Internet Routing: The goal is to find any route that is loop freeGlobal optimization is a distant dr

1
Lecture 18/19
Internet Routing The goal is to find any route
that is loop free-Global optimization is a
distant dream depending on economic and political
drivers Homework 4.1-35, 37-45
2
Distance-vector routing As routing table just
after link A-E failure
3
A
What happens when G advertises its table to D?
How does D get the message that A-E has failed?
F
G
4
Loops -A

• A advertises E, to B and B advertises E, 2
  to A (they cross intransit)
• A advertises E,3 and B advertises E,
• etc.

• B resets E, , - (since A is next hop) A
  updates E, 3, B since 3lt
• B updates E, 4,A and A resets E, , - since B
  is next hop
• etc.

Split horizon B should not advertise a route E,
2 it got from A (in the first step)
5
Loops -B

• A advertises E, to B C
• Advertisement to C is delayed
• B may advertise E, to C
• C advertises E, 2 to B
• As advertisement E, arrives at C
• B advertises E,3 to A
• C advertises E, to B
• A advertises E,4 to C
• B advertises E, to A
• C advertises E,5 to B

• B resets E, , - since A is next hop
• C will not update (why)
• B updates E, 3, C since 3lt
• C updates E, , - since A is next hop
• A updates E,4, B since 4lt
• B updates E, , - since C is next hop
• C updates E, 5, A since 5lt
• A updates E, , - since B is next hop
• B updates E, 6, A since 6lt

Split horizon will not solve this problem
6
Link State Routing
You have a global view routing table is spanning
tree as seen from root node
7
The Internet circa 1990 A hierarchical
collection of autonomous systems (AS)
8
Todays multiple backbone
Stub AS Multi-homed AS Transit AS
9
Inefficiency of IP address classes

• If you have 257 end users, you need class B and
  then you have 16K addresses. We need finer
  distinctions.
• Two issues
• How do you give different network addresses to
  physical networks within 1 class A, B or C
  network---subnetting
• How do you aggregate networks within an domain to
  simplifier routing outside the domain--supernettin
  g

10
(No Transcript)
11
(No Transcript)
12
Forwarding Table of R1
13
More Subnetting

• You can break the same physical network into
  subnetsforcing hosts to speak through a router
• Each host has its own subnet in the new
  engineering network

14
Classless Interdomain Routing (CIDR)

• We can get better utilization if we hand out
  Class C addresses
• This would increase the size of forwarding tables
• We aggregate contiguous class C blocks

15
Aggregation of 16 Class C network addresses into
a single 20 bit CIDER address
16
(No Transcript)
17
Intradomain Routing
Border Gateway RoutersDefault router for
outbound traffic
18
Intradomain routing issues

• Scale100,000 network addresses
• Autonomy
• Trust
• Flexibilityhot potato routing

19
BGP example
1 BGP speaker/AS Advertise routes-prevents
looping Withdrawn broken routes
20
Routing Areas in OSPF
21
End-to-end protocols-the transport layer

• Application requirements
• Delivery guarantee
• Ordered delivery
• Exactly 1 copy
• Any message size
• Synchronization between sender and receiver
• Support multiple applications on each host

• Network services
• Drop messages
• Reorder messages
• Duplicate copies
• Size limitations
• Long delays

22
Simple DemuxChecksum--UDP
23
UDP packet delivery
24
Transport Control Protocol

• Reliable, in-order delivery of a byte stream
• Full-duplex
• Flow control
• Demux
• Congestion-control (next semester)

25
TCP sliding window issues

• Indirect connection between hosts explicit
  connection establishment/tear down phases
• Variable RTTs-adaptive timouts for retransmission
• Packet Reordering and Long Delays-large sequence
  numbers
• Variable host resources-exchange of system
  information
• Congestion

26
TCP bytestream management
27
TCP Simplified
28
TCP flags

• SYN
• FIN
• RESET
• PUSH
• URG
• ACK

29
TCP 3-way handshake for connection establishment
and tear-down