ovs vlan tag管理

1、背景

当使用ovs管理虚机的网络时，ovs会为虚机的tap设备添加一个内部的vlan tag，如下图所示，存在两个bridge，br0-ovs、br-int，其中br0-ovs挂载物理网卡设备eth2，br-int挂载虚机tatp设备，br0-ovs与br-int之间通过一对patch口连接，ovs为tap设备分配了内部vlan 2和内部vlan 3，本文主要描述ovs是如何完成这些内部vlan与外部vlan的转换。

2、流表规则

首先看一下br-int的流表规则，可以看到从int-br0-ovs这个patch进来的外部报文，分成两种情况，如果收到的报文没有vlan，则添加一个内部vlan 2，然后normal查询actiton动作，如果收到的报文带vlan 1049，则转换成内部vlan 3，然后normal查找actiton动作；

然后看一下br0-ovs的流表规则，从phy-br0-ovs这个patch收到的报文，再发出去之前判断如果带内部vlan 2，则把vlan信息剥离，如果带内部vlan 3，则添加外部vlan 1049，然后normal查询action动作；

从这两个桥的流表规则大致可以看出，是在br-int收到外部报文时，将其vlan信息转成内部vlan，br0-ovs收到虚机往外发的报文时，将内部vlan转化成外部vlan；既然br-int收到的发往虚机的报文，无论是不是有vlan，都会添加一个内部vlan，那为什么在虚机内部转包的时候又没有vlan信息呢？这个mod_vlan_vid究竟是做什么用呢？

3、mod_vlan_vid

首先先看一下ovs的mod_vlan_vid的动作，看ovs的upcall流程里，mod_vlan_vid的时候是会修改ctx->xin->flow的vlan信息，当ovs触发upcall时，ovs会将skb的流信息保存到ctx->xin->flow里，然后通过用户态的流表规则修改flow信息（在recv_upcalls->dpif_recv->odp_flow_key_to_flow流程里赋值的），最终匹配好actiton动作后再通知给datapath处理，从这个mod_vlan_vid的处理逻辑看，应该是修改了flow的vlan信息；

do_xlate_actions(const struct ofpact *ofpacts, size_t ofpacts_len,
                 struct xlate_ctx *ctx, bool is_last_action,
                 bool group_bucket_action)
{
    struct flow_wildcards *wc = ctx->wc;
    struct flow *flow = &ctx->xin->flow;
    const struct ofpact *a;
     ...   
        case OFPACT_SET_VLAN_VID:
            wc->masks.vlans[0].tci |= htons(VLAN_VID_MASK | VLAN_CFI);
            if (flow->vlans[0].tci & htons(VLAN_CFI) ||
                ofpact_get_SET_VLAN_VID(a)->push_vlan_if_needed) {
                if (!flow->vlans[0].tpid) {
                    flow->vlans[0].tpid = htons(ETH_TYPE_VLAN);
                }
                flow->vlans[0].tci &= ~htons(VLAN_VID_MASK);
                flow->vlans[0].tci |=
                    (htons(ofpact_get_SET_VLAN_VID(a)->vlan_vid) |
                     htons(VLAN_CFI));
            }
            break;

    ...
}

4、xlate_normal

以br-int的流表规则为例，mod_vlan_vid之后就是normal查询：

static void
xlate_normal(struct xlate_ctx *ctx)
{
 
	/*校验flow的vlan信息与in_bound口是否匹配*/
    /* Check VLAN. */
	xvlan_extract(flow, &in_xvlan);
    if (!input_vid_is_valid(ctx, in_xvlan.v[0].vid, in_xbundle)) {
        xlate_report(ctx, OFT_WARN,
                     "disallowed VLAN VID for this input port, dropping");
        return;
    }
	
	/*根据in_xbound类型，做内部vlan转换*/
    xvlan_input_translate(in_xbundle, &in_xvlan, &xvlan);
    vlan = xvlan.v[0].vid;

    /* Check other admissibility requirements. */
    if (in_port && !is_admissible(ctx, in_port, vlan)) {
        return;
    }

    /* Learn source MAC. */
    bool is_grat_arp = is_gratuitous_arp(flow, wc);
    if (ctx->xin->allow_side_effects
        && flow->packet_type == htonl(PT_ETH)
        && in_port->pt_mode != NETDEV_PT_LEGACY_L3
    ) {
        update_learning_table(ctx, in_xbundle, flow->dl_src, vlan,
                              is_grat_arp);
    }
    if (ctx->xin->xcache && in_xbundle != &ofpp_none_bundle) {
        struct xc_entry *entry;

        /* Save just enough info to update mac learning table later. */
        entry = xlate_cache_add_entry(ctx->xin->xcache, XC_NORMAL);
        entry->normal.ofproto = ctx->xbridge->ofproto;
        entry->normal.in_port = flow->in_port.ofp_port;
        entry->normal.dl_src = flow->dl_src;
        entry->normal.vlan = vlan;
        entry->normal.is_gratuitous_arp = is_grat_arp;
    }

	/*发送到out_xbound*/
    /* Determine output bundle. */
    if (mcast_snooping_enabled(ctx->xbridge->ms)
        && !eth_addr_is_broadcast(flow->dl_dst)
        && eth_addr_is_multicast(flow->dl_dst)
        && is_ip_any(flow)) {
        struct mcast_snooping *ms = ctx->xbridge->ms;
        struct mcast_group *grp = NULL;
        struct dp_packet *p = CONST_CAST(struct dp_packet *,
                                         ctx->xin->packet);

        /* We will need the whole data for processing the packet below */
        if (p && !dp_packet_is_linear(p)) {
            dp_packet_linearize(p);
        }

        if (is_igmp(flow, wc)) {
            /*
             * IGMP packets need to take the slow path, in order to be
             * processed for mdb updates. That will prevent expires
             * firing off even after hosts have sent reports.
             */
            ctx->xout->slow |= SLOW_ACTION;

            memset(&wc->masks.tp_src, 0xff, sizeof wc->masks.tp_src);
            if (mcast_snooping_is_membership(flow->tp_src) ||
                mcast_snooping_is_query(flow->tp_src)) {
                if (ctx->xin->allow_side_effects && ctx->xin->packet) {
                    update_mcast_snooping_table(ctx, flow, vlan,
                                                in_xbundle, ctx->xin->packet);
                }
            }

            if (mcast_snooping_is_membership(flow->tp_src)) {
                struct mcast_output out = MCAST_OUTPUT_INIT;

                ovs_rwlock_rdlock(&ms->rwlock);
                xlate_normal_mcast_send_mrouters(ctx, ms, in_xbundle, &xvlan,
                                                 &out);
                /* RFC4541: section 2.1.1, item 1: A snooping switch should
                 * forward IGMP Membership Reports only to those ports where
                 * multicast routers are attached.  Alternatively stated: a
                 * snooping switch should not forward IGMP Membership Reports
                 * to ports on which only hosts are attached.
                 * An administrative control may be provided to override this
                 * restriction, allowing the report messages to be flooded to
                 * other ports. */
                xlate_normal_mcast_send_rports(ctx, ms, in_xbundle, &out);
                ovs_rwlock_unlock(&ms->rwlock);

                mcast_output_finish(ctx, &out, in_xbundle, &xvlan);
            } else {
                xlate_report(ctx, OFT_DETAIL, "multicast traffic, flooding");
                xlate_normal_flood(ctx, in_xbundle, &xvlan);
            }
            return;
        } else if (is_mld(flow, wc)) {
            ctx->xout->slow |= SLOW_ACTION;
            if (ctx->xin->allow_side_effects && ctx->xin->packet) {
                update_mcast_snooping_table(ctx, flow, vlan,
                                            in_xbundle, ctx->xin->packet);
            }
            if (is_mld_report(flow, wc)) {
                struct mcast_output out = MCAST_OUTPUT_INIT;

                ovs_rwlock_rdlock(&ms->rwlock);
                xlate_normal_mcast_send_mrouters(ctx, ms, in_xbundle, &xvlan,
                                                 &out);
                xlate_normal_mcast_send_rports(ctx, ms, in_xbundle, &out);
                ovs_rwlock_unlock(&ms->rwlock);

                mcast_output_finish(ctx, &out, in_xbundle, &xvlan);
            } else {
                xlate_report(ctx, OFT_DETAIL, "MLD query, flooding");
                xlate_normal_flood(ctx, in_xbundle, &xvlan);
            }
        } else {
            if (is_ip_local_multicast(flow, wc)) {
                /* RFC4541: section 2.1.2, item 2: Packets with a dst IP
                 * address in the 224.0.0.x range which are not IGMP must
                 * be forwarded on all ports */
                xlate_report(ctx, OFT_DETAIL,
                             "RFC4541: section 2.1.2, item 2, flooding");
                xlate_normal_flood(ctx, in_xbundle, &xvlan);
                return;
            }
        }

        /* forwarding to group base ports */
        struct mcast_output out = MCAST_OUTPUT_INIT;

        ovs_rwlock_rdlock(&ms->rwlock);
        if (flow->dl_type == htons(ETH_TYPE_IP)) {
            grp = mcast_snooping_lookup4(ms, flow->nw_dst, vlan);
        } else if (flow->dl_type == htons(ETH_TYPE_IPV6)) {
            grp = mcast_snooping_lookup(ms, &flow->ipv6_dst, vlan);
        }
        if (grp) {
            xlate_normal_mcast_send_group(ctx, ms, grp, in_xbundle, &out);
            xlate_normal_mcast_send_fports(ctx, ms, in_xbundle, &out);
            xlate_normal_mcast_send_mrouters(ctx, ms, in_xbundle, &xvlan,
                                             &out);
        } else {
            if (mcast_snooping_flood_unreg(ms)) {
                xlate_report(ctx, OFT_DETAIL,
                             "unregistered multicast, flooding");
                out.flood = true;
            } else {
                xlate_normal_mcast_send_mrouters(ctx, ms, in_xbundle, &xvlan,
                                                 &out);
                xlate_normal_mcast_send_fports(ctx, ms, in_xbundle, &out);
            }
        }
        ovs_rwlock_unlock(&ms->rwlock);

        mcast_output_finish(ctx, &out, in_xbundle, &xvlan);
    } else {
        ovs_rwlock_rdlock(&ctx->xbridge->ml->rwlock);
        mac = mac_learning_lookup(ctx->xbridge->ml, flow->dl_dst, vlan);
        mac_port = mac ? mac_entry_get_port(ctx->xbridge->ml, mac) : NULL;
        ovs_rwlock_unlock(&ctx->xbridge->ml->rwlock);

        if (mac_port) {
            struct xbundle *mac_xbundle = xbundle_lookup(ctx->xcfg, mac_port);
            if (mac_xbundle
                && mac_xbundle != in_xbundle
                && mac_xbundle->ofbundle != in_xbundle->ofbundle) {
                xlate_report(ctx, OFT_DETAIL, "forwarding to learned port");
                output_normal(ctx, mac_xbundle, &xvlan);
            } else if (!mac_xbundle) {
                xlate_report(ctx, OFT_WARN,
                             "learned port is unknown, dropping");
            } else {
                xlate_report(ctx, OFT_DETAIL,
                             "learned port is input port, dropping");
            }
        } else {
            xlate_report(ctx, OFT_DETAIL,
                         "no learned MAC for destination, flooding");
            xlate_normal_flood(ctx, in_xbundle, &xvlan);
        }
    }
}

从以上的noraml处理流程，可以看出ovs的normal逻辑主要包含以下几个动作：

1）、检查flow的vlan与in_xbound是否匹配；

这一步的校验主要就是获取flow的vlan信息，然后看跟in_xbound的类型是否匹配，比如access口不能发出带vlan的报文，ovs为每个port都分配了一个vlan mode类型，默认都是trunk类型，如果port带tag信息，则port为access口（可通过ovs-vsctl list port xxx查询）；

static void
xvlan_extract(const struct flow *flow, struct xvlan *xvlan)
{
    int i;
    memset(xvlan, 0, sizeof(*xvlan));
    for (i = 0; i < FLOW_MAX_VLAN_HEADERS; i++) {
        if (!eth_type_vlan(flow->vlans[i].tpid) ||
            !(flow->vlans[i].tci & htons(VLAN_CFI))) {
            break;
        }
        xvlan->v[i].tpid = ntohs(flow->vlans[i].tpid);
        xvlan->v[i].vid = vlan_tci_to_vid(flow->vlans[i].tci);
        xvlan->v[i].pcp = ntohs(flow->vlans[i].tci) & VLAN_PCP_MASK;
    }
}


static bool
input_vid_is_valid(const struct xlate_ctx *ctx,
                   uint16_t vid, struct xbundle *in_xbundle)
{
    /* Allow any VID on the OFPP_NONE port. */
    if (in_xbundle == &ofpp_none_bundle) {
        return true;
    }

    switch (in_xbundle->vlan_mode) {
    case PORT_VLAN_ACCESS:
        if (vid) {
            xlate_report_error(ctx, "dropping VLAN %"PRIu16" tagged "
                               "packet received on port %s configured as VLAN "
                               "%d access port", vid, in_xbundle->name,
                               in_xbundle->vlan);
            return false;
        }
        return true;

    case PORT_VLAN_NATIVE_UNTAGGED:
    case PORT_VLAN_NATIVE_TAGGED:
        if (!vid) {
            /* Port must always carry its native VLAN. */
            return true;
        }
        /* Fall through. */
    case PORT_VLAN_TRUNK:
        if (!xbundle_trunks_vlan(in_xbundle, vid)) {
            xlate_report_error(ctx, "dropping VLAN %"PRIu16" packet "
                               "received on port %s not configured for "
                               "trunking VLAN %"PRIu16,
                               vid, in_xbundle->name, vid);
            return false;
        }
        return true;

    case PORT_VLAN_DOT1Q_TUNNEL:
        if (!xbundle_allows_cvlan(in_xbundle, vid)) {
            xlate_report_error(ctx, "dropping VLAN %"PRIu16" packet received "
                               "on dot1q-tunnel port %s that excludes this "
                               "VLAN", vid, in_xbundle->name);
            return false;
        }
        return true;

    default:
        OVS_NOT_REACHED();
    }

}

2）、根据in_xbound类型，做内部vlan转；

这个流程主要是根据in_xbound口类型，设置内部vlan信息(xvlan)，如果报文是从access口来的，则内部vlan为in_xbound的内部vlan值，如果in_bound口trunk，则内部vlan为flow里解析得到的vlan值；这里计算得到的内部vlan值主要作用有两个：更新记录mac以及normal flood时保证只在同个vlan域flood；

static void
xvlan_input_translate(const struct xbundle *in_xbundle,
                      const struct xvlan *in_xvlan, struct xvlan *xvlan)
{

    switch (in_xbundle->vlan_mode) {
	//如果报文是从access口来的，则xvlan设置为in_xbound口的内部vlan信息；
    case PORT_VLAN_ACCESS:
        memset(xvlan, 0, sizeof(*xvlan));
        xvlan->v[0].tpid = in_xvlan->v[0].tpid ? in_xvlan->v[0].tpid :
                                                 ETH_TYPE_VLAN_8021Q;
        xvlan->v[0].vid = in_xbundle->vlan;
        xvlan->v[0].pcp = in_xvlan->v[0].pcp;
        break;

	//如果报文是从trunk口来的，则xvlan设置为in_xvlan的值,in_xvlan即为xvlan_extract里根据flow解析
	//得到的vlan信息
    case PORT_VLAN_TRUNK:
        xvlan_copy(xvlan, in_xvlan);
        break;

    case PORT_VLAN_NATIVE_UNTAGGED:
    case PORT_VLAN_NATIVE_TAGGED:
        xvlan_copy(xvlan, in_xvlan);
        if (!in_xvlan->v[0].vid) {
            xvlan->v[0].tpid = in_xvlan->v[0].tpid ? in_xvlan->v[0].tpid :
                                                     ETH_TYPE_VLAN_8021Q;
            xvlan->v[0].vid = in_xbundle->vlan;
            xvlan->v[0].pcp = in_xvlan->v[0].pcp;
        }
        break;

    case PORT_VLAN_DOT1Q_TUNNEL:
        xvlan_copy(xvlan, in_xvlan);
        xvlan_push_uninit(xvlan);
        xvlan->v[0].tpid = in_xbundle->qinq_ethtype;
        xvlan->v[0].vid = in_xbundle->vlan;
        xvlan->v[0].pcp = 0;
        break;

    default:
        OVS_NOT_REACHED();
    }
}

3）、更新mac表信息；

这里生成entry cache信息，后面revalidate使用该entry信息跟新mac表，mac表记录了某个mac地址是从哪个port来的，以及改port所属的内部vlan信息；

    if (ctx->xin->xcache && in_xbundle != &ofpp_none_bundle) {
        struct xc_entry *entry;

        /* Save just enough info to update mac learning table later. */
        entry = xlate_cache_add_entry(ctx->xin->xcache, XC_NORMAL);
        entry->normal.ofproto = ctx->xbridge->ofproto;
        entry->normal.in_port = flow->in_port.ofp_port;
        entry->normal.dl_src = flow->dl_src;
        entry->normal.vlan = vlan;
        entry->normal.is_gratuitous_arp = is_grat_arp;
    }

4）、根据目的mac，找到对应的out_xbound口；

如果normal的时候根据目的mac找不到out_xbound，就需要flood，flood的时候就会根据第（2）步计算得到的内部vlan，然后遍历bridge上的所有port，当port的vlan与计算得到的内部vlan匹配时，才会转发到对应的口；

static void
xlate_normal_flood(struct xlate_ctx *ctx, struct xbundle *in_xbundle,
                   struct xvlan *xvlan)
{
    struct xbundle *xbundle;

    LIST_FOR_EACH (xbundle, list_node, &ctx->xbridge->xbundles) {
        if (xbundle != in_xbundle
            && xbundle->ofbundle != in_xbundle->ofbundle
            //xvlan为xvlan_input_translate阶段计算得到的内部vlan
            && xbundle_includes_vlan(xbundle, xvlan)
            && xbundle->floodable
            && !xbundle_mirror_out(ctx->xbridge, xbundle)) {
            output_normal(ctx, xbundle, xvlan);
        }
    }
    ctx->nf_output_iface = NF_OUT_FLOOD;
}

5）、output_normal

当找到匹配的出口时，ovs会调用output_normal，在output_normal的时候，ovs还会根据实际out_xbound口的类型获取out_xvlan，这个out_xvlan最终会通过output_normal->compose_output_action->xlate_commit_actions->commit_odp_actions设置到真正要下发给datapath的缓存流表里，也就是说一开始mod_vlan_vid所修改的vlan信息并不一定真的会修改实际数据包的vlan信息，在normal流程里，是否修改实际数据包的vlan信息取决于out_xbound的类型。当out_xbound类型为access口时，out_xvlan为0，这时候就不去修改实际数据包的vlan信息，当out_xbound为trunk时，out_xvlan为xvlan的值，这里的xvlan即为in_xbound的值，也就是说当数据包从access口发给trunk口时，数据包到了trunk口就会添加上access口的内部vlan信息。

static void
xvlan_output_translate(const struct xbundle *out_xbundle,
                       const struct xvlan *xvlan, struct xvlan *out_xvlan)
{
    switch (out_xbundle->vlan_mode) {
	//如果是发往access口，则不设置vlan
    case PORT_VLAN_ACCESS:
        memset(out_xvlan, 0, sizeof(*out_xvlan));
        break;

	//如果是发往trunk口，则将in_xbound 口的内部vlan拷贝过来
    case PORT_VLAN_TRUNK:
    case PORT_VLAN_NATIVE_TAGGED:
        xvlan_copy(out_xvlan, xvlan);
        break;

    case PORT_VLAN_NATIVE_UNTAGGED:
        xvlan_copy(out_xvlan, xvlan);
        if (xvlan->v[0].vid == out_xbundle->vlan) {
            xvlan_pop(out_xvlan);
        }
        break;

    case PORT_VLAN_DOT1Q_TUNNEL:
        xvlan_copy(out_xvlan, xvlan);
        xvlan_pop(out_xvlan);
        break;

    default:
        OVS_NOT_REACHED();
    }
}

 

总结：

1）、流表规则normal前的mod_vlan_vid并不会真正修改数据包的vlan信息，这里的mod_vlan_vid主要作用是保证从trunk来的数据包，当需要做normal flood的时候，只在同一个vlan域内flood；

2）、当数据包从access口发往trunk口时，会带上access口的vlan信息；

3）、流表规则output前的mod_vlan_vid会修改数据包的vlan信息；