Post on 21-Dec-2015
Chapter 4IP Routing
Professor Rick HanUniversity of Colorado at Boulder
rhan@cs.colorado.edu
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, …
Prof. Rick Han, University of Colorado at Boulder
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)
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
Requesting Node
Destination Node
IPEthHdr
ARP query
Eth. Headerhas dest. MAC
ARP response
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
• 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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Routing Table atRouter 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
Prof. Rick Han, University of Colorado at Boulder
Forwarding Datagrams (4)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
• 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)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
• 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 wellRouter
C
RouterD
RouterB
RouterE
RouterX
RouterY
Host 1Host
2
Homogeneous IP routing fabric
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
C Sends Dist. Vector to B
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
Prof. Rick Han, University of Colorado at Boulder
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)
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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 sendsrouting update to B
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
Prof. Rick Han, University of Colorado at Boulder
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
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