Himanshu Shah
Ciena Networks
K.Arvind
Enterasys Networks
PPVPN Working Group
Internet Draft
Draft-shah-ppvpn-ipls-02.txt Eric Rosen
Francois Le Faucheur
June 2003 Cisco Systems
Expires: December 2003
Giles Heron
PacketExchange,Ltd
Vasile Radoaca
Nortel Networks
IP-Only LAN Service (IPLS)
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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Internet-Drafts are draft documents valid for a maximum of six
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reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
A Virtual Private LAN Service (VPLS) [VPLS] is used to interconnect
systems across a wide-area or metropolitan-area network, making it
appear to those systems as if they are interconnected on a private
LAN. The systems which are interconnected in this way may
themselves be LAN switches. If, however, the interconnected systems
are NOT LAN switches, but rather are IP hosts or IP routers, certain
simplifications are possible. We call this simplified type of
virtual private LAN service an "IP-only LAN Service" (IPLS). In
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IPLS, as in VPLS, LAN interfaces are run in promiscuous mode, and
frames are forwarded based on their MAC Destination Addresses.
However, the maintenance of the MAC forwarding tables is done via
signaling, rather than via the "MAC Address Learning" procedures of
IEEE 802.1D. Further, Address Resolution Protocol (ARP) messages
are proxied, rather than being carried transparently. This draft
specifies the protocols and procedures for support of the IPLS
service.
1.0 Boiler Plate for Sub-IP Area Drafts
RELATED DOCUMENTS
draft-ietf-ppvpn-l2-framework-03.txt
draft-lasserre-vkompella-ppvpn-vpls-04.txt
draft-ietf-pwe3-control-protocol-02.txt
draft-heinanen-inarp-uni-01.txt
WHERE DOES IT FIT IN THE PICTURE OF THE SUB-IP WORK
Belongs in PPVPN
WHY IS IT TARGETED AT THIS WG
This document describes a mechanism to assist in Provider-
Provisioned Layer 2 VPNs.
JUSTIFICATION
This document provides a detailed description for IP-only LAN
Service (IPLS), which is discussed in the L2 PPVPN Framework [PPVPN-
FWK]. The VPLS [VPLS] services of L2VPN require PE devices to
function as MAC learning bridges. IPLS is a solution for a specific
topology where MAC learning capabilities are not required for VPLS
services, because user data traffic is restricted to IP, and the CE
devices are not LAN switches.
2.0 Overview
As emphasized in [VPLS], Ethernet has become popular as an access
technology in Metropolitan and Wide Area Networks. [VPLS] describes
how geographically dispersed customer LANs can be interconnected
over a service providerÆs network using Layer 2 VPNs. The VPLS
service is provided by Provider Edge (PE) devices, and it is
provided to Customer Edge (CE) devices. The VPLS architecture
provides such services by incorporating bridging functions such as
MAC address learning in the PE devices.
There are Provider Edge platforms, both existing and forthcoming,
which have been designed primarily to be IP routers, rather than to
be LAN switches. It can be fairly straightforward to add a MAC
address lookup capability to these platforms, and to run their LAN
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interfaces in promiscuous mode, so that they can forward frames
based on the MAC Destination Address of the frame. It is less
straightforward to add the IEEE 802.1D MAC Address learning
capability to these platforms. However, as discussed in [L2VPN-
FWK], in scenarios where the CE devices are NOT LAN switches, but
rather are IP hosts or IP routers, it is possible to provide the
virtual private LAN service without requiring IEEE 802.1D MAC
address learning/aging on the PE. Due to these restrictions, such a
service is referred to as an "IP-Only LAN Service", or IPLS.
Requirements for such an IPLS service are presented in [L2VPN-
REQTS]. The purpose of this draft is to specify a solution optimized
for this IPLS service.
Consequently, IPLS allows a service provider to provide a VPLS-like
service by using PE routers that are not designed to perform general
LAN bridging functions. However one must be willing to accept the
restriction that the Virtual LAN service be used for IP traffic
only, and not used to interconnect CE devices that are themselves
LAN switches. This seems like an acceptable restriction in many
environments, given that IP is the predominant type of traffic in
today's networks.
In IPLS, a PE device implements multi-point LAN connectivity for IP
traffic using the following key functions:
1. Discovery: Each Provider Edge (PE) device discovers IP/MAC
address associations for the locally attached Customer Edge
(CE) devices, for each IPLS instance configured on the PE
device.
2. PseudowirePseudowires (PW) for Unicast Traffic: For each
locally attached CE device in a given IPLS instance, a PE
device sets up a pseudo-wire (VC-LSP) to each of the other PEs
that supports the same IPLS instance.
For instance, if PEx and PEy both support IPLS I, and PEy is
locally attached to CEw and CEz, PEy will initiate the setup of
two PseudowirePseudowires between itself and PEx. One of these
will be used to carry unicast traffic from any of PExÆs CE
devices to CEw. The other will be used to carry unicast
traffic from any of PExÆs CE devices to CEz.
Note that these PseudowirePseudowires carry traffic only in one
direction. Further, while the PseudowirePseudowire implicitly
identifies the destination CE of the traffic, it does not
identify the source CE; packets from many CEs may be freely
intermixed on a given PseudowirePseudowire.
3. Pseudowires for Multicast Traffic: In addition, every PE
supporting a given IPLS instance will set up a special
"multicast Pseudowire" to every other PE in that IPLS instance.
If, in the above example, one of PExÆs CE devices sends a
multicast packet, PEx would forward the multicast packet to PEy
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on the special multicast Pseudowire. PEy would then send a
copy of that packet to CEw and a copy to CEz.
Thus when a PE sends a multicast packet across the network, it
sends one copy to each remote PE (supporting the given IPLS
instance). If a particular remote PE has more than one CE
device in that IPLS instance, the remote PE must replicate the
packet and send one copy to each of its local CEs.
As with the Pseudowires that are used for unicast traffic,
packets travel in only one direction on these Pseudowires, and
packets from different sources may be freely intermixed.
4. Signaling: The necessary Pseudowires can be set up and
maintained using the LDP-based signaling procedures described
in [PWE3-CONTROL] and/or [ROSEN-SIG]. Use of other signaling
procedures is for further study.
A PE may assign the same label to each of the unicast
Pseudowires that lead to a given CE device, in effect creating
a multipoint-to-point Pseudowire.
Similarly, a PE may assign the same label to each of the
multicast Pseudowires for a given IPLS instances, in effect
creating a multipoint-to-point Pseudowire.
When setting up a Pseudowire to be used for unicast traffic,
the PE must also signal the IP address and the MAC address of
the corresponding CE device.
5. Proxy ARP: Distribution of IP/MAC address associations to
remote PE devices via PW signaling enables each PE device to
function as a proxy ARP server for CE devices attached to other
PE devices. This makes it possible for any CE device to ARP for
the MAC addresses of remote CE devices.
6. Forwarding: A PE device programs its Forwarding Information
Base using the CE MAC addresses and VC labels signaled through
the PW signaling. Unicast IP traffic from the local CEs is then
switched to the proper VC-LSP based on the destination MAC
address. Multicast IP traffic from the local CEs is replicated
by the local PE over all the Attachment Circuits (except the
one it came in) and all the multicast VC-LSPs for that IPLS
instance. Remote PEs that receive the multicast packets over
the multicast VC-LSPs then replicates onto all its Attachment
Circuits for that IPLS instance.
Both VPLS [VPLS] and IPLS require the ingress PE to forward a frame
based on its destination MAC address. However, two key differences
between VPLS and IPLS can already be noted from the above
description:
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. In VPLS, MAC entries are placed in the FIB of the ingress PE as
a result of IEEE 802.1D MAC address learning (which occurs in
the data plane) while in IPLS MAC entries are placed in the FIB
as a result of Pseudowire signaling operations (control plane).
. In VPLS, the egress PE looks up a frameÆs MAC destination
address to determine the customer-facing interface out which
the frame must be sent; in IPLS, the choice of interface is
based entirely on the VC-label.
The following sections describe the details of the IPLS scheme.
2.1 Terminology
IPLS IP-only LAN service (a type of Virtual Private
LAN Service that is restricted to IP traffic
only).
IPLS Network A collection of PE nodes supporting the IPLS
service and the associated mechanisms described
in this document, including the Extended LDP
based PW signaling between them.
IPLS Service A single service instance of IPLS emulating a
LAN segment for IP data traffic.
MPt-Pt PW Multipoint-to-Point Pseudowire. A Pseudowire
that carries traffic from remote PE devices to
a PE device that signals the Pseudowire. The
signaling PE device advertises the same VC-
label to all remote PE devices that participate
in the IPLS service instance. In IPLS, for a
given IPLS instance, a MPt-Pt PW used for IP
unicast traffic is established by a PE for each
CE device locally attached to that PE. It is a
unidirectional tree whose leaves consist of the
remote PE peers (which connect at least one
Attachment Circuit associated with the same
IPLS instance) and whose root is the signaling
PE. Traffic flows from the leaves towards the
root.
Multicast PW Multicast Pseudowire. A special kind of MPt-Pt
PW that carries only IP multicast/broadcast
traffic. In the IPLS architecture, for each
IPLS instance supported by a PE, that PE device
establishes exactly one Multicast PW.
CE Customer Edge device. In this document, a CE is
any IP node (host or router) connected to the
IPLS LAN service.
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Replication Tree The collection of all Multicast Pseudowires
and attachment circuits that are members of an
IPLS service instance on a given PE. When a
multicast/broadcast packet is received by the
PE on an attachment circuit, the PE device
sends a copy of the packet to every Multicast
Pseudowire and attachment circuit of the
replication tree, excluding the attachment
circuit on which the packet was received.
3.0 Topology
The Customer Edge (CE) devices are IP nodes (hosts or routers) that
are connected to PE devices either directly, or via an Ethernet
network. We assume that the PE/CE connection may be regarded by the
PE as an "interface" to which one or more CEs are attached. This
interface may be the physical LAN interface or a VLAN. The Provider
Edge (PE) routers are MPLS Label Edge Routers (LERs) that serve as
Pseudowire endpoints.
+----+ +----+
+ S1 +---+ ........................... +---| S2 |
+----+ | | . . | +----+
IPa | | +----+ +----+ | IPe
+ +---| PE1|---MPLS and/or IP---| PE2|---+
/ \ +----+ |Network +----+ |
+----+ +---+ . | . | +----+
+ S1 + | S1| . +----+ . +---| S2 |
+----+ +---+ ..........| PE3|........... +----+
IPb IPc +----+ IPf
|
|
+----+
| S3 |
+----+
IPd
In the above diagram, an IPLS instance is shown with three sites:
site S1, site S2 and site S3. In site S3, the CE device is directly
connected to its PE. In the other two sites, there are multiple CEs
connected to a single PE. More precisely, the CEs at these sites are
on an Ethernet (switched at site 1 and shared at site 2) network (or
VLAN), and the PE is attached to that same Ethernet network or
VLAN). We impose the following restriction: if one or more CEs
attach to a PE by virtue of being on a common LAN or VLAN, there
MUST NOT be more than one PE on that LAN or VLAN.
PE1, PE2 and PE3 are shown to be connected via an MPLS network;
however, other tunneling technologies, such as GRE, L2TP, etc.,
could also be used to carry the Pseudowires.
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An IPLS instance is a single broadcast domain, such that each IP end
station (e.g., IPa) appears to be co-located with other IP end
stations (e.g., IPb though IPf) on the same subnet. The IPLS service
is transparent to the CE devices and requires no changes to them.
4.0 Configuration
Each PE router is configured with one or more IPLS service
instances, and each IPLS service instance is associated with a
unique VPN-Id. For a given IPLS service instance, a set of
Attachment Circuits is identified. Each Attachment Circuit can be
associated with only one IPLS instance. An Attachment Circuit, in
this document, is either a customer-facing Ethernet port, or a
particular VLAN (identified by an IEEE 802.1Q VLAN ID) on a
customer-facing Ethernet port.
The PE router can optionally be configured with a local MAC address
to be used as source MAC address when packets are forwarded from a
Pseudowire to an Attachment Circuit. By default, a PE uses the MAC
address of the customer-facing Ethernet interface for this purpose.
5.0 Discovery
The discovery process includes:
. Remote PE discovery
. VPN (i.e., IPLS) membership discovery
. IP CE end station discovery
This draft does not discuss the remote PE discovery or VPN
membership discovery. This information can either be user configured
or can be obtained using auto-discovery techniques described in
[DNS-Discovery] or [BGP-Discovery]. However, the discovery of the CE
is an important operational step in the IPLS model and is described
below.
5.1 CE discovery
Each PE actively detects the presence of local CEs by snooping IP
and ARP frames received over the Attachment Circuits. During the
discovery phase, the PE examines each broadcast/multicast Ethernet
frame. For IP frames (for example IGP discovery/multicast/broadcast
packets), the CEÆs (source) MAC address is extracted from the
Ethernet header and the (source) IP address is obtained from the IP
header. For ARP frames, the source MAC and IP address are determined
from the ARP PDU.
For each CE, the PE maintains a <Attachment Circuit identification
info, VPN-Id, IP address, MAC address> tuple.
Once discovered, the presence/liveness of a CE is monitored
continuously by examining the received ARP frames and by
periodically generating ARP requests. The absence of an ARP response
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from a CE after a configurable number of such ARP requests, is
interpreted as a loss of connectivity with the CE.
6.0 Pseudowire Creation
6.1 Receive Unicast Multipoint-to-point Pseudowire
As the PE discovers each locally attached CE, a unicast Multipoint-
to-point Pseudowire (MPt-Pt PW) associated exclusively with that CE
is created by distributing the CEÆs IP address and MAC address along
with a VC-Label to all the remote PE peers that participate in the
same IPLS instance. Note that the same value of a VC-label should be
distributed to all the remote PE peers for a given CE. The MP-Pt PW
thus created is used by remote PEs to send unicast IP traffic to a
specific CE.
(The same functionality can be provided by a set of point-to-point
PWs, so the PE is not required to send the same VC-label to all the
other PEs. For convenience however, we will speak in the following
only of multipoint-to-point PWs, without pointing out each time that
a set of point-to-point PWs could be used instead.)
The PE forwards a frame received over this MPt-Pt PW to the
associated attachment circuit.
6.2 Receive Replication Multipoint-to-point Pseudowire
When a PE is configured to participate in an IPLS instance, it
advertises a "multicast" VC-label to every other PE that is a member
of the same IPLS. The advertised VC-label value is the same for each
PE, which creates a multipoint-to-point Pseudowire for IP multicast
traffic. There is only one multicast MPt-Pt PW per PE for each IPLS
instance and this Pseudowire is used exclusively to carry
multicast/broadcast IP traffic from the remote PEs to this PE for
this IPLS instance.
Note that no special functionality is expected from this Pseudowire.
We sometimes call it a "multicast Pseudowire" because we use it only
to carry multicast traffic. The Pseudowire itself need not provide
any different service than any of the unicast Pseudowires.
In particular, the Receive multicast MPt-PT PW does not perform any
replication of frames itself. Rather, it is there to signify to the
PE that the PE needs to replicate a copy of a frame received over
this MPt-Pt PW onto all the attachment circuits that are associated
with the IPLS instance of the MPt-Pt PW.
The use of Pseudowires, which are specially optimized for multicast,
is for further study.
6.3 Send Multicast Replication tree
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The PE creates a send replication tree for each IPLS instance, which
consists of the collection of all attachment circuits and all the
"multicast" Pseudowires of this IPLS instance.
Any broadcast/multicast frame received over an attachment circuit is
replicated to all the other attachment circuits and all Pseudowires
of the send replication tree of the IPLS instance of the incoming
Attachment Circuit.
7.0 Proxy ARP
As part of the signaling of the unicast multipoint-to-point pseudo-
wire (See Section 8), each PE distributes to its remote PE peers the
CE IP address/MAC address associations that it has discovered. The
remote PE peers then build and maintain a database of these
associations.
When a PE receives an ARP request from a local CE for a remote CE,
it searches for the destination IP address in the database
associated with the CEÆs IPLS instance. If a match is found, the PE
sends an ARP response with the MAC address of the remote CE. This
enables the local CE to send unicast IP frames addressed directly to
the MAC address of the remote CE.
8.0 Signaling
The [PWE3-CONTROL] uses Label Distribution Protocol (LDP) transport
to exchange VC-FEC in the label mapping message in a downstream
unsolicited mode. The VC-FEC comes in two flavors; Pwid and
Generalized ID FEC elements. These FEC elements define some fields
that are common between them. The discussions below refer to these
common fields for IPLS related extensions.
8.1 IPLS PW Signaling
The IPLS uses user IP data as payload over the Pseduowire. The use
of such encapsulation is identified by VC type field of the VC-FEC
as the value 0x000B [PWE3-IANA].
In addition, this document defines an IP MAC address TLV that must
be included in the optional TLV field of the label mapping message
when advertising VC-FEC for the IPLS. Such use of optional TLV in
the label mapping message to extend the attributes of the VC-FEC has
also been specified in the [PWE3-Control].
When processing a received VC-FEC, the PE matches the VC-Id and VC-
type with the locally configured VC-id to determine if the VC-FEC is
of type IPLS. If matched, it further checks the presence of IP
address TLV. If an IP MAC address TLV is absent, a label release
message is issued to reject the PW establishment.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| IP MAC address TLV (TBD) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address (0-3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Address (4-5) | Multicast Flag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Length field is defined as the sum of length of the IP address
(4) and length of MAC address (6) and multicast flag (2) and is set
to value 12.
The non-zero unicast value of the IP address field denotes IP
address of advertising PEÆs attached CE device.
The non-zero unicast value of the MAC address field denotes MAC
address of the advertising PEÆs attached CE device.
The Multicast Flag value of 1 indicates that the advertised VC-Label
represents a "multicast" PW. The Multicast Flag value of 0 indicates
that advertised VC-label represents a "unicast" PW. As explained
earlier, the term "multicast PW" only means that the PW carries IP
broadcast/multicast traffic and does not refer to a multicast LSP in
the traditional sense.
The Multicast Flag must be zero, if present, when the IP and MAC
address parameters are present (and their value is non-zero). When
Multicast Flag is set to 1, the values in IP and MAC Address fields
are set to null and are ignored.
8.2 Signaling Advertisement Processing
A PE should process a received [PWE3-CONTROL] advertisement with VC-
type of IPLS as follows,
- Verify the IPLS VPN membership by matching the VPN-Id
signaled in the AGI field or the PW-ID field with all the
VPN-Ids configured on the PE. Discard and release the VC
label if VPN-Id is not found.
- Distribute the received IP address-to-MAC address binding by
sending a gratuitous ARP response on all the attachment
circuits associated with the VPN-Id.
- Program the Forwarding Information Base (FIB) such that when
a packet is received from an attachment circuit with its
destination MAC address matching the advertised MAC address,
the packet is forwarded out over the tunnel to the
advertising PE with the advertised VC-label as the inner
label.
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- When the advertised VC-label is "multicast", add the VC-label
to the send multicast replication tree for the VPN-Id. This
enables sending a copy of a multicast/broadcast IP frame from
the attachment circuit to this Pseudowire.
8.3 Requesting for IP to MAC binding
It is possible that in some cases, some CEs may remain undetected in
the absence of any multicast/broadcast IP or ARP packet generation.
If a local CE needs to converse with a remote CE in this undetected
set, it will proceed to generate ARP requests. The Proxy ARP scheme
described so far will be unable to resolve the ARP request, since
the address to be resolved would not have been discovered (signaled)
yet.
In order to address such situations, an optional Address Resolution
Request TLV can be included in the LDPÆs Notification Message. This
TLV contains an IP address parameter that represents the destination
IP address that needs to be resolved. The PE may use some
intelligent mechanisms (e.g., the number of ARP requests received
for unknown IP destination within a certain interval exceeds a
threshold) to detect the need for such advertisement. When the need
is detected, the PE generates Notification Messages to all remote
PEs in the IPLS, with the IP address parameter in the Address
Resolution Request TLV set to the destination IP address to be
resolved.
A PE that supports the Address Resolution Request TLV must, on
receiving a notification message with this TLV, generate an ARP
request message using the received IP address as the destination,
and some already known IP and MAC address as the source (in the ARP
PDU) on all Attachment Circuits associated with the IPLS instance.
In essence, this is a request to remote PEs to generate an ARP
request on their Attachment Circuits to locate a specific CE and
advertise a Label Mapping message back to the requesting PE. This
can be seen as reverting to the usual full broadcasting of ARP
messages throughout the Emulated LAN in rare cases when Proxy ARP
fails.
The definition of Address Resolution Request TLV for the
Notification message is the subject of future study.
8.4 CE MAC Address
Throughout this document we have referenced remote CEÆs MAC address
to be the 48-bit physical MAC address. The MAC address is learned
and signaled by the remote PE while local PE uses the signaled MAC
address to proxy ARP request for remote CEÆs IP address, program the
address in the FIB and use it as a key to forward packet from the
Attachment Circuit to the Pseudowire.
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Alternatively, it is also desirable to allow local PE to generate a
unique 48-bit MAC address for the remote CE instead of using the
signaled MAC address by the remote PE. The local PE would then use
the generated MAC address for ARP proxy, programming the FIB and as
a key to forward packets from the Attachment Circuit to the
Pseudowire. By permitting address generation to represent each
remote CE, local PE can use a key lookup algorithm that is most
suitable for its architecture. For example, PE could use only 32-
bits of the 48-bit MAC DA as the key for fast table lookup.
Which mechanism local PE uses to represent remote CE (i.e. using
signaled MAC address or locally generated MAC address), is of local
matter to the PE and has no bearing on the IPLS functionality.
9 Forwarding
9.1 Non-IP traffic
In an IPLS VPN, only IP traffic is forwarded by a PE. ARP frames are
directed to the control plane in the PE and the rest of the frames
are dropped silently. If the CEs must pass non-IP traffic to each
other, they must do so through IP tunnels that terminate at the CEs
themselves.
9.2 Unicast IP Traffic
In IPLS, IP traffic is forwarded from the Attachment Circuit to the
PW based on the destination MAC address of the layer 2 frame (and
not based on the IP Header).
To do so, the PE associates a Forwarding Information Base (FIB) with
each IPLS instance and processes the FIB in the following manner:
- The PE programs its FIB when a CE (and its MAC address) is
discovered on one of its Attachment Circuit such that a frame
received on any other Attachment Circuit with destination to
this CE, the frame is forwarded to the corresponding
Attachment Circuit.
- The PE programs its FIB with the PW label such that a frame
received over that unicast PW is forwarded to the
corresponding Attachment Circuit prepended with CEÆs MAC
address as the destination.
- The PE programs its FIB when processing a received PW signal
such that a frame received from any of its Attachment
Circuits associated with the same IPLS instance, is forwarded
to the PW if the destination MAC address matches the one
advertised in the PW signal.
The PE identifies the FIB associated with an IPLS instance based on
the Attachment Circuit or the PW label. When a frame is received
from an Attachment Circuit, PE uses destination MAC address as the
lookup key. When a frame is received from PW, PE uses VC-Label as
the lookup key. The frame is dropped if the lookup fails.
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9.3 Broadcasts and Multicast forwarding
When the destination MAC address is either a broadcast or multicast,
a copy of the frame is sent to the control plane for CE discovery
purposes (see section 5.1).
When a multicast/broadcast IP frame is received from an Attachment
Circuit, a PE replicates it onto the Send Multicast Replication Tree
(See section 6.3). When a multicast/broadcast IP frame is received
from a Pseudowire, the PE forwards a copy of the frame to all
attachment circuits associated with the IPLS VPN instance involved.
It is important to note that PEs participating in an IPLS VPN are
responsible for translating a multicast IP address to a multicast
Ethernet MAC address when forwarding frames from a "multicast"
Pseudowire to the Attachment Circuits. (The translation consists of
recognizing the multicast IP address (224.x1.x2.x3) and appending
the least significant three bytes of the IP address to 0x01-00-05 to
construct the MAC address, e.g., 0x01-00-5E-x1-x2-x3 [RFC-1112]).
All other IP packets received over the "multicast" MPt-Pt PW (such
as directed broadcasts, subnet broadcasts, etc) are forwarded over
Attachment Circuits using a broadcast MAC address.
9.4 Encapsulation
The Ethernet MAC header of a frame received from an Attachment
Circuit is stripped before forwarding the frame to the appropriate
Pseudowire. However, the MAC header is retained when a unicast or
broadcast IP frame is directed to one or more Attachment Circuit(s).
An IP frame received over a Pseudowire is prepended with a MAC
header before transmitting it on the appropriate Attachment
Circuit(s). The fields in the MAC header are filled in as follows:
- The destination MAC address is the MAC address associated
with the VC label in the FIB when the Pseudowire is unicast
- The destination MAC address is a multicast MAC address
derived from the IP multicast address or the broadcast MAC
address when the VC label is "multicast"
- The source MAC address is the PEÆs own local MAC address or a
MAC address which has been specially configured on the PE for
this use.
- The Ethernet Type field is 0x0800
- The frame may get IEEE802.1Q tagged based on the VLAN
information associated with the Attachment Circuit.
An FCS field is appended to the frame.
10.0 Attaching to IPLS via ATM or FR
In addition to (i) an Ethernet port and a (ii) combination of
Ethernet port and a VLAN ID, an Attachment Circuit to IPLS may also
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be (iii) an ATM or FR VC carrying encapsulated bridged Ethernet
frames or (iv) the combination of an ATM or FR VC and a VLAN ID.
The ATM/FR VC is just used as a way to transport Ethernet frames
between a customer site and the PE. The PE terminates the ATM/FR VC
and operates on the encapsulated Ethernet frames exactly as if those
were received on a local Ethernet interface. When a frame is
propagated from Pseudowire to a ATM or FR VC, PE prepends the
Ethernet frame with the appropriate bridged encapsulation header as
define in [RFC 1487] and [RFC 1490] respectively. Operation of an
IPLS over ATM/FR VC is exactly as described above, with the
exception that the attachment circuit is then identified via the ATM
VCI/VPI or Frame Relay DLCI (instead of via a local Ethernet port
ID), or a combination of those with a VLAN ID.
11.0 VPLS vs IPLS
The VPLS approach proposed in [VPLS] provides VPN services for IP as
well as other protocols. The IPLS approach described in this draft
is similar to VPLS in many respects:
- It provides a Provider Provisioned Virtual LAN service with
multipoint capability where a CE connected via a single
attachment circuit can reach many remote CEs
- It appears as a broadcast domain and a single subnet
- forwarding is based on destination MAC addresses
However, unlike VPLS, IPLS is restricted to IP traffic only. By
restricting the scope of the service to the predominant type of
traffic in today's environment, IPLS eliminates the need for service
provider edge routers to implement some bridging functions such as
MAC address learning in the data path (by, instead, distributing MAC
information in the control plane). Thus this solution offers a
number of benefits:
- Facilitates Virtual LAN services in instances where PE
devices cannot or cannot efficiently (or are specifically
configured not to) perform MAC address learning.
- Does not require flooding of ARP frames normally.
- Encapsulation is more efficient (MAC header is stripped)
while traversing the backbone network.
- PE devices are not burdened with the processing overhead
associated with traditional bridging (e.g., STP processing,
etc.). Note however that some of these overheads (e.g., STP
processing) could optionally be turned-off with a VPLS
solution in the case where it is known that only IP devices
are interconnected.
- Loops (perhaps through backdoor links) are minimized since a
PE could easily reject (via label release) a duplicate IP to
MAC address advertisement.
12.0 IP Protocols
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The solution described in this document offers IPLS service for IPv4
traffic only. For this reason, the MAC Header is not carried over
the Pseudowire. It is reconstructed by the PE when receiving a
packet from a Pseudowire and the Ethertype 0x0800 is used in the MAC
Header since IPv4 is assumed.
However, this solution may be extended to carry other types of
important traffic such as ISIS and IPv6 which are not encapsulated
in Etherent with the use of Ethertype 0x0800. In order to permit the
propagation of such packets correctly, one may create a separate set
of Pseudowires, or pass protocol information in the "control word"
of a "multiprotocol" Pseudowire, or encapsulate the Ethernet MAC
Header in the Pseudowire. The selection of appropriate
multiplexing/demultiplexing scheme is the subject of future study.
The current document focuses on IPLS service for IPv4 traffic.
13.0 Dual Homing with IPLS
As stated in previous sections, IPLS prohibits connection of a
common LAN or VLAN to more than one PE. Alternatively, CE device by
itself can connect to more than one instance of IPLS through two
separate LAN or VLAN connections to separate PEs. To the CE IP
device, these separate connections appear as a connection to two IP
subnets. The failure of reachability through one subnet is then
resolved via other subnet by the IP protocols.
14.0 Acknowledgements
Authors would like to thank Nigel Burmeister and others at Tenor
Networks for their valuable comments.
15.0 Security Considerations
The security aspects of this solution will be discussed at a later
time.
16.0 References
[L2VPN-REQ] Augustyn, W. et.al "Requirements for Layer 2 Virtual
Private Network Services (L2VPN)", draft-augustyn-ppvpn-l2vpn-
requirements-02.txt, Work in Progress, Internet Draft, February
2003.
[L2VPN-FMWK] Andersson, draft-ietf-ppvpn-l2-framework-01.txt, PPVPN
L2 Framework, August 2002, (work in progress).
[PWE3-CONTROL] Martini et. Al., "Transport of Layer 2 Frames Over
MPLS", draft-ietf-pwe3-control-protocol-02.txt, February 2003 (work
in progress)
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[PWE3-IANA] Martini et. Al., "IANA Allocations for pseudo Wire Edge
to Edge Emulation (PWE3)", draft-ietf-pwe3-iana-allocation-01.txt,
June 2003 (work in progress)
[PWE3-ETH-ENCAP] Martini et. Al., "Encapsulation Methods for
Transport of Ethernet Frames over IP/MPLS Networks", draft-ietf-
pwe3-ethernet-encap-00.txt, August 2002 (work in progress)
[VPLS] Lasserre et al, "Virtual Private LN Service over MPLS",
draft-lasserre-vkompella-ppvpn-vpls-04.txt, March 2003 (work in
progress).
[DNS-Discovery] "DNS/LDP Based VPLS", Heinanen, draft-heinanen-dns-
ldp-vpls-00.txt, June 2002
[BGP-Discovery] "Using BGP as an Auto-Discovery Mechanism for
Network Based VPNs", Ould-Brahim et al., draft-ietf-ppvpn-bgpvpn-
auto-02.txt, February 2002, (work in progress).
[ARP] Plummer, D., "An Ethernet Address Resolution Protocol: Or
Converting Network Protocol Addresses to 48.bit Ethernet
Addresses for Transmission on Ethernet Hardware", STD 37, RFC 826,
November 1982.
[PROXY-ARP] Postel, J., "Multi-LAN Address Resolution", RFC 925,
October 1984.
[RFC-1112] Deering, S., "Host Extensions for IP Multicasting", RFC
1112, August, 1989.
7.0. Intellectual Property Considerations
Tenor/Enterasys Networks may seek patent or other intellectual
property protection for some or all of the technologies disclosed in
this document. It any standards arising from this document are or
become protected by one or more patents assigned to Tenor/Enterasys
Networks, Tenor/Enterasys intends to disclose those patents and
license them on reasonable and non-discriminatory terms.
Author's Address
Himanshu Shah
Ciena Networks
35 Nagog Park,
Acton, MA 01720
Email: hshah@rcn.com
K.Arvind
Enterasys Networks
50 Minuteman Rd, Suite 100
Andover, MA 01810
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Email: karvind@enterasys.com
Eric Rosen
Cisco Systems
300 Apollo Drive,
Chelmsford, MA 01824
Email: erosen@cisco.com
Giles Heron
PacketExchange Ltd.
The Truman Brewery
91 Brick Lane
LONDON E1 6QL
United Kingdom
Email: giles@packetexchange.net
Francois Le Faucheur
Cisco Systems, Inc.
Village d'Entreprise Green Side - Batiment T3
400, Avenue de Roumanille
06410 Biot-Sophia Antipolis
France
Email: flefauch@cisco.com
Vasile Radoaca
Nortel Networks
Email: vasile@nortelnetworks.com
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