Network working group X. Xu
Internet Draft S. Hares
Category: Informational Huawei Technologies
Expires: September 2012 March 12, 2012
Virtual Subnet: A Host Route based Subnet Extension Solution
draft-xu-virtual-subnet-07
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Abstract
This document describes a host route based subnet extension solution
referred to as Virtual Subnet, which mainly reuses existing BGP/MPLS
IP VPN [RFC4364] and ARP proxy [RFC925][RFC1027] technologies.
Virtual Subnet provides a scalable approach for interconnecting
geographically dispersed cloud data centers.
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 [RFC2119].
Table of Contents
1. Introduction ................................................ 3
2. Terminology ................................................. 5
3. Solution Description......................................... 5
3.1. Unicast ................................................ 5
3.1.1. Intra-subnet Unicast .............................. 5
3.1.2. Inter-subnet Unicast .............................. 6
3.2. Multicast/Broadcast .................................... 7
3.3. CE Host Discovery ...................................... 7
3.4. ARP Proxy .............................................. 8
3.5. CE Host Mobility ....................................... 8
3.6. Forwarding Table Scalability ........................... 8
3.6.1. MAC Table Reduction on DC Switches ................ 8
3.6.2. FIB Reduction on PE Routers ....................... 9
3.6.3. RIB Reduction on PE Routers ....................... 9
3.7. ARP Table Scalability on DC Gateways .................. 10
3.8. ARP/Unknown Uncast Flood Avoidance .................... 11
3.9. Active-active Multi-homing ............................ 11
3.10. Path Optimization .................................... 11
4. Future Work ................................................ 11
5. Security Considerations .................................... 11
6. IANA Considerations ........................................ 11
7. Acknowledgements ........................................... 12
8. References ................................................. 12
8.1. Normative References .................................. 12
8.2. Informative References ................................ 12
Authors' Addresses ............................................ 13
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1. Introduction
To achieve business continuance and disaster prevention, Virtual
Machine (VM) migration across data centers is increasingly adopted
in today's data centers. For instance, VM migration across data
centers can be used in many scenarios such as data center
maintenance, data center migration, data center consolidation, data
center expansion, or data center disaster avoidance.
For interconnecting multiple Infrastructure-as-a-Service (IaaS)
cloud data centers in a scalable manner, the following requirements
provided by data center and cloud operation groups SHOULD be
considered for any solution:
1) Subnet Extension
To allow seamlessly migration of a VM from one data center to
another without IP renumbering, the subnet that the VM belongs to
SHOULD be extended across those data centers.
2) VPN Instance Scalability
In a modern cloud data center environment, thousands of even tens
of thousands of tenants could be hosted over a shared network
infrastructure. For security and performance isolation
considerations, these tenants SHOULD be isolated from one another.
Hence, the data center interconnect solution SHOULD be capable of
providing a large enough VPN space, at least beyond 4K VPN
instances, for tenant isolation.
3) Forwarding Table Scalability
With the development of virtualization technologies, a single
cloud data center containing millions of VMs is not uncommon
today. [Gashinsky] noted in a NANOG 52 presentation there are 2.4
million VMs in a single data center. This number already implies
a big challenge for data center switches from the perspective of
forwarding table scalability. Hence an ideal data center
interconnect solution SHOULD avoid the forwarding table size from
growing by folds as the number of data centers to be
interconnected increases. Furthermore, the use of existing L2VPN
or L3VPN technology for interconnecting data centers needs to
take into account the scale of forwarding tables of PE routers.
4) ARP Table Scalability on DC Gateways
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[NARTEN-ARMD] notes that the ARP table on data center gateway
routers can be a "pain" point for cloud data centers that
contains millions of VMs. Therefore, an ideal data center
interconnection solution SHOULD avoid the ARP table size from
growing by multiples as the number of data centers to be
connected increases.
5) ARP/Unknown Unicast Flood Suppression or Avoidance
It's well-known that the flooding of ARP broadcast and unknown
unicast traffic within large Layer2 networks will lead to certain
performance impact on both networks and hosts. As multiple data
centers each containing millions of VMs are interconnected
together across the WAN, and more importantly, the bandwidth of
which is usually insufficient and costly, the problems mentioned
above will become even worse. As such, how to suppress or even
avoid the flooding of ARP broadcast and unknown unicast traffic
across data centers becomes increasingly desirable for avoiding
unnecessary consumption of the bandwidth resource and the CPU
resource.
6) Active-active Multi-homing
In order to utilize the bandwidth of all available upstream paths
while providing resilient connectivity between the data center
and the transport network, active-active multi-homing is
increasingly advocated by data center operators as a replacement
of the traditional active-standby multi-homing approach.
7) Path Optimization
A subnet usually represents a location in the network. However,
in a subnet that has been extended across multiple geographically
dispersed data centers, the location semantics of subnets are
lost. As a result, the client-to-server traffic between
enterprise applications (i.e. cloud users), may travel via sub-
optimal paths based on subnet routes. Furthermore, this traffic
may travel across the expensive cross-data center links.
This document describes a host route based subnet extension solution
referred to as Virtual Subnet (VS), which meets the requirements for
cloud data center interconnect as described above. Since VS mainly
reuses existing technologies including BGP/MPLS IP VPN [RFC4364] and
ARP proxy [RFC925][RFC1027], it allows service providers that are
offering IaaS cloud services to realize a scalable data center
interconnection based on their existing reliable MPLS/IP VPN
infrastructures and provisioning experiences.
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Please note that VS is only targeted at scenarios where the traffic
across data centers is just routable IP traffic. In such scenarios,
data center operators could benefit from the advantages of this host
route based data center interconnect solution over traditional VPLS
solutions [RFC 4761, RFC4762]. For instance, in VS, the flooding
domain associated with the IP subnet that has been extended across
multiple data centers, is confined into each data center and
therefore the performance impact on networks and servers imposed by
the flooding of ARP broadcast and unknown unicast traffic is greatly
alleviated. In addition, the forwarding table scalability of data
center switches and PE routers, and the ARP table scalability of
data center gateways are greatly improved as well.
2. Terminology
This memo makes use of the terms defined in [RFC4364], [RFC2338]
[MVPN] and [VA-AUTO].
3. Solution Description
3.1. Unicast
3.1.1. Intra-subnet Unicast
+--------------------+
+-----------------+ | | +------------------+
|VPN_A:10.0.0.0/8 | | | |VPN_A:10.0.0.0/8 |
| | | | | |
| +------+ ++---+-+ +-+---++ +------+ |
| |Host A+----+ PE-1 | | PE-2 +----+Host B| |
| +------+ ++-+-+-+ +-+-+-++ +------+ |
| 10.1.1.1/8 | | | IP/MPLS Backbone | | | 10.1.1.2/8 |
+-----------------+ | | | | +------------------+
| +--------------------+ |
| |
V V
+-------+------------+--------+ +-------+------------+--------+
|VRF ID |Destination |Next Hop| |VRF ID |Destination |Next Hop|
+-------+------------+--------+ +-------+------------+--------+
| VPN_A |10.1.1.1/32 | Local | | VPN_A |10.1.1.2/32 | Local |
+-------+------------+--------+ +-------+------------+--------+
| VPN_A |10.1.1.2/32 | PE-2 | | VPN_A |10.1.1.1/32 | PE-1 |
+-------+------------+--------+ +-------+------------+--------+
Figure 1: Intra-subnet Unicast
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As shown in Figure 1, CE hosts dispersed across different data
centers are actually within a single subnet (e.g., 10.0.0.0/8). PE
routers discover their locally connected CE hosts by some means such
as ARP or ICMP scan, and then create host routes for these CE hosts.
These host routes are distributed across PE routers by using the
existing BGP/MPLS IP VPN signaling. Meanwhile, ARP proxy is enabled
on each PE router. Thus, upon receiving from a local CE host an ARP
request for a remote CE host, PE router would return its own MAC
address as a response.
Assume host A sends an ARP request for host B before communicating
with each other. Upon receiving the ARP request, PE-1 lookups the
associated VRF table to find the corresponding host route for the
target host (i.e., host B). If found, PE-1 as an ARP proxy returns
its own MAC address to the above ARP request. Host A will then send
IP packets for host B towards PE-1, which in turn forwards the
received packets towards PE-2 according to existing L3VPN forwarding
procedures. PE-2 then forwards received packets to host B as normal.
3.1.2. Inter-subnet Unicast
+--------------------+
+-----------------+ | | +-----------------+
|VPN_A:10.0.0.0/8 | | | |VPN_A:10.0.0.0/8 |
| | | | | |
| +------+ ++---+-+ +-+---++ +-------++
| |Host A+----+ PE-1 | | PE-2 +--------+ GW |
| +------+ ++-+-+-+ +-+-+-++ +-------++
| 10.1.1.1/8 | | | IP/MPLS Backbone | | | 10.1.1.2/8 |
+-----------------+ | | | | +-----------------+
| +--------------------+ |
| |
V V
+-------+------------+--------+ +-------+------------+--------+
|VRF ID |Destination |Next Hop| |VRF ID |Destination |Next Hop|
+-------+------------+--------+ +-------+------------+--------+
| VPN_A |10.1.1.1/32 | Local | | VPN_A |10.1.1.2/32 | Local |
+-------+------------+--------+ +-------+------------+--------+
| VPN_A |10.1.1.2/32 | PE-2 | | VPN_A |10.1.1.1/32 | PE-1 |
+-------+------------+--------+ +-------+------------+--------+
| VPN_A |10.0.0.0/8 | NULL | | VPN_A |10.0.0.0/8 | NULL |
+-------+------------+--------+ +-------+------------+--------+
| VPN_A |0.0.0.0/0 | PE-2 | | VPN_A |0.0.0.0/0 | GW |
+-------+------------+--------+ +-------+------------+--------+
Figure 2: Inter-subnet Unicast
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As shown in Figure 2, to allow a CE host (e.g., host A) to
communicate with other hosts outside its own subnet, a PE router
(e.g., PE-2) which is connected to a CE gateway router (e.g., GW)
would either be configured with or learn from that CE gateway router
at least one route for the outside networks (e.g., a default route)
with next-hop pointing to that CE gateway router, and this route
would be distributed to other PE routers.
Now host A sends an ARP request for its default gateway (i.e., GW)
before communicating with a destination host outside its subnet.
Upon receiving this ARP request, PE-1 as an ARP proxy returns its
own MAC address as a response according to the same procedure as
described in the above section. Upon receiving a packet destined for
the outside from host A, PE-1 forwards it towards PE-2 according to
the default route, which in turn forwards that packet to GW
according to the default route as well.
To avoid forwarding packets destined for nonexistent hosts within
the scope of their configured VPN subnet mistakenly according to the
default route, PE routers each are suggested to be configured with a
null route for that VPN subnet.
3.2. Multicast/Broadcast
To support IP multicast and broadcast between CE hosts of the same
VPN instance, the MVPN technology [MVPN] could be reused in VS. For
example, PE routers attached to a given VPN join a default provider
multicast distribution tree which is dedicated for that VPN. Ingress
PE routers, upon receiving customer multicast or broadcast traffic
from their local CE hosts, forward such customer traffic towards
remote PE routers of the same VPN through the corresponding default
provider multicast distribution tree. More details about how to
support multicast and broadcast in VS will be explored in a later
version of this document.
3.3. CE Host Discovery
PE routers must be able to discovery their locally connected CE
hosts and keep the list of local CE hosts up to date in a timely
manner so as to keep the availability of the host route information,
especially after its rebooting up. PE routers could accomplish local
CE host discovery by some traditional host discovery means such as
ARP scan or ICMP scan. To keep their ARP cache fresh while avoiding
too frequent ARP/ICMP scan behaviors, PE routers could send unicast
ARP requests to those already discovered local CE hosts at shorter
intervals as a complement of the regular ARP or ICMP scan at longer
intervals.
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Furthermore, Link Layer Discovery Protocol (LLDP) described in
[802.1AB] or VSI Discovery and Configuration Protocol (VDP)
described in [802.1Qbg], or even interaction with the data center
orchestration system could also be considered as a means for local
CE host discovery.
3.4. ARP Proxy
A PE router as an ARP proxy SHOULD only respond to ARP requests for
those CE hosts for which there are matching host routes and those
host routes are learnt from remote PE routers. In other words, the
PE router SHOULD not respond to ARP requests for its local CE hosts
or those nonexistent CE hosts within the configured subnet (i.e., no
matching host routes found). In the CE multi-homing scenario where
VRRP is enabled on PE routers, only the PE router which has been
elected as the VRRP master is entitled to perform the ARP proxy
function.
3.5. CE Host Mobility
Once a CE host (e.g., a VM) moves from one VPN site to another, it
will usually send out a gratuitous ARP request or reply when
attaching to the new VPN site. Upon receiving that gratuitous ARP
packet, the PE router attached to the new VPN site will create a
host route for that CE host and then advertise that route to remote
PE routers. When the old PE router to which that CE host was
previously attached, receives the route update for that CE host, it
would immediately send an ARP request or ICMP echo request for that
CE host locally to check whether or not that CE host is still
locally connected to it. If no response is returned within a certain
period of time, the PE router would delete the ARP cache entry of
that CE host and withdraw the corresponding host route. Meanwhile,
the PE router would broadcast a gratuitous ARP packet on behalf of
that CE host, with the sender hardware address field being filled
with its own MAC addresses. As such, the ARP entry for that CE host
that is cached on any local CE host would be updated accordingly.
3.6. Forwarding Table Scalability
3.6.1. MAC Table Reduction on DC Switches
Since the MAC learning domain associated with a given subnet that
has been extended across multiple data centers, has been partitioned
and confined within each data center location, CE switches in data
centers only needs to learn MAC addresses of local CE hosts, without
any need for learning MAC addresses of remote CE hosts.
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3.6.2. FIB Reduction on PE Routers
To suppress the FIB of PE routers, VS reuses the Virtual Aggregation
[VA-AUTO] technology to achieve on-demand FIB installation.
The detailed procedures are described as follows:
1) The extended subnet is configured as a Virtual Prefix (VP) and a
Route-Reflector (RR) is configured as an Aggregation Point Router
(APR) for that VP. PE routers as RR clients advertise host routes
for their local CE hosts to the RR which in turn, as an APR,
installs those host routes into FIB and then attach the "can-
suppress" tag to those host routes before reflecting them to its
clients. Those host routes attached with that tag will not be
installed into FIB by clients since they are not APRs for those
host routes. In addition, the RR as an APR would advertise a VP
route (i.e., a route for the extended subnet) to all of its
clients which in turn would install this VP route into FIB as
normal. Upon receiving a packet from a local CE host, if no
matching host route found, the packet will be forwarded by the
ingress PE router to the RR according to the subnet route learnt
from the RR, which in turn forwards the packet to the egress PE
router according to the matching host route learnt from that
egress PE router. In a word, the FIB table size of PE routers can
be greatly reduced at the cost of path stretch.
2) Provided an ARP request for a remote CE host within the extended
subnet is received from a given local CE host, the ingress PE
router will immediately install the corresponding host route for
that remote CE host into FIB, in case that there exists a host
route for that CE host in the RIB but it has not yet been
installed into FIB. Therefore, the forwarding path for the traffic
destined for that remote CE host is optimized. Note that the FIB
entries of remote host routes could expire if there has been no
traffic using them for a certain period of time.
3.6.3. RIB Reduction on PE Routers
To suppress the RIB of PE routers, VS reuses the BGP Outbound Route
Filtering (ORF) mechanism to realize on-demand route announcement.
Note that to use this RIB reduction approach, the ARP proxy
functionality described before SHOULD be changed as: a PE router as
an ARP proxy SHOULD respond to the ARP request for any CE host
within the extended subnet, as long as the requested CE host is not
a local CE host.
The detailed procedures are described as follows:
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1) PE routers as RR clients advertise host routes for their local CE
hosts to a RR which in turn, however doesn't reflect these host
routes by default unless it receives corresponding ORF requests
for them from its clients. The RR is configured with a null route
for the extended subnet and that route is then advertised to all
of its clients. Upon receiving a packet from a local CE host, if
no matching host route found, the packet will be forwarded by the
ingress PE router to the RR according to the subnet route learnt
from the RR, which in turn forwards the packet to the egress PE
router according to the matching host route learnt from that
egress PE router. In a word, the RIB table size of PE routers can
be greatly reduced at the cost of path stretch.
2) Provided an ARP request for a remote CE host within the extended
subnet is received from a given local CE host, the ingress PE
router will request the corresponding host route from its RR by
using ORF (e.g., a group ORF containing Route-Target (RT) and
prefix information) in case that there is no host route for that
CE host yet in its RIB. Once the host route for the remote CE host
is received from the RR, the subsequent packets destined for that
CE host would be forwarded directly from the ingress PE router to
the egress PE router. Note that the RIB entries of remote host
routes could expire if there has been no traffic using their
corresponding FIB entries for a certain period of time. Once the
expiration time for a given RIB entry is approaching, the PE
router would notice its RR to withdraw the corresponding host
route by sending an ORF message. Upon receiving the corresponding
withdraw message from its RR, the PE router will delete that host
route from its RIB as normal.
3.7. ARP Table Scalability on DC Gateways
In VS, DC gateways are directly connected to PE routers of the VS.
Due to the usage of ARP proxy on PE routers, all CE hosts of a given
VPN subnet share one MAC address (i.e., the MAC address of the
connected PE router) from the DC gateways' points of view. Therefore,
ARP entries for all CE hosts of a given VPN subnet can be aggregated
into a single ARP entry (e.g., 10.0.0.0/8-> the MAC address of the
PE router). Accordingly, DC gateways are required to use the longest-
matching algorithm for ARP cache lookup instead of the existing
exact-matching algorithm. In this way, the ARP table size of DC
gateways can be reduced by several orders of magnitude.
The side-effect introduced by this approach is DC gateways could
possibly send packets destined for non-existing CE hosts to their
connected PE routers of the VS. Fortunately, once those packets
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arrive at the PE routers, which in turn will drop them due to route
missing.
3.8. ARP/Unknown Uncast Flood Avoidance
In VS, the flooding domain associated with a given subnet that has
been extended across multiple data centers, has been partitioned and
confined within each data center. Therefore, the performance impact
on networks and servers caused by the flooding of ARP broadcast and
unknown unicast traffic is alleviated for the IP-only traffic case.
3.9. Active-active Multi-homing
For the PE router redundancy purpose, a VPN site could be connected
to more than one PE router. In this case, VRRP [RFC2338] SHOULD be
enabled on these PE routers and only the PE router which has been
elected as the VRRP master could perform the ARP proxy functionality.
However, all PE routers, either as a VRRP master or a VRRP slave,
are allowed to perform the host discovery procedure and accordingly
advertise host routes for their local CE hosts. Hence, from the
perspective of remote PE routers, there will be multiple host routes
for a given CE host located within that multi-homed VPN site. In
other words, active-active multi-homing is available for the inbound
traffic of a given multi-homed VPN site.
3.10. Path Optimization
To optimize the path for traffic between enterprise VPN sites (e.g.,
cloud users) and cloud data centers, host routes for CE hosts within
cloud data centers could be directly propagated to the enterprise
VPN sites.
4. Future Work
How to support IPv6 CE hosts in VS is for future study.
5. Security Considerations
TBD.
6. IANA Considerations
There is no requirement for IANA.
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7. Acknowledgements
Thanks to Dino Farinacci, Himanshu Shah, Nabil Bitar, Giles Heron,
Ronald Bonica and Monique Morrow for their valuable comments and
suggestions on this document.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[RFC4364] Rosen. E and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[MVPN] Rosen. E and Aggarwal. R, "Multicast in MPLS/BGP IP VPNs",
draft-ietf-l3vpn-2547bis-mcast-10.txt, Work in Progress,
Janurary 2010.
[VA-AUTO] Francis, P., Xu, X., Ballani, H., Jen, D., Raszuk, R., and
L. Zhang, "Auto-Configuration in Virtual Aggregation",
draft-ietf-grow-va-auto-05.txt, Work in Progress, December
2011.
[RFC925] Postel, J., "Multi-LAN Address Resolution", RFC-925, USC
Information Sciences Institute, October 1984.
[RFC1027] Smoot Carl-Mitchell, John S. Quarterman, "Using ARP to
Implement Transparent Subnet Gateways", RFC 1027, October
1987.
[RFC2338] Knight, S., et. al., "Virtual Router Redundancy Protocol",
RFC 2338, April 1998.
[RFC4761] Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
(VPLS) Using BGP for Auto-Discovery and Signaling", RFC
4761, January 2007.
[RFC4762] Lasserre, M. and V. Kompella, "Virtual Private LAN Service
(VPLS) Using Label Distribution Protocol (LDP) Signaling",
RFC 4762, January 2007.
[802.1AB] IEEE Standard 802.1AB-2009, "Station and Media Access
Control Connectivity Discovery", September 17, 2009.
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[802.1Qbg] IEEE Draft Standard P802.1Qbg/D2.0, "Virtual Bridged
Local Area Networks -Amendment XX: Edge Virtual Bridging",
Work in Progress, December 1, 2011.
[NARTEN-ARMD] Narten, T., Karir, M., and I. Foo, "Problem Statement
for ARMD", draft-ietf-armd-problem-statement-01.txt, Work
in Progress, February 2012.
[Gashinsky] Igor Gashinsky, "Data Center Scalability Panel",
http://www.nanog.org/meetings/nanog52/presentations/Tuesda
y/Gashinsky-3-Y-Datacenter-scalability.pdf, June 14, 2010,
Authors' Addresses
Xiaohu Xu
Huawei Technologies,
Beijing, China.
Phone: +86 10 60610041
Email: xuxiaohu@huawei.com
Susan Hares
Huawei Technologies (FutureWei group)
2330 Central Expressway
Santa Clara, CA 95050
Phone: +1-734-604-0332
Email: Susan.Hares@huawei.com
shares@ndzh.com
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