Building TCP/IP packets

A look at the computation-steps which need to be performed for utilizing the TCP/IP protocol

NIC’s ‘offloading’ capabilities

• An advanced feature of Intel’s most recent PRO1000 gigabit ethernet controllers is an ability to perform many of the calculations associated with the processing of network packets which otherwise would have to be programmed by device-driver authors and then executed at runtime by a CPU that has much other vital work to attend to

How much work?

• To gain a concrete idea of the work that is required to set up a conventional network packet for transmission, and then to take it apart when it’s received, we have created a character-mode Linux driver that shows us all of the essential calculation-steps, as it does not rely on the usual networking software included within Linux’s kernel

TCP/IP over Ethernet

Here is a diagram, from Intel’s Software Developer’s Manual, of the widely deployed TCP/IP Ethernet packet format

UnixWare: online tutorial

<http://uw713doc.sco.com/en/NET_tcpip/tcpN.tcpip_stack.html>

© 2002 Caldera International, Inc. All rights reserved. UnixWare 7 Release 7.1.3 - 30 October 2002

“The TCP/IP Protocol Stack”

recommended background reading

TCP/IP Protocol Stack

by Caldera, Incorporated

Encapsulation and Decapsulation of Data within a network stack

by Caldera, Incorporated

Network stack support for TCP/IP

by Caldera, Incorporated

Internet Protocol Header

IP Header (Little Endian layout)

Transport Control Protocol Header

TCP Header (Little-Endian layout)

TCP Pseudo Header layout

Our ‘nictcp.c’ demo

• This device-driver’s ‘write()’ function will show the sequence of steps needed for building a packet with the TCP/IP format:

• Setup the packet’s data ‘payload’• Prepend a TCP packet-header • Then prepend a TCP ‘pseudo’ packet-header• Compute and insert the TCP checksum• Replace the pseudo-header with an IP header• Compute and insert the IP Header Checksum• Finally prepend the Ethernet Header

packet-data

Two checksum calculations

TCP HeaderTCP Pseudo-Header

The TCP Segment

The TCP Checksum is computed over this array of words

packet-dataTCP Header

The IP Header Checksum is computed over this array of words

IP HeaderFrame Header

The TCP Checksum gets inserted here

The IP Header Checksum gets inserted here

Sample checksum algorithm

// to compute and insert the TCP Checksumunsigned char *cp = phys_to_virt( txring[ txtail ].base_addr );unsigned short *wp = (unsigned short *)( cp + 22 );unsigned int nbytes = 12 + 20 + len;unsigned int nwords = (nbytes / 2) + (nbytes % 2);unsigned int cksum = 0;

if ( len & 1 ) cp[ 14 + 20 + 20 + len ] = 0; // padding

for (int i = 0; i < nwords; i++) cksum += htons( wp[ i ] );cksum += (cksum >> 16); // end-around-carriescksum & 0xFFFF; // mask for 16-bitscksum ^= 0xFFFF; // flip the low 16-bits*(unsigned short*)(cp + 50) = htons( cksum );

In-class demonstration

• We can use our ‘pktsplit.c’ demo-module to confirm the correctness of our TCP/IP packet-format as far as the NIC hardware is concerned – it reports (in its descriptor ‘status’ and ‘errors’ fields) that both of our checksums were computed by the NIC on reception -- and that neither was in error

In-class exercise

• Can you adapt these ideas in our ‘nictcp.c’ driver to produce the analogous ‘nicudp.c’ demonstration module?

• A similar ‘pseudo’ UDP Header is used to calculate the UDP Checksum value (see the Intel Software Developer’s Manual for the layout and size of the UDP Header)