Chapter 4
IP Routing
Professor Rick Han
University of Colorado at Boulder
rhan@cs.colorado.edu
Announcements
• Reminder: Programming assignment #1 is due
Feb. 19
• Part of Homework #2 available on Web site,
due Feb. 26
• Last week’s lecture are now on Web site
• Next, IP routing, …
Prof. Rick Han, University of
Colorado at Boulder
Recap of Previous Lecture
• Routing to connect remote LANs
•
Encapsulation
•
•
•
Connects Networks of Networks
“Best-Effort” Service
IP Packet Header – 20 bytes
• TTL
IP Addressing – 32 bit, heirarchy, 128.72.191.4
IP Fragmentation and Reassembly
Address Resolution Protocol (ARP)
• Internet Protocol (IPv4)
•
•
•
Prof. Rick Han, University of
Colorado at Boulder
Address Resolution Protocol
(ARP)
• Given a known IP address, ARP returns the
desired Ethernet MAC address
• If sending to a host on the same Ethernet,
•
•
First, check cache if address already present
If not, send an Ethernet’s broadcast query (all 1’s in
48-bit address) with “target IP” address
• Target host responds with its IP address
• ARP updates its cache
Destination Node
Eth
Eth. Header
IP
Hdr
has dest. MAC
ARP query
ARP response
Requesting Node
Prof. Rick Han, University of
Colorado at Boulder
ARP (2)
• What if destination host is on a remote LAN?
•
No local host will respond to broadcast ARP query
•
IP end host sends to IP network, which routes
packet to destination IP host
ARP is performed separately on LAN 1 and LAN 2
• Solution:
•
IP Router
LAN1
LAN2
Requesting
Destination
Node Prof. Rick Han, University ofNode
Colorado at Boulder
• On LAN 1:
•
•
•
ARP (3)
IP routers broadcast ICMP “router advertisements”
on local LAN or impatient end host broadcasts
“solicitations”
When IP end host wants to send outside of LAN, it
does ARP request to find MAC address of router’s
IP interface address to LAN, if not already cached
Sends a packet containing <src IP, dest. IP>
encapsulated by Eth. Header containing dest. MAC
address of IP router
IP Router
LAN1
LAN2
Requesting
Destination
Node Prof. Rick Han, University ofNode
Colorado at Boulder
• On LAN 2:
•
•
•
ARP (4)
IP packet with <src IP, dest IP> arrives at IP router
on LAN 2
IP router does an ARP request to find MAC address
of dest IP end host, if not already cached
Sends a packet containing <src IP, dest. IP>
encapsulated by Eth. Header containing dest. MAC
address of dest IP end host
• Proxy ARP when only one router between two
LANs
IP Router
LAN1
LAN2
Requesting
Destination
Node Prof. Rick Han, University ofNode
Colorado at Boulder
Forwarding Datagrams
Routing
Table at
Router
B
Destination
Host 1
Output Port
X-B link
Host 2
Host 3
Host 4
E-B link
D-B link
C-B link
Router
X
Router
B
Router
C
Router
Y
Host 1
Router
D
Host 3
Prof. Rick Han, University of
Colorado at Boulder
Host 4
Router
E
Host
2
Forwarding Datagrams (2)
Routing
Table at
Router
E
Destination
Host 1
Output Port
B-E link
Host 2
Host 3
Host 4
Y-E link
D-E link
C-E link
Router
X
Router
B
Router
C
Router
Y
Host 1
Router
D
Host 3
Prof. Rick Han, University of
Colorado at Boulder
Host 4
Router
E
Host
2
Forwarding Datagrams (3)
Routing
Table at
Router
E
Destination
Host 1
Output Port
B-E link
Host 2
Host 3
Host 4
Y-E link
D-E link
C-E link
• Only need to know the destination address to
route the datagram to output port. Compare to:
•
•
VC routing tables had 4 columns: input VC, input port,
output VC, output port
Ethernet Bridge tables store the source address and
source port/LAN, but forwards using destination
address
Prof. Rick Han, University of
Colorado at Boulder
Forwarding Datagrams (4)
Routing
Table at
Router
E
Destination
Host 1
Output Port
B-E link
Host 2
Host 3
Host 4
Y-E link
D-E link
C-E link
• Each datagram travels its own independent
path: There is no connection unlike VCs
•
•
“Connectionless” datagram networks
“Connection-oriented” virtual circuits
Prof. Rick Han, University of
Colorado at Boulder
Forwarding Datagrams (5)
Routing
Table at
Router
E
Destination
Host 1
Output Port
B-E link
Host 2
Host 3
Host 4
Y-E link
D-E link
C-E link
• Each routing table has to contain a complete
list of all of the hosts on the net and how to
get to them (next hop output port)
•
•
Implications on scalability
Compare to VC’s, where each switch only needed to
keep in its table the virtual circuits that ran
through the switch
Prof. Rick Han, University of
Colorado at Boulder
Internet Routing
• “Routing” helps to fill in the IP forwarding tables
• IP routing employs a distributed algorithm to
calculated the shortest path through a graph
•
Many challenges to make distributed algorithms work
well
Homogeneous IP routing fabric
Router
X
Host 1
Router
B
Router
C
Router
D
Prof. Rick Han, University of
Colorado at Boulder
Router
Y
Router
E
Host
2
Internet Routing (2)
• Routing algorithms view the network as a graph
• Problem: find lowest cost path between two
nodes. What info is required for solution?
•
•
Need complete topology info
Need link costs
•
•
Distance vector (RIP)
Link state (OSPF)
• Two types of distributed algorithms:
A
6
1
3
4
C
2
1
B
9
Prof. Rick Han, University of
Colorado at Boulder
E
1
D
F
Distance Vector (RIP)
• Employed in the early Arpanet
• RIP = Routing Information Protocol
•
A specific implementation of distance-vector
routing
• Distributed next hop computation
• Unit of information exchange
•
Vector of distances to destinations
• Distributed Bellman-Ford Algorithm
Prof. Rick Han, University of
Colorado at Boulder
Distance Vector (2)
• Start Conditions:
•
Each router starts with a vector of distances to all
directly attached networks
• Send step:
•
Each router advertises its current vector to all
neighboring routers
• Receive step:
•
•
•
•
Upon receiving vectors from each of its neighbors,
router computes its own distance to each neighbor
Then, for every network X, router finds that
neighbor who is closer to X than any other neighbor
Router updates its cost to X
After doing this for all X, router goes to send step
Prof. Rick Han, University of
Colorado at Boulder
Distance Vector (3)
• Example courtesy of Prof. Srini Seshan at CMU
1
B
C
7
8
A
1
2
2
E
D
Distance to Node
Info at
Node
A
B
C
D
E
A
0
7
~
~
1
B
7
0
1
~
8
C
~
1
0
2
~
D
~
~
2
0
2
E
1
8
~
2
0
Global minimum distance table,
each row is a condensed forwarding
Prof. Rick Han, University of
table for node i
Colorado at Boulder
Distance Vector (4)
Format of Distance Table in A
Dest.
Node
7
A
1
Distance
via
Neighbor
B
C
B
E
A
--
--
B
-- 8
C
Format of Routing/Forwarding
Table in A
Dest. at
Node
Distance
Via Neighbor
A
--
B
--
B
--
E
--
--
C
--
B
1D
--
--
D
--
B
E
E--
--
E
--
E
Prof. Rick Han, University of
Colorado at Boulder
E Receives D’s Routes; Updates Cost
Global minimum distance table,
Node i only sees info on its row,
not entire global view
1
B
C
7
8
A
1
2
2
E
D
Distance to Node
Info at
Node
A
B
C
D
E
A
0
7
~
~
1
B
7
0
1
~
8
C
~
1
0
2
~
D
~
~
2
0
2
E
1
8
4
2
0
Prof. Rick Han, University of
Colorado at Boulder
A receives B’s; Updates Cost
1
B
C
7
8
A
1
2
2
E
D
Distance to Node
Info at
Node
A
B
C
D
E
A
0
7
8
~
1
B
7
0
1
~
8
C
~
1
0
2
~
D
~
~
2
0
2
E
1
8
4
2
0
Prof. Rick Han, University of
Colorado at Boulder
A receives E’s routes; Updates Costs
For every dest. node X, router finds that neighbor who is
closer to X than any other neighbor & updates its cost to X
1
B
C
7
8
A
1
2
2
E
D
Distance to Node
Info at
Node
A
B
C
D
E
A
0
7
5
3
1
B
7
0
1
~
8
C
~
1
0
2
~
D
~
~
2
0
2
E
1
8
4
2
0
Prof. Rick Han, University of
Colorado at Boulder
Final Distances
• Topology/distance info ripples outward
from each node from every other node
1
B
C
7
8
A
1
2
2
E
D
Distance to Node
Info at
Node
A
B
C
D
E
A
0
6
5
3
1
B
6
0
1
3
5
C
5
1
0
2
4
D
3
3
2
0
2
E
1
5
4
2
0
Prof. Rick Han, University of
Colorado at Boulder
Link Failure Causes
“Bouncing” Effect
dest
B
C
cost via
1
2
B
B
dest
1
X
A
A
C
B
1
25
C
dest
A
B
Prof. Rick Han, University of
Colorado at Boulder
cost via
2
1
B
B
cost via
1
1
A
C
B Notices A-B Link Failure
dest
B
C
B notices failure, resets cost via A to
infinity in distance table (not shown), &
knows cost via C is 26
cost via
1
2
B
B
dest
A
A
C
B
1
25
C
dest
A
B
Prof. Rick Han, University of
Colorado at Boulder
cost via
2
1
B
B
cost via
26
1
C
C
C Sends Dist. Vector to B
dest
B
C
cost via
1
2
B
B
dest
A
A
C
B
1
25
C
dest
A
B
Prof. Rick Han, University of
Colorado at Boulder
cost via
3
1
C sends routing
update to B
cost via
2
1
B
B
C
C
B Updates Distance to A
dest
B
C
cost via
1
2
B
B
dest
A
A
C
B
1
25
C
dest
A
B
Prof. Rick Han, University of
Colorado at Boulder
cost via
3
C
1
C
Packet sent from C
to A bounces
between C and B
cost via until TTL=0!
2
1
B
B
B Sends Dist. Vector to C
dest
B
C
cost via
1
2
B
B
dest
A
B
1
25
C
dest
A
B
Prof. Rick Han, University of
Colorado at Boulder
A
C
cost via
3
C
1
C
C adds one to B’s
advertised distance
to A. (Why does C
cost via override
4
B
its stored
1
B
distance of 2
to A with 4,
larger value?)
C Sends Dist. Vector to B
dest
B
C
cost via
1
2
B
B
dest
A
B
25
C
A
C
cost via
5
C
1
C
B adds one to C’s
1 advertised distance
to A. (overrides
its stored
dest cost via distance of 3
A
4
to A with 5,
B
B
1
B
larger value)
Prof. Rick Han, University of
Colorado at Boulder
Link Failure: Bad News Travels
Slowly
dest
B
C
cost via
26
25
C
C
dest
A
A
C
B
Assume A has advertised
C
its link cost of 25 to C
during B<->C exchanges.
C stores this cost in its distance
table (not shown)
dest
A
B
Prof. Rick Han, University of
Colorado at Boulder
25
C
1
C
After 20+ exchanges,
routing tables look
like this:
1
25
cost via
cost via
24
1
B
B
Bad News Travels Slowly (2)
dest
B
C
cost via
26
25
C
C
dest
A
A
C
B
25
C
cost via
25
C
1
C
C increments B’s
1 update by 1, and
chooses 25 via A
to A, instead of 26
dest cost via Via B to A
A
B
Prof. Rick Han, University of
Colorado at Boulder
25
A
1
B
Bad News Travels Slowly (3)
dest
B
C
cost via
26
25
C
C
dest
A
A
C
B
C
dest
A
B
Prof. Rick Han, University of
Colorado at Boulder
26
C
1
C
After 25 B-C
exchanges, finally
converge to
stable routing
1
25
cost via
cost via
25
A
1
B
Link Failure Causes
“Counting to Infinity” Effect
dest
B
C
cost via
1
2
B
B
dest
1
X
A
A
C
B
1
25
C
dest
A
B
Prof. Rick Han, University of
Colorado at Boulder
cost via
2
1
B
B
cost via
1
1
A
C
B Notices A-B Link Failure
B notices failure,
resets cost to 26
dest
B
C
cost via
1
2
B
B
dest
A
A
C
B
1
25
C
dest
A
B
Prof. Rick Han, University of
Colorado at Boulder
cost via
2
1
B
B
cost via
26
1
C
C
C Sends Dist. Vector to B
dest
B
C
cost via
1
2
B
B
dest
A
A
C
B
1
25
C
dest
A
B
Prof. Rick Han, University of
Colorado at Boulder
cost via
3
C
1
C
C sends
routing update to B
cost via
2
1
B
B
A-C Link Fails
dest
A
A
C
B
C detects link to A has failed,
but no change in C’s
routing table (why?)
1
C
dest
A
B
Prof. Rick Han, University of
Colorado at Boulder
cost via
2
1
B
B
cost via
3
C
1
C
Now, B and C Count to Infinity
dest
A
A
C
B
1
C
dest
A
B
Prof. Rick Han, University of
Colorado at Boulder
cost via
4
1
B
B
cost via
3
C
1
C
How are These Loops
Caused?
• Observation 1:
– B’s metric increases
• Observation 2:
– C picks B as next hop to A
– But, the implicit path from C to A
includes itself (C ) !
Prof. Rick Han, University of
Colorado at Boulder
Solution 1: Holddowns
• If metric increases, delay propagating
information
– In our example, B delays advertising route
– C eventually thinks B’s route is gone, picks its
own route
– B then selects C as next hop
• Adversely affects convergence from
failures
Prof. Rick Han, University of
Colorado at Boulder
Other “Solutions”
• Split horizon
– C does not advertise route to B when it
sends its distance vector
• Poisoned reverse
– C advertises route to B with infinite
distance in its distance vector
• Works for two node loops
– Does not work for loops with more nodes
Prof. Rick Han, University of
Colorado at Boulder
Avoiding the Counting to
Infinity Effect
• Select loop-free paths
• One way of doing this:
– Each route advertisement carries entire
path
– If a router sees itself in path, it rejects
the route
• BGP does it this way
• Space proportional to diameter
Prof. Rick Han, University of
Colorado at Boulder
Loop Freedom at Every
Instant?
• Does bouncing effect avoid loops?
– No! Transient loops are still possible
– Why? Because implicit path information
may be stale
– See this in BGP convergence
• Only way to fix this
– Ensure that you have up-to-date
information by explicitly querying
Prof. Rick Han, University of
Colorado at Boulder
Distance Vector in Practice
• RIP and RIP2
– Uses split-horizon/poison reverse
• BGP
– Propagates entire path
– Path also used for effecting policies
Prof. Rick Han, University of
Colorado at Boulder
Example Where Split
Horizon Fails
1
A
B
1
1
C
1
D
• When link breaks, C marks D as
unreachable and reports that
to A and B
• Suppose A learns it first
– A now thinks best path to D
is through B
– A reports D unreachable to B
and a route of cost=3 to C
• C thinks D is reachable through
A at cost 4 and reports that to
B
• B reports a cost 5 to A who
reports new cost to C
• etc...
Prof. Rick Han, University of
Colorado at Boulder