11 replies to this topic

[Apprentice123]

What would be the 3-way handshake method for a sender and addressee that exchange packets of parallel?

[Momerath]

In general a 3-way handshake is where two machines exchange three messages to establish communications. For example, this is used in the TCP protocol. Machine A sends a 'SYN' message to machine B. Machine B sends back a 'SYN-ACK' message. Machine A then responds with an 'ACK' message.

[Apprentice123]

Momerath said:

In general a 3-way handshake is where two machines exchange three messages to establish communications. For example, this is used in the TCP protocol. Machine A sends a 'SYN' message to machine B. Machine B sends back a 'SYN-ACK' message. Machine A then responds with an 'ACK' message.

OK. Thanks

[Apprentice123]

How can I implement this presentation?

[Momerath]

Apprentice123 said:

How can I implement this presentation?

That question is too vague to answer in any way other than to point to the definition of 3-way handshake.

At the least you'll need to establish a protocol for sending messages, a message format, what information you are negotiating, ... Those are just off the top of my head, I'm sure there are more.

[Apprentice123]

Momerath said:

That question is too vague to answer in any way other than to point to the definition of 3-way handshake.

At the least you'll need to establish a protocol for sending messages, a message format, what information you are negotiating, ... Those are just off the top of my head, I'm sure there are more.

I'm thinking of using the algorithm Go-Back-N. But I did not understand very well the workings of this algorithm.

[Apprentice123]

In three-way handshake we have:
(1) client-------------------> server
(2) client<------------------- server
(3) client-------------------> server

(1) client sends a packet with the flag SYN (sequence) to the server
(2) If the server accepts the packet, it sends a packet to the client with the flags SYN and ACK
(3) Upon receiving the SYN + ACK from the server, the client sends another ACK packet to the server, thus establishing the connection
Are correct ? But why is SYN and ACK (what to send ?) ?

[Momerath]

SYN and ACK are part of the TCP protocol. You can develop any protocol you like and send whatever you like. If you are doing the 3-way handshake, then you'll need a 3 step method (like the one you described).

For example, your client could send: HELLO 382839
Server responds with: HI 382839 HELLO 289593
Client responds with : HI 289593

In this case HELLO is the SYN message, and HI is the ACK message. The number following is to ensure that they are responding to the correct client/server. If the number to a HI message is not the number sent by the HELLO message, then something is wrong. See, I just made up a protocol :)

[Apprentice123]

Momerath said:

SYN and ACK are part of the TCP protocol. You can develop any protocol you like and send whatever you like. If you are doing the 3-way handshake, then you'll need a 3 step method (like the one you described).

For example, your client could send: HELLO 382839
Server responds with: HI 382839 HELLO 289593
Client responds with : HI 289593

In this case HELLO is the SYN message, and HI is the ACK message. The number following is to ensure that they are responding to the correct client/server. If the number to a HI message is not the number sent by the HELLO message, then something is wrong. See, I just made up a protocol :)

Thank you, very good explanation.
About the algorithm Go Back N, when a packet is lost, a timer is created to send the package again. This timer is pre-defined or is it the time it takes to send packets successors of the lost packet?
For Example:

P1 --------> receives P1 and send ACK1
P2 -----Lost
P3 --------> discards P3 and send ACK1
P4 --------> discards P4 and send ACK1
.
.
.
P2 -------> receives P2 and send ACK2
P3 ------->
.
.
.

The time it takes to Resubmit P2 is the time to send and discard or P3 and P4 or is a set time?

[Momerath]

You set the frame for how many packets you are going to send before you check for the ACK statements (on the server). For example, let's say you check every 4 and number 2 was lost (like your example). This results in:

[B]Server                       Client[/B]

Send P1                      Send ACK 1

Send P2                      not recieved

Send P3                      dropped

Send P4                      dropped

Check ACKs received          ....

Send P2                      Send ACK 2

Send P3                      Send ACK 3

etc

The N (of the Go-Back-N) in this case is 4, as the server will never go back more than 4.

[Apprentice123]

OK. Thank you, the Go-Back-N I understand, thanks.
I have other problem: I have to "split" messages, to create the packages and package identification header (use fragmentation for packets larger than 20 bytes, using the idea of IPv4)

What I understand about fragmentation is: accurate identification, flag and displacement.
identification is to determine which package is fragmenting
flag is flag=1 for each segment, until the last fragment where flag=0
displacement is where data will be inserted in each fragment
I'm correct ?

And what's "create the packages and package identification header", would be the fragmentation?

[Apprentice123]

You understand my question doubt?