Transmission Control Protocol

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Transmission Control Protocol
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Review

• How does the router know where to send an IP
  packet?
• Whats in the packet? Whats in the router?
• How does a destination host know what order a
  series of packets was sent in?
• If we were to define a streaming audio protocol
  as a spanning layer
• Where would it have to run?
• Could people use different media players to
  listen to audio streamed with this protocol?

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Learning Objectives

• Understand TCP and relationship to IP
• Service
• Protocol
• Commercial Internet Provision
• Network Address Translation
• The End-to-End Principle

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Conceptual Layering of Internet Protocols
Application
HyperText Transport Protocol (HTTP)
Real-time Transport Protocol (RTP)
Application Layer
User Datagram Protocol (UDP)
Transmission Control Protocol (TCP)
Transport Layer
Todays topic
Internet Protocol (IP)
Network Layer
Subnetworks
Physical and Data Link Layer
Last sessions topic
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TCP Services

• TCP picks up where IP leaves off
• Message fragmentation and reassembly
• Packet sequence numbers
• Guarantees delivery
• Error detection
• ACKs and retransmits
• Maintains conversational context
• Sessions
• Specifies process at destination
• Port numbers in TCP packet header

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Layered View of TCP and IP
Reliable connection between peers
TCP
TCP
Best-effort (unreliable) delivery between peers
IP
IP
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Layered Headers

• Each layer adds its own header to the message
  that it receives from the layer above
• As far as the lower layer is concerned, the
  higher layers headers are just a part of the
  message to be delivered
• The higher layers never see the lower layer
  headers because the lower layers remove them
  before passing the message up

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Layered Headers In TCP/IP

• The TCP layer fragments a message into packets,
  puts headers on the packets, and passes them to
  the IP layer
• The IP layer adds its own header to each packet
  that it receives from TCP
• The IP layer removes its header before passing
  the delivered packet back to the TCP layer
• The TCP layer removes its headers and reassembles
  the message before passing the result back to the
  layer above

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Generalized View of Layered Communication
Protocols
Layer n1 protocol header
Layer n1
Layer n1
Peer-to-peer operation
Layer n
Layer n
Layer n protocol header
Encapsulated layer n1 packet, including header
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TCP Protocol Message Fragmentation And Reassembly

• TCP client at source divides message into packets
• Each packet gets a sequence number
• Stored in the header
• TCP packet becomes payload of IP packet
• TCP software at destination reassembles
• If arrive out of order, use sequence number

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Header
Payload
Original packet
TCP divides original into packet fragments
Adds headers to new packets and passes to IP to
deliver
At destination, TCP uses headers to correctly
reassemble original packet
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TCP Fragmentation And Sequencing Demo

• Same routing tables as on Thursday
• Dont forget to decrement HopLimit
• New message takes several index cards
• Each index card is a TCP fragment
• The card is placed in a small envelope that has a
  TCP header
• The small envelope is placed in a big envelope
  with an IP header

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Why Fragment Messages?

• If part of message is lost or garbled, you only
  have to resend the affected packet(s)
• Speed
• Store-and-forward delay is minimized
• A can send packet 1 to B while receiving packet 2
  from S
• Not possible if whole message sent at once

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TCP Protocol Guaranteed Delivery

• Error Detection on TCP Packets
• Checksum detects if IP packet is corrupt
• E.g., parity check even or odd number of 1s in
  payload
• If error detected, discard packet
• Sender remembers packets it sent
• Receiver ACKs each packet received
• Clever optimization piggyback ACK to data packet
  already flowing other direction
• If ACK not received within a specified timeout
  interval, the sender resends packet
• Receiver may get two copies!
• Just ignore the second one

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TCP Guaranteed Delivery Demo

• Same as before, except
• The sender will retransmit if ACK not received
  within the timeout interval
• When packet reaches destination, receiver must
  acknowledge by telling the class the sequence
  number of the packet received
• The TCP header includes error checking
  information.

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TCP Service Conversational Context

• Two machines must establish a connection before
  they can exchange data
• Must agree on a session ID before sending first
  message
• Each message includes the session ID
• At end of conversation the machines agree that
  the conversation is over
• Called session tear-down

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TCP Service Specify Process at Destination

• TCP packets specify a source and destination port
  number
• The source and destination port numbers do not
  have to be the same
• The port number is used to determine which
  process (application) will receive the message
• For example, port 80 specifies that the message
  should be sent to a web server, while port 23
  indicates that the message is destined for a
  Telnet server

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TCP Service Specify Process at Destination
Port
Port
IP host-to-host
Process
Process
Host
Host
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Identifying A Connection Another Use Of TCP
Ports

• Each side of a TCP connection is referred to as a
  socket, and can be identified by the IP address
  and port
• A logical connection between a source and
  destination host is uniquely identified by the
  two sockets involved

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Questions?

• About TCP services?
• About anything else?

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TCP Congestion

• If a router is overworked
• Its queues fill up
• It drops additional packets
• Because TCP provides guaranteed delivery through
  ACK, the source clients resend the packets
• Even more congestion

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TCP Congestion Control

• Start slow, using a low rate of packet sends
• Slowly increase the rate as long as ACKs are
  regularly received
• Quickly decrease if packet ACKs are slow (not
  received within timeout interval)
• Called exponential backoff
• The algorithm for determining when and how much
  to backoff is a potential opportunity for
  innovation
• E.g., FAST TCP in optional readings

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Flow Control

• Similar to congestion, but not due to network
  interference
• Destination hosts own traffic is too much
• Destination unable to receive so fast
• TCP flow control implementation is similar to
  solution for congestion control
• Receiver can explicitly request lower send rate
  in ACKs

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Summary of TCP/IP

• IP
• Best-effort delivery to remote destination
• Globally unique addresses
• Routers forward to next hop
• TCP
• Message fragmentation and reassembly
• Guaranteed delivery
• Sessions
• Port numbers
• Congestion and flow control

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Review

• Which layer in the network protocol stack is
  responsible for each of the following?
• Breaking a message into smaller pieces for
  transmission over the network
• Placing a header on the message
• Specifying the destination address
• Specifying the process that will receive the
  message
• Directing a packet as it travels over the network

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Commercial Provision of TCP/IP

• Internet Access Providers (IAPs)
• Customers connect computer or network to IAP
  (ISP)
• IAP connects to rest of the Internet
• Routes may involve intermediate transit carriers
• Transit traffic neither source nor destination
  address is a direct customer of the carrier
• Interconnection between transit carriers
• Peering
• NAPs (multilateral peering between IAPs)
• Paid interconnects (small providers to big ones
  that carry lots of transit traffic)
• Usually flat rate based on maximum bitrate, not
  actual bitrate of transfers

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Internet Interconnections
NAP
Hosts
Big IAPs Subnet
Point of Presence
Small IAP
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More Internet Interconnections
Transit carrier for small IAP
Peering
Hosts
Point of Presence
Small IAP
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NAT Sharing an IP Address

• Network Address Translationthat indirection
  thing again
• Some IP addresses are not globally unique
  designated for local use/reuse
• Host has local address behind the NAT device
• NAT translates outgoing packet headers
• Rewrites source IP address to be NATs IP address
• Rewrites source port
• Saves map of internal-external address/port
  mappings
• Reverse for incoming packet headers

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NAT Example

• Hypothetical example
• Sources internal socket is IP 192.168.0.101,
  port 1341
• Translated to IP 68.40.162.3 (address assigned to
  customer by IAP) port 5280
• Traffic from NAT device identifies translated
  address/port as source
• NAT forwards traffic sent to it (IP 68.40.162.3)
  with port 5280 to source host (192.168.0.101)
  using port 1341

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NAT Example
Messages sent between host B to another host on
the Internet Host B original source
socket 192.168.0.101 port 1341 Host B translated
socket 68.40.162.3 port 5280
IAPs Point of Presence
A
B
C
Router with NAT External IP 68.40.162.3 Internal
IP 192.168.0.0
Router assigns internal IPs to hosts on LAN A
192.168.0.100 B 192.168.0.101 C 192.168.0.102
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NAT Advantages and Disadvantages

• Rest of Internet unaffected
• Transparent benefit of indirection
• Internal addresses dont need to be globally
  unique
• Allows simultaneous use benefit of local
  namespace
• Multiple hosts can share one external IP address
• E.g., useful for home networks
• but protocols that depend on IP address of host
  (e.g., early versions of Kerberos) get confused
• Host says it has an internal IP address
• Different (external) address used for routing

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The End-to-End Principle

• If function needs to be done as
  endpoint-to-endpoint service, dont implement it
  link-to-link
• More generally, if function will need to be
  (re)done at higher layer, dont do it at lower
  layer
• Examples in TCP/IP
• Error detection
• Guaranteed delivery of packets
• Not encryption -- implemented at higher layer

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The End-to-End Principle
Layer n1
Layer n1
If you need it at this layer, implement it here
Layer n
Layer n
And dont implement it here.