[65798] in North American Network Operators' Group
Re: MTU path discovery and IPSec
daemon@ATHENA.MIT.EDU (Barney Wolff)
Wed Dec 10 16:06:16 2003
Date: Wed, 10 Dec 2003 16:01:21 -0500
From: Barney Wolff <barney@databus.com>
To: Joe Maimon <jmaimon@ttec.com>
Cc: nanog@merit.edu
In-Reply-To: <3FD7858F.1070500@ttec.com>
Errors-To: owner-nanog-outgoing@merit.edu
--jRHKVT23PllUwdXP
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On Wed, Dec 10, 2003 at 03:43:59PM -0500, Joe Maimon wrote:
>
> Packets are fragmented into equally sized units to prevent further
> downstream fragmentation.
For amusement's sake, in response to a challenge from Crist Clark,
here's code to do it right. Pretty simple, although I have no idea
how to do it in an ASIC.
--
Barney Wolff http://www.databus.com/bwresume.pdf
I'm available by contract or FT, in the NYC metro area or via the 'Net.
--jRHKVT23PllUwdXP
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// Simulate a fragmentation algorithm
// frag Omtu [Ihdrsz]
// Barney Wolff <barney@databus.com> 12/4/03
// Copyright (c) Databus Inc. Free to use with attribution.
#include <iostream>
#include <cstdlib>
using namespace std;
int
main(int argc, char **argv) {
int Ihdrsz=20, Omtu, MinIpaysz, MaxIpaysz, Opaylst, Opaysz;
switch (argc) {
default: cerr << "Usage: frag-alg Omtu [Ihdrsz]\n"; exit(1);
case 3: Ihdrsz = atoi(argv[2]);
case 2: Omtu = atoi(argv[1]);
}
MinIpaysz = Omtu - 1 - Ihdrsz;
MaxIpaysz = 65535 - Ihdrsz;
Opaylst = Omtu - Ihdrsz;
Opaysz = Opaylst & 0xfff8;
cout << "Output MTU " << Omtu << " Input header size " << Ihdrsz << endl;
cout << "Max payload of last packet: "<<Opaylst<<" others "<<Opaysz<<endl;
for (int Ipaysz=MinIpaysz; Ipaysz<=MaxIpaysz; Ipaysz++) {
cout << Ipaysz + Ihdrsz;
int Opayn = Opaysz, Nfrag, Maxin, Lastsz, Delta;
Nfrag = 1 + (Ipaysz - Opaylst + Opaysz - 1) / Opaysz;
Maxin = Nfrag * Opaysz;
if (Maxin > Ipaysz) Opayn -= ((Maxin - Ipaysz) / Nfrag) & 0xfff8;
Lastsz = Ipaysz - (Nfrag-1) * Opayn;
Delta = Opayn - Lastsz;
if (Delta >= 8) {
int Nfrag2 = Delta / 8;
Nfrag -= Nfrag2;
Lastsz += 8 * Nfrag2;
cout << " " << Nfrag2 << " " << Opayn-8+Ihdrsz;
}
if (Nfrag > 1) cout << " " << Nfrag-1 << " " << Opayn+Ihdrsz;
cout << " " << Lastsz+Ihdrsz;
if (Lastsz-Opayn>8 || Opayn-Lastsz>8) cout << " *";
cout << endl;
}
exit(0);
}
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