Point-to-Point Protocol Extensions Working Group Bruce Thompson Internet Draft Bruce Buffam February 22, 2001 Tmima Koren Expires September 2001 Cisco Systems draft-ietf-pppext-ppp-over-aal2-00.txt PPP over AAL2 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. Internet Drafts may be updated, replaced, or obsolete by other documents at any time. It is not appropriate 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.txt This draft is being submitted as a possible work item to the IETF Audio/Video Transport working group. To subscribe to the mailing list send a message to rem-conf-request@es.net with the line "subscribe" in the body of the message. Archives are available from: ftp://ftp.es.net/pub/mail-archive/rem-conf Copyright Notice Copyright (C) The Internet Society (1999-2000). 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 the use of ATM Adaptation Layer 2 (AAL2) for framing PPP encapsulated packets. Applicability This specification is intended for those implementations which desire to use the facilities which are defined for PPP, such as the Link Control Protocol, Network-layer Control Protocols, authentication, and compression. These capabilities require a point-to-point relationship between the peers, and are not designed for the multi- point relationships which are available in ATM and other multi-access environments. 1. Introduction PPP over AAL5 [2] describes the encapsulation format and operation of PPP when used with the ATM AAL5 adaptation layer. While this encapsulation format is well suited to PPP transport of IP, it is bandwidth inefficient when used for transporting small payloads such as voice. PPP over AAL5 is especially bandwidth inefficient when used with RTP header compression [3]. PPP over AAL2 addresses the bandwidth efficiency issues of PPP over AAL5 for small packet transport by making use of the AAL2 Common Part Sublayer (CPS)[4] to allow multiple PPP payloads to be multiplexed into a set of ATM cells. 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 [6]. 3. AAL2 Layer Service Interface The PPP layer treats the underlying ATM AAL2 layer service as a bit- synchronous point-to-point link. In this context, the PPP link corresponds to an ATM AAL2 virtual connection. The virtual connection MUST be full-duplex, point to point, and it MAY be either dedicated (i.e. permanent, set up by provisioning) or switched (set up on demand). In addition, the PPP/AAL2 service interface boundary MUST meet the following requirements: Interface Format - The PPP/AAL2 layer boundary presents an octet service interface to the AAL2 layer. There is no provision for sub-octets to be supplied or accepted. Transmission Rate - The PPP layer does not impose any restrictions regarding transmission rate or the underlying ATM layer traffic descriptor parameters. Control Signals - The AAL2 layer MUST provide control signals to the PPP layer which indicate when the virtual connection link has become connected or disconnected. These provide the "Up" and "Down" events to the LCP state machine [1] within the PPP layer. In the case of PPP over AAL2, the state of the link can be Derived from the type 3 fault management packets carried in-band within a given AAL2 CID flow. 4. PPP Operation with AAL2 PPP over AAL2 defines an encapsulation that uses the Segmentation and Reassembly Service Specific Convergence Sublayer (SSSAR) [5] for AAL type 2. The SSSAR sublayer is used to segment PPP packets into frames that can be transported using the AAL2 CPS. The SSSAR sublayer uses different AAL2 UUI code-points to indicate whether a segment is the last segment of a packet or not. The encapsulation used by PPP over AAL2 provides CRC support for PPP payloads. The base encapsulation of PPP over AAL2 provides a 16-bit CRC for PPP payloads. The use of a 16-bit CRC limits the MRU size for PPP over AAL2 implementations to 1500 bytes. In this document, the 16 bit CRC encapsulation for PPP over AAL2 is referred to as CRClong. There are 2 UUI code points assigned from SSSAR to indicate intermediate fragments of a packet and the last fragment of a packet encapsulated using CRClong. Code point 27 indicates intermediate frames of a fragmented packet and code point 26 indicates the last frame of a fragmented packet. The encapsulation format for CRClong is more fully described in section 6.2.1. An additional encapsulation is also defined that provides maximum bandwidth efficiency for small packets. This encapsulation may be negotiated by PPP over AAL2 endpoints as an alternative to CRClong for packets less than or equal to 32 bytes. This encapsulation uses an 8 bit CRC instead of a 16 bit CRC. In this document, the 8 bit CRC encapsulation is referred to as CRCshort. When CRCshort encapsulation is negotiated for PPP over AAL2 sessions, packets less than or equal to 32 bytes MAY be encapsulated using the CRCshort format. Packets greater than 32 bytes MUST still be encapsulated using CRClong format. Since CRCshort packets are less than the fragmentation length of SSSAR, they will never be fragmented. Because of this, the CRCshort encapsulation uses a single UUI code point. UUI code point 25 is used to indicate a CRCshort packet. The encapsulation format for CRCshort is more fully described in section 6.2.2. An implementation of PPP over AAL2 MAY use a single AAL2 Channel Identifier (CID) for transport of all PPP packets. A PPP over AAL2 implementation may also use multiple AAL2 CIDs to carry a single PPP session. Multiple CIDs could be used to implement a multiple class real time transport service for PPP using the AAL2 layer for link fragmentation and interleaving.A companion document [10] describes class extensions for PPP over AAL2 using multiple AAL2 CIDs. 5. Comparison of PPP over AAL2 with existing encapsulations This document proposes the substitution of AAL2 transport for PPP in scenarios where small packets are being transported over an ATM network. This is most critical in applications such as voice transport using RTP [9] where RTP Header compression [5] is used. In applications such as voice transport, both bandwidth efficiency and low delay are very important. This section provides justification for the PPP over AAL2 service for ATM transport by comparing it to existing PPP encapsulation formats used for transport over ATM. Two encapsulation formats that will be examined here. They are: PPP over AAL5 [2], and PPP with PPPMUX [8] over AAL5. 5.1 Comparison with PPP over AAL5 This proposal uses ATM AAL2 rather than AAL5 as the transport for PPP. The header efficiency of the short payload encapsulation with SSSAR and the AAL2 CPS provides for less ATM encapsulation overhead per PPP payload. The short payload encapsulation consists of a 1 byte CRC. The AAL2 CPS header consists of 3 bytes, and the Offset field is 1 byte. This is a total encapsulation overhead of 6 bytes. This compares to 8 bytes of overhead for the AAL5 trailer used for PPP over AAL5. The multiplexing function of the AAL2 CPS layer allows more bandwidth efficient transport of CRTP frames by multiplexing multiple CRTP frames into one or more ATM cells using the AAL2 CPS function. The removes the pad overhead of AAL5 when used to transport short frames. 5.2 Comparison with PPPMUX over AAL5 A new method for doing multiplexing in the PPP layer has been adopted in the PPP Extensions working group. The draft is called the PPP Multiplexed Frame Option [8]. PPP Multiplexing provides similar functionality to the CPS based multiplexing function of AAL2. Using PPP multiplexing, a PPP stack would look like PPP/PPPMUX/AAL5. Both PPP/PPPMUX/AAL5 and PPP/AAL2 use multiplexing to reduce the overhead of cell padding when frames are sent over an ATM virtual circuit. However, the bandwidth utilization of PPP/AAL2 will typically be better than the bandwidth used by PPP/PPPMUX/AAL5. This is because multiplexed frames in PPP/PPPMUX/AAL5 must always be encapsulated within an AAL5 frame before being sent. This encapsulation causes an additional 8 bytes of AAL5 trailer to be added to the PPPMUX encapsulation. In addition to the 8 bytes of AAL5 trailer, PPPMUX will incur an average of 24 additional bytes of AAL5 PAD. These 2 factors will end up reducing the effective efficiency of PPPMUX when it is used over AAL5. With PPP/AAL2, the AAL2 CPS layer treats individual PPP frames as a series of CPS payloads that can be multiplexed. As long as PPP frames arrive at the CPS layer before the CPS TIMER_CU expires, all ATM cells coming from the CPS layer will be filled. Under these conditions, PPP/AAL2 will have no PAD associated with it. When the AAL2 CPS function causes a PAD to be generated, PPP/AAL2 will be more bandwidth efficient than PPP/PPPMUX/AAL2. In PPP/PPPMUX/AAL5, the AAL5 SAR and the PPP MUX/DEMUX are performed in two different layers. Thus, the PPPMUX/AAL5 receiver must reassemble a full AAL5 frame from the ATM layer before the PPPMUX layer can extract the PPP payloads. To derive maximum PPP Multiplexing efficiency, many PPP payloads may be multiplexed together. This increases the size of the multiplexed frame to many ATM cells. If one of these ATM cells is lost, the whole PPPMUX packet will be discarded. Also, there may be a significant delay incurred while the AAL5 layer waits for many ATM cell arrival times until a full frame has been assembled before the full frame is passed up to the PPP Multiplexing layer where the inverse PPP demux then occurs. This same issue incurs for the PPPMUX/AAL5 frames progressing down the stack. With AAL2, both the segmentation and reassembly and multiplexing functions are performed in the AAL2 CPS layer. Because of the definition of the AAL2 CPS function, a multiplexed payload will be extracted as soon as it is received. The CPS receiver does not wait until the many payloads of an AAL2 multiplexed frame are received before removing payloads from the multiplexed stream. The same benefit also applies to AAL2 CPS sender implementations. Also, the loss of an ATM cell causes the loss of the packets that are included in that cell only. The AAL2 CPS function provides multiplexing in AAL2. This function often needs to be implemented in hardware for performance reasons. Because of this, a PPP/AAL2 implementation that takes advantage of an AAL-2 SAR implemented in hardware will have significant performance benefits over a PPP/PPPMUX/AAL5 implementation where PPPMUX is implemented in software. Also, the AAL2 specification has been available significantly longer than the PPP Multiplexing specification and because of this, optimized software and hardware implementations of the AAL2 CPS function are further in development than PPP Multiplexing implementations. 6. Detailed Protocol Operation Description 6.1 Background 6.1.1 AAL2 Multiplexing ITU-T I.363.2 specifies ATM Adaptation Layer Type 2. This AAL type provides for bandwidth efficient transmission of low-rate, short and variable length packets in delay sensitive applications. More than one AAL type 2 user information stream can be supported on a single ATM connection. There is only one definition for the sub-layer because it implements the interface to the ATM layer and is shared by more than one SSCS layer. 6.1.2 AAL2 Service Specific Convergence Sub-layers ITU-T I.366.1 and I.366.2 define Service Specific Convergence Sub- layers (SSCS) that operate above the Common Part Sub-layer defined in I.363.2. This layer specifies packet formats and procedures to encode the different information streams in bandwidth efficient transport. As the name implies, this sub-layer implements those elements of service specific transport. While there is only one definition of the Common Part Layer there can be more than one SSCS function defined to run over the CPS Layer. Different CIDs within an AAL-2 virtual circuit MAY run different SSCSs. This proposal uses the SSSAR sublayer of I.366.1 for transport. 6.1.3 AAL2 CPS-PKT Format The CPS-PKT format over AAL2 as defined in I.363.2: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | + + + + | | CID + LI + UUI + HEC + CPS-INFO | | + + + + | | + + + + | | (8) + (6) + (5) + (5) + (45/64 * 8) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Note: The size of the fields denote bit-width The Channel ID (CID) identifies the sub-stream within the AAL2 connection. The Length indication (LI) indicates the length of the CPS-INFO payload. The User-to-User Indication (UUI) carries information between the SSCS/Application running above the CPS. The SSSAR sublayer as defined in I.366.1 uses the following code points: UUI Code-point Packet Content ++++++++++++++ ++++++++++++++ 0-26 Framed mode data, final packet. 27 Framed mode data, more to come. This proposal uses three UUI code-points as follows: UUI Code-point Packet Content ++++++++++++++ ++++++++++++++ 27 Long CRC encapsulation, non-final packet. 26 Long CRC encapsulation, final packet. 25 Short CRC encapsulation. 6.1.4 AAL2 CPS-PDU Format The CPS-PDU format over AAL2 as defined in I.363.2: +-+-+-+~+~+-+-+ +CPS+ CPS-INFO+ +PKT+ + +HDR+ + +-+-+-+~+~+-+-+ | | | +-+-+-+~+~+-+-+ +CPS+ CPS-INFO+ | +PKT+ + +HDR+ + | +-+-+-+~+~+-+-+ | | +-+-+-+~+~+-+-+ +CPS+ CPS-INFO+ | | +PKT+ + +HDR+ + | | +-+-+-+~+~+-+-+ V V V V +-+-+-+-+-+-+-+~+~+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Cell + +S+ + | + Header + OSF + +P+ CPS-PDU Payload | PAD + + (6) +N+ + | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+~+~+ Note: The size of the fields denote bitwidth The CPS-PDU format is used to carry one or more CPS-PKT's multiplexed on a single CPS-PDU. The offset field (OSF) carries the binary value of the offset, measured in number of octets, between the end of the STF and the first start of a CPS-Packet, or in the absence of a first start, to the start of the pad field. The SN bit is used to number (mod 2) the CPS-PDUs. The Parity(P) bit is set to 1 if the parity over the 8 bit STF is odd. 6.2 PPP over AAL2 Encapsulation PPP encapsulation over AAL2 uses the AAL2 CPS with no change. Some PPP encapsulated protocols such as RTP header compression require that the link layer provide packet error detection. Because of this, PPP over AAL2 defines a CRC that is used along with the SSSAR sublayer of I.366.1 to provide packet error detection. Two CRC formats are defined. A base 16-bit CRC format and an optional 8-bit CRC format. These 2 encapsulation formats are described below. 6.2.1 Long Payload Encapsulation (CRClong) The CRClong format is the base encapsulation for PPP over AAL2. The long payload encapsulation mode MUST be used for PPP packets when the combined length of the PPP Protocol ID and information fields is larger than 32 bytes. The long payload encapsulation of PPP appends a two byte CRC to each PPP frame before using the SSSAR layer to send the PPP packet as a series of AAL2 frames. The CRC-16 field is computed using the polynomial x^16 + x^12 + x^5 + 1. The format of a PPP over AAL2 packet using the long payload encapsulation format is shown in the diagram below. Note that the diagram below shows the long payload encapsulation when the packet is not segmented (UUI=26). When the PPP packet is segmented, the PPP Protocol ID, Information field, and CRC-16 fields will be split across multiple SSSAR frames. In this case, the UUI field will be set to 27 for all frames except the last frame. In the last frame, the UUI field will be set to 26. Long Payload Encapsulation +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- | + + + + + + | | CID + LI + UUI + HEC + Protocol + + | | + + + + ID + Information + CRC-16 | | + + + + + + | | (8) + (6) + (5) + (5) + (8/16) + + (16) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Note: The size of the fields denote bit-width The CRC-16 field is filled with the value of a CRC calculation which is performed over the contents of the PPP packet, including the PPP Payload ID and the information field. The CRC field shall contain the ones complement of the sum (modulo 2) of: 1) the remainder of x^k (x^15 + x^14 + ... + x + 1) divided (modulo 2) by the generator polynomial, where k is the number of bits of the information over which the CRC is calculated; and 2) the remainder of the division (modulo 2) by the generator polynomial of the product of x^16 by the information over which the CRC is calculated. The CRC-16 generator polynomial is: G(x) = x^16 + x^12 + x^5 + 1 The result of the CRC calculation is placed with the least significant bit right justified in the CRC field. As a typical implementation at the transmitter, the initial content of the register of the device computing the remainder of the division is preset to all "1"s and is then modified by division by the generator polynomial (as described above) on the information over which the CRC is to be calculated; the ones complement of the resulting remainder is put into the CRC field. As a typical implementation at the receiver, the initial content of the register of the device computing the remainder of the division is preset to all "1"s. The final remainder, after multiplication by x^16 and then division (modulo 2) by the generator polynomial of the serial incoming PPP packet, will be (in the absence of errors): C(x) = x^15 + x^14 + x^12 + x^11 + x^10 + x^8 + x^6 + x^5 + x^4 + x^3 + x + 1 6.2.2 Short Payload Encapsulation (CRCshort) The short payload encapsulation is negotiated by the endpoints of a PPP over AAL2 session, the short payload encapsulation format MAY be used for PPP packets when the combined length of the PPP Protocol ID and information fields is less than or equal to 32 bytes. The short payload encapsulation of PPP appends a 1 byte CRC to each PPP frame before using the SSSAR layer to send the PPP packet as a series of AAL2 frames. The format of a PPP over AAL2 packet using the short encapsulation format is shown in the diagram below. Note that short packets are not segmented, and always use UUI=25. Short Payload Encapsulation +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- | + + + + + + | | CID + LI + UUI + HEC + Protocol + + | | + + + + ID + Information + CRC-8 | | + + + + + + | | (8) + (6) + (5) + (5) + (8/16) + + (8) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Note: The size of the fields denote bit-width The CRC-8 field is filled with the value of a CRC calculation which is performed over the contents of the PPP packet, including the PPP Payload ID and the information field. The CRC field shall contain the ones complement of the sum (modulo 2) of: 1) the remainder of x^k (x^7 + x^6 + ... + x + 1) divided (modulo 2) by the generator polynomial, where k is the number of bits of the information over which the CRC is calculated; and 2) the remainder of the division (modulo 2) by the generator polynomial of the product of x^8 by the information over which the CRC is calculated. The CRC-8 generator polynomial is: G(x) = x8 + x2 + x + 1 The result of the CRC calculation is placed with the least significant bit right justified in the CRC field. As a typical implementation at the transmitter, the initial content of the register of the device computing the remainder of the division is preset to all "1"s and is then modified by division by the generator polynomial (as described above) on the information over which the CRC is to be calculated; the ones complement of the resulting remainder is put into the CRC field. As a typical implementation at the receiver, the initial content of the register of the device computing the remainder of the division is preset to all "1"s. The final remainder, after multiplication by x^8 and then division (modulo 2) by the generator polynomial of the serial incoming PPP packet, will be (in the absence of errors): C(x) = x^6 + x^4 + x^2 + 1 6.3 Use of AAL2 CPS-PKT CIDs An implementation of PPP over AAL2 MAY use a single AAL2 Channel Identifier (CID) or multiple CIDs for transport of all PPP packets. In order for the endpoints of a PPP session to work with AAL2, they MUST both agree on the number, SSCS mapping, and values of AAL2 CIDs that will be used for a PPP session. The values of AAL2 CIDs to be used for a PPP session MAY be obtained from either static provisioning in the case of a dedicated AAL2 connection (PVC) or from Q.2630.1 [7] signaling in the case of an AAL2 switched virtual ciruit (SPVC or SVC). Extensions to Q.2630.1 are required to allow the specification of an application type in the Segmentation and Reassembly sublayers (SSSAR) field of the AAL type 2 signaling protocol parameters. The specification of this field can be found in sections 7.4.8 and 7.4.9 of Q.2630.1. Using this proposal it is possible to support the use of conventional AAL2 in CIDs that are not used to support PPP over AAL2. This proposal to allows the co-existence of multiple types of SSCS function within the same AAL2 VCC. 6.4 PPP over AAL2 Operation PPP operation with AAL2 will perform basic PPP encapsulation with the PPP protocol ID. If CRCshort encapsulation has been negoiated, it MAY then perform a test on the resulting payload to determine whether the payload will be sent using the CRCshort encapsulation or the CRClong encapsulation. Packets less than or equal to 32 bytes MAY be sent using CRCshort encapsulation. If CRCshort encapsulation has not been negotiated, all packets MUST be sent using the CRClong encapsulation. PPP frames greater than 32 bytes MUST be encapsulated using the CRClong encapsulation independent of whether CRCshort has been negotiated. Both payload encapsulations MUST use the SSSAR sublayer of AAL2 for transport. Applications implementing PPP over AAL2 MUST meet all the requirements of PPP [1]. 7. Example implementation of PPP/AAL2 This section describes an example implementation of how PPP can be carried over AAL2 using the short and long payload encapsulations described in this document. This example assumes that CRCshort encapsulation format has been negotiated by the endpoints of this PPP session. The example shows 2 application stacks generating IP packets that are sent to the same interface running PPP/AAL2. One application stack is generating RTP packets and another application is generating IP Datagrams. The PPP/AAL2 interface shown in this example is running an RFC 2508 compliant version of RTP header compression. Here are the paths an Application packet can take in this implementation: +---+---+---+---+--+ + | Application A | | +---+---+---+---+--+ | | RTP | | +---+---+---+---+--+ +---+---+---+---+---+ Application | UDP | | Application B | | +---+---+---+---+--+ +---+---+---+---+---+ | | IP | | IP | | +---+---+---+---+--+ +---+---+---+---+---+ + | | +---------------+------------+ | | +---+---+---+---+---+--+ + | Compression Filter | | +---+---+---+---+---+--+ | | | | | +---------+-----------+ | | | | Compression | | Non-Compression | Interface V | Interface | +---+---+---+---+---+---+ | | | CRTP | | | +---+---+---+---+---+---+---+---+---+---+---+---+ Transport | PPP | | +---+---+---+---+---+---+---+---+---+---+---+---+ | | | +---+---+---+---+---+--+ | | Length Filter | | +---+---+---+---+---+--+ | | | +-----------------+----------------------+ | | | | +---+---+---+---+---+---+---+ +--+---+---+---+---+--+--+-+ | |Short Encapsulation (SSSAR)| |Long Encapsulation (SSSAR)| | +---+---+---+---+---+---+---+ +--+---+---+---+---+--+--+-+ | +---+---+---+---+---+---+---+---+---+---+---+---+---+----+ | | AAL2 CPS | | +---+---+---+---+---+---+---+---+---+---+---+---+---+----+ | | ATM Layer | | +---+---+---+---+---+---+---+---+---+---+---+---+---+----+ + In the picture above, application A is an RTP application generating RTP packets. Application B is an IP application generating IP datagrams. Application A gathers the RTP data and formats an RTP packet. Lower level layers of application A add UDP and IP headers to form a complete IP packet. Application B is generating datagrams to the IP layer. These datagrams have neither a UDP header or an RTP header. In the above picture, a protocol stack is configured to apply CRTP/PPP/AAL2 compression on an interface to a destination host. All packets that are sent to this interface will be tested to see if they can be compressed using RTP header compression. As packets appear at the interface, they will be tested by a compression filter to determine if they are candidates for header compression. If the compression filter determines that the packet is a candidate for compression, the packet will be sent to the CRTP compressor. If the packet is not a candidate for compression, it will be sent directly to the PPP layer for encapsulation as an IP packet encapsulated in PPP. The destination UDP port number and packet length are examples of criteria that may be used by the compression filter to select the interface. Packets from application A will be sent to the compression interface. The compression interface applies RFC 2508 compliant header compression and then hands the compressed packet to the PPP layer for encapsulation as one of the compressed header types of CRTP. The PPP layer will add the appropriate CRTP payload type for the compressed packet. Packets from application B will be sent directly to the PPP layer for encapsulation as an IP/PPP packet. The PPP layer will add the PPP payload type for an IP packet encapsulated in PPP. The PPP layer next checks the size of the result PPP packet. PPP packets less than or equal to 32 bytes will use the short payload encapsulation and I.366.1 segmentation with SSSAR. PPP packets greater than 32 bytes will use the long payload encapsulation and I.366.1 segmentation with SSSAR. The resulting AAL2 frame mode PDU is passed down as a CPS SDU to the CPS Layer for multiplexing accompanied by the CPS-UUI and the CPS-CID. The CPS Layer multiplexes the CPS-PKT onto a CPS-PDU. CPS-PDUs are passed to the ATM layer as ATM SDUs to be carried end-to-end across the ATM network. At the receiving end, the ATM SDU's arrive and are passed up to the AAL2 CPS. As the AAL2 CPS PDU is accumulated, complete CPS-PKT's are reassembled by the SSSAR SSCS. Reassembled packets are checked for errors using the CRC algorithm for short or long payloads depending on the UUI code-point used in the frames in the SSSAR sequence. At this point, the PPP layer on the receiving side uses the PPP payload type to deliver the packet to either the CRTP decompressor or the IP layer depending on the value of the PPP payload type. 8. LCP Configuration Options By default, PPP over AAL2 will use the CRClong encapsulation for all packets. The CRClong encapsulation uses a 16 bit CRC. Optionally an implementation may wish to use the CRCshort encapsulation for small packets. To use the CRCshort encapsultion for small packets, an implementation MUST request the FCS option: Field Check Sequence (FCS) Alternatives The current suboptions are: 1 Null FCS 2 CCITT 16-bit FCS 4 CCITT 32-bit FCS Two new suboptions specific to PPP over AAL2 will be added: 32 8-bit CRC 64 16-bit CRC An implementation that wishes to use the CRCshort encapsulation must indicate this by setting the new 8-bit and 16-bit CRC flags in the options field of the Frame Check Sequence Configuration option. The Maximum-Receive-Unit (MRU) option MUST NOT be negotiated to a larger size than 1500. 9. Acknowledgements The authors would like to thank Rajesh Kumar, Mike Mclaughlin, Pietro Schicker, James Carlson and John O'Neil for their contributions to this proposal. 10. References [1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [2] Gross, G., Editor, "PPP over AAL5", STD 51, RFC 2364, July 1998. [3] S. Casner, V. Jacobson, "Compressing IP/UDP/RTP Headers for Low-Speed Serial Links", RFC2508, February 1999. [4] ITU-T, "BISDN ATM Adaptation layer specification: Type 2 AAL(AAL2)", September 1997. [5] ITU-T, "Segmentation and Reassembly Service Specific Convergence Sublayer for the AAL type 2", June 1998. [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [7] ITU-T, "DRAFT NEW ITU-T RECOMMENDATION Q.2630.1", July 1999 [8] R. Pazhyannur, I. Ali, Craig Fox, "PPP Multiplexed Frame Option", draft-ietf-pppext-pppmux-00.txt, January 2000. [9] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC1889, January 1996. [10] B Thompson, T Koren, B Buffam, "Class Extensions for PPP over AAL2", draft-brucet-pppext-ppp-over-aal2-class-00.txt, March 2001 11. Authors' Addresses Bruce Thompson Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 USA Phone: +1 408 527-0446 Email: brucet@cisco.com Bruce Buffam Cisco Systems, Inc. 365 March Road Kanata, Ontario, Canada, K2K-2C9 Phone: +1 613 271-3412 Email: bbuffam@cisco.com Tmima Koren Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 USA Phone: +1 408 527-6169 Email: tmima@cisco.com 12. Full Copyright Statement Copyright (C) The Internet Society (2000). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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draft-ietf-pppext-ppp-over-aal2-00
This is an older version of an Internet-Draft that was ultimately published as RFC 3336.
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