Network working group X. Xu
Internet Draft Huawei Technologies
Category: Standard Track
Expires: July 2011 January 24, 2011
Virtual Subnet: A Scalable Data Center Network Architecture
draft-xu-virtual-subnet-04
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Abstract
This document proposes a new IP-only L2VPN solution which uses
BGP/MPLS IP VPN technology [RFC4364] with some extensions, together
with some other proven technologies including ARP proxy
[RFC925][RFC1027] to provide a more scalable IP-only L2VPN services
across a MPLS/IP backbone. This solution is intended to be a
scalable data center network architecture which can be deployed
today as an alternative to the spanning tree protocol bridge
technology.
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. Problem Statement...........................................3
2. Terminology.................................................3
3. Design Goals................................................3
4. Architecture Description....................................4
4.1. Unicast................................................5
4.1.1. Unicast inside a Service Domain...................5
4.1.2. Unicast outside a Service Domain..................6
4.2. Multicast/Broadcast....................................7
4.3. CE Host Discovery......................................8
4.4. CE Host Multi-homing and Mobility......................8
4.5. APR Proxy..............................................9
4.6. DHCP Relay Agent.......................................9
5. Comparison..................................................9
5.1. VS vs VPLS.............................................9
5.2. VS vs IPLS............................................10
6. Conclusion.................................................11
7. Future work................................................11
8. Security Considerations....................................11
9. IANA Considerations........................................11
10. Acknowledgements..........................................11
11. References................................................12
11.1. Normative References.................................12
11.2. Informative References...............................12
Authors' Addresses............................................13
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1. Problem Statement
With the popularity of cloud services, the scale of today's data
centers expands larger and larger. In addition, virtual machine
migration technology, which allows a virtual machine to be able to
migrate to any physical server while keeping the same IP address, is
becoming more and more prevalent for achieving service agility in
data centers. As a result, large Layer 2 networks are needed for
server-to-server connectivity. Meanwhile, due to the huge-volume
traffic exchanged between servers, the Layer 2 networks SHOULD
provide enough capacity for server-to-server interconnections.
Unfortunately, today's data center network using the Spanning-Tree
Protocol (STP) bridge technology, can not address the above
challenges facing today's large-scale data centers in several ways.
First, STP can calculate out only one single forwarding tree for all
connected servers of a particular Virtual Local Area Network (VLAN)
and it can not support multi-path routing, e.g., Equal Cost Multi-
Path (ECMP), hence the available network capacity in data center
networks can't be highly utilized so as to provide enough bandwidth
between servers; Second, since the bridge forwarding is based on the
flat MAC addresses, the scalability of the bridge forwarding table
would become a big issue, especially when the existing large Layer 2
network scales even larger; Third, broadcast storm impacts imposed
by some protocols, e.g., Address Resolution Protocol (ARP) and the
flooding of unknown destination unicast frames become much more
serious and unpredictable in the continually growing large-scale STP
bridge networks.
2. Terminology
This memo makes use of the terms defined in [RFC4364], [MVPN],
[RFC2236] and [RFC2131]. Below are provided terms specific to this
document:
- Service Domain: A group of servers which are dedicated for a
given service and are usually located on a separate IP subnet.
3. Design Goals
To overcome the limitations of the STP bridge networks as mentioned
above, this document describes Virtual Subnet (VS), a new IP-only
L2VPN solution which is intended to be a practical and scalable data
center network architecture meeting the following objectives:
- Bandwidth Utilization Maximization
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To provide enough bandwidth between servers, the server-to-server
traffic SHOULD always be delivered along the shortest paths while
multi-path routing is used for load-balancing purpose.
- Layer 2 Connectivity
To be backwards compatible with existing applications and protocols
running in today's data centers (e.g., virtual machine migration),
servers of a given service domain SHOULD be connected as if they
were on a Local Area Network (LAN) or an IP subnet.
- Domain Isolation
To achieve performance and security isolation, servers belonging to
different service domains SHOULD be isolated just as if they were
located on separate Virtual LANs (VLAN) or IP subnets.
- Forwarding Table Scalability
To accommodate tens to hundreds of thousands of servers in a single
data center network, the forwarding tables of those forwarding
devices (e.g., routers or bridges) SHOULD be scalable enough.
- Broadcast Storm Suppression
To alleviate the serious impacts on network performance which are
imposed by broadcast storms, broadcast domains SHOULD be limited to
their smallest scopes.
4. Architecture Description
VS uses BGP/MPLS IP VPN technology [RFC4364] with some extensions,
together with other proven technologies including ARP proxy
[RFC925][RFC1027] to provide scalable IP-only L2VPN services across
a MPLS/IP backbone.
Since VS constructs large-scale IP subnets, rather than real LANs,
across the MPLS/IP backbone, the non-IP traffic would not be
supported in VS anymore. However, given that IP traffic is the
predominant type of traffic in today's data center networks and the
non-IP traffic will disappear from the data center networks with the
elapse of time, we believe that VS can be used as a practical data
center network solution in most cases.
The following sections describe VS in detail.
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4.1. Unicast
4.1.1. Unicast inside a Service Domain
+--------------------+
+-----------------+ | | +------------------+
|VPN_A:10/8 | | | |VPN_A:10/8 |
| | | | | |
| +------+ ++---+-+ +-+---++ +------+ |
| |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 |
+-------+------------+--------+ +-------+------------+--------+
| VPN_A |10.0.0.0/8 | NULL | | VPN_A |10.0.0.0/8 | NULL |
+-------+------------+--------+ +-------+------------+--------+
Figure 1: Unicast inside a Service Domain
As shown in Figure 1, BGP/MPLS IP VPN technology with some
extensions is deployed in a data center network. To maintain proper
isolation of one service domain from another, each service domain is
mapped to a distinct VPN and servers of a given service domain, as
Customer Edge (CE) hosts, are attached to Provider Edge (PE) routers
directly or through one or more Ethernet bridges. In addition, to
build large IP subnets across the MPLS/IP backbone, different sites
of a particular VPN are associated with an identical IP subnet. PE
routers create host routes for their local CE hosts automatically
according to the corresponding ARP entries. Instead of distributing
the routes for the configured VPN subnets, PE routers distribute
host routes for their local CE hosts to each other. In addition,
each PE router automatically creates a route for the configured VPN
subnet whose next-hop is pointed to a null interface. With such
special route, packets destined for the nonexistent hosts of that
subnet will be discarded directly by the ingress PE routers. APR
proxy is implemented on PE routers for every attached VPN, thus,
upon receiving from a local CE host an ARP request for a remote CE
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host, the PE as an ARP proxy returns its own MAC address as a
response.
Assume host A broadcasts an ARP request for host B before
communicating with B, upon the receipt of this ARP request, PE-1
lookups the associated VRF to find the host route for B. If found
and the route is learnt from a remote PE, PE-1 acting as an ARP
proxy returns its own MAC address in the response to that ARP
request. Otherwise, no ARP reply SHOULD be sent. After obtaining the
ARP reply from PE-1, A sends an IP packet to B with destination MAC
address of PE-1's MAC address. Upon receiving this packet, PE-1
acting as an ingress PE, tunnels the packet towards PE-2 which in
turn, as an egress PE, forwards the packet to B.
4.1.2. Unicast outside a Service Domain
+--------------------+
+-----------------+ | | +-------------+
|VPN_A:10/8 | | | |VPN_A:10/8 |
| | | | | |
| +------+ ++---+-+ +-+---++ +----+--+
| |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 |0.0.0.0/8 | GW |
+-------+------------+--------+ +-------+------------+--------+
Figure 2: Unicast between Service Domains
As shown in Figure 2, for a CE host (e.g., host A) to communicate
with other hosts outside its subnet, the PE router (e.g., PE-2)
which is connected to the default gateway router (e.g., GW) for that
VPN SHOULD be configured with a default route with the next-hop of
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the default gateway router and then advertise such default route to
other PE routers of the same VPN. Now host A sends an ARP request
for its default gateway (i.e., GW) before sending a packet to a host
outside its subnet. Upon receiving this ARP request, PE-1 lookups
the associated VRF to find the host route for GW. If found and that
found host route is learnt from a remote PE, PE-1 as an ARP proxy,
returns its own MAC address in the ARP reply. Host A then sends an
IP packet for the destination host with destination MAC address of
PE-1's MAC. Upon receiving this packet, PE-1 as an ingress PE,
tunnels it towards PE-2 according to the best route (i.e., the
default route learnt from PE-2) for the destination host. PE-2 as an
egress PE, in turn, forwards the received packet towards the default
gateway router (i.e., GW). Due to the null route for the subnet,
packets destined for those nonexistent CE hosts of that subnet would
not be mistakenly forwarded to the default gateway router of that
subnet.
In the scenario where more than one default gateway router running
Virtual Router Redundancy Protocol (VRRP) [RFC2338] is connected to
a given VPN for redundancy purpose, only the PE router which is
connected to the Virtual Router Master SHOULD be allowed to announce
a default route into that VPN. To achieve that goal, a default route
with the next-hop of the corresponding Virtual Router IP address is
configured for that VPN instance on each of the PE routers which are
connected to the VRRP routers. In addition, the default route SHOULD
not be deemed as valid until there is an active host route for its
next-hop address. Since only the Virtual Router Master is allowed to
respond to ARP requests for the Virtual Router IP address and
broadcast gratuitous ARP requests containing the Virtual Router IP
address, only the PE router which is connected to the Virtual Router
Master could have an active ARP entry for the Virtual Router IP
address and therefore could have an active host route for the
Virtual Router IP address (i.e., the next-hop address of the
configured default route). In this way, all packets destined for the
outside would be sent to the corresponding Virtual Router Master.
4.2. Multicast/Broadcast
The MVPN technology [MVPN], in particular, the Protocol-Independent-
Multicast (PIM) tree option with some extensions, is partially
reused here to support IP multicast and broadcast between CE hosts
of the same VPN. For example, PE routers attached to a given VPN
join a default provider multicast distribution tree which is
dedicated for that VPN. PE routers receiving customer multicast or
broadcast traffic from local CE hosts forward such traffic to other
remote PE routers over the corresponding default provider multicast
distribution tree. When customer multicast or broadcast traffic is
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received from a provider multicast distribution tree, PE routers
forward such traffic to the associated VRF attachment circuits.
For the customer multicast group of a particular VPN which carries
high-volume traffic and not all sites of that VPN need the traffic
of that customer multicast group, a dedicated provider multicast
distribution tree other than the default provider multicast
distribution tree for that VPN can be assigned optionally. As a
result, those PE routers of that VPN that have no local CE hosts
which are interested in that customer multicast group will not
receive such traffic from remote PE routers anymore.
More details about how to support multicast and broadcast traffic in
VS will be explored in a later version of this document.
4.3. CE Host Discovery
To discover all local CE hosts including gateway routers, PE routers
SHOULD perform at least once ARP scan on the attached VPN subnet
after rebooting. For example, a PE broadcasts an ARP request for
each IP address within the subnet of each attached VPN.
Alternatively, this PE could also broadcast an ARP request for a
directed broadcast address (i.e., 255.255.255.255) or an ALL-Systems
multicast group address (i.e., 224.0.0.1), that is to say, the
target protocol address field is filled with 2555.255.255.255 or
224.0.0.1. Any CE host receiving this ARP request SHOULD respond
with an ARP reply containing its IP and MAC addresses. After a round
of such ARP scan, the PE will discover all local CE hosts and cache
their ARP entries in its ARP table. After that, the PE could send
ARP requests in unicast to each already-learnt local CE host
periodically so as to check whether the CE host is still present on
the subnet. Using unicast ARP requests has the advantage that it is
quieter than using the broadcast because it won't be received by all
CE hosts on the subnet. When receiving a gratuitous ARP from a local
CE host, the PE SHOULD cache the ARP entry of that CE host in its
ARP table immediately if no ARP entry for that CE host exists yet.
Otherwise, the PE SHOULD just update the corresponding ARP entry of
that CE host. Most operating systems generate a gratuitous ARP
request when the host boots up, the host's network interface or
links comes up, or an address assigned to the interface changes. In
the scarce scenarios where a host does not generate a gratuitous ARP,
the PE would have to perform ARP scan periodically.
4.4. CE Host Multi-homing and Mobility
When a given PE receives a host route for one of its local CE hosts
from a remote PE, it SHOULD immediately send an ARP request for that
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CE host to the attached VPN subnet so as to determine whether that
CE host is still connected locally. If an ARP reply is received in a
short amount of time (imaging the CE host multi-homing scenario),
the PE just needs to update the ARP entry for that CE host as normal.
Otherwise (considering the virtual machine migration scenario), the
PE SHOULD delete the ARP entry corresponding to that host from its
APR table. Meanwhile, the PE SHOULD broadcast a gratuitous ARP on
the attached VPN subnet on behalf of that CE host, with the sender
hardware address field being filled with one of its own MAC
addresses. As a result, the ARP entry for that CE host which has
been cached on other local CE hosts is updated.
4.5. APR Proxy
The PE, acting as an ARP proxy, SHOULD only respond to the ARP
requests for those CE hosts which have been learnt from other remote
PE routers. Especially, the PE SHOULD not respond to ARP requests
for local CE hosts. Otherwise, in case that the ARP reply from the
PE covers that from the requested CE host, the packet for that local
CE host which is sent from another local CE would be unnecessarily
relayed by the PE.
When VRRP, together with ARP proxy is enabled on multiple PE routers
which are attached to the same VPN site, only the PE acting as
Virtual Router Master is delegated to perform ARP proxy function on
the shared VPN subnet. In addition, it SHOULD use the Virtual Router
MAC address in any ARP packet it sends.
4.6. DHCP Relay Agent
To avoid flooding Dynamic Host Configuration Protocol (DHCP)
[RFC2131] broadcast messages through the data center network, DHCP
Relay Agent can be implemented on PE routers for each attached VPN.
Thus, DHCP broadcast messages received from DHCP clients on local CE
hosts would be relayed by DHCP Relay Agents on PE routers to DHCP
servers in unicast.
5. Comparison
5.1. VS vs VPLS
Virtual Private LAN Service (VPLS) [RFC4761, RFC4762] provides
private LAN services for IP as well as other protocols. Since PE
routers in VPLS work much similar as STP bridges, broadcast storm
issues are intactly inherited from traditional STP bridge networks
to VPLS.
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At the cost of being lacking in support for non-IP traffic, VS
alleviates the broadcast storm issues by using Layer 3 routing and
ARP proxy technologies on PE routers.
In addition, if CE hosts of multiple VPNs are attached to a PE
router through an intermediate Ethernet bridge, in VPLS, this
intermediate bridge would have to learn the MAC addresses of both
local CE hosts and remote CE hosts of these attached VPNs. However,
in VS, such intermediate bridge only needs to learn MAC addresses of
local CE hosts and local PE routers due to the ARP proxy implemented
on PE routers.
5.2. VS vs IPLS
Both VS and IP LAN Service (IPLS) [IPLS] are IP only L2VPN
technologies.
However, IPLS is different from VS in several aspects. First, in
IPLS, ARP packets even including the unicast ARP reply packets are
forwarded from attachment circuits to "multicast" PWs and the
received APR packets from the "multicast" PWs will be flooded to all
CE hosts (although broadcast ARP request packets can be suppressed
by PE routers on which there are matching ARP entries for the ARP
requests in their ARP caches). As a result, the broadcast storm
imposed by ARP traffic is worsened to some extent, rather than being
alleviated. In contrast, by using ARP Proxy on PE routers in VS, ARP
traffic is limited within small network scopes. Second, as said in
[IPLS], "An IP frame received over a unicast PW is prepended with a
MAC header before transmitting it on the appropriate attachment
circuits and the source MAC address is the PE router's own local MAC
address or a MAC address which has been specially configured on the
PE router for this use." However, the destination MAC address of the
packet to a remote CE host which is sent from a local CE host is the
MAC of the remote CE host, rather than the local PE router's MAC.
Thus, flooding unknown destination unicast frames on the above
Ethernet bridges would not be avoided anymore unless these
intermediary bridges are configured to not age out the learned MAC
entries (whether such configuration has any side-effects is
uncertain). In contrast, such intermediate bridges in VS only need
to learn MAC addresses of local CE hosts and local PE routers. Third,
IPLS prohibits connection of a common LAN or VLAN to more than one
PE router. In other words, IPLS can not allow CE hosts to be multi-
homed to multiple PE routers for redundancy and load-balancing. In
contrast, VS can support CE multi-homing easily.
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6. Conclusion
By using Layer 3 routing on the backbone of the data center network
to replace the STP bridge forwarding, traffic between any two
servers is forwarded along shortest paths between them and multi-
path routing is easily achieved. Thus, the totally available
bandwidth in data center networks is utilized to the maximum extent.
By reusing the BGP/MPLS IP VPN technology to build large IP subnets
across the backbones of data center networks, servers of a given VPN
are allowed to communicate with each other just as if they were on
the same subnet.
Due to the BGP/MPLS IP VPN technology, forwarding tables of P
routers are sized to the number of PE routers rather than the total
number of CE hosts. Meanwhile, forwarding tables of PE routers can
also scale well by distributing VPN instances and their
corresponding routing tables among multiple PE routers. Especially,
thanks to the Outbound Route Filtering (ORF) capability of BGP, PE
routers only needs to maintain the routing tables of their attached
VPN instances. Thus, the forwarding table scalability issues with
today's data center networks are largely alleviated.
By enabling APR proxy function on PE routers, ARP broadcast messages
from local CE hosts are blocked by local PE routers. Thus, APR
broadcast messages will not flood the whole data center network.
Besides, by enabling DHCP Relay Agent function on PE routers, DHCP
broadcast messages from local CE hosts are intercepted by DHCP Relay
Agents and forwarded to DHCP servers in unicast. Thus, the broadcast
storms in data center networks are largely suppressed.
7. Future work
How to support IPv6 CE hosts in VS is for future study.
8. Security Considerations
TBD.
9. IANA Considerations
There is no requirement for IANA.
10. Acknowledgements
Thanks to Dino Farinacci for his valuable comments.
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11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. 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.
[MVPN-BGP] R. Aggarwal, E. Rosen, T. Morin, Y. Rekhter, C.
Kodeboniya, "BGP Encodings for Multicast in MPLS/BGP IP
VPNs", draft-ietf-l3vpn-2547bis-mcast-bgp-08.txt (work in
progress), September 2009.
[RFC826] Plummer, D., "An Ethernet Address Resolution Protocol or
Converting Network Protocol Addresses to 48-bit Ethernet
Addresses for Transmission on Ethernet Hardware", RFC-826,
Symbolics, November 1982.
[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.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[RFC2236] Fenner, W., "Internet Group Management Protocol, Version
2", RFC 2236, November 1997.
[RFC4761] Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
(VPLS) Using BGP for Auto-Discovery and Signaling", RFC
4761, January 2007.
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[RFC4762] Lasserre, M. and V. Kompella, "Virtual Private LAN Service
(VPLS) Using Label Distribution Protocol (LDP) Signaling",
RFC 4762, January 2007.
[IPLS] H. Shah., et. al., "IP-Only LAN Service (IPLS)", draft-ietf-
l2vpn-ipls-09.txt (work in progress), February 2010.
Authors' Addresses
Xiaohu Xu
Huawei Technologies,
Hai-Dian District, Beijing 100085, P.R. China
Phone: +86 10 82882573
Email: xuxh@huawei.com
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