TTL Values

silverp1

Hi folks,
I've run into a practice question that I'm having a bit of trouble answering, the question asks you to predict the TTL value given a topology, the topology is similar to this:

HOST1

---

(FE1)-R1-(S1)

---

(S1)-R2-(S2)

---

(S1)-R3-(FE1) ---- HOST2

The question asks you to predict the TTL value after HOST1 pings R3's S1 interface. I understand that TTL values are decremented when a router forwards a packet, so I guess the gist of my question here is, would a ping to R3's serial interface be considered a packet forward per say? Also, when does the decrement occur - when the router receives the packet, or when it is moved sent out of an interface?

Thanks for your help!

networker050184

TTL is decremented on the egress as the packet headers are rewritten. So, as R3 does not forward this packet it will not decrement the TTL.

silverp1

Got it, thanks! So in this case, the TTL when the packet reaches R3's serial interface would be 253, is that right?

networker050184

I believe the TTL starts 128 for an ICMP echo if I'm not mistaken.

aashan

yes the default value for ICMP Echo request is 128
But the return journey values are different depending on what OS is replying

Have a look at this website which has a very in-depth analysis and has some Wireshark captures as well

http://openmaniak.com/ping.php#ping-ttl

iamme4eva

Every day is a school day...I didn't know different OS's used different start TTL's.

EDIT:

Well that just killed a lunch break. I just spent my time going around google reading about TTL. Found this table, if it's interesting to anyone:

OS/Device
Version
Protocol
TTL


AIX

TCP
60


AIX

UDP
30


AIX
3.2, 4.1
ICMP
255


BSDI
BSD/OS 3.1 and 4.0
ICMP
255


Compa
Tru64 v5.0
ICMP
64


Cisco

ICMP
254


DEC Pathworks
V5
TCP and UDP
30


Foundry

ICMP
64


FreeBSD
2.1R
TCP and UDP
64


FreeBSD
3.4, 4.0
ICMP
255


FreeBSD
5
ICMP
64


HP-UX
9.0x
TCP and UDP
30


HP-UX
10.01
TCP and UDP
64


HP-UX
10.2
ICMP
255


HP-UX
11
ICMP
255


HP-UX
11
TCP
64


Irix
5.3
TCP and UDP
60


Irix
6.x
TCP and UDP
60


Irix
6.5.3, 6.5.8
ICMP
255


juniper

ICMP
64


MPE/IX (HP)

ICMP
200


Linux
2.0.x kernel
ICMP
64


Linux
2.2.14 kernel
ICMP
255


Linux
2.4 kernel
ICMP
255


Linux
Red Hat 9
ICMP and TCP
64


MacOS/MacTCP
2.0.x
TCP and UDP
60


MacOS/MacTCP
X (10.5.6)
ICMP/TCP/UDP
64


NetBSD

ICMP
255


Netgear FVG318

ICMP and UDP
64


OpenBSD
2.6 & 2.7
ICMP
255


OpenVMS
07.01.2002
ICMP
255


OS/2
TCP/IP 3.0

64


OSF/1
V3.2A
TCP
60


OSF/1
V3.2A
UDP
30


Solaris
2.5.1, 2.6, 2.7, 2.8
ICMP
255


Solaris
2.8
TCP
64


Stratus
TCP_OS
ICMP
255


Stratus
TCP_OS (14.2-)
TCP and UDP
30


Stratus
TCP_OS (14.3+)
TCP and UDP
64


Stratus
STCP
ICMP/TCP/UDP
60


SunOS
4.1.3/4.1.4
TCP and UDP
60


SunOS
5.7
ICMP and TCP
255


Ultrix
V4.1/V4.2A
TCP
60


Ultrix
V4.1/V4.2A
UDP
30


Ultrix
V4.2 – 4.5
ICMP
255


VMS/Multinet

TCP and UDP
64


VMS/TCPware

TCP
60


VMS/TCPware

UDP
64


VMS/Wollongong
1.1.1.1
TCP
128


VMS/Wollongong
1.1.1.1
UDP
30


VMS/UCX

TCP and UDP
128


Windows
for Workgroups
TCP and UDP
32


Windows
95
TCP and UDP
32


Windows
98
ICMP
32


Windows
98, 98 SE
ICMP
128


Windows
98
TCP
128


Windows
NT 3.51
TCP and UDP
32


Windows
NT 4.0
TCP and UDP
128


Windows
NT 4.0 SP5-

32


Windows
NT 4.0 SP6+

128


Windows
NT 4 WRKS SP 3, SP 6a
ICMP
128


Windows
NT 4 Server SP4
ICMP
128


Windows
ME
ICMP
128


Windows
2000 pro
ICMP/TCP/UDP
128


Windows
2000 family
ICMP
128


Windows
Server 2003

128


Windows
XP
ICMP/TCP/UDP
128

It's from http://www.binbert.com/blog/2009/12/default-time-to-live-ttl-values/