re: Source Route IP Options


postel@ISI.EDU
Fri, 18 Sep 87 10:04:49 PDT


Network Working Group J. Postel
Request for Comments: DRAFT J. Reynolds
                                                       XXXX 1987

                 Comments on the IP Source Route Option

Status of this Memo

   This RFC discusses a feature of the Internet Protocol (IP) used in
   the ARPA-Internet community, and requests discussion and suggestions
   for improvements of the description of this feature. Distribution of
   this memo is unlimited.

Introduction

   The purpose of this memo is to expand the discussion of the IP Source
   Route Option and to give some examples of its use.

Overview

   The IP Source Route option allows the originator (source host) of an
   IP datagram to specify a number of specific gateways the datagram
   must pass though in sequence before being delivered to the
   destination host.

The Source Route Concept

   The concept of the Source Route Option is to let the originator (or
   source) of the datagram specify a list of points (gateways) the
   datagram is to pass through on the way to its destination.

   This type of explicit routing is usually not necessary since the
   Internet gateways exchange routing information and route datagrams
   based only on the destination address (and possibly other factors
   such as type of service).

   One reason for using source routing might be to reach some part of
   the Internet via a path that the gateways somehow don't know about
   via their normal routing information exchange. Another reason may be
   to explicitly use or avoid certain networks for performance, or
   administrative reasons (such as privacy, access policy, or billing).
   A reason for using source routing that has been exercised with good
   results already is testing. Source routing allows sending datagrams
   that transit particular Internet paths testing either particular
   networks or particuar gateways (or both) from a remote monitoring
   host.

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RFC DRAFT XXXX 1987

   The source route is implemented by including an option in the IP
   header that lists additional addresses. That is, a source routed
   datagram starts out with the address of the first stop on the route
   in the IP header destination address field, and the addresses of
   subsequent stops as elements in the option list. At each stop, an
   address is taken from an element of the list in the option and placed
   in the destination address field and that element of the list is
   replaced by the address of that stop.

   There are three IP options: Strict Source Route (SSR), Loose Source
   Route (LSR) and Record Route (RR). Both SSR and LSR also record the
   route as well. The difference between SSR and LSR is that in SSR the
   route must specify gateway separated by only one network, but in LSR
   the path between stops on the specified route maybe any length and
   determined by normal gateway routing.

   The information in a source route is a list of 32-bit IP addresses
   and a pointer that indicates which address in the list is to be used
   next.

The following example shows the working of the option in simplified
form.

   Example 1:

      Suppose that the source is host A, the destination is host E and
      the gateways to be explicitly passed through are B, C, and D.

      +-----+ +-----+ +-----+ +-----+ +-----+
      | | | | | | | | | |
      | A |-----| B |-----| C |-----| D |-----| E |
      | | | | | | | | | |
      +-----+ +-----+ +-----+ +-----+ +-----+

      When the datagram is sent from the source host (A) into the
      Internet the Source Address (SA), Destination Address (DA), Source
      Route List (SRL) and Source Route Pointer (SRP) the fields are:

               SA: A
               DA: B
               SRL: C,D,E
               SRP: 0

      After the datagram arrives at gateway B, the gateway notices the
      source route option and transposes the address from the
      destination field and the next element of the source route list
      and increments the source route pointer. As the datagram leaves
      gateway B, the fields are:

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RFC DRAFT XXXX 1987

               SA: A
               DA: C
               SRL: B,D,E
               SRP: 1

      At gateway C, the processing is similar. The datagram leaves
      gateway C with its fields appearing:

               SA: A
               DA: D
               SRL: B,C,E
               SRP: 2

      At gateway D, the processing is similar. The datagram leaves
      gateway D with its fields appearing:

               SA: A
               DA: E
               SRL: B,C,D
               SRP: 3

      Finally, the datagram arrives at host E. Even though the source
      route option is still present, host E knows it is the final
      destination because the source route pointer now indicates that
      the source route list is exhausted.

   Example 1 was simplified to present the general concept. One detail
   that was omitted is that each gateway really has two (or more)
   addresses. When the option is processed, the address that must be
   stored back into the option field is the address for going in the
   reverse direction.

   Example 2:

      Suppose that the source is host A, with address IA on net I. The
      destination is host E, with address LE on net L. The gateways to
      be explicitly passed through are B, C, and D. Each gateway has
      two addresses, one on each directly connected network.

      +-----+ +-----+ +-----+ +-----+ +-----+
      | |IA IB| |JB JC| |KC KD| |LD LE| |
      | A |-------| B |-------| C |-------| D |-------| E |
      | | net I | | net J | | net K | | net L | |
      +-----+ +-----+ +-----+ +-----+ +-----+

      When the datagram is sent from the source host (A) into the
      Internet, the Source Address (SA), Destination Address (DA),
      Source Route List (SRL), and Source Route Pointer (SRP) the fields

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RFC DRAFT XXXX 1987

      are:

               SA: IA
               DA: IB
               SRL: JC,KD,LE
               SRP: 0

      After the datagram arrives at gateway B, the gateway notices the
      source route option and moves the address from the next element of
      the source route list to the destination field, places its own
      reverse direction address in the that element of the source route
      list, and increments the source route pointer. As the datagram
      leaves gateway B, the fields are:

               SA: IA
               DA: JC
               SRL: JB,KD,LE
               SRP: 1

      At gateway C, the processing is similar. The datagram leaves
      gateway C with its fields appearing:

               SA: IA
               DA: KD
               SRL: JB,KC,LE
               SRP: 2

      At gateway D, the processing is similar. The datagram leaves
      gateway D with its fields appearing:

               SA: IA
               DA: LE
               SRL: JB,KC,LD
               SRP: 3

      Finally, the datagram arrives at host E. Even though the source
      route option is still present, host E knows it is the final
      destination, because the source route pointer now indicates that
      the source route list is exhausted.

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RFC DRAFT XXXX 1987

An Explicit Detailed Example

   Recall the format of the IP header:

     0 1 2 3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Version| IHL |Type of Service| Total Length |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Identification |Flags| Fragment Offset |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Time to Live | Protocol | Header Checksum |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Source Address |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Destination Address |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Options | Padding |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format of the the source route option is shown in the extract from
RFC-791 in Appendix A.

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RFC DRAFT XXXX 1987

   Example 3:

      Suppose that the source is host ISI-ARK, with address 128.9.0.12
      on ISI-NET, the destination is host DMC-CRC, with address
      128.43.0.1 on DRENET and the gateways to be explicitly passed
      through are ISI-WB-GW, LL-GW, and CRC-GW. Each gateway has two
      addresses, one on each directly connected network.

      +-----+
      | | 128.9.0.12
      |ISI |-------
      | ARK| /ISI-NET
      +-----+ /
                /
               /
              / +-----+ / |ISI | 28.45.0.0
             -------| WB |-------
         128.9.0.25 | GW| /WBNET
                    +-----+ /
                              /
                             / +-----+
                            / | | 10.5.0.10
                           -------|LL-GW|-------
                        28.19.0.0 | | /ARPANET
                                  +-----+ /
                                            /
                                           / +-----+
                                          / | | 128.43.1.1
                                         -------|CRC |-------
                                      10.1.0.15 | GW| /DRENET
                                                +-----+ /
                                                          /
                                                         / +-----+
                                                        / | |
                                                       -------|DMC |
                                                   128.43.0.1 | CRC|
                                                              +-----+

      When the datagram is sent from the source host (ISI-ARK) into the
      Internet the Source Address (SA), Destination Address (DA), Source
      Route List (SRL), and Source Route Pointer (SRP) the fields are:

               SA: 128.9.0.12
               DA: 128.9.0.25
               SRL: 28.19.0.0, 10.1.0.15, 128.43.0.1
               SRP: 0

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RFC DRAFT XXXX 1987

    0 1 2 3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Version| IHL |Type of Service| Total Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Identification |Flags| Fragment Offset |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Time to Live | Protocol | Header Checksum |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 128 | 9 | 0 | 12 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 128 | 9 | 0 | 25 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 137 | 15 | 4 | 28 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 19 | 0 | 0 | 10 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1 | 0 | 15 | 128 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 43 | 0 | 1 | Padding |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      After the datagram arrives at gateway ISI-WB-GW, the gateway,
      notices the source route option and moves the address from the
      next element of the source route list to the destination field,
      places its own reverse direction address in the that element of
      the source route list, and increments the source route pointer.
      As the datagram leaves gateway ISI-WB-GW, the fields are:

               SA: 128.9.0.12
               DA: 28.19.0.0
               SRL: 28.45.0.0, 10.1.0.15, 128.43.0.1
               SRP: 1

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RFC DRAFT XXXX 1987

    0 1 2 3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Version| IHL |Type of Service| Total Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Identification |Flags| Fragment Offset |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Time to Live | Protocol | Header Checksum |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 128 | 9 | 0 | 12 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 28 | 19 | 0 | 0 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 137 | 15 | 8 | 28 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 45 | 0 | 0 | 10 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1 | 0 | 15 | 128 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 43 | 0 | 1 | Padding |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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RFC DRAFT XXXX 1987

      At gateway LL-GW, the processing is similar. The datagram leaves
      gateway LL-GW with its fields appearing:

               SA: 128.9.0.12
               DA: 10.1.0.15,
               SRL: 28.45.0.0, 10.5.0.10, 128.43.0.1
               SRP: 2

    0 1 2 3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Version| IHL |Type of Service| Total Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Identification |Flags| Fragment Offset |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Time to Live | Protocol | Header Checksum |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 128 | 9 | 0 | 12 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 10 | 1 | 0 | 15 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 137 | 15 | 12 | 28 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 45 | 0 | 0 | 10 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 5 | 0 | 10 | 128 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 43 | 0 | 1 | Padding |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      At gateway CRC-GW, the processing is similar. The datagram leaves
      gateway CRC-GW with its fields appearing:

               SA: 128.9.0.12
               DA: 128.43.0.1
               SRL: 28.45.0.0, 10.5.0.10, 128.43.1.1
               SRP: 3

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RFC DRAFT XXXX 1987

    0 1 2 3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Version| IHL |Type of Service| Total Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Identification |Flags| Fragment Offset |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Time to Live | Protocol | Header Checksum |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 128 | 9 | 0 | 12 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 128 | 43 | 0 | 1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 137 | 15 | 16 | 28 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 45 | 0 | 0 | 10 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 5 | 0 | 10 | 128 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 43 | 1 | 1 | Padding |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Finally, the datagram arrives at host DMC-CRC. Even though the
      source route option is still present, host DMC-CRC knows it is the
      final destination because the source route pointer now indicates
      that the source route list is exhausted.

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RFC DRAFT XXXX 1987

Summary *****-----> [words here]

References

   [1] Postel, J. (ed.), "Internet Protocol - DARPA Internet Program
         Protocol Specification," RFC 791, USC/Information Sciences
         Institute, September 1981.

   [2] DoD Military Standard, "Internet Protocol", MIL-STD-1777,
         Department of Defense, August 1983.

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Appendix A -- Extracts from RFC-791

   Note: Editors notes and corrections are enclosed in angle brackets
   ("<", ">"), and text that should be deleted is enclosed in braces
   ("{","}").

   Loose Source and Record Route

         +--------+--------+--------+---------//--------+
         |10000011| length | pointer| route data |
         +--------+--------+--------+---------//--------+
          Type=131

      The loose source and record route (LSRR) option provides a means
      for the source of an internet datagram to supply routing
      information to be used by the gateways in forwarding the datagram
      to the destination, and to record the route information.

      The option begins with the option type code. The second octet is
      the option length which includes the option type code and the
      length octet, the pointer octet, and length-3 octets of route
      data. The third octet is the pointer into the route data
      indicating the octet which begins the next source address to be
      processed. The pointer is relative to this option, and the
      smallest legal value for the pointer is 4.

      A route data is composed of a series of internet addresses. Each
      internet address is 32 bits or 4 octets. If the pointer is
      greater than the length, the source route is empty (and the
      recorded route full) and the routing is to be based on the
      destination address field.

      If the address in destination address field has been reached and
      the pointer is not greater than the length, the next address in
      the source route replaces the address in the destination address
      field, and the recorded route address replaces the source <route>
      address just used, and pointer is increased by four.

      The recorded route address is the internet module's own internet
      address as known in the environment into which this datagram is
      being forwarded.

      This procedure of replacing the source route with the recorded
      route (though it is in the reverse of the order, it must be in to
      be used as a source route) means the option (and the IP header as
      a whole) remains a constant length as the datagram progresses
      through the internet.

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RFC DRAFT XXXX 1987

      This option is a loose source route because the gateway or host IP
      is allowed to use any route of any number of other intermediate
      gateways to reach the next address in the route.

      Must be copied on fragmentation. Appears at most once in a
      datagram.

   Strict Source and Record Route

         +--------+--------+--------+---------//--------+
         |10001001| length | pointer| route data |
         +--------+--------+--------+---------//--------+
          Type=137

      The strict source and record route (SSRR) option provides a means
      for the source of an internet datagram to supply routing
      information to be used by the gateways in forwarding the datagram
      to the destination, and to record the route information.

      The option begins with the option type code. The second octet is
      the option length which includes the option type code and the
      length octet, the pointer octet, and length-3 octets of route
      dAata. The third octet is the pointer into the route data
      indicating the octet which begins the next source address to be
      processed. The pointer is relative to this option, and the
      smallest legal value for the pointer is 4.

      A route data is composed of a series of internet addresses. Each
      internet address is 32 bits or 4 octets. If the pointer is
      greater than the length, the source route is empty (and the
      recorded route full) and the routing is to be based on the
      destination address field.

      If the address in the destination address field has been reached
      and the pointer is not greater than the length, the next address
      in the source route replaces the address in the destination
      address field, and the recorded route address replaces the source
      <route> address just used, and pointer is increased by four.

      The recorded route address is the internet module's own internet
      address as known in the environment into which this datagram is
      being forwarded.

      This procedure of replacing the source route with the recorded
      route (though it is in the reverse of the order, it must be in to
      be used as a source route) means the option (and the IP header as
      a whole) remains a constant length as the datagram progresses
      through the internet.

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RFC DRAFT XXXX 1987

      This option is a strict source route because the gateway or host
      IP must send the datagram directly to the next address in the
      source route through only the directly connected network indicated
      in the next address to reach the next gateway or host specified in
      the route.

      Must be copied on fragmentation. Appears at most once in a
      datagram.

   Record Route

         +--------+--------+--------+---------//--------+
         |00000111| length | pointer| route data |
         +--------+--------+--------+---------//--------+
           Type=7

      The record route option provides a means to record the route of an
      internet datagram.

      The option begins with the option type code. The second octet is
      the option length which includes the option type code and the
      length octet, the pointer octet, and length-3 octets of route
      data. The third octet is the pointer into the route data
      indicating the octet which begins the next area to store a route
      address. The pointer is relative to this option, and the smallest
      legal value for the pointer is 4.

      A recorded route is composed of a series of internet addresses.
      Each internet address is 32 bits or 4 octets. If the pointer is
      greater than the length, the recorded route data area is full.
      The originating host must compose this option with a large enough
      route data area to hold all the address expected. The size of the
      option does not change due to adding addresses. The intitial
       contents of the route data area must be zero.

      When an internet module routes a datagram it checks to see if the
      record route option is present. If it is, it inserts its own
      internet address as known in the environment into which this
      datagram is being forwarded into the recorded route beginning at
      the octet indicated by the pointer, and increments the pointer by
      four.

      If the route data area is already full (the pointer exceeds the
      length) the datagram is forwarded without inserting the address
      into the recorded route. If there is some room but not enough
      room for a full address to be inserted, the original datagram is
      considered to be in error and is discarded. In either case, an
      ICMP parameter problem message may be sent to the source host [3].

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      Not copied on fragmentation, goes in first fragment only. Appears
      at most once in a datagram.

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Appendix B -- Extracts from MIL-STD-1777

   Note: Editors notes and corrections are enclosed in angle brackets
   ("<", ">"), and text that should be deleted is enclosed in braces
   ("{","}").

   9.2.1.2 Routing options.

      IP provides a mechanism, called source routing, to supplement the
      gateway's independent routing decisions. This mechanism allows an
      upper layer protocol to influence the gateway route in which a
      datagram traverses. The UPL can pass a list of internet
      addresses, called a source route list, as one of the SEND service
      request parameters. Each address in the list, except for the
      last, is an intermediate gateway destination. The last address on
      the list is the final destination. The source IP module uses its
      normal routing mechanism to transmit the datagram to the first
      address in the source route list. Then the gateway IP replaces
      source route list entry with its own address as known in the
      environment into which it is forwarding the datagram. Thus, the
      datagram follows the source route while recording its "inverse" or
      recorded route.

   9.2.1.2.1 Routing types.

      Two kinds of source routing are proviced by IP: loose and strict.
      With loose source routing, the host and gateway IP modules along
      the route may use any number of other intermediate gateways to
      reach the addresses in the source list. With strict source
      routing, the datagram must travel directly (i.e., through only the
      directly connected subnetwork indicated by each address) to each
      address on the source list. When the source route cannot be
      followed, the source host IP is notified with an error message.
      For testing or diagnostic purposes, a ULP can acquire a datagram's
      record route (independent of the source route option) by using the
      record route mechanism. The sending ULP supplies an empty record
      route list and indicates that the gateway route is to be recorded
      in transit. Then, as each gateway IP module on the gateway route
      relays the datagram, it adds its address as known in the
      succeeding environment to the record route list. The destination
      ULP receives the original datagram along with the record route
      list which, if reversed, provides a source route to the sending
      ULP. If more gateways are traversed than can be recorded in the
      list, the additional gateway addresses are not recorded. Problems
      with the record route option discovered in transit are reported to
      the source host IP. When using a routing otion, the source ULP
      must provide a large enough route list to accommodate all the
      routing information expected. The size of a routing option does

Postel & Reynolds [Page 16]

RFC DRAFT XXXX 1987

      not change due to adding addresses.

   9.3.15.4 Loose source and record route.

      option type: 131 option length: variable

      The loose source route option provides a way for the source ULP of
      a datagram to supply routing information to be used by IP modules
      along the gateway route. At the same time, the "inverse" route is
      recorded in the option field. This option {is not} <must be>
      copied on fragmentation. It appears at most once in a datagram.
      The option begins with the option type code. The second octet is
      the option length which includes the option type octet, the length
      octet, the pointer octet, and the source route list. The third
      octet is a pointer into the route data indicating the octet which
      begins the next source address to be processed. The pointer is
      relative to this option, and its smallest legal value is 4. A
      loose source route list is composed of one or more internet
      addresses identifying intermediate gateways to be visited in
      transit. Each internet address is 4 octets long. When a gateway
      in the source route list is visited, the gateway address (as known
      in the environment into which the datagram is being forwarded)
      replaces that list entry <while that list entry replaces the
      destination address>. The size of this option is fixed by the
      source. It cannot change to accommodate additional information.
      The routing options are described in section {9.2.1.1} <9.2.1.2>.

   9.3.15.5 Strict source and record route.

      option type: 137 option length: variable

      The strict source route option provides a way for the source ULP
      of a datagram to name the exact set of IP modules to be visited
      along the gateway route. At the same time, the "inverse" route is
      recorded in the option field. This option must be copied on
      fragmentation. It appears at most once in a datagram. The option
      begins with the option type code. The second octet is the option
      length which includes the option type octet, the length octet, the
      pointer octet, and the source route list. The third octet is a
      pointer into the route data indicating the octet which begins the
      next source address to be processed. The pointer is relative to
      this option, and its smallest legal value is 4. A strict source
      route list is composed of one or more internet addresses
      identifying the gateways to be visited in transit. The datagram
      must visit exactly the gateways listed, traversing only the
      directly connected subnetworks indicated in the route list
      addresses. When a gateway in the source route list is visited,
      the gateway address (as known in the environment into which the

Postel & Reynolds [Page 17]

RFC DRAFT XXXX 1987

      datagram is being forwarded) replaces that list entry <while that
      list entry replaces the destination address>. The size of this
      option is fixed by the source. It cannot change to accommodate
      additional information. Routing options are described in section
      {9.2.1.1} <9.2.1.2>.

   9.3.15.6 Record route.

      option type: 7 option length: variable

      The record route option provides a way to record a datagram's
      gateway route. This option is not copied on fragmentation. It
      appears at most once in a datagram. The option begins with the
      option type code. The second octet is the option length which
      includes the option type code, the length octet, and the return
      route list. The third octet is a pointer into the route data
      indicating the octet which begins the next area to store a route
      address. The pointer is relative to this option, and the smallest
      legal value for the pointer is 4. A record route list is composed
      of a series of internet addresses. Each internet address is 4
      octets long. The source ULP provides a route list with zero value
      entries. As each gateway is visited in transit, it registers its
      address in the next free entry (indicated by the pointer). When
      the pointer is greater than the length, the record route list is
      full. No additional addresses are recorded, even if more are
      visited before arriving at the destination. The size of this
      option is fixed by the source. It cannot cnange to accommodate
      additional information. The routing options are described in
      section {9.2.1.1} <9.2.1.2>.

   9.4.6.3.13 Route.

Editors' Notes

Postel & Reynolds [Page 18]



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