Chapter 4 IP Routing

1
Chapter 4IP Routing

• Professor Rick Han
• University of Colorado at Boulder
• rhan_at_cs.colorado.edu

2
Announcements

• Reminder Programming assignment 1 is due Feb.
  19
• Part of Homework 2 available on Web site, due
  Feb. 26
• Last weeks lecture are now on Web site
• Next, IP routing,

3
Recap of Previous Lecture

• Routing to connect remote LANs
• Encapsulation
• Internet Protocol (IPv4)
• 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)

4
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 Ethernets broadcast query (all
  1s in 48-bit address) with target IP address
• Target host responds with its IP address
• ARP updates its cache

Destination Node
Requesting Node
5
ARP (2)

• What if destination host is on a remote LAN?
• No local host will respond to broadcast ARP query
• Solution
• IP end host sends to IP network, which routes
  packet to destination IP host
• ARP is performed separately on LAN 1 and LAN 2

IP Router
LAN1
LAN2
Destination Node
Requesting Node
6
ARP (3)

• On LAN 1
• 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 routers
  IP interface address to LAN, if not already
  cached
• Sends a packet containing ltsrc IP, dest. IPgt
  encapsulated by Eth. Header containing dest. MAC
  address of IP router

IP Router
LAN1
LAN2
Destination Node
Requesting Node
7
ARP (4)

• On LAN 2
• IP packet with ltsrc IP, dest IPgt 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 ltsrc IP, dest. IPgt
  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
Destination Node
Requesting Node
8
Forwarding Datagrams
Routing Table at Router B
Host 4
Router C
Router X
Router B
Router Y
Host 1
Router E
Router D
Host 2
Host 3
9
Forwarding Datagrams (2)
Routing Table at Router E
Host 4
Router C
Router X
Router B
Router Y
Host 1
Router E
Router D
Host 2
Host 3
10
Forwarding Datagrams (3)
Routing Table at Router E

• 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

11
Forwarding Datagrams (4)
Routing Table at Router E

• Each datagram travels its own independent path
  There is no connection unlike VCs
• Connectionless datagram networks
• Connection-oriented virtual circuits

12
Forwarding Datagrams (5)
Routing Table at Router E

• 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 VCs, where each switch only needed to
  keep in its table the virtual circuits that ran
  through the switch

13
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 C
Router X
Router B
Router Y
Host 1
Router E
Router D
Host 2
14
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
• Two types of distributed algorithms
• Distance vector (RIP)
• Link state (OSPF)

15
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

16
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

17
Distance Vector (3)

• Example courtesy of Prof. Srini Seshan at CMU

1
Distance to Node
B
C
Info at Node
A
B
C
D
E
7
0
7


1
A
B
2
8
A
7
0
1

8
C

1
0
2

D


2
1
2
0
2
E
D
E
1
8

2
0
Global minimum distance table, each row is a
condensed forwarding table for node i
18
Distance Vector (4)
Format of Routing/Forwarding Table in A
Format of Distance Table in A
1
Distance via Neighbor
B
C
Dest. Node
Dest. at Node
B
E
Distance
Via Neighbor
7
--
--
--
B
A
A
B
B
8
A
--
--
--
E
C
--
--
C
--
B
D
--
--
D
--
B
1
E
E
--
--
E
--
E
19
E Receives Ds Routes Updates Cost
Global minimum distance table, Node i only sees
info on its row, not entire global view
1
Distance to Node
B
C
Info at Node
A
B
C
D
E
7
0
7


1
A
B
2
8
A
7
0
1

8
C

1
0
2

D


2
0
2
1
2
D
E
E
1
8
4
2
0
20
A receives Bs Updates Cost
1
Distance to Node
B
C
Info at Node
A
B
C
D
E
7
0
7
8

1
A
B
2
8
A
7
0
1

8
C

1
0
2

D


2
0
2
1
2
D
E
E
1
8
4
2
0
21
A receives Es 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
Distance to Node
B
C
Info at Node
A
B
C
D
E
7
0
7
5
3
1
A
B
2
8
A
7
0
1

8
C

1
0
2

D


2
0
2
1
2
D
E
E
1
8
4
2
0
22
Final Distances

• Topology/distance info ripples outward from each
  node from every other node

1
Distance to Node
B
C
Info at Node
A
B
C
D
E
7
0
6
5
3
1
A
B
2
8
A
6
0
1
3
5
C
5
1
0
2
4
D
3
3
2
0
2
1
2
D
E
E
1
5
4
2
0
23
Link Failure Causes Bouncing Effect
dest
cost
dest
cost
via
via
1
X
A
B
B
A
1
1
B
A
C
2
C
1
B
C
1
25
C
dest
cost
via
A
2
B
B
1
B
24
B Notices A-B Link Failure
dest
cost
dest
cost
via
via
A
B
B
A
26
1
B
C
C
2
C
1
B
C
1
25
C
dest
cost
via
A
2
B
B
1
B
25
C Sends Dist. Vector to B
dest
cost
dest
cost
via
via
A
B
B
A
3
1
B
C
C
2
C
1
B
C
1
25
C
dest
cost
via
A
2
B
B
1
B
26
B Updates Distance to A
dest
cost
dest
cost
via
via
A
B
B
A
3
C
1
B
C
2
C
1
B
C
Packet sent from C to A bounces between C and
B until TTL0!
1
25
C
dest
cost
via
A
2
B
B
1
B
27
B Sends Dist. Vector to C
dest
cost
dest
cost
via
via
A
B
B
A
3
C
1
B
C
2
C
1
B
C
C adds one to Bs advertised distance to A. (Why
does C override its stored distance of 2 to A
with 4, larger value?)
1
25
C
dest
cost
via
A
4
B
B
1
B
28
C Sends Dist. Vector to B
dest
cost
dest
cost
via
via
A
B
B
A
5
C
1
B
C
2
C
1
B
C
B adds one to Cs advertised distance to A.
(overrides its stored distance of 3 to A with
5, larger value)
1
25
C
dest
cost
via
A
4
B
B
1
B
29
Link Failure Bad News Travels Slowly
dest
cost
dest
cost
via
via
A
B
B
A
25
C
26
C
C
25
C
1
C
C
After 20 exchanges, routing tables look like
this
1
25
Assume A has advertised its link cost of 25 to C
during Blt-gtC exchanges. C stores this cost in
its distance table (not shown)
C
dest
cost
via
A
24
B
B
1
B
30
Bad News Travels Slowly (2)
dest
cost
dest
cost
via
via
A
B
B
A
25
C
26
C
C
25
C
1
C
C
C increments Bs update by 1, and chooses 25 via
A to A, instead of 26 Via B to A
1
25
C
dest
cost
via
A
25
A
B
1
B
31
Bad News Travels Slowly (3)
dest
cost
dest
cost
via
via
A
B
B
A
26
C
26
C
C
25
C
1
C
C
After 25 B-C exchanges, finally converge
to stable routing
1
25
C
dest
cost
via
A
25
A
B
1
B
32
Link Failure Causes Counting to Infinity Effect
dest
cost
dest
cost
via
via
1
X
A
B
B
A
1
1
B
A
C
2
C
1
B
C
1
25
C
dest
cost
via
A
2
B
B
1
B
33
B Notices A-B Link Failure
dest
cost
dest
cost
via
via
A
B
B
A
26
1
B
C
C
2
C
1
B
C
1
25
C
dest
cost
via
A
2
B
B
1
B
34
C Sends Dist. Vector to B
dest
cost
dest
cost
via
via
A
B
B
A
3
C
1
B
C
2
C
1
B
C
1
25
C
dest
cost
via
A
2
B
B
1
B
35
A-C Link Fails
dest
cost
via
A
B
A
3
C
C
1
C
1
C detects link to A has failed, but no change in
Cs routing table (why?)
C
dest
cost
via
A
2
B
B
1
B
36
Now, B and C Count to Infinity
dest
cost
via
A
B
A
3
C
C
1
C
1
C
dest
cost
via
A
4
B
B
1
B
37
How are These Loops Caused?

• Observation 1
• Bs metric increases
• Observation 2
• C picks B as next hop to A
• But, the implicit path from C to A includes
  itself (C ) !

38
Solution 1 Holddowns

• If metric increases, delay propagating
  information
• In our example, B delays advertising route
• C eventually thinks Bs route is gone, picks its
  own route
• B then selects C as next hop
• Adversely affects convergence from failures

39
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

40
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

41
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

42
Distance Vector in Practice

• RIP and RIP2
• Uses split-horizon/poison reverse
• BGP
• Propagates entire path
• Path also used for effecting policies

43
Example Where Split Horizon Fails

• 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
  cost3 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...

1
A
B
1
1
C
X
1
D