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TFTP端口问题
作者: TFTP 点击:
请教：TFTP端口问题？

从资料中TFTP默认使用UDP 69端口工作，但在实际中，从TFTP服务器上传或下载一个文件，只有第一个数据包的TFTP服务器会使用UDP 69端口，第二个数据包开始TFTP服务器端均不再使用69端口。
疑问：
1.为什么从第二个数据包开始，TFTP服务器就不再使用UDP 69？
2.不使用UDP 69，而使用其它的某个端口，这个端口是怎么得出来的，随机，还是什么算法？
 
 
回答：
 
仔细看这个里面的说明Initial Connection Protocol

   A transfer is established by sending a request (WRQ to write onto a
   foreign file system, or RRQ to read from it), and receiving a
   positive reply, an acknowledgment packet for write, or the first data
   packet for read.  In general an acknowledgment packet will contain
   the block number of the data packet being acknowledged.  Each data
   packet has associated with it a block number; block numbers are
   consecutive and begin with one.  Since the positive response to a
   write request is an acknowledgment packet, in this special case the
   block number will be zero.  (Normally, since an acknowledgment packet
   is acknowledging a data packet, the acknowledgment packet will
   contain the block number of the data packet being acknowledged.)  If
   the reply is an error packet, then the request has been denied.

   In order to create a connection, each end of the connection chooses a
   TID for itself, to be used for the duration of that connection.  The
   TID's chosen for a connection should be randomly chosen, so that the
   probability that the same number is chosen twice in immediate
   succession is very low.  Every packet has associated with it the two
   TID's of the ends of the connection, the source TID and the
   destination TID.  These TID's are handed to the supporting UDP (or
   other datagram protocol) as the source and destination ports.  A
   requesting host chooses its source TID as described above, and sends
   its initial request to the known TID 69 decimal (105 octal) on the
   serving host.  The response to the request, under normal operation,
   uses a TID chosen by the server as its source TID and the TID chosen
   for the previous message by the requestor as its destination TID.
   The two chosen TID's are then used for the remainder of the transfer.

   As an example, the following shows the steps used to establish a
   connection to write a file.  Note that WRQ, ACK, and DATA are the
   names of the write request, acknowledgment, and data types of packets
   respectively.  The appendix contains a similar example for reading a
   file.

      1. Host A sends  a  "WRQ"  to  host  B  with  source=  A's  TID,
         destination= 69.

      2. Host  B  sends  a "ACK" (with block number= 0) to host A with
         source= B's TID, destination= A's TID.

   At this point the connection has been established and the first data
   packet can be sent by Host A with a sequence number of 1.  In the
   next step, and in all succeeding steps, the hosts should make sure
   that the source TID matches the value that was agreed on in steps 1
   and 2.  If a source TID does not match, the packet should be
   discarded as erroneously sent from somewhere else.  An error packet
   should be sent to the source of the incorrect packet, while not
   disturbing the transfer.  This can be done only if the TFTP in fact
   receives a packet with an incorrect TID.  If the supporting protocols
   do not allow it, this particular error condition will not arise.

   The following example demonstrates a correct operation of the
   protocol in which the above situation can occur.  Host A sends a
   request to host B. Somewhere in the network, the request packet is
   duplicated, and as a result two acknowledgments are returned to host
   A, with different TID's chosen on host B in response to the two
   requests.  When the first response arrives, host A continues the
   connection.  When the second response to the request arrives, it
   should be rejected, but there is no reason to terminate the first
   connection.  Therefore, if different TID's are chosen for the two
   connections on host B and host A checks the source TID's of the
   messages it receives, the first connection can be maintained while
   the second is rejected by returning an error packet.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

如果不用新的端口，那tftp同时只能传一个文件。。。
这种把UDP端口直接应用为transaction ID的方式真是简洁
 
 
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