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What is Latency? (Latency and Tracert)  

What is Latency? (Latency and Tracert)
1) In a network, latency, a synonym for delay, is an expression of how much time it takes for a packet of data to get from one designated point to another. In some usages (for example, AT&T), latency is measured by sending a packet that is returned to the sender and the round-trip time is considered the latency.
The latency assumption seems to be that data should be transmitted instantly between one point and another (that is, with no delay at all). The contributors to network latency include:
  • Propagation: This is simply the time it takes for a packet to travel between one place and another at the speed of light.
  • Transmission: The medium itself (whether optical fiber, wireless, or some other) introduces some delay. The size of the packet introduces delay in a round trip since a larger packet will take longer to receive and return than a short one.
  • Router and other processing: Each gateway node takes time to examine and possibly change the header in a packet (for example, changing the hop count in the time-to-live field).
  • Other computer and storage delays: Within networks at each end of the journey, a packet may be subject to storage and hard disk access delays at intermediate devices such as switches and bridges. (In backbone statistics, however, this kind of latency is probably not considered.)
2) In a computer system, latency is often used to mean any delay or waiting that increases real or perceived response time beyond the response time desired. Specific contributors to computer latency include mismatches in data speed between the microprocessor and input/output devices and inadequate data buffers.
Within a computer, latency can be removed or "hidden" by such techniques as prefetching (anticipating the need for data input requests) and multithreading, or using parallelism across multiple execution threads.
Tracing route to pf2.pacfusion.com [] over a maximum of 30 hops:
  1    <1 ms    <1 ms    <1 ms
  2     1 ms    <1 ms     <1 ms
  3     1 ms     2 ms       1 ms  netblock-66-51-202-129.dslextreme.com []
  4    32 ms    20 ms     21 ms  netblock-66-218-58-113.dslextreme.com []
  5    54 ms    54 ms     22 ms  g11-0-1.gsr.cr1.lax1.dslextreme.com []
  6    22 ms    20 ms     22 ms  GigabitEthernet5-3.GW1.LAX1.ALTER.NET []
  7    26 ms    30 ms     22 ms  0.so-0-0-0.xl1.lax1.alter.net []
  8    34 ms    19 ms     20 ms  0.so-1-0-0.tl1.lax9.alter.net []
  9    23 ms    30 ms     20 ms  0.pos6-0.ir1.lax9.alter.net []
 10    23 ms    19 ms    20 ms  so-7-1-0.IR1.LAX12.ALTER.NET []
 11   227 ms   232 ms   231 ms  0.so-6-0-0.tr1.sin1.alter.net []
 12   223 ms   230 ms   230 ms  pos1-0-0.xr1.sin1.alter.net []
 13   394 ms   279 ms   416 ms  311.ATM5-0-0.GW1.SIN1.Alter.Net []
 14   284 ms   284 ms   286 ms  pacific-internet-gw.aspac.customer.alter.net []
 15   215 ms   206 ms   207 ms  scipk004-vl15.pacific.net.sg []
 16   208 ms   213 ms   259 ms  scipk006.pacific.net.sg []
 17   211 ms   210 ms   200 ms  pf2.pacfusion.com []
Trace complete.
Understanding a Tracert
The tracert utility determines the route taken to a destination by sending Internet Control Message Protocol (ICMP) echo packets with varying Time-To-Live (TTL) values to the destination. Each router along the path is required to decrement the TTL on a packet by at least 1 before forwarding it, so the TTL is effectively a hop count. When the TTL on a packet reaches 0, the router is supposed to send back an ICMP Time Exceeded message to the source system. The tracert command determines the route by sending the first echo packet with a TTL of 1 and incrementing the TTL by 1 on each subsequent transmission until the target responds or the maximum TTL is reached (usually 30 hops). The route is determined by examining the ICMP Time Exceeded messages sent back by the intermediate routers.
Using tracert
If you wanted to do a tracert to genealogy.org and save the results in a text file, just right click on the title bar and click on edit > select all. The screen should turn white. Again ,right click on the title bar and click on edit > mark
The results returned might look something like those below:
Tracing route to genealogy.org [] over a maximum of 30 hops:
1     257 ms    186 ms    183 ms
2     195 ms    177 ms    246 ms
3     317 ms    187 ms    241 ms
4     212 ms    443 ms    281 ms
5     *         *         *         Request timed out.
6     190 ms    187 ms    309 ms    Vienna5.VA.ALTER.NET []
7     194 ms    193 ms    198 ms    Fddi0-0.CR2.TCO1.ALTER.NET []
8     265 ms    325 ms    273 ms    189.Hssi6-0.CR2.SCL1.Alter.Net []
9     337 ms    302 ms    277 ms    107.Hssi0-0.BR1.NUQ1.Alter.Net []
10    271 ms    396 ms    270 ms    T3-CRL-SFO-01-H1/0.US.CRL.NET []
11    311 ms    267 ms    309 ms    T3-CRL-SFO-01-H3/0.US.CRL.NET []
12    263 ms    309 ms    261 ms    E0-CRL-SFO-05-E0X0.US.CRL.NET []
13    430 ms    281 ms    357 ms    znet-gw.ex.us.crl.net []
14    *         434 ms    *         znet-gw.ex.us.crl.net []
15    *         *         *         Request timed out.
16    295 ms    356 ms    301 ms    genealogy.org []
Understanding the results:
The first column is the hop number, which is the Time-To-Live (TTL) value set in the packet.
Each of the next three columns contains the round-trip times in milliseconds for an attempt to reach the destination with the TTL value.
The fourth column is the host name (if it was resolved) and IP address of the responding system.
In the results you will see the comment, "over a maximum of 30 hops". All this means is that the diameter of the Internet is roughly 30 hops and, therefore, many trace routes will only go that far out in trying to reach a destination.
This first hop is to the modem that answered your call and typically takes 200 to 300 milliseconds.
An asterisk, "*", followed by the words "request timed out" is not out of the ordinary. If the tracert had stopped here and continued to respond with "request timed out", then it might have been indicative of a problem. In the above example, the first "request timed out" is where the interface between our network and the rest of the Internet lies. The asterisk "*" is a normal consequence of our security implementation on that junction.
Generally, a response time of less than 500 milliseconds is considered acceptable. If the example above had indicated results larger than that, then there might have been a problem router at that point in the network.
The first 4 or 5 hops are internal to our network. These hops will not resolve to a domain name.
Hop 6 in the above example is where we connect to alter.net (UUNET). The Vienna5.VA.ALTER.NET hub is commonly referred to MAE east. This is a major junction point for the Internet at large. If you are having problems at this hop and live on the east coast, then you can pretty much count on having problems getting anywhere on the Internet. There are similar hubs on the west coast as well.
Hop 10 is where alter.net connects with crl.net (CRL Network Services) on router T3-CRL-SFO-01-H1/0.US.CRL.NET.
Finally, on hop 16, the connection to genealogy.org is made. The entire trip went through our network (4 hops), alter.net (4 hops), crl.net (5 hops) and genealogy.org (1 hop).


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