RFC 1716 (rfc1716) - Page 1 of 186
Towards Requirements for IP Routers
Alternative Format: Original Text Document
Network Working Group P. Almquist, Author
Request for Comments: 1716 Consultant
Category: Informational F. Kastenholz, Editor
FTP Software, Inc.
November 1994
Towards Requirements for IP Routers
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Almquist & Kastenholz [Page i]
RFC 1716 Towards Requirements for IP Routers November 1994
Table of Contents
0. PREFACE ....................................................... 1
1. INTRODUCTION .................................................. 2
1.1 Reading this Document ........................................ 4
1.1.1 Organization ............................................... 4
1.1.2 Requirements ............................................... 5
1.1.3 Compliance ................................................. 6
1.2 Relationships to Other Standards ............................. 7
1.3 General Considerations ....................................... 8
1.3.1 Continuing Internet Evolution .............................. 8
1.3.2 Robustness Principle ....................................... 9
1.3.3 Error Logging .............................................. 9
1.3.4 Configuration .............................................. 10
1.4 Algorithms ................................................... 11
2. INTERNET ARCHITECTURE ......................................... 13
2.1 Introduction ................................................. 13
2.2 Elements of the Architecture ................................. 14
2.2.1 Protocol Layering .......................................... 14
2.2.2 Networks ................................................... 16
2.2.3 Routers .................................................... 17
2.2.4 Autonomous Systems ......................................... 18
2.2.5 Addresses and Subnets ...................................... 18
2.2.6 IP Multicasting ............................................ 20
2.2.7 Unnumbered Lines and Networks and Subnets .................. 20
2.2.8 Notable Oddities ........................................... 22
2.2.8.1 Embedded Routers ......................................... 22
2.2.8.2 Transparent Routers ...................................... 23
2.3 Router Characteristics ....................................... 24
2.4 Architectural Assumptions .................................... 27
3. LINK LAYER .................................................... 29
3.1 INTRODUCTION ................................................. 29
3.2 LINK/INTERNET LAYER INTERFACE ................................ 29
3.3 SPECIFIC ISSUES .............................................. 30
3.3.1 Trailer Encapsulation ...................................... 30
3.3.2 Address Resolution Protocol - ARP .......................... 31
3.3.3 Ethernet and 802.3 Coexistence ............................. 31
3.3.4 Maximum Transmission Unit - MTU ............................ 31
3.3.5 Point-to-Point Protocol - PPP .............................. 32
3.3.5.1 Introduction ............................................. 32
3.3.5.2 Link Control Protocol (LCP) Options ...................... 33
3.3.5.3 IP Control Protocol (ICP) Options ........................ 34
3.3.6 Interface Testing .......................................... 35
4. INTERNET LAYER - PROTOCOLS .................................... 36
4.1 INTRODUCTION ................................................. 36
4.2 INTERNET PROTOCOL - IP ....................................... 36
Almquist & Kastenholz [Page ii]
RFC 1716 Towards Requirements for IP Routers November 1994
4.2.1 INTRODUCTION ............................................... 36
4.2.2 PROTOCOL WALK-THROUGH ...................................... 37
4.2.2.1 Options: RFC-791 Section 3.2 ............................. 37
4.2.2.2 Addresses in Options: RFC-791 Section 3.1 ................ 40
4.2.2.3 Unused IP Header Bits: RFC-791 Section 3.1 ............... 40
4.2.2.4 Type of Service: RFC-791 Section 3.1 ..................... 41
4.2.2.5 Header Checksum: RFC-791 Section 3.1 ..................... 41
4.2.2.6 Unrecognized Header Options: RFC-791 Section 3.1 ......... 41
4.2.2.7 Fragmentation: RFC-791 Section 3.2 ....................... 42
4.2.2.8 Reassembly: RFC-791 Section 3.2 .......................... 43
4.2.2.9 Time to Live: RFC-791 Section 3.2 ........................ 43
4.2.2.10 Multi-subnet Broadcasts: RFC-922 ........................ 43
4.2.2.11 Addressing: RFC-791 Section 3.2 ......................... 43
4.2.3 SPECIFIC ISSUES ............................................ 47
4.2.3.1 IP Broadcast Addresses ................................... 47
4.2.3.2 IP Multicasting .......................................... 48
4.2.3.3 Path MTU Discovery ....................................... 48
4.2.3.4 Subnetting ............................................... 49
4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP ..................... 50
4.3.1 INTRODUCTION ............................................... 50
4.3.2 GENERAL ISSUES ............................................. 50
4.3.2.1 Unknown Message Types .................................... 50
4.3.2.2 ICMP Message TTL ......................................... 51
4.3.2.3 Original Message Header .................................. 51
4.3.2.4 ICMP Message Source Address .............................. 51
4.3.2.5 TOS and Precedence ....................................... 51
4.3.2.6 Source Route ............................................. 52
4.3.2.7 When Not to Send ICMP Errors ............................. 53
4.3.2.8 Rate Limiting ............................................ 54
4.3.3 SPECIFIC ISSUES ............................................ 55
4.3.3.1 Destination Unreachable .................................. 55
4.3.3.2 Redirect ................................................. 55
4.3.3.3 Source Quench ............................................ 56
4.3.3.4 Time Exceeded ............................................ 56
4.3.3.5 Parameter Problem ........................................ 57
4.3.3.6 Echo Request/Reply ....................................... 57
4.3.3.7 Information Request/Reply ................................ 58
4.3.3.8 Timestamp and Timestamp Reply ............................ 58
4.3.3.9 Address Mask Request/Reply ............................... 59
4.3.3.10 Router Advertisement and Solicitations .................. 61
4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP .................... 61
5. INTERNET LAYER - FORWARDING ................................... 62
5.1 INTRODUCTION ................................................. 62
5.2 FORWARDING WALK-THROUGH ...................................... 62
5.2.1 Forwarding Algorithm ....................................... 62
5.2.1.1 General .................................................. 63
5.2.1.2 Unicast .................................................. 64
Almquist & Kastenholz [Page iii]
RFC 1716 Towards Requirements for IP Routers November 1994
5.2.1.3 Multicast ................................................ 65
5.2.2 IP Header Validation ....................................... 66
5.2.3 Local Delivery Decision .................................... 68
5.2.4 Determining the Next Hop Address ........................... 70
5.2.4.1 Immediate Destination Address ............................ 71
5.2.4.2 Local/Remote Decision .................................... 71
5.2.4.3 Next Hop Address ......................................... 72
5.2.4.4 Administrative Preference ................................ 77
5.2.4.6 Load Splitting ........................................... 78
5.2.5 Unused IP Header Bits: RFC-791 Section 3.1 ................. 79
5.2.6 Fragmentation and Reassembly: RFC-791 Section 3.2 .......... 79
5.2.7 Internet Control Message Protocol - ICMP ................... 80
5.2.7.1 Destination Unreachable .................................. 80
5.2.7.2 Redirect ................................................. 82
5.2.7.3 Time Exceeded ............................................ 84
5.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP .................. 84
5.3 SPECIFIC ISSUES .............................................. 84
5.3.1 Time to Live (TTL) ......................................... 84
5.3.2 Type of Service (TOS) ...................................... 85
5.3.3 IP Precedence .............................................. 87
5.3.3.1 Precedence-Ordered Queue Service ......................... 88
5.3.3.2 Lower Layer Precedence Mappings .......................... 88
5.3.3.3 Precedence Handling For All Routers ...................... 89
5.3.4 Forwarding of Link Layer Broadcasts ........................ 92
5.3.5 Forwarding of Internet Layer Broadcasts .................... 92
5.3.5.1 Limited Broadcasts ....................................... 94
5.3.5.2 Net-directed Broadcasts .................................. 94
5.3.5.3 All-subnets-directed Broadcasts .......................... 95
5.3.5.4 Subnet-directed Broadcasts ............................... 97
5.3.6 Congestion Control ......................................... 97
5.3.7 Martian Address Filtering .................................. 99
5.3.8 Source Address Validation .................................. 99
5.3.9 Packet Filtering and Access Lists .......................... 100
5.3.10 Multicast Routing ......................................... 101
5.3.11 Controls on Forwarding .................................... 101
5.3.12 State Changes ............................................. 101
5.3.12.1 When a Router Ceases Forwarding ......................... 102
5.3.12.2 When a Router Starts Forwarding ......................... 102
5.3.12.3 When an Interface Fails or is Disabled .................. 103
5.3.12.4 When an Interface is Enabled ............................ 103
5.3.13 IP Options ................................................ 103
5.3.13.1 Unrecognized Options .................................... 103
5.3.13.2 Security Option ......................................... 104
5.3.13.3 Stream Identifier Option ................................ 104
5.3.13.4 Source Route Options .................................... 104
5.3.13.5 Record Route Option ..................................... 104
5.3.13.6 Timestamp Option ........................................ 105
Almquist & Kastenholz [Page iv]
RFC 1716 Towards Requirements for IP Routers November 1994
6. TRANSPORT LAYER ............................................... 106
6.1 USER DATAGRAM PROTOCOL - UDP ................................. 106
6.2 TRANSMISSION CONTROL PROTOCOL - TCP .......................... 106
7. APPLICATION LAYER - ROUTING PROTOCOLS ......................... 109
7.1 INTRODUCTION ................................................. 109
7.1.1 Routing Security Considerations ............................ 109
7.1.2 Precedence ................................................. 110
7.2 INTERIOR GATEWAY PROTOCOLS ................................... 110
7.2.1 INTRODUCTION ............................................... 110
7.2.2 OPEN SHORTEST PATH FIRST - OSPF ............................ 111
7.2.2.1 Introduction ............................................. 111
7.2.2.2 Specific Issues .......................................... 111
7.2.2.3 New Version of OSPF ...................................... 112
7.2.3 INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM - DUAL IS-IS
.............................................................. 112
7.2.4 ROUTING INFORMATION PROTOCOL - RIP ......................... 113
7.2.4.1 Introduction ............................................. 113
7.2.4.2 Protocol Walk-Through .................................... 113
7.2.4.3 Specific Issues .......................................... 118
7.2.5 GATEWAY TO GATEWAY PROTOCOL - GGP .......................... 119
7.3 EXTERIOR GATEWAY PROTOCOLS ................................... 119
7.3.1 INTRODUCTION ............................................... 119
7.3.2 BORDER GATEWAY PROTOCOL - BGP .............................. 120
7.3.2.1 Introduction ............................................. 120
7.3.2.2 Protocol Walk-through .................................... 120
7.3.3 EXTERIOR GATEWAY PROTOCOL - EGP ............................ 121
7.3.3.1 Introduction ............................................. 121
7.3.3.2 Protocol Walk-through .................................... 122
7.3.4 INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL .............. 124
7.4 STATIC ROUTING ............................................... 125
7.5 FILTERING OF ROUTING INFORMATION ............................. 127
7.5.1 Route Validation ........................................... 127
7.5.2 Basic Route Filtering ...................................... 127
7.5.3 Advanced Route Filtering ................................... 128
7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE .................. 129
8. APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS .............. 131
8.1 The Simple Network Management Protocol - SNMP ................ 131
8.1.1 SNMP Protocol Elements ..................................... 131
8.2 Community Table .............................................. 132
8.3 Standard MIBS ................................................ 133
8.4 Vendor Specific MIBS ......................................... 134
8.5 Saving Changes ............................................... 135
9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS ................... 137
9.1 BOOTP ........................................................ 137
9.1.1 Introduction ............................................... 137
9.1.2 BOOTP Relay Agents ......................................... 137
10. OPERATIONS AND MAINTENANCE ................................... 139
Almquist & Kastenholz [Page v]
RFC 1716 Towards Requirements for IP Routers November 1994
10.1 Introduction ................................................ 139
10.2 Router Initialization ....................................... 140
10.2.1 Minimum Router Configuration .............................. 140
10.2.2 Address and Address Mask Initialization ................... 141
10.2.3 Network Booting using BOOTP and TFTP ...................... 142
10.3 Operation and Maintenance ................................... 143
10.3.1 Introduction .............................................. 143
10.3.2 Out Of Band Access ........................................ 144
10.3.2 Router O&M Functions ...................................... 144
10.3.2.1 Maintenance - Hardware Diagnosis ........................ 144
10.3.2.2 Control - Dumping and Rebooting ......................... 145
10.3.2.3 Control - Configuring the Router ........................ 145
10.3.2.4 Netbooting of System Software ........................... 146
10.3.2.5 Detecting and responding to misconfiguration ............ 146
10.3.2.6 Minimizing Disruption ................................... 147
10.3.2.7 Control - Troubleshooting Problems ...................... 148
10.4 Security Considerations ..................................... 149
10.4.1 Auditing and Audit Trails ................................. 149
10.4.2 Configuration Control ..................................... 150
11. REFERENCES ................................................... 152
APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS ................ 162
APPENDIX B. GLOSSARY ............................................. 164
APPENDIX C. FUTURE DIRECTIONS .................................... 169
APPENDIX D. Multicast Routing Protocols .......................... 172
D.1 Introduction ................................................. 172
D.2 Distance Vector Multicast Routing Protocol - DVMRP ........... 172
D.3 Multicast Extensions to OSPF - MOSPF ......................... 173
APPENDIX E Additional Next-Hop Selection Algorithms .............. 174
E.1. Some Historical Perspective .................................. 174
E.2. Additional Pruning Rules ..................................... 176
E.3 Some Route Lookup Algorithms ................................. 177
E.3.1 The Revised Classic Algorithm ............................... 178
E.3.2 The Variant Router Requirements Algorithm ................... 179
E.3.3 The OSPF Algorithm .......................................... 179
E.3.4 The Integrated IS-IS Algorithm .............................. 180
Security Considerations ........................................... 182
Acknowledgments ................................................... 183
Editor's Address .................................................. 186
Almquist & Kastenholz [Page vi]
RFC 1716 Towards Requirements for IP Routers November 1994
0. PREFACE
This document is a snapshot of the work of the Router Requirements
working group as of November 1991. At that time, the working group had
essentially finished its task. There were some final technical matters
to be nailed down, and a great deal of editing needed to be done in
order to get the document ready for publication. Unfortunately, these
tasks were never completed.
At the request of the Internet Area Director, the current editor took
the last draft of the document and, after consulting the mailing list
archives, meeting minutes, notes, and other members of the working
group, edited the document to its current form. This effort included
the following tasks: 1) Deleting all the parenthetical material (such as
editor's comments). Useful information was turned into DISCUSSION
sections, the rest was deleted. 2) Completing the tasks listed in the
last draft's To be Done sections. As a part of this task, a new "to be
done" list was developed and included as an appendix to the current
document. 3) Rolling Philip Almquist's "Ruminations on the Next Hop"
and "Ruminations on Route Leaking" into this document. These represent
significant work and should be kept. 4) Fulfilling the last intents of
the working group as determined from the archival material. The intent
of this effort was to get the document into a form suitable for
publication as an Historical RFC so that the significant work which went
into the creation of this document would be preserved.
The content and form of this document are due, in large part, to the
working group's chair, and document's original editor and author: Philip
Almquist. Without his efforts, this document would not exist.
Almquist & Kastenholz
