假设我有两个直接连接的路由器r1和r2. 例如，我确实看到r2发送Label Mapping Message到r1并提供了几个标签，包括21（0x15）：
r1将标签与LDP 标签数据库中的21前缀关联：192.0.2.7/32
Input label database, 192.0.2.1:0--192.0.2.4:0
Labels received: 7
Label Prefix
23 192.0.2.1/32
17 192.0.2.2/32
16 192.0.2.3/32
3 192.0.2.4/32
19 192.0.2.5/32
20 192.0.2.6/32
21 192.0.2.7/32
为什么r1将标签21与前缀相关联192.0.2.7/32？
FEC 元素是地址或前缀，当作为映射消息的一部分包含在 FEC TLV 中时，它们被映射到相关联的标签。
如果您展开数据包捕获的FEC 元素 1部分，它很可能会显示该元素192.0.2.7/32。这在FEC TLV中以RFC5036 §3.4.1中记录的格式编码，每个元素具有以下结构和字段：
Prefix FEC Element value encoding:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (2) | Address Family | PreLen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Address Family
Two octet quantity containing a value from ADDRESS FAMILY
NUMBERS in [ASSIGNED_AF] that encodes the address family for
the address prefix in the Prefix field.
PreLen
One octet unsigned integer containing the length in bits of
the address prefix that follows. A length of zero indicates
a prefix that matches all addresses (the default
destination); in this case, the Prefix itself is zero
octets).
Prefix
An address prefix encoded according to the Address Family
field, whose length, in bits, was specified in the PreLen
field, padded to a byte boundary.
映射消息本身记录在 §3.5.7 中。