Search Results

Search found 20116 results on 805 pages for 'network flow'.

facebook twitter digg google delicious technorati stumbleupon myspace wordpress linkedin gmail igoogle windows live tumblr viadeo yahoo buzz yahoo mail yahoo bookmarks favorites email print

Page 246/805 | < Previous Page | 242 243 244 245 246 247 248 249 250 251 252 253  | Next Page >

  • Oracle Magazine, March/April 2010

    Oracle Magazine March/April features articles on enterprise software applications, application grid architecture, partitions, Tom Kyte on Edition-Based Redefinition, Oracle Application Express and much more.

    Read the article

  • Portraits of Excellence: Editors' Choice Awards 2009

    Each year the editors of Oracle Magazine recognize men and women who exemplify leadership, vision, and dedication in working with and managing Oracle technology. This year, we are pleased to present the winners of our eighth annual Editors' Choice Awards, and we are honored to feature them in our pages.

    Read the article

  • DTracing TCP congestion control

    - by user12820842
    In a previous post, I showed how we can use DTrace to probe TCP receive and send window events. TCP receive and send windows are in effect both about flow-controlling how much data can be received - the receive window reflects how much data the local TCP is prepared to receive, while the send window simply reflects the size of the receive window of the peer TCP. Both then represent flow control as imposed by the receiver. However, consider that without the sender imposing flow control, and a slow link to a peer, TCP will simply fill up it's window with sent segments. Dealing with multiple TCP implementations filling their peer TCP's receive windows in this manner, busy intermediate routers may drop some of these segments, leading to timeout and retransmission, which may again lead to drops. This is termed congestion, and TCP has multiple congestion control strategies. We can see that in this example, we need to have some way of adjusting how much data we send depending on how quickly we receive acknowledgement - if we get ACKs quickly, we can safely send more segments, but if acknowledgements come slowly, we should proceed with more caution. More generally, we need to implement flow control on the send side also. Slow Start and Congestion Avoidance From RFC2581, let's examine the relevant variables: "The congestion window (cwnd) is a sender-side limit on the amount of data the sender can transmit into the network before receiving an acknowledgment (ACK). Another state variable, the slow start threshold (ssthresh), is used to determine whether the slow start or congestion avoidance algorithm is used to control data transmission" Slow start is used to probe the network's ability to handle transmission bursts both when a connection is first created and when retransmission timers fire. The latter case is important, as the fact that we have effectively lost TCP data acts as a motivator for re-probing how much data the network can handle from the sending TCP. The congestion window (cwnd) is initialized to a relatively small value, generally a low multiple of the sending maximum segment size. When slow start kicks in, we will only send that number of bytes before waiting for acknowledgement. When acknowledgements are received, the congestion window is increased in size until cwnd reaches the slow start threshold ssthresh value. For most congestion control algorithms the window increases exponentially under slow start, assuming we receive acknowledgements. We send 1 segment, receive an ACK, increase the cwnd by 1 MSS to 2*MSS, send 2 segments, receive 2 ACKs, increase the cwnd by 2*MSS to 4*MSS, send 4 segments etc. When the congestion window exceeds the slow start threshold, congestion avoidance is used instead of slow start. During congestion avoidance, the congestion window is generally updated by one MSS for each round-trip-time as opposed to each ACK, and so cwnd growth is linear instead of exponential (we may receive multiple ACKs within a single RTT). This continues until congestion is detected. If a retransmit timer fires, congestion is assumed and the ssthresh value is reset. It is reset to a fraction of the number of bytes outstanding (unacknowledged) in the network. At the same time the congestion window is reset to a single max segment size. Thus, we initiate slow start until we start receiving acknowledgements again, at which point we can eventually flip over to congestion avoidance when cwnd ssthresh. Congestion control algorithms differ most in how they handle the other indication of congestion - duplicate ACKs. A duplicate ACK is a strong indication that data has been lost, since they often come from a receiver explicitly asking for a retransmission. In some cases, a duplicate ACK may be generated at the receiver as a result of packets arriving out-of-order, so it is sensible to wait for multiple duplicate ACKs before assuming packet loss rather than out-of-order delivery. This is termed fast retransmit (i.e. retransmit without waiting for the retransmission timer to expire). Note that on Oracle Solaris 11, the congestion control method used can be customized. See here for more details. In general, 3 or more duplicate ACKs indicate packet loss and should trigger fast retransmit . It's best not to revert to slow start in this case, as the fact that the receiver knew it was missing data suggests it has received data with a higher sequence number, so we know traffic is still flowing. Falling back to slow start would be excessive therefore, so fast recovery is used instead. Observing slow start and congestion avoidance The following script counts TCP segments sent when under slow start (cwnd ssthresh). #!/usr/sbin/dtrace -s #pragma D option quiet tcp:::connect-request / start[args[1]-cs_cid] == 0/ { start[args[1]-cs_cid] = 1; } tcp:::send / start[args[1]-cs_cid] == 1 && args[3]-tcps_cwnd tcps_cwnd_ssthresh / { @c["Slow start", args[2]-ip_daddr, args[4]-tcp_dport] = count(); } tcp:::send / start[args[1]-cs_cid] == 1 && args[3]-tcps_cwnd args[3]-tcps_cwnd_ssthresh / { @c["Congestion avoidance", args[2]-ip_daddr, args[4]-tcp_dport] = count(); } As we can see the script only works on connections initiated since it is started (using the start[] associative array with the connection ID as index to set whether it's a new connection (start[cid] = 1). From there we simply differentiate send events where cwnd ssthresh (congestion avoidance). Here's the output taken when I accessed a YouTube video (where rport is 80) and from an FTP session where I put a large file onto a remote system. # dtrace -s tcp_slow_start.d ^C ALGORITHM RADDR RPORT #SEG Slow start 10.153.125.222 20 6 Slow start 138.3.237.7 80 14 Slow start 10.153.125.222 21 18 Congestion avoidance 10.153.125.222 20 1164 We see that in the case of the YouTube video, slow start was exclusively used. Most of the segments we sent in that case were likely ACKs. Compare this case - where 14 segments were sent using slow start - to the FTP case, where only 6 segments were sent before we switched to congestion avoidance for 1164 segments. In the case of the FTP session, the FTP data on port 20 was predominantly sent with congestion avoidance in operation, while the FTP session relied exclusively on slow start. For the default congestion control algorithm - "newreno" - on Solaris 11, slow start will increase the cwnd by 1 MSS for every acknowledgement received, and by 1 MSS for each RTT in congestion avoidance mode. Different pluggable congestion control algorithms operate slightly differently. For example "highspeed" will update the slow start cwnd by the number of bytes ACKed rather than the MSS. And to finish, here's a neat oneliner to visually display the distribution of congestion window values for all TCP connections to a given remote port using a quantization. In this example, only port 80 is in use and we see the majority of cwnd values for that port are in the 4096-8191 range. # dtrace -n 'tcp:::send { @q[args[4]-tcp_dport] = quantize(args[3]-tcps_cwnd); }' dtrace: description 'tcp:::send ' matched 10 probes ^C 80 value ------------- Distribution ------------- count -1 | 0 0 |@@@@@@ 5 1 | 0 2 | 0 4 | 0 8 | 0 16 | 0 32 | 0 64 | 0 128 | 0 256 | 0 512 | 0 1024 | 0 2048 |@@@@@@@@@ 8 4096 |@@@@@@@@@@@@@@@@@@@@@@@@@@ 23 8192 | 0

    Read the article

  • Mac won't boot into safe mode

    - by Stephen
    Mac boots fine normally, except when in safe mode. Holding down shift when booting gets me to the progress bar on the grey screen. Progress bar gets about half way before mac reboots. I modified nvram boot-args to get a better look: sudo nvram boot-args="-x -v" It definitely gets through fsck, skips loading kernel extensions (since it's in safe mode), does something with the network interfaces, then this is the last thing it wips through... Aug 22 11:56:21 Crockpot com.apple.SecurityServer[15]: Succeeded authorizing right 'com.apple.ServiceManagement.daemons.modify' by client '/usr/libexec/UserEventAgent' [10] for authorization created by '/usr/libexec/UserEventAgent' [10] (100012,0) Aug 22 11:56:22 Crockpot fseventsd[37]: event logs in /.fseventsd out of sync with volume. destroying old logs. (1 174 330) Aug 22 11:56:22 Crockpot fseventsd[37]: log dir: /.fseventsd getting new uuid: 5C379650-26FA-428F-B81F-4FE4349D50B3 Aug 22 11:56:23 Crockpot mDNSResponder[39]: mDNSResponder mDNSResponder-379.27 (Jun 20 2012 15:40:55) starting OSXVers 12 Aug 22 11:56:23 Crockpot systemkeychain[35]: done file: /var/run/systemkeychaincheck.done Aug 22 11:56:23 Crockpot configd[17]: network changed: DNS* Aug 22 11:56:24 --- last message repeated 1 time --- Aug 22 11:56:24 Crockpot mDNSResponder[39]: D2D_IPC: Loaded Aug 22 11:56:24 Crockpot mDNSResponder[39]: D2DInitialize succeeded Aug 22 11:56:24 Crockpot mDNSResponder[39]: Adding registration domain 273025955.members.btmm.icloud.com. Aug 22 11:56:24 Crockpot kernel[0]: MacAuthEvent en1 Auth result for: 00:23:69:35:dc:fe MAC AUTH succeeded Aug 22 11:56:24 Crockpot kernel[0]: MacAuthEvent en1 Auth result for: 00:23:69:35:dc:fe Unsolicited Auth Aug 22 11:56:24 Crockpot kernel[0]: wlEvent: en1 en1 Link UP virtIf = 0 Aug 22 11:56:24 Crockpot kernel[0]: AirPort: Link Up on en1 Aug 22 11:56:24 Crockpot kernel[0]: en1: BSSID changed to 00:23:69:35:dc:fe Aug 22 11:56:24 Crockpot kernel[0]: en1::IO80211Interface::postMessage bssid changed Aug 22 11:56:24 Crockpot kernel[0]: AirPort: RSN handshake complete on en1 Aug 22 11:56:25 Crockpot cfprefsd[19]: CFPreferences failed to read preferences data. Errno was 21 Aug 22 11:56:25 --- last message repeated 1 time --- Aug 22 11:56:25 Crockpot airportd[30]: _doAutoJoin: Already associated to “burnum”. Bailing on auto-join. Aug 22 11:56:25 Crockpot com.apple.kextd[11]: Can't load IOBluetoothSerialManager.kext - ineligible during safe boot. Aug 22 11:56:25 Crockpot com.apple.kextd[11]: Load com.apple.iokit.IOBluetoothSerialManager failed; removing personalities from kernel. Aug 22 11:56:25 Crockpot cfprefsd[19]: CFPreferences: error renaming file blued.plist.HXuEmQn to blued.plist. Aug 22 11:56:27 Crockpot awacsd[52]: Starting awacsd connectivity-77 (Jun 20 2012 15:40:49) Aug 22 11:56:27 Crockpot com.apple.SecurityServer[15]: Succeeded authorizing right 'system.services.systemconfiguration.network' by client '/System/Library/Frameworks/SystemConfiguration.framework/Versions/A/Resources/SCHelper' [54] for authorization created by '/usr/sbin/awacsd' [52] (100003,0) Aug 22 11:56:27 --- last message repeated 1 time --- Aug 22 11:56:27 Crockpot awacsd[52]: Configuring lazy AWACS client: 273025955.p04.members.btmm.icloud.com. Aug 22 11:56:28 Crockpot apsd[55]: CGSLookupServerRootPort: Failed to look up the port for "com.apple.windowserver.active" (1102) Aug 22 11:56:32 --- last message repeated 1 time --- Aug 22 11:56:32 Crockpot awacsd[52]: KV HTTP 0 Aug 22 11:56:38 --- last message repeated 1 time --- Aug 22 11:56:38 Crockpot apsd[55]: CGSLookupServerRootPort: Failed to look up the port for "com.apple.windowserver.active" (1102) Aug 22 11:56:47 Crockpot awacsd[52]: KV HTTP 0 Aug 22 11:56:49 Crockpot configd[17]: subnet_route: write routing socket failed, Network is unreachable Aug 22 11:56:51 Crockpot configd[17]: network changed: v4(en1+:169.254.80.161) DNS* Proxy+ SMB Aug 22 11:56:51 Crockpot UserEventAgent[10]: Captive: en1: Not probing 'burnum' (protected network) Aug 22 11:56:51 Crockpot configd[17]: network changed: v4(en1:169.254.80.161) DNS Proxy SMB Aug 22 11:57:07 Crockpot awacsd[52]: KV HTTP 0 Aug 22 11:57:23 Crockpot fseventsd[37]: Logging disabled completely for device:1: /Volumes/Recovery HD Aug 22 11:57:25 Crockpot kernel[0]: Kext loading now disabled. Aug 22 11:57:25 Crockpot kernel[0]: Kext unloading now disabled. Aug 22 11:57:25 Crockpot mDNSResponder[39]: mDNSResponder mDNSResponder-379.27 (Jun 20 2012 15:40:55) stopping Aug 22 11:57:25 Crockpot com.apple.SecurityServer[15]: Killing auth hosts Aug 22 11:57:25 Crockpot UserEventAgent[10]: dnssd_clientstub DNSServiceProcessResult called with DNSServiceRef with no ProcessReply function Aug 22 11:57:25 Crockpot configd[17]: dnssd_clientstub read_all(26) failed 0/28 0 Aug 22 11:57:25 Crockpot configd[17]: [0x7fb025119ff0] SCNetworkReachability _llq_callback w/error=-65563 Aug 22 11:57:25 Crockpot UserEventAgent[10]: dnssd_clientstub DNSServiceProcessResult called with DNSServiceRef with no ProcessReply function Aug 22 11:57:25 Crockpot mDNSResponder[39]: D2D_IPC: Terminated Aug 22 11:57:25 Crockpot mDNSResponder[39]: D2DTerminate succeeded Aug 22 11:57:25 Crockpot awacsd[52]: dnssd_clientstub read_all(4) failed 0/28 0 Aug 22 11:57:25 Crockpot UserEventAgent[10]: dnssd_clientstub DNSServiceProcessResult called with DNSServiceRef with no ProcessReply function Aug 22 11:57:25 --- last message repeated 2 times --- Aug 22 11:57:25 Crockpot apsd[55]: dnssd_clientstub read_all(4) failed 0/28 0 Aug 22 11:57:25 Crockpot configd[17]: SCNC: stop, triggered by configd, type PPPSerial, reason Terminated All Aug 22 11:57:25 Crockpot configd[17]: _d2dCallback: D2D connection to mDNSResponder lost Aug 22 11:57:25 Crockpot UserEventAgent[10]: dnssd_clientstub DNSServiceProcessResult called with DNSServiceRef with no ProcessReply function Aug 22 11:57:25 --- last message repeated 4 times --- Aug 22 11:57:25 Crockpot kernel[0]: Kext autounloading now disabled. Aug 22 11:57:25 Crockpot kernel[0]: Kernel requests now disabled. ... before rebooting in the middle of the safe mode startup sequence. Aug 22 12:01:10 localhost bootlog[0]: BOOT_TIME 1345662070 0 Aug 22 12:01:32 localhost kernel[0]: PMAP: PCID enabled Aug 22 12:01:32 localhost kernel[0]: Darwin Kernel Version 12.0.0: Sun Jun 24 23:00:16 PDT 2012; root:xnu-2050.7.9~1/RELEASE_X86_64 Any ideas what's causing the safe mode boot to fail? System Info MacBook Pro 8,2 2.2 Ghz Core i7 4 GM Ram Mountain Lion 10.8 500GB TOSHIBA MK5065GSXF Serial-ATA rotational disk

    Read the article

  • Oracle Magazine, November/December 2009

    Oracle Magazine November/December features articles on our Editors' Choice Awards 2009, Oracle Database 11g Release 2, ODP.NET and Oracle Streams Advanced Queuing, SQL tuning with Oracle Database 11g, Tom Kyte takes a first look at Oracle Database 11g Release 2 and much more.

    Read the article

  • Oracle Magazine, July/August 2006

    Oracle Magazine July/August 2006 features articles on Oracle Enterprise Manager, Oracle OpenWorld, Green Mountain Coffee Roasters, Retailers, Identity Management, XML, SQL, ODP.NET Performance, Oracle ADF, Oracle Application Express, and much more.

    Read the article

facebook twitter digg google delicious technorati stumbleupon myspace wordpress linkedin gmail igoogle windows live tumblr viadeo yahoo buzz yahoo mail yahoo bookmarks favorites email print

< Previous Page | 242 243 244 245 246 247 248 249 250 251 252 253  | Next Page >