1. UDP and TCP Basics
Rocky K. C. Chang October 2010

2. Transport protocols
Problem: How to extend the host-to-host packet delivery service to a process-to-process communication channel?
Best-effort service provided by IP:
drops messages
re-orders messages
duplicates messages
limits message size
delivers messages with delay

3. Transport protocols Expectations from transport layer services:
guarantees message delivery
delivers messages in order
delivers at most one copy of each message
supports arbitrarily large messages
supports synchronization between sender and receiver
allows receiver to apply flow control
supports multiple application processes on each host

4. Transport protocols
Different transport protocols provide somewhat different sets of services:
User Datagram Protocol (UDP): Mainly provide a demultiplexing service.
Transmission Control Protocol (TCP): Provide a reliable byte-stream service
Remote Procedure Call (RPC): Provide services to transaction-based applications.
Real Time Protocol (RTP): Provide services for transporting real-time data over UDP.

5. UDP (RFC 768) UDP adds a demultiplexing service to IP.
UDP optionally provides error detection but it becomes mandatory for UDP over IPv6.
UDP provides the demultiplexing service through UDP ports.
Process IDs can also be used if all systems run on the same OS.
A process is uniquely identified by (Port number, IP address), which is usually referred to as a socket.
A port is usually implemented by a message queue.

6. UDP (RFC 768) How does a process learn the port for the other party?
For a server process, it will know the client’s port whenever the client gets connected.
For a client process, one approach to learn server’s port is through the well-known port.

7. Pseudo-header for UDP
When a sender computes the checksum, a pseudo-header is first prepended to the UDP header. Why?

8. End-to-end issues
As shall see later, TCP uses the same sliding window protocol as in go-back-n ARQ to provide transport services.
packets
+ve ACK
packets
+ve ACK

9. End-to-end issues Hop-by-hop issues vs end-to-end issues:
Potentially connects many different hosts
need explicit connection establishment and termination
Potentially long delay in network
need to be prepared for arrival of very old packets
need to limit packet’s lifetime in the network, in additional to the TTL mechanism.
Potentially different buffering capacity at end nodes
unable to always allocate enough buffers for the bandwidth-delay product of the “connection.”

10. End-to-end issues Potentially different network capacity
need to accommodate different amounts of buffering, especially at the receiving nodes.
Potentially different network capacity
need to be prepared for network congestion
Potentially different round trip times
need highly adaptive timeout mechanism to cater for various round trip time.
need to cater for the additional delay resulting from network congestion.

11. End-to-end reliability and ordered delivery
An hop-by-hop approach
Reliable A-->B and reliable B-->C and reliable C-- >D ==> reliable A-->D?
An end-to-end approach
Does it mean that we don’t need the hop-by-hop ARQ?
ARQ
ARQ
ARQ A B C D
TCP A B C D

12. End-to-end argument
Question: which networking layer should a function (e.g., reliability and ordered delivery in our case) be implemented?
End-to-end argument
A function should not be provided in the lower levels of the system unless it can completely and correctly implemented at that level.
However, the argument does not exclude providing that function at the lower levels for the purpose of performance enhancement.

13. TCP (RFC 793) TCP uses connection as its fundamental abstraction.
A TCP connection is specified by a pair of sockets, each of which identifies an endpoint.
TCP supports full-duplex connections.
TCP provides a connection-oriented, reliable, ordered delivery, byte stream service to the upper layer.
TCP also provides end-to-end congestion control and flow control services.

14. TCP (RFC 793) Connection-oriented transport protocol
Need to obtain an explicit agreement from the other side before sending data.
Don’t be confused with the connection-oriented network protocol.
Reliable service through a sliding window mechanism, positive acknowledgment, timing-out and retransmission
Accumulative acknowledgment vs selective acknowledgment

15. TCP (RFC 793)
Ordered delivery: receiver delivers ordered data to the application.
Byte-stream service:
Each byte is identified by a number and a TCP receiver does not understand the relationship among bytes.
Byte-stream vs message-based
Permit a number of messages to be sent out as a larger message.
Flow control on the number of messages is difficult.

16. TCP connection establishment
TCP connection setup is explicit and asymmetric.
The one initiates the connection does an active open.
The other end does a passive open.
It involves a total of three special TCP messages (SYN segments): A three-way handshaking (1.5 round-trip time).
Connection establishment timeout
TCP client and sender resend a SYN segment with exponential backoff up to a certain time.
TCP SYN flooding attacks

17. TCP connection establishment
Information exchange during connection establishment:
Initial Sequence Numbers (ISN), which are the first SNs used by the two sides. Why don’t just fix them to, say, 0?
The SYN segment also advertises window size (buffer available for receiving data).
Each side may optionally announce the Maximum Segment Size (MSS) it expects to receive.
If the destination IP address is local, set MSS to the local network’s MTU  40 bytes.
Otherwise, usually set MSS to 536 bytes.

18. An example Client Server LISTEN SYN, SequenceNum = SYN_SENT x y
socket()
socket(), bind(), listen()
connect()
LISTEN
SYN, SequenceNum =
SYN_SENT
accept() x y ,
SYN_RCVD
SYN + ACK, SequenceNum = x + 1
Acknowledgment =
ESTABLISHED
connect()returns
ACK, Acknowledgment =
write() y
ESTABLISHED + 1
accept()returns
data
read()returns

19. TCP connection termination
TCP connection termination is explicit and symmetric:
Each side has to close the connection independently.
At most four FIN segments are needed.
By closing the connection, it means that no more data will be sent, but can still receive data.
A connection in the TIME_WAIT state cannot move to the CLOSED state until it has waited for 2Maximum Segment Lifetime (MSL).

20. An example Client Server close() FIN_WAIT_1 FIN, SequenceNum = x
CLOSE_WAIT
ACK x + 1
close()
LAST_ACK
FIN_WAIT_2
FIN, SequenceNum = y
TIME_WAIT
ACK y + 1
CLOSED
CLOSED

21. TCP state transition diagram
CLOSED
LISTEN
SYN_RCVD
SYN_SENT
ESTABLISHED
CLOSE_WAIT
LAST_ACK
CLOSING
TIME_WAIT
FIN_WAIT_2
FIN_WAIT_1
Passive open
Close
Send/
SYN
SYN/SYN + ACK
SYN + ACK/ACK
ACK
/FIN
FIN/ACK
ACK + FIN/ACK
Timeout after two
segment lifetimes
Active open
/SYN

22. TCP state transition diagram
The transition from LISTEN to SYN_SENT is legal but is not supported in practice.
The transition from SYN_RCVD back to LISTEN is valid only of the SYN_RCVD state was entered from the LISTEN state, not from the SYN_SENT state.
Other extraneous state transitions will be discussed in the TCP security paper.

23. Reset segments
A TCP reset segment (RST) is generated when a connection request for a nonactive port is received.
For the same error, UDP sends an ICMP port unreachable message.
An application may also send a RST segment to abort a connection.
Any queued data is thrown away.
The RST segment is not acknowledged.
Why?

24. Simultaneous open and close
SYN K
SYN _SENT
SYN J
SYN _SENT
SYN _RCVD
SYN _RCVD
SYN K, ACK J+1
SYN J, ACK K+1
ESTABLISHED
ESTABLISHED
FIN_WAIT_1
FIN J
FIN K
FIN_WAIT_1
CLOSING
CLOSING
ACK K+1
ACK J+1
TIME_WAIT
TIME_WAIT

25. TCP segments
| Source Port | Destination Port |
| Sequence Number |
| Acknowledgment Number |
|Header | |U|A|P|R|S|F| |
|length | Reserved |R|C|S|S|Y|I| Advertised Window |
| | |G|K|H|T|N|N| |
| Checksum | Urgent Pointer |
| Options | Padding |
| data |

26. TCP segments Data field is optional.
SN, AN, and advertised window are all involved in the sliding window algorithm.
SN refers to the number of the first data byte or the special segment, e.g., SYN, FIN.
Flags: special segment indication
Examine the AN if the ACK flag is on.
The TCP header is not of fixed length due to the options (MSS, timestamp, window scale, etc).

27. TCP segments
The checksum covers the header and the payload, i.e., end-to-end checksum.
A pseudo-header is first prepended to the TCP header before computing the checksum.
Both urgent pointer and PUSH can be used to serve as a signal to the receiver about the “record boundary.”
The receiving process needs to be notified when the URG bit or PUSH bit is set.
These mechanisms serve as end-of-record markers.

28. TCP segments TCP options
The PUSH bit can also be used to allow the sending process to send whatever bytes in the buffer (for interactive applications).
TCP options
Selective acknowledgement (SACK) option (RFCs 1072, 2018, and 2883)
TCP MD5 signature option (RFC 2385)
TCP extensions for high performance: window scaling and timestamp (RFC 1323)
MSS announcement (RFC 793)

29. Summary
UDP and TCP are the two primary transport protocols provided in the Internet today.
SCTP and DCCP
Similar to IP at the network layer, UDP provides a connectionless, transport-layer service (demultiplexing and error detection).
TCP provides
connection-oriented, reliable, ordered delivery, byte stream service to the upper layer, and
end-to-end congestion control and flow control services.

30. References
J. Saltzer, D. Reed, and D. Clark, “End-to-end arguments in system design,” web.mit.edu/Saltzer/www/publications/ endtoend/ANe2ecomment.html.
D. Clark, V. Jacobson, J. Romkey, and H. Salwen, “An analysis of TCP processing overhead,” IEEE Commun. Mag., vol. 27, no. 6, June 1989.
A special issue on TCP Performance in Future Networking Environments. IEEE Commun. Mag., vol. 39, no. 4, Apr