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More on the IP
Internet Protocol
Internet Layer Process
• Transport layer process passes EACH TCP
segment to the internet layer process for
delivery
Transport Layer
Process
TCP segment
Internet Layer
Process
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
Internet
Process
IP Packet
Internet
Process
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!)
Internet
Process
IP Packet
Internet
Process
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
Check Only Once
– Reduces router costs
Transport
Transport
Internet
Internet
Internet
Host
Router
Host
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
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)
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
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
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!
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
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
IP Addresses
• Each Organization is Given a Network
Part Number
• For the University of Hawaii, this is
128.171
– All IP Addresses in that Organization’s
Network Begin with that Network Part
Network Part
IP Address
128.171
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
IP Addresses
• Between different organization networks,
routers look first at the Network Part of an
arriving IP packet’s 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
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)
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
128.171
Firm
128.171.123.130
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 computer’s IP address became
128.171.17.13.
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
IP Address (32 bits total)
Host Part
Assigning Subnet Parts
• Organization Assigns Subnet Parts
– Assigns subnet parts to suborganizations
– Suborganization assigns host bits to hosts
128.171
Registrar
128.171.17.13
Firm
Suborganization
128.171.17.13
Host
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
IP Address (32 bits total)
Host Part
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)
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)
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
Masks
• Masks Begin with 1s, End with 0s
(111…00)
• 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
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
Router Delivery
• If Destination Host is On the Source
Host’s Subnet, Source Host Delivers the
Packet Directly
– No router is involved
Source
Host
Subnet
Subnet
Destination Host
Router Delivery
• If Destination Host is NOT On the
Source Host’s Subnet, Source Host Sends
the Packet to a Router for Delivery
Subnet
Subnet
Router Delivery
• If Destination Host is On One of the
Router’s Subnets, the Router Sends the
Packet to the Destination Host for
Delivery
Subnet
Subnet
Router Delivery
• If Destination Host is NOT On One of
the Router’s 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
Router Delivery
• Border Routers Connect Networks, Not
Subnets
– Select between next-hop router on own
network or on another network
Own Network
Other Network
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
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
IP Address
Delivery
128.171.17.13
Local
142.99.171.3
Next-Hop Router A
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 packet’s IP dest. addr.
– Compares to IP Address Part for that row
IP Address
Part
127.171.17
Mask
Destination
24
Local
142
8
Next-Hop
Router A
Router Forwarding Tables
• Also Has a Mask Column
– A network mask for a host outside the organization’s
network
– A subnet mask for an internal host
– Can’t tell which by looking at the mask
IP Address
Part
127.171.17
Mask
Destination
24
Local
142
8
Next-Hop
Router A
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
IP Address
Part
127.171.17
Mask
Destination
24
Local
142
8
Next-Hop
Router A
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 row’s IP address part, so use Local
delivery
IP Address
Part
127.171.17
Mask
Destination
24
Local
142
8
Next-Hop
Router A
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)
IP Address
Part
127.171.17
Mask
Destination
24
Local
128.171
16
Next-Hop
Router A
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
IP Address Mask
Part
128.171
16
Metric
(Cost)
23
Delivery
128.171
45
Next-Hop
Router A
16
Local
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)
IP Address
Delivery
128.171.17.13
Local
0.0.0.0
Next-Hop Router C
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
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
Interface
Delivery
2
Local
3
Next-Hop Router C
3
Next-Hop Router D
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
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
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
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