Re: Data Description Exchange

[Shreyas <[e-mail]>]

Thanks for your reply Acee.

But, I have one more question regarding the packet
exchange during exStart/exChange period.

During that transition phase, I am trying to send DD
packets like as I mentioned in this mail. But when I
sniff the network or looking at my OSPF router, I can
see it did not understand the packet properly and
network analyser only shows packet information upto
Data Description and simply ignores LSA Header. (I
know it may be due to checksum in LSA Header).

Any Idea/thoughts on this is really appreciated! In
other words, how I can include LSA header information
in this packet!


Thanks!
- SB

//
// ----- Ethernet II -----
//

define ospf [ 00 20 9c 23 73 e8
        08 00 20 ac 9d d5
        08 00

//
// ----- Internet Protocol -----
// ----- 20 bytes -----
//

45
00
00 34                     // Total Length
5a a0                     // Identification
00                        // Flags
00                        // Frame Offset
40                        // TTL
59                        // Protocol (OSPF)
00 00                     // Checksum
0a 65 01 43               // Source
0a 65 01 41               // Destination

//
// ----- OSPF Header -----
//

02                        // OSPF Version
02                        // OSPF Packet Type (Hello
Pkt)
00 20                     // Packet Length
0a 65 01 43                       // Source OSPF Router ID
0a 65 01 40                       // Area ID
00 00                     // Packet Checksum
00 00                     // Auth Type (None)
00 00 00 00               // Auth Date (None)
00 00 00 00

//
// ----- OSPF Data Description Info -----
//

05 dc                     // Interface MTU
02                        // OSPF Version
00                        // Master
00 00 00 02                       // DD Sequence

// ------- LSA Header Information ------
//

00 01                   // LS Age
02                      // Options
01                      // Router LSA
0a 65 01 43             // Link State ID
0a 65 01 43             // Adv. Router
0a 00 00 00             // LS Sequence
00 00                   // Checksum
00 14                   // Length
];

xsum ospf[24] 14 20;
xsum ospf[46] 34 32;
xsum ospf[82] 68 20;

send ospf;



--- Acee Lindem <[e-mail]> wrote:
> Shreayas Becker wrote:
>
> > Hi,
>
>
> Hi Shreayas,
>
>
> >
> > I have few questions regarding DD exchange to
> understand what we can
> > implement/others implemented while trying form an
> adjacency.
> >
> > Question:
> >
> > Before changing into FULL state, I would like to
> know whether we need to
> > send LSA header information in DD packets.
>
>
> Yes. You need to synchronize databases to reach FULL
> state.
>
>
> >
> > If so, how we are going to calculate checksum for
> this LSA header (i.e.
> > just omit LS Age Information) and
>
>
> No. The LSA checksum unmodified (i.e., for the
> entire LSA) is sent
> in the LSA header in the DD packets.
>
>
> >
> > What length one has to specify while mentioning
> its packet length (i.e.
> > even though, we are just going to send LSA Header,
> do we need to say Full
> > LSA packet size!)
>
>
> You always specify the real OSPF packet length in
> the OSPF packet header.
> In other words, you specify only take into account
> the 20 byte LSA header
> since that is what you actually put in the packet.
>
>
> >
> > Also, during exStart->exChange->Full transition,
> is it possible to form an
> > adjacency without sending LSA Header info in its
> DD packets.
>
> No. You'd always have at least your router LSA.
>
> >
> > Any Suggestions or comments are welcome!
>
> You might want to re-read the first 4 sections of
> RFC 2328 and then
> consult the details as necessary.
>
> Good Luck,
> Acee
>
> >
> > Thanks and looking forward to all your replies!
> >
> > - SB
> >
> >
>
>
> --
> Acee


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