Veranstaltung Internet und WWW

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Veranstaltung Internet und WWW
IP, Routing, DNS, Client/Server Prof. Dr. A.
Fischer, 15.12.2004
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ARP -- Remote Host Example (continued)

• The default gateway configuration instructs A
  that ALL remote hosts are reachable via the
  next-hop gateway R1.
• Host A will ARP for R1s local interface, NOT
  Host B.

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Summary of IP Host Communication

• The primary steps for all IP host communications
  
• Route determination
• Address resolution
• Remote destinations require a next-hop gateway
  lookup to find a router to reach the remote
  network or subnet.
• ARP requests are MAC layer broadcasts and
  therefore are not forwarded by routers (the
  router responds to the ARP request).
• Proxy ARP can be used to minimize the router
  knowledge required by IP hosts.

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Hop-by-Hop-Routing
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Static Routing

• All routing information is pre-computed and
  provided through manual configuration.
• Routing information must be recomputed and
  provided to the routers each time the network
  topology changes.
• Disadvantage Not well suited to large, dynamic
  internets that may experience constant
  topological changes.

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Distributed-Adaptive Routing

• Distributed-adaptive routing is more practical
  than static routing in a large, dynamic
  environment.
• With distributed-adaptive routing
• Routers use a common algorithm or a common set of
  rules for determining the best path.
• Routers dynamically sense their local
  environments and exchange this information
  amongst themselves in a distributed fashion.
• A system of routers participates in a distributed
  algorithm to determine the optimal route between
  end-stations in an internet.
• Two forms of distributed-adaptive routing are in
  common use
• Link State (Example OSPF)
• Distance Vector (Example RIP)

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Link-State Routing

• Link-state routing protocols (also called
  shortest-path-first protocols) require each
  router to maintain at least a partial map of the
  network.
• When a network link changes state (up to down, or
  vice versa), a notification, called a link-state
  advertisement (LSA) is flooded throughout the
  network. All the routers note the change and
  recompute their routes accordingly.
• Routers know more about the internetwork using
  link-state routing, than when using any
  distance-vector routing protocol.
• Link-state routing is more reliable, easier to
  debug, and less bandwidth-intensive than
  distance-vector routing.
• Link-state routing is also more complex and more
  compute- and memory-intensive.
• OSPF is link-state routing protocols.

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Distance-Vector Routing

• Distance-vector routing finds the best path to a
  remote network by judging distance.
• Each time a packet goes through a router, its
  called a hop. The route with the least number of
  hops to the network is determined to be the best
  route.
• Distance-vector routing requires that each router
  maintain information about the distance from
  itself to each possible destination.
• The term distance-vector comes from the
  information in the periodic update messages sent
  between routers.
• Each router in the internet learns about the
  network topology by exchanging routing
  information packets with its neighbor routers.
• When a router receives a routing information
  packet from a neighbor, it updates its routing
  table if
• The update contains routing information for a
  destination not known previously.
• The update contains a shorter route to a known
  destination.
• The receiving router is routing to a destination
  via the originator, and the update contains a
  distance change to that destination.

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More on Distance-Vector Routing

• Limitations
• Slow convergence due to the update period
  (impedes scalability).
• The formation of router loops can occur.
• Computational complexity of the algorithm grows
  rapidly as the internet grows in size.
• Advantages
• Simple to design
• Simple to use
• Examples of distance-vector routing protocols
• Routing Information Protocol (RIP)
• Interior Gateway Routing Protocol (IGRP)

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Routing Information Protocol (RIP)

• RIP is one form of distance-vector routing.
• Routing decision is based on hop count.
• Each router is one hop.
• RIP has a 15 hop-count limitation.
• RIP does not consider distance or bandwidth
  capacity.
• RIP updates occur every 30 seconds and contain
  the entire routing table contents.
• As the network size increases, convergence time
  increases, as does overhead (table sizes
  increase).
• Two versions of RIP
• RIP version 1, defined by RFC 1058 (STD 34) 6/88
• RIP version 2, defined by RFC 2453 (STD 56) 8/99

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Example of Distance-Vector RoutingPrior to
information exchange

• Prior to the exchange of routing information,
  routers are only aware of directly-connected
  networks.

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Example of Distance-Vector RoutingAfter first
information exchange

• The first exchange of routing information results
  in the additions to the routing tables shown
  below the line in the diagram.

Net Metric Via
Net Metric Via
Net Metric Via
Net Metric Via
1 0 DC 2 0 DC 3 1
B
2 0 DC 3 0 DC 1 1
A 4 1 C
3 0 DC 4 0 DC 2 1
B 5 1 D
4 0 DC 5 0 DC 3 1
C
30 sec
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Example of Distance-Vector RoutingNext periodic
update

• The next periodic update results in the changes
  below. Note A and D still do not have complete
  knowledge of the topology.

Net Metric Via
Net Metric Via
Net Metric Via
Net Metric Via
1 0 DC 2 0 DC 3 1
B 4 2 B
2 0 DC 3 0 DC 1 1
A 4 1 C 5 2 C
3 0 DC 4 0 DC 2 1
B 5 1 D 1 2 B
4 0 DC 5 0 DC 3 1
C 2 2 C
30 sec
60 sec
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Example of Distance-Vector RoutingThird iteration

• In this example, it took three iterations and 90
  seconds for this network to converge.

Net Metric Via
Net Metric Via
Net Metric Via
Net Metric Via
1 0 DC 2 0 DC 3 1
B 4 2 B 5 3 B
2 0 DC 3 0 DC 1 1
A 4 1 C 5 2 C
3 0 DC 4 0 DC 2 1
B 5 1 D 1 2 B
4 0 DC 5 0 DC 3 1
C 2 2 C 1 3 C
30 sec
60 sec
90 sec
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Populating the Route Table

• Route Sources
• Routes are delivered to the RIB from a variety of
  different sources.
• The RIB does not pass routes back to the sources
• Based on the routes it receives, it decides which
  routes to forward to the Route Table
• This decision is based on Preference
• Autonomous System Boundary Router (ASBR)
• Adds external routes to OSPF
• Called OSPF-ASE routes

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Private IP-Adressen
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TCP Transmission Control Protocol

• Nutzt IP
• Stellt sicher, dass Daten
• korrekt
• in der richtigen Reihenfolgen übertragen werden
• Verbindungsorientiert
• Zuverlässig

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Sockets und Ports

• Daten, die an einer IP-Adresse ankommen, müssen
  an das richtige Transportprotokoll und dann an
  die richtige Anwendung übertragen werden.
• IP nutzt Protokollnummern zur Identifikation der
  Transportprotokolle (z. B. TCP 6)
• TCP benutzt Ports zur Definition der
  Anwendungsprotokolle / Anwendungen
• Well known ports
• Dynamically allocated ports

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Well known ports

• 1 TCP Port Service Multiplexer (TCPMUX)
• 5 Remote Job Entry (RJE)
• 7 ECHO
• 18 Message Send Protocol (MSP)
• 20 FTP -- Data
• 21 FTP -- Control
• 22 SSH Remote Login Protocol
• 23 Telnet
• 25 Simple Mail Transfer Protocol (SMTP)
• 29 MSG ICP
• 37 Time
• 42 Host Name Server (Nameserv)
• 43 WhoIs
• 49 Login Host Protocol (Login)
• 53 Domain Name System (DNS)
• 69 Trivial File Transfer Protocol (TFTP)
• 70 Gopher Services
• 79 Finger
• 80 HTTP

• 137 NetBIOS Name Service
• 139 NetBIOS Datagram Service
• 143 Interim Mail Access Protocol (IMAP)
• 150 NetBIOS Session Service
• 156 SQL Server
• 161 SNMP
• 179 Border Gateway Protocol (BGP)
• 190 Gateway Access Control Protocol (GACP)
• 194 Internet Relay Chat (IRC)
• 197 Directory Location Service (DLS)
• 389 Lightweight Directory Access Protocol (LDAP)
• 396 Novell Netware over IP
• 443 HTTPS
• 444 Simple Network Paging Protocol (SNPP)
• 445 Microsoft-DS
• 458 Apple QuickTime
• 546 DHCP Client
• 547 DHCP Server
• 563 SNEWS

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IPv6

• 128 bit IP-Adressen
• 00000000000032100123456789ABCDEF
• 32100123456789ABCDEF
• Vereinfachte Struktur des Headers.
• Verkettete Header für den Transport von Optionen.
• Optionen für Verschlüsselung und Authentisierung
  auf IP-Ebene.
• Neue Klassifizierung von Datenströmen (Flows) für
  einen optimierten Transport von Audio- und
  Video-Daten.
• Vereinfachung der manuellen Konfiguration.
• Verbesserung der Flusskontrolle und der Erkennung
  von Engpässen.
• Spezielle Mechanismen zur Entdeckung und
  Überwachung von Nachbarn beim Einsatz auf Routern

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Domain Name Service (DNS)

• IP-Adressen ? ? Rechnernamen
• Weltweites verteiltes System
• Baumförmige Struktur
• Je Domain
• Primary name server
• Secondary name server

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Rekursive Anfragen
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Domain Name Service (DNS)
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Client/Server
Request
Reply
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Client/Server Beispiel E-Mail
Aufbau einer Mailnachricht
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Client/Server Beispiel E-Mail
Beispielschlüsselwörter einer Mailnachricht
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Client/Server Beispiel E-Mail

• MIME-Standard
• Ursprünglich nur für ASCII-Text
• Möglichkeit Binärdaten zu versenden (Als
  Hexzahlen)
• MIME Multipurpose Internet Mail Exchange
• z.B. im Mailkopf
• MIME-Version 1.0Content-Type Multipart/Mixed
  BoundaryMime_separator
• und vor jedem Teil der E-Mail-Nachricht der
  passende Separator
• z.B.Content-Type text/plain
• Damit sehr flexibel!

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Client/Server Beispiel E-Mail
Mail Transport
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Client/Server Beispiel E-Mail

• Mail Transport
• mit SMTP (Simple Mail Transport Protocol) wird
  die Mail transportiert
• Store-and-Forward-Konzept

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Client/Server Beispiel E-Mail

• Mail-Exploder, Listen und Forwarder
• Maillisten erlauben Mailtransport an Gruppen
• Der Exploder überprüft Empfangsmail in seiner
  Datenbank und Forwarded Kopien der Mail an viele
  Empfänger (Wieviele bei Freunde_at_wit.com?)

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Client/Server Beispiel E-Mail

• Mail-Gateways/Mail-Relay
• Mail-Gateway Exploder und Mailtransfer-Programm
  auf einem Rechner
• Der Exploder überprüft Empfangsmail in seiner
  Datenbank und Forwarded Kopien der Mail an viele
  Empfänger (Wieviele bei Freunde_at_wit.com?)

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Client/Server Beispiel E-Mail

• Mail-Listenmanager
• Abarbeiten von Routinearbeiten automatische
  Pflege von Maillistenadd ltmailboxgt to
  ltlistgt(oder subscribe und unsubscripe, usw.)
• Oder Vereinheitlichen von Mailadressen
• Menzelk_at_fh-brandenburg.de und fischer_at_fh-brandenbu
  rg.de sind gültige Email-Adressen. Diese kommen
  beim Mail-Gateway an und werden vom Exploder
  unterschiedlich an deren Mailbox weitergeleitet
  z.B. menzelk_at_zeus.fh-brandenburg.de und
  fischer_at_wotan.fh-brandenburg.de
• gt Datenbank für Mailbox-Identifizierer im
  Mail-Gateway.

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Client/Server Beispiel E-Mail

• Zugang zur Mailbox
• Eigenes Protokoll für den Zugang zur Mailbox
  Post Office Protocol (POP)