Zebra

Overview of the Zebra Protocol

The Zebra protocol (or ZAPI) is used by protocol daemons to communicate with the zebra daemon.

Each protocol daemon may request and send information to and from the zebra daemon such as interface states, routing state, nexthop-validation, and so on. Protocol daemons may also install routes with zebra. The zebra daemon manages which routes are installed into the forwarding table with the kernel. Some daemons use more than one ZAPI connection. This is supported: each ZAPI session is identified by a tuple of: {protocol, instance, session_id}. LDPD is an example: it uses a second, synchronous ZAPI session to manage label blocks. The default value for session_id is zero; daemons who use multiple ZAPI sessions must assign unique values to the sessions’ ids.

The Zebra protocol is a streaming protocol, with a common header. Version 0 lacks a version field and is implicitly versioned. Version 1 and all subsequent versions have a version field. Version 0 can be distinguished from all other versions by examining the 3rd byte of the header, which contains a marker value of 255 (in Quagga) or 254 (in FRR) for all versions except version 0. The marker byte corresponds to the command field in version 0, and the marker value is a reserved command in version 0.

Version History

• Version 0

  Used by all versions of GNU Zebra and all version of Quagga up to and including Quagga 0.98. This version has no version field, and so is implicitly versioned as version 0.

• Version 1

  Added marker and version fields, increased command field to 16 bits. Used by Quagga versions 0.99.3 through 0.99.20.

• Version 2

  Used by Quagga versions 0.99.21 through 0.99.23.

• Version 3

  Added vrf_id field. Used by Quagga versions 0.99.23 until FRR fork.

• Version 4

  Change marker value to 254 to prevent people mixing and matching Quagga and FRR daemon binaries. Used by FRR versions 2.0 through 3.0.3.

• Version 5

  Increased VRF identifier field from 16 to 32 bits. Used by FRR versions 4.0 through 5.0.1.

• Version 6

  Removed the following commands:

  • ZEBRA_IPV4_ROUTE_ADD

  • ZEBRA_IPV4_ROUTE_DELETE

  • ZEBRA_IPV6_ROUTE_ADD

  • ZEBRA_IPV6_ROUTE_DELETE

  Used since FRR version 6.0.

Zebra Protocol Definition

Zebra Protocol Header Field Definitions

Length

Total packet length including this header.

Marker

Static marker. The marker value, when it exists, is 255 in all versions of Quagga. It is 254 in all versions of FRR. This is to allow version 0 headers (which do not include version explicitly) to be distinguished from versioned headers.

Version

Zebra protocol version number. Clients should not continue processing messages past the version field for versions they do not recognise.

Command

The Zebra protocol command.

Current Version

Version 5, 6

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             Length            |     Marker    |    Version    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             VRF ID                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Command            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Past Versions

Version 0

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             Length            |    Command    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version 1, 2

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             Length            |     Marker    |    Version    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Command            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version 3, 4

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             Length            |     Marker    |    Version    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             VRF ID            |            Command            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Zebra Protocol Commands

The definitions of zebra protocol commands can be found at lib/zclient.h.

Zebra Dataplane

The zebra dataplane subsystem provides a framework for FIB programming. Zebra uses the dataplane to program the local kernel as it makes changes to objects such as IP routes, MPLS LSPs, and interface IP addresses. The dataplane runs in its own pthread, in order to off-load work from the main zebra pthread.

The zebra dataplane API is versioned; the version number must be updated along with API changes. Plugins can test the current version number and confirm that they are compatible with the current version.

Dataplane batching

Dataplane batching is an optimization feature that reduces the processing time involved in the user space to kernel space transition for every message we want to send.

Design

With our dataplane abstraction, we create a queue of dataplane context objects for the messages we want to send to the kernel. In a separate pthread, we loop over this queue and send the context objects to the appropriate dataplane. A batching enhancement tightly integrates with the dataplane context objects so they are able to be batch sent to dataplanes that support it.

There is one main change in the dataplane code. It does not call kernel-dependent functions one-by-one, but instead it hands a list of work down to the kernel level for processing.

Netlink

At the moment, this is the only dataplane that allows for batch sending messages to it.

When messages must be sent to the kernel, they are consecutively added to the batch represented by the struct nl_batch. Context objects are firstly encoded to their binary representation. All the encoding functions use the same interface: take a context object, a buffer and a size of the buffer as an argument. It is important that they should handle a situation in which a message wouldn’t fit in the buffer and return a proper error. To achieve a zero-copy (in the user space only) messages are encoded to the same buffer which will be passed to the kernel. Hence, we can theoretically hit the boundary of the buffer.

Messages stored in the batch are sent if one of the conditions occurs:

• When an encoding function returns the buffer overflow error. The context object that caused this error is re-added to the new, empty batch.

• When the size of the batch hits certain limit.

• When the namespace of a currently being processed context object is different from all the previous ones. They have to be sent through distinct sockets, so the messages cannot share the same buffer.

• After the last message from the list is processed.

As mentioned earlier, there is a special threshold which is smaller than the size of the underlying buffer. It prevents the overflow error and thus eliminates the case, in which a message is encoded twice.

The buffer used in the batching is global, since allocating that big amount of memory every time wouldn’t be most effective. However, its size can be changed dynamically, using hidden vtysh command: zebra kernel netlink batch-tx-buf (1-1048576) (1-1048576). This feature is only used in tests and shouldn’t be utilized in any other place.

For every failed message in the batch, the kernel responds with an error message. Error messages are kept in the same order as they were sent, so parsing the response is straightforward. We use the two pointer technique to match requests with responses and then set appropriate status of dataplane context objects. There is also a global receive buffer and it is assumed that whatever the kernel sends it will fit in this buffer. The payload of netlink error messages consists of a error code and the original netlink message of the request, so the batch response won’t be bigger than the batch request increased by some space for the headers.