Hi all ! (English is not my native langage but I will try to be as simple as possible.) The question is at the end ! ----- I am using Debian Woody (stable). Kernel 2.4.18 is patched with patch-o-matic, HTB, IMQ, ESFQ and some other things. At home, an old 486dx4-100mhz acts as a firewall/gateway. This PC is connected to Internet with an Ethernet ADSL Modem (Speed Touch Home). The protocol used is PPPOE. I am playing with Qos and HTB since two months and it works great ! All should be perfect in a perfect world except one thing : rates used by htb do not include (I think so) PPPOE overhead... From http://tldp.org/HOWTO/ADSL-Bandwidth-Management-HOWTO/implementation.html : > This means that if you are sending a typical TCP ACK packet > which consists of 0 bytes data + 20 bytes TCP header + 20 bytes > IP header + 18 bytes Ethernet header. In actuality, even though the > ethernet packet you are sending has only 40 bytes of payload (TCP > and IP header), the minimum payload for an Ethernet packet is 46 > bytes of data, so the remaining 6 bytes are padded with nulls. > This means that the actual length of the Ethernet packet plus header > is 18 + 46 = 64 bytes. This is for standard ethernet but with PPPOE, you must add 8 bytes. So, when sending "a typical TCP ACK packet", payload is 48 bytes (0 data +20 tcp + 20 ip + 8 pppoe) plus 18 bytes Ethernet header (14 header + 4 crc). Total transmited : 66 bytes. The problem is that HTB do not see these extra bytes (26). This is not a big problem with large packets but when you send many little packets (ssh keystokes) the overhead is quite big. ------ An example : Supose I want to limit total upload bandwidth to 10ko/s. (rate = ceil = 10000 bytes/s) When I send a (big) email, packets are truncated to 1420bytes (from ppp and htb point of view). so 10000bytes/s / 1420 = 7 packets/s If we add pppoe overhead (1420+26*7), 10122 bytes are *realy* transmitted per second. Now, imagine that while I am sending that big mail, I use ssh "intensively" (like holding "down" key to go further in a long man page...) When pressing a key, ssh send two packets : each one is 92 bytes long so if you keyboard rate is set to 30 keys/s, 60 packets are sent. 60 packets/s * 92 bytes = 5520 bytes/s. Because my ssh htb class has a lower prio than smtp htb class, ssh packets are sent first. As a result 9780 bytes/s are allowed to be sent by HTB (60 * 92 bytes (ssh) + 3 * 1420 (mail) ) But with pppoe overhead, 11418 bytes/s are transmited ( 60 * (92+26) + 3 * (1420+26) ) ------- Now, if I want to avoid that, I must limit total upload speed arround 8600 bytes/s :
