L2VPN Working Group Himanshu Shah Force10 Networks
Intended Status: Proposed Standard Eric Rosen Cisco System
Internet Draft Giles Heron British Telecom
Vach Kompella Alcatel-Lucent
June 2009
Expires: December 2009
ARP Mediation for IP Interworking of Layer 2 VPN
draft-ietf-l2vpn-arp-mediation-12.txt
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This Internet-Draft will expire on December 2009.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Abstract
The VPWS service [L2VPN-FRM] 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". 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.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described
in [RFC 2119].
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Table of Contents
Copyright Notice........................................... 2
1. Contributing Authors....................................... 4
2. Introduction............................................... 4
3. ARP Mediation (AM) function................................ 5
4. IP Layer 2 Interworking Circuit............................ 7
5. IP Address Discovery Mechanisms............................ 7
5.1. Discovery of IP Addresses of Locally Attached IPv4 CE. 8
5.1.1. Monitoring Local Traffic......................... 8
5.1.2. CE Devices Using ARP............................. 8
5.1.3. CE Devices Using Inverse ARP.....................10
5.1.4. CE Devices Using PPP............................ 10
5.1.5. Router Discovery method......................... 11
5.1.6. Manual Configuration............................ 11
5.2. How a CE Learns the IPv4 address of a remote CE...... 12
5.2.1. CE Devices Using ARP............................ 12
5.2.2. CE Devices Using Inverse ARP.................... 12
5.2.3. CE Devices Using PPP............................ 12
5.3. Discovery of IP Addresses of IPv6 CE Devices......... 13
5.3.1. Distinguishing factors between IPv4 and IPv6.... 13
5.3.2. Requirements for PE............................. 13
5.3.3. Processing of Neighbor Solicitations............ 14
5.3.4. Processing of Neighbor Advertisements........... 14
5.3.5. Processing of Inverse Neighbor Solicitations.... 15
5.3.6. Processing of Inverse Neighbor Advertisements... 16
5.3.7. Processing of Router Solicitations.............. 17
5.3.8. Processing of Router Advertisements............. 17
5.3.9. Duplicate Address Detection [RFC 2462].......... 17
6. CE IPv4 Address Signaling between PEs..................... 18
6.1. When to Signal an IPv4 address of a CE............... 18
7. IPv6 Capability Advertisement............................. 21
8. IANA Considerations....................................... 22
8.1. LDP Status messages.................................. 22
8.2. Interface Parameters................................. 23
9. Use of IGPs with IP L2 Interworking L2VPNs................ 23
9.1. OSPF................................................. 24
9.2. RIP.................................................. 24
9.3. IS-IS................................................ 24
10. Multi-domain considerations.............................. 25
11. Security Considerations.................................. 26
11.1. Control plane security.............................. 26
11.2. Data plane security................................. 27
12. Acknowledgements......................................... 27
13. References............................................... 28
13.1. Normative References................................ 28
13.2. Informative References.............................. 29
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14. Authors' Addresses....................................... 29
1. Contributing Authors
This document is the combined effort of the following
individuals and many others who have carefully reviewed the
document and provided the technical clarifications.
W. Augustyn consultant
T. Smith NetApps
A. Malis Verizon
S. Wright Bell South
T. Grigoriu Alcatel-Lucent
N. Hart Alcatel-Lucent
A. Dolganow Alcatel-Lucent
S. Amante Level3
A. Vishwanathan Force10 Networks
A. Moranganti Consultant
2. 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 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.
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+-----+
+------ -----| 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 IPv4 CE would use ARP
[ARP] and a FR-attached CE might use Inverse ARP [INVARP]. 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 for VPWS services,
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.
The draft uses "ARP" terminology to mean any protocol that is
used to resolve IP address to Link Layer address association
purposes. For instance in IPv4, ARP and InvArp protocols are
used for address resolution while in IPv6 Neighbor Discovery and
Inverse Neighbor Discovery protocol based on ICMPv6 is used for
address resolution.
3. 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
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via an VPWS 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, the
ARP Mediation procedure depends on whether the packets being
forwarded are IPv4 or IPV6. A PE that is to perform ARP
Mediation for IPv4 packets must perform the following logical
steps:
1. Discover the IP address of the locally attached CE device
2. Terminate, do not distribute ARP and Inverse ARP requests
from CE device at local PE.
3. Distribute the IP Address to the remote PE using
pseudowire control signaling.
4. Notify the locally attached CE of the IP address of the
remote CE.
5. Respond appropriately to ARP and Inverse ARP requests from
local CE device, using IP address of remote CE and
hardware address of local PE.
A PE that is to perform ARP Mediation for IPv6 packets must
perform the following logical steps:
1. Discover the IPv6 addresses of the locally attached CE device,
together with those of the remote CE device.
2. Intercept Neighbor Discovery and Inverse Neighbor Discovery
packets received from the local CE device, learning
information about the IPv6 configuration of the CE, before
forwarding the packets across the VPWS to the remote PE.
3. Intercept Neighbor Discovery and Inverse Neighbor Discovery
packets received over the VPWS from the remote PE, possibly
modifying them (if required for the type of outgoing AC)
before forwarding to the local CE, and also learning
information about the IPv6 configuration of the remote CE.
PEs MUST support ARP mediation for IPv4 L2 Interworking
circuits. Support for IPv6 L2 interworking circuits is OPTIONAL.
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Details for the above-described procedures are given in the
following sections.
4. IP Layer 2 Interworking Circuit
The IP Layer 2 interworking Circuit refers to interconnection of
the Attachment Circuit with the IP Layer 2 Transport pseudowire
that carries IP datagrams as the payload. The ingress PE removes
the data link header of its local Attachment Circuit and
transmits the payload (an IP packet) over the pseudowire with or
without the optional control word. In some cases, multiple data
link headers may exist, such as bridged Ethernet PDU on ATM
Attachment Circuit. In this case, ATM header as well as the
Ethernet header is removed to expose the IP packet at the
ingress. The egress PE encapsulates the IP packet with the data
link header used on its local Attachment Circuit.
The encapsulation for the IP Layer 2 Transport pseudowire is
described in [RFC4447]. The "IP Layer 2 interworking circuit"
pseudowire is also commonly referred to as "IP pseudowire".
In the case of an IPv6 L2 Interworking Circuit, the egress PE
may modify the contents of Neighbor Discovery or Inverse
Neighbor Discovery packets before encapsulating the IP packet
with the data link header.
5. IP Address Discovery Mechanisms
An IP Layer 2 Interworking Circuit enters monitoring state
immediately after the configuration. During this state it
performs two functions.
- Discovery of the CE IP device(s)
- Establishment of the PW
The establishment of the PW occurs independently from local CE
IP address discovery. During the period when the PW has been
established but the local CE IP device has not been discovered,
only broadcast/multicast IP frames are propagated between the
Attachment Circuit and pseudowire; unicast IP datagrams are
dropped. The IP destination address is used to classify
unicast/multicast packets.
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The unicast IP frames are propagated between AC and pseudowire
only when CE IP devices on both Attachment Circuits have been
discovered, notified and proxy functions have completed.
The need to wait for address resolution completion before the
unicast IP traffic can flow is simple.
. PEs do not perform routing operations
. Destination IP address in the packet is not necessarily
that of the attached CE
. On a broadcast link, there is no way to find out the MAC
address of the CE based on the Destination IP address of
the packet.
5.1. Discovery of IP Addresses of Locally Attached IPv4 CE
A PE MUST support manual configuration of IPv4 CE addresses.
This section also describes automated mechanisms by which a PE
MAY also discover an IPv4 CE address.
5.1.1. 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.
5.1.2. CE Devices Using ARP
If a CE device uses ARP to determine the IP address to MAC
address binding of its neighbor, the PE processes the ARP
requests to learn the IP address of local CE for the local
Attachment Circuit.
This document mandates that there MUST be only one CE per
Attachment Circuit. However, customer facing access topologies
may exist whereby more than one CE appears to be connected to
the PE on a single Attachment Circuit. For example, this could
be the case when CEs are connected to a shared LAN that connects
to the PE. In such case, the PE MUST select one local CE. The
selection could be based on manual configuration or the PE may
optionally use following selection criteria. In either case,
manual configuration of IP address of the local CE (and its MAC
address) MUST be supported.
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o Wait to learn the IP address of the remote CE (through PW
signaling) and then select the local CE that is sending
the request for IP address of the remote CE.
o Augment cross checking with the local IP address learned
through listening of link local multicast packets (as per
section 5.1.1 above)
o Augment cross checking with the local IP address learned
through the Router Discovery protocol (as described below
in section 5.1.5).
o 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 "the first ARP received"), 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 and remote CEs has been discovered for the given
Attachment Circuit, the local PE responds with its own MAC
address to any subsequent ARP requests from the local CE with a
destination IP address matching the IP address of the remote CE.
The local PE signals IP address of the CE to the remote PE and
may initiate an unsolicited ARP response to notify the IP
address to MAC address binding for the remote CE to local CE
(again using its own MAC address).
Once the ARP mediation function is completed (i.e. the PE device
knows both the local and remote CE IP addresses), unicast IP
frames are propagated between the AC and the established PW.
The PE may periodically generate ARP request messages for the IP
address of the CE as a means of verifying the continued
existence of the address and its MAC address binding. The
absence of a response from the CE device for a given number of
retries could be used as a trigger for withdrawal of the IP
address advertisement to the remote PE. The local PE would then
re-enter the address resolution phase to rediscover the IP
address of the attached CE. Note that this "heartbeat" scheme is
needed only for broadcast links (such as Ethernet AC), where the
failure of a CE device may otherwise be undetectable.
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5.1.3. 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
from the Attachment Circuit and responds with an Inverse ARP
reply containing the IP address of the remote CE, if the address
is known. If the PE does not yet have the IP address of the
remote CE, it does not respond, but records 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 of notifying the IP
address of the remote CE to the local CE.
This is the typical mode of operation for Frame Relay and ATM
Attachment Circuits. If the CE does not use Inverse ARP, the PE
can still discover the IP address of local CE using the
mechanisms described in section 5.1.1 and 5.1.5.
5.1.4. CE Devices Using PPP
The IP Control Protocol [PPP-IPCP] describes a procedure to
establish and configure IP on a point-to-point connection,
including the negotiation of IP addresses. When such Attachment
Circuit is configured for IP interworking, PPP negotiation is
not performed end-to-end between CE devices. Instead, PPP
negotiation takes place between the CE and its local PE. The PE
performs proxy PPP negotiation and informs the attached CE the
IP address of the remote CE during IPCP negotiation using the
IP-Address option (0x03).
When a PPP link completes LCP negotiations, the local PE MAY
perform the following IPCP actions:
o The PE learns the IP address of the local CE from the
Configure-Request received with the IP-Address option
(0x03). If the IP address is non-zero, PE records the
address and responds with Configure-Ack. However, if the
IP address is zero, PE responds with Configure-Reject (as
this is a request from CE to assign it an IP address).
Also, the IP address option is set with zero value in the
Configure-Reject response to instruct the CE to not
include that option in subsequent new Configure-Request.
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o If the PE receives Configure-Request without the IP-
Address option, it responds with a Configure-Ack. In this
case the PE is unable to learn the IP address of the local
CE using IPCP and hence must rely on other means as
described in sections 5.1.1 and 5.1.5. Note that in order
to employ other learning mechanisms, the IPCP negotiations
must have reached the open state.
o If the PE does not know the IP address of the remote CE,
it sends a Configure-Request without the IP-Address
option.
o If the PE knows the IP address of the remote CE, it sends
a Configure-Request with the IP-Address option containing
the IP address of the remote CE.
The IPCP IP-Address option MAY be negotiated between the PE and
the local CE device. Configuration of other IPCP options MAY be
rejected. Other NCPs, with the exception of the Compression
Control Protocol (CCP) and Encryption Control Protocol (ECP),
MUST be rejected. The PE device MAY reject configuration of the
CCP and ECP.
5.1.5. 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 -
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 - router advertisement) message
from the CE. It is possible that the response contains more than
one router addresses with the same preference level; in which
case, some heuristics (such as first on the list) is necessary.
The use of the Router Discovery method by the PE is optional.
5.1.6. Manual Configuration
In some cases, it may not be possible to discover the IP address
of the local CE device using the mechanisms described in section
5.1 above. In such cases manual configuration MAY be used. All
implementations of this draft MUST support manual configuration
of the IPv4 address of the local CE. This is the only REQUIRED
mode for a PE to support.
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5.2. How a CE Learns the IPv4 address of a remote CE
Once the local PE has received the IP address information of the
remote CE from the remote PE, it will either initiate an address
resolution request or respond to an outstanding request from the
attached CE device.
5.2.1. CE Devices Using ARP
When the PE learns IP address of the remote CE as described in
section 6.1 and 6.2, it may or may not already know IP address
of the local CE. If the IP address is not known, the PE must
wait until it is acquired through one of the methods described
in sections 5.1.1, 5.1.2 and 5.1.5. If 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 IP
address of the remote CE with the PE's own MAC address.
When the local CE generates an ARP request, the PE must proxy
the ARP response [PROXY-ARP] using its own MAC address as the
source hardware address and IP address of remote CE as the
source protocol address. The PE must respond only to those ARP
requests whose destination protocol address matches the IP
address of the remote CE.
5.2.2. CE Devices Using Inverse ARP
When the PE learns the IP address of the remote CE, it should
generate an Inverse ARP request. If the Attachment Circuit
requires activation (e.g. Frame Relay) the PE should activate it
first before the Inverse ARP request. It should be noted, that
PE might never receive the response to its own request, nor see
any Inverse ARP request from the CE, in cases where CE is pre-
configured with IP address of the remote CE or where the use of
Inverse ARP has not been enabled. In either case the CE has used
other means to learn the IP address of his neighbor.
5.2.3. CE Devices Using PPP
When the PE learns the IP address of the remote CE, it should
initiate a Configure-Request and set the IP-Address option to
the IP address of the remote CE to notify the IP address of the
remote CE to the local CE.
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5.3. Discovery of IP Addresses of IPv6 CE Devices
5.3.1. Distinguishing factors between IPv4 and IPv6
The IPv6 uses ICMPv6 extensions to resolve IP address and link
address associations. These are essentially IP packets as
compared to ARP and invARP in IPv4 which is a separate protocol
and not IP packets. The IP pseudowire can not be used to carry
the ARP/invARP packets and hence requires local processing of
these PDUs and signaling of IP address information between the
PEs using the Pseudowire control plane.
5.3.2. Requirements for PE
A PE device that supports IPv6 MUST be capable of,
- Intercepting ICMPv6 Neighbor Discovery [RFC 2461] and
Inverse Neighbor Discovery [RFC 3122] packets received
over the AC as well as over the PW.
- Record the IPv6 interface addresses and CE link-layer
addresses present in these packets
- Possibly modify these packets as dictated by the data link
type of the egress AC (described in the following
sections), and
- Forward them towards the original destination
The PE MUST also be capable of generating packets in order to
interwork between Neighbor Discovery (ND) and Inverse Neighbor
Discovery (IND). This is specified in Sections 5.3.3. to Section
5.3.6. below.
A PE device MUST also be capable of intercepting Router
Discovery packets. This is required in order to translate
between different link layer addresses. If a Router Discovery
message contains a link layer address, then the PE MAY also use
this message to discover the link layer address and IPv6
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interface address. This is described in more detail in Section
5.3.7. and Section 5.3.8.
The PE device MUST learn a list of CE IPv6 interface addresses
for its directly-attached CE and another list of CE IPv6
interface addresses for the far-end CE. The PE device MUST also
learn the link-layer address of the local CE and be able to use
it when forwarding traffic between the local and far-end CEs.
The PE MAY also wish to monitor the source link-layer address of
data packets received from the CE, and discard packets not
matching its learned CE link-layer address.
5.3.3. Processing of Neighbor Solicitations
A Neighbor Solicitation received on an AC from a local CE SHOULD
be inspected to determine and learn an IPv6 interface address
(if provided - this will not be the case for Duplicate Address
Detection) and any link-layer address provided. The packet MUST
then be forwarded over the pseudowire unmodified. A Neighbor
Solicitation received over the pseudowire SHOULD be inspected to
determine and learn an IPv6 interface address for the far-end
CE. If a source link-layer address option is present, the PE
MUST remove it. The PE MAY substitute an appropriate link-layer
address option, specifying the link-layer address of the local
AC. Note that if the local AC is Ethernet, failure to substitute
a link-layer address option may mean that the CE has no valid
link-layer address with which to transmit data packets.
When a PE with a local AC of the type point-to-point link
receives a Neighbor Solicitation over the pseudowire, after
learning the far-end CE's IP address, the PE may use either of
the following handling procedures:
1. Forward the Neighbor Solicitation to the local CE after
replacing the source link-layer address with the link-layer
address of the local AC.
2. Send an Inverse Neighbor Solicitation to the local CE,
specifying the far-end CE's IP address and the link-layer
address of the local AC.
5.3.4. Processing of Neighbor Advertisements
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A Neighbor Advertisement received on an AC from a local CE
SHOULD be inspected to determine and learn an IPv6 interface
address and any link-layer address provided. The packet MUST
then be forwarded over the pseudowire unmodified.
A Neighbor Advertisement received over the pseudowire SHOULD be
inspected to determine and learn an IPv6 interface address for
the far-end CE. If a source link-layer address option is
present, the PE MUST remove it. The PE MAY substitute an
appropriate link-layer address option, specifying the link-layer
address of the local AC. Note that if the local AC is Ethernet,
failure to substitute a link-layer address option may mean that
the local CE has no valid link-layer address with which to
transmit data packets.
When a PE with a local AC of the type point-to-point link
receives a Neighbor Advertisement over the pseudowire, it should
perform the following steps.
o Learn the IPv6 interface addresses of the far-end CE.
o If the PE had already processed an IND-SOL from local CE,
it should send on the local AC an IND-ADV using source IP
address information received in ND-ADV and its own link
information.
o If the PE had not received any IND-SOL from the local CE,
it should send on the local AC an IND-SOL using source IP
address information received in ND-ADV and its own link
information
5.3.5. Processing of Inverse Neighbor Solicitations
An Inverse Neighbor Solicitation received on an AC from a local
CE SHOULD be inspected to determine and learn an IPv6 interface
address and the link-layer addresses. The packet may optionally
contain a list of interface addresses for the local CE, and
these SHOULD also be learned. The packet MUST then be forwarded
over the pseudowire unmodified.
An Inverse Neighbor Solicitation received over the pseudowire
SHOULD be inspected to determine and learn one or more interface
addresses for the far-end CE. If the local AC supports Inverse
Neighbor Discovery (e.g., a Frame Relay AC), the packet may be
forwarded to the local CE, after modifying the link-layer
address options to match the type of the local AC.
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If the local AC does not support Inverse Neighbor Discovery
(IND), processing of the packet depends on whether the PE has
learned at least one interface address for its directly-attached
CE. If it has learned at least one interface address for the CE,
the PE MUST discard the Inverse Neighbor Solicitation (INS) and
generate an Inverse Neighbor Advertisement (INA) back into the
pseudowire. The destination address of the INA is the source
address from the INS, the source address is one of the local
CE's interface addresses, and all the local CE's interface
addresses that have been learned so far SHOULD BE included in
the Target Address List. The Source and Target Link-Layer
addresses are copied from the INS. In addition, the PE should
generate ND advertisement on the local AC using IP address of
the remote CE and MAC address of the local PE.
The INS MUST be discarded if the PE has not yet learned at least
one interface address for its directly-connected CE. This
processing continues until the PE learns an address from the
local CE (through receiving, for example, a Neighbor
Solicitation). After this has occurred, the PE will be able to
respond to INS messages received over the pseudowire.
5.3.6. Processing of Inverse Neighbor Advertisements
An Inverse Neighbor Advertisement (INA) received on an AC from a
local CE SHOULD be inspected to determine and learn one or more
interface addresses for the CE. It MUST then be forwarded
unmodified over the pseudowire.
An INA received over the pseudowire SHOULD be inspected to
determine and learn one or more interface addresses for the far-
end CE.
If the local AC supports Inverse Neighbor Discovery (e.g., a
Frame Relay AC), the packet MAY be forwarded to the local CE,
after modifying the link-layer address options to match the type
of the local AC.
If the local AC does not support Inverse Neighbor Discovery, the
PE MUST discard the INA and generate a Neighbor Advertisement
(NA) towards its local CE. The source address of the NA is the
source address from the INA, the destination address is the
destination address from the INA and the link-layer address is
that of the local AC on the PE.
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5.3.7. Processing of Router Solicitations
A Router Solicitation received on an AC from a local CE SHOULD
be inspected to determine and learn an interface address for the
CE, and, if present, the link-layer address of the CE. It MUST
then be forwarded unmodified over the pseudowire.
A Router Solicitation received over the pseudowire SHOULD be
inspected to determine and learn an interface address for the
far-end CE. If a source link-layer address option is present,
the PE MUST remove it. The PE MAY substitute a source link-layer
address option specifying the link-layer address of its local
AC. The packet is then forwarded to the local CE.
5.3.8. Processing of Router Advertisements
A Router Advertisement received on an AC from a local CE SHOULD
be inspected to determine and learn an interface address for the
CE, and, if present, the link-layer address of the CE. It MUST
then be forwarded unmodified over the pseudowire.
A Router Advertisement received over the pseudowire SHOULD be
inspected to determine and learn an interface address for the
far-end CE. If a source link-layer address option is present,
the PE MUST remove it. The PE MAY substitute a source link-layer
address option specifying the link-layer address of its AC. If
an MTU option is present, the PE MAY reduce the specified MTU if
the MTU of the pseudowire is less than the value specified in
the option. The packet is then forwarded to the local CE.
5.3.9. Duplicate Address Detection [RFC 2462]
Duplicate Address Detection allows IPv6 hosts and routers to
ensure that the addresses assigned to interfaces are unique
on a link. As with all Neighbor Discovery packets, those
used in Duplicate Address Detection will simply flow
through the pseudowire, being inspected at the PEs at each
end. Processing is performed as above. However, the source
address of Neighbor Solicitations used in Duplicate Address
Detection is the unspecified address, so the PEs can not
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learn the CE's interface address (nor would it make sense
to do so, given that at least one address is tentative at
that time).
6. CE IPv4 Address Signaling between PEs
6.1. When to Signal an IPv4 address of a CE
A PE device advertises the IPv4 address of the attached CE only
when the encapsulation type of the pseudowire is IP Layer2
Transport (the value 0x0000B, as defined in [PWE3-IANA]). It is
quite possible that the IPv4 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 IPv4 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 IPv4 address of the CE is known and hence uses
IPv4 address value zero. When the IPv4 address of the CE device
does become available, the PE re-advertises the PW FEC along
with the IPv4 address of the CE.
Similarly, if the PE detects that an IP address of a CE 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 IP address of the CE. The receiving PE then waits
for notification of the remote IP address. During this period,
propagation of unicast IPv4 traffic is suspended, but multicast
IPv4 traffic can continue to flow between the AC and the
pseudowire.
If two CE devices are locally attached to the PE on disparate AC
types (for example, one CE connected to an Ethernet port and the
other to a Frame Relay port), the IPv4 addresses are learned in
the same manner as described above. However, since the CE
devices are local, the distribution of IPv4 addresses for these
CE devices is a local step.
Note that the PEs discover the IPv6 addresses of the remote CE
by intercepting Neighbor Discovery and Inverse Neighbor
Discovery packets that have been passed in-band through the
pseudowire. Hence, there is no need to communicate the IPv6
addresses of the CEs through LDP signaling.
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If the pseudowire is only carrying IPv6 traffic, the address
specified in the IP Address List TLV will always be zero. If the
pseudowire is carrying both IPv4 and IPv6 traffic, the
mechanisms used for IPV6 and IPv4 should not overlap. In
particular, just because a PE has learned a link-layer address
for IPv6 traffic by intercepting a Neighbor Advertisement from
its directly-connected CE, it should not assume that it can use
that link-layer address for IPv4 traffic until that fact is
confirmed by reception of, for example, an IPv4 ARP message from
the CE.
LDP Based Distribution of CE IPv4 Addresses
[RFC4447] uses Label Distribution Protocol (LDP) transport to
exchange PW FECs 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 has some common fields between
them. The discussions below refer to these common fields for IP
L2 Interworking encapsulation.
In addition to PW-FEC, this document defines an IP address list
TLV that is to be included in the optional parameter field of
the Label Mapping message when advertising the PW FEC for the IP
Layer2 Transport. The use of optional parameters in the Label
Mapping message to extend the attributes of the PW FEC is
specified in the [RFC4447].
As defined in [RFC4447], when processing a received PW FEC, the
PE matches the PW ID and PW type with the locally configured PW
ID and PW Type. If there is a match and if the PW Type is IP
Layer2 Transport, the PE further checks for the presence of an
Address List TLV (as specified in [RFC 3036]) in the optional
parameter TLVs. The processing of the address list TLV is as
follows.
o If a pseudowire is configured for AC with IPv4 CEs only,
the PE should advertise address list tlv with address
family type to be of IPv4 address. The PE should process
the IPv4 address list TLV as described in this document.
o If a pseudowire is configured for AC with both IPv4 and
IPv6 CEs, the PE should advertise IPv6 capability using
the procedures described in Section 7. below.
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o If a PE does not receive any address list TLV or IPv6
capability advertisement, it MAY assume IPv4 behavior. The
address resolution for IPv4 MUST then depend on local
manual configuration.
We use the Address List TLV as defined in [RFC 3036] to signal
the IPv4 address of the local CE. This IP address list TLV is
included in the optional parameter field of the Label Mapping
message.
The Address List TLV is only used for IPv4 addresses.
Encoding of the IP Address List TLV is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| Address List (0x0101) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family | IP Address of CE ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ IP Address of CE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Length
6 bytes: 2 bytes for address family and 4 bytes of IPv4
address.
Address Family
Two octet quantity containing a value from the ADDRESS
FAMILY NUMBERS from ADDRESS FAMILY NUMBERS in [RFC 3232]
that encodes the address contained in the Address field.
IP Address of CE
IPv4 address of the CE attached to the advertising PE. The
encoding of the individual address depends on the Address
Family (which may be of value zero).
The following address encodings are defined by this version of
the protocol:
Address Family Address Encoding
IPv4 (1) 4 octet full IPv4 address
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The IP address field is set to all zeroes to denote that
advertising PE has not learned the IPv4 address of its local CE.
Any non-zero value of the IP address field denotes the IPv4
address of advertising PE's attached CE device.
The IPv4 address of the CE is also supplied in the optional
parameters field of the LDP Notification message along with the
PW FEC. The LDP Notification message is used to signal any
change in the status of the CE's IPv4 address.
The encoding of the LDP Notification message is as follows.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Notification (0x0001) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status (TLV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address List TLV (as defined above) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PWId FEC or Generalized ID FEC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Status TLV status code is set to 0x0000002C "IP address of
CE", to indicate that IP Address update follows. Since this
notification does not refer to any particular message the
Message Id, and Message Type fields are set to 0. [note: Status
Code 0x0000002C is pending IANA allocation].
The PW FEC TLV SHOULD not include the interface parameters as
they are ignored in the context of this message.
7. IPv6 Capability Advertisement
A 'Stack Capability' Interface Parameter sub-TLV is signaled by
the two PEs so that they can agree which stack(s) they should be
using. It is assumed by default that the IP PW will always be
capable of carrying IPv4 packets. Thus this capability sub-TLV
is used to indicate if other stacks need to be supported
concurrently with IPv4.
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The 'Stack Capability' sub-TLV is part of the interface
parameters of the PW FEC. The proposed format for the Stack
Capability interface parameter sub-TLV is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter ID | Length | Stack Capability |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Parameter ID = 0x16
Length = 4
Stack capability = 0x0001 to indicate IPv6 stack capability
The Value of Stack capability is dependent on the PW type
context. For IP PW type, a setting of 0x0001 indicates IPv6
stack capability.
A PE that supports IPv6 on an IP PW MUST signal the stack
capability sub-TLV in the initial label mapping message for the
PW. The PE nodes compare the value advertised by the remote PE
with the local configuration and only use a capability which is
advertised by both. If a PE does not receive a 'stack
capability' sub-TLV from the far-end PE in the initial label
mapping message, or one is received but it is set to a reserved
value, the PE MUST assume IPv4-only behavior.
If the stack capability value in the 'stack capability' sub-TLV
received from the far end PE is not set to all zeros, then the
sub-TLV MUST be treated as invalid and the PE MUST release the
PW label.
The behavior of a PE that does not understand an interface
parameter sub-TLV is specified in RFC4447 [RFC4447].
8. IANA Considerations
8.1. LDP Status messages
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This document uses new LDP status codes, IANA already maintains
a registry of name "STATUS CODE NAME SPACE" defined by [RFC
3036]. The following values are suggested for assignment:
0x0000002C "IP Address of CE"
8.2. Interface Parameters
This document proposes a new Interface Parameters sub-TLV, to be
assigned from the 'Pseudowire Interface Parameters Sub-TLV type
Registry'. The following value is suggested for the Parameter ID:
0x16 "Stack capability"
IANA is also requested to set up a registry of "L2VPN PE stack
capabilities". This is a 16 bit field. Stack capability values
0x0001 is specified in Section 7. of this document. The remaining
bitfield values (0x0002 to 0x8000) are to be assigned by IANA
using the "IETF Consensus" policy defined in [RFC2434].
L2VPN PE Stack Capabilities:
Bit (Value) Description
=============== ==========================================
Bit 0 (0x0001) - IPv6 stack capability
Bit 1 (0x0002) - Reserved
Bit 2 (0x0004) - Reserved
.
.
.
Bit 14 (0x4000) - Reserved
Bit 15 (0x8000) - Reserved
9. 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.
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9.1. 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 each other router on the link 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 the two cross-connected routers.
Finally, each OSPF router generates network LSAs when connected
to a broadcast link such as Ethernet, receipt of which by an
OSPF router which believes itself to be connected to a 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.
9.2. RIP
RIP protocol broadcasts RIP advertisements every 30 seconds. If
the multicast/broadcast traffic snooping mechanism is used as
described in section 5.1, the attached PE can learn the local CE
router's IP address from the IP header of its advertisements. No
special configuration is required for RIP in this type of Layer
2 IP Interworking L2VPN.
9.3. IS-IS
The IS-IS protocol does not encapsulate its PDUs in IP, and
hence cannot be supported in IP L2 Interworking L2VPNs.
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10. Multi-domain considerations
In a back-to-back configuration, when two PEs are connected with
Ethernet, the ARP proxy function has limited application as
there is no local CE.
|
Network A | Network B
CE-1 <---> PE-1 <---> PE-2 <===> PE-3 <---> PE-4 <---> CE-2
ATM LDP ETH LDP ETH
PW-1 PW-2
Consider a Multi-domain network topology as shown above where PW
segment 1 (PE1<->PE2) is in network A and PW segment 2 (PE3<-
>PE4) is in network B. In this configuration CE1 is connected to
PE1 and CE2 is connected to PE4. PE2 on network A is directly
connected to PE3 in network B with Ethernet. In this
configuration there needs to be a mechanism for PE2 and PE3 to
learn IP addresses of the CEs present in each other's network.
The two options to do this are as follows.
o Configure CE2's IP address as a local CE's IP address at
PE2 and CE1's IP address as local CE's IP address at PE3.
Additionally, PE2 and PE3 are required to generate ARP
requests using their own MAC addresses as the source
address. These PEs are in effect proxying for CEs present
in the each other's network. This is not a desirable
option as it requires configuration of IP address of a CE
that is present in others (possibly other service
provider's) network.
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o In the second option, PE2 and PE3 use gratuitous ARP which
eliminates configuration of IP addresses of the CEs. In
this scheme, when PE2 learns the IP address of CE1
(through LDP signaling), PE2 sends a gratuitous ARP to PE3
with the source and destination IP address field set to
CE1's IP address and the source MAC address field set to
PE2's MAC address. When PE3 learns the IP address of CE1
(from the gratuitous ARP), PE3 notifies PE4 of the IP
address of the CE1 through LDP signaling. Similarly, for
the traffic in the opposite direction, when PE3 learns the
IP address of CE2, it sends a gratuitous ARP to PE2. PE2
sends an IP address notification, via LDP, of CE2's IP
address to PE1 using the same procedures described above.
This allows PE2 and PE3 to dynamically learn the IP
addresses of the CEs present in each other's networks.
This is the preferred mode of operation as compared to the
option 1 above.
11. Security Considerations
The security aspect of this solution is addressed for two
planes; control plane and data plane.
11.1. Control plane security
Control plane security pertains to establishing the LDP
connection, and to pseudowire signaling and CE IP address
distribution over that LDP connection. The LDP connection
between two trusted PEs can be achieved by each PE verifying the
incoming connection against the configured address of the peer
and authenticating the LDP messages using MD5 authentication.
Pseudowire signaling 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.
Learning the IP address of the appropriate CE can be a security
issue. It is expected that the Attachment Circuit to the local
CE will be physically secured. If this is a concern, the PE must
be configured with IP and MAC address of the CE when connected
with Ethernet or IP and virtual circuit information (DLCI or
VPI/VCI when connected over Frame Relay or ATM and IP address
only when connected over PPP). During each ARP/inARP frame
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processing, the PE must verify the received information against
local configuration before forwarding the information to the
remote PE to protect against hijacking the connection.
For IPv6, the preferred means of security is Secure Neighbor
Discover (SEND) [RFC3971]. SEND provides a mechanism for
securing Neighbor Discovery packets over media (such as wireless
links) that may be insecure and open to packet interception and
substitution. SEND is based upon cryptographic signatures of
Neighbor Discovery packets. These signatures allow the receiving
node to detect packet modification and confirm that a received
packet originated from the claimed source node.
SEND is incompatible with the Neighbor Discovery packet
modifications described in this document. As such, SEND cannot
be used for Neighbor Discovery across an ARP Mediation
pseudowire. PEs taking part in IPv6 ARP Mediation must remove
all SEND packet options from Neighbor Discovery packets before
forwarding into the pseudowire. If the CE devices are configured
to only accept SEND Neighbor Discovery packets, this will lead
to Neighbor Discovery failing. Thus, the CE devices must be
configured to accept non-SEND packets, even if they treat them
with lower priority than SEND packets.
Because SEND cannot be used in combination with IPv6 ARP
Mediation, it is suggested that IPv6 ARP Mediation is only used
with secure Attachment Circuits.
11.2. 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 sever the connection with the local CE, tear down the PW
and start over.
12. Acknowledgements
The authors would like to thank Mathew Bocci, Yetik Serbest,
Prabhu Kavi, Bruce Lasley, Mark Lewis, Carlos Pignataro, Shane
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Amante and other folks who participated in the discussions
related to this draft.
13. References
13.1. 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".
[RFC4447] L. Martini et al., "Pseudowire Setup and
Maintenance using LDP", RFC 4447.
[PWE3-IANA] L. Martini et al,. "IANA Allocations for pseudo
Wire Edge to Edge Emulation (PWE3) ", RFC 4446.
[RFC 2119] S. Bradner, "Key words for use in RFCs to indicate
requirement levels"
[RFC 3036] L.Anderssen et al., "LDP Specification"
[RFC 2461] Narten, T., Nordmark, E. and W.Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461,
December, 1998.
[RFC 3122] Conta, A., "Extensions to IPv6 Neighbor Discovery
for Inverse Discovery Specification", RFC 3122,
June 2001.
[RFC 2462] Thomson, S. and Narten, T., "IPv6 Stateless
Address Autoconfiguration", RFC 2462, December
1998.
[RFC 3971] Arkko, J. et al., "Secure Neighbor Discovery
(SEND)", RFC 3971, March 2005.
[RFC2434] Narten, T et al., "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, Oct 1998
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13.2. 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)".
[PROXY-ARP] RFC 925, J. Postel, "Multi-LAN Address
Resolution".
[RFC 1256] S.Deering, "ICMP Router Discovery Messages".
[RFC 3232] Reynolds and Postel, "Assigned Numbers".
14. Authors' Addresses
Himanshu Shah
30 Nagog Park,
Acton, MA 01720
Email: hshah@force10networks.com
Eric Rosen
Cisco Systems
Email: erosen@cisco.com
Waldemar Augustyn
Email: waldemar@wdmsys.com
Giles Heron
BT
Email: giles.heron@gmail.com
Sunil Khandekar and Vach Kompella
Email: sunil@timetra.com
Email: vkompella@timetra.com
Toby Smith
Network Appliance, Inc.
EMail: tob@netapp.com
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Andrew G. Malis
Verizon
EMail: Andy.g.Malis@verizon.com
Steven Wright
Bell South Corp
Email: steven.wright@bellsouth.com
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