UDP/IP packet question

UDP/IP packet question
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Subject Author Date
UDP/IP packet question Kott 07-05-2004
Posted by Kott on July 5, 2004, 6:58 am
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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


NMFall 20%
Posted by on July 5, 2004, 2:17 pm
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> 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.


Posted by Albert Manfredi on July 5, 2004, 10:01 pm
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> 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



Posted by Kott on July 13, 2004, 5:39 am
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>
> > 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


Posted by Albert Manfredi on July 13, 2004, 3:59 pm
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> 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



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