IHA præsentation1 Protocol Design Lesson 4. IHA præsentation2 Outline for today Guidelines for...

IHA præsentation 1

Protocol Design

Lesson 4

IHA præsentation 2

Outline for today

• Guidelines for implementing protocols

• Protocol Design Patterns

IHA præsentation 3

Guidelines for implementing protocols


• Where is the protocol entity positioned in the complete protocol stack

• Clear picture of the services our protocol provides• Procedure/functionality descriptions

• Clear picture of which other protocol entities our protocol interfaces to• Service Access Points, service primitives or other

• Definition of messages/PDUs• ASN.1, Augmented BNF or other

• Protocol Specification Ready

• Requirements to lower layer servicesIHA præsentation 4


• Consider Task and module decomposition• Shall our protocol run as separate task?

• Interaction with the Operating System• Memory management

• Dynamic memory allocation needed?

• Timer management• Are timers needed?

• (Inter process communication)• Queues, etc.

• (Security management)• Passwords, authentication where are they stored?

IHA præsentation 5


• Protocol states• Finite State Machine (State-event machines)

• Identify all states and all internal events– How do we do this?

– Message sequence charts (sequence diagrams) for normal scenarios

– MSCs for timer expiry scenarios– MSC for abnormal scenarios

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Guidelines

• Task and module decomposition ?• Protocol functions that need to manage their own timers

may have their own task

• Protocol functions that need to have their own transmit/receive messages to/from peer protocol entities may also have their own task

• However, consider context switching overhead

Traditionally……

Tasks

IHA præsentation 9

Guidelines

•Designing for Reentrancy

Application

Presentation

Session

Transport

Network

Data Link

Physical

TCPUDP

IP

ip_send() to send packets

Service

ServiceUsers

AVOID U

SING G

LOBAL DATA STRUCTURES


• We have protocol related design patterns?

• Are they useful?

• Can they handle all kind of protocols?

IHA præsentation 10


Protocol Design Patterns


• Protocol Design Patterns

• Protocol Layer Design Pattern

• Protocol Packet Design Pattern

• Protocol Stack Design Pattern

• Receive Protocol Handler

• Transmit Protocol Handler

• State PatternState Pattern


Protocol Layer Design Pattern

• Motivation• A typical protocol layer interfaces with an upper and a lower layer of

the protocol stack

• In most designs there is a lot of dependency bewteen different layers of the protocol

=> inflexibility

The Protocol Layer Design Pattern addresses these limitations by decopling the individual protocol layers



Layer N+1

Layer N

Layer N-1

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Structure• Communication between layers takes place using standard interfaces defined by a Protocol Layer base class

• All implementations of the protocol layer inherit from this class

• The inheriting class should implement the standard interfaces:

• Transmit is invoked by the upper layer to transfer a packet to the lower layer

• Handle Receive is invoked by the lower layer to transfer a packet to the upper layer



Network Layer

DataLink Layer

Physical Layer

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ExampleTransmitting Direction: 1. The application invokes the Network layer's Transmit method.

2. The Network layer performs its actions and invokes the Transmit method for the lower layer.

3. This invokes the Datalink layers transmit method. The Datalink layer performs the layer specific actions and invokes the lower layer's Transmit method.

4.The Physical layer's Transmit method is invoked. This layer programs the appropriate hardware device and transmits the message.



Participants

• Protocol Layer: This is the base class for all protocol layers. The individual layers interface with each other via pointers to this class. The actual type of the upper layer and lower layer classes is not known to the implementers of a certain layer.

• Protocol Packet: This class manages addition and removal of headers and trailers for various protocol layers.



Layer N+1

Layer N

Layer N-1

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Consequences• The implementation of one layer is completely decoupled from the adjacent layers

• Layers can be added and removed without needing any changes to the code for individual layers

• an IPsec layer can be added between IP and physical layer without any changes to the IP or physical layer code

• A single layer could interface with multiple upper and lower layer protocols using the same interface

• an IP layer could interface with an ATM or Ethernet physical layer. No changes to the IP layer needed.


Protocol Layer Design Pattern#ifndef PROTOCOL_LAYER_H#define PROTOCOL_LAYER_H

#include <stdio.h>class Protocol_Packet;

class Protocol_Layer{ Protocol_Layer *m_p_Lower_Layer; Protocol_Layer *m_p_Upper_Layer; public: Protocol_Layer() { m_p_Lower_Layer = NULL; m_p_Upper_Layer = NULL; }

virtual void Transmit(Protocol_Packet *p_Packet) = 0; virtual void Handle_Receive(Protocol_Packet *p_Packet) = 0;

void Set_Upper_Layer(Protocol_Layer *p_Layer) { m_p_Upper_Layer = p_Layer; }

void Set_Lower_Layer(Protocol_Layer *p_Layer) { m_p_Lower_Layer = p_Layer; }

Protocol_Layer *Get_Upper_Layer() const { return m_p_Upper_Layer; }

Protocol_Layer *Get_Lower_Layer() const { return m_p_Lower_Layer; }

};#endif


Protocol Packet Design Pattern

• Motivation• A protocol stack generally handles multiple layers of a protocol• Each layer adds its own headers and trailers

• Size of the buffer containing the message keeps changing

• In most implementations this results in each layers allocating a new buffer to adjust the changed buffer size

The Protocol Packet Design Pattern addresses this issue with a simple and efficient buffering architecture



Structure

This pattern is implemented by just one class, the Protocol Packet. This class works on a raw buffer that is capable of holding the entire packet with protocol headers added for all the layers in the protocol stack. The raw buffer is dynamically partitioned into three regions:

• Header• Body• Trailer

As the message moves from one layer the other the location of the different regions is adjusted. Let see:



Transmitting a Packet

Application Body Region Layer 3 Body Region

Layer 3 Header Region

Layer 3 Trailer Region

Layer 2 Body Region



Layer 1 Body Region



Transmitted Body Region

1. The Protocol Packet object is constructed with just the application body. Notice that the body does not start from the first byte of the buffer. The application body is placed at an offset, leaving enough space for the protocol headers. At this point, the header and trailer regions are of zero size.

2. The packet is passed to Layer 3. This layer adds its own headers and trailers regions into the same buffer.

3. Layer 2 adds its own headers and trailers regions. The previous header and trailer regions get merged into the body

4. Layer 1 adds headers and trailers. Again, the body region grows to accommodate the headers and trailers for Layer 2

5. Finally, zero length header and trailers are added, resulting in the entire packet moving to the body region. At this point the header and trailer regions are of 0 length.



1. The received packet is created with all the bytes in the body region of the message. At this point, the header and trailer regions are of zero length.

2. Layer 1 extracts its headers and trailer regions. The two regions have been carved out of the received body region. The size of the body region is reduced.

3. Layer 2 also extracts its own header and trailer regions. Again shrinking the body region.

4. Layer 3 similarly extracts its header and trailer regions. Shrinking the body to the original application body.

5. The application extracts a zero length header and trailer. This leaves only the packet with only the body region.

Receiving a Packet

Application Body RegionLayer 3 Body Region



Layer 2 Body Region



Layer 1 Body Region



Received Bytes(Body Region)



Participants

This pattern is implemented by just one class, the Protocol Packet. The class has three internal constituent regions. These regions are defined by the Region private structure.

Collaboration

The Protocol Packet class contains the header, body and trailer regions. This relationship is shown in the following collaboration graph:


class Protocol_Packet { enum { MAXIMUM_PACKET_LENGTH = 1500}; struct Region { int offset; int length; }; Region m_header; Region m_body; Region m_trailer;

char m_buffer[MAXIMUM_PACKET_LENGTH]; …}



Consequences

• Using this pattern provides allows efficient handling of packets as different layers are added or extracted.

• A single buffer is used across layers. This reduces the overhead in buffer processing • In addition, this pattern brings uniformity to the design of the protocol stack.



Implementation

•Add_Header: Add the header to the transmit packet.

•Add_Trailer: Add the trailer to the transmit packet.

•Extract_Header: Extract the header from the received packet.

•Extract_Trailer: Extract the trailer from the received packet.

•Get_Header: Get a pointer to the current packet header.

•Get_Body: Get a pointer to the current packet body.

•Get_Trailer: Get a pointer to the current packet trailer.

•Get_Length: Get the total length. The length includes the header, body and trailer.

The Protocol Packet class consists of the following methods:


Protocol Stack Design Pattern

We have already seen that Protocol Layer and Protocol Packet provide a standardized interface between different layers of a protocol.

The Protocol Stack design pattern takes advantage of the layer decoupling and provides a mechanism for dynamic insertion and removal of protocol layers from a stack



• Motivation

• Protocol stacks tend to be rigid in design and protocol layers cannot be dynamically added or removed from a protocol stack

• Limits the use of protocol stacks in the even changing world of protocol standards

Example:

The user has enabled encryption and this requires the sandwiching of the encryptionlayer between the network layer and the data-link layer

The Protocol Stack Design Pattern addresses this issue and introduces a flexible architecture for dynamic addition and removal of protocol layers



Structure

The Protocol Stack Design Pattern is implemented by the Protocol Stack class. This class maintains a doubly linked list of active layers.

Participants

The key actors of this design pattern:

• Protocol Stack: This class maintains a doubly linked list of Protocol layers. It supports dynamic addition and removal of protocol layers.

• Protocol Layer: This is the base class for all protocol layers. The individual layers interface with each other via pointers to this class. The actual type of the upper layer and lower layer classes is not known to the implementers of a certain layer.



Consequences

The Protocol Stack design pattern breaks down the rigid protocol layer structure and provides a very flexible solution where layers can be dynamically added and removed from the stack.

Examples



A debug pass-through layer that displays the messages being exchanged between the datalink layer and the physical layer

Network Layer

Datalink Layer

Physical Layer

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Debug Layer

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A loopback layer that facilitates the testing of the datalink and network layers by just looping back all transmitted messages back for receive

Network Layer

Datalink Layer

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Loopback Layer

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An echo-back layer allows the protocol stack to emulate a node by just echoing back all higher layer messages back for transmission.

Network Layer

Datalink Layer

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Physical Layer

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An encryption layer sandwiched between the datalink and physical layers. This layers encrypts and decrypts data that is passed between these layers Datalink Layer

Encryption Layer

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Physical Layer

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Network Layer

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Implementation

• Handle_Transmit: This handler is invoked by the application to transmit messages using the protocol stack.

• Handle_Recieve: This handler is invoked by the device to pass received messages to the protocol stack.

• Add_Layer: Add a protocol layer at a specific position in the protocol stack.

• Remove_Layer: Remove a layer from the protocol stack.

The Protocol Stack is implemented as a single class. The class maintains a doubly linked list of Protocol Layers. Important methods of the class are:



Discussion:

Any issues in implementing these patterns?


PDCP

RLCRLC

RRC

PDCP

BMC

MAC

RLCRLC

RLC

RLCRLC

RLC

PHY

control

cont

rol

cont

rol

cont

rol

cont

rol

L2/RLC

L2/MAC

L1

L2/PDCP

L2/BMC

L3

TransportChannels

LogicalChannels

UMTS:

Protocol Stack Design PatternDECT

Protocol Stack Design PatternZigBee



PHY

MAC

Link

Network

010010100111000000

Timer running

Handle ReceiveTransmit

Timeout – send NWK PDU

When is control releasedso we can handle timeout?


Receive Protocol Handler Pattern

• Motivation• Different sliding window protocols have a lot of similarity. This similarity

can be captured in a common design pattern for their implementation. Here we will focus on the receive side of the protocol.

• Applicability• Receive Protocol Handler Pattern can be used to implement protocols at

any layer.



• StructureThis pattern is implemented by just one class, Receive Protocol

Handler. This class receives the packets from the other end and

performs the following operations:

• Check validity of the received packet • Ask Transmit Protocol Handler to acknowledge the received

packet • Check if the remote end has acknowledged that was sent by

Transmit Protocol Handler. • Inform Transmit Protocol Handler about acknowledged packet.


• To achieve this functionality, the following sequence numbers are maintained:

• Next Expected Sequence Number: Transmit sequence number expected in the next packet from the remote end.

• Last Acknowledged Sequence Number: Last receive sequence number received from the remote end. This sequence number is used by the remote end to acknowledge packets.

• Participants• The Transmit and Receive Protocol Handlers are the main

participants in this pattern. The received messages are added to the Receive Queue. The received message will be picked by the next higher layer.


Transmit Protocol Handler

• Motivation• Different sliding window protocols have a lot of

similarity. This similarity can be captured in a common design pattern for their implementation. Here we will focus on the transmit side of the protocol.

• Applicability• Transmit Protocol Handler Pattern can be used to

implement protocols at any layer.



• Structure• This pattern provides a framework for implementing a

sliding window protocol.

• The Transmit Protocol Handler receives a packet from the higher layer and transmits it to the lower layer after assigning a sequence number

• The packet is also stored in an internal retransmission buffer.

• The packet is removed from the retransmission queue if the remote end acknowledges the packet.

• The Transmit Protocol Handler retransmits the packet if it times out for an acknowledgement.



• Participants• The key actors of this design pattern:

• Transmit_Protocol_Handler: Class that manages the transmit end of the protocol. This class interfaces with the receive end and the retransmission queue.

• Transmit_Queue: Enqueues messages that wait for transmission when the window is full.

• Retransmission_Buffer: Manages buffers until an acknowledgement is received from the other end. The messages are retransmitted If no ack is received, .


Example – Data Link Layer

+Protocol_Layer()+Transmit()+Handle_Receive()+Set_Upper_Layer()+Set_Lower_Layer()+Get_Upper_Layer()+Get_Lower_Layer()

-m_p_Lower_Layer-m_p_Upper_Layer

Protocol_Layer

+DataLink_Layer()+Tranmit()+Handle_Receive()

-m_transmit_Protocol_Handler-m_receive_Protocol_Handler

DataLink_Layer

+Handle_Received_Packet()+Receive_Protocol_Handler()

-m_next_Expected_Sequence_Number-m_last_Acknowledged_Sequence_Number-m_p_Transmit_Protocol_Handler-m_p_Layer

Receive_Protocol_Handler

+Transmit_Protocol_Handler()+Handle_Transmit_Request()+Handle_Send_Ack_Request()+Handle_Received_Ack_Notification()+Transmit_Packet()

-m_next_Transmit_Sequence_Number-m_next_Receive_Sequence_Number-m_Retransmission_Buffer-m_Transmit_Queue-m_p_Layer

Transmit_Protocol_Handler

1

1

1

1

* *

1

1

1

1


• A packet is received from the upper layer1. The upper layer uses its "Lower Layer" pointer to invoke the

Transmit method for the lower layer. The "Protocol Packet" to be transmitted is passed to the Transmit method.

2. This invokes the Datalink Layer's Transmit method.

3. The Datalink Layer passes the "Protocol Packet" to the "Transmit Protocol Handler" object.

4. The "Transmit Protocol Handler" processes the "Protocol Packet" and adds the datalink layer header to the packet.

5. The "Transmit Protocol Handler" uses its parent layer to obtain a pointer to the lower layer.

6. The "Protocol Packet" is passed to the lower layer by invoking the Transmit method.



• A packet is received from the lower layer1. The lower layer uses its "Upper Layer" pointer to invoke the

"Handle Receive" method for the upper layer. The received "Protocol Packet" is passed to the "Handle Receive" method.

2. This invokes the Datalink Layer's "Handle Receive" method.

3. The Datalink Layer passes the "Protocol Packet" to the "Receive Protocol Handler" object.

4. The "Receive Protocol Handler" object uses the parent layer to obtain a pointer to the upper layer.

5. The "Protocol Packet" is passed to the higher layer by invoking the "Handle Receive" method.


State Pattern

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