TCP

TCP header
TCP connections
TCP checksum and pseudo-header
UDP

TCP header

    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |       Destination Port        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Acknowledgment Number                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data |           |U|A|P|R|S|F|                               |
   | Offset| Reserved  |R|C|S|S|Y|I|            Window             |
   |       |           |G|K|H|T|N|N|                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Checksum            |         Urgent Pointer        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Options                    |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
                            TCP Header Format

TCP Header fields

Source and Destination port: demultiplexing
Sequence and acknowledgement: reliable delivery
Data Offset: header size, options
Window: flow control
Checksum: correctness
Urgent Pointer: "special place" in the data stream

TCP Header bits

SYN: I want to establish a connection
FIN: I will never again send data on this connection
RST: kill this connection
PSH: immediate delivery of this data is probably a good idea
URG: the urgent pointer is valid
ACK: the acknowledgement field is valid (set in all but the first SYN packet)

TCP connections

sequence numbers, windows, etc. must be remembered and applied to incoming packets
remembering these numbers is a form of state
since TCP has state, designers decided to have the peers explicitly manage this state (called a connection)
the peers agree on when to establish (open) a connection, when to tear it down (close), and when the connection must be thrown away (reset)
the state on each system reflects an understanding about the state on the peer

TCP connection establishment

when I receive a request to establish a connection, I must check:
- that I don't already have this socket: one or more of the port numbers or IP numbers must differ from existing connections
- that an application on my end desires to be connected
- that I have sufficient resources to handle this connection
the purpose of the connection establishment phase is to set up consistent connection state on the two peers

TCP State Machine -- Open

                              +---------+ ---------\      active OPEN  
                              |  CLOSED |            \    -----------  
                              +---------+<---------\   \   create TCB  
                                |                  \   \  snd SYN    
                   passive OPEN |     |   CLOSE        \   \           
                   ------------ |     | ----------       \   \         
                    create TCB  |     | delete TCB         \   \       
                                V     |                      \   \     
                              +---------+            CLOSE    |    \   
                              |  LISTEN |          ---------- |     |  
                              +---------+          delete TCB |     |  
                   rcv SYN      |     |     SEND              |     |  
                  -----------   |     |    -------            |     V  
 +---------+      snd SYN,ACK  /       \   snd SYN          +---------+
 |         |<-----------------           ------------------>|         |
 |   SYN   |                    rcv SYN                     |   SYN   |
 |   RCVD  |<-----------------------------------------------|   SENT  |
 |         |                    snd ACK                     |         |
 |         |------------------           -------------------|         |
 +---------+   rcv ACK of SYN  \       /  rcv SYN,ACK       +---------+
   |           --------------   |     |   -----------                  
   |                  x         |     |     snd ACK                    
   |                            V     V                                
   |  CLOSE                   +---------+                              
   | -------                  |  ESTAB  |                              
   | snd FIN                  +---------+

TCP 3-way handshake

from state CLOSED:
- send SYN, enter state SYN SENT
- receive SYN and ACK, send ACK, enter state ESTAB, or
- receive SYN, send ACK, enter state SYN RCVD, then proceed as below
from state LISTEN:
- receive SYN, send SYN and ACK, enter state SYN RCVD
- receive ACK, enter state ESTAB
retransmissions in case any of these are dropped
last ack issue

TCP State Machine -- Close

   |  CLOSE                   +---------+                              
   | -------                  |  ESTAB  |                              
   | snd FIN                  +---------+                              
   |                   CLOSE    |     |    rcv FIN                     
   V                  -------   |     |    -------                     
 +---------+          snd FIN  /       \   snd ACK          +---------+
 |  FIN    |<-----------------           ------------------>|  CLOSE  |
 | WAIT-1  |------------------                              |   WAIT  |
 +---------+          rcv FIN  \                            +---------+
   | rcv ACK of FIN   -------   |                            CLOSE  |  
   | --------------   snd ACK   |                           ------- |  
   V        x                   V                           snd FIN V  
 +---------+                  +---------+                   +---------+
 |FINWAIT-2|                  | CLOSING |                   | LAST-ACK|
 +---------+                  +---------+                   +---------+
   |                rcv ACK of FIN |                 rcv ACK of FIN |  
   |  rcv FIN       -------------- |    Timeout=2MSL -------------- |  
   |  -------              x       V    ------------        x       V  
    \ snd ACK                 +---------+delete TCB         +---------+
     ------------------------>|TIME WAIT|------------------>| CLOSED  |
                              +---------+                   +---------+

TCP close, part 1

from state ESTAB:
- receive FIN, send ACK, enter state CLOSE WAIT
- application closes connection, send FIN, enter state LAST ACK
- receive ACK, enter state CLOSED
from state ESTAB:
- application closes connection, send FIN, enter state FINWAIT-1
- receive FIN, send ACK, enter state CLOSING
- receive ACK, enter state TIME WAIT

TCP close, part 2

from state FINWAIT-1, if we get an ACK:
- receive ACK, enter state FINWAIT-2
- receive FIN, send ACK, enter state TIME WAIT
from state TIME WAIT, enter state CLOSED after 4 minutes (2 Maximum Segment Lifetimes)

TCP close: example

TCP congestion control

TCP Reno:
- if a packet is lost, assume it's because of congestion
- slow down significantly (cut window by a factor of two)
TCP Vegas:
- measure the round-trip time of packets
- if significantly greater than the minimum, assume due to congestion, slow down slightly (linear)
- use Reno if packets are lost

Checksum

In TCP and UDP, the checksum covers the header, data, and a pseudo-header
Unlike TCP, the UDP checksum is optional, and sent as zero (0x0000) if not computed
the TCP and UDP pseudo-header has fields derived from the IP header:
this header is used in computing the checksum, but never sent

    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Source address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Destination address                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     zero      |   Protocol    |       TCP/UDP length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
                          Pseudo Header Format

UDP header format

RFC 768

    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |       Destination Port        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Length             |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
                            UDP Header Format