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From: brescia@CCV.BBN.COM (Mike Brescia)
Newsgroups: mod.protocols.tcp-ip
Subject: Re: IP source routing questions
Message-ID: <8610091126.AA09239@ucbvax.Berkeley.EDU>
Date: Tue, 7-Oct-86 12:39:55 EDT
Article-I.D.: ucbvax.8610091126.AA09239
Posted: Tue Oct  7 12:39:55 1986
Date-Received: Thu, 9-Oct-86 20:46:34 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 47
Approved: tcp-ip@sri-nic.arpa

Steve,

The following points are important:
- the IP source field remains constant, so that the higher level protocol can
  ignore the recorded route
- the IP destination field reflects the current goal, and not the ultimate
  destination, until the source route has been consumed
- the offset includes the first 3 bytes and is zero-based.  "... the smallest
  legal value for the pointer is 4."

          host A           gate B           gate C           host D
         +------+         +------+         +------+         +------+
         |      |         |      |         |      |         |      |
         |    A1|-------->|B1  B2|-------->|C2  C3|-------->|D3    |
         |      |  net 1  |      |  net 2  |      |  net 3  |      |
         +------+         +------+         +------+         +------+

    (..host..)    source:A1  destination:B1  route:B1,C2,D3  offset:4
     on net 1:    source:A1  destination:B1  route:A1,C2,D3  offset:8
     on net 2:    source:A1  destination:C2  route:A1,B2,D3  offset:12
     on net 3:    source:A1  destination:D3  route:A1,B2,C3  offset:16

What you show is the way TOPS20 has implemented it, where host A has put a
record of its outgoing interface into the packet.  The unix implementations
do not do this, and the source route option only has 2 hops.

Here is an example of unix flavor source routed packet, with an octal dump of
the packet on the way out of teh source host, and another on receipt at the
destination (the same host in this case).  Note also the extra pad (1) option.

A1=10.2.0.82
B1=10.2.0.5     B2=128.89.0.5
		C2=128.89.0.4   C3=10.3.0.82
				D3=10.2.0.82
Outgoing packet:
   110      0      0     54      0      0    100      0     31     24 
   263     53     12      2      0    122     12      2      0      5 
     1    203     13      4    200    131      0      4     12      2 
     0    122      1    226      0      0      0      0      0    305 
   147    244    226      0 

Incoming packet:
   110      0      0     54      0      0    100      0     27     24 
   264    324     12      2      0    122     12      2      0    122 
     1    203     13     14    200    131      0      5     12      3 
     0    122      1    226      0      0      0      0      0    305 
   147    244    226      0