Lecture 9
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Reminder: Homework 3, Programming Project
2 due today.
Questions?
Thursday, September 22
CS 475 Networks - Lecture 9
1
Outline
Chapter 3 - Internetworking
3.1 Switching and Bridging
3.2 Basic Internetworking (IP)
3.3 Routing
3.4 Implementation and Performance
3.5 Summary
Thursday, September 22
CS 475 Networks - Lecture 9
2
Routing
Forwarding consists of looking at a packets
destination, consulting a table and directing the
packet to the corresponding interface.
Routing is the process by which forwarding tables
are built.
Forwarding is a simple process that takes place at
a single node. Routing depends on complex
distributed algorithms.
Thursday, September 22
CS 475 Networks - Lecture 9
3
Routing
We distinguish between the forwarding and
routing tables although they may be the same in
practice. The forwarding table contains
information to forward a packet, while the routing
table contains the information to build the
forwarding table.
Example Routing
Table
Prefix/Length
Next Hop
18/8
171.69.245.10
Example Forwarding
Table
Prefix/Length
Interface MAC Address
18/8
if0
Thursday, September 22
CS 475 Networks - Lecture 9
8:0:2b:e4:b:1:2
4
Routing
A routing domain is an internetwork under the
same administrative control, e.g. an ISP or
university (a network of 10 - 100 networks).
Today we will be discussing intradomain routing
protocols or interior gateway protocols (IGPs).
The algorithms do not scale for use in larger
networks.
Next chapter we will discuss interdomain routing
protocols that can be used on larger networks
(i.e., the Internet).
Thursday, September 22
CS 475 Networks - Lecture 9
5
Network as a Graph
We will represent a network as a graph. The
nodes represent routers and the edges are links
between routers. A cost may be associated with
each link. The routing problem is to find the
lowest cost path between any two nodes in the
graph.
Thursday, September 22
CS 475 Networks - Lecture 9
6
Network as a Graph
For small networks routing tables could be
constructed manually. Statically constructed
routing tables must be re-built anytime the
network changes (node or link failure, addition of
nodes or links, cost changes).
For these reasons routing tables are usually
constructed by running routing protocols among
the nodes. The protocols provide a distributed,
dynamic means for construction of the routing
tables.
Thursday, September 22
CS 475 Networks - Lecture 9
7
Distance-Vector (RIP)
The distance vector (DV) algorithm constructs an
array (vector) containing the distances (costs) to
all other nodes in the network. The vector is then
distributed to its immediate neighbors.
Initially each node is assumed to know only the
cost to its directly connected neighbors. A link
that is down is assumed to have an infinite cost.
Thursday, September 22
CS 475 Networks - Lecture 9
8
Distance-Vector (RIP)
Example network with
cost of 1 along each link
(since all costs are equal
they are not shown).
Final Table at A
Initial Table at A
Dest
Cost
Next Hop
Dest
Cost
Next Hop
B
1
B
B
1
B
C
1
C
C
1
C
D
∞
-
D
2
C
E
1
E
E
1
E
F
1
F
F
1
F
G
∞
-
G
2
F
Thursday, September 22
CS 475 Networks - Lecture 9
9
Distance-Vector (RIP)
STOP HERE
SPOILER ALERT
Initial Table at C
Dest
Cost
Next Hop
Final Table at C
Dest
A
A
B
B
D
D
E
E
F
F
G
G
Thursday, September 22
CS 475 Networks - Lecture 9
Cost
Next Hop
10
Distance-Vector (RIP)
Initial Table at C
Dest Cost
Next Hop
Final Table at C
Dest Cost
Next Hop
A
1
A
A
1
A
B
1
B
B
1
B
D
1
D
D
1
D
E
∞
-
E
2
A
F
∞
-
F
2
A
G
∞
-
G
2
D
Thursday, September 22
CS 475 Networks - Lecture 9
11
Distance-Vector (RIP)
The table below shows a
global view of the initial
distances to each node.
Distance to Reach Node
Node
A
B
C
D
E
F
G
A
0
1
1
∞
1
1
∞
B
1
0
1
∞
∞
∞
∞
C
1
1
0
1
∞
∞
∞
D
∞
∞
1
0
∞
∞
1
E
1
∞
∞
∞
0
∞
∞
F
1
∞
∞
∞
∞
0
1
G
∞
∞
∞
1
∞
1
0
Thursday, September 22
CS 475 Networks - Lecture 9
12
Distance-Vector (RIP)
The table below shows
a global view of the final
distances to each node.
Distance to Reach Node
Node
A
B
C
D
E
F
G
A
0
1
1
2
1
1
2
B
1
0
1
2
2
2
3
C
1
1
0
1
2
2
2
D
2
2
1
0
3
2
1
E
1
2
2
3
0
2
3
F
1
2
2
2
2
0
1
G
2
3
2
1
3
1
0
Thursday, September 22
CS 475 Networks - Lecture 9
13
Distance-Vector (RIP)
The nodes send their distance vectors to their
directly connected neighbors allowing the
neighbors to update their vectors.
Initially A believes that it has no path to D. C has
a path of cost 1 to D and A and C are directly
connected. After A receives C's vector, A
determines that it can reach D through C at a cost
of 2 and updates its table. (A can reach B through
C at a cost of 2, but this is greater than the current
cost of 1, so A does not change the
corresponding entry.)
Thursday, September 22
CS 475 Networks - Lecture 9
14
Distance-Vector (RIP)
Updates are sent out periodically. They may also
be triggered due to a change in a routing table
caused by an update from a neighbor.
The routing tables converge fairly rapidly (both
initially and also due to any changes).
Thursday, September 22
CS 475 Networks - Lecture 9
15
Distance-Vector (RIP)
The count to infinity problem can occur if a link
goes down. Assume link A-to-E goes down. A
advertises a distance of infinity, but C might still
advertise a distance of two (through A). B
advertises a distance of three through C. A
advertises a distance of four through B. C
updates to advertise a distance of five through A
and so on.
The cycle stops only when the distances reach
infinity. During this time, all nodes think a path to
E still exists.
Thursday, September 22
CS 475 Networks - Lecture 9
16
Distance-Vector (RIP)
The count to infinity problem can be bounded by
using a small value to represent infinity (say 16
hops).
Alternatively, in the split horizon method, a node
does not send those routes it learned from a
neighbor back to that neighbor.
In the split horizon with poisoned reverse a node
sends back a distance of infinity to those
neighbors from which it learned a route.
Thursday, September 22
CS 475 Networks - Lecture 9
17
Distance-Vector (RIP)
The Routing Information
Protocol (RIP) is a very
widely used routing
protocol on IP networks.
RIP is based on the DV
algorithm. Distances to
networks instead of
distances to routers are
advertised in RIP.
Thursday, September 22
RIPv2 packet format. RIP was designed to
support other protocols in addition to IP
CS 475 Networks - Lecture 9
18
Link State (OSPF)
In DV, routers advertise the cost-to-each-router in
the network to their immediate neighbors. In a
link state (LS) algorithm, routers advertise the
cost-to-their-immediate-neighbors to each router
in the network.
In LS each node floods the network with a linkstate packet (LSP) that contains:
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the ID of the node that created the LSP
a list of directly connected nodes with costs
a sequence number for the packet
a time-to-live for the packet
Thursday, September 22
CS 475 Networks - Lecture 9
19
Link State (OSPF)
When a node X receives an LSP from node Y with
a larger sequence number than the one X has in
memory it stores the new LSP. If it is an older
LSP (smaller seq. num. it discards it)
X sends a new LSP out to all neighbors except
the one from which it received the LSP. Each
neighbor does the same, flooding the network
with the new LSP.
Each node will eventually have LSPs from all
other nodes stored in memory.
Thursday, September 22
CS 475 Networks - Lecture 9
20
Link State (OSPF)
Routing tables at each node are computed using
the forward search implementation of Dijkstra's
algorithm:
Each switch maintains Tentative and Confirmed
lists. The lists contain entries of the form
(Destination, Cost, NextHop).
1. The Confirmed list is initialized with an entry to
this switch with a cost of 0.
2. The node just added to Confirmed is called
Next. Select its LSP.
Thursday, September 22
CS 475 Networks - Lecture 9
21
Link State (OSPF)
3. For each neighbor (Neighbor) of Next,
calculate the cost (Cost) to each Neighbor
through Next.
a) If Neighbor is not in either list, add (Neighbor,
Cost, NextHop) to Tentative.
b) If Neighbor is in Tentative but the Cost
through Next is less, update entry.
4. If Tentative is empty, stop. Otherwise pick
entry from Tentative with lowest cost, move it to
the Confirmed list and return to step 2.
Thursday, September 22
CS 475 Networks - Lecture 9
22
Link State (OSPF)
Step Conf.
1
Tent.
(D,0,-)
Comments
Look at D's LSP
2
3
Example network used to
illustrate LS algorithm.
4
5
Steps at left illustrate
construction of
routing table for
node D.
6
7
Thursday, September 22
CS 475 Networks - Lecture 9
23
Link State (OSPF)
Step Conf.
1
(D,0,-)
2
(D,0,-)
Tent.
Comments
Look at D's LSP
(B,11,B)
(C,2,C)
D's LSP contains routes to B
and C that are smaller in costs
than any existing routes
3
Example network used to
illustrate LS algorithm.
4
5
Steps at left illustrate
construction of
routing table for
node D.
6
7
Thursday, September 22
CS 475 Networks - Lecture 9
24
Link State (OSPF)
Step Conf.
Tent.
Comments
1
(D,0,-)
Look at D's LSP
2
(D,0,-)
(B,11,B)
(C,2,C)
D's LSP contains routes to B
and C that are smaller in costs
than any existing routes
3
(D,0,-)
(C,2,C)
(B,11,B)
Move lowest cost member of
Tentative to Confirmed. Look
at C's LSP
4
Example network used to
illustrate LS algorithm.
5
Steps at left illustrate
construction of
routing table for
node D.
6
7
Thursday, September 22
CS 475 Networks - Lecture 9
25
Link State (OSPF)
Step Conf.
Tent.
Comments
1
(D,0,-)
2
(D,0,-)
(B,11,B)
(C,2,C)
D's LSP contains routes to B
and C that are smaller in costs
than any existing routes
3
(D,0,-)
(C,2,C)
(B,11,B)
Move lowest cost member of
Tentative to Confirmed. Look
at C's LSP
4
(D,0,-)
(C,2,C)
Look at D's LSP
(B,5,C)
C has lower cost route to B,
(A,12,C) replace existing route in
Tentative. Add A route.
Example network used to
illustrate LS algorithm.
5
Steps at left illustrate
construction of
routing table for
node D.
6
7
Thursday, September 22
CS 475 Networks - Lecture 9
26
Link State (OSPF)
Step Conf.
Tent.
Comments
1
(D,0,-)
Look at D's LSP
2
(D,0,-)
(B,11,B)
(C,2,C)
D's LSP contains routes to B
and C that are smaller in costs
than any existing routes
3
(D,0,-)
(C,2,C)
(B,11,B)
Move lowest cost member of
Tentative to Confirmed. Look
at C's LSP
4
(D,0,-)
(C,2,C)
(B,5,C)
C has lower cost route to B,
(A,12,C) replace existing route in
Tentative. Add A route.
5
(D,0,-)
(C,2,C)
(B,5,C)
(A,12,C) Move lowest cost member of
Tentative to Confirmed. Look
at B's LSP
6
7
Thursday, September 22
CS 475 Networks - Lecture 9
Example network used to
illustrate LS algorithm.
Steps at left illustrate
construction of
routing table for
node D.
27
Link State (OSPF)
Step Conf.
Tent.
Comments
1
(D,0,-)
Look at D's LSP
2
(D,0,-)
(B,11,B)
(C,2,C)
D's LSP contains routes to B
and C that are smaller in costs
than any existing routes
3
(D,0,-)
(C,2,C)
(B,11,B)
Move lowest cost member of
Tentative to Confirmed. Look
at C's LSP
4
(D,0,-)
(C,2,C)
(B,5,C)
C has lower cost route to B,
(A,12,C) replace existing route in
Tentative. Add A route.
5
(D,0,-)
(C,2,C)
(B,5,C)
(A,12,C) Move lowest cost member of
Tentative to Confirmed. Look
at B's LSP
6
(D,0,-)
(C,2,C)
(B,5,C)
(A,10,C) There is a lower cost path to A
through B
7
Thursday, September 22
CS 475 Networks - Lecture 9
Example network used to
illustrate LS algorithm.
Steps at left illustrate
construction of
routing table for
node D.
28
Link State (OSPF)
Step Conf.
Tent.
Comments
1
(D,0,-)
Look at D's LSP
2
(D,0,-)
(B,11,B)
(C,2,C)
D's LSP contains routes to B
and C that are smaller in costs
than any existing routes
3
(D,0,-)
(C,2,C)
(B,11,B)
Move lowest cost member of
Tentative to Confirmed. Look
at C's LSP
4
(D,0,-)
(C,2,C)
(B,5,C)
C has lower cost route to B,
(A,12,C) replace existing route in
Tentative. Add A route.
5
(D,0,-)
(C,2,C)
(B,5,C)
(A,12,C) Move lowest cost member of
Tentative to Confirmed. Look
at B's LSP
6
(D,0,-)
(C,2,C)
(B,5,C)
(A,10,C) There is a lower cost path to A
through B
7
(D,0,-)
(C,2,C)
(B,5,C)
(A,10,C)
Thursday, September 22
Move lowest cost member of
Tentative to Confirmed. We
are done.
CS 475 Networks - Lecture 9
Example network used to
illustrate LS algorithm.
Steps at left illustrate
construction of
routing table for
node D.
29
Link State (OSPF)
Step Conf.
Tent.
Comments
1
2
3
Example network used to
illustrate LS algorithm.
4
5
Steps at left illustrate
construction of
routing table for
node A.
STOP HERE
SPOILER ALERT
6
7
Thursday, September 22
CS 475 Networks - Lecture 9
30
Link State (OSPF)
Step Conf.
Tent.
Comments
1
(A,0,-)
Look at A's LSP
2
(A,0,-)
(B,5,B)
A's LSP contains routes to B
(C,10,C) and C that are smaller in costs
than any existing routes
3
(A,0,-)
(B,5,B)
(C,10,C) Move lowest cost member of
Tentative to Confirmed. Look
at B's LSP
4
(A,0,-)
(B,5,B)
(C,8,B)
B has lower cost route to C,
(D,16,B) replace existing route in
Tentative. Add D route.
5
(A,0,-)
(B,5,B)
(C,8,B)
(D,16,B) Move lowest cost member of
Tentative to Confirmed. Look
at C's LSP
6
(A,0,-)
(B,5,B)
(C,8,B)
(D,10,B) There is a lower cost path to D
through C
7
(A,0,-)
(B,5,B)
(C,8,B)
(D,10,B)
Thursday, September 22
Move lowest cost member of
Tentative to Confirmed. We
are done.
CS 475 Networks - Lecture 9
Example network used to
illustrate LS algorithm.
Steps at left illustrate
construction of
routing table for
node A.
31
Link State (OSPF)
Node
A
B
C
D
Routing Table
(A,0,-) (B,5,B) (C,8,B) (D,10,B)
(A,5,A) (B,0,-) (C,3,C) (D,5,C)
(A,8,B) (B,3,B) (C,0,-) (D,2,D)
(A,10,C) (B,5,C) (C,2,C) (D,0,-)
Example network used to
illustrate LS algorithm.
Routing tables at each node after
convergence of LS algorithm.
Thursday, September 22
CS 475 Networks - Lecture 9
32
Link State (OSPF)
The Open Shortest Path First (OSPF) protocol is
one of the most popular LS protocols. “Open”
refers to the fact that it is an open standard. SPF
is another name for link state.
OSPF allows for authentication of routing
messages, partitioning of a domain, and load
balancing along equal cost paths.
Details of the OSPF packet format can be found
in the text.
Thursday, September 22
CS 475 Networks - Lecture 9
33
Metrics
Many of the methods used to determine link-costs
(metrics) were developed on ARPANET. The
original ARPANET used the number of packets
waiting to be transmitted on a link as the link cost.
This method did not take either BW or latency
into account.
A second ARPANET algorithm used the packet
delay (from time of arrival to delivery at the next
hop) as the metric. This worked under light load,
but exhibited several problems under heavy load.
Thursday, September 22
CS 475 Networks - Lecture 9
34
Metrics
A third method used
a metric based on a
non-linear mapping
from load to costs.
Different links used
different mappings.
In practice today, most costs are statically
assigned. A common setting is
cost=Constant/BW.
Thursday, September 22
CS 475 Networks - Lecture 9
35
Implementation & Performance
It is possible to use an ordinary PC (with
additional Network Interface Cards (NICs)) as a
switch as shown below:
Thursday, September 22
CS 475 Networks - Lecture 9
36
Implementation & Performance
Performance of a PC based switch is limited since
all packets pass through a single point of
contention. Aggregate throughput (the total
sustainable data rate summed over all inputs) is
half the main memory BW or half the I/O bus BW.
A PC with a 133 MHz 64-bit I/O bus can transmit
at a peak rate of 8 Gbps giving an aggregate
throughput of 4 Gbps. Fast enough for several
100 Mbps Ethernet ports but not fast enough for a
high-end router.
Thursday, September 22
CS 475 Networks - Lecture 9
37
Ports
Many modern switches are based on
architectures similar to that shown below. Each
port typically contains logic and memory.
Thursday, September 22
CS 475 Networks - Lecture 9
38
Ports
Each input port typically contains a VC mapping
table (VC switches) or a forwarding table (packet
switches).
Ports also provide buffering. Most use either pure
output buffering or a mixture of output buffering
and internal buffering (buffering in the switching
fabric.
Simple input only buffers
can lead to head-of-line
blocking as shown at right.
Thursday, September 22
CS 475 Networks - Lecture 9
39
Fabrics
Switching fabric architectures can be categorized
as one of the following four types:
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
shared bus: This is the PC architecture
illustrated earlier. A single I/O bus is shared
between all input and output ports.
shared memory: packets are written to memory
by an input port and read directly from memory
by an output port (similar to shared bus but
uses a high speed memory bus instead of an
I/O bus)
Thursday, September 22
CS 475 Networks - Lecture 9
40
Fabrics

crossbar: This is a
matrix of pathways
that can connect any
input port to any
output port. In the
simplest design each
output has to be able
to accept packet from
all inputs at once.
A 4 x 4 Crossbar
Thursday, September 22
CS 475 Networks - Lecture 9
41
Fabrics

Thursday, September 22
self-routing: Each
input port adds a
header to each
packet to direct it to
its correct output
CS 475 Networks - Lecture 9
42
Fabrics
In the banyan fabric at
left each 2 x 2 switch
directs the packet up if
the header bit is 0 and
down if it is a 1.
The banyan fabric requires that the input packets
be sorted (by output port number). Multiple
packets can be simultaneously routed through the
fabric.
Thursday, September 22
CS 475 Networks - Lecture 9
43
Router Implementation
IP routers are more complicated than switches in
several respects.


Routers must be able to handle variable length IP
packets.
The forwarding algorithm is different because the
network part of the IP address is not fixed in size.
Routers may be implemented using network
processors instead of general purpose
microprocessors. High-performance routers can
handle 40 Gbps per interface.
Thursday, September 22
CS 475 Networks - Lecture 9
44
In-class Exercises
1) netstat utility
2) Start homework.
Thursday, September 22
CS 475 Networks - Lecture 9
45