1. Communication in Distributed Systems

2. Three popular models of communication:
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

3. 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

4. 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

5. 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

6. Main differences between connection-oriented and connectionless protocols:
Connection-oriented is reliable, while the other is not.
Connection-oriented is fully duplex
Connection-oriented is byte-stream service with no structure
Error checking an essential component of unreliable connectionless protocol
Connectionless protocol transfers packets of data and uses a protocol port address to specify receiver process

8. 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
Higher Level Protocols
FTP
HTTP

9. 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

10. Protocol Stack 4 Network Application Protocol Application 6 Middleware
Middleware Protocol 5
Transport Protocol
Transport 4
Network Protocol
Network 3
Data Link Protocol 2
Data Link
Physical Protocol 1
Physical
Network

11. 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.

12. Remote Procedure Calls
Remote procedure calls can simplify the way IPCs are conducted 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.

13. Synchronous RPC Operation:
Process on machine A calls procedure on machine B, the calling process 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

14. Passing Value Parameters
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

15. 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.

16. Asynchronous RPC:
Client continues immediately after issuing RPC request and receiving 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.

17. 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

18. Berkeley Sockets
Sockets interface introduced in 1970s in Berkeley Unix
Standardizes the interface of the transport layer to allow programmers the 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.

19. Bind primitive associates a local address with a newly-created socket
Bind primitive associates a local address with a newly-created socket. For example, 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

20. 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

21. 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

22. Assumes that serious failures such as process crashes or network 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

23. 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

24. Transient asynchronous communication is supported by MPI_bsend 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.

25. 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

26. 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

27. 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

28. 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.

29. Primitive
Meaning
Put
Append a message to a specified queue. A nonblocking call by sender.
Get
Block until specified queue is non-empty 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

30. 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.

31. 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

32. 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.

33. Systems
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
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 e- mail.

34. 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

35. 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 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

36. 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

37. 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-to- end 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 end- end delay (bounded (delay) jitter). Example are distributed multimedia systems such as audio/video

38. 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.

40. 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.

41. 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

42. 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.

43. 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

44. 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

45. 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.

46. 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 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.

47. The Stream Control Transmission Protocol (SCTP) is a new IP transport protocol, 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].

48. SCTP Transport Service SCTP Transport Service
SCTP User Application
SCTP User Application
SCTP Transport Service
SCTP Transport Service
One or more IP
Address
Appearances
One or more IP
Address
Appearances
IP Network Service
IP Network Service
SCTP Node B
SCTP Node A
Network Transport
An SCTP Association

49. Message Format
Common Header
Chunk No: 1 …
Chunk No: n

50. 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 …

51. SCTP Common Header Format
Source Port No: Destination Port No :
Verification Tag
Check Sum

52. Source Port Number: 16 bits (unsigned integer)
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.

53. Verification Tag: 32 bits (unsigned integer)
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.

54. Basic SCTP Features SCTP Multi-Streaming Feature
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.

55. 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.

56. 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.

57. 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.

58. 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.

59. - 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.

60. 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 zero Verification Tag.

61. 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 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.

62. 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 Mbit/s for 4x4 antennas, and 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)

63. 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

64. 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

65. 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.

66. 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

67. 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.

68. 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

69. Node that has not seen the data is called susceptible.
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

70. 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

71. In this presentation, following topics were discussed:
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

72. References: [1] Stewart, R. , Xie, Q. , Morneault, K. , Sharp, C
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 [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 [4] Jungmeier, Rescorla and Tuexen, "TLS Over SCTP", Work in Progress. [5] [6] RFC4960 [7] RFC3286 [8] RFC2196 [9] RFC1858 [10] RFC3314 [11] ftp://ftp.3gpp.org

73. Bibliography
1. A. Tanenbaum, M.V. Steen, Distributed Systems: Principles and Paradigms, Pearson(2nd Ed), 2007.