Nov 11, 2004CS573: Network Protocols and Standards1 IP Routing: OSPF Network Protocols and Standards...

Nov 11, 2004 CS573: Network Protocols and Standards

1

IP Routing: OSPF

Network Protocols and Standards

Autumn 2004-2005

Nov 11, 2004 CS573: Network Protocols and Standards 2

Issues Design of OSPF

Separating hosts and routers Broadcast networks (Ethernet, FDDI,

…) Non-broadcast networks (ATM, X.25,

…) Splitting very large networks into

areas


Separating Hosts and Routers

Instead of link-state records for hosts:R-H1R-H2R-H3

Use simplification based on subnet model- One link between the router and the subnet: “link to stub network”- Identified by its subnet number

R H1H2

H3


Stub Networks

Multiaccess/broadcast stub network Single router attached to the network N is network IP address and associated mask

N

RTRT

N

Graph Representation


Point to Point Networks

Unnumbered point-to-point network Interfaces to point-to-point network not

assigned IP addresses

RT1 RT1 RT2


RT2


Point to Point Networks

Numbered point-to-point network Ia and Ib are interface IP addresses

RT1 RT1 RT2


RT2Ia Ib

Ia Ib


Stub Networks

Host directly attached to a router Host route Mask is 0xFFFF FFFF (255.255.255.255)

Host

RTRT

Host



Broadcast Networks Such networks characterized by:

Full connectivity Broadcast capability

Issues (when N routers coexist on a broadcast network):

# Adjacencies: N(N-1)/2 Each router would advertise:

N-1 links to other routers One link to the subnetwork

Solution: reduce number of adjacencies to N One router is designated through election Election of the designated router is through “Hello

Protocol”


Broadcast Networks Reducing the number of link state

records using designated router Database will include two links per router

One link from router to virtual node Called router link type 2 (transit network) Advertised by the router itself Appropriate metric

Link from virtual node to router Called network link Advertised by designated router Network links have a null metric


Broadcast Networks

RT1

RT2

RT3

RT4

N

RT1 RT2

RT4RT3

Multiaccess/broadcast transit network - 3 or more routers attached to the network


Broadcast Networks Simplifying flooding

A router sends a link state advertisement to the designated router only using 224.0.0.6 “all designated routers” multicast address

If advertisement is new, designated router floods the link state on all its interfaces (including the network on which it is received!) using 224.0.0.5 “all OSPF routers” multicast address


Broadcast Networks Designated router forms adjacencies with all

routers in the broadcast network For reliability, there is one backup designated

router Backup designated router also forms adjacencies with

all routers in the broadcast network Listens to 224.0.0.6 but remains silent in terms of

flooding, etc. Failure of designated router is discovered by means of

Hello protocol Smoother transition occurs since the backup router

has already formed all the adjacencies


Non-broadcast Networks IP over X.25 networks

Popular in Europe in 1980’s IP over ATM

Static configurations Avoid N(N-1)/2 overhead

Use designated router On-demand circuits Permanent circuits are for links between

routers and the “designated router”


Link State DB records Five types of link state records

1. Router link2. Network link3. Summary link (IP network)4. Summary link (to a border router)5. External link

Types 3 and 4 are used when OSPF areas are used


OSPF Router ID Each OSPF router is assigned an

OSPF router ID 32 bit number uniquely identifying

the router within the OSPF domain When the router interfaces have IP

addresses assigned to them, then the OSPF router ID is one of the router’s IP address


LSA Header

LS AGE LS TYPE

LINK STATE ID

OPTIONS

ADVERTISING ROUTER

LS SEQUENCE NUMBER

LS CHECKSUM LENGTH

ADVERTISEMENT DATA (LINKS)

OSPF HEADERTYPE=4

#ADVERTISEMENTS

ADVERTISEMENT 1

ADVERTISEMENT N

…

4 BYTES

Advertise

men

tH

eader


LSA Header Fields Link state Type

Type of LS record (1, 2, 3, 4, or 5) Link state ID

Chosen by the advertising router Generally an IP address

Sequence Number Identifies one particular advertisement

Checksum Protects header as well as content

Length Total length of the record (including the 20-byte

header)


LSA Header Fields Advertising router

The OSPF ID of the sender Age

16-bit unsigned integer indicating the time in seconds since the link state record was first advertised

Options:

E: used in Hello protocol T: Set when router supports nonzero TOS

Removed from the latest version of the standard (RFC2328)

E TRFC 1583Definition

Type of ServiceExternal Links


Multiple Areas Hierarchical routing

Decreased routing overhead Size of link state DB Duration of route computation Volume of messages exchanged

Split the network into set of independent parts by a backbone

Each area operates like an independent network Database includes only the state of the area’s links Flooding stops at the boundaries Routers compute routes within the area

Cost of routing proportional to the size of the area


Multiple Areas How to glue the network together? Some routers belong to several areas

Typically to the backbone and to one lower-level area

At least one area border router in each area Area border routers

Maintain several link state databases (one for each area to which they belong)

Emit special link state records (summaries) to signal reachability of networks in each area


Stub Areas Areas where there is only one exit point, or the

exit point is not a function of the external destination

Stub area does not need to know the topology of the rest of the AS

All external traffic goes to the exit point Obviously, no AS boundary router can be

internal to the stub areas

R


OSPF Packet Formats OSPF directly over IP, using protocol number 89 OSPF does not explicitly support fragmentation,

but protocol messages can generally be split This should be used rather than IP fragmentation

OSPF packets are sent with an IP TOS of 0 OSPF packets are sent with IP precedence set

to Internetwork control All OSPF packets use the same OSPF header OSPF Multicast addresses (sent with TTL=1)

224.0.0.5: All OSPF routers 224.0.0.6: OSPF designated and backup routers


The Common OSPF Header

VERSION # TYPE PACKET LENGTH

ROUTER ID

AREA ID

AUTHENTICATION

AUTHENTICATION

CHECKSUM AUTYPE


OSPF Header Fields Version #: set to 2 (current version) Type: The OSPF packet type

1. Hello2. Database description3. Link state request4. Link state update5. Link state acknowledgement

Packet length: Number of bytes in the header including the header

Router ID: The IP selected for identifying the router Area ID: The value 0 is reserved for backbone area.

Commonly, an IP address is used to identify the areas Checksum: Computed on the whole OSPF packet,

excluding the 8-octet authentication field


OSPF Header Fields AUTYPE: Identifies the authentication algorithm. Only

three values are identified in the standard itself: 0: No authentication

Exchanges not authenticated Authentication field ignored; can be set to anything

1: Simple authentication “Clear password” type of authentication; all packets must

contain the right value, pre-configured for that area Used to prevent unconfigured routers from joining in

2: Cryptographic authentication Secret key is used to generate a digest of the packet Digest is added at the end of the packet; size not included in

the header 64-bit field is restructured to contain digest size, key ID, and

sequence number (to protect against replay attacks)

Nov 11, 2004CS573: Network Protocols and Standards1 IP Routing: OSPF Network Protocols and Standards...

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