BROCADE IP PRIMER

Transmission Control Protocol (TCP)Flags. This is a 6-bit field. Each bit represents a special message in TCP.• URG Urgent pointer• ACK Acknowledgment• PSH Push function• RST Reset the connection• SYN Synchronize the sequence numbers• FIN Request to close the connection (finished)Window. This is a 16-bit number. It is the size of the window in bytes.Checksum. This is a 16-bit number. This provides error checking for TCP. Eachsegment has a checksum that allows the client to verify the data.Urgent Pointer. This is a 16-bit number. If the URG flag is set, this number actsas an offset to the sequence number.Options. This is an optional field that must be a multiple of 32-bits. It is usedin conjunction with the Urgent pointer. It provides additional settings.Data. This is the data received by the Session Layer.The 3-Way HandshakeTCP uses what's referred to as a “3-way handshake” to initiate a conversation.Let's start by showing you what it looks like.TCP SYNTCP SYN/ACKTCP ACKThe client sends a TCP SYN segment. The SYN is short for synchronize. It synchronizesthe segment sequence numbers between the client and the server,but it also initiates the session. The first packet (the SYN) determines whatdestination port you're communicating with, and what source port the servershould reply on.The server sends back a TCP SYN/ACK segment. To me, this is really a misnomer.It might be easier to think of this as an “ACK/SYN” packet. The “ACK” partis acknowledging the SYN that the client sent. The “SYN” part is the server'sSYN packet back to the client. It's as if the client is saying, “Hey, do you want totalk?” And the server replies, “Yeah, I want to talk. Do you want to talk?”And the client replies, “Yeah.” He does this by sending the third segment: anACK. An ACK, put simply, is an “I got it” segment. It's TCP's method for communicatingthat you've received a segment.Brocade IP Primer 57