3.3. Message Architecture
OSPF messages and LSAs are all structured on 32-bit boundaries. The original intention was to make the messages easier to parse. But in these times of high-speed processors and abundant memory, such architecture does not matter much any more. The format of OSPF messages is fixed, so the protocol can only be extended by creating new LSAs.
IS-IS does not adhere to set boundaries, and all messages are constructed of type-specific headers followed by type/length/value (TLV) structures.[2] The type and length fields are each one octet, and specify the type and length (in bytes) of the data in the value field. Because the length field is one byte, the value field can vary from 0 to 254 bytes. They can also be nested—that ...
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