Outline

• Reliable transmission:
  • Alternating Bit Protocol
  • Sliding Window Protocol
  • sequence number size
• Virtual Circuits

Stop-and-Wait

• Send a packet, wait for ack.
• If ack is not received within timeout, retransmit frame.
• two possibilities:
  1. ack is lost, sender retransmits
  2. ack is received, sender transmits next packet
  receiver must be able to distinguish the two cases
• [ ==> ] we need a 1-bit sequence number to distinguish the two cases
• Alternating-Bit Protocol

Sliding Window

• Stop-and-Wait with multiple outstanding packets
• window size W
• last ack received by sender is LAR
• sender can send up to LAR + W
• new ack moves LAR, allowing sender to send more packets
• next packet expected by receiver is NPE
• maximum packet expected by receiver is NPE + W - 1
• any packet outside this range is discarded
• cumulative ack sent when NPE changes

Sliding Window

• reliable delivery
• if W >= bandwidth * delay, the "pipe" remains full of bits (if receiver is faster than the network)
• in-order packet delivery is easy
• flow control is easy:
  • receiver could withhold acks (leading to retransmissions)
  • receiver might tell sender to reduce W
• separation of concerns: one mechanism, many functions

Sequence Numbers

• why should receiver discard packets outside the window?
• Stop-and-Wait is sliding window with W=1
• Stop-and-Wait receiver must discard packet other than expected, since it is an old retransmission (due to lost ack)
• sliding window receiver must be able to distinguish W valid packets from W retransmitted packets
• [ ==> ] sliding window receiver must be able to distinguish 2W different packets
• [ ==> ] sequence number space must be at least 2W, e.g. we need at least 5 bits if W=16
• delayed acks require much more than 2W

in-class exercise

• how fast can we transmit data if we have:
  • network bandwidth 10Mb/s (10^7 bits/s)
  • network round-trip delay 100ms (10^{-1}s)
  • window size 12.5KB (1.25 * 10^4 bytes)
• assume:
  • the network does not lose any packets
  • cumulative acks
  • the acks do affect the amount of bandwidth available in the "forward" direction
• write down the number visibly on a sheet of paper and hold it up at the end of the exercise

Packet and Circuit switching

• packet switching: each packet is forwarded independently of other packets, even though they may have the same source and destination address (connectionless)
• examples: Ethernet, FDDI, IP
• circuit switching: each packet is forwarded as part of a connection. A connection must be established before any packet can be sent, and must be torn down to reclaim the resources associated with the connection (connection-oriented)
• examples: ATM, TCP, telephone

Packet and Circuit switching

• packet switching:
  • simpler to use
  • provides less information (both to the network and to the host)
  • has no persistent state -- must use caching for efficiency
• circuit switching:
  • provides more information -- e.g. bandwidth and delay requirements or guarantees, how many senders are using this link
  • has persistent state
  • requires connection setup and teardown, state management

Virtual Circuit Switching

• provides many circuits on a single link
• each packet on a link is tagged with a virtual circuit identifier (VCI)
• the VCI has local significance only, is not an end-to-end identifier: the same VCI can be reused for different connections on different links
• because the same VCI can be reused for different connections on different links, it can be allocated independently for each link (no co-ordination among switches is needed)
• forwarding means identifying the connection, rewriting the VCI, placing on output port

Virtual Circuit Example

• I am a switch with two ports, A and B
• port A is connected to host x
• port B is connected to a switch which is connected to hosts y and z
• I have two virtual circuits, one from x to y, one from x to z
• the VC from x to y has VCI 0 on port A, and VCI 1 on port B
• the VC from x to z has VCI 1 on port A, and VCI 0 on port B
• a packet on port A with VCI=0 must be from x to y, I place it on port B with VCI=1
• a packet on port B with VCI=0 must be from z to x