You the man Paul! Many thanks. "Paul Werner" <[EMAIL PROTECTED]> wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > > Excuse me. I meant to ask if a UDP Packet can be > fragmented > > > > > > Yes. > > > > > > > and what happens > > > > to the UDP Header if it can be fragmented? > > > > > > Nothing. It's sent unchanged in the first fragment. The > only header > > that > > > gets modified and replicated in each fragment is the IP > header. > > > > So will each fragment have a bit of the UDP Header in it or I > >am correct in saying that only the first fragment has the FULL > >UDP >Header in it ... > > how does the destination router know how to put the UDP paket > back together? > > Priscilla et al. have appropriately addressed your original > question and follow on questions. What you have not asked (and > what has not been addressed) is what layers are performing > which functionality (and why). > > The User Datagram Protocol is an OSI transport layer protocol > that is designed to give a low overhead, unreliable (best > effort) datagram delivery. Internet standards documents > describe UDP as a connectionless datagram PDU, while Cisco > refers to it as a segment. Understand Cisco's version for > their tests, and understand the Internet version for real > life. One of the principal reasons for UDP, in addition to its > low overhead, is the ability to deliver datagrams from one to > many hosts. TCP is always set up as a one to one connection. > If you need to deliver UDP datagrams to a host, you must depend > upon the lower layers for delivery (as you do for TCP). > > In the case of delivery over mutliple types of media, > fragmentation may be required. This is handled as a layer 3 > (network layer) function. IP has as part of its protocol, the > ability to fragment data that will not traverse a low MTU > (maximum transmissible unit) path. A typical MTU might be 1500 > bytes in an Ethernet environment, but it might be much larger > in a Token Ring environment. One of the layers must make > accomodation for this situation, otherwise you will have lost > data (you can't fit 10 lbs of manure in a five pound bag :-) > This process can be optimized to a degree. Many Internet hosts > and gateways support the ability to do Path MTU discovery which > will allow for advanced discovery of the Path MTU and set it as > the default for transmission. In some cases this capability > works well, in other cases it does not. Additionally, some > hosts will have a "Do not Fragment" bit set on the IP header to > not allow any form of fragmentation. If this occurs and > fragmentation is required but not possible, an ICMP error > message will be sent to the originator indicating fragmentation > was required, but not possible. In some instances, if the > amount of datagrams that need to be fragmented cannot be > buffered by the receiving host to completion of reassembly, you > may receive a different ICMP error message indicating a > fragmentation failure. Either situation will require a > retransmission of all fragments. > > I would strongly recommend reading TCP/IP Illustrated by the > late W. Richard Stevens. It is a must read to understand the > TCP/IP protocol suite. > > HTH, > > Paul Werner > > ________________________________________________ > Get your own "800" number - Free > Free voicemail, fax, email, and a lot more > http://www.ureach.com/reg/tag > > **NOTE: New CCNA/CCDA List has been formed. For more information go to > http://www.groupstudy.com/list/Associates.html > _________________________________ > UPDATED Posting Guidelines: http://www.groupstudy.com/list/guide.html > FAQ, list archives, and subscription info: http://www.groupstudy.com > Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] > **NOTE: New CCNA/CCDA List has been formed. For more information go to http://www.groupstudy.com/list/Associates.html _________________________________ UPDATED Posting Guidelines: http://www.groupstudy.com/list/guide.html FAQ, list archives, and subscription info: http://www.groupstudy.com Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
