CS 3830 Day 15 Introduction 1-1. Announcements r Quiz 3: Wednesday, Oct 10 r Prog3 due (in 1DropBox)...
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Transcript of CS 3830 Day 15 Introduction 1-1. Announcements r Quiz 3: Wednesday, Oct 10 r Prog3 due (in 1DropBox)...
Announcements
Quiz 3: Wednesday, Oct 10 Prog3 due (in 1DropBox) on Wednesday,
Oct 10 Prog4:
Parts A and B Work in roups of two. Sign-up for groups IN CLASS by
Wednesday, Oct 10• If not, I will assign random groups!
Transport Layer 3-2
Transport Layer 3-3
Chapter 3 outline
3.1 Transport-layer services
3.2 Multiplexing and demultiplexing
3.3 Connectionless transport: UDP
3.4 Principles of reliable data transfer
3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection
management
3.6 Principles of congestion control
3.7 TCP congestion control
Transport Layer 3-4
Principles of Reliable data transfer important in app., transport, link layers top-10 list of important networking topics!
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer 3-5
Principles of Reliable data transfer important in app., transport, link layers top-10 list of important networking topics!
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer 3-6
Principles of Reliable data transfer important in app., transport, link layers top-10 list of important networking topics!
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer 3-7
Reliable data transfer: getting started
sendside
receiveside
rdt_send(): called from above, (e.g., by app.). Passed data to deliver to receiver upper layer
udt_send(): called by rdt,to transfer packet over unreliable channel to
receiver
rdt_rcv(): called when packet arrives on rcv-side of channel
deliver_data(): called by rdt to deliver data to
upper
Transport Layer 3-8
Reliable data transfer: getting startedWe will: consider only unidirectional data transfer
but control info will flow on both directions!
assume data sent first are received first use finite state machines (FSM) to specify
sender, receiver
state1
state2
event causing state transitionactions taken on state transition
state: when in this “state” next state
uniquely determined by
next event
eventactions
Transport Layer 3-9
Rdt1.0: reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender, receiver: sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packet,data)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Reliable transfer over an unreliable channel Pkts may be corrupted or dropped! Solution:
Pkts should have sequence numbers Only two sequence numbers suffice: 0 and 1 Received pkts ACKed ACKs have sequence number of pkt being
ACKed Sender will have countdown timer
We will develop rdt3.0 from the book (see the book for intermediate rdt versions)
Transport Layer 3-10
Receiver
Always ACK last “good” pkt: If waiting for pkt0 and receive corrupt pkt, send ACK1 If waiting for pkt1 and receive corrupt pkt, send ACK0 If waiting for pkt0 and receive pkt0, send ACK0 If waiting for pkt0 and receive pkt1, send ACK1 If waiting for pkt1 and receive pkt0, send ACK0 If waiting for pkt1 and receive pkt1, send ACK1
Only 2 states needed!
Transport Layer 3-11
Transport Layer 3-12
rdt_rcv(rcvpkt) && (corrupt(rcvpkt) || has_seq0(rcvpkt))
Wait for 1 from below
Wait for 0 from below
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt)
extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(ACK1, chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) && (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
What if first pkt is corrupt?
rdt3.0 receiver
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq0(rcvpkt)
extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(ACK0, chksum)udt_send(sndpkt)
udt_send(sndpkt)
Transport Layer 3-13
Must check if received ACK corrupt If waiting for ACK0 and receive corrupt pkt, re-send pkt0 If waiting for ACK1 and receive corrupt pkt, re-send pkt1
Duplicate ACK: resend current pkt If waiting for ACK0 and receive ACK0, send pkt1 If waiting for ACK0 and receive ACK1, re-send pkt0 If waiting for ACK1 and receive ACK0, re-send pkt1 If waiting for ACK1 and receive ACK1, send pkt0
ACK gets dropped: If waiting for ACK0 and receive no ACK, re-send pkt0 If waiting for ACK1 and receive no ACK, re-send pkt1
Handle re-sending of all pkts with countdown timer
Sender
Transport Layer 3-14
sndpkt = make_pkt(0, data, checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isACK(rcvpkt,1) )
Wait for call 1 from
above
sndpkt = make_pkt(1, data, checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0)
rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) ||isACK(rcvpkt,0) )
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
rdt3.0 sender