CSCE 515: Computer Network Programming

1
CSCE 515Computer Network Programming

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

2
IP Routing

• Forward datagrams generated either on local host
  or on some other hosts toward their ultimate
  destination
• Static routing when network is small, single
  connection point to other networks, no redundant
  route existent
• Dynamic routing use routing daemon to run
  routing protocol in order to communicate with
  other routers

3
Routing Principles

• IP performs three steps when searching its
  routing table
• Search for a matching host address
• Search for a matching network address
• Search for a default entry

4
Initialize a Routing Table

• Three ways to create routing table entries
• Created by default when an interface is
  configured
• Added by route command
• Created by an ICMP redirect

5
route Command

• Explicitly add or remove routing table entry from
  configuration files at bootstrap time

6
ICMP Redirect Error

• Sent by a router to sender of an IP datagram when
  the datagram should have been sent to a different
  router
• Used only when the host has a choice of routers
  to send its datagram to

7
ICMP Redirect
0
7
8
15
16
31
type(5)
code(0-3)
checksum
router IP address that should be used
IP header first 8 bytes of original datagram
data
8
ICMP Router Solicitation
0
7
8
15
16
31
type(10)
code(0)
checksum
unused (sent as 0)
9
ICMP Router Advertisement
0
7
8
15
16
31
type(9)
code(0)
checksum
number of addresses
address entry size(2)
lifetime
router address 1
preference level 1
router address 2
preference level 2

10
Routing Protocols

• Executed by routing daemon to communicate routing
  information with other routers
• Two types of routing protocols
• Distance-vector protocols
• Link-state protocols

11
Distance-vector Protocols

• Maintain a vector of distances
• Each router updates its routing table based on
  vector of distances received from neighbors
• Example RIP

12
Link-state Protocols

• Each router maintains a complete routing table of
  the network
• Example OSPF

13
Routing Information Protocol (RIP)

• Most widely used routing protocol
• Carried in UDP datagrams

14
RIP Message Format
0
7
8
15
16
31
command(1-6)
version(1)
(must be 0)
(must be 0)
address family(2)
32-bit IP address
(must be 0)
20 bytes
(must be 0)
metric(1-16)
(up to 24 more routes)
15
RIP Metrics

• RIP uses hop count as its metric
• If there are multiple paths, router chooses the
  one with smallest hop count, and ignores other
  paths

16
RIP Operation

• Initialization
• Request received
• Response received
• Regular routing updates
• Triggered updates

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Problems with RIP

• Has no knowledge about subnet addressing
• Take long time to stabilize after a router or
  link failure
• Maximum of metric limits network size
• No security protection

18
RIP Version 2

• Fix some deficiencies of RIP
• Support multiple domain
• Include subnet mask
• Some simple authentication scheme added

19
RIP-2 Message Format
0
7
8
15
16
31
command(1-6)
version(2)
routing domain
route tag
address family(2)
32-bit IP address
32-bit subnet mask
20 bytes
32-bit next-hop IP address
metric(1-16)
(up to 24 more routes)
20
Next Class

• Routing protocols OSPF, BGP
• UDP
• Read TI Ch. 10, 11