Section 3.3. Message Architecture


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 nestedthat is, a TLV can exist inside another TLV.

[2] ISO terminology calls this construct a code/length/variable (CLV). The acronym TLV is much better known, and so is used throughout this book.

Extensibility of the Message Architecture

Because OSPF messages are built of set-length, well-defined fields, only the LSAs are extendableand these only by defining new LSAs when needed. As a result, OSPF can be difficult to extend. Not only must new LSAs be defined, but the OSPF processes on all routers where the new LSAs are used must be able to negotiate the acceptance of these LSAs. And in one caseextending OSPF to support IPv6an entirely new version of OSPF is required. Opaque LSAs, discussed in Chapter 10, help alleviate this extensibility problem in some cases.

The use of TLVs in both IS-IS messages and LSPs make IS-IS much more easily extendable. IPv6 support, for example, requires the simple addition of two TLVs to the existing messages. The comparative extensibility of the two protocols is discussed in more detail in Chapter 10, and specific extensions are covered in Chapters 11 through 13.


Figure 3.1 compares the general structure of OSPF and IS-IS messages. The types of messages depicted here, and the meanings of the fields in each message, are not important yet. For now, you only need to know that the formats shown are the same ones used throughout this book for depicting the messages of each protocol. OSPF messages are always depicted on their 32-bit boundaries, whereas IS-IS messages are always shown with labels to the right indicating the length in bytes of the variable-length fields. As Dave Katz says, OSPF's 32-bit alignment is mostly useful these days in that it provides tidy packet pictures.

Figure 3.1. The conventions used to depict OSPF and IS-IS messages throughout this book.





OSPF and IS-IS(c) Choosing an IGP for Large-Scale Networks
OSPF and IS-IS: Choosing an IGP for Large-Scale Networks: Choosing an IGP for Large-Scale Networks
ISBN: 0321168798
EAN: 2147483647
Year: 2006
Pages: 111
Authors: Jeff Doyle

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