Network Working Group B. Aboba
INTERNET-DRAFT D. Thaler
Category: Informational Loa Andersson
Expires: July 5, 2009 Stuart Cheshire
19 December 2008 Internet Architecture Board
Principles of Internet Host Configuration
draft-iab-ip-config-10.txt
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Abstract
This document describes principles of Internet host configuration.
It covers issues relating to configuration of Internet layer
parameters, as well as parameters affecting higher layer protocols.
Table of Contents
1. Introduction.............................................. 3
1.1 Terminology ........................................ 3
1.2 Internet Host Configuration ........................ 4
2. Principles ............................................... 6
2.1 Minimize Configuration ............................. 7
2.2 Less is More ....................................... 7
2.3 Minimize Diversity ................................. 8
2.4 Lower Layer Independence ........................... 9
2.5 Configuration is Not Access Control ................ 11
3. Additional Discussion .................................... 11
3.1 Reliance on General Purpose Mechanisms ............. 11
3.2 Relationship between IP Configuration and
Service Discovery .................................. 12
3.3 Discovering Names vs. Addresses .................... 14
3.4 Dual Stack Issues .................................. 15
3.5 Relationship between Per-Interface and
Per-Host Configuration ............................. 16
4. Security Considerations .................................. 17
4.1 Configuration Authentication ....................... 17
5. IANA Considerations ...................................... 19
6. References ............................................... 19
6.1 Informative References ............................. 19
Acknowledgments .............................................. 23
Appendix A - IAB Members ..................................... 23
Authors' Addresses ........................................... 24
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1. Introduction
This document describes principles of Internet host [STD3]
configuration. It covers issues relating to configuration of
Internet layer parameters, as well as parameters affecting higher
layer protocols.
In recent years, a number of architectural questions have arisen, for
which we provide guidance to protocol developers:
o What protocol layers and general approaches are most appropriate
for configuration of various parameters.
o The relationship between parameter configuration and
service discovery.
o The relationship between per-interface and per-host
configuration.
o The relationship between network access authentication and
host configuration.
o The desirability of avoiding parameter configuration or
supporting self-configuration.
o The role of link-layer protocols and tunneling protocols
in Internet host configuration.
The role of the link-layer and tunneling protocols is particularly
important, since it can affect the properties of a link as seen by
higher layers (for example, whether privacy extensions specified in
"Privacy Extensions for Stateless Address Autoconfiguration in IPv6"
[RFC4941] are available to applications).
1.1. Terminology
link A communication facility or medium over which nodes can
communicate at the link-layer, i.e., the layer immediately
below IP. Examples are Ethernets (simple or bridged), PPP
links, X.25, Frame Relay, or ATM networks as well as
Internet (or higher) layer "tunnels", such as tunnels over
IPv4 or IPv6 itself.
on link An address that is assigned to an interface on a specified
link.
off link The opposite of "on link"; an address that is not assigned
to any interfaces on the specified link.
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mobility agent
Either a home agent or a foreign agent.
1.2. Internet Host Configuration
1.2.1. Internet Layer Configuration
Internet layer configuration is defined as the configuration required
to support the operation of the Internet layer. This includes
configuration of per-interface and per-host parameters, including IP
address(es), subnet prefix(es), default gateway(s), mobility
agent(s), boot service configuration and other parameters:
IP address(es)
Internet Protocol (IP) address configuration includes both
configuration of link-scope addresses as well as global
addresses. Configuration of IP addresses is a vital step,
since practically all of IP networking relies on the
assumption that hosts have IP address(es) associated with
(each of) their active network interface(s). Used as the
source address of an IP packet, these IP addresses
indicate the sender of the packet; used as the destination
address of a unicast IP packet, these IP addresses
indicate the intended receiver.
The only common example of IP-based protocols operating
without an IP address involves address configuration, such
as the use of DHCPv4 [RFC2131] to obtain an address. In
this case, by definition, DHCPv4 is operating before the
host has an IPv4 address, so the DHCP protocol designers
had the choice of either using IP without an IP address,
or not using IP at all. The benefits of making IPv4 self-
reliant, configuring itself using its own IPv4 packets,
instead of depending on some other protocol, outweighed
the drawbacks of having to use IP in this constrained
mode. Use of IP for purposes other than address
configuration can safely assume that the host will have
one or more IP addresses, which may be self-configured
link-local addresses, or other addresses configured via
DHCP or other means.
Subnet prefix(es)
Once a subnet prefix is configured on an interface, hosts
with an IP address can exchange unicast IP packets
directly with on-link hosts within the same subnet prefix.
Default gateway(s)
Once a default gateway is configured on an interface,
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hosts with an IP address can send unicast IP packets to
that gateway for forwarding to off-link hosts.
Mobility agent(s)
While Mobile IPv4 [RFC3344] and Mobile IPv6 [RFC3775]
include their own mechanisms for locating home agents, it
is also possible for mobile nodes to utilize dynamic home
agent configuration.
Boot service configuration
Boot service configuration is defined as the configuration
necessary for a host to obtain and perhaps also to verify
an appropriate boot image. This is appropriate for disk-
less hosts looking to obtain a boot image via mechanisms
such as the Trivial File Transfer Protocol (TFTP)
[RFC1350], Network File System (NFS) [RFC3530] and
Internet Small Computer Systems Interface (iSCSI)
[RFC3720][RFC4173]. It also may be useful in situations
where it is necessary to update the boot image of a host
that supports a disk, such as in the Preboot eXecution
Environment (PXE) [PXE][RFC4578]. While strictly speaking
boot services operate above the Internet layer, where boot
service is used to obtain the Internet layer code, it may
be considered part of Internet layer configuration. While
boot service parameters may be provided on a per-interface
basis, loading and verification of a boot image affects
behavior of the host as a whole.
Other IP parameters
Internet layer parameter configuration also includes
configuration of per-host parameters (e.g. hostname) and
per-interface parameters (e.g. IP Time-To-Live (TTL) to
use in outgoing packets, enabling/disabling of IP
forwarding and source routing, and Maximum Transmission
Unit (MTU)).
1.2.2. Higher Layer Configuration
Higher layer configuration is defined as the configuration required
to support the operation of other components above the Internet
layer. This includes, for example:
Name Service Configuration
The configuration required for the host to resolve names.
This includes configuration of the addresses of name
resolution servers, including IEN 116 [IEN116], Domain
Name System (DNS), Windows Internet Name Service (WINS),
Internet Storage Name Service (iSNS) [RFC4171][RFC4174]
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and Network Information Service (NIS) servers [RFC3898],
and the setting of name resolution parameters such as the
DNS domain and search list [RFC3397], the NetBIOS node
type, etc. It may also include the transmission or
setting of the host's own name. Note that link local name
resolution services (such as NetBIOS [RFC1001], Link-Local
Multicast Name Resolution (LLMNR) [RFC4795] and multicast
DNS (mDNS) [mDNS]) typically do not require configuration.
Once the host has completed name service configuration, it
is capable of resolving names using name resolution
protocols that require configuration. This not only
allows the host to communicate with off-link hosts whose
IP address is not known, but to the extent that name
services requiring configuration are utilized for service
discovery, this also enables the host to discover services
available on the network or elsewhere. While name service
parameters can be provided on a per-interface basis, their
configuration will typically affect behavior of the host
as a whole.
Time Service Configuration
Time service configuration includes configuration of
servers for protocols such as the Simple Network Time
Protocol (SNTP) and the Network Time Protocol (NTP).
Since accurate determination of the time may be important
to operation of the applications running on the host
(including security services), configuration of time
servers may be a prerequisite for higher layer operation.
However, it is typically not a requirement for Internet
layer configuration. While time service parameters can be
provided on a per-interface basis, their configuration
will typically affect behavior of the host as a whole.
Other service configuration
This can include discovery of additional servers and
devices, such as printers, Session Initiation Protocol
(SIP) proxies, etc. This configuration will typically
apply to the entire host.
2. Principles
This section describes basic principles of Internet host
configuration.
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2.1. Minimize Configuration
Anything that can be configured can be misconfigured. "Architectural
Principles of the Internet" [RFC1958] Section 3.8 states: "Avoid
options and parameters whenever possible. Any options and parameters
should be configured or negotiated dynamically rather than manually."
That is, to minimize the possibility of configuration errors,
parameters should be automatically computed (or at least have
reasonable defaults) whenever possible. For example, the Path
Maximum Transmission Unit (PMTU) can be discovered, as described in
"Packetization Layer Path MTU Discovery" [RFC4821], "TCP Problems
with Path MTU Discovery" [RFC2923], "Path MTU discovery" [RFC1191]
and "Path MTU Discovery for IP version 6" [RFC1981].
Having a protocol design with many configurable parameters increases
the possibilities for misconfiguration of those parameters, resulting
in failures or other sub-optimal operation. Eliminating or reducing
configurable parameters helps lessen this risk. Where configurable
parameters are necessary or desirable, protocols can reduce the risk
of human error by making these parameters self-configuring, such as
by using capability negotiation within the protocol, or by discovery
of other hosts that implement the same protocol.
2.2. Less is More
The availability of standardized, simple mechanisms for general-
purpose Internet host configuration is highly desirable. RFC 1958
[RFC1958] states, "Performance and cost must be considered as well as
functionality" and "Keep it simple. When in doubt during design,
choose the simplest solution."
To allow protocol support in more types of devices, it is important
to minimize the footprint requirement. For example, IP-based
protocols are used on a wide range of devices, from supercomputers
to small low-cost devices running "embedded" operating systems.
Since the resources (e.g. memory and code size) available for host
configuration may be very small, it is desirable for a host to be
able to configure itself in as simple a manner as possible.
One interesting example is IP support in pre-boot execution
environments. Since by definition boot configuration is required in
hosts that have not yet fully booted, it is often necessary for pre-
boot code to be executed from Read Only Memory (ROM), with minimal
available memory. Many hosts do not have enough space in this ROM
for even a simple implementation of TCP, so in the Pre-boot Execution
Environment (PXE) the task of obtaining a boot image has to be
performed using the User Datagram Protocol over IP (UDP/IP) [RFC768]
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instead. This is one reason why Internet layer configuration
mechanisms typically depend only on IP and UDP. After obtaining the
boot image, the host will have the full facilities of TCP/IP
available to it, including support for reliable transport protocols,
IPsec, etc.
In order to reduce complexity, it is desirable for Internet layer
configuration mechanisms to avoid dependencies on higher layers.
Since embedded devices may be severely constrained on how much code
they can fit within their ROM, designing a configuration mechanism in
such a way that it requires the availability of higher layer
facilities may make that configuration mechanism unusable in such
devices. In fact, it cannot even be guaranteed that all Internet
layer facilities will be available. For example, the minimal version
of IP in a host's boot ROM may not implement IP fragmentation and
reassembly.
2.3. Minimize Diversity
The number of host configuration mechanisms should be minimized.
Diversity in Internet host configuration mechanisms presents several
problems:
Interoperability As configuration diversity increases, it becomes
likely that a host will not support the
configuration mechanism(s) available on the
network to which it has attached, creating
interoperability problems.
Footprint For maximum interoperability, a host would need to
implement all configuration mechanisms used on all
the link layers it supports. This increases the
required footprint, a burden for embedded devices.
It also leads to lower quality, since testing
resources (both formal testing, and real-world
operational use) are spread more thinly -- the
more different configuration mechanisms a device
supports, the less testing each one is likely to
undergo.
Redundancy To support diversity in host configuration
mechanisms, operators would need to support
multiple configuration services to ensure that
hosts connecting to their networks could configure
themselves. This represents an additional expense
for little benefit.
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Latency As configuration diversity increases, hosts
supporting multiple configuration mechanisms may
spend increasing effort to determine which
mechanism(s) are supported. This adds to
configuration latency.
Conflicts Whenever multiple mechanisms are available, it is
possible that multiple configuration(s) will be
returned. To handle this, hosts would need to
merge potentially conflicting configurations.
This would require conflict resolution logic, such
as ranking of potential configuration sources,
increasing implementation complexity.
Additional traffic To limit configuration latency, hosts may
simultaneously attempt to obtain configuration by
multiple mechanisms. This can result in
increasing on-the-wire traffic, both from use of
multiple mechanisms as well as from
retransmissions within configuration mechanisms
not implemented on the network.
Security Support for multiple configuration mechanisms
increases the attack surface without any potential
benefit.
2.4. Lower Layer Independence
"Architectural Principles of the Internet" [RFC1958] states,
"Modularity is good. If you can keep things separate, do so."
It is becoming increasingly common for hosts to support multiple
network access mechanisms, including dialup, wireless and wired local
area networks, wireless metropolitan and wide area networks, etc.
The proliferation of network access mechanisms makes it desirable for
hosts to be able to configure themselves on multiple networks without
adding configuration code specific to a new link layer.
As a result, it is highly desirable for Internet host configuration
mechanisms to be independent of the underlying lower layer. That is,
only the link layer protocol (whether it be a physical link, or a
virtual tunnel link) should be explicitly aware of link-layer
parameters (although it may configure them). Introduction of lower
layer dependencies increases the likelihood of interoperability
problems and adds Internet layer configuration mechanisms that hosts
need to implement.
Lower layer dependencies can be best avoided by keeping Internet host
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configuration above the link layer, thereby enabling configuration to
be handled for any link layer that supports IP. In order to provide
media independence, Internet host configuration mechanisms should be
link-layer protocol independent.
While there are examples of Internet layer configuration within the
link layer (such as in the Point-to-Point Protocol (PPP) IPv4CP
[RFC1332] and "Mobile radio interface Layer 3 specification; Core
network protocols; Stage 3 (Release 5)" [3GPP-24.008]), this approach
has disadvantages. This includes the extra complexity of
implementing different mechanisms on different link layers, and the
difficulty in adding new parameters which would require defining a
mechanism in each link layer protocol.
For example, "Internet Protocol Control Protocol (IPCP) Extensions
for Name Service Configuration" [RFC1877] was developed prior to the
definition of the DHCPINFORM message in "Dynamic Host Configuration
Protocol" [RFC2131]; at that time Dynamic Host Configuration Protocol
(DHCP) servers had not been widely implemented on access devices or
deployed in service provider networks. While the design of IPv4CP
was appropriate in 1992, it should not be taken as an example that
new link layer technologies should emulate. Indeed, in order to
"actively advance PPP's most useful extensions to full standard,
while defending against further enhancements of questionable value",
"IANA Considerations for the Point-to-Point Protocol (PPP)" [RFC3818]
changed the allocation of PPP protocol numbers (including IPv4CP
extensions) so as to no longer be "first come first served."
In IPv6 where link layer independent mechanisms such as "IPv6
Stateless Address Autoconfiguration" [RFC4862] and "Stateless Dynamic
Host Configuration Protocol (DHCP) Service for IPv6" [RFC3736] are
available, PPP IPv6CP [RFC5072] configures an Interface-Identifier
which is similar to a MAC address. This enables PPP IPv6CP to avoid
duplicating DHCPv6 functionality.
However, Internet Key Exchange Version 2 (IKEv2) [RFC4306] utilizes
the same approach as PPP IPv4CP by defining a Configuration Payload
for Internet host configuration for both IPv4 and IPv6. While the
IKEv2 approach reduces the number of exchanges, "Dynamic Host
Configuration Protocol (DHCPv4) Configuration of IPsec Tunnel Mode"
[RFC3456] points out that leveraging DHCP has advantages in terms of
address management integration, address pool management,
reconfiguration and fail-over.
Extensions to link layer protocols for the purpose of Internet,
transport or application layer configuration (including server
configuration) should be avoided. Such extensions can negatively
affect the properties of a link as seen by higher layers. For
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example, if a link layer protocol (or tunneling protocol) configures
individual IPv6 addresses and precludes using any other addresses,
then applications that desire "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6" [RFC4941] may not function well.
Similar issues may arise for other types of addresses, such as
Cryptographically Generated Addresses [RFC3972].
Avoiding lower layer dependencies is desirable even where the lower
layer is link independent. For example, while the Extensible
Authentication Protocol (EAP) may be run over any link satisfying its
requirements (see [RFC3748] Section 3.1), many link layers do not
support EAP and therefore Internet layer configuration mechanisms
with EAP dependencies would not be usable on all links that support
IP.
2.5. Configuration is Not Access Control
Network access authentication and authorization is a distinct problem
from Internet host configuration. Therefore network access
authentication and authorization is best handled independently of the
Internet and higher layer configuration mechanisms.
Having an Internet (or higher) layer protocol authenticate clients is
appropriate to prevent resource exhaustion of a scarce resource on
the server (such as IP addresses or prefixes), but not for preventing
hosts from obtaining access to a link. If the user can manually
configure the host, requiring authentication in order to obtain
configuration parameters (such as an IP address) has little value.
Network administrators who wish to control access to a link can
achieve this better using technologies like Port Based Network Access
Control [IEEE-802.1X]. Note that client authentication is not
required for Stateless DHCPv6 [RFC3736] since it does not result in
allocation of any limited resources on the server.
3. Additional Discussion
3.1. Reliance on General Purpose Mechanisms
Protocols should either be self-configuring (especially where fate
sharing is important), or use general-purpose configuration
mechanisms (such as DHCP or a service discovery protocol, as noted in
Section 3.2). The choice should be made taking into account the
architectural principles discussed in Section 2.
Taking into account the availability of existing general-purpose
configuration mechanisms, we see little need for development of
additional general-purpose configuration mechanisms.
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When defining a new host parameter, protocol designers should first
consider whether configuration is indeed necessary (see Section 2.1).
If configuration is necessary, in addition to considering fate
sharing (see Section 3.2.1), protocol designers should consider:
1. The organizational implications for administrators. For
example, routers and servers are often administered by
different sets of individuals, so that configuring a router
with server parameters may require cross-group collaboration.
2. Whether the need is to configure a set of interchangeable
servers or to select a server satisfying a particular set
of criteria. See Section 3.2.
3. Whether IP address(es) should configured or name(s).
See Section 3.3.
4. If IP address(es) are configured, whether IPv4 and
IPv6 addresses should be configured simultaneously or
separately. See Section 3.4.
5. Whether the parameter is a per-interface or a per-host
parameter. For example, configuration protocols
such as DHCP run on a per-interface basis and hence
are more appropriate for per-interface parameters.
6. How per-interface configuration affects host-wide behavior.
For example, whether the host should select a subset
of the per-interface configurations, or whether the
configurations are to merged, and if so, how this is
done. See Section 3.5.
3.2. Relationship between IP Configuration and Service Discovery
Higher-layer configuration often includes configuring server
addresses. The question arises as to how this differs from "service
discovery" as provided by Service Discovery protocols such as the
Service Location Protocol Version 2 (SLPv2) [RFC2608] or DNS-Based
Service Discovery (DNS-SD) [DNS-SD].
In Internet host configuration mechanisms such as DHCP, if multiple
server instances are provided, they are considered interchangeable.
For example, in a list of time servers, the servers are considered
interchangeable because they all provide the exact same service --
telling you the current time. In a list of local caching DNS
servers, the servers are considered interchangeable because they all
should give you the same answer to any DNS query. In service
discovery protocols, on the other hand, a host desires to find a
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server satisfying a particular set of criteria, which may vary by
request. When printing a document, it is not the case that any
printer will do. The speed, capabilities, and physical location of
the printer matter to the user.
Information learned via DHCP is typically learned once, at boot time,
and after that may be updated only infrequently (e.g. on DHCP lease
renewal), if at all. This makes it appropriate for information that
is relatively static and unchanging over these time intervals. Boot-
time discovery of server addresses is appropriate for service types
where there are a small number of interchangeable servers that are of
interest to a large number of clients. For example, listing time
servers in a DHCP packet is appropriate because an organization may
typically have only two or three time servers, and most hosts will be
able to make use of that service. Listing all the printers or file
servers at an organization is a lot less useful, because the list may
contain hundreds or thousands of entries, and on a given day a given
user may not use any of the printers in that list.
Service discovery protocols can support discovery of servers on the
Internet, not just those within the local administrative domain. For
example, see "Remote Service Discovery in the Service Location
Protocol (SLP) via DNS SRV" [RFC3832] and DNS-Based Service Discovery
[DNS-SD]. Internet host configuration mechanisms such as DHCP, on
the other hand, typically assume the server(s) in the local
administrative domain contain the authoritative set of information.
For the service discovery problem (i.e., where the criteria varies on
a per-request basis, even from the same host), protocols should
either be self-discovering (if fate sharing is critical), or use
general purpose service discovery mechanisms.
In order to avoid a dependency on multicast routing, it is necessary
for a host to either restrict discovery to services on the local link
or to discover the location of a Directory Agent (DA). Since the DA
may not be available on the local link, service discovery beyond the
local link is typically dependent on a mechanism for configuring the
DA address or name. As a result, service discovery protocols can
typically not be relied upon for obtaining basic Internet layer
configuration, although they can be used to obtain higher-layer
configuration parameters.
3.2.1. Fate Sharing
If a server (or set of servers) is needed to get a set of
configuration parameters, "fate sharing" ([RFC1958], Section 2.3) is
preserved if the servers are ones without which the parameters could
not be used, even if they were obtained via other means. The
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possibility of incorrect information being configured is minimized if
there is only one machine which is authoritative for the information
(i.e., there is no need to keep multiple authoritative servers in
sync). For example, learning default gateways via Router
Advertisements provides perfect fate sharing. That is, gateway
addresses can be obtained if and only if they can actually be used.
Similarly, obtaining DNS server configuration from a DNS server would
provide fate sharing since the configuration would only be obtainable
if the DNS server were available.
While fate sharing is a desirable property of a configuration
mechanism, in a number of situations fate sharing may not be
available. When utilized to discover services on the local link,
service discovery protocols typically provide for fate sharing, since
hosts providing service information typically also provide the
services. However, this is no longer the case when service discovery
is assisted by a Directory Agent (DA). First of all, the DA's list
of operational servers may not be current, so that it is possible for
the DA to provide clients with service information that is out of
date. For example, a DA's response to a client's service discovery
query may contain stale information about servers that are no longer
operational. Similarly, recently introduced servers might not yet
have registered themselves with the DA. Furthermore, the use of a DA
for service discovery also introduces a dependency on whether the DA
is operational, even though the DA is typically not involved in the
delivery of the service.
Similar limitations exist for other server-based configuration
mechanisms such as DHCP. Typically DHCP servers do not check for the
liveness of the configuration information they provide, or do not
discover new configuration information automatically. As a result,
there is no guarantee that configuration information will be current.
"IPv6 Host configuration of DNS Server Information Approaches"
[RFC4339] Section 3.3 discusses the use of well-known anycast
addresses for discovery of DNS servers. The use of anycast addresses
enables fate sharing, even where the anycast address is provided by
an unrelated server. However, in order to be universally useful,
this approach would require allocation of one or more well-known
anycast addresses for each service. Configuration of more than one
anycast address is desirable to allow the client to fail over faster
than would be possible from routing protocol convergence.
3.3. Discovering Names vs. Addresses
In discovering servers other than name resolution servers, it is
possible to either discover the IP addresses of the server(s), or to
discover names, each of which may resolve to a list of addresses.
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It is typically more efficient to obtain the list of addresses
directly, since this avoids the extra name resolution steps and
accompanying latency. On the other hand, where servers are mobile,
the name to address binding may change, requiring a fresh set of
addresses to be obtained. Where the configuration mechanism does not
support fate sharing (e.g. DHCP), providing a name rather than an
address can simplify operations, assuming that the server's new
address is manually or automatically updated in the DNS; in this case
there is no need to re-do parameter configuration, since the name is
still valid. Where fate sharing is supported (e.g. service discovery
protocols), a fresh address can be obtained by re-initiating
parameter configuration.
In providing the IP addresses for a set of servers, it is desirable
to distinguish which IP addresses belong to which servers. If a
server IP address is unreachable, this enables the host to try the IP
address of another server, rather than another IP address of the same
server, in case the server is down. This can be enabled by
distinguishing which addresses belong to the same server.
3.4. Dual Stack Issues
One use for learning a list of interchangeable server addresses is
for fault tolerance, in case one or more of the servers are
unresponsive. Hosts will typically try the addresses in turn, only
attempting to use the second and subsequent addresses in the list if
the first one fails to respond quickly enough. In such cases, having
the list sorted in order of expected likelihood of success will help
clients get results faster. For hosts that support both IPv4 and
IPv6, it is desirable to obtain both IPv4 and IPv6 server addresses
within a single list. Obtaining IPv4 and IPv6 addresses in separate
lists, without indicating which server(s) they correspond to,
requires the host to use a heuristic to merge the lists.
For example, assume there are two servers, A and B, each with one
IPv4 address and one IPv6 address. If the first address the host
should try is (say) the IPv6 address of server A, then the second
address the host should try, if the first one fails, would generally
be the IPv4 address of server B. This is because the failure of the
first address could either be due to server A being down, or due to
some problem with the host's IPv6 address, or due to a problem with
connectivity to server A. Trying the IPv4 address next is preferred
since the reachability of the IPv4 address is independent of all
potential failure causes.
If the list of IPv4 server addresses were obtained separate from the
list of IPv6 server addresses, a host trying to merge the lists would
not know which IPv4 addresses belonged to the same server as the IPv6
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address it just tried. This can be solved either by explicitly
distinguishing which addresses belong to which server or, more
simply, by configuring the host with a combined list of both IPv4 and
IPv6 addresses. Note that the same issue can arise with any
mechanism (e.g. DHCP, DNS, etc.) for obtaining server IP addresses.
Configuring a combined list of both IPv4 and IPv6 addresses gives the
configuration mechanism control over the ordering of addresses, as
compared with configuring a name and allowing the host resolver to
determine the address list ordering. See "DHCP Dual-Stack Issues"
[RFC4477] for more discussion of dual-stack issues in the context of
DHCP.
3.5. Relationship between Per-Interface and Per-Host Configuration
Parameters that are configured or acquired on a per-interface basis
can affect behavior of the host as a whole. Where only a single
configuration can be applied to a host, the host may need to
prioritize the per-interface configuration information in some way
(e.g. most trusted to least trusted). If the host needs to merge
per-interface configuration to produce a host-wide configuration, it
may need to take the union of the per-host configuration parameters
and order them in some way (e.g. highest speed interface to lowest
speed interface). Which procedure is to be applied and how this is
accomplished may vary depending on the parameter being configured.
Examples include:
Boot service configuration
While boot service configuration can be provided on
multiple interfaces, a given host may be limited in the
number of boot loads that it can handle simultaneously.
For example, a host not supporting virtualization may only
be capable of handling a single boot load at a time, or a
host capable of supporting N virtual machines may only be
capable of handling up to N simultaneous boot loads. As a
result, a host may need to select which boot load(s) it
will act on, out of those configured on a per-interface
basis. This requires that the host prioritize them (e.g.
most trusted to least trusted).
Name service configuration
While name service configuration is provided on a per-
interface basis, name resolution configuration typically
will affect behavior of the host as a whole. For example,
given the configuration of DNS server addresses and
searchlist parameters on each interface, the host
determines what sequence of name service queries is to be
sent on which interfaces.
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Since the algorithms used to determine per-host behavior based on
per-interface configuration can affect interoperability, it is
important for these algorithms to be understood by implementers. We
therefore recommend that documents defining per-interface mechanisms
for acquiring per-host configuration (e.g. DHCP or IPv6 Router
Advertisement options) include guidance on how to deal with multiple
interfaces. This may include discussions of the following items:
1. Merging. How are per-interface configurations combined to
produce a per-host configuration? Is a single configuration
selected, or is the union of the configurations taken?
2. Prioritization. Are the per-interface configurations
prioritized as part of the merge process? If so, what are
some of the considerations to be taken into account in
prioritization?
4. Security Considerations
Secure IP configuration presents a number of challenges. In addition
to denial-of-service and man-in-the-middle attacks, attacks on
configuration mechanisms may target particular parameters. For
example, attackers may target DNS server configuration in order to
support subsequent phishing or pharming attacks such as those
described in "New trojan in mass DNS hijack" [DNSTrojan]. A number
of issues exist with various classes of parameters, as discussed in
Section 2.6, "IPv6 Neighbor Discovery (ND) Trust Models and Threats"
[RFC3756] Section 4.2.7, "Authentication for DHCP Messages" [RFC3118]
Section 1.1, and "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)" [RFC3315] Section 23. Given the potential vulnerabilities,
hosts often restrict support for DHCP options to the minimum set
required to provide basic TCP/IP configuration.
Since boot configuration determines the boot image to be run by the
host, a successful attack on boot configuration could result in an
attacker gaining complete control over a host. As a result, it is
particularly important that boot configuration be secured.
Approaches to boot configuration security are described in
"Bootstrapping Clients using the Internet Small Computer System
Interface (iSCSI) Protocol" [RFC4173] and "Preboot Execution
Environment (PXE) Specification" [PXE].
4.1. Configuration Authentication
The techniques available for securing Internet layer configuration
are limited. While it is technically possible to perform a very
limited subset of IP networking operations without an IP address, the
capabilities are severely restricted; a host without an IP address
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can only receive IP packets sent to the broadcast or a multicast
address. Configuration of an IP address enables the use of IP
fragmentation; packets sent from the unknown address cannot be
reliably reassembled, since fragments from multiple hosts using the
unknown address might be reassembled into a single IP packet.
Without an IP address, it is not possible to take advantage of
security facilities such as IPsec, specified in "Security
Architecture for the Internet Protocol" [RFC4301], or Transport Layer
Security (TLS) [RFC5246]. As a result, configuration security is
typically implemented within the configuration protocols themselves.
PPP [RFC1661] does not support secure negotiation within IPv4CP
[RFC1332] or IPv6CP [RFC5072], enabling an attacker with access to
the link to subvert the negotiation. In contrast, IKEv2 [RFC4306]
provides encryption, integrity and replay protection for
configuration exchanges.
Where configuration packets are only expected to originate on
particular links or from particular hosts, filtering can help control
configuration spoofing. For example, a Network Access Server (NAS)
acting as a DHCP relay can only permit incoming DHCP packets sent to
the client port originating from DHCP server addresses. To prevent
spoofing, communication between the DHCP Relay and Server can be
authenticated and integrity protected using a mechanism such as
IPsec. Where configuration packets can only originate on a wired
link, incoming configuration packets on wireless links can be
discarded, such as IPv6 Router Advertisement packets (ICMP Type 134),
DHCPv4 packets sent to the client port (68), and DHCPv6 packets sent
to the client port (546).
Internet layer secure configuration mechanisms include SEcure
Neighbor Discovery (SEND) [RFC3971] for IPv6 stateless address
autoconfiguration [RFC4862], or DHCP authentication for stateful
address configuration. DHCPv4 [RFC2131] initially did not include
support for security; this was added in "Authentication for DHCP
Messages" [RFC3118]. DHCPv6 [RFC3315] included security support.
However, DHCP authentication is not widely implemented for either
DHCPv4 or DHCPv6.
Higher layer configuration can make use of a wider range of security
techniques. When DHCP authentication is supported, higher-layer
configuration parameters provided by DHCP can be secured. However,
even if a host does not support DHCPv6 authentication, higher-layer
configuration via Stateless DHCPv6 [RFC3736] can still be secured
with IPsec.
Possible exceptions can exist where security facilities are not
available until later in the boot process. It may be difficult to
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secure boot configuration even once the Internet layer has been
configured, if security functionality is not available until after
boot configuration has been completed. For example, it is possible
that Kerberos, IPsec or TLS will not be available until later in the
boot process; see "Bootstrapping Clients using the Internet Small
Computer System Interface (iSCSI) Protocol" [RFC4173] for discussion.
Where public key cryptography is used to authenticate and integrity-
protect configuration, hosts need to be configured with trust anchors
in order to validate received configuration messages. For a node
that visits multiple administrative domains, acquiring the required
trust anchors may be difficult. This is left as an area for future
work.
5. IANA Considerations
This document has no actions for IANA.
6. References
6.1. Informative References
[3GPP-24.008]
3GPP TS 24.008 V5.8.0, "Mobile radio interface Layer 3
specification; Core network protocols; Stage 3 (Release 5)",
June 2003.
[DNSTrojan]
Goodin, D., "New trojan in mass DNS hijack", The Register,
December 5, 2008, http://www.theregister.co.uk/2008/12/05/
new_dnschanger_hijacks/
[IEN116] J. Postel, "Internet Name Server", IEN 116, August 1979,
http://www.ietf.org/rfc/ien/ien116.txt
[IEEE-802.1X]
Institute of Electrical and Electronics Engineers, "Local and
Metropolitan Area Networks: Port-Based Network Access
Control", IEEE Standard 802.1X-2004, December 2004.
[DNS-SD] Cheshire, S., and M. Krochmal, "DNS-Based Service Discovery",
Internet-Draft (work in progress), draft-cheshire-dnsext-dns-
sd-05.txt, September 2008.
[mDNS] Cheshire, S. and M. Krochmal, "Multicast DNS", June 2005.
http://files.multicastdns.org/draft-cheshire-dnsext-
multicastdns.txt
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[PXE] Henry, M. and M. Johnston, "Preboot Execution Environment
(PXE) Specification", September 1999,
http://www.pix.net/software/pxeboot/archive/pxespec.pdf
[RFC768] Postel, J., "User Datagram Protocol", RFC 768, August, 1980.
[RFC1001] NetBIOS Working Group in the Defense Advanced Research
Projects Agency, Internet Activities Board, and End-to-End
Services Task Force, "Protocol standard for a NetBIOS service
on a TCP/UDP transport: Concepts and methods", STD 19, RFC
1001, March 1987.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1332] McGregor, G., "PPP Internet Control Protocol", RFC 1332,
Merit, May 1992.
[RFC1350] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC
1350, July 1992.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
1661, July 1994.
[RFC1877] Cobb, S., "PPP Internet Protocol Control Protocol Extensions
for Name Server Addresses", RFC 1877, December 1995.
[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.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[RFC2608] Guttman, E., et al., "Service Location Protocol, Version 2",
RFC 2608, June 1999.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923,
September 2000.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[RFC3315] Droms, R., Ed., Bound, J., Volz,, B., Lemon, T., Perkins, C.
and M. Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
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[RFC3344] Perkins, C., "IP Mobility Support for IPv4", RFC 3344, August
2002.
[RFC3397] Aboba, B. and S. Cheshire, "Dynamic Host Configuration
Protocol (DHCP) Domain Search Option", RFC 3397, November
2002.
[RFC3456] Patel, B., Aboba, B., Kelly, S. and V. Gupta, "Dynamic Host
Configuration Protocol (DHCPv4) Configuration of IPsec Tunnel
Mode", RFC 3456, January 2003.
[RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R., Beame,
C., Eisler, M. and D. Noveck, "Network File System (NFS)
version 4 Protocol", RFC 3530, April 2003.
[RFC3720] Satran, J., Meth, K., Sapuntzakis, C. Chadalapaka, M. and E.
Zeidner, "Internet Small Computer Systems Interface (iSCSI)",
RFC 3720, April 2004.
[RFC3736] Droms, R., "Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6", RFC 3736, April 2004.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
[RFC3756] Nikander, P., Kempf, J. and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.
[RFC3775] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[RFC3818] Schryver, V., "IANA Considerations for the Point-to-Point
Protocol (PPP)", RFC 3818, BCP 88, June 2004.
[RFC3832] Zhao, W., Schulzrinne, H., Guttman, E., Bisdikian, C. and W.
Jerome, "Remote Service Discovery in the Service Location
Protocol (SLP) via DNS SRV", RFC 3832, July 2004.
[RFC3898] Kalusivalingam, V., "Networking Information Service (NIS)
Configuration Options for Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3898, October 2004.
[RFC3971] Arkko, J., Kempf, J., Sommerfeld, B., Zill, B. and P.
Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March
2005.
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[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)", RFC
3972, March 2005.
[RFC4171] Tseng, J., Gibbons, K., Travostino, F., Du Laney, C. and J.
Souza, "Internet Storage Name Service (iSNS), RFC 4171,
September 2005.
[RFC4173] Sarkar, P., Missimer, D. and C. Sapuntzakis, "Bootstrapping
Clients using the iSCSI Protocol", RFC 4173, September 2005.
[RFC4174] Monia, C., Tseng, J. and K. Gibbons, "The IPv4 Dynamic Host
Configuration Protocol (DHCP) Option for the Internet Storage
Name Service", RFC 4174, September 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, December 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
4306, December 2005.
[RFC4339] Jeong, J., "IPv6 Host Configuration of DNS Server Information
Approaches", RFC 4339, February 2006.
[RFC4477] Chown, T., Venaas, S. and C. Strauf, "Dynamic Host
Configuration Protocol (DHCP): IPv4 and IPv6 Dual-Stack
Issues", RFC 4477, May 2006.
[RFC4578] Johnston, M. and S. Venaas, "Dynamic Host Configuration
Protocol (DHCP) Options for the Intel Preboot eXecution
Environment (PXE)", RFC 4578, November 2006.
[RFC4795] Aboba, B., Thaler, D. and L. Esibov, "Link-Local Multicast
Name Resolution (LLMNR)", RFC 4795, January 2007.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC4862] Thomson, S., Narten, T. and T. Jinmei, "IPv6 Stateless Address
Autoconfiguration", RFC 4862, September 2007.
[RFC4941] Narten, T., Draves, R. and S. Krishnan, "Privacy Extensions
for Stateless Address Autoconfiguration in IPv6", RFC 4941,
September 2007.
[RFC5072] Varada, S., Haskins D. and E. Allen, "IP Version 6 over PPP",
RFC 5072, September 2007.
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[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[STD3] Braden, R., "Requirements for Internet Hosts -- Communication
Layers", STD 3, RFC 1122, and "Requirements for Internet Hosts
-- Application and Support", STD 3, RFC 1123, October 1989.
Acknowledgments
Elwyn Davies, Bob Hinden, Pasi Eronen, Jari Arkko, Pekka Savola,
James Kempf, Ted Hardie and Alfred Hoenes provided valuable input on
this document.
Appendix A - 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
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Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
EMail: bernarda@microsoft.com
Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
EMail: dthaler@microsoft.com
Loa Andersson
Acreo AB
EMail: loa@pi.nu
Stuart Cheshire
Apple Computer, Inc.
1 Infinite Loop
Cupertino, CA 95014
EMail: chesire [at] apple [dot] com
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