Computer Networks
The Network Layer
1
Main Functions
• Routing.
• Forwarding.
2
Design Issues
• Services provided to transport layer.
• How to design network-layer protocols.
3
Store-and-Forward Packet Switching
Subnet
fig 5-1
. Host sends packet to nearest router.
. Packet forwarded to next router.
. Until packet reaches destination.
4
Services
• What kind of services provided to transport
layer?
• Connection-oriented versus connectionless
service?
5
Connectionless Service
• Datagram network.
• “Move all intelligence to the edges”.
– Routers just route.
– Everything else should be done end-to-end.
• No ordering, no flow/congestion control, no
reliable delivery.
• Best-effort service model.
• Packets are routed independently…
• E.g., Internet.
6
Connection-Oriented Service
•
•
•
•
Virtual circuit networks.
`A la telephone network.
Reliable, ordered service.
Virtual connection established from source to
destination.
• E.g., X-25, ATM.
7
Datagram Network Operation
• How does it work?
• Data from transport layer is broken into
packets, or datagrams.
• Network layer at host adds network-layer
header and forwards packets to directlyconnected router.
8
Datagram Network: Example
• Routing within a diagram subnet.
9
Virtual Circuit Network Operation
• Connection-establishment before sending
data.
– All traffic for that connection follows same
route.
10
Virtual Circuit Network: Example
• Routing within a virtual-circuit subnet.
11
Virtual-Circuit versus Datagram Subnets
5-4
12
Routing
13
Routing
• One of the main functions of network layer.
• Routing versus forwarding?
• Datagram versus VC networks?
14
Routing Algorithm
• Computes routing tables.
• Properties:
–
–
–
–
Correctness.
Robustness.
Stability.
Optimality.
• Try to optimize a certain metric.
15
Optimality Principle
• General statement about optimal routes
(topology, routing algorithm independent).
• If router J is on optimal path between I
and K, then the optimal path from J to K
also falls along the same route.
– Proof by contradiction.
• Corollary:
– Set of optimal routes from all sources to
destination form a tree rooted at destination.
– Sink tree.
16
Types of Routing Algorithms
• Non-adaptive versus adaptive.
17
Adaptive and Non-adaptive Routing
• Non-adaptive routing:
– Fixed routing, static routing.
– Do not take current state of the network (e.g.,
load, topology).
– Routes are computed in advance, off-line, and
downloaded to routers when booted.
• Adaptive routing:
– Routes change dynamically as function of current
state of network.
– Algorithms vary on how they get routing
information, metrics used, and when they change
routes.
18
Static Algorithms
(Non-Adaptive)
1.Shortest-path routing.
2.Flooding.
19
Shortest-Path Routing
• Problem: Given a graph, where nodes
represent routers and edges, links, find
shortest path between a given pair of nodes.
• What is shortest in shortest path?
– Depends on the routing metric in use.
– Example: number of hops (static), geographic
distance (static), delay, bandwidth (raw versus
available), combination of a subset of these.
• Dijkstra’s shortest-path algorithm (1959).
20
Dijkstra’s Shortest-Path Algorithm
• Initially, links are assigned costs.
• As the algorithm executes, nodes are labeled
with its distance to source along best known
path.
• Initially, no routes known, so all nodes are
labeled with infinity.
• Labels change as the algorithm proceeds.
• Labels can be temporary or permanent.
– Initially all labels are tentative.
– A label becomes permanent if it represents the
shortest path from the source to the node.
21
Shortest Path Routing
Find shortest-path from A to D:
Label each adjacent node
with distance to
A.
Start
B is made
permanent.
22
Flooding
• Every incoming packet forwarded on every
outgoing link except the one it arrived on.
• Problem: duplicates.
• Constraining the flood:
– Hop count.
– Keep track of packets that have been flooded.
• Robust, shortest delay (picks shortest path as
one of the paths).
23
Flooding: Example
•Stallings Figure 12.4
(hop-count=3)
24
Dynamic Routing Algorithms
•
(Adaptive Routing)
– Distance vector routing.
– Link state routing.
25
Distance Vector Routing
• Aka, Bellman-Ford (1957), Ford-Fulkerson
(1962).
• Original ARPANET routing; also used by
Internet’s RIP.
• Each router keeps routing table (or routing
vector) with best known distance to each
destination and corresponding outgoing
interface.
• Routing tables are updated by exchanging
routing information with neighbors.
26
Distance Vector (Cont’d)
• Routing table at each router:
– One entry per participating router.
– Each entry contains outgoing interface and distance to
corresponding destination.
– Metric: number of hops, delay, queue length.
– Each router knows distance to its neighbors.
• Old ARPANET algorithm: DV where cost metric
is outgoing link queue length.
27
Distance Vector Routing
•
(a) A subnet. (b) Input from A, I, H, K, and the
new routing table for J.
28
Routing Updates
•
Every T interval, routers exchange
routing updates.
Routing update from router X consists of
a vector with all destinations and the
corresponding distance from X to them.
When router Y receives an update from
X, it can estimate its distance to router Z
through X as Dyz = Dyx + Dxz.
Router Y receives update from all its
neighbors and builds a new RT.
•
•
•
29
Distance Vector: Example
3
2
5
2
2 9
1
1
4
79
3
3
1
6
5 2
Node Distance Next
1
0
-
2
3
2
3
2
4
4
5
6
1
2
4
4
4
4
T=T2
1
Node Distance Next
1
0
-
2
3
2
5
2
3
4
5
6
1
6
8
4
3
3
T=T0
2
3
4
3
0
7
5
4
0
2
1
3
2
3
2
3
5
2
0
1
3
T=T1
30
Problems
1.Routing loops.
2.Slow convergence.
3.Counting to infinity.
31
Count-to-Infinity
• Good news propagate faster.
A
Initially, A down:
A comes up:
B
infinity
1
1
1
1
C
D
E
infinity infinity infinity
infinity infinity infinity
2
infinity infinity
2
3
infinity
2
3
4
(after 1 exchange)
(after 2 exchanges)
(after 3 exchanges)
(after 4 exchanges)
32
Count-to-Infinity (Cont’d)
• But, bad news propagate slower!
A
Initially, all up:
A goes down:
B
1
3
3
5
5
7
7
C
2
2
4
4
6
6
8
E
D
3
3
3
5
5
7
7
4
4
4
4
6
6
8
(after 1 exchange)
(after 2 exchanges)
(after 3 exchanges)
(after 4 exchanges)
(after 5 exchanges)
(after 6 exchanges)
….
infinity
33
Count-to-Infinity (Cont’d)
• Gradually routers work their way up to infinity.
• Number of exchanges depends on how large is
infinity.
• To reduce number of exchanges, if metric is
number of hops, infinity=maximum path+1.
34
Solution
•
Routing loops:
–
–
•
Path vector: record actual path used in the DV.
Previous hop tracing: records preceding router.
Count-to-infinity:
–
Split horizon: router reports to neighbor cost “infinity”
for destination if route to that destination is through
that neighbor.
35
Split Horizon
• Tries to make bad news spread faster.
• A node reports infinity as distance to node
X on link packets to X are sent.
• Example, in the first exchange, C tells D
its distance to A but tells B its distance to
A is infinity.
– So B discovers its link to A is down and C’s
distance to A is infinity; so it sets its distance to A
to infinity.
36
Link State Routing
• DV routing used in the ARPANET until 1979,
when it was replaced by link state routing.
• Used by the Internet’s OSPF.
• Based on Dijkstra’s “all pairs shortest path”
algorithm.
• Plus link state updates.
37
Link State Routing (Cont’d)
• Link state routing is based on:
– Discover your neighbors and measure the
communication cost to them.
– Send updates about your neighbors to all other routers.
– Compute shortest path to every other router.
38
Finding Neighbors
• When router is booted, its first task is to find
who its neighbors are.
• Special single-hop “hello” packets.
• Cost metric:
– Number of hops: in this case, always 1.
– Delay: “echo” packets and measure RTT/2.
– Load?
39
Generating Link State Updates
• Link state packets (LSP).
–
–
–
–
Sender identity.
Sequence number.
TTL.
List of (neighbor, cost).
• When to send updates?
– Proactive: periodic updates; how often?
– Reactive: whenever some significant event is detected,
e.g., link goes down.
• Where to send them? Everywhere: flood.
40
Processing Updates
• When LSP is received:
– Check sequence number.
– If higher than current sequence number, keep it and
flood it; otherwise, discard it.
– Periodically decrement TTL.
• When TTL=0, purge LSP.
41
Computing Routes
• Routers have global view of network.
– They receive updates from all other routers with their
cost to their neighbors.
– Build network graph.
• Use Dijkstra’s shortest-path algorithm to
compute shortest paths to all other nodes.
42
Measuring Line Cost
• A subnet in which the East and West parts
are connected by two lines.
43
Building Link State Packets
•
(a) A subnet. (b) The link state
packets for this subnet.
44
Distributing the Link State
Packets
B’s LSP buffer: each row corresponds to a
recently LSP that hasn’t been processed yet.
45
Link State Routing: Problems
• Scalability:
– Storage: kn, where n is number of routers and
k is number of neighbors.
– Computation time.
– LSP propagation via flooding.
46
DV versus LS
• DV:
– Node tells its neighbors what it knows about everybody.
– Based on other’s knowledge, node chooses best route.
– Distributed computation.
• LS:
– Node tells everyone what it knows about its neighbors.
– Every node has global view.
– Compute their own routes.
47
Hierarchical Routing
•
For scalability:
–
•
As network grows, so does RT size, routing update
generation, processing, and propagation overhead,
and route computation time and resources.
Divide network into routing regions.
–
–
–
Routers within region know how to route packets to
all destinations within region.
But don’t know how to route within other regions.
“Border” routers: route within regions.
48
Hierarchical Routing: Example
Flat routing:
1A Dest. Next Hops
1B
2A
2B
1C
1A
4A
2D
2C
5B
5A
3A
3B
5C
4B 4C 5E
5D
1A
1B
1C
2A
2B
2C
2D
3A
3B
4A
4B
4C
5A
5B
5C
5D
5E
1B
1C
1B
1B
1B
1B
1C
1C
1C
1C
1C
1C
1C
1B
1C
1C
1
1
2
3
3
4
3
2
3
4
4
4
5
5
6
5
49
Hierarchical Routing: Example
1A Dest. Next Hops
Hierarchy:
1B
1A
2A
1C
4A
3A
2B
3B
2D
2C
1A
1B
1C
2
3
4
5
1B
1C
1B
1C
1C
1C
1
1
2
2
3
4
5B
5A
5C
4B 4C
5E
5D
50
Hierarchical Routing
• Optimal paths are not guaranteed.
– Example: 1A->5C should be via 2 and not 3.
• How many hierarchical levels?
– Example: 720 routers.
• 1 level: each router needs 720 RT entries.
• 2 levels: 24 regions of 30 routers: each router’s
RT has 30+23 entries.
• 3 levels: 8 clusters of 9 regions with 10 routers:
each router’s RT 10+8+7.
51
Intra-AS and Inter-AS routing
C.b
a
C
Gateways:
B.a
A.a
b
A.c
d
A
a
b
c
a
c
B
b
•perform inter-AS
routing amongst
themselves
•perform intra-AS
routers with other
routers in their
AS
network layer
inter-AS, intra-AS
routing in
gateway A.c
link layer
physical layer
52
Intra-AS and Inter-AS routing
C.b
a
C
Host
h1
b
A.a
a
Inter-AS Internet: BGP
routing
between
B.a
A and B
Host
h2
c
A.c
a
b
B
d
c
b
A
Intra-AS routing
within AS A
Intra-AS routing
within AS B
Internet: OSPF, IS-IS, RIP
53
Many-to-Many Routing
• Support many-to-many communication.
• Example applications: multi-point data
distribution, multi-party teleconferencing.
54
Broadcasting
• Send to ALL destinations.
• Several possible routing mechanisms to broadcasting.
• Simplistic approach: send separate packet to
each destination.
– Simple but expensive.
– Source needs to know about all destinations.
• Flooding:
– May generate too many duplicates (depending on
node connectivity).
55
Multidestination Routing
•
•
Packet contains list of destinations.
Router checks destinations and determines on
which interfaces it will forward packet.
–
–
Router generates new copy of packet for each output
line and includes in packet only the appropriate set of
destinations.
Eventually, packets will only carry 1 destination.
56
Spanning Tree Routing
• Use spanning tree (sink tree) rooted at
broadcast initiator.
• No need for destination list.
• Each on spanning tree forwards packets on all
lines on the spanning tree (except the one the
packet arrived on).
• Efficient but needs to generate the spanning
tree and routers must have that information.
57
Reverse Path Forwarding
•
•
Routers don’t have to know spanning tree.
Router checks whether broadcast packet
arrived on interface used to send packets to
source of broadcast.
–
–
If so, it’s likely that it followed best route and thus not
a duplicate; router forwards packet on all lines.
If not, packet discarded as likely duplicate.
58
Broadcast Routing
Reverse path forwarding. (a) A subnet. (b) a Sink tree. (c) The
tree built by reverse path forwarding.
59
Multicasting
• Special form of broadcasting:
– Instead of sending messages to all nodes, send
messages to a group of nodes.
• Multicast group management:
– Creating, deleting, joining, leaving group.
– Group management protocols communicate group
membership to appropriate routers.
60
Multicast Routing
• Each router computes spanning tree covering all
other participating routers.
– Tree is pruned by removing that do not contain
any group members.
2
2
1
1,2
1,2
2
1
2
1
1
2
1
2
1
1
1
1,2
2
1
1,2
2
2
2
1
2
1
61
Shared Tree Multicasting
• Source-rooted tree approaches don’t scale well!
– 1 tree per source, per group!
– Routers must keep state for m*n trees, where m is number
of sources in a group and n is number of groups.
• Core-based trees: single tree per group.
– Host unicast message to core, where message is
multicast along shared tree.
– Routes may not be optimal for all sources.
– State/storage savings in routers.
62
Internetworking
63
Internetworking
• What is it?
– Connecting networks together forming a single
“internet”.
64
Connecting Networks
• A collection of interconnected networks.
65
How Networks Differ
5-43
66
How Networks Can Be Connected
•
•
(a) Two Ethernets connected by a switch.
(b) Two Ethernets connected by routers.
67
How to Internet?
• Connection-oriented versus connectionless
internetworking.
• Connection oriented internetworking:
– Based on VC concatenation.
• Connectionless internetworking follows the
datagram model.
68
Concatenated Virtual Circuits
Gateway
. Builds VC crossing the different networks.
. Use of gateways to perform necessary conversions.
69
Connectionless Internetworking
. Follows datagram model.
. Packets from Host X to Host Y may follow different routes.
. Gateways make routing decisions and perform translations.
70
Translating versus “Gluing”
• Translation: converting between different
protocols.
• Hard!
• Alternative: “gluing”.
– I.e., using the same network layer protocol
everywhere.
– That’s what IP does!
71
Tunneling
• Interconnecting source and destination on
separate networks but of the same type.
S
D
72
Tunneling Analogy
73
More Tunneling …
74
Internetworking
75
Internetwork Routing
• Inherently hierarchical.
– Routing within each network: interior gateway
protocol (IGP).
– Routing between networks: exterior gateway
protocol (EGP).
• Within each network, different routing
algorithms can be used.
• Each network is autonomously managed
and independent of others: autonomous
system (AS).
76
Internetwork Routing: Example
• (a) An internetwork. (b) A graph of the internetwork.
77
Internetwork Routing (Cont’d)
• Typically, packet starts in its LAN. Gateway
receives it (broadcast on LAN to “unknown”
destination).
• Gateway sends packet to gateway on the
destination network using its routing table. If it
can use the packet’s native protocol, sends
packet directly. Otherwise, tunnels it.
78
Fragmentation
• Happens when internetworking.
• Network-specific maximum packet size.
– Width of TDM slot.
– OS buffer limitations.
– Protocol (number of bits in packet length field).
• Maximum payloads range from 48 bytes (ATM
cells) to 64Kbytes (IP packets).
79
Problem
• What happens when large packet wants to
travel through network with smaller maximum
packet size? Fragmentation.
• Gateways break packets into fragments; each
sent as separate packet.
• Gateway on the other side have to reassemble
fragments into original packet.
• 2 kinds of fragmentation: transparent and nontransparent.
80
Types of Fragmentation
• (a) Transparent fragmentation.
Nontransparent fragmentation.
(b)
81
Transparent Fragmentation
• Small-packet network transparent to other
subsequent networks.
• Fragments of a packet addressed to the same
exit gateway, where packet is reassembled.
– OK for concatenated VC internetworking.
• Subsequent networks are not aware
fragmentation occurred.
• ATM networks (through special hardware)
provide transparent fragmentation.
82
Problems with Transparent
Fragmentation
• Exit gateway must know when it received all the
pieces.
– Fragment counter or “end of packet” bit.
• Some performance penalty but requiring all
fragments to go through same gateway.
• May have to repeatedly fragment and
reassemble through series of small-packet
networks.
83
Non-Transparent
Fragmentation
• Only reassemble at destination host.
– Each fragment becomes a separate packet.
– Thus routed independently.
• Problems:
– Hosts must reassemble.
– Every fragment must carry header until it reaches
destination host.
84
Keeping Track of Fragments
•
•
Fragments must be numbered so that original
data stream can be reconstructed.
Tree-structured numbering scheme:
–
–
–
–
Packet 0 generates fragments 0.0, 0.1, 0.2, …
If these fragments need to be fragmented later on, then
0.0.0, 0.0.1, …, 0.1.0, 0.1.1, …
But, too much overhead in terms of number of fields
needed.
Also, if fragments are lost, retransmissions can take
alternate routes and get fragmented differently.
85
Keeping Track of Fragments
(Cont’d)
• Another way is to define elementary
fragment size that can pass through every
network.
• When packet fragmented, all pieces equal
to elementary fragment size, except last
one (may be smaller).
• Packet may contain several fragments.
86
Fragmentation: Example
•
•
•
Fragmentation when the elementary data size is 1 byte.
(a) Original packet, containing 10 data bytes.
(b) Fragments after passing through a network with maximum
packet size of 8 payload bytes plus header.
(c) Fragments after passing through a size 5 gateway.
•
87
Keeping Track of Fragments
• Header contains packet number, number of first
fragment in the packet, and last-fragment bit.
27 0
1 A B
C
D
Number of
first fragment
Packet number
27 0
0 A B
Last-fragment bit
E F G H I
C
D
E
F
G
H
1 byte
J
(a) Original packet
with 10 data bytes.
27 8 1 I
J
(b) Fragments after passing through network
with maximum packet size = 8 bytes.
88
The Internet
89
Design Principles for Internet
•
•
•
•
•
•
•
Keep it simple.
Exploit modularity.
Expect heterogeneity.
Think robustness.
Avoid static options and parameters.
Think about scalability.
Consider performance and cost.
90
Internet as Collection of
Subnetworks
91
IP (Internet Protocol)
• Glues Internet together.
• Common network-layer protocol spoken
by all Internet participating networks.
• Best effort datagram service:
– No reliability guarantees.
– No ordering guarantees.
92
IP
• Transport layer breaks data streams into
datagrams; fragments transmitted over Internet,
possibly being fragmented.
• When all packet fragments arrive at destination,
reassembled by network layer and delivered to
transport layer at destination host.
93
Encapsulation & Demultiplexing
•
•
•
•
•
When an application sends data using TCP, the data
is sent down the protocol stack, through each
layer, until it is sent as a stream of bits across the
network. Each layer adds information to the data
by prepending headers (and sometimes adding
trailer information) to the data that it receives.
The unit of data that TCP sends to IP is called a
TCP segment. The unit of data that IP sends to the
network interface is called an IP datagram. The
stream of bits that flows across the Ethernet is
called a frame.
A physical property of an Ethernet frame is that
the size of its data must be between 46 and 1500
bytes
We could draw a nearly identical picture for UDP
data. The only changes are that the unit of
information that UDP passes to IP is called a UDP
datagram, and the size of the UDP header is 8
bytes.
When an Ethernet frame is received at the
destination host it starts its way up the protocol
stack and all the headers are removed by the
appropriate protocol box.
94
IP Versions
• IPv4: IP version 4.
– Current, predominant version.
– 32-bit long addresses.
• IPv6: IP version 6 (aka, IPng).
– Evolution of IPv4.
– Longer addresses (16-byte long).
95
IP Datagram Format
• IP datagram consists of header and data (or
payload).
• Header:
– 20-byte fixed (mandatory) part.
– Variable length optional part.
96
The IP v4 Header
97
The format of an IP datagram
•
•
•
•
•
The current protocol version is 4, so IP is
sometimes called IPv4.
The header length is the number of 32bit words in the header, including any
options. Since this is a 4-bit field, it
limits the header to 60 bytes.
The type-of-service field (TOS) is
composed of a 3-bit precedence field
(which is ignored today), 4 TOS bits, and
an unused bit that must be 0. The 4 TOS
bits are: minimize delay, maximize
throughput, maximize reliability, and
minimize monetary cost.
The total length field is the total length
of the IP datagram in bytes. Since this is
a 16-bit field, the maximum size of an IP
datagram is 65535 bytes.
The identification, flags and fragment
offset fields are used when the packet is
fragmented.
•
•
The time-to-live field, or TTL, sets an
upper limit on the number of routers
through which a datagram can pass. It
limits the lifetime of the datagram.
The header checksum is calculated over the
IP header only. It does not cover any data
that follows the header.
98
IP Options
5-54
99
IP Fragmentation & Reassembly
•
network links have MTU
(max.transfer size) - largest
possible link-level frame.
– different link types,
different MTUs
•
fragmentation:
in: one large datagram
out: 3 smaller datagrams
large IP datagram divided
(“fragmented”) within net
– one datagram becomes
several datagrams
– “reassembled” only at
final destination
– IP header bits used to
identify, order related
fragments
reassembly
100
IP Fragmentation & Reassembly
Example
• 4000 byte datagram
• MTU = 1500 bytes
length ID fragflag offset
=4000 =x
=0
=0
One large datagram becomes
several smaller datagrams
length ID fragflag offset
=1500 =x
=1
=0
1480 bytes in
data field
length ID fragflag offset
=1500 =x
=1
=185
offset =
1480/8
length ID fragflag offset
=1040 =x
=0
=370
101
IP Addresses
• IP address formats.
102
IP Addresses (Cont’d)
• Class A: 128 networks with 16M hosts each.
• Class B: 16,384 networks with 64K hosts
each.
• Class C: 2M networks with 256 hosts each.
• More than 500K networks connected to the
Internet.
• Network numbers centrally administered by
ICANN.
103
IP Addresses (Cont’d)
• Special IP addresses.
104
Scalability of IP Addresses
• Problem: a single A, B, or C address refers to
a single network.
• As organizations grow, what happens?
105
Example: A Campus Network
106
Solution
• Subnetting: divide the organization’s address
space into multiple “subnets”.
• How? Use part of the host number bits as the
“subnet number”.
• Example: Consider a university with 35
departments.
– With a class B IP address, use 6-bit subnet
number and 10-bit host number.
– This allows for up to 64 subnets each with
1024 hosts.
107
Subnets
• IP address:
– subnet part (high
order bits)
– host part (low order
bits)
• What’s a subnet ?
– device interfaces
with same subnet
part of IP address
– can physically reach
each other without
intervening router
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
LAN
223.1.3.1
223.1.3.2
network consisting of 3 subnets
Subnet mask: /24
108
Subnets
• A class B network subnetted into 64 subnets.
109
Subnet Mask
• Indicates the split between network and subnet
number + host number.
Subnet Mask: 255.255.252.0 or
/22 (network + subnet part)
110
Subnetting: Observations
• Subnets are not visible to the outside world.
• Thus, subnetting (and how) is a decision
made by local network admin.
111
Subnet: Example
• Subnet 1: 10000010 00110010 000001|00 00000001
– 130.50.4.1
• Subnet 2: 10000010 00110010 000010|00 00000001
– 130.50.8.1
• Subnet 3: 10000010 00110010 000011|00 00000001
– 130.50.12.1
112
Problem with IPv4
• IPv4 is running out of addresses.
• Problem: class-based addressing scheme.
– Example: Class B addresses allow 64K hosts.
• More than half of Class B networks have fewer
than 50 hosts!
113
Solution: CIDR
• CIDR: Classless Inter-Domain Routing.
– RFC 1519.
• Allocate remaining addresses in variablesized blocks without considering classes.
• Example: if an organization needs 2000
addresses, it gets 2048-address block.
• Forwarding had to be modified.
– Routing tables need an extra entry, a 32-bit
mask, which is ANDed with the destination IP
address.
– If there is a match, the packet is forwarded on
that interface.
114
IP addressing: CIDR
CIDR: Classless InterDomain Routing
– subnet portion of address of arbitrary length
– address format: a.b.c.d/x, where x is # bits in
subnet portion of address
subnet
part
host
part
11001000 00010111 00010000 00000000
200.23.16.0/23
115
IP addresses: how to get one?
Q: How does host get IP address?
• hard-coded by system admin in a file
– Windows: control-panel->network>configuration->tcp/ip->properties
– UNIX: /etc/rc.config
• DHCP: Dynamic Host Configuration Protocol:
dynamically get address from as server
– “plug-and-play”
(more in next chapter)
116
IP addresses: how to get one?
Q: How does network get subnet part of IP
addr?
A: gets allocated portion of its provider ISP’s
address space
ISP's block
11001000 00010111 00010000 00000000
200.23.16.0/20
Organization 0
Organization 1
Organization 2
...
11001000 00010111 00010000 00000000
11001000 00010111 00010010 00000000
11001000 00010111 00010100 00000000
…..
….
200.23.16.0/23
200.23.18.0/23
200.23.20.0/23
….
Organization 7
11001000 00010111 00011110 00000000
200.23.30.0/23
117
Hierarchical addressing: route
aggregation
Hierarchical addressing allows efficient advertisement of routing
information:
Organization 0
200.23.16.0/23
Organization 1
200.23.18.0/23
Organization 2
200.23.20.0/23
Organization 7
.
.
.
.
.
.
Fly-By-Night-ISP
“Send me anything
with addresses
beginning
200.23.16.0/20”
Internet
200.23.30.0/23
ISPs-R-Us
“Send me anything
with addresses
beginning
199.31.0.0/16”
118
Hierarchical addressing: more
specific routes
ISPs-R-Us has a more specific route to Organization 1
Organization 0
200.23.16.0/23
Organization 2
200.23.20.0/23
Organization 7
.
.
.
.
.
.
Fly-By-Night-ISP
“Send me anything
with addresses
beginning
200.23.16.0/20”
Internet
200.23.30.0/23
ISPs-R-Us
Organization 1
200.23.18.0/23
“Send me anything
with addresses
beginning 199.31.0.0/16
or 200.23.18.0/23”
119
IP addressing: the last word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers
– allocates addresses
– manages DNS
– assigns domain names, resolves disputes
120
Routing: Store-and-Forward
• Store-and-Forward Packet Switching
A host with a packet to send transmits it to the nearest router,
either on its own LAN or over a point-to-point link to the carrier.
The packet is stored there until it has fully arrived so the
checksum can be verified. Then it is forwarded to the next
router along the path until it reaches the destination host,
where it is delivered.
121
Router Architecture
Overview
Two key router functions:
• run routing algorithms/protocol
• forwarding datagrams from incoming to outgoing link
122
Input Port Functions
Physical layer:
bit-level reception
Data link layer:
e.g., Ethernet
Decentralized switching:
• given datagram dest., lookup output port
using forwarding table in input port
memory
• goal: complete input port processing at
‘line speed’
• queuing: if datagrams arrive faster than
forwarding rate into switch fabric
123
Three types of switching
fabrics
124
Switching Via Memory
First generation routers:
• traditional computers with switching under direct control of CPU
• packet copied to system’s memory
• speed limited by memory bandwidth (2 bus crossings per datagram)
Input
Port
Memory
Output
Port
System Bus
125
Switching Via a Bus
• datagram from input port memory
to output port memory via a shared
bus
• bus contention: switching speed
limited by bus bandwidth
• 1 Gbps bus, Cisco 1900: sufficient
speed for access and enterprise
routers (not regional or backbone)
126
Switching Via An Interconnection
Network (CrossBar)
• overcome bus bandwidth limitations
• Banyan networks, other interconnection nets
initially developed to connect processors in
multiprocessor
• Advanced design: fragmenting datagram into
fixed length cells, switch cells through the
fabric.
• Cisco 12000: switches Gbps through the
interconnection network
127
Output Ports
• Buffering required when datagrams arrive from
fabric faster than the transmission rate
• Scheduling discipline chooses among queued
datagrams for transmission
128
Output port queueing
• buffering when arrival rate via switch exceeds
output line speed
• queueing (delay) and loss due to output port
buffer overflow!
129
Network Address Translation
• Another “quick fix” to the address shortage in IP v4.
• Specified in RFC 3022.
• Each organization gets a single (or small number of)
IP addresses.
– This is used for Internet traffic only.
– For internal traffic, each host gets its own “internal” IP
address.
• Three IP ranges have been declared as “private”.
– 10.0.0.0 – 10.255.255.255/8
– 172.16.0.0 – 172.31.255.255/12
– 192.168.0.0 – 192.168.255.255/16
• No “private” IP address can show up on the Internet,
i.e., outside the organization’s network.
130
NAT: Network Address Translation
rest of
Internet
local network
(e.g., home network)
10.0.0/24
10.0.0.4
10.0.0.1
10.0.0.2
138.76.29.7
10.0.0.3
All datagrams leaving local
network have same single source
NAT IP address: 138.76.29.7,
different source port numbers
Datagrams with source or
destination in this network
have 10.0.0/24 address for
source, destination (as usual)
131
NAT: Network Address Translation
(Cont’d)
• Motivation: local network uses just one IP
address as far as outside word is concerned:
– no need to be allocated range of addresses from
ISP: - just one IP address is used for all devices
– can change addresses of devices in local network
without notifying outside world
– can change ISP without changing addresses of
devices in local network
– devices inside local net not explicitly
addressable, visible by outside world (a security
plus).
132
NAT: Network Address Translation
(Cont’d)
Implementation: NAT router must:
– outgoing datagrams: replace (source IP address, port #)
of every outgoing datagram to (NAT IP address, new port
#)
. . . remote clients/servers will respond using (NAT
IP address, new port #) as destination addr.
– remember (in NAT translation table) every (source IP
address, port #) to (NAT IP address, new port #)
translation pair
– incoming datagrams: replace (NAT IP address, new port
#) in dest fields of every incoming datagram with
corresponding (source IP address, port #) stored in NAT
table
133
NAT: Network Address Translation
(Cont’d)
2: NAT router
changes datagram
source addr from
10.0.0.1, 3345 to
138.76.29.7, 5001,
updates table
2
NAT translation table
WAN side addr
LAN side addr
1: host 10.0.0.1
sends datagram to
128.119.40, 80
138.76.29.7, 5001 10.0.0.1, 3345
……
……
S: 10.0.0.1, 3345
D: 128.119.40.186, 80
S: 138.76.29.7, 5001
D: 128.119.40.186, 80
138.76.29.7
S: 128.119.40.186, 80
D: 138.76.29.7, 5001
3: Reply arrives
dest. address:
138.76.29.7, 5001
3
1
10.0.0.4
S: 128.119.40.186, 80
D: 10.0.0.1, 3345
10.0.0.1
10.0.0.2
4
10.0.0.3
4: NAT router
changes datagram
dest addr from
138.76.29.7, 5001 to 10.0.0.1, 3345
134
NAT: Network Address Translation
(Cont’d)
• 16-bit port-number field:
– 60,000 simultaneous connections with a
single LAN-side address!
• NAT is controversial:
– routers should only process up to layer 3
– violates end-to-end argument
• NAT possibility must be taken into account
by app designers, eg, P2P applications
– address shortage should instead be solved
by IPv6
135
Internet Control Protocols
• “Companion” protocols to IP.
• Control protocols used mainly for signaling
and exchange of control information.
• Examples: ICMP, ARP, RARP, BOOTP, and
DHCP.
136
ICMP: Internet Control Message
Protocol
•
used by hosts & routers to
communicate network-level
information
– error reporting:
unreachable host,
network, port,
protocol
– echo request/reply
(used by ping)
•
network-layer “above” IP:
– ICMP msgs carried
in IP datagrams
•
ICMP messages encapsulated
within an IP datagram
ICMP message
ICMP message: type, code plus
first 8 bytes of IP datagram
causing error
137
ICMP: Internet Control
Message Protocol (Cont’d)
•
•
ICMP error messages are
sometimes handled specially. For
example, an ICMP error message
is never generated in response to
an ICMP error message.
When an ICMP error message is
sent, the message always contains
the IP header and the first 8
bytes of the IP datagram that
caused the ICMP error to be
generated. This lets the receiving
ICMP module associate the
message with one particular
protocol and one particular user
process.
138
Example of ICMP —— ping
• The name "ping" is taken from the sonar operation to locate
objects. The Ping program was written by Mike Muuss and it
tests whether another host is reachable.
• The program sends an ICMP echo request message to a host,
expecting an ICMP echo reply to be returned.
• Normally if you can't Ping a host, you won't be able to Telnet
or FTP to that host. Conversely, if you can't Telnet to a
host, Ping is often the starting point to determine what the
problem is. Ping also measures the round-trip time to the
host, giving us some indication of how "far away" that host
is.
• With the increased awareness of security on the Internet,
ping may show a host as being unreachable, yet we might be
able to Telnet.
139
Example of ICMP —— ping
(Cont’d)
•
•
Format of ICMP message for echo request and echo reply.
Example of ping output
bsdi % ping svr4
PING svr4 (140.252.13.34): 56 data bytes
64 bytes from 140.252.13.34: icmp_seq=0 ttl=255 time=0 ms
64 bytes from 140.252.13.34: icmp_seq=l ttl=255 time=0 ms
64 bytes from 140.252.13.34: icmp_seq=2 ttl=255 time=0 ms
64 bytes from 140.252.13.34: icmp_seq=3 ttl=255 time=0 ms
64 bytes from 140.252.13.34: icmp_seq=4 ttl=255 time=0 ms
64 bytes from 140.252.13.34: icmp_seq=5 ttl=255 time=0 ms
64 bytes from 140.252.13.34: icmp_seq=6 ttl=255 time=0 ms
64 bytes from 140.252.13.34: icmp_seq=7 ttl=255 time=0 ms
^?
type interrupt key to stop
--- svr4 ping statistics --8 packets transmitted, 8 packets received, 0% packet loss
round-trip min/avg/max = 0/0/0 ms
140
Example of ICMP —— ping
(Cont’d)
• IP Record Route Option
– The ping program gives us an opportunity to look at the
IP record route (RR) option.
– It causes ping to set the IP RR option in the outgoing
IP datagram. This causes every router that handles the
datagram to add its IP address to a list in the options
field. When ping receives the echo reply it prints the list
of IP addresses.
– Most systems today do support these optional
features, but some systems don't reflect the IP list.
– Example ……
141
Example of ICMP —— ping
(Cont’d)
• Broadcast ping
– For example, %ping –b 211.69.193.255
WARNING: pinging broadcast address
PING 211.69.193.255 (211.69.193.255) from 211.69.193.53 : 56(84) bytes of data.
Warning: time of day goes back, taking countermeasures.
64 bytes from 211.69.193.53: icmp_seq=0 ttl=255 time=649 usec
64 bytes from 211.69.193.62: icmp_seq=0 ttl=255 time=1.243 msec (DUP!)
64 bytes from 211.69.193.13: icmp_seq=0 ttl=64 time=1.298 msec (DUP!)
64 bytes from 211.69.193.12: icmp_seq=0 ttl=64 time=1.316 msec (DUP!)
64 bytes from 211.69.193.52: icmp_seq=0 ttl=255 time=1.332 msec (DUP!)
64 bytes from 211.69.193.52: icmp_seq=0 ttl=255 time=1.348 msec (DUP!)
64 bytes from 211.69.193.11: icmp_seq=0 ttl=64 time=1.364 msec (DUP!)
64 bytes from 211.69.193.15: icmp_seq=0 ttl=64 time=1.380 msec (DUP!)
64 bytes from 211.69.193.25: icmp_seq=0 ttl=64 time=1.396 msec (DUP!)
64 bytes from 211.69.193.26: icmp_seq=0 ttl=64 time=1.412 msec (DUP!)
64 bytes from 211.69.193.41: icmp_seq=0 ttl=64 time=1.428 msec (DUP!)
64 bytes from 192.168.1.216: icmp_seq=0 ttl=64 time=1.485 msec (DUP!)
142
MAC Addresses and ARP
• 32-bit IP address:
– network-layer address
– used to get datagram to destination IP subnet
• MAC (or LAN or physical or Ethernet)
address:
– used to get datagram from one interface to
another physically-connected interface (same
network).
– 48 bit MAC address (for most LANs)
burned in the adapter ROM.
143
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
1A-2F-BB-76-09-AD
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Broadcast address =
FF-FF-FF-FF-FF-FF
= adapter
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
144
ARP: Address Resolution Protocol
Question: how to determine
MAC address of B
knowing B’s IP address?
237.196.7.78
1A-2F-BB-76-09-AD
237.196.7.23
237.196.7.14
• Each IP node (Host,
Router) on LAN has
ARP table
• ARP Table: IP/MAC
address mappings for
some LAN nodes
< IP address; MAC
address; TTL>
LAN
71-65-F7-2B-08-53
237.196.7.88
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
– TTL (Time To Live):
time after which
address mapping will be
forgotten (typically 20
min)
145
ARP Operation
• Host 1 broadcasts an ARP request on the
Ethernet asking who owns host 2’s IP
address.
• Host 2 replies with its Ethernet address.
• Some optimizations:
– ARP caches.
– Piggybacking host’s own Ethernet address on
ARP requests.
– Proxy ARP: services ARP requests for hosts
on separate LANs.
146
• “ftp bsdi”
–
–
–
–
–
–
–
–
–
An Example of ARP
The application, the FTP client, calls the
function gethostbyname to convert the
hostname (bsdi) into its 32-bit IP address.
This function is called a resolver in the
DNS (Domain Name System).
The FTP client asks its TCP to establish a
connection with that IP address
TCP sends a connection request segment to
the remote host by sending an IP datagram
to its IP address.
the IP datagram is sent to a host or router
on a locally attached network.
Assuming an Ethernet, the sending host
must convert the 32-bit IP address into a
48-bit Ethernet address.
ARP sends an Ethernet frame called an ARP
request to every host on the network. This
is called a broadcast.
The destination host's ARP layer receives
this broadcast, recognizes that the sender
is asking for its hardware address, and
replies with an ARP reply. This reply
contains the IP address and the
corresponding hardware address.
The ARP reply is received and the IP
datagram that forced the ARP requestreply to be exchanged can now be sent.
The IP datagram is sent to the destination
host.
147
Other problems about ARP
• The fundamental concept behind ARP is that the
network interface has a hardware address (a 48-bit
value for an Ethernet or token ring interface).
Frames exchanged at the hardware level must be
addressed to the correct interface.
• Knowing a host's IP address doesn't let the kernel
send a frame to that host. The kernel (i.e., the
Ethernet driver) must know the destination's
hardware address to send it data.
• Point-to-point links don't use ARP.
• RARP (reverse address resolution protocol)
– This protocol performs the exactly reverse
functionality than ARP
• Important Command about ARP
– arp (in both Windows and Unix)
148
DHCP
•
•
•
•
Dynamic Host Configuration Protocol.
RFCs 2131 and 2132.
Assigns IP addresses to hosts dynamically.
DHCP server may not be on the same LAN
as requesting host.
• DHCP relay agent.
149
DHCP Operation
• Newly booted host broadcasts a DHCP
DISCOVER message.
• DHCP relay agent intercepts DHCP
DISCOVERs on its LAN and unicasts them to
DHCP server.
150
DHCP Operation
151
DHCP: Address Reuse
• How long should an IP address be allocated?
• Issue: hosts come and go.
• IP addresses may be assigned on a “Lease”
basis.
• Hosts must renew their leases.
152