8 IP: Error Processing (ICMP)• Any computer using IP must accept ICMP messages

and change behavior in response to the reported error.• ICMP messages are type-dependent• All ICMP messages begin with a fixed header:

– ic_type: message type

– ic_code: message subtype

– ic_cksum: message checksum

• next 32-bit in an ICMP message depend on the message type (using union to declare)

/* icmp.h *//* Internet Control Message Protocol Constants */

/* ic_type field */#define ICT_ECHORP 0 /* Echo reply */#define ICT_DESTUR 3 /* Destination unreachable*/#define ICT_SRCQ 4 /* Source quench */#define ICT_REDIRECT 5 /* Redirect message type*/#define ICT_ECHORQ 8 /* Echo request */#define ICT_TIMEX 11 /* Time exceeded */#define ICT_PARAMP 12 /* Parameter Problem */#define ICT_TIMERQ 13 /* Timestamp request */#define ICT_TIMERP 14 /* Timestamp reply */#define ICT_INFORQ 15 /* Information request */#define ICT_INFORP 16 /* Information reply */#define ICT_MASKRQ 17 /* Mask request */#define ICT_MASKRP 18 /* Mask reply */

/* ic_code field */#define ICC_NETUR 0

/* dest unreachable, net unreachable */#define ICC_HOSTUR 1

/* dest unreachable, host unreachable */#define ICC_PROTOUR 2

/* dest unreachable, proto unreachable */#define ICC_PORTUR 3

/* dest unreachable, port unreachable */#define ICC_FNADF 4

/* dest unr, frag needed & don't frag */#define ICC_SRCRT 5

/* dest unreachable, src route failed */

#define ICC_NETRD 0/* redirect: net */

#define ICC_HOSTRD 1/* redirect: host */

#define IC_TOSNRD 2/* redirect: type of service, net */

#define IC_TOSHRD 3/* redirect: type of service, host */

#define ICC_TIMEX 0 /* time exceeded, ttl */#define ICC_FTIMEX 1 /* time exceeded, frag */

#define IC_HLEN 8 /* octets */#define IC_PADLEN 3 /* pad length (octets) */

#define IC_RDTTL 300 /* ttl for redirect routes */

/* ICMP packet format (following the IP header) */struct icmp { /* ICMP packet */

char ic_type; /* type of message (ICT_* above)*/char ic_code; /* code (ICC_* above) */short ic_cksum; /* checksum of ICMP header+data*/union {

struct {int ic1_id:16; /* echo type, a message id */int ic1_seq:16;/* echo type, a seq. number*/

} ic1;IPaddr ic2_gw; /* for redirect, gateway*/struct {

char ic3_ptr;/* pointer, for ICT_PARAMP*/char ic3_pad[IC_PADLEN];

} ic3;int ic4_mbz; /* must be zero */

} icu;char ic_data[1]; /* data area of ICMP message*/

};

/* format 1 */#define ic_id icu.ic1.ic1_id#define ic_seq icu.ic1.ic1_seq

/* format 2 */#define ic_gw icu.ic2_gw

/* format 3 */#define ic_ptr icu.ic3.ic3_ptr#define ic_pad icu.ic3.ic3_pad

/* format 4 */#define ic_mbzicu.ic4_mbz

Received Frame Processing flow

• Driver p.39 ni_in()->ip_in() p.82: enqueue to IP process

->arp_in() p.54

->rarp_in()• IP process p.68 ipproc(): routing rtget(), put the datagram

to appropriate interface, ipputp() p.110– Ipputp() decides whether need to fragment and call netwrite()

– P. 49 netwrite()-> local_out() for upper layer

->write() write to network interface

/*------------------------------------------------------------------------ * local_out - handle an IP datagram headed for a local process *------------------------------------------------------------------------ */Int local_out(struct ep *pep){ struct netif *pni = &nif[NI_LOCAL];

struct ip *pip = (struct ip *)pep->ep_data;int rv;ipnet2h(pip);pep->ep_order |= EPO_IP;pep = ipreass(pep);if (pep == 0) return OK;pip = (struct ip *)pep->ep_data;ipdstopts(pni, pep); /* do IP option processing */switch (pip->ip_proto) {case IPT_ICMP: rv = icmp_in(pni, pep); break;case IPT_IGMP: rv = igmp_in(pni, pep); break;case IPT_TCP: rv = tcp_in(pni, pep); break;case IPT_UDP: rv = udp_in(pni, pep); break;default:

IpInUnknownProtos++;icmp(ICT_DESTUR, ICC_PROTOUR, pip->ip_src, pep, 0);return OK;

}IpInDelivers++;return rv;

}

8.4 handling Incoming ICMP messagesWhen receive an ICMP message, Net-Interface => IP => icmp_in

/*------------------------------------------------------- *icmp_in - handle ICMP packet coming in from the network *------------------------------------------------------*/int icmp_in(struct netif *pni, struct ep *pep){ struct ip *pip;

struct icmp *pic;int i, len;pip = (struct ip *)pep->ep_data;pic = (struct icmp *) pip->ip_data;

len = pip->ip_len - IP_HLEN(pip);if (cksum((WORD *)pic, len)) {

IcmpInErrors++;freebuf(pep);return SYSERR; }

IcmpInMsgs++;

switch(pic->ic_type) {case ICT_ECHORQ:

IcmpInEchos++;return icmp(ICT_ECHORP, 0, pip->ip_src, pep, 0);

case ICT_MASKRQ:IcmpInAddrMasks++;if (!gateway) {

freebuf(pep);return OK;

}pic->ic_type = (char) ICT_MASKRP;*(IPaddr *)pic->ic_data = netmask(pip->ip_dst);break;

case ICT_MASKRP:IcmpInAddrMaskReps++;for (i=0; i<Net.nif; ++i)

if (nif[i].ni_ip == pip->ip_dst)break;

if (i != Net.nif) {setmask(i, *(IPaddr *)pic->ic_data);send(pic->ic_id, ICT_MASKRP); }

freebuf(pep);return OK;

case ICT_ECHORP:IcmpInEchoReps++;if (send(pic->ic_id, (int)pep) != OK)

freebuf(pep);return OK;

case ICT_REDIRECT:IcmpInRedirects++;icredirect(pep);return OK;

case ICT_DESTUR: IcmpInDestUnreachs++;freebuf(pep); return OK;

case ICT_SRCQ: IcmpInSrcQuenchs++; freebuf(pep); return OK;

case ICT_TIMEX: IcmpInTimeExcds++;freebuf(pep); return OK;

case ICT_PARAMP: IcmpInParmProbs++;freebuf(pep); return OK;

case ICT_TIMERQ: IcmpInTimestamps++;freebuf(pep); return OK;

case ICT_TIMERP: IcmpInTimestampReps++;freebuf(pep); return OK;

default:IcmpInErrors++;freebuf(pep);return OK;

}

icsetsrc(pip);

len = pip->ip_len - IP_HLEN(pip);

pic->ic_cksum = 0;pic->ic_cksum = cksum((WORD *)pic, len);

IcmpOutMsgs++;ipsend(pip->ip_dst, pep, len, IPT_ICMP, IPP_INCTL, IP_TTL);return OK;

}