One document matched: draft-ietf-dhc-dna-ipv4-03.txt
Differences from draft-ietf-dhc-dna-ipv4-02.txt
DHC Working Group Bernard Aboba
INTERNET-DRAFT Microsoft Corporation
Category: Proposed Standard
<draft-ietf-dhc-dna-ipv4-03.txt>
9 October 2003
Detection of Network Attachment (DNA) in IPv4
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other groups
may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
The time required to detect movement (or lack of movement) between
subnets, and to obtain (or continue to use) a valid IPv4 address may
be significant as a fraction of the total delay in moving between
points of attachment. This specification synthesizes experience
garnered over the years in the deployment of hosts supporting ARP,
DHCP and IPv4 Link-Local addresses, in order to optimize detection of
network attachment by mobile hosts.
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Table of Contents
1. Introduction.............................................. 3
1.1 Requirements .................................... 4
1.2 Terminology ..................................... 4
2. Framework ................................................ 4
2.1 Reachability test ............................... 5
2.2 Packet format ................................... 5
2.3 IPv4 Address Acquisition ........................ 7
3. IANA Considerations ...................................... 8
4. Security Considerations .................................. 8
5. References ............................................... 8
5.1 Normative references ............................ 8
5.2 Informative references .......................... 9
Acknowledgments .............................................. 10
Authors' Addresses ........................................... 10
Appendix A - Hints ........................................... 11
Intellectual Property Statement .............................. 12
Full Copyright Statement ..................................... 12
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1. Introduction
The time required to detect movement (or lack of movement) between
subnets, and to obtain (or continue to use) a valid IPv4 address may
be significant as a fraction of the total delay in moving between
points of attachment. As a result, optimizing detection of network
attachment is important for mobile hosts.
This document synthesizes experience in the deployment of hosts
supporting ARP [RFC826], DHCP [RFC2131], and Link-Local IPv4
addresses [IPv4LL], specifying a procedure to be performed for IPv4
detection of network attachment. The procedure consists of three
phases: determination of the "most likely" point of attachment,
reachability testing, and IPv4 address acquisition.
On disconnection from a network, there is no need to take action
until the host is reconnected, since it is typically not possible for
a host to communicate until it has obtained connectivity. Therefore,
contrary to [RFC2131] Section 3.7, no action need be taken on network
disconnection.
For Detection of Network attachment, the following basic principles
are suggested:
[a] Utilization of link layer hints. Link layers such as
IEEE 802 [IEEE802] provide hints about whether a host
remains on the same subnet despite changing its point of
attachment, or even whether the host is connected to an
adhoc or infrastructure network. Where available, these
hints can be used to guide host behavior - with the
understanding that they are not infallible and therefore
that the host should be capable of making the correct
determination even in the presence of misleading hints.
Link layer hints are described in more detail in
Appendix A.
[b] Link-Local IPv4 addressing as a mechanism of last resort.
Experience has shown that Link-Local IPv4 addresses are
often assigned inappropriately. For example, an IPv4 host
assigning an Link-Local IPv4 address may not be connected
to any network, in which case assignment of a Link-Local
IPv4 address does no good; or the host may be attached to a
network with a DHCPv4 server but may not receive a response
to a DHCPREQUEST or DHCPDISCOVER. As described in Appendix A
of [IPv4LL] once a Link-Local IPv4 address is assigned,
existing implementations do not query the DHCPv4 server
again for 5 minutes. As noted in Section 2.3, this behavior
is in violation of [RFC2131] Section 4.1. For hosts changing
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their point of attachment more frequently than this,
inappropriate assignment of an Link-Local IPv4 address can
result in an ongoing inability to connect. As a result,
this document suggests that hosts behave conservatively with
respect to assignment of Link-Local IPv4 addresses, using
them only as a last resort.
1.1. Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. 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:
DHCP client
A DHCP client or "client" is an Internet host using DHCP 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.
Routable address
In this specification, the term "routable address" refers to any
address other than an IPv4 Link-Local address. This includes
private addresses as specified in [RFC1918].
2. Framework
On connecting to a new point of attachment, the host attempts to
determine the "most likely" configuration associated with the new
point of attachment.
In order to determine the "most likely" point of attachment it is
assumed that the host is capable of obtaining and writing to stable
storage parameters relating to networks that it connects to,
including:
[1] IP and link layer hints associated with each network. For
details, see Appendix A.
[2] The IP and MAC address of the primary default gateway on
each network.
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By matching the received hints against information previously
collected, the host may be able to make an educated guess of which
network it has attached to. Where no additional information is
available, by default the host assumes that the "most likely" point
of attachment is the network to which it was most recently connected.
If the host has a valid routable IPv4 address on the "most likely"
point of attachment, the host performs a reachability test as
described below. If the reachability test is not successful, or if
the host does not have a valid routable IPv4 address on the "most
likely" point of attachment, the host proceeds to the IPv4 address
acquisition phase.
2.1. Reachability Test
The purpose of the reachability test is to determine whether the host
is connected to a network on which it had previously obtained a still
valid routable IPv4 address. The test is performed by attempting to
verify reachability of a previously configured primary default
gateway on a former point of attachment. If the test is successful,
the host may continue to use a valid routable IPv4 address without
having to re-acquire it. This reduces roaming latency by allowing
the host to bypass the DHCP exchange as well as subsequent Duplicate
Address Detection (DAD). In contrast to a DHCP exchange, which may
be between a DHCP client and an offlink DHCP server, the reachability
test occurs between a host and its next hop router.
The host skips the reachability test and proceeds to the address
acquisition phase in the following circumstances:
[a] If the host does not have information on the default
gateway on the network.
[b] If the host does not have a valid routable IPv4 address
on the network. Since Link-Local IPv4 addresses are a
last resort, these addresses do not count as a valid
routable IPv4 address.
2.2. Packet Format
To perform the reachability test, an ARP Request SHOULD be sent,
using the host's MAC address as the source, and the broadcast MAC
address as the destination. The host sets the target protocol
address (ar$tpa) to the IPv4 address of the primary default gateway,
and uses its own MAC address in the sender hardware address field
(ar$sha).
If the host has a valid globally routable IP address on the most
likely point of attachment, then it SHOULD set the sender protocol
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address field (ar$spa) to that address. It is assumed that the host
had previously done duplicate address detection so that an address
conflict is unlikely.
However if the host has a private address as defined in [RFC1918],
then it SHOULD set the protocol address field (ar$spa) to 0.0.0.0.
This is to avoid an address conflict in the case where the host has
changed its point of attachment from one private network to another.
Note: Some routers may refuse to answer an ARP Request sent with
the protocol address field (ar$spa) set to the unspecified
address. In this case the reachability test will fail.
If a valid ARP Response is received, the MAC address in the target
hardware address field (ar$tha) and the IPv4 address in the target
protocol address field (ar$tpa) are matched against the list of
networks and associated default gateway parameters. If a match is
found, then if the host has a valid IPv4 address lease 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.
Checking for a match on both the IPv4 and MAC addresses of the
default gateway allows the host to confirm reachability even where
the host moves between two private networks. In this case the IPv4
address of the default gateway could remain the same, while the MAC
address would change, so that both addresses need to be checked.
Sending an ICMP Echo Request to the default gateway IPv4 address does
not provide the same level of assurance since this requires an ARP
Request/Response to be sent first, and typically does not allow the
MAC address to be checked as well. It therefore SHOULD NOT be used
as a substitute. 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.
If the initial ARP Request does not elicit a Response, the host waits
200ms and then sends another ARP Request. If no ARP Response is
received in response to this second Request, the host proceeds to the
IPv4 address acquisition phase. If a valid ARP Response is received,
but cannot be matched against known networks, the host assumes it has
moved subnets and moves on to the address acquisition phase.
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2.3. IPv4 Address Acquisition
If the host has a valid cached configuration on the "most likely"
point of attachment, but is unable to confirm reachability to the
primary default gateway, then the host seeks to verify the cached
configuration by entering the INIT-REBOOT state, and sending a
DHCPREQUEST to the broadcast address as specified in [RFC2131]
Section 4.4.2.
If the host does not have a valid cached configuration, or had not
previously obtained a routable IPv4 address on the "most likely"
point of attachment, then 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 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 IP broadcast address.
However, sending a DHCPREQUEST to the unicast address when in INIT-
REBOOT state is not appropriate since it is possible that the client
has moved to another subnet, and therefore the DHCPREQUEST needs to
be forwarded to and from the DHCP server by a DHCP Relay so that the
response can be broadcast. This ensures that the host will receive a
response regardless of whether the cached IP address is correct for
the network to which it has connected.
As described in [IPv4LL] Section 1.7, use of a routable address is
preferred to a Link-Local IPv4 address whenever it is available.
[RFC2131] Section 3.2 states that if the host possesses a valid
routable IPv4 address on the "most likely" network and does not
receive a response after employing the retransmission algorithm, the
client MAY choose to use the previously allocated network address and
configuration parameters for the remainder of the unexpired lease.
This is preferable to assigning a Link-Local IPv4 address if the host
has good reason to believe that it remains connected to a network on
which it possesses a valid IPv4 address lease. This would be the
case, for example, where a host has received hints that it believes
to be "strong". See Appendix A for details.
If the host does not have a valid IPv4 address lease on the "most
likely" network and does not receive a response after employing the
retransmission algorithm, it MAY assign a Link-Local IPv4 address.
Since a Link-Local IPv4 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 Link-Local IPv4 address
assignment as soon as a valid IPv4 address lease can be obtained.
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Where a Link-Local IPv4 address is assigned, experience has shown
that five minutes (see [IPv4LL] Appendix A.2) is too long an interval
to wait until retrying to obtain a routable IPv4 address using DHCP.
According to [RFC2131] Section 4.1:
The retransmission delay SHOULD be doubled with
subsequent retransmissions up to a maximum of 64 seconds.
As a result, a DHCP client compliant with [RFC2131] will continue to
retry every 64 seconds, even after allocating a Link-Local IPv4
address. Should the DHCP client succeed in obtaining a routable
address, then as noted in [IPv4LL], the Link-Local IPv4 address is
deprecated. In order to avoid inappropriate assignment of an IPv4
Link-Local address, it is RECOMMENDED that such an address not be
assigned until the DHCP client has retransmitted at least 3 times.
3. IANA Considerations
Guidelines for IANA considerations are specified in [RFC2434]. This
specification does not request the creation of any new parameter
registries, nor does it require any other IANA assignments.
4. Security Considerations
Detection of Network Attachment (DNA) is typically insecure, so that
it is inadvisable for a host to adjust its security based on which
network it believes it is attached to. For example, it would be
inappropriate for a host to disable its personal firewall based on
the believe that it had connected to a home network.
ARP [RFC826] traffic is inherently insecure, so that the reachability
test described in Section 1.3 can be easily spoofed by an attacker,
leading a host to conclude that it remained attached to a former
network. Similarly, where DHCP [RFC2131] traffic is not secured, an
attacker could masquerade as a DHCP server, in order to convince the
host that it was attached to a particular network.
Where secure detection of network attachment is required, a host MAY
wish to skip the ARP-based reachability test entirely since it cannot
be secured, and go immediately to the IPv4 address acquisition phase,
utilizing authenticated DHCP [RFC3118].
5. References
5.1. Normative References
[RFC792] Postel, J., "Internet Control Message Protocol", RFC 792,
USC/Information Sciences Institute, September 1981.
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[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.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[RFC2132] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell
University, March 1997.
[RFC2434] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[IPv4LL] Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration
of IPv4 Link-Local Addresses", Internet draft (work in
progress), draft-ietf-zeroconf-ipv4-linklocal-10.txt, October
2003.
5.2. Informative References
[RFC1661] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, Daydreamer, July 1994.
[RFC1918] Rekhter, Y., et al., "Address Allocation for Private
Internets", RFC 1918, February 1996.
[RFC2284] Blunk, L. and J. Vollbrecht, "PPP Extensible Authentication
Protocol (EAP)", RFC 2284, March 1998.
[RFC3580] Congdon, P., et al., "IEEE 802.1X Remote Authentication Dial
In User Service (RADIUS) Usage Guidelines", RFC 3580,
September 2003.
[IEEE8021AB]
IEEE Standards for Local and Metropolitan Area Networks:
Station and Media Access Control - Connectivity Discovery,
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IEEE Std 802.1AB/D5, September 2003.
[IEEE8021X]
IEEE Standards for Local and Metropolitan Area Networks: Port
based Network Access Control, IEEE Std 802.1X-2001, June 2001.
[IEEE802] IEEE Standards for Local and Metropolitan Area Networks:
Overview and Architecture, ANSI/IEEE Std 802, 1990.
[IEEE8021Q]
IEEE Standards for Local and Metropolitan Area Networks: Draft
Standard for Virtual Bridged Local Area Networks, P802.1Q,
January 1998.
[IEEE80211]
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. 802.11-1999, 1999.
Acknowledgments
The authors would like to acknowledge Greg Daley of Monash
University, Erik Guttman and Erik Nordmark of Sun Microsystems, Ted
Lemon of Nominum and Thomas Narten of IBM for contributions to this
document.
Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
EMail: bernarda@microsoft.com
Phone: +1 425 706 6605
Fax: +1 425 936 7329
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Appendix A - Hints
In order to assist in IPv4 network attachment detection, information
associated with each network may be retained by the host. Based on
IP and link-layer information, the host may be able to make an
educated guess as to whether it has moved between subnets, or
remained on the same subnet. If it is likely to have moved between
subnets, the host may have an educated guess as to which subnet it
has moved to. The term "strong hint" refers to information which
provides an unambiguous indication of the network to which a host has
connected. "Weak hints" involve information which is inconclusive.
IPv4 ICMP Router Discovery messages [RFC1256] provide information
directly relevant to determining the network to which a host has
connected. As such, information gleaned from Router Advertisements
can be considered a "strong" hint.
For networks running over PPP [RFC1661], "weak" hints include the
link characteristics negotiated in LCP, and the associated phone
number. The IP parameters negotiated in IPCP are considered a
"strong" hint.
On IEEE 802 [IEEE802] wired networks, hints include link-layer
discovery traffic as well as information exchanged as part of IEEE
802.1X authentication [IEEE8021X]. Link-layer discovery traffic
includes Link Layer Discovery Protocol [IEEE8021AB] traffic as well
as network identification information passed in the EAP-
Request/Identity or within an EAP method exchange, as defined in EAP
[RFC2284].
For example, LLDP advertisements can provide information on the IP
address or VLANs supported by the device. These hints, if provided,
are considered "strong". When used with IEEE 802.1X authentication
[IEEE8021X], the EAP-Request/Identity exchange may contain the name
of the authenticator, also providing information on the potential
network. Similarly, 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 [IEEE8021Q] to be assigned
dynamically, so this static information may not prove definitive. As
a result, these hints are considered "weak".
In IEEE 802.11 [IEEE80211] stations provide information in Beacon
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 Adhoc mode. As described in
[RFC3580], it is possible to assign a Station to a VLAN dynamically,
based on the results of IEEE 802.1X [IEEE8021X] authentication. This
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implies that a single SSID may offer access to multiple VLANs, and in
practice most large WLAN deployments offer access to multiple
subnets.
Thus, associating to the same SSID is a necessary, but not
necessarily a sufficient condition, for remaining within the same
subnet. While a Station associating to the same SSID may not
necessarily remain within the same subnet; on the other hand, a
Station associating to a different SSID is likely to have changed
subnets.
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. Such hints are considered
"strong"; all other IEEE 802.11 hints are considered "weak".
Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
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The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
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this standard. Please address the information to the IETF Executive
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Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
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included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English. The limited permissions granted above are perpetual and
will not be revoked by the Internet Society or its successors or
assigns. This document and the information contained herein is
provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Open issues
Open issues relating to this specification are tracked on the
following web site:
http://www.drizzle.com/~aboba/DNA/dnaissues.html
Expiration Date
This memo is filed as <draft-ietf-dhc-dna-ipv4-03.txt>, and expires
April 22, 2004.
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