Service mappings
How IP quality-of-service settings map to individual network types.
Postel, J., "Service mappings", RFC 795, DOI 10.17487/RFC0795, September 1981, <https://www.rfc-editor.org/info/rfc795>.
@misc{rfc795,
series = {Request for Comments},
number = 795,
howpublished = {RFC 795},
publisher = {RFC Editor},
doi = {10.17487/RFC0795},
url = {https://www.rfc-editor.org/info/rfc795},
author = {J. Postel},
title = {{Service mappings}},
pagetotal = 4,
year = 1981,
month = sep,
} RFC text
Contents
Network Working Group J. Postel
Request for Comments: 795 ISI
September 1981
SERVICE MAPPINGS
----------------
This memo describes the relationship between the Internet
Protocol (IP) [1] Type of Service and the service parameters of specific
networks.
The IP Type of Service has the following fields:
Bits 0-2: Precedence.
Bit 3: 0 = Normal Delay, 1 = Low Delay.
Bits 4: 0 = Normal Throughput, 1 = High Throughput.
Bits 5: 0 = Normal Relibility, 1 = High Relibility.
Bit 6-7: Reserved for Future Use.
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
| | | | | | |
| PRECEDENCE | D | T | R | 0 | 0 |
| | | | | | |
+-----+-----+-----+-----+-----+-----+-----+-----+
111 - Network Control
110 - Internetwork Control
101 - CRITIC/ECP
100 - Flash Override
011 - Flash
010 - Immediate
001 - Priority
000 - Routine
The individual networks listed here have very different and specific
service choices.
RFC 795 Service Mappings
AUTODIN II
The service choices are in 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.
The Application types can be mapped into the remaining IP TOS fields
as follows.
TA DELAY THROUGHPUT RELIABILITY
--- ----- ---------- -----------
I/A 1 0 0
Q/R 0 0 0
B1 0 1 0
B2 0 1 1
DTR TA
--- ---
000 Q/R
001 Q/R
010 B1
011 B2
100 I/A
101 I/A
110 I/A
111 error
RFC 795 Service Mappings
ARPANET
The service choices are in quite limited. There is one priority bit
that can be mapped to the high order bit of the IP TOS precedence.
The other choices are to use the regular ("Type 0") messages vs. the
uncontrolled ("Type 3") messages, or to use single packet vs.
multipacket messages. The mapping of ARPANET parameters into IP TOS
parameters can be as follows.
Type Size DELAY THROUGHPUT RELIABILITY
---- ---- ----- ---------- -----------
0 S 1 0 0
0 M 0 0 0
3 S 1 0 0
3 M not allowed
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
RFC 795 Service Mappings
PRNET
There is no priority indication. The two choices are to use the
station routing vs. point-to-point routing, or to require
acknowledgments vs. having no acknowledgments. The mapping of PRNET
parameters into IP TOS parameters can be as follows.
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
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
There is no priority indication. The four choices are to use the
block vs. stream type, to select one of four delay catagories, to
select one of two holding time strategies, or to request one of three
reliability levels. The mapping of SATNET parameters into IP TOS
parameters can thus quite complex there being 2*4*2*3=48 distinct
possibilities.
References
----------
[1] Postel, J. (ed.), "Internet Protocol - DARPA Internet Program
Protocol Specification," RFC 791, USC/Information Sciences
Institute, September 1981.
AI ELI5
On this page
§1IP Type of Service Fields
The Internet Protocol (RFC 791) includes an 8-bit Type of Service field in each packet header. Bits 0–2 specify Precedence (8 levels from Routine to Network Control). Bit 3 controls Delay (Normal or Low). Bit 4 controls Throughput (Normal or High). Bit 5 controls Reliability (Normal or High). Bits 6–7 are reserved. These fields allow hosts and routers to express quality-of-service requirements, but the meaning is abstract: actual networks support different options with different names.
IP packets can ask for different service levels: fast or slow, reliable or quick, important or routine. But each network underneath understands these requests differently.
§2Network-Specific Mappings
RFC 795 documents how to map IP Type of Service to four major networks. AUTODIN II distinguishes between traffic acceptance categories (Immediate/Addressed, Query/Response, and two Background levels) and application types, offering different combinations of delay, throughput, and reliability. ARPANET offers limited choices: a priority bit, message type (Type 0 regular or Type 3 uncontrolled), and message size (single packet or multipacket). PRNET (packet radio network) offers point-to-point or station routing and optional acknowledgments. SATNET (satellite network) provides the most complex options, with block vs. stream type, four delay categories, two holding-time strategies, and three reliability levels, creating 48 distinct combinations. For each network, the RFC provides tables mapping DTR (Delay-Throughput-Reliability) bit combinations to the network's native options.
Each network has its own buttons to press: AUTODIN has priority and application type, ARPANET has message type and size, PRNET has routing style, and SATNET has many detailed options. The tables show how to press the right buttons.
§3Status and Application
RFC 795 is a historic document reflecting the diversity of early 1980s military and research networks. Many of the networks it describes (ARPANET, PRNET, SATNET, AUTODIN II) are no longer in operation. The approach of mapping abstract service parameters to network-specific options remains sound and has evolved into modern QoS mechanisms in Internet routing protocols like DiffServ, but the specific networks and mappings in this RFC are primarily of historical interest.
This memo is about old military and research networks from the 1980s. The idea of translating requests into what each network offers is still useful, but the specific networks have changed.