1. Multiple Encapsulation Methods
Draft-iab-link-encaps-05.txt
Bernard Aboba
IETF 67
San Diego, CA

2. Outline
History
Questions for 16ng

3. History Ethernet vs. IEEE 802 Encapsulation
IPv4 over Ethernet: RFC 894
IPv4 over IEEE 802: RFC 1042
Trailer Encapsulation: RFC 893

4. Ethernet Header
The 7 byte Preamble and 1 byte Start Frame Delimiter are not shown.
Data field is at least 46 bytes, to bring the minimum frame size to 64 bytes.
Ethernet Jumbo frames can be as large ~9000 bytes
802.1Q adds an additional 4 bytes of header (2 for EtherType=0x8100, 2 for Tag Control Information).
With 1500 bytes of data this brings the total frame size to 1522.

5. IEEE 802 Header Data and FCS omitted for brevity
How is IEEE 802 Length distinguished from Ethernet Type?
If the value of the field is less than or equal to 1500, then it indicates the Length in bytes of the subsequent MAC Client Data field (IEEE 802 encapsulation)
If the value of the field is greater than or equal to 1536, then it indicates the EtherType (Ethernet encapsulation).
IEEE and Ethernet have different MTUs!
IEEE MTU is 1492 (1518 – 18 (MAC header + FCS) – 8 (LLC + SNAP header), not including IP headers)

6. EtherTypes http://standards.ieee.org/regauth/ethertype/eth.txt
EtherType
Protocol
0x x05DC
IEEE Length Field
0x0600
Xerox NS
0x0800
Internet Protocol, Version 4 (IPv4)
0x0806
Address Resolution Protocol (ARP)
0x1000
Berkeley Trailer Negotiation
0x F
Berkeley Trailer encaps/IP
0x8035
Reverse Address Resolution Protocol (RARP)
0x809b
AppleTalk (Ethertalk)
0x80f3
AppleTalk Address Resolution Protocol (AARP)
0x8100
IEEE 802.1Q-tagged frame
0x8137
Novell IPX (alt)
0x8138
Novell
0x86DD
Internet Protocol, Version 6 (IPv6)
0x876B
RFC 1144 TCP/IP Compression
0x8847
MPLS unicast
0x8848
MPLS multicast
0x8863
PPPoE Discovery Stage
0x8864
PPPoE Session Stage
0x88A2
ATA over Ethernet
0xFFFF
Reserved

7. Implications of Mixed Environments (From RFC 1042)
A host can potentially receive both Ethernet and IEEE frames
If it does, it must keep track of which link protocol was used with each host and send using the right encapsulation.
Link layer broadcasts will not reach all hosts, just those who can receive the encapsulation used in the broadcast.
To enable hosts reading and sending only one encapsulation to communicate with each other, an IP gateway is recommended.
The MTU of Ethernet (1500) is different from the IEEE MTU (1492).

8. Recommendations from RFC 1122
It is not useful or even possible for a dual-format host to discover automatically which format to send, because of the link-layer broadcast issue.
Every host:
MUST be able to send and receive using RFC 894 encapsulation (Ethernet)
SHOULD be able to receive RFC 1042 (IEEE 802) packets intermixed with RFC 894 packets
MAY be able to send packets using RFC 1042 encapsulation
A host that implements sending both RFC 894 and 1042 encapsulation MUST provide a configuration switch to select which is sent, and this switch MUST default to RFC 894.

9. Comparison of 1042 & 1122 Send & Receiving Format discovery
1024: It is assumed that most computers will read and send using only one protocol
1122: Sending and receiving RFC 894 a MUST implement
1122: Receiving RFC 1042 a SHOULD implement, sending a MAY implement
1122: RFC 894 a MUST use by default
Format discovery
1024: a host receiving both 894 & 1024 must keep track and send using the right encapsulation
1122: Automatic discovery is not useful or even possible
1122 guarantees that a host can receive RFC 894, so no need to keep track or send using the right encapsulation

10. Trailer Encapsulation (RFC 893)
Enabled to minimize memory-to-memory copy operations performed by a receiving host
Done by moving variable length headers (e.g. IP + TCP) after the data segment
Enables reception on a page aligned boundary
Packets using trailers utilize a distinct EtherType [RFC893].
Type is calculated as 0x1000 plus the number of 512-byte pages of data (maximum of 16 pages or 8192 bytes)
Packet formulation
L2 header as normal
Data minus IP and TCP header always a multiple of 512 bytes
Trailer: Original Type (2) + Header Length (2) + IP and TCP headers
Frame Check Sequence

11. Negotiation Potential for four encapsulations! Trailer negotiation
IEEE 802 w or w/o trailers
Ethernet w or w/o trailers
Trailer negotiation
ARP exchange completed using the IP protocol type
Host that wants to speak trailers will send an additional “trailer ARP reply”
Receiver can add the host to the list of machines that understand trailers (e.g. marking the ARP cache entry).
Receiving host replies to “trailer ARP reply” with a “trailer ARP reply”
4.2 BSD implementation
Did not implement trailer negotiation
Configured trailers at boot time, assumed that all hosts either did or did not implement it.

12. RFC 1122 on Trailers
“The trailer protocol for link-layer encapsulation MAY be used, but only when it has been verified that both systems (host or gateway) involved in the link-layer communication implement trailers. If the system does not dynamically negotiate use of the trailer protocol on a per-destination basis, the default configuration MUST disable the protocol.”

13. What We Learn From History
Multiple encapsulations should be avoided wherever possible
After effects of DIX vs. IEEE 802 are still with us today
Mitigating approaches are not completely satisfactory
Automated discovery of link encapsulation is complex and inefficient
Best done at the link layer
Requires keeping state for each destination (not a shopstopper)
Results in higher bandwidth overhead and latency plus implementation complexity
IP gateways are not a panacea
IP gateways need to support separate virtual interface for each encapsulation.
Separate prefixes needed for each encapsulation so that hosts can avoid keeping state.

14. Questions for 16ng Roaming Interoperability
What if an MS supporting only Ethernet CS roams to a network supporting only IPv4 or IPv6 CS?
Interoperability
What if the Mobile Station (MS) and Base Station (BS) CS negotiate more than one way to send a given packet?
Example: Ethernet CS + IPv4 CS
What if MS and BS do not have any CSes in common?

15. Feedback?