I am a newbie to networking,I was analysing WireShark TCP dumps.I found TCP header timestamp value of 4016140 which dates back to 1970.Am i missing something?
TCP timestamps are reset to 0 on host reboot and incremented by OS dependent algorithm (for example it's incremented 100 times per sec ref ), these are not based on host clock/wall time.
Agree with what #VenkatC said. Note that standard (rfc1323) doesn't specify the unit of timestamp value. It's possible to measure the unit of timestamp this way: with Wireshark, capture a tcp session, look at the two packets sent by your PC, compare the difference between the wireshark timestamp and the difference between the TCP timestamp value field of the two packets.
In my case, the Wireshark timestamp difference is 160ms, the difference in TCP header timestamp values is 40. So the unit is 4ms. It may be different on your system.
Related
If TCP runs out of its sequence number, what will happen?
If it again turns to 0 as the sequence number of the next byte, won't that be considered "duplicate" by the receiver?
If yes, then it has to ignore that byte.
If not, why?
I think, i found the answer.
The answer of this query, lies on one of the TCP option field known as "timestamp". It's in every TCP segment (including data and ACK segments).
Therefore to identify a unique tcp segment, we look for a combination of "timestamp" and "sequence number".
The basic idea is that a segment can be discarded as an old duplicate if it is
received with a timestamp less than some timestamp recently received on this connection.
Example :
Two segments 400:12001 and 700:12001 definitely belongs to two different incarnations.
And this mechanism is known as "PAWS" or protection against wrapped sequence numbers.
Reference: https://www.rfc-editor.org/rfc/rfc1323#page-17
I am using Wireshark to capture and analyse the data from the radiotap header and the frame metadata. I would like to know if the epoch time from the frame metadata is the arrival time on the router or arrival time on my device.
I know epoch time is the time in microseconds since Epoch ( 1st January 1970 ). So i am using it as the time my device has captured the packet.
I am making a project that uses the difference between the MAC timestamp ( Stamp given in the router ) and the time my device captured the packet and i am using those two fields to get said difference.
The doubt came from the fact that this difference is always in the interval of 0.25 to 2.75 Microseconds, independent of where my computer was capturing the packets. So i wanted to make sure that:
The MAC Timestamp is a timestamp that the Access Point ( My router ) puts into the packet before sending it out and the Epoch Time is a timestamp my computer puts on the frame metadata when said packet was captured.
Is that correct? If not, how can i determine the time my packet was captured?
Edit: Fixing field name for Epoch time.
As it turns out, both are the same. The Mac timestamp is part of the Metadata added by the capturing software, as is epoch time and arrival time.
The timestamp that the access point prints into the packet is a on a base-16 and is contained under the frame tab, on fixed paramers on Beacon and Probre Frames. On wireshark, the field is simply labed "Timestamp:".
In TCP, Initial Sequence Numbers are set on a clock and are incremented every 4 microseconds.
Suppose (IP#1, port#1) and (IP#2, port#2) set up a TCP connection and with the respective Initial Sequence Number-s as
ISN1=100, ISN2=500.
A millisecond after, (IP#1, port#1) and (IP#2, port#2) are setting up another TCP connection
with the applying ISN numbers. In this case, the sequence numbers of the first and the
second connections are likely to coincide.
How does TCP handle this? It can reset and thus re-synchronize a connection under
certain circumstances along the way when the sequence numbers are odd (not synchronizing), but this may not cover every case.
How are the sequence numbers handled in such a case when these numbers of
2 different TCP connections may interfere with those of one another?
TIA.
Each TCP connection is identified by a tuple of [ local address, remote address, local port, remote port]. The sequence number is part of the state associated with that key therefore it doesn't matter at all if sequence numbers overlap.
Your particular example is impossible. You cannot setup two different TCP connections with identical local and remote addresses and ports.
In TCP how is the time for a time out to happen determined? I was told it is randomly selected from an interval that doubles after each time out, but nothing I found on Google mentions anything about random selection and instead says it's calculated used Smoothed Round Trip Time after the first acknowledgment is received. Does it do this for each packet or is there some randomness to the design?
An initial value of the RTT is calculated during the TCP 3-way handshake that starts a connection. It is updated thereafter when qualifying send/acks are seen.
Most modern implementations don't use this method directly but rather using a statistical analysis of the maximum time it should take to get an ACK and retransmit after that interval. The "exponential backoff" (the doubling of the wait interval) happens for further retransmissions of the same data.
A connection "times out" after some number of transmissions with no ACK being received.
For the TCP stream, i want to get the Network Jitter values as shown for the RTP packets under RTP statistics as Max Jitter and Mean Jitter in Wireshark.
Is there any option in Wireshark which gives the Jitter values for a TCP stream.
I have only found graphs related to RTT, Throughput, Seq. no. and Windows scaling but nothing related to Jitter is there for TCP streams.
Anyone has idea how can i get the Jitter values in Wireshark for TCP stream?
Regards
Nitin
I don't know of a way to calculate TCP jitter in Wireshark (or any other tool) but I assume that you could do this by comparing the deltas between the TCP timestamp fields and comparing to the actual arrival times.
You can export received timestamp values by adding a custom column with the expression tcp.options.timestamp.tsval then exporting as CSV or XML