the end-to-
   end principle of Section 2.1. However, any change made to a packet in
   transit that requires per-flow state information to be kept at an
   intermediate point would violate the fate-sharing aspect of the end-
   to-end principle.

   The second consequence, using addresses as unique labels, was in a
   sense a side-effect of the catenet concept. However, it was a side-
   effect that came to be highly significant. The uniqueness and
   durability of addresses have been exploited in many ways, in
   particular by incorporating them in transport identifiers.  Thus they
   have been built into transport checksums, cryptographic signatures,
   Web documents, and proprietary software licence servers. [RFC 2101]
   explores this topic in some detail. Its main conclusion is that IPv4
   addresses can no longer be assumed to be either globally unique or
   invariant, and any protocol or applications design that assumes these
   properties will fail unpredictably. Work in the IAB and the NAT
   working group [NAT-ARCH] has analysed the impact of one specific
   cause of non-uniqueness and non-invariance, i.e., network address
   translators. Again the conclusion is that many applications will
   fail, unless they are specifically adapted to avoid the assumption of
   address transparency. One form of adaptation is the insertion of some
   form of application level gateway, and another form is for the NAT to
   modify payloads on the fly, but in either case the adaptation is

   Non-transparency of addresses is part of a more general phenomenon.
   We have to recognise that the Internet has lost end-to-end
   transparency, and this requires further analysis.

3. Multiple causes of loss of transparency

   This section describes various recent inventions that have led to the
   loss of end-to-end transparency in the Internet.

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3.1 The Intranet model

   Underlying a number of the specific developments mentioned below is
   the concept of an "Intranet", loosely defined as a private corporate
   network using TCP/IP technology, and connected to the Internet at
   large in a carefully controlled manner. The Intranet is presumed to
   be used by corporate employees for business purposes, and to
   interconnect hosts that carry sensitive or confidential information.
   It is also held to a higher standard of operational availability than
   the Internet at large. Its usage can be monitored and controlled, and
   its resources can be better planned and tuned than those of the
   public network. These arguments of security and resource management
   have ensured the dominance of the Intranet model in most corporations
   and campuses.

   The emergence of the Intranet model has had a profound effect on the
   notion of application transparency. Many corporate network managers
   feel it is for them alone to determine which applications can
   traverse the Internet/Intranet boundary. In this world view, address
   transparency may seem to be an unimportant consideration.

3.2 Dynamic address allocation

3.2.1 SLIP and PPP

   It is to be noted that with the advent of vast numbers of dial-up
   Internet users, whose addresses are allocated at dial-up time, and
   whose traffic may be tunneled back to their home ISP, the actual IP
   addresses of such users are purely transient. During their period of
   validity they can be relied on end-to-end, but they must be forgotten
   at the end of every session. In particular they can have no permanent
   association with the domain name of the host borrowing them.

3.2.2 DHCP

   Similarly, LAN-based users of the Internet today frequently use DHCP
   to acquire a new address at system restart, so here again the actual
   value of the address is potentially transient and must not be stored
   between sessions.

3.3 Firewalls

3.3.1 Basic firewalls

   Intranet managers have a major concern about security: unauthorised
   traffic must be kept out of the Intranet at all costs. This concern
   led directly to the firewall concept (a system that intercepts all
   traffic between the Internet and the Intranet, and only lets through

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   selected traffic, usually belonging to a very limited set of
   applications). Firewalls, by their nature, fundamentally limit

3.3.2 SOCKS

   A footnote to the effect of firewalls is the SOCKS mechanism [RFC
   1928] by which untrusted applications such as telnet and ftp can
   punch through a firewall.  SOCKS requires a shim library in the
   Intranet client, and a server in the firewall which is essentially an
   application level relay. As a result, the remote server does not see
   the real client; it believes that the firewall is the client.

3.4 Private addresses

   When the threat of IPv4 address exhaustion first arose, and in some
   cases user sites were known to be "pirating" addresses for private
   use, a set of official private addresses were hurriedly allocated
   [RFC 1597] and later more carefully defined [BCP 5].  The legitimate
   existence of such an address allocation proved to very appealing, so
   Intranets with large numbers of non-global addresses came into
   existence. Unfortunately, such addresses by their nature cannot be
   used for communication across the public Internet; without special
   measures, hosts using private addresses are cut off from the world.

   Note that private address space is sometimes asserted to be a
   security feature, based on the notion that outside knowledge of
   internal addresses might help intruders. This is a false argument,
   since it is trivial to hide addresses by suitable access control
   lists, even if they are globally unique - indeed that is a basic
   feature of a filtering router, the simplest form of firewall. A
   system with a hidden address is just as private as a system with a
   private address.  There is of course no possible point in hiding the
   addresses of servers to which outside access is required.

   It is also worth noting that the IPv6 equivalent of private
   addresses, i.e. site-local addresses, have similar characteristics to
   BCP 5 addresses, but their use will not be forced by a lack of
   globally unique IPv6 addresses.

3.5 Network address translators

   Network address translators (NATs) are an almost inevitable
   consequence of the existence of Intranets using private addresses yet
   needing to communicate with the Internet at large. Their
   architectural implications are discussed at length in [NAT-ARCH], the
   fundamental point being that address translation on the fly destroys
   end-to-end address transparency and breaks any middleware or

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   applications that depend on it. Numerous protocols, for example
   H.323, carry IP addresses at application level and fail to traverse a
   simple NAT box correctly. If the full range of Internet applications
   is to be used, NATs have to be coupled with application level
   gateways (ALGs) or proxies. Furthermore, the ALG or proxy must be
   updated whenever a new address-dependent application comes along.  In
   practice, NAT functionality is built into many firewall products, and
   all useful NATs have associated ALGs, so it is difficult to
   disentangle their various impacts.

3.6 Application level gateways, relays, proxies, and caches

   It is reasonable to position application level gateways, relays,
   proxies, and caches at certain critical topological points,
   especially the Intranet/Internet boundary.  For example, if an
   Intranet does not use SMTP as its mail protocol, an SMTP gateway is
   needed. Even in the normal case, an SMTP relay is common, and can
   perform useful mail routing functions, spam filtering, etc. (It may
   be observed that spam filtering is in some ways a firewall function,
   but the store-and-forward nature of electronic mail and the
   availability of MX records allow it to be a distinct and separate

   Similarly, for a protocol such as HTTP with a well-defined voluntary
   proxy mechanism, application proxies also serving as caches are very
   useful. Although these devices interfere with transparency, they do
   so in a precise way, correctly terminating network, transport and
   application protocols on both sides. They can however exhibit some
   shortfalls in ease of configuration and failover.

   However, there appear to be cases of "involuntary" applications level
   devices such as proxies that grab and modify HTTP traffic without
   using the appropriate mechanisms, sometimes known as "transparent
   caches", or mail relays that purport to remove undesirable words.
   These devices are by definition not transparent, and may have totally
   unforeseeable side effects.  (A possible conclusion is that even for
   non-store-and-forward protocols, a generic diversion mechanism
   analogous to the MX record would be of benefit. The SRV record [RFC
   2052] is a step in this direction.)

3.7 Voluntary isolation and peer networks

   There are communities that think of themselves as being so different
   that they require isolation via an explicit proxy, and even by using
   proprietary protocols and addressing schemes within their network. An
   example is the WAP Forum which targets very small phone-like devices
   with some capabilities for Internet connectivity. However, it's not

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   the Internet they're connecting directly to. They have to go through
   a proxy. This could potentially mean that millions of devices will
   never know the benefits of end-to-end connectivity to the Internet.

   A similar effect arises when applications such as telephony span both
   an IP network and a peer network layer using a different technology.
   Although the application may work end-to-end, there is no possibility
   of end-to-end packet transmission.

3.8 Split DNS

   Another consequence of the Intranet/Internet split is "split DNS" or
   "two faced DNS", where a corporate network serves up partly or
   completely different DNS inside and outside its firewall. There are
   many possible variants on this; the basic point is that the
   correspondence between a given FQDN (fully qualified domain name) and
   a given IPv4 address is no longer universal and stable over long

3.9 Various load-sharing tricks

   IPv4 was not designed to support anycast [RFC 1546], so there is no
   natural approach to load-sharing when one server cannot do the job.
   Various tricks have been used to resolve this (multicast to find a
   free server, the DNS returns different addresses for the same FQDN in
   a round-robin, a router actually routes packets sent to the same
   address automatically to different servers, etc.). While these tricks
   are not particularly harmful in the overall picture, they can be
   implemented in such a way as to interfere with name or address

4. Summary of current status and impact

   It is impossible to estimate with any numerical reliability how
   widely the above inventions have been deployed. Since many of them
   preserve the illusion of transparency while actually interfering with
   it, they are extremely difficult to measure.

   However it is certain that all the mechanisms just described are in
   very widespread use; they are not a marginal phenomenon. In corporate
   networks, many of them are the norm. Some of them (firewalls, relays,
   proxies and caches) clearly have intrinsic value given the Intranet
   concept. The others are largely artefacts and pragmatic responses to
   various pressures in the operational Internet, and they must be
   costing the industry very dearly in constant administration and
   complex fault diagnosis.

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   In particular, the decline of transparency is having a severe effect
   on deployment of end-to-end IP security. The Internet security model
   [SECMECH] calls for security at several levels (roughly, network,
   applications, and object levels).  The current network level security
   model [RFC 2401] was constructed prior to the recognition that end-
   to-end address transparency was under severe threat.  Although
   alternative proposals have begun to emerge [HIP] the current reality
   is that IPSEC cannot be deployed end-to-end in the general case.
   Tunnel-mode IPSEC can be deployed between corporate gateways or
   firewalls.  Transport-mode IPSEC can be deployed within a corporate
   network in some cases, but it cannot span from Intranet to Internet
   and back to another Intranet if there is any chance of a NAT along
   the way.

   Indeed, NAT breaks other security mechanisms as well, such as DNSSEC
   and Kerberos, since they rely on address values.

   The loss of transparency brought about by private addresses and NATs
   affects many applications protocols to a greater or lesser extent.
   This is explored in detail in [NAT-PROT]. A more subtle effect is
   that the prevalence of dynamic addresses (from DHCP, SLIP and PPP)
   has fed upon the trend towards client/server computing.  Today it is
   largely true that servers have fixed addresses, clients have dynamic
   addresses, and servers can in no way assume that a client's IP
   address identifies the client. On the other hand, clients rely on
   servers having stable addresses since a DNS lookup is the only
   generally deployed mechanism by which a client can find a server and
   solicit service.  In this environment, there is little scope for true
   peer-to-peer applications protocols, and no easy solution for mobile
   servers. Indeed, the very limited demand for Mobile IP might be
   partly attributed to the market dominance of client/server
   applications in which the client's address is of transient
   significance. We also see a trend towards single points of failure
   such as media gateways, again resulting from the difficulty of
   implementing peer-to-peer solutions directly between clients with no
   fixed address.

   The notion that servers can use precious globally unique addresses
   from a small pool, because there will always be fewer servers than
   clients, may become anachronistic when most electrical devices become
   network-manageable and thus become servers for a management protocol.
   Similarly, if every PC becomes a telephone (or the converse), capable
   of receiving unsolicited incoming calls, the lack of stable IP
   addresses for PCs will be an issue. Another impending paradigm shift
   is when domestic and small-office subscribers move from dial-up to
   always-on Internet connectivity, at which point transient address
   assignment from a pool becomes much less appropriate.

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   Many of the inventions described in the previous section lead to the
   datagram traffic between two hosts being directly or indirectly
   mediated by at least one other host. For example a client may depend
   on a DHCP server, a server may depend on a NAT, and any host may
   depend on a firewall. This violates the fate-sharing principle of
   [Saltzer] and introduces single points of failure. Worse, most of
   these points of failure require configuration data, yet another
   source of operational risk. The original notion that datagrams would
   find their way around failures, especially around failed routers, has
   been lost; indeed the overloading of border routers with additional
   functions has turned them into critical rather than redundant
   components, even for multihomed sites.

   The loss of address transparency has other negative effects.  For
   example, large scale servers may use heuristics or even formal
   policies that assign different priorities to service for different
   clients, based on their addresses. As addresses lose their global
   meaning, this mechanism will fail. Similarly, any anti-spam or anti-
   spoofing techniques that rely on reverse DNS lookup of address values
   can be confused by translated addresses. (Uncoordinated renumbering
   can have similar disadvantages.)

   The above issues are not academic. They add up to complexity in
   applications design, complexity in network configuration, complexity
   in security mechanisms, and complexity in network management.
   Specifically, they make fault diagnosis much harder, and by
   introducing more single points of failure, they make faults more
   likely to occur.

5. Possible future directions

5.1 Successful migration to IPv6

   In this scenario, IPv6 becomes fully implemented on all hosts and
   routers, including the adaptation of middleware, applications, and
   management systems. Since the address space then becomes big enough
   for all conceivable needs, address transparency can be restored.
   Transport-mode IPSEC can in principle deploy, given adequate security
   policy tools and a key infrastructure.  However, it is widely
   believed that the Intranet/firewall model will certainly persist.

   Note that it is a basic assumption of IPv6 that no artificial
   constraints will be placed on the supply of addresses, given that
   there are so many of them. Current practices by which some ISPs
   strongly limit the number of IPv4 addresses per client will have no
   reason to exist for IPv6. (However, addresses will still be assigned
   prudently, according to guidelines designed to favour hierarchical

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   Clearly this is in any case a very long term scenario, since it
   assumes that IPv4 has declined to the point where IPv6 is required
   for universal connectivity. Thus, a viable version of Scenario 5.3 is
   a prerequisite for Scenario 5.1.

5.2 Complete failure of IPv6

   In this scenario, IPv6 fails to reach any significant level of
   operational deployment, IPv4 addressing is the only available
   mechanism, and address transparency cannot be restored. IPSEC cannot
   be deployed globally in its current form. In the very long term, the
   pool of globally unique IPv4 addresses will be nearly totally
   allocated, and new addresses will generally not be available for any

   It is unclear exactly what is likely to happen if the Internet
   continues to rely exclusively on IPv4, because in that eventuality a
   variety of schemes are likely to promulgated, with a view toward
   providing an acceptable evolutionary path for the network. However,
   we can examine two of the more simplistic sub-scenarios which are
   possible, while realising that the future would be unlikely to match
   either one exactly:

5.2.1 Re-allocating the IPv4 address space

   Suppose that a mechanism is created to continuously recover and re-
   allocate IPv4 addresses. A single global address space is maintained,
   with all sites progressively moving to an Intranet private address
   model, with global addresses being assigned temporarily from a pool
   of several billion. A sub-sub-scenario of this is generalised use of NAT and NAPT
           (NAT with port number translation). This has the disadvantage
           that the host is unaware of the unique address being used for
           its traffic, being aware only of its ambiguous private
           address, with all the problems that this generates. See
           [NAT-ARCH]. Another sub-sub-scenario is the use of realm-specific IP
           addressing implemented at the host rather than by a NAT box.
           See [RSIP]. In this case the host is aware of its unique
           address, allowing for substantial restoration of the end-to-
           end usefulness of addresses, e.g. for TCP or cryptographic

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RFC 2775                 Internet Transparency             February 2000 A final sub-sub-scenario is the "map and encapsulate" model
           in which address translation is replaced by systematic
           encapsulation of all packets for wide-area transport.  This
           model has never been fully developed, although it is
           completely compatible with end-to-end addressing.

5.2.2 Exhaustion

   Suppose that no mechanism is created to recover addresses (except
   perhaps black or open market trading). Sites with large address
   blocks keep them.  All the scenarios of 5.2.1 appear but are
   insufficient.  Eventually the global address space is no longer
   adequate.  This is a nightmare scenario - NATs appear within the
   "global" address space, for example at ISP-ISP boundaries. It is
   unclear how a global routing system and a global DNS system can be
   maintained; the Internet is permanently fragmented.

5.3 Partial deployment of IPv6

   In this scenario, IPv6 is completely implemented but only deploys in
   certain segments of the network (e.g. in certain countries, or in
   certain areas of application such as mobile hand-held devices).
   Instead of being transitional in nature, some of the IPv6 transition
   techniques become permanent features of the landscape. Sometimes
   addresses are 32 bits, sometimes they are 128 bits. DNS lookups may
   return either or both. In the 32 bit world, the scenarios 5.2.1 and
   5.2.2 may occur. IPSEC can only deploy partially.

6. Conclusion

   None of the above scenarios is clean, simple and straightforward.
   Although the pure IPv6 scenario is the cleanest and simplest, it is
   not straightforward to reach it. The various scenarios without use of
   IPv6 are all messy and ultimately seem to lead to dead ends of one
   kind or another. Partial deployment of IPv6, which is a required step
   on the road to full deployment, is also messy but avoids the dead

7. Security Considerations

   The loss of transparency is both a bug and a feature from the
   security viewpoint. To the extent that it prevents the end-to-end
   deployment of IPSEC, it damages security and creates vulnerabilities.
   For example, if a standard NAT box is in the path, then the best that
   can be done is to decrypt and re-encrypt IP traffic in the NAT.  The
   traffic will therefore be momentarily in clear text and potentially
   vulnerable. Furthermore, the NAT will possess many keys and will be a
   prime target of attack.  In environments where this is unacceptable,

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   encryption must be applied above the network layer instead, of course
   with no usage whatever of IP addresses in data that is
   cryptographically protected. See section 4 for further discussion.

   In certain scenarios, i.e. 5.1 (full IPv6) and (RSIP), end-
   to-end IPSEC would become possible, especially using the "distributed
   firewalls" model advocated in [Bellovin].

   The loss of transparency at the Intranet/Internet boundary may be
   considered a security feature, since it provides a well defined point
   at which to apply restrictions. This form of security is subject to
   the "crunchy outside, soft inside" risk, whereby any successful
   penetration of the boundary exposes the entire Intranet to trivial
   attack. The lack of end-to-end security applied within the Intranet
   also ignores insider threats.

   It should be noted that security applied above the transport level,
   such as SSL, SSH, PGP or S/MIME, is not affected by network layer
   transparency issues.


   This document and the related issues have been discussed extensively
   by the IAB. Special thanks to Steve Deering for a detailed review and
   to Noel Chiappa. Useful comments or ideas were also received from Rob
   Austein, John Bartas, Jim Bound, Scott Bradner, Vint Cerf, Spencer
   Dawkins, Anoop Ghanwani, Erik Guttmann, Eric A. Hall, Graham Klyne,
   Dan Kohn, Gabriel Montenegro, Thomas Narten, Erik Nordmark, Vern
   Paxson, Michael Quinlan, Eric Rosen, Daniel Senie, Henning
   Schulzrinne, Bill Sommerfeld, and George Tsirtsis.


   [Bellovin]    Distributed Firewalls, S. Bellovin, available at
                 http://www.research.att.com/~smb/papers/distfw.pdf or
                 http://www.research.att.com/~smb/papers/distfw.ps (work
                 in progress).

   [Berners-Lee] Weaving the Web, T. Berners-Lee, M. Fischetti,
                 HarperCollins, 1999, p 208.

   [Saltzer]     End-To-End Arguments in System Design, J.H. Saltzer,
                 D.P.Reed, D.D.Clark, ACM TOCS, Vol 2, Number 4,
                 November 1984, pp 277-288.

   [IEN 48]      Cerf, V., "The Catenet Model for Internetworking,"
                 Information Processing Techniques Office, Defense
                 Advanced Research Projects Agency, IEN 48, July 1978.

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RFC 2775                 Internet Transparency             February 2000

   [CATENET]     Pouzin, L., "A Proposal for Interconnecting Packet
                 Switching Networks," Proceedings of EUROCOMP, Brunel
                 University, May 1974, pp. 1023-36.

   [STD 7]       Postel, J., "Transmission Control Protocol", STD 7, RFC
                 793, September 1981.

   [RFC 1546]    Partridge, C., Mendez, T. and  W. Milliken,  "Host
                 Anycasting Service", RFC 1546, November 1993.

   [RFC 1597]    Rekhter, Y., Moskowitz, B., Karrenberg, D. and G. de
                 Groot, "Address Allocation for Private Internets", RFC
                 1597, March 1994.

   [RFC 1633]    Braden, R., Clark, D. and S. Shenker, "Integrated
                 Services in the Internet Architecture: an Overview",
                 RFC 1633, June 1994.

   [RFC 1889]    Schulzrinne, H., Casner, S., Frederick, R. and V.
                 Jacobson, "RTP: A Transport Protocol for Real-Time
                 Applications", RFC 1889, January 1996.

   [BCP 5]       Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot,
                 G.  and E. Lear, "Address Allocation for Private
                 Internets", BCP 5, RFC 1918, February 1996.

   [RFC 1928]    Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D.
                 and L. Jones, "SOCKS Protocol Version 5", RFC 1928,
                 March 1996.

   [RFC 1958]    Carpenter, B., "Architectural Principles of the
                 Internet", RFC 1958, June 1996.

   [RFC 2018]    Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
                 Selective Acknowledgement Options", RFC 2018, October

   [RFC 2052]    Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying
                 the location of services (DNS SRV)", RFC 2052, October

   [RFC 2101]    Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4
                 Address Behaviour Today", RFC 2101, February 1997.

   [RFC 2210]    Wroclawski, J., "The Use of RSVP with IETF Integrated
                 Services", RFC 2210, September 1997.

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RFC 2775                 Internet Transparency             February 2000

   [RFC 2309]    Braden, B., Clark, D., Crowcroft, J., Davie, B.,
                 Deering, S., Estrin, D., Floyd, S., Jacobson, V.,
                 Minshall, G., Partridge, C., Peterson, L.,
                 Ramakrishnan, K., Shenker, S., Wroclawski, J. and L.
                 Zhang, "Recommendations on Queue Management and
                 Congestion Avoidance in the Internet", RFC 2309, April

   [RFC 2326]    Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time
                 Streaming Protocol (RTSP)", RFC 2326, April 1998.

   [RFC 2401]    Kent, S. and R. Atkinson, "Security Architecture for
                 the Internet Protocol", RFC 2401, November 1998.

   [RFC 2475]    Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
                 and W. Weiss, "An Architecture for Differentiated
                 Service", RFC 2475, December 1998.

   [RFC 2581]    Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
                 Control", RFC 2581, April 1999.

   [NAT-ARCH]    Hain, T., "Architectural Implications of NAT", Work in

   [NAT-PROT]    Holdrege, M. and P. Srisuresh, "Protocol Complications
                 with the IP Network Address Translator (NAT)", Work in

   [SECMECH]     Bellovin, S., "Security Mechanisms for the Internet",
                 Work in Progress.

   [RSIP]        Lo, J., Borella, M. and D. Grabelsky, "Realm Specific
                 IP: A Framework", Work in Progress.

   [HIP]         Moskowitz, R., "The Host Identity Payload", Work in

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RFC 2775                 Internet Transparency             February 2000

Author's Address

   Brian E. Carpenter
   c/o iCAIR
   Suite 150
   1890 Maple Avenue
   Evanston, IL 60201

   EMail: brian@icair.org

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RFC 2775                 Internet Transparency             February 2000

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