[lkml]   [1999]   [Feb]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    SubjectRe: A router bug?
    HP-UX 11 has a tunable, ip_pmtu_strategy, that has a workaround for
    problems with PMTU. I think this scheme is from Mentat, but I'm not
    sure. And I'm not sure how well it works in practice, but it might be
    something to consider/think about.

    From ``ndd -h ip_pmtu_strategy'':


    Set the Path MTU Discovery strategy: 0 disables Path MTU
    Discovery; 1 enables Strategy 1; 2 enables Strategy 2.

    Because of problems encountered with some firewalls, hosts,
    and low-end routers, IP provides for selection of either
    of two discovery strategies, or for completely disabling the
    algorithm. The tunable parameter ip_pmtu_strategy controls
    the selection.

    Strategy 1: All outbound datagrams have the "Don't Fragment"
    bit set. This should result in notification from any intervening
    gateway that needs to forward a datagram down a path that would
    require additional fragmentation. When the ICMP "Fragmentation
    Needed" message is received, IP updates its MTU for the remote
    host. If the responding gateway implements the recommendations
    for gateways in RFC 1191, then the next hop MTU will be included
    in the "Fragmentation Needed" message, and IP will use it.
    If the gateway does not provide next hop information, then IP
    will reduce the MTU to the next lower value taken from a table
    of "popular" media MTUs.

    Strategy 2: When a new routing table entry is created for a
    destination on a locally connected subnet, the "Don't Fragment"
    bit is never turned on. When a new routing table entry for a
    non-local destination is created, the "Don't Fragment" bit is
    not immediately turned on. Instead,

    o An ICMP "Echo Request" of full MTU size is generated and
    sent out with the "Don't Fragment" bit on.

    o The datagram that initiated creation of the routing table
    entry is sent out immediately, without the "Don't Fragment"
    bit. Traffic is not held up waiting for a response to the
    "Echo Request".

    o If no response to the "Echo Request" is received, the
    "Don't Fragment" bit is never turned on for that route;
    IP won't time-out or retry the ping. If an ICMP "Fragmentation
    Needed" message is received in response to the "Echo Request",
    the Path MTU is reduced accordingly, and a new "Echo Request"
    is sent out using the updated Path MTU. This step repeats as

    o If a response to the "Echo Request" is received, the
    "Don't Fragment" bit is turned on for all further packets
    for the destination, and Path MTU discovery proceeds as for
    Strategy 1.

    Assuming that all routers properly implement Path MTU Discovery,
    Strategy 1 is generally better - there is no extra overhead for the
    ICMP "Echo Request" and response. Strategy 2 is available
    only because some routers, or firewalls, or end hosts have been
    observed simply to drop packets that have the DF bit on without
    issuing the "Fragmentation Needed" message. Strategy 2 is more
    conservative in that IP will never fail to communicate when using
    it. [0,2] Default: Strategy 2

    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to
    Please read the FAQ at

     \ /
      Last update: 2005-03-22 13:50    [W:0.021 / U:13.720 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site