Chapter 4 - TCP and UDP

1. What is TCP?
   1. connection-oriented protocol responsible for ensuring the transfer of a datagram from source to destination (end-to-end communications)
   2. enables an application in the upper layers to send a datagram via the network layer
   3. ensures that priorities and security are properly respected
   4. virtual circuit - communications between two "end machines"
   5. TCP is only necessary on machines that "process" datagrams (not on gateways)

2. Following a Message
   1. message is passed to TCP from an upper-layer protocol as a stream
   2. stream of bytes is assembled into TCP segments
   3. header information (checksums and sequence numbers) is attached to the front of the data
   4. in 2-way communications (i.e. ftp & telnet), a connection (virtual circuit) is established between the sending and receiving machines
      1. TCP software issues a request for a TCP connection (sending it it's socket number)
      2. The receiving TCP software returns its own socket number
   5. TCP sends the segment down to the network layer software (usually IP) which sends the message over the network
   6. the receiving machine's IP passes the received segment to it's TCP software which strips the header info and passes the message up
   7. if the message is more than one segment, the receiving TCP reassembles the segments requesting a resend of any corrupted segments
   8. The receiving TCP software may generate an ACK back to the sender
   9. Some TCP implementations use a sliding window (buffer) where the receiver sends a window size before each message is sent
   10. a timer is used to monitor the time between the sending of a message and the ACK back

3. Ports and Sockets
   • Port Numbers - identifies applications:

     Frequently used TCP port numbers

     Port Number	Process Name	Description
     1	TCPMUX	TCP Port Service Multiplexer
     5	RJE	Remote Job Entry
     17	Quote	Quote of the Day
     20	FTP-DATA	File Transfer Protocol--Data
     21	FTP	File Transfer Protocol--Control
     23	TELNET	Telnet
     25	SMTP	Simple Mail Transfer Protocol
     43	NICNAME	Who Is
     79	FINGER	Finger

   • WELL KNOWN PORT NUMBERS
   • Socket - IP address of the machine & its port number

4. TCP Communications with the Upper Layers

   ULP to TCP Service Request Primitives

   Command	Parameters Expected
   ABORT	Local connection name
   ACTIVE-OPEN	Local port, remote socket Optional: ULP timeout, timeout action, precedence, security, options
   CLOSE	Local connection name
   SEND	Local connection name, buffer address, data length, push flag, urgent flag Optional: ULP timeout, timeout action

   TCP to ULP Service Request Primitives

   Command	Parameters Expected
   CLOSING	Local connection name
   DELIVER	Local connection name, buffer address, data length, urgent flag
   ERROR	Local connection name, error description
   OPEN-ID	Local connection name, remote socket, destination address

5. Passive and Active Ports
   1. active connection - TCP issues a request for a connection, based on an instruction from an ULP that provides the socket number
   2. passive connection - ULP instructs TCP to wait for the arrival of connection requests from a remote system
      1. specified passive open - connection created when the precedence and security levels are acceptable
      2. unspecified passive open - connection created for any request

6. TCP Timers
   • The Retransmission Timer - manages retransmission timeouts (RTOs) [if the time between the send and ACK is exceeded, the message is resent with an increased RTO] (RTT - round-trip time)
   • The Quiet Timer - used to prevent just-closed ports from reopening too soon and receiving datagrams that were delayed [usually = twice the TTL]
   • The Persistence Timer - used when the receive window = 0
   • The Keep-Alive Timer and the Idle Timer - sends empty packets to keep the connection open

7. Transmission Control Blocks (TCB) and Flow Control

   TCP send and receive variables

   Variable Name	Description
   Send Variables
   SND.UNA	Send Unacknowledged
   SND.NXT	Send Next
   SND.WND	Send window
   SND.UP	Sequence # of last urgent set
   SND.WL1	Sequence # for last window update
   SND.WL2	Ack # for last window update
   SND.PUSH	Seq. # of last pushed set
   ISS	Initial send sequence number
   Receive Variables
   RCV.NXT	Seq. # of next received set
   RCV.WND	# of sets that can be received
   RCV.UP	Seq. # of last urgent data
   RCV.IRS	Initial receive sequence number

8. TCP Protocol Data Units

   TCP Protocol Data Unit (PDU)

   Source Port (16 bits)	Destination (16 bits)
   Sequence Number (32 bit)
   Acknowledge Number (32 bit)
   Data Offset and Flags	Window (16 bits)
   Checksum (16 bits)	Urgent Pointer (16 bits)
   Options and Padding

9. TCP and Connections
   • Establishing a Connection [can be established only if no connection between the two sockets, both machines agree to the connection, and both machines have adequate TCP resources] (see figure 4.6)
   • Data Transfer (see figure 4.7)
     TCP data transport subservices:
     • full duplex - enables both ends of a connection to transmit at any given time
     • timeliness - uses timers
     • ordered - TCP reassembles the message in the correct order
     • labeled - all connections have an agreed upon precedence and security
     • controlled flow - uses buffers and windows to control the flow of information
     • error correction - uses checksums to ensure data is error-free
   • Closing Connections (see figure 4.8)

     TCP uses a connection table to keep trace of all of the connections

10. User Datagram Protocol (UDP)

    UDP is connectionless and has no error-recovery capabilities

    UDP Header

    Source Port(16 bits)	Destination Port(16 bits)
    Length(16 bits)	Checksum(16 bits)
    Data

Tools for Tracing TCP Packets

• tcptrace - a TCP dump file analysis tool

LAND - TCP Bug Threatens Networked Computers