本文档展示如何修改ndn-cxx和NFD的源码，添加一个值基本类型的Tag，下面的过程将展示添加一个值为uint64_t类型的Tag SrcAddress。

添加Tag的含义

在NDNLPv2协议中定义了LpPacket，具体的格式如下：

LpPacket = LP-PACKET-TYPE TLV-LENGTH
             *LpHeaderField
             [Fragment]

LpHeaderField = Sequence

Sequence = SEQUENCE-TYPE TLV-LENGTH 8OCTET

Fragment = FRAGMENT-TYPE TLV-LENGTH 1*OCTET
  • 其中Fragment放置的可以是一个NDN报的分片，或者是一个完整的NDN包（NDN包足够小，不需要分片的情况）；
  • *LpHeaderField就是一系列的LpPacket的Header，类似一个变长的数组，所以可以自己定义新的LpHeader；
  • 所以添加一个新的Tag实际上就是添加一个新的LpHeader。

修改ndn-cxx

  • 修改ndn-cxx/lp/tlv.hpp，为将要添加的新Tag分配一个新的TLV类型
enum {
    LpPacket = 100,
    Fragment = 80,
    Sequence = 81,
    FragIndex = 82,
    FragCount = 83,
    PitToken = 98,
    Nack = 800,
    NackReason = 801,
    NextHopFaceId = 816,
    IncomingFaceId = 817,
    CachePolicy = 820,
    CachePolicyType = 821,
    CongestionMark = 832,
    Ack = 836,
    TxSequence = 840,
    NonDiscovery = 844,
    PrefixAnnouncement = 848,
    / add by qjm, for add new tag
    SrcAddress = 852
    /
};

如上所示，我们给新添加的Tag分配了一个TLV type，这个type是唯一标识一个TLV的。并且该值的分配也是需要遵循一定准则的，具体的分配规则可以参考：https://redmine.named-data.net/projects/nfd/wiki/NDNLPv2

Reserved Blocks

Two blocks of TLV-TYPEs have been reserved by link protocols:

  • [80, 100]: 1-octet encoding
  • [800, 1000]: 3-octet encoding

TLV-TYPE numbers for LpHeaderField SHOULD be assigned according to the following rules:

  1. if the field can be safely ignored by a receiver that doesn’t understand the field, pick an unused number in the range [800, 959] whose two least significant bits are 00.
  2. if the field would occur frequently, pick an unused number in the range [81, 99].
  3. otherwise, pick an unused number in the range [800, 959] whose two least significant bits are效果 01.

Note: number assignment for a TLV-TYPE nested within a LpHeaderField is not restricted by the above rules.

  • 修改ndn-cxx/lp/tags.hpp，定义一个新的Tag：
/// add by qjm, for add new tag
typedef SimpleTag<uint64_t, 16> SrcAddressTag;
///
  • 修改ndn-cxx/lp/fields.hpp，添加新Tag对应的Field：
// add by qjm, for add new tag
typedef FieldDecl<field_location_tags::Header,
        uint64_t, tlv::SrcAddress> SrcAddressField;
BOOST_CONCEPT_ASSERT((Field<SrcAddressField>));
//

/** \brief Set of all field declarations.
*/
typedef boost
::mpl::set<
        FragmentField,
        SequenceField,
        FragIndexField,
        FragCountField,
        PitTokenField,
        NackField,
        NextHopFaceIdField,
        IncomingFaceIdField,
        CachePolicyField,
        CongestionMarkField,
        AckField,
        TxSequenceField,
        NonDiscoveryField,
        PrefixAnnouncementField,
        /// add by qjm, for add new tag
        SrcAddressField
        ///
> FieldSet;
  • 修改ndn-cxx/face.cpp => extractLpLocalFields：
template<typename NetPkt>
static void
extractLpLocalFields(NetPkt &netPacket, const lp::Packet &lpPacket) {
    addTagFromField<lp::IncomingFaceIdTag, lp::IncomingFaceIdField>(netPacket, lpPacket);
    addTagFromField<lp::CongestionMarkTag, lp::CongestionMarkField>(netPacket, lpPacket);

    // add by qjm, for add new tag
    addTagFromField<lp::SrcAddressTag, lp::SrcAddressField>(netPacket, lpPacket);
    //
}

上面的静态函数是在一个Face从Transport接收到一个Element，进行解析时调用的，功能是 将收到的LpPacket中的LpHeaderField提取，并保存到指定包的Tag列表当中。

  • 如果插入的Tag要在Interest包中生效，则需要修改ndn-cxx/impl/face-impl.hpp => asyncExpressInterest：
void
asyncExpressInterest(RecordId id, shared_ptr<const Interest> interest,
                     const DataCallback &afterSatisfied,
                     const NackCallback &afterNacked,
                     const TimeoutCallback &afterTimeout) {
    NDN_LOG_DEBUG("<I " << *interest);
    this->ensureConnected(true);

    const Interest &interest2 = *interest;
    auto &entry = m_pendingInterestTable.put(id, std::move(interest), afterSatisfied, afterNacked,afterTimeout, ref(m_scheduler));

    lp::Packet lpPacket;

    addFieldFromTag<lp::NextHopFaceIdField, lp::NextHopFaceIdTag>(lpPacket, interest2);
    addFieldFromTag<lp::CongestionMarkField, lp::CongestionMarkTag>(lpPacket, interest2);

    // add by qjm, for add new tag
    addFieldFromTag<lp::SrcAddressField, lp::SrcAddressTag>(lpPacket, interest2);
    //

    entry.recordForwarding();
    m_face.m_transport->send(finishEncoding(std::move(lpPacket), interest2.wireEncode(),
                                            'I', interest2.getName()));
    dispatchInterest(entry, interest2);
}

上面新添的代码的功能是将一个要发送的包（Interest、Data等继承自TagHost）中的Tag提取处理出来，并作为一个LpHeaderField写入新构造的lpPacket当中。

  • 如果插入的Tag要在Data包中生效，则需要修改ndn-cxx/impl/face-impl.hpp => asyncPutData：
void
asyncPutData(const Data &data) {
    NDN_LOG_DEBUG("<D " << data.getName());
    bool shouldSendToForwarder = satisfyPendingInterests(data);
    if (!shouldSendToForwarder) {
        return;
    }

    this->ensureConnected(true);

    lp::Packet lpPacket;
    addFieldFromTag<lp::CachePolicyField, lp::CachePolicyTag>(lpPacket, data);
    addFieldFromTag<lp::CongestionMarkField, lp::CongestionMarkTag>(lpPacket, data);
    
   /// add by qjm, for add new tag
    addFieldFromTag<lp::SrcAddressField, lp::SrcAddressTag>(lpPacket, data);
    //

    m_face.m_transport->send(finishEncoding(std::move(lpPacket), data.wireEncode(),
                                            'D', data.getName()));
}

上面新添的代码的功能是将一个要发送的包（Interest、Data等继承自TagHost）中的Tag提取处理出来，并作为一个LpHeaderField写入新构造的lpPacket当中。

修改NFD

NFD中需要修改daemon/face/generic-link-service.cpp中的两处代码：

  • GenericLinkService::decodeInterest
void
GenericLinkService::decodeInterest(const Block &netPkt, const lp::Packet &firstPkt,
                                   const EndpointId &endpointId) {
    .....

    /// add by qjm, for add srcAddress Tag
    if (firstPkt.has<lp::SrcAddressField>()) {
        interest->setTag(make_shared<lp::SrcAddressTag>(firstPkt.get<lp::SrcAddressField>()));
    }
    ///

    this->receiveInterest(*interest, endpointId);
}

上面的代码在GenericLinkService解析Interest的时候，提取对应LpPacket中的Field，写入Interest的tags列表当中。

ps: 由于此处演示的是添加一个Tag，只对于Interest生效，所以需要修改这个函数，如果要对Data包或者Nack等包生效，类比上述代码，对GenericLinkService::decodeData，GenericLinkService::decodeNack之类的进行修改即可

  • GenericLinkService::encodeLpFields
void
GenericLinkService::encodeLpFields(const ndn::PacketBase &netPkt, lp::Packet &lpPacket) {
   .....

    /// add by qjm, for add srcAddress Tag
    auto srcAddressTag = netPkt.getTag<lp::SrcAddressTag>();
    if (srcAddressTag != nullptr) {
        lpPacket.add<lp::SrcAddressField>(*srcAddressTag);
    }
    ///

    auto pitToken = netPkt.getTag<lp::PitToken>();
    if (pitToken != nullptr) {
        lpPacket.add<lp::PitTokenField>(*pitToken);
    }
}

上面新添的代码会在发送一个包的时候调用，目的是将Tag提取出来，座位一个LpHeaderField 添加到LpPacket中。

添加成功后的效果

ndef nfc ndef nfcc tag_ndef nfc