Jumbo Frame（巨帧）

IEEE 802.3以太网标准仅规定支持1500Byte的帧MTU，总计1518Byte的帧大小。（使用IEEE 802.1Q VLAN/QoS标签时，增加至1522Byte）而巨型帧往往采用9000Byte的帧MTU，合计9018/9022Byte的帧大小。

目前巨型帧尚未成为官方的IEEE 802.3以太网标准的一部分。所以不同硬件厂商的设备支持程度可能不尽相同。

使用巨型帧，增大的有效报文长度提升了带宽使用效率的提升（如下图）。与此同时，增长的报文也带来传输时延的增加，时延敏感型数据并不适合使用巨型帧传输。

neutron中的MTU配置项

从配置项的描述总结而言，global_physnet_mtu与physical_network_mtus共同定义了underlay physical network的MTU，path_mtu定义了overlay network的MTU。

**调整MTU的3个用例

单MTU值物理网络体系**

在neutron.conf中

1.[DEFAULT]

2.global_physnet_mtu?=?900

在ml2.ini中

1.[ml2]??

2.path_mtu?=?9000

该配置定义了所有underlay网络（flat，vlan）与overlay网络（vxlan，gre）的MTU值均为9000。

多MTU值物理网络体系

在neutron.conf中

1.[DEFAULT]??

2.global_physnet_mtu?=?9000?

在ml2.ini中

1.? [ovs]??

2.??bridge_mappings?=?provider1:eth1,provider2:eth2,provider3:eth3??

3.? [ml2]??

4.??physical_network_mtus?=?provider2:4000,provider3:1500??

5.??path_mtu?=?9000??

该配置定义了underlay网络provider2的MTU值为4000，provider3的MTU值为1500，其他如provider1的MTU值为9000。而overlay网络的MTU值为9000。

Overlay网络MTU

在neutron.conf中

1. [DEFAULT]??

2.?global_physnet_mtu?=?9000?

在ml2.ini中

1. [ml2]??

2.?path_mtu?=?4000?

该配置定义了所有underlay网络MTU值为9000，overlay网络的MTU值均为4000。

代码浅析

创建network resource时的MTU处理

flat和vlan网络，根据实际的物理网络映射与physical_network_mtus、global_physnet_mtu信息，获取最小可用MTU值。

1.??def?get_deployment_physnet_mtu():??

2.??return?cfg.CONF.global_physnet_mtu??

3.? ??

4.??class?BaseTypeDriver(api.ML2TypeDriver):??

5.??def?init(self):??

6.??try:??

7.? self.physnet_mtus = helpers.parse_mappings(??

8.? cfg.CONF.ml2.physical_network_mtus, unique_values=False??

9.? ? )??

10.??except?Exception as e:??

11.? LOG.error("Failed to parse physical_network_mtus: %s", e)??

12.? self.physnet_mtus = []??

13.? ?

14.??def?get_mtu(self, physical_network=None):??

15.???return?p_utils.get_deployment_physnet_mtu()??

16.? ?

17.?class?FlatTypeDriver(helpers.BaseTypeDriver):??

18.? ? ?...??

19.??def?get_mtu(self, physical_network):??

20.? seg_mtu = super(FlatTypeDriver, self).get_mtu()??

21.??mtu?= []??

22.? if seg_mtu > 0:??

23.? ?mtu.append(seg_mtu)??

24.???if?physical_network in self.physnet_mtus:??

25.? mtu.append(int(self.physnet_mtus[physical_network]))??

26.??return?min(mtu) if mtu else 0??

27.? ?

28.?class?VlanTypeDriver(helpers.SegmentTypeDriver):??

29.? ? ?...??

30.??def?get_mtu(self, physical_network):??

31.? seg_mtu = super(VlanTypeDriver, self).get_mtu()??

32.? mtu = []??

33.??if?seg_mtu > 0:??

34.? mtu.append(seg_mtu)??

35.??if?physical_network in self.physnet_mtus:??

36.? mtu.append(int(self.physnet_mtus[physical_network]))??

37.??return?min(mtu) if mtu else 0??

Geneve，Gre，Vxlan类型网络，则根据global_physnet_mtu与path_mtu中选取最小的可用MTU值，减去各类型报文头部开销，获取实际可用MTU值。

1.? class _TunnelTypeDriverBase(helpers.SegmentTypeDriver):??

2.? ...??

3.? def get_mtu(self, physical_network=None):??

4.? seg_mtu = super(_TunnelTypeDriverBase, self).get_mtu()??

5.? mtu = []??

6.? if seg_mtu > 0:??

7.? mtu.append(seg_mtu)??

8.? if cfg.CONF.ml2.path_mtu > 0:??

9.? mtu.append(cfg.CONF.ml2.path_mtu)??

10.? version = cfg.CONF.ml2.overlay_ip_version??

11.? ip_header_length = p_const.IP_HEADER_LENGTH[version]??

1. return min(mtu) - ip_header_length if mtu else 0??

13.? ?

1. class GeneveTypeDriver(type_tunnel.EndpointTunnelTypeDriver):??

15.? ...??

1. def get_mtu(self, physical_network=None):??
2. mtu = super(GeneveTypeDriver, self).get_mtu()??
3. return mtu - self.max_encap_size if mtu else 0??

19.? ?

1. class GreTypeDriver(type_tunnel.EndpointTunnelTypeDriver):??

21.? ? ?...??

1. def get_mtu(self, physical_network=None):??

23.? mtu = super(GreTypeDriver, self).get_mtu(physical_network)??

24.? return mtu - p_const.GRE_ENCAP_OVERHEAD if mtu else 0??

25.? ? ? ? ? ?

1. class VxlanTypeDriver(type_tunnel.EndpointTunnelTypeDriver):??

27.? ...??

1. def get_mtu(self, physical_network=None):??
2. mtu = super(VxlanTypeDriver, self).get_mtu()??
3. return mtu - p_const.VXLAN_ENCAP_OVERHEAD if mtu else 0??

在用户实际创建network资源时，若未显式指定网络MTU值，则使用该网络类型下系统定义的最大可用MTU。若显式指定MTU，neutron会检查用户定义MTU是否小于等于该网络类型下系统定义的最大可用MTU。

1.? def _get_network_mtu(self, network_db, validate=True):??

2.? mtus = []??

3.? ...??

4.? for s in segments:??

5.? segment_type = s.get(‘network_type‘)??

6.? if segment_type is None:??

7.? ?...??

8.? else:??

9.? mtu = type_driver.get_mtu(s[‘physical_network‘])??

1. Some drivers, like ‘local‘, may return None; the assumption??

11.? # then is that for the segment type, MTU has no meaning or??

1. unlimited, and so we should then ignore those values.??

13.? if mtu:??

14.? mtus.append(mtu)??

15.? ?

1. max_mtu = min(mtus) if mtus else p_utils.get_deployment_physnet_mtu()??
2. net_mtu = network_db.get(‘mtu‘)??

18.? ?

19.? if validate:??

20.? # validate that requested mtu conforms to allocated segments??

21.? if net_mtu and max_mtu and max_mtu < net_mtu:??

1. msg = _("Requested MTU is too big, maximum is %d") % max_mtu??
2. raise exc.InvalidInput(error_message=msg)??

24.? ?

25.? # if mtu is not set in database, use the maximum possible??

1. return net_mtu or max_mtu??

虚拟机tap设置MTU

在使用Linux Bridge实现的Neutron网络中，Linux Bridge Agent在侦测到新的device后，会通过ip link set 操作，根据network中的MTU值，设置虚拟机绑定至Linux Bridge的tap设备的MTU值。反观Openvswitch实现的网络中却没有相关的设置。实际在使用过程中需要通过ovs-vsctl set Interface <tap name> mtu_request=<MTU Value>命令人工去设置tap设备的MTU值。

1.? class LinuxBridgeManager(amb.CommonAgentManagerBase):??

2.? def plug_interface(self, network_id, network_segment, tap_name,??

3.? device_owner):??

4.? return self.add_tap_interface(network_id, network_segment.network_type,??

5.? network_segment.physical_network,??

6.? network_segment.segmentation_id,??

7.? tap_name, device_owner,??

8.? network_segment.mtu)??

9.? ??

10.? def _set_tap_mtu(self, tap_device_name, mtu):??

1. ip_lib.IPDevice(tap_device_name).link.set_mtu(mtu)??

网络设备tap设置MTU

dhcp和router相关的tap设备在plug时，neutron会根据网络的MTU，在各tap设备所在的namespace内运行“ip link set <tap name> mtu <MTU value>”设置tap设备的MTU值。

1.? class OVSInterfaceDriver(LinuxInterfaceDriver):??

1. def plug_new(self, network_id, port_id, device_name, mac_address,??

3.? bridge=None, namespace=None, prefix=None, mtu=None):??

4.? ...??

5.? # NOTE(ihrachys): the order here is significant: we must set MTU after??

6.? # the device is moved into a namespace, otherwise OVS bridge does not??

7.? # allow to set MTU that is higher than the least of all device MTUs on??

8.? # the bridge??

9.? if mtu:??

10.? self.set_mtu(device_name, mtu, namespace=namespace, prefix=prefix)??

11.? else:??

12.? LOG.warning("No MTU configured for port %s", port_id)??

13.? ...??

14.? ?

1. def set_mtu(self, device_name, mtu, namespace=None, prefix=None):??

16.? if self.conf.ovs_use_veth:??

1. tap_name = self._get_tap_name(device_name, prefix)??
2. root_dev, ns_dev = _get_veth(??

19.? tap_name, device_name, namespace2=namespace)??

20.? root_dev.link.set_mtu(mtu)??

1. else:??
2. ns_dev = ip_lib.IPWrapper(namespace=namespace).device(device_name)??

23.? ns_dev.link.set_mtu(mtu)??

24.? ?

1. class IpLinkCommand(IpDeviceCommandBase):??
2. COMMAND = ‘link‘??

27.? ? ?...??

28.? def set_mtu(self, mtu_size):??

29.? self._as_root([], (‘set‘, self.name, ‘mtu‘, mtu_size))?

bridge间veth设置MTU

Openstack从J版以后，neutron使用ovs patch port代替了linux veth实现OVS网桥之间的连接（出于性能提升的目的）。但依旧保留了veth连接的方式。在openvswitch_agent.ini中可以通过配置use_veth_interconnection=true启用veth连接网桥的功能。如果开启这项配置，默认的veth_mtu值为9000。当配置链路MTU大于9000时，需要修改openvswitch_agent.ini配置文件中veth_mtu的值，以免发生瓶颈效应。

1.? class OVSNeutronAgent(l2population_rpc.L2populationRpcCallBackTunnelMixin,??

2.? dvr_rpc.DVRAgentRpcCallbackMixin):??

3.? def init(self, bridge_classes, ext_manager, conf=None):??

4.? ...??

5.? self.use_veth_interconnection = ovs_conf.use_veth_interconnection??

6.? self.veth_mtu = agent_conf.veth_mtu??

7.? ?...??

8.? def setup_physical_bridges(self, bridge_mappings):??

9.? ?‘‘‘‘‘Setup the physical network bridges.?

10.??

11.? Creates physical network bridges and links them to the?

12.? integration bridge using veths or patch ports.?

13.??

14.? ?:param bridge_mappings: map physical network names to bridge names.?

15.? ?‘‘‘??

16.? self.phys_brs = {}??

17.? self.int_ofports = {}??

18.? self.phys_ofports = {}??

19.? ip_wrapper = ip_lib.IPWrapper()??

20.? ovs = ovs_lib.BaseOVS()??

21.? ovs_bridges = ovs.get_bridges()??

22.? for physical_network, bridge in bridge_mappings.items():??

23.? ?...??

24.? ?if self.use_veth_interconnection:??

25.? ?# enable veth to pass traffic??

26.? int_veth.link.set_up()??

27.? ?phys_veth.link.set_up()??

28.? ?if self.veth_mtu:??

29.? ?# set up mtu size for veth interfaces??

30.? int_veth.link.set_mtu(self.veth_mtu)??

31.? phys_veth.link.set_mtu(self.veth_mtu)??

32.? else:??

33.? # associate patch ports to pass traffic??

34.? self.int_br.set_db_attribute(‘Interface‘, int_if_name,??

35.? ?‘options‘, {‘peer‘: phys_if_name})??

36.? br.set_db_attribute(‘Interface‘, phys_if_name,??

37.? ? ‘options‘, {‘peer‘: int_if_name})??

虚拟机网卡如何设置MTU

虚拟机内部网卡配置MTU则是通过虚拟机DHCP请求IP地址时，顺便请求MTU值。在RFC2132 DHCP Option and BOOTP Vendor Extensions里明确定义了Interface MTU Option。DHCP Option Code 26 用两个字节的MTU数据，定义了网络接口的MTU值。如下表所示。

在DHCP agent中，dnsmasq的spawn_process会根据network的MTU值调整自身的启动参数。从而使虚拟机在DHCP过程中能正确地配置自身网卡的MTU值。

1.? class Dnsmasq(DhcpLocalProcess):??

2.? def _build_cmdline_callback(self, pid_file):??

3.? # We ignore local resolv.conf if dns servers are specified??

4.? # or if local resolution is explicitly disabled.??

5.? ...??

6.? mtu = getattr(self.network, ‘mtu‘, 0)??

7.? ?# Do not advertise unknown mtu??

8.? ?if mtu > 0:??

9.? cmd.append(‘--dhcp-option-force=option:mtu,%d‘ % mtu)??

10.? ?...??

11.? return cmd??

探测MTU

通过指定ICMP报文内容size以及IP报文不分片来探测MTU值设置是否正确。注意这里的size是指icmp data size。该size并不包含ICMP报文头部长度（8Byte）以及IP头部长度（20Byte）。

windows下：

1.? ping -f -l <size> <target_name/target_ip>??

linux下：

1. ping -M do -s <size> <target_name/target_ip>

原文地址：https://blog.51cto.com/99cloud/2403060