Internet Engineering Task Force Juha Heinanen
INTERNET DRAFT Song Networks
Expires September 2002 January, 2002
DNS/LDP Based VPLS
<draft-heinanen-dns-ldp-vpls-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
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Abstract
This memo describes how provider provisioned Virtual Private LAN
Service (VPLS) can be implemented using DNS and LDP for PE discovery
and label distribution.
1. Introduction
This memo describes a simple mechanism to implement provider based
Virtual Private LAN Service (VPLS) [1] using DNS [2] and LDP [3] for
PE discovery and label distribution.
An advantage of a DNS/LDP based solution for provider based VPNs is
that it doesn't require BGP implementation or configuration
complexity in the PE routers and can be easily deployed also in
inter-AS cases where the VPN sites are attached to PEs in more than
one AS. Another advantage of DNS is that it has been in wide use for
years and can thus be deployed without any new standardization
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effort. See [2] for more discussion on the use of DNS for VPN
discovery.
A similar DNS/LDP based solution can also be applied to provider
based Virtual Circuit and Virtual Router VPNs.
2. Addition of Sites
2.1 Configuration Actions
DNS/LDP based VPLS is very easy to provision. Only the following two
configuration actions are needed when a new site (CE device) is added
to a VPN:
(1) If the PE device (PE for short) does not previously connect any
sites of this VPN, the IP address (A record) of the PE is added
to DNS under domain name
vpn-name.domain
The label "vpn-name" uniquely identifies the VPN within
"domain", which belongs to the administrative "owner" of the
VPN. An example of the domain name of a VPN is
bobsVpn.serviceProvider.net.
(2) The "interface" of the PE to which the site is connected to is
configured to belong to the VPN. This is done by specifying
the domain name and type of the VPN. This document covers the
case where the type of the VPN is "LAN".
Note that also in the case of a multi-provider VPN, the
administrative "owner" of the VPN is the single body that operates
the master DNS server for the VPN zone. The "owner" of a VPN MAY
choose to make all updates to the zone data of the VPN by itself or
MAY allow other providers to dynamically update the zone data.
2.2 Protocol Actions
After the above configuration actions, the following protocol actions
take place in sequence at the PE of the new site if the PE of the new
site doesn't previously connect site(s) of the VPN:
(1) The PE of the new site checks that its own IP address has
become available in the DNS under the domain name of the VPN.
(2) The PE of the new site queries DNS for IP addresses of the
other (remote) PEs of the VPN.
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(3) The PE of the new site establishes an LDP session with each of
the remote PEs unless one already exists.
(4) The PE of the new site sends a Label Mapping Message to each of
the remote PEs that advertises a label to be used when a remote
PE sends packets to the sites of the VPN at the PE of the new
site. Each such label MUST uniquely identify at the PE of the
new site both the VPN and the sending PE.
The Label Mapping Message uses the following VPN ID FEC TLV:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VPN ID TLV | Address Family | VPN ID Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Domain name of the VPN padded with spaces to the next |
+ four octet boundary +
...
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Element type name: VPN ID
Type: TBD by IANA
Address Family: set to zero
VPN ID Length: Variable
The following protocol actions take place in sequence at a PE when it
receives a Label Mapping message from another PE:
(1) The PE checks from the DNS that the other PE belongs to the
VPN of the Label Mapping Message and that it itself has at
least one site in that VPN. If not, the PE responds to the
Label Mapping Message with a Label Release Message and no other
protocol actions take place at the PE.
(2) The PE checks if it already has a label for the VPN and PE of
the Label Mapping Message. If so, the PE responds to the Label
Mapping Message with a Label Release Message and no other
protocol actions take place at the PE.
(3) The PE checks if it already has itself advertised a label to
the other PE for the VPN of the Label Mapping Message. If not,
the PE sends a Label Mapping Message to the other PE to be used
when the other PE sends packets to the sites of the VPN at the
PE. The advertised label MUST again uniquely identify at the PE
both the VPN and the other PE.
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If the PE of the new site already connects site(s) of this VPN, no
protocol actions take place at either the PE of the new site or at
the remote PEs.
3. Removal of Sites
3.1 Configuration Actions
The following configuration actions are needed when an existing site
(CE device) is removed from a VPN:
(1) If the site to be removed is the last site of the VPN at the
PE, the IP address of the PE is removed from DNS under the
domain name of the VPN.
(2) The site is removed from the VPN by unconfiguring the VPN from
the "interface" of the PE to which the site is connected to.
3.2 Protocol Actions
After the above configuration actions, the following protocol actions
take place at the PE of the removed site if the removed site was the
last site of the VPN at the PE:
(1) The PE checks that its IP address does not anymore exist in the
DNS under the domain name of the VPN.
(2) The PE removes any existing labels of the VPN that it had
advertised to the remote PEs by sending them a Label Withdraw
Message.
In addition to processing the Label Withdraw Message, the following
protocol actions take place when a PE receives a Label Withdraw
Message from another PE:
(1) The PE removes the label that it had advertised to the other PE
for the VPN of the Label Withdraw Message by sending it a Label
Withdraw Message.
(2) If there is no remaining need to keep the LDP session up
between the PE and the other PE, the PE MAY terminate the LDP
session with the other PE.
4. Failure Recovery
If a PE looses its LDP session with another PE having site(s) in a
common VPN, the PE releases the label it has advertised to the other
PE for this VPN. The PE then tries to re-establish the LDP session
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until (a) the session gets established or (b) this PE or the other PE
no longer have site(s) in this VPN. Once the LDP session gets
established, the PE advertises to the other PE a label to be used to
send packets to the site(s) of the VPN at this PE as described in
section 2.2.
When a PE recovers from a crash, it adds each of the configured VPN
site(s) to their respective VPN(s) as described in section 2.2.
5. Exponential Back-off Behavior
If any protocol action does not succeed immediately, the normal
behavior is that the PE keeps on trying with exponential back-off
until the action either succeeds or becomes invalid due to a change
in VPN configuration. If the protocol action fails for an
implementation specific prolonged period of time, the PE SHOULD
notify the "owner" of the VPN about the problem via a management
action.
6. Data Plane
The PEs that host the sites of a VPN act as fully connected learning
bridges.
When PE A needs to forward an Ethernet packet to PE B, PE A
encapsulates the Ethernet packet into a label stack entry [4] as
described in [5]. The label of the label stack entry is the one that
PE B has advertised to PE A for this VPN. The optional control word
MUST NOT be used.
PE A then sends the resulting frame to PE B in any available tunnel,
such as a HIP, GRE, IPSec, VLAN, or MPLS. The selection of the
tunneling protocol is outside the scope of this memo.
7. DNS Zone Update Latency
In order to make addition and removal of VPN PEs as fast as possible,
it is important to try to minimize the latency of VPN zone updates.
This can be achieved by turning on DNS NOTIFY [6] in the master
server for each VPN zone and/or by configuring refresh times of VPN
zones small, e.g., zero.
8. DNS Message Size
Correct operation of directory/LDP based VPNs requires that IP
addresses of all PE routers of a VPN fit into a single DNS response.
As described in [7], if a PE receives a response that has the
Truncated Header bit (TC) set, it MUST ignore that response, and
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query again, using a TCP connection that will permit larger replies.
9. Security Considerations
Security of directory/LDP based VPNs depends on security of the
directory (DNS), LDP, and the tunneling protocol(s). Security of LDP
is covered in the security section of [3]. Also the various
tunneling protocol specifications have their own security sections.
Regarding DNS security, the important issues related to this memo are
security of zone transfers, security of possible dynamic updates, as
well as integrity and authentication of DNS queries and responses.
In case of dynamic updates, it is RECOMMENDED that secure dynamic
updates [8] are used. Security of zone transfers as well as
integrity of queries and responses are addressed by DNS extensions
[9] and [10].
No DNS extensions exist for providing confidentiality for queries or
responses. It is thus possible that if a party knows the domain name
of a VPN, the party can find out the IP addresses of PE routers that
connect sites of that domain. Depending on the situation, that may
or may not be an acceptable security risk.
In a single-provider VPN, DNS servers that host VPN zones can be
easily fire-walled from all public access. Another way to prevent
outside parties from accessing VPN information is to use DNS access
lists that allow VPN zone related queries only from trusted PE
routers.
Acknowledgements
I would like to thank Joel Halpern of Longitude Systems and Matt
Squire of Hatteras Networks for their constructive comments on an
earlier versions of this memo.
References
[1] Augustyn, et al., "Requirements for Virtual Private LAN Services
(VPLS)". draft-augustyn-vpls-requirements-00.txt, October 2001.
[2] Luciani et al., "Using DNS for VPN Discovery". draft-luciani-
ppvpn-vpn-discovery-01.txt, September 2001.
[3] Andersson, et al., "LDP Specification". RFC 3036, January 2001.
[4] Rosen et al., "MPLS Label Stack Encoding". RFC 3032, January
2001.
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[5] Martini, et al., "Encapsulation Methods for Transport of Layer 2
Frames Over IP and MPLS Networks". draft-martini-l2circuit-encap-
mpls-04.txt, November 2001.
[6] Vixie, "A Mechanism for Prompt Notification of Zone Changes (DNS
NOTIFY)". RFC 1996, August 1996.
[7] Elz and Bush, "Clarifications to the DNS Specification". RFC
2181, July 1997.
[8] Wellington, "Secure Domain Name System (DNS) Dynamic Update".
RFC 3007, November 2000.
[9] Vixie, et al., "Secret Key Transaction Authentication for DNS
(TSIG)". RFC 2845, May 2000.
[10] Eastlake, "Domain Name System Security Extensions". RFC 2535,
March 1999.
Author's Address
Juha Heinanen
Song Networks, Inc.
Hallituskatu 16
33200 Tampere, Finland
Email: jh@song.fi
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