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Network Layer II (routing)Routing Styles:
Static vs. Dynamic RoutingRouting Protocols/Algorithms
Routing Table Routing Information Protocol (RIP) & Distance Vector Routing
(DVR) Open Shortest Path First (OSPF) & Link State Routing (LSR) Dijkstra’s “Shortest Path” Algorithm Border Gateway Protocol (BGP) and Path Vector Routing (PVR)
Routing Protocol & Routing Algorithm
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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 for deciding which output line and incoming packet should be transmitted on.
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Routinga) Routing requires a host or a router to have a routing table.b) 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.
c) However, this simple solution is impossible in today’s Internet world because the number of entries in the routing table makes the table lookups inefficient.
d) 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.
e) Next-hop routing, Network-specific routing, host specific routingf) Static versus Dynamic Routingg) Routing Protocols: RIP, OSPF, BGPh) Routing Algorithms: DVR, LSR, PVR
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Next-hop routing
Next-hop routing holds only the information that leads to the next hop instead of complete route.
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Host-specific & Network-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.
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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
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General Routing Table
FlagsU 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.
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Routing tablea) Generally, a routing table needs a minimum of 4
columns: mask, destination network address, next hop address and interface.
b) 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.
c) So, the mask serves as essential tool to match destination address in routing table and the address it receives.
d) 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.
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Configuration for routing example
Mask Dest. Next Hop I.
255.0.0.0 111.0.0.0 -- m0
255.255.255.224 193.14.5.160 - m2
255.255.255.224 193.14.5.192 - m1
255.255.255.255 194.17.21.16 111.20.18.14 m0
255.255.255.0 192.16.7.0 111.15.17.32 m0
255.255.255.0 194.17.21.0 111.20.18.14 m0
0.0.0.0 0.0.0.0 111.30.31.18 m0
Standard delivery
Host-specific
Network-specific
Default
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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:
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
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
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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
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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
200.34.12.34 & 0.0.0.0 0.0.0.0. match
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Example 4 Make the routing table for router R1 in figure below
Solution
Mask Destination Next Hop I.
255.255.0.0 134.18.0.0 -- m0
255.255.0.0 129.8.0.0 222.13.16.40 m1
255.255.255.0 220.3.6.0 222.13.16.40 m1
0.0.0.0 0.0.0.0 134.18.5.2 m0
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Example 5 Make the routing table for router R1 in figure below
Subnet mask Destination Next HopI.
255.255.255.0 200.8.4.0 ---- m2
255.255.255.0 80.4.5.0 201.4.10.3 m1
or 200.8.4.12 or m2
255.255.255.0 80.4.6.0 201.4.10.3 m1
or 200.4.8.12 or m2
0.0.0.0 0.0.0.0 m0
Solution
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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:MaskedAddress := EntryMask (bitAND) IPDatagramDestinationAddress;if (MaskedAddress == EntryDestination)
Mark the entry;
Choose the marked entry with the longest Mask prefix.
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Make a routing table for router R1, using the configuration in Figure belowExample 7a
Routing table for router R1 in Figure aboveSolution
m3
The table is sorted from the longest mask to the shortest mask.
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Show the forwarding process if a packet arrives at R1 with the destination address 180.70.65.140.
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.
Example 7b
Solution
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Show the forwarding process if a packet arrives at R1 with the destination address 201.4.22.35.
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).
Example 7c
Solution
3. The third mask (/24) is applied to the destination address. The result is 201.4.22.0, which matches the corresponding network address..
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Show the forwarding process if a packet arrives at R1 with the destination address 18.24.32.78.
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.
Example 7d
Solution
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Routing/routersa) An internet is a combination of networks connected by routers.b) 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.
c) 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.
d) 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.
e) 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.
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Routing Protocol:Interior Vs Exterior
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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. • Let’s use one routing protocol to exchange routing information
among AS.
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Routing Architecture in the Internet
An AS is a group of networks and routers under the authority of a single administrator.
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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.
Static versus Dynamic Routing
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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.
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Interior Routing Protocol 1:Routing Information Protocol
(RIP)
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Distance Vector Routing (DVR)
a) 3 keys to understand how this algorithm works:• Sharing knowledge about the entire AS. Each
router shares whatever it has.• Sharing only with immediate neighbours. • Sharing at regular intervals. e.g. every 30 sec.
b)Problems: Tedious comparing/updating process, slow response to infinite loop problem, huge list to be maintained!!
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Initialization of tables in distance vector routing (DVR)
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Updating in distance vector routing example: C to A
A to A via C: ACA = AC+ CA = 2+2
A to B via C: ACB = AC + CB = 2+4
From C From A
A to D via C: ACD = AC + CD = 2+ inf.
A to C via C: ACC = AC + CC = 2+0
A to E via C: ACD = AC + CE = 2+4
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Final Distance vector routing tables
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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).
Choose the minimum value
B A D F
A 6 A B(A)A = 6+0 B(D)A = 9+6 B(F)A = 11+21
B 16 D B(A)B = 6+13 B(D)B = 9+7 B(F)B = 11+10
C 16 F B(A)C = 6+11 B(D)C= 9+9 B(F)C = 11+5
D 9 D B(A)D = 6+5 B(D)D = 9+0 B(F)D = 11+9
E 16 D B(A)E = 6+15 B(D)E = 9+7 B(F)E = 11+7
F 11 F B(A)F = 6+22 B(D)F= 9+9 B(F)F = 11+0
G 10 A B(A)G = 6+4 B(D)G = 9+21 B(F)G = 11+12
H 12 A B(A)H = 6+6 B(D)H = 9+18 B(F)H = 11+16
I 15 F B(A) I = 6+21 B(D) I = 9+11 B(F) I = 11+4
J 13 A B(A) J = 6+7 B(D) J = 9+17 B(F) J = 11+9
K 7 A B(A)K = 6+1 B(D)K = 9+11 B(F)K = 11+2
B
A
D
F
6
9
11
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Solution
B
A
D
F
6
9
11
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Routing Information Protocol (RIP)a) RIP is based on distance vector routingb) RIP treats all network equals; the cost of passing thru a network
is the same: one hop count per network.c) Each router/node maintains a table of minimum number of hop-
count.d) Path costs are based on number of hops.e) In distance vector routing, each router periodically shares its
knowledge about the entire internet with its neighbour. f) Each router keeps a routing table that has one entry for each
destination network of which the router is aware. g) The entry consists of Destination Network Address/id, Hop-
Count and Next-Router.
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Example of Initial routing tables (RIP) in a small autonomous system
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Example of Final routing tables
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Example of a domain using RIP
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Interior Routing Protocol 2:Open Shortest Path First
Protocol (OSPF)
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Open Shortest Path First (OSPF)
a) OSPF uses link state routing to update the routing table in an area; (OSPF divides an AS into different areas).
b) Unlike RIP, OSPF treats the entire network with different philosophy; depending on the types, cost (metric) and condition of each link: to define the ‘state’ of a link.
c) 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)
d) 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.
e) Each router needs to advertise to the neighbourhood of every other routers involved in an Area. (flood)
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Areas in an Autonomous System
Open Shortest Path First (OSPF)
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.
(AS>Areas)
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Link State Routing (LSR)a) Like RIP, in link state routing, each router also shares its knowledge
about its neighbourhood with every routers in the area. b) 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.
c) 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.
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Link State Routing (LSR)a) LSR differs from DVR in the following:
a) Can use different cost/metric instead of just hop-countsb) Routing update is only performed when there is a change in topology or after a
long period (every 30 minutes) c) Each router has an ‘overall map’ or knowledge of the entire network topology
within the AS or an area of the ASd) 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.
e) This solve the problem of infinity-loop as all routers will be informed instantly by LSA and paths are recalculated immediately.
b) From the received LSPs and knowledge of entire topology, a router can then calculate the shortest path between itself and each network.
c) Usually works better for large networks.
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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.
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1. 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.
Dijkstra’s Algorithm
Shortest Path Search
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Dijkstra algorithm
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Shortest Path SearchThe 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
The label on each node can be TENTATIVE or PERMANENT
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
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Example of formation of shortest path tree
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Example of an internet
Graphical representation of an internet8
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Shortest path calculation
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Shortest path calculation
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Exterior Routing Protocol:Border Gateway Protocol
(BGP)
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BGP & Path Vector Routing (PVR)a) Border Gateway Protocol (BGP) is an inter-domain or inter-
autonomous system routing protocol: routing between different ASs.b) BGP uses path vector routing to update the routing table in an area.c) 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.
d) 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.
e) Security and Political issues involved: more desired to avoid ‘unsaved’ paths/routes/ASs than to take a shorter route.
f) The AS boundary router that participate in PVR advertise the routes of the networks in their own AS to neighbour AS boundary routers.
g) Solve the count-to-infinity problem
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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.
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Path Vector Routing Policya) Policy routing can be easily implemented through path vector routing. b) 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.
c) If one of the ASs listed in the path is against its policy, the router can ignore that path entirely and that destination.
d) 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.
e) 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.
f) 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)
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Big picture: RIP/OSPF/BGP
The relation between ASs, backbones, and areas.
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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 there’s a simpler explanation of all these issues.
