#LyX 1.3 created this file. For more info see http://www.lyx.org/
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\layout Chapter
Assessment on how INET processes TCP/IP packets
\begin_inset LatexCommand \label{cha:A3-Assessment on TCP}
\end_inset
\layout Standard
\begin_inset ERT
status Collapsed
\layout Standard
\backslash
thispagestyle{empty}
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\begin_inset ERT
status Collapsed
\layout Standard
\backslash
renewcommand
\backslash
figurename{Fig.}
\end_inset
The last section was about the UML tool.
This section is about how Linux's TCP suite, INET, Handles TCP packets.
This section will give a slight overview of the TCP/IP layers and then
in the next section go into more detail over some of the major structures
in the INET.
These next two sections are helpful to understanding the kernel modifications,
and how the Linux kernel handles the TCP protocol.
The INET (Linux implementation of the TCP IP suite) is purely the same
as the RFC's and some explanation of the differences are needed.
\layout Section
Overview of TCP
\layout Standard
The TCP is the transport communication protocol.
In the protocol stack it is four (transport) right above the Internet Protocol
(IP).
The TCP guarantees the arrival of the packets.
The way the Linux kernel uses TCP is the packet arrives at the network
card and the information (digital signals) is transferred to memory.
The Linux kernel creates a structure in memory (called a sk_buff) which
has pointers to the newly received digital transmission now in memory.
Each transmission is a packet â a part of the original transmitted data
with the headers attached.
Each TCP packet consist of three different types of headers (Ethernet,
IP, TCP) and the data .
The need for TCP and other transport layer protocols is to allow the partitioni
ng of data.
\layout Standard
If a user is downloads a modestly size file, say the latest UML root_fs
from sourceforge (the file would be roughly 16 megs).
Transferring all this file in one big packet (one set of headers) would
be cumbersome.
If another user else wants to use the bandwidth, they would have to wait
until the transmission of 16 megabytes is complete.
With a transport layer protocol, like TCP, the users share bandwidth.
\layout Standard
The data is partition into small packets (roughly 1500 bits a piece) each
having their own set of headers.
The TCP protocol was developed for the department of defense.
If a network connection was broke and other routes were available, the
department of defense wanted a way to ensure the data's arrival.
These headers tell the packet where to go and ensures the packet gets there.
This is what the Ethernet and IP headers do.
The Ethernet and IP headers contain sets of address (kind of like street
address) to tell the packet where to go.
\layout Standard
he data packets are divided into pieces, and the Internet is not all the
same bandwidth and reliability.
Some packets will arrived mixed up, corrupted, twice or not at all.
The TCP headers contain information to manage the arrival of the packets
and makes sure the application receives the data in order.
\layout Section
TCP Header and Sk_buff
\layout Standard
Fig.\SpecialChar ~
\begin_inset LatexCommand \ref{fig:TCP Header}
\end_inset
shows the TCP header and Fig.\SpecialChar ~
illistrates the TCP header structure in the
Linux code.
The total length of the TCP header is 20 bytes; four bytes for each row.
Eight bits equal one byte; the TCP header is 160 bits.
Of those 160 bits, 64 bits are used for the sequence and acknowledgment
numbers (32 bits for the sequence and 32 bits for the acknowledgment number).
The Seq and Ack numbers are how the TCP keeps packets in order.
\layout Standard
\begin_inset Float figure
placement H
wide false
collapsed false
\layout Standard
\align center
\begin_inset Graphics
filename Figures/a1tcpHeader.eps
display none
\end_inset
\layout Caption
\begin_inset LatexCommand \label{fig:TCP Header}
\end_inset
Tcp header
\end_inset
\layout Standard
The header has all sorts of information.
In the kernel, TCP header is accessed by a pointer in the sk_buff (stands
for socket buffer).
The sk_buff is the most important structure in the networking of the kernel
because it references the incoming network packet and a cental hub for
routing information.
In Fig.\SpecialChar ~
\begin_inset LatexCommand \ref{fig: headers of skbuf}
\end_inset
is a code sniped of the sk_buff:
\layout Standard
\begin_inset Float figure
placement H
wide false
collapsed false
\layout Standard
\align center
\begin_inset Tabular
|
\begin_inset Text
\layout Standard
\paragraph_spacing single
\align left
struct sk_buff {
\newline
\SpecialChar ~
\SpecialChar ~
/* These two members must be first.
*/
\newline
\SpecialChar ~
\SpecialChar ~
struct sk_buff * next; /* Next buffer in list */
\newline
\SpecialChar ~
\SpecialChar ~
struct sk_buff * prev; /* Previous buffer in list */
\newline
\SpecialChar ~
\SpecialChar ~
struct sk_buff_head * list; /* List we are on */
\newline
\SpecialChar ~
\SpecialChar ~
struct sock *sk; /* Socket we are owned by */
\newline
\SpecialChar ~
\SpecialChar ~
struct timeval stamp; /* Time we arrived */
\newline
\SpecialChar ~
\SpecialChar ~
struct net_device *dev; /* Device we arrived on/are leaving by */
\newline
\SpecialChar ~
\SpecialChar ~
/* Transport layer header */
\newline
\SpecialChar ~
\SpecialChar ~
union {
\newline
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
struct tcphdr *th;
\newline
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
struct udphdr *uh;
\newline
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
struct icmphdr *icmph;
\newline
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
struct igmphdr *igmph;
\newline
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
struct iphdr *ipiph;
\newline
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
struct spxhdr *spxh;
\newline
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
unsigned char *raw;
\newline
\SpecialChar ~
\SpecialChar ~
} h;
\newline
\SpecialChar ~
\SpecialChar ~
/* Network layer header */
\newline
\SpecialChar ~
\SpecialChar ~
union {
\newline
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
struct iphdr *iph;
\newline
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
struct ipv6hdr *ipv6h;
\newline
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
struct arphdr *arph;
\newline
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
struct ipxhdr *ipxh;
\newline
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
unsigned char *raw;
\newline
\SpecialChar ~
\SpecialChar ~
} nh;
\newline
\SpecialChar ~
\SpecialChar ~
/* Link layer header */
\newline
\SpecialChar ~
\SpecialChar ~
union {
\newline
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
struct ethhdr *ethernet;
\newline
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
\SpecialChar ~
unsigned char *raw;
\newline
\SpecialChar ~
\SpecialChar ~
} mac;
\newline
\SpecialChar ~
\SpecialChar ~
struct dst_entry *dst;
\newline
\SpecialChar ~
\newline
\end_inset
|
\end_inset
\layout Caption
\begin_inset LatexCommand \label{fig: headers of skbuf}
\end_inset
header section of the sk_buff
\end_inset
\layout Standard
\begin_inset Float figure
placement H
wide false
collapsed false
\layout Standard
\align center
\begin_inset Tabular
|
\begin_inset Text
\layout Standard
\paragraph_spacing single
\align left
struct tcphdr {
\newline
\SpecialChar ~
\SpecialChar ~
__u16 source;
\newline
\SpecialChar ~
\SpecialChar ~
__u16 dest;
\newline
\SpecialChar ~
\SpecialChar ~
__u32 seq;
\newline
\SpecialChar ~
\SpecialChar ~
__u32 ack_seq;
\newline
\SpecialChar ~
\SpecialChar ~
#if defined(__LITTLE_ENDIAN_BITFIELD)
\newline
\SpecialChar ~
\SpecialChar ~
__u16 res1:4,
\newline
\SpecialChar ~
\SpecialChar ~
doff:4,
\newline
\SpecialChar ~
\SpecialChar ~
fin:1,
\newline
\SpecialChar ~
\SpecialChar ~
syn:1,
\newline
\SpecialChar ~
\SpecialChar ~
rst:1,
\newline
\SpecialChar ~
\SpecialChar ~
psh:1,
\newline
\SpecialChar ~
\SpecialChar ~
ack:1,
\newline
\SpecialChar ~
\SpecialChar ~
urg:1,
\newline
\SpecialChar ~
\SpecialChar ~
ece:1,
\newline
\SpecialChar ~
\SpecialChar ~
cwr:1;
\newline
\SpecialChar ~
\SpecialChar ~
#elif defined(__BIG_ENDIAN_BITFIELD)
\newline
\SpecialChar ~
\SpecialChar ~
__u16 doff:4,
\newline
\SpecialChar ~
\SpecialChar ~
res1:4,
\newline
\SpecialChar ~
\SpecialChar ~
cwr:1,
\newline
\SpecialChar ~
\SpecialChar ~
ece:1,
\newline
\SpecialChar ~
\SpecialChar ~
urg:1,
\newline
\SpecialChar ~
\SpecialChar ~
ack:1,
\newline
\SpecialChar ~
\SpecialChar ~
psh:1,
\newline
\SpecialChar ~
\SpecialChar ~
rst:1,
\newline
\SpecialChar ~
\SpecialChar ~
syn:1,
\newline
\SpecialChar ~
\SpecialChar ~
fin:1;
\newline
\SpecialChar ~
\SpecialChar ~
#else
\newline
\SpecialChar ~
\SpecialChar ~
#error "Adjust your defines"
\newline
\SpecialChar ~
\SpecialChar ~
#endif
\newline
\SpecialChar ~
\SpecialChar ~
__u16 window;
\newline
\SpecialChar ~
\SpecialChar ~
__u16 check;
\newline
\SpecialChar ~
\SpecialChar ~
__u16 urg_ptr;
\newline
};
\newline
\SpecialChar ~
\newline
\end_inset
|
\end_inset
\layout Caption
\begin_inset LatexCommand \label{fig:IP-error messages}
\end_inset
IP tunnel error messages
\end_inset
\layout Section
Establishing a TCP connection
\layout Standard
The sequence and acknowledgment numbers mentioned earlier manages the arrival
and order of the packets.
The Linux kernel labels each packet that is sent with a beginning sequence
number on each side.
This sequence number is incremented by a known value.
If the packets become out of sequence, the receiver can tell by the sequence
number.
\layout Standard
The initial sequence number is determined by the sender.
It creates a packet (sk_buff) and sends it down the protocol stack and
out to the Internet.
The TCP header has a section of one bit FLAGS.
There are six flags: URG, ACK, PSH, RST, SYN, and FIN.
The initial packet send by the sender not only has the initial sequence
number, but also has the SYN flag set.
When the receiver gets the initial packet.
It processes the packet and sends its own sequence number and places the
just received sequence number in the acknowledgment field.
The sender then sets two flags in the TCP header: SYN and ACK.
\layout Standard
How the INET starts the connection is discussed in Section\SpecialChar ~
\layout Standard
\the_end