The IP over ATM Mailing List Archive by date

[Craig Partridge <craig@aland.bbn.com>]

    Craig also, can you give some comment on the fixed ATM address sizes
    in the ATMARP packets that was raised as an issue last week or so?

Well, it is a classical "save space" vs "save time" issue.  Fixed sized
fields are very nice for quick packet parsing.  To illustrate the point,
if we had a rule that said that the fields were fixed sized and if not
in use, were, say, stuffed with all zeros but still there, one could
define an ATM-ARP structure something like this:

struct  aarphdr {
        u_short aar_hrd;        /* format of hardware address */
#define ARP_ATM_ADDR 19
        u_short aar_pro;        /* format of protocol address */
#define AA_TMASK 0100
#define AA_LMASK 077
#define AA_RESV  0200
#define AAT_E164 0100
#define HTL_NULL 0x40   /* null value of HTL */
        u_char  aar_shtl;       /* type & whether source ATM present */
        u_char  aar_sstl;       /* type & whether source ATM sub addr present */
        u_short aar_op;         /* one of: */
#define ARPOP_INARP     8
#define ARPOP_INARP_REPLY       9
#define ARPOP_NACK      10
        u_char  aar_thtl;       /* type & whether target ATM present */
        u_char  aar_tstl;       /* type & whether target ATM sub addr present */
	u_char  aar_atmsrc[20];
	u_char  aar_atmsrc_sub[20];
	struct in_addr aar_srcip;
	u_char  aar_atmtgt[20];
	u_char  aar_atmtgt_sub[20];
	struct in_addr aar_tgtip;
};

and the result, when you want to grab, say, the target ATM address is that
you simply check to see if aar_thtl is not HTL_NULL (or some such) and
then just access msg->aar_atmtgt

Currently you have to do something more tricky, for example:

get_tgtaddr(amp,samp)
struct arpmsg *amp;
struct sockaddr_atm *samp;
{
    register int i = AM_HDR_BYTES;
    register int j;

    /*
     * skip over source ATM address and source ATM subaddress
     * and source IP addr
     */  

    i += amp->am_hdr.aar_shtl & ~0xC0;
    i += amp->am_hdr.aar_sstl & ~0xC0;
    i += amp->am_hdr.aar_spln;

    j = amp->am_hdr.aar_thtl & ~0xC0;
    if (i+j > amp->am_len)
        goto runt;

    if (samp)
        bcopy((char *)&(amp->am_data[i]),(char *)&(samp->satm_addr),j);
    i += j;

    if (j = (amp->am_hdr.aar_tstl & ~0xC0))
    {
        if (i+j > amp->am_len)
            goto runt;
        if (samp)
            bcopy((char *)&(amp->am_data[i]),(char *)&(samp->satm_subaddr),j);
        i += j;
    }

    return(0);
runt:
    syslog(LOG_WARNING,"runt ATM ARP packet");
    return(1);
}

One can do this (and I have, in great quantity in my ARP server -- the problems
get even more fun when stuffing fields into an outbound packet -- if you
are not careful you have to bcopy already filled in data to make space for
new data, etc, plus play these offset games).

Craig