ICS 156: Networking Lab
Magda El Zarki
Professor, ICS
UC, Irvine

Course Outline
 Ch 1: Introduction
 Ch 2:Bridges
 Ch 3:Routers
 Ch 4: Transport Protocols

4. Transport Layer Protocols

Introduction

Transport Layer Protocols:
– Transport Control Protocol (TCP) and
– User Datagram Protocol (UDP)

4.1 Introduction
Host Host
4
3
2
1
Router
3
2
1
End-to-End Transport
Router
3
2
1
Router
3
2
1
Subnet 1 Subnet 2 Subnet 3 Subnet 4
4
3
2
1

4.1.1 TCP and UDP

There are two kinds of Transport Protocols used in the Internet:
– UDP : User Datagram Protocol
– TCP : Transport Control Protocol

They both operate over IP.

They both use PORT numbers for connecting to applications.

4.1.2 Port Numbers (1/3)

The concatenation of IP address and Port Number provides a unique identifier for transferring information between a source and a destination.
This is known as a socket , the door between an application and the underlying transport system.

Each application service is allocated a unique port number:
– FTP - 21
– Telnet - 23
– Domain Name Server - 53, etc.

4.1.2 Port Numbers (2/3)

There are 1023 port numbers available for application services.

Not all port numbers are well known, in that case a request is sent to a catch all port number. The response packet contains the correct port number.

4.1.2 Port Numbers (3/3)
2
1
4
3
Physical Address
Subnet 1
Port Number
Network IP Address
3
2 2
1 1
Subnet 2
4
3
2
1

4.1.3 TCP vs UDP

The tuple <Source Port, Destination Port> is used in the addressing of the Transport Data Unit
(TDU).

Most applications that require a reliable transport mechanism use TCP, otherwise UDP is used because it is simpler.

UDP is the preferred choice for real-time services as it does not incorporate a lot of the overhead that is associated with TCP.

4.2 UDP (1/3)

Uses only 8 bytes of header:
– 2 bytes for source port
– 2 bytes for destination port
– 2 bytes for length of data unit
– 2 bytes for checksum (covers IP addresses and
IP length field included)

4.2 UDP (2/3)

Provides connectionless service - no sequencing, no end to end error control (only error detection, includes data), no flow control.

NFS, some routing protocols (e.g., RIP), RTP
(Real-Time Protocol), TFTP, etc., use UDP as it is simple and fast if reliability is not an issue.

IP will do fragmentation if the UDP packet is too large for the path (i.e., > path MTU).

4.2 UDP (3/3)

A UDP packet can be as large as the max.
IP packet minus the headers. Generally the application will limit the packet size.

UDP leaves most of the work to the upper
(application) layer. This can slow things down, as ACKs, re-ordering, etc., has to be done by the application.

4.3 TCP (1/2)
Source Port
Header Length
Checksum
32 bits
Dest. Port
Sequence Number
Acknowledgement Number
Flags Advertised Window
Urgent Pointer
Options
Data
Flags: 1) SYN - for connection set-up
2) FIN - last segment connection tear down
3) URG - urgent data carried
4) ACK - read ack field
5) RESET, 6) PSH - pass data to application asap
Advertised Window: amount of data sender can send.
Urgent Pointer: Points to where urgent data starts in data field
Checksum: for whole segment

4.3 TCP (2/2)

20 byte header (without options)

Header plus data referred to as a transport segment

Segment size chosen to accommodate the smallest IP packet size on path to avoid fragmentation

4.3.1 Connection Establishment and Tear Down

TCP goes through a 3 phase set-up:
– Connection set-up (3 way hand-shake): SYN,
SYN ACK, ACK
– Data transfer
– Connection tear down: FIN, ACK, FIN, ACK
(two way or one way)

4.3.2 Sequence Numbers and

ACKs
TCP is connection oriented - uses sequence numbers
(always increasing, numbers not consecutive, reflect the number of bytes not segments !). For example: if TCP wants to send 2000 bytes and sends them in two segments of 1000 bytes each, if first segment has sequence no 102, then sequence number of second segment will be
102+1000= 1102.

The ACKs on the other hand reflect the sequence number of the next byte that the receiver is expecting . If the receiver received segment with sequence number 102 of length 1000, then ACK will use number: 102+ 1000 =
1102.

4.3.3 Out of Order Arrivals

The RFC does not specify what to do with out of order packets. TCP will only acknowledge ordered transmissions, but the question is: should it keep out of order packets in its buffer or just discard them? The easiest implementation is to discard and wait for all packets to arrive in order.

An ACK is sent in response to an out of order packet, it contains the “expected” sequence number.


4.3.4 Group and Delayed ACKs
Group ACKs: TCP will use group ACKs, also referred to as delayed ACKs. in otherwords, it will not ACK each received segment, it will wait for a short while (differs on each system, 200msecs for
WANs, 20msec for LANs) and then ACK all segments (max. 2 segments) that it has received so far.

It does that so that: 1) it can do group ACKing which is more efficient, and 2) it hopes to be able to piggyback an ACK on data going in the reverse direction. If there is data in the buffer that needs to be sent, it will not do a delayed ACK. Only uses delayed ACK if send buffer is empty.

4.3.5 Timers & Retransmissions

Uses error detection and recovery - ACKs and retransmissions. If an ACK is not received before a timer expires, the segment is resent.

The retransmission timer is not a static value. It is calculated based upon current network status. The timer must be greater than the roundtrip delay!

4.3.6 Flow Control (1/2)

Sliding window is used as the flow control technique (matching of sender and receiver data rates): Each end system will advertise the largest window it is willing to receive without ACKs, i.e., packets that can be outstanding between source and destination.

This value is dynamic , it changes based upon how fast the receiver can read the arriving data. It is always advertised in the messages sent from the destination to the source. Note however that the
TCP congestion control mechanism does kick in too and affects the amount of data that can be sent.

4.3.6 Flow Control (2/2)

If an ACK packet comes back with a receiver window size of “0”, that means that the destination wants the source to hold off with transmission. In this case the sending host can only send segments with 1 byte of data. This is to keep the connection alive between the two ends, i.e., keep ACKs flowing from destination to source indicating the window size.

4.3.7 Congestion Control

TCP has imbedded congestion control . When ACKs come back with a delay (i.e. timers expire) the window size is reduced by the sender. The sender will only send data = min{congestion window, receiver (or sliding) window}.

TCP also uses something called slow start : The sender starts off with only sending 1 (in some cases 2) packet, then doubles that if the ACK comes back before the retransmit timer expires. It keeps on doubling until it reaches a threshold then goes into a linear increase (i.e., adds one to the window size). If at anytime an ACK is delayed, it sets the threshold to half the current congestion window size and starts again with slow start.

4.3.8 Applications and TCP
 All applications that are concerned with reliable transport use TCP
 SMTP (email), ftp, RIP, http, etc. use
TCP.
 TCP hides the underlying network from the application: it segments,it re-orders packets, does error control and handles flow matching and congestion control.