UDP/IP packet question

Discussion in 'Cisco' started by Kott, Jul 5, 2004.

  1. Kott

    Kott Guest

    Hello -
    Could any one tell me which all fields in Ip header and UDP header
    does a Router typically process before doing a Routing across a routed
    interface ?

    Because I connect aross a Extreme box( with IP forwarding enabled) and
    the UDP/IP packet generated by my device crosses a routed boundary.
    But when I connect a Zyxel Presige 700 router and send the same UDP/IP
    packet from my device it does not cross the routed boundary.

    The size of the UDP/IP packet on the wire is 240 bytes.
    TOS field in IP header = 0x00
    IP version is =4


    Could any one throw some light on the way Layer 3 and Layer 4 fields
    are processed ?

    Thanks in advance
    Kott
     
    Kott, Jul 5, 2004
    #1
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  2. Kott

    Guest

    In comp.dcom.lans.ethernet Kott <> wrote:
    > Hello -
    > Could any one tell me which all fields in Ip header and UDP header
    > does a Router typically process before doing a Routing across a routed
    > interface ?


    > Because I connect aross a Extreme box( with IP forwarding enabled) and
    > the UDP/IP packet generated by my device crosses a routed boundary.
    > But when I connect a Zyxel Presige 700 router and send the same UDP/IP
    > packet from my device it does not cross the routed boundary.


    > The size of the UDP/IP packet on the wire is 240 bytes.
    > TOS field in IP header = 0x00
    > IP version is =4



    > Could any one throw some light on the way Layer 3 and Layer 4 fields
    > are processed ?


    Routers don't care about layer 4 ( or 2 )

    ip-addresses and-ed with netmask tells which net to send a packet,
    the router in your case only examines dest-address.

    See "ftp://ftp.cs.rutgers.edu/runet/tcp-ip-intro.txt" for an introduction
    of IP protocol.

    > Thanks in advance
    > Kott


    --
    Peter Håkanson
    IPSec Sverige ( At Gothenburg Riverside )
    Sorry about my e-mail address, but i'm trying to keep spam out,
    remove "icke-reklam" if you feel for mailing me. Thanx.
     
    , Jul 5, 2004
    #2
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  3. "Kott" <> wrote:

    > Hello -
    > Could any one tell me which all fields in Ip header and UDP header
    > does a Router typically process before doing a Routing across a routed
    > interface ?
    >
    > Because I connect aross a Extreme box( with IP forwarding enabled) and
    > the UDP/IP packet generated by my device crosses a routed boundary.
    > But when I connect a Zyxel Presige 700 router and send the same UDP/IP
    > packet from my device it does not cross the routed boundary.
    >
    > The size of the UDP/IP packet on the wire is 240 bytes.
    > TOS field in IP header = 0x00
    > IP version is =4
    >
    > Could any one throw some light on the way Layer 3 and Layer 4 fields
    > are processed ?


    In principle, routers do not look at anything Layer 4, and therefore
    should not care. However, many routers are set up with packet filters,
    to prevent unwanted packets from crossing the subnet boundary. Could the
    Prestige router possibly be set up to filter out UDP/IP packets? This
    would not be uncommon. The filters should be configurable.

    In IPv4, the Protocol byte, the second byte in the third header word,
    can be used by routers to determine what Transport Layer protocol is
    being carried over the IP packet. If the byte is set to (decimal) 17,
    it's UDP. Or, possibly, certain Port IDs are being filtered as well.

    Bert
     
    Albert Manfredi, Jul 5, 2004
    #3
  4. Kott

    Kott Guest

    "Albert Manfredi" <> wrote in message news:<>...
    > "Kott" <> wrote:
    >
    > > Hello -
    > > Could any one tell me which all fields in Ip header and UDP header
    > > does a Router typically process before doing a Routing across a routed
    > > interface ?
    > >
    > > Because I connect aross a Extreme box( with IP forwarding enabled) and
    > > the UDP/IP packet generated by my device crosses a routed boundary.
    > > But when I connect a Zyxel Presige 700 router and send the same UDP/IP
    > > packet from my device it does not cross the routed boundary.
    > >
    > > The size of the UDP/IP packet on the wire is 240 bytes.
    > > TOS field in IP header = 0x00
    > > IP version is =4
    > >
    > > Could any one throw some light on the way Layer 3 and Layer 4 fields
    > > are processed ?

    >
    > In principle, routers do not look at anything Layer 4, and therefore
    > should not care. However, many routers are set up with packet filters,
    > to prevent unwanted packets from crossing the subnet boundary. Could the
    > Prestige router possibly be set up to filter out UDP/IP packets? This
    > would not be uncommon. The filters should be configurable.
    >
    > In IPv4, the Protocol byte, the second byte in the third header word,
    > can be used by routers to determine what Transport Layer protocol is
    > being carried over the IP packet. If the byte is set to (decimal) 17,
    > it's UDP. Or, possibly, certain Port IDs are being filtered as well.
    >
    > Bert



    thanks for the responses.
    It is now working, in that that my packet is crossing a Cisco router
    interface. The change was , the TOS field in IP header was set to
    0x00. It was earlier set to 0xF0, not 0x00 as mentioned in earlier
    posting.
    Any ideas on why it was crossing Extreme routed interface with TOS
    set to 0xf0 ,but not a Cisco ?

    Also I have no policy or filters set on the boxes.



    Kott
    Thanks
     
    Kott, Jul 13, 2004
    #4
  5. "Kott" <> wrote:

    > It is now working, in that that my packet is crossing a Cisco router
    > interface. The change was , the TOS field in IP header was set to
    > 0x00. It was earlier set to 0xF0, not 0x00 as mentioned in earlier
    > posting.
    > Any ideas on why it was crossing Extreme routed interface with TOS
    > set to 0xf0 ,but not a Cisco ?


    In principle, the TOS byte should only be used as a suggestion to
    routers as to what kind of service the datagram requires. In the
    original design, IP was supposed to be a best effort protocol, the TOS
    byte should be used to select the best route when options exist, but not
    to block a datagram entirely.

    The TOS byte was set to 0xF0, or 111 100 00. According to the original
    meaning of these bits, RFC 1349, the first three bits mean that the
    datagram requires the highest "precedence," which would imply a "network
    control" datagram. These packets should therefore get the highest
    priority. The next three bits request the lowest delay, but not
    necessarily the highest throughput or the highest reliability. (Not sure
    that 111 100 makes a lot of sense. Seems to me that 111 would be
    followed by x x 1.) The lowest two bits are unused.

    The TOS byte gets redefined with RFC 2474, where the top 6 bits are now
    called a Differentiated Services Code Point. But the meaning of the
    first three bits remains compatible with the old precedence bits. So in
    principle, setting these to 1 values should not cause datagrams to be
    dropped. If anything, the opposite. These packets ought to bump off
    others, potentially.

    The last two bits are defined in RFC 3168, as "explicit congestion
    notification" (ECN) bits. Set to 00, the packet is not using ECN. So
    this is normal.

    So to cut to the chase, I don't know why the packets were being dropped.
    Unless the router got upset that the reliability bit was set to 0 when
    the precedence bits are set to 7.

    Bert
     
    Albert Manfredi, Jul 13, 2004
    #5
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