Transcript 223.1.1.4

Introduction to Networks
and the Internet
CMPE 80N
Winter 2004
Lecture 18
CMPE 80N - Introduction to Networks and the Internet
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Announcements
• Fourth quiz on Monday, March 1st.
• Third HTML discussion session today by
Kiran (before class).
• Summary posted on the Web page.
• FINAL HTML discussion session by
Debasree on Thu 3-4pm in BE 109.
• Quiz review session on Friday by Kiran in BE
354I from 4:45-5:45.
– Practice quiz.
• In order to schedule a second review session,
we will wait to hear from you first.
CMPE 80N - Introduction to Networks and the Internet
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Networks and IP addressing
• IP address:
223.1.1.1
– Network part + Host part
223.1.1.2
223.1.1.4
• Network:
– Any host can physically
be reached by any other
host without intervening
router
– All hosts in the same
network have the same
network number
223.1.2.1
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.27
223.1.3.1
223.1.3.2
network consisting of 3 IP networks
(for IP addresses starting with 223
(class C), first 24 bits are network
address).
CMPE 80N - Introduction to Networks and the Internet
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Routing Table (revisited)
routing table in A
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
A
223.1.1.4
223.1.1.4
0
1
1
223.1.1.1
223.1.2.1
B
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.3.1
CMPE 80N - Introduction to Networks and the Internet
223.1.2.2
E
223.1.3.2
4
Routing Example 1
misc
fields 223.1.1.1 223.1.1.3 data
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
Datagram from A to B:
•look up net. address of B
•find B is on same net. as A
•link layer will send datagram
directly to B inside link-layer frame
•B and A are directly connected
A
223.1.1.4
223.1.1.4
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
B
223.1.1.3
223.1.3.1
CMPE 80N - Introduction to Networks and the Internet
0
1
1
223.1.2.9
223.1.3.27
223.1.2.2
E
223.1.3.2
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Routing Example 2
misc
fields 223.1.1.1 223.1.2.2 data
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
Datagram from A to E:
• look up network address of E
•E on different network
•A, E not directly attached
•routing table: next hop router to E
is 223.1.1.4
•link layer sends datagram to
router 223.1.1.4 inside link-layer
frame
•datagram arrives at 223.1.1.4
•continued…..
A
223.1.1.4
223.1.1.4
0
1
1
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
B
223.1.1.3
CMPE 80N - Introduction to Networks and the Internet
223.1.3.1
223.1.2.9
223.1.3.27
223.1.2.2
E
223.1.3.2
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Routing Example 2 (cont’d)
misc
fields 223.1.1.1 223.1.2.2 data
Arriving at 223.1.4, destined
for 223.1.2.2
•look up network address of E
•E on same network as router’s
interface 223.1.2.9
Dest.
next
network router Nhops interface
223.1.1
223.1.2
223.1.3
A
1
1
1
223.1.1.4
223.1.2.9
223.1.3.27
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
•router, E directly attached
•link layer sends datagram to
223.1.2.2 inside link-layer frame
via interface 223.1.2.9
•datagram arrives at 223.1.2.2!!!
(hooray!)
-
B
223.1.2.9
223.1.2.2
223.1.1.3
CMPE 80N - Introduction to Networks and the Internet
223.1.3.1
E
223.1.3.27
223.1.3.2
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Internetwork Routing
• Internetworks are collections on networks.
• The Internet interconnects autonomous
systems (ASs).
• Hierarchical structure.
Transcontinental
links
US
backbone
Regional
network
Transcontinental
links
European
backbone
National
network
CMPE 80N - Introduction to Networks and the Internet
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Internetwork Routing
• 2-level hierarchy:
– Routing within each network: interior gateway
protocol.
– Routing between networks: exterior gateway
protocol.
• Within each network, different routing algorithms
can be used.
• Each network is autonomously managed and
independent of others: autonomous system
(AS).
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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.
CMPE 80N - Introduction to Networks and the Internet
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The Internetworking Layer
• The Internet as a collection on networks or
autonomous systems (ASs).
• Hierarchical structure.
Transcontinental
links
US
backbone
Regional
network
Transcontinental
links
European
backbone
National
network
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The Internet Protocol: IP
• Glues Internet together.
• Common network-layer protocol spoken by all
Internet participating networks.
• Best effort datagram service:
– No reliability guarantees.
– No ordering guarantees.
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IP (cont’d)
• IP is responsible for datagram routing.
• Important: each datagram is routed
independently!
– Two different datagrams from same source to same
destination can take different routes!
– Why?
– Implications?
CMPE 80N - Introduction to Networks and the Internet
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IP (cont’d)
• IP provides a best effort delivery mechanism
– Does not guarantee to prevent duplicate
datagrams, delayed and out-of-order delivery,
corruption of data or datagram loss
• Reliable delivery is provided by the transport
layer, not the network layer (IP)
• Network layer (IP) can detect and report errors
without actually fixing them
CMPE 80N - Introduction to Networks and the Internet
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Datagrams
• Transport layer breaks data streams into
datagrams which are transmitted over
Internet, possibly being fragmented.
• When all datagram fragments arrive at
destination, reassembled by network layer
and delivered to transport layer at destination
host.
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IP Datagram Format
• IP datagram consists of header and data (or
payload).
• Header:
– 20-byte fixed (mandatory) part.
– Variable length optional part.
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IP Versions
• IPv4: IP version 4.
– Current, predominant version.
– 32-bit long addresses.
• IPv6: IP version 6.
– Evolution of IPv4.
– Longer addresses (16-byte long).
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Payload
Header
IP(v4) Header Format
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Encapsulation Revisited
• Each datagram is encapsulated within a data link
layer frame
– The whole datagram is placed in the data area of
the frame.
– The data link layer addresses for source and
destination included in the frame header.
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Encapsulation - Example
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Encapsulation Across
Multiple Hops
• Each router in the path from source to
destination:
– Decapsulates datagram from incoming frame.
– Forwards datagram - determines next hop.
– Encapsulate datagram in outgoing frame.
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Encapsulation Across
Multiple Hops - Example
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Maximum Transfer Unit
• Each data link layer technology specifies the
maximum size of a frame.
– Called the Maximum Transfer Unit (MTU).
• Ethernet: 1,500 bytes.
• Token Ring: 2048 or 4096 bytes.
• What happens when large packet wants to travel
through network with smaller MTU?
• Maximum payloads (data portion of datagram)
range from 48 bytes (ATM cells) to 64Kbytes (IP
packets).
CMPE 80N - Introduction to Networks and the Internet
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MTU (cont’d)
• A possible solution:
– The sender may limit the size of the datagrams to
the MTU of the network
– What if there are other networks in the path to
destination with smaller MTU?
CMPE 80N - Introduction to Networks and the Internet
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Fragmentation
• Another solution (used by IP): fragmentation.
• Gateways break packets into fragments to fit the
network’s MTU; each sent as separate datagram.
• Gateway on the other side have to reassemble
fragments into original datagram.
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Keeping Track of Fragments
• Fragments must be numbered so that
original data stream can be reconstructed.
• 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).
• Datagram may contain several fragments.
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Fragmentation - Example
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