1 / 18

Communication Networks

Communication Networks. Recitation 7 RDT, CLOS & DV Revisited. Reliable Transmission. How do we send a packet reliably when it can be lost and/or corrupted in the network? Mechanisms Acknowledgements Timeouts Simplest reliable protocol: Simple Stop and Wait send a packet

edan-bullock
Télécharger la présentation

Communication Networks

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Communication Networks Recitation 7 RDT, CLOS & DV Revisited Comnet 2010

  2. Reliable Transmission • How do we send a packet reliably when it can be lost and/or corrupted in the network? • Mechanisms • Acknowledgements • Timeouts • Simplest reliable protocol: Simple Stop and Wait • send a packet • stop and wait until an ACK arrives from the receiver • retransmit if timeout occurs before ACK arrives • Is this protocol CORRECT ?! Comnet 2010

  3. Simple Stop and Wait Comnet 2010

  4. Recovering from Errors packet duplicate duplicate +missing Comnet 2010

  5. Problems with Simple Stop and Wait • Duplicate packets: • How does the receiver recognize a duplicate transmission (e.g. in the case of a lost ACK)? • Missing Packet: • In early timeout case. • Duplicate ACK. • Solution: Put sequence number in packet Comnet 2010

  6. Sequence Numbers • Detect retransmissions using sequence numbers • both packets and ACKs • Simple: unbounded # • Realistic: sequence # in packet is finite • Assuming FIFO channel • How many bits do we need? • 1 bit for stop and wait • Won’t send seq #1 until receive ACK for seq #0 Comnet 2010

  7. Sender: Init: n=1 Wait for data(n) Send data(n) Receive ACK(m) If m=n Then n = n+1 When timeout Send data(n) Receiver Init: n=1 Receive data(k) IF k=n then Deliver data(n) n = n+1 Send ACK(n-1) Stop and Wait: unbounded seq # Comnet 2010

  8. What if packets are reordered? • Never reuse a seq #? Finite… • Require in order delivery? Unreliable network … • Prevent very late delivery? • TTL: Decrement hop count per packet, discard if exceeded • Seq # not reused within delay bound Comnet 2010

  9. Problems with Stop and Wait • Performance • No pipeline effect • Example • For a l.5 Mbps link with 45 ms RTT • Bandwidth-delay product = 67.5 Kb (~8KB) • 1KB packets imply 1/8th link utilization Comnet 2010

  10. rdt3.0 sender: Stop and Wait Comnet 2010 Reliable Data Transfer #10

  11. rdt 3.0 receiver: Stop and Wait rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq0(rcvpkt) Extract(rcvpkt,data) deliver_data(data) udt_send(ACK[0]) rdt_rcv(rcvpkt) && corrupt(rcvpkt) rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(ACK[0]) udt_send(ACK[1]) Wait for 1 Wait for 0 rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq0(rcvpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) udt_send(ACK[0]) udt_send(ACK[1]) Extract(rcvpkt,data) deliver_data(data) udt_send(ACK[1]) Comnet 2010 Reliable Data Transfer #11

  12. Distance Vector Routing: Basic Idea At node i, the basic update rule for dest. v where - di(v) denotes the distance estimation from i to the destination v, - N(i) is set of neighbors of node i, and - dij is the distance of the direct link from i to j;assume positive destination v j i Comnet 2010 Lecture 6: Network Layer

  13. Distance Vector: poisoned reverse 60 4 1 50 X Z Y If Z routes through Y to get to X : • Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z) • will this completely solve count to infinity problem? algorithm terminates Comnet 2010 Lecture 6: Network Layer

  14. A B 1 1 1 C 1 D Example: poisoned reverse fails • When link breaks, C marks D as unreachable and reports that to A and B • Suppose A learns it first. A now thinks best path to D is through B. A reports a route of cost=3 to C. • C thinks D is reachable through A at cost 4 and reports that to B. • B reports a cost 5 to A who reports new cost to C. • etc... Comnet 2010

  15. Clos Network 2x2 2x2 2x2 Clos(N, n , k) : N - inputs/outputs; cross-points: 2 (N/n)nk + k(N/n)2 kxn nxk (N/n)x(N/n) 2x2 3x3 N=6 n=2 k=2 2x2 N 3x3 2x2 k N/n N/n Comnet 2010

  16. Example 2x3 4x4 3x2 2x3 4x4 3x2 N=8 n=2 k=3 2x3 3x2 4x4 2x3 3x2 Clos(8,2,3) Need to route a new call Comnet 2010

  17. Clos Network Why is k=n NOT strict sense non-blocking? Proof gives counter example: kxn nxk (N/n)x(N/n) 2x2 3x3 2x2 N=6 n=2 k=2 2x2 2x2 3x3 2x2 2x2 Blocking means NOT rearrangeable (NOT strict sense non-blocking) Comnet 2010

  18. Clos Network Why is k<n NOT rearrangeable? Proof gives counter example: kxn nxk (N/n)x(N/n) 3x2 3x3 2x3 N=6 n=3 k=2 3x2 2x3 3x3 3x2 2x3 Blocking means NOT rearrangeable (NOT strict sense non-blocking) Comnet 2010

More Related