- Packet
time out:
- Each time a
IP packet is
forwarded by a
router:
- The TTL header field is decremented by 1
- When the TTL (Time To Live)
field in the IP packet header
becomes zero:
- The IP packet is discarded ("times out").
- Each time a
IP packet is
forwarded by a
router:
-
Reporting
the
Packet "timeout" error
:
- A router
that discards
an IP packet
must:
- send an ICMP Time Exceeded message back to the sender !!!
Example:
- A router
that discards
an IP packet
must:
- Note:
- The ICMP: time exceeded
error
packet
contains:
- Source IP address = IP address of the router that has dropped the IP packet !!!!
- The ICMP: time exceeded
error
packet
contains:
- Deliberate
time outs:
- Suppose we
deliberately set
the TTL value of
the IP packets as follows:
- Transmit the
first
IP packet
with TTL = 1:
Result:
- The first router on the path to the destination will send back a ICMP time exceeded message.
- Next.
transmit another
(dummy) IP message
with TTL = 2
Result:
- Now the second router on the path to the destination will respond with a ICMP time exceeded message.
- Transmit the
first
IP packet
with TTL = 1:
- And so on:
- We can discover the identity of the routers on the path to the destination !!!!
- Suppose we
deliberately set
the TTL value of
the IP packets as follows:
- The traceroute utility:
- tracroute = a network utility that exploits the Time Exceeded ICMP mechanism to find the routers on the path to a destination
Example: traceroute www.google.com
cheung@shangchun(1146)> traceroute www.google.com traceroute: Warning: www.google.com has multiple addresses; using 74.125.45.99 traceroute to www.google.com (74.125.45.99), 30 hops max, 40 byte packets 1 170.140.14.33 (170.140.14.33) 0.898 ms 0.442 ms 0.327 ms 2 COX1-academic-ve1162.emory.net (170.140.0.9) 0.549 ms 0.463 ms 0.732 ms 3 CMONT1-academic-ve1142.emory.net (170.140.0.6) 0.436 ms 0.416 ms 0.329 ms 4 NDB1-academic-ve1262.emory.net (170.140.0.41) 0.578 ms 0.420 ms 0.463 ms 5 FWNDB2-academic-ve3934.emory.net (170.140.0.169) 0.451 ms 0.456 ms 0.459 ms 6 NDB1-academic-iborder-ve3944.emory.net (163.246.200.126) 1.214 ms 0.797 ms 0.714 ms 7 BNDB1-eborder-ve3369.emory.net (170.140.0.1) 0.828 ms 0.690 ms 0.785 ms 8 SoX-Internet2.emory.net (163.246.200.86) 1.377 ms 1.278 ms 1.190 ms 9 74.125.48.33 (74.125.48.33) 1.575 ms 1.471 ms 1.797 ms 10 72.14.239.100 (72.14.239.100) 1.476 ms 1.741 ms 1.574 ms 11 72.14.232.213 (72.14.232.213) 1.943 ms 1.965 ms 209.85.254.241 (209.85.254.241) 2.193 ms 12 209.85.253.133 (209.85.253.133) 5.937 ms 209.85.253.141 (209.85.253.141) 2.355 ms 9.245 ms 13 yx-in-f99.1e100.net (74.125.45.99) 98.039 ms 53.444 ms 44.371 ms