BIERv6 Ping/Tracert

On a BIERv6 network, the control plane (responsible for BIERv6 establishment) cannot detect failures to forward data over the multicast tunnel, making network maintenance difficult. BIERv6 ping and tracert can be used to detect such failures and quickly locate faulty nodes. Both BIERv6 ping and tracert check network connectivity and host reachability, and BIERv6 tracert can also locate failure points.

BIERv6 Ping

On the network shown in Figure 1, PEs are edge nodes on the BIERv6 network. PE1 connects to the multicast source, PE2 through PE6 connect to multicast users, and P is a transmission node. PE2 can function as either a transmission node or a leaf node.

Figure 1 Using ping or tracert to test a BIERv6 network

The BIERv6 ping process from PE1 to PE2 and from PE1 to PE4 is as follows:

PE1 initiates a BIERv6 ping test and encapsulates a BIERv6 ping request packet in the format shown in Figure 2. The source address encapsulated into the IPv6 header is an IPv6 address of the VPN to which the multicast traffic belongs, and the destination address is the End.BIER SID of the next-hop device. The specified target BFR-ID is encapsulated into BIER-Header and BIER OAM. BIER-Header is used to guide packet replication and forwarding, and BIER OAM is used for verification on the control plane.
- Because the BitString length cannot be dynamically negotiated, you need to specify the length manually.
- Currently, only detection based on a single BitString is supported. Therefore, the target BFR-IDs entered by users must be within the same SI range.
When PE2 functions as a transmission node, it searches the IPv6 FIB table based on the destination address and finds that this address is a local End.BIER SID. PE2 then performs the following actions according to the BIER forwarding process:
- Checks whether the NextHdr field in the IPv6 header is 60. If not, PE2 considers the packet abnormal and discards it.
- If the NextHdr value is 60, PE2 obtains the BIFT-ID value from the packet and searches the Incoming Label Map (ILM) of BIERv6 for the next-hop label forwarding entry (NHLFE). It then calculates the set of outbound interfaces on which traffic needs to be replicated and sent from the obtained NHLFE table based on the BitString information in the packet and performs the corresponding operations. When sending the packet, PE2 changes the destination address of the packet to the End.BIER SID of the next-hop node, and keeps the source address unchanged.
When PE2 functions as a leaf node, after receiving a packet, PE2 or PE4 matches the BFR-ID of the local device according to the BitString information in the BIER-Header. PE2 or PE4 then determines that the packet needs to be sent to the control plane for processing according to the destination address in the inner IPv6 Header. After receiving the packet, the control plane returns a response packet to PE1 through a route.
PE1 displays the detection result.
- If no response is received within the timeout period after a request packet is sent, the message "Request time out" is displayed.
- If a response is received within a specified period after a request packet is sent, statistics are collected and displayed based on the BFR-ID of the received packet.

Figure 2 BIERv6 ping/tracert request packet

For details about the fields in a BIERv6 ping request packet, see Table 1.

**Table 1** Fields in a BIERv6 ping/tracert request packet
Field	Description
Ver	Version number.
TC	Traffic type.
FLOW LABEL	Flow label, which is used to differentiate packets at the network layer.
Payload Length	Payload length, indicating the length of the packet excluding the IPv6 packet header.
NextHdr	Next header.
Hop Limit	Hop limit.
Source Address	Source IP address of the packet.
Dst Address	Destination IP address of the packet.
NextHdr Len	Length of the next header.
Option Type	Option type.
Option Len	Option length.
BIFT-ID	BIER forwarding table ID.
S	Bottom of a label stack.
TTL	Time to live. For ping packets, the default value is 255. For tracert packets, the value starts from 1.
Nibble	Half byte.
BSL	Length of BitString.
Entropy	Entropy value.
OAM	Management and maintenance identifier.
Rsv	Reserved bit, which must be 0.
DSCP	DiffServ Codepoint (DSCP).
Proto	Next protocol type.
BFIR-ID	BFIR ID.
BitString	Bit string.
Dest Port	Destination port number of a packet.
Src Port	Source port number of a packet.
Len	Packet length.
CheckSum	Verification field.
Req/Rep	Packet type. The value 1 indicates a request packet, and the value 2 indicates a response packet.
QTF	Format of the timestamp in a request packet. The value 2 indicates the NTP format, and the value 3 indicates the Precision Time Protocol (PTP) format. If other values are used, an error is reported.
RTF	Format of the timestamp in a response packet. The value 2 indicates the NTP format, and the value 3 indicates the Precision Time Protocol (PTP) format. If other values are used, an error is reported.
Reply Mode	Reply mode. The value 1 indicates no reply, the value 2 indicates reply through IPv4/IPv6 UDP, and the value 4 indicates reply through BIER. Currently, only UDP is supported.
Return Code	The request packet must be set to 0. The response packet has the following settings: 0: No return code. No value is returned. 1: Malformed Echo Request received. An incorrect Echo Request packet is received. 2: One or more of the TLVs was not understood, indicating that an unknown TLV exists. 3: Replying BFR is the only BFER in header BitString. The responder is the destination node and does not forward packets any more. 4: Replying BFR is one of the BFER in header Bitstring. The responder is not the destination node and needs to forward the packet. 5: Packet-Forward-Success. The tracert packet is successfully forwarded by the transit node. 6: Invalid Multipath Info Request, which indicates that multipath information is incorrect (Multipath points to multiple BFERs). 8: No matching entry in forwarding table. The specified BitString cannot be found in the forwarding table. 9: Set-Identifier Mismatch. BIER-MPLS cannot be found in SI-BitString. 10: DDMAP mismatch. DDMAP mismatch occurs.
Sender's Handle	Sender handle, which is set in the request packet and returned without any change in the response packet.
Sequence Number	Sequence number, which is set in the request packet and returned without any change in the response packet.
TimeStamp Sent	Sending timestamp.
TimeStamp Received	Receiving timestamp.
TLVs	TLV encapsulated into the data area.

BIERv6 Tracert

As shown in Figure 1, PEs are edge nodes on the BIERv6 network. PE1 connects to the multicast source, PE2 through PE6 connect to multicast users, and P is a transmission node. PE2 can function as either a transmission node or a leaf node. The process of the BIERv6 tracert test from PE1 to PE4 is as follows:

PE1 initiates a BIERv6 tracert test and encapsulates a BIERv6 tracert request packet in the format shown in Figure 2. The source address encapsulated into the IPv6 header is an IPv6 address of the VPN to which the multicast traffic belongs, and the destination address is the End.BIER SID of the next-hop device. The specified target BFR-ID is encapsulated into BIER-Header and BIER OAM. BIER-Header is used to guide packet replication and forwarding, and BIER OAM is used for verification on the control plane. The Hop Limit field (equivalent to TTL in IPv4) in the outer IPv6 header is set to 64, and the TTL field in BIER-Header is initially set to 1. Each time a BIERv6 tracert packet passes through a BIERv6 device, the value of the TTL field in BIER-Header decreases by 1. Each time a BIERv6 tracert packet passes through a non-BIERv6 device, the value of the Hop Limit field in the IPv6 Header decreases by 1.
- Because the BitString length cannot be dynamically negotiated, you need to specify the length manually.
- Currently, only detection based on a single BitString is supported. Therefore, the target BFR-IDs entered by users must be within the same SI range.
After receiving the packet, PE2 (transmission node) searches the IPv6 FIB table based on the destination address and finds that this address is a local End.BIER SID. In this case, the subsequent actions are performed according to the BIER forwarding process. If the TTL field in BIER-Header is 0, PE2 needs to send the packet to the control plane for processing. After receiving the packet, the control plane returns a response packet to PE1 through a route.
After receiving the response packet from PE2, PE1 increases the value of the TTL field in BIER-Header by 1 (the value is now set to 2) and continues to send the BIERv6 tracert packet.
After receiving the packet, PE4 (leaf node) searches the IPv6 FIB table based on the destination address and finds that this address is a local End.BIER SID. In this case, the subsequent actions are performed according to the BIER forwarding process. If the TTL field in BIER-Header is 0, PE4 needs to send the packet to the control plane for processing. After receiving the packet, the control plane returns a response packet to PE1 through a route.
After receiving the response packet from PE4, PE1 parses the packet and finds that the BFR-ID in the response packet is the destination address of the BIERv6 tracert. The BIERv6 tracert ends.
PE1 displays the detection result.
- If no response is received within the timeout period after a request packet is sent, the message "Request time out" is displayed.
- If a response is received within a specified period after a request packet is sent, statistics are collected and displayed based on the BFR-ID of the received packet.

For details about the fields in a BIERv6 tracert request packet, see Table 1.