Embedded Web Server for the CR16
National Semiconductor
Jeff Wright
20
2.
IPv4_T holds the IP header information.
typedef struct ipv4_t{
UBYTE Ver_HL;
// Version and Header length byte
UBYTE ToS; // Type Of Service
UWORD Length; // Total datagram length
UWORD Ident;
// Fragment Identification field
UBYTE FlgsOffst; // Flags and MS offset bits
UBYTE Offst; // LS offset bits
UBYTE TTLive; // Time-To-Live byte
UBYTE Protocol; // Transport Protocol byte
UWORD HdCkSum; // One's compliment header checksum
QUADB_T SrcIP; // Source IP address
QUADB_T DestIP; // Destination IP address
} IPv4_T;
Figure 13. IP’s Header structure
Two IPv4_T structures are created during initialization – one for transmit and one for receive.
Operation is straightforward: the flags IPSEND and IPRECV are monitored for input from TCP,
while also monitoring the flags DLRECV and DLSEND for the Link layer. When a segment is readied
to send by TCP (IPSEND flag is set), IP will:
1.
Compute the total datagram length and record this number in the Length field…
2.
Compute the header checksum and record it in the HdCkSum field…
3.
Flag the Link layer by setting the DLSEND flag in DLAPI_T to send the datagram.
Upon receiving a datagram from the Link layer (DLRECV flag set), IP will:
1.
Confirm that the header checksum is correct. If incorrect, the datagram is ignored…
2.
Compute the TCP segment length for use by the Transport layer (TCP or UDP), or ICMP
3.
Copy the protocol number for multiplexing…
4.
Flag the appropriate layer (Transport or ICMP) that a datagram has been received.
NOTE: This IP does not currently support fragmentation.
ICMP (Internet Control Message Protocol)
ICMP (Internet Control Message Protocol) is a required and integral part of IP, although it logically
sits above it. ICMP messages are transmitted in IP datagrams just like TCP and UDP messages. All
IP’s are required to implement certain minimum ICMP features. This implementation supports the
following ICMP message types:
·
8
-
Echo Request (used by ping)
·
10
-
Router Solicitation
To facilitate future expansion, ICMP functions are conveniently tabulated as shown in Figure 15.
Every host MUST implement an ICMP Echo server function that receives Echo Requests and sends
matching Echo Replies. The IP source address in an ICMP Echo Reply MUST be the same as the
