Chapter 6 Delivery Forwarding, and Routing of IP Packets

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Chapter 6Delivery Forwarding, and Routing of IP
Packets
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Introduction

• Delivery
• Meaning the physical forwarding of the packets
• Connectionless and connection-oriented services
• Direct and indirect delivery
• Routing
• Related to finding the route (next hop) for a
  datagram

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6.1 Delivery

• Connection Types
• Connection-oriented service
• Using same path
• The decision about the route of a sequence of
  packets with the same source and destination
  addresses can be made only once, when the
  connection is established
• Connectionless service
• Dealing with each packet independently
• Packets may not travel the same path to their
  destination
• IP is
• Connectionless protocol

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Direct versus Indirect Delivery

• Two methods delivering a packet to its final
  destination
• Direct
• Indirect
• Direct delivery
• The final destination of the packet is a host to
  the same physical network as the deliverer or the
  delivery is between the last router and the
  destination host
• Decision making whether delivery is direct or not
• Extracting the network address of the destination
  packet (setting the hostid part to all 0s)
• Then, comparing the addresses of the network to
  which it is connected

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Direct versus Indirect Delivery (contd)

• Direct delivery

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Direct versus Indirect Delivery (contd)

• Indirect delivery
• The destination host is not on the same network
  as the deliverer
• The packet goes from router to router until
  finding the final destination
• Using ARP to find the next physical address
• Mapping between the IP address of next router and
  the physical address of the next router

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Direct versus Indirect Delivery (contd)

• Indirect delivery

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6.3 Forwarding

• Forwarding means to place the packet in its
  route to its destination. So, it requires a host
  or a router a routing table.
• Routing table
• Used to find the route to the final destination

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Forwarding Techniques

• Next-hop Method
• A technique to reduce the contents of a routing
  table
• The routing table holds only the address of the
  next hop instead of holding information about the
  complete route
• The entries of a routing table must be
  consistent with each other

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Forwarding Techniques (contd)
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Forwarding Techniques (contd)

• Network-Specific Method
• Having only one entry to define the address of
  network itself

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Forwarding Techniques (contd)

• Host-Specific Method
• Destination host addresses is given in the
  routing table
• The efficiency is sacrificed for the advantages
  
• Giving to administrator more control over
  routing
• Ex) if the administrator wants all packets
  arriving for host B delivered to router R3
  instead of R1, one single entry in the routing
  table of host A can explicitly define the route

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Routing methods (contd)
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Routing methods (contd)

• Default Method
• Instead of listing all networks in the entire
  Internet, host A can just have one entry called
  the default (network address 0.0.0.0)

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Forwarding with Classful Addressing

• Forwarding without Subnetting

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Example 1

• Figure 6.8 shows an imaginary part of the
  Internet. Show the routing tables for router R1.

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Example 1 - Solution
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Example 2

• Router R1 in Figure 6.8 receives a packet with
  destination address 192.16.7.14. Show how the
  packet is forwarded.
• Solution

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Example 2 - Solution

• The destination address in binary is 11000000
  00010000 00000111 00001110. A copy of the address
  is shifted 28 bits to the right. The result is
  00000000 00000000 00000000 00001100 or 12. The
  destination network is class C. The network
  address is extracted by masking off the leftmost
  24 bits of the destination address the result is
  192.16.7.0. The table for Class C is searched.
  The network address is found in the first row.
  The next-hop address 111.15.17.32. and the
  interface m0 are passed to ARP.

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Example 3

• Router R1 in Figure 6.8 receives a packet with
  destination address 167.24.160.5. Show how the
  packet is forwarded
• Solution
• The destination address in binary is 10100111
  00011000 10100000 00000101. A copy of the address
  is shifted 28 bits to the right. The result is
  00000000 00000000 00000000 00001010 or 10. The
  class is B. The network address can be found by
  masking off 16 bits of the destination address,
  the result is 167.24.0.0. The table for Class B
  is searched. No matching network address is
  found. The packet needs to be forwarded to the
  default router (the network is somewhere else in
  the Internet). The next-hop address 111.30.31.18
  and the interface number m0 are passed to ARP.

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Forwarding with Subnetting
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Example 4

• Figure 6.11 shows a router connected to four
  subnets.

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Example 5

• The router in Figure 6.11 receives a packet with
  destination address 145.14.32.78. Show how the
  packet is forwarded.
• Solution
• The mask is /18. After applying the mask, the
  subnet address is 145.14.0.0. The packet is
  delivered to ARP with the next-hop address
  145.14.32.78 and the outgoing interface m0.

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Example 6

• A host in network 145.14.0.0 in Figure 6.11 has
  a packet to send to the host with address
  7.22.67.91. Show how the packet is routed.
• Solution
• The router receives the packet and applies the
  mask (/18). The network address is 7.22.64.0. The
  table is searched and the address is not found.
  The router uses the address of the default router
  (not shown in figure) and sends the packet to
  that router.

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Forwarding with Classless Addressing

• In classful addressing we can have a routing
  table with three columns in classless
  addressing, we need at least four columns.

Figure 6.12 Simplified forwarding module in
classless address
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Example 7

• Make a routing table for router R1 using the
  configuration in Figure 6.13.

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Example 7 - Solution

• SolutionTable 6.1 shows the corresponding table

Table 6.1 Routing table for router R1 in Figure
6.13
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Example 8

• Show the forwarding process if a packet arrives
  at R1 in Figure 6.13 with the destination address
  180.70.65.140.

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Example 8 - Solution

• SolutionThe 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 (the destination address of
  the packet in this case) and the interface number
  m0 are passed to ARP for further processing.

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Example 9

• Show the forwarding process if a packet arrives
  at R1 in Figure 6.13 with the destination address
  201.4.22.35.

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Example 9 - Solution

• SolutionThe 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 (row 1).
• 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.
  The destination address of the package and the
  interface number m3 are passed to ARP.

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Example 10

• Show the forwarding process if a packet arrives
  at R1 in Figure 6.13 with the destination address
  18.24.32.78.
• SolutionThis time all masks are applied to the
  destination address, but no matching network
  address is found. When it reaches the end of the
  table, the module gives the next-hop address
  180.70.65.200 and interface number m2 to ARP.
  This is probably an outgoing package that needs
  to be sent, via the default router, to some place
  else in the Internet.

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Example 11

• Now let us give a different type of example. Can
  we find the configuration of a router, if we know
  only its routing table? The routing table for
  router R1 is given in Table 6.2. Can we draw its
  topology?
• Table 6.2 Routing table for Example 11

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Example 11 - Solution
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Address Aggregation

• Figure 6.15 Address aggregation

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Longest Mask Matching

• The routing table is sorted from the longest
  mask to the shortest mask.

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Hierarchical Routing

• To solve the problem of gigantic routing tables,
  creating a sense of the routing tables
• Routing table can decrease in size

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Example 12

• As an example of hierarchical routing, let us
  consider Figure 6.17. A regional ISP is granted
  16,384 addresses starting from 120.14.64.0. The
  regional ISP has decided to divide this block
  into four subblocks, each with 4096 addresses.
  Three of these subblocks are assigned to three
  local ISPs, the second subblock is reserved for
  future use. Note that the mask for each block is
  /20 because the original block with mask /18 is
  divided into 4 blocks.

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Example 12
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6.3 Routing - Static versus Dynamic Routing

• Static routing table
• Containing information entered manually
• Cannot update automatically when there is a
  change in the internet
• Used in small internet that does not change very
  much, or in an experimental internet for
  troubleshooting
• Dynamic routing table
• is updated periodically using one of the dynamic
  routing protocols such RIP, OSPF, or BGP (see
  Chap. 14)
• Updating the routing table corresponding to
  shutdown of a router or breaking of a link

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Routing Module
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Routing Table

• Routing table
• In classless addressing, routing table has a
  minimum of four columns.
• - Some routers have even more columns

Flags U (up) The router is up and running. G
(gateway) The destination is in another
network. H Host-specific address. D Added
by redirection. M Modified by redirection.
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Routing Table (contd)

• Flags
• U (Up) indicating the routers running
• G (Gateway) meaning that the destination is
  another network
• H (Host-specific) indicating that the entry in
  the destination is a host-specific address
• D (Added by redirection) indicating that
  routing information for this destination has been
  added to the host routing table by a redirection
  message from ICMP
• M (Modified by redirection) indicating that
  routing information for this destination has been
  modified by a redirection message from ICMP
• Reference count giving the number of users that
  are using this route at any moment
• Use showing the number of packets transmitted
  through this router for the corresponding
  destination
• Interface showing the name of the interface

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Example 13

• One utility that can be used to find the contents
  of a routing table for a host or router is
  netstat in UNIX or LINUX. The following shows the
  listing of the contents of the default server. We
  have used two options, r and n. The option r
  indicates that we are interested in the routing
  table and the option n indicates that we are
  looking for numeric addresses. Note that this is
  a routing table for a host, not a router.
  Although we discussed the routing table for a
  router throughout the chapter, a host also needs
  a routing table.

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Example 13 (contd)
netstat -rnKernel IP routing table Destination
Gateway Mask Flags
Iface 153.18.16.0 0.0.0.0 255.255.240.0 U
eth0 127.0.0.0 0.0.0.0 255.0.0.0 U
lo 0.0.0.0 153.18.31. 254 0.0.0.0 UG eth0.
Loopback interface
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Example 13 (contd)
More information about the IP address and
physical address of the server can be found using
the ifconfig command on the given interface
(eth0).
ifconfig eth0 eth0 Link encapEthernet
HWaddr 00B0D0DF095D inet addr153.18.17.11
Bcast153.18.31.255
Mask255.255.240.0 ....
From the above information, we can deduce the
configuration of the server as shown in Figure
6.19.
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Example 13 (contd)
Ifconfig command gives us the IP address and the
physical address (hardware) address of the
interface
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6.4 Structure of a Router
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Components

• Input port
• Output port

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Components (contd)

• Routing Processor
• performing the functions of the network layer
• destination address is used to find the address
  of the next hop and output port number table
  lookup

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Switching fabrics

• Crossbar switch

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Switching Fabrics

• Banyan switch
• log2 (n) stages with n/2 microswitches

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Switching Fabrics (contd)

• Examples of routing in a banyan switch

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Switching Fabrics (contd)

• Possibility of internal collision even when two
  packets are not heading for the same output port
  in banyan switch
• solving the problem by sorting the arriving
  packets based on their destination port
• Trap module preventing duplicate packets
  (packets with the same output destination) from
  passing to the banyan switch simultaneously

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Switching Fabrics (contd)

• Batcher-banyan switch

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Summary(1)

• In a connection-oriented service, the local
  network layer protocol first makes a connection
  with the network layer protocol at the remote
  site before sending a packet.
• In a connectionless service, the network layer
  protocol treats each packet independently, with
  each packet having no relationship to any other
  packet. The packets in a message may or may not
  travel the same path to their destination. The IP
  protocol is a connectionless protocol.
• The delivery of a packet is called direct if the
  deliverer(host or router) and the destination are
  on the same network.
• The delivery of a packet is called indirect if
  the deliverer(host or router) and the destination
  are on different networks.
• In the next-hop method, instead of a complete
  list of the stops the packet must make, only the
  address of the next hop is listed in the routing
  table.
• In the network-specific method, all hosts on a
  network share one entry in the routing table.
• In the host-specific method, the full IP address
  of a host is given in the routing table.

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Summary(2)

• In the default method, a router is assigned to
  receive all packets with no match in the routing
  table.
• The routing table for classful forwarding can
  have three columns.
• The routing table for classless addressing needs
  at least four columns.
• The number of columns in a routing table is
  vendor dependent.
• Address aggregation simplifies the forwarding
  process in classless addressing.
• Longest mask matching is required in classless
  addressing.
• Classless addressing requires hierarchical and
  geographical routing to prevent immense routing
  tables.
• Search algorithms for classful addressing are not
  efficient for classless addressing.
• A static routing table's entries are updated
  manually by an administrator.
• A dynamic routing table's entries are updated
  automatically by a routing protocol.
• A router is normally made of four components
  input ports, output ports, the routing processor,
  and the switching fabric.