One document matched: draft-meyer-gre-update-00.txt
Network Working Group David Meyer
Internet Draft Cisco Systems
Category Standards Track
draft-meyer-gre-update-00.txt November, 1999
Generic Routing Encapsulation (GRE)
<draft-meyer-gre-update-00.txt>
1. Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
2. Abstract
This document specifies a protocol for performing encapsulation of an
arbitrary network layer protocol over another arbitrary network layer
protocol.
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3. Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved
4. Introduction
A number of different proposals [RFC1234, RFC1226] currently exist
for the encapsulation of one protocol over another protocol. Other
types of encapsulations [RFC1241, SDRP, RFC1479] have been proposed
for transporting IP over IP for policy purposes. This memo describes
a protocol which is very similar to, but is more general than, the
above proposals. In attempting to be more general, many protocol
specific nuances have been ignored. The result is that this proposal
is may be less suitable for a situation where a specific "X over Y"
encapsulation has been described. It is the attempt of this protocol
to provide a simple, general purpose mechanism which is reduces the
problem of encapsulation from its current O(n^2) problem to a more
manageable state. This proposal also attempts to provide a
lightweight encapsulation for use in policy based routing. This memo
explicitly does not address the issue of when a packet should be
encapsulated. This memo acknowledges, but does not address problems
with other encapsuations such as mutual encapsulation [RFC1326] or
MPLS [MPLS].
In the most general case, a system has a packet that needs to be
encapsulated and delivered to some destination. We will call this
the payload packet. The payload is first encapsulated in a GRE
packet, which possibly also includes a route. The resulting GRE
packet can then be encapsulated in some other protocol and then
forwarded. We will call this outer protocol the delivery protocol.
The algorithms or processing this packet are discussed later.
4.1. Overall packet
The entire encapsulated packet would then have the form:
---------------------------------
| |
| Delivery Header |
| |
---------------------------------
| |
| GRE Header |
| |
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---------------------------------
| |
| Payload packet |
| |
---------------------------------
4.2. Packet header
The GRE packet header 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|R|K|S|s|G| Reserved | Ver | Protocol Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum (optional) | Offset (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Routing (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note that a compliant implemenation MUST accept and process optional
fields.
4.3. Flags and version (2 octets)
The GRE flags are encoded in the first two octets. Bit 0 is the most
significant bit, bit 15 is the least significant bit. Bits 13
through 15 are reserved for the Version field. Bits 6 through 12 are
reserved for future use and MUST be transmitted as zero.
4.4. Checksum Present (bit 0)
If the Checksum Present bit is set to 1, then the Checksum field is
present and contains valid information.
If either the Checksum Present bit or the Routing Present bit are
set, BOTH the Checksum and Offset fields are present in the GRE
packet.
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4.5. Routing Present (bit 1)
If the Routing Present bit is set to 1, then it indicates that the
Offset and Routing fields are present and contain valid information.
If either the Checksum Present bit or the Routing Present bit are
set, BOTH the Checksum and Offset fields are present in the GRE
packet.
4.6. Key Present (bit 2)
If the Key Present bit is set to 1, then it indicates that the Key
field is present in the GRE header. Otherwise, the Key field is not
present in the GRE header. In general, the key is used to match
packets that come over a point to multipoint tunnel.
4.7. Sequence Number Present (bit 3)
If the Sequence Number Present bit is set to one, then it indicates
that the Sequence Number field is present. Otherwise, the Sequence
Number field is not present in the GRE header. The Sequence Number is
used to enforce packet ordering on the tunnel.
4.8. Strict Source Route (bit 4)
The meaning of the Strict Source route bit is defined in other
documents. It is recommended that this bit only be set to one if all
of the the Routing Information consists of Strict Source Routes.
4.9. Generic Routing Field (bit 5)
The Generic Routing Field is provided to generalize the Routing Field
to provide additional flexibiity for future use. If the Generic
Routing Field (G-bit) bit is set to one, then the Routing Field is
encoded as a sequence of TLVs. The sequence of TLVs is terminated by
a TLV with type 0x0000 and length 4. If an implemenation does not
understand a TLV in the range 0-127, the TLV SHOULD be ignored. If an
implemenation does not understand a TLV in the range 128-255, the
packet MUST be dropped.
Since the Routing field is used when either the R-bit or the G-bit is
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set, the R-bit takes precedence. That is if both bits are set, the
Routing field is used in accordance to section 4.5.
The idea behind the G-bit is that when you have the G-bit set and the
R-bit is not set, you escape from format specified in section 4.17.
This provides some degree of backward compatability.
4.9.1. GRE TLV Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (8 bits)
Describes the format of the Value field.
Length (16 bits)
Length of Type, Length, and Value fields in octets. Minimum
length required is 3 octets, except in the case of the NULL
TLV, which has length 4.
Value (variable length)
Format is based on the Type value. The length of the value
field is Length field minus 3.
With the exception of the NULL TLV (Type 0x00 and and Length 4), this
document does not define or document any TLVs for use with GRE.
4.10. Reserved (bits 6-12)
These bits are reserved for future use and MUST contain the value
zero.
4.11. Version Number (bits 13-15)
The Version Number field MUST contain the value zero.
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4.12. Protocol Type (2 octets)
The Protocol Type field contains the protocol type of the payload
packet. In general, the value will be the Ethernet protocol type
field for the packet. Currently defined protocol types are listed
below [ETYPES].
4.13. Offset (2 octets)
The offset field indicates the octet offset from the start of the
Routing field to the first octet of the active Source Route Entry to
be examined. This field is present if the Routing Present or the
Checksum Present bit is set to one, and contains valid information
only if the Routing Present bit is set to one. If the R-bit is not
set, this field should be set to zero.
4.14. Checksum (2 octets)
The Checksum field contains the IP (one's complement) checksum of the
GRE header and the payload packet. This field is present if the
Routing Present bit, the Checksum Present bit, or the Generic Routing
Field bit are set to one, and contains valid information only if the
Checksum Present bit is set to one.
4.15. Key (4 octets)
The Key field contains a four octet number which was inserted by the
encapsulator. It is used to identify a security association and its
associated keying information for the purposes of authenticating the
source of the packet. The Key field is only present if the Key
Present field is set to one.
4.16. Sequence Number (4 octets)
The Sequence Number field contains an unsigned 32 bit integer which
is inserted by the encapsulator. It may be used by the receiver to
establish the order in which packets have been transmitted from the
encapsulator to the receiver. It is not necessary to specify an
exact algorithm other than to say that both sides should use
montonically increasing sequence numbers.
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4.17. Routing (variable)
The Routing field is optional and is present only if the Routing
Present bit is set to one (note that the Generic Routing Field bit
may also be set. However, the Routing field is interpreted in the
same way if set, independent of value of the Generic Routing Field).
The Routing field is a list of Source Route Entries (SREs). Each SRE
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Routing Information ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The routing field is terminated with a "NULL" SRE containing an
address family of type 0x0000 and a length of 0.
4.18. Address Family (2 octets)
Type is a two octet field that encodes an RFC1700 [RFC1700] address
family.
4.19. SRE Offset (1 octet)
The SRE Offset field indicates the octet offset from the start of the
Routing Information field to the first octet of the active entry in
Source Route Entry to be examined.
4.20. SRE Length (1 octet)
The SRE Length field contains the number of octets in the SRE. If
the SRE Length is zero, this indicates this is the last SRE in the
Routing field.
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4.21. Routing Information (variable)
The Routing Information field contains data which may be used in
routing this packet. If the R-bit is set, the packet is processed as
follows:
if (RBIT(GREPAK) { /* R-Bit present? */
SRE = GETSRE(GREPAK);
if (LENGTH(SRE) == 0) /* packet is for us */
PROCESS_GREPAK(GREPAK);
} else {
switch (ADDRESS_FAMILY(SRE)) {
case GRE_PROTO_TYPE_X: /* Processing for AF = <X> */
DATA = GRESREDATA(SRE);
AJUST_SRE_OFFSET(SRE); /* process next element */
if (OFFSET(SRE) == LENGTH(SRE)) /* done with this SRE */
AJUST_GRE_OFFSET(GREPAK); /* get next SRE */
SOURCE = IPADDRESS(GREPAK);
DEST = DATA;
AF_SENDGREPACKET(GREPAK, SOURCE, DEST); /* Per AF forward the packet */
RETURN;
}
}
Note that RFC1702 [RFC1702] describes GRE operation over IPv4
networks.
5. Forwarding of GRE packets
Normally, a system which is forwarding delivery layer packets will
not differentiate GRE packets from other packets in any way. However,
a GRE packet may be received by a system. In this case, the system
should use some delivery-specific means to determine that this is a
GRE packet. Once this is determined, the Key, Sequence Number and
Checksum fields if they contain valid information as indicated by the
corresponding flags may be checked. If the Routing Present bit is
set to one, then the Address Family field should be checked to
determine the semantics and use of the SRE Length, SRE Offset and
Routing Information fields. The exact semantics for processing a SRE
is dependent on the Address Family. However, for the IPv4 address
family, see the psuedo-code above.
Once all SREs have been processed, then the source route is complete,
the GRE header should be removed, the payload's TTL MUST be
decremented (if one exists) and the payload packet should be
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forwarded as a normal packet.
6. Current List of Protocol Types
The following are currently assigned protocol types for GRE. Future
protocol types must be taken from DIX ethernet encoding. For
historical reasons, a number of other values have been used for some
protocols. The following table of values MUST be used to identify
the following protocols:
Protocol Family PTYPE
--------------- -----
Reserved 0000
SNA 0004
OSI network layer 00FE
PUP 0200
XNS 0600
IP 0800
Chaos 0804
RFC 826 ARP 0806
Frame Relay ARP 0808
VINES 0BAD
VINES Echo 0BAE
VINES Loopback 0BAF
DECnet (Phase IV) 6003
Transparent Ethernet Bridging 6558
Raw Frame Relay 6559
Apollo Domain 8019
Ethertalk (Appletalk) 809B
Novell IPX 8137
RFC 1144 TCP/IP compression 876B
IP Autonomous Systems 876C
Secure Data 876D
Reserved FFFF
See [ETYPES] for the complete list of these values.
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7. Security Considerations
Security in a network using GRE should be relatively similar to
security in a normal IP network, as routing using GRE follows the
same routing that IP uses natively. Route filtering will remain
unchanged. However packet filtering requires either that a firewall
look inside the GRE packet or that the filtering is done on the GRE
tunnel endpoints. In those environments in which this is considered
to be a security issue it may be desirable to terminate the tunnel to
the firewall.
8. Acknowledgements
This document is derrived from the original ideas of the authors of
RFC1701 and RFC1702. Bill Fenner, Thomas Narten, and Dino Farinacci
provided constructive and insightful comments.
9. References
[ETYPES] ftp://ftp.isi.edu/in-notes/iana/assignments/ethernet-
numbers
[MPLS] Rosen, Eric, et. al, "Multiprotocol Label Switching
Architecture", draft-ietf-mpls-arch-06.txt, August,
1999.
[RFC1479] Steenstrup, M. "Inter-Domain Policy Routing Protocol
Specification: Version 1", RFC1479, BBN Systems and
Technologies, July 1993.
[RFC1226] Kantor, B. "Internet Protocol Encapsulation of AX.25
Frames", RFC1226, University of California, San Diego,
May 1991.
[RFC1234] Provan, D. "Tunneling IPX Traffic through IP Networks",
RFC1234, Novell, Inc., June 1991.
[RFC1241] Woodburn, R., and D. Mills, "Scheme for an Internet
Encapsulation Protocol: Version 1", RFC1241, SAIC,
University of Delaware, July 1991.
[RFC1326] Tsuchiya, P., "Mutual Encapsulation Considered
Dangerous", RFC1326, Bellcore, May 1992.
[RFC1700] J. Reynolds and J. Postel, "Assigned Numbers",
RFC1700, October 1994.
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[RFC1701] Hanks, S., Li, T, Farinacci, D., and P. Traina, "Generic
Routing Encapsulation", RFC1701, NetSmiths, Ltd., and
cisco Systems, October 1994.
[RFC1702] Hanks, S., Li, T., Farinacci, D., and P. Traina,
"Generic Routing Encapsulation over IPv4 networks",
RFC1702, NetSmiths, Ltd., cisco Systems, October 1994.
[SDRP] Estrin, D., Li, T., and Y. Rekhter, "Source Demand
Routing Protocol Specification (Version 1)", Work in
Progress.
10. Authors' Addresses
David Meyer
Cisco Systems, Inc.
170 W. Tasman Drive
San Jose, CA 95134-1706
United States
EMail: dmm@cisco.com
David Meyer [Page 11]
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