chapter 4 ip routing professor rick han university of colorado at boulder [email protected]

Chapter 4IP Routing

Professor Rick HanUniversity of Colorado at Boulder

[email protected]

Prof. Rick Han, University of Colorado at Boulder

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, …


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)


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

Requesting Node

Destination Node

IPEthHdr

ARP query

Eth. Headerhas dest. MAC

ARP response


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

DestinationNode

RequestingNode

IP Router

LAN1 LAN2


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 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

DestinationNode

RequestingNode

IP Router

LAN1 LAN2


ARP (4)• On LAN 2:• 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

DestinationNode

RequestingNode

IP Router

LAN1 LAN2


Forwarding Datagrams

RouterC

RouterD

RouterB

RouterE

RouterX

RouterY

Host 1

Host2

Host 3

Destination Output Port

Host 1 X-B link

Host 2 E-B link

Host 3 D-B link

Host 4 C-B link

Routing Table atRouter B

Host 4


Forwarding Datagrams (2)

RouterC

RouterD

RouterB

RouterE

RouterX

RouterY

Host 1

Host2

Host 3

Destination Output Port

Host 1 B-E link

Host 2 Y-E link

Host 3 D-E link

Host 4 C-E link

Routing Table atRouter E

Host 4


Forwarding Datagrams (3)Destination Output Port

Host 1 B-E link

Host 2 Y-E link

Host 3 D-E link

Host 4 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



Host 1 B-E link

Host 2 Y-E link

Host 3 D-E link

Host 4 C-E link


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



Host 1 B-E link

Host 2 Y-E link

Host 3 D-E link

Host 4 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


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 wellRouter

C

RouterD

RouterB

RouterE

RouterX

RouterY

Host 1Host

2

Homogeneous IP routing fabric


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)

4

3

6

21

9

1

1

D

A

FE

B

C


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


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


Distance Vector (3)• Example courtesy of Prof. Srini Seshan at

CMU

A

B

E

C

D

Info atNode

A

B

C

D

A B C

0 7 ~

7 0 1

~ 1 0

~ ~ 2

7

1

1

2

28

Distance to Node

D

~

~

2

0

E 1 8 ~ 2

1

8

~

2

0

E

Global minimum distance table,each row is a condensed forwarding

table for node i


Distance Vector (4)

A

B

E

CDest. atNode

A

B

C

D

Distance Via Neighbor

-- B

-- E

-- B

-- B

7

1

1

8

E -- E

Dest.Node

A

B

C

D

B E

-- --

-- --

-- --

-- --

Distance via Neighbor

E -- --

Format of Distance Table in AFormat of Routing/Forwarding

Table in A


E Receives D’s Routes; Updates Cost

Info atNode

A

B

C

D

A B C

0 7 ~

7 0 1

~ 1 0

~ ~ 2

Distance to Node

D

~

~

2

0

E 1 8 4 2

1

8

~

2

0

E

A

B

E

C

D

7

1

1

2

28

Global minimum distance table,Node i only sees info on its row,

not entire global view


A receives B’s; Updates Cost

Info atNode

A

B

C

D

A B C

0 7 8

7 0 1

~ 1 0

~ ~ 2

Distance to Node

D

~

~

2

0

E 1 8 4 2

1

8

~

2

0

E

A

B

E

C

D

7

1

1

2

28


A receives E’s routes; Updates Costs

Info atNode

A

B

C

D

A B C

0 7 5

7 0 1

~ 1 0

~ ~ 2

Distance to Node

D

3

~

2

0

E 1 8 4 2

1

8

~

2

0

E

A

B

E

C

D

7

1

1

2

28

For every dest. node X, router finds that neighbor who is closer to X than any other neighbor & updates its cost to X


Final Distances

Info atNode

A

B

C

D

A B C

0 6 5

6 0 1

5 1 0

3 3 2

Distance to Node

D

3

3

2

0

E 1 5 4 2

1

5

4

2

0

E

A

B

E

C

D

7

1

1

2

28

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


Link Failure Causes “Bouncing” Effect

A

25

1

1

B

C

B

C 21

dest cost

XBB

via

A

C 11

dest cost

CA

via

A

B 12

dest cost

BB

via


B Notices A-B Link Failure

A

25 1

B

C

B notices failure, resets cost via A toinfinity in distance table (not shown), &knows cost via C is 26

B

C 21

dest cost

BB

via

A

C 126

dest cost

CC

via

A

B 12

dest cost

BB

via


C Sends Dist. Vector to B

A

25 1

B

C

B

C 21

dest cost

BB

via

A

C 13

dest cost

CC

via

A

B 12

dest cost

BB

via

C sends routing update to B


B Updates Distance to A

A

25 1

B

C

Packet sent from Cto A bounces between C and B

until TTL=0!

B

C 21

dest cost

BB

via

A

C 13

dest cost

CC

via

A

B 12

dest cost

BB

via


B Sends Dist. Vector to C

A

25 1

B

C

C adds one to B’sadvertised distanceto A. (Why does C

overrideits storeddistance of 2to A with 4,larger value?)

B

C 21

dest cost

BB

via

A

C 13

dest cost

CC

via

A

B 14

dest cost

BB

via



A

25 1

B

C

B adds one to C’sadvertised distanceto A. (overrides

its storeddistance of 3to A with 5,larger value)

B

C 21

dest cost

BB

via

A

C 15

dest cost

CC

via

A

B 14

dest cost

BB

via


Link Failure: Bad News Travels Slowly

A

25 1

B

C

After 20+ exchanges,routing tables looklike this:

B

C 2526

dest cost

CC

via

A

C 125

dest cost

CC

via

A

B 124

dest cost

BB

viaAssume A has advertisedits link cost of 25 to C during B<->C exchanges.C stores this cost in its distancetable (not shown)


Bad News Travels Slowly (2)

A

25 1

B

C

C increments B’supdate by 1, andchooses 25 via Ato A, instead of 26

Via B to A

B

C 2526

dest cost

CC

via

A

C 125

dest cost

CC

via

A

B 125

dest cost

BA

via


Bad News Travels Slowly (3)

A

25 1

B

C

After 25 B-Cexchanges, finallyconverge tostable routing

B

C 2526

dest cost

CC

via

A

C 126

dest cost

CC

via

A

B 125

dest cost

BA

via


Link Failure Causes “Counting to Infinity” Effect

A

25

1

1

B

C

B

C 21

dest cost

XBB

via

A

C 11

dest cost

CA

via

A

B 12

dest cost

BB

via


B Notices A-B Link Failure

A

25 1

B

C

B notices failure,resets cost to 26

B

C 21

dest cost

BB

via

A

C 126

dest cost

CC

via

A

B 12

dest cost

BB

via



A

25 1

B

C

B

C 21

dest cost

BB

via

A

C 13

dest cost

CC

via

A

B 12

dest cost

BB

via

C sendsrouting update to B


A-C Link Fails

A

1

B

C

C detects link to A has failed,but no change in C’srouting table (why?)

A

C 13

dest cost

CC

via

A

B 12

dest cost

BB

via


Now, B and C Count to Infinity

A

1

B

C

A

C 13

dest cost

CC

via

A

B 14

dest cost

BB

via


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 ) !


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


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


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


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


Distance Vector in Practice

• RIP and RIP2– Uses split-horizon/poison reverse

• BGP– Propagates entire path– Path also used for effecting policies


Example Where Split Horizon Fails

1

11

1

A

• 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...

X

B

C

D

chapter 4 ip routing professor rick han university of colorado at boulder [email protected]

Documents

router e host

router b host

destination ip host

destination host

ip address arp

eb link host

be link host

local host