The Network Layer

1
Concept of Routing in Network Layer
2
Network Layer II (routing)

• Routing Styles
• Static vs. Dynamic Routing
• Routing Protocols/Algorithms
• Routing Table
• Routing Information Protocol (RIP) Distance
  Vector Routing (DVR)
• Open Shortest Path First (OSPF) Link State
  Routing (LSR)
• Dijkstras Shortest Path Algorithm
• Border Gateway Protocol (BGP) and Path Vector
  Routing (PVR)

3
Routing Protocol Routing Algorithm

• A Routing Protocol is a combination of rules and
  procedures that lets routers in an internet
  inform each other of changes. It allows routers
  to share whatever they know about the internet or
  their neighbourhood.
• A Routing Algorithm is that part of network layer
  software responsible fro deciding which output
  line and incoming packet should be transmitted on.

4
Routing

1. Routing requires a host or a router to have a
   routing table.
2. Usually when a host has a packet to send or when
   a router has received a packet to be forwarded,
   it looks at this table to find the route to the
   final destination.
3. However, this simple solution is impossible in
   todays Internet world because the number of
   entries in the routing table makes the table
   lookups inefficient.
4. Need to make the size of table manageable and
   handles issues such security at the same time.
   The key question is how to design the routing
   table.
5. Next-hop routing, Network-specific routing, host
   specific routing
6. Static versus Dynamic Routing
7. Routing Protocols RIP, OSPF, BGP
8. Routing Algorithms DVR, LSR, PVR

5
Next-hop routing
Next-hop routing holds only the information that
leads to the next hop instead of complete route.
6
Network-specific host-specific routing
Instead of having an entry for every host
connected to the same network, only one entry is
needed to defined the address of the network
itself. All host connected to the same network as
one single entity.
The destination host address is given in the
routing table to have greater control over
routing.
7
Default routing
R1 is used to route packets to hosts connected to
N2.
However, R2 is used to as default to route other
packets to the rest of Internet without listing
all the networks involved Only one default
routing is allowed with network address 0.0.0.0
8
General Routing Table
Flags U The router is up and running. G
The destination is in another network. H
Host-specific address. D Added by
redirection. M Modified by redirection.
9
Routing table

1. Generally, a routing table needs a minimum of 4
   columns mask, destination network address, next
   hop address and interface.
2. When a packet arrives, the router applies the
   mask to the destination address it receives
   (one-by-one until a match is found) in order to
   find the corresponding destination network
   address.
3. So, the mask serves as essential tool to match
   destination address in routing table and the
   address it receives.
4. If found, the packet is sent out from the
   corresponding interface in the table. If not
   found, the packet is delivered to the default
   interface which carries the packet to default
   router.

10
Configuration for routing example
Standard delivery
Host-specific
Network-specific
Default
11
Example 1
Router R1 receives 500 packets for destination
192.16.7.14 the algorithm applies the masks row
by row to the destination address until a match
(with the value in the second column of Dest. in
table) is found
Rule of thumb Apply the individual mask (from
Routing table) to the received destination
address (row-by-row) and see if its matches any
of the DEST address stated in its routing table.
If match is found, then stop
Solution
Direct delivery 192.16.7.14 255.0.0.0
? 192.0.0.0 no match to
111.0.0.0 192.16.7.14 255.255.255.224 ?
192.16.7.0 no match to 193.14.5.160 192.16.7.
14 255.255.255.224 ? 192.16.7.0 no match
to 193.14.5.192
Host-specific 192.16.7.14 255.255.255.255
?192.16.7.14 no match to 194.17.21.16 Network-
specific 192.16.7.14 255.255.255.0
?192.16.7.0 match to 192.16.7.0
12
Example 2
Router R1 receives 100 packets for destination
193.14.5.176 the algorithm applies the masks row
by row to the destination address until a match
is found
Solution
Direct delivery 193.14.5.176 255.0.0.0
? 193.0.0.0 no match 193.14.5.176
255.255.255.224 ?193.14.5.160 match
13
Example 3
Router R1 receives 20 packets for destination
200.34.12.34 the algorithm applies the masks row
by row to the destination address until a match
is found
Solution
200.34.12.34 255.0.0.0
?200.0.0.0 no match 200.34.12.34
255.255.255.224 ?200.34.12.32 no
match 200.34.12.34 255.255.255.224
?200.34.12.32 no match 200.34.12.34
255.255.255.255 ?200.34.12.34 no
match
200.34.12.34 255.255.255.0 ? 200.34.12.0
no match 200.34.12.34 255.255.255.0 ?
200.34.12.0 no match Default 200.34.1
2.34 0.0.0.0 ? 0.0.0.0.
match
14
Example 4
Make the routing table for router R1 in figure
below
15
Example 5
Make the routing table for router R1 in figure
below
Solution
16
In classless addressing, we need at least four
columns in a routing table.
17
Routing Tables in IP with CIDR(Classless
InterDomain Routing)
Mask Destination Next Hop
/12 128.96.0.0 145.12.56.29
/17 128.125.0.0 153.202.12.128
/12 128.112.0.0 153.202.14.1
/26 128.105.14.64 153.2.45.101
/32 128.105.14.66 153.2.45.101
For each entry in the routing table MaskedAddres
s EntryMask (bitAND) IPDatagramDestinationAddre
ss if (MaskedAddress EntryDestination) Mark
the entry Choose the marked entry with the
longest Mask prefix.
18
Make a routing table for router R1, using the
configuration in Figure below
Example 7a
Solution
Routing table for router R1 in Figure above
The table is sorted from the longest mask to the
shortest mask.
19
Show the forwarding process if a packet arrives
at R1 with the destination address 180.70.65.140.
Example 7b
Solution
The router performs the following steps 1. The
first mask (/26) is applied to the destination
address. The result is 180.70.65.128, which
does not match the corresponding network
address. 2. The second mask (/25) is applied to
the destination address. The result is
180.70.65.128, which matches the
corresponding network address. The next-hop
address and the interface number m0 are
passed on for further processing.
20
Show the forwarding process if a packet arrives
at R1 with the destination address 201.4.22.35.
Example 7c
Solution
The router performs the following steps 1. The
first mask (/26) is applied to the destination
address. The result is 201.4.22.0, which does not
match the corresponding network address. 2. The
second mask (/25) is applied to the destination
address. The result is 201.4.22.0, which does not
match the corresponding network address (row 2).
3. The third mask (/24) is applied to the
destination address. The result is 201.4.22.0,
which matches the corresponding network address..
21
Show the forwarding process if a packet arrives
at R1 with the destination address 18.24.32.78.
Example 7d
Solution
This time all masks are applied, one by one, to
the destination address, but no matching network
address is found. When it reaches the end of the
table, the module gives the default next-hop
address 180.70.65.200 (because it could not find
the match) . This is probably an outgoing package
that needs to be sent, via the default router, to
someplace else in the Internet.
22
Routing/routers

1. An internet is a combination of networks
   connected by routers.
2. When a packet goes from a source to a
   destination, it will pass through many routers
   until it reaches the router attached to
   destination network.
3. A router consults a routing table when a packet
   is ready to be forwarded. The routing table
   specifies the optimum path for the packet and can
   be either static of dynamic. Dynamic routing is
   more popular.
4. Static table does not change frequently. Dynamic
   table is updated automatically when there is a
   change somewhere in the network i.e when a route
   is down or a better route has been created.
5. Routing protocols is a combination of
   rules/procedures that lets routers in the
   internet inform one another when changes occur
   mostly based on sharing/combining information
   between routers at different networks.

23
Unicast Routing

1. Unicast one source and one destination. (1-to-1
   relationship).
2. In Unicast routing, when a router receives a
   packet, it forwards the packet thru only one of
   its ports as defined in the routing table. The
   router may discard the packet if it cannot find
   the destination address
3. Questions In dynamic routing, how does the
   router decides to which network should it pass
   the packet next? What routing algorithm is the
   routing based on? The decision is based on
   optimisation which of the available pathways is
   the best/optimum path?
4. But how to measure? A metric is a cost assigned
   for passing thru a network and the total metric
   of a particular route is equal to the sum of the
   metrics of networks that comprise the route.
5. Simple protocols such as Routing Information
   Protocol (RIP), treat all network equally cost
   of passing each network is the same as one hop
   count per network.
6. Other sophisticated protocols e.g. OSPF, based on
   services required and using different metrics
   max throughput, minimum delay.

24
Routing Protocol Interior Vs Exterior
25
Routing Architecture in the Internet
Fact Nobody owns the whole Internet. However,
parts of the Internet are owned and administered
by commercial and public organisations (such as
ISPs, universities, governmental offices,
research institutes, companies etc.).

• Idea
• Divide the Internet in Autonomous Systems (AS)
  that are independently administered by individual
  organisations.
• Let each administrative authority use its own
  routing protocol within the AS.
• Lets use one routing protocol to exchange
  routing information among AS.

26
Routing Architecture in the Internet
An AS is a group of networks and routers under
the authority of a single administrator.
27
Static versus Dynamic Routing
A static routing table contains information
entered manually Usually remained unchanged.
A dynamic routing table is updated periodically
or whenever necessarily using one of the dynamic
routing protocols such as RIP, OSPF, or BGP.
28
Routing Protocols Interior vs Exterior

• Routing inside an AS is referred to as interior
  routing whereas routing between ASs is referred
  to as exterior routing.
• Each AS can choose one or more interior routing
  protocols inside an AS.
• Only one exterior routing protocol is usually
  chosen to handle routing between ASs.
• To know the next path (or router) a packet
  should be pass-on, the decision is based on some
  optimisation rule/protocol, e.g. using different
  assignment of the cost (metric) for each passing
  through a network for different routing Protocol
  above.

29
Interior Routing Protocol 1 Routing Information
Protocol (RIP)
30
Distance Vector Routing (DVR)

• 3 keys to understand how this algorithm works
• Sharing knowledge about the entire AS. Each
  router shares its knowledge about the entire AS
  with neighbours. It sends whatever it has.
• Sharing only with immediate neighbours. Each
  router sends whatever knowledge it has thru all
  its interface.
• Sharing at regular intervals. sends at fixed
  intervals, e.g. every 30 sec.
• Problems Tedious comparing/updating process,
  slow response to infinite loop problem, huge list
  to be maintained!!

31
Initialization of tables in distance
vector routing (DVR)
32
Updating in distance vector routing example C to
A
From A
From C
A to A via C ACA AC CA 22
A to B via C ACB AC CB 24
A to D via C ACD AC CD 2 inf.
A to E via C ACD AC CE 24
A to C via C ACC AC CC 20
33
Final Distance vector routing tables
34
Example-1
Distance Vectors below that are received at
node-B in a network. Given the estimated distance
to its neighbours node-A, node-D and node-F are
6, 9, and 11 hops, respectively. Find the new
distance vector at B. (Note The new vector must
include the next hop and the estimated cost).
34
35
Solution
35
36
Example-2
Distance Vectors below that are received at
node-A in a network. Given that the estimated
delay to its neighbours node-B, node-F and node-H
are 6, 10, and 8 units, respectively. Find the
new distance vector at A. (The new distance
vector must indicate the next hop and the
estimated delay)
36
37
Solution
37
38
DVR extra example from Tenenbaum (with estimated
delay)
Neighbour routers
JA,8
JB, JAB, 812
JC, JIC, 1018
JD, JHD, 128
JE, JIE. 107
JF, JIF, 1020
JG, JHG, 126
JH, 12
JI. 10
JJ, 0
JK, 6
JL, JKL, 69
Each router maintain a table (a vector) giving
the best known metric (or delay) to each
destination and which line to use. These tables
are then updated by exchanging information with
the neighbours (direct link, 1 hop)

• A subnet. (b) Input from A, I, H, K, and the new
  routing table for J.
• 1st DRAWBACK VERY SLOW!!!

39
Routing Information Protocol (RIP)

1. RIP is based on distance vector routing, which
   uses the Bellman-Ford algorithm for calculating
   the routing table.
2. RIP treats all network equals the cost of
   passing thru a network is the same one hop count
   per network.
3. Each router/node maintains a vector (table) of
   minimum distances to every node. (the least-cost
   route btw any nodes is the route with the minimum
   number of hop-count).
4. The hop-count is the number of networks that a
   packet encounters to reach its destination. Path
   costs are based on number of hops.
5. In distance vector routing, each router
   periodically shares its knowledge about the
   entire internet with its neighbour.
6. Each router keeps a routing table that has one
   entry for each destination network of which the
   router is aware.
7. The entry consists of Destination Network
   Address/id, Hop-Count and Next-Router.

40
Example of Initial routing tables (RIP) in a
small autonomous system
41
Example of Final routing tables
42
Example of a domain using RIP
43
Infinite loop problem
Initially, X was running before the failure and
the number of hop count from X is available in
each node A and B. After the failure of X, the
connection is broken and A changes its table to
infinity hop count about X, while B is still
preserving the same count. In the subsequent
update, if B sends its table before A, then A
assumes B has found a way to reach X, while B in
turn assumes that A has changed it table and
update accordingly. The hop count continues to
increase gradually until infinity.
44
Infinite loop problem in DVR
A initially up then down
A initially down hence
The count-to-infinity problem!
Good news (a) travels faster than bad news
(b) React rapidly to good news but slowly to bad
news Although it will eventual converge to
correct answer, they adapt slowly, they must be
told to change. Convergence to the correct answer
is slow.
45
Interior Routing Protocol 2 Open Shortest Path
First Protocol (OSPF)
46
Open Shortest Path First (OSPF)

1. OSPF uses link state routing to update the
   routing table in an area (OSPF divides an AS
   into different areas).
2. Unlike RIP, OSPF treats the entire network within
   differently with different philosophy depending
   on the types, cost (metric) and condition of each
   link to define the state of a link.
3. OSPF allows the administrator to (only) assign a
   cost for passing through a network based on the
   type of service required. e.g. minimum delay,
   maximum throughput. (but not stating exact path)
4. Each router should have the exact topology of the
   AS network (a picture of entire AS network) at
   every moment. The topology is a graph consisting
   of nodes and edges.
5. Each router needs to advertise to the
   neighbourhood of every other routers involved in
   an Area. (flood)

47
Open Shortest Path First (OSPF)
Areas in an Autonomous System
(ASgtAreas)
OSPF divides an AS into areas. An area is a
collection of network, hosts and routers all
contained within an AS. Routers inside an area
flood the area with routing info. At the border
of an Area, special routers called Area Border
routers summarize the info. about the area and
send it to other area. Among the areas inside an
AS is a special area called the Backbone
connecting all areas through Backbone routers and
serves as a primary area to the outside (other
ASs) via the AS Boundary router.
48
Link State Routing (LSR)

• Like RIP, in link state routing, each router also
  shares its knowledge about its neighbourhood with
  every routers in the area.
• However, in LSR, the link-state packet (LSP)
  defines the best known network topology (of an
  area) is sent to every routers (of other area)
  after it is constructed locally. Whereas RIP
  slowly converge to final routing list based
  information received from immediate neighbours.
• 3 keys to understand how this algorithm works
• Sharing knowledge about the neighbourhood. Each
  router sends the state of its neighbourhood to
  every other router in the area.
• Sharing with every other routers. Thru process of
  flooding. each router sends the state of its
  neighbourhood thru all its output ports and each
  neighbour sends to every other neighbours and so
  on until all routers received same full
  information eventually. (DO NOT SEND UPDATE
  FREQUENTLY)
• Sharing when there is a change. Each router share
  its state of its neighbour only when there is a
  change contrasting DVR results in lower traffic.

49
Link State Routing (LSR)

1. LSR differs from DVR in the following
2. Can use different cost/metric instead of just
   hop-counts
3. Routing update is only performed when there is a
   change in topology or after a long period (every
   30 minutes)
4. Each router has an overall map or knowledge of
   the entire network topology within the AS or an
   area of the AS
5. Because the network-topology is known in
   advanced, routers can work out which is the best
   route to choose between two nodes if there is
   more than two alternative routes/paths by
   shortest path algorithm.
6. This solve the problem of infinity-loop as all
   routers will be informed instantly by LSA and
   paths are recalculated immediately.
7. From the received LSPs and knowledge of entire
   topology, a router can then calculate the
   shortest path between itself and each network.
8. Usually works better for large networks.

49
50
When the link between two routers is broken, the
administrator may create a virtual link between
them using longer path that probably goes through
several routers
51
Link State Advertisement (LSA)
To share information about the neighbourhood,
each entity distribute link state advertisements
(LSAs).
5 Types of LSAs
Info exchange between different Areas inside an AS
Info exchange outside across different AS
Info exchange to external internet
Info. exchange within inside an Area
52
Router link
A router link advertisement defines the links of
a true router. A true router uses this
advertisement to announce information about all
its links and what is at the other side of the
link (neighbour).
53
Network link
A network link advertisement defines the links of
a network. A designated router on behalf of the
transient network distributes this types of LSA
packet. The packet announces the existence of all
the routers connected to the network.
54
Summary link to network
area border router R2
area border router R1
Backbone network
Router and network link advertisements flood each
area with info about the router links and network
links within/inside an area. But a router must
also know about the networks outside its area,
and the area border routers can provide this
information. An area border router is active in
more than one area. It receives router link and
network link advertisements and creates a routing
table for each area.
55
Summary link to AS boundary router
The previous advertisement lets every router know
the cost to reach all networks within/inside an
AS. But what about the network outside the AS? If
a router inside an area wants to send a packet
outside the autonomous system, it should first
know the route to an AS boundary router the
summary link to AS boundary router provides this
information. The border routers can then flood
their areas with this information.
56
External link
Although the previous advertisement lets each
router know the route to different AS boundary
router, this information is not enough. A router
inside an AS also wants to know which networks
are available outside the AS i.e. the external
internet. The external link advertisement
provide this information. The AS boundary router
floods the AS with cost of each network outside
the AS, using a routing table created by an
exterior routing table protocol. Each
advertisement announces one single network. If
there is more than one network. Separate
announcements are made.
57
Example
In the figure below, which router(s) sends out
router link LSAs? and which router(s) sends out
network link LSAs?
Solution
All routers advertise router link LSAs.
R1 has two links, Net1 and Net2. R2
has one link, Net2 in this AS. R3 has
two links, Net2 and Net3.
58
Solution Continue
All three network must advertise network link
LSAs Advertisement for Net1 is done by R1
because it is the only router and therefore the
designated router. Advertisement for Net2 can be
done by either R1, R2, or R3, depending on which
one is chosen as the designated router.
Advertisement for Net3 is done by R3 because it
is the only router and therefore the designated
router.
59
In OSPF, all routers have the same Link State
database.

• Every router in an area receives the router link
  and network link LSAs and form a link state
  database.
• Every router in the same area has the same link
  state database.
• A link state database is a tabular
  representation of the topology of the internet
  inside an area. It shows the relationship between
  each router and its neighbors including the
  metrics used.
• To calculate its next-route in the routing
  table, each router applies the Dijkstra algorithm
  to its state database, to find the shortest path
  between 2 points on a network, using a graph
  (nodes and edges).
• The algorithm divides the nodes into two sets
  tentative and permanent. It chooses nodes, makes
  them tentative, examines them, and if they pass
  the criteria, makes permanent.

60
Graph representation of AS nodes and edges

• (a) An autonomous system. (b) A graph
  representation of (a).

61
Shortest Path Search
Dijkstras Algorithm

• Start with the local node (router) the root of
  the tree.
• 2. Assign a cost of 0 to this node and make it
  the first permanent node.
• 3. Examine each neighbour node of the node that
  was the last permanent node.
• 4. Assign a cumulative cost to each node and
  make it tentative.
• 5. Among the list of tentative nodes a. Find
  the node with the smallest cumulative cost and
  make it permanent.
• b. If a node can be reached from more
  than one direction
• i. Select the direction with the
  shortest cumulative cost.
• 6. Repeat steps 3 to 5 until every node becomes
  permanent.

62
Dijkstra algorithm
63
Shortest Path Search

• The steps used in computing the shortest path
  from A to D. The arrows indicate the working
  node permanent label.

The cost can relates to delay
Start search and compare with tentative label
Mark permanent when shortest node found Once
permanent never changed Tentative node can
always be search and relabelled
Tentative label change
The label on each node can be TENTATIVE or
PERMANENT
64
Example of formation of shortest path tree
65
Example of an internet
Graphical representation of an internet
66
Shortest path calculation
67
Shortest path calculation
14
68
Shortest path calculation
69
Exterior Routing Protocol Border Gateway
Protocol (BGP)
70
BGP Path Vector Routing (PVR)

• Border Gateway Protocol (BGP) is an inter-domain
  or inter-autonomous system routing protocol
  routing between different ASs.
• BGP uses path vector routing to update the
  routing table in an area.
• DVR and LSR are not suitable candidates for
  inter-AS routing
• DVR there are occasions in which the route with
  the smallest hop count is not the preferred
  route non-secure path although the shortest
  route taken.
• LSR internet is too big for this routing method
  to require each router to have a huge link state
  database. Taking very long time to calculate the
  routing table.
• PVR defines the exact paths as an ordered list of
  ASs that a packet should travel thru to reach the
  destination (besides having the destination
  network and next router info.) in its routing
  table.
• Security and Political issues involved more
  desired to avoid unsaved paths/routes/ASs than
  to take a shorter route.
• The AS boundary router that participate in PVR
  advertise the routes of the networks in their own
  AS to neighbour AS boundary routers.
• Solve the count-to-infinity problem

71
Path vector packets

• Each AS has its speaker router/node that acts
  on behalves of the AS. Only speaker router can
  communicate with other speaker routers.
• R1 send a path vector message advertising its
  reachability of N1. R2 receives the message,
  updates its routing table and after adding its
  AS to the path and inserting itself as next
  router, send message to R3. R3 receives the
  message, updates its routing table, make changes
  and sends the message to R4.

72
BGP the Exterior Gateway Routing Protocol
PVR

• Instead of periodically advertise to its
  neighbours the cost to each destination, each BGP
  router tells its neighbour the exact path it is
  using. e.g. F receives information from its
  neighbour routers to reach D.
• Can solve count-infinity problem suppose G is
  down then IFGCD and EFGCD routes are discarded
  since Gs state will be know immediately render
  BCD as only choice.

73
Path Vector Routing Policy

1. Policy routing can be easily implemented through
   path vector routing.
2. When a router receives a message from its
   neighbour, the speaker node or AS boundary router
   can check the path with its approved list of ASs.
   
3. If one of the ASs listed in the path is against
   its policy, the router can ignore that path
   entirely and that destination.
4. For any unapproved paths, the router does not
   update its routing table with this path, and it
   does not send the PV message to its neighbours.
5. This means that the routing table in path vector
   routing are not based on the smallest hop count
   (as in distance vector routing) or the minimum
   delay metric (as in open shortest path first
   routing) they are based on the policy imposed on
   the router by the administrator.
6. The path was presented as a list of ASs, but is
   in fact, a list of attributes. Each attributes
   gives some information about the path. The list
   of attributes helps the receiving router make a
   better decision when applying its policy.
   (Well-known Optional)

74
Types of BGP messages

1. Open To create a relationship, a router running
   BGP opens a connection with a neighbouring AS and
   sends an open message. If the neighbour accepted,
   it responds with a Keep-alive message to
   establish relationship between the two routers.
2. Update The heart of BGP protocol used by router
   to withdraw destination that have been advertised
   previously, announce a route to a new destination
   or do both. (Withdraw several but advertise only
   one).
3. Notification sent by a router whenever an error
   condition is detected or router wants to close
   the connection (down).

75
Initial routing tables in path vector routing
Stabilized tables for three autonomous systems
76
Big picture RIP/OSPF/BGP

• The relation between ASs, backbones, and areas.

77
Further Reading
1- Computer Networks, Andrew Tanenbaum, 4th Ed.
to learn more about the generic network
layer. 2- Internetworking with TCP/IP vol.1,
Douglas Comer, 4th Ed., provides a detailed and
comprehensive presentation of TCP/IP. 3- Data
Communications and Networking, Behrouz Forouzan,
4th Ed., when you get confused and wonder if
theres a simpler explanation of all these
issues.