RFC1702 - Generic Routing Encapsulation over IPv4 networks

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Network Working Group S. Hanks

Request for Comments: 1702 NetSmiths, Ltd.

Category: Informational T. Li

D. Farinacci

P. Traina

cisco Systems

October 1994

Generic Routing Encapsulation over IPv4 networks

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.

IntrodUCtion

In an earlier memo [RFC1701], we described GRE, a mechanism for

encapsulating arbitrary packets within an arbitrary transport

protocol. This is a companion memo which describes the use of GRE

with IP. This memo addresses the case of using IP as the delivery

protocol or the payload protocol and the special case of IP as both

the delivery and payload. This memo also describes using IP

addresses and autonomous system numbers as part of a GRE source

route.

IP as a delivery protocol

GRE packets which are encapsulated within IP will use IP protocol

type 47.

IP as a payload protocol

IP packets will be encapsulated with a Protocol Type field of 0x800.

For the Address Family value of 0x800, the Routing Information field

will consist of a list of IP addresses and indicates an IP source

route. The first octet of the Routing Information field constitute a

8 bit integer offset from the start of the Source Route Entry (SRE),

called the SRE Offset. The SRE Offset indicates the first octet of

the next IP address. The SRE Length field consists of the total

length of the IP Address List in octets.

This has the form:

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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Address Family SRE Offset SRE Length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IP Address List ...

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

For the Address Family value of 0xfffe, the Routing Information field

will consist of a list of Autonomous System numbers and indicates an

AS source route. The third octet of the Routing Information field

contains an 8 bit unsigned integer offset from the start of the

Source Route Entry (SRE), called the SRE Offset. The SRE Offset

indicates the first octet of the next AS number. THe SRE Length

field consists of the total length of the AS Number list in octets.

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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Address Family SRE Offset SRE Length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

AS Number List ...

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IP as both delivery and payload protocol

When IP is encapsulated in IP, the TTL, TOS, and IP security options

MAY be copied from the payload packet into the same fields in the

delivery packet. The payload packet's TTL MUST be decremented when

the packet is decapsulated to insure that no packet lives forever.

IP source routes

When a system is processing a SRE with an Address Family indicating

an IP source route, it MUST use the SRE Offset to determine the next

destination IP address. If the next IP destination is this system,

the SRE Offset field should be increased by four (the size of an IP

address). If the SRE Offset is equal to the SRE Length in this SRE,

then the Offset field in the GRE header should be adjusted to point

to the next SRE (if any). This should be repeated until the next IP

destination is not this system or until the entire SRE has been

processed.

If the source route is incomplete, then the Strict Source Route bit

is checked. If the source route is a strict source route and the

next IP destination is NOT an adjacent system, the packet MUST be

dropped. Otherwise, the system should use the IP address indicated

by the Offset field to replace the destination address in the

delivery header and forward the packet.

Autonomous system source routes

When a system is processing a SRE with an Address Family indicating

an AS source route, it MUST use the SRE Offset field to determine the

next autonomous system. If the next autonomous system is the local

autonomous system, the SRE Offset field should be increased by two

(the size of an autonomous system number). If the SRE Offset is

equal to the SRE Length in this SRE, then the Offset field in the GRE

header should be adjusted to point to the next SRE (if any). This

should be repeated until the next autonomous system number is not

equal to the local autonomous system number or until the entire SRE

has been processed.

If the source route is incomplete, then the Strict Source Route bit

is checked. If the source route is a strict source route and the

next autonomous system is NOT an adjacent autonomous system, the

packet should be dropped. Otherwise, the system should use the

autonomous system number indicated by the SRE Offset field to replace

the destination address in the delivery header and forward the

packet. The exact mechanism for determining the next delivery

destination address given the AS number is outside of the scope of

this document.

Security Considerations

Security issues are not discussed in this memo.

Authors' Addresses

Stan Hanks

NetSmiths, Ltd.

2025 Lincoln Highway

Edison, NJ 08817

EMail: stan@netsmiths.com

Tony Li

cisco Systems, Inc.

1525 O'Brien Drive

Menlo Park, CA 94025

EMail: tli@cisco.com

Dino Farinacci

cisco Systems, Inc.

1525 O'Brien Drive

Menlo Park, CA 94025

EMail: dino@cisco.com

Paul Traina

cisco Systems, Inc.

1525 O'Brien Drive

Menlo Park, CA 94025

EMail: pst@cisco.com

References

RFC1701

Hanks, S., Li, T, Farinacci, D., and P. Traina, "Generic Routing

Encapsulation", RFC1701, NetSmiths, Ltd., and cisco Systems,

October 1994.

 
 
 
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