Internet DRAFT - draft-armitage-ipp-security
draft-armitage-ipp-security
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Internet-Draft Grenville Armitage
Lucent Technologies
July 28th, 1997
Security issues for ION protocols
<draft-armitage-ion-security-00.txt>
Status of this Memo
This document was submitted to the IETF Internetworking over NBMA
(ION) WG. Publication of this document does not imply acceptance by
the ION WG of any ideas expressed within. Comments should be
submitted to the ion@nexen.com mailing list.
Distribution of this memo is unlimited.
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Abstract
This document aims to assist people attempting to understand the
security limitations of existing ION working group protocols RFC 1577
(ATMARP), RFC 2022 (MARS), and RFC xxxx (NHRP). As RFC 2022 and RFC
xxxx share their basic control message protocol(s), this document
also identifies common areas amenable to improvement using additional
security TLVs.
Change History
July 1997
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Initial release. Begins to describe the problems. Solutions still
TBD.
1. Introduction
Security is a broad term, and often used subjectively when a given
protocol is said to be 'secure' or 'insecure'. The context and prior
assumptions need to be clearly understood for each assertion of an
overall system's level of security. The ION working group and its
predecessors (the IP-ATM and ROLC working groups) are responsible for
three key protocols to support unicast and multicast IP over ATM (and
other NBMA) networks - RFC 1577 (ATMARP) [1], RFC 2022 (MARS) [2],
and RFC xxxx (NHRP) [3]. The development of these protocols focussed
on achieving a set of goals that did not include specific security
issues.
As deployment of IP begins to occur using these protocols, it is
important for the industry to be aware of the various security risks,
and what can be done to reduce these risks.
[cites to prior public work, if any, on each of RFC1577, RFC2022, and
NHRP security are solicited here.]
A common architectural component of all three protocols is the use of
query/response to establish IP to ATM address mappings. MARS and NHRP
both add to this model by having the clients accept unsolicited
control messages that result in asynchronous modifications of client
behavior. There is an implicit trust that only legitimate clients or
servers are generating messages at each point in the control message
exchanges that make up these protocols. Typically most security can
summarised as 'no-one would find me interesting enough to bother'.
Unfortunately, innocent hackers and/or malicious crackers will find
most IP/ATM installations 'interesting' at some point. Whether you
are victim of a denial-of-service attack, or denial-of-service
screw-up, the effect is similarly annoying.
The term 'attacker' will be used in this document to mean any
application actively utilizing the ATM network and IP/ATM protocol
elements to perform activities outside the intended scope of RFC
1577, RFC 2022, or RFC xxxx.
The rest of this document is structured in the following manner.
Section 2 briefly notes the limited assumptions you can make about
ATM level security, section 3 summarizes the known security
limitations of RFC 1577, section 4 summarizes the known issues with
RFC 2022, and section 5 summarizes the known issues with RFC xxxx.
Section 6 briefly outlines the mechanism shared by RFC 2022 and RFC
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xxxx for adding security related option fields to control messages.
2. ATM level security
All three protocols assume that the underlying ATM network itself is
trustworthy. There is an implicit assumption that if a SETUP message
arrives at your node, the Calling Party Information Element (IE)
contains a legitimate address correctly identifying the SETUP's
originator. (In line with the 'surely I'm too un-interesting to
bother' philosophy, there is an additional assumption that the SETUP
message came from someone with a right to establish the VC,
regardless of the address in the Calling Party IE.)
Unfortunately, UNI 3.0/3.1 ATM signaling does not utilize any form of
end-node authentication. This leaves the SETUP phase vulnerable to
'man in the middle' attacks (where a switch somewhere in the ATM
network is compromised, or a link is broken and an additional entity
introduced that is capable of intercepting and modifying UNI or NNI
signaling traffic on the fly).
Even if end points were authenticated, UNI 3.0/3.1 does not support
the notion of closed user groups. This allows anyone with UNI access
to your underlying ATM cloud to establish VCs to any entity within
your LIS [1], Cluster [2], or LAG [3]. This can become a problem if
UNI access to your ATM cloud is possible through poorly restricted
physical access (e.g. spare switch ports), or logical access through
machines running insecure OSes. An insecure OS environment can be
anything that allows user space applications direct access to either
the local hosts's UNI signaling stack, or the underlying ATM NIC
itself.
Weak access controls to a host's UNI signaling stack may allow a
local user application to establish VCs using Calling Party numbers
with arbitrary SEL values (the choice of the other 19 octets of a
Calling Party AESA is limited by the ESIs initially registered by the
client with the local switch using ILMI). Sufficiently weak access
controls might even allow an application to choose Calling Party
numbers that clash with other local ATM-based applications. Weak
access controls to the host's ATM NIC itself could allow user
deployment of a complete ILMI/UNI signaling stack of their own,
customized for whatever task is required.
A single PC attached to a campus-wide ATM network meets all the
criteria for a weakly controlled access point.
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3. RFC 1577 (Classical IP over ATM)
The RFC 1577 protocol is query/response based. ATMARP Clients
initiate activity that leads to responses from the ATMARP Server
(either by establishing a VC, or issuing an ATMARP Request). The
ATMARP Server trusts mapping information it receives from ATMARP
Clients. ATMARP Clients never change their behavior based on
asynchronous control messages from the ATMARP Server.
[Editors note: this section is from memory. corrections or
clarifications solicited. impact of Classic2 has not been
considered.]
3.1 IP/ATM address spoofing
Address spoofing involves the insertion of incorrect {IP,ATM}
mappings into the ATMARP Server's database. This is trivial to
achieve. An attacker establishes a VC to the ATMARP Server for a
given LIS, the Server issues a pre-emptive InARP REQUEST, and the
attacker provides a fake {IP,ATM} mapping in its InARP Reply.
This sort of spoofing can be used either as a denial of service
attack or a stepping stone to subsequent hi-jacking of higher level
IP services (e.g. register the IP address of the local NFS server or
router immediately after an ARP Server reboot).
If the ATM addresses of LIS members are known, an attacker can
attempt to directly insert misleading {IP,ATM} mappings into another
LIS member's ATMARP Client. ATMARP Client implementations that
attempt to learn {IP,ATM} mappings from the InARP exchange with other
clients can be fooled in this way. The attacker simply establishes a
VC to a known member and passes back an arbitrary IP address in its
reply to the target Client's InARP Request. If the supplied IP
address matches that of an important node in the LIS (e.g. a router)
to which the target client has yet to establish legitimate
communication, the attack can be the prelude to hi-jacking of higher
level IP services.
There is little value in an attacker registering IP addresses outside
the scope of the LIS, as intra-LIS clients will never query the
ATMARP Server for them.
3.2 Scanning of the LIS.
It is usually undesirable for outsiders to know the entire set of IP
addresses (and associated ATM addresses) that make up your LIS.
However, there is little to stop an attacker from establishing a VC
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to your ATMARP Server, registering an innocuous {IP,ATM} mapping, and
the proceeding to issue ATMARP_REQUESTs for a range (or ranges) of
'interesting' IP addresses.
The ATM addresses thus learned might be used in subsequent denial of
service attacks against specific hosts. Depending on range of IP
addresses chosen during the scan, and the speed with which the
repeated ATMARP_REQUESTs are issued, this scanning can itself keep
the ATMARP Server so busy as to constitute a denial of service
attack. Not knowing the LIS address range makes the attack less
efficient, but not impossible since the server actively responds to
bad guesses with ATMARP_NAKs. A selective search would make
intelligent guesses as to the probable range of net/subnet numbers to
scan.
3.3 Denial of Service attacks.
Denial of service is any action that subverts the normal and timely
operation of the total IP/ATM system. Attacks may aim to either slow
down normal operations, or cause a cessation of operations altogether
by exploting implementation weaknesses.
An attacker can present two types of overload to an ATMARP Server -
repetative VC setup/teardown events without actually registering any
{IP,ATM} mappings (simply to consume cpu cycles in the ATMARP
Server's underlying UNI stack), and repeated VC
setup/teardown/registration events (where the attacker responds to
the Server's initial InARP Request with a different {IP,ATM} mapping
each time).
The first type of attack wastes time at the ATMARP Server, and causes
additional loading of the control processor in the switch to which
the target is attached (and other switches along the path between the
attacker and target).
The second type of attack will be slower (although parallel VC setup
attempts can be used to keep the average rate up), but results in
additional database manipulation activity in the Server. This can
result in collisions with IP addresses already registered, or set the
stage for later collisions when the legitimate owners of a particular
IP address attempt to register.
Depending on the ATMARP Server's design, it may happily accept
{IP,ATM} mappings outside the range of IP addresses of the LIS it is
nominally supporting. (This is not strictly illegal, since the
'Classical IP' restrictions on resolving addresses outside the LIS
actually derive from host-side behavior not server-side behavior.)
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This would have the affect of avoiding collisions while filling up
the Server's database with useless information, possibly avoiding
detection of the attack until the Server's database collapses.
An attacker may also choose to launch similar attacks on LIS Clients
whose addresses were learned through previous scanning of the ATMARP
Server's database.
3.4 ATMARP Server spoofing
ATMARP Clients never receive asynchronous updates from server. This
makes it unlikely that a client implementation would listen to a
faked ARP Reply, ARP Nak, or InARP Reply arriving on a VC that the
client did not believe to be connected to the ATMARP Server (or
another client). An attack on the identity of the ATMARP Server would
either involve compromising the security of the Client's local
configuration file, or compromising the ATM network itself (to
redirect a client's SETUP towards the attacker's own substitute
ATMARP Server). These are both feasible, but do not depend on
characteristics of the RFC 1577 protocol itself.
3.5 Hiding the ATMARP Server
Keeping the address of the ATMARP Server secret can help discourage
many of the preceding attacks. However, few RFC 1577 implementations
make any attempt to hide the configuration information from users.
If the person behind the attacks has user level access to any machine
on the LIS, they will have access to the ATMARP Server address.
Even if the ATMARP Server's address could be kept a secret, a
determined search would make use of the fact that an ATMARP Server
announces its existence with a pre-emptive InARP Request whenever a
new VC is established to it. An attacker with complete or partial
knowledge of the AESAs in your region of the cloud can poll possible
AESA variations (varying the SEL field) looking for an endpoint that
blurts out InARP Requests.
An attacker with physical access to your ATM cloud can place a tap on
any link, parse the UNI signalling traffic going past, and draw
conclusions from the AESAs it sees in SETUP messages.
4. RFC 2022 (MARS)
As with RFC 1577, the RFC 2022 protocol is primarily query/response.
However, MARS clients also expect to receive asynchronous updates
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from their MARS, which opens up possibilities for an attacker to
trigger client misbehavior.
4.1 Joining the cluster to eavesdrop and interject
MARS Clients automatically add new ATM level leaf nodes to their
outgoing pt-mpt VCs upon receipt of valid MARS_JOIN messages. An
attacker wishing to eavesdrop on any multicast group's traffic within
a Cluster need only register as a cluster member, and issue a
MARS_JOIN to the MARS for the groups of interest. The MARS will
inform all other cluster members, and all current senders will add
the attacker as a new leaf node. An attacker who was interested in
all traffic within the cluster need only issue a block MARS_JOIN to
cover the entire multicast address space (a promiscuous client) in
one operation.
Aside from the fact that information is leaking from your cluster,
such eavesdropping may have a debilitating effect on the wider ATM
cloud. If the attacker is topologically distant at the ATM level,
this action results in a sudden increase in traffic along the ATM
path from your cluster to the attacker's own access point.
Having registered with the MARS as a legitimate cluster member, the
attacker is also free to begin transmitting its own data to the
members of any group it chooses.
4.2 Joining as an MCS to eavesdrop and interject
An alternative approach to eavesdropping is for an attacker to
register as an MCS and claim to support the group or groups of
interest. The attacker then becomes the focal point of transmissions
from legitimate MARS Clients, and is in a position to make copies of
packets sent to it.
A non-disruptive attacker would actually provide MCS functionality,
to ensure packets from legitimate cluster members are distributed
around the cluster as expected. A disruptive attacker could simply
black-hole the group's traffic, or creatively modify the packet
stream flowing through itself. A totally debilitating attacker would
register to support all multicast groups, and then black-hole the
traffic. Since the MARS trusts the MCS, and the MARS Clients trust
the MARS, this makes an effective denial of service attack. (The
fact that MCS cannot issue a block join provides minimal defense. A
creative attacker would register as a normal client in promiscuous
mode, watch for new groups being joined, and then simultaneously
register as an MCS for the new group. A single application running on
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an unsecured PC can conceivably emulate two distinct ATM entities.)
Aside from the fact that information is leaking from your cluster,
such eavesdropping may have a debilitating effect on the wider ATM
cloud. If the attacker is topologically distant at the ATM level,
this action results in a sudden increase in traffic along the ATM
path from your cluster to the attacker's own access point.
4.3 Bypassing or hi-jacking the MARS
The eavesdropping techniques described in the previous two sections
involve the attacker registering itself with the MARS as either a
legitimate client or MCS. If the MARS Clients within the Cluster fail
to perform sanity checks on the source of the MARS_JOIN messages they
listen to, it may be feasible for an attacker to target particular
senders directly. The attacker establishes a VC to the target MARS
Client and sends it a properly formatted MARS_JOIN. In this case,
the attacker only receives a copy of the traffic from the targetted
MARS Client.
A MARS Client that fails to sanity check MARS_LEAVE messages can be
effectively shut down by an attacker. The attacker finds the group
members by querying the MARS, then begins issuing MARS_LEAVEs to the
target MARS Client. Each MARS_LEAVE causes the target MARS client to
drop another leaf node from its forwarding VC. Eventually the VC is
closed down.
An MCS may be similarly vulnerable to fake MARS_SJOIN/SLEAVE messages
coming directly from an attacker.
This vulnerability may be partially closed if the MARS Client checks
the source of the VC on which MARS_JOIN/LEAVE messages arrive.
Messages to update outgoing pt-mpt VCs arrive on ClusterControlVC.
However, the current protocol does not provide Clients with a
definite mechanism for determining which incoming SVC represents
ClusterControlVC. If the ATM signaling is trusted, the sanity check
would be to only accept MARS_JOIN/LEAVE messages arriving on VCs
whose Calling Party number is that of the MARS entity. If the VCs
Calling Party number is unavailable, the target MARS Client can, at
best, make an assumption that the VC on which it first receives a
MARS_REDIRECT_MAP must be ClusterControlVC.
MARS_REDIRECT_MAP itself provides trouble for vulnerable MARS
Clients. An attacker who is prepared to emulate a complete MARS can
transmit a fake MARS_REDIRECT_MAP to all (or some subset) of the
Cluster's members, forcing them to voluntarily switch from the
current MARS. If a 'hard redirect' is demanded by the attacker, the
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MARS Clients will also assist the attacker by re-MARS_JOINing every
group they were members of. Once redirected, cluster operation
continues as though nothing has happened. (Clients that depend on
seeing a MARS_REDIRECT_MAP to decide which VC is ClusterControlVC are
vulnerable to this attack if the first MARS_REDIRECT_MAP that they
see is from the attacker.) Having taken over as the cluster's MARS,
the attacker is pretty much free to cause further havoc.
4.4 Denial of Service
As with RFC 1577, an attacker could disrupt or slow down MARS service
by repetitive VC setup/teardown events. An attacker who repeatedly
registered and de-registered as a cluster member would cause even
more ATM signaling activity, as the target MARS updates
ClusterControlVC. Similarly, an attacker could repeatedly register
and deregister as an MCS to force updates of ServerControlVC.
A direct attack on the MARS itself might involve explicitly joining,
in sequence, every possible multicast group, in the hope that the
internal database will overflow. Some MARS implementations may react
by crashing, providing a useful denial of service mechanism. Being
able to force a crash has additional uses - the attack can force a
restart, and while the clients are re-registering with the restarted
MARS, the attacker injects false MARS_REDIRECT_MAPs as described in
section 4.3.
Faking of an MCS by the attacker (as described in section 4.2) can be
used to create full or partial black-holes for traffic.
4.5 Hiding the MARS
Many scenarios involve the attacker knowing who the cluster members
are. In most situations this will be trivial to achieve, since there
is usually some well known multicast group joined by all cluster
members (e.g. 224.0.0.1 under IPv4). A MARS_REQUEST on this group
will provide the required information. (Interestingly, since MARS
Clients and ATMARP Clients are typically co-located, this search will
reveal the locations of all ATMARP Clients in the LIS as well.
Probing each client by opening a direct VC is likely to elicit an
InARP Request that reveals the client's unicast IP address.)
Keeping the address of the MARS secret can help discourage many of
the preceding attacks. However, experience with RFC 1577
implementations suggests that there will be little attempt to hide
the configuration information from users. If the person behind the
attacks has user level access to any machine on the LIS, they will
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have access to the MARS address.
Even if the MARS address could be kept a secret, a determined search
would make use of the fact that a MARS reacts in a predictable way
when it is sent a correctly formatted registration MARS_JOIN. An
attacker with complete or partial knowledge of the AESAs in your
region of the cloud can poll possible AESA variations (varying the
SEL field) looking for an endpoint that reacts correctly to
MARS_JOIN.
An attacker with physical access to your ATM cloud can place a tap on
any link, parse the UNI signalling traffic going past, and draw
conclusions from the AESAs it sees in SETUP messages.
5. RFC xxxx (NHRP)
As with RFC 1577, the RFC xxxx protocol is primarily query/response.
However, NHRP clients also expect to receive asynchronous updates
from their NHS, which opens up possibilities for an attacker to
trigger client misbehavior.
Although RFC xxxx provides an optional TLV for authentication
purposes, its use is not mandated. Therefore, this section assumes
NHRP installations where no use is being made of the authentication
TLV.
[insert cites to any prior work here.]
[This section is very much under construction.]
5.1 IP/ATM address spoofing
Address spoofing involves the insertion of incorrect {IP,ATM}
mappings into the NHS' database. An attacker establishes a VC to the
NHS for a given LIS/LAG, and provides a fake {IP,ATM} mapping in its
NHRP Register Request. An attacker may also attempt to register fake
{IP,ATM} mappings in NHSes serving remote LIS/LAGs, by specifying a
remote NHS as the Destination of the NHRP Register Request.
This sort of spoofing can be used either as a denial of service
attack or a stepping stone to subsequent hi-jacking of higher level
IP services (e.g. register the IP address of the local NFS server or
router immediately after an NHS reboot). As fake mappings can be
inserted into NHSes outside the immediate LIS/LAG, the scope for
damage is far greater than that presented in ATMARP installations.
An attacker may find value in attempting to register mappings with a
target NHS that represent IP addresses from a LIS/LAG not served by
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the target NHS, in case the NHS implementation fails to sanity check
the mapping. If the fake registration succeeds, no other NHSes
(including the NHS that actually serves the target IP address) will
know that the compromised NHS is handing out false information. The
compromised NHS may also respond to non-authoritative requests for
the affected IP addresses mapping from remote clients if their NHRP
Requests are routed through it.
(Note that registering with an NHS outside the local LIS/LAG only
requires that the IP address of the remote NHS be placed into the
attacker's NHRP Registration Request. Thus the attacker need only
know the ATM address of one NHS. The IP addresses of potential
target NHS(es) may be surmised by checking the IP addresses of
routers in the region, as reported by tools such as 'traceroute'.)
5.2 Scanning the {IP,ATM} mapping space.
It is usually undesirable for outsiders to know the entire set of IP
addresses (and associated ATM addresses) that make up your LIS/LAG.
However, there is little to stop an attacker from establishing a VC
to your NHS, registering an innocuous {IP,ATM} mapping, and
proceeding to issue NHRP Requests for a range (or ranges) of
'interesting' IP addresses.
Since the NHRP service's design goal is the resolution of IP
addresses outside the LIS/LAG, the attacker can also choose to scan
the mappings for other LIS/LAGs through the local NHS. Knowing the
address of any one NHS opens up an entire set of LIS/LAGs.
The ATM addresses thus learned might be used in subsequent denial of
service attacks against specific hosts. Depending on range of IP
addresses chosen during the scan, and the speed with which the
repeated NHRP Requests are issued, this scanning can itself keep the
NHS so busy as to constitute a denial of service attack. If the
target IP addresses are outside the local LIS/LAG, the request
traffic propagates to other NHSes too.
Not knowing the LIS/LAG address range makes the attack less
efficient, but not impossible since the server actively responds to
bad guesses with NHRP Reply indicating failure. A selective search
would make intelligent guesses as to the probable range of net/subnet
numbers to scan.
5.3 Denial of Service attacks.
An attacker can present two types of overload to an NHS - repetative
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VC setup/teardown events without actually registering any {IP,ATM}
mappings (simply to consume cpu cycles in the NHS' underlying UNI
stack), and repeated VC setup/teardown/registration events (where the
attacker explicitly NHRP Registers with a different {IP,ATM} mapping
each time).
The first type of attack wastes time at the NHS, and causes
additional loading of the control processor in the switch to which
the target is attached (and other switches along the path between the
attacker and target).
The second type of attack will be slower (although parallel VC setup
attempts can be used to keep the average rate up), but results in
additional database manipulation activity in the NHS. This can result
in collisions with IP addresses already registered, or set the stage
for later collisions when the legitimate owners of a particular IP
address attempt to register.
As noted in section 5.1, the second type of attack can be launched
against remote NHSes without even knowing their ATM addresses.
Harrassment through repetative VC setup/teardown may also be launched
against NHCs whose addresses were learned through scanning of the NHS
database.
An attacker can transmit fake NHRP Purge messages to both NHSes and
NHCs. At minimum this is likely to result in unnecessary VC
teardown/setup sequences between NHCs whose current short-cuts are
prematurely purged.
5.5 Hiding the NHS
Keeping the ATM addresses of NHS secret would help discourage many of
the preceding attacks. However, this is unlikely to be even remotely
achievable.
As noted in section 5.1, if only one NHS ATM address is known this is
sufficient to cover all NHSes. Their IP addresses can be guessed from
the IP addresses of routers in the network, and if required the
attacker can query the initial NHS to discover the matching ATM
addresses.
Even if all NHS ATM addresses could be kept a secret, a determined
search would make use of the fact that an NHS reacts in a predictable
way when it is sent a correctly formatted NHRP messages. An attacker
with complete or partial knowledge of the AESAs in your region of the
cloud can poll possible AESA variations (varying the SEL field)
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looking for an endpoint that reacts correctly to NHRP Registration
Request.
An attacker with physical access to your ATM cloud can place a tap on
any link, parse the UNI signalling traffic going past, and draw
conclusions from the AESAs it sees in SETUP messages.
6. Common extensions to MARS and NHRP
The RFC1577 control packet format cannot be extended to support an
authentication field in a backward compatible manner. However, both
MARS and NHRP share a common control packet syntax that supports
optional TLV-based fields. RFC xxxx contains an authentication TLV,
and it would seem reasonable to build on this TLV for MARS.
[TBD]
[TLV exists, but key distribution is a problem.]
7. Conclusion
[TBD]
Security Consideration
This document is all about security considerations.
Acknowledgments
Author's Address
Grenville Armitage
Bell Labs, Lucent Technologies.
101 Crawfords Corner Rd,
Holmdel, NJ, 07733
USA
Email: gja@lucent.com
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References
[1] Laubach, M., "Classical IP and ARP over ATM", RFC1577, Hewlett-
Packard Laboratories, December 1993.
[2] G. Armitage, "Support for Multicast over UNI 3.0/3.1 based ATM
Networks.", Bellcore, RFC 2022, November 1996.
[3] J. Luciani, et al, "NBMA Next Hop Resolution Protocol (NHRP)",
INTERNET DRAFT, draft-ietf-rolc-nhrp-11.txt, February 1997.
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