> > > 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]
