L2VPN Working Group H. Shah Ciena Networks
Internet Draft E. Rosen Cisco Systems
W. Augustyn consultant
October 2003 G. Heron PacketExchange,Ltd
Expires: April 2004 T. Smith Laurel Networks
A. Moranganti ADC Telecom
S. Khandekar Timetra Networks
V. Kompella Timetra Networks
A. Malis Vivace Networks
S. Wright Bell South
V. Radoaca Nortel Networks
A. Vishwanathan Force10 Networks
ARP Mediation for IP Interworking of Layer 2 VPN
draft-shah-l2vpn-arp-mediation-03.txt
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
The VPWS service [L2VPN Framework] provides point-to-point
connections between pairs of Customer Edge (CE) devices. It does
so by binding two Attachment Circuits (each connecting a CE device
with a Provider Edge, PE, device) to a Pseudowire (connecting the
two PEs). In general, the Attachment Circuits must be of the same
technology (e.g., both ethernet, both ATM), and the Pseudowire must
carry the frames of that technology. However, if it is known that
the frames' payload consists solely of IP datagrams, it is possible
to provide a point-to-point connection in which the Pseudowire
connects Attachment Circuits of different technologies. This
requires the PEs to perform a function known as "ARP Mediation".
This document specifies the ARP Mediation function, and specifies
draft-shah-l2vpn-arp-mediation-03.txt
the encapsulation used to carry the IP datagrams on the Pseudowires
when ARP mediation is used.
Table of Contents
1 .0 Introduction..................................................2
2 .0 ARP Mediation (AM) function...................................3
3 .0 IP Layer 2 Interworking Circuits..............................4
4 .0 Discovery of IP Addresses of Locally Attached CE Device.......4
4.1 Monitoring Local Traffic.......................................4
4.2 CE Devices Using ARP...........................................4
4.3 CE Devices Using Inverse ARP...................................5
4.4 CE Devices Using PPP...........................................5
4.5 Proactive method..............................................65
5 .0 IP Address Distribution Between PE............................6
5.1 When To Distribute IP Address..................................6
5.2 LDP Based Distribution.........................................6
5.3 Out-of-band Distribution, Manual Configuration.................7
5.4 Single sided ARP mediation....................................87
6 .0 How CE Learns The Remote CE's IP address......................8
6.1 CE Devices Using ARP...........................................8
6.2 CE Devices Using Inverse ARP...................................9
6.3 CE Devices Using PPP...........................................9
7 .0 Use of IGPs with IP L2 Interworking L2VPNs....................9
7.1 OSPF...........................................................9
7.2 IS-IS.........................................................10
7.3 RIP...........................................................10
8 .0 Security Considerations....................................1110
9 .0 Acknowledgements...........................................1110
10 .0 References..................................................11
10.1 Normative References.........................................11
10.2 Informative References.......................................11
11 .0 Authors' Addresses........................................1211
1.0 Introduction
Layer 2 Virtual Private Networks (L2VPN) are constructed with the
use of a Service Provider IP backbone but are presented to the
Customer Edge (CE) devices as Layer 2 networks. In theory, L2VPNs
can carry any Layer 3 protocol, but in many cases, the only Layer 3
protocol is IP. Thus it makes sense to consider procedures that
are either optimized for IP or are outright dedicated to IP traffic
only.
In a typical implementation, illustrated in the diagram below, the
CE devices are connected to the Provider Edge (PE) devices via
Attachment Circuits (AC). The ACs are Layer 2 links. In a pure
L2VPN, if traffic sent from CE1 via AC1 reaches CE2 via AC2, both
ACs would have to be of the same type (i.e., both ethernet, both
FR, etc.). However, if it is known that only IP traffic will be
carried, the ACs can be of different technologies, provided that
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the PEs provide the appropriate procedures to allow the proper
transfer of IP packets.
+-----+
+--------------------| CE3 |
| +-----+
+-----+
........| PE3 |.........
. +-----+ .
. | .
. | .
+-----+ AC1 +-----+ Service +-----+ AC2 +-----+
| CE1 |-----| PE1 |--- Provider ---| PE2 |-----| CE2 |
+-----+ +-----+ Backbone +-----+ +-----+
. .
........................
A CE, which is connected via a given type of AC, may use an IP
Address Resolution procedure that is specific to that type of AC.
For example, an ethernet-attached CE would use ARP, a FR-attached
CE might use Inverse ARP. If we are to allow the two CEs to have a
layer 2 connection between them, even though each AC uses a
different layer 2 technology, the PEs must intercept and "mediate"
the technology-specific address resolution procedures.
In this draft, we specify the procedures which the PEs must
implement in order to mediate the IP address resolution mechanism.
We call these procedures "ARP Mediation".
Consider a Virtual Private Wire Service (VPWS) constructed between
CE1 and CE2 in the diagram above. If AC1 and AC2 are of different
technologies, e.g. AC1 is Ethernet and AC2 is Frame Relay (FR),
then ARP requests coming from CE1 cannot be passed transparently to
CE2. PE1 must interpret the meaning of the ARP requests and
mediate the necessary information with PE2 before responding.
2.0 ARP Mediation (AM) function
The ARP Mediation (AM) function is an element of a PE node
operation that deals with the IP address resolution for CE devices
connected via a L2VPN. By placing this function in the PE node, ARP
Mediation can be made completely transparent to the CE devices.
For a given point-to-point connection between a pair of CEs, a PE
must perform three logical steps as part of the ARP Mediation
procedure:
1. Discover the IP addresses of the locally attached CE device
2. Distribute those IP Addresses to the remote PE
3. Notify the locally attached CE of the remote CE's IP address.
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This information is gathered using the mechanisms described in the
following sections.
3.0 IP Layer 2 Interworking Circuits
The IP Layer 2 Interworking Circuits refer to Pseudowires that
carry IP datagram as the payload. At ingress, data link header of
an IP frame is removed and dispatched over the Pseudowire with or
without the optional control word. At the egress, PE encapsulates
the IP packet with the data link header used on the local
Attachment Circuit.
The use of this encapsulation is determined by the exchange of
value 0x000B as the VC type during Pseudowire establishment as
described in [PWE3-Control].
4.0 Discovery of IP Addresses of Locally Attached CE Device
An IP Layer 2 Interworking Circuit enters monitoring state right
after the configuration. During this state it performs two
functions.
. Discovery of locally attached CE IP device
. Establishment of the PW
The establishment of PW occurs independently from local CE IP
address discovery. During the period when (bi-directional) PW has
been established but local CE IP device has not been detected, only
datagrams inside of broadcast/multicast frames are propagated; IP
datagrams inside unicast frames are dropped. The IP datagrams from
unicast frames flow only when IP end systems on both Attachment
Circuits have been discovered, notified and proxy functions have
completed.
4.1 Monitoring Local Traffic
The PE devices may learn the IP addresses of the locally attached
CEs from any IP traffic, such as local multicast (e.g. 224.x.x.x)
packets, that CE may generate irrespective of reacting to specific
address resolution queries described below.
4.2 CE Devices Using ARP
If a CE device uses ARP to determine the IP address of its
neighbor, the PE processes the ARP requests for the stated locally
attached circuit and responds with ARP replies containing the
remote CE's IP address, if the address is known. If the PE does not
yet have the remote CE's IP address, it does not respond, but notes
the IP address of the local CE and the circuit information,
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including related MAC address. Subsequently, when the IP address of
the remote CE becomes available, the PE may initiate the ARP
response as a means to notify the local CE, the IP address of the
remote CE.
This is a typical operation for Ethernet attachment circuits. It is
important to note that IP L2 Interworking circuit function is
restricted to only one end station per Ethernet Attachment Circuit.
The PE may periodically generate ARP request messages to the CE's
IP address as a means to verify the continued existence of the
address and its binding to the stated MAC address. The absence of a
response from the CE device for a given number of retries could be
used as a cause for a withdrawal of the IP address advertisement to
the remote PE and entering into the address resolution phase to
rediscover the attached CE's IP address. Note that such "heartbeat"
scheme is needed only for broadcast links, as a loss of CE may
otherwise be undetectable.
4.3 CE Devices Using Inverse ARP
If a CE device uses Inverse ARP to determine the IP address of its
neighbor, the attached PE processes the Inverse ARP request for
stated circuit and responds with an Inverse ARP reply containing
the remote CE's IP address, if the address is known. If the PE does
not yet have the remote CE's IP address, it does not respond, but
notes the IP address of the local CE and the circuit information.
Subsequently, when the IP address of the remote CE becomes
available, the PE may initiate the Inverse ARP request as a means
to notify the local CE, the IP address of the remote CE.
This is a typical operation for Frame Relay and ATM attachment
circuits. In the cases where the CE does not use Inverse ARP, PE
could still discover the CE as described in section 4.1 and 4.5.
4.4 CE Devices Using PPP
If a CE device uses PPP to determine the IP address of its
neighbor, a PE takes part in the IPCP [PPP-IPCP] exchange and
supplies the IP address of the remote CE if the address is known.
If the PE does not have the remote CE's IP address, it does not
respond to the local CE's IPCP request but simply notes its IP
address. Subsequently, when the IP address of the remote CE becomes
available, the PE generates IPCP Configure-Request to the local CE.
The PE must deny configurations such as header compression and
encryptions in the NCP packets with such options.
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4.5 Proactive method
In order to learn the IP address of the CE device for a given
Attachment Circuit, the PE device may execute Router Discovery
Protocol [RFC 1256] whereby a Router Discovery Request (ICMP û
router solicitation) message is sent using a source IP address of
zero. The IP address of the CE device is extracted from the Router
Discovery Response (ICMP û router advertisement) message from the
CE.
The use of the router discovery mechanism by the PE is optional.
5.0 IP Address Distribution Between PE
5.1 When To Distribute IP Address
A PE device advertises the IP address of the attached CE only when
the encapsulation type of the Pseudowire is IP L2 interworking. It
is quite possible that the IP address of a CE device is not
available at the time the PW labels are advertised. For example, in
Frame Relay the CE device dispatches inverse ARP request only when
the DLCI is active; if the PE signals the DLCI to be active only
when it has received the IP address along with the VC-FEC from the
remote PE, a chicken and egg situation arises. In order to avoid
such problems, the PE must be prepared to advertise the VC-FEC
before the CE's IP address is known. When the IP address of the CE
device does become available, the PE re-advertises the VC-FEC along
with the IP.
Similarly, if the PE detects invalidation of the CE's IP address
(by methods described above) the PE must re-advertise the VC-FEC
with null IP address to denote the withdrawal of the CE's IP
address. The receiving PE then waits for the notification of remote
IP address. During this period, propagation of unicast IP traffic
is suspended while continuing to let multicast IP traffic flow.
If two CE devices are locally attached to the PE where, one CE is
connected to an Ethernet data link and the other to a Frame Relay
interface, for example, the IP addresses are learned in the same
manner described above. However, since the CE devices are local,
the distribution of IP addresses for these CE devices is a local
step.
5.2 LDP Based Distribution
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 and shares some fields that are common
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between them. The discussions below refer to these common fields
for IP L2 Interworking Circuits.
The IP L2 Interworking uses IP datagram 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-Control].
In addition, this document defines an IP address TLV that must be
included in the optional TLV field of the Label Mapping message
when advertising VC-FEC for the IP L2 Interworking Circuit. 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 IP L2 Interworking. If matched, it further checks the
presence of IP address TLV. If an IP address TLV is absent, a Label
Release message is issued to reject the PW establishment.
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 address TLV (TBD) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Length field is defined as the length of the IP address and is
set to value 4.
The IP address field is set to value null to denote that
advertising PE has not learned the IP address of his local CE
device. The non-zero value of the IP address field denotes IP
address of advertising PEÆs attached CE device.
The IP address TLV is also used in the LDP notification message
along with the VC-FEC. The IP address TLV in Notification message
is used as an update mechanism to notify the changes in the IP
address of the local CE device as described in [SHAH-CONTROL].
5.3 Out-of-band Distribution, Manual Configuration
In some cases, it may not be possible to deduce the IP addresses
from the VPN traffic nor induce remote PEs to supply the necessary
information on demand. For those cases, out-of-band methods, such
as manual configuration, could be used. The use of these types of
methods is useful only to handle corner cases.
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5.4 Single sided ARP mediation
In this configuration, one PE device treats the Pseudowire as a
homogeneous circuit, while the other PE device treats it as a
heterogenous circuit. For example, if PE1 is connected to an
Ethernet Attachment Circuit and PE2 is connected to an ATM
Attachment Circuit, PE1 and PE2 would both treat the Pseudowire as
of type Ethernet. From PE1's point of view, the circuit is
homogeneous, since the Attachment Circuit and the Pseudowire are
both Ethernet. Hence PE1 does no ARP mediation. From PE2's point of
view, the circuit is heterogeneous, so PE2 performs ARP mediation.
That is,
o PE2 signals to PE1 that the PW Type is Ethernet,
o PE2 learns the IP address of remote CE from Ethernet
frames received over the PW,
o PE2 learns the IP address of locally attached ATM CE,
o PE2 proxies the IP address of each CE to the other,
o PE2 decapsulates the ATM data link header and
reencapsulates with an Ethernet header before forwarding
the IP data frames from its local CE over the PW. The
information used to build the Ethernet data link header
is obtained through ARP mediation functions. Similar
header manipulation is performed when Ethernet IP frames
are forwarded to ATM Attachment Circuit,
o Drop all non IP Ethernet frames received over Ethernet
PW.
In summary, single sided configuration handles ARP mediation as PE
would typically when managing two locally attached heterogenous
Attachment Circuits.
6.0 How CE Learns The Remote CE's IP address
Once the PE has received the remote CE's IP address information
from the remote PE, it will either initiate an address resolution
request or respond to an outstanding request from the attached CE
device.
6.1 CE Devices Using ARP
When the PE learns the remote CE's IP address as described in
section 5.1 and 5.2, it may or may not know the local CE's IP
address. If the local CE's IP address is not known, the PE must
wait until it is acquired through one of the methods described in
sections 4.1, 4.3 and 4.5. If the IP address of the local CE is
known, the PE may choose to generate an unsolicited ARP message to
notify the local CE about the binding of the remote CE's IP address
with the PE's own MAC address.
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When the local CE generates an ARP request, the PE must proxy the
ARP response using its own MAC address as the source hardware
address and remote CE's IP address as the source protocol address.
The PE must respond only to those ARP requests whose destination
protocol address matches the remote CE's IP address.
6.2 CE Devices Using Inverse ARP
When the PE learns the remote CE's IP address, it should generate
an Inverse ARP request. In case, the local circuit requires
activation e.g. Frame Relay, PE should activate it first before
sending Inverse ARP request. It should be noted, that PE might
never receive the response to its own request, nor see any CE's
Inverse ARP request in cases where CE is pre-configured with remote
CE IP address or the use of Inverse ARP is not enabled. In either
case CE has used other means to learn the IP address of his
neighbor.
6.3 CE Devices Using PPP
When the PE learns the remote CE's IP address, it must initiate the
Configure-Request using the remote CE's IP address or respond to
pending Configure-Request from the local CE. As noted earlier, all
other configuration options related to compression, encryptions,
etc., should be rejected.
7.0 Use of IGPs with IP L2 Interworking L2VPNs
In an IP L2 interworking L2VPN, when an IGP on a CE connected to a
broadcast link is cross-connected with an IGP on a CE connected to
a point-to-point link, there are routing protocol related issues
that must be addressed. The link state routing protocols are
cognizant of the underlying link characteristics and behave
accordingly when establishing neighbor adjacencies, representing
the network topology, and passing protocol packets.
7.1 OSPF
The OSPF protocol treats broadcast link type with a special
procedure that engages in neighbor discovery to elect a designated
and a backup designated router (DR and BDR respectively) with which
it forms adjacencies. However, these procedures are neither
applicable nor understood by OSPF running on a point-to-point link.
By cross-connecting two neighbors with disparate link types, an IP
L2 interworking L2VPN has the potential to experience connectivity
issues.
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Additionally, the link type specified in the router LSA will not
match for two routers that are supposedly sharing the same link
type. Finally, each OSPF router generates network LSAs when
connected to a broadcast link such as Ethernet, receipt of which by
an OSPF router on the point-to-point link further adds to the
confusion.
Fortunately, the OSPF protocol provides a configuration option
(ospfIfType), whereby OSPF will treat the underlying physical
broadcast link as a point-to-point link.
It is strongly recommended that all OSPF protocols on CE devices
connected to Ethernet interfaces use this configuration option when
attached to a PE that is participating in an IP L2 Interworking
VPN.
7.2 IS-IS
The IS-IS protocol sends a LAN Hello PDU (IIH packet) with the MAC
address and the IP address of the intermediate system (i.e., CE
device) when attached to Ethernet links. The CE device expects its
neighbor to insert its own MAC and IP address in the response. If
the neighbor is connected via a point-to-point link type, the LAN
Hello PDU will be silently discarded. Similarly, Hello PDUs on the
point-to-point link do not contain any MAC address, which will
confuse a neighbor on an Ethernet link, if these two neighbors were
cross-connected via above described mechanisms.
Thus, use of the IS-IS protocol on CE devices presents problems
when interconnected by disparate data link types in an IP L2
Interworking VPN environment. There are some mechanisms defined in
draft-ietf-isis-igp-p2p-over-lan-00.txt to accommodate point-to-
point behavior over broadcast networks. The feasibility of such
techniques to solve this problem is under review.
It is important to note that the use of the IS-IS protocol in
enterprise networks (i.e., CE routers) is less common. The IS-IS
related difficulties for IP L2 Interworking VPNs, hence are
minimized.
7.3 RIP
RIP protocol broadcasts RIP advertisements every 30 seconds. If the
group/broadcast address snooping mechanism is used as described
above, the attached PE can learn the advertising (CE) router's IP
address from the IP header of the advertisement. No special
configuration is required for RIP in this type of Layer 2 IP
Interworking L2VPN.
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8.0 Security Considerations
The security aspects of this solution will be discussed at a later
time.
9.0 Acknowledgements
The authors would like to thank Prabhu Kavi, Bruce Lasley and other
folks who participated in the discussions related to this draft.
10.0 References
10.1 Normative References
[ARP] RFC 826, STD 37, D. Plummer, "An Ethernet Address Resolution
Protocol: Or Converting Network Protocol Addresses to 48.bit
Ethernet Addresses for Transmission on Ethernet Hardware".
[INVARP] RFC 2390, T. Bradley et al., "Inverse Address Resolution
Protocol".
10.2 Informative References
[L2VPN-REQ] W. Augustyn et al., "Service Requirements for Layer 2
Provider Provisioned Virtual Private Networks", February 2003, work
in progress.
[L2VPN-FRM] L. Andersson et al., "L2VPN Framework", January 2003,
work in progress.
[PPP-IPCP] RFC 1332, G. McGregor, "The PPP Internet Protocol
Control Protocol (IPCP)".
[L2VPN-Kompella] K. Kompella et al., "Layer 2 VPNs Over Tunnels",
June 2002, work in progress.
[PWE3-CONTROL] L. Martini et al., "Transport of Layer 2 Frames Over
MPLS", November 2002, work in progress.
[L2VPN-Signaling] E. Rosen et al., "LDP-based Signaling for
L2VPNs", September 2002, work in progress.
[PROXY-ARP] RFC 925, J. Postel, "Multi-LAN Address Resolution".
[SHAH-CONTROL] H. Shah et al., ôDynamic Parameters Signaling for
MPLS-based Pseudowiresö, June 2003, work in progress
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11.0 Authors' Addresses
Himanshu Shah
35 Nagog Park,
Acton, MA 01720
Email: hshah@ciena.com
Eric Rosen
Cisco Systems
1414 Massachusetts Avenue,
Boxborough, MA 01719
Email: erosen@cisco.com
Waldemar Augustyn
Email: waldemar@nxp.com
Giles Heron
PacketExchange Ltd.
The Truman Brewery
91 Brick Lane
LONDON E1 6QL
United Kingdom
Email: giles@packetexchange.net
Sunil Khandekar and Vach Kompella
TiMetra Networks
274 Ferguson Dr.
Mountain View, CA 94043
Email: sunil@timetra.com
Email: vkompella@timetra.com
Toby Smith
Laurel Networks
Omega Corporate Center
1300 Omega drive
Pittsburgh, PA 15205
Email: jsmith@laurelnetworks.com
Arun Vishwanathan
Force10 Networks
1440 McCarthy Blvd.,
Milpitas, CA 95035
Email: arun@force10networks.com
Ashwin Moranganti
Appian Communications
35 Nagog Park,
Acton, MA 01720
Email: amoranganti@appiancom.com
Andrew G. Malis
Vivace Networks, Inc.
2730 Orchard Parkway
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San Jose, CA 95134
Email: Andy.Malis@vivacenetworks.com
Steven Wright
Bell South Corp
Email: steven.wright@bellsouth.com
Vasile Radoaca
Nortel Networks
Email: vasile@nortelnetworks.com
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