Computer Science 328 Distributed Systems

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Computer Science 328Distributed Systems

• Lecture 4
• NETWORKING (Contd)

2
Congestion Control

• Is achieved by informing nodes along a route that
  congestion has occurred and asking them to reduce
  their packet transmission rate.
• Congestion information can be supplied by
• explicit transmission of special messages (called
  choked packets).
• implementation of a specific transmission control
  protocol (e.g., TCP takes packet loss as an
  indication of congestion and will hence reduce
  its congestion window the number of packets
  allowed to be transmitted before receipt of a
  positive acknowledgement).

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Mobile IP
Sender
Subsequent IP packets
Mobile host MH
tunnelled to FA
Address of FA
returned to sender
First IP packet
addressed to MH
Internet
Foreign agent FA
Home
First IP packet
agent
tunnelled to FA
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Transport Layer

• Function 1 (Message decomposition and
  reassembly) Breaks messages into packets at the
  transmitting end and reassembles packets into
  messages at the receiving end.
• Function 2 (Multiplexing and demultiplexing)
  Multiplexes several lower-rate sessions, all from
  the same source and all going to the same
  destination, into one session at the network
  layer.
• Function 3 (Reliable communication) Performs
  message retransmission if the underlying network
  layer is not reliable.
• Function 4 (End-to-end congestion/flow control)
  Avoids sending data faster than the destination
  can absorb it and cooperates with network layer
  entities (e.g., routers) for congestion control.

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TCP Header
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TCP Window Mechanism
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TCP Congestion Control
Congestion avoidance
Slow start
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TCP Congestion Control -- AIMD

• Slow start phase
• During slow start, a TCP increments cwnd by at
  most one TCP segment for each ACK received.
• Slow start ends when cwnd exceed ssthresh, or
  when congestion is observed.
• Congestion avoidance phase
• During congestion avoidance, cwnd is incremented
  by 1 segment per round-trip time. In practice,
  TCP does not wait for an entire window's worth of
  ACKs to add one segment to the congestion window,
  but instead increments cwnd by a little for each
  ACK that arrives, i.e.,
• cwnd ? cwnd 1/ cwnd.
• Upon arrival of 3 duplicate ACKs (i.e., 4
  identical ACKs),
• cwnd ? cwnd/2.
• Upon timeout
• cwnd ? 1.
• Why does TCP not perform well in wireless
  environments?
• Uses packet losses as indication of congestion.

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TCP Error Control Packet Retransmission

• A TCP receiver sends an immediate duplicate ACK
  when an
• out-of-order packet arrives, informing the
  sender of the SN expected.
• Upon arrival of 3 duplicate ACKs (i.e., 4
  identical ACKs) or timeout,
• The sender performs a retransmission of the
  missing packet.

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TCP Flow Control

• From time to time the receiver may include a
  receiver advertisement window RcvWin telling the
  sender its maximum allowable buffer for this
  connection.
• The sender makes sure
• LastByteSent LastByteAcked lt min(CongWin,
  RcvWin)

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Session/Presentation/Application Layers

• The session layer deals with access rights in
  setting up sessions and other interactions
  between the two end points in setting up
  sessions.
• The presentation layer deals with data
  encryption, data compression, and code
  conversion.
• The application layer actually does the work
  required by the users, e.g., FTP, telnet, WWW.
• The application/presentation/session layers are
  not clearly distinguished in the Internet
  protocol stack.

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DNS Domain Name System

• Domain Name System
• distributed database implemented in the hierarchy
  of many name servers
• application-layer protocol that is responsible
  for resolving names (address/name translation)

• People many identifiers
• SSN, name, Passport
• Internet hosts, routers
• IP address (32 bit) - used for addressing
  datagrams
• name, e.g., sal.cs.uiuc.edu - used by humans
• Q map between IP addresses and name ?

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DNS Name Servers

• no server has all name-to-IP address mappings
• local name servers
• each ISP, company has local (default) name server
• host DNS query first goes to local name server
• authoritative name server
• for a host stores that hosts IP address, name
• can perform name/address translation for that
  hosts name

• Why not centralize DNS?
• single point of failure
• traffic volume
• distant centralized database
• maintenance
• doesnt scale!

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DNS Root Name Servers

• contacted by local name server that can not
  resolve name
• root name server
• contacts authoritative name server if name
  mapping not known
• gets mapping
• returns mapping to local name server
• dozen root name servers worldwide

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Simple DNS Example
root name server
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4

• host surf.eurecom.fr wants IP address of
  dragon.cs.uiuc.edu
• 1. Contacts its local DNS server, dns.eurecom.fr
• 2. dns.eurecom.fr contacts root name server, if
  necessary
• 3. root name server contacts authoritative name
  server, dns.cs.uiuc.edu, if necessary

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5
authorititive name server dns.cs.uiuc.edu
1
6
requesting host surf.eurecom.fr
dragon.cs.uiuc.edu
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DNS Example
root name server

• Root name server
• may not know the authoritiative name server
• may know intermediate name server who to contact
  to find authoritative name server

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2
3
7
5
4
1
8
authoritative name server dns.cs.uiuc.edu
requesting host surf.eurecom.fr
dragon.cs.uiuc.edu
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DNS Iterated Queries
root name server

• recursive query
• puts burden of name resolution on contacted name
  server
• heavy load?
• iterated query
• contacted server replies with name of server to
  contact
• I dont know this name, but ask this server

iterated query
2
3
4
7
5
6
1
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authoritative name server dns.cs.uiuc.edu
requesting host surf.eurecom.fr
dragon.cs.uiuc.edu
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DNS Caching and Updating Records

• once (any) name server learns mapping, it caches
  mapping
• cache entries timeout (disappear) after some time
• update/notify mechanisms under design by IETF
• RFC 2136
• http//www.ietf.org/html.charters/dnsind-charter.h
  tml

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Firewalls

• A set of processes that monitor control all
  comm. Into and out of an interanet, for
• Service control
• Behavior control
• User control
• Firewall filtering can be done at diff. Levels
• IP packet filtering
• TCP gateway filtering
• Application gateway filtering

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Firewall Configurations