[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-thaler-ip-model-evolution) 00 01 02 03 04 RFC 6250

Network Working Group                                          D. Thaler
Internet-Draft                                                       IAB
Expires: May 7, 2009                                    November 3, 2008


                       Evolution of the IP Model
                  draft-iab-ip-model-evolution-01.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on May 7, 2009.

Abstract

   This draft attempts to document various aspects of the IP service
   model and how it has evolved over time.  In particular, it attempts
   to document the properties of the IP layer as they are seen by upper-
   layer protocols and applications, and especially properties which
   were (and at times still are) incorrectly perceived to exist, as well
   as properties that would cause problems if changed.  Finally, it
   provides some guidance to protocol designers and implementers.









Thaler                     Expires May 7, 2009                  [Page 1]


Internet-Draft          Evolution of the IP Model          November 2008


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  The IP Service Model . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Links and Subnets  . . . . . . . . . . . . . . . . . . . .  5
   3.  Common Application Assumptions . . . . . . . . . . . . . . . .  6
     3.1.  Assumptions about routing  . . . . . . . . . . . . . . . .  6
       3.1.1.  Reachability is symmetric  . . . . . . . . . . . . . .  6
       3.1.2.  Reachability is transitive . . . . . . . . . . . . . .  6
       3.1.3.  Multicast is supported within a link . . . . . . . . .  7
       3.1.4.  IPv4 broadcast is supported  . . . . . . . . . . . . .  7
       3.1.5.  Multicast/broadcast is less expensive than
               replicated unicast . . . . . . . . . . . . . . . . . .  8
       3.1.6.  The end-to-end latency of the first packet to a
               destination is typical . . . . . . . . . . . . . . . .  8
       3.1.7.  Reordering is rare . . . . . . . . . . . . . . . . . .  9
       3.1.8.  Loss is rare and probabilistic, not deterministic  . .  9
       3.1.9.  An end-to-end path exists at a single point in time  . 10
       3.1.10. Discussion . . . . . . . . . . . . . . . . . . . . . . 10
     3.2.  Assumptions about addressing . . . . . . . . . . . . . . . 11
       3.2.1.  Addresses are stable over long periods of time . . . . 11
       3.2.2.  A host has only one address on one interface . . . . . 11
       3.2.3.  A non-multicast/broadcast address identifies a
               single host over a long period of time . . . . . . . . 12
       3.2.4.  An address used by an application is the same as
               the address used for routing . . . . . . . . . . . . . 12
       3.2.5.  A subnet is smaller than a link  . . . . . . . . . . . 13
       3.2.6.  Selecting a local address selects the interface  . . . 13
       3.2.7.  An address is part of an on-link subnet  . . . . . . . 14
       3.2.8.  Discussion . . . . . . . . . . . . . . . . . . . . . . 14
     3.3.  Assumptions about upper-layer extensibility  . . . . . . . 14
       3.3.1.  New transport-layer protocols can work across the
               Internet . . . . . . . . . . . . . . . . . . . . . . . 14
       3.3.2.  If one stream between a pair of addresses can get
               through, then so can another . . . . . . . . . . . . . 14
       3.3.3.  Discussion . . . . . . . . . . . . . . . . . . . . . . 15
     3.4.  Assumptions about security . . . . . . . . . . . . . . . . 15
       3.4.1.  Packets are unmodified in transit  . . . . . . . . . . 15
       3.4.2.  Packets are private  . . . . . . . . . . . . . . . . . 16
       3.4.3.  Source addresses are not forged  . . . . . . . . . . . 16
       3.4.4.  Discussion . . . . . . . . . . . . . . . . . . . . . . 16
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   6.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   8.  IAB Members at the time of this writing  . . . . . . . . . . . 18
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 18



Thaler                     Expires May 7, 2009                  [Page 2]


Internet-Draft          Evolution of the IP Model          November 2008


     9.2.  Informative References . . . . . . . . . . . . . . . . . . 19
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22
   Intellectual Property and Copyright Statements . . . . . . . . . . 23
















































Thaler                     Expires May 7, 2009                  [Page 3]


Internet-Draft          Evolution of the IP Model          November 2008


1.  Introduction

   Since the Internet Protocol was first published as [IEN028] in 1978,
   IP has provided a network-layer connectivity service to upper-layer
   protocols and applications.  The basic IP service model was
   documented in the original IEN's (and subsequently in the RFC's that
   obsolete them).  However, since the mantra has been "Everything Over
   IP", the IP service model has evolved significantly over the past 30
   years to enable new behaviors that the original definition did not
   envision.  For example, by 1989 there was already some confusion and
   so [RFC1122] clarified many things and extended the model.  In 2004,
   [RFC3819] gave advice to link-layer protocol designers on a number of
   issues that affect upper layers and is the closest in intent to this
   document.  Today's IP service model is not well documented in a
   single place, but is either implicit or discussed piecemeal in many
   different RFCs.  As a result, today's IP service model is actually
   not well known, or at least is often misunderstood.

   In the early days of IP, changing or extending the basic IP service
   model was easier since it was not as widely deployed and there were
   fewer implementations.  Today, the ossification of the Internet makes
   evolving the IP model even more difficult.  Thus it is important to
   understand the evolution of the IP model for two reasons:

   1.  To make it clear what upper-layer protocols and applications can
       and cannot depend on.  There are many myths (or at least beliefs
       that are no longer true) on which applications may be based, and
       which are problematic.
   2.  To document lessons for future evolution to take into account.
       It is important that the service model remain consistent, rather
       than evolving in two opposing directions.  It is sometimes the
       case in IETF Working Groups today that directions are considered
       or even taken which would change the IP service model.  Doing
       this without understanding the implications on applications can
       be dangerous.

   This draft attempts to document various aspects of the IP service
   model and how it has evolved over time.  In particular, it attempts
   to document the properties of the IP layer, as seen by upper-layer
   protocols and applications, especially properties that were (and at
   times still are) incorrectly perceived to exist.  It also highlights
   properties which would cause problems if changed.


2.  The IP Service Model

   In this document, we use the term "IP Service Model" to refer to the
   model exposed by IP to higher-layer protocols and applications.  This



Thaler                     Expires May 7, 2009                  [Page 4]


Internet-Draft          Evolution of the IP Model          November 2008


   is depicted in Figure 1 by the horizontal line.

    +-------------+                                  +-------------+
    | Application |                                  | Application |
    +------+------+                                  +------+------+
           |                                                |
    +------+------+                                  +------+------+
    | Upper-Layer |                                  | Upper-Layer |
    |  Protocol   |                                  |  Protocol   |
    +------+------+                                  +------+------+
           |                                                |
   ------------------------------------------------------------------
           |                                                |
        +--+--+                  +-----+                 +--+--+
        | IP  |                  | IP  |                 | IP  |
        +--+--+                  +--+--+                 +--+--+
           |                        |                       |
     +-----+------+           +-----+------+          +-----+------+
     | Link Layer |           | Link Layer |          | Link Layer |
     +-----+------+           +--+-----+---+          +-----+------+
           |                     |     |                    |
           +---------------------+     +--------------------+

         Source                                        Destination

                             IP Service Model

                                 Figure 1

   The foundation of the IP service model today is documented in
   [RFC0791] section 2.2.  Generally speaking, IP provides a
   connectionless delivery service for variable size packets, which does
   not guarantee ordering, delivery, or lack of duplication, but is
   merely best effort (although some packets may get better service than
   others).  Senders can send to a destination address without signaling
   a priori, and receivers just listen on an already provisioned
   address, without signaling a priori.

   Architectural principles of the IP model are further discussed in
   [RFC1958] and in [NEWARCH] sections 5 and 6.

2.1.  Links and Subnets

   Section 2.1 of [RFC4903] discusses the terms "link" and "subnet" with
   respect to the IP model.

   A "link" in the IP service model refers to the topological area
   within which a packet with an IPv4 TTL or IPv6 Hop Limit of 1 can be



Thaler                     Expires May 7, 2009                  [Page 5]


Internet-Draft          Evolution of the IP Model          November 2008


   delivered.  That is, where no IP-layer forwarding (which entails a
   TTL/Hop Limit decrement) occurs between two nodes.

   A "subnet" in the IP service model refers to the topological area
   within which addresses from the same subnet prefix are assigned to
   interfaces.


3.  Common Application Assumptions

   Below is a list of properties which are often assumed by applications
   and upper-layer protocol, but which have become less true over time.

3.1.  Assumptions about routing

3.1.1.  Reachability is symmetric

   Many applications assume that if a host A can contact a host B, then
   the reverse is also true.  Examples of this behavior include request-
   response patterns, which require reverse reachability only after
   forward reachability, as well as callbacks (e.g., as used by the File
   Transfer Protocol (FTP) [RFC0959]).

   Originally it was the case that reachability was symmetric (although
   the path taken may not be), both within a link and across the
   Internet.  With the advent of technologies such as Network Address
   Translators (NATs) and firewalls, this can no longer be assumed.
   However, it is still the case that if a request can be sent, then a
   reply to that request can generally be received, but an unsolicited
   request in the other direction may not be received.  [RFC2993]
   discusses this in more detail.

   There are also links (e.g., satellite) that were defined as
   unidirectional links and hence an address on such a link results in
   asymmetric reachability.  [RFC3077] explicitly addresses this problem
   for multi-homed hosts by tunneling packets over another interface in
   order to restore symmetric reachability.

   Finally, even with common wireless networks such as 802.11, this
   assumption may not be true, as discussed in [WIRELESS] section 5.5.

3.1.2.  Reachability is transitive

   Many applications assume that if a host A can contact host B, and B
   can contact C, then host A can contact C. Examples of this behavior
   include applications and protocols that use referrals.

   Originally it was the case that reachability was transitive, both



Thaler                     Expires May 7, 2009                  [Page 6]


Internet-Draft          Evolution of the IP Model          November 2008


   within a link and across the Internet.  With the advent of
   technologies such as NATs and firewalls, this can no longer be
   assumed across the Internet, but it is often still true within a
   link.  As a result, upper-layer protocols and applications may be
   relying on transitivity within a link.  However, some radio
   technologies, such as 802.11 ad-hoc mode, violate this assumption
   within a link.

3.1.3.  Multicast is supported within a link

   [RFC1112] introduced multicast to the IP service model.  In this
   evolution, senders still just send to a destination address without
   signaling a priori, but in contrast to the original IP model,
   receivers must signal to the network before they can receive traffic
   to a multicast address.

   Today, many applications and protocols use multicast addresses,
   including protocols for address configuration, service discovery,
   etc.  (See [MCAST4] and [MCAST6] for those that use well-known
   addresses.)

   Most of these only assume that multicast works within a link, and may
   or may not function across a wider area.  While network-layer
   multicast works over most link types, there are Non-Broadcast Multi-
   Access (NBMA) links over which multicast does not work (e.g., X.25,
   ATM, frame relay, 6to4, ISATAP, Teredo) and this can interfere with
   some protocols and applications.  Similarly, there are links such as
   802.11 ad-hoc mode where multicast packets may not get delivered to
   all receivers on the link.  [RFC2461] and its successor [RFC4861]
   both state:
      "Note that all link types (including NBMA) are expected to provide
      multicast service for applications that need it (e.g., using
      multicast servers)."

   However, not all link types today meet this expectation.

3.1.4.  IPv4 broadcast is supported

   IPv4 broadcast support was originally defined on a link, across a
   network, and for subnet directed broadcast, and is used by many
   applications and protocols.  For security reasons, however, [RFC2644]
   deprecated forwarding of broadcast packets.  Thus, since 1999,
   broadcast can only be relied on within a link.  Still, there exist
   NBMA links over which broadcast does not work, and there exist some
   "semi-broadcast" links (e.g., 802.11 ad-hoc mode) where broadcast
   packets may not get delivered to all nodes on the link.  Another case
   where broadcast fails to work is when a /32 or /31 is assigned to a
   point-to-point interface (e.g., [RFC3021]), leaving no broadcast



Thaler                     Expires May 7, 2009                  [Page 7]


Internet-Draft          Evolution of the IP Model          November 2008


   address available.

   The addition of link-scoped multicast to the IP service model to a
   large extent obsoleted the need for broadcast.  It is also worth
   noting that the broadcast API model used by most platforms allows
   receivers to just listen on an already provisioned address, without
   signaling a priori, but in contrast to the unicast API model, senders
   must signal to the local IP stack (SO_BROADCAST) before they can send
   traffic to a broadcast address.  However, from the network's
   perspective, the host still sends without signaling a priori.

3.1.5.  Multicast/broadcast is less expensive than replicated unicast

   Some applications and upper-layer protocols that use multicast or
   broadcast do so not because they do not know the addresses of
   receivers, but simply to avoid sending multiple copies of the same
   packet over the same link.

   In wired networks, sending a single multicast packet on a link is
   generally less expensive than sending multiple unicast packets.  This
   may not be true for wireless networks, where implementations can only
   send multicast at the basic rate, regardless of the negotiated rates
   of potential receivers.  As a result, replicated unicast may achieve
   much higher throughput across such links than multicast/broadcast
   traffic.

3.1.6.  The end-to-end latency of the first packet to a destination is
        typical

   Many applications and protocols choose a destination address by
   sending a message to each of a number of candidates, picking the
   first one to respond, and then using that destination for subsequent
   communication.  If the end-to-end latency of the first packet to each
   destination is atypical, this can result in a highly non-optimal
   destination being chosen, with much longer paths (and hence higher
   load on the Internet) and lower throughput.

   Today, protocols such as Mobile IPv6 [RFC3775], Protocol Independent
   Multicast - Sparse Mode (PIM-SM) [RFC4601], and the Multicast Source
   Discovery Protocol (MSDP) [RFC3618] send packets on one path, and
   then allow immediately switching to a shorter path, resulting in a
   large latency difference.  There are various proposals currently
   being evaluated by the IETF Routing Research Group that result in
   similar path switching.







Thaler                     Expires May 7, 2009                  [Page 8]


Internet-Draft          Evolution of the IP Model          November 2008


3.1.7.  Reordering is rare

   As discussed in [REORDER], [RFC2991], and [RFC3819] section 15, there
   are a number of effects of reordering.  For example, reordering
   increases buffering requirements (and jitter) in many applications,
   and in devices that do packet reassembly.  TCP [RFC0793] in
   particular is adversely affected by reordering, since it enters fast-
   retransmit when three packets are received before a late packet,
   which drastically lowers throughput.  Finally, some NATs and
   firewalls assume that the initial fragment arrives first, resulting
   in packet loss when this is not the case.

   Today there are number of things that cause reordering.  For example,
   some routers do per-packet round-robin load balancing, which,
   depending on the topology, can result in a great deal of reordering.
   As another example, when a packet is fragmented at the sender, some
   hosts send the last fragment first.  Finally, as discussed in
   Section 3.1.6, protocols that do path switching after the first
   packet result in deterministic reordering within the first burst of
   packets.

3.1.8.  Loss is rare and probabilistic, not deterministic

   In the original IP model, senders just send, without signaling the
   network a priori.  This works to a degree.  In practice, the last hop
   (and in rare cases, other hops) of the path needs to resolve next hop
   information (e.g., the link-layer address of the destination) on
   demand which results in queuing traffic, and if the queue fills up,
   some traffic gets dropped.  This means that bursty sources can be
   problematic (and indeed a single large packet that gets fragmented
   becomes such a burst).  The problem is rarely observed in practice
   today, either because the resolution within the last hop happens very
   quickly, or because bursty applications are rarer.  However, any
   protocol that significantly increases such delays or adds new
   resolutions would be a change to the classic IP model and may
   adversely impact upper-layer protocols and applications that result
   in bursts of packets.

   In addition, mechanisms that simply drop the first packet, rather
   than queuing it, also break this assumption.  Similar to the result
   of reordering, they can result in a highly non-optimal destination
   being chosen by applications that use the first one to respond.  Two
   examples of mechanisms that appear to do this are network interface
   cards that support a "Wake-on-LAN" capability where any packet that
   matches a specified pattern will wake up a machine in a power-
   conserving mode, but only after dropping the matching packet, and
   MSDP (since encapsulating data packets is optional).




Thaler                     Expires May 7, 2009                  [Page 9]


Internet-Draft          Evolution of the IP Model          November 2008


3.1.9.  An end-to-end path exists at a single point in time

   In classic IP, applications assume that either an end-to-end path
   exists to a destination, or that the packet will be dropped.  In
   addition, IP today tends to assume that the packet delay is
   relatively short (since the "Time"-to-live is just a hop count).  In
   IP's earlier history, the TTL field was expected to also be
   decremented each second (not just each hop).

   The IRTF Delay Tolerant Networking Research Group is investigating
   changing this assumption.

3.1.10.  Discussion

   The reasons why assumptions listed above are increasingly less true
   can be divided into two categories: effects caused by attributes of
   link-layer technologies, and effects caused by network-layer
   technologies.

   RFC 3819 [RFC3819] gives advice to link-layer protocol designers to
   minimize these effects.  Generally the link-layer causes are not
   intentionally trying to break IP, but rather adding IP over the
   technology introduces the problem.  Hence where the link-layer
   protocol itself does not do so, designers of how IP is defined over
   such a link should compensate to the maximum extent possible.  As
   examples, [RFC3077] and [RFC2491] compensate for lack of symmetric
   reachability and lack of link-layer multicast, respectively.  That
   is, when IP is defined over a link type, the protocol designers
   should attempt to restore the assumptions listed in this document.
   For example, since an implementation can distinguish between 802.11
   ad hoc mode vs. infrastructure mode, it may be possible to define a
   mechanism below IP to compensate for the lack of transitivity over
   such links.

   At the network layer, as a general principle, we believe that
   reachability is good.  For security reasons ([RFC4948]), however, it
   is desirable to restrict reachability by unauthorized parties; indeed
   IPsec, an integral part of the IP model, provides one means to do so.
   Where there are issues with asymmetry, non-transitivity, and so
   forth, which are not direct results of restricting reachability to
   only authorized parties (for some definition of authorized), the IETF
   should attempt to avoid or solve such issues.  Similar to the
   principle outlined in [RFC1958] section 3.9, the general theme when
   defining a protocol is to be liberal in what effects you accept, and
   conservative in what effects you cause.






Thaler                     Expires May 7, 2009                 [Page 10]


Internet-Draft          Evolution of the IP Model          November 2008


3.2.  Assumptions about addressing

3.2.1.  Addresses are stable over long periods of time

   Originally addresses were manually configured on fixed machines, and
   hence addresses were very stable.  With the advent of technologies
   such as DHCP, roaming, and wireless, addresses can no longer be
   assumed to be stable for long periods of time ([RFC3775] section
   4.2).  However, the APIs provided to applications today typically
   still assume stable addresses (e.g., address lifetimes are not
   exposed to applications that get addresses).  This can cause problems
   today when addresses become stale.

   For example, many applications resolve names to addresses and then
   cache them without any notion of lifetime.  In fact, the classic name
   resolution APIs do not even provide applications with the lifetime of
   entries.

   Proxy Mobile IPv6 [RFC5213] tries to restore this assumption to some
   extent by preserving the same address while roaming around a local
   area.  The issue of roaming between different networks has been known
   since at least 1980 when [IEN135] proposed a mobility solution that
   attempted to restore this assumption by adding an additional address
   that can be used by applications which is stable while roaming
   anywhere with Internet connectivity.  More recent protocols such as
   Mobile IPv6 (MIP6) [RFC3775] and the Host Identity Protocol (HIP)
   [RFC4423] follow in this same vein.

3.2.2.  A host has only one address on one interface

   Although many applications assume this (e.g., by calling a name
   resolution function such as gethostbyname and then just using the
   first address returned), it was never really true to begin with, even
   if it was the common case.  Even [RFC0791] states:
      "provision must be made for a host to have several physical
      interfaces to the network with each having several logical
      internet addresses".

   However today this assumption is increasingly less true, with the
   advent of multiple interfaces (e.g., wired and wireless), dual-IPv4/
   IPv6 nodes, multiple IPv6 addresses on the same interface (e.g.,
   link-local and global), etc.  Similarly, many protocol specifications
   such as DHCP only describe operations for a single interface, whereas
   obtaining host-wide configuration from multiple interfaces presents a
   merging problem for nodes in practice.  Too often this problem is
   simply ignored by Working Groups, and applications and users suffer
   as a result from poor merging algorithms.




Thaler                     Expires May 7, 2009                 [Page 11]


Internet-Draft          Evolution of the IP Model          November 2008


   One use of protocols such as MIP6 and HIP is to make this assumption
   somewhat more true by adding an additional "address" that can be the
   one used by such applications, and the protocol will deal with the
   complexity of multiple physical interfaces and addresses.

3.2.3.  A non-multicast/broadcast address identifies a single host over
        a long period of time

   Many applications and upper-layer protocols maintain a communication
   session with a destination over some period of time.  If that address
   is reassigned to another host, or if that address is assigned to
   multiple hosts and the host at which packets arrive changes, such
   applications can have problems.

   [RFC1546] introduced the notion of anycast to the IP service model.
   It states:
      Because anycasting is stateless and does not guarantee delivery of
      multiple anycast datagrams to the same system, an application
      cannot be sure that it is communicating with the same peer in two
      successive UDP transmissions or in two successive TCP connections
      to the same anycast address.
      The obvious solutions to these issues are to require applications
      which wish to maintain state to learn the unicast address of their
      peer on the first exchange of UDP datagrams or during the first
      TCP connection and use the unicast address in future
      conversations.

   The issues with anycast are further discussed in [RFC4786].

   Another mechanism by which multiple hosts use the same address is as
   a result of scoped addresses, as defined for both IPv4 [RFC1918]
   [RFC3927] and IPv6 [RFC4007].  Because such addresses can be reused
   within multiple networks, hosts in different networks can use the
   same address.  As a result, a host that is multihomed to two such
   networks cannot use the destination address to uniquely identify a
   peer.  For example, a host can no longer use a 5-tuple to uniquely
   identify a TCP connection.  This is why IPv6 added the concept of a
   "zone index".

   See [RFC2101] and [RFC2775] for additional discussion on address
   uniqueness.

3.2.4.  An address used by an application is the same as the address
        used for routing

   Some applications assume that the address the application uses is the
   same as that used by routing.  For example, some applications use raw
   sockets to read/write packet headers, including the source and



Thaler                     Expires May 7, 2009                 [Page 12]


Internet-Draft          Evolution of the IP Model          November 2008


   destination addresses in the IP header.  As another example, some
   applications make assumptions about locality (e.g., whether the
   destination is on the same subnet) by comparing addresses.

   Protocols such as Mobile IPv6 and HIP specifically break this
   assumption (in an attempt to restore other assumptions as discussed
   above).  Recently, the IRTF Routing Research Group has been
   evaluating a number of possible mechanisms, some of which would also
   break this assumption, while others preserve this assumption near the
   edges of the network and only break it in the core of the Internet.

   Breaking this assumption is sometimes referred to as an "identifier/
   locator" split.  As originally defined in 1978 ([IEN019], [IEN023]),
   however, an address was originally defined as only a locator, whereas
   names were defined to be the identifiers.  However, the TCP protocol
   then used addresses as identifiers.

   Finally, in a liberal sense, any tunneling mechanism might be said to
   break this assumption, although in practice applications that make
   this assumption will continue to work.  Since the address of the
   inside of the tunnel is still used for routing as expected.

3.2.5.  A subnet is smaller than a link

   In the classic IP model, a "subnet" is smaller than, or equal to, a
   "link".  Destinations with addresses in the same subnet can be
   reached with TTL (or Hop Count) = 1.  Link-scoped multicast packets,
   and all-ones broadcast packets will be delivered (in a best effort
   fashion) to all listening nodes on the link.  Subnet broadcast
   packets will be delivered (in a best effort fashion) to all listening
   nodes in the subnet.  There have been some efforts in the past (e.g.,
   [RFC0925], [RFC3069]) to allow multi-link subnets and change the
   above service model, but the adverse impact on applications that have
   such assumptions recommend against changing this assumption.
   [RFC4903] discusses this topic in more detail and surveys a number of
   protocols and applications that depend on this assumption.

3.2.6.  Selecting a local address selects the interface

   Some applications assume that binding to a given local address
   constrains traffic reception to the interface with that address, and
   that traffic from that address will go out on that address's
   interface.  However, [RFC1122] section 3.3.4.2 defines two models:
   the Strong End System (or Strong host) model where this is true, and
   the Weak End System (or Weak host) model where this is not true.  In
   fact any router is inherently a weak host implementation, since
   packets can be forwarded between interfaces.




Thaler                     Expires May 7, 2009                 [Page 13]


Internet-Draft          Evolution of the IP Model          November 2008


3.2.7.  An address is part of an on-link subnet

   To some extent, this was never true, in that there were cases in IPv4
   where the "mask" was 255.255.255.255, such as on a point-to-point
   link where the two endpoints had addresses out of unrelated address
   spaces.  However, this didn't stop many platforms and applications
   from assuming that every address had a "mask" (or prefix) that was
   on-link.  The assumption of whether a subnet is on-link (in which
   case one can send directly to the destination after using ARP/ND) or
   off-link (in which case one just sends to a router) has evolved over
   the years, and it can no longer be assumed that an address has an on-
   link prefix.  In 1998, [RFC2461] introduced the distinction as part
   of the core IPv6 protocol suite.  This topic is discussed further in
   [I-D.wbeebee-on-link-and-off-link-determination], and [RFC4903] also
   touches on this topic with respect to the service model seen by
   applications.

3.2.8.  Discussion

   RFC 1958 [RFC1958] section 4.1 states: "In general, user applications
   should use names rather than addresses."

   We emphasize the above point, which is too often ignored.  Many
   commonly used APIs unnecessarily expose addresses to applications
   that already use names.  Similarly, some protocols are defined to
   carry addresses, rather than carrying names (instead of or in
   addition to addresses).  Protocols and applications that are already
   dependent on a naming system should be designed in such a way that
   they avoid or minimize any dependence on the notion of addresses.

3.3.  Assumptions about upper-layer extensibility

3.3.1.  New transport-layer protocols can work across the Internet

   IP was originally designed to support the addition of new transport-
   layer protocols, and [PROTOCOLS] lists many such protocols.

   However, as discussed in [I-D.rosenberg-internet-waist-hourglass],
   NATs and firewalls today break this assumption and often only allow
   UDP and TCP (or even just HTTP).

3.3.2.  If one stream between a pair of addresses can get through, then
        so can another

   Some applications and protocols use multiple upper-layer streams of
   data between the same pair of addresses, and initiated by the same
   party.  Passive-mode FTP [RFC0959], and RTP [RFC3550], are two
   examples of such protocols, which use separate streams for data vs.



Thaler                     Expires May 7, 2009                 [Page 14]


Internet-Draft          Evolution of the IP Model          November 2008


   control channels.

   Today, there are many reasons this may not be true.  Firewalls, for
   example, may selectively allow/block specific protocol numbers and/or
   values in upper-layer protocol fields (such as port numbers).
   Similarly, middleboxes such as NATs that create per-stream state may
   cause other streams to fail once they run out of space to store
   additional stream state.

3.3.3.  Discussion

   [NEWARCH] section 5.1 discusses the primary requirements of the
   original internet architecture, including Service Generality.  It
   states:

   "This goal was to support the widest possible range of applications,
   by supporting a variety of types of service at the transport level.
   Services might be distinguished by speed, latency, or reliability,
   for example.  Service types might include virtual circuit service,
   which provides reliable, full-duplex byte streams, and also datagram
   service, which delivers individual packets with no guarantees of
   reliability or ordering.  The requirement for datagram service was
   motivated by early ARPAnet experiments with packet speech (using IMP
   Type 3 messages)."

   The reasons the assumptions in this section are becoming less true
   are due to network-layer (or higher-layer) techniques being
   introduced that interfere with the original requirement.  Generally
   these are done either in the name of security, or as a side effect of
   solving some other problem such as address shortage.  Work is needed
   to investigate ways to restore the original behavior while still
   meeting today's security requriements.

3.4.  Assumptions about security

3.4.1.  Packets are unmodified in transit

   Some applications and upper-layer protocols assume that a packet is
   unmodified in transit, except for a few well-defined fields (e.g.,
   TTL).  Examples of this behavior include protocols that define their
   own integrity protection mechanism such as a checksum.

   This assumption is broken by NATs as discussed in [RFC2993] and other
   middleboxes that modify the contents of packets.  There are many
   tunneling technologies (e.g., [RFC4380]) that attempt to restore this
   assumption to some extent.

   The IPsec architecture [RFC4301] added security to the IP model,



Thaler                     Expires May 7, 2009                 [Page 15]


Internet-Draft          Evolution of the IP Model          November 2008


   providing a way to address this problem without changing
   applications, although it is not currently widely used over the
   Internet.

3.4.2.  Packets are private

   The assumption that data is private has never really been true.
   However, many old applications and protocols (e.g., FTP) transmit
   passwords or other sensitive data in the clear.

   IPsec provides a way to address this problem without changing
   applications, although it is not yet widely deployed, and doing
   encryption/decryption for all packets can be computationally
   expensive.

3.4.3.  Source addresses are not forged

   Most applications and protocols use the source address of some
   incoming packet when generating a response, and hence assume that it
   has not been forged (and as a result can often be vulnerable to
   reflection attachs).

   Various mechanisms that restore this assumption include, for example,
   IPsec and Cryptographically Generated Addresses (CGAs) [RFC3972].

3.4.4.  Discussion

   A good discussion of threat models and common tools can be found in
   [RFC3552].  Protocol designers and applications developers are
   encouraged to be familiar with that document.


4.  Security Considerations

   This document discusses assumptions about the IP service model made
   by many applications and upper-layer protocols.  Whenever these
   assumptions are broken, if the application or upper-layer protocol
   has some security-related behavior that is based on the assumption,
   then security can be affected.

   For example, if an application assumes that binding to the IP address
   of a "trusted" interface means that it will never receive traffic
   from an "untrusted" interface, and that assumption is broken (as
   discussed in Section 3.2.6) then an attacker could get access to
   private information.

   As a result, great care should be taken when expanding the extent to
   which an assumption is false.  On the other hand, application and



Thaler                     Expires May 7, 2009                 [Page 16]


Internet-Draft          Evolution of the IP Model          November 2008


   upper-layer protocol developers should carefully consider the impact
   of basing their security on any of the assumptions enumerated in this
   document.

   It is also worth noting that many of the changes that have occurred
   over time (e.g., firewalls, dropping directed broadcasts, etc.) that
   are discussed in this document were done in the interest of improving
   security at the expense of breaking some applications.


5.  IANA Considerations

   This document has no IANA Actions.


6.  Conclusion

   Because a huge number of applications already exist that use TCP/IP
   for business-critical operations, any changes to the service model
   need to be done with extreme care.  Extensions that merely add
   additional optional functionality without impacting any existing
   applications are much safer than extensions which change one or more
   of the core assumptions discussed above.  Any changes to the above
   assumptions should only be done in accordance with some mechanism to
   minimize or mitigate the risks of breaking mission-critical
   applications.  Historically, changes have been done without regard to
   such considerations and as a result the situation for applications
   today is already problematic.  Key to maintaining an interoperable
   Internet is documenting and maintaining invariants that higher layers
   can depend on, and being very judicious with changes.


7.  Acknowledgements

   Bernard Aboba, Chris Hopps, Dow Street, and others provided helpful
   discussion on various points that led to this document.  Iain Calder
   and Brian Carpenter also provided valuable feedback.














Thaler                     Expires May 7, 2009                 [Page 17]


Internet-Draft          Evolution of the IP Model          November 2008


8.  IAB Members at the time of this writing

   Loa Andersson
   Gonzalo Camarillo
   Stuart Cheshire
   Russ Housley
   Olaf Kolkman
   Gregory Lebovitz
   Barry Leiba
   Kurtis Lindqvist
   Andrew Malis
   Danny McPherson
   David Oran
   Dave Thaler
   Lixia Zhang


9.  References

9.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, August 1989.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1546]  Partridge, C., Mendez, T., and W. Milliken, "Host
              Anycasting Service", RFC 1546, November 1993.

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              December 1998.

   [RFC2644]  Senie, D., "Changing the Default for Directed Broadcasts
              in Routers", BCP 34, RFC 2644, August 1999.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.





Thaler                     Expires May 7, 2009                 [Page 18]


Internet-Draft          Evolution of the IP Model          November 2008


9.2.  Informative References

   [I-D.rosenberg-internet-waist-hourglass]
              Rosenberg, J., "UDP and TCP as the New Waist of the
              Internet Hourglass",
              draft-rosenberg-internet-waist-hourglass-00 (work in
              progress), February 2008.

   [I-D.wbeebee-on-link-and-off-link-determination]
              Singh, H., Beebee, W., and E. Nordmark, "IPv6 Subnet
              Model",
              draft-wbeebee-on-link-and-off-link-determination-02 (work
              in progress), February 2008.

   [IEN019]   Shoch, J., "A note on Inter-Network Naming, Addressing,
              and Routing", IEN 19, January 1978,
              <ftp://ftp.rfc-editor.org/in-notes/ien/ien19.txt>.

   [IEN023]   Cohen, D., "On Names, Addresses and Routings", IEN 23,
              January 1978,
              <ftp://ftp.rfc-editor.org/in-notes/ien/ien23.txt>.

   [IEN028]   Postel, J., "Draft Internetwork Protocol Specification",
              IEN 28, February 1978,
              <ftp://ftp.rfc-editor.org/in-notes/ien/ien-index.html>.

   [IEN135]   Sunshine, C. and J. Postel, "Addressing Mobile Hosts in
              the ARPA Internet Environment", IEN 135, March 1980,
              <ftp://ftp.rfc-editor.org/in-notes/ien/ien135.txt>.

   [MCAST4]   Internet Assigned Numbers Authority, "IPv4 Multicast
              Addresses",
              <http://www.iana.org/assignments/multicast-addresses>.

   [MCAST6]   Internet Assigned Numbers Authority, "INTERNET PROTOCOL
              VERSION 6 MULTICAST ADDRESSES",
              <http://www.iana.org/assignments/
              ipv6-multicast-addresses>.

   [NEWARCH]  Clark, D., et al., "New Arch: Future Generation Internet
              Architecture", Air Force Research Laboratory Technical
              Report AFRL-IF-RS-TR-2004-235, August 2004, <http://
              www.dtic.mil/cgi-bin/
              GetTRDoc?AD=ADA426770&Location=U2&doc=GetTRDoc.pdf>.

   [PROTOCOLS]
              Internet Assigned Numbers Authority, "Protocol Numbers",
              <http://www.iana.org/assignments/protocol-numbers>.



Thaler                     Expires May 7, 2009                 [Page 19]


Internet-Draft          Evolution of the IP Model          November 2008


   [REORDER]  Bennett, J., Partridge, C., and N. Shectman, "Packet
              reordering is not pathological network behavior", IEEE/ACM
              Transactions on Networking, Vol. 7, No. 6, December 1999.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC0925]  Postel, J., "Multi-LAN address resolution", RFC 925,
              October 1984.

   [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 959, October 1985.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC1958]  Carpenter, B., "Architectural Principles of the Internet",
              RFC 1958, June 1996.

   [RFC2101]  Carpenter, B., Crowcroft, J., and Y. Rekhter, "IPv4
              Address Behaviour Today", RFC 2101, February 1997.

   [RFC2491]  Armitage, G., Schulter, P., Jork, M., and G. Harter, "IPv6
              over Non-Broadcast Multiple Access (NBMA) networks",
              RFC 2491, January 1999.

   [RFC2775]  Carpenter, B., "Internet Transparency", RFC 2775,
              February 2000.

   [RFC2991]  Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
              Multicast Next-Hop Selection", RFC 2991, November 2000.

   [RFC2993]  Hain, T., "Architectural Implications of NAT", RFC 2993,
              November 2000.

   [RFC3021]  Retana, A., White, R., Fuller, V., and D. McPherson,
              "Using 31-Bit Prefixes on IPv4 Point-to-Point Links",
              RFC 3021, December 2000.

   [RFC3069]  McPherson, D. and B. Dykes, "VLAN Aggregation for
              Efficient IP Address Allocation", RFC 3069, February 2001.

   [RFC3077]  Duros, E., Dabbous, W., Izumiyama, H., Fujii, N., and Y.
              Zhang, "A Link-Layer Tunneling Mechanism for
              Unidirectional Links", RFC 3077, March 2001.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.



Thaler                     Expires May 7, 2009                 [Page 20]


Internet-Draft          Evolution of the IP Model          November 2008


              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              July 2003.

   [RFC3618]  Fenner, B. and D. Meyer, "Multicast Source Discovery
              Protocol (MSDP)", RFC 3618, October 2003.

   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.

   [RFC3819]  Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, July 2004.

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927,
              May 2005.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005.

   [RFC4007]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
              B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
              March 2005.

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              February 2006.

   [RFC4423]  Moskowitz, R. and P. Nikander, "Host Identity Protocol
              (HIP) Architecture", RFC 4423, May 2006.

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, August 2006.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, December 2006.

   [RFC4903]  Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
              June 2007.

   [RFC4948]  Andersson, L., Davies, E., and L. Zhang, "Report from the
              IAB workshop on Unwanted Traffic March 9-10, 2006",



Thaler                     Expires May 7, 2009                 [Page 21]


Internet-Draft          Evolution of the IP Model          November 2008


              RFC 4948, August 2007.

   [RFC5213]  Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
              and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.

   [WIRELESS]
              Kotz, D., Newport, C., and C. Elliott, "The mistaken
              axioms of wireless-network research", Dartmouth College
              Computer Science Technical Report TR2003-467, July 2003,
              <http://pdos.csail.mit.edu/decouto/papers/kotz03.pdf>.


Author's Address

   Dave Thaler
   IAB
   One Microsoft Way
   Redmond, WA  98052
   USA

   Phone: +1 425 703 8835
   Email: dthaler@microsoft.com





























Thaler                     Expires May 7, 2009                 [Page 22]


Internet-Draft          Evolution of the IP Model          November 2008


Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.











Thaler                     Expires May 7, 2009                 [Page 23]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/