Chapter 4 Network Layer 3: The Internet Protocol (IP)

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Chapter 4Network Layer 3The Internet Protocol
(IP)

• Professor Rick Han
• University of Colorado at Boulder
• rhan_at_cs.colorado.edu

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Announcements

• Reminder Programming assignment 1 is due Feb.
  19
• Part of Homework 2 available later today on Web
  site, the traceroute part will be available
  Monday
• Homework 1 solutions when we hand back graded
  Homework 1
• Reading Chapter 4
• 4.1 today added material
• 4.2, 4.3, 4.4 in same order
• Next, IP network, packets, ARP, RARP,

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Recap of Previous Lecture

• Interconnecting Ethernet LANs
• Ethernet Bridges/Switches Layer 2
• Loops can form, causing
• Packet multiplication
• Endless Looping
• Solution Create Spanning Trees
• Eliminates Loops and Spanning Trees
• Interconnecting Hosts and Switches via
  Point-to-Point Links
• Asynchronous Transfer Mode (ATM)
• Virtual circuits to route packets

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ATM Network

• Switch packets via virtual circuit routing
• Lost to Ethernet in LAN, Losing to Gig. Eth./
  SONET in MAN, SONET/MPLS in WAN
• Cost and complexity
• But, some customers (DSL) want AALs guaranteed
  QOS for voice/video

Switch C
Host A
Switch B
Host F
Switch E
Switch D
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Frame Relay and X.25

• Frame Relay
• Like ATM, uses permanent virtual circuits (PVCs
  more common) and SVCs
• Widely deployed in 1990s
• No error recovery per link not necessary over
  optical fiber
• X.25 is an old 1970s public packet switching
  technology
• Like ATM, uses virtual circuits to interconnect
  dumb terminals
• Error recovery on each link, due to noisy copper
  phone lines

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Bridging to Connect Remote LANs?

• Network Cloud could be one giant bridge
• Switch B keeps Ethernet MAC header, encapsulates
  Ethernet frame with network header, Switch E
  strips away network header
• spanning tree and a bridge table within cloud

ATM or Frame Relay Network
Ethernet 1
Ethernet 2
Switch C
Switch B
Switch E
Switch D
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Bridging to Connect Remote LANs? (2)

• Problems
• Many different types of LANs, e.g. Token Ring
  and FDDI, with completely different addressing
  schemes
• Spanning tree doesnt scale well

ATM or Frame Relay Network
Undecipherable?
Ethernet 1
Switch C
Switch B
Token Ring
Switch E
Switch D
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Routing to Connect Remote LANs

• Internet Protocol (IP) addressing is the glue
  that spans heterogeneous LANs and WANs
• IP hosts send IP packets via IP routers (shown in
  yellow)

ATM/Frame Relay
Switch C
Router X
Switch B
Router Y
Host 1
Switch E
Switch D
Host 2
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Routing to Connect Remote LANs (2)
Host 1
Router X
Host 2
Router Y
IP
IP
IP
IP
Tok R MAC
Eth. MAC
Eth. MAC
Tok R MAC
Phys.
Phys.
Phys.
Phys.
ATM Net.
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Routing to Connect Remote LANs (3)

• Alternatively, IP directly over SONET (MANs)
• Link-layer framing over fiber
• Less overhead (IP over SONET) vs. (IP over ATM
  over link layer (could be SONET))

IP over SONET
Router C
SONET
Router X
Router B
SONET
Router Y
SONET
SONET
SONET
SONET
Host 1
Router E
SONET
Router D
Host 2
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Routing to Connect Remote LANs (4)
Host 1
Router X
Host 2
Router Y
IP
IP
IP
IP
Tok R MAC
Eth. MAC
SONET framing
Eth. MAC
Tok R MAC
SONET framing
IP Net.
Opt Fbr OC-?
Opt Fbr OC-?
Phys.
Phys.
Phys.
Phys.

• OC3155 Mbps, OC12622 Mbps, OC482.488 Gbps,
  OC19210 Gbps
• Competitors to SONET in MAN Gigabit Ethernet

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Internet Backbone
Take this with a grain of salt can be a
highly political prediction of what someone wants
to happen
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• UUNet/WorldCom
• Backbone Provider
• To ISPs
• Leader at 28
• market share
• Claim theres a
• bandwidth glut on
• the backbone
• 1 bandwidth
• utilization

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ATT SONET Backbone
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Internet Topology
Internet Service Provider
ISP
ISP
Host 2
POP
Host 1
POP
Point of Presence
Network Access Point
Backbone Provider
Backbone Provider
NAP
Also called NSP Network Service Provider
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Internet Routing

• For simplicity, assume an Internet with a
  homogeneous IP backbone. IP provides
• Unreliable out-of-order datagram delivery, also
  called best-effort service - no QOS guarantees,
  just First-Come-First-Serve (FCFS) routing

IP backbone
Router C
Router X
Router B
Router Y
Host 1
Router E
Router D
Host 2
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Internet Protocol Packet Format
IP Datagram
IP Header
Data (variable length)
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IP Packet Header

• Big endian/network byte order send lower order
  bytes first
• Send bits 0-7, then 8-15, then
• Version current version is 4, I.e. IPv4
• proposal for IPv6, which will have a different
  header

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IP Packet Header (2)

• IHL header length in 32-bit words
• Normally 5, i.e. 20 byte IP headers
• Max 60 bytes
• Header can be variable length

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IP Packet Header (3)

• Type of Service 3-bit precedence field (unused),
  4 TOS bits, 1 unused bit set to 0
• TOS bit 1 (min delay), 2 (max throughput), 3 (max
  reliability), 4 (min cost) only one can be set
• typically all are zero, for best-effort service
• DiffServ proposes to use TOS for IP QOS

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IP Packet Header (4)

• Total Length of datagram, in bytes
• Max size is 65535 bytes
• Identification uniquely identifies each datagram
  sent by a host
• Used for fragmentation and reassembly

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IP Packet Header (5)

• Flags Fragment Offset for fragmentation
• Time To Live upper limit on routers that a
  datagram may pass through
• Initialized by sender, and decremented by each
  router. When zero, discard datagram. Stops
  looping

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IP Packet Header (6)

• Protocol IP needs to know to what protocol it
  should hand the received IP datagram
• demultiplexes incoming IP datagrams into either
  UDP, TCP, ARP,

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IP Packet Header (7)

• Header Checksum calculated only over header
• At sender, set to 0. Compute ones complement
  16-bit sum. Insert 16-bit ones complement of
  this sum.
• At receiver, compute 16-bit ones complement sum
  of header should be all 1s. If not, discard

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IP Packet Header (8)

• Source and Destination IP address 32 bits long
  each
• Often see written like, 12.244.92.161
• 127.0.0.1 is localhost loopback address, i.e.
  yourself
• Various classes of IP addresses

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IP Addressing

• Destination address is the key to packet routing
• IP routers only look at where the packet is
  headed, rather than where it came from
• Source address is useful
• At receiver, to decide whether to accept incoming
  packet
• At receiver, to send acknowledgement back to
  sender, e.g. TCP sends its acknowledgements
• IP address is per interface, so a given router
  with N interfaces can have N IP addresses

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IP Addressing (2)

• IP addresses are hierarchical 12.244.92.161
• Class A
• Class B
• Class C
• Hierarchy to handle WANs, MANs, and LANs
• Class C allows for only 256 local hosts, but 221
  Class C networks for small office nets
• Class A allows many 224 local hosts, few 27
  networks

7
24
0
Network
Host
14
16
1
Network
Host
0
21
8
1
1
0
Network
Host
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IP Addressing (3)

• Classes impose fixed-size network sub-fields that
  may not suit an organizations needs gt waste
  much address space
• Phase out fixed classes A, B, C
• Solution classless routing, or Classless
  Interdomain Routing (CIDR), 1993
• Network sub-field can have any number of bits
• a.b.c.d/x is CIDR notion for an IP address
  a.b.c.d with first x bits as network address

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IP Addressing (4)

• Assigning IP addresses
• Automatically via Dynamic Host Configuration
  Protocol (DHCP) well study it later
• Manually
• Contact your ISP
• an organization contacts its ISP for a block of
  allocated IP addresses
• An ISP contacts one of several well-known global
  registries (originally managed by IANA alone)
• 4 billion possible addresses
• Running out?
• NAT (Network Address Translation) ease the
  pressure well study it later
• IPv6

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IP Fragmentation and Reassembly

• Fragmentation occurs when datagram exceeds MTU of
  underlying network
• Ethernet MTU is 1500 bytes, FDDI MTU is 4500
  bytes
• Identifier field uniquely identifies a datagram
  sent from a source
• Set M bit in Flags field to one to indicate more
  fragments to follow
• Set Offset to 0 for first fragment
• For second fragment, set Offset length of data
  in first fragment
• For Nth fragment, set Offset sum of lengths of
  data in N-1 fragments

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IP Fragmentation and Reassembly (2)

• For last fragment, set M in Flags field to 0, to
  indicate no more fragments
• Each IP fragment is a full-fledged datagram
• Reassembly
• Fragments can be lost
• After waiting a reasonable amount of time, an
  IP end host will stop reassembly
• To avoid this waiting delay due to lost
  fragments, the sending host should perform path
  MTU discovery prior to sending IP packets, and
  then send at the MTU of the path

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Address Resolution Protocol (ARP)

• How does IP sends its packet over Ethernet?
• Ethernet doesnt understand 32-bit addresses
• Need to map 32-bit to Ethernets physical
  48-bit addresses
• Each host builds a cache that maps IP addresses
  to Ethernet addresses distributed, not
  centralized
• If sending to a host on the same Ethernet,
• First, check cache if address already present
• If not, send an Ethernets broadcast query (all
  1s in 48-bit address), frames Type field set to
  ARP
• Query contains target IP address, and link
  layer address of sending host

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Address Resolution Protocol (2)

• Each host receives broadcast query and checks to
  see if target IP address matches its own
• If match, sends a response to link-layer address
  of originator, containing its own link-layer
  address
• When another host hears an ARP request
• If requester is in cache, then refresh its own
  cache
• Entries in ARP cache time out every 15 min
• If requester is not in cache
• If host is target, then add to cache
• Otherwise dont add to cache, to keep ARP table
  clean