weave/site/using-weave/fastdp.md

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Using Fast Datapath	96

The most important thing to know about fast datapath is that you don't need to configure anything before using this feature. If you are using Weave Net 1.2 or greater, fast datapath (fastdp) is automatically enabled.

When Weave Net cannot use the fast data path between two hosts, it falls back to a slower packet forwarding approach called sleeve. Selecting the fastest forwarding approach is automatic, and is determined on a connection-by-connection basis. For example, a Weave network spanning two data centers might use fast data path within the data centers, but not for the more constrained network link between them.

See How Fastdp Works for a more in-depth discussion of this feature.

###Disabling Fast Datapath

You can disable fastdp by enabling the WEAVE_NO_FASTDP environment variable at weave launch:

$ WEAVE_NO_FASTDP=true weave launch

###Fast Datapath and Encryption

Encryption does not work with fast datapath. If you enable encryption using the --password option to launch weave (or you use the WEAVE_PASSWORD environment variable), fast datapath will by default be disabled.

When encryption is not in use there may be other conditions in which the fast datapath reverts to sleeve mode. Once these conditions pass, Weave Net reverts back to using fastdp. To view which mode Weave Net is using, run weave status connections.

###Viewing Connection Mode Fastdp or Sleeve

Weave Net automatically uses the fastest datapath for every connection unless it encounters a situation that prevents it from working. To ensure that Weave Net can use the fast datapath:

• Avoid Network Address Translation (NAT) devices
• Open UDP port 6784 (This is the port used by the Weave routers)
• Ensure that WEAVE_MTU fits with the MTU of the intermediate network (see below)

The use of fast datapath is an automated connection-by-connection decision made by Weave Net, and because of this, you may end up with a mixture of connection tunnel types. If fast datapath cannot be used for a connection, Weave Net falls back to the sleeve "user space" packet path.

Once a Weave network is set up, you can query the connections using the weave status connections command:

$ weave status connections
<-192.168.122.25:43889  established fastdp a6:66:4f:a5:8a:11(ubuntu1204)

Where fastdp indicates that fast datapath is being used on a connection. If fastdp is not shown, the field displays sleeve indicating Weave Net's fall-back encapsulation method:

$ weave status connections
<- 192.168.122.25:54782  established sleeve 8a:50:4c:23:11:ae(ubuntu1204)

###Packet size (MTU)

The Maximum Transmission Unit, or MTU, is the technical term for the limit on how big a single packet can be on the network. Weave Net defaults to 1410 bytes, but you can set a smaller size if your underlying network has a tighter limit, or set a larger size for better performance.

The underlying network must be able to deliver packets of the size specified plus overheads of around 50 bytes. This requirement applies to every path between peers.

To specify a different MTU, before launching Weave Net set the environment variable WEAVE_MTU. For example, for a typical "jumbo frame" configuration:

$ WEAVE_MTU=8950 weave launch host2 host3

See Also

• Using Weave Net
• How Fastdp Works
• Performance measurements