RFC2520 - NHRP with Mobile NHCs

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　　Network Working Group J. Luciani
Request for Comments: 2520 Nortel Networks
Category: Experimental H. Suzuki
Cisco Systems
N. Doraswamy
Nortel Networks
D. Horton
CiTR Pty Ltd
February 1999

NHRP with Mobile NHCs

Status of this Memo

This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

Abstract

This document describes an extension to NHRP [1] which would allow
Mobile NHCs to perform a registration with and attach to an NHS in
their home LIS in an authenticated manner.

As described in this document, Mobile NHCs are NHCs which are not
configured with enough information to find a specific serving NHS in
their home LIS, but which have a mechanism to find an NHS (which may
or may not be a serving NHS) to which they will attach. As described
in [1], an NHC may attach to a 'surrogate' NHS by using a mechanism
such as an anycast address. In this case, the NHC may use the
surrogate NHS to send a NHRP Registration Request toward the NHC's
home LIS where a serving NHS resides. However, as defined in [1],
packet authentication is performed on a hop by hop basis. In the
mobile NHC case, it is not practical for the mobile NHC be in a
security relationship with every surrogate NHS, thus it is presumably
desirable to have some form of end to end only authentication for the
case of a mobile NHC's registration. This document describes an
authentication extension for NHRP which has such end to end only
semantics.

1. Introduction

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [4].

This document describes an extension for Mobile NHCs to use when they
wish to register with their home LIS but initially connect to a non-
serving NHS to do so. The reader is encouraged to read [1] for more
details on the NHRP registration process.

2.0 Definition of the NHRP Mobile NHC Authentication Extension

Compulsory = 1
Type = 10 (proposed)
Length = variable

The NHRP Mobile NHC Authentication Extension is carried in NHRP
Registration packets to convey end to end authentication Information.
This extension is defined in contrast to the NHRP Authentication
Extension defined in [1] which has hop by hop semantics.

This new extension is used when a mobile NHC initially connects to an
NHS which is not one of its serving NHSs and the mobile NHC and
nonserving NHS are not in a security relationship. The mobile NHC
does this in order to send an NHRP Registration Request, via normal
routing and forwarding processes, to one of its serving NHSs with
which it does have a security relationship. As defined in [1], a
serving NHS is an NHS in the NHC's home LIS with which the NHC will
register. Upon receiving such an NHRP Registration Request, the
serving NHS will do the following: authenticate the sender NHC, set
up a VC to the NHC, and then send an NHRP Resolution Reply in
response on that new VC.

Note that, as defined in [1], a transit NHS (such as the one to which
the mobile NHC initially connects) must ignore an extension which it
does not understand and that an NHS must not change the order of
extensions in an NHRP packet. Thus, the end to end semantics of this
extension are preserved without causing changes to existing
implementations.

If a serving NHS receives a packet which fails the hop by hop
authentication test defined in [1] then the NHS MUST generate an
Error Indication of type 'Authentication Failure' and discard the
packet. However in the case where the NHRP Mobile NHC Authentication
Extension is used as described above, sending an Error Indication is
not possible since no route exists back toward the mobile NHC
assuming a VC does not already exist between the mobile NHC and the

serving NHS which received the NHRP Registration Request. In this
case, the NHRP Registration Request is merely dropped.

2.1 Header Format

The authentication header has the following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Security Parameter Index (SPI)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src Addr... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+ Authentication Data... -+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Security Parameter Index (SPI) can be thought of as an index into a
table that maintains the keys and other information such as a hash
algorithm. Src and Dst communicate either offline using manual keying
or online using a key management protocol to populate this table. The
sending NHRP entity always allocates the SPI and the parameters
associated with it.

The Src Addr field is a variable length field which contains the
address assigned to the outgoing interface. The length of the field
is obtained from the source protocol length field in the mandatory
part of the NHRP header. The tuple <spi, src addr> uniquely
identifies the key and the other parameters that are used in
authentication.

The length of the authentication data field is dependent on the hash
algorithm used. The Authentication Data field contains the keyed hash
calculated over the following fields: fixed part (with hop count,
packet size and checksum being treated as if set to zero), mandatory
part, and extensions up to and including the Mobile NHC
Authentication extension.

Note that [1] defines an explicit ordering of extensions such that:

(a) If the Responder Address extension exists then it must appear
before the Authentication Extension.

(b) Any extensions that may be modified in transit (e.g., Forward
Transit Extension, Hop by Hop Authentication Extension) must
appear after the Mobile NHC Authentication Extension.

2.2 SPI and Security Parameters Negotiation

SPI's can be negotiated either manually or using an Internet Key
Management protocol. Manual keying MUST be supported. The following
parameters are associated with the tuple <SPI, src>- lifetime,
Algorithm, Key. Lifetime indicates the duration in seconds for which
the key is valid. In case of manual keying, this duration can be
infinite. Also, in order to better support manual keying, there may
be multiple tuples active at the same time (Dst being the same).

Algorithm specifies the hash algorithm agreed upon by the two
entities. HMAC-MD5-128 [2] is the default algorithm and MUST be
implemented. Other algorithms MAY be supported by defining new
values. IANA will assign the numbers to identify the algorithm being
used as described in [1].

Any Internet standard key management protocol MAY so be used to
negotiate the SPI and parameters.

2.3 Message Processing

Unauthenticated 'Mobile' Registration Request processing proceeds as
follows [1]:

- the NHC inserts the internetwork address of a serving NHS in the
'Destination Protocol Address' field; If the NHS address is
unknown, then the NHC inserts its own internetwork address. A '
responder address' extension is optionally added.
- the non-serving NHS forwards the packet along the routed path
based on the contents of the 'Destination Protocol Address'
field.
- the serving NHS which receives the NHRP Registration Request
will set up a direct VCC to NHC after authenticating the request
- the serving NHS will then send the NHRP Registration Reply back
to the NHC on that new VCC. Note that the NHS MUST wait some
configured interval before doing this reply in order to prevent
a race condition from occurring between the VC setup and sending
the NHRP reply packet.
- the NHC will subsequently send all NHRP traffic to the serving
NHS on the direct VCC.

When the NHC adds the authentication extension header, it performs a
table look up in order to fetch the SPI and the security parameters
based on the outgoing interface address. If there are no entries in
the table and if there is support for key management, the NHC
initiates the key management protocol to fetch the necessary
parameters. The NHC constructs the Authentication Extension payload
and calculates the hash by zeroing out the authentication data field.
The result is placed in the authentication data field. The src
address field in the payload is the internetwork address assigned to
the outgoing interface.

If key management is not supported and authentication is mandatory,
the packet is dropped and this information is logged.

On the receiving end, the serving NHS fetches the parameters based on
the SPI and the internetwork address in the authentication extension
payload. The authentication data field is extracted before being
zeroed out in order to calculate the hash. It computes the hash on
the entire payload and if the hash does not match, then an "abnormal
event" has occurred.

The keys used by the mobile NHC for communicating with the serving
NHS in NHRP Registration Requests can be used in subsequent
resolution and purge requests made directly to the serving NHS after
receiving the NHRP Registration Reply. However, the authentication
extension defined in [1] MUST be used when these keys are applied to
resolution and purge packets.

Hop by Hop Authentication[1] and End to End authentication MAY be
used in combination to provide protection against both spoofing and
denial of service attacks. If only an end-to-end Mobile NHC
Authentication Extension is present, it MAY be the policy of each
transit NHS to reject the NHRP Registration Request based on the
requirement for having a Hop by Hop authentication present. Such a
requirement is a local matter.

2.4 Security Considerations

It is important that the keys chosen are strong since the security of
the entire system depends on the keys being chosen properly.

End-to-end authentication counters spoofing attacks on the home
subnet through not relying on the potentially compromised chain of
trust. The use of end-end authentication is further described in [3].

Hop-by-hop authentication prevents denial of service attacks by
introducing access control at the first point of contact to the NHRP
infrastructure.

The security of this extension is performed on an end to end basis.
The data received can be trusted only so much as one trusts the end
point entities in the path traversed. A chain of trust is established
amongst NHRP entities in the path of the NHRP Message. If the
security in an NHRP entity is compromised, then security in the
entire NHRP domain is compromised.

Data integrity covers the entire NHRP payload up to and including the
Mobile NHC Authentication Extension. This guarantees that the data
and extensions covered by this authentication hash were not modified
and that the source is authenticated as well. If the authentication
extension is not used or if the security is compromised, then NHRP
entities are liable to both spoofing attacks, active attacks, and
passive attacks.

There is no mechanism to encrypt the messages. It is assumed that a
standard layer 3 confidentiality mechanism will be used to encrypt
and decrypt messages. It is recommended to use an Internet standard
key management protocol to negotiate the keys between the neighbors.
Transmitting the keys in clear text, if other methods of negotiation
is used, compromises the security completely.

References

[1] Luciani, J., Katz, D., Piscitello, D., Cole, B. and N. Doraswamy,
"NBMA Next Hop Resolution Protocol (NHRP)", RFC2332, April 1998.

[2] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed Hashing
for Message Authentication", RFC2104, February 1997.

[3] Perkins, C., "IP Mobility Support", RFC2002, October 1996.

[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.

Authors' Addresses

James V. Luciani
Nortel Networks
3 Federal Street
Mail Stop: BL3-03
Billerica, MA 01821

Phone: +1 978 916 4734
EMail: luciani@baynetworks.com

Hiroshi Suzuki
Cisco Systems
170 West Tasman Dr.
San Jose, CA 96134

Phone: +1 408 525 6006
EMail: hsuzuki@cisco.com

Naganand Doraswamy
Nortel Networks
3 Federal Street
Mail Stop: BL3-03
Billerica, MA 01821

Phone: +1 978 916 4734
EMail: naganand@baynetworks.com

David Horton
CiTR PTY Ltd
Level 2 North Tower
339 Coronation Drive
Milton, Australia 4064

Phone: +61 7 32592222
EMail: d.horton@citr.com.au

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Copyright (C) The Internet Society (1999). All Rights Reserved.

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