Network Working Group D. Thaler
Internet-Draft IAB
Intended status: Informational October 25, 2010
Expires: April 28, 2011
Evolution of the IP Model
draft-iab-ip-model-evolution-02.txt
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.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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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."
This Internet-Draft will expire on April 28, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. The IP Service Model . . . . . . . . . . . . . . . . . . . . . 5
2.1. Links and Subnets . . . . . . . . . . . . . . . . . . . . 6
3. Common Application Assumptions . . . . . . . . . . . . . . . . 7
3.1. Assumptions about routing . . . . . . . . . . . . . . . . 7
3.1.1. Reachability is symmetric . . . . . . . . . . . . . . 7
3.1.2. Reachability is transitive . . . . . . . . . . . . . . 8
3.1.3. Error messages can be received in response to
data packets . . . . . . . . . . . . . . . . . . . . . 8
3.1.4. Multicast is supported within a link . . . . . . . . . 9
3.1.5. IPv4 broadcast is supported . . . . . . . . . . . . . 9
3.1.6. Multicast/broadcast is less expensive than
replicated unicast . . . . . . . . . . . . . . . . . . 10
3.1.7. The end-to-end latency of the first packet to a
destination is typical . . . . . . . . . . . . . . . . 10
3.1.8. Reordering is rare . . . . . . . . . . . . . . . . . . 10
3.1.9. Loss is rare and probabilistic, not deterministic . . 11
3.1.10. An end-to-end path exists at a single point in time . 11
3.1.11. Discussion . . . . . . . . . . . . . . . . . . . . . . 12
3.2. Assumptions about addressing . . . . . . . . . . . . . . . 13
3.2.1. Addresses are stable over long periods of time . . . . 13
3.2.2. A host has only one address on one interface . . . . . 13
3.2.3. A non-multicast/broadcast address identifies a
single host over a long period of time . . . . . . . . 14
3.2.4. An address can be used as an indication of
physical location . . . . . . . . . . . . . . . . . . 15
3.2.5. An address used by an application is the same as
the address used for routing . . . . . . . . . . . . . 15
3.2.6. A subnet is smaller than a link . . . . . . . . . . . 16
3.2.7. Selecting a local address selects the interface . . . 16
3.2.8. An address is part of an on-link subnet prefix . . . . 16
3.2.9. Discussion . . . . . . . . . . . . . . . . . . . . . . 17
3.3. Assumptions about upper-layer extensibility . . . . . . . 17
3.3.1. New transport-layer protocols can work across the
Internet . . . . . . . . . . . . . . . . . . . . . . . 17
3.3.2. If one stream between a pair of addresses can get
through, then so can another . . . . . . . . . . . . . 17
3.3.3. Discussion . . . . . . . . . . . . . . . . . . . . . . 18
3.4. Assumptions about security . . . . . . . . . . . . . . . . 18
3.4.1. Packets are unmodified in transit . . . . . . . . . . 18
3.4.2. Packets are private . . . . . . . . . . . . . . . . . 19
3.4.3. Source addresses are not forged . . . . . . . . . . . 19
3.4.4. Discussion . . . . . . . . . . . . . . . . . . . . . . 19
4. Security Considerations . . . . . . . . . . . . . . . . . . . 19
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 20
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7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
8. IAB Members at the time of this writing . . . . . . . . . . . 21
9. IAB Members at expected time of approval . . . . . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . . 22
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 26
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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
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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
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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 (as in the following example
figure), this can no longer be assumed. Today, host-to-host
connectivity is challenging if not impossible in general. It is
relatively easy to initiate communication from hosts (A-E in the
example diagram) to servers (S), but not vice versa, nor between
hosts A-E. For a longer discussion on peer-to-peer connectivity see
[RFC5694] Appendix A.
__________ ___ ___
/ \ ___ ___ / \ ____|FW |__A
/ \ ___ / \ _____|NAT|__| | |___|
| |__|NAT|__| | |___| | |__B
| | |___| | |__C \___/
| | \___/ ___
S__| Internet | ___ ___ / \
| | ___ / \ _____|NAT|__| |__D
| |__|FW |__| | |___| | |
| | |___| | |__E \___/
\ / \___/
\__________/
Figure 2
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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
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. Error messages can be received in response to data packets
Some upper-layer protocols and applications assume that ICMP error
messages will be received in response to packets sent that cannot be
delivered. Examples of this include the use of Path MTU Discovery
[RFC1191][RFC1981]) relying on messages indicating packets were too
big, and traceroute and the use of expanding ring search [RFC1812]
relying on messages indicating packets reached their maximum hop
count limit.
Originally this assumption largely held, but many ICMP senders then
chose to rate-limit responses, and many firewalls now block ICMP
messages. For a longer discussion, see [RFC2923] section 2.1.
This led to an alternate mechanism for Path MTU Discovery that does
not rely on this assumption being true [RFC4821], and guidance to
firewall administrators ([RFC2979] section 3.1.1 and [RFC4890]).
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3.1.4. 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.
3.1.5. 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
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
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traffic to a broadcast address. However, from the network's
perspective, the host still sends without signaling a priori.
3.1.6. 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.
3.1.7. 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, there are a number of reasons this is not true. First, when
sending to a new destination there may be some startup latency
resulting from the link-layer or network-layer mechanism in use, such
as ARP resolution for instance. In addition, the first packet may
follow a different path from subsequent packets. For example,
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.
3.1.8. 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
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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 3.1.7, protocols that do path switching after the first
packet result in deterministic reordering within the first burst of
packets.
3.1.9. 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, where encapsulating data packets is optional, but doing so
enables bursty sources to be accommodated while a multicast tree is
built back to the source's domain.
3.1.10. 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
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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).
This assumption is still true in general today. However, the IRTF
Delay Tolerant Networking Research Group is investigating ways for
applications to use IP in networks where this assumption is not true,
such as store-and-forward networks (e.g., packets carried by vehicles
or animals).
3.1.11. 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, when specifying how IP is defined
over such a link protocol, designers 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.
However, in being liberal in what effects you accept, it is also
important to remember that diagnostics are important, and being too
liberal can mask problems. Thus a tussle exists between the desire
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to provide a better experience to one's own users or applications and
thus be more successful ([RFC5218]), vs. the desire to put pressure
on getting problems fixed. One solution is to provide a separate
"pedantic mode" that can be enabled to see the problems rather than
mask them.
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
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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.
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.
In addition, many security mechanisms and configurations assume that
one can block traffic by IP address, implying that a single attacker
can be identified by IP address. If that IP address can also
identify many legitimate hosts, apply such a block can result in
denial-of-service.
[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] and
[I-D.iab-anycast-arch-implications].
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
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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".
Yet another example is that, in some high-availability solutions, one
host takes over the IP address of another failed host.
See [RFC2101], [RFC2775], and
[I-D.ietf-intarea-shared-addressing-issues] for additional discussion
on address uniqueness.
3.2.4. An address can be used as an indication of physical location
Some applications attempt to use an address to infer some information
about the physical location of the host with that address. For
example, geo-location services are often used to provide targeted
content or ads.
Various forms of tunneling have made this assumption less true, and
this will become increasingly less true as the use of IPv4 NATs for
large networks continues to increase. See
[I-D.ietf-intarea-shared-addressing-issues] section 7 for a longer
discussion.
3.2.5. 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
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
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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.
3.2.6. 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 on-link subnet
prefix 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.
Specifically, some applications assume that, if a destination address
is in the same on-link subnet prefix as the local machine, then
therefore packets can be sent with TTL=1, or that packets can be
received with TTL=255, or link-scoped multicast or broadcast can be
used to reach the destination.
3.2.7. 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.
3.2.8. An address is part of an on-link subnet prefix
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, and no on-link subnet prefix exists on the link. 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
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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.
3.2.9. 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.
One challenge is that many hosts today do not have names that can be
resolved. For example, a host may not have a fully-qualfied domain
name (FQDN) or a Domain Name System (DNS) server that will host its
name.
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.
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).
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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,
providing a way to address this problem without changing
applications, although transport-mode IPsec is not currently widely
used over the Internet.
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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
various types of attacks such as reflection attacks).
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 3.2.7) 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
upper-layer protocol developers should carefully consider the impact
of basing their security on any of the assumptions enumerated in this
document.
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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.
In general, lower-layer protocols should document the contract they
provide to higher layers; that is, what assumptions the upper layer
can rely on (sometimes this is done in the form of an applicability
statement). Conversely, higher-layer protocols should document the
assumptions they rely on from the lower layer (sometimes this is done
in the form of requirements).
We must also recognize that a successful architecture often evolves
as success brings growth and as technology moves forward. As a
result, the various assumptions made should be periodically reviewed
when updating protocols.
7. Acknowledgements
Chris Hopps, Dow Street, Phil Hallam-Baker, and others provided
helpful discussion on various points that led to this document. Iain
Calder, Brian Carpenter, Jonathan Rosenberg, Erik Nordmark, Alain
Durand, and Iljitsch van Beijnum also provided valuable feedback.
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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. IAB Members at expected time of approval
Bernard Aboba
Marcelo Bagnulo
Ross Callon
Spencer Dawkins
Vijay Gill
Russ Housley
John Klensin
Olaf Kolkman
Danny McPherson
Jon Peterson
Andrei Robachevsky
Dave Thaler
Hannes Tschofenig
10. References
10.1. 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
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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.
10.2. Informative References
[I-D.iab-anycast-arch-implications]
McPherson, D. and D. Oran, "Architectural Considerations
of IP Anycast", draft-iab-anycast-arch-implications-00
(work in progress), February 2010.
[I-D.ietf-intarea-shared-addressing-issues]
Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
Roberts, "Issues with IP Address Sharing",
draft-ietf-intarea-shared-addressing-issues-02 (work in
progress), October 2010.
[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>.
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[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>.
[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.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
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BCP 5, RFC 1918, February 1996.
[RFC1958] Carpenter, B., "Architectural Principles of the Internet",
RFC 1958, June 1996.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 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.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, September 2000.
[RFC2979] Freed, N., "Behavior of and Requirements for Internet
Firewalls", RFC 2979, October 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.
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.
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[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.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
ICMPv6 Messages in Firewalls", RFC 4890, May 2007.
[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",
RFC 4948, August 2007.
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[RFC5213] Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.
[RFC5218] Thaler, D. and B. Aboba, "What Makes For a Successful
Protocol?", RFC 5218, July 2008.
[RFC5694] Camarillo, G. and IAB, "Peer-to-Peer (P2P) Architecture:
Definition, Taxonomies, Examples, and Applicability",
RFC 5694, November 2009.
[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
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