Understanding TCP/IP Model Internals and Interfaces

1508 Understanding TCP/IP Model Internals and Interfaces
Identifying an IP Packet
When the simple_mac process model receives a stream interrupt, it needs to decide whether the
packet is from the ARP layer or from the receiver. This is done based on the input stream index.
Refer to the transition macros PACKET_FROM_IP and PACKET_FROM_NTWK defined in the Header
block (Code Blocks / Header Block or button) of the process model. The relevant lines are
reproduced below.
/***** Transition Macros ******/
#define PACKET_FROM_IP (in_strm == SMC_IN_STRM_INDEX_FROM_ARP)
#define PACKET_FROM_NTWK (in_strm == SMC_IN_STRM_INDEX_FROM_RCV)
/* Macro corresponding to stream indices */
#define SMC_OUT_STRM_INDEX_TO_ARP 0
#define SMC_OUT_STRM_INDEX_TO_XMT 1
#define SMC_IN_STRM_INDEX_FROM_ARP 0
#define SMC_IN_STRM_INDEX_FROM_RCV 1
Note: The stream indices do not have to be hard-coded like this. Instead the MAC process can
perform a graph walk at initialization and determine these values and store them as state variables.
This approach was not used for this lab for the sake of simplicity.
Determining the Destination MAC Address
Every packet from the ARP layer will have an ICI of type ip_mac_req associated with the stream
interrupt. The destination address of the packet will be specified in the dest_addr field of the ICI.
Note:
• Since the ARP module reuses the ICI, the MAC layer SHOULD NOT destroy the ICI after
handling the packet.
• For broadcast packets, the dest_addr field will be set to IPC_PHYS_ADDR_BROADCAST.
CONFIDENTIAL – RESTRICTED ACCESS: This information may not be disclosed, copied, or transmitted in any format without the prior written consent of OPNET Technologies, Inc.
© 2010 OPNET Technologies, Inc.
34