DNA WG JinHyeock Choi
Internet-Draft Samsung AIT
Expires: December 10, 2004 Greg Daley
CTIE Monash University
June 11, 2004
Detecting Network Attachment in IPv6 Goals
draft-ietf-dna-goals-00.txt
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Abstract
At the time a host establishes a new link-layer connection, it may or
may not have a valid IP configuration for Internet connectivity. The
host may check for link change, i.e. determine whether a link change
has occurred, and then, based on the result, it can automatically
decide whether its IP configuration is still valid or not. While
checking for link change, the host may also collect necessary
information to initiate a new IP configuration for the case that the
IP subnet has changed. In this memo, this procedure is called
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Detecting Network Attachment (DNA).
Rapid attachment detection is required when a host has on-going data
traffic. Current DNA schemes are inadequate to support real-time
applications. The existing procedures for advertising network
information incur reception delays and do not provide enough
information to properly determine the identity of links. For
to-be-defined, efficient DNA schemes, a way to correctly represent a
link change, a complete and consistent procedure for network
information advertisement and a rapid delivery of the advertisements
will be necessary.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Detecting Network Attachment in Existing IPv6 Systems . . . . 5
3. Problems in Detecting Network Attachment . . . . . . . . . . . 7
3.1 Wireless link properties . . . . . . . . . . . . . . . . . 7
3.2 Inadequacies in RA information . . . . . . . . . . . . . . 7
3.3 Delays . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Goals for Detecting Network Attachment . . . . . . . . . . . . 10
4.1 Goals list . . . . . . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1 Normative References . . . . . . . . . . . . . . . . . . . . 15
8.2 Informative References . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . 17
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1. Introduction
When a host has established a new link-layer connection, it can send
and receive some IP packets at the link, particularly those used for
configuration. On the other hand, the host has full Internet
connectivity only when it is able to exchange packets with arbitrary
destinations.
When a link-layer connection is established or re-established, the
host doesn't know whether its existing IP configuration is still
valid for Internet connectivity. A subnet change might have occurred
when the host changed its attachment point.
In practice, the host doesn't know which of its addresses are valid
on the newly attached link. A host knows neither if its existing
default router is on this link, nor whether its neighbor cache
entries are valid. Correct configuration of each of these components
are necessary in order to send packets on and off the link.
While other information for IP connectivity (such as DNS server
configuration) is also important, obtaining most of such information
depends on the determination of correct global addressing and valid
default router configuration.
Hence, to determine the need of a further configuration, a host needs
to check at least that:
1) Whether it knows a (partially) reachable default router.
2) Whether it possesses a valid IP address.
Partial reachability indicates that the host is able to receive
packets from the router, but not necessarily vice versa.
To examine the status of the existing configuration, a host may check
whether a 'link change' has occurred.
Today a link change necessitates an IP configuration change.
Whenever the host detects that it has remained at the same link, it
can usually assume its IP configuration is still valid. Otherwise,
the existing one is no longer valid and a new configuration must be
acquired. Hence a host only needs to check for link change to
examine the validity of its IP configuration.
In the process of checking for link change, a host may collect some
of the necessary information for a new IP configuration, for example
on-link prefixes. So, when an IP subnet change has occurred, a host
can immediately initiate the process of getting a new IP
configuration. This may reduce handoff delay and minimize signaling.
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Rapid attachment detection is required for a device that changes
subnet while having on-going sessions. This may be the case if a
host is connected intermittently, is a mobile node, or has urgent
data to transmit upon attachment to a link.
The process by which a host collects the appropriate information,
detects the identity of its currently attached link, and ascertains
the validity of its IP configuration, is called Detecting Network
Attachment (DNA).
It is important to note that DNA process does not include the actual
IP configuration procedure. For example, with respect to DHCP, the
DNA process may determine that the host needs to get some
configuration information from a DHCP server. However, the process
of actually retrieving the information from a DHCP server falls
beyond the scope of DNA.
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2. Detecting Network Attachment in Existing IPv6 Systems
When a host changes its attachment point, for efficiency, a host
should examine whether its IP configuration is still valid before
initiating the process of acquiring a new IP configuration.
DNA process aims to examine whether a host's current IP configuration
is still valid. To achieve this goal, DNA process checks whether the
host is still at the same link. Also, DNA process collects necessary
information for a new IP configuration.
For this, the following mechanisms and data are available to hosts:
1) Router Solicitation/Router Advertisement (RS/RA) messages
2) Neighbor Solicitation/Neighbor Advertisement (NS/NA) messages
3) Information provided by the link-layer
Currently there is no way to represent the identity of links such
that link changes can be detected instantly. The information in the
above messages cannot be used to unambiguously identify links.
Section 3 gives the detail.
Though the set of all the assigned prefixes may be used to represent
a link identity, it is difficult for a host to attain and retain all
the prefixes with certainty. Moreover there may be links without
any advertised prefix.
Hence, to check for link change, as of today, a host must resort to
guessing, based on Router Discovery and Neighbor Unreachability
Detection (NUD). A host can examines whether 1) its existing default
router is still reachable and 2) its IP addresses are still valid.
After a network attachment, a host receives new (solicited or
unsolicited) RAs and compares them with the previously received ones.
It checks whether the information in the new RAs, the prefixes and
the router addresses, matches with the stored ones, the configured IP
addresses and the default router addresses.
If Router Discovery fails to update the existing router's
information, it indicates that the router is unreachable and should
not be used.
For Internet connectivity, a host needs to have an IP address whose
prefix is assigned on the current link. To check the validity of its
IP addresses, a host examines whether the prefix of its IP addresses
are present in the Prefix Information Option of received RAs.
Even if an address prefix is valid, an individual address is known to
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be valid only after the Duplicate Address Detection (DAD) procedure
[5] has been completed.
If RAs indicate that a subnet change has occured, the invalidity of
other IP subnet information is implied.
At the same time, from the received RAs, a host may collect some of
the necessary information for a new configuration: a prefix to form a
new IP address and router addresses to select a new default router.
For rapid attachment detection, it is necessary for a host to receive
an RA quickly enough.
A host may also use NS/NA exchange to examine the reachability of the
existing default router. Upon detecting a new link-layer connection,
the host may probe the router with an NS message. If it receives an
appropriate NA message, the router is still reachable. Otherwise, if
the host receives no replies to Neighbor Solicitations, it may assume
that its default router is no longer reachable.
If a host receives a message from a router with which it has
previously been able to exchange packets, then the Neighbor Discovery
procedure may be used to establish full bi-directional reachability.
In some environments, link-layer information regarding IP
connectivity may be considered as a strong hint that change of
link-layer attachment implies change of IP subnet. While this is
sometimes the case, not all IP implementations at the network layer
will be able to understand the indication from the link-layers, nor
will those indications necessarily be always sufficient to make a
proper decision.
If the information from the link layer is available, but it is not
considered authoritative, the information may still be used as a
'Link-layer hint'. Link-layer hints are indications from lower
layers that IP connectivity may have changed. With suitable
hysteresis, these hints may be used to initiate IP based reachability
checks.
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3. Problems in Detecting Network Attachment
There are a number of issues that make DNA complicated. First,
wireless connectivity is not as clear-cut as wired one. Second the
information contained in RA messages is not adequate for efficient
DNA. Third, Router Discovery or NUD may take too long and result in
service disruption.
3.1 Wireless link properties
1) Unclear boundary
Unlike wired environments, what constitutes wireless link is variable
in both time and space. It doesn't have clear boundaries. This may
be illustrated by the fact that a host may contact multiple (802.11)
access points at the same time. Moreover reachability on a wireless
link is very volatile, which may make link detection hard.
2) Asymmetric reachability
In some wireless environments, it may be possible to receive
periodically multicast advertisement information without being able
to send IP packets to the network. In these cases, it is
insufficient to rely upon reception of unsolicited advertisement
information as confirmation of router reachability.
3.2 Inadequacies in RA information
Usually a host receives the information necessary for IP
configuration from RA messages. Based on the current definition [4],
the information contained in RA messages is inadequate to represent a
link change. RA messages are not designed to represent link
identities and have inherent ambiguities.
1) Link local scope of Router Address
Usually a router address is contained in the source address field of
RA messages. That router address is link-local scope and its
uniqueness can't be guaranteed out side of a link.
So if it happens that two different router interfaces have the same
link-local address, a host can't detect that it has moved from one
interface to another by checking the router address in RA messages.
On the other hand, a host can't be sure that its default router is
reachable, even if it can communicate with the router that has the
same address as its existing router address. That router may be a
different one, which happens to have the same link-local address as
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its default router address.
2) Omission of Prefix Information Option
To check the validity of its IP address, a host should examine
whether the prefix of its IP address is advertised on the link to
which it is currently attached.
The host checks whether the prefix of its IP addresses are present in
the Prefix Information Option of incoming RA messages. But an
unsolicited RA message can omit some prefixes for convenience, for
example to save bandwidth [4].
Hence, the host can't be sure that the prefix of its current IP
address is not supported on the current link, even though the prefix
is not contained in a received RA.
3.3 Delays
The following issues cause DNA delay that may result in communication
disruption.
1) Delay for receiving a hint
For rapid attachment detection, hints can be used to tell a host that
a link change might have happened. This hint itself doesn't confirm
a link change, but can be used to initiate the appropriate
procedures.
Hints come in various forms, and differ in how they indicate a new
attachment. They can be link-layer indications, the lack of RA from
the default router or the receipt of a new RA. The time taken to
receive a hint also varies.
As soon as a new link-layer connection has been made, the link-layer
may send a link up notification to the IP layer. A host may
interpret a new network attachment as a hint for a possible link
change. With link-layer support, a host can receive a hint almost
instantly.
[9] defines the use of RA Interval Timer expiry for a hint. A host
keeps monitoring periodic RAs and interprets the lack of them as a
hint. It may implement its own policy to determine the number of
missing RAs for a hint. Hence the delay depends on the Router
Advertisement interval.
Without the schemes such as above, a host must receive a new RA from
a new router to detect a possible link change. The detection time
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then also depends on the Router advertisement frequency.
Periodic RA beaconing transmits packets within an interval varying
randomly between MinRtrAdvInterval to MaxRtrAdvInterval seconds.
Because a network attachment is unrelated to the advertisement time
on the new link, the host is expected to arrive on average half way
through the interval. This is approximately 1.75 seconds with [4]
advertisement rates.
2) Delay for checking current default router Unreachability
When a host examines the reachability of the current default router,
a certain delay occurs if the current default router is not
reachable.
Usually it's easier to detect a node's presence than its absence. A
host sends a solicitation message and, upon the receipt of a reply,
it can assume that it's there.
To be sure that a node is absent, time needs to be taken to ensure
that the lack of a reply is not due to another reasons (for example,
packet loss, MAC latency, or processing delay). So it takes time to
verify the unreachability of the current router.
After a host moves to another link, if it uses NUD for detection, it
will take more than 3 seconds to recognize that the current router is
no longer reachable [4].
3) Random delay execution for RS/ RA exchange
Router Solicitation and Router Advertisement messages are used for
Router Discovery. According to [4], it is sometimes necessary for a
host to wait a random amount of time to send an RS and for a router
to wait to reply an RA.
Before a host sends an initial solicitation, it SHOULD delay the
transmission for a random amount of time between 0 and
MAX_RTR_SOLICITATION_DELAY (1 second). Also Router Advertisements
sent in response to a Router Solicitation MUST be delayed by a random
time between 0 and MAX_RA_DELAY_TIME (0.5 seconds).
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4. Goals for Detecting Network Attachment
DNA solutions should be 1) Precise, 2) Sufficiently fast and 3) Of
limited signaling. The solutions should correctly determine whether
a link change has occurred. They also should be sufficiently fast
lest there should be service disruption. They should not flood the
link with related signaling.
It will be necessary to investigate the usage of available tools, NS/
NA messages, RS/RA messages, link-layer hints and other features.
This will allow precise description of procedures for efficient DNA
Schemes.
4.1 Goals list
G1 DNA schemes should detect the identity of the currently attached
link to ascertain the validity of the existing IP configuration.
They should recognize and determine whether a link change has
occurred and initiate the process of acquiring a new configuration
if necessary.
G2 When upper-layer protocol sessions are being supported, DNA
schemes should detect the identity of an attached link rapidly,
with minimal latency.
G3 In the case where a host has not changed a link or subnet, an IP
configuration change should not occur.
G4 DNA schemes should not cause undue signaling on a link.
G5 DNA schemes should make use of existing signaling mechanisms where
available.
G6 DNA schemes should make use of signaling within the link
(particularly link-local scope messages), since communication
off-link may not be achievable in the case of a link change.
G7 DNA schemes should be compatible with security schemes such as
Secure Neighbour Discovery [8] and IPSec [7].
G8 A host configured for DNA should not expose itself to additional
man in the middle or identity revealing attacks.
G9 A host or router configured for DNA should not expose itself or
other devices on the link to additional denial of service attacks.
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G10 The Routers supporting DNA should work appropriately with hosts
using unmodified configuration schemes, such as [4] and [6].
G11 The Hosts supporting DNA should be able to work with unmodified
routers and hosts which do not support DNA schemes.
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5. IANA Considerations
No new message formats or services are defined in this document.
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6. Security Considerations
Because DNA schemes are based on Neighbor Discovery, its trust models
and threats are similar to the ones presented in [10]. Nodes
connected over wireless interfaces may be particularly susceptible to
jamming, monitoring and packet insertion attacks. Use of [8] to
secure Neighbor Discovery are important in achieving reliable
detection of network attachment. DNA schemes SHOULD incorporate the
solutions developed in IETF SEND WG if available, where assessment
indicates such procedures are required.
Even in the case where authoritative information (routing and prefix
state) are advertised, wireless network attackers may still prevent
soliciting nodes from receiving packets. This may cause unnecessary
IP configuration change in some devices. Such attacks may be used to
make a host preferentially select a particular configuration or
network access.
Devices receiving confirmations of reachability (for example from
upper-layer protocols) should be aware that unless these indications
are sufficiently authenticated, reachability may falsely be asserted
by an attacker. Similarly, such reachability tests, even if known to
originate from a trusted source should be ignored for reachability
confirmation if duplicates or stale. This may reduce the effective
window for attackers replaying otherwise authentic data.
It may be dangerous to receive link-change indications from
link-layer and network-layer, if they are received from devices which
are insufficiently authenticated. In particular, indications that
authentication has completed at the link-layer may not imply that a
security relationship is available at the network-layer. Additional
authentication may be required at the network layer to justify
modification of IP configuration.
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7. Acknowledgment
Erik Nordmark has contributed significantly [11] to work predating
this draft. Also Ed Remmell's comments on the inconsistency of RA
information were most illuminating. The authors wish to express our
appreciation to Pekka Nikander for valuable feedback. We gratefully
acknowledge the generous assistance we received from Shubhranshu
Singh for clarifying the structure of the arguments. Thanks to Brett
Pentland, Nick Moore, Youn-Hee Han, JaeHoon Kim and Alper Yegin for
their contributions to this draft.
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8. References
8.1 Normative References
[1] Bradner, S., "IETF Rights in Contributions", BCP 78, RFC 3667,
February 2004.
[2] Bradner, S., "Intellectual Property Rights in IETF Technology",
BCP 79, RFC 3668, February 2004.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461, December 1998.
[5] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[6] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 3315, July 2003.
[7] Thayer, R., Doraswamy, N. and R. Glenn, "IP Security Document
Roadmap", RFC 2411, November 1998.
[8] Arkko, J., Kempf, J., Sommerfeld, B., Zill, B. and P. Nikander,
"SEcure Neighbor Discovery (SEND)", draft-ietf-send-ndopt-05
(work in progress), April 2004.
8.2 Informative References
[9] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[10] Nikander, P., Kempf, J. and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.
[11] Nordmark, E., "MIPv6: from hindsight to foresight?",
draft-nordmark-mobileip-mipv6-hindsight-00 (work in progress),
November 2001.
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Authors' Addresses
JinHyeock Choi
Samsung AIT
i-Networking Lab
P.O.Box 111 Suwon 440-600
KOREA
Phone: +82 31 280 9233
EMail: jinchoe@samsung.com
Greg Daley
CTIE Monash University
Centre for Telecommunications and Information Engineering
Monash University
Clayton 3800 Victoria
Australia
Phone: +61 3 9905 4655
EMail: greg.daley@eng.monash.edu.au
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