1. 3. Internetworking (part 3: IP)
Rocky K. C. Chang
Department of Computing
The Hong Kong Polytechnic University
21 February 2017

2. 1. The internetworking problem
Problem: How to interconnect heterogeneous networks effectively?
Three problems with interconnection at the data-link layer:
Do not scale to the number of data-link technologies.
Do not scale to the number of hosts (or networks).
Do not have a common addressing space.

3. 1. The internetworking problem
Network 1 (Ethernet) H7 S3 H8 H1 H2 H3
Network 4
(point-to-point)
Network 2 (Ethernet) S1 S2 H4
Network 3 (FDDI) H5 H6

4. 1.1 A layer-three internetworking solution
Use IP, XNS, IPX, etc on top of the networks.
Replace LAN switches with layer-three switches, more commonly known as routers.
Add IP software to each end host (with the whole protocol suite software).
Assign an IP address to each network interface.

5. 1.1 A layer-three internetworking solution
Network 1 (Ethernet) H7 R3 H8 H1 H2 H3
Network 4
(point-to-point)
Network 2 (Ethernet) R1 R2 H4
Network 3 (FDDI) H5 H6

6. 2. Encapsulation and address binding
To transmit IP datagrams over any data-link network, two requirements are needed:
A standard way to encapsulate IP datagrams
Address resolution between IP addresses and MAC addresses
Standard RFCs for specifying datagram encap-sulations and possibly address resolutions, e.g., Ethernet (RFC 894), IEEE 802 (RFC 1042), etc.
A shared medium uses an Address Resolution Protocol (ARP) for address binding.

7. 2.1 Data encapsulation You have seen from chapter 2
IP over DIX Ethernet
IP over IEEE 802.3
IP over PPP
Others are in the RFC documents

8. 2.2 Address resolution protocol
An ARP request message is data-link broadcasted on the LAN with the target IP address.
Every IP host picks up a copy of the message and examine the target IP address.
If matching its IP address, send an ARP reply message back to the sender with its MAC address.
Else, drop the message.
To reduce broadcast traffic, each host uses an ARP cache to remember the recent binding.

9. 2.2 Address resolution protocol8 16 31
Hardware type = 1
ProtocolT
ype = 0x0800
HLen = 48
PLen = 32
Operation
SourceHardwareAddr (bytes 0 – 3)
SourceHardwareAddr (bytes 4 – 5)
SourceProtocolAddr (bytes 0 – 1)
SourceProtocolAddr (bytes 2 – 3) T
argetHardwareAddr (bytes 0 – 1) T
argetHardwareAddr (bytes 2 – 5) T
argetProtocolAddr (bytes 0 – 3)

10. 2.3 An internetworking example
On each “hop or link,” both data encapsulation and address resolution occur. H1 H8
TCP
TCP R1 R2 R3 IP IP IP IP IP
ETH
ETH
FDDI
FDDI
PPP
PPP
ETH
ETH

11. 3. The IP service model The IP service model consists of
an addressing scheme to identify an IP host, and
a datagram (connectionless) model of data delivery.
IP provides a best-effort service.
IP makes its best effort to send a datagram to its destination.
The best-effort service does not guarantee reliable datagram delivery, i.e., an unreliable service.

12. 3.1 Internet protocol suite (incomplete)
Application
Ping
FTP
HTTP
DNS NV
TFTP
RTP
SSL
Transport
TCP
UDP
Network
ICMP IP
IGMP
ARP & RARP
Is ARP a data-link or network layer protocol? In terms of encapsulation, it is on the network layer, above the data-link. But in terms of service, it provides the binding service to IP.
Data-link …
NET
NET
NET 1 2 n

13. 4. IP datagram 4 8 16 19 31 V ersion HLen TOS Length Ident Flags4 8 16 19 31 V
ersion
HLen
TOS
Length
Ident
Flags
Offset
TTL
Protocol
Checksum
SourceAddr
DestinationAddr
Pad
Options (variable)
(variable)
Data

14. 4. IP datagram Version: 4 for the current IP.
Type of service (TOS) for specifying how a router should handle this datagram.
Header length handles a variable-length header.
20-byte IP header without IP options
A 16-bit length limits the size of an IP datagram to 65,535 bytes, including the IP header.
Identification, flags, and offset are used for packet fragmentation and reassembly.

15. 4. IP datagram
Time to live (TTL) limits the the number of times that a datagram processed by routers.
Protocol specifies the type of payload, e.g., 6 for TCP and 17 for UDP.
Checksum is a 16-bit word checksum.
IP options, e.g.,
Source routing
Record route

16. 5. MTU and packet fragmentation
Each network chooses a maximum packet size that can be sent on it, Maximum Transmission Unit (MTU). For example,
1500 bytes for 10-Mbps Ethernet
4352 bytes for FDDI
17914 bytes for 16-Mbps token ring
Note that all MTUs are smaller than IP datagram’s maximum size.
One internetworking problem is to accommodate various MTU values.

17. 5. MTU and packet fragmentation
To send datagrams to a directly attached host, use the network’s MTU.
To send datagrams to a nondirectly attached host, use the path MTU.
Path MTU is the minimum of the networks’ MTUs on the path from the source to destination.
If the actual MTU used is larger than the path MTU, packet fragmentation occurs.
Fragmentation occurs when a router attempts to forward it to a network with a smaller MTU.

18. 5. MTU and packet fragmentationH1 R1 R2 R3 H8
ETH IP
(1400)
FDDI IP
(1400)
PPP IP
(512)
ETH IP
(512)
PPP IP
(512)
ETH IP
(512)
PPP IP
(376)
ETH IP
(376)

19. Start of header
Ident
= x
Offset
= 0
(a)
Rest of header
1400 data bytes
Start of header
Ident
= x 1
Offset
= 0
(b)
Rest of header
512 data bytes
Start of header
Ident
= x 1
Offset
= 512
Rest of header
512 data bytes
Start of header
Ident
= x
Offset
= 1024
Rest of header
376 data bytes

20. 5. MTU and packet fragmentation
Each IP fragment contains enough information for forwarding to the destination.
A fragmented IP datagram will be reassembled only at the destination node.
If any fragments do not arrive within a certain time, other received fragments in the datagram will be discarded.
Fragmentation could occur multiple times to an IP datagram.

21. 6. IP subnets
IP subnets introduce additional levels within an IP network:
A network address, a subnet ID, and a host ID.
IP subnets offer flexibility in allocating addresses to different sizes of sub-networks.
A subnet mask is used to indicate which bits are referred to the network and subnet ID.
Each network interface stores subnet mask and its unicast IP address.

22. 6. IP subnets Subnetting for a class B address: Network number
Host number
Class B address
Subnet mask ( )
Network number
Subnet ID
Host ID
Subnetted address

23. 6. IP subnets Subnet mask: 255.255.255.128 Subnet number: 128.96.34.0 H1 R1
Subnet mask:
Subnet number: H2 R2 H3
Subnet mask:
Subnet number:

24. 7. IP forwarding mechanisms
Assume that both routers and hosts already have appropriate routing tables in place.
Routing tables for routers are constructed from routing protocols.
Routing tables for hosts are constructed from other means.
Problem: Given a routing table, how do hosts and routers forward datagrams?

25. 7.1 Examples of routing tables
For example, R1’s routing table:
Network/Subnet Subnet Mask Next Hop
upper int.
lower int.
For example, H1’s routing table:

26. 7.2 Host’s forwarding mechanisms
A host sends a datagram to another host on the same LAN or not.
In the former, it sends the datagram to the destination directly.
In the latter, it sends the datagram to a default router.
In both cases, the host uses ARP cache or ARP to find out the corresponding MAC addresses.

27. 7.3 A general forwarding mechanism

28. 7.4 Characteristics of IP forwarding
Both hosts and routers are involved in forwarding.
Compared with routers, a host makes a much simpler binary decision.
IP forwarding is done on a hop-by-hop basis.
It is assumed that the next-hop router is really closer to the destination.
IP forwarding is able to specify a route to a network, and not have to specify a route to every host.

29. 8. The routing problem
Problem: How does a router construct its routing table for IP forwarding?
Forwarding vs routing
Routing is the process by which forwarding tables are built.
Forwarding table vs routing table
A routing table is built by routing protocols as a precursor to building the forwarding table.
A forwarding table consists of detail enough information to speed up datagram forwarding.

30. 8.1 Internet topology Large corporation “ Consumer ” ISP Peering point
Backbone service provider
Peering
point “
Consumer ”
ISP “
Consumer ”
ISP
Large corporation
Small
corporation

31. 8.1 Internet topology Major components in the Internet topology:
Autonomous system (AS), e.g., polyu.edu.hk, ibm.com, etc.
Internet service providers (ISPs): Local ISPs, regional ISPs, National ISPs, Backbone ISPs.
Exchange networks: For local traffic interchange, e.g., HKIX.
Some special networks, like Harnet in Hong Kong.
Routers (plus other networks) are usually used to connect these components together.

32. 8.1 Not all routers are equal
Interior routers: Only know how to route datagrams to destinations within the same AS.
Border routers: Interface between its AS and other AS:
A nonbackbone router usually has a “default route” to another “more knowledgeable” router for “unknown destinations.”
A backbone router is supposed to know every IP network in the Internet.
Intradomain routing vs Interdomain routing

33. 8.2 Distance vector routing protocols
Each node does two things:
It constructs a one-dimensional array (a vector) containing the “distances” (costs) to all other nodes.
It distributes the vector to its immediate neighbors.
Each node’s vector initially consists of
a distance of 0 for reaching itself, and
a distance of infinity for reaching other nodes.
When the algorithm converges, each node knows for each destination node
(1) the next node closer to the destination, and
(2) the associated cost for this path.

34. 8.2.1 An example

35. 8.2.1 An example
Node A’s routing table (using hop count as the cost)

36. 8.2.2 Dynamic routing
Each node periodically sends its distance vector to its neighbor (periodic updates).
If link A-C fails,
The cost in A’s entry to C becomes infinity.
B will advertise to A a path to C with cost 1.
F will advertise to A a path to C with cost 2.
Therefore, A’ entry to C is updated to: Next hop = B and cost = 2.

37. 8.2.2 Dynamic routing
Each node may send an updated distance vector to its neighbor, triggered by external events (triggered updates).
If link A-C fails,
The cost in A’s entry to C becomes infinity.
A will immediately send its updated vector to B, E, F.
This update does not affect B’s routing table.
However, E will update its entry to C from 2 to infinity, and then from infinity to 3; and similarly for F.

38. 8.2.3 Routing loops If the link A-E fails,
The corresponding entry in A is updated.
A triggered update from A, and periodic updates from B, C, and F.
Possible timing (>: earlier than):
Case 1: A > B and A > C and A > F
Case 2: A > B and A > C but A < F
Case 3: A > B and A > F, but A < C
In case 1, all nodes will eventually conclude that E is unreachable.
In case 2, a routing loop between A and F forms.

39. 8.2.3 Routing loops
In case 3, a routing loop between A and C forms.
In both cases 2 and 3, the cost to E keeps on increasing.
One solution to this problem is to declare the link unusable when the cost reaches, say, 16 (count to infinity).
Split horizon is another solution to solving 2-node routing loop.
A node will not advertise a route back to another node that serves as the next hop for that route.
For example, B, C, F will not advertise their routes to E back to A.

40. 8.2.4 Routing information protocol (RIP)
RIP implements the distance vector approach.
A hop count of 16 is interpreted as infinity.
Each RIP router broadcasts its distance vectors to its neighbors every 30 seconds.
RIP is implemented at the application level.
Common daemons used on the Unix systems are the programs routed and gated.
RIP packets are carried over UDP and IP.

41. 8.3 Link state routing protocols
In this approach, every nodes maintains the network topology information in a link state database.
Thus, this approach relies on two mechanisms:
A reliable flooding for dissemination of link-state information, and
a shortest-path algorithm for computing routes.

42. 8.3.1 An example

43. 8.3.1 An example
Link state database:

44. 8.3.2 Link state updates
The link state can be based on any metric, including hop count, latency, throughput, monetary cost, etc.
When a link state is changed, say from 1 to 2 for AE, A will send this update to all other nodes through a reliable flooding scheme.
A sends the update to B, C, F.
A ensures the reliable transmission of the update through positive acknowledgment and retransmission.

45. 8.3.2 Link state updates
B, C, F, upon receiving the update, compare the sequence number of the update and that in their databases.
If the sequence number in the update is higher, update the link state in the database, and forward it to other interfaces other than the one where the update is received.
Otherwise, drop the update and no change in the database.
Although C receives two copies of the update, it forwards only one copy to D and the other is discarded.
The new link state database becomes

46. 8.3.2 Link state updates

47. 8.3.3 Computing optimal paths
Given a link state database for the network topology, each node can apply any shortest-path algorithms to find optimal paths from itself to other nodes in the network.
For example, using the hop count as the metric, we have for node A:

48. 8.3.3 Computing optimal pathsB C D A E F G

49. 8.3.4 Open shortest path first (OSPF) protocol
OSPF implements a link state approach.
OSPF supports different type-of-service routing by having different sets of metric for route computation.
OSPF supports equal-cost routes to a destination.
OSPF reduces the amount of routing update messages as compared with RIP.
OSPF provides fast and loopless convergence.