ISCW实验1：了解PPPoE Session的建立过程

试验预备知识：

 

PPP协议要求进行通信的双方之间是点到点的关系，不适于广播型的以太网和另外一些多点访问型的网络，于是就产生了PPPOE协议(Point-to-Point Protocol Over Ethernet)。它不仅为使用桥接以太网接入的用户提供了一种宽带接入手段，同时还能提供方便的接入控制和计费。每个接入用户均建立一个独一无二PPP的会话，因此会话建立之前必需知道远端访问集中设备的MAC地址，PPPOE协议可通过发现协议来获取到。

    正如RFC2516中定义的，PPPoE有两个明显的阶段，发现阶段（PPPoE Discovery stage）和PPP会话阶段（PPPoE Session Stage）。

    当主机希望开始一个PPPOE会话时，它首先要执行一个发现过程来识别对方的MAC地址，然后建立一个唯一的PPPOE会话ID。PPPOE使用一个发现协议来解决这个问题，它是基于客户/服务器模型的。由于以太网的广播特性，在这个过程中主机(客户)能发现所有的访问集中器(服务器)，并选择其中一个，根据所获信息在两者之间建立点对点的连接。当一个PPP会话被建立起来之后，就完成了PPPOE的整个发现阶段。

    发现阶段有四个步骤。当它完成时，PPPoE双方都知道了PPPoE SESSION_ID和对方的以太网地址，它们一起唯一定义了PPPoE会话：
    1.  主机广播一个初始数据包PADI（PPPOE Active Discovery Initiation）
    2.  接入集中器发送有效发现提供数据包响应请求PADO（PPPOE Active Discovery Offer）
    3.  用户主机向接入集中器以单播发送会话请求数据包PADR（PPPOE Active Discovery Request）
    4.  被选中的接入集中器发送一个确认数据包PADS（PPPOE Active Discovery Session-confirmation）

    然后PPP经历标准的LCP和NCP（IPCP）过程，下图显示了PPPoE是如何工作的：

    下图是PPPoE连接时抓到的数据包：

   

    PPPOE的会话阶段开始后，主机和访问集中器之间就依据PPP协议传送PPP数据，进行PPP的各项协商和数据传输。在这一阶段传输的数据包中必须包含在发现阶段确定的会话标识并保持不变。正常情况下，会话阶段的结束是由PPP协议控制成的，但在PPPOE中定义了一个PADT （PPPOE Active Discovery Terminate）包用来结会话，主机或者访问集中器可以在PPP会话开始的任何时候通过发送这个数据包来结束会话。

    PPPoE会话过程分为三个阶段：
    1.  主机和接入集中器进行LCP协商
    2.  由接入集中器对用户身份进行认证
    3.  进行IPCP协商并为主机分配一个IP地址

附：RFC2516 - A Method for Transmitting PPP Over Ethernet (PPPoE)

Network Working Group L. Mamakos
Request for Comments: 2516 K. Lidl
Category: Informational J. Evarts
UUNET Technologies, Inc.
D. Carrel
D. Simone
RedBack Networks, Inc.
R. Wheeler
RouterWare, Inc.
February 1999

A Method for Transmitting PPP Over Ethernet (PPPoE)

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

Abstract

The Point-to-Point Protocol (PPP) [1] provides a standard method for
transporting multi-protocol datagrams over point-to-point links.

This document describes how to build PPP sessions and encapsulate PPP
packets over Ethernet.

Applicability

This specification is intended to provide the facilities which are
defined for PPP, such as the Link Control Protocol, Network-layer
Control Protocols, authentication, and more. These capabilities
require a point-to-point relationship between the peers, and are not
designed for the multi-point relationships which are available in
Ethernet and other multi-access environments.

This specification can be used by multiple hosts on a shared,
Ethernet to open PPP sessions to multiple destinations via one or
more bridging modems. It is intended to be used with broadband
remote access technologies that provide a bridged Ethernet topology,
when access providers wish to maintain the session abstraction
associated with PPP.

This document describes the PPP Over Ethernet encapsulation that is
being deployed by RedBack Networks, RouterWare, UUNET and others.

1. Introduction

Modern access technologies are faced with several conflicting goals.
It is desirable to connect multiple hosts at a remote site through
the same customer premise access device. It is also a goal to
provide access control and billing functionality in a manner similar
to dial-up services using PPP. In many access technologies, the most
cost effective method to attach multiple hosts to the customer
premise access device, is via Ethernet. In addition, it is desirable
to keep the cost of this device as low as possible while requiring
little or no configuration.

PPP over Ethernet (PPPoE) provides the ability to connect a network
of hosts over a simple bridging access device to a remote Access
Concentrator. With this model, each host utilizes it's own PPP stack
and the user is presented with a familiar user interface. Access
control, billing and type of service can be done on a per-user,
rather than a per-site, basis.

To provide a point-to-point connection over Ethernet, each PPP
session must learn the Ethernet address of the remote peer, as well
as establish a unique session identifier. PPPoE includes a discovery
protocol that provides this.

2. Conventions

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [2].

3. Protocol Overview

PPPoE has two distinct stages. There is a Discovery stage and a PPP
Session stage. When a Host wishes to initiate a PPPoE session, it
must first perform Discovery to identify the Ethernet MAC address of
the peer and establish a PPPoE SESSION_ID. While PPP defines a
peer-to-peer relationship, Discovery is inherently a client-server
relationship. In the Discovery process, a Host (the client)
discovers an Access Concentrator (the server). Based on the network
topology, there may be more than one Access Concentrator that the
Host can communicate with. The Discovery stage allows the Host to
discover all Access Concentrators and then select one. When
Discovery completes successfully, both the Host and the selected
Access Concentrator have the information they will use to build their
point-to-point connection over Ethernet.

The Discovery stage remains stateless until a PPP session is
established. Once a PPP session is established, both the Host and
the Access Concentrator MUST allocate the resources for a PPP virtual
interface.

4. Payloads

The following packet formats are defined here. The payload contents
will be defined in the Discovery and PPP sections.

An Ethernet frame is as follows:

1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DESTINATION_ADDR |
| (6 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SOURCE_ADDR |
| (6 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ETHER_TYPE (2 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ payload ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CHECKSUM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The DESTINATION_ADDR field contains either a unicast Ethernet
destination address, or the Ethernet broadcast address (0xffffffff).
For Discovery packets, the value is either a unicast or broadcast
address as defined in the Discovery section. For PPP session
traffic, this field MUST contain the peer's unicast address as
determined from the Discovery stage.

The SOURCE_ADDR field MUST contains the Ethernet MAC address of the
source device.

The ETHER_TYPE is set to either 0x8863 (Discovery Stage) or 0x8864
(PPP Session Stage).

The Ethernet payload for PPPoE is as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VER | TYPE | CODE | SESSION_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LENGTH | payload ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The VER field is four bits and MUST be set to 0x1 for this version of
the PPPoE specification.

The TYPE field is four bits and MUST be set to 0x1 for this version
of the PPPoE specification.

The CODE field is eight bits and is defined below for the Discovery
and PPP Session stages.

The SESSION_ID field is sixteen bits. It is an unsigned value in
network byte order. It's value is defined below for Discovery
packets. The value is fixed for a given PPP session and, in fact,
defines a PPP session along with the Ethernet SOURCE_ADDR and
DESTINATION_ADDR. A value of 0xffff is reserved for future use and
MUST NOT be used

The LENGTH field is sixteen bits. The value, in network byte order,
indicates the length of the PPPoE payload. It does not include the
length of the Ethernet or PPPoE headers.

5. Discovery Stage

There are four steps to the Discovery stage. When it completes, both
peers know the PPPoE SESSION_ID and the peer's Ethernet address,
which together define the PPPoE session uniquely. The steps consist
of the Host broadcasting an Initiation packet, one or more Access
Concentrators sending Offer packets, the Host sending a unicast
Session Request packet and the selected Access Concentrator sending a
Confirmation packet. When the Host receives the Confirmation packet,
it may proceed to the PPP Session Stage. When the Access
Concentrator sends the Confirmation packet, it may proceed to the PPP
Session Stage.

All Discovery Ethernet frames have the ETHER_TYPE field set to the
value 0x8863.

The PPPoE payload contains zero or more TAGs. A TAG is a TLV (type-
length-value) construct and is defined as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TAG_TYPE | TAG_LENGTH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TAG_VALUE ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

TAG_TYPE is a sixteen bit field in network byte order. Appendix A
contains a list of all TAG_TYPEs and their TAG_VALUEs.

TAG_LENGTH is a sixteen bit field. It is an unsigned number in
network byte order, indicating the length in octets of the TAG_VALUE.

If a discovery packet is received with a TAG of unknown TAG_TYPE, the
TAG MUST be ignored unless otherwise specified in this document.
This provides for backwards compatibility if/when new TAGs are added.
If new mandatory TAGs are added, the version number will be
incremented.

Some example Discovery packets are shown in Appendix B.

5.1 The PPPoE Active Discovery Initiation (PADI) packet

The Host sends the PADI packet with the DESTINATION_ADDR set to the
broadcast address. The CODE field is set to 0x09 and the SESSION_ID
MUST be set to 0x0000.

The PADI packet MUST contain exactly one TAG of TAG_TYPE Service-
Name, indicating the service the Host is requesting, and any number
of other TAG types. An entire PADI packet (including the PPPoE
header) MUST NOT exceed 1484 octets so as to leave sufficient room
for a relay agent to add a Relay-Session-Id TAG.

5.2 The PPPoE Active Discovery Offer (PADO) packet

When the Access Concentrator receives a PADI that it can serve, it
replies by sending a PADO packet. The DESTINATION_ADDR is the
unicast address of the Host that sent the PADI. The CODE field is
set to 0x07 and the SESSION_ID MUST be set to 0x0000.

The PADO packet MUST contain one AC-Name TAG containing the Access
Concentrator's name, a Service-Name TAG identical to the one in the
PADI, and any number of other Service-Name TAGs indicating other
services that the Access Concentrator offers. If the Access
Concentrator can not serve the PADI it MUST NOT respond with a PADO.

5.3 The PPPoE Active Discovery Request (PADR) packet

Since the PADI was broadcast, the Host may receive more than one
PADO. The Host looks through the PADO packets it receives and
chooses one. The choice can be based on the AC-Name or the Services
offered. The Host then sends one PADR packet to the Access
Concentrator that it has chosen. The DESTINATION_ADDR field is set
to the unicast Ethernet address of the Access Concentrator that sent
the PADO. The CODE field is set to 0x19 and the SESSION_ID MUST be
set to 0x0000.

The PADR packet MUST contain exactly one TAG of TAG_TYPE Service-
Name, indicating the service the Host is requesting, and any number
of other TAG types.

5.4 The PPPoE Active Discovery Session-confirmation (PADS) packet

When the Access Concentrator receives a PADR packet, it prepares to
begin a PPP session. It generates a unique SESSION_ID for the PPPoE
session and replies to the Host with a PADS packet. The
DESTINATION_ADDR field is the unicast Ethernet address of the Host
that sent the PADR. The CODE field is set to 0x65 and the SESSION_ID
MUST be set to the unique value generated for this PPPoE session.

The PADS packet contains exactly one TAG of TAG_TYPE Service-Name,
indicating the service under which Access Concentrator has accepted
the PPPoE session, and any number of other TAG types.

If the Access Concentrator does not like the Service-Name in the
PADR, then it MUST reply with a PADS containing a TAG of TAG_TYPE
Service-Name-Error (and any number of other TAG types). In this case
the SESSION_ID MUST be set to 0x0000.

5.5 The PPPoE Active Discovery Terminate (PADT) packet

This packet may be sent anytime after a session is established to
indicate that a PPPoE session has been terminated. It may be sent by
either the Host or the Access Concentrator. The DESTINATION_ADDR
field is a unicast Ethernet address, the CODE field is set to 0xa7
and the SESSION_ID MUST be set to indicate which session is to be
terminated. No TAGs are required.

When a PADT is received, no further PPP traffic is allowed to be sent
using that session. Even normal PPP termination packets MUST NOT be
sent after sending or receiving a PADT. A PPP peer SHOULD use the
PPP protocol itself to bring down a PPPoE session, but the PADT MAY
be used when PPP can not be used.

6. PPP Session Stage

Once the PPPoE session begins, PPP data is sent as in any other PPP
encapsulation. All Ethernet packets are unicast. The ETHER_TYPE
field is set to 0x8864. The PPPoE CODE MUST be set to 0x00. The
SESSION_ID MUST NOT change for that PPPoE session and MUST be the
value assigned in the Discovery stage. The PPPoE payload contains a
PPP frame. The frame begins with the PPP Protocol-ID.

An example packet is shown in Appendix B.

7. LCP Considerations

The Magic Number LCP configuration option is RECOMMENDED, and the
Protocol Field Compression (PFC) option is NOT RECOMMENDED. An
implementation MUST NOT request any of the following options, and
MUST reject a request for such an option:

Field Check Sequence (FCS) Alternatives,

Address-and-Control-Field-Compression (ACFC),

Asynchronous-Control-Character-Map (ACCM)

The Maximum-Receive-Unit (MRU) option MUST NOT be negotiated to a
larger size than 1492. Since Ethernet has a maximum payload size of
1500 octets, the PPPoE header is 6 octets and the PPP Protocol ID is
2 octets, the PPP MTU MUST NOT be greater than 1492.

It is RECOMMENDED that the Access Concentrator ocassionally send
Echo-Request packets to the Host to determine the state of the
session. Otherwise, if the Host terminates a session without sending
a Terminate-Request packet, the Access Concentrator will not be able
to determine that the session has gone away.

When LCP terminates, the Host and Access concentrator MUST stop using
that PPPoE session. If the Host wishes to start another PPP session,
it MUST return to the PPPoE Discovery stage.

8. Other Considerations

When a host does not receive a PADO packet within a specified amount
of time, it SHOULD resend it's PADI packet and double the waiting
period. This is repeated as many times as desired. If the Host is
waiting to receive a PADS packet, a similar timeout mechanism SHOULD
be used, with the Host re-sending the PADR. After a specified number
of retries, the Host SHOULD then resend a PADI packet.

The ETHER_TYPEs used in this document (0x8863 and 0x8864) have been
assigned by the IEEE for use by PPP Over Ethernet (PPPoE). Use of
these values and the PPPoE VER (version) field uniquely identify this
protocol.

UTF-8 [5] is used throughout this document instead of ASCII. UTF-8
supports the entire ASCII character set while allowing for
international character sets as well. See [5] for more details.

9. Security Considerations

To help protect against Denial of Service (DOS) attacks, the Access
Concentrator can employ the AC-Cookie TAG. The Access Concentrator
SHOULD be able to uniquely regenerate the TAG_VALUE based on the PADR
SOURCE_ADDR. Using this, the Access Concentrator can ensure that the
PADI SOURCE_ADDR is indeed reachable and can then limit concurrent
sessions for that address. What algorithm to use is not defined and
left as an implementation detail. An example is HMAC [3] over the
Host MAC address using a key known only to the Access > Concentrator.
While the AC-Cookie is useful against some DOS attacks, it can not
protect against all DOS attacks and an Access Concentrator MAY employ
other means to protect resources.

While the AC-Cookie is useful against some DOS attacks, it can not
protect against all DOS attacks and an Access Concentrator MAY employ
other means to protect resources.

Many Access Concentrators will not wish to offer information
regarding what services they offer to an unauthenticated entity. In
that case the Access Concentrator should employ one of two policies.
It SHOULD never refuse a request based on the Service-Name TAG, and
always return the TAG_VALUE that was sent to it. Or it SHOULD only
accept requests with a Service-Name TAG with a zero TAG_LENGTH
(indicating any service). The former solution is RECOMMENDED.

10. Acknowledgments

This document is based on concepts discussed in several forums,
including the ADSL forum.

Copious amounts of text have been stolen from RFC1661, RFC1662 and
RFC2364.

11. References

[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51,
RFC1661, July 1994

[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.

[3] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC2104, February 1998.

[4] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC1700,
October 1994. See also: [url]http://www.iana.org/numbers.html[/url]

[5] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.

Appendix A

TAG_TYPES and TAG_VALUES

0x0000 End-Of-List

This TAG indicates that there are no further TAGs in the list. The
TAG_LENGTH of this TAG MUST always be zero. Use of this TAG is
not required, but remains for backwards compatibility.

0x0101 Service-Name

This TAG indicates that a service name follows. The TAG_VALUE is
an UTF-8 string that is NOT NULL terminated. When the TAG_LENGTH
is zero this TAG is used to indicate that any service is
acceptable. Examples of the use of the Service-Name TAG are to
indicate an ISP name or a class or quality of service.

0x0102 AC-Name

This TAG indicates that a string follows which uniquely identifies
this particular Access Concentrator unit from all others. It may
be a combination of trademark, model, and serial id information,
or simply an UTF-8 rendition of the MAC address of the box. It is
not NULL terminated.

0x0103 Host-Uniq

This TAG is used by a Host to uniquely associate an Access
Concentrator response (PADO or PADS) to a particular Host request
(PADI or PADR). The TAG_VALUE is binary data of any value and
length that the Host chooses. It is not interpreted by the Access
Concentrator. The Host MAY include a Host-Uniq TAG in a PADI or
PADR. If the Access Concentrator receives this TAG, it MUST
include the TAG unmodified in the associated PADO or PADS
response.

0x0104 AC-Cookie

This TAG is used by the Access Concentrator to aid in protecting
against denial of service attacks (see the Security Considerations
section for an explanation of how this works). The Access
Concentrator MAY include this TAG in a PADO packet. If a Host
receives this TAG, it MUST return the TAG unmodified in the
following PADR. The TAG_VALUE is binary data of any value and
length and is not interpreted by the Host.

0x0105 Vendor-Specific

This TAG is used to pass vendor proprietary information. The
first four octets of the TAG_VALUE contain the vendor id and the
remainder is unspecified. The high-order octet of the vendor id
is 0 and the low-order 3 octets are the SMI Network Management
Private Enterprise Code of the Vendor in network byte order, as
defined in the Assigned Numbers RFC[4].

Use of this TAG is NOT RECOMMENDED. To ensure inter-operability,
an implementation MAY silently ignore a Vendor-Specific TAG.

0x0110 Relay-Session-Id

This TAG MAY be added to any discovery packet by an intermediate
agent that is relaying traffic. The TAG_VALUE is opaque to both
the Host and the Access Concentrator. If either the Host or
Access Concentrator receives this TAG they MUST include it
unmodified in any discovery packet they send as a response. All
PADI packets MUST guarantee sufficient room for the addition of a
Relay-Session-Id TAG with a TAG_VALUE length of 12 octets.

A Relay-Session-Id TAG MUST NOT be added if the discovery packet
already contains one. In that case the intermediate agent SHOULD
use the existing Relay-Session-Id TAG. If it can not use the
existing TAG or there is insufficient room to add a Relay-
Session-Id TAG, then it SHOULD return a Generic-Error TAG to the
sender.

0x0201 Service-Name-Error

This TAG (typically with a zero-length data section) indicates
that for one reason or another, the requested Service-Name request
could not be honored.

If there is data, and the first octet of the data is nonzero, then
it MUST be a printable UTF-8 string which explains why the request
was denied. This string MAY NOT be NULL terminated.

0x0202 AC-System-Error

This TAG indicates that the Access Concentrator experienced some
error in performing the Host request. (For example insufficient
resources to create a virtual circuit.) It MAY be included in
PADS packets.

If there is data, and the first octet of the data is nonzero, then
it MUST be a printable UTF-8 string which explains the nature of
the error. This string MAY NOT be NULL terminated.

0x0203 Generic-Error

This TAG indicates an error. It can be added to PADO, PADR or
PADS packets when an unrecoverable error occurs and no other error
TAG is appropriate. If there is data then it MUST be an UTF-8
string which explains the nature of the error. This string MUST
NOT be NULL terminated.

Appendix B

The following are some example packets:

A PADI packet:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xffffffff |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xffff | Host_mac_addr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host_mac_addr (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ETHER_TYPE = 0x8863 | v = 1 | t = 1 | CODE = 0x09 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SESSION_ID = 0x0000 | LENGTH = 0x0004 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TAG_TYPE = 0x0101 | TAG_LENGTH = 0x0000 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

A PADO packet:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host_mac_addr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host_mac_addr (cont) | Access_Concentrator_mac_addr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Access_Concentrator_mac_addr (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ETHER_TYPE = 0x8863 | v = 1 | t = 1 | CODE = 0x07 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SESSION_ID = 0x0000 | LENGTH = 0x0020 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TAG_TYPE = 0x0101 | TAG_LENGTH = 0x0000 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TAG_TYPE = 0x0102 | TAG_LENGTH = 0x0018 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x47 | 0x6f | 0x20 | 0x52 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x65 | 0x64 | 0x42 | 0x61 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x63 | 0x6b | 0x20 | 0x2d |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x20 | 0x65 | 0x73 | 0x68 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x73 | 0x68 | 0x65 | 0x73 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x68 | 0x6f | 0x6f | 0x74 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

A PPP LCP packet: The PPP protocol value is shown (0xc021) but the
PPP payload is left to the reader. This is a packet from the Host to
the Access Concentrator.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Access_Concentrator_mac_addr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Access_Concentrator_mac_addr(c)| Host_mac_addr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host_mac_addr (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ETHER_TYPE = 0x8864 | v = 1 | t = 1 | CODE = 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SESSION_ID = 0x1234 | LENGTH = 0x???? |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PPP PROTOCOL = 0xc021 | PPP payload ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authors' Addresses

Louis Mamakos
UUNET Technologies, Inc.
3060 Williams Drive
Fairfax, VA 22031-4648
United States of America

EMail: [email protected]

Kurt Lidl
UUNET Technologies, Inc.
3060 Williams Drive
Fairfax, VA 22031-4648
United States of America

EMail: [email protected]

Jeff Evarts
UUNET Technologies, Inc.
3060 Williams Drive
Fairfax, VA 22031-4648
United States of America

EMail: [email protected]

David Carrel
RedBack Networks, Inc.
1389 Moffett Park Drive
Sunnyvale, CA 94089-1134
United States of America

EMail: [email protected]

Dan Simone
RedBack Networks, Inc.
1389 Moffett Park Drive
Sunnyvale, CA 94089-1134
United States of America

EMail:[email protected]

Ross Wheeler
RouterWare, Inc.
3961 MacArthur Blvd., Suite 212
Newport Beach, CA 92660
United States of America

EMail: [email protected]

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