Lecture 4: Dynamic routing protocols
Today:1. Overview of router architecture2. RIP, OSPF, BGP3. Notes on Lab 44. Midterm review
Router Architectures
An overview of router architectures.
3
Two key router functions
Control plane: run routing protocols (RIP, OSPF, BGP)
Data plane: forwarding packets from incoming to outgoing link
routingtable
Routingfunctions
IPForwarding
routing tablelookup
routing tableupdates
incoming IPdatagrams
outgoing IPdatagrams
routingprotocol
routingprotocol
4
Routing and Forwarding
Routing functions include:– route calculation– maintenance of the routing table– execution of routing protocols
• On commercial routers handled by a single general purpose processor, called route processor
IP forwarding is per-packet processing• On high-end commercial routers, IP forwarding is distributed• Most work is done on the interface cards
5
Router Hardware Components
• Hardware components of a router:– Network interfaces– Switching fabrics– Processor with a memory
and CPU
Interface Card
Switching fabric
Interface Card Interface Card
Processor
CPUMemory
6
PC Router versus commercial router
• On a PC router: – Switching fabric is the (PCI)
bus – Interface cards are NICs (e.g.,
Ethernet cards)– All forwarding and routing is
done on central processor
• On Commercial routers:– Switching fabrics and
interface cards can be sophisticated
– Central processor is the route processor (only responsible for control functions)
Interface Card
Switching fabric
Interface Card Interface Card
Processor
CPUMemory
7
Basic Architectural ComponentsPer-packet processing
8
Evolution of Router Architectures
• Early routers were essentially general purpose computers • Today, high-performance routers resemble supercomputers
• Exploit parallelism• Special hardware components
• Until 1980s (1st generation): standard computer• Early 1990s (2nd generation): delegate to interfaces• Late 1990s (3rd generation): Distributed architecture• Today: Distributed over multiple racks
9
1st Generation Routers (switching via memory)
• This architecture is still used in low end routers
• Arriving packets are copied to main memory via direct memory access (DMA)
• Switching fabric is a backplane (shared bus)
• All IP forwarding functions are performed in the central processor.
• Routing cache at processor can accelerate the routing table lookup.
Memory
Shared Bus
DMA
MAC
DMA
MAC
InterfaceCard
DMA
MAC
Route Processor
InterfaceCard
InterfaceCard
CacheCPU
10
Drawbacks of 1st Generation Routers
• Forwarding Performance is limited by memory and CPU
• Capacity of shared bus limits the number of interface cards that can be connected
InputPort
OutputPort
Memory
System Bus
11
SharedBus
InterfaceCards
DMA
MAC
DMA
MAC
DMA
MAC
Route Cache
Memory
Route Cache
Memory
Route Cache
Memory
Route Processor
MemoryCacheCPU
2nd Generation Routers (switching via a shared bus)
• Keeps shared bus architecture, but offloads most IP forwarding to interface cards
• Interface cards have local route cache and processing elements
Fast path: If routing entry is found in local cache, forward packet directly to outgoing interface
Slow path: If routing table entry is not in cache, packet must be handled by central CPU
slow pathfast path
12
CPU
Cache
Memory
MAC MAC
Memory
Forwarding Bus(IP headers only)
InterfaceCards
Data Bus
Control Bus
Memory
MAC
Memory
ForwardingEngine
CPU
Cache
Memory
ForwardingEngine Route Processor
CPU
Memory
Another 2nd Generation Architecture
• IP forwarding is done by separate components (Forwarding Engines)
Forwarding operations:1. Packet received on interface:
Store the packet in local memory. Extracts IP header and sent to one forwarding engine
2. Forwarding engine does lookup, updates IP header, and sends it back to incoming interface
3. Packet is reconstructed and sent to outgoing interface.
13
Drawbacks of 2nd Generation Routers
SharedBus
InterfaceCards
DMA
MAC
DMA
MAC
DMA
MAC
Route Cache
Memory
Route Cache
Memory
Route Cache
Memory
Route Processor
MemoryCacheCPU
Bus contentionlimits throughput
14
3rd Generation Architecture
• Switching fabric is an interconnection network (e.g., a crossbar switch)
• Distributed architecture: – Interface cards operate
independent of each other – No centralized processing for
IP forwarding• These routers can be scaled to
many hundred interface cards and to aggregate capacity of > 1 Terabit per second
CPU
Memory
RouteProcessor
Memory
RouteProcessing
MAC
SwitchFabric
Interface
SwitchFabric
Memory
RouteProcessing
MAC
SwitchFabric
Interface
15
Slotted Chassis
• Large routers are built as a slotted chassis– Interface cards are inserted in the slots– Route processor is also inserted as a slot
• This simplifies repairs and upgrades of components
Dynamic Routing ProtocolsPart 1: RIP
Relates to Lab 4.
The first module on dynamic routing protocols. This module introduces RIP.
17
Routing
• Recall: There are two parts to routing IP packets:1. How to pass a packet from an input interface to the output interface of a router (packet forwarding) ? 2. How to find and setup a route ?
• We already discussed the packet forwarding part– Longest prefix match
• There are two approaches for calculating the routing tables:– Static Routing (Lab 3)– Dynamic Routing: Routes are calculated by a routing protocol
18
Routing protocols versus routing algorithms
• Routing protocols establish routing tables at routers.• A routing protocol specifies
– What messages are sent between routers– Under what conditions the messages are sent– How messages are processed to compute routing tables
• At the heart of any routing protocol is a routing algorithm that determines the path from a source to a destination
19
What routing algorithms common routing protocols use
Routing information protocol (RIP) Distance vectorInterior Gateway routing protocol (IGRP, cisco proprietary)
Distance vector
Open shortest path first (OSPF) Link stateIntermediate System-to-Intermediate System (IS-IS
Link state
Border gateway protocol (BGP) Path vector
Routing protocol Routing algorithm
20
Intra-domain routing versus inter-domain routing
• Recall Internet is a network of networks.• Administrative autonomy
– internet = network of networks– each network admin may want to control routing in its own
network• Scale: with 200 million destinations:
– can’t store all dest’s in routing tables!– routing table exchange would swamp links
21
Autonomous systems
• aggregate routers into regions, “autonomous systems” (AS) or domain• routers in the same AS run the same routing protocol
– “intra-AS” or intra-domain routing protocol– routers in different AS can run different intra-AS routing protocol
Ethernet
Router
Ethernet
Ethernet
RouterRouter
Ethernet
Ethernet
EthernetRouterRouter
Router
AutonomousSystem 2
AutonomousSystem 1
22
Autonomous Systems
• An autonomous system is a region of the Internet that is administered by a single entity.
• Examples of autonomous regions are:• UCI’s campus network• MCI’s backbone network• Regional Internet Service Provider
• Routing is done differently within an autonomous system (intradomain routing) and between autonomous system (interdomain routing).
• RIP, OSPF, IGRP, and IS-IS are intra-domain routing protocols.
• BGP is the only inter-domain routing protocol.
23
RIP and OSPF computes shortest paths
• Shortest path routing algorithms• Goal: Given a network where each link is assigned a
cost. Find the path with the least cost between two nodes.
a
b
c d3 1
6
2
24
Distance vector algorithm
• A decentralized algorithm– A router knows physically-connected neighbors and link
costs to neighbors– A router does not have a global view of the network
• Path computation is iterative and mutually dependent.– A router sends its known distances to each destination
(distance vector) to its neighbors.– A router updates the distance to a destination from all its
neighbors’ distance vectors– A router sends its updated distance vector to its neighbors.– The process repeats until all routers’ distance vectors do
not change (this condition is called convergence).
25
A router updates its distance vectors using bellman-ford equation
Bellman-Ford EquationDefinedx(y) := cost of the least-cost path from x to y
Then• dx(y) = minv{c(x,v) + dv(y) }, where min is taken over all
neighbors of node x
26
Distance vector algorithm: initialization
• Let Dx(y) be the estimate of least cost from x to y• Initialization:
– Each node x knows the cost to each neighbor: c(x,v). For each neighbor v of x, Dx(v) = c(x,v)
– Dx(y) to other nodes are initialized as infinity.• Each node x maintains a distance vector (DV):
– Dx = [Dx(y): y 2 N ]
27
Distance vector algorithm: updates
• Each node x sends its distance vector to its neighbors, either periodically, or triggered by a change in its DV.
• When a node x receives a new DV estimate from a neighbor v, it updates its own DV using B-F equation:– If c(x,v) + Dv(y) < Dx(y) then
• Dx(y) = c(x,v) + Dv(y)• Sets the next hop to reach the destination y to the
neighbor v• Notify neighbors of the change
• The estimate Dx(y) will converge to the actual least cost dx(y)
28
Distance vector algorithm: an example
• t = 0• a = ((a, 0), (b, 3), (c, 6))• b = ((a, 3), (b, 0), (c,1))• c = ((a, 6), (b, 1), (c, 0) (d, 2))• d = ((c, 2), (d, 0))
a
b
c d3 1
6
2
• t = 1• a = ((a, 0), (b, 3), (c, 4), (d, 8))• b = ((a, 3), (b, 0), (c,1), (d, 3))• c = ((a, 4), (b, 1), (c, 0), (d, 2))• d = ((a, 8), (b, 3), (c, 2), (d,0))
• t = 2• a = ((a, 0), (b, 3), (c, 4), (d, 6))• b = ((a, 3), (b, 0), (c,1), (d, 3))• c = ((a, 4), (b, 1), (c, 0), (d, 2))• d = ((a, 6), (b, 3), (c, 2), (d,0))
29
How to map the abstract graph to the physical network
• Nodes (e.g., v, w, n) are routers, identified by IP addresses, e.g. 10.0.0.1• Nodes are connected by either a directed link or a broadcast link (Ethernet)• Destinations are IP networks, represented by the network prefixes, e.g.,
10.0.0.0/16– Net(v,n) is the network directly connected to router v and n.
• Costs (e.g. c(v,n)) are associated with network interfaces.– Router1(config)# router rip– Router1(config-router)# offset-list 0 out 10 Ethernet0/0– Router1(config-router)# offset-list 0 out 10 Ethernet0/1
n
vw
Net
Net(v,w)
Net(v,n)
c(v,w)
c(v,n)
30
Distance vector routing protocol: Routing Table
Dest
n
vw
D (v,Net)n
costvia(next hop)
Net
RoutingTable of node v
Net
Net(v,w)c(v,w)
Net(v,n)c(v,n)
Net(v,w): Network address of the network between v and w
c(v,w): cost to transmit on the interface to network Net(v,w)
D(v,net) is v’s cost to Net
31
Distance vector routing protocol: Messages
Dest
D (v,Net)n
costvia(next hop)
Net
RoutingTable of node v
• Nodes send messages to their neighbors which contain distance vectors• A message has the format: [Net , D(v,Net)] means“My cost to go to Net is D (v,Net)”
v n[Net , D(v,Net)]
32
Distance vector routing algorithm: Sending Updates
Dest
D (v,Net 2)n
costvia(next hop)
Net 2
RoutingTable of node v
D (v,Net 1)mNet 1
D (v,Net N)wNet N
Periodically, each node v sends the content of its routing table to its neighbors:
n
v wm
[Net N,D(v,Net N)]
[Net 1,D(v,Net 1)]
[Net N,D(v,Net N)]
[Net 1,D(v,Net 1)]
[Net N,D(v,Net N)]
[Net 1,D(v,Net 1)]
33
Initiating Routing Table I
Destc (v,w)
Net(v,w)
0m
costvia(next hop)
Net(v,m)
RoutingTablec(v,m)
Net(v,m)
c(v,n)Net(v,n) 0wNet(v,w)
0nNet(v,n)n
v wm
• Suppose a new node v becomes active.• The cost to access directly connected networks is zero:
– D (v, Net(v,m)) = 0– D (v, Net(v,w)) = 0– D (v, Net(v,n)) = 0
34
Initiating Routing Table II
Dest
0m
costvia(next hop)
Net(v,m)
RoutingTable
0wNet(v,w)
0nNet(v,n)
• Node v sends the routing table entry to all its neighbors:
n
v wm[w,0][n,0 ] [n,0 ]
[m,0]
[m,0][w,0]
n
v wm[Net(v,w),0][Net(v,n),0] [Net(v,n),0]
[Net(v,m),0]
[Net(v,w),0][Net(v,m),0]
n
v wm[Net(v,w),0][Net(v,n),0] [Net(v,n),0]
[Net(v,m),0]
[Net(v,w),0][Net(v,m),0]
35
n
v wm
[Net N,D(n,Net N)]
[Net 1,D(n,Net 1)]
[Net N,D(m,Net N)]
[Net 1,D(m,Net 1)]
[Net N,D(w,Net N)]
[Net 1,D(w,Net 1)]
Initiating Routing Table III
• Node v receives the routing tables from other nodes and builds up its routing table
36
Updating Routing Tables I
c(v,m)Net(v,m)
n
v wmNet[Net,D(m,Net)]
• Suppose node v receives a message from node m: [Net,D(m,Net)]
if ( D(m,Net) + c (v,m) < D (v,Net) ) {Dnew (v,Net) := D (m,Net) + c (v,m);Update routing table;send message [Net, Dnew (v,Net)] to all neighbors
}
Node v updates its routing table and sends out further messages if the message reduces the cost of a route:
37
Updating Routing Tables II
c(v,m)Net(v,m)
n
v wmNet[Net,D(m,Net)]
• Before receiving the message:
Dest
D(v,Net)??
costvia(next hop)
Net
RoutingTable
c(v,m)Net(v,m)
n
v wmNet[Net,D new (v,Net)]
[Net,D new (v,Net)]
Dest
m
costvia(next hop)
Net
RoutingTable
D new (v,Net)
• Suppose D (m,Net) + c (v,m) < D (v,Net):
38
Example
Router A Router B Router C Router D
10.0.2.0/24 10.0.3.0/24 10.0.4.0/24 10.0.5.0/2410.0.1.0/24
.1.2.2.2.2 .1.1.1
Assume: - link cost is 1, i.e., c(v,w) = 1 - all updates, updates occur simultaneously - Initially, each router only knows the cost of connected interfaces
t=0:10.0.1.0 - 010.0.2.0 - 0
Net via cost
t=0:10.0.2.0 - 010.0.3.0 - 0
Net via cost
t=0:10.0.3.0 - 010.0.4.0 - 0
Net via cost
t=0:10.0.4.0 - 010.0.5.0 - 0
Net via cost
t=1:10.0.1.0 - 010.0.2.0 - 0 10.0.3.0 10.0.2.2 1
t=2:10.0.1.0 - 010.0.2.0 - 0 10.0.3.0 10.0.2.2 110.0.4.0 10.0.2.2 2
t=2:10.0.1.0 10.0.2.1 1 10.0.2.0 - 010.0.3.0 - 010.0.4.0 10.0.3.2 110.0.5.0 10.0.3.2 2
t=1:10.0.1.0 10.0.2.1 1 10.0.2.0 - 010.0.3.0 - 010.0.4.0 10.0.3.2 1
t=2:10.0.1.0 10.0.3.1 2 10.0.2.0 10.0.3.1 1 10.0.3.0 - 010.0.4.0 - 010.0.5.0 10.0.4.2 1
t=1:10.0.2.0 10.0.3.1 1 10.0.3.0 - 010.0.4.0 - 010.0.5.0 10.0.4.2 1
t=2:10.0.2.0 10.0.4.1 210.0.3.0 10.0.4.1 110.0.4.0 - 010.0.5.0 - 0
t=1:10.0.3.0 10.0.4.1 110.0.4.0 - 010.0.5.0 - 0
39
Example
Router A Router B Router C Router D
10.0.2.0/24 10.0.3.0/24 10.0.4.0/24 10.0.5.0/2410.0.1.0/24
.1.2.2.2.2 .1.1.1
t=3:10.0.1.0 - 010.0.2.0 - 0 10.0.3.0 10.0.2.2 110.0.4.0 10.0.2.2 210.0.5.0 10.0.2.2 3
Net via cost
t=3:10.0.1.0 10.0.2.1 1 10.0.2.0 - 010.0.3.0 - 010.0.4.0 10.0.3.2 110.0.5.0 10.0.3.2 2
Net via cost
t=3:10.0.1.0 10.0.3.1 2 10.0.2.0 10.0.3.1 1 10.0.3.0 - 010.0.4.0 - 010.0.5.0 10.0.4.2 1
Net via cost
t=3:10.0.1.0 10.0.4.1 310.0.2.0 10.0.4.1 210.0.3.0 10.0.4.1 110.0.4.0 - 010.0.5.0 - 0
Net via cost
Now, routing tables have converged !
t=2:10.0.1.0 - 010.0.2.0 - 0 10.0.3.0 10.0.2.2 110.0.4.0 10.0.2.2 2
t=2:10.0.1.0 10.0.2.1 1 10.0.2.0 - 010.0.3.0 - 010.0.4.0 10.0.3.2 110.0.5.0 10.0.3.2 2
t=2:10.0.1.0 10.0.3.1 2 10.0.2.0 10.0.3.1 1 10.0.3.0 - 010.0.4.0 - 010.0.5.0 10.0.4.2 1
t=2:10.0.2.0 10.0.4.1 210.0.3.0 10.0.4.1 110.0.4.0 - 010.0.5.0 - 0
40
Characteristics of Distance Vector Routing Protocols
• Periodic Updates: Updates to the routing tables are sent at the end of a certain time period. A typical value is 30 seconds.
• Triggered Updates: If a metric changes on a link, a router immediately sends out an update without waiting for the end of the update period.
• Full Routing Table Update: Most distance vector routing protocol send their neighbors the entire routing table (not only entries which change).
• Route invalidation timers: Routing table entries are invalid if they are not refreshed. A typical value is to invalidate an entry if no update is received after 3-6 update periods.
41
The Count-to-Infinity Problem
A B C1 1
A's Routing Table B's Routing Table
C
to costvia(next hop)
2B C
to costvia(next hop)
1C
now link B-C goes down
C 2 C
C 1-C 2B
C C 3
C 3AC -
C 4 C
C -C 4B
11
11
1
42
Count-to-Infinity
• The reason for the count-to-infinity problem is that each node only has a “next-hop-view”
• For example, in the first step, A did not realize that its route (with cost 2) to C went through node B
• How can the Count-to-Infinity problem be solved?
43
Count-to-Infinity
• The reason for the count-to-infinity problem is that each node only has a “next-hop-view”
• For example, in the first step, A did not realize that its route (with cost 2) to C went through node B
• How can the Count-to-Infinity problem be solved?• Solution 1: Always advertise the entire path in an update
message to avoid loops (Path vectors)– BGP uses this solution
44
Count-to-Infinity
• The reason for the count-to-infinity problem is that each node only has a “next-hop-view”
• For example, in the first step, A did not realize that its route (with cost 2) to C went through node B
• How can the Count-to-Infinity problem be solved?• Solution 2: Never advertise the cost to a neighbor if this
neighbor is the next hop on the current path (Split Horizon)– Example: A would not send the first routing update to B, since B
is the next hop on A’s current route to C– Split Horizon does not solve count-to-infinity in all cases!
» You can produce the count-to-infinity problem in Lab 4.
45
RIP - Routing Information Protocol
• A simple intradomain protocol• Straightforward implementation of Distance Vector Routing• Each router advertises its distance vector every 30 seconds
(or whenever its routing table changes) to all of its neighbors• RIP always uses 1 as link metric• Maximum hop count is 15, with “16” equal to “”• Routes are timeout (set to 16) after 3 minutes if they are not
updated
46
RIP - History
• Late 1960s : Distance Vector protocols were used in the ARPANET
• Mid-1970s: XNS (Xerox Network system) routing protocol is the ancestor of RIP in IP (and Novell’s IPX RIP and Apple’s routing protocol)
• 1982 Release of routed for BSD Unix• 1988 RIPv1 (RFC 1058)
- classful routing• 1993 RIPv2 (RFC 1388)
- adds subnet masks with each route entry - allows classless routing
• 1998 Current version of RIPv2 (RFC 2453)
47
RIPv1 Packet Format
IP header UDP header RIP Message
Command Version Set to 00...0
32-bit address
Unused (Set to 00...0)
address family Set to 00.00
Unused (Set to 00...0)
metric (1-16)
one
rout
e en
try(2
0 by
tes)
Up to 24 more routes (each 20 bytes)
32 bits
One RIP message can have up to 25 route entries
1: request2: response
2: for IP
Address of destination
Cost (measured in hops)
1: RIPv1
48
RIPv2
• RIPv2 is an extends RIPv1:– Subnet masks are carried in the route information– Authentication of routing messages– Route information carries next-hop address– Uses IP multicasting
• Extensions of RIPv2 are carried in unused fields of RIPv1 messages
49
RIPv2 Packet Format
IP header UDP header RIP Message
Command Version Set to 00...0
32-bit address
Unused (Set to 00...0)
address family Set to 00.00
Unused (Set to 00...0)
metric (1-16) one
rout
e en
try(2
0 by
tes)
Up to 24 more routes (each 20 bytes)
32 bits
One RIP message can have up to 25 route entries
1: request2: response
2: for IP
Address of destination
Cost (measured in hops)
2: RIPv2
50
RIPv2 Packet Format
IP header UDP header RIPv2 Message
Command Version Set to 00.00
IP address
Subnet Mask
address family route tag
Next-Hop IP address
metric (1-16) one
rout
e en
try(2
0 by
tes)
Up to 24 more routes (each 20 bytes)
32 bits
Used to provide a method of separating "internal" RIP routes (routes for networks within the RIP routing domain) from "external" RIP routes
Identifies a better next-hop address on the same subnet than the advertising router, if one exists (otherwise 0….0)
2: RIPv2
Subnet mask for IP address
51
RIP Messages
• This is the operation of RIP in routed. Dedicated port for RIP is UDP port 520.
• Two types of messages: – Request messages
• used to ask neighboring nodes for an update– Response messages
• contains an update
52
Routing with RIP
• Initialization: Send a request packet (command = 1, address family=0..0) on all interfaces:
• RIPv1 uses broadcast if possible, • RIPv2 uses multicast address 224.0.0.9, if possible
requesting routing tables from neighboring routers • Request received: Routers that receive above request send their entire
routing table• Response received: Update the routing table
• Regular routing updates: Every 30 seconds, send all or part of the routing tables to every neighbor in an response message
• Triggered Updates: Whenever the metric for a route change, send entire routing table.
53
RIP Security
• Issue: Sending bogus routing updates to a router• RIPv1: No protection• RIPv2: Simple authentication scheme
IP header UDP header RIPv2 Message
Command Version Set to 00.00
Password (Bytes 0 - 3)
Password (Bytes 4 - 7)
0xffff Authentication Type
Password (Bytes 8- 11)
Password (Bytes 12 - 15) Auth
etic
atio
nUp to 24 more routes (each 20 bytes)
32 bits
2: plaintext password
54
RIP Problems
• RIP takes a long time to stabilize– Even for a small network, it takes several minutes until the
routing tables have settled after a change• RIP has all the problems of distance vector algorithms, e.g.,
count-to-Infinity » RIP uses split horizon to avoid count-to-infinity
• The maximum path in RIP is 15 hops
Relates to Lab 4. This module covers link state routing and the Open Shortest Path First (OSPF) routing protocol.
Dynamic Routing Protocols IIOSPF
56
Distance Vector vs. Link State Routing
• With distance vector routing, each node has information only about the next hop:
• Node A: to reach F go to B• Node B: to reach F go to D• Node D: to reach F go to E• Node E: go directly to F
• Distance vector routing makespoor routing decisions if directions are not completelycorrect (e.g., because a node is down).
• If parts of the directions incorrect, the routing may be incorrect until the routing algorithms has re-converged.
A B C
D E F
57
Distance Vector vs. Link State Routing
• In link state routing, each node has a complete map of the topology
• If a node fails, each node can calculate the new route
• Difficulty: All nodes need to have a consistent view of the network
A B C
D E F
A B C
D E F
A B C
D E F
A B C
D E F
A B C
D E F
A B C
D E F
A B C
D E F
58
Link State Routing: Properties
• Each node requires complete topology information• Link state information must be flooded to all nodes• Guaranteed to converge
59
Link State Routing: Basic principles
1. Each router establishes a relationship (“adjacency”) with its neighbors2. Each router generates link state advertisements (LSAs) which are
distributed to all routers LSA = (link id, state of the link, cost, neighbors of the link)
Each router sends its LSA to all routers in the network (using a method called reliable flooding)
3. Each router maintains a database of all received LSAs (topological database or link state database), which describes the network has a graph with weighted edges
4. Each router uses its link state database to run a shortest path algorithm (Dijikstra’s algorithm) to produce the shortest path to each network
60
Link state routing: graphical illustration
a
b
c d3 1
6
2
a
3
6
b
c
a
b
c
3 1
a
b
c d
1
6
c d2
a’s view
b’s view
c’s view
d’s view
Collecting all pieces yielda complete view of the network!
61
Operation of a Link State Routing protocol
ReceivedLSAs
IP Routing Table
Dijkstra’s
Algorithm
Link StateDatabase
LSAs are flooded to other interfaces
62
Dijkstra’s Shortest Path Algorithm for a Graph
Input: Graph (N,E) with N the set of nodes and E the set of edges
cvw link cost (cvw = 1 if (v,w) E, cvv = 0)s source node.
Output: Dn cost of the least-cost path from node s to node n
M = {s};for each n M
Dn = csn;while (M all nodes) do
Find w M for which Dw = min{Dj ; j M};Add w to M;for each neighbor n of w and n M
Dn = min[ Dn, Dw + cwn ];Update route;
enddo
63
OSPF
• OSPF = Open Shortest Path First• The OSPF routing protocol is the most important link state routing
protocol on the Internet (another link state routing protocol is IS-IS (intermediate system to intermediate system)
• The complexity of OSPF is significant– RIP (RFC 2453 ~ 40 pages)– OSPF (RFC 2328 ~ 250 pages)
• History:– 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)
64
Features of OSPF
• Provides authentication of routing messages• Enables load balancing by allowing traffic to be split evenly
across routes with equal cost• Type-of-Service routing allows to setup different routes
dependent on the TOS field• Supports subnetting• Supports multicasting• Allows hierarchical routing
65
Hierarchical OSPF
66
Hierarchical OSPF
• Two-level hierarchy: local area, backbone.
– Link-state advertisements only in area – each nodes has detailed area topology; only know
direction (shortest path) to nets in other areas.• Area border routers: “summarize” distances to nets in own area,
advertise to other Area Border routers.• Backbone routers: run OSPF routing limited to backbone.
67
Example Network
Router IDs can be selected independent of interface addresses, but usually chosen to be the smallest interface address
3
4 2
5
1
1
32
•Link costs are called Metric
• Metric is in the range [0 , 216]
• Metric can be asymmetric
10.1.1.0 / 24
.1 .2 .2
10.1.1.1
10.1.4.0 / 24
10.1.2.0 / 24
.1
.4
10.1.7.0 / 24
10.1
.6.0
/ 24
10.1
.3.0
/ 24
10.1.5.0/24
10.1.
8.0 / 2
4
.3
.3 .5
.2
.3
.5
.5
.4
.4
.6
.6
10.1.1.2 10.1.4.4 10.1.7.6
10.1.2.3 10.1.5.5
68
Link State Advertisement (LSA)
• The LSA of router 10.1.1.1 is as follows:
• Link State ID: 10.1.1.1 = Router ID
• Advertising Router: 10.1.1.1 = Router ID• Number of links: 3 = 2 links plus router itself
• Description of Link 1: Link ID = 10.1.1.2, Metric = 4• Description of Link 2: Link ID = 10.1.2.2, Metric = 3• Description of Link 3: Link ID = 10.1.1.1, Metric = 0
10.1.1.0 / 24
.1 .2 .210.1.1.1
10.1.4.0 / 24
10.1.2.0 / 24
.1
.410.1.7.0 / 24
10.1
.6.0
/ 24
10.1
.3.0
/ 24
10.1.5.0/24
10.1.
8.0 / 2
4
.3.3 .5
.2
.3
.5
.5
.4
.4
.6
.610.1.1.2 10.1.4.4 10.1.7.6
10.1.2.3 10.1.5.5
4
3 2
69
Network and Link State Database
Each router has a database which contains the LSAs from all other routers
LS Type Link StateID Adv. Router Checksum LS SeqNo LS Age
Router-LSA 10.1.1.1 10.1.1.1 0x9b47 0x80000006 0
Router-LSA 10.1.1.2 10.1.1.2 0x219e 0x80000007 1618
Router-LSA 10.1.2.3 10.1.2.3 0x6b53 0x80000003 1712
Router-LSA 10.1.4.4 10.1.4.4 0xe39a 0x8000003a 20
Router-LSA 10.1.5.5 10.1.5.5 0xd2a6 0x80000038 18
Router-LSA 10.1.7.6 10.1.7.6 0x05c3 0x80000005 1680
10.1.1.0 / 24
.1 .2 .210.1.1.1
10.1.4.0 / 24
10.1.2.0 / 24
.1
.410.1.7.0 / 24
10.1
.6.0
/ 24
10.1
.3.0
/ 24
10.1.5.0/24
10.1.
8.0 / 2
4
.3.3 .5
.2
.3
.5
.5
.4
.4
.6
.610.1.1.2 10.1.4.4 10.1.7.6
10.1.2.3 10.1.5.5
70
Link State Database
• The collection of all LSAs is called the link-state database• Each router has an 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
71
OSPF Packet Format
OSPF MessageIP header
Body of OSPF MessageOSPF MessageHeader
Message TypeSpecific Data LSA LSALSA ...
LSAHeader
LSAData
...
Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters)
TTL: set to 1 (in most cases)
OSPF packets are not carried as UDP payload!OSPF has its own IP protocol number: 89
72
OSPF Packet Format
source router IP address
authentication
authentication32 bits
version type message length
Area ID
checksum authentication type
Body of OSPF MessageOSPF MessageHeader
2: current version is OSPF V2
Message types:1: Hello (tests reachability)2: Database description3: Link Status request4: Link state update5: Link state acknowledgement
ID of the Area from which the packet originated
Standard IP checksum taken over entire packet
0: no authentication1: Cleartext password2: MD5 checksum(added to end packet)
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)
Prevents replay attacks
73
OSPF LSA Format
Link State ID
link sequence number
advertising router
Link Age Link Type
checksum length
Link ID
Link Data
Link Type Metric#TOS metrics
LSA
LSAHeader
LSAData
Link ID
Link Data
Link Type Metric#TOS metrics
LSA Header
Link 1
Link 2
74
Discovery of Neighbors
• Routers multicasts OSPF Hello packets on all OSPF-enabled interfaces.
• If two routers share a link, they can become neighbors, and establish an adjacency
• After becoming a neighbor, routers exchange their link state databases
OSPF Hello
OSPF Hello: I heard 10.1.10.2
10.1.10.1 10.1.10.2
Scenario:Router 10.1.10.2 restarts
75
Neighbor discovery and database synchronization
OSPF Hello
OSPF Hello: I heard 10.1.10.2
Database Description: Sequence = X
10.1.10.1 10.1.10.2
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
Database Description: Sequence = X+1
Sends empty database description
Scenario:Router 10.1.10.2 restarts
Discovery of adjacency
Sends database description. (description only contains LSA headers)
Database description of 10.1.10.2Acknowledges
receipt of description
After neighbors are discovered the nodes exchange their databases
76
Regular LSA exchanges
10.1.10.2 explicitly requests each LSA from 10.1.10.1
10.1.10.1 sends requested LSAs
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,
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
77
Routing Data Distribution
• LSA-Updates are distributed to all other routers via Reliable Flooding
• Example: Flooding of LSA from 10.10.10.1
LSA
LSA
Updatedatabase
Updatedatabase
ACK
ACK
LSA
LSA
LSA
LSA A
CK
AC
K
ACK
ACK
LSA
LSA
LSA
LSA
Updatedatabase
Updatedatabase
ACK
AC
K
ACK
AC
K
Updatedatabase
10.1.1.1 10.1.2.2 10.1.3.4 10.1.7.6
10.1.1.2 10.1.4.5
78
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 not new changes.
• Acknowledgements of LSA-updates:• explicit ACK, or• implicit via reception of an LSA-Update
• Question: If a new node comes up, it could build the database from regular LSA-Updates (rather than exchange of database description). What role do the database description packets play?
Border Gateway protocol (BGP)
80
BGP
• BGP = Border Gateway Protocol . Currently in version 4, specified in RFC 1771. (~ 60 pages)
• Note: In the context of BGP, a gateway is nothing else but an IP router that connects autonomous systems.
• Interdomain routing protocol for routing between autonomous systems
• Uses TCP to establish a BGP session and to send routing messages over the BGP session
• BGP is a path vector protocol. Routing messages in BGP contain complete routes.
• Network administrators can specify routing policies
81
BGP policy routing
• BGP’s goal is to find any path (not an optimal one). Since the internals of the AS are never revealed, finding an optimal path is not feasible.
• Network administrator sets BGP’s policies to determine the best path to reach a destination network.
82
BGP basics
• A route is defined as a unit of information that pairs a destination with the attributes of a path to that destination.
• EBGP session is a BGP session between two routers in different ASes.
• IBGP session is a BGP session between internal routers of an AS.
83
EBGP and IBGP
• IBGP is organized into a full mesh topology, or IBGP sessions are relayed using a route reflector.
128.195.0.0/16 0 128.195.0.0/16 0
128.195.0.0/16 1 0AS 0
AS 1
AS 2
AS 3128.195.0.0/16 2 1 0
R1R2 R3
R4
R5R6
R7
R8
84
Commonly BGP attributes
• Origin: whether it is an internal prefix or an prefix learned from BGP peers• AS path• Next hop• Multi_Exit_Disc (MED, multiple exit discriminator): used to distinguish
routes learned from different peers of the same neighboring AS• Local_pref• Community: group routes to communities
85
BGP route selection process
• Input/output engine may filter routes or manipulate their attributes
InputPolicyEngine
Decisionprocess
Bestroutes
OutPolicyEnigne
Routes recved from peers Routes sentto peers
86
Best path selection algorithm
1. If next hop is inaccessible, ignore routes2. Prefer the route with the largest local preference value.3. If local prefs are the same, prefer route with the shortest AS
path4. If AS_path is the same, prefer route with lowest origin (IGP
< EGP < incomplete)5. If origin is the same, prefer the route with lowest MED6. IF MEDs are the same, prefer EBGP paths to IBGP paths7. If all the above are the same, prefer the route that can be
reached via the closest IGP neighbor.8. If the IGP costs are the same, prefer the router with lowest
router id.
87
Example of BGP route selection
InputPolicyEngine
Decisionprocess
Bestroutes
OutPolicyEnigne
AS1
AS2
AS 5
AS3
AS4
128.195.0.0/16
0/0
128.195.0.0/16
0/0
•Deny 0/0 from AS1•Give 128.195.0.0/16•From AS1 higher•Local_pref•Accept other routes
•Accept 0/0 from AS2•Use AS1 to reach 128.195.0.0/16
0/0 AS2128.195.0.0/16 AS1
•Do not propagate 0/0 .
128.195.0.0/16
128.195.0.0/16
88
Summary
• Router architectures• Dynamic routing protocols: RIP, OSPF, BGP• RIP uses distance vector algorithm, and converges slow (the
count-to-infinity problem)• OSPF uses link state algorithm, and converges fast. But it is
more complicated than RIP.• Both RIP and OSPF finds lowest-cost path.• BGP uses path vector algorithm, and its path selection
algorithm is complicated, and is influenced by policies.
Lab 4: dynamic routing protocols
90
Exercise (4B): count-to-infinity is optional
• Time consuming to reproduce, but interesting.• Why does count-to-infinity still exist with split horizon?• Lab report due after midterm
Router2
Router4
Router3
Router1101
1
11
1
91
Why does count-to-infinity still exist with split horizon?
Router2Router4
Router3
Router1101
1
11X1
10.0.1.0/24
Router3’s routing table:10.0.1.0/24 ?? 1
Router2’s routing table:10.0.1.0/24 ?? 1
Router4’s routing table:10.0.1.0/24 Router3 3
Router2 is not Router4’s next hop.Router4 sends to router2 the routing update
Router2’s routing table:10.0.1.0/24 Router 4 4
This lie will be told to Router3 andCirculates in the system count-to-infinity
Suppose updates happen in the following sequence:1. The update from PC3 arrives at Router2. The update from Router 3 arrives at Router 23. The update from Router 4 arrives at Router 2
PC3
Midterm review
93
What you’ll be tested on
• Basic lab commands– E.g., ping, traceroute, tcpdump, ethereal, ifconfig, how to
copy a file, how to list a directory• Basic trouble shooting
– E.g., I cannot ping 128.195.1.150, why?• Basic networking concepts
– E.g., layering principle, multiplexing, and encapsulation• Protocols we’ve covered so far
– ARP– ICMP– IP
94
Address translation protocol
• What is it used for?• What is an ARP cache used for?
95
ICMP
• What is it used for?– E.g. error reporting, route redirect
• When will an ICMP message be triggered?
96
IP
• Network order versus host order• CIDR addressing• Route aggregation• Longest prefix match• Fragmentation