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<h1>20040101 - Simple IP Reference</h1>
<br>
<pre>________________________________________________________________
SIMPLE IP REFERENCE - USE RFC'S FOR FULL REFERENCE
This is a minimal reference for programming a simple operating
system's internet related code. Note network byte order is big
endian (most significant byte first), keep this in mind when
sending multibyte sized numbers. This assumes some basic
knowledge of IP networking, and is not a complete reference.
________________________________________________________________
SMTP - SIMPLE MAIL TRANSPORT PROTOCOL
In progress...
________________________________________________________________
POP3 - POST OFFICE PROTOCOL
Access a maildrop via TCP port 110. The following are used to
describe,
[optional] -> optional arguments
(grouping) -> used only to describe repeating
... -> repeats last grouping
<SPACE> -> decimal 32
<DOT> -> decimal 46
<LF> -> decimal
<CR> -> decimal
"text" -> text to be sent as is
The formatting is as follows,
client sends: (command)...
command: keyword [(<SPACE> argument)...] <CR> <LF>
keywords: are 3-4 characters long and case insensitive
arguments: are up to 40 characters long
server sends: (responce)...
responce: first [(extra)... <DOT> <CR> <LF>]
first: status_indicator <SPACE> keyword
[(<SPACE argument)...] <CR> <LF>
extra: data <CR> <LF>
first: may be up to 512 characters long
status_indicator: will be "+OK" or "-ERR"
always in upper case
data: if first character is <DOT> then it
will actually be <DOT> <DOT>
The connection happens in states,
client: connects to server
server: "+OK" SPACE "POP3" safe_to_ignore_data <CR> <LF>
AUTHORIZATION STATE
The client needs to identify itself before accessing mail.
client: "USER" <SPACE> name <CR> <LF>
server: responds if good, "+OK" safe_to_ignore_data <CR> <LF>
server: responds if bad, "-ERR" safe_to_ignore_data <CR> <LF>
then,
client: "PASS" <SPACE> password <CR> <LF>
server: responds if good, "+OK" safe_to_ignore_data <CR> <LF>
server: responds if bad, "-ERR" safe_to_ignore_data <CR> <LF>
loop if bad
TRANSACTION STATE
In this state the mailbox is locked, all messages get
assigned a decimal number starting with "1" and going in
incremental order. All messages are flagged as not deleted.
client: "STAT" <CR> <LF>
server: "+OK" <SPACE> number_of_messages <SPACE>
mail_size_in_bytes safe_to_ignore_data <CR> <LF>
server: responds if bad, "-ERR" safe_to_ignore_data <CR> <LF>
server: does not count deleted messages in the total
client: "LIST" [<SPACE> message_number] <CR> <LF>
server: for message_number option,
"+OK" <SPACE> size_of_message safe_to_ignore_data
<CR> <LF>
server: otherwise gives multiline responce,
"+OK" safe_to_ignore_data <CR> <LF>
[(message_number <SPACE> message_size safe_to_ignore
<CR> <LF>)...] <DOT> <CR> <LF>
server: responds if bad, "-ERR" safe_to_ignore_data <CR> <LF>
server: does not list deleted messages
client: "RETR" <SPACE> message_number <CR> <LF>
server: "+OK" safe_to_ignore <CR> <LF>
lines_of_message_remember_special_dot_dot_encoding
<DOT> <CR> <LF>
server: responds if bad,
"-ERR" safe_to_ignore_data <CR> <LF>
server: should not send a deleted message
client: "DELE" <SPACE> message_number <CR> <LF>
server: marks message as deleted,
"+OK" safe_to_ignore <CR> <LF>
server: responds if bad,
"-ERR" safe_to_ignore_data <CR> <LF>
server: will respond in error if already deleted
server: does not actualy delete message until update state
client: "NOOP" <CR> <LF>
server: "+OK" <CR> <LF>
client: "RSET" <CR> <LF>
server: messages set not deleted,
"+OK" safe_to_ignore <CR> <LF>
server: responds if bad, "-ERR"
safe_to_ignore_data <CR> <LF>
client: "QUIT" <CR> <LF>
server: changes to update state
UPDATE STATE
server: "+OK" safe_to_ignore_data <CR> <LF>
server: responds if bad, "-ERR"
safe_to_ignore_data <CR> <LF>
server: will perminatly delete messages marked as deleted
Reported email message size may be different than actual sent
size of message, the <DOT> characters should not be counted
twice even though they are sent that way.
________________________________________________________________
SLIP - SERIAL LINE IP
Use 1006 byte datagrams (not including the adjusted characters
below). To send a packet just send data. If data byte is 192,
then send 219 and 220 instead. If data byte is 219, then send
219 and 221 instead. When the last byte of the packet is sent
then send 192.
________________________________________________________________
ETHERNET
Ethernet Header
offset bytes meaning
0 6 destination 48bit Ethernet MAC
6 6 source 48bit Ethernet MAC
12 2 type, ARP=0x806, IP=0x800
14 total size (offset of IP header)
When get IP from network, save source MAC to IP mapping if
source IP is on the local network (hence the requirement of a
netmask). When put IP to network, set destination MAC to
correct MAC for destination IP (if on local network) or
address of default router (not on local network).
Ethernet ARP (Address Resolution Protocol) Header
offset bytes meaning
14 2 hardware type: 1=ethernet
16 2 protocol: IP=0x800
18 1 hardware length: 6 (for 48bit Ethernet MAC)
19 1 protocol length: 4 (for 32bit IP)
20 2 opcode: request=1, reply=2
22 6 source MAC
28 4 source IP
32 6 destination MAC
38 4 destination IP
These packets are used to resolve IP addresses into MAC
addresses. After initial resolution the translation should be
cached. Probably should use 20 minutes as default for ARP
cache entry timeout (this is BSD default). If ARP request, and
destination IP equals host IP, then send reply with source set
to host info and destination set to source info from request.
To send an ARP request set ethernet header destination MAC to
all 0xFF, ARP header destination MAC to all 0x00, and set
source to host.
________________________________________________________________
DNS HOST NAME TO IP ADDRESS RESOLUTION
DNS uses UDP packets at port 53. This only gives the minimal
amount of into to write a simple resolver. DNS messages
encode host names with a special format, host dot seperated
substring,
offset bytes meaning
0 1 n, size of string or 0 for end of string
1 n the characters for the string (skips .)
This may be repeated multiple times to encode a string
with many '.' characters. So to encode "this.name.com",
4,'this',4,'name',3,'com',0,
DNS Message Header
offset bytes meaning
0 2 id (could be used for resolver table entry)
2 1 flags
0x80 response
0x01 query
3 1 flags
0xf error mask
0x3 error name
0x0 error none
4 2 number of questions, 1
6 2 number of answers, 0 for query
8 2 number of authority, 0
10 2 number of additional, 0
12 total size
for query
special formated host name follows
for request
special formated host name follows
then DNS answer messages follows
DNS Answer Message First Part (2 options)
0 1 c0 (compressed record)
1 1 skip
or a DNS encoded host name
DNS Answer Message Second Part
offset bytes meaning
0 2 type, 1
2 2 class, 1
4 2 time to live
6 2 length, 4
8 4 IP
12 total size
To query for IP address for hostname, the hostname must be
encoded after the message header in the special dot broken
encoding, followed by the following sequence of bytes:
0,1,0,1. So to encode "this.name.com",
4,'this',4,'name',3,'com',0, 0,1,0,1
When reading a DNS responce, and the second flag byte to error
mask to check for error. The question name is entered
immediately following the message header. To get to the
answers this name must be parsed by reading a number and then
skipping that number of characters, until the number read is
zero. You should probably check that the question matches the
hostname under question for security. The answers follow. The
answer may contain compressed name records or normal host name
records (with the dot broken encoding). Choose an answer with
a valid IP address.
________________________________________________________________
IP PACKETS
Offsets from Link Level Header (ethernet=14),
IP HEADER
offset bytes meaning
0 1 vhl
1 1 type of service
2 2 length of packet
4 2 ip id
6 2 ip offset
8 1 time to live, set to maximum 255 hops
9 1 protocol, 1=ICMP, 6=TCP, 17=UDP
10 2 ip checksum
12 4 source ip address
16 4 destination ip address
20 total size
TCP HEADER
offset bytes meaning
20 2 source port
22 2 destination port
24 4 sequence number
28 4 acknowledge number
32 1 tcp offset
33 1 flags
34 2 window
36 2 tcp checksum
38 2 urgp
40 total size
________________________________________________________________
ICMP - INTERNET CONTROL MESSAGE PROTOCOL
ICMP Header
offset bytes meaning
0 1 type (8=echo, 0=reply)
1 1 icode (0)
2 2 icmp checksum
4 2 id (0 or number to help match echos and replies)
6 2 sequence number (0 or like id byte)
8 total size
This is only enough to do a ping. The ICMP checksum is the
16bit one's complement of the one's complement of the sum of
the ICPM message starting at the type byte. What ever data
sent after the ICMP header in a echo should be returned in the
reply.
________________________________________________________________
UDP - USER DATAGRAM PROTOCOL
UDP Header
offset bytes meaning
0 2 source port
2 2 destination port
4 2 udp length (udp header + data)
6 2 udp checksum (use zero here when computing this)
8 total size
The UPD packet sits on top of the IP packet. The length for a
zero data size packet is 8. The checksum is the 16bit one's
complement of the one's complement sum of a pseudo header of
information from the IP header, the UDP header, and the data
padded with zeros if necessary to make a multiple of 2 bytes.
The pseudo header is as follows,
offset bytes meaning
0 4 source IP address
4 4 destination IP address
8 1 zero
9 1 protocol (17 for UDP)
10 2 udp length
If the computed checksum is zero it is sent as all ones. An
all zero checksum means that there is no checksum.
________________________________________________________________
FTP - FILE TRANSFER PROTOCOL
Work in progress...
________________________________________________________________
TCP
Work in progress...
________________________________________________________________
</pre>
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