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Embedded Web Server for the CR16
National Semiconductor
Jeff Wright
a segment.  On a dial-up connection, the MTU is essentially irrelevant.  PPP does impose an
arbitrary MTU of 1500 bytes, presumably to mimic that of Ethernet.   Since our TCP advertises
its MSS, the peer should never decide to fragment a segment.  This IP does not examine the
More Fragments (MF) bit.  Should a datagram be received with this bit set, this IP will not
treat it any differently that any other datagram.  This will eventually lead to checksum errors. 
IP Options 
IP defines several options, many or most of which never find any real use in common
applications, let alone an embedded one.  These Options provide IP with useful control functions
needed in some situations, but for the most part, are simply not implemented in many stacks. 
Options include - provisions for timestamps, security, and special routing.   
No IP options are supported in this stack, although provision is made to receive them.  If
received they are ignored.
ICMP (Internet Control Message Protocol) was defined to be integral to IP, although logically it
sits above it.  What that means is that ICMP messages are transmitted in IP datagrams just
like TCP and UDP messages.  Nonetheless, ICMP is technically part of IP and all IP’s are
required to implement certain of its features.  ICMP messages are grouped into two classes:
Error messages:
Destination Unreachable
Source Quench
Time Exceeded
Parameter Problem 
Query messages:
Address Mask
·  Echo Request/Reply:
Types 8 and 0.  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 specific-destination address (defined in Section of the
corresponding ICMP Echo Request message.
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