MPLS Working Group B. Jamoussi
Internet Draft D. Jamieson
Expiration Date: February 1999 P. Beaubien
Nortel (Northern Telecom) Ltd.
August 1998
MPLS-VNS Interworking
<draft-jamoussi-mpls-vns-00.txt>
Status of this Memo
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Abstract
This document specifies MPLS [1,2] to VNS [3] interworking in an
efficient manner that preserves the label switching property when
crossing an MPLS/VNS boundary. The interworking function also ensures
that COS characteristics of an LSP are preserved when going from VNS
to MPLS and vice versa.
Table of Contents
1 Introduction ............................................ 2
2 Interworking Through VNS ................................ 3
2.1 Label Distribution ...................................... 3
2.2 Label Stack Encoding .................................... 4
3 Interworking Between MPLS and VNS ....................... 5
3.1 Label Distribution ...................................... 5
3.2 Label Stack Encoding .................................... 5
4 Summary ................................................. 5
5 Security Considerations ................................. 6
6 Acknowledgement ......................................... 6
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7 References .............................................. 6
8 Authors' Addresses ...................................... 6
1. Introduction
Nortel's Virtual Network Switching (VNS) is defined in [3]. VNS
offers several unique capabilities such as the transport of IP, IPX
and Bridging traffic in a multi-service network (voice, video, and
data). It has been deployed in many live networks around the globe.
Multi-Protocol Label Switching (MPLS) architecture and framework are
defined in [1] and [2] respectively. MPLS is an emerging protocol
being standardized by the IETF. As the development of MPLS progresses
and its deployment in customer networks takes place, it becomes
necessary to provide a solution for interworking MPLS and VNS
networks.
MPLS and VNS are two technologies that forward IP traffic based on a
fixed size label to avoid processing IP headers at tandem nodes
between the source and the destination.
This document specifies MPLS-VNS interworking in an efficient manner
that preserves the fast forwarding of packets based on labels when
crossing an MPLS/VNS boundary. The interworking function also ensures
that COS characteristics of an LSP are preserved when going from VNS
to MPLS and vice versa.
It is possible to interwork MPLS and VNS at the IP layer by
terminating an MPLS label switched path (LSP), mapping the IP
destination address to a VNS label, and forwarding packets inside the
VNS domain based on the VNS label. However, this solution would
invoke L3 forwarding at the boundary between MPLS and VNS.
The solution described in this draft ensures that label forwarding is
preserved at the interworking point between MPLS and VNS. Two
interworking scenarios are identified. In the first scenario, traffic
is exchanged between two MPLS nodes through a VNS network. For
example between nodes 1 and 4 shown in Figure 1. In the second
scenario, traffic is exchanged between an MPLS node and a VNS node
(e.g., between nodes 1 and 2 of Figure 3).
Interworking through VNS is described in Section 2. Interworking
between VNS and MPLS is described in section 3. Section 4 concludes
this draft.
This document should be read along with a companion document,
Nortel's Virtual Network Switching (VNS) Overview [3].
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2. Interworking through VNS
This section describes the interworking functions that are required
in order to connect two MPLS nodes through a VNS network. Section 2.1
specifies the label distribution protocol. Section 2.2 specifies the
label stack encoding.
LDP Sessions
+-------------------+
| |
MPLS Domain | VNS Domain | MPLS Domain
+------+ +------+ +------+ +------+
| | | | +-----+ | | | |
| | |M V| | | |V M| | |
| |---+-----|P N|--- ----|N P|---+-----| |
| | |L S| | | |S L| | |
| | |S | +-----+ | S| | |
| 1 | | 2 | | 3 | | 4 |
+------+ +------+ +------+ +------+
Figure 1. MPLS--VNS Interworking
2.1 Label Distribution Protocol
In a VNS Network, three separate nodal functions are defined. An
ingress function, an egress function, and a tandem (or core)
function. The ingress and egress nodes define the boundary between an
MPLS domain and the VNS domain as shown in Figure 1 (nodes 2 and 3).
In MPLS, label to stream binding information is communicated through
a label distribution protocol [4] between peer Label Switching
Routers (LSRs). In the example of Figure 1, nodes 2 and 3 are LDP
peers. Therefore, in order for MPLS label information to be
communicated across a VNS domain, an LDP session is established
between all the ingress and egress VNS nodes of a logical network.
Tandem (or core) VNS nodes do not need to participate in LDP.
VNS supports a multicast forwarding service for traffic within a
Logical Network (LN) [3] at the VNS layer. Multicast packets are
delivered to all nodes supporting the logical network to which the
multicast packet belongs.
The LDP session establishment takes advantage of this VNS multicast
capability to send "Hello" packets. Edge nodes performing the VNS-
MPLS interworking function are able to dynamically discover each
other through VNS multicast.
Inside the VNS network, VNS uses it own label distribution mechanism
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which is based on a distributed serverless topology driven approach.
Standard ARP is used to distribute a mapping between network layer
addresses and VNS labels.
As described in [3], a VNS Label is composed of the destination node
ID and the Logical Network Number (LNN). When an ingress VNS node
receives the ARP reply that maps an IP prefix to a VNS label, it
initiates an LDP session with that destination node as specified in
[4]. This LDP session is used to exchange MPLS label mappings to FECs
between the two VNS edge nodes.
2.2 Label Stack Encoding
When packets are carried in an MPLS domain, the standard label stack
encoding defined in [5] is used. When packets enter a VNS network, a
VNS label defined in [3] is pushed on top of the MPLS stack resulting
in a stack depth of at least two labels. The top label is the VNS
label. The bottom label is the MPLS shim encoding defined in [5].
Packets are forwarded inside the VNS network based on the VNS header
as defined in [3]. When a packet is about to leave a VNS network, the
VNS header is popped and MPLS-based label forwarding is resumed.
Figure 2. shows the label stack encoding of an IP packet as it
traverses a VNS domain.
+--------------+----+-------------+------------+
| Data | IP | MPLS Header | VNS Header |
+--------------+----+-------------+------------+
Figure 2. MPLS/VNS Label Stack Encoding
A Protocol Type field in the VNS header indicates the type of
protocol being carried in the VNS packet. Examples include IP, IPX,
and Bridging. If the packet is a multicast packet then this is
indicated in this field.
A new codepoint is defined in this Protocol Type field to indicate
that the packet being carried by VNS is an MPLS packet.
The MPLS shim encoding includes a 3-bit COS field used to indicate
the Class of Service of the packet. The VNS header also includes a
3-bit COS field. A mapping function between the MPLS and the VNS COS
fields ensures that packets receive a consistent queuing and
scheduling treatment in both the MPLS and the VNS domains.
In addition, the VNS header includes a Discard Priority field that
indicates the level of congestion at which the packet should be
dropped. The MPLS shim encoding does not have a field that indicates
the discard eligibility of a packet. Therefore, a mapping to the MPLS
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COS field is necessary.
3. Interworking between VNS and MPLS
This section describes the interworking functions required to
preserve the label switching path when traffic is terminated on an
MPLS node on one end and a VNS node on the other.
MPLS Domain VNS Domain
+------+ +------+
| | +------+ | |
| | |M V| | |
| |-------|P N|-------| |
| | |L S| | |
| 1 | |S | | 2 |
+------+ +------+ +------+
Figure 3. MPLS--VNS Interworking
3.1 Label Distribution
In this interworking mode, labels are distributed within the MPLS
domain as defined in [4] and within the VNS domain as defined in [3]
independently of each other. At the node of intersection of the VNS
and MPLS domains, the lack of an LDP session with a remote MPLS peer
for a given stream indicates that label swapping is to take place at
that node. Therefore, the forwarding table is populated accordingly.
3.2 Label Stack Encoding
Since in this mode of operation, traffic is terminating on a VNS node
on one end and on an MPLS node on the other, label stack encoding
defined in [5] is used within the MPLS domain and label encoding
defined in [3] is used in the VNS domain. At the point of
intersection, a swapping operation is performed between the VNS and
MPLS labels.
4. Summary
VNS uses a label switching scheme to forward IP packets in a VNS
domain. Many live networks are running VNS to switch their IP
traffic. MPLS is an emerging standard that also uses label switching
to carry IP traffic. As MPLS networks get deployed, it becomes
necessary to provide an MPLS-VNS Interworking solution.
This draft describes an architectural view of how MPLS and VNS
interworking can be done in an efficient manner that preserves the
label switching property at the MPLS/VNS boundary nodes.
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Two interworking scenarios are identified. In the first scenario,
traffic is exchanged between two MPLS nodes through a VNS network. In
this case, LDP is used to carry label bindings between MPLS peer
nodes across a VNS domain. VNS uses its label distribution protocol
to map IP reachability to VNS labels. At least a two-label-stack is
used to carry traffic across a VNS domain. The top label is a VNS
label (as defined in [3]) and the bottom label is an MPLS label (as
defined in [5]).
In the second interworking scenario, traffic is exchanged between an
MPLS node and a VNS node. In this case, a label swapping function is
invoked at the VNS-MPLS boundary.
5. Security Considerations
Security issues are not discussed in this memo.
6. Acknowledgements
The authors would like to acknowledge the valuable comments of Jerry
Wu, Denis Fortier, Robert Eros, and Pierre Cousineau.
7. References
[1] E. Rosen et al, "Multiprotocol Label Switching Architecture",
draft-ietf-mpls-arch-01.txt, March 1998.
[2] R. Callon, et. al., "A Framework for Multiprotocol Label
Switching", draft-ietf-mpls-framework-02.txt, November 21, 1997.
[3] B. Jamoussi, et. al., "Nortel's Virtual Network Switching (VNS)
Overview", RFC 2340, May 1998.
[4] L. Anderson, et. al., "Label Distribution Protocol", draft-mpls-
ldp-00.txt, March 1998.
[5] E. Rosen, et. al., "MPLS Label Stack Encoding", draft-ietf-mpls-
label-encaps-01.txt, February 1998.
8. Authors' Addresses
Bilel Jamoussi
Nortel (Northern Telecom), Ltd.
PO Box 3511 Station C
Ottawa ON K1Y 4H7
Canada
EMail: jamoussi@nortel.com
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Dwight Jamieson
Nortel (Northern Telecom), Ltd.
PO Box 3511 Station C
Ottawa ON K1Y 4H7
Canada
EMail: djamies@nortel.com
Paul Beaubien
Nortel (Northern Telecom), Ltd.
PO Box 3511 Station C
Ottawa ON K1Y 4H7
Canada
EMail: beaubien@nortel.com
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