CSC 600 Internetworking with TCP/IP

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CSC 600Internetworking withTCP/IP

• Unit 5 IP, IP Routing, and ICMP (ch. 7, ch. 8,
  ch. 9, ch. 10)
• Dr. Cheer-Sun Yang
• Spring 2001

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Internet Protocol (IP)

• Part of TCP/IP
• Used by the Internet
• Specifies interface with higher layer
• e.g. TCP
• Specifies protocol format and mechanisms

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IP Services

• Unreliable
• Connectionless
• Best-effort delivery

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IP Protocol Specification

• IP datagram format
• Routing function
• Fragmentation and reassembly
• Internet control message protocol (ICMP) network
  level error message handling

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Header Fields (1)

• Version
• Currently 4
• IP v6 - see later
• Internet header length
• In 32 bit words
• Including options
• Type of service(next slide)
• Total length
• Of datagram, in octets

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Type of Service

• Precedence
• 8 levels
• Reliability
• Normal or high
• Delay
• Normal or low
• Throughput
• Normal or high

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Type of Service

• In the late 1990s, the IETF redefiend the meaning
  of the 8-bit SERVICE TYPE field to accommodate a
  set of differential services (DS).

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Header Fields (2)

• Total Length
• Identification
• Sequence number
• Used with addresses and user protocol to identify
  datagram uniquely
• Flags
• More bit
• Dont fragment

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Header Fields (3)

• Dont fragment indicator
• Can IP fragment data
• If not, may not be possible to deliver
• Send only
• Time to Live
• Protocol
• next higher layer to receive data field at
  destination
• Header checksum
• Reverified and recomputed at each router
• 16 bit ones complement sum of all 16 bit words in
  header
• Set to zero during calculation

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Header Fields (4)

• Source address
• Destination address
• Options(next slides)
• Padding
• To fill to multiple of 32 bits long

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Options

• Security
• Source routing
• Route recording
• Stream identification
• Timestamping

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Data Field

• Carries user data from next layer up
• Integer multiple of 8 bits long (octet)
• Max length of datagram (header plus data) 65,535
  octets

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Design Issues

• Routing (later)
• Fragmentation and re-assembly
• Datagram lifetime
• Error control
• Flow control

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Reassembly of Fragments

• Maximum Transfer Unit (MTU)

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Fragmentation Control

• Identification
• Flags
• Fragment Offset

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Time to Live (TTL)

• TTL specifies how long, in seconds, a datagram
  is allowed to remain in the internet system.

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Other Header Fields

• Protocol
• Header Checksum
• Source IP Address
• Destination IP Address
• Data

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Internet Datagram Options

• Record Route Option
• Use ping -R on taz.cs.wcupa.edu
• Source Route Options
• Timestamp Option
• Processing Options During Fragmentation

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Record Route Option
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Source Route Option
The strict source route option specifies an exact
route by giving a list of IP addresses the data
gram must follow.
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Timestamp Option
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Routing

• End systems and routers maintain routing tables
• Indicate next router to which datagram should be
  sent
• Static
• May contain alternative routes
• Dynamic
• Flexible response to congestion and errors
• Source routing
• Source specifies route as sequential list of
  routers to be followed
• Security
• Priority
• Route recording

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Datagram Lifetime

• Datagrams could loop indefinitely
• Consumes resources
• Transport protocol may need upper bound on
  datagram life
• Datagram marked with lifetime
• Time To Live field in IP
• Once lifetime expires, datagram discarded (not
  forwarded)
• Hop count
• Decrement time to live on passing through a each
  router
• Time count
• Need to know how long since last router
• (Aside compare with Logans Run)

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Fragmentation and Re-assembly

• Different packet sizes
• When to re-assemble
• At destination
• Results in packets getting smaller as data
  traverses internet
• Intermediate re-assembly
• Need large buffers at routers
• Buffers may fill with fragments
• All fragments must go through same router
• Inhibits dynamic routing

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IP Fragmentation (1)

• IP re-assembles at destination only
• Uses fields in header
• Data Unit Identifier (ID)
• Identifies end system originated datagram
• Source and destination address
• Protocol layer generating data (e.g. TCP)
• Identification supplied by that layer
• Data length
• Length of user data in octets

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IP Fragmentation (2)

• Offset
• Position of fragment of user data in original
  datagram
• In multiples of 64 bits (8 octets)
• More flag
• Indicates that this is not the last fragment

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Fragmentation Example
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Dealing with Failure

• Re-assembly may fail if some fragments get lost
• Need to detect failure
• Re-assembly time out
• Assigned to first fragment to arrive
• If timeout expires before all fragments arrive,
  discard partial data
• Use packet lifetime (time to live in IP)
• If time to live runs out, kill partial data

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Error Control

• Not guaranteed delivery
• Router should attempt to inform source if packet
  discarded
• e.g. for time to live expiring
• Source may modify transmission strategy
• May inform high layer protocol
• Datagram identification needed
• (Look up ICMP)

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Flow Control

• Allows routers and/or stations to limit rate of
  incoming data
• Limited in connectionless systems
• Send flow control packets
• Requesting reduced flow
• e.g. ICMP

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Chapter 8 IP Routing Overview

• Router perform IP forwarding as its main
  function
• Host a multi-homed host also forward IP datagrams

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Routing IP Datagrams

• Routing in an Internet
• Direct and Indirect Delivery
• Table-Driven IP Routing
• Next-Hop Routing
• Default Routers
• Host-Specific Routers
• The IP Routing Algorithms

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Datagram Delivery Over a Single Network

• A machine can send a frame directly to another
  machine on the same network.

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Datagram Delivery Over a Single Network

• How does a machine know if another machine is
  located in a directly-connected network?

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Indirect Delivery

• How does a machine deliver a datagram indirectly
  to another host?
• It encapsulate the datagram
• sends it to the nearest router
• The IP software on the router selects the next
  router towards the destination
• How does a router know where to send next?

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Indirect Delivery

• Table-Driven IP Routing
• Next-Hop Routing
• Default Routers
• Host-Specific Routes

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Indirect Delivery

• We ignored the routing table initialization and
  maintenance as network changes.

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Chapter 9ICMP

• Internet Control Message Protocol
• RFC 792 (get it and study it)
• Transfer of (control) messages from routers and
  hosts to hosts
• Feedback about problems
• e.g. time to live expired
• Encapsulated in IP datagram
• Not reliable

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ICMP Message Formats
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Chapter 10

• Subnetting and Routing

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Subnetting
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Routing in the Presence of Subnets

• The standard IP routing must be modified to work
  with subnet addresses.
• All hosts and routers that attach to the subnet
  must use the modified algorithms, called subnet
  routing.

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Subnet Routing

• A conventional routing table contains entries of
  the form (network address, next hop address).
• A subnetting routing table consists of entries of
  the form (subnet mask, network address, next hop
  address).

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