Transcript Communication in Distributed Systems

Communication in Distributed
Systems
 Communication in Distributed Systems based on low
level message passing offered by underlying network
 Three popular models of communication:
 Remote Procedure Calls (RPC)
 Message-oriented Middleware (MOM)
 Data Streaming, discussion of SCTP, 3GPP
 Sending data to multiple receivers or Multicasting
 Interprocess Communication is part of the Client/Server
Computing Model
 A client is an application or a process that requests a service
from some other application or process.
 A server is an application or a process that responds to a client
request.
 Both client/server applications perform specialized functions
in a distributed computing environment.
 Intraprocess communication used between homogeneous
systems. Example : using named pipes, named queues, shared
memory etc
 Focus of this presentation is Interprocess Communication
 In between the end users and large pool of computing
resources, many applications act as both a client and a
server, depending on the situation.
 Interprocess communication is necessary to communicate
between heterogeneous systems
 Protocols govern the format, contents and meaning of
messages
Two main types of Protocols:
 Connection-oriented: Sender and receiver establish a connection
and negotiate the protocol to use before exchanging data. At the end
of commiunication, terminate or release the connection. An
example: TCP
 Connectionless: No connection is set up in advance. Sender
transmits message when ready. An example: UDP
Main differences between connection-oriented and
connectionless protocols:
1. Connection-oriented is reliable, while the other is not.
2. Connection-oriented is fully duplex
3. Connection-oriented is byte-stream service with no
structure
4. Error checking an essential component of unreliable
connectionless protocol
5. Connectionless protocol transfers packets of data and uses
a protocol port address to specify receiver process
 Lower-Level Protocols
 Implemented in physical layer and data link layer of the stack. Groups data bits
into frames and adds a pattern called checksum at either end of frame
 Network layer chooses best path from sender to receiver by routing
 Transport Protocols
• TCP
• UDP
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Higher Level Protocols
• FTP
• HTTP
Middleware Protocols:
 Examples are Authentication and Authorization protocols,
commit protocols in transaction databases
 Middleware protocols support high level communication
services
• Protocols that allow a process to call a procedure or invoke an
object on a remote machine transparently. An example RPC/RMI
• Protocols that support the setting up and synchronizing of streams
for transferring real-time data such as multimedia applications. An
example: SCTP
• Protocols that support reliable multicast services to a WAN
Protocol Stack
Application Protocol
Application
Middleware
Transport
Network
6
Middleware Protocol
5
Transport Protocol
4
Network Protocol
3
Data Link Protocol
2
Data Link
Physical Protocol
Physical
Network
1
Types of Communication
• Persistent communication: Message submitted for transmission stored
by communication middleware as long as it takes to deliver it to the
receiver. Neither sending application nor receiving application need to
be executing.
• Transient communication: Message stored by communication system
only as long as sending and receiving application are executing.
• Asynchronous communication: Sender continues immediately after
submitting message for transmission. Message temporarily stored by
middleware on submission.
• Synchronous communication: Sender blocks until message received
and processed and receiver returns acknowledgement.
Remote Procedure Calls
 Remote procedure calls can simplify the way IPCs are conducted
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through the network.
Client applications use the client side network API to call RPCs.
Server side of the network API turns the RPCs into local function
calls.
Return value being transmitted back to the client application again
through the network.
The OSI layers are transparent to the client application, as if it is
making a local function call.
The RPC provided by Windows enable applications that use RPC to
communicate with applications running with other operating systems
that support DCE (Distributed Computing Environment). RPC
automatically supports data conversion to account for different
hardware architectures and for byte-ordering between dissimilar
environments.
Synchronous RPC Operation:
 Process on machine A calls procedure on machine B, the calling process
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on A is suspended and execution of the called procedure takes place on
B.
Information is transported from caller to callee in the parameters and
comes back in the procedure result
No message passing visible to programmer
Client and Server stubs are used
Client stub takes its parameters and packs them into a message
(parameter marshalling) and sends them to the server stub
Parameter Passing:
 Passing Value Parameters
• Client stub takes parameters and puts them in the message. It also puts
the name or number of the procedure to be called in the message
• When message arrives at server, server stub examines the message to
see which procedure is needed and then makes appropriate call. Stub
takes the result and packs it into a message. Message is sent back to
client stub.
• Client stub unpacks the message to extract the result and returns it to
waiting client procedure
 Passing Reference Parameters
• Pointers and references passed by copying the data structure such as
array into message and sent to server
• Server stub calls server with a pointer to this array
• Server makes changes using this pointer that also affects the message
buffer inside server stub
• Server finishes its work, original message sent back to client stub
which copies it back to the client. This is similar to copy/restore.
Asynchronous RPC:
 Client continues immediately after issuing RPC request and receiving
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acknowledgement from server, it is not blocked.
Server sends immediately a reply or acknowledgement to the client the
moment RPC request is received
Server then calls requested procedure
One-way RPC: Client does not wait for even an acknowledgement
from server. Reliability not guaranteed as client has no
acknowledgement from server.
Deferred synchronous RPC is a combination of two asynchronous
RPCs, where client polls the server periodically to see whether results
are available yet rather than server calling back the client.
Message-Oriented Communication
 RPC assumes that receiver process is executing at the time a request is
issued
 Message-oriented communication such as message-queuing systems is
therefore needed to allow processes to exchange information even if
one party is not executing at the time the communication is initiated.
 Message-oriented-model (MOM) offered by transport layer, as part of
middleware solution
Berkeley Sockets
 Sockets interface introduced in 1970s in Berkeley Unix
 Standardizes the interface of the transport layer to allow programmers the
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use of messaging protocols through simple set of primitives
Another interface XTI stands for X/Open Transport Interface, also formerly
called Transport Layer Interface (TLI) developed by AT&T
Sockets and XTI similar in their model of network programming but differ
in their set of primitives
Socket forms an abstraction over the communication end point to which an
application can write data that are sent over the underlying network and
from which incoming data can be read.
Servers execute the first four primitives in table
When calling the socket primitive, the caller creates a new communication
end point for a specific transport protocol. Internally, the OS reserves
resources to accommodate sending and receiving messages for the specific
protocol.
• Bind primitive associates a local address with a newly-created socket. For example,
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server should bind the IP address of its machine together with a known port # to a
socket
Binding tells the OS that the server wants to receive messages only on the specified
address and port
Listen primitive is called only in the case of connection-oriented communication. It is
a nonblocking call that allows the local OS to reserve enough buffers for a specified #
of connections that the caller is willing to accept
A call to accept primitive blocks the caller until a connection request arrives. When
the request arrives, the local OS creates a new socket with same properties as the
original one and returns it to caller. Server can wait for another connection request on
the original socket
Connect primitive on client requires that the caller specifies the transport level
address to which a connection request is to be sent
Client is blocked until a connection is set up, after which info can be exchanged using
send/receive
Closing the connection is symmetric as both server and client call the close primitive
Primitive
Meaning
Socket
Create a new communication end point
Bind
Attach a local address to a socket
Listen
Announce willingness to accept
connections
Accept
Block caller until connection request
arrives
Connect
Actively attempt to establish a connection
Send
Send some data over the connection
Receive
Receive some data over the connection
Close
Release the connection
Message-Passing Interface
 A standard defined for message passing
 Hardware and platform independent
 Designed for parallel applications and transient
communication.
 Makes use of underlying network
• Assumes that serious failures such as process crashes or network
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partitions are fatal and do not require automatic recovery
MPI assumes communication takes place within a known group of
processes
Each group is assigned an identifier
Each process within a group is also assigned a local identifier
A (groupID, processID) pair uniquely identifies the source or
destination of a message, and is used instead of a transport-level address
Several possibly overlapping groups of processes involved in a
computation, executing at the same time
MPI has messaging primitives to support transient communication
shown in next table
Primitive
Meaning
MPI_bsend
Append outgoing message to a local send
buffer
MPI_send
Send a message and wait until copied to
local or remote buffer
MPI_ssend
Send a message and wait until receipt starts
MPI_sendrecv
Send a message and wait for reply
MPI_isend
Pass reference to outgoing message, and
continue
MPI_issend
Pass reference to outgoing message, and
wait until receipt starts
MPI_recv
Receive a message; block if there is none
MPI_irecv
Check if there is an incoming message, but
do not block
• Transient asynchronous communication is supported by MPI_bsend
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primitive
Sender submits a message for transmission which is copied to a local
buffer in MPI runtime system. Sender continues after message is copied.
Local MPI runtime system will remove the message from its local buffer
and transmit as soon as a receiver has called a receive primitive
MPI_send is a blocking send operation with implementation dependent
semantics
It may either block the caller until the specified message has been copied
to the MPI runtime system at the sender’s side, or until the receiver has
initiated a receive operation
MPI_ssend implements synchronous communication by which the sender
blocks until its request is accepted for further processing
MPI_sendrecv when called by sender, sends a request to the receiver and
blocks until the latter returns a reply. This corresponds to normal RPC.
• MPI_isend allows the sender to pass a pointer to the message
and the MPI runtime system takes care of the
communication. Sender continues.
• MPI_issend allows the sender to pass a pointer to MPI
runtime system. When runtime system indicates it has
processed the message, sender knows that receiver has
accepted the message.
• Caller is blocked until message arrives when MPI_recv is
called to receive a message
• Asynchronous variant MPI_irecv called by receiver indicates
it is prepared to accept message
Message-Oriented Persistent Communication
 Message Oriented Middleware(MOM) or message queuing
systems provide support for persistent asynchronous
communication
 Intermediate-term storage capacity for messages. Does not
require sender/receiver to be active during message
transmission
 Slower than Berkeley sockets and MPI
Message-Queuing Model
 Applications communicate by inserting messages in specific
queues
 Messages are forwarded over a series of communication
servers and delivered to the destination, even if it was down
when message was sent
 Each application has its own private queue to which other
applications can send messages
 Sender is informed that the message will be eventually
inserted in the receiver’s queue. No time is specified.
 Neither sender nor receiver need to be executing when
message is placed in the queue
 Loosely coupled communication with sender and receiver
executing independent of each other.
Primitive
Meaning
Put
Append a message to a specified
queue.A nonblocking call by
sender.
Get
Block until specified queue is nonempty and remove the first
message
Poll
Check a specified queue for
messages and remove the first.
Never block.
Notify
Install a handler as a callback
function to be called when a
message is put into a specified
queue
Architecture of Message Queuing System
 Source queue to put message is local to sender
 Message can be read only from local queue
 Message put into a queue will contain the specification of a
destination queue to which it should be transferred
 Message queuing system responsible to provide the queues
to sender/receiver and transfer messages from source to
destination queue.
 Message queuing system maintains mapping of queues
distributed across multiple machines to network locations –
a distributed database of queue names to network locations,
similar to DNS.
 Queues managed by Queue Managers, who interact with the
application that is sending or receiving a message. Special
queue managers operate as routers/relays that forward
incoming messages to other queue managers
 Message-queuing system may grow into a complete
application level overlay network on top of existing
computer network
 Relays/routers help build scalable messaging systems
 Relays allow secondary processing of messages, for example,
a message may need to be logged for reasons of fault
tolerance or security
 Relays can be used for multicasting purposes. Incoming
message put into each send queue.
E-mail Systems
• E-mail systems use underlying transport services. Example,
mail protocol for the Internet-SMTP, a message is transferred
by setting up a direct TCP connection to the destination mail
server. Generally no routing is used.
• Provide direct support for end users when compared to
message-queuing system
• E-mail systems have specific requirements such as automatic
message filtering, support for advanced messaging databases
to retrieve old messages etc
• Message queuing system enables persistent communication
between processes. Wide range of applications including email.
Stream-oriented Communication
 Communication discussed till now dealt with independent,
complete units of information and time had no effect on
correctness of the communication
 Stream communication such as audio or video stream in
contrast to above are time-dependent
 Information is represented in different formats such as
GIF/JPEG for images, audio streams are encoded by taking
16-bit samples using PCM
 In continuous representation media, temporal relationship
between data is retained in order to correctly interpret the
data. For example, motion can be represented by a series of
images with successive images displayed at uniform spacing
T in time (about 30-40 msec per image)
 In discrete representation media, temporal relationships
between data is not fundamental to correctly interpreting the
data. Examples are text, still images etc
 Data stream is a sequence of data units that can be applied to
discrete as well as continuous media. Examples are UNIX
pipes, TCP/IP connections of byte-oriented discrete data
streams. Playing an audio file requires setting up a
continuous data stream between the file and the audio device
 Time is crucial to continuous data streams
 Three types of transmission modes exist for data streams
 Asynchronous transmission mode where data items are transmitted
one after the other but no time constraints as to when the
transmission of each item takes place. Example for discrete data
streams : file transmission
 Synchronous transmission mode where there is a maximum end-toend delay defined for each data unit in stream. Example is
Temperature sampled by sensors and passed over network
 Isochronous transmission mode where time constraint is rigid and
data units are transferred subject to maximum and minimum endend delay (bounded (delay) jitter). Example are distributed
multimedia systems such as audio/video
 Focus in the presentation is on continuous data streams
(streams) using isochronous transmission
 Simple stream consists of single sequence of data
 Complex stream consists of several related simple streams
called substreams that are interdependent on each other
based on time. Example is a video stream such as movie
where two substreams continuously synchronized to transmit
audio for the movie, a video substream, and a substream
containing subtitles for the deaf or different language
translation. All substreams are synchronized.
 Architecture in the figure reveals some issues such as
compression needed to reduce required storage and network
capacity especially more for video than audio
 Quality of transmission and synchronization to be controlled
 Timing requirements expressed by Quality of Service(QoS)
QoS for continuous data streams concerns the timeliness,
volume and reliability of transmission.
QoS Specification
 The required bit rate at which data is transported
 Maximum delay until a session is set up(i.e., when an
application can start sending the data).
 Maximum end-to-end delay
 Maximum delay variance or jitter
Enforcing QoS
• Use buffers to reduce jitter
• Use forward error correction to compensate for lost packets- encode the
outgoing packets such that any k out of n received packets is enough to
reconstruct k correct packets
• Many distributed systems for stream-oriented communication are built on
top of Internet protocol stack. Internet provides differentiating classes for
data using differentiated services. Sending host can mark outgoing packets as
belonging to one of several classes. Expedited forwarding class specifies that
the packet should be forwarded by the router with absolute priority. With
assured forwarding class, traffic is divided into 4 subclasses along with three
ways to drop packets if the network gets congested. This means a range of
priorities can be assigned to the packets to differentiate time-critical packets
from non-critical ones.
Stream Synchronization
 Maintains temporal relationships between
streams/substreams , for example between discrete data
stream and continuous data stream, or between continuous
data streams
 Synchronization takes place at the level of data units
 Synchronization mechanisms concerned with synchronizing
data streams and distribution of the mechanism in a
networked environment
 Multimedia middleware offers a collection of interfaces for
controlling audio and video streams including interfaces for
controlling devices such as monitors, cameras, microphones
etc.
 Each device and stream has its own high level interface
including interfaces for notifying an application when some
event occurred. Latter used for writing handlers for
synchronizing streams.
 Distribution of synchronization mechanism - receiving side has
to have complete synchronization specification locally available
 This approach is followed by MPEG streams
 MPEG standards form a collection of algorithms for compressing
video and audio
 MPEG -2 designed for compressing broadcast quality video into 4 to 6
Mbps. Unlimited number of continuous and discrete streams merged
into single stream. Input stream turned into a stream of packets that
carry timestamp based on 90kHz clock. These streams multiplexed
into a program stream consisting of variable length packets with
common time base. Receiving side demultiplexes the stream using
timestamp for interstream synchronization. Better to do
synchronization at the sender rather than at receiver.
Stream Control Transmission Protocol
(SCTP)
• SCTP is a reliable transport protocol operating on top of a connectionless packet
network such as IP. It is described in RFC 4960, RFC 3286. It offers the following
services to its users:
-- acknowledged error-free non-duplicated transfer of user data
-- data fragmentation to conform to discovered path size
-- sequenced delivery of user messages within multiple streams, with an option for
order-of-arrival delivery of individual user messages
-- optional bundling of multiple user messages into a single SCTP packet
-- network-level fault tolerance through supporting of multi-homing at either or both
ends of an association
• The design of SCTP includes appropriate congestion avoidance behavior and resistance
to flooding and masquerade attacks.
 The Stream Control Transmission Protocol (SCTP) is a new IP transport protocol,
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existing at an equivalent level with UDP (User DatagramProtocol) and TCP
(Transmission Control Protocol), which provide transport layer functions to many
Internet applications.
SCTP has been approved by the IETF as a Proposed Standard
Like TCP, SCTP provides a reliable transport service, ensuring that data is transported
across the network without error and in sequence.
Like TCP, SCTP is a session-oriented mechanism, meaning that a relationship is
created between the endpoints of an SCTP association prior to data being
transmitted, and this relationship is maintained until all data transmission has been
successfully completed.
Unlike TCP, SCTP provides a number of functions that are critical for telephony
signaling transport, and at the same time can potentially benefit other applications
needing transport with additional performance and reliability. The original framework
for the SCTP definition is described in [3].
SCTP User
Application
SCTP User
Application
SCTP
Transport
Service
SCTP
Transport
Service
IP Network
Service
One or more
IP
Address
Appearances
One or more
IP
Address
Appearances
IP Network
Service
SCTP Node B
SCTP Node A
Network Transport
An SCTP Association
Message Format
Common Header
Chunk No: 1
…
Chunk No: n
Chunk ID : 0 through 255. Each ID has Chunk Type defined as follows:
 0 - Payload Data (DATA)
 1 - Initiation (INIT)
 2 - Initiation Acknowledgement (INIT ACK)
 3 - Selective Acknowledgement (SACK)
 4 - Heartbeat Request (HEARTBEAT)
 5 - Heartbeat Acknowledgement (HEARTBEAT ACK)
 6 - Abort (ABORT)
 7 - Shutdown (SHUTDOWN)
 8 - Shutdown Acknowledgement (SHUTDOWN ACK)
 9 - Operation Error (ERROR)
 Etc …
SCTP Common Header Format
Source Port No:
Destination Port No :
Verification Tag
Check Sum
 Source Port Number: 16 bits (unsigned integer). This is the SCTP
sender’s port number. It can be used by the receiver in combination
with the source IP address, the SCTP destination port, and possibly the
destination IP address to identify the association to which this packet
belongs. The port number 0 MUST NOT be used.
 Destination Port Number: 16 bits (unsigned integer). This is the SCTP
port number to which this packet is destined. The receiving host will
use this port number to de-multiplex the SCTP packet to the correct
receiving endpoint/application. The port number 0 MUST NOT be
used.
• Verification Tag: 32 bits (unsigned integer). The receiver of this packet
uses the Verification Tag to validate the sender of this SCTP packet. On
transmit, the value of this Verification Tag must be set to the value of
the Initiate Tag received from the peer endpoint during the association
initialization, with the following exceptions:
- A packet containing an INIT chunk MUST have a zero Verification Tag.
- A packet containing a SHUTDOWN COMPLETE chunk with the T bit
set MUST have the Verification Tag copied from the packet with the
SHUTDOWN ACK chunk.
- A packet containing an ABORT chunk may have the verification tag
copied from the packet that caused the ABORT to be sent.
An INIT chunk MUST be the only chunk in the SCTP packet carrying it.
• Checksum: 32 bits (unsigned integer). This field contains the checksum
of this SCTP packet.
Basic SCTP Features
• SCTP is a unicast protocol, and supports data exchange between exactly 2 endpoints,
although these may be represented by multiple IP addresses.
• SCTP provides reliable transmission, detecting when data is discarded, reordered,
duplicated or corrupted, and retransmitting damaged data as necessary. SCTP
transmission is full duplex.
• SCTP is message oriented and supports framing of individual message boundaries. In
comparison, TCP is byte oriented and does not preserve any implicit structure
within a transmitted byte stream without enhancement.
• SCTP is rate adaptive similar to TCP, and will scale back data transfer to the
prevailing load conditions in the network. It is designed to behave cooperatively with
TCP sessions attempting to use the same bandwidth
SCTP Multi-Streaming Feature
• The name Stream Control Transmission Protocol is derived from the multi-
streaming function provided by SCTP. This feature allows data to be partitioned into
multiple streams that have the property of independently sequenced delivery, so that
message loss in any one stream will only initially affect delivery within that stream,
and not delivery in other streams.
SCTP accomplishes multi-streaming by creating independence between data transmission
and data delivery. In particular, each payload DATA "chunk" in the protocol uses two
sets of sequence numbers, a Transmission Sequence Number that governs the
transmission of messages and the detection of message loss, and the Stream ID/Stream
Sequence Number pair, which is used to determine the sequence of delivery of
received data.
This independence of mechanisms allows the receiver to determine immediately when
a gap in the transmission sequence occurs (e.g., due to message loss), and also whether
or not messages received following the gap are within an affected stream.
SCTP Multi-Homing Feature
 Another core feature of SCTP is multi-homing, or the ability for a single SCTP
endpoint to support multiple IP addresses. The benefit of multi-homing is potentially
greater survivability of the session in the presence of network failures. To support
multi-homing, SCTP endpoints exchange lists of addresses during initiation of the
association. Each endpoint must be able to receive messages from any of the addresses
associated with the remote endpoint; in practice, certain operating systems may utilize
available source addresses in round robin fashion, in which case receipt of messages
from different source addresses will be the normal case. A single port number is used
across the entire address list at an endpoint for a specific session.
Security Objectives
 As a common transport protocol designed to reliably carry time-sensitive user
messages, such as billing or signaling messages for telephony services, between two
networked endpoints, SCTP has the following security objectives.
- availability of reliable and timely data transport services
- integrity of the user-to-user information carried by SCTP
SCTP Responses to Potential Threats
• SCTP may potentially be used in a wide variety of risk situations. It is important for
operators of systems running SCTP to analyze their particular situations and decide
on the appropriate countermeasures.
• Operators of systems running SCTP should consult [RFC2196] for guidance in
securing their site.
Countering Insider Attacks
• The principles of [RFC2196] should be applied to minimize the risk of theft of
information or sabotage by insiders. Such procedures include publication of security
policies, control of access at the physical, software, and network levels, and separation
of services.
 Protecting against Data Corruption in the Network
 Protecting Confidentiality
 As with the supplementary checksum service, user data encryption MAY be performed by
the SCTP user application.
 Alternately, the user application may use an implementation-specific API to request that the
IP Encapsulating Security Payload (ESP) [RFC4303] be used to provide confidentiality and
integrity.
 Protecting against Blind Denial-of-Service Attacks
A blind attack is one where the attacker is unable to intercept or otherwise see the content of
data flows passing to and from the target SCTP node. Blind denial-of-service attacks may take
the form of flooding, masquerade, or improper monopolization of services
• Flooding
– The objective of flooding is to cause loss of service and incorrect behavior at target systems
through resource exhaustion, interference with legitimate transactions, and exploitation of
buffer-related software bugs. Flooding may be directed either at the SCTP node or at
resources in the intervening IP Access Links or the Internet. Where the latter entities are
the target, flooding will manifest itself as loss of network services, including potentially the
breach of any firewalls in place.
– In general, protection against flooding begins at the equipment design level, where
it includes measures such as:
- avoiding commitment of limited resources before determining that the request for
service is legitimate.
- giving priority to completion of processing in progress over the acceptance of new
work.
- identification and removal of duplicate or stale queued requests for service.
- not responding to unexpected packets sent to non-unicast addresses
• Network equipment should be capable of generating an alarm and log if a suspicious
increase in traffic occurs.
• Blind Masquerade
Masquerade can be used to deny service in several ways:
- by tying up resources at the target SCTP node to which the impersonated node has
limited access. For example, the target node may by policy permit a maximum of
one SCTP association with the impersonated SCTP node. The masquerading
attacker may attempt to establish an association purporting to come from the
impersonated node so that the latter cannot do so when it requires it.
- by deliberately allowing the impersonation to be detected, thereby provoking
counter-measures that cause the impersonated node to be locked out of the target
SCTP node.
- by interfering with an established association by inserting extraneous content such as
a SHUTDOWN request.
 SCTP reduces the risk of blind masquerade attacks through IP spoofing by use of the
four-way startup handshake. Because the initial exchange is memory-less, no lockout
mechanism is triggered by blind masquerade attacks. In addition, the INIT ACK
containing the State Cookie is transmitted back to the IP address from which it
received the INIT. Thus, the attacker would not receive the INIT ACK containing the
State Cookie. SCTP protects against insertion of extraneous packets into the flow of
an established association by use of the Verification Tag.
Logging of received INIT requests and abnormalities such as unexpected INIT ACKs
might be considered as a way to detect patterns of hostile activity.
 Improper Monopolization of Services
 Attacks under this heading are performed openly and legitimately by the attacker.
They are directed against fellow users of the target SCTP node or of the shared
resources between the attacker and the target node. Possible attacks include the
opening of a large number of associations between the attacker’s node and the target,
or transfer of large volumes of information within a legitimately established
association.
 Policy limits should be placed on the number of associations per adjoining SCTP node.
SCTP user applications should be capable of detecting large volumes of illegitimate or
"no-op" messages within a given association and either logging or terminating the
association as a result, based on local policy.
 SCTP Interactions with Firewalls
 It is helpful for some firewalls if they can inspect just the first fragment of a
fragmented SCTP packet and unambiguously determine whether it corresponds to
an INIT chunk (for further information, refer to [RFC1858]).
 Accordingly, the requirements, (1) an INIT chunk MUST NOT be bundled with
any other chunk in a packet, and (2) a packet containing an INIT chunk MUST have
a zeroVerification Tag.
3GPP
 The 3rd Generation Partnership Project (3GPP) is a collaboration
between groups of telecommunications associations, to make a globally
applicable third-generation (3G) mobile phone system specification
within the scope of the International Mobile Telecommunications-2000
project of the International Telecommunication Union (ITU). 3GPP
specifications are based on evolved Global System for Mobile
Communications (GSM) specifications. 3GPP standardization
encompasses Radio, Core Network and Service architecture. Some
details of 3 GPP can found in RFC 3314.
 Much of the standard addresses upgrading 3G UMTS to 4G mobile
communications technology, which is essentially a mobile broadband
system with enhanced multimedia services built on top.
 The standard includes:
 Peak download rates of 326.4 Mbit/s for 4x4 antennas, and 172.8
Mbit/s for 2x2 antennas (utilizing 20 MHz of spectrum).
 Peak upload rates of 86.4 Mbit/s for every 20 MHz of spectrum
using a single antenna.
 Five different terminal classes have been defined from a voice centric
class up to a high end terminal that supports the peak data rates. All
terminals will be able to process 20 MHz bandwidth.
 At least 200 active users in every 5 MHz cell. (Specifically, 200 active
data clients)
Security Issues
 Security documents can be located at ftp://ftp.3gpp.org
 GSM was the first public telephone system to use integrated
cryptographic mechanisms
GSM security features
• Secure user access to telecommunications services
Identity of user authenticated by network operator
• User and signaling traffic confidentiality
Protects user voice and data traffic, and signaling data from
eavesdropping on radio path
• User anonymity
Attacker who knows user’s IMSI can be prevented from tracking location
of user and eavesdropping on radio path
GSM security mechanisms
• Cryptographic authentication verifies the subscription with the
home network when service is requested
– Challenge / response authentication protocol based on a
subscriber specific secret authentication key
• Radio interface encryption prevents eavesdropping and
authenticates the use of the radio channel
– The encryption mechanism is based on a symmetric stream
cipher
– The key for encryption is established as part of the
authentication protocol
• The allocation and use of temporary identities helps to provide
user anonymity
Multicast Communication
 Sending data to multiple receivers
 Explicit communication paths set up could be at Application
Level for peer-to-peer solutions
 Without
explicit communication paths, gossip based
information dissemination provides simple but less efficient
way to implement multicasting.
Application level multicasting
 Nodes organize into an overlay network that is used to
disseminate information to its members
 Network routers not involved in group membership
 Overlay network could be organized into a tree or a mesh
network. Former provides a unique overlay path between
every pair of nodes, while latter has each node connected to
multiple neighbors – higher robustness in the event a
connection breaks
Multicast Session
 Node generates a multicast identifier mid (randomly chosen 160 bit key).
It then looks up succ(mid) that is the node responsible for this key and
promotes it to become the root of the multicast tree that is used to send
data to interested nodes
 To join the tree, a node P executes operation LOOKUP(mid) that
allows a lookup message with request to join the multicast group mid to
be routed from P to succ(mid).
 On the way up to the root, join request will add forwarder nodes or
helpers for the group
 Multicasting implemented by a node sending a multicast message
towards the root by executing LOOKUP(mid) after which message can
be sent along the tree.
Gossip based data dissemination
 Spreading information without explicit communication paths
in large distributed systems using epidemic protocols.
 These protocols rapidly propagate information among large
collection of nodes using only local information without any
central component to coordinate information dissemination
 Avoid write conflicts by allowing only a single node to initiate
updates for a specific data item
 Node is infected if it holds data that it is willing to spread to
other nodes.
 Node that has not seen the data is called susceptible.
 Updated node that is not willing or able to spread the data is
said to be removed.
 Data is timestamped
 Anti-entropy model of propagation has three approaches for
updates for node P propagating to random node Q:
 P only pushes its own updates to Q
 P only pulls its own updates from Q
 P and Q send updates to each other (push-pull approach) – This is the best
 Rumor spreading or gossiping allows node P to push update
to arbitrary node Q if it is not yet updated. If Q already
updated by another node, P loses interest in spreading the
update further- it becomes removed.
 Deletion of data item needs the use of death certificates
to be recorded and spread through all the nodes.
 Death certificates are time-stamped and they are removed
after a maximum propagation time has elapsed
Conclusion
In this presentation, following topics were discussed:
 Three popular models of communication in distributed
systems:
 Remote Procedure Calls (RPC)
 Message-oriented Middleware (MOM)
 Data Streaming, discussion of SCTP, 3GPP
 Sending data to multiple receivers or Multicasting
References:
[1] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer, H., Taylor, T., Rytina,
I., Kalla, M., Zhang, L. and V. Paxson, "Stream Control Transmission Protocol", RFC
2960, October 2000.
[2] Stewart, Sharp, et. al., "SCTP Checksum Change", Work in Progress.
[3] Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene, L., Lin, H., Juhasz, I.,
Holdrege, M. and C. Sharp, "Framework Architecture for Signaling Transport", RFC
2719, October 1999.
[4] Jungmeier, Rescorla and Tuexen, "TLS Over SCTP", Work in Progress.
[5] www.ietf.org
[6] RFC4960
[7] RFC3286
[8] RFC2196
[9] RFC1858
[10] RFC3314
[11] ftp://ftp.3gpp.org
Bibliography
1. A. Tanenbaum, M.V. Steen, Distributed Systems: Principles and
Paradigms, Pearson(2nd Ed), 2007.