CSCE 515: Computer Network Programming

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CSCE 515Computer Network Programming

• Chin-Tser Huang
• huangct_at_cse.sc.edu
• University of South Carolina

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Open Shortest Path First (OSPF)

• A link-state routing protocol
• Each router tests status of all its links and
  broadcasts its link status to all routers on
  network
• Use IP to carry its message
• Provide features superior to RIP

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OSPF Message Format
1
1
2
4
4
2
2
8
ver- sion
type
packet length
router ID
area ID
checksum
auth type
authentication
data
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Autonomous Systems

• A collection of networks administered by single
  entity, e.g. a corporation or a campus
• Three categories of AS
• Stub AS
• Multihomed AS
• Transit AS

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Example of Autonomous System
Source Internetworking Technologies Handbook by
Cisco Press
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IGP and EGP

• Each AS selects its interior gateway protocol
  (IGP) for communications between routers in this
  AS
• E.g. RIP, OSPF
• Multiple ASs use exterior gateway protocol (EGP)
  for communications between routers in different
  ASs
• E.g. EGP, BGP

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Border Gateway Protocol (BGP)

• An exterior gateway protocol
• Distance-vector protocol but enumerates route to
  each destination
• Allow policy-based routing
• Use TCP to transport its messages

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Classless Interdomain Routing (CIDR)

• Solve two problems
• Shortage of class B network IDs
• Explosion in routing table size due to allocation
  of multiple class C network IDs
• Summarize multiple IP addresses into a smaller
  number of routing table entries

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Requirements of CIDR

• Multiple IP addresses to be summarized need to
  share same high-order bits
• Routing decisions need to be based on 32-bit IP
  address and 32-bit mask
• Routing protocols need to carry 32-bit mask in
  addition to 32-bit address

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Example of CIDR

• CIDR Block Prefix Equivalent Class C of Host
  Addresses
• /27 1/8th of a Class C 32 hosts
• /26 1/4th of a Class C 64 hosts
• /25 1/2 of a Class C 128 hosts
• /24 1 Class C 256 hosts
• /23 2 Class C 512 hosts
• /15 512 Class C 131,072 hosts
• /14 1,024 Class C 262,144 hosts
• /13 2,048 Class C 524,288 hosts

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User Datagram Protocol (UDP)

• A datagram-oriented transport layer protocol
• Each output operation by a process produces
  exactly one UDP datagram
• Provide no reliability

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UDP Header
0
15
16
31
8 bytes
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UDP Checksum

• Cover UDP header and UDP data
• End-to-end checksum calculated by sender and
  verified by receiver
• Should always be enabled
• 16-bit ones complement sum of header (including
  a pseudo header) and data is calculated

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Computation of UDP Checksum
0
15
16
31
source IP address
destination IP address
UDP pseudo header
UDP length
zero
protocol(17)
destination port number
source port number
UDP header
UDP checksum
UDP length
data (if any)
pad byte(0)
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Fragmentation

• Need to fragment if size of resulting IP datagram
  exceeds MTU
• Can take place at source or at an intermediate
  router
• Fragments are reassembled at destination

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Example of UDP Fragmentation
IP header
UDP header
UDP data
20
8
1473
IP header
UDP header
IP header
20
8
1472
20
1
17
ICMP Unreachable Error (Fragmentation Required)
0
7
8
15
16
31
type(3)
code(0-15)
checksum
unused (must be 0)
MTU of next-hop network
IP header first 8 bytes of original datagram
data
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Next Class

• UDP
• Broadcasting and multicasting
• Class MulticastSocket
• Read TI Ch. 11, 12, JNP Ch. 22