Download Routing Table Lookup

Link State Routing Algorithm
• Use a routing protocol to collect the whole network topology
• Obtain destination reachability information as well as link
weights/states
•Compute shortest paths using Dijkstra’s algorithm from a
node to all other nodes
•Construct routing tables that show the destination addresses
and the next hop addresses
•Note that while Dijkstra’s algorithm gives you end-to-end
routes, the routing tables may only store the next hop
address.
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
1
Shortest path calculations
Calculate shortest paths for node s to all other nodes
Dijkstra’s Algorithm:
s
source node.
Dn
cost of the least-cost path from node s to node n
M = {s};
for each n  M
Dn = dsn;
while (M  all nodes) do
Find w  M for which Dw = min{Dj ; j  M};
Add w to M;
for each n  M
Dn = minw [ Dn, Dw + dwn ];
Update route;
enddo
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
2
Example
• Assume node 1 has obtained the entire network topology
using some link state routing protocol
•Construct the routing table at node 1 using Dijkstra’s
algorithm to determine shortest paths from node 1 to all
other nodes in the network
5
2
5
2
1
1
3
1
2
6
2
4
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
3
3
1
5
3
Example (at node 1)
Iteration
M
D1
D2
D3
D4
D5
D6
Init
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
4
Solution
• Use Dijkstra’s algorithm described in slide 2
• Start with M consisting of only node 1.
0
1
2
3
4
M
{1}
{1,4}
{1,4,2,5}
{1,4,2,5,3}
{1,4,2,5,3,6}
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
D1
0
0
0
0
0
D2
2
2
2
2
2
D3
5
4
3
3
3
D4
1
1
1
1
1
D5
inf
2
2
2
2
D6
inf
inf
4
4
4
5
Resulting Routing Tree
2
2
3
1
1
1
4
1
5
6
2
•The tree is translated into a routing table at node 1:
Destination
2
3
4
5
6
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
Next Hop
2
4
4
4
4
6
Link State Discussion
• Each node requires complete topology information.
• Link state Information must be flooded to all nodes.
Guaranteed to converge.
• Each node must maintain a global database.
• Convergence of the algorithm is guaranteed.
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
7
IP Routing
• IP Forwarding is performed by IP (in OS kernel)
• IP Routing is performed by a user-level process
• In Unix, by the daemon processes routed and gated
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
8
IP Routing
routing
daemon
route
command
netstat
command
UDP
TCP
YES
ct
e
dir
P
For me ?
ICMP
Re
M
IC
routing
table
IP Output: Calculate
Next Hop Router
led
b
:
NO g ena
n
rdi
a
rw
fi fo Source
ing
Rout
Process IP
Options
IP Input
Queue
Network Interfaces
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
IP Layer
9
Routing Table Lookup
• For each IP packet, there is one routing table lookup.
1. Find matching host address
2. Find matching network address
3. Find default entry
• Routing table printout with netstat -rn
• Example:
Destination
140.252.23.32
127.0.0.1
default
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
Gateway
140.252.23.1
127.0.0.1
140.252.13.33
Flags Refcnt Use Interface
UGH
UH
UG
3
1
0
25000
0
0
emd0
lo0
emd0
10
Explanation of the printout
• Flags:
– U: route is up
– G: route is to a gateway (router); if flag is not set, destination is directly
connected
– H: route is to a host, I.e., destination address is the complete host
address; if flag is not set, route is to a network and destination address
is netID or subnetID
– D: route created by redirect
– M: route modified by redirect
• Refcnt: reference count: number of active uses of each route; for each
TCP connection passing through route, this is increased by 1
• Use: number of packets sent on route
• Interface: local interface
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
11
Statically Setting IP Routing Tables
• There are several ways for setting IP routing tables
without a routing protocol (=Static Routing)
1. Automatic creation of entry during initialization of a local
interface (with ifconfig)
2. During bootstrap with route command
3. Via ICMP redirect messages
4. Via ICMP router advertisement/router
discovery messages
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
12
Route Command
• Route commands are put in a system file that is read during
system bootstrap
• System file is:
/etc/rc2.d/S69inet in Solaris,
/etc/netstart in FreeBSD.
• Example:
“default:” destination “sun:” gateway or router 1: metric
route add default sun 1
route add slip bsdi 1
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
13
ICMP Redirect
• Based on routing data in host, it does an arp for router 1 and sends packet
to router 1
• When router 1 detects that an IP datagram should have gone to a different
router, the router:
• forwards the IP datagram to the correct router
• sends an ICMP redirect message to the host
• Host uses ICMP message to update its routing table
(1) IP datagram
(3) ICMP Redirect
(2) IP datagram
Router 1
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
Router 2
14
ICMP Router Solicitation
ICMP Router Advertisement
• Specified in RFC 1256 (1991)
• After bootstrapping a host broadcasts an ICMP router
solicitation message.
• In response, routers send an ICMP router
advertisement message
• Also, routers periodically broadcast ICMP router
advertisement
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
15
Dynamic IP Routing Protocols
• In Unix systems, the dynamic setting of routing tables is done
by the routed or gated daemons
•
The routing daemons execute the following intradomain and
interdomain routing protocols
interdomain
intradomain
Daemon
Hello
rrouted
Gated
(Version 3)
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
RIP
OSPF
EGP
BGP
V2
Yes
V2, V3
V1
Yes
V1
V2
16
RIP - Routing Information Protocol
• A simple intradomain protocol
• Straightforward implementation of Distance Vector Routing
• Each router advertises its minimum distances to destinations
every 30 seconds (or whenever its routing table changes)
• RIP always uses the hop-count as link metric. Maximum hop
count is 15, with “16” equal to “”.
• Routes timeout after 3 minutes if they are not updated. Route
metric is set to  (16) and marked for deletion
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
17
RIP Packet Format
RIP Message
IP header UDP header
1: request
2: reply
3, 4: unused
5: poll
6: poll entry
addr. family: 2 for IP
0
7 8
1516
Command Version
(1-6)
(1)
address family
31
Set to 00...0
Set to 00.00
32-bit address
20 bytes
long
IP address for which a
route is requested
Unused (Set to 00...0)
Unused (Set to 00...0)
metric (1-16)
Up to 24 more routes (each 20 bytes)
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
18
Routing with RIP
• This is the operation of RIP in routed. Dedicated port for
RIP is UDP port 520.
• Initialization: Broadcast a request packet (command = 1, metric=16; address
family=0, metric=16) on the interfaces requesting current routing tables from
routers.
• Request received: Routers that receive above request send their entire
routing table.
• Response received: Update the routing table (see distance vector
algorithm).
• Regular routing updates: Every 30 seconds, send all or part of the
routing tables to every neighbor.
• Triggered Updates: Whenever the metric for a route changes, send data
that has changed.
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
19
RIPv2
RIPv2 Message
IP header UDP header
Process ID of
routing daemon
Command Version
(1-6)
(=2)
address family
routing domain
route tag
32-bit address
Subnet Mask (32 bits)
Support of EGP
and BGP
Subnet Mask of IP address
(RIP version 1 is not aware of
subnet masks)
Next-Hop IP address (32 bits)
Metric (1-16)
Identifies next hop; value of 0 means packets
Identifies next hop: value of 0 means packets should be
should be sent to node sending this RIP message
sent to node sending this RIP message
Up to 24 more routes (each 20 bytes)
•RIPv2 also supports multicast and provides authentication
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
20
OSPF
• OSPF = Open Shortest Path First
• RFC 1247 from 1991
• Alternative solution to RIP as interior gateway protocol
• OSPF is a link state protocol, i.e., each node has complete
topology information
• OSPF messages are sent directly in IP (and not as payload of
UDP packets)
• Hellos and Link State Advertisements (LSAs)
– To get the topology of the network
• Shortest-path algorithm, e.g., Dijkstra’s to precompute routing
tables.
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
21
Features of OSPF
• Provides authentication of routing messages
• Enables load balancing by allowing traffic to be split evenly
across routes with equal cost
• Supports subnetting
• Supports multicasting
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
22
BGP
• BGP = Border Gateway Protocol
• Currently in version 4
• 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 send routing messages
• BGP is a distance vector protocol, but unlike in RIP, routing
messages in BGP contain complete routes.
• Network administrators can specify routing policies
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
23
BGP
• 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.
• For each autonomous system (AS), BGP distinguishes:
• local traffic = traffic with source or destination in AS
• transit traffic = traffic that passes through the AS
• Stub AS
= has connection to only one AS, only
carry local traffic
• Multihomed AS = has connection to >1 AS, but does
not carry transit traffic
• Transit AS
= has connection to >1 AS and carries
transit traffic
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
24
BGP
AS 1
AS 2
Router
Router
Router
AS 3
Router
Router
Router
Router
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
AS 4
25
Exercise
135.60.81
.1
R1
161.6.5
.1
11.4.6
.2
.1
R2
180.11.22
.4
.1
190.4.6
•Assume route commands in the bootstrap sequence
in both routers allows them to immediately recognize
their directly connected subnets
•Show the routing tables at nodes R1 and R2 initially
before they exchange RIP messages and after.
•See Page 131 of your textbook for guidance.
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
26
Exercise solution
Routing table at router R1 soon after reboot
Destination Gateway
Flags
135.60.81
135.60.81.1 U
RIP
Metric
1
161.6.5
161.6.5.1
U
1
180.11.22
180.11.22.1 U
1
•Table created by route commands executed during boot-up
procedure - directly-connected interfaces have a route command
metric of 0 (pg. 116) but a RIP metric of 1 (pg. 131)
•See page 116, 114 - it states that the metric is 0 if the G flag is
not set, which means a direct route. The gateway column has the
IP address of the outgoing interface.
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
27
Exercise solution contd.
Routing table at router R2 soon after reboot
Destination Gateway
Flags
190.4.6
190.4.6.1
U
11.4.6
180.11.22
11.4.6.4
U
180.11.22.2 U
RIP
Metric
1
1
1
•Same as on previous slide
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
28
Exercise solution contd.
• RIP messages sent. As per fig. 10.4 on page 131 of your text
book each router advertises routes to each subnet on all other
subnets with a hop count of 1.
• Thus, each router listens to the broadcasts about the other
router’s subnets.
• Updated routing tables are shown on the next two slides.
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
29
Exercise solution contd.
Routing table at router R1 after receiving
RIP messages from R2
Destination Gateway
Flags
RIP
Metric
135.60.81 135.60.81.1 U
1
161.6.5
180.11.22
190.4.6
11.4.6
161.6.5.1
180.11.22.1
180.11.22.2
180.11.22.2
U
U
UG
UG
1
1
2
2
•See page 131 of your textbook for RIP metrics for
subnets on adjacent routers
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
30
Exercise solution contd.
Routing table at router R2 after receiving
RIP messages from R1
Destination Gateway
Flags
RIP
Metric
190.4.6
190.4.6.1
U
1
11.4.6
11.4.6.4
U
1
180.11.22
180.11.22.2 U
1
135.60.81
180.11.22.1 UG
2
161.6.5
180.11.22.1 UG
2
Malathi Veeraraghavan
(originals by Jörg Liebeherr)
31