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INTERNETWORK
SIMPLE INTERNETWORKING
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INTERNETWORK Internetwork or “internet” refers to an
arbitrary collection of network interconnected to provide some sort of host-to-host packet delivery service
Internet is different from internet Also called “network of networks”
made up of lots smaller networks Device: router or gateway
used to interconnect the networks
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SIMPLE INTERNETWORK
Net1 : Ethernet
Host1 Host2 Host3
Host4Router1
Net2 : Token-ring
Host5 Host6 Host7
Net4 : Ethernet
Host8 Host9 Host10
Host11Router3
Router2
Net3 : Point-to-point
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SIMPLE INTERNETWORK
Protocol layers used to connect Host 3 to Host 9:
ETH Ethernet TR Token Ring P2P Point to Point
TCP
IP
ETH ETH TR
IP
H3
IP
TR P2P
R1 R2
IP
P2P ETH
R3
TCP
IP
ETH
H9
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SERVICE MODEL
Packet Delivery 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
IPv4 Header: 192 bits (24 bytes)
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DATAGRAM FORMAT (IPV4)
Version (4): currently 4 Hlen (4): number of 32-bit words in header TOS (8): type of service (not widely used) Length (16): number of bytes in this datagram Ident (16): used by fragmentation Flags/Offset (16): used by fragmentation TTL (8): number of hops this datagram has
travelled Protocol (8): demux key (TCP=6, UDP=17) Checksum (16): of the header only DestAddr & SrcAddr (32)
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FRAGMENTATION & REASSEMBLY
Each network has some MTU (Maximum Transfer Unit)
Strategyfragment when necessary (MTU <
Datagram)try to avoid fragmentation at source hostrefragmentation is possiblefragments are self-contained datagramsdelay reassembly until destination hostdo not recover from lost fragments
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EXAMPLE
H3 R1 R2 R3 H9
ETH IP (1400) TR IP (1400) P2P IP (512)
P2P IP (512)
P2P IP (376)
ETH IP (512)
ETH IP (512)
ETH IP (376)
Start of Header
Ident = x 0 Offset = 0
Rest of Header
1400 Data bytes
Start of Header
Ident = x 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
a)
b)
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GLOBAL ADDRESSES Properties
globally uniquehierarchical: network + host
Format
Dot notation10.3.2.4 ; 128.96.33.81 ; 192.12.69.77
0 Network Id Host Id
7 24
1 0 Network Id Host Id
14 16
1 0 Network Id Host Id
21 8
1
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DATAGRAM FORWARDING
Strategy 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
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EXAMPLE (ROUTER R2)
NetworkNumber
Next_Hop
1 Router1
2 Interface 0
3 Interface 1
4 Router3
Net1 : Ethernet
Host1 Host2 Host3
Host4Router1
Net2 : Token-ring
Host5 Host6 Host7
Net4 : Ethernet
Host8 Host9 Host10
Host11Router3
Router2
Net3 : Point-to-point
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GLOBAL INTERNET
Scalability Issues IP “hides” hosts in address hierarchy,
but... Inefficient use of address space
class C network with 2 hosts (2/255 = 0.78% efficient)
class B network with 256 hosts (256/65535 = 0.39% efficient)
Too many networks today's Internet has tens of thousands of networks routing tables do not scale route propagation protocols do not scale
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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• table entries timeout in about 10 minutes
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ARP OPERATION
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ICMP
IP protocol is best-effort delivery service It has 2 deficiencies:
Lack of error controls (no error-correcting) Lack of assistance mechanisms (no error-
reporting) A host needs to determine if a another node
is alive use ICMP ICMP is a companion to the IP protocol
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Error Reporting messages : Destination unreachable Source quench Time exceeded Redirection
• Query :– Echo request or reply– Timestamp request and
reply– Address mask request
and reply– Router solicitation and
advertisement
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DYNAMIC HOST CONFIGURATION PROTOCOL (DHCP)
Extension of BOOTP, and its compatible DHCP provides temporary IP addresses for a
limited period of time DCHP has 2 database:
1. Database that binds physical @ with IP @2. Database with pool of available IP @
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SUBNETTING Add another level to address/routing
hierarchy: subnet Subnet masks define variable partition of
host part of class A and B addresses Subnets visible only within site
Network Id Host Id
111111111111111111111111 00000000
Network Id Host IdSubnet Id
16 8 8
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SUBNET EXAMPLE
Host1
Subnet mask : 255.255.255.128Subnet number : 135.50.21.0
Router1
Subnet mask : 255.255.255.128Subnet number : 135.50.21.128
135.50.21.1
135.50.21.130Host2
135.50.21.139
Router2
135.50.45.1
135.50.21.129
Subnet mask : 255.255.255.0Subnet number : 135.50.45.0
Host3
135.50.50.33
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SUBNET EXAMPLE
Forwarding table at router R1
Subnet number Subnet mask Next_Hop
135.50.21.0 255.255.255.128 Interface 0
135.50.21.128 255.255.255.128 Interface 1
135.50.45.0 255.255.255.0 Router2
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FORWARDING ALGORITHM
D = destination IP addressfor each entry < SubnetNum, SubnetMask,
NextHop>D1 = SubnetMask & D
if D1 = SubnetNum if NextHop is an interface
deliver datagram directly to destination
else deliver datagram to NextHop (a
router)
Notes: Would use a default router if nothing matches Can put multiple subnets on one physical network Subnets not visible from the rest of the Internet
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SUPERNETTING
Assign block of contiguous network numbers to near-by networks
Called CIDR: Classless Inter-Domain Routing Represent blocks with a single pair
<first_network_address, count> Restrict block sizes to powers of 2 Use a bit mask (CIDR mask) to identify block
size All routers must understand CIDR
addressing
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ROUTE PROPAGATION Idea:
Impose a second hierarchy on the network that limits what routers talk to each other. (The first hierarchy is the address hierarchy that governs how packets are forwarded.)
Autonomous System (AS) corresponds to an administrative domain examples: University, company, backbone
network assign each AS a 16-bit number
Two-level route propagation hierarchy interior gateway protocol (each AS selects its
own) exterior gateway protocol (Internet-wide
standard)
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INTRA AS
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ROUTING
Forwarding versus Routing forwarding: to select an output port
based on destination address and routing table
routing: process by which routing table is built
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NETWORK AS A GRAPH
AB
D
C E
F4
19
3
1
1
6
2
Problem: Find the lowest cost path between any two nodes
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ROUTING PROTOCOL
Factors: Static: topology Dynamic: load
Classes: Distance Vector Link State
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DISTANCE VECTOR Each node maintains a set of triples:
(Destination, Cost, NextHop)
Each node sends updates to (and receives updates from) its directly connected neighbors
periodically (on the order of several seconds) whenever its 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
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EXAMPLE
Information at node
Distance to reach node
A B C D E F G
A 0 1 ∞ ∞ 1 ∞ ∞
B 1 0 1 1 ∞ 1 ∞
C ∞ 1 0 ∞ ∞ ∞ ∞
D ∞ 1 ∞ 0 ∞ 1 ∞
E 1 ∞ ∞ ∞ 0 ∞ 1
F ∞ 1 ∞ 1 ∞ 0 1
G ∞ ∞ ∞ ∞ 1 1 0
A
BC
D
E
F
G
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…. CONT’D
Information at node
Distance to reach node
A B C D E F G
A 0 1 2 2 1 2 2
B 1 0 1 1 2 1 2
C 2 1 0 2 3 2 3
D 2 1 2 0 3 1 2
E 1 2 3 3 0 2 1
F 2 1 2 1 2 0 1
G 2 2 3 2 1 1 0
A
BC
D
E
F
G
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TOPOLOGY CHANGES
Example 1 A detects that link to E has
failed A sets distance to E to infinity
and sends update to B B sets distance to E to infinity
since it uses A to reach E B receives periodic update
from F with 2-hop path to E B sets distance to E to 3 and
sends update to A A decides it can reach E in 4
hops via B
A
BC
D
E
F
G
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ROUTING LOOPS ….CONT’D
Example 2: Link from B to C fails B advertises distance of
infinity to C D and F advertise a
distance of 2 to C D decides it can reach C in
3 hops; advertises this to B
B decides it can reach C in 4 hops; advertises this to F
F decides that it can reach C in 5 hops......
A
BC
D
E
F
G
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ROUTING LOOPS ….CONT’D
Heuristics to break routing loops: set infinity to 16 split horizon split horizon with poison reverse hold-down timer
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LINK STATE
Strategy: Send to all nodes (not just neighbors) information about directly connected links (not entire routing table).
Link State Packet (LSP) id of the node that created the LSP cost of link to each directly connected neighbor sequence number (SEQNO) time-to-live (TTL) for this packet
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LINK STATE …CONT’D
Reliable Flooding: store most recent LSP from each node forward LSP to all nodes but one that
sent it generate new LSP periodically;
increment SEQNO start SEQNO at 0 when reboot decrement TTL of each stored LSP;
discard when TTL=0
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LINK STATE …CONT’D
Route Calculation (in theory) Dijkstra's shortest path algorithm N denotes set of nodes in the graph l(i,j) denotes non-negative cost
(weight) for edge (i,j) s in N denotes this node M denotes the set of nodes
incorporated so far C(n) denotes cost of the path from s
to node n
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DIJKSTRA'S ALGORITHM
M = {s}for each n in N - {s} C(n) = l(s,n)while (N ≠ M) M = M {w} such that C(w) is the minimum for all w in (N-
M) for each n in (N-M) C(n) = MIN (C(n), C(w)+l(w,n))
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LINK STATE
Route Calculation (in practice) Forward search algorithm Each switch maintains two lists:
Tentative and Confirmed Each list contains a set of triples:
(Destination, Cost, NextHop)
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FORWARD SEARCH ALGORITHM
1. Initialize Confirmed with entry for me; cost = 0.
2. For the node just added to Confirmed (call it Next) select its LSP.
3. For each Neighbor of Next, calculate the Cost to reach this Neighbor as the sum of the cost from me to Next and from Next to Neighbor
3.1. If Neighbor is currently in neither Confirmed or Tentative, add (Neighbor, Cost, NextHop) to Tentative, where NextHop is the direction to reach Next.
3.2. If Neighbor is currently in Tentative and Cost is less that current cost for Neighbor, then replace current entry with (Neighbor, Cost, NextHop), where NextHop is the direction to reach Next.
4. If Tentative is empty, stop. Otherwise, pick entry from Tentative with the lowest cost, move it to Confirmed, and return to step 2.
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EXAMPLE:
A
B
C
D
5
3
11
10
2
Langkah-langkah pembentukan tabel ruting untuk node C:
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Step Confirmed Tentative Comments
1 (C,0,-) Karena C merupakan satu-satunya anggota baru dari Confirmed, maka dilihat LSP-nya
2
(C,0,-) (B,11,B)(D,2,D)
LSP dari C menyatakan bahwa B dapat dicapai melalui B dengan biaya 11, yang lebih baik (kecil) dibanding entri lain dalam list sehingga dimasukkan dalam Tentative. Hal yang sama juga berlaku untuk D
3(C,0,-)(D,2,D)
(B,11,B) Masukkan entri Tentative dengan biaya terkecil (D) ke Confirmed. Kemudian lihat LSP dari anggota Confirmed yang baru tersebut (D)
4
(C,0,-)(D,2,D)
(B,5,D)(A,12,D)
Biaya untuk mencapai B melalui D adalah 5, sehingga entri (B,11,B) digantikan oleh (B,5,D). LSP dari C juga memberikan informasi bahwa A dapat dicapai dengan biaya 12.
5(C,0,-)(D,2,D)(B,5,D)
(A,12,D) Pindahkan anggota Tentative dengan biaya terkecil (B) ke Confirmed, kemudian lihat LSP-nya
6(C,0,-)(D,2,D)(B,5,D)
(A,10,D) Karena A dapat dicapai dengan biaya 5 dari B maka entri dari Tentative di-update
7
(C,0,-)(D,2,D)(B,5,D)
(A,10,D)
Pindahkan anggota Tentative dengan biaya terkecil (A) ke Confirmed. Karena Tentative telah kosong (jalur terbaik ke seluruh node telah diketahui) maka eksekusi algoritma selesai
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INTER AS
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INTER AND INTRA AS
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EGP: EXTERIOR GATEWAY PROTOCOL
Overview designed for tree-structured Internet concerned with reachability, not optimal routes
Protocol messages neighbor acquisition: one router requests that
another routers could be its peer; peers exchange reachability information
neighbor reachability: one router periodically tests to see if the other router is still reachable; exchange HELLO/ACK messages;
routing updates: peers periodically exchange their routing tables (similar to distance-vector)
Formal specification : RFC-904
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BGP-4: BORDER GATEWAY PROTOCOL
Assumes the Internet is an arbitrarily interconnected set of AS's.
Define local traffic as traffic that originates at or terminates on nodes within an AS, and transit traffic as traffic that passes through an AS
We can classify AS's into three types: Stub AS: an AS that has only a single connection to
one other AS; such an AS will only carry local traffic. Multi-homed AS: an AS that has connections to
more than one other AS, but refuses to carry transit traffic.
Transit AS: an AS that has connections to more than one other AS, and is designed to carry both transit and local traffic.
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BGP-4: BORDER GATEWAY PROTOCOL
Each AS has: One or more border routers One BGP speaker that advertises:
local networks other reachable networks (transit AS only) gives path information
BGP-4 : RFC-1771
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BGP EXAMPLE
Speaker for AS 2 advertises reachability to A and BNetwork 128.96, 192.4.153, 192.4.32, and 192.4.3, can be reached directly from AS 2.
Speaker for backbone network then advertisesNetworks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached along the path <AS 1, AS 2>.
Speaker can also cancel previously advertised paths
Backbone(AS1)
Provider X(AS 2)
Provider Y(AS 3)
Customer A(AS 4)
Customer B(AS 5)
Customer C(AS 6)
Customer D(AS 7)
128.96192.4.153
192.4.32192.4.3
192.12.69
192.4.54192.4.23
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EXERCISE
Suppose a TCP message that contains 2028 bytes of data and 20 bytes of TCP header is passed to IP for delivery across two networks of the Internet. The first network uses 14 bytes headers and has an MTU of 1024 bytes; the second uses 8-byte header with an MTU of 512. Each network’s MTU gives the sizes of largest IP datagram that can be carried in a link-layer frame. Give the sizes and offsets of the sequence of fragment delivered to the network layer at the destination host. Assume all IP headers are 20 bytes.
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REFERENCES
1. Peterson, Larry L. Computer Networks: A Systems Approach. 5th edition. Morgan Kaufmann.
2. Forouzan, Behrouz A. TCP/IP Protocol Suite. Mc Graw Hill.