Underlying best-effort network drop messages reorder messages
deliver duplicate copies of a given message limit messages to some
finite size deliver messages after an arbitrarily long delay
Transport layer protocol support communication between the end
application programs also called end-to-end protocol
Slide 3
Physical Data Link Network Transport Application Signals
Physical Data Link Network framing, error detection, reliability
control, media-access control Transport Application Src Dst
host-to-host packet delivery global addressing routing
Slide 4
Common properties that a transport protocol should provide
guarantee message delivery deliver messages in the same order they
are sent deliver at most one copy of each message support
arbitrarily large messages support synchronization between the
sender and the receiver allow the receiver to apply flow control to
the sender support multiple application processes on each host
Slide 5
5.1 Simple Demultiplexer (UDP) Unreliable and unordered
datagram service Support multiplexing allows multiple application
processes on each host to share the network No flow control
Slide 6
Endpoints identified by ports servers have well-known ports,
examples domain name server (DNS) receives messages at well-known
port 53 on each host mail service listens for messages at port 25
the Unix talk program accepts messages at port 517 this mapping is
published periodically in an RFC (Request for Comments) and is
available on most Unix systems in file /etc/services Internet's
User Datagram Protocol (UDP) is an example of such a transport
protocol
Slide 7
The header for an end-to-end protocol that implements this
demultiplexing function typically contains an identifier (port) for
both the sender (source) and the receiver (destination) of the
message For example, the UDP header UDP header format
Slide 8
UDP header format UDP port field is 16 bits long i.e., up to
64K (=2 16 ) possible ports ports are not interpreted across the
entire Internet, but only on a single host a process is identified
by a port on some particular host - a (port, host) pair UDP header
consists of only 4 fields two of them is optional (Source Port
& Checksum)
Slide 9
Fields Source port identifies the sending port when meaningful
and should be assumed to be the port to reply to if needed if not
used, then it should be zero Destination port identifies the
destination port and is required
Slide 10
Length (16-bit) specifies the length in bytes of the entire
datagram: header and data the minimum length is 8 bytes since
that's the length of the header the field size sets a theoretical
limit of 65,535 bytes for the data carried by a single UDP datagram
the practical limit for the data length which is imposed by the
underlying IPv4 protocol is 65,507 bytes Checksum (16-bit) used for
error-checking of the header and data
Slide 11
A port is purely an abstraction exactly how it is implemented
differs from system to system, or more precisely, from OS to OS
example, the socket API described is an example implementation of
ports
Slide 12
A port is implemented by a message queue when a message
arrives, the protocol (e.g., UDP) appends the message to the end of
the queue should the queue be full, the message is discarded there
is no flow-control mechanism that tells the sender to slow down
when an application process wants to receive a message, one is
removed from the front of the queue if the queue is empty, the
process blocks until a message becomes available
Slide 13
UDP message queue
Slide 14
UDP computes its checksum over UDP header the contents of the
message body pseudoheader Pseudoheader consists of protocol number
source IP address destination IP address UDP length field
Slide 15
UDP uses the same checksum algorithm as IP (section 2.4.2)
Motivation behind the pseudoheader verify this message has been
delivered between the correct two endpoints example, if the
destination IP address was modified while the packet was in
transit, causing the packet to be misdelivered, this fact would be
detected by the UDP checksum
Slide 16
5.2 Reliable Byte Stream (TCP) In contrast to a simple
demultiplexing protocol like UDP, a more sophisticated transport
protocol is one that offers a reliable, connection-oriented,
byte-stream service it frees the application from having to worry
about missing or reordered data Internet's Transmission Control
Protocol (TCP) is probably the most widely used protocol of this
type
Slide 17
TCP guarantees reliable, in-order delivery of a stream of bytes
a full-duplex protocol each TCP connection supports a pair of byte
streams, one flowing in each direction a flow-control mechanism
allows the receiver to limit how much data the sender can transmit
at a given time keep sender from overrunning receiver
Slide 18
a demultiplexing mechanism allows multiple application programs
on any given host to simultaneously carry on a conversation with
their peers a highly-tuned congestion-control mechanism keep sender
from overrunning network
Slide 19
5.2.2 Segment Format TCP a byte-oriented protocol, which means
that the sender writes bytes into a TCP connection and the receiver
reads bytes out of the TCP connection "byte stream" describes the
service TCP offers to application processes app writes bytes TCP
sends segments app reads bytes
Slide 20
TCP does not itself transmit individual bytes over the Internet
TCP on the source host buffers enough bytes from the sending
process to fill a reasonably sized packet and then sends this
packet to its peer on the destination host TCP on the destination
host then empties the contents of the packet into a receive buffer,
and the receiving process reads from this buffer at its
leisure
Slide 21
Application process Write bytes TCP Send buffer Segment
Transmit segments Application process Read bytes TCP Receive buffer
How TCP manages a byte-stream
Slide 22
The packets exchanged between TCP peers are called segments
Each TCP segment contains a header TCP header format
