Path Recovery Performances
of Routing Protocols
Chia Hoo Hon
HT023558M
chiahoohon@yahoo.com
Ng Hian James
HT035267U
nghianja@comp.nus.edu.sg
Abstract
of router nodes is selected to make up the
path which the data packets will travel in.
This is the report for the project worked on
by two students for the module CS5224:
High Speed Networks and Multimedia
Networking. In this project, the two general
routing protocols, link state routing and
distance vector routing, are being
investigated for their path recovery
performances due to link failures. By
running simulations using the NS2 simulator,
it can be deduced that link state routing is a
better routing protocol than distance vector
routing when it comes to throughput and
delay but it also incurs a higher overhead
for the total number of data transmitted.
However, when there are an increasing
number of failed links, distance vector
routing is better with smaller increases of
overheads and delay.
Like all hardware, routers are not infallible.
A router may not be functioning due to
power failures (blackouts), damages to
internal hardware (circuitry), system
malfunctions, or software errors (in the case
of a first generation router with a general
CPU and OS). A backup router is usually
available for switchovers for each node in
critical networks but for regular networks,
no backup router is, more often than not,
available. In the absence of one, a new relay
path, or at least a sub-path bypassing the
failed router, has to be created. This is called
path recovery. The efficiency of path
recovery depends on the routing protocol
employed in the network concerned.
1.
Introduction
In the world of computer networks, every
node is important in having the task of
relaying data packets from one end-point to
another end-point. The source end-point will
send its data packets to the nearest router
node that is closer to the destination endpoint, if the source cannot reach the
destination directly. The router node will
forward the data packets to the next router
one step closer to the destination, which will
in turn do the same. Eventually, the router
node that is connected to the destination
end-point will receive the data packets and
forward them to the destination. Depending
on the routing protocol being deployed, a set
In this project, we are investigating the two
general routing protocols: link state routing
and distance vector routing. We have
obtained sufficient results for the
comparison, evaluation and analysis on the
performance of link state routing and
distance vector routing in the presence of
link failure(s) through simulation. We hope
that our simulation can help to answer the
question about which routing protocol is
better in dealing with link failures, both
small and big numbers of them.
The rest of the paper is organized as follows:
Section 2 gives a brief overview of the
routing protocols we are looking into.
Section 3 describes our simulation and
Section 4 contains our analysis of the results
obtained. In Section 5 we provide a
conclusion on our findings.
2.
Routing Protocols
Routing is the process of finding a path from
a source to every destination in the network.
It is accomplished by means of routing
protocols that establish mutually consistent
routing tables in every router in the network.
A routing table contains at least two
columns: the first is the address of a
destination endpoint or a destination
network, and the second is the address of the
network element that is the next hop in the
“best” path to this destination. In a network,
links and routers are unreliable, alternative
paths are scarce, and traffic patterns can
change unpredictably. It is not surprising
that routing follows a different path. The
two fundamental routing algorithms in
packet-switched networks are distancevector and link-state.[1]
a. Link State Routing
The notion of link state routing was first
introduced and deployed on the Arpanet
back in the late 1970s.[2] Subsequent
revisions and improvements to the basic
function now give network engineers a
choice of link state routing protocols to
deploy. Two such protocols that have
emerged as the preferred choices are the
Open Shortest Path First and its close
cousin,
Intermediate
System
to
Intermediate System (IS-IS).[3]
In link state routing, a router knows the
entire network topology (or at least a
partial map of the network) and is able
to compute the shortest path by itself.
These are possible because of link state
packets (LSPs, figure 1), which are
controllably flooded throughout the
network and stored in the routers’
topology databases. A LSP contains
information regarding a router’s
neighbors. Whenever a router receives a
LSP, it will check the LSP with the
entries in its database. If the LSP is
found to be new, the router then
forwards it to every interface other than
the incoming one. It will reach all
routers that are connected to this one
and will be forwarded to other routers in
the similar fashion. The Dijkstra
algorithm is then used to calculate
routes which are belongs to the shortest
paths.
In the event of a router failure,
information is passed to other routers
with the use of HELLO packets. With
the rest of the routers knowing about the
failed router, an alternative sub-path
around the failed router can be found.
LSPs CREATED BY A
B
1
A
1
1
B
1
A
C
4
D
4
4
A
C
Figure 1: Link-state packets. Each router participating
in link-state routing creates and distributes a set of linkstate packets that contain the router’s cost to reach each
neighbor.[1]
b. Distance Vector Routing
This type of routing protocol requires
that each router simply inform its
neighbors of its routing table. For each
network path, the receiving routers pick
the neighbor advertising the lowest cost,
then add this entry into its routing table
for re-advertisement.[4] One such
common protocol that has been widely
used is known as Routing Information
Protocol (RIP).
In distance vector routing, a router tells
its neighbors its best idea of distance to
every other router in the network. It
does this by sending and receiving
distance vectors to and from its
neighbors. Upon receiving such distance
vectors, a router then updates its notion
of best path to each destination, and the
next hop for this destination (figure 2).
This is in a way like having each router
casting its entire routing table to its
neighbors. This mode of getting routes
is
known
as
the
Distributed
Asynchronous Bellman-Ford algorithm.
Routers detect router failures with the
exchange of periodic “Hello” or distance
vector messages. In the event of a router
failure, path recovery is done by the
routers around the failed one. Since all
routers know the routing tables of their
neighbors, the neighbors of the failed
router can always find an alternative
path linking the routers that the failed
one would have used.
B
1
A
1
1
4
4
C
INITIAL
B C
d. Coping with Link Failures
D
A
0
1
4
∞
D B
1
0
1
1
C
4
1
0
2
D
∞
1
2
0
COMPUTATION AT A WHEN
DV FROM B ARRIVES
AB = 1
} COST TO GO TO B
+
1
0
1
1
widely used. While link state algorithms
send small updates everywhere, distance
vector algorithms send larger updates
only to neighboring routers. Because
they converge more quickly, link-state
algorithms are somewhat less prone to
routing loops than distance vector
algorithms. On the other hand, link state
algorithms require more CPU power and
memory than distance vector algorithms.
Link-state algorithms, therefore, can be
more expensive to implement and
support. Link-state protocols are
generally more scalable than distance
vector protocols.[5]
} COST TO DESTN FROM B
=
2
1
2
2
} COST TO DESTN VIA B
0
1
4
∞
} CURRENT COST FROM A
0
1
2
2
 MIN OF THE ABV TWO
NEXT
HOP { B
B
NEW COST=NEW DV FOR A
Figure 2: Distance-vector algorithm at node A. A
receives a distance vector from its neighbor B. It uses
this information to find that it can reach nodes C and D
at a lower cost. It therefore updates its own distance
vector and chooses B as its next hop to C and D.[1]
c. Link State vs. Distance Vector
Both protocols are evenly matched and
When links are subject to failure, the
problem in routing, which is the transfer
of control information from points in the
network where it is collected to other
points where it is needed, becomes very
challenging. Getting routing information
reliably to the places where it is needed
in the presence of potential link failures
involves some difficulties. These
difficulties also arise in the context of
broadcasting information related to the
congestion status of each link.
As it is impossible to for every node to
know the correct network topology at all
times, an algorithm for broadcasting
routing information must at least can
cope successfully with any finite
number of topological changes within
finite time. As the network will not
always remain connected, a topology
update algorithm working with some
protocol for bringing up links should be
capable of starting up the network from
a state where all links are down. Hence
there is the development of basic
algorithms that broadcast routing
information resulting in the flooding
algorithm using sequence numbers, the
flooding algorithm without periodic
updates, and the broadcast algorithm
without sequence numbers.[6]
3.
n6 – n12
n7 – n13
n8 – n14
Simulation
In our simulations, we are using a 41-node
network topology (figure 3) with uni-cast
routing to test both Distance Vector and
Link State routing. The application protocol
we used is the File Transfer Protocol (FTP)
which sends data from the following source
nodes to the following destination nodes:
Source Node Destination Node
n0
n38
n1
n39
n2
n40
n3
n41
For Scenario 2 (figure 5), there will be links
that will be down, and is a further extension
from Scenario 1. Like Scenario 1, the
following set of links will be down from
time 5 sec onwards and up from time 7 sec.
However, before the following first set of
links is up, a second set of links will be
down from 6 sec and up from 9 sec.
First Set of
Links
n4 – n10
n5 - n11
n6 – n12
n7 – n13
n8 – n14
We will be testing the 2 routing protocols
based on 2 scenarios. For Scenario 1 (figure
4), the following set of links will be down
from time 5 sec onwards and then up from 7
sec:
For both scenarios, the FTP applications will
start at 0.1 sec and stop at 10 sec. Both
simulations will stop at 10.5 sec.
First set of Links
n4 – n10
n5 - n11
n9
n15
n21
n27
n4
n0
n33
n10
n16
n22
n28
n5
n1
n17
n23
n29
n6
n39
n35
n12
n18
n24
n30
n7
n3
n38
n34
n11
n2
Second Set of
Links
n16 – n21
n17 - n22
n18 – n23
n19 – n24
n20 – n25
n40
n36
n13
n19
n25
n31
n8
n41
n37
n14
n20
n26
n32
Figure 3: The original network topology
Down when T= 5 sec
Up when T = 7 sec
n9
n15
n21
n27
n4
n0
n33
n10
n16
n22
n28
n5
n1
n34
n11
n17
n23
n29
n6
n2
n39
n35
n12
n18
n24
n30
n7
n3
n38
n40
n36
n13
n19
n25
n31
n8
n41
n37
n14
n20
n26
n32
Figure 4: Scenario 1 network topology
Down when T = 6 sec
Up when T = 9 sec
Down when T = 5 sec
Up when T = 7 sec
n9
n15
n21
n27
n4
n0
n33
n10
n16
n22
n28
n5
n1
n34
n11
n17
n23
n29
n6
n2
n39
n35
n12
n18
n24
n30
n7
n3
n38
n40
n36
n13
n19
n25
n31
n8
n41
n37
n14
n20
n26
n32
Figure 5: Scenario 2 network topology
4.
Analysis
Results of the simulations are obtained from
data in the form of traces contained in trace
files. We are interested in the performances
of the two routing protocols in doing path
recovery. This translated to reading the trace
files for information regarding the amount of
bytes and delay occurred for reestablishing
paths. We did step-by-step analysis of what
the information we wanted and can be
extracted from the trace files. Once we had
settled on the information we wanted and
the methods of extracting them from the
data, a Java™ program with the appropriate
data extraction methods is written to parse
the trace files for link state routing and
distance vector routing.
Firstly, we are able to extract information
about how much data has been passing
around the network within the simulated
time under link state routing (LSR) or
distance vector routing (DVR). To do this,
we start by keeping counts on the number of
packets and bytes passing through the
network. The results are given in table 1 and
from them we can notice that within the
same simulated time, DVR transmitted less
packets and bytes in the network than LSR
for all three scenarios.
However, such results are insufficient for us
to give a conclusive deduction of the
performances of the routing protocols. The
smaller number of packets used in DVR
does not necessary mean that it was efficient
in having less message exchanges between
for setting or recovering routes. Similarly,
the bigger number of bytes transmitted in
LSR does not necessary mean that it was
better in data transmission.
Nonetheless, one observation can be drawn
from the results and that is both the routing
protocols saw an increase in the number of
packets and bytes transmitted when there
were link failures. There is also a trend that
more packets and bytes were transmitted as
more links failed. The increase is also about
28-58% lesser for DVR than for LSR. This
implies that DVR responds better than LSR
in the face of link failures by incurring fewer
overheads.
Since the results obtained so far are not
conclusive, we need to parse the trace files
again to capture other data. For better
analysis, we would need to know the
throughput and delay of only the FTP data
packets the routing protocols provide under
the different scenarios. Therefore we
modified the Java™ program to allow the
capturing of data pertaining to packet type,
throughput and delay when the trace files
are parsed in. The statistics for the FTP data
packets is given in table 2 while the results
of the throughput and delay are discussed in
the following subsections.
w/o link
failures
scenario
1
link failures
(% diff w.r.t.
w/o failures)
scenario
2
link failures
(% diff w.r.t.
w/o failures)
LSR
DVR
12852
8256
18226
(41.81%)
8735
(5.80%)
22072
(71.74%)
9366
(13.44%)
Table 1(a): Total number of packets
w/o link
failures
scenario
1
link failures
(% diff w.r.t.
w/o failures)
scenario
2
link failures
(% diff w.r.t.
w/o failures)
LSR
DVR
3729040
3347456
4052480
(8.67%)
3367574
(0.60%)
4284240
(14.89%)
3394076
(1.39%)
Table 1(b): Total number of bytes
w/o link
failures
scenario
1
link failures
(% diff w.r.t.
w/o failures)
scenario
2
link failures
(% diff w.r.t.
w/o failures)
LSR
tcp/ack
DVR
tcp/ack
3080/1624
3038/1610
3080/1624
(0%)
3038/1610
(0%)
3080/1624
(0%)
3038/1610
(0%)
more links failed.
a. Throughput
To record the throughput of the routing
protocols, we kept track of the number
of packets and bytes arriving at the
destination nodes. This is simple to
implement as we already defined the
destination nodes (38, 39, 40, and 41) in
the network setup. The results of this
round of data capture and information
extraction are given in table 3.
Table 2(a): Number of FTP packets
w/o link
failures
scenario
1
link failures
(% diff w.r.t.
w/o failures)
scenario
2
link failures
(% diff w.r.t.
w/o failures)
LSR
tcp/ack
3175200
/64960
DVR
tcp/ack
3131520
/64400
3175200
/64960
(0%)
3131520
/64400
(0%)
3175200
/64960
(0%)
3131520
/64400
(0%)
Table 2(b): Number of FTP bytes
Looking at the statistics for the FTP data
packets given, it can be seen that both
routing protocols transmitted the same
amount of packets and bytes despite the
different numbers of link failures. However,
comparing the protocols against each other,
we can see that LSR was able to transmit
more data than DVR within the simulated
time across all three scenarios.
If we compare table 1 against table 2, we
can observe that there were more additional
data packets transmitted using LSR than
DVR. This implies that LSR incurred a
higher overhead for setting up the routes as
the additional data clearly belongs to the
messages passed between the network nodes.
We can further deduce from the results that
the overhead increased more for LSR when
Destination
nodes only
w/o link
failures
scenario
1
link failures
(% diff w.r.t.
w/o failures)
scenario
2
link failures
(% diff w.r.t.
w/o failures)
LSR
DVR
440
434
440
(0%)
434
(0%)
440
(0%)
434
(0%)
Table 3(a): Number of FTP packets
Destination
nodes only
w/o link
failures
scenario
1
link failures
(% diff w.r.t.
w/o failures)
scenario
2
link failures
(% diff w.r.t.
w/o failures)
LSR
DVR
453600
447360
453600
(0%)
447360
(0%)
453600
(0%)
447360
(0%)
Table 3(b): Number of FTP bytes
From the results we can observe that the
throughput of LSR is much greater than
the throughput of DVR. Thus we may
conclude that LSP is a more efficient
routing protocol in terms of transmitting
data than DVR.
incurred by the two routing protocols.
Figure 7 shows the throughputs and average
delays of LSR and DVR.
b. Delay
To record the delay of packets under the
different routing protocols is a little
harder. We have to make a simplifying
assumption that packets received are in
the same order as they are sent. This
makes it easier for us to keep track of
the timings and perform the necessary
computations. We are safe to make this
assumption because we are looking at
the average timing of all the packets
transmitted from source to destination.
The order of the packets is thus not
important. The results obtained are
given in table 4.
w/o link
failures
scenario
1
link failures
(% diff w.r.t.
w/o failures)
scenario
2
link failures
(% diff w.r.t.
w/o failures)
LSR
DVR
0.09298909
0.09513528
Having provided our analysis, we think it is
necessary and prudent to give a brief
discussion on the limitation of our project
due to time constraint. We should have run a
couple of simulations more and run them
longer for better results and a more complete
analysis. We also cannot rule out completely
the possibility of the simulations being
flawed. There is no easy way for us to know
whether the trace files contained all the path
recoveries and that all the data transmissions
giving the throughput values are captured
correctly. A situation that may arise is that
the routing tables of all the nodes affected
by the failed links have yet to be updated
and data packets for the updates are still in
the network. In this way, the timing results
will not be accurate.
5.
0.09305478
0.09513627
0.09341024
0.09514401
Table 4: Average delay
From the results we can observe that the
average delay of packet transmission is
longer for DVR than for LSR. Therefore
we may conclude that LSR is a more
efficient routing protocol in terms of
timing. However, when the number of
failed links increased, the average delay
for LSR increased more than that of
DVR.
Our observations, discussion and analysis so
far are summarized graphically in figure 6
and 7. Figure 6 shows the overheads
Conclusion
For this project, we have designed
simulations to find out the performances of
link state routing and distance vector routing
when dealing with link failures. The
observations we have gotten are that both
the routing protocols have their own merits.
The link state routing gives a higher
throughput and average delay but the
distance vector routing gives a smaller
number of data overheads and a smaller
increase in average delay for more link
failures. Therefore we can conclude that
distance vector routing be used if the
network is highly unstable with link failures
occurring frequently and link state routing
be used if the network does not change
much at all.
Figure 6: Overheads for the routing protocols
Figure 7: Throughputs and average delays of the routing protocols
References
[1] S. Keshav, "An Engineering Approach to Computer Networking", Addison-Wesley.
[2] J. McQuillan et al., “An Overview of the New Routing Algorithm for the Arpanet”, Proc.
Sixth Data Comm. Symp., Nov. 1979.
[3] C. Metz, “At the core of IP networks: link-state routing protocols”, Internet Computing,
IEEE, Volume: 3, Issue: 5, Sept.-Oct. 1999, Pages: 72 – 77.
[4] http://www.freesoft.org/CIE/Topics/117.htm
[5] http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/routing.htm#xtocid14
[6] Bertsekas and Gallager, "Data Networks", 2nd Edition, Prentice Hall.