Link State & OSPF
Spring 2013
CE 151 - Advanced Networks
1
Administrativia
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Static Routing lab
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How are the labs going?
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Cruzio… I’m waiting to hear back
NMO Software Development for Cisco Advanced Services… waiting for applications
Expect more from campus network operations group…
Next week
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I will start upgrades of VMs today… will send e-mail w/ new password when done
Opportunities
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Wording problems
VM stability issues
Link Layer lab due Wednesday, 4/24
Link-State Routing quiz Thursday, 4/25
Project proposal due Tuesday 4/30
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Spring 2013
Topic
Draft outline
What you need to investigate
CE 151 - Advanced Networks
2
Routing Introduction
•
Remember… delivery of IP packets implemented by two processes
– Forwarding
– Routing
•
Dynamic routing process is a distributed computation
– Triggered by topology changes
– Processing defined by a routing protocol
– Output of the computation is forwarding state
•
Goal of computation is convergence
– After finite sequence of topology changes
– Process should terminate updates to forwarding state
– Forwarding state should be correct
• Loop-free (after convergence vs. instantaneous)
• With desired characteristics: performance, possibly conform to policy
Spring
2013
3
CE 151 - Advanced Networks
“Fate-sharing” Principle
•
“The fate-sharing model suggests that it is acceptable to lose the state
information associated with an entity if, at the same time, the entity itself is
lost.” Dave Clark, “Design Philosophy of the DARPA Internet Protocols”,
SIGCOMM ‘88.
•
Benefits
– Ensures the failure of any single component of an internet does not invalidate
state located elsewhere in the internet
– Localizing the effects of any failures
– More robust system
•
Internet routing architecture co-locates
– Forwarding state
– Routing process that computes the state
•
Compared with virtual-circuit routing?
Spring
2013
4
CE 151 - Advanced Networks
Routing Protocol
•
A set of algorithms and messages that are used to exchange topology information
and populate the forwarding table with the routing protocol’s choice of best paths.
•
Purpose:
– Discover remote subnets
– Maintain up-to-date forwarding tables
– Choose the best path to destination subnets
•
Components of a routing protocol:
– Algorithm:
• Procedures for
– Processing routing information
– Selecting best-paths
• The data structures needed for these steps.
– Routing protocol messages:
• Discover neighboring routers
• Exchange topology information
Spring
2013
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CE 151 - Advanced Networks
Classifying Routing Protocols
• Function:
– Intra-domain/Interior Gateway Protocol (IGP)
– Inter-domain/Exterior Gateway Protocol (EGP)
• Algorithm… distinguished by information exchanged:
– Distance-Vector
– Link-State
– Path-Vector
Spring
2013
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CE 151 - Advanced Networks
Functional Classification
• An autonomous system (AS) or routing domain is a region of the
Internet that is administered by a single entity
– UCSC’s network
– IBM’s corporate network
– AT&T’s ISP network
Ethernet
Router
Ethernet
Autonomous
System 1
Router
Router
• Routing inside an AS
– Focus is on performance
Ethernet
– Popular protocols: RIP, OSPF, IS-IS
– Called intra-domain or internal gateway (IGP) routing
• Routing between ASs
– Focus is on policy
– Popular protocol: BGP
– Called inter-domain or external gateway (EGP) routing
Spring
2013
7
Ethernet
CE 151 - Advanced Networks
Router
Router
Ethernet
Autonomous
System 2
Router
Ethernet
How ensure correct routes?
•
Recall requirement for correctness of routing protocol
– Loop-free
– Desired path characteristics
•
Two strategies for ensuring correctness
– Use identical algorithm for selecting paths
• Share minimal topology information
• Use identical path selection algorithm at all nodes
• Used for IGP/Intra-domain routing
• Use link-state or distance vector protocol
– Use custom (private) algorithm for selecting paths
• Share full path information
• Use policy-specific path selection algorithm at each node
• Used for EGP/Inter-domain routing
• Use path-vector protocol
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CE 151 - Advanced Networks
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Algorithm Classification
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Distance-Vector
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Vectors of destination and distance sent to neighbors
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Destination in terms of a network prefix
Distance in terms of a metric: hop count, delay, bandwidth
Use Distributed Bellman-Ford path selection algorithm
Popular protocol: Routing Information Protocol (RIP)
Link-State
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Flood description of your links (link state)
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“Tell the rest of the network about your neighbors”
Links described by
•
•
•
“Tell your neighbors about the rest of the network”
End-point routers of subnet in internet
Cost of subnet: delay, bandwidth
Use Dijkstra path selection algorithm
Popular protocol: Open Shortest Path First (OSPF)
Path-Vector
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Routes advertised as full-paths
Paths described by sequence of ASs
Popular protocol is Border Gateway Routing Protocol (BGP)
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2013
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CE 151 - Advanced Networks
Destination-Based Forwarding
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Internet routing uses a single path per destination
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Destination-based forwarding is a restricted version of single-path
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A path through a node to a destination…
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…must be an extension of the path from the node to the destination.
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This is coming back to haunt us…
Spring 2013
CE 151 - Advanced Networks
10
Review
•
The Internet implements a distributed routing architecture that is triggered
by topology change events.
•
Convergence of routing in the Internet depends on the stability of the
network topology for a sufficient period of time.
•
The Fate-Sharing Principle – “It is acceptable to lose the state information
associated with an entity if, at the same time, the entity itself is lost.”
– Ensures the failure of any single component of an internet does not invalidate
state located elsewhere in the internet
– Localizing the effects of any failures
– Results in a more robust system
– Achieved in Internet by co-locating
• Forwarding state
• Routing process that computes the state
Spring
11 2013
CE 151 - Advanced Networks
Review
•
An Autonomous System (AS) is a region of the Internet that is administered
by a single entity and follows a single routing policy.
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Functional classification of routing protocols
– IGP - routing inside an AS
• Share minimal topology information
• Use identical path selection algorithm at all nodes
• Optimize performance
– EGP - routing between ASs
• Share full path information
• Use custom path selection algorithm at each node to implement desired policies
• Enforce policies
Spring
12 2013
CE 151 - Advanced Networks
Review
•
Algorithmic classification of routing protocols
– Distance-vector
• Send vectors of distances to destinations to neighbors
• “Tell your neighbors about the rest of the network”
• “Distributed computation”
– Link-state
• Flood description of your links to all routers
• “Tell the rest of the network about your neighbors”
• “Distributed database”
– Path-vector
• Distance-vector with full paths
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Review
•
Destination-based forwarding
– A path through a node to a destination… must be an extension of the path from
the node to the destination
– Tends concentrates traffic on a subset of the network topology.
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Link-State
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Routing Algorithms
•
Distance-Vector
– Vectors of destination and distance sent to neighbors
•
•
– Destination in terms of a network prefix
– Distance in terms of a metric: hop count, delay, bandwidth
– Use Distributed Bellman-Ford path selection algorithm
– Popular protocol: Routing Information Protocol (RIP)
Link-State
– Flood description of your links (link state)
•
–
“Tell the rest of the network about your neighbors”
Links described by
•
•
•
“Tell your neighbors about the rest of the network”
End-point routers of subnet in internet
Cost of subnet: delay, bandwidth
– Use Dijkstra path selection algorithm
– Popular protocol: Open Shortest Path First (OSPF)
Path-Vector
– Routes advertised as full-paths
– Paths described by sequence of ASs
– Popular protocol is Border Gateway Routing Protocol (BGP)
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CE 151 - Advanced Networks
16
Routing Algorithms
•
Distance-Vector
– Vectors of destination and distance sent to neighbors
•
•
– Destination in terms of a network prefix
– Distance in terms of a metric: hop count, delay, bandwidth
– Use Distributed Bellman-Ford path selection algorithm
– Popular protocol: Routing Information Protocol (RIP)
Link-State
– Flood description of your links (link state)
•
–
“Tell the rest of the network about your neighbors”
Links described by
•
•
•
“Tell your neighbors about the rest of the network”
End-point routers of subnet in internet
Cost of subnet: delay, bandwidth
– Use Dijkstra path selection algorithm
– Popular protocol: Open Shortest Path First (OSPF)
Path-Vector
– Routes advertised as full-paths
– Paths described by sequence of ASs
– Popular protocol is Border Gateway Routing Protocol (BGP)
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17
How ensure correct routes?
• Recall requirement for correctness of routing protocol
– Loop-free
– Desired path characteristics
• Two strategies for ensuring correctness
– Use identical algorithm for selecting paths
•
•
•
•
Share minimal topology information
Use identical path selection algorithm at all nodes
Used for IGP/Intra-domain routing
Use link-state or distance vector protocol
– Use custom (private) algorithm for selecting paths
•
•
•
•
Spring 2013
Share full path information
Use policy-specific path selection algorithm at each node
Used for EGP/Inter-domain routing
Use path-vector protocol
CE 151 - Advanced Networks
18
How ensure correct routes?
• Recall requirement for correctness of routing protocol
– Loop-free
– Desired path characteristics
• Two strategies for ensuring correctness
– Use identical algorithm for selecting paths
•
•
•
•
Share minimal topology information
Use identical path selection algorithm at all nodes
Used for IGP/Intra-domain routing
Use link-state or distance vector protocol
– Use custom (private) algorithm for selecting paths
•
•
•
•
Spring 2013
Share full path information
Use policy-specific path selection algorithm at each node
Used for EGP/Inter-domain routing
Use path-vector protocol
CE 151 - Advanced Networks
19
Link-State Protocols
• Are Interior-Gateway Protocols (IGPs)
• Exchange link-state information
– Pair of routers connected by a subnet
– Cost of subnet (hop count, delay, etc.)
• Conceptually, very simple…
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Link-State Protocols
• Maintains a topology database of all the links it has heard of
– Initialize with the subnets it is connected to.
• Floods link-state updates describing its directly connected subnets,
including any changes to these links.
– “Tell the rest of the network about your neighbors”
• Participates in the flooding of link-state updates from other routers.
• On update of its topology database
– Runs a shortest-path algorithm on the database to compute routes
• Dijkstra is most efficient
– Updates its forwarding table with any changes.
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Characterizing Link State
Link-State
• # updates per link change?
– One.
• How far propagate updates?
– Flooded to all nodes.
• One update, global distribution.
• Scaling problems due to flooding
•
As we’ll see next lecture, the characteristics of distance vector…
– …are very different
– …hint at a much better solution
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Dijstra Shortest-Path Algorithm
•
Breadth-first search of paths, by increasing path cost, for best paths to all
destinations. Terminate when path has been found for all destinations.
•
Maintain two sets
– Destinations for which shortest paths have been found.
• Permanently labeled destinations P
• Initialize with self
– Destinations for which candidate shortest paths have been found.
• Temporarily labeled destinations T
• Initialize with my neighbors.
•
Iterate
– Move shortest path in T, say for destination D, to P
– Add routes for D’s neighbors, that are extensions of the path to T, to T if they are shorter
than the current path in T for each neighbor. The “relaxation” step.
– Repeat until a route has been added to P for all destinations
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More formally…
•
E is the set of edges.
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wij is the weight of the link between
nodes i and j.
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P and T… see previous slide.
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P and T entries are triples, <d, p, w>:
– d is the destination
– p is the predecessor
– w is the link weight
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CE 151 - Advanced Networks
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Dijkstra… path cost
B,2
C,
7
2
3
2
3
2
A,0
D,
F,
E,
2
2
6
1
4
G,
H,
B,2
C,
A,0
E,4
G,5
B,2
A,0
D,
F,
A,0
E,4
C,
E,4
A,0
H,
C,
E,4
D,
A,0
H,8
B,2
A,0
G,5
H,
C,9
E,4
G,5
C,9
E,4
D,
F,6
G,5
B,2
F,6
D,
F,
G,
B,2
H,
C,
G,5
Spring 2013
B,2
D,
F,6
H,8
D,10
F,6
H,8
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Translating to a Protocol
• Dijkstra requires a “centralized” implementation
– Maintain a full graph of the network, on an event-driven basis
– Re-compute routes as the graph changes
– Flood changes to your links
• “Brute-force” protocol.
– Straight-forward, easy to understand
– Inefficient… lots of overhead
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Review
• Dijkstra
– Iterates on “next shortest path”
– Requires centralized computation
• LS protocols
– Is an IGP
– Implements a centralized routing model
– Floods link-state updates describing current state of its links
• “Tell the rest of the network about your neighbors”
– Use Dijkstra algorithm because it is most efficient shortest-path algorithm
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The Challenge of Internet Routing
• Independent routing computations at each router…
• Need to compute paths that…
– Support destination-based forwarding
– Are shortest
– Are loop-free
• This is trickier than it looks…
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Examples illustrating challenges
of distributed routing…
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Correct solution depends on
combination of routing algorithm
and algebra used for metrics.
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OSPF
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OSPF
• OSPF = Open Shortest Path First
• The most widely used routing protocol
• The complexity of OSPF is significant
• History:
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1989: RFC 1131 OSPF Version 1
1991: RFC1247 OSPF Version 2
1994: RFC 1583 OSPF Version 2 (revised)
1997: RFC 2178 OSPF Version 2 (revised)
1998: RFC 2328 OSPF Version 2 (current version)
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What We Cover…
• Messages
• Router IDs
• Flooding process
• Metrics
• Designated routers
• Areas
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OSPF Messages
• An OSPF message can contain one of five packet types.
• OSPF is embedded directly in an IP frame (doesn’t use UDP)
– Protocol field is set to 89 (OSPF)
– Destination address is typically set to one of two multicast addresses:
• 224.0.0.5 (“AllSPFRouters”… Hello messages) or
• 224.0.0.6 (“AllDRouters”… routing info to “Designated Routers”).
• If the OSPF packet is encapsulated in an Ethernet frame, the destination
MAC address is also a multicast address:
– 01-00-5E-00-00-05 or 01-00-5E-00-00-06
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OSPF Packet Types
• Hello: Used to establish and maintain adjacency with other OSPF routers.
• DBD: The database description (DBD) packet contains an abbreviated list
of the sending router’s link-state database and is used by receiving routers
to check against the local link-state database.
• LSR: Receiving routers can then request more information about any entry
in the DBD by sending a link-state request (LSR).
• LSU: Link-state update (LSU) packets are used to reply to LSRs and to
announce new information. LSUs contain seven different types of linkstate advertisements (LSA).
• LSAck: When an LSU is received, the router sends a link-state
acknowledgment (LSAck) to confirm receipt of the LSU.
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OSPF Message Header
OSPF Message
Header
2: current version
is OSPF V2
Message types:
1: Hello (tests reachability)
2: Database description
3: Link Status request
4: Link state update
5: Link state acknowledgement
Standard IP checksum taken
over entire packet
Body of OSPF Message
version
message length
type
source router IP address
Area ID
authentication type
checksum
authentication
authentication
32 bits
Authentication passwd = 1: 64 cleartext password
Authentication passwd = 2: 0x0000 (16 bits)
KeyID (8 bits)
Length of MD5 checksum (8 bits)
Nondecreasing sequence number (32 bits)
Spring 2013
CE 151 - Advanced Networks
ID of the Area
from which the
packet originated
0: no authentication
1: Cleartext
password
2: MD5 checksum
(added to end
packet)
Prevents replay
attacks
36
RouterIDs
•
Router
ID
Router ID plays an important role in OSPF
– Uniquely identifies each router in a routing domain
– Used in Designated Router election process
(explained later)
•
Router ID is an IP address of a router
•
Cisco routers use following algorithm to
determine Router ID
Router
ID
– IP address configured with OSPF router-id
command
– If not configured, use highest IP address of a
loopback interface
– If no loopback interfaces, use highest active IP
address of physical interface
•
Advantage of loopback interface is it cannot fail
Spring 2013
Router
ID
Router
ID
Router
ID
CE 151 - Advanced Networks
Router
ID
37
OSPF Hello Packets
• Used to
– Discover OSPF neighbors and establish neighbor adjacencies
– Elect the Designated Router and Backup Designated Router on
multiaccess networks such as Ethernet and Frame Relay
– Negotiate
• Hello interval (e.g. 10 sec on Ethernet segments)
• Dead interval: time to declare neighbor down (4x Hello interval)
• Network type
• Five network types
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Point-to-point
Point-to-multipoint
Broadcast multiaccess (Ethernet)
Nonbroadcast multiaccess (Frame Relay)
Virtual links
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OSPF LSU Packets
• Link State Update (LSU) packets
– Used for OSPF routing updates
– Contain one or more LSAs
• Link State Advertisements (LSAs)
– Contain route information for destination networks
– There are 11 types of LSAs
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Link State Advertisement (LSA)
10.10.10.1
10.10.10.2
.1
• The LSA of router 10.10.10.1 is as follows:
.2
10.1.1.0 / 24
.2
10
. 1.
2. 0
/2
4
10.10.10.1 = can be Router ID
10.10.10.1 = Router ID
3 = 2 links plus router itself
10.1.4.0 / 24
10.1.3.0 / 24
.1
• Link State ID:
• Advertising Router:
• Number of links:
.2
.3
.3
.3
• Description of Link 1: Link ID = 10.1.1.1, Metric = 4
• Description of Link 2: Link ID = 10.1.2.1, Metric = 3
• Description of Link 3: Link ID = 10.10.10.1, Metric = 0
10.1.5.0/24
10.10.10.3
Each router sends its LSA to all routers in the network
(using a method called reliable flooding)
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LSA Format
LSA
Link Age
Link State ID
LSA
Header
LSA
Header
LSA
Data
Link Type
advertising router
link sequence number
checksum
length
Link ID
Link Data
Link 1
Link Type #TOS metrics
Metric
Link ID
Link Data
Link 2
Link Type #TOS metrics
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CE 151 - Advanced Networks
Metric
41
OSPF Metrics
•
The OSPF metric is called cost. The following passage is from RFC 2328:
–
A cost is associated with the output side of each router interface. This cost is configurable
•
RFC 2328 does not specify which values should be used to determine the cost.
•
In Cisco IOS
–
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Spring 2013
Link cost is 108 ÷ link bandwidth.
Reference bandwidth (108) can be changed with auto-cost referencebandwidth
Modify link bandwidth value with bandwidth
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Link State Database
• The collection of all LSAs is called the link-state database
• Each router has and identical link-state database
– Useful for debugging: Each router has a complete description of the network
• If neighboring routers discover each other for the first time, they will
exchange their link-state databases
• The link-state databases are synchronized using reliable flooding
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Link State Database
10.10.10.2
.2
.2
.4
10.1.1.0 / 24
.4
10.1.4.0 / 24
.6
10.1.7.0 / 24
.4
.6
. 1.
4
/2
10
4
10
/2
.1
2. 0
10.1.6.0 / 24
.2
10.1.3.0 / 24
.1
Each router has a
database which
contains the LSAs
from all other routers
10.10.10.6
.0
.1
10.10.10.4
.8
10.10.10.1
.3
.5
.3
.3
.5
.5
10.1.5.0/24
10.10.10.2
10.10.10.5
LS Type
Link StateID
Adv. Router
Checksum
LS SeqNo
LS Age
Router-LSA
10.1.10.1
10.1.10.1
0x9b47
0x80000006
0
Router-LSA
10.1.10.2
10.1.10.2
0x219e
0x80000007
1618
Router-LSA
10.1.10.3
10.1.10.3
0x6b53
0x80000003
1712
Router-LSA
10.1.10.4
10.1.10.4
0xe39a
0x8000003a
20
Router-LSA
10.1.10.5
10.1.10.5
0xd2a6
0x80000038
18
Router-LSA
10.1.10.6
10.1.10.6
0x05c3
0x80000005
1680
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Neighbor Discovery
• Router multicasts OSPF Hello packets on all OSPF-enabled interfaces.
• If two routers share a link, they can become neighbors, and establish an
adjacency
10.1.10.1
10.1.10.2
Scenario:
Router 10.1.10.2 restarts
OSPF Hello
OSPF Hello: I heard 10.1.10.2
• After becoming a neighbor, routers exchange their link state databases
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Neighbor discovery and
database synchronization
10.1.10.1
Discovery of
adjacency
Scenario:
Router 10.1.10.2 restarts
10.1.10.2
OSPF Hello
OSPF Hello: I heard 10.1.10.2
After neighbors are discovered the nodes exchange their databases
Database Description: Sequence = X
Sends database
description.
(description only
contains LSA
headers)
Acknowledges
receipt of
description
Spring 2013
Database Description: Sequence = X, 5 LSA headers =
Router-LSA, 10.1.10.1, 0x80000006
Router-LSA,
10.1.10.2, 0x80000007
Router-LSA,
10.1.10.3, 0x80000003
Router-LSA,
10.1.10.4, 0x8000003a
Router-LSA,
10.1.10.5, 0x80000038
Router-LSA,
10.1.10.6, 0x80000005
Database Description: Sequence = X+1, 1 LSA header=
Router-LSA,
10.1.10.2, 0x80000005
Sends empty
database
description
Database
description of
10.1.10.2
Database Description: Sequence = X+1
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Regular LSA exchanges
10.1.10.1
10.1.10.2
Link State Request packets, LSAs =
Router-LSA,
10.1.10.1,
Router-LSA,
10.1.10.2,
Router-LSA,
10.1.10.3,
Router-LSA,
10.1.10.4,
Router-LSA,
10.1.10.5,
Router-LSA,
10.1.10.6,
10.1.10.1 sends
requested LSAs
Link State Update Packet, LSAs =
Router-LSA, 10.1.10.1, 0x80000006
Router-LSA, 10.1.10.2, 0x80000007
Router-LSA, 10.1.10.3, 0x80000003
Router-LSA, 10.1.10.4, 0x8000003a
Router-LSA, 10.1.10.5, 0x80000038
Router-LSA, 10.1.10.6, 0x80000005
10.1.10.2 explicitly
requests each LSA
from 10.1.10.1
10.1.10.2 has more
recent value for
10.0.1.6 and sends it
to 10.1.10.1
(with higher sequence
number)
Link State Update Packet, LSA =
Router-LSA,
10.1.1.6, 0x80000006
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Dissemination of LSA-Update
• A router sends and refloods LSA-Updates, whenever the topology or link
cost changes. (If a received LSA does not contain new information, the
router will not flood the packet)
• Exception: Infrequently (every 30 minutes), a router will flood LSAs even if
there are no new changes.
• Acknowledgements of LSA-updates:
– explicit ACK, or
– implicit via reception of an LSA-Update
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Why Designated Routers?
• Large number of adjacencies
– Full mesh of adjacencies
– n(n - 1)/2 adjacencies
• Excessive load from flooding LSAs
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Electing Designated Router
• OSPF elects a Designated Router (DR) on multiaccess networks
• DR is collection and distribution point for LSAs on network
• Backup Designated Router (BDR) also elected for case where DR fails
224.0.0.5
224.0.0.6
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Electing Designated Router
• Election held through exchange of Hello messages
• DR/BDR election criteria
– DR is router with highest interface priority
– BDR is router with second-highest interface priority
– If interface priorities are equal, use Router ID
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Electing Designated Router
• Further elections only occur on failure of DR/BDR
• On DR failure
– BDR promoted to DR
– New BDR elected
• On BDR failure
– New BDR elected
• Live (B)DR not replaced by election.
– If DR dies
• BDR replaces DR
• Election held to replace BDR
– If BDR dies
• Election held to replace BDR
Spring 2013
CE 151 - Advanced Networks
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OSPF Areas
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An OSPF Area is a set of routers that share link state information
•
Goal is to address scalability problem with OSPF flooding
•
Area 0 is the backbone area
– All areas must be connected to he backbone area
– Best practice to use area 0 in single-area OSPF
– Eases conversion to multi-area
Spring 2013
CE 151 - Advanced Networks
53
Review
• OSPF message types
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–
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–
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Hello - establish and maintain adjacency with OSPF routers
DataBase Description (DBD) - summary of my database
Link-State Request (LSR) - request for detailed entry
Link-State Update (LSU) - response to LSR
Link-State Ack (LSAck) - acknowledge receipt of LSU
• Router IDs…
– Uniquely identifies each router in a routing domain
– Used in Designated Router election process
– Best practice is to use loopback interface... it can’t fail
Spring 2013
CE 151 - Advanced Networks
54
Review
• OSPF metrics…
– “Cost is associated with the output side of each router interface.”
– In IOS is 108 / link bandwidth (inversely proportional to bandwidth)
• Designated routers…
– Mitigate impact of “full-mesh” on # of adjacencies in topology (n(n-1)/2)
– DR is collection and distribution point for LSAs on network
• OSPF deals with scaling problem by using “areas,” area 0 is the backbone.
Spring 2013
CE 151 - Advanced Networks
55