Layer_Protocol

Iterfaces and Buses / Networking / Layer_Protocol

Block Name: Layer_Protocol

Code File Location: VisualSim/Actor/lib/Layer_Protocol

Block Overview

  o  Used to describe the connectivity and attributes of each layer of the protocol stack.

  o  Connections between this layer and the one's above/below can be done using connected wires or connectionless

  o  Detailed logic and timing information can be augmented to the layer by associating with external definitions.

Description

• This block is used to define an Open System Interconnect (OSI) layer on top of a NODE block.  The Layer_Table block defines the attributes of this block.  A layer block can add/remove the necessary overhead bytes for header and trailers, delay the block for the processing time, queue, force a retry, and add custom logic and timing.  The user can define the probability of retrying a packet that may occur due to noise, buffer overflow, or data corruption from other layers. If a packet is flagged for retry, then a short message is sent to the same layer of the source node to retransmit the packet.  'Retry_' is added as a prefix to the Type_Message field.
• There are two forms of latency through this block- data transfer and processing delay (internal or external).  The data transfer latency is the Task_Size/Layer_MBytes_Sec.  The Task_Size used is after the Header/Trailer overhead has been added.  The processing delay is added from the up_internal_delay and dn_internal_delay.  If the Layer_Configuration is set to 'External_Delay", then the up_internal_delay and dn_internal_delay are ignored. 

• This block can perform additional layer level processing by setting the Layer_Table block parameter Layer_Configuration to "External_Delay".  In this case the internal (up and down) is ignored. All other operations function normally.  The Layer_Protocol block send the data structure to the ports called 'up_ext_output' (going up the stack) and 'dn_ext_output' (going down the stack) to implement the external delay.  This external process must be terminated with a Layer_Complete block, which returns the packet to the Layer_Protocol block to resume either up or down the layer stack.

The Layer_Protocol block processes data by appending header (H) and trailer (T) bytes to each protocol layer, see below.  The Layer_Header_Trailer_Bytes is a total sum of Header and Trailer quantity.  The overhead is added to the Task_Overhead and the Task_Size fields. The overhead is computed by first dividing the incoming Task_Size by the Layer_Frame_Size bytes.  The overhead is the Layer_Header_Trailer_Bytes* Number of Frames.  If the addition of the overhead exceeds the last frame size, then overhead is added for an additional frame. 

A Protocol Layer accepts data structures from a NODE actor or intermediate (Child or Parent) Protocol Layer. The Data Structures are passed to/from a Protocol Layer by either a direct connection, or a virtual connection. The connection flow is for the lower block output to be connected to the ds_up_input port while the output from this block to the upper block is the ds_up_output.  The connection from the upper layer ouput is to ds_dn_input port and the connection to the lower layer input is from the  ds_dn_output.   Virtual Connections are via Parent_Name (going up), or Child_Name (going down) parameter references.  A 'Layer_Table' Block is required to define the parameter of the 'Layer_Protocol' Block.  The Layer_Table provides the common layer parameters values.  Multiple Layer_Protocol block can reference a single Layer_Table.  users can add delay and other logic operations between the Layer_Protocols and the Nodes.

The block works as following in the Up direction:

• If the Parent_Name = "none" in the lower level, the data arrives on the ds_up_input port. If the Parent_Name = "Value" in the lower level, the data arrives virtually to this block and the port is ignored.  If the Multicast block is connected at the lower or upper level of this block, the next name must be "none".
• The packet overhead is subtracted from the Task_Size and Task_Overhead
• The packet is tested for Retry.  A random number is generated and if it is below the Layer_Retry_Probability, then the Network_Message is prefixed with "Retry_".  This is set back to the Layer with the same Layer_Number at the Source_Node.  None of the data structures fields are modified before sending this out.  The size of the message is specified in the first index of the Routing_Table Message_Bytes parameter.  At the source node, the packet is retransmitted.  The retry can occur on the Upward and downward paths.  If it occurs in the downward path of the Source Node, it is immediately send to the Upward path and retransmitted in the same Layer.  When the retry is generated, the Task_Retry is updated.  If the number of retries for a packet exceeds the Up_Retransmissions, then packet is dropped. The Network_Message = "Drop_Packet" and sent out the ds_up_output of the top Layer of the Source Node.  The block uses the Task_Layer field at the Source Node to determine where to re-generate.  The re-generated layer must be at the same level as the layer that caused the retry.
• The delay for the data transfer is scheduled.  The delay is Task_Size / Layer_MBytes_Sec.
• The Data Structure is queued.  If the number of packets exceeds the Layer_Queue_Size_Frames, this new data structure is dropped.  The number of dropped packets will be visible in the Layer Statistics as a delta between the Up_Number_Entered and Up_Number_Exited.
• If the Layer is not busy, the data structure is taken out of the Queue and delayed by the Up_Internal_Delay.  If the Layer_Configuration is set to "External_Delay", then this value is ignored and send to the up_ext_output port.  When the packet is sent out on the up_ext_output port, the data structure must encounter a Layer_Complete block to return to the Layer_Protocol to be transfered to the next above Layer_Protocol.
• The Task_ID is updated by this block if the value is -1.  A unique incremental number is generated.  When the ID has reached the maximum, the counter is reset to 1.  This unique number is used to remove the item from the internal Queue.

The block works as following in the Down direction:

• If the Child_Name = "none" in the upper level, the data arrives on the ds_dn_input port. If the Child_Name = "Value" in the upper level, the data arrives virtually to this block and the port is ignored.  If the Multicast block is connected at the lower or upper level of this block, the next name must be "none".
• The packet overhead is added to both Task_Size and Task_Overhead.  The overhead is computed by first dividing the incoming Task_Size by the Layer_Frame_Size bytes.  The overhead is the Layer_Header_Trailer_Bytes* Number of Frames.  If the addition of the overhead exceeds the last frame size, then one Layer_Header_Trailer_Bytes is added for an additional frame.  The Layer_Header_Trailer_Bytes contains the Header + Trailer bytes.
• The packet is tested for Retry.  A random number is generated and if it is below the Layer_Retry_Probability, then the Network_Message is prefixed with "Retry_".  This is set back to the Layer with the same Layer_Number at the Source_Node.  None of the data structures fields are modified before sending this out.  The size of the message is specified in the first index of the Routing_Table Message_Bytes parameter.  At the source node, the packet is retransmitted.  The retry can occur on the Upward and Downward paths.  If the retry is for a packet originating at this Source Node, it is immediately send to the Upward path and retransmitted in the same Layer.  When the retry is generated, the Task_Retry is updated.  If the number of retries for a packet exceeds the Dn_Retransmissions, then packet is dropped. The Network_Message = "Drop_Packet" and sent out the ds_up_output of the top Layer of the Source Node.  The block uses the Task_Layer field at the Source Node to determine where to re-generate.  The re-generated layer must be at the same level as the layer that caused the retry.
• The delay for the data transfer is scheduled.  The delay is Task_Size / Layer_MBytes_Sec.
• The Data Structure is queued.  If the number of packets exceeds the Layer_Queue_Size_Frames, this new data structure is dropped.  The number of dropped packets will be visible in the Layer Statistics as a delta between the Dn_Number_Entered and Dn_Number_Exited.
• If the Layer is not busy, the data structure is taken out of the Queue and delayed by the Dn_Internal_Delay.  If the Layer_Configuration is set to "External_Delay", then this value is ignored and send to the dn_ext_output port.  When the packet is sent out on the dn_ext_output port, the data structure must encounter a Layer_Complete block to return to the Layer_Protocol to be transfered to the next below Layer_Protocol or Node. 
• The Task_ID is updated by this block if the value is -1.  A unique incremental number is generated.  When the ID has reached the maximum, the counter is reset to 1.  This unique number is used to remove the item from the internal Queue.

Statistics

    Users can obtain statistics for each Layer_Protocol block through the use of RegEx functions shown below.  The statistics output
    looks like:

    DISPLAY AT TIME          ------ 4999.99999999990 sec ------
{A_Layer                            = "MAC3",
A_Layer_Table                    = "LT",
BLOCK                             = "Layer_MAC2",
DELTA                                = 0.0,
DS_NAME                         = "Layer_Stats",
Dn_MBps                                = 9.9616053598169,
Dn_Max_Delay            = 1.00000141,
Dn_Max_Occupancy        = 1.0,
Dn_Mean_Delay            = 1.000001161506,
Dn_Mean_Occupancy        = 0.0783582089552,
Dn_Min_Delay            = 1.00000016,
Dn_Min_Occupancy        = 0.0,
Dn_Number_Entered        = 499,
Dn_Number_Exited        = 498,
Dn_StDev_Delay            = 5.1305080090995E-7,
Dn_StDev_Occupancy        = 0.268734441493,
Dn_Utilization            = 9.9616053598169,
ID                = 1,
INDEX                = 0,
TIME                = 4999.9999999999,
Up_MBps                = 9.9816080693188,
Up_Max_Delay            = 1.00000111,
Up_Max_Occupancy        = 1.0,
Up_Mean_Delay            = 1.00000111,
Up_Mean_Occupancy        = 0.0118811881188,
Up_Min_Delay            = 1.00000111,
Up_Min_Occupancy        = 0.0,
Up_Number_Entered        = 499,
Up_Number_Exited        = 499,
Up_StDev_Delay            = 0.0,
Up_StDev_Occupancy        = 0.1083513981806,
Up_Utilization            = 9.9816080693188}

    This shows that each Layer_Protocol collects the up/dn layer number entered, number exited, throughput, layer delay,
    queue occupancy, and utilization statistics.  The RegEx function allows one to collect all of the statistics for each
    Layer_Protocol block, using the following expression, assuming the Layer_Protocol name is "MAC_1":

    getBlockStatus("MAC_1","Any Value", "stats", 1,"Any Integer")       // Returns 'MAC_1' statistics
    getBlockStatus("MAC_1","Any Value", "stats", -1,"Any Integer")      // Resets 'MAC_1' statistics

    getBlockStatus("MAC_1","Any Value", "length", 1,Any Integer)    // Returns 'MAC_1' up queue length

 getBlockStatus("MAC_1","Any Value", "length", 2,Any Integer)    // Returns 'MAC_1' down queue length

 Using getBlockStatus to collect the layer queue length returns the current queue length.  To learn more, click on the getBlockStatus example.

Fields

User Fields:

Task_Source: Message Source Node.  If the Task_Source does not have a name, it sets it to the current Node Name.  If it does have a name, it is not modified. If this layer generates a Retry or this block is the original source for a Retry, then the Task_Source=Task_Destination.  This field is modified for Network_Message=="Retry_*" and if the field is empty (or -1).

Task_Hop: Next Node in the data transfer.  The user will set it to the current node.  This field is not modified by this block.

Task_Destination: Final Destination Node Name.  If this layer generates a Retry or this block is the original source for a Retry, then the Task_Destination=Task_Source.  This field is modified for Network_Message=="Retry_*". only.

Task_Size: Size in Bytes of the data, excluding Header and Trailers.  This field is modified by this block to add the overhead.

Task_ID: First Layer_Protocol block encountered will generated a sequence number.  This is a single sequence is for all Layers in all the Nodes.  When a new packets arrives a Layer for the first time, the sequence number is incremented.  For the Task_ID to be updated, the entering Task_ID=-1.  This field is modified if the Task_ID ==-1.

Network_Message: Currently used only for identifying Retry_ and Drop_Packet.  This field is modified if this block generates a Retry.

Internal Fields:

Task_Retry: Number of retries for this packet.  When this number exceeds the threshold, the packet is dropped by updating Network_Message="Drop_Packet", and then returned to the top Layer_Protocol of the Source_Node.  This field is incremented by this block every time a Retry is generated for a packet.  This field is modifiedl.

Task_Trace: This is a array of Node names that this packet has encountered.  This field is not used and this field is not modified.

Task_Layer: It is used to set the layer where the retry occurs.  It is used to match the layer number when the packet gets back to the source.  This block modifies this field when a retry is generated.

Task_Overhead: This block will maintain the amount of overhead bytes accumulated.  Subtracting this from Task_Size will give the original data size.  This block adds the overhead size to this field on the downpath and subtracts the overhead size from this field on the upward movement.  This field is modified.

Demonstrations

Retry_Layer1 Demo Model shows the connection between the Layers and to the Node.  It also shows the association of the Layer_Table to the Layer_Protocol.  This models shows the use of multiple Layer_Tables in the model and the setting of the retry.

Network_External_Defn Demo Model shows the Configuration Layer setting to External Delay and the conenctivity for this.

Mulitcast Demo model showing the use of the Multicast function along with the Node block.

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