Routers Host A Host B Host C Host D Host E Modern routers do not ‘forward’ broadcast datagrams, but they can be configured to ‘forward’ any multicast datagrams Router X Router Y Host I Host J Host K Host L Host M Host F Host N Host G Host O Host H (Older routers can use ‘tunneling’) Host P
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Sending ICMP packets Echo Request to D (TTL=1) Host A 10.0.1.1 10.0.1.2 Host B 10.0.2.1 10.0.2.2 10.0.2.1 10.0.2.2 Host C 10.0.3.1 10.0.3.2 10.0.3.1 10.0.3.2 10.0.3.1 10.0.3.2 Host D Time Exceeded to A Echo Request to D (TTL=2) Host A 10.0.1.1 10.0.1.2 Host B Host C Host D Time Exceeded to A Echo Request to D (TTL=3) Host A 10.0.1.1 10.0.1.2 Host B 10.0.2.1 10.0.2.2 Host C Host D Echo Reply to A
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Sending UDP packets UDP message to D (TTL=1) Host A 10.0.1.1 10.0.1.2 Host B 10.0.2.1 10.0.2.2 10.0.2.1 10.0.2.2 Host C 10.0.3.1 10.0.3.2 10.0.3.1 10.0.3.2 10.0.3.1 10.0.3.2 Host D Time Exceeded to A UDP message to D (TTL=2) Host A 10.0.1.1 10.0.1.2 Host B Host C Host D Time Exceeded to A UDP message to D (TTL=3) Host A 10.0.1.1 10.0.1.2 Host B 10.0.2.1 10.0.2.2 Host C Host D Destinnation Unreachable to A
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Internet Access and Flows T Host A Host M X W Z Host L Y 2 nodes 11 15 13 9 Host E W3 4 W4 5 16 12 10 Host D 3 14 W2 AP 1 Host B Host C W1 Host J 17 6 7 Host H 8 Host G Host F Computer Networks Introduction 10
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T X W Host L 2 W3 AP 3 W4 4 5 nodes 16 14 11 Host J 17 12 15 6 13 10 Host D Y 1 Host B Host C W2 Host M Host A W1 Z Host H 7 9 Host E 8 Host G Host F Networks: Sample Performance Problems 2
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Internet Access and Flows T Host A Host M X W Z Host L Y 2 nodes 11 15 13 9 Host E W3 4 W4 5 16 12 10 Host D 3 14 W2 AP 1 Host B Host C W1 Host J 17 6 7 Host H 8 Host G Host F Computer Networks Introduction 4
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Computer Networks T Host A Host M X W Z Host L Y 2 nodes 11 15 13 9 Host E W3 4 W4 5 16 12 10 Host D 3 14 W2 AP 1 Host B Host C W1 17 Host J 6 7 Host H 8 Host G Host F Computer Networks Performance Metrics 3
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The ‘multicast’ concept Host A Host B Host C Host D Host E Local Area Network With IP-broadcasting, a datagram whose destination-address is equal to 255.255.255.255 will be received by every host on the local network Host A Host B Host C Host D Host E Local Area Network With IP-multicasting, a datagram whose destination-address is equal to 224.3.3.6 would be received only by those hosts on the local network that have chosen to become ‘members’ of this specific multicast-group
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‘Local’ networks host host host host host host ‘ring’ topology host HUB host host ‘star’ topology Why do you think the star-topology is more widely deployed nowadays?
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The ‘Router’ device switch router host switch host switch host host host host host host host
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Fast Restarts with Local State Durable : Input Stream Task-Container-Host Mapping Task 1, Task 4 -> Host-A Task 2 -> Host-B Task 3 -> Host-C Samza Job Task-1 Task-4 Host-A Change-log Task-2 Host-B Task-3 Host-C Host Affinity in YARN : - Try to place task on same host after upgrade - Minimize state rebuilding Overhead
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Subnetting CLASS “B” e.g. Company e.g. Site 2 10 2 10 Net ID 0000 e.g. Dept 10 Subnet ID (22) Host-ID 16 14 Net ID 2 10 Subnet Host ID (12) Subnet ID (20) 2 Host-ID Net ID 16 14 16 14 000000 Subnet Host ID (10) Net ID 1111 Host-ID Subnet Host ID (12) Subnet ID (20) 2 Host-ID 16 14 10 16 14 Net ID 1111011011 Subnet ID (26) Host-ID Subnet Host ID (6) 4-22
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The Wine Network Video Host M 2 Host K 3 4 1 Host B Host C W1 Host L 5 nodes W2 AP 16 14 11 17 12 15 6 W4 13 10 7 9 Web Server W3 8 Host G Host H Host F Peer-to-Peer 2 Research Byte October 22, 2004
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Hostname, Host, and Nslookup • Hostname utility – Provides client’s host name • Administrator may change • Host utility – Learn IP address from host name – No switches: returns host IP address or host name • Nslookup – Query DNS database from any network computer • Find the device host name by specifying its IP address – Verify host configured correctly; troubleshoot DNS resolution problems Network+ Guide to Networks, 5th Edition 51
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‘Hub’ versus ‘Switch’ host host host host host host host Why are ‘switches’ preferred over ‘hubs’ nowadays? host
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Reclaiming Memory  ESX maintains four host free memory states: high, soft, hard, and low, which are reflected by four thresholds: 6%, 4%, 2%, and 1% of host memory respectively.  When to use ballooning or swapping (which activates memory compression) to reclaim host memory is largely determined by the current host free memory state. In the high state, the aggregate virtual machine guest memory usage is smaller than the host memory size. Whether or not host memory is overcommitted, the hypervisor will not reclaim memory through ballooning or swapping.  If host free memory drops towards the soft threshold, the hypervisor starts to reclaim memory using ballooning. Ballooning happens before free memory actually reaches the soft threshold because it takes time for the balloon driver to allocate and pin guest physical memory. Usually, the balloon driver is able to reclaim memory in a timely fashion so that the host free memory stays above the soft threshold.
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Reclaiming Memory  If ballooning is not sufficient to reclaim memory or the host free memory drops towards the hard threshold, the hypervisor starts to use swapping in addition to using ballooning. During swapping, memory compression is activated as well. With host swapping and memory compression, the hypervisor should be able to quickly reclaim memory and bring the host memory state back to the soft state.  In a rare case where host free memory drops below the low threshold, the hypervisor continues to reclaim memory through swapping and memory compression, and additionally blocks the execution of all virtual machines that consume more memory than their target memory allocations.  In certain scenarios, host memory reclamation happens regardless of the current host free memory state. For example, even if host free memory is in the high state, memory reclamation is still mandatory when a virtual machine’s memory usage exceeds its specified memory limit. If this happens, the hypervisor will employ ballooning and, if necessary, swapping and memory compression to reclaim memory from the virtual machine until the virtual machine’s host memory usage falls back to its specified limit.
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Datagram • Every packet contains enough information to enable any switch to decide how to get to its destination • Each packet is forwarded independently of previous packet that might have been sent to the same destination Host D 0 3 Host C Host E Switch 1 1 2 3 Host F 2 Switch 2 1 0 Host A Host G 1 0 Switch 3 Host B 3 Destination Port A B C D E F G H 3 0 3 3 2 1 0 0 Forwarding table for switch 2 2 Host H 3
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