--OXfL5xGRrasGEqWY
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On Thu, Nov 07, 2002 at 03:17:13PM -0600, David Young wrote:
> 
> On Thu, Nov 07, 2002 at 08:53:39PM +0100, Jochen Kunz wrote:
> > On 2002.11.07 17:47 Joel Wilsson wrote:
> > 
> > > Heh, not really. I do have a good idea about how an IP packet goes
> > > through the kernel, how timeouts, fragmentation, and a few other 
> > > things work, which I didn't know before.
> > What about writing a documentation about that? ;-)
> > "IP Packet Processing in NetBSD" 
> 
>   I would love to read such a document.

Okay, you can read it, but only if you all point out the errors
I'm sure exist in this first draft ;)

It's a bit short, I think, so if you could point out anything you think
is missing that'd be great.


--OXfL5xGRrasGEqWY
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=2ETL=0AIPv4 Packet Processing in NetBSD=0A=0A.AU=0AJoel Wilsson=0A=0A.AB=
=0AWhile extensive documentation is available for the 4.4BSD-Lite kernel's=
=0AIP stack, it is not freely available and not completely accurate for the=
=0ANetBSD code base. The basic design is the same, but the code has changed=
=0Aa bit over the last 8 years. This document is a brief introduction to=0A=
how IP version 4 packets are processed by the NetBSD kernel.=0A.AE=0A=0A.2C=
=0A.NH=0AA packet for you (maybe).=0A.PP=0AWe will limit the discussion to =
ethernet, IP version 4 and UDP.=0AThe ethernet network interface card (NIC)=
 driver is the first part of=0Athe kernel that sees the packet, which cause=
s an interrupt to be made.=0AWhen the kernel has time, hopefully very soon,=
 it calls the driver's=0Ainterrupt handler routine. The interrupt handler c=
opies the packet into=0Aan mbuf, which is a special data structure used a l=
ot by the networking=0Asubsystem. They are quite complicated, and have many=
 uses, but for now=0Ayou can think of the mbuf as representing a single pac=
ket, since we=0Aassume the packet is small enough to fit in a single mbuf.=
=0A=0AThe first few bytes in the packet is the basic ethernet header, which=
=0Ais defined as struct ether_header in sys/net/if_ether.h=0A.TS=0Acenter e=
xpand ;=0Ac s=0Ac c=0A| l | r | .=0AEthernet header=0AField	Size=0A_=0ASour=
ce	6 bytes=0A_=0ADestination	6 bytes=0A_=0AType	2 bytes=0A_=0A.TE=0A=0AThe =
destination address is usually the receiving NIC's MAC (media access=0Acont=
rol) address. All NICs have a unique MAC address, although it can be=0Achan=
ged.=0AWhen the interrupt handler has copied the the packet into an mbuf, i=
t=0Acalls the function ether_input. Unless the kernel has been configured t=
o=0Asupport bridging, the packet is dropped if the destination address=0Ado=
esn't match the with the NIC's address. Then, after saving the type=0Afield=
, the ethernet header is stripped off. What happens next will=0Adepend on t=
he value of that field: it tells us whether the packet is an=0AIP version 4=
 packet or something else. Other supported protocols include=0AAppleTalk, I=
PX and IP version 6.=0AIf the packet is of an unknown or unsupported type, =
it is dropped.=0AIn our case, it's an IPv4 packet, so ether_input puts it o=
n the IPv4=0Apacket queue and schedules a software interrupt.=0A=0A.NH=0AEn=
tering the IP stack.=0A.PP=0ASoftware interrupts are different from normal =
interrupts in that they=0Aare not generated by a physical event, like the c=
ompletion of a disk=0Asector read, but they are handled in almost the same =
way by the kernel.=0ASo eventually, when the kernel isn't busy with hardwar=
e interrupts, it=0Awill=0Acall the interrupt handler that ether_input decid=
ed should take care of=0Athe packet. For IPv4, that's ipintr. All ipintr do=
es is take packets=0Afrom the IPv4 packet queue and passes them on to ip_in=
put. This is where=0Athe real fun begins. First of all, ip_input makes a wh=
ole lot of checks=0Ato see if the packet is valid, most of which has someth=
ing to do with=0Athe IP header, which is defined as struct ip in sys/netine=
t/ip.h and=0Alooks like this:=0A.bp=0A.TS=0Acenter expand ;=0Ac s=0Ac c=0A|=
 l | r | .=0AIP header=0AField	Size=0A_=0AIP version	4 bits=0A_=0AHeader le=
ngth	4 bits=0A_=0AType of Service	1 byte=0A_=0ALength	2 bytes=0A_=0AIdentif=
ication	2 bytes=0A_=0AFragment offset	2 bytes=0A_=0ATime To Live	1 byte=0A_=
=0AProtocol	1 byte=0A_=0AHeader checksum	2 bytes=0A_=0ASource address	2 byt=
es=0A_=0ADestination address	2 bytes=0A_=0A.TE=0A=0AIf no IP addresses have=
 been configured, the packet can't be for us, so=0Ait's dropped. If it's sm=
aller than the minimum IP packet size, it's=0Adropped.  If it has a multica=
st reserved source address, bad header=0Achecksum, wrong IP version or isn'=
t allowed by the packet filter, it's=0Adropped. If it passes all those test=
s, it is either forwarded to another=0Ahost (if the kernel has been configu=
red for router functionality), or it=0Ais destined for this host. IP packet=
s can be fragmented, so ip_input=0Achecks if this packet is a fragment of a=
 bigger packet. If it is, we=0Alook in the queue of fragments to see if we =
have all the other fragments=0Aof that packet, and try to reassemble the fr=
agments into the original=0Apacket if we do.=0AIn any other case, the fragm=
ent is put on the fragment queue, where it=0Awill stay until it either time=
s out or all other fragments arrive, and=0Aip_input returns.=0AStill here? =
Then use the protocol field in the IP header to pass it up=0Athrough the st=
ack to the next level. The packet we received was a UDP=0Apacket, so ip_inp=
ut calls udp_input.=0A=0A.NH=0AUDP processing=0A.PP=0Audp_input looks at th=
e UDP header, which is called struct udphdr. The=0Adefinition can be found =
in sys/netinet/udp.h, but this is how it looks:=0A.TS=0Acenter expand ;=0Ac=
 s=0Ac c=0A| l | r | .=0AUDP header=0AField	Size=0A_=0ASource port	2 bytes=
=0A_=0ADestination port	2 bytes=0A_=0AUDP length	2 bytes=0A_=0AUDP checksum=
	2 bytes=0A_=0A.TE=0A=0AYou might recall that the IP header also had a leng=
th and checksum field=0Ain its header and wonder why UDP also has them. The=
 checksum here is=0Adifferent from the IP checksum, because it covers the w=
hole UDP=0Adatagram, while the IP checksum covers only the IP header. The l=
ength is=0Aincluded because there could be non-UDP data in the IP packet af=
ter this=0Adatagram.=0AAfter making sure the UDP length and checksum fields=
 are valid,=0Audp_input uses the IP header's source and destination fields =
together=0Awith the source fields in the UDP header to create source and=0A=
destination socket address structures. It then calls udp4_realinput,=0Awith=
 the socket adresses, the mbuf, and the header length as arguments.=0Audp4_=
realinput has an easy job, it just looks up which processes that=0Aare list=
ening on the socket and then calls udp4_sendup. The data=0Astructure that r=
epresents a process's binding to a particular port is=0Acalled a socket.=0A=
If the packet's destination is a multicast address, there can be more=0Atha=
n one listener, but that's usually not the case. udp4_sendup is=0Aanother v=
ery simple function; it copies the mbuf, puts the copy on the=0Asocket's re=
ceive queue, and notifies the socket that new data has=0Aarrived. If the pr=
ocess has used ioctl to indicate it wants to do=0Aasynchronous I/O on the s=
ocket, the kernel will send it a SIGIO signal.=0AThe packet can finally be =
read from userland.=0A
--OXfL5xGRrasGEqWY
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27=0A(interrupt handler that ether_input decided should)302.4 517.2 R(tak)=
=0A302.4 529.2 Q 2.904(ec)-.1 G .404(are of the pack)-2.904 F .404(et. F)=
=0A-.1 F .405(or IPv4, that')-.15 F 2.905(si)-.55 G(pintr)-2.905 E 2.905=0A=
(.A)-.55 G(ll)-2.905 E 1.492(ipintr does is tak)302.4 541.2 R 3.991(ep)=0A-=
.1 G(ack)-3.991 E 1.491(ets from the IPv4 pack)-.1 F(et)-.1 E 2.457=0A(queu=
e and passes them on to ip_input. This is)302.4 553.2 R 1.958=0A(where the =
real fun be)302.4 565.2 R 1.957(gins. First of all, ip_input)=0A-.15 F(mak)=
302.4 577.2 Q .193=0A(es a whole lot of checks to see if the pack)-.1 F .19=
3(et is)-.1 F -.25=0A(va)302.4 589.2 S .206(lid, most of which has somethin=
g to do with the)=0A.25 F 4.662(IP header)302.4 601.2 R 7.162(,w)-.4 G 4.66=
3=0A(hich is de\214ned as struct ip in)-7.162 F=0A(sys/netinet/ip.h and loo=
ks lik)302.4 613.2 Q 2.5(et)-.1 G(his:)-2.5 E=0AEP=0A%%Page: 2 2=0A%%BeginP=
ageSetup=0ABP=0A%%EndPageSetup=0A/F0 10/Times-Roman@0 SF(-2-)282.17 48 Q(IP=
 header)153.78 84 Q 88.849=0A(Field Size)119.898 96 R .4 LW 273.6 100.5 72 =
100.5 DL(IP v)90.878 110 Q=0A98.714(ersion 4)-.15 F(bits)2.5 E 273.6 114.5 =
72 114.5 DL(Header length)=0A90.878 124 Q 2.5(4b)85.524 G(its)-2.5 E 273.6 =
128.5 72 128.5 DL -.8(Ty)=0A90.878 138 S(pe of Service).8 E 2.5(1b)76.054 G=
(yte)-2.5 E 273.6 142.5=0A72 142.5 DL 104.404(Length 2)90.878 152 R(bytes)2=
.5 E 273.6 156.5 72=0A156.5 DL 79.404(Identi\214cation 2)90.878 166 R(bytes=
)2.5 E 273.6 170.5=0A72 170.5 DL(Fragment of)90.878 180 Q 69.384(fset 2)-.2=
5 F(bytes)2.5 E=0A273.6 184.5 72 184.5 DL -.35(Ti)90.878 194 S(me T).35 E 2=
.5(oL)-.8 G=0A-2.15 -.25(iv e)-2.5 H 2.5(1b)85.374 G(yte)-2.5 E 273.6 198.5=
 72 198.5=0ADL 102.734(Protocol 1)90.878 208 R(byte)2.5 E 273.6 212.5 72 21=
2.5 DL=0A(Header checksum)90.878 222 Q 2.5(2b)63.874 G(ytes)-2.5 E 273.6 22=
6.5 72=0A226.5 DL(Source address)90.878 236 Q 2.5(2b)74.974 G(ytes)-2.5 E 2=
73.6=0A240.5 72 240.5 DL(Destination address)90.878 250 Q 2.5(2b)56.634 G(y=
tes)=0A-2.5 E 273.6 254.5 72 254.5 DL 273.6 100.5 273.6 254.5 DL 197.795 10=
0.5=0A197.795 254.5 DL 72 100.5 72 254.5 DL 3.619(If no IP addresses ha)72 =
282=0AR 3.918 -.15(ve b)-.2 H 3.618(een con\214gured, the).15 F(pack)72 294=
 Q=0A2.835(et can')-.1 F 5.335(tb)-.18 G 5.335(ef)-5.335 G 2.836=0A(or us, =
so it')-5.335 F 5.336(sd)-.55 G 2.836(ropped. If it')-5.336 F(s)=0A-.55 E 3=
.022(smaller than the minimum IP pack)72 306 R 3.021=0A(et size, it')-.1 F(=
s)-.55 E 5.335(dropped. If)72 318 R 2.836=0A(it has a multicast reserv)5.33=
5 F 2.836(ed source)-.15 F 1.133=0A(address, bad header checksum, wrong IP =
v)72 330 R(ersion)-.15 E .943=0A(or isn')72 342 R 3.443(ta)-.18 G(llo)-3.44=
3 E .944(wed by the pack)-.25=0AF .944(et \214lter)-.1 F 3.444(,i)-.4 G(t')=
-3.444 E 3.444(sd)-.55 G=0A(ropped.)-3.444 E .367(If it passes all those te=
sts, it is either forw)=0A72 354 R .367(arded to)-.1 F .218(another host \(=
if the k)72 366 R .219=0A(ernel has been con\214gured for)-.1 F .201=0A(rou=
ter functionality\), or it is destined for this host.)72 378 R .645=0A(IP p=
ack)72 390 R .646(ets can be fragmented, so ip_input checks)-.1 F=0A.118(if=
 this pack)72 402 R .117(et is a fragment of a bigger pack)-.1 F=0A.117(et.=
 If it)-.1 F .452=0A(is, we look in the queue of fragments to see if we)72 =
414 R(ha)72 426 Q=0A.348 -.15(ve a)-.2 H .048(ll the other fragments of tha=
t pack).15 F .047=0A(et, and try)-.1 F 3.882(to reassemble the fragments in=
to the original)=0A72 438 R(pack)72 450 Q 1.015(et if we do.)-.1 F 1.014(In=
 an)6.015 F=0A3.514(yo)-.15 G 1.014(ther case, the fragment)-3.514 F 1.477=
=0A(is put on the fragment queue, where it will stay)72 462 R 2.701=0A(unti=
l it either times out or all other fragments)72 474 R(arri)72 486 Q=0A-.15(=
ve)-.25 G 3.345(,a).15 G .845(nd ip_input returns.)-3.345 F .846=0A(Still h=
ere? Then use)5.846 F 1.717=0A(the protocol \214eld in the IP header to pas=
s it up)72 498 R .378=0A(through the stack to the ne)72 510 R .378(xt le)-.=
15 F -.15(ve)-.25 G=0A.378(l. The pack).15 F .378(et we)-.1 F(recei)72 522 =
Q -.15(ve)-.25 G=0A5.598(dw).15 G 3.097(as a UDP pack)-5.698 F 3.097(et, so=
 ip_input calls)=0A-.1 F(udp_input.)72 534 Q/F1 10/Times-Bold@0 SF 2.5(3. U=
DP)72 570 R(pr)=0A2.5 E(ocessing)-.18 E F0 1.123(udp_input looks at the UDP=
 header)97=0A585.6 R 3.624(,w)-.4 G(hich)-3.624 E 3.406(is called struct ud=
phdr)72=0A597.6 R 5.906(.T)-.55 G 3.405(he de\214nition can be)-5.906 F 3.7=
43=0A(found in sys/netinet/udp.h, b)72 609.6 R 3.743(ut this is ho)-.2 F=0A=
6.243(wi)-.25 G(t)-6.243 E(looks:)72 621.6 Q(UDP header)148.225 639.6 Q=0A9=
0.237(Field Size)115.734 651.6 R 273.6 656.1 72 656.1 DL(Source port)=0A93.=
654 665.6 Q 2.5(2b)83.302 G(ytes)-2.5 E 273.6 670.1 72 670.1 DL=0A(Destinat=
ion port)93.654 679.6 Q 2.5(2b)64.962 G(ytes)-2.5 E 273.6 684.1=0A72 684.1 =
DL(UDP length)93.654 693.6 Q 2.5(2b)82.182 G(ytes)-2.5 E 273.6=0A698.1 72 6=
98.1 DL(UDP checksum)93.654 707.6 Q 2.5(2b)67.192 G(ytes)-2.5=0AE 273.6 712=
.1 72 712.1 DL 273.6 656.1 273.6 712.1 DL 190.855 656.1=0A190.855 712.1 DL =
72 656.1 72 712.1 DL -1.1(Yo)302.4 84 S 4.687(um)1.1 G=0A2.187(ight recall =
that the IP header also had a)-4.687 F .702=0A(length and checksum \214eld =
in its header and w)302.4 96 R(on-)-.1 E=0A.872(der wh)302.4 108 R 3.372(yU=
)-.05 G .871=0A(DP also has them. The checksum here)-3.372 F .36(is dif)302=
.4 120 R .36=0A(ferent from the IP checksum, because it co)-.25 F(v-)-.15 E=
 .449=0A(ers the whole UDP datagram, while the IP check-)302.4 132 R 3.052=
=0A(sum co)302.4 144 R -.15(ve)-.15 G 3.052(rs only the IP header).15 F=0A5=
.553(.T)-.55 G 3.053(he length is)-5.553 F .513=0A(included because there c=
ould be non-UDP data in)302.4 156 R 1.558=0A(the IP pack)302.4 168 R 1.559(=
et after this datagram.)-.1 F 1.559=0A(After making)6.559 F 3.13=0A(sure th=
e UDP length and checksum \214elds are)302.4 180 R -.25(va)302.4=0A192 S 1.=
3(lid, udp_input uses the IP header').25 F 3.8(ss)-.55 G 1.3=0A(ource and)-=
3.8 F .031=0A(destination \214elds together with the source \214elds in)302=
.4 204 R=0A1.307(the UDP header to create source and destination)302.4 216 =
R(sock)=0A302.4 228 Q .8(et address structures. It then calls udp4_real-)-.=
1 F=0A.622(input, with the sock)302.4 240 R .623(et adresses, the mb)-.1 F=
=0A.623(uf, and the)-.2 F 1.752(header length as ar)302.4 252 R 4.252=0A(gu=
ments. udp4_realinput)-.18 F(has)4.252 E .797=0A(an easy job, it just looks=
 up which processes that)302.4 264 R 5.683=0A(are listening on the sock)302=
.4 276 R 5.683(et and then calls)-.1 F=0A.598(udp4_sendup. The data structu=
re that represents a)302.4 288 R=0A(process')302.4 300 Q 4.093(sb)-.55 G 1.=
592=0A(inding to a particular port is called a)-4.093 F(sock)302.4 312 Q 3.=
99=0A(et. If)-.1 F 1.49(the pack)3.99 F(et')-.1 E 3.99(sd)-.55 G 1.49=0A(es=
tination is a multicast)-3.99 F 1.184=0A(address, there can be more than on=
e listener)302.4 324 R 3.683(,b)-.4 G=0A(ut)-3.883 E(that')302.4 336 Q 7.72=
1(su)-.55 G 5.222=0A(sually not the case. udp4_sendup is)-7.721 F .996(anot=
her v)302.4 348 R=0A.996(ery simple function; it copies the mb)-.15 F(uf,)-=
.2 E 1.217=0A(puts the cop)302.4 360 R 3.717(yo)-.1 G 3.717(nt)-3.717 G 1.2=
18=0A(he sock)-3.717 F(et')-.1 E 3.718(sr)-.55 G(ecei)-3.718 E 1.518 -.15=
=0A(ve q)-.25 H 1.218(ueue, and).15 F .143(noti\214es the sock)302.4 372 R=
=0A.142(et that ne)-.1 F 2.642(wd)-.25 G .142(ata has arri)-2.642 F -.15=0A=
(ve)-.25 G .142(d. If the).15 F 1.518=0A(process has used ioctl to indicate=
 it w)302.4 384 R 1.518(ants to do)=0A-.1 F 1.849(asynchronous I/O on the s=
ock)302.4 396 R 1.848(et, the k)-.1=0AF 1.848(ernel will)-.1 F .532(send it=
 a SIGIO signal.)302.4 408 R .532=0A(The pack)5.532 F .533(et can \214nally=
 be)-.1 F(read from userland.)=0A302.4 420 Q EP=0A%%Trailer=0Aend=0A%%EOF=0A
--OXfL5xGRrasGEqWY--