L2VPN Working Group H. Shah Ciena
Internet Draft E. Rosen Cisco Systems
W. Augustyn consultant
April 2005 G. Heron Tellabs
Expires: September 2005 T. Smith Laurel Networks
A. Moranganti Big Band Networks
S. Khandekar Alcatel
V. Kompella Alcatel
A. Malis Tellabs
S. Wright Bell South
V. Radoaca Westridge Networks
A. Vishwanathan Force10 Networks
ARP Mediation for IP Interworking of Layer 2 VPN
draft-ietf-l2vpn-arp-mediation-01.txt
Status of this memo
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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
draft-ietf-l2vpn-arp-mediation-01.txt
connects Attachment Circuits of different technologies. This
requires the PEs to perform a function known as "ARP Mediation".
ARP Mediation refers to the process of resolving Layer 2 addresses
when different resolution protocols are used on either Attachment
Circuit. The methods described in this document are applicable even
when the CEs run a routing protocol between them, as long as the
routing protocol runs over IP. In particular, the applicability of
ARP mediation to ISIS is not addressed.
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.........................................6
4.5 Router Discovery method......................................6
5 .0 CE IP Address Signaling between PEs.........................7
5.1 When to Signal a CEÆs IP Address.............................7
5.2 LDP Based Distribution.......................................7
5.3 Out-of-band Distribution Configuration.......................8
6 .0 How a 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 RIP.........................................................10
8 .0 Security Considerations....................................10
8.1 Control plane security......................................10
8.2 Data plane security.........................................11
9 .0 Acknowledgements...........................................11
10 .0 References................................................11
10.1 Normative References.......................................11
10.2 Informative References.....................................11
11 .0 Authors' Addresses........................................12
1.0 Introduction
Layer 2 Virtual Private Networks (L2VPN) are constructed over 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 Layer 3 protocol is IP.
Thus it makes sense to consider procedures that are optimized for
IP.
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
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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
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 Layer 2 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 that
deals with the IP address resolution for CE devices connected via
an L2VPN. By placing this function in the PE node, ARP Mediation is
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
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2. Distribute those IP Addresses to the remote PE
3. Notify the locally attached CE of the remote CE's IP address.
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 datagrams as payload. The ingress PE removes the data
link header of its local Attachment Circuit and transmits the
payload (an IP frame) over the Pseudowire with or without the
optional control word. The egress PE encapsulates the IP packet
with the data link header used on its local Attachment Circuit.
The IP Pseudowire encapsulation is described in [PWE3-IANA].
4.0 Discovery of IP Addresses of Locally Attached CE Device
An IP Layer 2 Interworking Circuit enters monitoring state
immediately after the configuration. During this state it performs
two functions.
. Discovery of locally attached CE IP device
. Establishment of the PW
The establishment of the PW occurs independently from local CE IP
address discovery. During the period when the (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 link local multicast packets
(e.g., destined to 224.0.0.x), and are not restricted to the
operations below.
4.2 CE Devices Using ARP
If a CE device uses ARP to determine the MAC address to IP address
binding of its neighbor, the PE processes the ARP requests to learn
the IP address of local CE for the stated locally attached circuit.
If we observe the strict topology restriction whereby only one IP
router CE can exist for a given attachment circuit then the PE can
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assume that ARP request received is from the candidate IP CE and
can learn the IP to MAC address binding of the local CE.
However, if this topology restriction is relaxed, the PE can learn
the MAC address to IP address binding of the local CE but can not
assume that this CE (possibly amongst many) is the candidate IP
device that is to be interworked with the remote attachment
circuit. In this case, the PE may select the local CE device using
following criteria.
. Wait to learn the IP address of the remote CE (through PW
signaling) and then select the local CE that is sending the
ARP request for the remote CEÆs IP address.
. Augment cross checking with the local IP address learned
through listening of link local multicast packets (as per
section 4.1 above)
. Augment cross checking with the local IP address learned
through the Router Discovery protocol (as described below in
section 4.5).
. There is still a possibility that the local PE may not receive
an IP address advertisement from the remote PE and there may
exist multiple local IP routers that attempt to 'connect' to
remote CEs. In this situation, the local PE may use some other
criteria to select one IP device from many (such as ôthe first
ARP receivedö), or an operator may configure the IP address of
local CE. Note that the operator does not have to configure
the IP address of the remote CE (as that would be learned
through Pseudowire signaling).
Once the local CE has been discovered for the given Attachment
Circuit, the local PE responds to subsequent ARP requests from that
device with its own MAC address. The local PE signals the CEÆs IP
address to the remote PE and may initiate an unsolicited ARP
response to notify local CE MAC address to IP address binding of
the remote CE. Once this is completed, unicast traffic between two
CEs can start flowing.
The PE may periodically generate ARP request messages to the CE's
IP address as a means of verifying the continued existence of the
address and its binding to the MAC address. The absence of a
response from the CE device for a given number of retries could be
used as cause for withdrawal of the IP address advertisement to the
remote PE. The local PE would then enter into the address
resolution phase to rediscover the attached CE's IP address. Note
that this "heartbeat" scheme is needed only for broadcast links, as
the loss of a CE may otherwise be undetectable.
4.3 CE Devices Using Inverse ARP
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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 about the IP address of the remote CE.
This is a typical operation for Frame Relay and ATM attachment
circuits. When the CE does not use Inverse ARP, PE could still
discover the local CEÆs IP address as described in section 4.1 and
4.5.
4.4 CE Devices Using PPP
When a PPP link becomes operational after the LCP negotiations, the
local PE performs following actions
. If the local PE does not know the IP address of the local CE,
it generates a configure-request without the configure IP
address TLV. The response from CE is accepted as IP address of
the local CE.
. If PE knows the IP address of the remote CE, it sends an IPCP
configure-request with the IP address of the remote CE in the
configure IP address TLV
. If the local PE receives an IPCP configure-request without the
configure IP address TLV, and if it knows the IP address of
the remote CE, it responds with configure NAK with the
configure IP address TLV set with remote CEÆs IP address.
However, if PE does not know the remote CEÆs IP address yet,
it responds with a configure NAK.
. If the local PE does not know the IP address of the remote CE,
it sends an IPCP configure-request with IP address as zero.
The response from CE is recorded and used to validate the
incoming remote CEÆs IP address via PW signaling.
The local PE must deny configurations such as header compression
and encryptions in the NCP packets with such options.
4.5 Router Discovery 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.
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The use of the Router Discovery method by the PE is optional.
5.0 CE IP Address Signaling between PEs
5.1 When to Signal a CEÆs 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 signaled. For example, in
Frame Relay the CE device sends an 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 PW 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 PW 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 PW FEC along
with the IP.
Similarly, if the PE detects a CE's IP address is no longer
valid(by methods described above), the PE must re-advertise the PW
FEC with null IP address to denote the withdrawal of the CE's IP
address. The receiving PE then waits for notification of the remote
IP address. During this period, propagation of unicast IP traffic
is suspended, but multicast IP traffic can continue to flow.
If two CE devices are locally attached to the PE where one CE is
connected to an Ethernet port and the other to a Frame Relay port,
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 PW FEC in the Label Mapping message in the Downstream
Unsolicited (DU) mode. The PW FEC comes in two flavors; PWid and
Generalized ID FEC elements and have some common fields between
them. The discussions below refer to these common fields for IP L2
Interworking Circuits.
The IP L2 Interworking uses an IP datagram as payload over the
Pseudowire[PWE3-IANA].
In addition, this document defines an IP address TLV that must be
included as an optional parameter in the Label Mapping message
when advertising the PW FEC for the IP L2 Interworking Circuit. The
use of optional parameters in the Label Mapping message to extend
the attributes of the PW FEC is specified in the [PWE3-Control].
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When processing a received PW FEC, the PE matches the PW Id and PW
type with the locally configured PW Id to determine if the PW FEC
is of type IP L2 Interworking. If there is a match, it further
checks the presence of IP address optional parameter. If absent, a
Label Release message is issued to reject the PW establishment.
The optional parameter of the Label Mapping message is defined as
follows.
Optional Parameter type length value
IP address TBD 04 CEÆs IP address
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 CEÆs IP address is also supplied in the optional parameter
field of the LDPÆs Notification message along with the PW FEC. The
LDP Notification message is used to signal the change in CEÆs IP
address.
5.3 Out-of-band Distribution Configuration
In some cases, it may not be possible either 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, MAY be used.
6.0 How a CE Learns the Remote CE's IP address
Once the local 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.
When the local CE generates an ARP request, the PE must proxy the
ARP response using its own MAC address as the source hardware
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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 should 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 a 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 may experience connectivity issues.
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
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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 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.
8.0 Security Considerations
The security aspect of this solution is addressed for two planes;
control plane and data plane.
8.1 Control plane security
The control plane security pertains to establishing the LDP
connection, Pseudowire establishment and CEÆs IP address
distribution. The LDP connection between two trusted PEs can be
achieved by each PE verifying the incoming connection against the
configured peerÆs address and authenticating the LDP messages using
MD5 authentication. The Pseudowire establishments between two
secure LDP peers do not pose security issue but mis-wiring could
occur due to configuration error. Some checks, such as, proper
Pseudowire type and other Pseudowire options may prevent mis-wiring
due to configuration errors.
The learning of the appropriate CEÆs IP address can be a security
issue. It is expected that the local attachment circuit to CE is
physically secured. If this is a concern, the PE must be configured
with CEÆs IP and MAC address when connected with Ethernet or CEÆs
IP and virtual circuit information (e.g. DLCI or VPI/VCI). During
each ARP/inARP frame processing, PE must verify the received
information against the configuration before accepting to protect
against hijacking the connection.
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8.2 Data plane security
The data traffic between CE and PE is not encrypted and it is
possible that in an insecure environment, a malicious user may tap
into the CE to PE connection and generate traffic using the spoofed
destination MAC address on the Ethernet Attachment Circuit. In
order to avoid such hijacking, local PE may verify the source MAC
address of the received frame against the MAC address of the
admitted connection. The frame is forwarded to PW only when
authenticity is verified. When spoofing is detected, PE must severe
the connection with the local CE, tear down the PW and start over.
9.0 Acknowledgements
The authors would like to thank Yetik Serbest, 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-FRM] L. Andersson et al., "Framework for L2VPN", June 2004,
work in progress.
[PPP-IPCP] RFC 1332, G. McGregor, "The PPP Internet Protocol
Control Protocol (IPCP)".
[PWE3-CONTROL] L. Martini et al., "Pseudowire Setup and Maintenance
using LDP", February 2005, work in progress.
[PWE3-IANA] L. Martini et al,. ôIANA Allocations for pseudo Wire
Edge to Edge Emulation (PWE3)ö, February 2005, work in progress.
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[PROXY-ARP] RFC 925, J. Postel, "Multi-LAN Address Resolution".
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
Email: giles@tellabs.com
Sunil Khandekar and Vach Kompella
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
Andrew G. Malis
Tellabs
2730 Orchard Parkway
San Jose, CA 95134
Email: Andy.Malis@vivacenetworks.com
Steven Wright
Bell South Corp
Email: steven.wright@bellsouth.com
Vasile Radoaca
Email: vasile@westridgenetworks.com
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