More on the IP

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More on the IP

• Internet Protocol

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Internet Layer Process

• Transport layer process passes EACH TCP segment
  to the internet layer process for delivery

Transport LayerProcess
TCP segment
Internet LayerProcess
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IP Connectionless Service

• The Internet Protocol (IP)
• Internet layer protocol
• IP messages are called IP packets
• No connections are established
• No open, close, error correction, flow control
• Low overhead

IP Packet
Internet Process
Internet Process
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IP Connectionless Service

• IP is unreliable
• No error handling (Let TCP catch errors!)
• No sequence numbers, so no way to put arriving IP
  packets in order (Let TCP put the TCP segments
  these IP packets contain in order!)

IP Packet
Internet Process
Internet Process
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TCP/IP Partnership

• TCP checks for errors once, at the destination
  host
• IP is used in many hops between routers
• Not checking for errors at each step greatly
  reduces overall processing work
• Reduces router costs

Check Only Once
Transport
Transport
Internet
Internet
Internet
Host
Router
Host
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Cobnnectionless IP

• IP is unreliable (does not catch errors)
• But this is not bad
• First, errors are caught--at the next-higher
  layer (transport) if TCP is used
• Second, avoiding error checking at each hop
  between routers lowers router costs
• Far less expensive to check for errors on one
  destination host than on many routers along the
  way

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IP is a Best-Effort Service

• IP Only Offers Best-Effort Service
• Does its best to get packets through
• No guarantees of delivery
• No way to give priority to time-sensitive
  traffic, such as voice
• Overall, low overhead but limited Quality of
  Service (QoS)
• QoS should change in the future (see Chapter 8)

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IP Address

• 32-bit Strings
• Often given in dotted decimal notation
  128.171.17.13
• Fits into 32-bit source and destination address
  field of IP headers

IP Packet
32-bit Source and Destination Addresses
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IP Addresses and Router Forwarding

• Routers use the destination IP address of an
  incoming packet in the router forwarding
  decision, that is, to decide what output port to
  use to send the packet back out to the
  destination host or to another router

B
B?
D?
Router A
D
Packet
C?
C
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IP Addresses

• Many Addressing Systems Use Hierarchical
  Addressing
• Postal delivery city, street address
• Post office looks at city first
• If not P.O.s city, put in bag for other city
• If in P.O.s city, put in bag for sorting by
  street address
• Hierarchical addressing greatly speeds sorting at
  each post office
• Imagine if we needed a sorting bin for each
  address in the country!

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IP Addresses

• For IP, Routers Take the Place of Post Offices
• There are hundreds of millions of IP addresses on
  the Internet
• Routers cannot store decision rules for reaching
  each address individually
• So router simply asks if a destination IP address
  is that of a host on one one of the networks or
  subnets connected to the router or must be passed
  on to another router
• This is the router forwarding decision

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IP Addresses

• To Simplify Router Decisions, IP Addresses are
  Hierarchical
• The Internet is Made of Many Individual Networks
  Owned by Different Organizations
• First route packets to a single network only
  need one sorting bin for each network!
• In the next step, route packet to host on the
  network

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IP Addresses

• Each Organization is Given a Network Part Number
• For the University of Hawaii, this is 128.171
• All IP Addresses in that Organizations Network
  Begin with that Network Part

Network Part
IP Address
128.171
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IP Addresses

• Network Parts can be 8 to 24 bits long
• For University of Hawaii, it is 16 bits long
• 16 bits is only an example

Network Part (8 to 24 bits)
IP Address
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IP Addresses

• Between different organization networks, routers
  look first at the Network Part of an arriving IP
  packets destination address
• If the network part is not that of the
  organization, the router cannot deliver the IP
  packet locally
• Passes the IP packet on to another router, called
  a next-hop router, to move the IP packet closer
  to the destination host

Network Part
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IP Addresses

• Local Part
• The part of the IP address after the network part
  is called the local part
• Total address is 32 bits, so if the network part
  is 8, the local part is 24

Network Part
Local Part
IP Address (32 bits total)
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Assigning Network Parts

• Organization applies to an Internet IP address
  registrar
• Registrar gives organization a network part
• Organization assigns the local part to its hosts
  internally
• Only large organizations and ISPs get network
  parts

128.171.17.13
Registrar
Firm
128.171
128.171.123.130
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Assigning Parts

• Example
• IP address registrar gave the University of
  Hawaii the network part 128.171
• UH gave the College of Business Administration
  the subnet part 17
• College of Business Administration gave the host
  part 13 to a computer it later gave the host name
  voyager.cba.hawaii.edu
• So the computers IP address became 128.171.17.13.

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IP Addresses

• Most Organizations Have Multiple Subnets within
  the Organizational Network
• Usually represent each as a Subnet Part within
  the Local Part
• Remaining Bits are the Host Part, designating a
  particular host on that subnet

Local Part
Network Part
Subnet Part
Host Part
IP Address (32 bits total)
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Assigning Subnet Parts

• Organization Assigns Subnet Parts
• Assigns subnet parts to suborganizations
• Suborganization assigns host bits to hosts

128.171
128.171.17.13
Suborganization
Registrar
Firm
128.171.17.13
Host
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IP Addresses

• Within an organizational Network
• Router looks at Network Plus Subnet Part Combined
• If destination host is on a subnet attached to
  the router, delivers the IP packet to the host
• Otherwise, passes the packet on to a next-hop
  router

Local Part
Network Part
Subnet Part
Host Part
IP Address (32 bits total)
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IP Addresses

• In IP Addresses, Network and Subnet are
  Organizational Concepts, not Technical Concepts
• Network is the collection of individual networks
  and routers owned by an organization
• Subnet is a collection of individual networks and
  routers owned by a suborganization
• Often a single physical network (subnet)

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Importance of Part Sizes

• Determine Number of Possible Networks, Subnets,
  or Hosts
• If There are N Bits in the Part, there can be 2N
  possible Networks, Subnets, or Hosts
• Actually, 2N-2
• All zeros cannot be used for a part
• All ones cannot be used for a part
• Example if part has 8 bits, 28-2 possibilities
  (254)

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Masks

• IP Addresses are Always Paired with a Second
  32-bit Number Called a Mask
• Two Types Network Masks and Subnet Masks
• Network Mask Tells the Length of the Network Part
• Subnet Mask Tells the length of the Network Plus
  Subnet Parts (not just subnet part)
• IP Address will be paired with one or the other,
  but not both simultaneously

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Masks

• Masks Begin with 1s, End with 0s (11100)
• For network masks, 1s are in Network Part bits
  0s are in Subnet and Host Parts
• For subnet masks, 1s are in Network and Subnet
  Parts 0s are in Host part

11111111111111110000000000000000
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Masks

• IP Address-Mask Pairs often Written with Prefix
  Notation
• 128.171.17.13/16
• 16 means that the mask has 16 initial 1s
• Total number of bits is 32 in an IP address, so
  there must be 16 trailing 0s

11111111111111110000000000000000
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Router Delivery

• If Destination Host is On the Source Hosts
  Subnet, Source Host Delivers the Packet Directly
• No router is involved

Source Host
Subnet
Subnet
Destination Host
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Router Delivery

• If Destination Host is NOT On the Source Hosts
  Subnet, Source Host Sends the Packet to a Router
  for Delivery

Subnet
Subnet
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Router Delivery

• If Destination Host is On One of the Routers
  Subnets, the Router Sends the Packet to the
  Destination Host for Delivery

Subnet
Subnet
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Router Delivery

• If Destination Host is NOT On One of the Routers
  Subnets, the Router Sends the Packet to a
  Next-Hop Router for Delivery
• May have to choose among several possible
  next-hop routers for delivery

Subnet
Subnet
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Router Delivery

• Border Routers Connect Networks, Not Subnets
• Select between next-hop router on own network or
  on another network

Own Network
Other Network
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Router Forwarding Tables

• Allow Routers to Decide Whether Local Delivery is
  Possible to Destination Host
• Allow Routers to Select Next-Hop Router if Local
  Delivery is Not Possible

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Router Forwarding Tables

• Router Compares Destination IP Address to Each
  Row in Router Forwarding Table
• If matches IP address, delivers according to
  Delivery rule
• So if destination address of IP packet is
  128.171.17.13, router delivers packet locally

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Router Forwarding Tables

• Also Has a Mask Column
• Masks destination IP Address of packet
• If Mask in a row is 24 bits long, router only
  compares first 24 bits of packets IP dest. addr.
• Compares to IP Address Part for that row

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Router Forwarding Tables

• Also Has a Mask Column
• A network mask for a host outside the
  organizations network
• A subnet mask for an internal host
• Cant tell which by looking at the mask

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Router Forwarding Tables

• Also Has a Mask Column
• Masks destination IP Address of packet
• IP Address Part really is a network part or
  network plus subnet part of a network or subnet,
  respectively

Not in the Book
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Router Forwarding Tables

• Matching Destination IP Addresses
• Example Destination IP Address is 127.171.17.13
• Mask is 24, so only look at 127.171.17
• Matches rows IP address part, so use Local
  delivery

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Router Forwarding Tables

• Longest Match Principle
• Must select one row to determine delivery
• If two rows match, use longest match, that is
  match to greatest number of bits
• For 127.171.17.13, use local delivery (24-bit
  match)

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Router Forwarding Tables

• Metric
• If same length of match, turn to metric column
• Metric describes the desirability of a choice
• If metric is cost, choose lowest cost
• For other metrics (speed, etc.), may chose
  largest value

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Router Forwarding Tables

• There May be No Matches
• One IP Address Part is Always 0.0.0.0
• If there is no match, choose its next-hop router
  (called the Default Router)

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Router Forwarding Tables

• Recap of Selection Rules
• Compare destination IP address of an arriving
  packet against ALL rows within the router
  forwarding table because there may be multiple
  matches
• Select the single row that matches
• If multiple rows match, select the longest match
• If multiple rows tie on the longest match, select
  the row with the largest or smallest metric,
  depending on the specific metric
• If there is no match, select the default router
  row

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Router Forwarding Tables

• Delivery
• Table not only designates local delivery or a
  next hop router
• Also designates the router interface (port) that
  will be used for delivery

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Dynamic Routing Protocols

• How Do Routers Get Information for their Router
  Forwarding Tables?
• Share router forwarding table information
• Standards for these exchanges are called dynamic
  routing protocols

Router Forwarding Table Information
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Dynamic Routing Protocols

• How Do Routers Get Information for their Router
  Forwarding Tables?
• Thanks to dynamic routing protocols, the Internet
  needs no central point of control
• Routers create their router forwarding tables
  strictly by information from peers and their own
  knowledge

Router Forwarding Table Information
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IP Version 6

• Current Version of IP is IP Version 4
• This is the version we have been discussing
• Has 32-bit IP address fields
• Not long enough running out of IP addresses
• Next Version will be IP Version 6
• Will have 128-bit IP address fields
• Will allow vast numbers of IP addresses (2128)
• Being adopted slowly

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Terminology Confusion

• TCP/IP is a Standards Architecture
• Includes not only TCP and IP but also UDP, HTTP,
  and many other protocols
• May not even use TCP (UDP instead) or IP (ARP
  instead, as discussed in Module A)
• TCP and IP are Individual Standards
• Within the TCP/IP Architecture