Computer Networks

1
Computer Networks

• Section1. Routing Algorithms and Network Layer
  Protocol
• Presenter Shu-Ping Lin

2
Outline

• Routing Algorithms
• The Network Layer in The Internet

3
Outline

• Routing Algorithms
• The Network Layer in The Internet

4
Store-and-Forward Packet Switching
5
Routing Algorithm

• Routing algorithm
• Datagram
• Virtual circuit
• Differences between routing and forwarding
• Routing versus forwarding

6
Routing Algorithm (contd)

• Requirement of routing algorithm
• Correctness and Simplicity
• Robustness
• Stability
• Fairness
• Optimality

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Datagram Routing
8
Virtual-Circuit Routing
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Comparison
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Routing Algorithm (contd)

• Nonadaptive (static routing)
• Do not base routing decisions on measurement of
  the current traffic and topology.
• The route used to get from node to node is
  computed in advance.
• Adaptive (dynamic routing)
• Change routing decisions to reflect changes in
  topology and traffic.

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Optimality Principle

• If router J is on the optimal path from router I
  to router K, the the optimal path from J to K
  also falls along the same route.
• Sink tree
• The set of optimal routes from all sources to a
  given destination
• The goal of all routing algorithms is to discover
  and use the sink trees for all routers.

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Sink Tree
13
Shortest Path Routing

• Link metric
• Dijkstra algorithm

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Flooding

• Every incoming packet is sent out on every
  outgoing link except the one it arrived on.
• Termination of flooding process
• Hop counter
• Record of packet which has been flooded
• Applications of flooding
• Military application
• Comparison

15
Distance Vector Routing

• Each router maintains a table giving the best
  known distance to each destination and which line
  to be used.
• Tables are updated by exchanging information with
  the neighbors.
• Items in the table
• Preferred outgoing line
• Estimate of the distance to that distance

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Distance Vector Routing (contd)
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Distance Vector Routing (contd)

• Converge to the correct answer, but do so slowly.
• It reacts rapidly to good news, but leisurely to
  bad news.
• The count-to-infinity problem

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Count-to-Infinity Problem
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Problem of DVR

• Does not take line bandwidth into account.
• Take too long to converge.

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Link State Routing

• Learning about the neighbors
• Sending a special HELLO packet on each
  point-to-point line
• Measuring line cost to each of its neighbors
• Round-trip time
• Traffic load
• Line bandwidth

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Link State Routing (contd)

• Building link state packet

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Link State Routing (contd)

• Distributing the link state packets
• Flooding is used to distribute the link state
  packet.
• Each packet contains a sequence number that is
  incremented for each new packet sent.
• Computing the new routes
• Dijkstras algorithm can be run locally to
  construct the shortest path to all destinations.

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Hierarchical Routing

• As networks grow in size, the router routing
  tables grow proportionally.
• Each router has responsible for its region and
  knows nothing about the internal structure of
  other router.

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Hierarchical Routing (contd)
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Hierarchical Routing (contd)

• Penalty of increased path length
• The optimal number of levels for an N router
  subnets is ln N.

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Broadcasting Routing

• The source simply send a distinct packet to each
  destination.
• Waste bandwidth
• Have to know complete list of all destinations
• Flooding
• Generate too many packets
• Consume too much bandwidth

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Broadcasting Routing (contd)

• Multidestination routing
• Each packet contains a list of destinations.
• The destination set is partitioned among the
  output lines.
• Using spanning tree
• Routers must know which of its lines belong to
  the spanning tree in advance.
• Copy an incoming packet onto all the spanning
  lines except the on it arrived on.

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Broadcasting Routing (contd)

• Reverse path forwarding
• Router checks to see if the packet arrived on the
  line that is normally used for sending packets to
  the source of the broadcast.

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Multicast Routing

• Each router computes a spanning tree covering all
  other routers.
• When a process sends a multicast packet to a
  group
• Examining the spanning tree of this group
• Removing all lines that do not lead to hosts that
  are members of group

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Multicast Routing (contd)
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Multicast Routing (contd)

• Pruning spanning tree
• Reverse path forwarding
• Router with no hosts interested in a particular
  group sends a PRUNE message.
• The subnet is recursively pruned.

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Multicast Routing (contd)

• Disadvantage of algorithm, suppose that network
  has n groups, each with an average of m members.
• For each group, m pruned spanning trees must be
  stored
• Total of mn trees
• Core-based tree
• A host wanting to multicast sends packets to the
  core, which does the multicast along the spanning
  tree.

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Routing for Mobile Hosts
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Routing for Mobile Hosts (contd)

• Mobile host
• Migratory hosts
• Roaming hosts
• All hosts are assumed to have a permanent home
  location that never changes.
• Hosts also have a permanent home address that can
  be used to determine their home location.

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Routing for Mobile Hosts (contd)

• Each area has a home agent which keeps track of
  hosts whose home is in the area, but who are
  currently visiting another area.
• Each area has foreign agent which keeps track of
  all mobile hosts visiting this area.

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Routing for Mobile Hosts (contd)

• Registration procedure
• Foreign agent searching
• Registration
• Foreign agent contacts the mobile hosts home
  agent.
• Verification of mobile hosts home agent
• Acknowledgement from the mobile hosts home agent

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Routing for Mobile Hosts (contd)

• Tunneling
• Encapsulate the original packet in the payload
  field of an outer packet and sends the latter to
  the foreign agent.

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Routing for Mobile Hosts (contd)
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Routing in Ad Hoc Networks

• AODV (Ad hoc On-demand Distance Vector)
• Distant relative of the Bellman-Ford distance
  vector algorithm
• Considering the limited bandwidth and low battery
  life
• On-demand algorithm

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Routing in Ad Hoc Networks (contd)

• AODV algorithm maintains a table at each node,
  keyed by destination and which neighbor to send
  packets in order to reach destination.
• Route request packet

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Routing in Ad Hoc Networks (contd)

• When a route request packet arrives at a node
• The (Source address, request ID) pair is looked
  up in a local history table too see if this
  request has already been seen and processed.
• Receiver looks up the destination in its route
  table.
• If receiver does not know a fresh route to the
  destination, it increments the Hop count field
  and rebroadcast the REQUEST packet.

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Routing in Ad Hoc Networks (contd)
43
Routing in Ad Hoc Networks (contd)

• Destination constructs a ROUTE REPLY packet which
  follows the reverse path to source.
• Each intermediate node enters this packet into
  local routing table when
• No route to destination is known.
• Sequence number for destination in REPLY is
  greater than the value in the routing table.
• Sequence number is equal but the new route is
  shorter.

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Routing in Ad Hoc Networks (contd)

• Problem of mobility
• Route maintenance
• Periodically, each node broadcasts a Hello
  message.
• If no response is returned, router prune this
  link.
• Active neighbor

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Routing in Ad Hoc Networks (contd)
46
Routing in Ad Hoc Networks (contd)

• Critical difference between AODV and DVR
• Nodes do not send out periodic broadcasts
  containing their entire routing table.

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Node Lookup in Peer-to-Peer Networks

• One of p2p routing algorithmChord
• Each user node has an IP address that can be
  hashed to an m-bit number call node identifier.
• Successor (k) is the node identifier of the first
  actual node following k around the circle
  clockwise.

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Node Lookup in Peer-to-Peer Networks (contd)

• Finger table
• Start field k 2i (modulo 2m)
• If key falls between k and successort (k), then
  the node holding information about key is
  successor (k).
• Otherwise, the entry whose start field is the
  closest predecessor of key is tried.

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Node Lookup in Peer-to-Peer Networks (contd)
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Node Lookup in Peer-to-Peer Networks (contd)

• Node r joining
• New node r asks successor (r) for its
  predecessor.
• Insert r in between successor and predecessor.
• Successor should hand over those keys in the
  range predecessor (r)-r, which now belong to r.
• Every node runs a background process that
  periodically recomputes each finger by calling
  successor.

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Outline

• Routing Algorithm
• The Network Layer in The Internet

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The Network Layer in The Internet

• Principles for network design
• Make sure it works.
• Keep it simple.
• Make clear choices.
• Exploit modularity.
• Expect heterogeneity.
• Avoid static options and parameters.
• Look for a good design it need not be perfect.

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The Network Layer in The Internet (contd)

• Think about scalability.
• Consider performance and cost.
• Network layer provides best-efforts way to
  transport datagrams from source to destination.

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The IP Protocol

• The IPv4 header

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IP Address

• IP address formats

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IP Address (contd)

• Special IP address

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Subnet

• Split a network into several parts for internal
  use.

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Subnet (contd)

• Some bits are taken away from the host number to
  create a subnet number.
• For example, a university can use a 6-bit subnet
  number and a 10-bit host number, allowing fro up
  to 64 subnets.
• Outside the network, the subnet is not visible,
  so allocating a new subnet does not require
  contacting ICANN.

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Subnet (contd)

• Subnet mask indicates the split between network
  subnet number and host.

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Network Address Translation

• Shortage of IP address
• Within the company, every computer gets a unique
  IP address, which is used for routing intramural
  traffic.
• An address translation takes place when a packet
  exits the company.

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Network Address Translation (contd)
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Network Address Translation (contd)

• Use TCP port to distinguish the traffic.
• TCP source port field is replaced by an index
  into the NAT boxs translation table.
• Each entry contains original IP address and
  original source port.

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Network Address Translation (contd)

• Violation of the architectural model of IP
• Every IP address uniquely identifies a machine.
• NAT changes the Internet from a connectionless
  network to a kind of connection-oriented network.
• Violation of protocol layering
• layer k may not make any assumptions about what
  layer k1 has put into the payload field.

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Internet Control Protocols

• Internet Control Message Protocol (ICMP)

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Address Resolution Protocol

• How do IP address get mapped onto data link layer
  addresses, such as Ethernet?
• Configuration file
• ARP
• Broadcast a packet onto the Ethernet asking Who
  owns IP address xxx.xxx.xxx.xxx.
• Correspondent will reply with its Ethernet address

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Address Resolution Protocol (contd)
67
Address Resolution Protocol (contd)

• Make ARP work more efficiently
• Local cache
• ARP request with requestors Ethernet address
• Transmit data to remote network.

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RARP, BOOTP, and DHCP

• When a computer is booted how does it get the IP
  address.
• RARP
• Broadcast a packet to ask RARP server.
• Packet cannot be forwarded by routers, so RARP
  server is needed on each network.

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RARP, BOOTP, and DHCP (contd)

• BOOTP
• Use UDP message, which are forwarded over
  routers.
• Require manual configuration of tables mapping IP
  address to Ethernet address.
• DHCP
• Both manual IP address assignment and automatic
  assignment

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RARP, BOOTP, and DHCP (contd)

• DHCP

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OSPFThe Interior Gateway Routing Protocol

• Routing algorithm within as autonomous system
  (AS) is called an interior gateway protocol.
• The original Internet interior gateway protocol
  was distance vector protocol (RIP).
• Link state protocol replaced RIP in 1979.
• OSPF began work in 1988.

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OSPFThe Interior Gateway Routing Protocol
(contd)

• Internet is made up of a large number of
  autonomous systems.
• Each AS is operated by a different organization
  and can use its own routing algorithm.
• Every AS has a backbone area.
• All areas are connected to the backbone and can
  communicate each other via backbone.

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OSPFThe Interior Gateway Routing Protocol
(contd)
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OSPFThe Interior Gateway Routing Protocol
(contd)

• Using flooding, each router informs all the other
  router in its area of its neighbors and costs.
• This information allows each router to construct
  the graph for its area and compute the shortest
  path.
• Backbone routers accept information from the area
  border routers in order to compute the best route
  from each backbone router to every other router.

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BGPThe Exterior Gateway Routing Protocol

• All interior gateway protocol has to do is to
  move packets as efficiently as possible without
  worrying about politics.
• Exterior gateway protocol routers have to worry
  about politics.
• No transit traffic through certain ASes.
• Traffic starting or ending at IBM should not
  transit Microsoft.
• Policies are typically manually configured into
  each BGP router and are not part of protocol.

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BGPThe Exterior Gateway Routing Protocol (contd)

• Stub networks
• Has only one connection to BGP graph and cannot
  be used for transit traffic.
• Multiconnected networks
• Could be used for transit traffic, except that
  they refuse
• Transit networks
• Willing to handle third-party packet

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BGPThe Exterior Gateway Routing Protocol (contd)

• Border Gateway Protocol (BGP)
• Fundamentally a distance vector protocol, but
  each BGP router keeps track of the path used.

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Internet Multicasting

• IP uses class D address to support multicast.
• Two kinds of group address
• Permanent address
• Temporary address

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Internet Multicasting (contd)

• 224.0.0.1 All systems on a LAN
• 224.0.0.2 All routers on a LAN
• 224.0.0.5 All OSPF routers on a LAN
• 224.0.0.6 All designated OSPF routers on a LAN
• Temporary groups must be created before they can
  be used.

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Internet Multicasting (contd)

• Multicasting routing is done using spanning tree.
• Each multicast router exchanges information with
  its neighbors, using a modified distance vector
  protocol.

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Mobile IP

• Increment of portable computers.
• Major goals
• Mobile host must be able to use its home IP
  address anywhere.
• Changes to software and router are not permitted.
• No overhead should be incurred when a mobile host
  is at home.

82
Mobile IP (contd)

• See section 5.2.9 (Routing for Mobile Hosts)
• When foreign shows up at a foreign site, it
  contacts the foreign host there and registers.
• The foreign host contacts the users home agent
  and gives it a care-of-address.

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Mobile IP (contd)

• Each foreign agent periodically broadcast its
  address and type of service, which called
  advertisement.
• Registration for impolite mobile hosts that leave
  without saying goodbye.

84
IPv6

• While NAT may buy a few more years time, IP in
  its current form (IPv4) is numbered.
• IETF issued a call for proposal and discussion in
  RFC 1550.
• IPv6 is not compatible with IPv4, but it is
  compatible with other auxiliary Internet protocol.

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IPv6 (contd)

• Main features
• Longer addresses
• Simplification of the header
• Better support for options
• Security
• Quality of service

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IPv6 (contd)

• The IPv6 fixed header

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IPv6 (contd)

• New notation of 16-byte address
• 80000000000000000123456789ABCDEF
• 8000123456789ABCDEF
• IPv4 can be written as 192.168.20.46
• Vanishment of IPv4 headers
• IHL
• Protocol
• Checksum

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IPv6 (contd)

• Extension header is encoded as (Type, Length,
  Value) tuple.
• Type is a 1-byte field telling which option this
  is.
• Length is a 1-byte field telling how long the
  value is.
• Value is any information required.

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IPv6 (contd)

• IPv6 extension header

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IPv6 (contd)

• Controversies
• Hop limit field length
• Maximum packet size
• Checksum
• Mobile issue

91

• Section2. The Internet Transport Protocols TCP

92
Introduction to TCP

• Function of providing reliable end-to-end byte
  stream over an unreliable internetwork.
• Lack of IP
• No guarantee that packets will be delivered
  properly.
• Packets may arrive in the wrong order.

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TCP Service Model

• TCP service is obtained by both sender and
  receiver creating sockets.
• Each socket has IP address and a 16-bit number
  called port.
• Port number below 1024 are called well-known
  ports and reserved for standard service.

94
TCP Service Model (contd)

• Well-Known Ports

95
TCP Service Model (contd)

• Characteristics of TCP connections
• Full duplex
• Point-to-point
• Byte stream, not a message stream

96
TCP Service Model (contd)

• A key feature of TCP is that every packet on a
  TCP connection has its own 32-bit sequence
  number.
• TCP segment consisting of fixed 20-byte header is
  used for exchanging data between sender and
  receiver.

97
TCP Service Model (contd)

• Segment size issue
• Fit in the 65515-byte IP paylo.ad
• Cant exceed maximum transfer unit (MTU).
• MTU is generally 1500 bytes.
• The basic protocol used by TCP is the sliding
  window protocol.
• Timer
• Acknowledgement number

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TCP Segment Header
99
TCP Connection Establishment

• Three-way handshake.

100
TCP Connection Release

• To release a connection , either party can send a
  TCP segment with the FIN bit set.
• When the FIN is acknowledged, that direction is
  shut down for new data.
• Normally, four TCP segments are required to
  release a connection.

101
TCP Connection Management Modeling
102
TCP Transmission Policy
103
TCP Transmission Policy (contd)

• When the window size is 0, the sender stop
  sending data to receiver except
• Urgent data may be sent.
• Send a 1-byte segment to make the receiver
  reannounce the next byte expected and window size.

104
TCP Transmission Policy (contd)

• Consider the worst case in performance issue.
• For receivers
• Delay acknowledgements and window updates for 500
  msec in the hope of acquiring some data.
• For senders
• Nagles algorithm

105
TCP Transmission Policy (contd)

• Silly window syndrome
• Data are passed to the sending TCP entity in
  large blocks, but an interactive application on
  the receiving side reads data 1 byte at a time.

106
TCP Transmission Policy (contd)

• Clarks algorithm
• It forces window update to wait until it has a
  decent amount of space available.
• Combination of Nagles and Clarks algorithm.

107
TCP Congestion Control

• When the load offered to any network is more than
  it can handle, congestion builds up.
• TCP achieves congestion control by dynamically
  manipulating the window size.
• First step of congestion control detection.
• All the Internet TCP algorithms assume that
  timeouts are caused by congestion.

108
TCP Congestion Control (contd)
109
TCP Congestion Control (contd)

• Two potential problemsnetwork capacity and
  receiver capacity.
• Each sender maintains two windowsreceiver window
  and congestion window.
• The number of bytes that may be sent is the
  minimum of the two windows.

110
TCP Congestion Control (contd)

• Slow start

111
TCP Timer Management

• Retransmission timer
• Difficulties of setting retransmission timer

112
TCP Timer Management (contd)

• Estimate RTT
• Where is a smothing factor that determines how
  much weight is given to the old value.
• TCP uses ßRTT to estimate retransmission timer.

113
TCP Timer Management (contd)

• Jacobson proposed proposed to use mean deviation
  as a cheap estimator of the standard deviation.

114
TCP Timer Management (contd)

• Potential problem
• When the ack of retransmission packet comes in,
  it is unclear whether the ack refers to the first
  transmission or a later one.
• Karns algorithm
• Persistence timer is design to prevent the
  deadlock.
• Keepalive timer.

115
Wireless TCP and UDP

• Problem due to congestion control.
• TCP assumes that timeouts are caused by
  congestion, not by lost packets.
• A packet is lost on a wired network, the sender
  should slow down.
• When one is lost on a wireless network, the
  sender should try harder.

116
Wireless TCP and UDP (contd)

• Bakne and Badrinath propose indirect TCP which
  split the TCP connection into two separate
  connections.

117
Wireless TCP and UDP (contd)

• The advantage of this scheme is that
  bothconnections are now homogeneous.
• Timeouts on the first connection can slow the
  sender down, whereas timeouts on the second one
  can speed it up.z
• The disadvantage of the scheme is that it
  violates the semantics of TCP.

118
Transactional TCP

• Efficiency problem of TCP