Requirements for
Internet Routers (Gateways) and Hosts
Relates to Lab 3. (Supplement)
Covers the compliance requirements of Internet routers and hosts.
1
What does it mean to be IP compliant?
• Requirements for Internet Routers and hosts have been
document in Request for Comments (RFCs) of the Internet
Engineering Task Force (IETF):
• RFC 985:
Requirements for Internet
Gateways – Draft (1986)
• RFC 1009:
Requirements for Internet
Gateways (1987)
• RDC 1716:
Towards Requirements for
IP Routers (1994)
• RFC 1812:
Requirements for IP
Version 4 Routers (1995)
• RFC 1122 (STD 3): Requirements for Internet Hosts - Communication
Layers (1989)
• RFC 1123 (STD 3): Requirements for Internet Hosts - Application and
2
Language of Requirements
MUST – absolute requirement of the specification.
MUST IMPLEMENT – Implementation is required, but need
not be enabled by default
MUST NOT – absolute prohibition of the specification.
SHOULD – there may exist valid reasons to, but full
implications must be understood and weighed
(recommended)
SHOULD NOT – there may exist valid reasons to include, but
full implications must be understood and weighed (not
recommended)
MAY / OPTIONAL – optional
3
Robustness Principle
"Be liberal in what you accept, and
conservative in what you send”
• The principle should be applied by all protocols to increase
robustness and interoperability
• Other hosts and routers may use deficient implementations
”Be Adaptive to Change"
• Assume that protocol will be extended in the future, and
options will be added
4
HOST REQUIREMENTS: IPv4 (partial list)
MUST NOT
SHOULD
NOT
✔
Multiple IP addresses (“multihoming”)
- local host
- remote host
✔
✔
Forward datagram received from connected network
Silently discard version number ≠ 4
MAY
SHOULD
MUST
Implement IP and ICMP
✔
✔
Able to log discarded datagrams
✔
Src address must be host's own IP address
✔
Support reassembly
✔
5
HOST REQUIREMENTS: IPv4 (partial list)
ICMP Echo Request and Reply (client and server)
Discard Echo Request to broadcast address
MUST NOT
Generate ICMP Dest Unreachable (code 2/3)
Send ICMP Redirect
Update routing table for incoming ICMP Redirect
SHOULD
NOT
Send ICMP for ICMP Error message
Send ICMP for broadcast or multicast IP datagram
Send ICMP error message (if not prohibited)
MAY
Send packet with TTL of 0
Discard received packets with TTL < 2
Transport layer can set TTL
SHOULD
MUST
Timestamp IP option
Source route IP option
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
6
HOST REQUIREMENTS: IPv4 (partial list)
Use default gateway if no match in routing table
✔
MUST NOT
✔
SHOULD
NOT
Operate without router or connected network
MAY
✔
SHOULD
MUST
Use netmask in local/remote decision for outgoing
packets
Ping routers continuously
✔
Use broadcast address as src IP address
✔
7
Definition of an IP router (gateway)
1. Conforms to specific Internet protocols, including IP
and ICMP
2. Interfaces to two or more packet networks
– Encapsulation/decapsulation, MTU, address translation
3. Receives and forwards Internet datagrams.
– Recognizes error conditions, Drop when TTL=0, perform
fragmentation
4. Chooses next hop
5. Supports a routing protocol
6. System Support (debugging, status report, etc.)
8
ROUTER REQUIREMENTS: IPv4 (partial list)
Support IP fragmentation
First fragment is smallest
Send fragments in order
Generate least possible number of fragments
✔
Decrement TTL by at least one
Discard packet when TTL is zero
Send ICMP Error due to dropped packet with TTL=0
Generate packet with TTL=0
✔
✔
MUST NOT
✔
SHOULD
NOT
Check IP header checksum
MAY
✔
SHOULD
MUST
Drop packets because unused IP header field values
Ignore unrecognized IP options
✔
✔
✔
✔
✔
✔
9
ROUTER REQUIREMENTS: IPv4 (partial list)
Forward destination 255.255.255.255
Forward destination with host number all `1’
Discard packets with host number all `1’
MUST NOT
✔
✔
SHOULD
NOT
Be able to reboot from NOC
Have mechanism for retrieving status
MAY
✔
SHOULD
MUST
Router must operate as a stand-alone device
✔
✔
✔
✔
See RFC’s for more rules.
10
HOST REQUIREMENTS: IPv4 (partial list)
MUST NOT
SHOULD
NOT
✔
Multiple IP addresses (“multihoming”)
- local host
- remote host
✔
✔
Forward datagram received from connected network
Silently discard version number ≠ 4
MAY
SHOULD
MUST
Implement IP and ICMP
✔
✔
Able to log discarded datagrams
✔
Src address must be host's own IP address
✔
Support reassembly
✔
11
HOST REQUIREMENTS: IPv4 (partial list)
ICMP Echo Request and Reply (client and server)
Discard Echo Request to broadcast address
MUST NOT
Generate ICMP Dest Unreachable (code 2/3)
Send ICMP Redirect
Update routing table for incoming ICMP Redirect
SHOULD
NOT
Send ICMP for ICMP Error message
Send ICMP for broadcast or multicast IP datagram
Send ICMP error message (if not prohibited)
MAY
Send packet with TTL of 0
Discard received packets with TTL < 2
Transport layer can set TTL
SHOULD
MUST
Timestamp IP option
Source route IP option
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
12
HOST REQUIREMENTS: IPv4 (partial list)
Use default gateway if no match in routing table
✔
MUST NOT
✔
SHOULD
NOT
Operate without router or connected network
MAY
✔
SHOULD
MUST
Use netmask in local/remote decision for outgoing
packets
Ping routers continuously
✔
Use broadcast address as src IP address
✔
13
Definition of an IP router (gateway)
1. Conforms to specific Internet protocols, including IP
and ICMP
2. Interfaces to two or more packet networks
– Encapsulation/decapsulation, MTU, address translation
3. Receives and forwards Internet datagrams.
– Recognizes error conditions, Drop when TTL=0, perform
fragmentation
4. Chooses next hop
5. Supports a routing protocol
6. System Support (debugging, status report, etc.)
14
ROUTER REQUIREMENTS: IPv4 (partial list)
Support IP fragmentation
First fragment is smallest
Send fragments in order
Generate least possible number of fragments
✔
Decrement TTL by at least one
Discard packet when TTL is zero
Send ICMP Error due to dropped packet with TTL=0
Generate packet with TTL=0
✔
✔
MUST NOT
✔
SHOULD
NOT
Check IP header checksum
MAY
✔
SHOULD
MUST
Drop packets because unused IP header field values
Ignore unrecognized IP options
✔
✔
✔
✔
✔
✔
15
ROUTER REQUIREMENTS: IPv4 (partial list)
Forward destination 255.255.255.255
Forward destination with host number all `1’
Discard packets with host number all `1’
MUST NOT
✔
✔
SHOULD
NOT
Be able to reboot from NOC
Have mechanism for retrieving status
MAY
✔
SHOULD
MUST
Router must operate as a stand-alone device
✔
✔
✔
✔
See RFC’s for more rules.
16