1. ECE544: Communication Networks-II, Spring 2016
D. Raychaudhuri
Lecture 4
Includes teaching materials from L. Peterson

2. Today’s Lecture IP basics Routing principles
Service model, packet syntax
IP address structure
ARP, DHCP
IP packet forwarding
Routing principles
Bellman-Ford algorithm & distance vector (RIP)
Dijkstra algorithm and link state (OSPF)

3. IP Basics Best Effort Service Model Global Addressing Scheme
ARP & DHCP
IP Packet forwarding

4. IP Internet Concatenation of Networks Protocol Stack R1 ETH FDDI IP
Network 2 (Ethernet)
Network 1 (Ethernet) H6
Network 3 (FDDI)
Network 4
(point-to-point) H7 R3 H8 R1
ETH
FDDI IP
TCP R2
PPP R3 H1 H8

5. Service Model Connectionless (datagram-based)
Best-effort delivery (unreliable service)
packets are lost
packets are delivered out of order
duplicate copies of a packet are delivered
packets can be delayed for a long time
Datagram format V
ersion
HLen
TOS
Length
Ident
Flags
Offset
TTL
Protocol
Checksum
SourceAddr
DestinationAddr
Options (variable)
Pad
(variable) 4 8 16 19 31
Data

6. Fragmentation and Reassembly
Each network has some MTU
Strategy
fragment when necessary (MTU < Datagram)
try to avoid fragmentation at source host
re-fragmentation is possible
fragments are self-contained datagrams
use CS-PDU (not cells) for ATM
delay reassembly until destination host
do not recover from lost fragments

7. Example Ident = x Offset = 0 Start of header Rest of header
Rest of header
1400 data bytes
Ident
= x
Offset
= 0
Start of header 1
Rest of header
512 data bytes
= 512
= 1024
376 data bytes

8. Global Addresses Properties Dot Notation globally unique
hierarchical: network + host
Dot Notation
Network
Host 7 24 A: 14 16 1 B: 21 8 C:

9. Datagram Forwarding Strategy Example (R2) Network Number Next Hop
every datagram contains destination’s address
if directly connected to destination network, then forward to host
if not directly connected to destination network, then forward to some router
forwarding table maps network number into next hop
each host has a default router
each router maintains a forwarding table
Example (R2) Network Number Next Hop
1 R3
2 R1
3 interface 1
4 interface 0

10. Address Translation Map IP addresses into physical addresses
destination host
next hop router
Techniques
encode physical address in host part of IP address
table-based
ARP
table of IP to physical address bindings
broadcast request if IP address not in table
target machine responds with its physical address
table entries are discarded if not refreshed

11. ARP Details Request Format Notes
HardwareType: type of physical network (e.g., Ethernet)
ProtocolType: type of higher layer protocol (e.g., IP)
HLEN & PLEN: length of physical and protocol addresses
Operation: request or response
Source/Target-Physical/Protocol addresses
Notes
table entries timeout in about 10 minutes
update table with source when you are the target
update table if already have an entry
do not refresh table entries upon reference

12. ARP Packet Format T argetHardwareAddr (bytes 2 – 5)
argetProtocolAddr (bytes 0 3)
SourceProtocolAddr (bytes 2
Hardware type = 1
ProtocolT
ype = 0x0800
SourceHardwareAddr (bytes 4
argetHardwareAddr (bytes 0 1)
SourceProtocolAddr (bytes 0
HLen = 48
PLen = 32
Operation
SourceHardwareAddr (bytes 0 8 16 31

13. ATM ARP ATM ARP for mapping IP<->ATM addr
medium is not a broadcast type unlike Ethernet
requires servers which maintain ARP tables
concept of multiple “logical IP subnets” (LIS)

14. Dynamic Host Control Protocol (DHCP)
DHCP server per network for IP address assignment
Static list of IP<->physical addr or dynamic binding from common pool
Host boot-up via well-known address
DHCP “relay agent” can be used to avoid one server per network

15. Dynamic Host Control Protocol (DHCP)
DHCP packet format (runs over UDP)
Operation
HType
HLen
Hops
Xid
Secs
Flag
ciaddr
yiaddr
siaddr
giaddr
chaddr (16B)
....

16. Internet Control Message Protocol (ICMP)
Echo (ping)
Redirect (from router to source host)
Destination unreachable (protocol, port, or host)
TTL exceeded (so datagrams don’t cycle forever)
Checksum failed
Reassembly failed
Cannot fragment

17. Routing Basics

18. Routing Problem Network as a Graph
Problem: Find lowest cost path between two nodes
Factors
static: topology
dynamic: load

19. Two main approaches DV: Distance-vector protocols
LS: Link state protocols
Variations of above methods applied to:
Intra-domain routing (small/med networks)
RIP, OSPF
Inter-domain routing (large/global networks)
BGP-4

20. Distance Vector Protocols
Employed in the early Arpanet
Distributed next hop computation
adaptive
Unit of information exchange
vector of distances to destinations
Distributed Bellman-Ford Algorithm

21. Distance Vector Each node maintains a set of triples
(Destination, Cost, NextHop)
Exchange updates directly connected neighbors
periodically (on the order of several seconds)
whenever table changes (called triggered update)
Each update is a list of pairs:
(Destination, Cost)
Update local table if receive a “better” route
smaller cost
came from next-hop
Refresh existing routes; delete if they time out

22. Distributed Bellman-Ford
Start Conditions:
Each router starts with a vector of (zero) distances
to all directly attached networks
Send step:
Each router advertises its current vector to all
neighboring routers.
Receive step:
Upon receiving vectors from each of its neighbors,
router computes its own distance to each neighbor.
Then, for every network X, router finds that neighbor
who is closer to X than to any other neighbor.
Router updates its cost to X. After doing this
for all X, router goes to send step.

23. Example - initial distances1
Distance to node B C
Info at
node A B C D E 7 A 7 ~ ~ 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 ~ 2

24. E receives D’s routes 1 Distance to node B C Info at node A B C D E 77 ~ ~ 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 ~ 2

25. E updates cost to C 1 Distance to node B C Info at node A B C D E 7 A7 ~ ~ 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 4 2

26. A receives B’s routes 1 Distance to node B C Info at node A B C D E 77 ~ ~ 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 4 2

27. A updates cost to C 1 Distance to node B C Info at node A B C D E 7 A7 8 ~ 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 4 2

28. A receives E’s routes 1 Distance to node B C Info at node A B C D E 77 8 ~ 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 4 2

29. A updates cost to C and D 1 Distance to node B C Info at node A B C D7 A 7 5 3 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 4 2

30. Final distances 1 Distance to node B C Info at node A B C D E 7 A 6 56 5 3 1 A 8 2 B 6 1 3 5 C 5 1 2 4 1 2 D 3 3 2 2 D E E 1 5 4 2

31. Final distances after link failure1
Distance to node B C
Info at
node A B C D E 7 A 7 8 10 1 A 8 2 B 7 1 3 8 C 8 1 2 9 1 2 D 10 3 2 11 D E E 1 8 9 11

32. View from a node E’s routing table 1 Next hop B C dest A B D 7 A 1 145 B A 8 2 7 8 5 C 6 9 4 D 4 11 2 1 2 D E

33. Distance Vector Example2 R3 9 R5 R1 4 R2 1 4 R4
Info at Node R1 R2 R3 R4 R5 4 - 2 1 9

34. DV Example (cont.) Info at Node R1 R2 R3 R4 R5 4 6 8 - 2 3 11 1 9 104 6 8 - 2 3 11 1 9 10

35. DV Example (cont.) Info at Node R1 R2 R3 R4 R5 4 6 7 15 2 3 11 1 9 104 6 7 15 2 3 11 1 9 10

36. DV Example – after link R2-R3 breaks9 R5 R1 4 R2 1 4 R4
Info at Node R1 R2 R3 R4 R5 4 6 7 15 - 3 11 1 9 10

37. DV Example – after link R2-R3 breaks9 R5 R1 4 R2 1 4 R4
Info at Node R1 R2 R3 R4 R5 4 - 5 14 1 9 10

38. DV Example – after link R2-R3 breaks9 R5 R1 4 R2 1 4 R4
Info at Node R1 R2 R3 R4 R5 4 9 8 18 5 14 1 10

39. The bouncing effect dest cost dest cost 1 A 1 B A B 1 C 1 C 2 25 1 C

40. C sends routes to B dest cost dest cost A ~ B A B 1 C 1 C 2 25 1 C

41. B updates distance to A dest cost dest cost A 3 B A B 1 C 1 C 2 25 1 C

42. B sends routes to C dest cost dest cost A 3 B A B 1 C 1 C 2 25 1 C4 B 1

43. C sends routes to B dest cost dest cost A 5 B A B 1 C 1 C 2 25 1 C4 B 1

44. How are these loops caused?
Observation 1:
B’s metric increases
Observation 2:
C picks B as next hop to A
But, the implicit path from C to A includes itself!

45. Avoiding the Bouncing Effect
Select loop-free paths
One way of doing this:
each route advertisement carries entire path
if a router sees itself in path, it rejects the route
BGP does it this way
Space proportional to diameter
Cheng, Riley et al

46. Computing Implicit Paths
To reduce the space requirements
propagate for each destination not only the cost but also its predecessor
can recursively compute the path
space requirements independent of diameter v x z y w u

47. Distance Vector in Practice
RIP and RIP2
uses split-horizon/poison reverse
BGP/IDRP
propagates entire path
path also used for effecting policies

48. RIP Packet Format Command, Ver Zeros Net 1 Family Net 1 Addr
Net 1 Addr (cont.)
(net addr, distance pairs)
Distance to Net 1
Net 2 Family
Net 2 Addr
Updates sent ~30 sec
Distance to Net 2 ……

49. Link State Routing
Each node assumed to know state of links to its neighbors
Step 1: Each node broadcasts its state to all other nodes
Step 2: Each node locally computes shortest paths to all other nodes from global state

50. Link State Routing: Building blocks
Reliable broadcast mechanism
flooding
sequence number issues
Shortest path tree (SPT) algorithm
Dijkstra’s SPT algorithm

51. Link state packets (LSPs)
Periodically, each node creates a Link state packet containing:
Node ID
List of neighbors and link cost
Sequence number
Time to live (TTL)
Node outputs LSP on all its links

52. Reliable flooding When node i receives LSP from node j:
If LSP is the most recent LSP from j that i has seen so far, i saves it in database and forwards a copy on all links except link LSP was received on.
Otherwise, discard LSP.

53. SPT algorithm (Dijkstra)
for all nodes v
if v adjacent to a then D(v) = cost (a, v)
else D(v) = infinity
Loop
find w not in SPT, where D(w) is min
add w in SPT
for all v adjacent to w and not in SPT
D(v) = min (D(v), D(w) + C(w, v))
until all nodes are in SPT

54. Link State Algorithm Flooding: 1) Periodically distribute link-state
advertisement (LSA) to neighbors
- LSA contains delays to each
neighbor
2) Install received LSA in LS database
3) Re-distribute LSA to all neighbors
Path Computation
1) Use Dijkstra’s shortest path algorithm
to compute distances to all destinations
2) Install <destination, nexthop> pair in
forwarding table

55. Forward Search Algorithm (Dijkstra)
Initialize Confirmed list with self entry 0
For node just added to confirmed list, call it “next” and select is LSP
Calculate distance to each neighbor of next
If neighbor is not on either confirmed or tentative list, add (neighbor, cost, nexthop) to tentative list
If neighbor is current on tentative list and cost is less than currently cost, then replace current entry with (neighbor, cost, nexthop)
If tentative list is empty stop, otherwise pick entry from tentative list with lowest cost and move to confirmed list. Return to step 2

56. Dijkstra/OSPF Method 1 Step # Confirmed Tentative 1 (R1,0,-) - 29 R5 R1 4 R2 1 4 R4
Step #
Confirmed
(R#, cost, next hop)
Tentative 1
(R1,0,-) - 2
(R2,4,R2) 3
(R3,6,R2)
(R4,8,R2)

57. Dijkstra/OSPF Method 1 Step # Confirmed Tentative R3 R5 R1 R2 R49 R5 R1 4 R2 1 4 R4
Step #
Confirmed
(R#, cost, next hop)
Tentative 4
(R1,0,-)
(R2,4,R2)
(R3,6,R2)
(R4,8,R2)
(R4,7,R3) 5
(R5,15,R3)

58. Dijkstra/OSPF Method 1 Step # Confirmed Tentative R3 R5 R1 R2 R49 R5 R1 4 R2 1 4 R4
Step #
Confirmed
(R#, cost, next hop)
Tentative 6
(R1,0,-)
(R2,4,R2)
(R3,6,R2)
(R4,7,R3)
(R5,15,R3)

59. Dijkstra SPT Method 2 5 3 B C 2 5 1 A 2 F 3 1 2 D E 1 B C D E F

60. Dijkstra SPT Method 2 5 3 B C 2 5 1 A 2 F 3 1 2 D E 1 B C D E F

61. Dijkstra SPT Method 2 5 3 B C 2 5 1 A 2 F 3 1 2 D E 1 B C D E F

62. Dijkstra SPT Method 2 5 3 B C 2 5 1 A 2 F 3 1 2 D E 1 B C D E F

63. Dijkstra SPT Method 2 5 3 B C 2 5 1 A 2 F 3 1 2 D E 1 B C D E F

64. Dijkstra SPT Method 2 5 3 B C 2 5 1 A 2 F 3 1 2 D E 1 B C D E F

65. Link State in Practice OSPF (Open Shortest Path First Protocol)
most commonly used routing protocol in the Internet
support for authentication, addl hierarchy, load balancing

66. OSPF Packets LS Age Options T ype=1 Link state ID Advertising router
LS sequence number
LS checksum
Length
Flags
Number of links
Link ID
Link data
Link type
Num_TOS
Metric
Optional TOS information
More links

67. Link State Characteristics
With consistent LSDBs, all nodes compute consistent loop-free paths
Limited by Dijkstra computation overhead, space requirements
Can still have transient loops B 1 1 3 A C
Packet from C->A
may loop around BDC 5 2 D

68. OSPF Sequencing and Aging
32-bit sequence number field, does not wrap
LSP’s compared on basis of sequence number
LSP’s purged after about an hour
Synchronized expiration of LSPs
expired LSP reflooded with age zero
On startup, router need not wait
can start with lowest sequence number
will be informed if its own LSP is in network

69. Problem: Router Failure
A failed router and comes up but does not remember the last sequence number it used before it crashed
New LSPs may be ignored if they have lower sequence number

70. One solution: LSP Aging
Nodes periodically decrement age (TTL) of stored LSPs
LSPs expire when TTL reaches 0
LSP is re-flooded once TTL = 0
Rebooted router waits until all LSPs have expired
Trade-off between frequency of LSPs and router wait after reboot

71. Today’s Homework Peterson & Davie - 4.13,14,16,17,21 (4th ed)
Download and browse RIP and OSPF RFC’s
Due on Fri (2/19)