Lecture #19: Transport Protocols.
Elements and Examples
Contents
OSI Transport Layer 2
Transport Service Primitives 4
Addressing
Connection Control
Transport Layer in Internet 11
TCP Addressing 12
TCP protocol 13
UDP 18
6
9
1
OSI Transport Layer
Application layer
Presentation layer
Session layer
6/1
User application 1
Encryption/
decryption
Session
control
Session
synch.
...
compression/
expansion
Choice of
syntax
Session to transport
mapping
Session
management
Transport layer
Layer and flow
control
Error
recovery
Multiplexing
Network layer
Connection
control
Routing
Addressing
Link layer
Data link
establishment
Error
control
Physical layer
Access to
transm. media
Physical and
electrical interface
Flow
control
Synch
Framing
Activation/
deactivation of con.
Functions within the transport layer may include:
•
•
•
•
•
mapping transport-address onto network-address;
multiplexing (end-to-end) transport-connections onto network-connections;
establishment and release of transport-connections;
end-to-end sequence control on individual connections;
end-to-end error detection and any necessary monitoring of the quality of 2
service; etc.
Services of the Transport Layer
Services to the session (or application) layer:
– unique identification of each session entity by its transport
address. The transport-service provides the means to establish,
maintain and release transport-connections (case the service is
connection-oriented)
– duplex transmission between a pair of transport addresses (one
or more transport connection can be established between the
same pair of transport addresses)
– provides endpoint identifiers that distinguish the transport
connection endpoints
– the quality of service provided on a transport connection depends
on the service class requested by the session entities (the
selected quality of service is maintained throughout the lifetime of
the transport connection)
– the following services are provided by the transport layer:
• transport-connection establishment
• data transfer
• transport-connection release
3
Transport service provider and transport service user.
6/3
Access primitives provide interface to user applications and
sessions
Example set of transport service’s access primitives
Transport Protocol Data Unit (TPDU) - standard data entity
exchanged between transport peers. TPDU payload is packed with
header in Network Layer packet. NL packet is cut into Data Link Layer frames
with frame’s headers
6/4
UNIX implementation of transport service primitives: as any other
interface in UNIX the network transport layer data has file presentation
established by SOCKET and assigned by BIND. Unblocking LISTEN allocates
buffer for remote requests.
6/6
6/5
Transport
Service
Primitives
Transport services are provided by the access primitives
State-transition diagram of the transport layer connection
process:
– event driven control; events: local execution of a
transport primitive or incoming packet
4
Transport Protocol
Elements
Transport protocols implement transport layer
functionality
Example protocol hierarchy (TCP/IP model)
Transport protocol environment:
19/1
– host-to-host communication over the subnet (instead the Network layer
communication between subnet components - routers, over data link)
6/7
– source/destination addressing (instead addressing by selection of router’s
outgoing line by the Network Layer
– possibility to store (and delay) packets in the subnet by datagram model -->
special transport protocols for real-time and interactive communications
– dynamic allocation of connection’s buffers as the number of connections is not
predefined (in contrast to the Network Layer where the number of input/output
lines is fixed and static allocation of the buffer space is applicable).
5
Addressing
Addressing is based on TSAP (Transport Service Access Point) that
corresponds to some port of a local process. TSAP is defined by pairs
(IP address, local port i)
where NSAP (Network Layer SAP) is presented by IP address (or another
adequate addressing - AAL…)
TSAP are multiple for any transport entity while NSAP are either one or few in
a machine/router
Connection phases:
6/8
– server OS assigns a TSAP to an active server process upon its request
– application process on a remote machine requests connection to server’s TSAP via its
local TSAP
– transport entities use the network link to process the communication request either
as connection oriented service (session) or as connectionless data delivery
6
Addressing
6/9
Constant TSAP address services
Dynamic TSAP assignment to the service
processes
– Initial connection protocol by process server
– process server acts as a proxy between a set of
server processes and remote client processes
– the requested server is split out from the process
server set and communicates autonomously to the
client during the session
7
Classes of the transport protocol
Protocol Network
class
type
Functions
0
A
Minimal functions
1
B
Network error recovery
2
A
Connection bonding
3
B
Network error recovery,
connection bonding
4
C
Error detection and recovery,
connection bonding
8
Connection Establishment
Three-way handshaking
– Normal operation:
6/11a
• control parameters:
– sequential number of initiator - x
– sequential number of the party - y
• CR - connection request from initiating host
• ACK - connection accepted from
• Data transfer
– delayed duplicate request TPDU - the delayed request is
6/11b
rejected by the initiator
– delayed duplicate request and delayed duplicate acknowledge:
6/11c
the initiator rejects the delayed request acknowledge as in
previous case; the party rejects the delayed duplicate dataacknowledge because of imparity of its sequential number
(z instead y)
9
Connection Release
• Asymmetric release: connection release initiative of any of the
parties:
• simple and quick
6/12
• possible lost of data
• Symmetric release
• treats the connection as two independent unidirectional connections
and release each of them separately
• no data losses
6/13
• synchronization problem (two-army problem)
10
Transport Layer in Internet
• Connection-oriented and connectionless oriented protocols
in Internet
19/1
• TCP (Transport Control Protocol) features:
• connection oriented protocol supporting end-to-end fullduplex byte stream over network links with losses and
errors
• upper interface to user processes or OS kernel requests
• lower interface to IP
• brakes user/OS data into IP datagrams of 1500-64kB or
restore the IP datagrams into upper level byte stream
• controls the timeout periods, retransmission events,
sequencing, etc, over the unreliable IP transfer
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• Addressing based on sockets:
TCP addressing
• a socket unifies NSAP end TSAP
• the socket number consists of:
• IP address (NSAP) - 4 bytes
• local port (TSAP) - 16b (up to 64k ports addressing space)
• a socket pair uniquely identifies a transport connection: (socket1,
socket2)
• several connections may ended in a common socket
• socket calls from the OS:
6/6
• port address space:
• 0 - 255 - well-known ports:
7 - Echo; 13 - Daytime; 21 - FTP File Transfer; 23 - Telnet; 49
- Login Host Protocol; 79 - Finger; 80 - World Wide Web HTTP;
110 - Post Office Protocol - Version 3 (POP3)
12
TCP protocol
•
•
•
32-bit numbering (0 - 4G) of the byte sequence
TPDUs: segments
Segmentation according the network properties (user data
stream can be split into smaller segments or collected in
larger ones).
• Segment structure:
• header (20B)
• header options
• data (optional, up to 64k)
• Segment header:
• 5 * 32-bit words
6/24 • Source/Destination port - 16b - local end points of the
connection
• Sequence Number/Acknowledgement Number - 32b (4G ) the byte numbers in the stream (last received and next expected) 13
TCP protocol
• Segment Header (cont.)
6/24
• TCP header length - (4b) - the header length in 32b words - up
to 16 words of which 5 are standard header and up to 11 are
optional header words
• reserved field of 6b (not used in TCP versions up to now)
• 6 flags field; the value 1 of the flags means respectively:
• URG: Urgent pointer in use. Urgent pointer is a facility that points the
urgent data field. Urgent data contains interrupt messages that are
interpret by the upper levels but not by TCP itself
• ACK: Valid Acknowledgement number. If ACK=0 the contents of the
Acknowledgement number field is ignored
• PSH: Pushed data indicator. “Pushed data” is data that has to be
delivered to the application layer without buffering (i.e. without delay)
• RST: Reset connection. Used in cases of host failure or refusal of
connection opening attempt
14
TCP protocol
• Segment Header - Flags’ field (cont.)
6/24
• SYN: Establish connection. Connection request/replay indicator. SYN=1 and
ACK=0 means connection request (no piggybacked acknowledgement) and
SYN=1 and ACK=1 means connection replay (acceptance) with a
acknowledgement piggybacked in the same segment. SYN segments are
ordered by the sequence numbers in order to prevent delayed duplications.
• FIN: Release connection request. The sender has finished the data
transmission. FIN segments also are ordered by the sequence numbers in
order to prevent delayed duplications.
• Window size. 16b - Indicates the free buffer space of the receiver
after the last received byte (which is Acknowledgement number - 1).
The maximum buffer buffer indicated is 64kB. 0 buffer space means
temporarily to stop data transmission to the receiver.
• Checksum. 16b - checksum over header, data and the pseudoheader:
(set of segment parameters)
• pseudoheader: set of transmission parameters that includes
6/25
source/destination IP addresses and the segment length (Note that the control
over IP addresses is not a functionality of TCP but of IP)
15
TCP protocol
• Segment Header (cont.)
6/24
• Urgent pointer. 16b - Used in compination with URG flag.
• Header Options. Variable length - optional extension of the
header facilities e.g. maximum segment length. The segment
length has to be compromised between the transmission
effectiveness and the host/application requirements.
16
TCP connection management
• TCP uses 3-way handshaking for connection establishment:
• the server host (host2) waits for incoming having issued LISTEN or
ACCEPT primitive
6/26
• the client host (host1) issued CONNECT primitive specifying TSAP by (IP
addr., port) couple; the flags SYN=1, ACK=0
• the server host acknowledges the connection if there is active server
process listening the specified port
• simultaneous attempts to open connection lead to failure because of
different sequence numbering (6/26b)
• simultaneous attempts to close connection are not in a collision; FIN
segments are acknowledged (or timed-out) and the connection is closed
• Finite state-machine of a TCP connection:
• set of states
6/28
6/27
17
• set of transitions (the transitions are invoced by event/action pair)
UDP
• UDP (User Datagram Protocol) implements
connectionless unreliable services of the Transport
Layer
• The establishment/release of the connection is skipped
• UDP segment:
6/34
• 8B (2x32b words) header
• extends IP datagram with
• source/destination ports fields
• UDP segment length field (data + 8B header)
• UDP checksum field over header, pseudoheader and data
• UDP is generally the protocol used in transmitting voice and video
across a network. In some instant messengers/chat programs too.
18
Miscellaneous Transport protocols
Stream Control Transmission Protocol (SCTP)
– RFC 4960 defines the protocol, with RFC 3286 providing an
introductory text.
– Provides reliable, in-sequence transport of message
streams with congestion control.
Feature Name
TCP
UDP
SCTP
Connection oriented
Yes
No
Yes
Reliable transport
Yes
No
Yes
Preserve message
boundary
No
Yes
Yes
Ordered delivery
Yes
No
Yes
Unordered delivery
No
Yes
Yes
Data checksum
Yes
Yes
Yes
Checksum size (bits)
16
16
32
Path MTU
Yes
No
Yes
Congestion control
Yes
No
Yes
Multiple streams
No
No
Yes
MultiMulti-homing support
No
No
Yes
Bundling
No
No
Yes
19
Miscellaneous Transport protocols (2)
Real-time Transport Protocol (RTP) defines a
standardized packet format for delivering audio and video
over the Internet. It was developed by the Audio-Video
Transport Working Group of the IETF and first published in
1996 as RFC 1889 which was made obsolete in 2003 by
RFC 3550. Real time transport protocol can also be used in
conjunction with RSVP protocol which enhances the field
of multimedia applications.
Resource ReSerVation Protocol (RSVP), described in
RFC 2205. Designed to reserve resources across a network
for an integrated services Internet (QoS).
"RSVP does not transport application data but is rather an
Internet control protocol, like ICMP, IGMP, or routing
protocols.“
RSVP provides receiver-initiated setup of resource
reservations for multicast or unicast data flows with
scaling and robustness.
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