RFC2735 - NHRP Support for Virtual Private Networks

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　　Network Working Group B. Fox
Request for Comments: 2735 Equipe Communications
Category: Standards Track B. Petri
Siemens AG
December 1999

NHRP Support for Virtual Private Networks

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

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

Abstract

The NBMA Next Hop Resolution Protocol (NHRP) is used to determine the
NBMA subnetwork addresses of the "NBMA next hop" towards a public
internetworking layer address (see [1]). This document describes the
enhancements necessary to enable NHRP to perform the same function
for private internetworking layer addresses available within the
framework of a Virtual Private Network (VPN) service on a shared NBMA
network.

1. Introduction

NHRP is a public internetworking layer based resolution protocol.
There is an implicit understanding in [1] that a control message
applies to the public address space.

Service Providers of Virtual Private Network (VPN) services will
offer VPN participants specific service level agreements (SLA) which
may include, for example, dedicated routing functions and/or specific
QoS levels. A particularly important feature of a VPN service is the
ability to use a private address space which may overlap with the
address space of another VPN or the Public Internet. Therefore, such
an internetworking layer address only has meaning within the VPN in
which it exists. For this reason, it is necessary to identify the
VPN in which a particular internetworking layer address has meaning,
the "scope" of the internetworking layer address.

As VPNs are deployed on shared networks, NHRP may be used to resolve
a private VPN address to a shared NBMA network address. In order to
properly resolve a private VPN address, it is necessary for the NHRP
device to be able to identify the VPN in which the address has
meaning and determine resolution information based on that "scope".

As VPN services are added to an NBMA network using NHRP devices, it
may be necessary to support the service with legacy NHRP devices that
do not have VPN knowledge and so do not explicitly support VPNs.
This document describes requirements for "VPN-aware" NHRP entities to
support VPN services while communicating with both "VPN-aware" and
"non-VPN-aware" NHRP entities.

2. Overview of NHRP VPN Support

2.1 Terminology

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 [4].

In addition to the terminology specified in section 2.1 of [1], the
following definitions and acronyms are used:

Default Routing Instance -- In the presence of VPNs, all packets are
processed (e.g., routed) within the context of a specific VPN. In the
case where no VPN is indicated, a packet is processed according to a
default VPN, i.e., a Default Routing Instance. This routing instance
may be the Public Internet, a particular VPN, etc. The term only has
meaning for "VPN-aware" NHRP entities.

Virtual Private Network (VPN) -- in the context of this
specification, this term is used as described in [3].

VPN-aware -- a "VPN-aware" NHRP entity is an NHRP entity that
implements the NHRP enhancements for VPNs as defined in this
document.

Non-VPN-aware -- a "non-VPN-aware" NHRP entity is an NHRP entity
which is deployed as part of a single VPN, but is not VPN-aware.
Restrictions applying to non-VPN-aware NHRP entities are outlined
below. NHRP devices as specified in [1] are examples of non-VPN-
aware entities.

VPN encapsulation -- An LLC/SNAP encapsulation of a PDU with an
indication of the VPN to which the PDU belongs. In the case that the
underlying NBMA network is an ATM network, VPN encapsulation is
specified in section 8 of [2].

VPN identifier (VPN-ID) -- in the context of this specification, this
term is used as specified in [3].

VPN signalling -- in the context of this specification, this term is
used to denote a method to indicate the VPN-ID via control signalling
or similar ways in the control path.

2.2 VPN Support Overview

When supporting NHRP for a VPN, it is necessary to specify to which
VPN the NHRP message applies in order to comply with the VPN service
level agreement applicable to that VPN.

On some NBMA networks, it is possible to establish a VPN-specific
control path between NHRP devices. This is sufficient to identify
the NHRP control packets as belonging to the "inherited" VPN.
However, when that alternative is not used, the NHRP device must
specify the VPN to which an NHRP packet applies in the PDU.

It is not useful to add a VPN extension to NHRP control messages
because transit NHRP Servers are not required to process the
extensions to an NHRP control message (see 5.3 in [1]). NHRP Servers
already deployed might resolve the control packet within the scope of
the public internetworking layer address space instead of the private
address space causing problems in routing.

Instead, an LLC/SNAP header with a VPN indication (as specified in
Section 4.1 below) will be prepended to the NHRP control message.
This solution allows the same VPN-specific LLC/SNAP header to be
prepended to PDUs in both the control and data paths.

3. NHRP VPN Operation

3.1 VPN-Aware NHRP Operation

When a VPN-aware NHRP device forwards a packet pertaining to a
particular VPN, that device MUST be able to indicate the VPN either:

a) explicitly through use of the VPN-specific LLC/SNAP header or
b) implictly through an indication via VPN signalling.

This applies to NHC-NHS, NHS-NHS, and NHS-NHC control messages as
well as NHC-NHC shortcut traffic.

For case a), the indication of the VPN-ID is via a VPN-specific
LLC/SNAP header specified in section 4.2 below. In the case of an
underlying ATM network, see also section 8 of [2].

For case b), the method used to indicate the VPN-ID via VPN
signalling depends on the mechanisms available in the underlying
network and is outside the scope of this memo. A VPN-aware NHRP
entity using VPN signalling SHOULD NOT also indicate the VPN-ID
explicity for any PDU on the related path.

In transiting an NHRP Server, the VPN identification MAY be forwarded
in a different format than was received, however, the same VPN-ID
MUST be indicated for the message. For example, a PDU received with
an LLC/SNAP header containing a VPN identifier may be forwarded on a
control path which was established with an indication of the same VPN
without the VPN-specific LLC/SNAP header.

When a VPN capable NHRP entity receives an NHRP message from a VPN-
aware NHRP device without a VPN indication via VPN encapsulation or
VPN signalling, the message applies to the default routing instance
supported by the shared infrastructure. The public Internet or a
particular VPN routing realm may be configured as the default routing
instance.

3.2 Interactions of VPN-aware and non-VPN-aware NHRP entities

A VPN-aware NHRP entity MUST be able to indicate the VPN-ID in one of
the ways specified in section 3.1 above. It MAY participate in more
than one VPN.

Because a non-VPN-aware NHRP device does not understand the concept
of VPNs, it only supports a single routing instance. Therefore, a
non-VPN-aware NHRP entity belongs to exactly one VPN without being
aware of it. All internetworking packets sent by that entity are
assumed to belong to that VPN (Note that if the current IPv4-based
Internet is regarded as just one big VPN, attached IPv4 hosts may
e.g. be regarded as being "contained" in that VPN).

In order for a non-VPN-aware NHRP entity to interact with a VPN-aware
NHRP entity, the VPN-aware NHRP entity MUST be configured to
associate the correct VPN-ID with information received from the non-
VPN-aware entity. In other words, the VPN-aware NHRP entity acts as
in the case of option b) from section 3.1 where the VPN-ID was
indicated via VPN signalling. However, this association is
provisioned using administrative means that are beyond the scope of
this document instead of via VPN signalling. Further, it MUST be
ensured by administrative means that non-VPN-aware NHRP entities only
communicate either with other NHRP entities contained in the same
VPN, or with VPN-aware NHRP entities with pre- configured information
about the related VPN-ID of those non-VPN-aware entities.

VPN-aware NHRP entities SHALL only send information to non-VPN-aware
NHRP entities if that information belongs to the VPN in which the
non-VPN-aware entity is contained. Information sent to a non-VPN-
aware NHRP entity MUST not include any indication of the VPN-ID.

In order to correctly transfer data packets, it is necessary for
VPN-aware ingress NHRP clients to know whether their partner is also
VPN-aware. If the egress is VPN-aware, the ingress NHC will also use
the means described in section 3.1 on an NBMA shortcut to that egress
NHC to specify the VPN to which the data packet belongs.

For this purpose, a further NHRP extension (in addition to those
specified in section 5.3 of [1]) is specified which is called NHRP
Device Capabilities extension (see section 4.2 below). This extension
currently indicates the VPN capabilities of NHRP source and
destination entities, but may also be used in the future for further
additions to NHRP to indicate other capabilities as well.

3.3 Handling of the NHRP Device Capabilities extension

The NHRP Device Capabilities extension MUST be attached to all NHRP
Resolution Requests generated by a VPN-aware source NHRP entity. The
device SHOULD set the Source Capabilities field to indicate that it
supports VPNs. The compulsory bit MUST be set to zero, so that a
non-VPN-aware NHS may safely ignore the extension when forwarding the
request. In addition, the A-bit (see section 5.2.1 of [1]) SHOULD be
set to indicate that only authoritative next hop information is
desired to avoid non-authoritative replies from non-VPN-aware NHRP
servers.

Since a non-VPN-aware NHS is not able to process the NHRP Device
Capability extension, Network Admistrators MUST avoid configurations
in which a VPN-aware NHRP Client is authoritatively served by a non-
VPN-aware NHRP Server.

If an egress NHS receives an NHRP Resolution Request with an NHRP
Device Capability Extension included, it returns an NHRP Resolution
Reply with an indication of whether the destination is VPN-aware by
correctly setting the target capabilities flag [see Section 4.2].

If an egress NHS receives an NHRP Resolution Request without an NHRP
Device Capability Extension included or with the source capabilities
flag indicating that the source NHRP device is non-VPN-aware, it MAY
act in one of the following ways:

- It MAY reject the NHRP Resolution Request; this is because the
VPN-aware destination will be unable to determine the context
of information received on an NBMA shortcut from a non-VPN-
aware NHRP source. This is the default case.

- If the destination is also non-VPN-aware, it MAY accept the
request and return an NHRP Resolution Reply. By default, the
two non-VPN-aware NHRP clients will interact correctly.

- It MAY offer itself as a destination and resolve the request
using its own NBMA address, if it has the related capabilities.

- If the indicated VPN-ID identifies the default routing instance
of the destination, the NHS MAY accept the request and send a
corresponding NHRP Resolution Reply.

The NHRP Device Capabilities extension SHOULD NOT be included in the
NHRP Register Request and Reply messages.

3.4 Error handling procedures

If an NHRP entity receives a PDU with a VPN-ID indicated via VPN
encapsulation which is in conflict to a VPN-ID earlier allocated to
that communication (e.g. via VPN signalling or administratively via
configuration), it SHOULD send back an NHRP error indication (see
5.2.7 of [1]) to the sender indicating error code 16 (VPN mismatch).
However, in order to avoid certain security issues, an NHRP entity
MAY instead silently drop the packet.

If a VPN-aware NHRP entity receives a packet for a VPN that it does
not support, it SHOULD send back an NHRP error indication to the
sender with an error code 17 (VPN not supported). However, in order
to avoid certain security issues, an NHRP entity MAY instead silently
drop the packet.

If a VPN-aware NHS cannot find a route to forward a VPN-related NHRP
message, it SHOULD send back an NHRP error indication to the sender
with error code 6 (protocol address unreachable). However, in order
to avoid certain security issues, an NHRP entity MAY instead silently
drop the packet.

In all cases, where an NHRP error indication is returned by a VPN-
aware NHRP entity, the incorrect VPN-ID related to this indication
SHALL be indicated via VPN encapsulation or VPN signalling, except
when sending it to a non-VPN-aware NHRP device (see 3.1 / 3.2 above).

4. NHRP Packet Formats

4.1 VPN encapsulation

The format of the VPN encapsulation header is as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xAA | 0xAA | 0x03 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | 0x5E | 0x00 | 0x08 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PAD | OUI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPN Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LLC encapsulated PDU (up to 2^16 - 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

It consists of the following parts:

- LLC/SNAP indication (0xAA-AA-03)
- OUI (of IANA) (0x00-00-5E)
- PID allocated by IANA for VPN encapsulation (0x00-08)
- PAD field (inserted for 32-bit alignment)
this field is coded as 0x00, and is ignored on receipt
- VPN related OUI (see [3])
- VPN Index (see [3]).

When this encapsulation header is used, the remainder of the PDU MUST
be structured according to the appropriate LLC/SNAP format (i.e. that
would have been used without the additional VPN encapsulation
header). Correspondingly, the following figure shows how NHRP
messages are transferred using VPN encapsulation:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xAA | 0xAA | 0x03 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | 0x5E | 0x00 | 0x08 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PAD | OUI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPN Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xAA | 0xAA | 0x03 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | 0x5E | 0x00 | 0x03 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NHRP message |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The following example shows how IP packets are transferred by VPN
encapsulation:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xAA | 0xAA | 0x03 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | 0x5E | 0x00 | 0x08 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PAD | OUI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPN Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xAA | 0xAA | 0x03 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | 0x00 | 0x08 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP PDU (up to 2^16 - 24 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.2 NHRP device capabilities extension

The format of the NHRP device capabilities extension is as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|u| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Capabilities |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Target Capabilities |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

C: Compulsory = 0 (not a compulsory extension)
u: Unused and MUST be set to zero.
Type = 0x0009
Length = 0x0008

Source Capabilities field:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused |V|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

V bit:

0x0 - the source NHRP device is non-VPN-aware
0x1 - the source NHRP device is VPN-aware

The unused bits MUST be set to zero on transmission
and ignored on receipt.

Target Capabilities field:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused |V|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

V bit:

0x0 - the destination NHRP device is non-VPN-aware
0x1 - the destination NHRP device is VPN-aware

The unused bits MUST be set to zero on transmission
and ignored on receipt.

4.3 Error Codes

The following further Error Codes are defined in addition to those
specified in section 5.2.7 of [1]):

16 - VPN mismatch

This error code is returned by a VPN-capable NHRP device, if it
receives a PDU with a VPN-ID in the LLC/SNAP header different
from the VPN-ID which had been specified earlier via VPN
signalling.

17 - VPN not supported

This error code is returned by a VPN-capable NHRP device, if it
receives an NHRP message for a VPN that it does not support.

5. Security Considerations

For any VPN application, it is important that VPN-related information
is not misdirected to other VPNs and is not accessible when being
transferred across a public or shared infrastructure. It is therefore
RECOMMENDED to use the VPN support functions specified in this
document in combination with NHRP authentication as specified in
section 5.3.4 of [1]. Section 5.3.4.4 of [1] also provides further
information on general security considerations related to NHRP.

In cases where the NHRP entity does not trust all of the NHRP
entities, or is uncertain about the availability of the end-to-end
NHRP authentication chain, it may use IPsec for confidentiality,
integrity, etc.

6. IANA Considerations

The LLC/SNAP protocol ID 0x00-08 for VPN encapsulation had already
been allocated by IANA in conjunction with [2]. This specification
does not require the allocation of any additional LLC/SNAP protocol
IDs beyond that.

It should be noted that IANA - as the owner of the VPN-related OUI:
0x00-00-5E - is itself also a VPN authority which may allocate VPN
indices to identify VPNs. The use of these particular VPN indices
within the context of this specification is reserved, and requires
allocation and approval by the IESG in accordance with RFC2434.

References

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

[2] Grossman, D. and J. Heinanen, "Multiprotocol Encapsulation over
ATM Adaptation Layer 5", RFC2684, September 1999.

[3] Fox, B. and B. Gleeson, "Virtual Private Networks Identifier",
RFC2685, September 1999.

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

Authors' Addresses

Barbara A. Fox
Equipe Communications
100 Nagog Park
Acton, MA 01720

Phone: +1-978-795-2009
EMail: bfox@equipecom.com

Bernhard Petri
Siemens AG
Hofmannstr. 51
Munich, Germany, D-81359

Phone: +49 89 722-34578
EMail: bernhard.petri@icn.siemens.de

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