VMware's SDN Dilemma: VXLAN or Nicira?

Posted by Greg Ferro
January 31, 2013

VMware has a technology problem: It's backing two competing standards for overlay networks: Nicira's STT and the IETF draft standard VXLAN. An overlay network enables network virtualization, which is a core component of VMware's software-defined data center initiative. Both STT and VXLAN have upsides and downsides. I'll look at each protocol and speculate on which direction VMware may go.

First, a little background. Before being acquired by VMware, Nicira developed the Stateless Transport Tunneling (STT) protocol for tunneling between open source software switches in the Openvswitch project.

VXLAN, which is now an IETF draft standard, was originally proposed by Cisco. Cisco sources say that the company then got VMware involved (although the IETF draft has a lot of names on it). The end result is VMware is telling everyone that it has this great VXLAN overlay network technology that removes any hypervisor dependency from physical network devices. Even better, it's configured and managed from vCenter.

The question is, which protocol will win?

Nicira and STT

Prior to acquisition, Nicira had a software controller for managing tunnels between virtual switches, and used OpenFlow-like commands to configure the vSwitch. STT is a tunneling protocol that connects the virtual switches, thus forming a virtual network.

STT performs this task well enough. It uses the TCP protocol for encapsulation. Supposedly, operating systems can use the TCP offload function of modern network adapters for better performance.

However, STT also has several limitations. One problem is that the limited entropy in the STT header means it doesn't balance loads evenly over Ethernet port bundles in network backbones. Depending on your network design, this could be a significant limitation.

Second, STT currently works only with the Openvswitch software switch on Linux hypervisors such as Xen or KVM. That's not necessarily a problem for cloud providers and very large organizations; for instance, eBay is using Nicira in its OpenStack deployment. However, VMware is more common in enterprise data centers. It's possible VMware could add STT to the ESXi vSwitch, and thus deliver a multicloud network overlay strategy, but the VXLAN protocol already has a lot of momentum.

VXLAN's Multicast Issues

VXLAN depends heavily on a multicast-enabled underlay network to handle broadcast/unicast/multicast Ethernet protocols. (I use the term "underlay network" to describe the physical devices that pass Ethernet frames and IP packets.) What's not well understood is that IP multicast is complex and risky to operate.

Each VXLAN-enabled device is known as a VXLAN Tunnel End Point (VTEP). When the VTEP is configured with VXLANs, it will be configured to join an IP multicast group. Joining the multicast tree is the method for VTEPs to discover the MAC of each host in the VXLAN in a self-configuring and autonomous method. Direct server-to-server data flows are transported through the VXLAN overlay in unicast packets.

IP multicast also provides an efficient way to broadcast Ethernet frames to all servers as is required--for example, for unknown MAC address flooding and IP ARP Requests.

VMware recommends a separate multicast group for each VXLAN; thus, 50 VXLANs would require 50 separate multicast trees in an attempt to control L2 Ethernet flooding problems. L2 loops remain a problem in VXLAN networks, but the failure domain is reduced to an individual VXLAN itself. The problem is that each of those multicast trees requires state to be held in the network layer, which consumes CPU, memory and TCAM space. TCAM size is a serious limitation on network diameter, and overloaded TCAM is serious network threat.

A lesser performance problem is the frame replication silicon in the switches. At its core, multicast is a method for duplicating Ethernet frames inside the hardware of your network. One multicast frame must be sent out of every Ethernet port that needs to receive it. On a data center core switch, this could mean replicating one received frame to 300 ports (thus, 1 Gbps of inbound multicast packets results in 300 Gbps output). Network switches require dedicated silicon to handle the duplication process. For example, this is an approximation of silicon pathways inside a single M1-series line card from a Nexus 7000 showing the replication engines on the blade:

(click image for larger view)
Internal Architecture of Single Line Card Nexus 7000. Source: Cisco Systems

There are a number of IP multicast routing protocols that maintain the multicast trees, including PIM-SM, PIM-DM, BiDir and ASM multicast. In general terms, PIM-SM will be the default choice because it's got the widest vendor support, but that isn't saying much. Most data center switches do not support multicast protocols today. This can make VXLAN hard to deploy in existing networks and usually requires new network hardware.

Next page: Picking a Winner

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