RFC 795

SERVICE MAPPINGS ✨

Network Working Group · J. Postel (ISI) · September 1981 🌸 Request for Comments: 795 Category/status: not stated in the title block 💅 Obsoletes/Updates: nothing listed here 🫶

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Okay bestie, so this memo is all about how the Internet Protocol (IP) [1] Type of Service lines up with the service settings of specific networks. It's giving "translation guide" and I'm obsessed 🦋

The IP Type of Service Fields ✨

Okay so the IP Type of Service has a few fields 💖

• Bits 0-2: Precedence.
• Bit 3: 0 = Normal Delay, 1 = Low Delay.
• Bit 4: 0 = Normal Throughput, 1 = High Throughput.
• Bit 5: 0 = Normal Relibility, 1 = High Relibility.
• Bits 6-7: Reserved for Future Use 🔮

Here's how it looks all lined up 👑

      0     1     2     3     4     5     6     7
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |                 |     |     |     |     |     |
   |   PRECEDENCE    |  D  |  T  |  R  |  0  |  0  |
   |                 |     |     |     |     |     |
   +-----+-----+-----+-----+-----+-----+-----+-----+

And the precedence values, from the highest-key to the lowest-key 🌈

   111 - Network Control
   110 - Internetwork Control
   101 - CRITIC/ECP
   100 - Flash Override
   011 - Flash
   010 - Immediate
   001 - Priority
   000 - Routine

The individual networks down below have really different, really specific service choices, so each one gets its own little mapping 🧚

AUTODIN II 💅

Okay so AUTODIN II splits its service choices into two parts: Traffic Acceptance Catagories and Application Type 🎀 The Traffic Acceptance Catagories can be mapped into and out of the IP TOS precedence reasonably directly, which is honestly so clean 😭 The Application types get mapped into the leftover IP TOS fields like this:

      TA    DELAY    THROUGHPUT    RELIABILITY
      ---   -----    ----------    -----------
      I/A     1           0             0
      Q/R     0           0             0
      B1      0           1             0
      B2      0           1             1

And the DTR-to-TA mapping, which is giving puzzle energy 🧩

      DTR    TA
      ---   ---
      000   Q/R
      001   Q/R
      010    B1
      011    B2
      100   I/A
      101   I/A
      110   I/A
      111   error

ARPANET 🛸

ARPANET is lowkey limited, not gonna lie 🙈 There is one priority bit, and it can be mapped to the high order bit of the IP TOS precedence. The other choices are regular ("Type 0") messages vs. uncontrolled ("Type 3") messages, or single packet vs. multipacket messages. The mapping can go like this:

      Type   Size   DELAY    THROUGHPUT    RELIABILITY
      ----   ----   -----    ----------    -----------
        0      S      1           0             0
        0      M      0           0             0
        3      S      1           0             0
        3      M      not allowed

And the DTR side of things 🍓

      DTR   Type   Size
      ---   ----   ----
      000     0      M
      001     0      M
      010     0      M
      011     0      M
      100     3      S
      101     0      S
      110     3      S
      111       error

PRNET 🌸

No priority indication here, bestie 💜 The two choices are station routing vs. point-to-point routing, or acknowledgments vs. no acknowledgments. The mapping:

      Routing   Acks    DELAY    THROUGHPUT    RELIABILITY
      -------   ----    -----    ----------    -----------
        ptp      no       1           0             0
        ptp      yes      1           0             1
      station    no       0           0             0
      station    yes      0           0             1

And the DTR breakdown ✨

      DTR   Routing   Acks
      ---   -------   ----
      000   station    no
      001   station    yes
      010   station    no
      011   station    yes
      100     ptp      no
      101     ptp      yes
      110     ptp      no
      111     ptp      yes

SATNET 🛰️

Also no priority indication, sorry girl 😭 SATNET has four choices: block vs. stream type, one of four delay categories, one of two holding time strategies, or one of three reliability levels. So the mapping of SATNET parameters into IP TOS parameters is honestly complex, with 2*4*2*3=48 distinct possibilities. Main character energy, but also a lot 💀

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References 📚

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