Solved

TCP/IP: 3-way Handshake

Posted on 2007-04-08
5
3,409 Views
Last Modified: 2012-06-21
Hello Experts,

I have a question regarding the 3-way tcp handshake.  Where does the fault lies when the sender does not send back an ACK?  This is the situation that I am currently getting.  We have two sender, A and B.  

A --> B     [SYN]
B --> A     [SYN, ACK]
A --> B     No response

Does this mean that A did not get the [SYN, ACK] from the second phase of the handshake?  Or did "A" block the third phase?  Can someone clarify this for me.  As always, thank you for your time on this matter...it is always appreciated.
0
Comment
Question by:coperator
  • 2
  • 2
5 Comments
 
LVL 8

Accepted Solution

by:
RGRodgers earned 250 total points
ID: 18873513
A should either ACK (called a SYN ACK ACK) or a NACK for either a bad message or timeout if A gets nothing from B.  A appears at fault.
0
 
LVL 5

Expert Comment

by:skaap2k
ID: 18885714
There could be a issue where one of these devices is using TCP SYN Cookies and the other does not know how to handle it (unlikely)

The best way to find out what is going on, is to take a Ethereal/Wireshark trace from both devices, and see whether A is indeed receiving the SYN ACK, and if it is sending the ACK/SYNACKACK to B ..

RN
0
 
LVL 8

Expert Comment

by:RGRodgers
ID: 18885845
All true, especially the Wireshark comment.  

However, whether A knew how to handle it or not, or received the ACK or not, A was obligated to respond with a SYN/ACK/ACK or a NACK.  No response is never correct.

But, do the trace and tell us what you see!  Thanks...
0
 
LVL 1

Author Comment

by:coperator
ID: 18888560
Unfortunately, I have no control of the other device.  It belongs to a client.  One thing I'd like to mention is the physical layout and where I was able to do a tcpdump.  Will call the client router A and my PIX B (OUTSIDE interface) and both are interconnected thru a switch C.

A -> C -> B

I was able to do a tcpdump from C.  I discovered that there was a Linux box setup to sniff and the port was configured for spanning.  So, that's were I was able to capture the packets.  Now, base on what I had described above, it seems that it is leaving my OUTSIDE interface of my PIX but I am not able to get an ACK from A.  Base on the conversation above, does this confirm that A is at fault?

As always, thank you for your time on this matter.
0
 
LVL 5

Assisted Solution

by:skaap2k
skaap2k earned 250 total points
ID: 18888587
More than likely, yes, that would be the issue.
0

Featured Post

Technology Partners: We Want Your Opinion!

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

I know for anybody starting from Beginner to Expert in Networking knows what OSI model. But this tutorial is for freshers or those who are new to networking world. Why I am putting OSI in such simple and compact manner is because it enables you to k…
The article explains the protocols and technology which is involved when two computers on different TCP/IP networks communicate with each other. In the diagram, a router is used to segregate two networks. The networks are 192.168.1.0/24 and 192…
After creating this article (http://www.experts-exchange.com/articles/23699/Setup-Mikrotik-routers-with-OSPF.html), I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…
After creating this article (http://www.experts-exchange.com/articles/23699/Setup-Mikrotik-routers-with-OSPF.html), I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…

740 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question