Operational Aspects of Proxy-ARP/ND in EVPN Networks
draft-ietf-bess-evpn-proxy-arp-nd-08
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9161.
|
|
|---|---|---|---|
| Authors | Jorge Rabadan , Senthil Sathappan , Kiran Nagaraj , Greg Hankins , Thomas King | ||
| Last updated | 2019-11-26 (Latest revision 2019-07-08) | ||
| Replaces | draft-snr-bess-evpn-proxy-arp-nd | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-09)
by Russ Housley
Almost ready
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Matthew Bocci | ||
| Shepherd write-up | Show Last changed 2019-07-09 | ||
| IESG | IESG state | Became RFC 9161 (Proposed Standard) | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Martin Vigoureux | ||
| Send notices to | Matthew Bocci <matthew.bocci@nokia.com> |
draft-ietf-bess-evpn-proxy-arp-nd-08
BESS Workgroup J. Rabadan, Ed.
Internet Draft S. Sathappan
K. Nagaraj
Intended status: Informational G. Hankins
Nokia
T. King
DE-CIX
Expires: January 9, 2020 July 8, 2019
Operational Aspects of Proxy-ARP/ND in EVPN Networks
draft-ietf-bess-evpn-proxy-arp-nd-08
Abstract
The EVPN MAC/IP Advertisement route can optionally carry IPv4 and
IPv6 addresses associated with a MAC address. Remote PEs can use this
information to reply locally (act as proxy) to IPv4 ARP requests and
IPv6 Neighbor Solicitation messages (or 'unicast-forward' them to the
owner of the MAC) and reduce/suppress the flooding produced by the
Address Resolution procedure. This EVPN capability is extremely
useful in Internet Exchange Points (IXPs) and Data Centers (DCs) with
large broadcast domains, where the amount of ARP/ND flooded traffic
causes issues on routers and CEs. This document describes how the
EVPN Proxy-ARP/ND function may be implemented to help IXPs and other
operators deal with the issues derived from Address Resolution in
large broadcast domains.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." The list
of current Internet-Drafts can be accessed at
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http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This Internet-Draft will expire on January 9, 2020.
Copyright Notice
Copyright (c) 2019 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. The DC Use-Case . . . . . . . . . . . . . . . . . . . . . . 5
2.2. The IXP Use-Case . . . . . . . . . . . . . . . . . . . . . 5
3. Solution Requirements . . . . . . . . . . . . . . . . . . . . . 6
4. Solution Description . . . . . . . . . . . . . . . . . . . . . 7
4.1. Learning Sub-Function . . . . . . . . . . . . . . . . . . . 9
4.1.1. Proxy-ND and the NA Flags . . . . . . . . . . . . . . . 10
4.2. Reply Sub-Function . . . . . . . . . . . . . . . . . . . . 11
4.3. Unicast-forward Sub-Function . . . . . . . . . . . . . . . 12
4.4. Maintenance Sub-Function . . . . . . . . . . . . . . . . . 13
4.5. Flooding (to Remote PEs) Reduction/Suppression . . . . . . 14
4.6. Duplicate IP Detection . . . . . . . . . . . . . . . . . . 15
5. Solution Benefits . . . . . . . . . . . . . . . . . . . . . . . 17
6. Deployment Scenarios . . . . . . . . . . . . . . . . . . . . . 17
6.1. All Dynamic Learning . . . . . . . . . . . . . . . . . . . 17
6.2. Dynamic Learning with Proxy-ARP/ND . . . . . . . . . . . . 18
6.3. Hybrid Dynamic Learning and Static Provisioning with
Proxy-ARP/ND . . . . . . . . . . . . . . . . . . . . . . . 18
6.4 All Static Provisioning with Proxy-ARP/ND . . . . . . . . . 18
6.5 Deployment Scenarios in IXPs . . . . . . . . . . . . . . . . 18
6.6 Deployment Scenarios in DCs . . . . . . . . . . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 20
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8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1. Normative References . . . . . . . . . . . . . . . . . . . 21
9.2. Informative References . . . . . . . . . . . . . . . . . . 22
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
BUM: Broadcast, Unknown unicast and Multicast layer-2 traffic.
ARP: Address Resolution Protocol.
GARP: Gratuitous ARP message.
ND: Neighbor Discovery Protocol.
NS: Neighbor Solicitation message.
NA: Neighbor Advertisement.
IXP: Internet eXchange Point.
IXP-LAN: it refers to the IXP's large Broadcast Domain to where
Internet routers are connected.
DC: Data Center.
IP->MAC: it refers to an IP address associated to a MAC address. The
entries may be of three different types: dynamic, static or EVPN-
learned.
SN-multicast address: Refers to the Solicited-Node IPv6 multicast
address used by NS messages.
NUD: Neighbor Unreachability Detection, as per [RFC4861].
DAD: Duplicate Address Detection, as per [RFC4861].
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SLLA: Source Link Layer Address, as per [RFC4861].
TLLA: Target Link Layer Address, as per [RFC4861].
R-bit: Router Flag in NA messages, as per [RFC4861].
O-bit: Override Flag in NA messages, as per [RFC4861].
S-bit: Solicited Flag in NA messages, as per [RFC4861].
RT2: EVPN Route type 2 or MAC/IP Advertisement route, as per
[RFC7432].
MAC or IP DA: MAC or IP Destination Address.
MAC or IP SA: MAC or IP Source Address.
AS-MAC: Anti-spoofing MAC.
LAG: Link Aggregation Group.
BD: Broadcast Domain.
This document assumes familiarity with the terminology used in
[RFC7432].
2. Introduction
As specified in [RFC7432] the IP Address field in the MAC/IP
Advertisement route may optionally carry one of the IP addresses
associated with the MAC address. A PE may learn local IP->MAC pairs
and advertise them in EVPN MAC/IP routes. The remote PEs may add
those IP->MAC pairs to their Proxy-ARP/ND tables and reply to local
ARP requests or Neighbor Solicitations (or 'unicast-forward' those
packets to the owner MAC), reducing and even suppressing in some
cases the flooding in the EVPN network.
EVPN and its associated Proxy-ARP/ND function are extremely useful in
Data Centers (DCs) or Internet Exchange Points (IXPs) with large
broadcast domains, where the amount of ARP/ND flooded traffic causes
issues on routers and CEs. [RFC6820] describes the Address Resolution
problems in Large Data Center networks.
This document describes how the [RFC7432] Proxy-ARP/ND function may
be implemented to help IXPs, DCs and other operators deal with the
issues derived from Address Resolution in large broadcast domains.
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2.1. The DC Use-Case
As described in [RFC6820] the IPv4 and IPv6 Address Resolution can
create a lot of issues in large DCs. In particular, the issues
created by the IPv4 Address Resolution Protocol procedures may be
significant.
On one hand, ARP Requests use broadcast MAC addresses, therefore any
Tenant System in a large Broadcast Domain will see a large amount of
ARP traffic, which is not addressed to most of the receivers.
On the other hand, the flooding issue becomes even worse if some
Tenant Systems disappear from the broadcast domain, since some
implementations will persistently retry sending ARP Requests. As
[RFC6820] states, there are no clear requirements for retransmitting
ARP Requests in the absence of replies, hence an implementation may
choose to keep retrying endlessly even if there are no replies.
The amount of flooding that Address Resolution creates can be
mitigated with the use of EVPN and its Proxy-ARP/ND function.
2.2. The IXP Use-Case
The implementation described in this document is especially useful in
IXP networks.
A typical IXP provides access to a large layer-2 peering network,
where (hundreds of) Internet routers are connected. Because of the
requirement to connect all routers to a single layer-2 network the
peering networks use IPv4 layer-3 addresses in length ranges from /21
to /24 (and even bigger for IPv6), which can create very large
broadcast domains. This peering network is transparent to the
Customer Edge (CE) devices and therefore floods any ARP request or NS
messages to all the CEs in the network. Unsolicited GARP and NA
messages are flooded to all the CEs too.
In these IXP networks, most of the CEs are typically peering routers
and roughly all the BUM traffic is originated by the ARP and ND
address resolution procedures. This ARP/ND BUM traffic causes
significant data volumes that reach every single router in the
peering network. Since the ARP/ND messages are processed in "slow
path" software processors and they take high priority in the routers,
heavy loads of ARP/ND traffic can cause some routers to run out of
resources. CEs disappearing from the network may cause Address
Resolution explosions that can make a router with limited processing
power fail to keep BGP sessions running.
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The issue may be better in IPv6 routers, since ND uses SN-multicast
address in NS messages, however ARP uses broadcast and has to be
processed by all the routers in the network. Some routers may also be
configured to broadcast periodic GARPs [RFC5227]. The amount of
ARP/ND flooded traffic grows exponentially with the number of IXP
participants, therefore the issue can only go worse as new CEs are
added.
In order to deal with this issue, IXPs have developed certain
solutions over the past years. One example is the ARP-Sponge daemon
[ARP-Sponge], which can reduce significantly the amount of ARP
messages sent to an absent router. While these solutions may mitigate
the issues of Address Resolution in large broadcasts domains, EVPN
provides new more efficient possibilities to IXPs. EVPN and its
Proxy-ARP/ND function may help solve the issue in a distributed and
scalable way, fully integrated with the PE network.
3. Solution Requirements
The distributed EVPN Proxy-ARP/ND function described in this document
meets the following requirements:
o The solution supports the learning of the CE IP->MAC entries on the
EVPN PEs via the management, control or data planes. An
implementation should allow to intentionally enable or disable
those possible learning mechanisms.
o The solution may suppress completely the flooding of the ARP/ND
messages in the EVPN network, assuming that all the CE IP->MAC
addresses local to the PEs are known or provisioned on the PEs from
a management system. Note that in this case, the unknown unicast
flooded traffic can also be suppressed, since all the expected
unicast traffic will be destined to known MAC addresses in the PE
BDs.
o The solution reduces significantly the flooding of the ARP/ND
messages in the EVPN network, assuming that some or all the CE
IP->MAC addresses are learned on the data plane by snooping ARP/ND
messages issued by the CEs.
o The solution provides a way to refresh periodically the CE IP->MAC
entries learned through the data plane, so that the IP->MAC entries
are not withdrawn by EVPN when they age out unless the CE is not
active anymore. This option helps reducing the EVPN control plane
overhead in a network with active CEs that do not send packets
frequently.
o The solution provides a mechanism to detect duplicate IP addresses.
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In case of duplication, the detecting PE should not reply to
requests for the duplicate IP. Instead, the PE should alert the
operator and may optionally prevent any other CE from sending
traffic to the duplicate IP.
o The solution should not change any existing behavior in the CEs
connected to the EVPN PEs.
4. Solution Description
Figure 1 illustrates an example EVPN network where the Proxy-ARP/ND
function is enabled.
BD1
Proxy-ARP/ND
+------------+
IP1/M1 +----------------------------+ |IP1->M1 EVPN|
GARP --->Proxy-ARP/ND | |IP2->M2 EVPN|
+---+ +----+---+ RT2(IP1/M1) | |IP3->M3 sta |
|CE1+------+ BD1 | ------> +------+---|IP4->M4 dyn |
+---+ +--------+ | +------------+
PE1 | +--------+ Who has IP1?
| EVPN | | BD1 | <----- +---+
| EVI1 | | | | |CE3|
IP2/M2 | | | | -----> +---+
GARP --->Proxy-ARP/ND | +--------+ | IP1->M1
+---+ +--------+ RT2(IP2/M2) | |
|CE2+----+ BD1 | ------> +--------------+
+---+ +--------+ PE3| +---+
PE2 | +----+CE4|
+----------------------------+ +---+
<---IP4/M4 GARP
Figure 1 Proxy-ARP/ND network example
When the Proxy-ARP/ND function is enabled in a BD (Broadcast Domain)
of the EVPN PEs, each PE creates a Proxy table specific to that BD
that can contain three types of Proxy-ARP/ND entries:
a) Dynamic entries: learned by snooping CE's ARP and ND messages. For
instance, IP4->M4 in Figure 1.
b) Static entries: provisioned on the PE by the management system.
For instance, IP3->M3 in Figure 1.
c) EVPN-learned entries: learned from the IP/MAC information encoded
in the received RT2's coming from remote PEs. For instance, IP1-
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>M1 and IP2->M2 in Figure 1.
As a high level example, the operation of the EVPN Proxy-ARP/ND
function in the network of Figure 1 is described below. In this
example we assume IP1, IP2 and IP3 are IPv4 addresses:
1. Proxy-ARP/ND is enabled in BD1 of PE1, PE2 and PE3.
2. The PEs start adding dynamic, static and EVPN-learned entries to
their Proxy tables:
a. PE3 adds IP1->M1 and IP2->M2 based on the EVPN routes received
from PE1 and PE2. Those entries were previously learned as
dynamic entries in PE1 and PE2 respectively, and advertised in
BGP EVPN.
b. PE3 adds IP4->M4 as dynamic. This entry is learned by snooping
the corresponding ARP messages sent by CE4.
c. An operator also provisions the static entry IP3->M3.
3. When CE3 sends an ARP Request asking for the MAC address of IP1,
PE3 will:
a. Intercept the ARP Request and perform a Proxy-ARP lookup for
IP1.
b. If the lookup is successful (as in Figure 1), PE3 will send an
ARP Reply with IP1->M1. The ARP Request will not be flooded to
the EVPN network or any other local CEs.
c. If the lookup is not successful, PE3 will flood the ARP Request
in the EVPN network and the other local CEs.
As PE3 learns more and more host entries in the Proxy-ARP/ND table,
the flooding of ARP Request messages is reduced and in some cases it
can even be suppressed. In a network where most of the participant
CEs are not moving between PEs and they advertise their presence with
GARPs or unsolicited NA messages, the ARP/ND flooding as well as the
unknown unicast flooding can practically be suppressed. In an EVPN-
based IXP network, where all the entries are Static, the ARP/ND
flooding is in fact totally suppressed.
The Proxy-ARP/ND function can be structured in six sub-functions or
procedures:
1. Learning sub-function
2. Reply sub-function
3. Unicast-forward sub-function
4. Maintenance sub-function
5. Flooding reduction/suppression sub-function
6. Duplicate IP detection sub-function
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A Proxy-ARP/ND implementation MAY support all those sub-functions or
only a subset of them. The following sections describe each
individual sub-function.
4.1. Learning Sub-Function
A Proxy-ARP/ND implementation SHOULD support static, dynamic and
EVPN-learned entries.
Static entries are provisioned from the management plane. The
provisioned static IP->MAC entry SHOULD be advertised in EVPN with an
ARP/ND extended community where the Immutable ARP/ND Binding Flag
flag (I) is set to 1, as per [EVPN-ARP-ND-FLAGS]. When the I flag in
the ARP/ND extended community is 1, the advertising PE indicates that
the IP address MUST NOT be associated to a MAC, other than the one
included in the MAC/IP route. The advertisement of I=1 in the ARP/ND
extended community is compatible with any value of the Sticky bit (S)
or Sequence Number in the [RFC7432] MAC Mobility extended community.
Note that the I bit in the ARP/ND extended community refers to the
immutable configured association between the IP and the MAC address
in the IP->MAC binding, whereas the S bit in the MAC Mobility
extended community refers to the fact that the advertised MAC address
is not subject to the [RFC7432] mobility procedures.
An entry MAY associate a configured static IP to a list of potential
MACs, i.e. IP1->(MAC1,MAC2..MACN). When there is more than one MAC in
the list of allowed MACs, the PE will not advertise any IP->MAC in
EVPN until a local ARP/NA message or any other frame is received from
the CE. Upon receiving traffic from the CE, the PE will check that
the source MAC is included in the list of allowed MACs. Only in that
case, the PE will activate the IP->MAC and advertise it in EVPN.
EVPN-learned entries MUST be learned from received valid EVPN MAC/IP
Advertisement routes containing a MAC and IP address.
Dynamic entries are learned in different ways depending on whether
the entry contains an IPv4 or IPv6 address:
a) Proxy-ARP dynamic entries:
They SHOULD be learned by snooping any ARP packet (Ethertype
0x0806) received from the CEs attached to the BD. The Learning
function will add the Sender MAC and Sender IP of the snooped ARP
packet to the Proxy-ARP table. Note that MAC and IPs with value 0
SHOULD NOT be learned.
b) Proxy-ND dynamic entries:
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They SHOULD be learned out of the Target Address and TLLA
information in NA messages (Ethertype 0x86DD, ICMPv6 type 136)
received from the CEs attached to the BD. A Proxy-ND
implementation SHOULD NOT learn IP->MAC entries from NS messages,
since they don't contain the R-bit Flag required by the Proxy-ND
reply function. See section 4.1.1 for more information about the
R-bit flag.
Note that if the O-bit is zero in the received NA message, the
IP->MAC SHOULD only be learned in case IPv6 'anycast' is enabled
in the EVI.
The following procedure associated to the Learning sub-function is
RECOMMENDED:
o When a new Proxy-ARP/ND EVPN or static active entry is learned (or
provisioned), the PE SHOULD send an unsolicited GARP or NA message
to the access CEs. The PE SHOULD send an unsolicited GARP/NA
message for dynamic entries only if the ARP/NA message creating the
entry was NOT flooded before. This unsolicited GARP/NA message
makes sure the CE ARP/ND caches are updated even if the ARP/NS/NA
messages from remote CEs are not flooded in the EVPN network.
Note that if a Static entry is provisioned with the same IP as an
existing EVPN-learned or Dynamic entry, the Static entry takes
precedence.
4.1.1. Proxy-ND and the NA Flags
[RFC4861] describes the use of the R-bit flag in IPv6 Address
Resolution:
o Nodes capable of routing IPv6 packets must reply to NS messages
with NA messages where the R-bit flag is set (R-bit=1).
o Hosts that are not able to route IPv6 packets must indicate that
inability by replying with NA messages that contain R-bit=0.
The use of the R-bit flag in NA messages has an impact on how hosts
select their default gateways when sending packets off-link:
o Hosts build a Default Router List based on the received RAs and NAs
with R-bit=1. Each cache entry has an IsRouter flag, which must be
set based on the R-bit flag in the received NAs. A host can choose
one or more Default Routers when sending packets off-link.
o In those cases where the IsRouter flag changes from TRUE to FALSE
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as a result of a NA update, the node MUST remove that router from
the Default Router List and update the Destination Cache entries
for all destinations using that neighbor as a router, as specified
in [RFC4861] section 7.3.3. This is needed to detect when a node
that is used as a router stops forwarding packets due to being
configured as a host.
The R-bit and O-bit will be learned in the following ways:
o Static entries SHOULD have the R-bit information added by the
management interface. The O-bit information MAY also be added by
the management interface.
o Dynamic entries SHOULD learn the R-bit and MAY learn the O-bit from
the snooped NA messages used to learn the IP->MAC itself.
o EVPN-learned entries SHOULD learn the R-bit and MAY learn the O-bit
from the ND Extended Community received from EVPN along with the
RT2 used to learn the IP->MAC itself. Please refer to [EVPN-ARP-ND-
FLAGS]. If no ND extended community is received, the PE will add
the default R-bit/O-bit to the entry. The default R-bit SHOULD be
an administrative choice. The default O-bit SHOULD be 1.
Note that the O-bit SHOULD only be learned if 'anycast' is enabled in
the EVI. If so, Duplicate IP Detection must be disabled so that the
PE is able to learn the same IP mapped to different MACs in the same
Proxy-ND table. If 'anycast' is disabled, NA messages with O-bit = 0
will not create a Proxy-ND entry, hence no EVPN advertisement with ND
extended community will be generated.
4.2. Reply Sub-Function
This sub-function will reply to Address Resolution
requests/solicitations upon successful lookup in the Proxy-ARP/ND
table for a given IP address. The following considerations should be
taken into account:
a) When replying to ARP Request or NS messages, the PE SHOULD use the
Proxy-ARP/ND entry MAC address as MAC SA. This is RECOMMENDED so
that the resolved MAC can be learned in the MAC FIB of potential
layer-2 switches sitting between the PE and the CE requesting the
Address Resolution.
b) A PE SHOULD NOT reply to a request/solicitation received on the
same attachment circuit over which the IP->MAC is learned. In this
case the requester and the requested IP are assumed to be
connected to the same layer-2 switch/access network linked to the
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PE's attachment circuit, and therefore the requested IP owner will
receive the request directly.
c) A PE SHOULD reply to broadcast/multicast Address Resolution
messages, that is, ARP-Request, NS messages as well as DAD NS
messages. A PE SHOULD NOT reply to unicast Address Resolution
requests (for instance, NUD NS messages).
d) A PE SHOULD include the R-bit learned for the IP->MAC entry in the
NA messages (see section 4.1.1). The S-bit will be set/unset as
per [RFC4861]. The O-bit will be included if IPv6 'anycast' is
enabled in the EVI and it is learned for the IP->MAC entry. If
'anycast' is enabled and there are more than one MAC for a given
IP, the PE will reply to NS messages with as many NA responses as
'anycast' entries are in the Proxy-ND table.
e) A PE SHOULD NOT reply to ARP probes received from the CEs. An ARP
probe is an ARP request constructed with an all-zero sender IP
address that may be used by hosts for IPv4 Address Conflict
Detection [RFC5227].
f) A PE SHOULD only reply to ARP-Request and NS messages with the
format specified in [RFC0826] and [RFC4861] respectively. Received
ARP-Requests and NS messages with unknown options SHOULD be either
forwarded (as unicast packets) to the owner of the requested IP
(assuming the MAC is known in the Proxy-ARP/ND table and BD) or
discarded. An administrative option to control this behavior
('unicast-forward' or 'discard') SHOULD be supported. The
'unicast-forward' option is described in section 4.3.
4.3. Unicast-forward Sub-Function
As discussed in section 4.2. in some cases the operator may want to
'unicast-forward' certain ARP-Request and NS messages as opposed to
reply to them. The operator SHOULD be able to activate this option
with one of the following parameters:
a) unicast-forward always
b) unicast-forward unknown-options
If 'unicast-forward always' is enabled, the PE will perform a Proxy-
ARP/ND table lookup and in case of a hit, the PE will forward the
packet to the owner of the MAC found in the Proxy-ARP/ND table. This
is irrespective of the options carried in the ARP/ND packet. This
option provides total transparency in the EVI and yet reduces the
amount of flooding significantly.
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If 'unicast-forward unknown-options' is enabled, upon a successful
Proxy-ARP/ND lookup, the PE will perform a 'unicast-forward' action
only if the ARP-Request or NS messages carry unknown options, as
explained in section 4.2. As an example, this would allow to enable
Proxy-ND and Secure ND [RFC3971] in the same EVI. The 'unicast-
forward unknown-options' configuration allows the support of new
applications using ARP/ND in the EVI while still reducing the
flooding at the same time.
4.4. Maintenance Sub-Function
The Proxy-ARP/ND tables SHOULD follow a number of maintenance
procedures so that the dynamic IP->MAC entries are kept if the owner
is active and flushed if the owner is no longer in the network. The
following procedures are RECOMMENDED:
a) Age-time
A dynamic Proxy-ARP/ND entry MUST be flushed out of the table if
the IP->MAC has not been refreshed within a given age-time. The
entry is refreshed if an ARP or NA message is received for the
same IP->MAC entry. The age-time is an administrative option and
its value should be carefully chosen depending on the specific
use-case: in IXP networks (where the CE routers are fairly static)
the age-time may normally be longer than in DC networks (where
mobility is required).
b) Send-refresh option
The PE MAY send periodic refresh messages (ARP/ND "probes") to the
owners of the dynamic Proxy-ARP/ND entries, so that the entries
can be refreshed before they age out. The owner of the IP->MAC
entry would reply to the ARP/ND probe and the corresponding entry
age-time reset. The periodic send-refresh timer is an
administrative option and is RECOMMENDED to be a third of the age-
time or a half of the age-time in scaled networks.
An ARP refresh issued by the PE will be an ARP-Request message
with the Sender's IP = 0 sent from the PE's MAC SA. If the PE has
an IP address in the subnet, for instance on an IRB interface,
then it MAY use it as a source for the ARP request (instead of
Sender's IP = 0). An ND refresh will be a NS message issued from
the PE's MAC SA and a Link Local Address associated to the PE's
MAC.
The refresh request messages SHOULD be sent only for dynamic
entries and not for static or EVPN-learned entries. Even though
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the refresh request messages are broadcast or multicast, the PE
SHOULD only send the message to the attachment circuit associated
to the MAC in the IP->MAC entry.
The age-time and send-refresh options are used in EVPN networks to
avoid unnecessary EVPN RT2 withdrawals: if refresh messages are sent
before the corresponding BD FIB and Proxy-ARP/ND age-time for a given
entry expires, inactive but existing hosts will reply, refreshing the
entry and therefore avoiding unnecessary MAC and MAC-IP withdrawals
in EVPN. Both entries (MAC in the BD and IP->MAC in Proxy-ARP/ND) are
reset when the owner replies to the ARP/ND probe. If there is no
response to the ARP/ND probe, the MAC and IP->MAC entries will be
legitimately flushed and the RT2s withdrawn.
4.5. Flooding (to Remote PEs) Reduction/Suppression
The Proxy-ARP/ND function implicitly helps reducing the flooding of
ARP Request and NS messages to remote PEs in an EVPN network.
However, in certain use-cases, the flooding of ARP/NS/NA messages
(and even the unknown unicast flooding) to remote PEs can be
suppressed completely in an EVPN network.
For instance, in an IXP network, since all the participant CEs are
well known and will not move to a different PE, the IP->MAC entries
may be all provisioned by a management system. Assuming the entries
for the CEs are all provisioned on the local PE, a given Proxy-ARP/ND
table will only contain static and EVPN-learned entries. In this
case, the operator may choose to suppress the flooding of ARP/NS/NA
to remote PEs completely.
The flooding may also be suppressed completely in IXP networks with
dynamic Proxy-ARP/ND entries assuming that all the CEs are directly
connected to the PEs and they all advertise their presence with a
GARP/unsolicited-NA when they connect to the network.
In networks where fast mobility is expected (DC use-case), it is NOT
RECOMMENDED to suppress the flooding of unknown ARP-Requests/NS or
GARPs/unsolicited-NAs. Unknown ARP-Requests/NS refer to those
ARP-Request/NS messages for which the Proxy-ARP/ND lookups for the
requested IPs do not succeed.
In order to give the operator the choice to suppress/allow the
flooding to remote PEs, a PE MAY support administrative options to
individually suppress/allow the flooding of:
o Unknown ARP-Request and NS messages.
o GARP and unsolicited-NA messages.
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The operator will use these options based on the expected behavior in
the CEs.
4.6. Duplicate IP Detection
The Proxy-ARP/ND function SHOULD support duplicate IP detection so
that ARP/ND-spoofing attacks or duplicate IPs due to human errors can
be detected.
ARP/ND spoofing is a technique whereby an attacker sends "fake"
ARP/ND messages onto a broadcast domain. Generally the aim is to
associate the attacker's MAC address with the IP address of another
host causing any traffic meant for that IP address to be sent to the
attacker instead.
The distributed nature of EVPN and Proxy-ARP/ND allows the easy
detection of duplicated IPs in the network, in a similar way to the
MAC duplication function supported by [RFC7432] for MAC addresses.
Duplicate IP detection monitors "IP-moves" in the Proxy-ARP/ND table
in the following way:
o When an existing active IP1->MAC1 entry is modified, a PE starts an
M-second timer (default value of M=180), and if it detects N IP
moves before the timer expires (default value of N=5), it concludes
that a duplicate IP situation has occurred. An IP move is
considered when, for instance, IP1->MAC1 is replaced by IP1->MAC2
in the Proxy-ARP/ND table. Static IP->MAC entries, that is, locally
provisioned or EVPN-learned entries (with I=1 in the ARP/ND
extended community), are not subject to this procedure. Static
entries MUST NOT be overridden by dynamic Proxy-ARP/ND entries.
o In order to detect the duplicate IP faster, the PE MAY send a
CONFIRM message to the former owner of the IP. A CONFIRM message is
a unicast ARP-Request/NS message sent by the PE to the MAC
addresses that previously owned the IP, when the MAC changes in the
Proxy-ARP/ND table. The CONFIRM message uses a sender's IP 0.0.0.0
in case of ARP (if the PE has an IP address in the subnet then it
MAY use it) and an IPv6 Link Local Address in case of NS. If the PE
does not receive an answer within a given timer, the new entry will
be confirmed and activated. In case of spoofing, for instance, if
IP1->MAC1 moves to IP1->MAC2, the PE may send a unicast ARP-
Request/NS message for IP1 with MAC DA= MAC1 and MAC SA= PE's MAC.
This will force the legitimate owner respond if the move to MAC2
was spoofed, and make the PE issue another CONFIRM message, this
time to MAC DA= MAC2. If both, legitimate owner and spoofer keep
replying to the CONFIRM message, the PE will detect the duplicate
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IP within the M timer:
- If the IP1->MAC1 pair was previously owned by the spoofer and the
new IP1->MAC2 was from a valid CE, then the issued CONFIRM
message would trigger a response from the spoofer.
- If it were the other way around, that is, IP1->MAC1 was
previously owned by a valid CE, the CONFIRM message would trigger
a response from the CE.
Either way, if this process continues, then duplicate detection
will kick in.
o Upon detecting a duplicate IP situation:
a) The entry in duplicate detected state cannot be updated with new
dynamic or EVPN-learned entries for the same IP. The operator
MAY override the entry though with a static IP->MAC.
b) The PE SHOULD alert the operator and stop responding ARP/NS for
the duplicate IP until a corrective action is taken.
c) Optionally the PE MAY associate an "anti-spoofing-mac" (AS-MAC)
to the duplicate IP. The PE will send a GARP/unsolicited-NA
message with IP1->AS-MAC to the local CEs as well as an RT2
(with IP1->AS-MAC) to the remote PEs. This will force all the
CEs in the EVI to use the AS-MAC as MAC DA for IP1, and prevent
the spoofer from attracting any traffic for IP1. Since the AS-
MAC is a managed MAC address known by all the PEs in the EVI,
all the PEs MAY apply filters to drop and/or log any frame with
MAC DA= AS-MAC. The advertisement of the AS-MAC as a "black-hole
MAC" that can be used directly in the BD to drop frames is for
further study.
o The duplicate IP situation will be cleared when a corrective action
is taken by the operator, or alternatively after a HOLD-DOWN timer
(default value of 540 seconds).
The values of M, N and HOLD-DOWN timer SHOULD be a configurable
administrative option to allow for the required flexibility in
different scenarios.
For Proxy-ND, Duplicate IP Detection SHOULD only monitor IP moves for
IP->MACs learned from NA messages with O-bit=1. NA messages with
O-bit=0 would not override the ND cache entries for an existing IP.
Duplicate IP Detection for IPv6 SHOULD be disabled when IPv6
'anycast' is activated in a given EVI.
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5. Solution Benefits
The solution described in this document provides the following
benefits:
a) The solution may suppress completely the flooding of the ARP/ND
and unknown-unicast messages in the EVPN network, in cases where
all the CE IP->MAC addresses local to the PEs are known and
provisioned on the PEs from a management system.
b) The solution reduces significantly the flooding of the ARP/ND
messages in the EVPN network, in cases where some or all the CE
IP->MAC addresses are learned on the data plane by snooping ARP/ND
messages issued by the CEs.
c) The solution reduces the control plane overhead and unnecessary
BGP MAC/IP Advertisements and Withdrawals in a network with active
CEs that do not send packets frequently.
d) The solution provides a mechanism to detect duplicate IP addresses
and avoid ARP/ND-spoof attacks or the effects of duplicate
addresses due to human errors.
6. Deployment Scenarios
Four deployment scenarios with different levels of ARP/ND control are
available to operators using this solution, depending on their
requirements to manage ARP/ND: all dynamic learning, all dynamic
learning with Proxy-ARP/ND, hybrid dynamic learning and static
provisioning with Proxy-ARP/ND, and all static provisioning with
Proxy-ARP/ND.
6.1. All Dynamic Learning
In this scenario for minimum security and mitigation, EVPN is
deployed in the peering network with the Proxy-ARP/ND function
shutdown. PEs do not intercept ARP/ND requests and flood all
requests, as in a conventional layer-2 network. While no ARP/ND
mitigation is used in this scenario, the IXP can still take advantage
of EVPN features such as control plane learning and all-active
multihoming in the peering network. Existing mitigation solutions,
such as the ARP-Sponge daemon [ARP-Sponge] MAY also be used in this
scenario.
Although this option does not require any of the procedures described
in this document, it is added as baseline/default option for
completeness. This option is equivalent to VPLS as far as ARP/ND is
concerned. The options described in 6.2, 6.3 and 6.4 are only
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possible in EVPN networks in combination with their Proxy-ARP/ND
capabilities.
6.2. Dynamic Learning with Proxy-ARP/ND
This scenario minimizes flooding while enabling dynamic learning of
IP->MAC entries. The Proxy-ARP/ND function is enabled in the BDs of
the EVPN PEs, so that the PEs intercept and respond to CE requests.
The solution MAY further reduce the flooding of the ARP/ND messages
in the EVPN network by snooping ARP/ND messages issued by the CEs.
PEs will flood requests if the entry is not in their Proxy table. Any
unknown source MAC->IP entries will be learnt and advertised in EVPN,
and traffic to unknown entries is discarded at the ingress PE.
6.3. Hybrid Dynamic Learning and Static Provisioning with Proxy-ARP/ND
Some IXPs want to protect particular hosts on the peering network
while allowing dynamic learning of peering router addresses. For
example, an IXP may want to configure static MAC->IP entries for
management and infrastructure hosts that provide critical services.
In this scenario, static entries are provisioned from the management
plane for protected MAC->IP addresses, and dynamic learning with
Proxy-ARP/ND is enabled as described in section 6.2 on the peering
network.
6.4 All Static Provisioning with Proxy-ARP/ND
For a solution that maximizes security and eliminates flooding and
unknown unicast in the peering network, all MAC-IP entries are
provisioned from the management plane. The Proxy-ARP/ND function is
enabled in the BDs of the EVPN PEs, so that the PEs intercept and
respond to CE requests. Dynamic learning and ARP/ND snooping is
disabled so that traffic to unknown entries is discarded at the
ingress PE. This scenario provides an IXP the most control over
MAC->IP entries and allows an IXP to manage all entries from a
management system.
6.5 Deployment Scenarios in IXPs
Nowadays, almost all IXPs installed some security rules in order to
protect the IXP-LAN. These rules are often called port security. Port
security summarizes different operational steps that limit the access
to the IXP-LAN, to the customer router and controls the kind of
traffic that the routers are allowed to be exchange (e.g., Ethernet,
IPv4, IPv6). Due to this, the deployment scenario as described in 6.4
"All Static Provisioning with Proxy-ARP/ND" is the predominant
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scenario for IXPs.
In addition to the "All Static Provisioning" behavior, in IXP
networks it is recommended to configure the Reply Sub-Function to
'discard' ARP-Requests/NS messages with unrecognized options.
At IXPs, customers usually follow a certain operational life-cycle.
For each step of the operational life-cycle specific operational
procedures are executed.
The following describes the operational procedures that are needed to
guarantee port security throughout the life-cycle of a customer with
focus on EVPN features:
1. A new customer is connected the first time to the IXP:
Before the connection between the customer router and the IXP-LAN
is activated, the MAC of the router is white-listed on the IXP's
switch port. All other MAC addresses are blocked. Pre-defined IPv4
and IPv6 addresses of the IXP's peering network space are
configured at the customer router. The IP->MAC static entries
(IPv4 and IPv6) are configured in the management system of the IXP
for the customer's port in order to support Proxy-ARP/ND.
In case a customer uses multiple ports aggregated to a single
logical port (LAG) some vendors randomly select the MAC address of
the LAG from the different MAC addresses assigned to the ports. In
this case the static entry will be used associated to a list of
allowed MACs.
2. Replacement of customer router:
If a customer router is about to be replaced, the new MAC
address(es) must be installed in the management system besides the
MAC address(es) of the currently connected router. This allows the
customer to replace the router without any active involvement of
the IXP operator. For this, static entries are also used. After
the replacement takes place, the MAC address(es) of the replaced
router can be removed.
3. Decommissioning a customer router
If a customer router is decommissioned, the router is disconnected
from the IXP PE. Right after that, the MAC address(es) of the
router and IP->MAC bindings can be removed from the management
system.
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6.6 Deployment Scenarios in DCs
DCs normally have different requirements than IXPs in terms of Proxy-
ARP/ND. Some differences are listed below:
a) The required mobility in virtualized DCs makes the "Dynamic
Learning" or "Hybrid Dynamic and Static Provisioning" models more
appropriate than the "All Static Provisioning" model.
b) IPv6 'anycast' may be required in DCs, while it is not a
requirement in IXP networks. Therefore if the DC needs IPv6
'anycast' it will be explicitly enabled in the Proxy-ND function,
hence the Proxy-ND sub-functions modified accordingly. For
instance, if IPv6 'anycast' is enabled in the Proxy-ND function,
Duplicate IP Detection must be disabled.
c) DCs may require special options on ARP/ND as opposed to the
Address Resolution function, which is the only one typically
required in IXPs. Based on that, the Reply Sub-function may be
modified to forward or discard unknown options.
7. Security Considerations
The procedures in this document reduce the amount of ARP/ND message
flooding, which in itself provides a protection to "slow path"
software processors of routers and Tenant Systems in large BDs. The
ARP/ND requests that are replied by the Proxy-ARP/ND function (hence
not flooded) are normally targeted to existing hosts in the BD.
ARP/ND requests targeted to absent hosts are still normally flooded,
however the suppression of Unknown ARP-Requests and NS messages
described in Section 4.5. can provide an additional level of security
against ARP-Requests/NS messages issued to non-existing hosts.
The solution also provides protection against Denial Of Service
attacks that use ARP/ND-spoofing as a first step. The Duplicate IP
Detection and the use of an AS-MAC as explained in Section 4.6. will
definitely protect the BD against ARP/ND spoofing.
When EVPN and its associated Proxy-ARP/ND function are used in IXP
networks, they provide ARP/ND security and mitigation. IXPs MUST
still employ additional security mechanisms that protect the peering
network and SHOULD follow established BCPs such as the ones described
in [Euro-IX BCP].
For example, IXPs should disable all unneeded control protocols, and
block unwanted protocols from CEs so that only IPv4, ARP and IPv6
Ethertypes are permitted on the peering network. In addition, port
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security features and ACLs can provide an additional level of
security.
8. IANA Considerations
No IANA considerations.
9. References
9.1. Normative References
[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based Ethernet
VPN", RFC 7432, DOI 10.17487/RFC7432, February 2015,
<https://www.rfc-editor.org/info/rfc7432>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI
10.17487/RFC4861, September 2007, <https://www.rfc-
editor.org/info/rfc4861>.
[RFC0826] Plummer, D., "Ethernet Address Resolution Protocol: Or
Converting Network Protocol Addresses to 48.bit Ethernet Address for
Transmission on Ethernet Hardware", STD 37, RFC 826, DOI
10.17487/RFC0826, November 1982, <https://www.rfc-
editor.org/info/rfc826>.
[RFC6820] Narten, T., Karir, M., and I. Foo, "Address Resolution
Problems in Large Data Center Networks", RFC 6820, DOI
10.17487/RFC6820, January 2013, <https://www.rfc-
editor.org/info/rfc6820>.
[RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971, DOI 10.17487/RFC3971,
March 2005, <https://www.rfc-editor.org/info/rfc3971>.
[RFC5227] Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227,
DOI 10.17487/RFC5227, July 2008, <https://www.rfc-
editor.org/info/rfc5227>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March
1997, <http://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC2119
Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017,
<http://www.rfc-editor.org/info/rfc8174>.
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9.2. Informative References
[ARP-Sponge] Wessel M. and Sijm N., Universiteit van Amsterdam,
"Effects of IPv4 and IPv6 address resolution on AMS-IX and the ARP
Sponge", July 2009.
[EVPN-ARP-ND-FLAGS] Sathappan S., Nagaraj K. and Rabadan J.,
"Propagation of ARP/ND Flags in EVPN", draft-ietf-bess-evpn-na-flags-
04, Work in Progress, July 2019.
[Euro-IX BCP] https://www.euro-ix.net/pages/28/1/bcp_ixp.html
10. Acknowledgments
The authors want to thank Ranganathan Boovaraghavan, Sriram
Venkateswaran, Manish Krishnan, Seshagiri Venugopal, Tony Przygienda,
Robert Raszuk and Iftekhar Hussain for their review and
contributions. Thank you to Oliver Knapp as well, for his detailed
review.
11. Contributors
In addition to the authors listed on the front page, the following
co-authors have also contributed to this document:
Wim Henderickx
Nokia
Daniel Melzer
DE-CIX Management GmbH
Erik Nordmark
Zededa
Authors' Addresses
Jorge Rabadan (Editor)
Nokia
777 E. Middlefield Road
Mountain View, CA 94043 USA
Email: jorge.rabadan@nokia.com
Senthil Sathappan
Nokia
Email: senthil.sathappan@nokia.com
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Kiran Nagaraj
Nokia
Email: kiran.nagaraj@nokia.com
Greg Hankins
Nokia
Email: greg.hankins@nokia.com
Thomas King
DE-CIX Management GmbH
Lichtstrasse 43i, Cologne 50825, Germany
Email: thomas.king@de-cix.net
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