Network Layer

1
Network Layer
2
The context
3
Store and Forward Packet Switching

• A packet is stored in entirety, checksum
  recomputed at every hop and forwarded.

4
Services provided to the transport layer

• Services should be independent of the router
  technology.
• Transport layer be shielded from the topology of
  the routers.
• The network addresses made available to the TL
  should use a uniform numbering plan, even across
  LANs and WANs.

5
Two schools of thought

• Whether NL provides a CL service to the TL
  (Internet Community), or
• Whether It provides a CO service to the TL
  (telephone companies)

6
CL VS CO

• CL Since subnet is inherently unreliable, host
  should do the error , flow control, packet
  ordering etc themselves.
• CO subnet must provide reliable service QoS is
  imporatnt

7
Implementation of CL

• Packets are called datagrams and the subnet is
  called datagram subnet.

8
Implementation of Connectionless Service

• Routing within a diagram subnet.

9
Implementation of Connection-Oriented Service

• Routing within a virtual-circuit subnet.

10
Comparison of Virtual-Circuit and Datagram Subnets
5-4
11
Comparison of Virtual-Circuit and Datagram Subnets
5-4
12
Routing Algorithms desirable properties

• Correctness
• Simplicity
• Robustness
• Stability converge to equilibrium
• Fairness
• Optimality Minimize mean packet delay, maximize
  throughput conflicting, since queuing near full
  capacity implies long delays
• Minimize the number of hops improves delays as
  well as throughput

13
Equilibrium
14
Fairness Vs Optimality

• Conflict between fairness and optimality.

15
Routing Algorithms Adaptive or Non-adaptive

• Non-adaptive Static
• Adaptive Dynamic

16
Routing Algorithms

• The Optimality Principle
• Shortest Path Routing
• Flooding
• Distance Vector Routing
• Link State Routing
• Hierarchical Routing
• Broadcast Routing
• Multicast Routing
• Routing for Mobile Hosts
• Routing in Ad Hoc Networks

17
The Optimality Principle

• (a) A subnet. (b) A sink tree for router B.

18
Shortest Path Routing Static

• The first 5 steps used in computing the shortest
  path from A to D. The arrows indicate the
  working node.

19
Flooding

• Problems Jamming/Congestion
• Solutions
• Hop count in the header
• Sequence number for every source discard
  duplicate packets.
• Adv Though not practical for routine routing but
  useful when a system starts afresh.

20
Distance Vector Routing Dynamic

• (a) A subnet. (b) Input from A, I, H, K, and the
  new
• routing table for J.

21
Distance Vector Routing (2)
The count-to-infinity problem.
22
Link State Routing

• Each router must do the following
• Discover its neighbors, learn their network
  address.
• Measure the delay or cost to each of its
  neighbors.
• Construct a packet telling all it has just
  learned.
• Send this packet to all other routers.
• Compute the shortest path to every other router.

23
Discovering the neighbours

• Send a HELLO packet
• The other router sends a reply telling who he is

24
Problem

• (a) Nine routers and a LAN. (b) A graph model of
  (a).

25
Measuring the delay to neighbours

• Send an ECHO packet
• The receiver sends it back immediately with a
  time stamp

26
Building Link State Packets

• (a) A subnet. (b) The link state packets for
  this subnet.

27
Distributing the Link State Packets

• Packets are flooded,
• Seq no.s to check jamming

28
Distributing the Link State Packets

• The packet buffer for router B in the previous
  slide (Fig. 5-13).

29
Hierarchical Routing

• Hierarchical routing.

30
Broadcasting in point to point subnet

• Individual packet for each destination
• Flooding
• Multidestination routing each packet has to
  contain either a list of destinations or a bit
  map indicating all the destinations.
• Spanning tree best in terms of channel
  utilization minimum number of copies are
  generated . But knowledge of some spanning tree
  at every router is required ..which is sometimes
  available (as in LSR) but sometimes not(as in
  DVR)
• Reverse path forwarding

31
Reverse path forwarding

• Approximates the behaviour of Spanning tree algo
  .. When routers have no knowledge of spanning
  trees.

32
Reverse path forwarding
Reverse path forwarding. (a) A subnet. (b) a
Sink tree. (c) The tree built by reverse path
forwarding.
33
Reverse path forwarding

• No knowledge of spanning trees required
• No bit map etc required
• Simple and easy to implement
• No additional CPU time required
• Not the best but in practice reasonably
  efficient.

34
Multicast Routing in the subnet

• MR sending a message to members within a group

35
Multicast Routing
(a) A network. (b) A spanning tree for the
leftmost router. (c) A multicast tree for
group 1. (d) A multicast tree for group 2.
36
Congestion Control
37
What is congestion?

• Too many packets in a part of the subnet, too
  many to handle.

38
Some of the Causes of congestion

• Lot of packets arriving at 3-4 lines of a router,
  all needing the same output line. As a result, a
  queue will build up on the outgoing line. When
  the Q is full, packets will start dropping.
• Increasing the size of the buffer may help
  initially but after a certain limit may have
  adverse effect--- By the time a packet moves up
  in the Q, it times out and retransmitted
  increasing the load on the network.

39
Causes of congestion

• Slow processors If the computational powers of a
  router are weak, it will take time in
  book-keeping and processing, again resulting in
  building up of Qs.
• Low bandwidth
• In fact, Slow processors and High bandwidth, or
• Fast processors but Low Bandwidth
• a good combination of fast processors and high
  bandwidth is required to improve the situation.

40
Solutions to Congestion

• Classified into two categories
• Open Loop solutions Static Solutions, take
  preventive measures by good design, but no
  corrections are done once the system is up, that
  is do not take the current state of the system
  into account. Adv simplicity
• Closed Loop Solutions Based on feedback
• a router detects congestion,
• pass the information (feedback) to nodes where
  action can be taken, say the sender, and
• adjust the system parameters to fix the problem.

41
Open Loop Solutions

• Policy-Decisions at various levels

42
Congestion Prevention Policies

• Policies that affect congestion.

5-26
43
Open Loop Solutions contd..

• Flow Control --- window size
• Acknowledgement policy --- piggyback or not
• Out-of-order policy --- Go back N/Selective
  repeat
• Retransmission Policy --- Time out etc

44
Closed Loop Solutions

• Detecting/Predicting congestion Let u be any
  parameter being monitored say, output line
  utilization or queue length or buffer
  utilization Let a be a constant between 0 and
  1
• U_new is predicted as folllows
• u_new a u_old (1-a)u_new
• A value of u going above a certain threshold is
  considered as a situation for congestion
• Congestion Control in Virtual-Circuit Subnets
• Congestion Control in Datagram Subnets

45
Congestion Control in Virtual-Circuit Subnets

• Admission Control No more new VCs
• Allow new VCs but route them around the problem
  area.

46
Congestion Control in Virtual-Circuit Subnets

• A congested subnet. (b) A redrawn subnet,
  eliminates congestion and a virtual circuit from
  A to B.
• Back

47
Congestion Control in Datagram Subnets

• Warning Bit is set in the forwarded packet and
  copied en-route by the destination in the
  acknowledgement packet.
• Choke Packets are sent back to the sender as a
  feedback.
• The corrective measures (slow down the
  transmission) are taken only at the source, Fig
  a.
• The corrective measures (slow down the forwarding
  of the packets coming from the source) are taken
  at every Hop en-route the Choke Packet, Fig b.

48
Hop-by-Hop Choke Packets

• (a) A choke packet that affects only the source.
• (b) A choke packet that affects each hop it
  passes through.
• Back

49
Load Shedding in Datagram Subnet

• Throw the packet when nothing else works
• Select the packet to be dropped randomly
• Select intelligently For example
• In a file transfer, older is better so drop the
  later ones. Dropping older would cause a gap at
  the destination and more packets would have to be
  retransmitted.
• In audio/video file loosing few bits is not
  important, so dropping older is better, no
  retransmission may be asked by the destination.
• Or, sender may specify the priority

50
Internetworking
51
Connecting Networks of different type by routers

• So far we have assumed that our subnet is
  connected to LANs of same type. So the only role
  of routers was to route the packets.
• As the h/w and n/w gets cheaper, the place where
  decisions are made move downwards in the
  hierarchy in an organization. For eg ..in a univ
  .. Each department decides on its own what type
  of LAN do they want. Hence Maths deptt may have
  an Ethernet LAN whereas CS may have a wireless
  LAN.
• Now the univ. must be able to provide a subnet to
  connect these two LANs so that a host on one can
  communicate with the host on another.

52
Connecting LANs of different types by bridge

• Sort of assumes that their NL are same..
• If that is not the case, then the scheme studied
  earlier will not suffice.

53
Connecting Networks

• A collection of interconnected networks.

54
How Networks Differ
5-43

• Some of the many ways networks can differ.

55
How Networks Can Be Connected

• (a) Two Ethernets connected by a switch.
• (b) Two Ethernets connected by routers.

56
Concatenated Virtual Circuits

• Internetworking using concatenated virtual
  circuits.

57
Connectionless Internetworking

• A connectionless internet.

58
Tunneling

• Tunneling a packet from Paris to London.

59
Tunneling (2)

• Tunneling a car from France to England.

60
Autonomous System

• Each network in an Internet is independent and
  hence it is called an Autonomous System.

61
Internetwork Routing

• (a) An internetwork. (b) A graph of the
  internetwork.

62
Two level routing algorithm in IN

• Once a graph of multi-protocol routers(gateways)
  is constructed , routing algorithms such as DVR
  and LSR can be applied. This leads to 2-level
  routing in internetworks
• Interior Gateway Protocol (within a network).
• Exterior Gateway Protocol (across the networks)

63
Routing in Internet contd..

• A host H1 on LAN 1 wants to send a packet to a
  host H2 on LAN 2.
• A packet is prepared by NL of H1 with Network
  address of H2 but,
• Encapsulated in a frame by the DLLwith MAC
  address of the multi-protocol router connected to
  LAN 1
• Packet arrives at MPR1 on LAN1
• NL at MPR1 uses the Network address to decide
  which MPR2 to forward the packet to.
• If the Network Protocol used by the Network
  through which MPR1 sends a packet to MPR2 is same
  as that used by LAN1, the packet is sent directly
  with no change in the packet.
• Else, the packet is encapsulated in the payload
  field of the packet of the Network Protocol used
  by the connecting Network and tunneled.
• Of course, assuming that the connecting network
  uses MAC addressing, DLL of MPR1 puts the MAC
  address of MPR2.
• The process is repeated at MPR2
• Until the packet reaches the destination network.

64
Fragmentation

• (a) Transparent fragmentation. (b)
  Nontransparent fragmentation.

65
Fragmentation (2)

• Fragmentation when the elementary data size is 1
  byte.
• (a) Original packet, containing 10 data bytes.
• (b) Fragments after passing through a network
  with maximum packet size of 8 payload bytes plus
  header.
• (c) Fragments after passing through a size 5
  gateway.

66
The Network Layer in the Internet

• The IP Protocol
• IP Addresses
• Internet Control Protocols
• OSPF The Interior Gateway Routing Protocol
• BGP The Exterior Gateway Routing Protocol
• Internet Multicasting
• Mobile IP
• IPv6

67
Design Principles for Internet

• Make sure it works.
• Keep it simple.
• Make clear choices.
• Exploit modularity.
• Expect heterogeneity.
• Avoid static options and parameters.
• Look for a good design it need not be perfect.
• Be strict when sending and tolerant when
  receiving.
• Think about scalability.
• Consider performance and cost.

68
Collection of Subnetworks

• The Internet is an interconnected collection of
  many networks.

69
The IP Protocol

• The IPv4 (Internet Protocol) header.

70
IP Protocol contd..

• Version
• IHL
• Type of Service
• Total Length, Identification, DF, MF, Fragment
  Offset
• TTL
• Protocol
• Header Checksum
• Options

71
Version Number

• To let several versions to work
  simultaneouslyactually two during a transition
  period which takes years.

IP Protocol contd..
72
IHL IP Header Length

• Header length is variable
• specified as number of 32 bit words
• 20 bytes (5 32 bit words) to 60 bytes (15 32 bit
  words)

IP Protocol contd..
73
Type of Service

• Various combinations of reliability and speed can
  be specified here. For Example, digitized voice
  prefers fast over error-free transmission and
  file transfer prefers error-free over fast
  transmission. Routers on the way use this
  information to choose a path. If the shortest
  path is error-prone, it may use an alternate path
  to transfer a packet for file-transfer whereas
  for voice packet it will choose the shortest path
  even if it is error-prone.

74
Type of Service contd..

• First 3 bits precedence or priority bits
• Next 3 are flags D (delay), T (Throughput) and
  R (Reliability)
• Allow the routers to make a choice between high
  throughput and high delay link like satellite and
  low throughput, low delay link like leased line.

IP Protocol contd..
75
Other fields

• Total Length Header Data 16 bits ..65,535
  bytes
• Identification No. To know which datagram the
  fragment belongs to
• DF, MF Dont Fragment and More Fragment
• Fragment Offset
• specified in number of elementary fragment unit
  i.e. 8 bytes i.e multiple of 8 bytes.
• 13 bits 2138192
• Total Datagram 8192 8.

IP Protocol contd..
76
Time to Live

• Specified in seconds and decremented on every Hop
  and even when in the queue.
• In practice, Number of Hops is used.

IP Protocol contd..
77
Protocol

• Mentions the number of Transport Layer Protocol
  to which the packet must be handed over. For eg.
  TCP/UDP or any other.
• These TP are assigned numbers (called ports) are
  unique across the globe.

IP Protocol contd..
78
Header Checksum

• Computed at every hop
• to take care of error that might creep in due to
  bad bits in the router memory.
• TTL field changes at every hop

IP Protocol contd..
79
Some of the IP Options

• .

5-54
80
IP Addresses 32 bit number

• Assigned by central naming authority ICANN
  Internet Corporation for Assigned Names and
  Numbers

81
IP Addresses

• IP address formats.

82
Dotted decimal Notation

• 202.14.13.1
• 192.133.13.5
• Each 8 bit block is written as its decimal eqvt.

83
IP Addresses (2)

• Special IP addresses.

84
Routing Tables

• Routing tables at each router has two types of
  entries
• (network,0)
• (this network, host)
• Note the first type of entry. By keeping only the
  network number and not all the IP addresses
  belonging to a distant network, the size of the
  routing table is greatly reduced.

85
Problems in Class based Addressing

• What to do when the network grows beyond the
  current maximum.
• Solution To allow a network to be split into
  several parts (called subnet) for internal use
  but appear to be a single network to the outside
  world.
• Note The word subnet has been used again now
  to define a different context. The difference
  will be clear from the context.

86
Subnets

• A campus network consisting of LANs for various
  departments.

87
Subnets (2)

• A class B network subnetted into 64 subnets.

88
Routing Tables

• Routing tables at each router now has three types
  of entries
• (network,0)
• (this network, subnet, 0)
• (this network, subnet, host)

89
CDR Classless InterDomain Routing

• A set of IP address assignments.

5-59
90
IP Addresses are scarce

• Most of the people are opting for broadband
  Internet Connection i.e. a permanent IP address
• One solution IPV6 128 bit address, but it
  will take years to come.
• A quick solution is needed NAT

91
NAT Network Address Translation

• Placement and operation of a NAT box.

92
Three reserved ranges of IP addresses for
Internal Use

• 10.0.0.0 -- 10.255.255.255/8
• 172.16.0.0 172.31.255.255/12
• 192.168.0.0 192.168.255.255/16
• For example Delhi University Intranet
• IP addresses are of the form 10.25.2.23
• Gateway 10.25.1.4
• DNS 10.2.1.13, 10.2.1.16

93
Working of NAT

• Before a packet from internal host exits the
  company (connected to ISP through say a leased
  line) as shown in the figure or
• a packet from a home/ business user connected
  through broadband to ISP exits ISP
• local IP address (10.x.y.z etc) is mapped to the
  company's/ ISP' s true IP address and sent out.
• What when a packet comes from Internet? How does
  a router decide whom to hand it over to?

94
NAT contd..

• NAT must remember the internal addresses. How?
• It uses Header of the Transport Layer.
• Source Port field is replaced by a pointer to an
  entry in a table (maintained by NAT box)
  containing the local address.
• How does it remember the Source Port then?
• The table entry contains the source port also,
  which is copied into the destination port on
  return.

95
Other Network Layer Protocols used in Internet
Internet Control Protocols

• ICMP Internet Control Message Protocol used
  by the routers to monitor the Internet for
  unexpected events, and also to test the Internet
  from time to time.
• ARP Address Resolution Protocol maps an IP
  address to a unique DLL address
• RARP Reverse ARP DLL address to IP
• BOOTP, DHCP and others

96
Internet Control Message Protocol

• The principal ICMP message types.

5-61
97
ARP

• Although every machine on the Internet has one or
  more IP addresses, they are not sufficient for
  sending packets as the DLL h/w doesnt understand
  the IP addresses.
• How are IP addresses mapped to DLL addresses?

98
ARP The Address Resolution Protocol

• Three interconnected /24 networks two Ethernets
  and an FDDI ring.

99
ARP How does it work

• Suppose H1 wants to send a packet to H2.
• It sends a broadcast packet (broadcast address in
  the DLL address for destination) on its LAN
  asking who owns the IP address 192.31.65.5?
• Everyone on LAN1 gets it but only H2 replies with
  its DLL address.
• H1 now prepares the data packet meant for H2 and
  sends it.

100
ARP How does it work

• Now Suppose H1 wants to send a packet to H4.
• It sends a broadcast packet (broadcast address in
  the DLL address for destination) on its LAN
  asking who owns the IP address 192.31.63.8?
• Everyone on LAN1 gets it but this time the router
  replies with its DLL address.
• H1 now prepares the data packet meant for H4 with
  the DLL address of the router and sends it to the
  router.
• The process is repeated on the FDDI ring and so
  on.

101
DHCP Dynamic Host Configuration Protocol

• Maps DLL address to IP address
• When a diskless workstation boots from a remote
  machine, how does it get its IP address?
  Remember,
• IP addresses are assigned in the s/w and,
• DLL address in h/w
• When a machine boots from a local OS it learns
  its IP address from the settings already done
  (settings in TCP/IP etc) but,
• When it boots from a remote machine it gets the
  binary image of its OS from a remote file server.
  The IP address cannot be included in this binary
  image for then a separate binary image will be
  required to boot each host.

102
DHCP contd..

• Such a host (say H1) asks a question My DLL
  address is..Does anyone know my IP address?
• Another machine running DHCP server responds back
  with the IP address of H1.
• DHCP Server maintains a table of (DLL address, IP
  address) of the nodes it serves.
• How does H1 get down (IP address of) to the DHCP
  server?
• If the DHCP server is on the same LAN as the host
  H1, there is no problem
• A broadcast packet from H1 is enough.
• However, if DHCP server is on a remote machine, a
  machine called DHCP relay agent (who knows the IP
  address of the DHCP server) is required on each
  LAN.
• DHCP relay agent relays the packet from H1 to the
  DHCP server and back.

103
Dynamic Host Configuration Protocol

• Operation of DHCP.

104
Routing Protocols in Internet

• OSPF Open Shortest Path First (Interior Gateway
  Protocol)
• BGP Border Gateway Protocol (Exterior Gateway
  Protocol)

105
OSPF Open Shortest Path First Interior Gateway
Protocol

• Routing algorithm within an AS
• Initially when As were small .. A variant of
  DVR(RIP) was used.
• DVR suffered from count to infinity problem and
  was replaced by Link State Routing algorithm in
  1979.

106
OSPF Basic form

• AS is small same as LSR.
• To understand OSPF lets see how an AS looks like
  AS is a collection of routers and networks.

107
OSPF hierarchical structure when ASes became
large

• When ASes became large they were further divided
  into areas
• Each area is a collection of networks and routers
  now.

108
OSPF works in an AS

• Three types of Routers
• Internal routers (used for routing within an area
  hence keep the Link State Database for routers
  within an area and run Shortest Path Algorithm
  locally )
• Area border routers (Inter-area routing through
  backbone routers, keeps the LSDB for all the
  areas(2 areas in case of non-backbone area
  router and may be more than 2 areas for a
  backbone area router) each is connected to and
  runs a possibly different SPA for each area
  separately.
• Backbone routers could be internal or area
  border routers of Area 0

109
OSPF - Hierarchical Structure
The relation between ASes, backbones, and areas
in OSPF.
110
OSPF Hierarchical Structure contd..
111
Internal Routers

• These are routers that are only connected to
  other routers or networks within a single area.
  They maintain an LSDB for only that area, and
  really have no knowledge of the topology of other
  areas.

112
Area Border Routers

• These are routers that connect to routers or
  networks in more than one area. They maintain an
  LSDB for each area of which they are a part. They
  also participate in the backbone.

113
Backbone Routers

• These are routers that are part of the OSPF
  backbone. By definition, this includes all area
  border routers, since those routers pass routing
  information between areas.
• However, a backbone router may also be a router
  that connects only to other backbone (or area
  border) routers, and is therefore not part of any
  area (other than Area 0).
• Back

114
BGP Border Gateway ProtocolExterior Gateway
Protocol

• Issues
• No transit traffic thru certain Ases
• Never put Iraq on a route starting at Pentagon
• DO not use US to get from British Coumbia to
  Ontario
• Only transit Albania if there is no alternative
  to the destination
• Traffic starting or ending at IBM should not
  transit Microsoft
• These kind of issues cannot be solved by
  computing the Shortest Path trees.

115
BGP cntd

• BGP is basically a DVR protocol.
• However it maintains and advertise the entire
  path.
• Since choice of entire paths are available ..
  decisions around the routers can be taken on a
  datagram basis.
• Hence, it takes care of the count to infinity
  problem also.

116
BGP contd..

• (a) A set of BGP routers. (b) Information
  sent to F.

117
The Main IPv6 Header

• The IPv6 fixed header (required).

118
Extension Headers
5-69

• IPv6 extension headers.

119
Extension Headers (2)

• The hop-by-hop extension header for large
  datagrams (jumbograms).

120
Extension Headers (3)

• The extension header for routing.