rfc823.txt

来自「RFC 的详细文档！」· 文本 代码 · 共 2,611 行 · 第 1/5 页

     DARPA Internet Gateway                             September 1982
     RFC 823



     gateway is directly connected to the network.  A neighbor gateway

     is  one  which  shares  a  common network with this gateway.  The

     distance metric that is  used  to  determine  which  neighbor  is

     closest  is  defined  as the "number of hops," where a gateway is

     considered to be zero hops from its directly connected  networks,

     one  hop  from a network that is reachable via one other gateway,

     etc.  The Gateway-to-Gateway Protocol (GGP) is used to update the

     Routing  Table (see Section 4.4 describing the Gateway-to-Gateway

     Protocol).


          The gateway tries to match the destination  network  address

     in  the IP header of the datagram to be forwarded, with a network

     in its Routing Table.  If no match is found,  the  gateway  drops

     the datagram and sends an ICMP Destination Unreachable message to

     the IP source.  If the gateway does find an entry for the network

     in  its  table,  it  will use the network address of the neighbor

     gateway entry as the local network  destination  address  of  the

     datagram.   However, if the final destination network is one that

     the gateway is directly connected to, the destination address  in

     the  local network header is created from the destination address

     in the IP header of the datagram.







                                    -8-





     DARPA Internet Gateway                             September 1982
     RFC 823



     3.4  Redirects


          If the routing procedure decides that an IP datagram  is  to

     be  sent back out the same network interface that it was read in,

     then this gateway is not on the shortest path  to  the  IP  final

     destination.   Nevertheless, the datagram will still be forwarded

     to the next address chosen by  the  routing  procedure.   If  the

     datagram  is  not  using  the  IP Source Route Option, and the IP

     source network of the datagram is the same as the network of  the

     next  gateway  chosen  by the routing procedure, an ICMP Redirect

     message will be sent  to  the  IP  source  host  indicating  that

     another  gateway  should  be used to send traffic to the final IP

     destination.




     3.5  Fragmentation


          The datagram is passed to the  fragmentation  routine  after

     the  routing decision has been made.  If the next network through

     which the datagram must pass has a maximum message size  that  is

     smaller  than  the  size  of  the  datagram, the datagram must be

     fragmented.   Fragmentation  is  performed   according   to   the

     algorithm  described  in the Internet Protocol Specification [1].

     Certain IP options must be copied  into  the  IP  header  of  all



                                    -9-





     DARPA Internet Gateway                             September 1982
     RFC 823



     fragments, and others appear only in the first fragment according

     to the IP specification.  If a datagram must be  fragmented,  but

     the  Don't  fragment bit is set, the datagram is discarded and an

     ICMP error message is sent to the IP source of the datagram.




     3.6  Header Rebuild


          The datagram (or the fragments of the original  datagram  if

     fragmentation  was  needed)  is  next  passed  to  a routine that

     rebuilds  the  Internet  header.  The  Time  to  Live  field   is

     decremented by one and the IP checksum is recomputed.


          The  local  network  header  is  now   built.    Using   the

     information  obtained  from  its  routing  procedure, the gateway

     chooses the network interface it considers  proper  to  send  the

     datagram  and  to  build  the  destination  address  in the local

     network header.




     3.7  Output


          The datagram is now enqueued on an output queue for delivery

     towards  its destination.  A limit is enforced on the size of the

     output queue for each network interface so that  a  slow  network



                                   -10-





     DARPA Internet Gateway                             September 1982
     RFC 823



     does  not  unfairly  use  up  all of the gateway's buffers.  If a

     datagram cannot be enqueued due to the limit on the output  queue

     length, it is dropped and an HMP trap is sent to the INOC.  These

     traps, and others of a similar nature, are  used  by  operational

     personnel to monitor the operations of the gateways.






































                                   -11-





     DARPA Internet Gateway                             September 1982
     RFC 823



     4  PROTOCOLS SUPPORTED BY THE GATEWAY


          A number of  protocols  are  supported  by  the  gateway  to

     provide   dynamic   routing,  monitoring,  debugging,  and  error

     reporting.  These protocols are described below.




     4.1  Cross-Net Debugging Protocol


          The Cross-Net Debugging Protocol (XNET) [8] is used to  load

     the  gateway  and  to  examine  and  deposit  data.   The gateway

     supports the following XNET op-codes:


          o  NOP
          o  Debug
          o  End Debug
          o  Deposit
          o  Examine
          o  Create Process




     4.2  Host Monitoring Protocol


          The Host Monitoring Protocol (HMP) [6] is  used  to  collect

     measurements   and   status   information   from   the  gateways.

     Exceptional conditions in the gateways are reported in HMP traps.

     The status of a gateway's interfaces, neighbors, and the networks

     which it can reach are reported in the HMP status message.



                                   -12-





     DARPA Internet Gateway                             September 1982
     RFC 823



          Two types of gateway statistics, the Host Traffic Matrix and

     the  gateway  throughput,  are currently defined by the HMP.  The

     Host Traffic Matrix records the number  of  datagrams  that  pass

     through  the  gateway  with  a  given IP source, destination, and

     protocol number.   The  gateway  throughput  message  collects  a

     number  of  important counters that are kept by the gateway.  The

     current gateway reports the following values:


          o  Datagrams dropped because destination net unreachable

          o  Datagrams dropped because destination host unreachable


          o  Per Interface:
                  Datagrams received with IP errors
                  Datagrams received for this gateway
                  Datagrams received to be forwarded
                  Datagrams looped
                  Bytes received
                  Datagrams sent, originating at this gateway
                  Datagrams sent to destination hosts
                  Datagrams dropped due to flow control limitations
                  Datagrams dropped due to full queue
                  Bytes sent

          o  Per Neighbor:
                  Routing updates sent to
                  Routing updates received from
                  Datagrams sent, originating here
                  Datagrams forwarded to
                  Datagrams dropped due to flow control limitations
                  Datagrams dropped due to full queue
                  Bytes sent







                                   -13-





     DARPA Internet Gateway                             September 1982
     RFC 823



     4.3  ICMP


          The gateway will generate the following ICMP messages  under

     appropriate  circumstances  as  defined by the ICMP specification

     [4]:


          o  Echo Reply
          o  Destination Unreachable
          o  Source Quench
          o  Redirect
          o  Time Exceeded
          o  Parameter Problem
          o  Information Reply




     4.4  Gateway-to-Gateway Protocol


          The gateway uses the Gateway-to-Gateway  Protocol  (GGP)  to

     determine  connectivity  to networks and neighbor gateways; it is

     also used in  the  implementation  of  a  dynamic,  shortest-path

     routing  algorithm.  The current GGP message formats (for release

     1003 of the gateway software) are presented in Appendix A.




     4.4.1  Determining Connectivity to Networks


          When a gateway  starts  running  it  assumes  that  all  its

     neighbor  gateways  are  "down,"  that  it  is  disconnected from




                                   -14-





     DARPA Internet Gateway                             September 1982
     RFC 823



     networks to which it is attached, and that the distance  reported

     in  routing  updates  from  each  neighbor  to  each  network  is

     "infinity."


          The gateway first determines the state of  its  connectivity

     to  networks  to  which it is physically attached.  The gateway's

     connection to a network is declared up if it can send and receive

     internet  datagrams  on its interface to that network.  Note that

     the method that the gateway uses to determine its connectivity to

     a  network  is network-dependent.  In some networks, the host-to-

     network protocol determines whether or not datagrams can be  sent

     and  received  on  the  host  interface.   In these networks, the

     gateway simply checks-status information provided by the protocol

     in order to determine if it can communicate with the network.  In

     other networks, where  the  host-to-network  protocols  are  less

     sophisticated,  it  may  be  necessary  for  the  gateway to send

     datagrams to itself to determine if it can communicate  with  the

     network.   In  these networks, the gateways periodically poll the

     network using GGP network interface status messages [Appendix  A]

     to determine if the network interface is operational.


          The gateway has two rules relevant to computing distances to

     networks:   1) if the gateway can send and receive traffic on its




                                   -15-





     DARPA Internet Gateway                             September 1982
     RFC 823



     network interface, its distance to the network is zero;  2) if it

     cannot send and receive traffic on the interface, its distance to

     the network is "infinity."  Note  that  if  a  gateway's  network

     interface is not working, it may still be able to send traffic to

     the network on  an  alternate  route  via  one  of  its  neighbor

     gateways.




     4.4.2  Determining Connectivity to Neighbors


          The gateway determines connectivity to neighbors using a  "K

     out  of  N"  algorithm.   Every 15 seconds, the gateway sends GGP

     Echo messages  [Appendix  A]  to  each  of  its  neighbors.   The

     neighbors  respond  by  sending GGP echo replies.  If there is no

     reply to K out of  N  (current  values  are  K=3  and  N=4)  echo

     messages sent to a neighbor, the neighbor is declared down.  If a

     neighbor is down and J out of M (current values are J=2 and  M=4)

     echo  replies  are  received,  the neighbor is declared to be up.

     The values of J,K,M,N  and  the  time  interval  are  operational

     parameters which can be adjusted as required.










                                   -16-