RFC2520 - NHRP with Mobile NHCs

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.

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

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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