One document matched: draft-ietf-dhc-dna-ipv4-15.txt
Differences from draft-ietf-dhc-dna-ipv4-14.txt
DHC Working Group Bernard Aboba
INTERNET-DRAFT Microsoft Corporation
Category: Proposed Standard
<draft-ietf-dhc-dna-ipv4-15.txt>
9 August 2005
Detecting Network Attachment (DNA) in IPv4
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Copyright Notice
Copyright (C) The Internet Society 2005.
Abstract
The time required to detect movement between networks, and to obtain
(or continue to use) an IPv4 configuration may be significant as a
fraction of the total handover latency in moving between points of
attachment. This document synthesizes from experience in the
deployment of hosts supporting ARP, DHCP, and IPv4 Link-Local
addresses a set of steps known as Detecting Network Attachment for
IPv4 (DNAv4), in order to decrease the handover latency in moving
between points of attachment.
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Table of Contents
1. Introduction.............................................. 3
1.1 Requirements .................................... 3
1.2 Terminology ..................................... 3
2. Overview ................................................. 5
2.1 Most Likely Network(s) .......................... 6
2.2 Reachability Test ............................... 6
2.3 IPv4 Address Acquisition ........................ 8
2.4 IPv4 Link-Local Addresses ....................... 9
3. Constants ................................................ 10
4. IANA Considerations ...................................... 10
5. Security Considerations .................................. 11
6. References ............................................... 11
6.1 Normative references ............................ 11
6.2 Informative references .......................... 12
Acknowledgments .............................................. 13
Authors' Addresses ........................................... 13
Appendix A - Hints ........................................... 14
A.1 Introduction .................................... 14
A.2 Link Layer Hints ................................ 15
A.3 Internet Layer Hints ............................ 16
Intellectual Property Statement .............................. 17
Disclaimer of Validity ....................................... 17
Copyright Statement .......................................... 18
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1. Introduction
The time required to detect movement between networks and to obtain
(or continue to use) an operable IPv4 configuration may be
significant as a fraction of the total handover latency in moving
between points of attachment.
This document synthesizes from experience in the deployment of hosts
supporting ARP [RFC826], DHCP [RFC2131], and IPv4 Link-Local
addresses [RFC3927] a set of steps known as Detecting Network
Attachment for IPv4 (DNAv4). DNAv4 optimizes the (common) case of
reattachment to a network that one has been connected to previously
by attempting to re-use a previous (but still valid) configuration,
reducing the reattachment time to a few milliseconds on LANs. Since
this procedure is dependent on the ARP protocol, it is not suitable
for use on media that do not support ARP.
1.1. Requirements
In this document, several words are used to signify the requirements
of the specification. The key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
1.2. Terminology
This document uses the following terms:
ar$sha
ARP packet field: Sender Hardware Address [RFC826]. The hardware
(MAC) address of the originator of an ARP packet.
ar$spa
ARP packet field: Sender Protocol Address [RFC826]. For IP Address
Resolution this is the IPv4 address of the sender of the ARP
packet.
ar$tha
ARP packet field: Target Hardware Address [RFC826]. The hardware
(MAC) address of the target of an ARP packet.
ar$tpa
ARP packet field: Target Protocol Address [RFC826]. For IPv4
Address Resolution, the IPv4 address for which one desires to know
the hardware address.
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DHCP client
A DHCP client or "client" is an Internet host using the Dynamic
Host Configuration protocol (DHCP) [RFC2131] to obtain
configuration parameters such as a network address.
DHCP server
A DHCP server or "server" is an Internet host that returns
configuration parameters to DHCP clients.
Link A communication facility or medium over which network nodes can
communicate. Each link is associated with a minimum of two
endpoints. Each link endpoint has a unique link-layer identifier.
Link Down
An event provided by the link layer that signifies a state change
associated with the interface no longer being capable of
communicating data frames; transient periods of high frame loss are
not sufficient. DNAv4 does not utilize "Link Down" indications.
Link Layer
Conceptual layer of control or processing logic that is responsible
for maintaining control of the data link. The data link layer
functions provide an interface between the higher-layer logic and
the data link. The link layer is the layer immediately below IP.
Link Up
An event provided by the link layer that signifies a state change
associated with the interface becoming capable of communicating
data frames.
Most Likely Networks (MLNs)
The attached network(s) determined by the host to be most likely.
Point of Attachment
The link endpoint on the link to which the host is currently
connected.
Routable address
In this specification, the term "routable address" refers to any
IPv4 address other than an IPv4 Link-Local address. This includes
private addresses as specified in [RFC1918].
Operable address
In this specification, the term "operable address" refers to either
a static IPv4 address, or an address assigned via DHCPv4 which has
not been relinquished, and whose lease has not yet expired.
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2. Overview
DNAv4 consists of three phases: determination of the Most Likely
Networks (MLNs), reachability testing, and IPv4 address acquisition.
On connecting to a new point of attachment, the host responds to a
"Link Up" indication from the link layer by carrying out the DNAv4
procedure. Based on the networks that the host has most recently
connected to as well as hints available from the link and Internet
layers, the host determines the "Most Likely Networks" (MLNs) and
determines whether it has an operable IPv4 configuration associated
with each of them.
If the host believes that it has an operable IPv4 configuration on a
MLN, it performs a reachability test in order to confirm that
configuration. The reachability test is designed to verify bi-
directional connectivity to the default gateway(s) on the MLN. If
the reachability test is successful, the host SHOULD continue to use
an operable routable IPv4 address without needing to re-acquire it,
thereby allowing the host to bypass DHCPv4 as well as Duplicate
Address Detection (DAD).
Since DNAv4 represents a performance optimization, it is important to
avoid compromising robustness. In some circumstances, DNAv4 may
result in a host successfully verifying an existing IPv4
configuration where attempting to obtain configuration via DHCPv4
would fail (such as when the DHCPv4 server is down).
To improve robustness, this document suggests that hosts behave
conservatively with respect to assignment of IPv4 Link-Local
addresses [RFC3927], configuring them only in situations in which
they can do no harm. Experience has shown that IPv4 Link-Local
addresses are often assigned inappropriately, compromising both
performance and connectivity.
In implementations where MLN selection is dependent on hints provided
to the client, the performance of DNAv4 may be dependent on the
reliability of the hints. However, the host will ultimately
determine the correct IPv4 configuration even in the presence of
misleading hints.
Where there is more than one MLN, the host can test reachability to
the MLNs in serial or in parallel. An implementation can also
attempt to obtain IPv4 configuration via DHCPv4 in parallel with one
or more reachability tests, with the host using the first answer
returned. These optimizations improve performance and reduce the
reliance on link and Internet layer hints, which may not be present
or may be misleading.
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Attempting to obtain IPv4 configuration via DHCPv4 in parallel with
reachability testing is particularly valuable in implementations that
only test reachability of a single MLN. Since confirming failure of
a reachability test requires a timeout, mistakes are costly and
sending a DHCPREQUEST from the INIT-REBOOT state, as described in
[RFC2131] Section 3.2 and 4.3.2 may complete more quickly than the
reachability test.
DNAv4 does not increase the likelihood of an address conflict. The
DNAv4 procedure is only carried out when the host has an operable
IPv4 configuration on one or more MLNs, implying that duplicate
address detection has previously been completed. Restrictions on
sending ARP Requests and Responses are described in Section 2.2.1.
2.1. Most Likely Networks (MLNs)
In order to determine the MLN(s), it is assumed that the host saves
to stable storage parameters relating to the networks it connects to:
[1] The IPv4 and MAC address of the default gateway(s) on
each network.
[2] The link type, such as whether the link utilizes
Ethernet, or 802.11 adhoc or infrastructure mode.
[3] Link and Internet layer hints associated with each
network. Appendix A discusses hints useful for the
determination of MLNs.
An implementation may select one or more MLNs by matching received
hints against network parameters previously stored, by including
networks it has most recently connected to, or by some combination of
these strategies.
2.2. Reachability Test
If the host has an operable routable IPv4 address on a MLN, a host
conforming to this specification SHOULD perform a reachability test
for that MLN, in order to confirm the configuration.
The host skips the reachability test for a MLN if any of the
following conditions are true:
[a] The host does not have an operable routable IPv4
address on a MLN. In this case, the reachability
test cannot confirm that the host has an operable
routable IPv4 address, so completing the
reachability test would serve no purpose.
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A host MUST NOT use the reachability test to
confirm configuration of an IPv4 Link-Local
address.
[b] The host does not know the default gateway(s) on
a MLN. In this case, insufficient information
is available to carry out the reachability test.
[c] If secure detection of network attachment is required.
The reachability test utilizes ARP which is insecure,
whereas DHCPv4 can be secured via DHCPv4 authentication,
described in [RFC3118]. See Section 5 for details.
For a particular MLN, the host MAY test the reachability of the
primary default gateway, or it MAY test reachability of the primary
and secondary default gateways in series or in parallel. In order to
ensure configuration validity, the host SHOULD only configure
default gateway(s) which pass the reachability test.
In situations where more than one network is available on a given
link, and more than one reachability test is performed in parallel,
potentially with an attempt to obtain IPv4 configuration via DHCPv4,
it is possible for the host to confirm more than one configuration.
In this case, a DNAv4 implementation SHOULD prefer the configuration
provided via DHCPv4.
2.2.1. Packet Format
The reachability test is performed by sending an ARP Request. The
host MUST set the target protocol address (ar$tpa) to the IPv4
address of the default gateway being tested, and the sender protocol
address field (ar$spa) to its own IPv4 address. The ARP Request MUST
use the host's MAC address as the source, and the default gateway MAC
address as the destination. The host includes its MAC address in the
sender hardware address field (ar$sha), and sets the target hardware
address field (ar$tha) to 0.
If a valid ARP Reply is received, the MAC address in the sender
hardware address field (ar$sha) in the ARP Reply is matched against
the target hardware address field (ar$tpa) in the ARP Request, and
the and the IPv4 address in the sender protocol address field
(ar$spa) of the ARP Reply is matched against the target protocol
address field (ar$tpa) in the ARP Request. If a match is found, then
if the host has an operable routable IPv4 address on the matched
network, the host continues to use that IPv4 address, subject to the
lease re- acquisition and expiration behavior described in [RFC2131],
Section 4.4.5.
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The risk of an address conflict is greatest when the host moves
between private networks, since in this case the completion of
Duplicate Address Detection on the former network does not provide
assurance against an address conflict on the new network. Until a
host with a private address has confirmed the operability of its IPv4
configuration, it SHOULD NOT respond to ARP Requests, and SHOULD NOT
broadcast ARP Requests containing its address within the sender
protocol address field (ar$spa). However, where the host has an
operable routable non-private address on a MLN, it MAY send ARP
Requests using its address within the sender protocol address field
(ar$spa) prior to confirming its IPv4 configuration, and MAY respond
to ARP Requests.
Sending an ICMP Echo Request [RFC792] to the default gateway IPv4
address does not provide the same level of assurance since this may
require an ARP Request/Reply exchange. Where the host has moved
between two private networks, this could result in ARP cache
pollution.
Where a host moves from one private network to another, an ICMP Echo
Request can result in an ICMP Echo Response even when the default
gateway has changed, as long as the IPv4 address remains the same.
This can occur, for example, where a host moves from one home
network using prefix 192.168/16 to another one. In addition, if the
ping is sent with TTL > 1, then an ICMP Echo Response can be received
from an off-link gateway. As a result, if the MAC address of the
default gateway is not checked, the host can mistakenly confirm
attachment to a MLN, potentially resulting in an address conflict.
As a result, sending of an ICMP Echo Request SHOULD NOT be used as a
substitute for the DNAv4 procedure.
If the initial ARP Request does not elicit a response, the host waits
for REACHABILITY_TIMEOUT. Where IPv4 address acquisition occurs in
parallel, the host MAY retransmit; otherwise the host SHOULD move on
to the IPv4 address acquisition phase. If a valid ARP Reply is
received, but cannot be matched against known networks, the host
assumes it does not have an operable IPv4 configuration.
2.3. IPv4 Address Acquisition
If the host has an operable routable IPv4 address on one or more
MLNs, but the reachability test(s) fail, the host SHOULD attempt to
revalidate the configuration by entering the INIT-REBOOT state, and
sending a DHCPREQUEST to the broadcast address as specified in
[RFC2131] Section 4.4.2. As noted in Section 2, it is also possible
for IPv4 address acquisition to occur in parallel with the
reachability test.
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If the host does not have an operable routable IPv4 address on any
MLN, the host enters the INIT state and sends a DHCPDISCOVER packet
to the broadcast address, as described in [RFC2131] Section 4.4.1.
If the host supports the Rapid Commit Option [RFC4039], it is
possible that the exchange can be shortened from a 4-message exchange
to a 2-message exchange.
If the host does not receive a response to a DHCPREQUEST or
DHCPDISCOVER, then it retransmits as specified in [RFC2131] Section
4.1.
As discussed in [RFC2131], Section 4.4.4, a host in INIT or REBOOTING
state that knows the address of a DHCP server may use that address in
the DHCPDISCOVER or DHCPREQUEST rather than the IPv4 broadcast
address. In the INIT-REBOOT state a DHCPREQUEST is sent to the
broadcast address so that the host will receive a response regardless
of whether the previously configured IPv4 address is correct for the
network to which it has connected.
Sending a DHCPREQUEST to the unicast address in INIT-REBOOT state is
not appropriate, since if the DHCP client has moved to another
subnet, a DHCP server response cannot be routed back to the client
since the DHCPREQUEST will bypass the DHCP relay and will contain an
invalid source address.
2.4. IPv4 Link-Local Addresses
To avoid inappropriate assignment of IPv4 Link-Local addresses, it is
recommended that hosts behave conservatively, assigning them only
when they can do no harm. As described in [RFC3927] Section 1.9, use
of a routable address is preferred when it is available:
2. If a host finds that an interface that was previously
configured with an IPv4 Link-Local address now has an operable
routable address available, the host MUST use the routable
address when initiating new communications, and MUST cease
advertising the availability of the IPv4 Link-Local address
through whatever mechanisms that address had been made known to
others.
Where the host does not have an operable routable IPv4 address on any
MLN, the host MAY configure an IPv4 Link-Local address prior to
entering the INIT state and sending a DHCPDISCOVER packet, as
described in [RFC2131] Section 2.3. However, should a routable IPv4
address be obtained, the IPv4 Link-Local address is deprecated, as
noted in [RFC3927] Section 1.9.
Where a host has an operable routable IPv4 address on one or more
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MLNs, but the DHCP client does not receive a response after employing
the retransmission algorithm, [RFC2131] Section 3.2 states that the
client MAY choose to use the previously allocated network address and
configuration parameters corresponding to one of the MLNs for the
remainder of the unexpired lease. Where a host can confirm that it
remains connected to a network on which it possesses an operable
routable IPv4 address, that address SHOULD be used, rather than
assigning a IPv4 Link-Local address.
Since a IPv4 Link-Local address is often configured because a DHCP
server failed to respond to an initial query or is inoperative for
some time, it is desirable to abandon the IPv4 Link-Local address
assignment as soon as an IPv4 address lease can be obtained.
As described in [RFC3927] Appendix A, once a Link-Local IPv4 address
is assigned, existing implementations do not query the DHCPv4 server
again for five minutes. This behavior violates [RFC2131] Section
4.1:
The retransmission delay SHOULD be doubled with
subsequent retransmissions up to a maximum of 64 seconds.
Instead of waiting for five minutes, a DHCP client should continue to
retry every 64 seconds, even after allocating a IPv4 Link-Local
address. If the DHCP client succeeds in obtaining a routable
address, then the IPv4 Link-Local address is deprecated, as noted in
[RFC3927] Section 1.9.
Since it is inevitable that hosts will inappropriately configure IPv4
Link-Local addresses at some point, hosts with routable IPv4
addresses need to be able to respond to peers with IPv4 Link-Local
addresses, as per [RFC3927] Section 1.8. For example, a host
configured with a routable address may receive a datagram from a
link-local source address. In order to respond, the host will use
ARP to resolve the target hardware address and send the datagram
directly, not to a router for forwarding.
3. Constants
The suggested default value of REACHABILITY_TIMEOUT is 200 ms. This
value was chosen so as to accommodate the maximum retransmission
timer likely to be experienced on an Ethernet network.
4. IANA Considerations
This specification does not request the creation of any new parameter
registries, nor does it require any other IANA assignments.
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5. Security Considerations
Detecting Network Attachment for IPv4 (DNAv4) is based on ARP and
DHCP and inherits the security vulnerabilities of these two
protocols.
ARP [RFC826] traffic is not secured, so that an attacker gaining
access to the network can spoof a response to the reachability test
described in Section 2.2, leading the querier to falsely conclude
that it is attached to a network that it is not connected to.
Similarly, where DHCPv4 traffic is not secured, an attacker could
masquerade as a DHCPv4 server, in order to convince the host that it
was attached to a particular network. This and other threats
relating to DHCPv4 are described in "Authentication for DHCP
Messages" [RFC3118].
The effect of these attacks will typically be limited to denial of
service, unless the host utilizes its IP configuration for other
purposes, such as security configuration or location determination.
For example, a host that disables its personal firewall based on
evidence that it had attached to a home network could be compromised
by spoofing of the DNAv4 reachability test. In general, adjustment
of the security configuration based on DNAv4 is NOT RECOMMENDED.
Hosts that depend on secure IP configuration SHOULD NOT use DNAv4,
but SHOULD instead utilize DHCP authentication [RFC3118], possibly in
combination with the Rapid Commit Option [RFC4039].
6. References
6.1. Normative References
[RFC792] Postel, J., "Internet Control Message Protocol", RFC 792,
USC/Information Sciences Institute, September 1981.
[RFC826] D. Plummer, "An Ethernet Address Resolution Protocol -or-
Converting Network Addresses to 48-bit Ethernet Address for
Transmission on Ethernet Hardware", STD 37, RFC 826, November
1982.
[RFC1256] Deering, S., "ICMP Router Discovery Messages", RFC 1256, Xerox
PARC, September 1991.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March, 1997.
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[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[RFC3927] Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration
of IPv4 Link-Local Addresses", RFC 3927, May 2005.
6.2. Informative References
[DNALINK] Yegin, A., Njedjou, E., Veerepalli, S., Montavont, N. and T.
Noel, "Link-layer Event Notifications for Detecting Network
Attachments", draft-ietf-dna-link-information-01.txt, February
2005.
[RFC1058] Hedrick, C., "Routing Information Protocol", RFC 1058, June
1988.
[RFC1661] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, Daydreamer, July 1994.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and
E. Lear, "Address Allocation for Private Internets", RFC 1918,
February 1996.
[RFC2453] Malkin, G., "RIP Version 2", RFC 2453, STD 56, November 1998.
[RFC3580] Congdon, P., Aboba, B., Smith, A., Zorn, G., and J. Roese,
"IEEE 802.1X Remote Authentication Dial In User Service
(RADIUS) Usage Guidelines", RFC 3580, September 2003.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
[RFC4039] Park, S., Kim, P., and B. Volz, "Rapid Commit Option for the
Dynamic Host Configuration Protocol version 4 (DHCPv4)", RFC
4039, March 2005.
[IEEE-802.1AB]
IEEE Standards for Local and Metropolitan Area Networks:
Station and Media Access Control - Connectivity Discovery,
IEEE Std 802.1AB, March 2005.
[IEEE-802.1X]
IEEE Standards for Local and Metropolitan Area Networks: Port
based Network Access Control, IEEE Std 802.1X-2004, December
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2004.
[IEEE-802]
IEEE Standards for Local and Metropolitan Area Networks:
Overview and Architecture, ANSI/IEEE Std 802, 1990.
[IEEE-802.1Q]
IEEE Standards for Local and Metropolitan Area Networks: Draft
Standard for Virtual Bridged Local Area Networks, P802.1Q,
January 1998.
[IEEE-802.11]
Information technology - Telecommunications and information
exchange between systems - Local and metropolitan area
networks - Specific Requirements Part 11: Wireless LAN Medium
Access Control (MAC) and Physical Layer (PHY) Specifications,
IEEE Std. 802.11-2003, 2003.
Acknowledgments
The authors would like to acknowledge Greg Daley of Monash
University, Erik Guttman, James Carlson, and Erik Nordmark of Sun
Microsystems, Ralph Droms of Cisco Systems, Ted Lemon of Nominum,
John Loughney of Nokia, Thomas Narten of IBM, Stuart Cheshire of
Apple Computer and David Hankins of ISC for contributions to this
document.
Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
EMail: bernarda@microsoft.com
Phone: +1 425 818 4011
Fax: +1 425 936 7329
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Appendix A - Hints
A.1 Introduction
In order to assist in detecting network attachment, information
associated with each network may be retained by the host. Based on
Internet and link-layer information, the host may be able to make an
educated guess as to whether it has moved between networks, or has
remained on the same network, as well as whether it has connected to
an infrastructure or adhoc network.
If the host is likely to have moved between networks, it may be
possible to make an educated guess as to which network it has moved
to. Since an educated guess may be incorrect, prior to concluding
that the host remains on the same network, further tests (such as a
reachability test or a DHCPREQUEST sent from the INIT-REBOOT state)
are REQUIRED.
In practice, it is necessary for hints to be highly reliable before
they are worth considering, if the penalty paid for an incorrect hint
is substantial. For this reason, implementations may wish to test
reachability to multiple MLNs simultaneously, or attempt IPv4 address
acquisition in parallel with one or more reachability tests.
In order to examine the tradeoffs in implementations that only test
reachability to a single MLN, assume that a DHCPREQUEST requires
DHCPREQUEST_TIME to determine if a host has remained on the same
network, while a reachability test typically completes in REACH_TIME
and times out in REACHABILITY_TIMEOUT, after which a DHCPREQUEST is
sent.
If a hint that the host has remained on the same network cannot be
confirmed x fraction of the time, then it is only worth considering
if:
DHCPREQUEST_TIME > (1 - x) * REACH_TIME +
x * (REACHABILITY_TIMEOUT + DHCPREQUEST_TIME)
x < DHCPREQUEST_TIME - REACH_TIME
----------------------------------------------------
REACHABILITY_TIMEOUT + DHCPREQUEST_TIME - REACH_TIME
If we assume that DHCPREQUEST_TIME = 50 ms, REACH_TIME = 10 ms, and
REACHABILITY_TIMEOUT = 200ms, then:
x < (50 - 10)/(200 + 50 - 10) = 16.67 percent
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In this example, if the hint cannot be confirmed more than one sixth
of the time, it is not worth considering. A hint may not be
confirmable because it is wrong (the host has changed networks) or
because of packet loss in the reachability test.
If instead in the above example IPv4 address acquisition were carried
out simultaneously with the reachability test, then performance would
not suffer, even where hints are unreliable.
A.2 Link-Layer Hints
"Link-layer Event Notifications for Detecting Network Attachments"
[DNALINK] discusses the definition of link layer events on various
media. Therefore this section focuses solely on hints useful in
determining MLN(s).
For networks running IPv4 over PPP [RFC1661], IPv4 parameters
negotiated in IPCP provide direct information on whether a previously
obtained address remains operable on the link.
On media supporting EAP [RFC3748], network identification information
may be passed within the EAP-Request/Identity or within an EAP method
exchange. For example, the EAP-Request/Identity may contain the name
of the authenticator. During the EAP method exchange the
authenticator may supply information that may be helpful in
identifying the network to which the device is attached. However,
as noted in [RFC3580], it is possible for the VLANID defined in
[IEEE-802.1Q] to be assigned dynamically, so that static
advertisements may not prove definitive.
On IEEE 802 [IEEE-802] wired networks, hints can be obtained via the
Link Layer Discovery Protocol (LLDP) defined in [IEEE-802.1AB]. LLDP
advertisements can include the chassis ID, which represents the
authenticator's chassis identification, enabling a host to determine
if it has attached to a previously encountered device. However,
since a device may support dynamic VLANs, re-attachment does not
necessarily imply that the VLAN has remained the same, although this
is likely.
LLDP also enables advertisement of the port's VLAN identifier, as
well as a VLAN name, allowing the host to determine whether it has
attached to a VLAN on which it had previously obtained an operable
IPv4 configuration. Since such an advertisement cannot be heard
until 802.1X authentication has completed, the advertised VLAN will
reflect a dynamic VLAN assignment if one has been made, so that it is
likely to be definitive.
In IEEE 802.11 [IEEE-802.11] stations provide information in Beacon
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and/or Probe Response messages, such as the SSID, BSSID, and
capabilities, as well as information on whether the station is
operating in Infrastructure or Ad hoc mode. As described in
[RFC3580], it is possible to assign a Station to a VLAN dynamically,
based on the results of IEEE 802.1X [IEEE-802.1X] authentication.
This implies that a single SSID may offer access to multiple VLANs,
and in practice most large WLAN deployments offer access to multiple
subnets. While a Station associating to the same SSID may not remain
within the same subnet, a Station associating to a different SSID is
likely to have changed subnets.
In IEEE 802.11, the SSID is a non-unique identifier, and SSIDs such
as "default", "linksys" and "tsunami" are often configured by
manufacturers by default. As a result, matching an advertised SSID
against those of previously encountered networks may be misleading.
Where an SSID known to be configured by default is encountered, it is
recommended that the BSSID be stored and subsequently compared
against the advertised BSSID to determine whether a match exists.
In order to provide additional guidance on the subnets to which a
given AP offers access, additional subnet-related Information
Elements (IEs) have been proposed for addition to the IEEE 802.11
Beacon and Probe Response messages. As noted earlier, VLANs may be
determined dynamically so that these information elements may not be
reliable.
In IEEE 802.11, the presence of an IBSS can be used as a hint that a
link supports adhoc networking, and therefore that assignment of a
IPv4 Link-Local address is likely. When running IPv4 over PPP, if an
IPv4 address is not statically configured or assigned via IPv4CP,
this can also be taken as a hint that assignment of an IPv4 Link-
Local address is likely. Media such as USB or IEEE 1394 may be
considered inherently more likely to support adhoc operation, so that
attachment to these media may by itself be considered a hint.
A.3 Internet Layer Hints
Aside from utilizing link layer indications, a host may also be able
to utilize Internet layer information in order to determine MLN(s).
IPv4 ICMP Router Discovery messages [RFC1256] provide information
relating to prefix(es) available on the link, as well as the routers
that serve those prefix(es). A host may use ICMP Router Discovery to
conclude that an advertised prefix is available; however it cannot
conclude the converse -- that prefixes not advertised are
unavailable.
However, since [RFC1256] is not widely implemented, it is NOT
RECOMMENDED that hosts utilize ICMP Router Discovery messages as an
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alternative to the reachability test outlined in Section 2.2.
Instead, ICMP Router Advertisements can be used to obtain information
on available prefixes and default gateway(s). This can provide
additional resilience in the case where default gateway(s) become
unavailable.
Similarly hosts that support routing protocols such as RIP [RFC2453]
can use these protocols to determine the prefix(es) available on a
link and the default gateway(s) that serve those prefixes. Full
support is not required to glean this information. A host that
passively observes routing protocol traffic may deduce this
information without supporting a fully conformant routing protocol
implementation. For a description of "Silent RIP", see [RFC1058]
Section 3.1.
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This document and the information contained herein are provided on an
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INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Open issues
Open issues relating to this specification are tracked on the
following web site:
http://www.drizzle.com/~aboba/DNA/
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