Syn Syn Ack Ack

Syn, Syn-Ack, Ack: The Three-Way Handshake of TCP

The reliable transmission of data across a network relies heavily on the Transmission Control Protocol (TCP). Unlike UDP (User Datagram Protocol), which offers a connectionless, faster but less reliable service, TCP establishes a connection before data transmission using a process called the three-way handshake. This handshake involves three packets: SYN (synchronize), SYN-ACK (synchronize-acknowledge), and ACK (acknowledge). Understanding the sequence and function of these packets is crucial for comprehending how TCP ensures reliable data delivery. This article will delve into the details of the "SYN, SYN-ACK, ACK" process, explaining each packet's role and highlighting common scenarios.

1. The SYN Packet: Initiating the Connection

The three-way handshake begins with the client sending a SYN packet to the server. This packet signifies the client's desire to establish a connection. The SYN packet contains several crucial pieces of information:

Source Port: The port number on the client machine initiating the connection.
Destination Port: The port number on the server machine the client is trying to connect to (e.g., port 80 for HTTP).
Sequence Number: A randomly chosen 32-bit number used for reliable data ordering. This number is incremented for each data segment sent.
SYN Flag: Set to 1, indicating that this is a synchronization request.

Think of the SYN packet as a polite knock on the server's door, asking for permission to establish a connection. It's important to note that the SYN packet itself doesn't transmit any data; its sole purpose is to initiate the connection request.

Example: A web browser (client) initiates a connection to a web server (server) by sending a SYN packet to port 80.

2. The SYN-ACK Packet: Acknowledging and Synchronizing

Upon receiving the SYN packet, the server responds with a SYN-ACK packet. This packet serves two critical purposes:

Acknowledgement (ACK): It acknowledges receipt of the client's SYN packet by including the client's sequence number plus one (acknowledging the received sequence number).
Synchronization (SYN): It also initiates its own synchronization process by including its own sequence number.

The SYN-ACK packet, therefore, acts as both an acknowledgement and a synchronization request from the server. It’s like the server opening the door and saying, "Yes, I received your request, and I'm ready to connect; here's my sequence number."

Example: The server receives the SYN packet and responds with a SYN-ACK packet, containing the acknowledged sequence number (client's sequence number + 1) and the server's own randomly chosen sequence number.

3. The ACK Packet: Completing the Handshake

The final step in the three-way handshake involves the client sending an ACK packet to the server. This packet acknowledges the server's SYN-ACK packet, confirming that the connection is established. The ACK packet contains:

Acknowledgement Number: The server's sequence number plus one, acknowledging the received sequence number from the server.
Sequence Number: The client's sequence number incremented from the initial SYN packet.
ACK Flag: Set to 1, indicating this packet is an acknowledgement.

At this point, the connection is fully established, and data can begin flowing bidirectionally between the client and server. The ACK packet is like the client saying, "Okay, I received your response and we're connected!"

Example: The client receives the SYN-ACK packet and sends an ACK packet back to the server, acknowledging the server's sequence number.

4. Data Transmission and Connection Termination

Once the three-way handshake is complete, data transmission can begin. Each data packet sent will have its own sequence number, allowing for reliable ordering and error detection. When the connection is no longer needed, it's terminated through a four-way handshake involving FIN (finish) and ACK packets. This process ensures that all data is properly transferred and acknowledged before closing the connection.

5. Common Scenarios and Implications

Understanding the three-way handshake is crucial for troubleshooting network issues. For example, a failure at any stage of the handshake will prevent a connection from being established. This could manifest as a website failing to load or a network application not connecting. Furthermore, SYN floods, a type of Denial-of-Service (DoS) attack, exploit the handshake by sending a large number of SYN packets without responding to the SYN-ACK packets, overwhelming the server's resources.

Summary

The SYN, SYN-ACK, ACK exchange is the foundation of reliable communication in TCP. This three-way handshake ensures that both the client and the server are ready to communicate before any data is transmitted, guaranteeing ordered and error-free data transfer. Understanding this process is critical for network administrators and developers alike in troubleshooting connectivity issues and developing robust network applications.

FAQs:

1. What happens if the SYN packet is lost? If the SYN packet is lost, the server won't receive the connection request, and no connection will be established. The client will typically retransmit the SYN packet after a timeout.

2. What happens if the SYN-ACK packet is lost? The client won't receive the server's acknowledgement. After a timeout period, the client will retransmit the SYN packet. This retry mechanism is a crucial part of TCP's reliability.

3. What is a half-open connection? A half-open connection occurs when a SYN packet is sent, and the server responds with a SYN-ACK, but the client fails to send the final ACK. This leaves a connection hanging on the server, consuming resources.

4. How does TCP ensure reliable data delivery beyond the three-way handshake? TCP uses sequence numbers and acknowledgements for every data packet to ensure reliable ordering and error detection. It also incorporates mechanisms for retransmission of lost packets.

5. Can the three-way handshake be bypassed? No, the three-way handshake is fundamental to TCP's connection establishment. Bypassing it would compromise the reliability guarantees provided by TCP. However, some optimized techniques exist to minimize the handshake overhead in specific scenarios, but they don't completely bypass the essential steps.

Search Results:

RST after SYN-ACK - Wireshark Q&A The acknowledgment number in the SYN/ACK packet in frame 3 should be 3839424769, but instead frame 3 has an acknowledgment number of 14744888. Because of this, the SYN/ACK packet from 10.223.100.100 does not correspond to a …

SYN - SYN/ACK - ACK - Wireshark Q&A 6 Mar 2013 · One more question, by using the filter tcp.flags == 0x0012, I'm able to find all [ SYN,ACK ] packets in one capture, so now wireshark lists all SYN,ACK, but how can I do a look up in the other sniffer capture that I took, to find out what syn,ack corresponds to …

Identify SYN packets without SYN/ACK - Wireshark Q&A 26 Sep 2011 · a. a read filter to find all the SYN frames: -R tcp.flags.syn == 1. b. ouput fields: -T fields -e ip.src -e ip.dst -e tcp.srcport -e tcp.dstport -e tcp.flags.ack. With some scripting (or maybe just some clever sorting) I think you should be able to identify SYNs without SYN/ACKS. You'll probably also want to output the frame number field (frame ...

Wireshark Q&A 20 Sep 2010 · When you are not only interested in the SYN packets, but also the SYN/ACK packets this changes to: tcp.flags.syn==1 tcp[0xd]&2=2. If I read your question in another way, you are looking for "all packets belonging to a TCP session for which the SYN packet is actually in the capture file". If this is your question, this can't be done directly ...

TCP SYN/SYN ACK/PSH ACK/ACK - Wireshark Q&A 4 Nov 2011 · Therefore the client will abandon Packet #9 since Packet #9 is not the ACK packet which the client expected to receive. So the client will re-transmit Packet #11 to the server, and the sequence number in the Packet #12 is 7, which is the correct ACK packet. This symptom happened after every packet sent from the client. Best Regards, Jimmy

No ACK after SYN/ACK - Wireshark Q&A 25 Aug 2013 · The captures have been taken ON the client itself. This clearly indicates that the SYN/ACK is reaching the phone but its still not responding with the ACK. Well, then either the capture process is broken (not recording the ACK - rather unlikely as there are several SYN-ACK because of the missing ACK) or the TCP stack is broken (not sending the ...

Wireshark Q&A 1 Apr 2016 · The SYN packets are the only location where the WS option may appear And the formula for the window size in the ack segment is for Wireshark info column displays calculated window size = window size * window scale factor ( 262140 = 65535 *4 )

Wireshark Q&A 22 Sep 2010 · You might even want to add "... and tcp.flags.ack==0" to make sure you only select the SYN packets and not the SYN/ACK packets. Now, back to the capture filter. You can use the filter "tcp[0xd]&2=2" which will capture all the frames with the SYN bit set (SYN as well as SYN/ACK). Or use "tcp[0xd]&18=2" to capture only SYN packets.

SYN - SYNACK- RST Reason - Wireshark Q&A Each SYN/ACK has a different sequence number, so the server is considering these SYN's to be of a different TCP session due to the fact that each previous SYN/ACK was answered with a TCP/RST. The TCP/RST packets all have a ip.id of 0x0000, while the TCP/SYN packets have ip.id's increasing by 1.

Windows sends RST after SYN-ACK on a TCP connection I ran the test on two machines with Windows 7 64-bit, and on one of them everything works as expected, but on the other one, after SYN-ACK is received Windows sends a RST, and I don't understand why. Here is a Wireshark capture file recorded on the physical interface. The test consist in running a web browser and try access a website.