rfc2328.hastabs.txt

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	referred to as variable	length subnetting.  A packet is	routed
	to the best (i.e., longest or most specific) match.  Host routes
	are considered to be subnets whose masks are "all ones"
	(0xffffffff).

	All OSPF protocol exchanges are	authenticated.	This means that
	only trusted routers can participate in	the Autonomous System's
	routing.  A variety of authentication schemes can be used; in
	fact, separate authentication schemes can be configured	for each
	IP subnet.

	Externally derived routing data	(e.g., routes learned from an
	Exterior Gateway Protocol such as BGP; see [Ref23]) is
	advertised throughout the Autonomous System.  This externally
	derived	data is	kept separate from the OSPF protocol's link
	state data.  Each external route can also be tagged by the
	advertising router, enabling the passing of additional
	information between routers on the boundary of the Autonomous
	System.




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    1.2.  Definitions of commonly used terms

	This section provides definitions for terms that have a	specific
	meaning	to the OSPF protocol and that are used throughout the
	text.  The reader unfamiliar with the Internet Protocol	Suite is
	referred to [Ref13] for	an introduction	to IP.


	Router
	    A level three Internet Protocol packet switch.  Formerly
	    called a gateway in	much of	the IP literature.

	Autonomous System
	    A group of routers exchanging routing information via a
	    common routing protocol.  Abbreviated as AS.

	Interior Gateway Protocol
	    The	routing	protocol spoken	by the routers belonging to an
	    Autonomous system.	Abbreviated as IGP.  Each Autonomous
	    System has a single	IGP.  Separate Autonomous Systems may be
	    running different IGPs.

	Router ID
	    A 32-bit number assigned to	each router running the	OSPF
	    protocol.  This number uniquely identifies the router within
	    an Autonomous System.

	Network
	    In this memo, an IP	network/subnet/supernet.  It is	possible
	    for	one physical network to	be assigned multiple IP
	    network/subnet numbers.  We	consider these to be separate
	    networks.  Point-to-point physical networks	are an exception
	    - they are considered a single network no matter how many
	    (if	any at all) IP network/subnet numbers are assigned to
	    them.

	Network	mask
	    A 32-bit number indicating the range of IP addresses
	    residing on	a single IP network/subnet/supernet.  This
	    specification displays network masks as hexadecimal	numbers.





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	    For	example, the network mask for a	class C	IP network is
	    displayed as 0xffffff00.  Such a mask is often displayed
	    elsewhere in the literature	as 255.255.255.0.

	Point-to-point networks
	    A network that joins a single pair of routers.  A 56Kb
	    serial line	is an example of a point-to-point network.

	Broadcast networks
	    Networks supporting	many (more than	two) attached routers,
	    together with the capability to address a single physical
	    message to all of the attached routers (broadcast).
	    Neighboring	routers	are discovered dynamically on these nets
	    using OSPF's Hello Protocol.  The Hello Protocol itself
	    takes advantage of the broadcast capability.  The OSPF
	    protocol makes further use of multicast capabilities, if
	    they exist.	 Each pair of routers on a broadcast network is
	    assumed to be able to communicate directly.	An ethernet is
	    an example of a broadcast network.

	Non-broadcast networks
	    Networks supporting	many (more than	two) routers, but having
	    no broadcast capability.  Neighboring routers are maintained
	    on these nets using	OSPF's Hello Protocol.	However, due to
	    the	lack of	broadcast capability, some configuration
	    information	may be necessary to aid	in the discovery of
	    neighbors.	On non-broadcast networks, OSPF	protocol packets
	    that are normally multicast	need to	be sent	to each
	    neighboring	router,	in turn. An X.25 Public	Data Network
	    (PDN) is an	example	of a non-broadcast network.

	    OSPF runs in one of	two modes over non-broadcast networks.
	    The	first mode, called non-broadcast multi-access or NBMA,
	    simulates the operation of OSPF on a broadcast network. The
	    second mode, called	Point-to-MultiPoint, treats the	non-
	    broadcast network as a collection of point-to-point	links.
	    Non-broadcast networks are referred	to as NBMA networks or
	    Point-to-MultiPoint	networks, depending on OSPF's mode of
	    operation over the network.






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	Interface
	    The	connection between a router and	one of its attached
	    networks.  An interface has	state information associated
	    with it, which is obtained from the	underlying lower level
	    protocols and the routing protocol itself.	An interface to
	    a network has associated with it a single IP address and
	    mask (unless the network is	an unnumbered point-to-point
	    network).  An interface is sometimes also referred to as a
	    link.

	Neighboring routers
	    Two	routers	that have interfaces to	a common network.
	    Neighbor relationships are maintained by, and usually
	    dynamically	discovered by, OSPF's Hello Protocol.

	Adjacency
	    A relationship formed between selected neighboring routers
	    for	the purpose of exchanging routing information.	Not
	    every pair of neighboring routers become adjacent.

	Link state advertisement
	    Unit of data describing the	local state of a router	or
	    network. For a router, this	includes the state of the
	    router's interfaces	and adjacencies.  Each link state
	    advertisement is flooded throughout	the routing domain. The
	    collected link state advertisements	of all routers and
	    networks forms the protocol's link state database.
	    Throughout this memo, link state advertisement is
	    abbreviated	as LSA.

	Hello Protocol
	    The	part of	the OSPF protocol used to establish and	maintain
	    neighbor relationships.  On	broadcast networks the Hello
	    Protocol can also dynamically discover neighboring routers.

	Flooding
	    The	part of	the OSPF protocol that distributes and
	    synchronizes the link-state	database between OSPF routers.

	Designated Router
	    Each broadcast and NBMA network that has at	least two
	    attached routers has a Designated Router.  The Designated



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	    Router generates an	LSA for	the network and	has other
	    special responsibilities in	the running of the protocol.
	    The	Designated Router is elected by	the Hello Protocol.

	    The	Designated Router concept enables a reduction in the
	    number of adjacencies required on a	broadcast or NBMA
	    network.  This in turn reduces the amount of routing
	    protocol traffic and the size of the link-state database.

	Lower-level protocols
	    The	underlying network access protocols that provide
	    services to	the Internet Protocol and in turn the OSPF
	    protocol.  Examples	of these are the X.25 packet and frame
	    levels for X.25 PDNs, and the ethernet data	link layer for
	    ethernets.


    1.3.  Brief	history	of link-state routing technology

	OSPF is	a link state routing protocol.	Such protocols are also
	referred to in the literature as SPF-based or distributed-
	database protocols.  This section gives	a brief	description of
	the developments in link-state technology that have influenced
	the OSPF protocol.

	The first link-state routing protocol was developed for	use in
	the ARPANET packet switching network.  This protocol is
	described in [Ref3].  It has formed the	starting point for all
	other link-state protocols.  The homogeneous ARPANET
	environment, i.e., single-vendor packet	switches connected by
	synchronous serial lines, simplified the design	and
	implementation of the original protocol.

	Modifications to this protocol were proposed in	[Ref4].	 These
	modifications dealt with increasing the	fault tolerance	of the
	routing	protocol through, among	other things, adding a checksum
	to the LSAs (thereby detecting database	corruption).  The paper
	also included means for	reducing the routing traffic overhead in
	a link-state protocol.	This was accomplished by introducing
	mechanisms which enabled the interval between LSA originations
	to be increased	by an order of magnitude.




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	A link-state algorithm has also	been proposed for use as an ISO
	IS-IS routing protocol.	 This protocol is described in [Ref2].
	The protocol includes methods for data and routing traffic
	reduction when operating over broadcast	networks.  This	is
	accomplished by	election of a Designated Router	for each
	broadcast network, which then originates an LSA	for the	network.

	The OSPF Working Group of the IETF has extended	this work in
	developing the OSPF protocol.  The Designated Router concept has
	been greatly enhanced to further reduce	the amount of routing
	traffic	required.  Multicast capabilities are utilized for
	additional routing bandwidth reduction.	 An area routing scheme
	has been developed enabling information
	hiding/protection/reduction.  Finally, the algorithms have been
	tailored for efficient operation in TCP/IP internets.


    1.4.  Organization of this document

	The first three	sections of this specification give a general
	overview of the	protocol's capabilities	and functions.	Sections
	4-16 explain the protocol's mechanisms in detail.  Packet
	formats, protocol constants and	configuration items are
	specified in the appendices.

	Labels such as HelloInterval encountered in the	text refer to
	protocol constants.  They may or may not be configurable.
	Architectural constants	are summarized in Appendix B.
	Configurable constants are summarized in Appendix C.

	The detailed specification of the protocol is presented	in terms
	of data	structures.  This is done in order to make the
	explanation more precise.  Implementations of the protocol are
	required to support the	functionality described, but need not
	use the	precise	data structures	that appear in this memo.


    1.5.  Acknowledgments

	The author would like to thank Ran Atkinson, Fred Baker, Jeffrey
	Burgan,	Rob Coltun, Dino Farinacci, Vince Fuller, Phanindra
	Jujjavarapu, Milo Medin, Tom Pusateri, Kannan Varadhan,	Zhaohui



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	Zhang and the rest of the OSPF Working Group for the ideas and
	support	they have given	to this	project.

	The OSPF Point-to-MultiPoint interface is based	on work	done by
	Fred Baker.

	The OSPF Cryptographic Authentication option was developed by
	Fred Baker and Ran Atkinson.


2.  The	Link-state Database: organization and calculations

    The	following subsections describe the organization	of OSPF's link-
    state database, and	the routing calculations that are performed on
    the	database in order to produce a router's	routing	table.


    2.1.  Representation of routers and	networks

	The Autonomous System's	link-state database describes a	directed
	graph.	The vertices of	the graph consist of routers and
	networks.  A graph edge	connects two routers when they are
	attached via a physical	point-to-point network.	 An edge
	connecting a router to a network indicates that	the router has
	an interface on	the network. Networks can be either transit or
	stub networks. Transit networks	are those capable of carrying
	data traffic that is neither locally originated	nor locally
	destined. A transit network is represented by a	graph vertex
	having both incoming and outgoing edges. A stub	network's vertex
	has only incoming edges.

	The neighborhood of each network node in the graph depends on
	the network's type (point-to-point, broadcast, NBMA or Point-
	to-MultiPoint) and the number of routers having	an interface to
	the network.  Three cases are depicted in Figure 1a.  Rectangles
	indicate routers.  Circles and oblongs indicate	networks.
	Router names are prefixed with the letters RT and network names
	with the letter	N.  Router interface names are prefixed	by the
	letter I.  Lines between routers indicate point-to-point
	networks.  The left side of the	figure shows networks with their
	connected routers, with	the resulting graphs shown on the right.




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