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3. INTERNETWORKING
(PART 3: IP)
Rocky K. C. Chang
Department of Computing
The Hong Kong Polytechnic University
18 February 2016
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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.
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1. The internetworking problem
Network 1 (Ethernet)
H7
H2
H1
H3
Network 4
(point-to-point)
Network 2 (Ethernet)
S1
S2
H4
Network 3 (FDDI)
H5
S3
H6
H8
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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.
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1.1 A layer-three internetworking solution
Network 1 (Ethernet)
H7
H2
H1
H3
Network 4
(point-to-point)
Network 2 (Ethernet)
R1
R2
H4
Network 3 (FDDI)
H5
R3
H6
H8
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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.
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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
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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.
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2.2 Address resolution protocol
0
8
16
Hardware type = 1
HLen = 48
PLen = 32
31
ProtocolType = 0x0800
Operation
SourceHardwareAddr (bytes 0 – 3)
SourceHardwareAddr (bytes 4 – 5) SourceProtocolAddr (bytes 0 – 1)
SourceProtocolAddr (bytes 2 – 3) TargetHardwareAddr (bytes 0 – 1)
TargetHardwareAddr (bytes 2 – 5)
TargetProtocolAddr (bytes 0 – 3)
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2.3 An internetworking example
• On each “hop or link,” both data encapsulation and
address resolution occur.
H1
H8
TCP
R1
IP
IP
ETH
R2
ETH
R3
IP
FDDI
FDDI
IP
PPP
PPP
TCP
IP
ETH
ETH
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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.
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3.1 Internet protocol suite (incomplete)
Application
FTP
Ping
DNS
HTTP
NV
TFTP
RTP
SSL
Transport
Network
UDP
TCP
ICMP
IGMP
IP
ARP & RARP
Data-link
NET1
NET2
…
NETn
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4. IP datagram
0
4
Version
8
HLen
16
TOS
31
Length
Ident
TTL
19
Flags
Protocol
Offset
Checksum
SourceAddr
DestinationAddr
Options (variable)
Data
Pad
(variable)
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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.
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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
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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.
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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.
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5. MTU and packet fragmentation
H1
ETH IP (1400)
R1
R2
FDDI IP (1400)
R3
H8
PPP IP (512)
ETH IP (512)
PPP IP (512)
ETH IP (512)
PPP IP (376)
ETH IP (376)
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Start of header
Ident = x
(a)
0
Offset = 0
Rest of header
1400 data bytes
Start of header
Ident = x
(b)
1
Offset = 0
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
0 Offset = 1024
Rest of header
376 data bytes
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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.
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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.
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6. IP subnets
• Subnetting for a class B address:
Network number
Host number
Class B address
1111111111111111 11111111
00000000
Subnet mask (255.255.255.0)
Network number
Subnet ID
Subnetted address
Host ID
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6. IP subnets
Subnet mask: 255.255.255.128
Subnet number: 128.96.34.0
128.96.34.15
128.96.34.1
H1
R1
Subnet mask: 255.255.255.128
Subnet number: 128.96.34.128
128.96.34.130
128.96.34.139
128.96.34.129
H2
R2
H3
128.96.33.1
128.96.33.14
Subnet mask: 255.255.255.0
Subnet number: 128.96.33.0
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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?
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7.1 Examples of routing tables
• For example, R1’s
• Network/Subnet
• 128.96.34.0
• 128.96.34.128
• 128.96.33.0
routing table:
• For example, H1’s
• Network/Subnet
• 128.96.34.0
• 0.0.0.0
routing table:
Subnet Mask Next Hop
255.255.255.128
upper int.
255.255.255.128
lower int.
255.255.255.0
128.96.34.129
Subnet Mask Next Hop
255.255.255.128
upper int.
0.0.0.0
128.96.34.1
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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.
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7.3 A general forwarding mechanism
D = Destination IP address
for each entry (Network/Subnet ID, Subnet Mask, Next Hop)
D1 = Subnet mask & D
if D1 = Network/Subnet ID
if Next Hop is an interface
deliver datagram directly to destination
else
deliver datagram to Next Hop (a router)
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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.
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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.
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8.1 Internet topology
Large corporation
“Consumer ” ISP
Peering
point
Backbone service provider
“ Consumer” ISP
Large corporation
Small
corporation
“Consumer”ISP
Peering
point
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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.
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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
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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.
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8.2.1 An example
B
C
A
D
E
F
G
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8.2.1 An example
• Node A’s routing table (using hop count as the cost)
1. Initially
Destination
A
Cost
0
Next hop
A
3. After convergence
Destination
A
B
2. After talking with its neighbors
C
D
Destination Cost
Next hop
E
A
0
A
F
B
1
B
G
C
1
C
E
1
E
F
1
F
Cost
0
1
1
2
1
1
2
Next hop
A
B
C
C
E
F
F
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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.
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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.
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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.
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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.
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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.
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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.
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8.3.1 An example
B
C
A
D
E
F
G
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8.3.1 An example
• Link state database:
From
A
A
A
A
A
A
B
B
B
B
B
B
:
:
To
B
C
D
E
F
G
A
C
D
E
F
G
:
:
Metric
1
1
1
1
1
1
1
1
1
1
1
1
:
:
Seq. Number
1
1
1
1
1
1
1
1
1
1
1
1
:
:
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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.
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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
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8.3.2 Link state updates
From
A
A
A
A
A
A
B
B
B
B
B
B
:
:
To
B
C
D
E
F
G
A
C
D
E
F
G
:
:
Metric
1
1
1
2
1
1
1
1
1
1
1
1
:
:
Seq. Number
1
1
1
2
1
1
1
1
1
1
1
1
:
:
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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:
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8.3.3 Computing optimal paths
B
C
D
A
E
F
G
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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.