CPS-356- Computer Networks
Class 9: IP-Continued
Theophilus Benson
Based partly on lecture notes by Rodrigo Fonseca, David Mazières, Phil Levis, John Jannotti
Administrivia
• Slides Updates
– Class slides 08 updated
– Class slides 06 updated
– Class slides 07 uploaded
–
• HW #1 put up on site
• Can we move exams?
Distance Vector Recap
A
4
A
C
1 C
B
4
B
4
B
C
1
0
C
1
A
C
10
A
10
A
B
1
B
• Neighbor information is preconfigured
– The link cost to a neigh and the link to that neighbor
• Nodes exchange message with neighbors
– (cost, dest)
• Pick the shortest paths
Distance Vector Recap
A
4
A
C
1 C
(A,4)
(C,1)
B
(B,4)
(C,10)
4
B
4
B
C
1
0
C
(A,10)
(B,1)
1
A
C
10
A
10
A
B
1
B
• Neighbor information is preconfigured
– The link cost to a neigh and the link to that neighbor
• Nodes exchange message with neighbors
– (cost, dest)
• Pick the shortest paths
Distance Vector Recap
From A
(B,4), (C,10)
From C
(A,10), (B,1)
4
C
1 C
B
4
B
C
1
0
C
A
B
4
From C
(A,10), (B,1)
From B
(A,4), (C,1)
A
From B
(A,4), (C,1)
From A
(B,4), (C,10)
1
A
C
10
• Pick the shortest paths
A
10
A
B
1
B
Distance Vector Recap
From A
(B,4), (C,10)
From C
(A,10), (B,1)
4
C
1 C
B
4
B
C
1
0
C
A
B
4
From C
(A,10), (B,1)
From B
(A,4), (C,1)
A
From B
(A,4), (C,1)
From A
(B,4), (C,10)
1
A
C
10
• Pick the shortest paths
A
10
A
B
1
B
Distance Vector Recap
From A
(B,4), (C,10)
From C
(A,10), (B,1)
4
C
1 C
B
4
B
C
5
B
A
B
4
From C
(A,10), (B,1)
From B
(A,4), (C,1)
A
From B
(A,4), (C,1)
From A
(B,4), (C,10)
1
A
C
10
• Pick the shortest paths
A
5
B
B
1
B
Distance Vector Recap
A
4
A
C
1 C
B
(B,4)
(C,5)
4
B
4
B
C
5
B
(A,5)
(B,1)
1
A
C
10
A
5
B
B
1
B
• Pick the shortest paths
• Update message are triggered by
– Change in Routing Table
– Or periodically every N seconds, by a timer
Good news travels fast
A
4
A
C
1 C
B
1
B
4
B
C
5
B
4
1
A
C
10
A
5
B
B
1
B
• A decrease in link cost has to be fresh information
• Network converges at most in O(diameter) steps
Good news travels fast
A
1
A
C
1 C
(A,1)
(C,1)
B
1
(B,1)
(C,10)
B
1
B
C
5
B
4
1
A
C
10
A
5
B
B
1
B
• A decrease in link cost has to be fresh information
• Network converges at most in O(diameter) steps
Good news travels fast
A
1
A
C
1 C
B
1
B
1
B
C
5
B
4
1
A
(A,1)
(C,1)
C
10
(B,1)
(C,10)
A
5
B
B
1
B
• A decrease in link cost has to be fresh information
• Network converges at most in O(diameter) steps
Good news travels fast
A
1
A
C
1 C
B
1
B
1
B
C
5
B
4
1
A
(A,1)
(C,1)
C
10
(B,1)
(C,10)
A
2
B
B
1
B
• A decrease in link cost has to be fresh information
• Network converges at most in O(diameter) steps
Good news travels fast
A
1
A
C
1 C
B
1
(B,1)
(C,2)
B
1
B
C
2
B
4
(A,2)
(B,1)
1
A
C
10
A
2
B
B
1
B
• A decrease in link cost has to be fresh information
• Network converges at most in O(diameter) steps
Bad news travels slowly
A
1
A
C
1 C
B
4
B
1
B
C
5
B
1
A
C
10
A
5
B
B
1
B
• An increase in cost may cause confusion with old information, may
form loops
• Consider routes to A
• Initially, B:A,4,A; C:A,5,B
• Then B:A,12,A, selects C as next hop -> B:A,6,C
• C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
• C finally chooses C:A,10,A, and B -> A,11,C!
Bad news travels slowly
A
6
C
12
B
1
B
C
5
B
A
4
A
C
1 C
B
4
1
A
C
10
A
5
B
B
1
B
• An increase in cost may cause confusion with old information, may
form loops
• Consider routes to A
• Initially, B:A,4,A; C:A,5,B
• Then B:A,12,A, selects C as next hop -> B:A,6,C
• C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
• C finally chooses C:A,10,A, and B -> A,11,C!
Bad news travels slowly
12
B
1
2
B
A
12
A
C
1
C
B
4
1
A
C
10
A
5
B
•
B 1
B
C
1
B
An increase in cost may cause confusion with old information, may
form loops 3
•
•
•
•
•
Consider routes to A
Initially, B:A,4,A; C:A,5,B
Then B:A,12,A, selects C as next hop -> B:A,6,C
C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
C finally chooses C:A,10,A, and B -> A,11,C!
Bad news travels slowly
12
B
1
2
B
A
12
A
C
1
C
B
4
(A,5)
(B,1)
1
A
C
10
A
5
B
•
B 1
B
C
1
B
An increase in cost may cause confusion with old information, may
form loops 3
•
•
•
•
•
Consider routes to A
Initially, B:A,4,A; C:A,5,B
Then B:A,12,A, selects C as next hop -> B:A,6,C
C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
C finally chooses C:A,10,A, and B -> A,11,C!
Bad news travels slowly
12
B
1
1
C
A
6
C
C
1
C
B
4
(A,5)
(B,1)
1
A
C
10
A
5
B
•
B 1
B
C
1
C
An increase in cost may cause confusion with old information, may
form loops 0
•
•
•
•
•
Consider routes to A
Initially, B:A,4,A; C:A,5,B
Then B:A,12,A, selects C as next hop -> B:A,6,C
C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
C finally chooses C:A,10,A, and B -> A,11,C!
Bad news travels slowly
12
B
1
1
C
A
6
C
C
1
C
(A,6)
(C,1)
B
4
1
A
C
10
A
5
B
•
B 1
B
C
1
C
An increase in cost may cause confusion with old information, may
form loops 0
•
•
•
•
•
Consider routes to A
Initially, B:A,4,A; C:A,5,B
Then B:A,12,A, selects C as next hop -> B:A,6,C
C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
C finally chooses C:A,10,A, and B -> A,11,C!
Bad news travels slowly
12
B
1
1
C
A
6
C
C
1
C
(A,6)
(C,1)
B
4
1
A
C
10
A 7
B
•
B 1
B
C
1
C
An increase in cost may cause confusion with old information, may
form loops 0
•
•
•
•
•
Consider routes to A
Initially, B:A,4,A; C:A,5,B
Then B:A,12,A, selects C as next hop -> B:A,6,C
C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
C finally chooses C:A,10,A, and B -> A,11,C!
Bad news travels slowly
12
B
1
1
C
A
6
C
C
1
C
B
4
(A,7)
(B,1)
1
A
C
10
A
7
B
•
B 1 B
C
1
C
An increase in cost may cause confusion with old information, may
form loops 0
•
•
•
•
•
Consider routes to A
Initially, B:A,4,A; C:A,5,B
Then B:A,12,A, selects C as next hop -> B:A,6,C
C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
C finally chooses C:A,10,A, and B -> A,11,C!
Bad news travels slowly
12
B
1
1
C
A
8
C
C
1
C
(A,8)
(C,1)
B
4
1
A
C
10
A
7
B
•
B 1 B
C
1
C
An increase in cost may cause confusion with old information, may
form loops 0
•
•
•
•
•
Consider routes to A
Initially, B:A,4,A; C:A,5,B
Then B:A,12,A, selects C as next hop -> B:A,6,C
C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
C finally chooses C:A,10,A, and B -> A,11,C!
Bad news travels slowly
12
B
1
1
C
A
8
C
C
1
C
B
4
(A,9)
(B,1)
1
A
C
10
A 9
B
•
B 1
B
C
1
C
An increase in cost may cause confusion with old information, may
form loops 0
•
•
•
•
•
Consider routes to A
Initially, B:A,4,A; C:A,5,B
Then B:A,12,A, selects C as next hop -> B:A,6,C
C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
C finally chooses C:A,10,A, and B -> A,11,C!
Bad news travels slowly
12
B
1
1
C
A
10
C
C
1
C
(A,10)
(C,1)
B
4
1
A
C
10
A
9
B
•
B 1 B
C
1
C
An increase in cost may cause confusion with old information, may
form loops 0
•
•
•
•
•
Consider routes to A
Initially, B:A,4,A; C:A,5,B
Then B:A,12,A, selects C as next hop -> B:A,6,C
C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
C finally chooses C:A,10,A, and B -> A,11,C!
Bad news travels slowly
12
B
1
1
C
A
10
C
C
1
C
(A,10)
(C,1)
B
4
(A,10)
(B,1)
1
A
C
10
A 10
A
•
B 1
B
C
1
C
An increase in cost may cause confusion with old information, may
form loops 0
•
•
•
•
•
Consider routes to A
Initially, B:A,4,A; C:A,5,B
Then B:A,12,A, selects C as next hop -> B:A,6,C
C -> A,7,B; B -> A,8,C; C -> A,9,B; B -> A,10,C;
C finally chooses C:A,10,A, and B -> A,11,C!
How to avoid loops
• IP TTL field prevents a packet from living
forever
– Does not repair a loop
• Simple approach: consider a small cost n (e.g.,
16) to be infinity
– After n rounds decide node is unavailable
– But rounds can be long, this takes time
• Problem: distance vector based only on local
information
Bad news travels slowly
12
B
1
1
C
C
1
0
C
A
6
C
C
1
C
(A,6)
(C,1)
B
4
(A,5)
(B,1)
1
A
C
10
A
5
B
B
1
B
• Why did it take a while to converge?
– Nodes are unknowingly using each other
Better loop avoidance
• Split Horizon
– When sending updates to node A, don’t include
routes you learned from A
– Prevents B and C from sending cost 2 to A
• Split Horizon with Poison Reverse
– Rather than not advertising routes learned from A,
explicitly include cost of ∞.
– Faster to break out of loops, but increases
advertisement sizes
Split Horizon
A
4
A
C
1
C
(A,4)
(C,1)
B
(B,4)
(C,10)
4
B
4
B
C
1
0
C
(A,10)
(B,1)
1
A
C
10
A
10 A
B
1
• Why did it take a while to converge?
– Nodes are unknowingly using each other
B
Split Horizon
A
4
A
C
1
C
B
(B,4)
(C,10)
4
B
4
B
C
5
B
(A,10)
(B,1)
1
A
C
10
(B,4)
(C,5)
(A,5)
(B,1)
A
5
B
B
1
B
• Why did it take a while to converge?
– Nodes are unknowingly using each other
Split Horizon
12
(B,4)
(C,10)
B
1
2
B
C
1
3
B
A
12
A
C
1
C
B
4
(A,10)
(B,1)
1
A
C
10
(B,4)
(C,5)
(A,5)
(B,1)
A
5
B
B
1
B
• Why did it take a while to converge?
– Nodes are unknowingly using each other
Split Horizon
12
(B,4)
(C,10)
B
1
1
C
C
1
0
C
A
12
A
C
1
C
B
4
(A,10)
(B,1)
1
A
C
10
(B,4)
(C,5)
(A,5)
(B,1)
A
5
B
B
1
B
• Why did it take a while to converge?
– Nodes are unknowingly using each other
Split Horizon
12
B
1
1
C
C
1
0
C
A
12
A
C
1
C
(A,12)
(C,1)
B
4
(A,10)
(B,1)
1
A
C
10
A
5
B
B
1
B
• Why did it take a while to converge?
– Nodes are unknowingly using each other
Split Horizon
12
B
1
1
C
C
1
0
C
A
12
A
C
1
C
(A,12)
(C,1)
B
4
(A,10)
(B,1)
1
A
C
10
A
10 B
B
1
• Why did it take a while to converge?
– Nodes are unknowingly using each other
B
Split Horizon
12
B
1
1
C
C
1
0
C
A
12
A
C
1
C
(A,12)
(C,1)
B
4
(A,10)
(B,1)
1
A
C
10
A
10 B
B
1
• Why did it take a while to converge?
– Nodes are unknowingly using each other
B
Split Horizon
12
B
1
1
C
C
1
0
C
A
11
A
C
1
C
(A,12)
(C,1)
B
4
(A,10)
(B,1)
1
A
C
10
A
10 B
B
1
• Why did it take a while to converge?
– Nodes are unknowingly using each other
B
Warning
• Split horizon/split horizon with poison reverse
only help between two nodes
– Can still get loop with three nodes involved
– Might need to delay advertising routes after
changes, but affects convergence time
Other approaches
• DSDV: destination sequenced distance vector
– Uses a ‘version’ number per destination message
– Avoids loops by preventing nodes from using old
information from descendants
– But, you can only update when new version
comes from root
• Path Vector: (BGP)
– Replace ‘distance’ with ‘path’
– Avoids loops with extra cost
Link State Routing
• Strategy:
– send to all nodes information about directly
connected neighbors
• Link State Packet (LSP)
– ID of the node that created the LSP
– Cost of link to each directly connected neighbor
– Sequence number (SEQNO)
– TTL
Time 0: Generate LSP from Config File
and Send out Updates
B
4
C
1
5
B
4
A
C
1
6
10
D
• Directly Connected Neighbors are preconfigured into the device
Time 0: Generate LSP from Config File
and Send out Updates
B
4
C
1
5
B
4
A
C
1
6
10
D
• Directly Connected Neighbors are preconfigured into the device
Reliable Flooding
• Store most recent LSP from each node
– Ignore earlier versions of the same LSP
• Forward LSP to all nodes but the one that
sent it
• Generate new LSP periodically
– Increment SEQNO
• Start at SEQNO=0 when reboot
– If you hear your own packet with SEQNO=n, set
your next SEQNO to n+1
• Decrement TTL of each stored LSP
– Discard when TTL=0
Time 1: Receive LSPs
B
4
C
1
B
4
C
1
5
B
4
A
C
1
6
10
D
• When a node gets an LSA, it floods it on all
ports except the port it received it from.
Time 1: Receive LSPs
B
4
C
1
B
4
C
1
5
B
4
A
C
1
6
10
D
• When a node gets an LSA, it floods it on all
ports except the port it received it from.
Time 10: Regenerate LSP and send out
5
B
4
A
C
1
6
10
D
• Generate new LSP periodically
– Increment SEQNO
Calculating best path
• Djikstra’s single-source shortest path algorithm
– Each node computes shortest paths from itself
• Let:
– N denote set of nodes in the graph
– l(i,j) denote the non-negative link between i,j
• ∞ if there is no direct link between i and j
– C(n) denote the cost of path from s to n
– s denotes yourself (node computing paths)
• Initialize variables
– M = {s} (set of nodes incorporated thus far)
– For each n in N-{s}, C(n) = l(s,n)
– Next(n) = n if l(s,n) < ∞, – otherwise
Djikstra’s Algorithm
• While N≠M
– Let w ∈(N-M) be the node with lowest C(w)
– M = M  {w}
– Foreach n ∈ (N-M), if C(w) + l(w,n) < C(n)
then C(n) = C(w) + l(w,n), Next(n) =
Next(w)
• Example: D: (D,0,-) (C,2,C) (B,5,C) (A,10,C)
Draw Backs of Link State Protocols
Draw Backs of Link State Protocols
• Scalability
– A lot of Network packets (Flooding of LSPs)
– A lot computation overhead, for each router:
• Router Memory: to store entire network
• Router CPU: to calculate Dijkstra
To Scale Add Areas (Hierarchy)
Backbone
(Area 0)
B
C
A
D
X
Area 1
Z
F
Y
E
Area 2
• Divide into backbone and other areas
To Scale Add Areas (Hierarchy)
Backbone
(Area 0)
B
C
A
D
X
Area 1
Z
F
Y
E
Area 2
• Divide into backbone and other areas
• An area must know how to route within itself
– LSPs are contained to an area: never flood outside of your area
– This provides scaling properties
• To route between two areas, you must go through the
backbone
To Scale Add Areas (Hierarchy)
Backbone
(Area 0)
B
C
A
Area 1
Area 2
• Divide into backbone and other areas
• An area must know how to route within itself
– LSPs are contained to an area: never flood outside of your area
– This provides scaling properties in all areas except Area 0
• To route between two areas, you must go through the backbone
• Backbone know how to route between areas but not how to route
within other areas.
– Backbone views each area as one node
– Allows scaling in the backbones
Distance Vector vs. Link State
Distance Vector
Link State
# of Messages
O(d), where d is
degree of node
O(nd) for n nodes
in system
Computation
convergence time
varies (e.g., countto-infinity)
O(n2) with O(nd)
messages
Robustness: what
happens with
malfunctioning
router?
Nodes can
advertise incorrect
path cost
Nodes can
advertise incorrect
link cost
Others can use the
cost, propagates
through network
May get correct link
information from
another node
Metrics
• Original ARPANET metric
– measures number of packets enqueued in each link
– neither latency nor bandwidth in consideration
• New ARPANET metric
– Stamp arrival time (AT) and departure time (DT)
– When link-level ACK arrives, compute
Delay = (DT – AT) + Transmit + Latency
– If timeout, reset DT to departure time for retransmission
– Link cost = average delay over some time period
• Fine Tuning
– Compressed dynamic range
– Replaced Delay with link utilization
• Today: commonly set manually to achieve specific
goals
Examples of
Distance Vector & Link State
• Distance Vector: RIPv2
– Fairly simple implementation of DV
– RFC 2453 (38 pages)
• Link State: OSPF (Open Shortest Path First)
– More complex link-state protocol
– Adds notion of areas for scalability
– RFC 2328 (244 pages)
RIPv2: Default on Several Cisco Routers
• Runs on UDP port 520
• Link cost = 1
• Periodic updates every 30s, plus triggered
updates
• Relies on count-to-infinity to resolve loops
– Maximum diameter 15 (∞ = 16)
– Supports split horizon, poison reverse
• Deletion
– If you receive an entry with metric = 16 from
parent OR
– If a route times out
OSPFv2: Costs more to activate on
Several Cisco Routers
• Link state protocol
• Runs directly over IP (protocol 89)
– Has to provide its own reliability
• All exchanges are authenticated
• Adds notion of areas for scalability
Other Routing Protocols
Routing Protocol Name
Type of Protocol
IS-IS (intermediate system –to- intermediate system)
Link State Protocol
IGRP (Interior Gateway Routing Protocol)
Distance Vector Protocol
BGP (border Gateway Protocol)
Distance-Vector (with Paths)
“Path-Vector”
Now You Have Your Routes, what
next?
Bring it All Together: Ethernet + IP
Step 0: Operator preconfigures routers with IP addresses
128.35.8.*/24
128.35.7.*/24
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
Router C
128.35.6.*/24
Router B
128.35.9.*/24
128.35.7.*/24
G
H
G
H
Bring it All Together: Ethernet + IP
Step 1: Router builds Routing Table using Routing Protocol
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
128.35.8.*/24
Router C
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
Router B
128.35.9.*/24
128.35.7.*/24
G
H
G
H
Bring it All Together: Ethernet + IP
Step 2: Get Your IP Address (DHCP)
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
128.35.8.*/24
Router C
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
Router B
128.35.7.*/24
G
128.35.9.*/24
H
G
H
Bring it All Together: Ethernet + IP
Step 2: Bonus Switch Learns (Learning Switch)
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
128.35.8.*/24
Router C
MAC_Y
MAC_A
MAC_B
MAC_Y
G
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
Router B
MAC_Z
MAC_A
MAC_B
MAC_Y
H
128.35.7.*/24
128.35.9.*/24
MAC_E
MAC_E
MAC_Z
MAC_Z
G
H
Bring it All Together: Ethernet + IP
Step 2: Bonus Switch Learns (Learning Switch)
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
128.35.8.*/24
Router C
MAC_Y
MAC_A
MAC_B
MAC_Y
G
Router B
MAC_Z
MAC_A
MAC_B
MAC_Y
H
128.35.9.*/24
MAC_E
MAC_E
MAC_Z
MAC_Z
G
H
128.35.7.*/24
MAC_A: 128.35.7.2
MAC_B: 128.35.7.5
MAC_E: 128.35.9.9
Bring it All Together: Ethernet + IP
Step 3: Get Dest-IP Address (DNS)
Source: MAC_A:128.35.7.2 Dest:UNKnown MAC: 128.35.5
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
128.35.8.*/24
Router C
MAC_Y
MAC_A
MAC_B
MAC_Y
G
Router B
MAC_Z
MAC_A
MAC_B
MAC_Y
H
128.35.9.*/24
MAC_E
MAC_E
MAC_Z
MAC_Z
G
H
128.35.7.*/24
MAC_A: 128.35.7.2
MAC_B: 128.35.7.5
MAC_E: 128.35.9.9
Bring it All Together: Ethernet + IP
Step 4: Get Dest MAC Address (ARP)
Source: MAC_A:128.35.7.2 Dest:UnKnown MAC: 128.35.5
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
128.35.8.*/24
Router C
MAC_Y
MAC_A
MAC_B
MAC_Y
G
Router B
MAC_Z
MAC_A
MAC_B
MAC_Y
H
128.35.9.*/24
MAC_E
MAC_E
MAC_Z
MAC_Z
G
H
128.35.7.*/24
MAC_A: 128.35.7.2
MAC_B: 128.35.7.5
MAC_E: 128.35.9.9
Bring it All Together: Ethernet + IP
Step 4: Get Dest MAC Address (ARP)
Source: MAC_A:128.35.7.2 Dest:UnKnown MAC: 128.35.5
128.35.6.*/24
128.35.6.*/24
128.35.9.*/24
128.35.9.*/24
128.35.8.*/24
128.35.8.*/24
128.35.7.*/24
your IP address with destination
IP address128.35.6.*/24
128.35.7.*/24
128.35.8.*/24 Compare
Source IP: 128.35.7.2
Router C
Router B
You netmaskMAC_Y
is 128.35.7.*/24 so you network
has: 128.35.7.0-128.35.7.255
MAC_Z
Dest IP: 128.35.7.5
Destination is in your network range, so you have to send to the destination!!!
MAC_ASo you ARPMAC_A
for the MAC address of the destination.
128.35.9.*/24
MAC_E
MAC_B
MAC_E
MAC_B
MAC_Z
MAC_Y
MAC_Z
MAC_Y
G
H
G
H
128.35.7.*/24
MAC_A: 128.35.7.2
MAC_B: 128.35.7.5
MAC_E: 128.35.9.9
Bring it All Together: Ethernet + IP
Step 4: Get Dest MAC Address (ARP)
Source: MAC_A:128.35.7.2 Dest: MAC_B: 128.35.5
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
128.35.8.*/24
Router C
MAC_Y
MAC_A
MAC_B
MAC_Y
G
Router B
MAC_Z
MAC_A
MAC_B
MAC_Y
H
128.35.9.*/24
MAC_E
MAC_E
MAC_Z
MAC_Z
G
H
128.35.7.*/24
MAC_A: 128.35.7.2
MAC_B: 128.35.7.5
MAC_E: 128.35.9.9
Bring it All Together: Ethernet + IP
Step 5: Send packets
Source: MAC_A:128.35.7.2 Dest: MAC_B: 128.35.5
128.35.8.*/24
Router C
MAC_Y
MAC_A
MAC_B
MAC_Y
IP: A->B
MAC: A->B
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
G
Router B
MAC_Z
MAC_A
MAC_B
MAC_Y
H
128.35.9.*/24
MAC_E
MAC_E
MAC_Z
MAC_Z
G
H
128.35.7.*/24
MAC_A: 128.35.7.2
MAC_B: 128.35.7.5
MAC_E: 128.35.9.9
Bring it All Together: Ethernet + IP
Step 5: Send packets
Source: MAC_A:128.35.7.2 Dest: MAC_B: 128.35.5
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
128.35.8.*/24
Router C
MAC_Y
MAC_A
MAC_B
MAC_Y
G
Router B
MAC_Z
MAC_A
MAC_B
MAC_Y
H
128.35.7.*/24
MAC_A: 128.35.7.2
IP: A->B
MAC: A->B
MAC_B: 128.35.7.5
128.35.9.*/24
MAC_E
MAC_E
MAC_Z
MAC_Z
G
H
MAC_E: 128.35.9.9
Bring it All Together: Ethernet + IP
Step 3: Get Dest-IP Address (DNS)
Alice to Eve
Source: MAC_A:128.35.7.2 Dest:UNKnown MAC: 128.35.9.9
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
128.35.8.*/24
Router C
MAC_Y
MAC_A
MAC_B
MAC_Y
G
Router B
MAC_Z
MAC_A
MAC_B
MAC_Y
H
128.35.9.*/24
MAC_E
MAC_E
MAC_Z
MAC_Z
G
H
128.35.7.*/24
MAC_A: 128.35.7.2
MAC_B: 128.35.7.5
MAC_E: 128.35.9.9
Bring it All Together: Ethernet + IP
Step 4: Get Dest MAC Address (ARP)
Alice to Eve
Source: MAC_A:128.35.7.2 Dest:UnKnown MAC: 128.35.9.9
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
128.35.8.*/24
Router C
MAC_Y
MAC_A
MAC_B
MAC_Y
G
Router B
MAC_Z
MAC_A
MAC_B
MAC_Y
H
128.35.9.*/24
MAC_E
MAC_E
MAC_Z
MAC_Z
G
H
128.35.7.*/24
MAC_A: 128.35.7.2
MAC_B: 128.35.7.5
MAC_E: 128.35.9.9
Bring it All Together: Ethernet + IP
Step 4: Get Dest MAC Address (ARP)
Alice to Eve
Source: MAC_A:128.35.7.2 Dest:UnKnown MAC: 128.35.9.9
128.35.6.*/24
128.35.6.*/24
128.35.9.*/24
128.35.9.*/24
128.35.8.*/24
128.35.8.*/24
128.35.7.*/24
your IP address with destination
IP address128.35.6.*/24
128.35.7.*/24
128.35.8.*/24 Compare
Source IP: 128.35.7.2
Router C
Router B
You netmaskMAC_Y
is 128.35.7.*/24 so you network
has: 128.35.7.0-128.35.7.255
MAC_Z
Dest IP: 128.35.9.9
Destination is not in your network range, so you have to send to router!!!
MAC_A So you ARP
for the MAC address of the router. 128.35.9.*/24
MAC_A
MAC_E
MAC_B
MAC_E
MAC_B
MAC_Z
MAC_Y
MAC_Z
MAC_Y
G
H
G
H
128.35.7.*/24
MAC_A: 128.35.7.2
MAC_B: 128.35.7.5
MAC_E: 128.35.9.9
Bring it All Together: Ethernet + IP
Step 4: Get Dest MAC Address (ARP)
Source: MAC_A:128.35.7.2 Dest: MAC_Y: 128.35.9.9
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
128.35.8.*/24
Router C
MAC_Y
MAC_A
MAC_B
MAC_Y
G
Router B
MAC_Z
MAC_A
MAC_B
MAC_Y
H
128.35.9.*/24
MAC_E
MAC_E
MAC_Z
MAC_Z
G
H
128.35.7.*/24
MAC_A: 128.35.7.2
MAC_B: 128.35.7.5
MAC_E: 128.35.9.9
Bring it All Together: Ethernet + IP
Step 5: Send packets
Source: MAC_A:128.35.7.2 Dest: MAC_B: 128.357.5
128.35.8.*/24
Router Y
MAC_Y
IP: A->E
MAC: A->Y
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
MAC_A
MAC_B
MAC_Y
G
Router Z
MAC_Z
MAC_A
MAC_B
MAC_Y
H
128.35.9.*/24
MAC_E
MAC_E
MAC_Z
MAC_Z
G
H
128.35.7.*/24
MAC_A: 128.35.7.2
MAC_B: 128.35.7.5
MAC_E: 128.35.9.9
Bring it All Together: Ethernet + IP
Step 5: Send packets
Source: MAC_A:128.35.7.2 Dest: MAC_B: 128.357.5
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
128.35.8.*/24
IP: A->E
Router C MAC: A->Z
MAC_Y
MAC_A
MAC_B
MAC_Y
G
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
Router B
MAC_Z
MAC_A
MAC_B
MAC_Y
H
128.35.9.*/24
MAC_E
MAC_E
MAC_Z
MAC_Z
G
H
128.35.7.*/24
MAC_A: 128.35.7.2
MAC_B: 128.35.7.5
MAC_E: 128.35.9.9
Bring it All Together: Ethernet + IP
Step 5: Send packets
Source: MAC_A:128.35.7.2 Dest: MAC_B: 128.357.5
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
128.35.8.*/24
Router C
MAC_Y
MAC_A
MAC_B
MAC_Y
G
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
IP: A->E
Router B
MAC_Z MAC: A->E
MAC_A
MAC_B
MAC_Y
H
128.35.9.*/24
MAC_E
MAC_E
MAC_Z
MAC_Z
G
H
128.35.7.*/24
MAC_A: 128.35.7.2
MAC_B: 128.35.7.5
MAC_E: 128.35.9.9
Bring it All Together: Ethernet + IP
Step 5: Send packets
Source: MAC_A:128.35.7.2 Dest: MAC_B: 128.357.5
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24 128.35.6.*/24
128.35.6.*/24
128.35.9.*/24
128.35.8.*/24
128.35.7.*/24
128.35.8.*/24
Router C
MAC_Y
MAC_A
MAC_B
MAC_Y
G
Router B
MAC_Z
MAC_A
MAC_B
MAC_Y
H
128.35.7.*/24
MAC_A: 128.35.7.2
MAC_B: 128.35.7.5
128.35.9.*/24
MAC_E
MAC_E
MAC_Z
MAC_Z
G
H
IP: A->E
MAC: A->
MAC_E: 128.35.9.9
What Limitations Does Ethernet Have?
• Minimum Packet Size
– 64Bytes (512 bits): To ensure that collisions are
detected!
– Bandwidth-Delay-Product*2
• Maximum Ethernet LAN size
– 2500 meters: Due to signal decay, any longer and
packets would not be delivered
A Router Does Two Things
Forwarding (per-node operation):
– how to pass a packet from an input interface to the
output interface of a router
– Destination based forwarding
• Check destination IP and look it up in your forwarding table
Routing (distributed algorithm):
– how to find and setup a route? Implemented by
dynamic routing protocols
– OSPF, RIP
Forwarding
• Due to CIDR, Routers use longest prefix match
Router Forwarding Table
After OSPF or RIP
128.35.6.*/24, fwd port 1
128.35.*.*/16, fwd port 3
128.35.6.3/32, fwd port 6
128.35.7.*/24, fwd port 18
From
From
From
From
128.35.6.0
128.35.0.0
128.35.6.3
128.35.7.0
To: 123.35.6.255
To: 123.35.255.255
To: 123.35.6.3
To: 128.35.7.255
Forwarding
• Due to CIDR, Routers use longest prefix match
Router Forwarding Table
After OSPF or RIP
128.35.6.*/24, fwd port 1
128.35.*.*/16, fwd port 3
128.35.6.3/32, fwd port 6
128.35.7.*/24, fwd port 18
From
From
From
From
128.35.6.0
128.35.0.0
128.35.6.3
128.35.7.0
To: 123.35.6.255
To: 123.35.255.255
To: 123.35.6.3
To: 128.35.7.255
Forwarding
• Due to CIDR, Routers use longest prefix match
Router Forwarding Table
After OSPF or RIP
128.35.6.*/24, fwd port 1
128.35.*.*/16, fwd port 3
128.35.6.3/32, fwd port 6
128.35.7.*/24, fwd port 18
From
From
From
From
Packet with destination: 128.35.6.3.
Which entry does it match? What port
does it get forwarded on?
128.35.6.0
128.35.0.0
128.35.6.3
128.35.7.0
To: 123.35.6.255
To: 123.35.255.255
To: 123.35.6.3
To: 128.35.7.255
Forwarding
• Due to CIDR, Routers use longest prefix match
Router Forwarding Table
After OSPF or RIP
128.35.6.*/24, fwd port 1
128.35.*.*/16, fwd port 3
128.35.6.3/32, fwd port 6
128.35.7.*/24, fwd port 18
From
From
From
From
Packet with destination: 128.35.6.8
Which entry does it match? What port
does it get forwarded on?
128.35.6.0
128.35.0.0
128.35.6.3
128.35.7.0
To: 123.35.6.255
To: 123.35.255.255
To: 123.35.6.3
To: 128.35.7.255
Router Versus Switches
Router
Switches
• Runs multiple switching
protocols: Ethernet, ATM
• Runs one switching protocol
– Switches between techs
• Runs routing protocols
• Runs DHCP
• Needs a common address
across techs: IP address
– E.g. Ethernet addresses make
no sense to ATM hosts
– Can only work with same tech
• Dictates how bits become
signals
• Dictates how bits becomes a
packet/frame
• Needs hardware addresses to
identify hosts/switches
Summary
• This Class
– Link State protocols
– Link State V. Distance Vector
– Routing V Forwarding
– How routing works within one ISP/AS
• Next Class
– Inter-Domain routing
– Routing over the Wide Area Network